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Dynamic Article LinksC<Journal ofMaterials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 11268
www.rsc.org/materials PAPER
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View Article Online / Journal Homepage / Table of Contents for this issue
Triphenyl moieties as building blocks for obtaining molecular glasses withnonlinear optical activity†
Kaspars Traskovskis,*a Igors Mihailovs,ab Andrejs Tokmakovs,b Andrejs Jurgis,b Valdis Kokarsa
and Martins Rutkisb
Received 12th February 2012, Accepted 13th April 2012
DOI: 10.1039/c2jm30861d
The incorporation of trityl and triphenylsilyl groups into low molecular weight molecules allows the
formation of stable molecular glasses. A series of materials based on the N-phenyldiethanolamine core
was synthesized bearing different azobenzenes and benzylydene-1,3-indandione as active
chromophores. Molecular hyperpolarizability of the synthesized compounds was calculated by
a restricted Hartree–Fock method with basis 6-31G(d,p) and measured in solutions by hyper-Rayleigh
scattering. Non-linear optical (NLO) activity of the thin glassy films was confirmed after a corona
poling procedure. Thermal sustainability of the NLO response of up to 85 �C was achieved. Quantum
chemical calculations of the compounds revealed increased steric bulk and conformational freedom of
the triphenylsilyl moiety. While the presence of the triphenylsilyl group results in more stable glasses
and increased material nonlinearity, in the case of trityl groups, measured glass transition temperatures
are higher.
Introduction
During the past decade the research towards development of
organic nonlinear optical (NLO) materials has resulted in
considerable developments, improving prospects for practical
applications relevant in telecommunications, computing and
defense.1 Standard design for such materials requires the pres-
ence of push–pull type chromophores with large molecular
hyperpolarizability (b), high optical transparency, good solu-
bility and chemical and thermal stabilities.1–5 A wide range of
modification possibilities for organic compounds has provided
numerous approaches regarding the placement of NLO active
chromophores, most commonly polymer,6–8 host polymer
matrix9,10 and dendrimeric11,12 systems.
By contrast, small molecular weight amorphous phase forming
materials (molecular glasses), a new emerging class of electro-
optical materials13–15 are less studied. While general principles
linking molecular structure and material, thermal, and amor-
phous phase stability characteristics remain unresolved,16
molecular glasses have several considerable advantages such as
relatively simple synthesis and purification, increased
aRiga Technical University, Faculty of Materials Science and AppliedChemistry, 14/24 Azenes Street, Riga, LV-1048, Latvia. E-mail: [email protected]; Tel: +371 29148070bInstitute of Solid State Physics, University of Latvia, 8 Kengaraga Street,Riga, LV-1063, Latvia
† Electronic Supplementary Information (ESI) available: detailed DSCthermograms, initial atomic coordinates for the quantum calculationsand additional sample microscope pictures. See DOI:10.1039/c2jm30861d
11268 | J. Mater. Chem., 2012, 22, 11268–11276
chromophore density and well defined structure. A widespread
strategy for obtaining molecular compounds capable of forming
stable glasses involves preventing the molecules from interacting
together in a strong and directional fashion. In particular the
presence of arene-rich starburst structural fragments is success-
fully used to obtain such materials,17,18 where the crystallization
and aggregation process is hindered by the steric demands of
conformationally rigid bulky substitutes or insufficient solid
phase packing due to the shape of the molecules. A promising
strategy for obtaining molecular glasses is the modular
approach, where a core molecule not capable of glass formation
is further functionalized with building blocks preventing the
crystallization. Substituents like N,N-diphenylhydrazone19 and
triazines20 are successfully used to obtain amorphous materials.
While minimizing the molecular interactions is one of the key
aspects for designing molecular glasses, it also plays a significant
role regarding the NLO properties of material. Conversion of
high chromophore b values into good macroscopic NLO
efficiencies is often problematic due to strong dipole–dipole
interactions causing aggregation and diminishing acentric order
of the chromophores, resulting in the decay of the electro-optic
coefficient.21 As the solution to this problem, the site isolation
principle proposed by Dalton et al. suggests designing the
molecule in a way so as to minimize dipole–dipole interactions.
Among the applied methods reported, materials involving the
introduction of sterically bulky spacer groups22,23 have shown
a noticeable increase of NLO efficiency.
In this work we present a novel modular approach in the
design of low molar weight molecular glasses for electro-optical
applications. During our research it came to our attention that
This journal is ª The Royal Society of Chemistry 2012
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the presence of triphenylmethyl and triphenylsilyl substituents
noticeably enhances amorphous phase formation of low molec-
ular weight molecules. While the pseudo-spherical shape and
steric demands of these groups prevent the crystallization
process, incorporating them relatively close to the photoactive
chromophore could provide effective shielding of dipole–dipole
interactions. A series of molecular glasses based on triaryl
functionalized N-phenyldiethanolamine was synthesized bearing
different azobenzenes and benzylydene-1,3-indandione as active
chromophores. The influence of the chosen triaryl group on
material properties was closely studied.
