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Kristina Kisel
DissertationsDepartment of ChemistryUniversity of Eastern Finland
No. 152 (2019)
122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium dichloride and its performance as a support in the Ziegler-Natta catalytic system123/2014 PIRINEN Sami: Studies on MgCl2/ether supports in Ziegler–Natta catalysts for ethylene polymerization124/2014 KORPELA Tarmo: Friction and wear of micro-structured polymer surfaces125/2014 HUOVINEN Eero: Fabrication of hierarchically structured polymer surfaces126/2014 EROLA Markus: Synthesis of colloidal gold and polymer particles and use of the particles in preparation of hierarchical structures with self-assembly127/2015 KOSKINEN Laura: Structural and computational studies on the coordinative nature of halogen bonding128/2015 TUIKKA Matti: Crystal engineering studies of barium bisphosphonates, iodine bridged ruthenium complexes, and copper chlorides129/2015JIANGYu:Modificationandapplicationsofmicro-structuredpolymersurfaces130/2015 TABERMAN Helena: Structure and function of carbohydrate-modifying enzymes 131/2015KUKLINMikhailS.:Towardsoptimizationofmetaloceneolefinpolymerizationcatalystsvia structuralmodifications:acomputationalapproach132/2015SALSTELAJanne:Influenceofsurfacestructuringonphysicalandmechanicalpropertiesof polymer-cellulosefibercompositesandmetal-polymercompositejoints133/2015 CHAUDRI Adil Maqsood: Tribological behavior of the polymers used in drug delivery devices134/2015 HILLI Yulia: The structure-activity relationship of Pd-Ni three-way catalysts for H2S suppression135/2016 SUN Linlin: The effects of structural and environmental factors on the swelling behavior of Montmorillonite-Beidellite smectics: a molecular dynamics approach136/2016 OFORI Albert: Inter- and intramolecular interactions in the stabilization and coordination of palladium and silver complexes: DFT and QTAIM studies137/2016 LAVIKAINEN Lasse: The structure and surfaces of 2:1 phyllosilicate clay minerals138/2016 MYLLER Antti T.: The effect of a coupling agent on the formation of area-selective monolayers of iron a-octabutoxy phthalocyanine on a nano-patterned titanium dioxide carrier139/2016KIRVESLAHTIAnna:Polymerwettabilityproperties:theirmodificationandinfluencesupon water movement140/2016 LAITAOJA Mikko: Structure-function studies of zinc proteins141/2017 NISSINEN Ville: The roles of multidentate ether and amine electron donors in the crystal structure formation of magnesium chloride supports 142/2017 SAFDAR Muhammad: Manganese oxide based catalyzed micromotors: synthesis, characterization and applications143/2017 DAU Thuy Minh: Luminescent coinage metal complexes based on multidentate phosphine ligands144/2017AMMOSOVALena:Selectivemodificationandcontrolleddepositiononpolymersurfaces145/2017 PHILIP Anish: PEI-mediated synthesis of gold nanoparticles and their deposition on silicon oxide supports for SERS and catalysis applications146/2017 RAHMAN Muhammad Rubinur: Structure and function of iron-sulfur cluster containing pentonate dehydratases147/2018 ANKUDZE Bright: Syntheses of gold and silver nanoparticles on support materials for trace analyses using surface-enhanced Raman spectroscopy148/2018 PENTTINEN Leena: Structure determination of enzymes with potential in processing of cell wall compounds149/2018 SIVCHIK Vasily: Tuning the photoluminescence of cyclometalated platinum(II) compounds via axial and non-axial interactions150/2019 MIELONEN Kati: Hierarchically structured polymer surfaces: Curved surfaces and sliding behavior on ice151/2019 CHAKKARADHARI Gomathy: Tuning the emission properties of oligophosphine copper and silver complexes with ancillary ligands
Probing the effect of coordination environment on the photophysical behavior of rhenium(I) luminophores
Kristina K
Isel: Probing the effect of coordination environm
ent on the photophysical behavior of rhenium(I) lum
inophores
152
Probing the effect of coordination environment on the
photophysical behavior of rhenium(I) luminophores
Kristina Kisel
Department of Chemistry
University of Eastern Finland
Joensuu 2019
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Kristina Kisel
Department of Chemistry, University of Eastern Finland
P.O. Box 111, FI-80101 Joensuu, Finland
Supervisors
Prof. Igor Koshevoy, University of Eastern Finland
Assoc. Prof. Elena Grachova, Saint Petersburg State University, Saint Petersburg,
Russia
Referees
Prof. Risto Laitinen, University of Oulu, Oulu, Finland
Prof. Andreas Steffen, Technical University of Dortmund, Dortmund, Germany
Opponent
Kari Rissanen, University of Jyväskylä, Jyväskylä, Finland
To be presented with the permission of the Faculty of Science and Forestry of the
University of Eastern Finland for public criticism in Auditorium N100, Yliopistokatu
7, Joensuu, on May 16th at 12 noon.
Copyright © 2019 Kristina Kisel
ISBN: 978-952-61-3079-8
ISSN: 2242-1033
Grano Oy
Jyväskylä 2019
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ABSTRACT The development of versatile light-emitting compounds and materials is crucial in
various technological fields including optoelectronic devices, photodynamic therapy,
sensing, photocatalysis, bio-imaging, and high-performance solar cells. As a result, a
large number of luminescent dyes were created over the years. Much of the research to
date has focused on transition metal complexes, due to the variety of their molecular
configurations that allows significant freedom in molecular design to attain unique
photophysical characteristics. Among the transition and rare earth metal
photofunctional compounds, the rhenium(I) luminophores have remarkable stability,
low toxicity, and appealing physical properties. Thus, they have attracted ever-
increasing interest from both academic and industrial researchers.
This work explores the synthetic approaches to new rhenium(I)-based luminescent
compounds with enriched photophysical behavior. Taking the archetypal emitter
[Re(CO)3(NN)Cl] (NN = diimine) as a starting point, I attempted to alter the optical
characteristics through two approaches: (i) modification of the coordination sphere of
Re(I) center by means of the diimine and auxiliary ligands, and (ii) introduction of
secondary chromophore units to the parent rhenium motif.
The first strategy, meant to improve the photophysical properties, is based on
incorporation of the peripheral ancillary ligands into the {Re(CO)2(NN)} core, which
bears a common chelating diimine (NN = phenanthroline). A series of
alkynylphosphines served as auxiliary axial ligands with different donor ability and
stereochemistry, which were accessibly modulated by changing the number and length
of alkynyl-phenylene substituents.
The second route of structural modification aims at extending the range of
luminescence colors to the whole visible spectrum. Such wide color-tuning of
[Re(CO)3(NN)Cl] emissions was achieved by integrating secondary chromophore
moieties into the pyridyl-phenanthroimidazole ligand.
Another feasible way to manipulate the photophysical behavior relies on the
construction of homo- and heterometallic rhenium-containing coordination assemblies
through merging two or more metal chromophores. The utilization of flexible
connecting bridges diminishes the communication between the constituting
components, while noncovalent intermolecular interactions substantially diversify the
optical properties of the bimetallic assemblies. On the other hand, coordination-driven
self-assembly using rigid cyanide linkers illustrates a facile way to attain
supramolecular aggregates decorated with multiple {Re(CO)3(NN)} chromophores.
Results from the present project should contribute to a more comprehensive
understanding of the structure-property relationships for the family of rhenium(I)
chromophores. This, in turn, should assist in elaborating novel molecular systems with
advanced photophysical functionality.
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LIST OF ORIGINAL PUBLICATIONS This dissertation is a summary of original publications I−IV.
(1) Kondrasenko, I.; Kisel, K. S.; Karttunen, A. J.; Jänis, J.; Grachova, E. V.;
Tunik, S. P.; Koshevoy, I. O. Rhenium(I) Complexes with Alkynylphosphane
Ligands: Structural, Photophysical, and Theoretical Studies. Eur. J. Inorg.
Chem. 2015, 2015 (5), 864–875.
(2) Kisel, K. S.; Eskelinen, T.; Zafar, W.; Solomatina, A. I.; Hirva, P.; Grachova, E.
V.; Tunik, S. P.; Koshevoy, I. O. Chromophore-Functionalized Phenanthro-
Diimine Ligands and Their Re(I) Complexes. Inorg. Chem. 2018, 57 (11),
6349–6361.
(3) Kisel, K. S.; Melnikov, A. S.; Grachova, E. V.; Hirva, P.; Tunik, S. P.;
Koshevoy, I. O. Linking ReI and Pt
II Chromophores with Aminopyridines: A
Simple Route to Achieve a Complicated Photophysical Behavior. Chem. - A
Eur. J. 2017, 23 (47), 11301–11311.
(4) Kisel, K. S.; Melnikov, A. S.; Grachova, E. V.; Karttunen, A. J.; Doménech-
Carbó, A.; Monakhov, K. Y.; Semenov, V. G.; Tunik, S. P.; Koshevoy, I. O.
Supramolecular Construction of Cyanide-Bridged ReI Diimine
Multichromophores. Inorg. Chem. 2019, 58 (3), 1988–2000.
Author’s contribution The research material for the listed publications was obtained by the author and the
collaborating groups, who performed theoretical, magnetic and electrochemical
investigations. The author was responsible for the synthesis of all the compounds in
publications I-IV (except compounds L9, 7, 9, 11 in publication I). The author carried
out NMR, IR and structural characterization of all novel compounds. The author also
carried out the photophysical measurements for all the complexes discussed in the
thesis. The author participated in analyzing the computational results and in writing the
original publications.
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CONTENTS ABSTRACT .................................................................................................................. 3
LIST OF ORIGINAL PUBLICATIONS ................................................................... 4
ABBREVIATIONS ....................................................................................................... 6
1 INTRODUCTION .............................................................................................. 7
1.1 Modulation of the photophysical behavior of rhenium(I) luminophores through
variation of the ligand environment ............................................................................. 8
1.2 Diimine rhenium(I) complexes: Diimine variation ................................................ 9
1.3 Diimine rhenium(I) complexes: Variation of ancillary ligands ........................... 13
1.3.1 Phosphine derivatives ..................................................................................... 13
1.3.2 Nitrogen-donor derivatives ............................................................................. 16
1.4 Polynuclear complexes built from rhenium(I) diimine blocks ............................ 18
1.4.1 Homometallic rhenium(I) complexes ............................................................. 18
1.4.2 Heterometallic rhenium(I) complexes ............................................................ 20
1.5 Applications ......................................................................................................... 23
1.5.1 Organic light emitting diodes (OLEDs) ......................................................... 23
1.5.2 Sensors............................................................................................................ 23
1.5.3 Photo-activated carbon monoxide releasing molecules (PhotoCORMs) ....... 24
1.5.4 Electrocatalysts ............................................................................................... 25
1.5.5 Bio-imaging .................................................................................................... 26
1.6 Aims of the study ................................................................................................. 27
2 EXPERIMENTAL ........................................................................................... 29
2.1 Syntheses ........................................................................................................... 29
2.1.1 Ligands ........................................................................................................... 29
2.1.2 Metal complexes ............................................................................................. 29
2.2 Characterization ................................................................................................... 33
3 RESULTS AND DISCUSSION ....................................................................... 34
3.1 Alkynylphosphine rhenium(I) complexes ............................................................ 34
3.2 Rhenium(I) complexes based on the extended diimine ligands ........................... 38
3.3 Bichromophore rhenium(I)–platinum(II) complexes based on the rhenium(I)
aminopyridine predecessors ....................................................................................... 44
3.4 Multichromophore rhenium(I) complexes of different nuclearity ....................... 48
4 CONCLUSIONS............................................................................................... 54
ACKNOWLEDGEMENTS
REFERENCES
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ABBREVIATIONS CIE coordinates Chromaticity coordinates
D-A Donor-acceptor
DFT Density functional theory
DMF Dimethylformamide
DMSO Dimethyl sulfoxide
dppz Dipyridophenazine
dpyb Dipyridinebenzene
EL Electroluminescence
ESI-MS Electrospray ionization mass spectrometry
HOMO Highest occupied molecular orbital
ILCT Intraligand charge transfer
IR Infrared
ISC Intersystem crossing
LC Ligand-centered
LLCT Ligand-to-ligand charge transfer
LUMO Lowest unoccupied molecular orbital
MLCT Metal-to-ligand charge transfer
NMR Nuclear magnetic resonance
OLED Organic light emitting diode
PhotoCORM Photo-activated carbon monoxide releasing molecule
PNI 4-(1-Piperidinyl)naphthalene-1,8-dicarboximide
ppy 2-Phenylpyridine
S0 Singlet ground state
SOC Spin-orbit coupling
T1 Lowest triplet excited state
TD-DFT Time-dependent density functional theory
THF Tetrahydrofuran
UV/vis Ultraviolet/visible
XRD X-ray diffraction
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1 INTRODUCTION The chemistry of rhenium began in 1925, when the vacant slot below manganese in the
periodic table was filled by Walter Noddack, Ida Tacke and Otto Berg, who were the
first to isolate this new element from the mineral gadolinite. Occupying the slot below
manganese in the periodic table, rhenium was the last-discovered element that has a
stable isotope,1 namely
185Re with a cosmic abundance of 37.07%. Meanwhile,
187Re
(mole fraction: 62.93%) is radioactive with a half-life of 41 billion years. Rhenium is
among the least abundant elements both in the Earth’s crust and in the solar system.
Natural sources that have a relatively high Re content for its economic extraction as the
primary commodity are extremely scarce.2,3
Currently, Re is commercially obtained as
a by-product in the process of molybdenum refining.
Although rhenium is the congener of manganese, these two elements have very
different chemical properties due to the phenomenon of secondary (supplementary)
periodicity. An important chemical characteristic of rhenium is the presence of several
oxidation states that can readily interconvert under mild conditions. The lower
oxidation states (1, 0, and -1) are typically found in a large number of carbonyl
derivatives and related organometallic compounds, which is similar to the family of
manganese relatives.4
The first coordination compounds of rhenium were discovered in the 1930s. In 1939,
Walter Hieber and colleagues reported the synthetic route to the distorted octahedral
rhenium(I) complexes [Re(CO)5Hal] by treating the corresponding
hexahalogenorhenate(IV) with carbon monoxide.5 This work laid the foundation of a
successful research direction for rhenium(I) carbonyl compounds. In 1958, G.
Wilkinson described the substitution reactions that convert [Re(CO)5I] into the di-
pyridine derivative fac-[Re(CO)3(py)2I].6 Further, R. Pietropaolo confirmed the
preferential fac-configuration of the rhenium(I) tricarbonyl compounds (Figure 1) by
IR spectroscopic measurements, and rationalized the result by analysis of the π backdonation between the rhenium atom and the carbonyl ligands.
7
Figure 1. fac-Configuration of the complex [Re(CO)3(py)2I].
6
In the 1970s, Wrighton and coworkers prepared the tricarbonyl diimine rhenium(I)
complexes [Re(NN)(CO)3L] (L = axial ligand) and performed the first systematic study
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of their photophysical behaviors. As a result, they discovered a diverse class of Re-
based organometallic luminescent materials.8
1.1 Modulation of the photophysical behavior of rhenium(I) luminophores through variation of the ligand environment
Upon photoexcitation, most rhenium(I) carbonyl diimine complexes can undergo
several types of electronic transitions, such as ligand-centered9,10
(LC) transitions,
metal-to-ligand,8,11,12
intraligand,13,14
and ligand-to-ligand15,16
charge transfers (MLCT,
ILCT, and LLCT, respectively).17
The intense high-energy absorption bands in the near-UV region (200–300 nm) for
these compounds are ascribed to the π→π* LC transition, while the lower energy
bands correspond to the dπ(Re)→π*(diimine) MLCT transitions. The 1MLCT and
1LC
excited states can be populated simultaneously. The presence of a heavy atom like
rhenium in organometallic compounds increases spin-orbit coupling (SOC) and
thereby favors intersystem crossing (ISC) from the singlet to the lowest triplet state
(T1). The T1 state is responsible for the radiative de-excitation process of T1→S0
(Scheme 1).18
As a rule, the 3MLCT emitters exhibit broad, structureless emission bands, whereas an
appreciable 3LC contribution leads to the appearance of a vibronic progression in the
emission profiles. The 3MLCT and
3LC states often have comparable energies and
therefore their mixture accounts for the photophysical behavior of rhenium
luminophores.
Scheme 1. The state energy diagrams.
1MLCT and
1LC are the lowest excited singlet
states, 3MLCT and
3LC are the lowest excited triplet states, kISC is the intersystem-
crossing rate constant, kr and knr are the radiative and non-radiative rate constants,
respectively.19
Strong ILCT electronic transitions often originate in complexes that contain extended
donor-acceptor (D-A) diimines as chelating ligands. The charge transfer characteristics
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depend on the energy levels of both donor and acceptor units, and can be fine-tuned by
altering one or both of them or by modulating the donor-acceptor communication.20
The co-existence of reducing- and oxidizing ligands in rhenium(I) complexes gives rise
to a low-lying LLCT excited state, by virtue of charge transfer from the donor
(ancillary ligand that predominantly accommodates the highest occupied molecular
orbital, HOMO) to the acceptor (diimine, where the lowest unoccupied molecular
orbital, LUMO, is localized). The large structural distortion of the excited state and its
low energy typically lead to a fast non-radiative deactivation process, making the 3LLCT states very short-lived. The LLCT states, which are “dark” in general, are
mainly studied by transient spectroscopy or indirectly by analyzing their influence on
the MLCT excited state lifetimes of the emissive species.17,21
In general, the electronic properties and the physical behavior of rhenium(I)
coordination compounds can be substantially influenced by changing the nature of the
ligand environment. In other words, variations of the non-carbonyl bidentate as well as
the ancillary ligands considerably affect the photoluminescence energies, lifetimes, and
quantum efficiencies; thereby allowing delicate tuning of the photophysical properties.
1.2 Diimine rhenium(I) complexes: Diimine variation
Many studies have tried to establish the relationships between the
structure/composition and the electronic/photophysical characteristics of luminescent
rhenium(I) species. A majority of these studies focus on the photophysics and
photochemistry of rhenium(I) diimine complexes.
Compounds of the general formula [Re(NN)(CO)3X] are conventionally prepared by
heating [Re(CO)5X] in the presence of the corresponding diimine. These carbonyl
derivatives are probably the most extensively investigated family of Re complexes
during the last 40 years, due to their facile synthesis, high stability, and tunability of
the photophysical characteristics by accessible modification of the diimine ligand.
In 1974, Wrighton and Morse published a pioneering report on the photophysics of
rhenium(I) systems based on 1,10-phenanthroline and related ligands. They
demonstrated that those chromophores undergo efficient radiative relaxation in their
lowest electronically excited state. The observed luminescence characteristics
suggested that the emission is associated with the Re → π*(diimine) charge transfer
state. In addition, the luminescence lifetimes and the quenching experiments were
largely consistent with the triplet → singlet character of the radiative transition. The
authors also described the unusual phenomenon of luminescence rigidochromism—a
strong dependence of the emission energy on the rigidity of the environment.8
In the mid-1980s, Kalyanasundaram presented a thorough account on the absorption
and emission properties of a series of polypyridyl Re(I) complexes, and studied the
sensitivity of their photophysical characteristics to changes in the polypyridyl ligand
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and solvent. Namely, electron-donating substituents in the diimine structure induced
moderate hypsochromic shifts of the low-energy absorption and emission bands,
compared with complexes based on the polypyridyl ligands bearing electron-
withdrawing substituents (Figure 2).
Figure 2. Shift of the emission to changes in the polypyridyl ligands in complexes 1−5
in CH2Cl2.22
Furthermore, increasing the solvent polarity led to an obvious blue-shift in the
absorption. The shorter emission lifetimes and reduced quantum yields in more polar
solvents were ascribed to an increased rate of non-radiative decay to the ground state.
In the early 1990s, Demas and coworkers embarked on a detailed study of the isonitrile
rhenium(I) photosensitizers [Re(CO)3(NN)(NC(CH2)nCH3)]+, obtained by substituting
the chloride ion in the [Re(CO)3(NN)Cl] species with the isonitrile ligand. The
isonitrile complexes showed extremely high luminescence quantum yields up to 80%
and long lifetimes of greater than 130 µs at room temperature, illustrating the
possibility to adjust the photophysical properties of rhenium(I) complexes by varying
the ancillary ligands. The given synthetic approach demonstrated that accessible
structural variations of these types of compounds permit the design of systems with
remarkable optical behavior.23
Functionalization of the diimine ligands remains a popular strategy in the molecular
design of novel rhenium(I) luminophores. The rhenium-based phosphors exhibiting
circularly polarized luminescence are notable examples of this methodology (Figure 3).
The incorporation of rhenium ion into an extended helical π-conjugated bipyridine has
a profound effect on the chiroptical and photophysical properties of the organic motif,
leading to unprecedented circularly polarized phosphorescence.
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Figure 3. Synthesis of rhenium(I) complexes 6 and enantio-enriched 7: (i) Re(CO)5Cl,
toluene, reflux; (ii) AgCF3SO3, EtOH/THF, then 2,6-dimethylphenyl isocyanide, THF,
NH4PF6, 75–80%; (iii) AgBF4, CH3CN, reflux, then pyridine, THF, 80%.24
Although the dRe orbitals do not strongly interact with the electronic π-systems of the
helical polyaromatic fragments, coordination of the metal increases the π-conjugation
area and promotes charge transfer excitations within the extended diimine ligand.24
An interesting case of dramatic alteration in the emission energy was observed for two
isomeric triazole-based rhenium(I) complexes, which contain two forms of the C2N3
heterocycle and consequently different dispositions of the 2-phenylbenzoxazole
substituents. As a result of these stereochemical alterations, the emission band of 8 in
solution is bathochromically shifted by 2390 cm-1
(82 nm) with respect to 9 (Figure 4).
Theoretical calculations pointed out that 8 has a smaller HOMO-LUMO gap than 9, in
line with the experimental observations.25
Figure 4. Absorption (black dotted lines), excitation (blue lines), and emission (red
and green lines) spectra of complexes 8 (top) and 9 (bottom) in dichloromethane;
photographs show the solutions under UV excitation, λ = 365 nm.25
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An impressive case of prolonged excited state lifetime was found in the rhenium(I)
complex bearing the modified 1,10-phenanthroline with the naphthalimide moiety
(Figure 5). The equilibrium between the lower 3LC state of the phen-based diimine and
the 3MLCT emitting states greatly extended the excited state lifetime from 197 ns for
the parent Re(phen)(CO)3Cl to 651 µs for complex 10, but retained the same quantum
yield (1.3%).26
Figure 5. Proposed energy level diagram describing the photophysical processes of 10
in solution at room temperature. Empty arrow: absorption, filled arrow: radiative
transition, and wavy arrow: non-radiative transition.26
Rhenium complexes with bipolar chromophore scaffolds are photophysically diverse
species featuring intraligand charge transfer. The use of donor-functionalized diimines
promoted population in the low-lying ILCT states that are lower in energy than the
MLCT states.
In a series of compounds based on dppz-type ligands (Figure 6), the spectroscopic
properties were dominated by strong ILCT transitions from the electron-donating
amino groups to the electron deficient dppz moiety, while the MLCT transitions were
not perceptible. Unlike the non-coordinated functionalized diimines, these complexes
were weakly emissive.14,20
Figure 6. Schematic representation of complexes 11, 1214
and 13, 1420
featuring ILCT
processes.
The luminescent complexes 15−18 with substituted 1,10-phenanthroline retained the
dominating ligand-centered nature of the low-energy electronic transitions. However, it
was noted that enhancing the electron withdrawing properties of the auxiliary ligand
leads to a significant red-shift of the emission (Figure 7).27
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Figure 7. Schematic representation of complexes 15−18 and their emission maxima in
CH2Cl2 at 293 K.27
Evidently, careful design of diimine ligands, which themselves act as organic dyes,
provides precious means for efficient modulation of the physical behavior of
rhenium(I)-containing luminophores.
Thus, the majority of reported rhenium complexes are represented by either well-
studied 3MLCT chromophores or weakly luminescent emitters with diverse nature in
their excited state (MLCT/ILCT, MLCT/LC, ILCT, and LC).
1.3 Diimine rhenium(I) complexes: Variation of ancillary ligands
In addition to the versatile variations of the diimine framework, changing the ancillary
ligands also proved to be another feasible way to influence the physical and chemical
properties of rhenium diimine species. As a consequence, a large number of these
derivatives bearing carbenes,28,29
alkynyl-,11,30,31
cyano/isocyano-,15,32
and thiolate33,34
groups, phosphines,35–37
and N-heterocycles38–40
have been synthesized. In particular, a
relatively simple modification of the neutral P- and N-donor ligands along with
straightforward coordination chemistry offers a convenient pathway to tune the steric
and electronic properties of the target rhenium(I) chromophores.