Experimental
Synthesis
Azobenzenes used in the study were obtained according to the
given route (Scheme 1) by conventional azo coupling reactions
between N-phenyldiethanolamine and aryldiazonium tetra-
fluoroborate salts followed by functionalization of free hydroxyl
groups with trityl or triphenylsilyl substituents. To better eval-
uate the chosen material design three different azochromophores
were used. In particular, the Disperse Red (DR) dye frag-
ment,24–26 a frequently used chromophore in NLO studies, con-
taining compounds A-1 and A-2 as well as its ortho-brominated
derivative A-3 were synthesized. Attempts to acquire stable
organic glass material using the trityl derivative of compound
A-3 were unsuccessful and this compound will not be discussed
here. Additionally tritylazobenzenes A-5 and A-6 were synthe-
sized to evaluate the influence of the presence of an additional
triaryl group on material characteristics.
Scheme 1 Synthesis of azobenzene containing molecular glasses: (a)
KBr, NaBO3$4H2O, AcOH, rt; (b) HCl/H2O, NaNO2, 5�C then HBF4;
(c) triphenylcarbinol, AcOH, HCl, reflux; (d) H2SO4/AcOH, NaNO2,
5 �C then HBF4; (e) N-phenyldiethanolamine, DMF, 0 �C; (f) TrCl,
pyridine, Et3N, 60 �C; (g) Ph3SiCl, pyridine, Et3N, rt.
This journal is ª The Royal Society of Chemistry 2012
Based on our previous studies showing the high NLO response
of 1,3-indandione derivatives,27 trityl and triphenylsilyl func-
tionalized benzylydene-1,3-indandiones I-1 and I-2 were
synthesized. The synthesis was realized by the condensation of
a known aldehyde 1028 with 1,3-indandione29 followed by
functionalization with triaryl substituents (Scheme 2).
General procedures and starting materials
The reagents 4-nitroaniline, sodium perborate tetrahydrate,
2-nitroaniline, tetrafluoroboric acid (50 wt% water solution),
triphenylcarbinol, N-phenyldiethanolamine, trityl chloride, tri-
phenylsilyl chloride and triethyl amine were obtained commer-
cially from Alfa Aesar and used as received. Solvents used were
reagent grade, DMF was distilled from P2O5 and pyridine from
CaH2 before use.1H NMR spectra were obtained using a Bruker
Avance 300MHz spectrometer using TMS as the inner standard.
Low-resolution mass spectra were acquired on a Waters EMD
1000 MS detector (ESI+ mode, cone voltage 30 V). The
elemental analysis was carried out with Costech Instruments
ECS 4010 CHNS-O Elemental Combustion System.
4-Nitrobenzenediazonium tetrafluoroborate (3a). Aniline 1
(10.0 g, 72 mmol) was dissolved in HCl (30 mL) and water
(15 mL). The reaction mixture was cooled to 0–5 �C using an ice-
bath and NaNO2 (6.0 g, 87 mmol) in water (20 mL) was added
dropwise. After the reaction mixture turned transparent the
remaining insoluble solid was filtered off and 10 mL of HBF4
(50 wt% water solution) was added. The white precipitate formed
was filtered and washed with water (30mL). After drying at room
temperature, the product was purified by precipitation from
nitromethane with methyl tert-butyl ether, yielding 3a (12.8 g,
76%) as white needle shaped crystals. The product was used in
subsequent reactions without characterization.
2-Bromo-4-nitrobenzenediazonium tetrafluoroborate (3b). The
compound was obtained from aniline 230 following the procedure
given for compound 3a, yielding 3b (69%) as white crystals. The
product was used in subsequent reactions without characterization.
2-Chloro-4-tritylbenzenediazonium tetrafluoroborate (6).
Aniline 531 (10.0 g, 26 mmol) was dissolved in sulfuric acid
(20 mL) and acetic acid (10 mL). The reaction mixture was
cooled to 0–5 �C using an ice-bath and NaNO2 (2.20 g, 32 mmol)
Scheme 2 Synthesis of indane-1,3-dione fragment containing molecular
glasses: (a) indane-1,3-dione, EtOH, piperidine, reflux; (b) TrCl, pyridine,
Et3N, 60 �C; (c) Ph3SiCl, pyridine, Et3N, rt.
J. Mater. Chem., 2012, 22, 11268–11276 | 11269
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in water (10 mL) was added dropwise. After stirring for 1 h, 8 mL
of HBF4 (50 wt% water solution) was added. The slurry
precipitate formed was decanted and dissolved in acetone (5 mL).
After pouring into methyl tert-butyl ether (100 mL), the white
precipitate was filtered and purified by additional precipitation
from nitromethane with methyl tert-butyl ether, yielding 6
(10.1 g, 82%) as white crystals. The product was used in subse-
quent reactions without characterization.
4-((4-Nitrophenyl)diazenyl)-N,N-bis(2-(trityloxy)ethyl)aniline
(A-1)
General procedure A. To a solution of N-phenyldiethanol-
amine (1.00 g, 5.0 mmol) in DMF (5 mL) at 0 �C diazonium salt
3a (1.56 g, 6.6 mmol) in DMF (2 mL) was added dropwise. After
2 h of stirring the mixture was poured into water (50 mL) and the
resulting red precipitate was filtered, washed with additional
amount of water, dried in air and crystallized from isopropanol.
Without further purification the technical product 7 (1.44 g,
4.3 mmol) was dissolved in dry pyridine (5 mL) after which
triethylamine (2.38 mL, 17.0 mmol) and tritylchloride (4.84 g,
17.0 mmol) were added. The mixture was stirred at 60 �C for 2 h
and then poured into isopropanol (50 mL). The resulting red
precipitate was filtered, washed with methanol (3 � 15 mL) and
dried in air. Purification was accomplished by column chroma-
tography over silica gel using DCM/petroleum ether (3 : 1) as
eluent. The resulting red substance was dissolved in DCM (5 mL)
and ethanol (50 mL) was added. After removal of DCM under
reduced pressure and filtration of the formed precipitate,A-1was
obtained as a red powder (3.04 g, 67% over 2 steps). 1H NMR (d,
CDCl3, 400 MHz): 3.31 (4H, t, 3J ¼ 5.50 Hz), 3.60 (4H, t, 3J ¼5.63 Hz), 6.56 (2H, d, 3J ¼ 9.27 Hz), 7.14–7.30 (30H, m), 7.73
(2H, d, 3J¼ 9.14 Hz), 7.86 (2H, d, 3J¼ 9.06 Hz), 8.26 (2H, d, 3J¼9.11 Hz). Elemental analysis (C, H, N): (calc. for C54H46N4O4: C,
82.84, H, 5.92, N, 7.16. Found: C, 79.39, H, 5.59, N, 6.83%).
4-((4-Nitrophenyl)diazenyl)-N,N-bis(2-(triphenylsilyloxy)ethyl)
aniline (A-2)
General procedure B. Compound 7 (0.52 g, 1.6 mmol), obtained
following the procedure described above, was dissolved in dry
pyridine (3mL) after which triethylamine (0.5mL, 3.69mmol) and
triphenylsilyl chloride (1.16 g, 3.69 mmol) were added. After stir-
ring at room temperature for 2 h, pyridine was removed under
reduced pressure and the resulting wet precipitate was washed with
ethanol (50 mL). Purification by column chromatography over
silica gel using DCM/petroleum ether (3 : 1) as eluent gave A-2 as
an orange powder (0.47 g, 21% over 2 steps). 1H NMR (d, CDCl3,
400 MHz): 3.53 (4H, t, 3J ¼ 5.84 Hz), 3.92 (4H, t, 3J ¼ 5.84 Hz),
6.35 (2H, d, 3J¼ 9.04 Hz), 7.30–7.60 (30H, m), 7.68–7.76 (3H, m),
8.19 (1H, dd, 3J ¼ 8.84 Hz, 4J ¼ 2.24 Hz), 8.58 (1H, d, 4J ¼ 2.24
Hz). Elemental analysis (C, H, N): (calc. for C52H46N4O4Si2: C,
73.73, H, 5.47, N, 6.61. Found: C, 73.81, H, 5.48, N, 6.59%). MS
(ESI+) m/z: 847.6 (M+, requires 847.1).
4-((2-Bromo-4-nitrophenyl)diazenyl)-N,N-bis(2-(triphenylsily-
loxy)ethyl)aniline (A-3). Prepared according to general procedure
B described above. Purification by column chromatography over
silica gel using DCM/petroleum ether (3 : 1) as eluent gaveA-3 as
a dark red powder (0.97 g, 42% over 2 steps). 1H NMR
(d, CDCl3, 400 MHz): 3.53 (4H, t, 3J ¼ 6.26 Hz), 3.92 (4H, t,
11270 | J. Mater. Chem., 2012, 22, 11268–11276
3J ¼ 6.26 Hz), 6.37 (2H, d, 3J ¼ 8.99 Hz), 7.30–7.60 (30H, m),
7.68–7.74 (3H, m), 8.20 (1H, dd, 3J ¼ 8.99, 4J ¼ 2.34 Hz) 8.58
(1H, d, 4J ¼ 2.34 Hz). Elemental analysis (C, H, N): (calc. for
C52H45BrN4O4Si2: C, 67.45, H, 4.90, N, 6.05. Found: C, 67.94,
H, 5.03, N, 5.77%). MS (ESI+) m/z: 927.2 (M+, requires 927.0).
4-((2-Chloro-4-tritylphenyl)diazenyl)-N,N-bis(2-(trityloxy)
ethyl)aniline (A-4). Prepared according to general procedure A
described above. Purification by column chromatography over
silica gel using DCM/petroleum ether (2 : 1) as eluent gaveA-4 as
a yellow powder (0.46 g, 49% over 2 steps). 1H NMR (d, CDCl3,
300 MHz): 3.26 (4H, t, 3J ¼ 5.84 Hz), 3.60 (4H, t, 3J ¼ 5.84 Hz),
6.51 (2H, d, 3J ¼ 9.23 Hz), 7.07 (1H, dd, 3J ¼ 8.66 Hz, 4J ¼ 2.07
Hz), 7.10–7.32 (45H, m), 7.36 (1H, d, 4J ¼ 2.07 Hz), 7.70 (2H,
d, 3J ¼ 9.23 Hz). Elemental analysis (C, H, N): (calc. for
C73H61N3O2: C, 83.76, H, 5.78, N, 4.01. Found: C, 83.23, H,
5.72, N, 3.93%). MS (ESI+) m/z: 1046.5 (M+, requires 1046.7).