1.3.1 Phosphine derivatives
Three practical synthetic methods have been described for preparing the mono- and
disubstituted phosphine rhenium(I) complexes:
1) photochemical substitution;
2) thermal substitution;
3) carbonyl removal using trimethylamine N-oxide.
Originally, Wrighton discovered that 436-nm irradiation of the deoxygenated
acetonitrile solution of [Re(CO)3(phen)(CH3CN)]+ containing PPh3 and [n-
Bu4N]PF6/[n-Bu4N]ClO4 led to formation of the tricarbonyl species
[Re(CO)3(phen)(PPh3)]+(PF6/ClO4)
−.41
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In 2000, the group of Ishitani presented a series of dicarbonyl compounds
[Re(NN)(CO)2(PR3)(L)]+ (L = NCCH3, py, PR´3), which were generated
photochemically from the monosubstituted precursor [Re(NN)(CO)3(PR3)]+ using
higher-energy excitation (λ > 330 nm). This protocol has been particularly suitable for
the synthesis of dicarbonyl complexes that contain two different phosphine ligands.35
Importantly, the reaction can be performed in a two-step manner, first producing the
labile acetonitrile intermediates [Re(NN)(CO)2(PR3)(CH3CN)]+, which can be further
thermally converted into a range of heteroleptic derivatives.
A decade later, the same group used photochemical substitution to synthesize a family
of diimine rhenium(I) complexes bearing two phosphine ligands with a variable
number of phenyl groups, and studied the related photophysical processes.
Figure 8. Rhenium(I) complexes 19−25 with variable number of alkyl and/or phenyl
groups on the phosphine ligands and their emission spectra.37
Upon increasing the number of phenyl substituents NPh from 0 to 6, the intensity of the
π–π* absorption band at around 260 nm was enhanced, and a blue-shift from 427 to
402 nm was observed for the MLCT absorption. Simultaneously, this structural
alteration resulted in visible growth in both the emission energies (Figure 8) and
quantum yields to 600 nm and 9%, respectively.37
An alternative synthetic route was suggested by Sullivan, who synthesized dicarbonyl
rhenium(I) complexes with monodentate or chelating phosphines via a thermal
substitution reaction. A new class of long-lived luminescent compounds was prepared
by refluxing either neutral [Re(CO)5Cl] or cationic [Re(CO)3(NN)(CF3SO3)] starting
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materials with a stoichiometric amount of the appropriative phosphine (Scheme 2). All
the obtained complexes were emissive in the range of 600–640 nm.42
Scheme 2. Routes to the phosphine-diimine dicarbonyl rhenium complexes (i: o-
dichlorobenzene, reflux; ii: AgOTf, dichloromethane).42
Another pathway towards the phosphine-containing dicarbonyl halide complexes of
rhenium(I) was presented in 2015 by Felton and co-authors (Figure 9). It involves
treatment of [Re(CO)3(NN)PR3](PF6) species with 15 equiv. of tetraethylammonium
chloride (NEt4Cl) in the presence of 1 equiv. of trimethylamine N-oxide. This non-
photochemical ligand substitution should be generally applicable for preparing light-
sensitive Re(I) diimine carbonyl compounds.43
Figure 9. Synthesis of rhenium(I) complexes 26−33 using trimethylamine N-oxide.43
The influence of axial ligands on the electronic structures and optical properties of
related rhenium(I) systems showed that, in complexes 34−37, substitution of the
acetonitrile ligand by chloride and that of the carbonyl ligand by a phosphine led to
significant bathochromic shifts of the absorption and emission bands (Figure 10).44
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Figure 10. Left: synthesis of rhenium(I) compounds 34−37 (i: AgOTf, CH3CN, 295 K,
then [NH4][PF6]; ii: AgOTf, acetone, reflux, then PMe3, acetone, reflux, after
[NH4][PF6]; iii: Me3NO, CH3CN, reflux; iv: [BnMe3N]Cl, CH2Cl2, reflux). Right: their
absorption (solid lines) and emission (dashed lines) spectra in CH3CN.44
1.3.2 Nitrogen-donor derivatives
At the end of the 1980s, Zipp, Demas, and DeGraff described various rhenium diimine
complexes decorated with substituted pyridines. These compounds of the general
composition [Re(NN)(CO)3L](ClO4), where L = substituted pyridine, were synthesized
by refluxing the starting chloride precursors [Re(NN)(CO)3Cl] with excess amount of
the appropriate pyridine in the presence of AgClO4. A detailed study of the
photophysical properties showed that the auxiliary pyridine derivatives exerted some
noticeable influence on the luminescence behavior, although the effect was less
pronounced than that caused by modification of the diimine ligands. For example,
small shifts of the emission bands as well as minor changes in the luminescence
lifetimes were observed.38
Variation of the ancillary ligands in the coordination sphere of rhenium complexes can
be realized by modification of the pyridine ligands after their coordination to the
metallocenter. For instance, the weakly emissive diamine parent species
[Re(CO)3(NN)(3,4-diaminopyridine)](CF3SO3) was converted into the triazole
counterparts by reacting with gaseous NO (Figure 11). As a result, an substantial
emission enhancement was observed, meaning that rhenium(I) complexes 38−41 could
be considered as sensors for nitric oxide (NO).40
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Figure 11. Conversion of 3,4-diaminopyridine complexes 38–41 into their
triazolopyridine derivatives 42–45.40
Thus, the introduction of an additional chromophore center or a functional group into
the axial position of rhenium(I) diimine unit may be an easy way to produce emissive
materials with particular activities, e.g. luminescent probes.
In this respect, there is a keen interest in rhenium compounds containing ligands
capable of reversible photoisomerization (e.g. trans→cis) due to their potential uses as
photoinduced molecular sensors and switches. For example, the cis-isomers of
complexes with 4-styrylpyridine are luminescent showing broad and poorly structured
emission profiles, while the majority of the trans−forms undergo nonradiative
relaxation of the excited state.39,45
Upon excitation, the photochemical reaction could
proceed through the population of 1,3
MLCT Re→diimine state and the subsequent
intramolecular energy transfer to 3IL trans−L (photoswitchable ancillary ligand) state
(triplet pathway), or population of the 1IL trans−L state (singlet pathway). Highly
efficient trans→cis isomerization (81%) was obtained for rhenium compound 46 by
irradiation at 365 nm (Figure 12).45
Figure 12. Spectral changes of complex 46 in CH3CN under 356-nm irradiation.45
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1.4 Polynuclear complexes built from rhenium(I) diimine blocks
The multimetallic compounds constitute an interesting class of efficient luminophores.
The construction of such assemblies is commonly based on a “complexes-as-ligands”
concept. In this approach, a di- or polydentate bridging ligand binds the metal cores in
a stepwise fashion. The nature of the linker and the coordination mode strongly
influence the electronic communication between the constituting fragments. For weak
interactions, the photophysical behavior combines properties of the two independent
chromophore centers. In the case of strong interactions, the compounds can be
considered as delocalized electronic systems with characteristics completely different
from their monomeric analogs. The vast majority of reported multi-chromophoric
rhenium metal complexes are based on {Re(NN)(CO)3} units, because of their unique
electronic and steric properties. By changing the linked metal components together
with the stereochemistry of bridging ligands, it is possible to rationally tune the
photophysical and photochemical characteristics.
1.4.1 Homometallic rhenium(I) complexes
The first work devoted to the synthesis of dinuclear homometallic complexes of
rhenium(I) dated from the beginning of 1990s. Kalyanasundaram described the
homonuclear cyano-bridged polypyridyl complex [(CO)3(bpy)Re–CN–Re(bpy)
(CO)3]+, obtained by reaction between the triflate precursor [Re(bpy)(CO)3(CF3SO3)]
and the cyanide complex [Re(bpy)(CO)3(CN)], which served as a metalloligand. It was
found that the charge transfer absorption band in the dinuclear complex was blue-
shifted and considerably broadened, with molar absorptivities being nearly two times
larger than those of the parent mononuclear species. Both the mono- and dinuclear
complexes displayed luminescence with single exponential decay, indicating that there
was no separate emissions from two different chromophores.32
Later, Yam presented a dinuclear rhenium(I) compound containing a bridging
butadiynyl ligand. The target complex 47 (Figure 13), synthesized by cross-coupling
two [Re(tBu2bpy)(CO)3(C≡C)2H] units in the presence of Cu(OAc)2, demonstrated
long-lived red emission from the 3MLCT (dπ(Re)→π*(
tBu2-bpy)) excited state.
Interestingly, upon excitation at λ ≥ 450 nm, an emission band appeared in the NIR
region and was assigned to 3MLCT (d(Re)→π*(C≡C)2) or
3LLCT (π(C≡C)2→π*(
tBu2-
bpy)) transitions.46
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Figure 13. Dirhenium(I) complex 47 with the acetylenic bridging ligand.46
Complexes 48 and 49 (Figure 14) exemplify a dinuclear architecture, where the
rhenium fragments are connected through a flexible bridging bis-diimine ligand. In
low‐polarity solvents, these neutral compounds exhibited an unusual folded
conformation due to the interaction between the amide and one rhenium-carbonyl
group.
Figure 14. Synthesis of the flexible bridging ligands and the corresponding
dirhenium(I) complexes 48 and 49: (i) 1,2-bis(2-aminoethoxy)ethane, NEt3, dry THF,
stirring, 16 h; (ii) [Re(CO)5Br], toluene, reflux, 16 h.47
It was observed that the quantum yields of the complexes were dramatically affected
by the substitution pattern within the diimine ligand. Introducing two electron-
withdrawing groups into the 2,2’-bipyridine motifs turned the weakly luminescent
complex 48 into the non-emissive system 49.47
A substantial contribution to the development of supramolecular chemistry of rhenium
complexes was made by the group of Ishitani, who developed a strategy to synthesize
series of linear- and ring-shaped molecular assemblies.48–50
Polynuclear structures were
constructed by linking the rhenium fragments {Re(CO)2(NN)} with the bidentate
phosphine (PP) ligands. Depending on the flexibility of the PP diphosphines, two main
aggregation scenarios were realized: oligomerization of the linear rhenium(I) systems
and oligonuclear cycle formation. For instance, a multistep synthesis of the trinuclear
Re(I) ring with a rigid bridging PP ligand is illustrated in Scheme 3.
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Scheme 3. Synthesis of the trinuclear Re(I) ring (PP = rigid bidentate phosphine
ligands; i: hν (λ > 330 nm), acetone/H2O; ii: CH3CN; iii: acetone, reflux).49
It is essential that these phosphine-bridged molecular assemblies are triplet emitters,
whose photophysical characteristics are remarkably improved by the strong interligand
π-π interactions. Specifically, in the Re-rings with ethynylene-containing PP ligands,
interactions between the unsaturated chains and the bpy ligands resulted in a 3-fold
enhancement of the emission intensity compared with analogues with alkyl
fragments.48–50
1.4.2 Heterometallic rhenium(I) complexes
The appealing physical properties and rather facile coordination chemistry of
rhenium(I) diimine blocks determined their popularity in the construction of
heterometallic assemblies. A variety of d- and f-metals were connected to the Re(I)
chromophore by means of diverse ligand systems to extend the optical functionality of
the target supramolecular compounds.
In the beginning of the 2000s, Lee and co-workers utilized a coordination-driven
assembly approach for the synthesis of luminescent heterometallic palladium(II)
(50−52) and platinum(II) (53, 54) tetranuclear square-like complexes (Figure 15).
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Figure 15. Synthesis of heterometallic rhenium(I)–platinum/–palladium complexes
50−54.51
Their photophysical properties were dominated by the conjugated bridging ligands. It
was found that squares 50−54 comprising rigid ligands demonstrated weaker
luminescence than that predicted by the energy gap law. The small quantum yields
were interpreted in terms of quenching via Pt(II)- or Pd(II) oxidation, and energy
transfer from the emitting 3MLCT state to lower-lying non-emissive states localized on
the ferrocene groups.51
Yam described bichromophoric polypyridine alkynyl-bridged molecular rods
consisting of rhenium and platinum fragments (Figure 16). These complexes were
prepared by coupling [Re(NN)(CO)3(C≡C-(C6H4)n-C≡CH)] and [Pt(NNN)Cl](CF3SO3)
compounds, where NNN = terpyridine derivatives, in the presence of CuI and the base
Et3N.
Figure 16. The chemical structures of complexes 55−58.52
All dyads displayed orange to red photoluminescence both in fluid media and in the
solid state. The emission energies of complexes 55−58 were similar irrespectively of
the nature of the substituents on the diimine ligand on the rhenium(I) metal. It was
assumed that the observed emissions occur from the 3MLCT excited states, d
(Pt)→π*(NNN), perturbed by the rhenium diimine fragment and perhaps mixed with
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the LLCT π(C≡C-R)→π*(NNN) or the metalloligand-to-ligand charge transfer
π(C≡C–C6H4-C≡C)→π*(NNN).52
Unlike compounds 55–58, the mixed-metal rhenium(I)–gold(I) families 59–62 (Figure
17) displayed luminescence assigned to the 3MLCT d(Re)→π*(NN) excited state.
Figure 17. Chemical structures of gold(I)–rhenium(I) systems with metallocenters
linked through the ancillary (59−62)53
and diimine (63 and 64)54
ligands.
The lack of emission from the alkynylgold(I) unit was explained by the efficient
intramolecular energy transfer from the gold(I) to the rhenium(I) moiety.53,54
Similar energy transfer processes arose in the dinuclear [Re(phen)(CO)3(µ-
CN)Ir(F2ppy)2(CN)] and trinuclear [{Re(phen)(CO)3(µ-CN)}2Ir(F2ppy)2](PF6)
rhenium(I)-iridium(III) systems, in which the chromophore units were linked by the
cyanide bridges.55
A different situation was described for the linear multinuclear rhenium systems
connected to a ruthenium fragment by a bis-diimine ligand (Figure 18). In this case,
energy transfer occurred from the rhenium unit (a photon absorber) to the ruthenium
motif (energy acceptor). Therefore, the emission behavior of these heterometallic
oligomers was mostly determined by the Ru(II) motif.56
Figure 18. The Ru(II)-Re(I) multinuclear complexes 65−67.
56
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1.5 Applications
Feasible modulation of the photophysical properties along with the functionalities and
reactivity of rhenium(I) complexes has made them candidates for a range of practical
applications.
1.5.1 Organic light emitting diodes (OLEDs)
One of the strategies to achieve high-performance OLEDs is to utilize phosphorescent
compounds that have the intrinsic advantage of maximum (100%) internal quantum
efficiency of electroluminescence.57–59
Thermally stable mononuclear rhenium(I) compounds with efficient luminescence
have been used as transition metal complexes in the fabrication of phosphorescent
OLEDs.60–62
Li and co-workers designed neutral Re(I) systems that showed
significantly improved electroluminescent properties63–66
and exhibited strong
electrogenerated emission (Figure 19). Complex 70 has a maximum current, power,
external quantum efficiency, and CIE coordinates of 36.5 cd A−1
, 31.7 lm W−1
, 12.0%,
and (0.46, 0.52), respectively. An EL device based on it was among the best orange
OLEDs with rhenium(I) dopant emitters reported so far.63
Figure 19. Structures of complexes 68−70 used as dopant emitters in
electroluminescent devices.63
1.5.2 Sensors
Luminescent rhenium(I) complexes containing recognition units that interact with
analytes have emerged as a versatile class of molecular sensors.67–70
The examples of rhenium-based probes are designed for selective determination of
metal cations. The chelating coordination function in the ancillary (Figure 20A) or
diimine ligand (Figure 20B) of the rhenium chromophores makes them responsive to
the presence of selected divalent metal cations. The complexation of Cd(II) or Zn(II)
ions by the tetrazolato-quinoline ligands blue-shifts the emission of the rhenium(I)
compound 71, and causes a substantial increase of phosphorescence intensity and
elongation of the emission lifetimes.70
Similarly, the addition of Cu(II) ion to the
chemosensor 72 leads to a significant enhancement in the emission intensity and
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excited state lifetime of the Re(I) complex due to the suppression of C=N
isomerization.67
Figure 20. A: Complexation of complex 71 with Zn(II), and the corresponding
photographs of CH2Cl2 solutions of 71 before and after the addition of Zn(II) salt.70
B:
Complexation of complex 72 with Cu(II).67
1.5.3 Photo-activated carbon monoxide releasing molecules (PhotoCORMs)
Recently, a promising approach to the controlled delivery of CO molecules for
therapeutic applications is using carbonyl organometallic compounds, which are stable
in the dark in aqueous solution for an extended period of time to afford sufficient
accumulation in the target biological structures.
The low toxicity and tunable properties of Re(I)-based carbonyls ensured their growing
usage as PhotoCORMs.71–74
For example, the photolytic liberation of carbon monoxide
from complex 73 (Figure 21) is accompanied by a change of the orange
phosphorescence (605 nm) to blue fluorescence (400 nm). This allows one to track the
entry and delivery of this CO donor within the target localization.72
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Figure 21. Time-resolved luminescence traces at 2 min intervals of 73 in CH3CN upon
UV excitation (λ < 310 nm).72
1.5.4 Electrocatalysis
High-performance catalysts can facilitate reduction of CO2 emission into the
environment. The advantages of utilizing rhenium(I) complexes for electrocatalytic and
photocatalytic reductions are first and foremost determined by the availability of at
least two oxidation states, the presence of accessible coordination site(s), as well as
high selectivity in the conversion of CO2 to CO.75–77
Water solubility played a
substantial role in the development of compound 74 as an electrocatalytic system for
the reduction of CO2, where aqueous solutions acted as the electron source. The titled
complex was used as a molecular catalyst in the electrochemical cell for CO2-to-CO
conversion (Figure 22), which proceeded with high total faradaic efficiency (81%) and
high selectivity of CO2 reduction (98%) even in acidic solution.75
Figure 22. Schematic illustration of the full electrochemical cell for CO2-to-CO
conversion.75
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1.5.5 Bio-imaging
The major reason of exploiting triplet emissive rhenium(I) compounds with the general
formula [Re(diimine)(CO)3(L)]0/1+
as cellular imaging probes is that their
photophysical properties are mainly guided by the bidentate diimine ligand. Thus, the
cellular uptake and localization properties of these complexes can be conveniently
adjusted via functionalization of the monodentate auxiliary ligand L without affecting
the luminescence behavior.78–81
Rhenium(I) complex 75 containing a fructose pendant group displayed the
appropriative photoluminescent characteristics (λems = 553 nm, τ = 2.9 µs, Φ = 18% in
phosphate-buffered saline) as well as high cell permeability and organelle selectivity. It
was successfully applied for imaging breast adenocarcinoma cell lines (Figure 23).
Additionally, the given compound exhibited photocytotoxic activity caused by 1O2
generation upon irradiation at λ>365 nm, making it an efficient photodynamic therapy
agent.82
Figure 23. Confocal images of four different types of cancer cells upon incubation
with complex 75 (50 μM, 1 h) at 310 K.82
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1.6 Aims of the study The rational design and synthesis of rhenium(I) carbonyl species with adjustable
electronic parameters and tailored functional groups are very important for prospective
applications that require molecular materials with certain photophysical and chemical
behavior. The foregoing survey of literature data shows that the diimine ligands
generally play a key role in the photophysics of the archetypal rhenium-based systems
of [Re(CO)3(NN)(L)]. Therefore, electronic changes of the NN constituents are
primarily responsible for variation in the optical characteristics of rhenium derivatives.
However, it is clear that their luminescence can also be substantially influenced by
altering the ancillary ligand(s) or by coupling the emissive Re(I) motifs with additional
chromophore units.
My research focused on the development of rhenium-based luminescent compounds,
with an aim of establishing the relationship between their structures and photophysical
properties. The chosen molecular design relied on the electronic and stereochemical
modifications of the constituting fragments, following the strategy summarized in
Scheme 4.
Scheme 4. Illustration of the research objectives.
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(I) The first objective deals with the methodological approach to efficiently
luminescent rhenium dicarbonyl compounds that are axially substituted with ancillary
phosphine ligands having variable donor ability and the conjugated substituents.
(II) In the second task, I considered systematic alteration of the new type of diimine
ligands tailored to the secondary chromophore system for constructing bichromophore
rhenium dyes containing donor−acceptor organic motifs.
(III) In the third direction, the rhenium diimine tricarbonyl block was utilized to design
series of homo- and heterometallic assemblies through merging two or more metal-
based chromophores. Here, I intended to investigate two approaches, in which the
target polynuclear compounds would differ in the rigidity and degree of conjugation
between luminescent units.
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2 EXPERIMENTAL All reactions (except the heterometallic systems described in publication IV) were
performed under an inert atmosphere of nitrogen using standard Schlenk techniques.
2.1 Syntheses
2.1.1 Ligands
Commercially available diimines (1,10-phenanthroline (phen) and 2,2’-bipyridine
(bpy)) and aminopyridines (4-aminopyridine (pyN1), 4-(aminomethyl)pyridine
(pyN2), 4-(2-aminoethyl)pyridine (pyN3)) were used as received. Ancillary alkynyl-
phosphine ligands (P1,83
P4,84
PP1,85
and PP285
) were prepared according to the
published procedures, which involve lithiation of the terminal alkynes and subsequent
treatment with diphenylchlorophosphine (Scheme 5). The novel phosphines P2, P3,
P5, and P6 were obtained following this general protocol as described in publication I.
Scheme 5. Synthesis of alkynyl-phosphine ligands: (i) n-BuLi dropwise, Et2O/hexane,
195 K, stirring, 5 min, then 243 K, 1.5 h; (ii) PPh2Cl dropwise, 195 K, stirring, then
298 K, stirring, 12 h; (iii) PPhCl2 dropwise, 195 K, stirring, then 298 K, stirring, 2 h.
2.1.2 Metal complexes
Neutral rhenium(I) complexes [Re(NN)(CO)3Cl] (NN = bipyridine, phenanthroline)
were prepared according to the literature procedure8,86
by reacting the corresponding
diimine with the commercially available pentacarbonyl [Re(CO)5Cl]. The cationic
starting materials [Re(NN)(CO)3(NCMe)](CF3SO3)87
and
[Re(NN)(CO)3(H2O)](CF3SO3)88
were obtained by treating their rhenium(I) chloride
precursors with AgCF3SO3 in acetonitrile and wet acetone solvents, respectively.
To prepare the heterobimetallic Re–Pt species presented in publication III, the labile
platinum compounds Pt(ppy)Cl(dmso)89
and [Pt(dpyb)(acetone)](CF3SO3)90
were
obtained as described in the corresponding literature.
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All novel homo- and heterometallic rhenium(I) complexes summarized in Table 1 were
synthesized using the general approaches of coordination chemistry. The details are
given in the original publications I−IV and in the following chapters.
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Tab
le 1. R
hen
ium
(I) com
plex
es obtain
ed in
this stu
dy
N
ame
Com
plex
D
iimin
e
ligan
d
Ancillary
ligan
d
Publica-
tion
Cry
stal
structu
re
1.
R
e(P1)
2 [R
e(phen
)(CO
)2 (P
1)
2 ](CF
3 SO
3 ) phen
P
1
I
2.
R
e(P2)
2 [R
e(phen
)(CO
)2 (P
2)
2 ](CF
3 SO
3 ) phen
P
2
I
3.
R
e(P3)
2 [R
e(phen
)(CO
)2 (P
3)
2 ](CF
3 SO
3 ) phen
P
3
I
4.
R
e(P4)
2 [R
e(phen
)(CO
)2 (P
4)
2 ](PF
6 ) phen
P
4
I
5.
R
e(P5)
2 [R
e(phen
)(CO
)2 (P
5)
2 ](PF
6 ) phen
P
5
I
6.
R
e(P6)
2 [R
e(phen
)(CO
)2 (P
6)
2 ](PF
6 ) phen
P
6
I
7.
(R
ePP
1)
2 [{
Re(p
hen
)(CO
)2 }
2 (PP
1)
2 ](CF
3 SO
3 )2
phen
P
P1
I
8.
(R
ePP
2)
2 [{
Re(p
hen
)(CO
)2 }
2 (PP
2)
2 ](CF
3 SO
3 )2
phen
P
P2
I
9.
R
eNN
1
[Re(C
O)
3 (NN
1)C
l] N
N1
Cl
II
10.