4-((2-Chloro-4-tritylphenyl)diazenyl)-N,N-bis(2-(triphenylsily-
loxy)ethyl)aniline (A-5). Prepared according to general procedure
A described above. Purification by column chromatography over
silica gel using DCM/petroleum ether (2 : 1) as eluent gaveA-5 as
a yellow powder (0.66 g, 39% over 2 steps). 1H NMR (d, CDCl3,
300 MHz): 3.43 (4H, t, 3J ¼ 5.84 Hz), 3.83 (4H, t, 3J ¼ 5.84 Hz),
6.24 (2H, d, 3J ¼ 9.04 Hz), 7.07 (1H, dd, 3J ¼ 8.47 Hz, 4J ¼ 2.26
Hz), 7.12–7.39 (34H, m), 7.45 (1H, d, 3J ¼ 8.47 Hz), 7.49–7.53
(12H, m), 7.57 (2H, d, 3J ¼ 9.04 Hz). Elemental analysis (C, H,
N): (calc. for C71H61N3O2Si2: C, 79.04, H, 5.61, N, 3.89. Found:
C, 78.89, H, 5.58, N, 3.78%). MS (ESI+) m/z: 1078.5 (M+,
requires 1078.8).
2-(4-(Bis(2-(trityloxy)ethyl)amino)benzylidene)-1H-indene-
1,3(2H)-dione (I-1). Aldehyde 1028 (2.0 g, 9.6 mmol) was dis-
solved in ethanol (120 mL) after which indane-1,3-dione29 and 3
drops of piperidine were added. The dark orange solution was
stirred at room temperature for 3 h and refluxed for an additional
1 h. After cooling, the resulting precipitate was filtered and
recrystallized from isopropanol, yielding 11 as orange crystals
(2.1 g, 65%). Compound 11 (0.74 g, 2.2 mmol) was then dissolved
in dry pyridine (5 mL) after which triethylamine (0.85 mL,
5.0 mmol) and tritylchloride (1.4 g, 5.0 mmol) were added. The
mixture was stirred at 60 �C for 2 h and then poured into iso-
propanol (50 mL). The resulting yellow precipitate was filtered,
washed with methanol (3 � 15 mL) and dried in air. Purification
by column chromatography over silica gel using DCM/petro-
leum ether (2 : 1) as eluent gave I-1 as a yellow powder (1.60 g,
88%). 1H NMR (d, CDCl3, 300 MHz): 3.40 (4H, t, 3J ¼ 6.03 Hz),
3.72 (4H, t, 3J ¼ 6.03 Hz), 6.63 (2H, d, 3J ¼ 9.04 Hz), 7.16–7.37
(m, 30H), 7.74 (m, 2H), 7.78 (s, 1H) 7.95 (m, 2H), 8.44 (2H, d,3J ¼ 8.85 Hz). Elemental analysis (C, H, N): (calc. for
C58H47NO4: C, 84.75, H, 5.76, N, 1.70. Found: C, 83.82, H, 5.68,
N, 1.45%). MS (ESI+) m/z: 822.6 (M+, requires 822.0).
2-(4-(Bis(2-(triphenylsilyloxy)ethyl)amino)benzylidene)-1H-
indene-1,3(2H)-dione (I-2). Prepared from compound 11
according to general procedure B described above. Purification
by column chromatography over silica gel using DCM/petro-
leum ether (2 : 1) as eluent gave I-2 as yellow powder (0.84 g,
76%). 1H NMR (d, CDCl3, 300 MHz): 3.46 (4H, t, 3J ¼ 6.03 Hz),
This journal is ª The Royal Society of Chemistry 2012
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3.85 (4H, t, 3J ¼ 6.03 Hz), 6.24 (2H, d, 3J ¼ 9.04 Hz), 7.22–7.52
(m, 30H), 7.62–7.68 (m, 3H), 7.82–7.90 (m, 2H), 8.22 (2H, d, 3J¼9.04 Hz). Elemental analysis (C, H, N): (calc. for C56H47NO4Si2:
C, 78.75, H, 5.55, N, 1.64. Found: C, 76.51, H, 5.36, N, 1.49%).
MS (ESI+) m/z: 854.5 (M+, requires 854.2).
Measurements of optical properties of chromophores in CHCl3solution
The extinction coefficient spectra and molecular hyper-
polarizabilities (bHRS) were measured in dilute CHCl3 (Sigma-
Aldrich, analytical grade) solutions (10�6–10�5 mol l�1) of the
investigated compounds. Spectra were obtained using an Ocean
Optics HR4000CG-UV-NIR based spectroscopic system. The
hyper-Rayleigh scattering (HRS) setup was based on a 1064 nm
excitation source (NL640 by EKSPLA) delivering pulses with
10 ns duration at a repetition rate of 1000 Hz. Pulse energy was
adjusted by a half wave plate and a pair of Glan-Taylor polarizers
within a range 0–80 mJ. The incident beam was focused on the
sample solution (f¼ 3 cm) in a fused quartz 10� 2mm fluorimeter
cuvette (Starna Scientific Ltd.). Scattered light was collected by
two plano-convex lenses in 90� and 270� angle geometry with
respect to the incident beam. By means of two custom built gated
photon counting channels, double frequency light intensity was
measured in a time window synchronized with the laser pulse.
Both channels are similar, except for the final second harmonic
(SH) 532 nm interference filter with full width at half maximum
(FWHM) 3 and 10 nm. To eliminate errors caused by laser pulse
energy variation, both obtained SH intensity signals are normal-
ized to a reference signal from the potassium dihydrogen phos-
phate crystal. Such a detection system allows us to separate HRS
and two photon luminescence (TPL) signals by two procedures.
The first of them is based on temporal discrimination of these
phenomena based on the property that the HRS signal is coinci-
dent in time with excitation pulse, but TPL has some decay time.
The efficiency of this procedure drops in the case where the excited
state life time becomes comparable with excitation pulse width.
Another possibility is discrimination based on TPL and HRS
spectral bandwidth characteristics. FWHM of TPL emission is
usually a number of times wider than HRS. Therefore the TPL
and HRS contribution in the measured SH intensity depends on
the spectral bandwidth of the detection system.Without going into
detail, let us consider that our spectral deconvolution of HRS and
TPL signals is based on the assumption that bHRS obtained by the
internal reference method using data acquired from both channels
should be the same independent of detection system bandwidth.
Such a TPL deconvolution procedure applied to our HRS
measurements of DR1 (Sigma-Aldrich) yielded a value
bHRS(532)¼ 780� 10�30 esu. This value, obtained with an internal
reference (CHCl3, b ¼ 0.49 � 10�30 esu), is lower than reported
previously.32 The HRS light intensity obtained after this decon-
volution was used with the external reference (DR1) method to
obtain the bHRS values presented here.
Thin film sample preparation
An appropriate amount of glass forming compound was dis-
solved in analytical grade chloroform with a typical concentra-
tion of 100 mg ml�1. The films from solutions were spin-coated
This journal is ª The Royal Society of Chemistry 2012
with a Laurell WS-400B-6NPP/LITE spin-coater (starting speed
0 rpm, terminal speed 300 rpm, acceleration 200 rpm s�2, spin-
ning time 40 s) on indium tin oxide covered glass substrates. The
obtained good optical quality films were 0.7 to 1.3 mm thick.
Determination of thermal properties
The glass transition temperatures of the investigated organic
glasses were determined by DSC thermograms acquired using
a Mettler Toledo DSC-1/200W at a heating rate of 5 �C min�1,
initially heating the samples above their melting temperatures
(200 �C) and cooling at the rate 50 �C min�1. The temperature
corresponding to the half NLO activity level, TSHI50 was evalu-
ated from temperature scans at 10 �C min�1. Decomposition
temperatures were obtained using a Perkin Elmer STA 6000
thermal analyzer.
Corona poling procedure
To produce NLO active media, a thermo-assisted electrical field
poling procedure was applied to thin film samples on ITO via
a custom built corona triode setup. The corona discharge was
generated by a 9 kV voltage drop over a 15 mm gap between
a tungsten wire needle (diameter 25 mm) and a control grid. At
a distance of 10 mm below the grid the ITO covered slide with
spin coated glassy film was placed on a temperature controlled
heater. The grid-to-ITO layer potential was kept constant at
2.7 kV. The corona discharge started in advance of heating the
sample to TSHI50. At the point when the sample charging current
reached an approximate steady state the poling was considered to
be finished and the sample was cooled to room temperature
under applied corona triode discharge. The samples were poled
through a mask with diameter of 0.8 cm. Some minor crystalli-
zation appeared in the samples during corona poling at
temperatures exceeding Tg values.
Measurements of linear and nonlinear optical properties of thin
films
The absorption spectra of the thin films were measured with an
Ocean Optics HR4000CG-UV-NIR based spectroscopic system.
The thickness of investigated thin films was measured using
a Dektac 150 profilometer and refractive indexes were deter-
mined by a prism coupler Metricon 2010. In some cases thickness
and refractive indices were evaluated by a procedure described
elsewhere33 using interference fringe separation in the sample
reflection spectrum obtained with the same spectroscopic system.
The experimental set-up for second harmonic generation is
described elsewhere.29 To avoid an electric field-induced second
harmonic generation (EFISHG) signal from charges trapped on
the film surface, the nonlinear coefficients were usually measured
2 days after poling. The second harmonic intensities were
recorded as functions of the fundamental light incidence angle
and polarization (Maker fringe technique). For corona poled
films the CNn symmetry was assumed and the material could be
characterized by three nonzero NLO coefficients – d33, d31 and
d15. As it is usually done for poled polymer films, we assume that
d31 ¼ d15,34 according to Kleinman symmetry. The NLO coeffi-
cients were obtained by a least squares fit of the experimental
curves to the theoretical approximation. The theoretical value of
J. Mater. Chem., 2012, 22, 11268–11276 | 11271
Fig. 1 Trityl and triphenylsilyl group phenyl ring rotation potential
energy wells according to RHF 6-31G(p,d) simulation. Deflection angle is
CPh2–CPh1–C–O or CPh2–CPh1–Si–O torsion angle detune from equilib-
rium position.
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second harmonic intensity (SHI) was calculated using the Her-
man–Hayden17 approach, taking the absorption of the film into
account. The fitting was carried out in two steps: the value of d31was evaluated from experimental s-p polarized SHI, then the d33was calculated from the p-p SHI. An x-cut quartz crystal was
used as reference (d11 ¼ 0.3 pm V�1) to calibrate the instrument
response function.35
Quantum chemical modeling
All molecular modeling calculations were done by Gaussian 09W
software package.36 Molecular geometries of the investigated
amorphous phase forming chromophores were optimized by
restricted Hartree–Fock (RHF) ab initio calculations with basis set
6-31G(d,p). Static and frequency-dependent (for 532 nm SH)
molecular hyperpolarizability tensor components are obtained by
G-09W taking into account solvent (CHCl3) effect (keywords
SCRF¼ IEFPCM andPolar¼DCSHG). Two hyperpolarizability
quantities of experimental interest could be calculated from these
tensor components. The vector component along dipole moment
bm37 is important for EFISHG and external electrical field poling
experiments. Another value, bHRS,38 is related to HRS measure-
ments of molecular hyperpolarizability in the solutions.