ReN
N1-C
N
[Re(C
O)
3 (NN
1)C
N]
NN
1
CN
II
11.
ReN
N2
[Re(C
O)
3 (NN
2)C
l] N
N2
C
l II
12.
ReN
N3
[Re(C
O)
3 (NN
3)C
l] N
N3
Cl
II
13.
ReN
N4
[Re(C
O)
3 (NN
4)C
l] N
N4
C
l II
14.
ReN
N5
[Re(C
O)
3 (NN
5)C
l] N
N5
C
l II
15.
Re(p
yN
1)
[Re(p
hen
)(CO
)3 (p
yN
1)](C
F3 S
O3 )
phen
pyN
1
III
16.
Re(p
yN
2)
[Re(p
hen
)(CO
)3 (p
yN
2)](C
F3 S
O3 )
phen
pyN
2
III
17.
Re(p
yN
3)
[Re(p
hen
)(CO
)3 (p
yN
3)](C
F3 S
O3 )
phen
pyN
3
III
18.
Re(p
yN
2)P
t(pp
y)
[Re(p
hen
)(CO
)3 (p
yN
2)P
t(ppy)C
l]
(CF
3 SO
3 )
phen
pyN
2
III
19.
Re(p
yN
2)P
t(dp
yb
) [R
e(phen
)(CO
)3 (p
yN
2)P
t(dpyb)]
(CF
3 SO
3 )2
phen
pyN
2
III
20.
Re(p
yN
3)P
t(pp
y)
[Re(p
hen
)(CO
)3 (p
yN
3)P
t(ppy)C
l]
(CF
3 SO
3 )
phen
pyN
3
III
21.
Re(p
yN
3)P
t(dp
yb
) [R
e(phen
)(CO
)3 (p
yN
3)P
t(dpyb)]
(CF
3 SO
3 )2
phen
pyN
3
III
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32
22.
Re
2 −b
py
[{R
e(bpy)(C
O)
3 }2 C
N](C
F3 S
O3 )
bpy
CN
IV
23.
Re
2 −p
hen
[{
Re(p
hen
)(CO
)3 }
2 CN
](CF
3 SO
3 ) phen
C
N
IV
24.
Ph
3 PR
e2 −
ph
en
[{R
e(phen
)(CO
)2 (P
Ph
3 )}2 C
N]
(CF
3 SO
3 )
phen
C
N
IV
25.
ReA
uP
Ph
3 −p
hen
[{
Re(p
hen
)(CO
)3 }
NC
Au(P
Ph
3 )]
(CF
3 SO
3 )
phen
C
N
IV
26.
Re
2 Au
−b
py
[{R
e(bpy)(C
O)
3 NC
}2 A
u](C
F3 S
O3 )
bpy
CN
IV
27.
Re
2 Au
−p
hen
[{
Re(p
hen
)(CO
)3 N
C}
2 Au](C
F3 S
O3 )
phen
C
N
IV
28.
Re
4 Pt−
bp
y
[{R
e(bpy)(C
O)
3 NC
}4 P
t](CF
3 SO
3 )2
bpy
CN
IV
29.
Re
4 Pt−
ph
en
[{R
e(phen
)(CO
)3 N
C}
4 Pt](C
F3 S
O3 )
2 phen
C
N
IV
30.
Re
6 Fe
2−b
py
[{R
e(bpy)(C
O)
3 NC
}6 F
e](CF
3 SO
3 )2
bpy
CN
IV
31.
Re
6 Fe
3−b
py
[{R
e(bpy)(C
O)
3 NC
}6 F
e](CF
3 SO
3 )3
bpy
CN
IV
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33
2.2 Characterization
All novel complexes were studied in solution by NMR and FT-IR spectroscopies and
ionization electrospray mass spectrometry (ESI-MS). Most of the rhenium compounds
in Table 1 were isolated as crystalline materials suitable for single-crystal X-ray
diffraction analysis. Their molecular structures in the solid state were in agreement
with spectroscopic data obtained in the fluid medium. The only exception was the
complex ReAuPPh3−phen (publication IV), which demonstrated reversible
dissociation and rearrangement in solution as confirmed by NMR and ESI-MS
measurements. Phase identification of the ReAuPPh3−phen crystalline material was
carried out by X-ray powder diffraction analysis. The purity of the studied species was
confirmed by microanalyses carried out at the analytical laboratory of the University of
Eastern Finland.
The photophysical measurements (UV/vis absorption, excitation and emission spectra,
excited state lifetimes, and emission quantum yields) for all titled compounds were
carried out using the core facilities of St. Petersburg State University and the scientific
equipment of Peter the Great St. Petersburg Polytechnic University with the support of
Dr. Alexei Melnikov.
Supplementary magnetic and electrochemical studies were performed by collaborating
groups at RWTH Aachen University, Germany (Prof. Kirill Monakhov, now at Leibniz
Institute of Surface Engineering) and University of Valencia, Spain (Prof. Antonio
Doménech-Carbó). The computational analysis was done at the University of Eastern
Finland (Dr. Pipsa Hirva, MSc. Toni Eskelinen) and Aalto University, Finland (Prof.
Antti J. Karttunen).
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3 RESULTS AND DISCUSSION 3.1 Alkynylphosphine rhenium(I) complexes
The prototype moiety {Re(CO)3(phen)}, which generally offers high luminescence
efficiency15,91
but often suffers from limited photostability,92–94
was utilized for
constructing robust rhenium phosphine-disubstituted platform. An expeditious route to
the target complexes relied on the displacement of carbonyl ligands in the metal
coordination sphere by thermally stable phosphines under extreme temperature
conditions. Luminescence behavior of the final Re(I) compounds was modulated
through the extension of conjugated substituents in the alkynylphosphine ligands.
The family of monodentate P1–P6 and bidentate PP1 and PP2 phosphine ligands were
readily obtained following a conventional protocol shown in Scheme 6. Alkynyl-
phosphines were incorporated into the coordination sphere of rhenium(I) species by
one-step thermal substitution, reacting [Re(CO)3(phen)(NCMe)](CF3SO3) compound
with excess amount of the corresponding P-donor ligand. Following this strategy,
phosphine dicarbonyl rhenium(I) complexes Re(P1)2−Re(P6)2, (RePP1)2, and
(RePP2)2 were generated using an autoclave technique under a nitrogen pressure of 30
atm. For selective isolation of the disubstituted species, the synthetic route was
performed under optimized conditions, where the mononuclear complexes were
obtained at 175−200 °C, while dinuclear congeners required elevated temperatures
(220 °C). Note that in contrast to the titled complexes with mono- and
dialkynylphosphines, no target compounds bearing the trialkynylphosphines P(C≡CR)3
(R = Ph, biphenyl) were obtained due to degradation of the reagents.
Scheme 6. Synthesis of the alkynylphosphine dicarbonyl rhenium(I) complexes.
All rhenium(I) complexes are air- and moisture-stable compounds. Single-crystal XRD
analysis was carried out for Re(P1)2−Re(P3)2 and (RePP1)2 species for unequivocal
identification of their molecular structures. These dicarbonyl compounds revealed
octahedral coordination geometry of the metal centers. The phosphine ligands are
located at the axial positions, i.e. above and below the equatorial plane defined by the
phenanthroline fragment and the CO groups (Figure 24), in line with the previous
studies.35–37
A visible deviation of the P–C≡C fragments from linear structure (with
angles of 162−179°) is eventually induced by intermolecular interactions in
Re(P1)2−Re(P3)2 and (RePP1)2, and is not exceptional.36,37
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Figure 24. Molecular views of complexes Re(P1)2 and (RePP1)2. Hydrogens and
CF3SO3− counter ions were omitted for clarity.
In solution, the synthesized phosphine ligands and the related complexes were
unambiguously characterized by NMR, FT-IR, and ESI-MS spectroscopic methods.
For all complexes, two intense IR bands at 1880–1896 and 1950–1963 cm−1
were
attributed to symmetric and asymmetric stretching vibrations of the two adjacent CO
groups. The C≡O stretches in Re(P4)2−Re(P6)2 occurred at higher frequencies in
comparison with those in Re(P1)2−Re(P3)2, due to decreased electron density on the
Re(I) centers caused by the more π–accepting phosphines. The 1H and
31P NMR
spectroscopic patterns reflected the presence of a symmetrical stereochemical
environment around the central rhenium atom in the final compounds.
All rhenium(I) species were intensely luminescent, with the emission mainly
determined by 3MLCT rhenium→diimine electronic transitions (Table 2), as is typical
for the rhenium(I) chromophores.8,95,96
In dichloromethane solutions at ambient temperature, the positions of the absorption
and emission maxima as well as the luminescence intensity of rhenium(I) complexes
(Re(P1)2−Re(P6)2, (RePP1)2, and (RePP2)2) were virtually identical, pointing to the
innocent character of the phosphines with respect to the energies of the electronic
transitions. Referenced to the previous spectroscopic investigations of related Re(I)
systems,36,37,97,98
the phosphorescence quantum yields of the titled family are among the
highest (up to 44%) for reported phosphine-containing emitters.
In contrast to the solution medium, the solid-state photophysical characteristics of the
described rhenium-based systems were affected by the ancillary alkynylphosphines.
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Figure 25. Normalized emission spectra of the crystalline complexes Re(P1)2−Re(P3)2
and Re(P1)2 in solution (CH2Cl2) at 298 K.
For instance, the emission bands were gradually red-shifted upon extending the
phenylene chain in the phosphine ligands (Figure 25). Owing to the phenomenon of
luminescence rigidochromism,99–101
a blue-shift in the solid state relative to the solution
was observed for this series of rhenium(I) emitters.
Table 2. Photophysical properties of complexes Re(P1)2−Re(P6)2, (RePP1)2, and
(RePP2)2 in solution (CH2Cl2) and in the solid state at 298 K.
λabs, nm
(ε, .104 M
−1 cm
−1)
a λem,
nm
b λem,
nm
a Φ, %
(aer/degas) a τ, μs
Re(P1)2 271 (2.9), 298 (1.8),
384 (0.4) 608 533 15/34 1.04
Re(P2)2 295 (4.0), 383 (0.3) 608 562 17/34 1.03
Re(P3)2 274 (2.7), 313 (5.3),
383 (0.3) 607 577 15/30 1.03
Re(P4)2 277 (3.4), 300 (1.9),
382 (0.4) 601 581 17/42 0.99
Re(P5)2 297 (6.3), 310 (6.6),
379 (0.6), 415 (0.4) 601 588 16/44 0.97
Re(P6)2 275 (4.6), 325 (8.2),
410 (0.4) 601 619 10/27 0.98
(RePP1)2 277 (6.4), 295 (7.4),
410 (0.7) 600 598 11/30
1.08 (90 %),
0.30 (10 %)
(RePP2)2 274 (5.2), 316 (9.0),
408 (0.7) 610 608 7/13
0.23 (26 %)
0.90 (74 %)
aCH2Cl2 solution (λext 385 nm).
bSolid state.
To provide further insight on the optical properties, quantum chemical analysis was
carried out for the electronic structures. The calculation results for complexes
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Re(P1)2−Re(P6)2 show some overestimation of the absorption and emission
wavelengths, while the simulated T1→S0 emission wavelengths for complexes
(RePP1)2 and (RePP2)2 are rather close to the experimental data (Table 3).
Table 3. Computational estimation of the photophysical properties for complexes
Re(P1)2−Re(P6)2, (RePP1)2, and (RePP2)2.
a λabs S0→S1,
nm
λem T1→S0,
nm
Re(P1)2 438 (0.01) 661
Re(P2)2 446 (0.01) 669
Re(P3)2 477 (<0.01) 623
Re(P4)2 435 (0.04) 680
Re(P5)2 448 (0.02) 689
Re(P6)2 470 (0.01) 687
(RePP1)2 459 (0.23) 615
(RePP2)2 467 (0.25) 628
a Oscillator strengths are given in parentheses.
For all rhenium(I) species, the corresponding transitions are mainly of dπ(Re)–
π*(phen) 1,3
MLCT character, with negligible contributions from the alkynyl fragments
of the phosphines to the emissive excited states (Figure 26).
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Figure 26. Electron density difference plots for the lowest-energy singlet excitation
(S0→S1) of systems Re(P1)2, Re(P4)2, and (RePP2)2. During the electronic transition,
the electron density increases in the blue areas and decreases in the red areas.