Results and discussion
Materials
It should be noted that the trityl and triphenylsilyl substituents
used in the study as the amorphous phase formation enhancers
are widely used in organic chemistry as easily cleavable pro-
tecting groups for alcohols. This restricts the use of materials
because during both the processing and practical application
several precautions need to be taken. In particular, these
compounds are labile towards acidic hydrolysis (both trityloxy
and triphenylsilyloxy), basic hydrolysis and fluorine ion presence
(triphenylsilyloxy). The chloroform used in our spectral
measurements and spin-coating process needed to be analytical
grade and chemically stabilised to avoid the presence of trace
amounts of HCl. No decomposition of the compounds was
observed when storing the compounds in solid form. It is worth
mentioning that in order to improve the materials’ chemical
stability structurally new molecular glasses have been obtained
where the trityl moiety is C–C bonded to a chromophore core.
The results regarding those materials will be reported elsewhere.
Quantum chemical modeling
Optimization of molecular structures revealed noticeable differ-
ences between the shapes of molecules depending on whether
trityl or triphenylsilyl groups were present. A fan-shaped triaryl
pseudo-sphere is sterically bulkier when Si is present in the core
due to the Si–CPh bond (1.884 �A) being significantly longer than
the C–CPh bond (1.544 �A). At the same time, according to our
modelling by means of RHF ab initio calculations (Fig. 1), such
an elongated bond allows more rotational freedom for phenyl
groups. The combination of increased size and less defined
conformation of the substituent allows us to conclude that the
triphenylsilyl group would be a more suitable substituent for
amorphous phase formation enhancement. Further results
11272 | J. Mater. Chem., 2012, 22, 11268–11276
revealed that this was the case. The results of quantum chemical
(QC) modeling of synthesized compounds and reference chro-
mophores (DR1, DR19 and DMABI (N,N-dimethylamino-
benzylidene 1,3-indandione)) in CHCl3 solution are presented
(Table 1). According to our quantum chemical calculations the
trans configuration for all azo dyes is energetically preferable, so
the given values correspond to this conformer.
Introduction of trityl and triphenylsilyl groups just slightly
changes the chromophore NLO properties. If compounds A-1
and A-2 are compared with DR19, the dipole moment as well as
hyperpolarizability values of experimental interest are within less
than 3% variation. Replacement of the nitro group with an
additional trityl moiety in the acceptor part (A-4 and A-5)
decreases conjugation effects in the chromophore. As a result of
the weakened acceptor, these molecules have a lower dipole
moment and molecular hyperpolarizability (bHRS) as well as
hypsochromically shifted CT absorption band maxima (Table 2).
Additionally, the presence of chlorine in the ortho position for
A-4 and A-5 causes a spatial mismatch of hyperpolarizability
with the dipole moment—the angle between them is 65 and
78 degrees, respectively. Due to this the external poling field
vector component along the dipole moment bm is additionally
diminished. Obviously, in such a situation one could not expect
to achieve highly NLO active films by external electrical field
poling as it is demonstrated later in this paper. As a comple-
mentary example, in the case of A-3, ortho substitution with
bromine in the presence of a strong acceptor group (NO2) at the
para position has no negative impact on the spatial alignment of
dipole moment and hyperpolarizability. It is worth mentioning
that at the same time bromine enhances overall acceptor
strength, and therefore a higher hyperpolarizability and a bath-
ochromic shift of the charge transfer (CT) band takes place for
A-3 in comparison with A-1 and A-2.
Linear and nonlinear optical properties in solutions
The CT band maxima wavelength and extinction coefficients of
diluted solutions of the compounds (10�6–10�5 mol l�1) in CHCl3
This journal is ª The Royal Society of Chemistry 2012
Table 1 Results of RHF 6-31G(p,d) quantum chemical calculations in CHCl3
Compound m, D cos<bm>bm(0)� 1030 esu
bHRS(0)� 1030 esu
bm(532)� 1030 esu
bHRS(532)� 1030 esu
DR1 10.2 0.99 56.4 41.0 81.9 58.6DR19 9.3 1.00 55.0 39.4 79.4 55.9A-1 9.2 1.00 56.5 40.6 83.4 58.9A-2 9.5 1.00 56.9 40.5 83.4 58.7A-3 9.4 0.99 64.7 46.6 97.5 69.4A-4 2.6 0.43 13.5 22.9 20.7 33.6A-5 2.1 0.21 6.0 22.1 9.6 31.6DMABI 4.3 0.99 38.9 28.3 54.5 39.5I-1 2.8 0.95 39.9 29.9 57.1 42.9I-2 3.1 0.97 41.4 30.0 59.2 43.0
Table 2 Results of UV-vis spectral and HRS measurements of investi-gated compounds in CHCl3
Compoundlmax,nm 3max
bHRS(532)� 1030 esu
bHRS(0)� 1030 esu
DR1 481 35700 780 113A-1 486 32900 710 93A-2 485 30900 870 116A-3 506 33100 870 64A-4 433 32300 320 90A-5 431 32900 290 83DMABI 482 70800 950 135I-1 485 70900 1750 224I-2 485 72200 1890 253
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are given (Table 2). In each case a single wide absorption peak
characteristic of the given chromophore classes is observed.
Comparison of triaryl substituted compounds (A-1 and A-2, I-1
and I-2) with the corresponding reference chromophores DR1
and DMABI shows no noticeable differences in the absorption
bands (Fig. 2), indicating that no interactions occur between the
chromophore core and modifying groups.
The molecular hyperpolarizabilities of the azo compounds
obtained by HRS are in good correlation (0.97) with quantum
chemical calculation results. At the same time, probably as
a result of underestimation of resonance effects by RHF calcu-
lations, a scaling factor (�12) between QC-calculated and HRS-
measured values takes place. In the case where indanedione
derivatives were included in the comparison of measured and
QC-calculated values this correlation breaks down. Values
measured by HRS for indanedione derivatives are much higher
than one would expect from QC calculations. This disparity
could be explained by the high proportion of TPL signal in the
measured 532 nm light intensity. Numeric simulations showed
that when TPL has an equal or larger intensity than HRS
intensity, our deconvolution procedure fails, resulting in an
overestimation of bHRS values. Clarification of bHRS values for
indanedione derivatives will be reported in the future after more
detailed spectroscopic HRS investigations.
Fig. 2 UV-vis absorption bands of triaryl functionalized chromophores
and their non-modified counterparts. No changes are observed in
absorption properties.
Linear and nonlinear optical properties of films
The UV-vis absorption spectra of thin films are presented in
Fig. 3. Given the fact that the chromophores under investigation
have high dipole moment values, under normal conditions
This journal is ª The Royal Society of Chemistry 2012
head-to-tail stacking (J-aggregation) was expected to dominate,
producing a bathochromic shift. The lack of such shifts in the
spectra leads us to assume that sterically bulky triaryl substitu-
ents do not allow the preferred solid phase packing of chromo-
phores. Furthermore, final packing in the solid phase determined
by p–p stacking of aromatic rings causes the apparent presence
of hypsochromically shifted shoulder peaks, characteristic of the
formation of H-aggregates where linear chromophores are
stacked face-to-face with each other.
The absorbance maxima wavelengths, corresponding absorp-
tion coefficient, as well as refractive indices and NLO coefficients
J. Mater. Chem., 2012, 22, 11268–11276 | 11273
Fig. 3 UV-vis absorption coefficient spectra of several thin films.
Hypsochromic shift is observed in all cases.
Table 4 Decomposition, glass transition and TSHI50 temperatures ofmaterials
Compound Tdec,�C Tg,
�C TSHI50,�C
A-1 292 75 49A-2 295 51 52A-3 315 53 53A-4 300 —a 84A-5 330 71 56I-1 330 88 85I-2 340 62 56
a No Tg peak was observed in DSC thermograms.
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for the investigated compounds are presented in Table 3. This
table presents the experimental d33 values corrected to zero
frequency according to the two level model,39 which is usually
used for chromophore comparison and benchmarking. The
compound A-1 is an exception, as this sample underwent fast
crystallization after the corona poling procedure at temperatures
close to Tg.
The obtained NLO coefficients are within expectations
based on QC calculations. The highest nonlinearity d33(532) ¼300 pm V�1 was achieved in the case of A-3. The high value may
be explained by more pronounced resonance effects due to the
proximity of the chromophore absorbance band maximum to the
SH frequency. In any case, value d33(0) ¼ 19.6 pm V�1 corrected
to zero frequency is still the highest observed among all investi-
gated structures. The weakest nonlinearity among the azochro-
mophore based glasses is observed for compounds A-4 and A-5.
As discussed above, this is caused by the lost acceptor strength
and the spatial mismatch of hyperpolarizability with the dipole
moment. Just a slightly higher nonlinearity was achieved for
indanedione based films. In comparison with A-2, smaller bm and
dipole moment value characteristics for I-2 result in an almost
five times lower d33(0) value under similar poling conditions.
Based on QC calculation results one could expect quite similar
behaviour for I-1, but that is not the case. For these films, the
lowest ratio of d33/d31 ¼ 1.76 was observed indicating incomplete
poling. For the structurally similar chromophore pairs A-4, A-5
and I-1, I-2, under the applied poling conditions a higher
nonlinearity was observed for the triphenylsilyl compounds. This
can be expected from the higher conformational flexibility
predicted by QC modelling. During chromophore poling this
enhanced flexibility allows easier alignment of molecular dipoles
Table 3 Linear and nonlinear optical properties of amorphous phase
Compoundlmax,nm
amax
� 10�4, cm�1 RI532
A-1 490 3.23 —A-2 487 3.41 1.93A-3 509 4.85 2.34A-4 437 2.71 1.84A-5 438 2.09 1.79I-1 490 5.03 1.90I-2 489 5.56 1.94
11274 | J. Mater. Chem., 2012, 22, 11268–11276
along the external electrical field direction. It is probable that, for
trityl moiety containing materials, a longer poling time is
necessary to achieve higher polar order.