Research discussed in this section focused on the new phosphine-substituted rhenium
complexes to improve their photoluminescence properties. Although the ancillary
phosphine ligands showed little influence on the emission energy in solution, they
clearly affect the behavior in the solid state. Moreover, the developed synthetic
approach demonstrates a quite facile route to the rhenium compounds with high
quantum efficiency and remarkable stability, which are the prominent advantages of
these systems over most of other rhenium emitters.
3.2 Rhenium(I) complexes based on the extended diimine ligands
One of the approaches to alter the photophysical properties of rhenium(I)-based
molecular architectures relied on the coordination of the presynthesized chromophore
diimines to the metal center.20,25,102
For this purpose, the pyridyl-phenanthroimidazole
core was selected as the ligating unit due to its straightforward synthesis and
functionalization, as well as efficient binding toward the transition metal ions.103
Integration of secondary chromophore moieties into the aforementioned bidentate
platform was anticipated to tune the optical properties of the resulting
[Re(CO)3(NN)Cl] species.
Modular platforms NN1 and NN2, conventionally prepared by Debus-Radziszewski
condensation reaction, were converted into the bichromophore compounds NN3−NN5
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containing electron-rich anthracene-derived functional groups via Sonogashira cross-
coupling reaction (Scheme 7).
Scheme 7. Synthesis of NN1−NN5 and schematic structures of rhenium complexes
ReNN1 to ReNN5 and ReNN1-CN.
The target tricarbonyl complexes ReNN1−ReNN5 were obtained following the
standard procedure.8 Exchange of the chloride ligand by cyanide was accomplished by
chloride abstraction with silver cyanide to give the compound ReNN1-CN.
In accordance with the X-ray crystallographic studies of NN1 and NN5, the nearly flat
phenanthro-imidazole fragment was almost perpendicular to the N-bounded phenyl
ring, which probably diminished the conjugation between the polyaromatic units in the
bichromophore ligands NN3−NN5. The coordination of the bulky diimines to the
rhenium(I) metallocenter through the pyridyl-imidazole chelating moiety caused steric
hindrance between the equatorial carbonyl ligand and the adjacent H−C group of
phenanthrene. The result was substantial geometry distortion that shifted the metal
atom out of the plane of the diimine. In all these complexes (ReNN1 to ReNN5 and
ReNN1-CN), the rhenium ions had the distorted octahedral environment typical for the
tricarbonyl diimine compounds (Figure 27).104–106
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Figure 27. Molecular views of the complex ReNN5.
Spectroscopic measurements confirmed that the given diimine ligands and the
corresponding complexes retained their structures in solution. Particularly, the 1H
NMR spectra demonstrated a number of clearly resolved multiplets in the aromatic
region that pointed to the stereochemical rigidity of the titled compounds (Figure 28).
Figure 28. 1H NMR spectra of the ligand NN2 and complexes ReNN2 (400 MHz,
DMSO-d6, 298 K).
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The photophysical studies of these species mainly concerned their solution behavior
(Table 4, Figures 29–31). The extended π-conjugated phenanthrene skeleton endowed
NN1 and NN2 with 1ππ*phenanthrene LC fluorescence, which was virtually invariant to the
para-substituent in N-bounded phenyl ring. Both ligands produced similar vibronically
structured deep blue emission with a maximum at around 380 nm and the quantum
yields of 37% and 22%, respectively. Tailoring various anthracene-containing units to
the phenanthro-imidazole core differentiated NN3, NN4, and NN5 chromophores
according to their optical properties.
Figure 29. Normalized emission spectra of NN1−NN5 (solid lines) and ReNN5
(dotted line) (CH2Cl2, 298 K).
Compound NN3 displayed luminescence nearly identical to that of reported anthracene
derivatives,107,108
with a quantum efficiency of 62%. Expansion of the π-system and
introduction of the electron-donating groups in dyes NN4 and NN5 changed the
character of the electronic transitions and significantly decreased the optical band
gap.20,109,110
In CH2Cl2 solutions, NN4 and NN5 showed highly intense structureless 1ILCT luminescence (Φem up to 92%) in the yellow (566 nm) to orange (602 nm)
region. Moreover, diimines NN4 and NN5 exhibited distinct fluorescence
solvatochromism attributed to the intramolecular charge transfer due to their donor-
acceptor (D-A) architecture.20,111–113
The emission bands of the latter displayed
substantial red shifts with increasing solvent polarity from cyclohexane to DMF
(Figure 30A).
The N-donor functions in NN1−NN5 enabled reversible switching of the photophysical
properties by protonation, which was also confirmed by NMR spectroscopy. For
instance, upon treating NN1 with CF3COOH, the luminescence spectrum showed two
new broad bands at ca. 413 and 517 nm that were bathochromically shifted with
respect to the initial signal (380 nm). This observation was attributed to the protonation
of the pyridyl-imidazole site, which presumably activated the emissive 1ILCT
phen→py excited state.114
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Figure 30. (A) Normalized emission spectra of NN5 at 298 K in various solvents, with
the inset showing their visual appearance under UV light (λext = 365 nm). (B) Effect of
protonation on the photoemission spectrum of NN5 (c = 10−5
M, CH2Cl2, 298 K).
The protonation site, which mainly affected the optical characteristics in NN5, was the
Namino
donor. The addition of CF3COOH to a solution of NN5 (Figure 30B) converted
the broad 1ILCT emission band centered at 602 nm to the structured
1ππ*anthracene LC
fluorescence band at around 490 nm. The original luminescence of the titled
compounds was restored upon neutralizing the acid with a stronger base, e.g. 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU).
Table 4. Photophysical properties of ligands NN1 to NN5 and related complexes
ReNN1 to ReNN5 and ReNN1-CN in solution (CH2Cl2, 298 K).
a λem,
nm
a Φ,
% τ, ns a λem, nm a Φ, % τ, ns
NN1 370,
388 37 1.9
ReNN1 369, 392,
408, 616 2 (616 nm)
2.6 (392 nm)
186 (616 nm)
ReNN1-CN 582 20 3550
NN2 370,
388 22 1.9 ReNN2
371, 392,
407, 624 2 (624 nm)
2.1 (405 nm)
142 (624 nm)
NN3
433,
459,
486
65 3.3 ReNN3 436, 460,
486(sh) 3 4.1
NN4
446w,
475w,
566
52 2.1 (446 nm)
1.4 (566 nm) ReNN4 572 10 4.5
NN5 484w,
602 92
2.6 (484 nm)
3.8 (602 nm) ReNN5 614 22 2.7
aλext = 260 nm (NN1, NN2), 360 nm (NN3), 365 nm (NN4, NN5, ReNN1 to ReNN5,
and ReNN1-CN).
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In the fluid medium, coordination compounds ReNN1 to ReNN5 and ReNN1-CN
demonstrated moderate photoluminescence with Φem up to 22%. Remarkably, the
excitation of ReNN1 and ReNN2 gave rise to dual luminescence, with two bands at ca.
400 and 620 nm corresponding to 1LC and
3ML’LCT origins, respectively (Figure 31).
The proposed assignment was based on the strikingly different dependence of the
intensity of these bands on the presence of molecular oxygen and the shapes of the
emission profiles. Notably, the phenomenon of dual emission is scarce among
rhenium(I) diimine systems.115–118
Figure 31. Normalized emission spectra of ReNN1-CN and ReNN2, the
corresponding spectrum of NN2 (filled) is shown for comparison (degassed CH2Cl2,
298 K).
In the case of ReNN1-CN, the dual luminescence disappeared, apparently as a result of
a change in the nature of the emissive excited state from mainly 3ML’LCT to the
mixture 3(LC+ML´LCT). Compared to ReNN1, ReNN1-CN revealed a blue-shifted
(by 34 nm, 950 cm-1
) and poorly structured emission profile (Figure 31) together with a
10-fold increase in quantum efficiency (from 2% to 20%).
Interestingly, complexes ReNN3−ReNN5 demonstrated optical characteristics that are
very similar to those of the parent ligands NN3−NN5 (Figure 29). This observation
implied that the emissions of ReNN4 and ReNN5 were dominated by the 1ILCT
transitions, which was also confirmed by the sensitivity to protonation and to the
solvent polarity. In contrast, most of previously reported rhenium(I) luminophores
principally displayed 3MLCT/
3IL emission
105,119 or demonstrated the formation of dark
3ILCT state,
14,120 which fundamentally distinguish them from ReNN3−ReNN5
complexes showing relatively efficient intra-ligand fluorescence.
These results illustrated that the pyridyl-imidazole core combined with tricarbonyl
rhenium(I) unit can produce complexes with versatile photophysical properties.
Compounds ReNN1 and ReNN2 displayed excitation-dependent singlet/triplet dual
emission, while changing the ancillary X ligand switched the luminescence of ReNN1-
CN to exclusively triplet in origin. The 1ILCT emissive states, which are
unconventional for rhenium luminophores, were realized for ReNN4 and ReNN5
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species by using the extended ligands with secondary chromophore fragments of a
donor-acceptor nature. Furthermore, this family of chromophore-functionalized
diimines can potentially serve as useful building blocks to generate complexes of many
other transition metals.
3.3 Bichromophore rhenium(I)–platinum(II) complexes based on the rhenium(I) aminopyridine predecessors
Another direction in the development of di-component emitters was compounds
comprising both the rhenium(I) and platinum(II) phosphorescent units. Tethering the
emissive cycloplatinated fragments {Pt(ppy)Cl}121,122
and {Pt(dpyb)}+ 90,123
to the
{Re(phen)(CO)3} chromophore was achieved by utilizing the bifunctional
aminopyridine ligands with different lengths in the spacers. This approach assumed
that the photophysical features of each component are preserved in the bichromophore
complex.
The preparation of rhenium(I) complexes Re(pyN1)−Re(pyN3) relied on site-selective
coordination of the heterodentate aminopyridines to the
[Re(CO)3(phen)(NCMe)](CF3SO3) precursor (Scheme 8).
Scheme 8. Synthesis of the aminopyridine rhenium complexes and the rhenium(I)–
platinum(II) complexes.
The 1H NMR spectra showed a distinct difference in chemical shifts of the low-field
signals of the diimine protons for Re(pyN1) and the pair Re(pyN2), Re(pyN3),
consistent with two types of coordination of aminopyridines to the Re(I) center. As
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indicated by X-ray crystallography data (Figure 32), linking pyN1 to rhenium diimine
unit occurred through the pyridine N-donor, while pyN2 and pyN3 were bonded to the
metal through the primary amine group. It should be noted that the Namine
coordination
mode has been found in only a few cases of Re(I) complexes.124,125
The mononuclear rhenium complexes Re(pyN1)−Re(pyN3) were 3MLCT-type
emitters (Figure 33 and Table 5), similar to other rhenium(I) species with the ancillary
N-heterocycles.38,40,126
Re(pyN1) displayed a bathochromic shift of the emission
maximum (by 10 nm) and reduced quantum efficiency (around 2 times) with respect to
Re(pyN2) and Re(pyN3), while the photophysical characteristics of the latter two
complexes were almost identical. However, in the solid phase, the different rigidity of
the crystal packing led to the dissimilar photophysical behaviors of these rhenium
congeners (Table 5).
Figure 32. Molecular views of complexes Re(pyN2)Pt(dpyb) and Re(pyN3)Pt(ppy).
CF3SO3− counter ions were omitted for clarity.
Further construction of bimetallic complexes Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb),
Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb) was achieved by merging
Re(pyN2)/Re(pyN3) predecessors with the platinum(II) building blocks via Npy
vacant
coordination function of the aminopyridine ligands (Scheme 8). The compositions and
structures of the dinuclear assemblies were established by means of IR and NMR
spectroscopies, ESI-MS, and X-ray crystallography. The single-crystal structures
confirmed the assembly of the heterometallic cations Re(pyN2)Pt(ppy),
Re(pyN2)Pt(dpyb), Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb). These assemblies
consisted of the distorted octahedral rhenium moieties and the square-planar platinum
fragments bridged by pyN2 and pyN3 (Figure 32). Thus, spatially separated metal ions
appeared in their typical geometries.122,126–128
The solution 1H NMR spectroscopic data
of the rhenium(I)–platinum(II) architectures clearly indicated retention of the
molecular structures in solution.
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Table 5. Photophysical properties of complexes Re(pyN1)−Re(pyN3),
Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb), Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb) in
solution (CH2Cl2) and in the solid state at 298 K.
λabs, nm
(ε, .104 M
−1 cm
−1)
a λem,
nm
a Φ, %
(aer/
degas)
a τ, μs b λem, nm
b Φ,
%
b c τ, μs
Re(pyN1) 360 (0.4), 271 (4.6),
262 (4.4) 580 6/14 0.7 550 9 1.4
Re(pyN2) 362 (0.3), 324 (0.5),
276 (2.1), 258 (2.0) 570 15/29 1.5 517 20 2.5
Re(pyN3) 360 (0.3), 322 (0.5),
275 (2.0), 257 (2.0) 570 14/25 1.4 571 4 1.2
Re(pyN2)
Pt(ppy)
369 (0.5), 340 (0.9),
324 (1.1), 275 (3.5),
258 (4.2), 247 (3.8)
566 6/10 0.5 (85%)
1.4 (15%)
479, 502,
516, 537 2
d 1.1
e 1.3
Re(pyN2)
Pt(dpyb)
399 (0.5), 377 (0.8),
361 (0.7), 329 (1.0),
278 (3.4), 256 (3.6)
491,
524,
559
f 8/17 2.7
490, 525,
565, 611 2
d 0.6
e 1.4
Re(pyN3)
Pt(ppy)
371 (0.5), 340 (0.8),
324 (1.0), 274 (3.6),
257 (4.5), 249 (4.2)
570 4/8 0.4 (88%)
1.3 (12%)
492, 526,
555 1
d 0.7
e 1.3
Re(pyN3)
Pt(dpyb)
400 (0.6), 378 (0.8),
363 (0.7), 330 (1.0),
287 (2.6), 276 (3.1)
491,
527,
561
f 8/28 1.6 493, 600 5
d 1.2
e 1.3
aCH2Cl2 solution (λext 365 nm).
bSolid state.
cAverage emission lifetimes for the two
exponential decays determined by the equation: τav = (A1τ12 + A2τ22
)/(A1τ1 + A2τ2); Ai is
the weight of the i-th exponent. dRhenium fragment.
ePlatinum fragment.
fcmax =
1.7×10−5
M for Re(pyN2)Pt(dpyb) and 1.2×10−5
M for Re(pyN3)Pt(dpyb).
Different from reported heterometallic species with "antenna"-type constituting
fragments,129–131
these complexes (Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb),
Re(pyN3)Pt(ppy), and Re(pyN3)Pt(dpyb)) demonstrated reasonably efficient 3MLCT
photoluminescence with quantum yields up to 28%. The luminescence behavior of
bimetallic assemblies Re(pyN2)Pt(ppy), Re(pyN2)Pt(dpyb), Re(pyN3)Pt(ppy), and
Re(pyN3)Pt(dpyb) was altered by means of the platinum-containing units, whilst
contribution of the rhenium chromophores remained invariable (Table 5).
Re(pyN2)Pt(ppy) and Re(pyN3)Pt(ppy) in fluid media demonstrated
phosphorescence, which included featureless rhenium-dominated emission with high-
energy shoulders at ca. 490 nm of the weakly emissive platinum motif132
(Figure 33).
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Figure 33. Normalized emission spectra of complexes Re(pyN1), Re(pyN2),
Re(pyN2)Pt(ppy), and Re(pyN2)Pt(dpyb) in solution (CH2Cl2) at 298 K.
The other pair of Re(pyN2)Pt(dpyb)/Re(pyN3)Pt(dpyb) species revealed a
substantially increased contribution of the platinum chromophore to the observed
emission. The key feature in both the absorption and emission spectra of Pt(dpyb)-
containing assemblies was their concentration dependence. In dilute solutions, the
emission profiles of Re(pyN2)Pt(dpyb) and Re(pyN3)Pt(dpyb) showed an intense
high-energy band with clear vibronic progression that corresponds to the {Pt(dpyb)}
unit, and a weaker red-shifted broad band of the {Re(phen)} chromophore (Figure
34A). For Re(pyN3)Pt(dpyb), apparent changes in the intensities of high- and low-
energy emission bands upon concentration increase were accompanied by the
appearance of a subtle shoulder at around 650 nm (Figure 34B). This new long-
wavelength phosphorescence band was assumed to originate from intermolecular
interactions.
Figure 34. A: Concentration dependence of the emission spectra of
Re(pyN3)Pt(dpyb) in solution (CH2Cl2) at 298 K. B: Deconvolution of the emission
spectrum of Re(pyN3)Pt(dpyb) at c = 6×10−5
M into three independent bands.
In order to rationalize the possible aggregates, TD-DFT studies were performed to
simulate the observed concentration-dependent alterations in the UV-visible spectra. In
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contrast to the homoleptic {Re}−{Re} and {Pt}−{Pt} aggregates, the predicted
absorption spectra for {Re(phen)}·· ·{Pt(dpyb)} stacks reasonably agree with the
experimental data, as evidenced from Figure 35. When the aromatic systems of
{Re(phen)} and {Pt(dpyb)} fragments were separated by 3.4 Å (mimicking the
association via π-stacking at higher concentrations), there is an intensity reduction for
the lower energy absorption peak, in line with experimental data. This stacking model
portrays an unprecedented case of heteroleptic π-π interaction involving platinum
square planar complexes.
Figure 35. A: UV-vis absorption spectra of Re(pyN3)Pt(dpyb) at variable
concentrations (CH2Cl2, 298 K). B: TD-DFT simulated absorption spectra of the {Re}–
{Pt} intermolecular stacking motif to model the aggregation of complex
Re(pyN3)Pt(dpyb).
The solid-state photophysical behavior of the rhenium(I)-platinum(II) architectures
generally resembled the luminescent behavior in solution, expressed by a combination
of emissions from both chromophoric metallocenters.
In this study, I investigated the photophysical behavior of phosphorescent
bichromophores by implementing the Re(I)−Pt(II) combination. The variation of the
platinum cyclometalated fragments allowed tuning of the contributions of metal
constituents to the observed emission. Importantly, the cationic complexes
Re(pyN2)Pt(dpyb) and Re(pyN3)Pt(dpyb) demonstrated a concentration-dependent
tendency to aggregate through unconventional π–π stacking heteroleptic interactions
{Re(phen)}·· ·{Pt(dpyb)}, which occur in the ground state and result in the appearance
of panchromatic luminescence.
3.4 Multichromophore rhenium(I) complexes of different nuclearity
The synthesis of multichromophore Re(I) supramolecular structures was performed via
a coordination-driven self-assembly route. The chosen molecular design featured
terminal {Re(CO)3(phen/bpy)} units around the central [M(CN)y]n−
scaffold (M = Au,
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Pt, Fe; y = 2−6). The nature of the assembling cyanometallates determined the
nuclearity of the resulting coordination architectures.
The construction of the multinuclear structures was based on the "metalloligand"
strategy,133–135
which consists of decorating the cyanide or cyanometallates by two or
more rhenium(I) fragments [Re(CO)3−x(phen/bpy)(PPh3)x(H2O)]+ (x = 0, 1) under mild
conditions (Scheme 9).
Scheme 9. Synthetic approach to homo- and heterometallic rhenium(I) complexes. The
detailed synthetic procedures are described in publication IV.
The stoichiometry of all compounds was confirmed by ESI-MS measurements.
However, due to reduction of Fe3+
to Fe2+
under the ESI-MS conditions, the signal of
triply charged cation Re6Fe3−bpy could not be detected.
136 Nevertheless, the cyanide
stretching frequencies of Re6Fe2−bpy and Re6Fe
3−bpy in their IR spectra (at 2089 and
2154 cm−1
, respectively) provided conclusive support for the existence of two different
Fe centers. The different CN band frequencies could be explained by the different π-basicities of the iron centers. In the
1H NMR spectra of Re2−NN and Ph3PRe2−phen
(where NN = bpy, phen), the presence of two sets of diimine signals with 1:1 ratio
suggests the formation of dyads with non-equivalent rhenium units. The 1H NMR
spectroscopic profiles of Re2Au−NN, Re4Pt−NN, and Re6Fem
−bpy (m = 2, 3)
correspond to only one type of Re(I) fragments, indicating high symmetry of the
architectures.
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The obtained multinuclear complexes were analyzed by single-crystal XRD
techniques. In all these assemblies, the metal atoms in the diimine rhenium units had a
slightly distorted octahedral surrounding (Figure 36).137–139
The diimine and carbonyl
ligands supplied five donor atoms, while the sixth coordination vacancy was filled with
the cyanide group.
Two {Re(phen)(CO)3-x(PPh3)x} fragments connected through the cyanide bridge in a
nearly linear fashion afforded dimeric Re2−phen and Ph3PRe2−phen complexes. They
adopt nearly eclipsed conformations along the Re-CN-Re axis (torsion angles < 3°),
resembling that of previously reported Re2−bpy.32,139
The structure of ReAuPPh3−phen revealed a heterobimetallic architecture, which
consisted of neutral [Au(PPh3)(CN)] and cationic {Re(phen)(CO)3}+ subunits held
together by the cyanide.
In aggregates Re2Au−NN, Re4Pt−NN, and Re6Fem
−bpy, the central [M(CN)y]n−
scaffolds (M = Au, Pt, Fe; y = 2−6) acted as anionic polydentate ligands toward the
pendant {Re(NN)(CO)3} blocks (Figure 36).
Figure 36. Molecular views of complexes Re2Au−phen, Re4Pt−bpy, and
Re6Fe2−bpy. CF3SO3
− counter ions were omitted for clarity.
Two {Re(NN)(CO)3} units were linked to one [Au(CN)2]− core to form trinuclear
species Re2Au−bpy and Re2Au−phen with eclipsed and staggered conformations,
respectively. The resulting Re–NCAuCN–Re arrays deviated from linearity, with the
Re(I)–Au(I)–Re(I) angles of 165.4–175.4°. Assemblies involving the anionic building
block [Pt(CN)4]2−
formed square-like pentanuclear Re4Pt−bpy and Re4Pt−phen
architectures. The decoration of [Fe(CN)]4−/3−
cores by six {Re(CO)3(bpy)} fragments
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produced the almost spherical complexes Re6Fe
2−bpy and Re6Fe
3−bpy. Each iron
center adopted the expected octahedral geometry consisting of six C atoms from
bridging cyanide ligands. Despite equivalence of the rhenium fragments in solution, in
the crystal steric requirements of the diimine ligands caused certain deformations of the
ideal arrangements of Re4Pt−NN and Re6Fem
−bpy compounds.137,140,141
All these homo- and heterometallic rhenium(I) complexes were good absorbers of
visible light. With the exception of Re−Fe assemblies, they also featured relatively
intense and long-lived luminescence associated with the triplet nature of the excited
state (Table 6). In fluid media, compounds Re2−NN, Re2Au−NN, and Re4Pt−NN
containing tricarbonyl rhenium moieties demonstrated green to yellow emission with
quantum yields up to 48%. A hypsochromic shift of the emission maxima and
systematic enhancement of the luminescence intensity were detected after replacing the
bpy ligand for the related phen. This observation was in concordance with previously
published data for [Re(NN)(CO)3L] emitters.22,142,143
In addition, phosphorescence
bands of the phen-based chromophores showed stronger sensitivity to the presence of
O2 compared to the bpy-analogues. This might be due to the longer excited state
lifetimes and larger size of the phenanathroline systems, making them more susceptible
to collisional quenching.
Table 6. Photophysical properties of complexes Re2−NN, Ph3PRe2−phen,
ReAuPPh3−phen, Re2Au−NN, Re4Pt−NN, and Re6Fem
−bpy (m = 2, 3) in solution
(CH2Cl2) and in the solid state at 298 K.
λabs, nm (ε, .10
4 M
−1
cm−1
)
a λem,
nm
a Φ, %
(aer/
degas)
a τ, μs
b
λem,
nm
b Φ,
%
b τ, μs
Re2−bpy 287 (4.6), 315 (2.4),
368 (1.0) 571 7/10 0.4 523 19 1.0
Re2−phen 260 (8.8), 287 (4.4),
355 (1.2) 564 12/30 1.6 528 22 2.5
Ph3PRe2−
phen
267 (9.8), 321 (2.5),
456 (1.2) 737 –/<1 0.023 637 2 0.1
ReAuPPh3−
phen − − − − 534 25 2.4
Re2Au−
bpy
246 (8.4), 281 (4.7),
317 (2.9), 359 (1.1) 554 11/19 0.6 522 31 1.0
Re2Au−
phen
249 (9.6), 254 (9.5),
273 (7.7), 330 (1.6) 543 15/48 1.9 562 3 0.6
Re4Pt−bpy
246 (10.3), 281
(9.0), 305 (5.7), 318
(5.1), 355 (1.8)
560 10/21 0.5 547 14 0.5
Re4Pt−phen 257 (12.5), 274
(12.5), 338 (2.2) 549 12/42 2.2 549 19 1.5
aCH2Cl2 solution (λext 365 nm).
bSolid state (λext 340 nm).
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The [Au(CN)2]
− and [Pt(CN)4]
2− cyanometallate linkers did not significantly affect the
photophysical behavior of the polynuclear rhenium(I) systems, which was clearly
supported by DFT calculations (Figure 37).
The conversion of tricarbonyl rhenium complex Re2−phen into dicarbonyl phosphine
derivative Ph3PRe2−phen shifted the emission to the near-IR region and decreased the
quantum efficiency. In comparison to the literature precedents, Ph3PRe2−phen is an
uncommon example of structurally simple rhenium(I) NIR emitter.13,144,145
Photophysical characteristics of the crystalline samples were very similar to those
found in solution, indicating that the emissive excited state had the same origin in both
phases. The DFT analysis underscored the nature of the low-energy electronic
transitions, which are localized on the {Re(NN)(CO)3} fragments and principally
correspond to the dRe→π*NN 3MLCT process (Figure 37).
Figure 37. Electron density difference plots of complexes Re2Au−phen (S0→S1 and
T1→S0) and Re4Pt−bpy (S0→S1). During the electronic transition, the electron density
increases in the blue areas and decreases in the red areas.
None of the Re−Fe complexes showed appreciable luminescence. In contrast to other
aggregates where the [M(CN)y]n−
bridges were electronically innocent, the
[Fe(CN)6]4−/3−
fragments played a major role in the lowest-energy excitation and
therefore evidently provided non-radiative relaxation pathways, which account for the
lack of emission.
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The electrochemical and magnetic properties of Re6Fe
m−bpy were investigated. The
complexes exhibited irreversible ReI to Re
II oxidation along with a quasi-reversible
reduction of [FeII/III
(CN)6]4−/3−
moieties. The magnetic data for compound Re6Fe3−bpy
clearly indicated a low-spin octahedral configuration of the FeIII
ion, which is virtually
unaffected by the coordination of terminal {Re(bpy)} blocks.
The described coordination-driven self-assembly of the Re(I) aggregates demonstrated
an efficient synthetic route to supramolecular systems comprising several chromophore
fragments. Depending on the electronic features of the connecting cyanometallate,
optical properties of these polynuclear species are defined either by the rhenium
diimine components, or by the central heterometal ion and the energy transfer between
the constituents. Notably, despite their multichromophore nature that is often
detrimental to luminescence, most of the studied compounds retain intense emission,
which can be extended into the NIR region upon a simple modification of the ligand
sphere.
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4 CONCLUSIONS In this work, I have investigated the structure-property relationships for a series of
organometallic luminescent complexes based on the rhenium(I) diimine-carbonyl
chromophore units. The prototype motif [Re(CO)3(diimine)L] (where L = auxiliary
ligand) offers substantial synthetic freedom along with appealing optical
characteristics. Therefore, it has been used to design several new families of Re(I)-
containing emitters. In order to manipulate the behavior of the excited state, I
implemented a number of stereochemical modifications that involve changes of the
ligand coordination sphere and the combination of two or more chromophore units in
one molecular entity.
As the first step, I studied the phenanthroline rhenium complexes, which incorporated
alkynylphosphine ligands with different electron-donor strength and stereochemistry.
In fluid medium, varying the alkynyl-phenylene substituents had negligible effect on
the electronic absorption and emission energies of the final Re(I) systems. However,
the incorporation of two phosphines into the diimine-dicarbonyl fragment led to high
quantum yields and remarkable stability in comparison with conventional tricarbonyl
systems. In contrast to the solution behavior, the solid-state emission spectra featured a
gradual bathochromic shift upon extension of the phenylene chain in the phosphine
substituents, which presumably influences rigidity of the environment of the
chromophore motif.
In the second direction, I have employed phenanthrene imidazole-pyridyl moiety as a
chelating diimine. The neutral rhenium(I) tricarbonyl derivatives displayed dual
fluorescence/phosphorescence, which can be controlled by the excitation wavelength.
The straightforward functionalization of this diimine with an additional fluorophore
fragment changed the emission origin to the singlet intra-ligand character, which is not
typical for Re(I) emitters. The energy of the corresponding electronic transitions was
readily influenced by the donor-acceptor nature of the extended ligands, which resulted
in the charge transfer emissive states. Consequently, the fluorescence of the parent
dyes and their Re(I) compounds with electron-donating pendant groups exhibited
pronounced red-shifts with increasing solvent polarity, indicating positive
solvatochromic properties. Furthermore, optical properties of these emitters can be
switched in the presence of a protonating agent in a reversible manner.
The third part of my research considered cationic dyads constructed from the Re(I) and
Pt(II) chromophores and linked through the bifunctional aminopyridine ligands. These
bimetallic systems might help clarify the delicate balance between preserving the
independent performance of each component (i.e. maintaining dual emission) and the
energy transfer between two different metal fragments. The cycloplatinated motifs
were used to regulate the phosphorescent properties, whereas the emission
characteristics of the rhenium centers remained unaltered in these assemblies.
Remarkably, the heterometallic species based on the {Pt(NCN)} moiety displayed
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concentration dependence in both the absorption and emission spectra, which was
assigned to unusual π-stacking interactions of the heteroleptic type
{Re(NN)}·· ·{Pt(NCN)}.
In the fourth chapter of the thesis, I designed multicomponent self-assemblies
constructed from the Re(I) building blocks. The supramolecular architectures
comprised two or more rhenium(I) fragments in tandem with the cyanometallates
[M(CN)y]n−
(M = Au, Pt, Fe; y = 2−6) as bridging ligands. The photophysical
properties of the gold and platinum-centered aggregates were governed by the nature of
the diimine-rhenium platforms, while the nuclearity of the metal complexes had a
minor influence on the absorption and emission behaviors. Interestingly, the
polynuclear rhenium(I) emitters attained high luminescence with quantum efficiencies
up to 48%, despite the presence of several photofunctional components.
In total, the judicious design of multichromophore structures offers wide opportunities
for elegantly tuning of the photophysical properties of rhenium(I) diimine-based
complexes. These complexes can be used to develop versatile luminescent molecular
materials.
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ACKNOWLEDGEMENT This work was carried out at the Department of Chemistry, University of Eastern
Finland during the years 2015-2019. Financial support provided by the Finnish
National Agency for Education (CIMO Fellowship), the Academy of Finland and the
University of Eastern Finland (SCITECO grant) is gratefully acknowledged.
Primarily, I would like to express my profound appreciation to my supervisor, Prof.
Igor Koshevoy. His solid expertise, professional guidance and unwavering support
during this period have been invaluable. I am indebted to my second supervisor, Assoc.
Prof. Elena Grachova, for giving me opportunity to work on this project.
Secondly, I am most grateful to the group of Prof. Sergey Tunik (St. Petersburg State
University), to Dr. Pipsa Hirva (UEF), to Prof. Antti Karttunen (Aalto University), to
Dr. Alexei Melnikov (Peter the Great St.-Petersburg Polytechnic University) and to
Ilya Kondrasenko for their resourceful and productive collaboration.
I would also like to extend my gratitude to all personnel at Department of Chemistry,
especially to Prof. Tapani Pakkanen, Prof. Mika Suvanto, Dr. Sari Suvanto, Ms. Mari
Heiskanen, Ms. Taina Nivajarivi, Ms. Paivi Inkinen and Ms. Eija Faari-Kapanen for
their friendly cooperation and assistance during my doctoral studies.
Furthermore, I would like to acknowledge Prof. Risto Laitinen (University of Oulu)
and Prof. Andreas Steffen (Technical University of Dortmund) for critically reading
this thesis and for valuable comments.
Additionally, I sincerely thank my colleagues and friends, especially, Andrey Belyaev,
Anastasia Solomatina and Julia Shakirova for their advice regarding my scientific work
and for their immense encouragement over the years.
At last but not the least, I would like to extend the heartfelt thanks to my family, who
are always by my side and give a precious counterbalance to the studies.
Joensuu, 2019 Kristina Kisel
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Kristina Kisel
DissertationsDepartment of ChemistryUniversity of Eastern Finland
No. 152 (2019)
122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium dichloride and its performance as a support in the Ziegler-Natta catalytic system123/2014 PIRINEN Sami: Studies on MgCl2/ether supports in Ziegler–Natta catalysts for ethylene polymerization124/2014 KORPELA Tarmo: Friction and wear of micro-structured polymer surfaces125/2014 HUOVINEN Eero: Fabrication of hierarchically structured polymer surfaces126/2014 EROLA Markus: Synthesis of colloidal gold and polymer particles and use of the particles in preparation of hierarchical structures with self-assembly127/2015 KOSKINEN Laura: Structural and computational studies on the coordinative nature of halogen bonding128/2015 TUIKKA Matti: Crystal engineering studies of barium bisphosphonates, iodine bridged ruthenium complexes, and copper chlorides129/2015JIANGYu:Modificationandapplicationsofmicro-structuredpolymersurfaces130/2015 TABERMAN Helena: Structure and function of carbohydrate-modifying enzymes 131/2015KUKLINMikhailS.:Towardsoptimizationofmetaloceneolefinpolymerizationcatalystsvia structuralmodifications:acomputationalapproach132/2015SALSTELAJanne:Influenceofsurfacestructuringonphysicalandmechanicalpropertiesof polymer-cellulosefibercompositesandmetal-polymercompositejoints133/2015 CHAUDRI Adil Maqsood: Tribological behavior of the polymers used in drug delivery devices134/2015 HILLI Yulia: The structure-activity relationship of Pd-Ni three-way catalysts for H2S suppression135/2016 SUN Linlin: The effects of structural and environmental factors on the swelling behavior of Montmorillonite-Beidellite smectics: a molecular dynamics approach136/2016 OFORI Albert: Inter- and intramolecular interactions in the stabilization and coordination of palladium and silver complexes: DFT and QTAIM studies137/2016 LAVIKAINEN Lasse: The structure and surfaces of 2:1 phyllosilicate clay minerals138/2016 MYLLER Antti T.: The effect of a coupling agent on the formation of area-selective monolayers of iron a-octabutoxy phthalocyanine on a nano-patterned titanium dioxide carrier139/2016KIRVESLAHTIAnna:Polymerwettabilityproperties:theirmodificationandinfluencesupon water movement140/2016 LAITAOJA Mikko: Structure-function studies of zinc proteins141/2017 NISSINEN Ville: The roles of multidentate ether and amine electron donors in the crystal structure formation of magnesium chloride supports 142/2017 SAFDAR Muhammad: Manganese oxide based catalyzed micromotors: synthesis, characterization and applications143/2017 DAU Thuy Minh: Luminescent coinage metal complexes based on multidentate phosphine ligands144/2017AMMOSOVALena:Selectivemodificationandcontrolleddepositiononpolymersurfaces145/2017 PHILIP Anish: PEI-mediated synthesis of gold nanoparticles and their deposition on silicon oxide supports for SERS and catalysis applications146/2017 RAHMAN Muhammad Rubinur: Structure and function of iron-sulfur cluster containing pentonate dehydratases147/2018 ANKUDZE Bright: Syntheses of gold and silver nanoparticles on support materials for trace analyses using surface-enhanced Raman spectroscopy148/2018 PENTTINEN Leena: Structure determination of enzymes with potential in processing of cell wall compounds149/2018 SIVCHIK Vasily: Tuning the photoluminescence of cyclometalated platinum(II) compounds via axial and non-axial interactions150/2019 MIELONEN Kati: Hierarchically structured polymer surfaces: Curved surfaces and sliding behavior on ice151/2019 CHAKKARADHARI Gomathy: Tuning the emission properties of oligophosphine copper and silver complexes with ancillary ligands
Probing the effect of coordination environment on the photophysical behavior of rhenium(I) luminophores
Kristina K
Isel: Probing the effect of coordination environm
ent on the photophysical behavior of rhenium(I) lum
inophores
152
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