Thermal and phase behaviour properties
The thermal properties of the synthesized compounds were
explored using differential scanning calorimetry (DSC) and
complementary data showing temperature dependent decay of
second harmonic generation intensity TSHI50. The results of these
measurements are given in Table 4.
The DSC thermograms analyzed (Fig. 4) were taken during
the second heating of the samples that underwent initial heating
above their melting temperature, followed by rapid cooling
(50 �C min�1). Although compounds form glass during slow
cooling of melt (10 �Cmin�1) fast cooling was necessary to obtain
distinct Tg peaks. Even with fast cooling and repeated
measurements no Tg signal was obtained in the case of
compound A-4 (see Supplementary Information† for full heat-
ing–cooling–heating cycles). In the all cases, except for
compound I-1, the absence of any additional peaks associated
with the crystallization or melting processes in the temperature
range up to 200 �C indicates the formation of a thermodynami-
cally stable amorphous state. Compound I-1 shows an
exothermic crystallization peak during both heating and slow
(10 �C min�1) cooling at 170 �C. This peak disappears during
a fast cooling (50 �C min�1).
With the DSC measured glass transition temperature values,
Tg well above ambient, the materials are considered stable at
room temperature. They are in the good agreement with the half
NLO activity temperatures (TSHI50), evidently linking the loss of
acentric molecular order and increased conformational freedom.
Considering the chemical structures of the compounds, it is
apparent that the Tg values are greatly influenced by the chosen
RI1064
d31(532),pm V�1
d33(532),pm V�1
d33(0),pm V�1
— — — —1.61 31.9 83.5 10.71.69 135.0 300.0 19.61.65 3.2 9.5 2.571.62 3.7 11.5 3.081.60 9.3 16.4 1.961.59 12.6 34.1 4.17
This journal is ª The Royal Society of Chemistry 2012
Fig. 4 DSC thermograms showing Tg peaks.
Fig. 6 Optical microscope images of corona poled samples for
compounds I-1 (a) and I-2 (b) after a month of storage. Sample (a)
contains numerous growing crystals while sample (b) reveals no visible
signs of crystallization.
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triaryl group. Structurally similar chromophore pairs A-4, A-5
and I-1, I-2 in both cases show increased material thermal
stability by up to 30 �C when a trityl group is present as
demonstrated in Fig. 5.
This observation once more confirms our QC calculation
results, showing that the C–CPh bond allows less rotational
freedom for phenyl groups than the Si–CPh bond, thus increasing
energy requirements for phase transition processes. While
rigidness of the molecule may seem a counter-intuitive property
for obtaining amorphous material, the steric demands of trityl
group seem sufficient to inhibit aggregation processes under
supercooled liquid conditions. Nevertheless, the general
assumption, that a stand-alone triphenylsilyl group is better
suited as an amorphous phase formation enhancer, is still valid
according to our observations. Quality evaluation of the
prepared samples with an optical microscope after a month of
storage showed signs of crystallization processes in compounds
containing the trityl group. Comparative illustrations of the
samples made from compounds I-1 and I-2 are given in Fig. 6.
Moreover, in the case of compound A-1, complete crystallization
was observed in a matter of days after the samples underwent
poling close to Tg. While the trityl group can generally be
considered less suitable for stable glass formation, it is interesting
to note that the examination of samples made from the
compound containing an additional trityl moiety in the acceptor
Fig. 5 Polar order thermal stability of structurally comparable chro-
mophore pairs. Trityl group containing compounds show significantly
higher thermal sustainability.
This journal is ª The Royal Society of Chemistry 2012
part, A-4, revealed no signs of crystallization, indicating
increased amorphous phase stability in the presence of bulky
substituents at both ends of the linear chromophore.
Conclusions
In this paper we have presented a new structural design of low
molecular organic glasses suitable for photonics studies. The
synthesized triaryl functionalized N-phenyldiethanolamine
derivatives have shown excellent solubility in non-polar organic
solvents and are able to form good optical quality glassy films
without mixing in a polymer matrix. The stability of the films was
sufficient to undergo corona discharge poling at elevated
temperatures, making the materials NLO active.
Triphenyl moieties as crystallization preventing structural
elements have shown no effect on electronic properties of push–
pull NLO chromophores. QC calculations have revealed
noticeable differences in the chosen triphenyl moiety structure
regarding the conformational freedom and occupied volume.
Due to these dissimilarities the replacement of carbon with
silicon in the triphenyl core has a conflicting impact on the
properties of the investigated NLO active organic glasses. On the
one hand, the amorphous phase formation favours the presence
of a bulkier and conformationally less defined triphenylsilyl
group which was most evident in the case of azochromophore
containing compounds. At the same time this structural element
reduces the thermal stability of polar order in corona poled films.
For structurally comparable pairs (A-4 and A-5, I-1 and I-2) the
temperature corresponding to the half NLO activity, TSHI50, is
lowered by �30 �C. It is noteworthy that the presence of an
additional trityl group in the acceptor part of azochromophores
enhanced the amorphous phase stability and the polar order
thermal stability of corona poled films. At the same time the
increased rigidness of triphenyl groups resulted in lowered
nonlinearity due to obstructed alignment of molecular dipoles
along the externally applied electrical field direction during
corona discharge poling.
Acknowledgements
This work has been supported by the European Social Fund
within the project ‘‘Support for the implementation of doctoral
studies at Riga Technical University’’ and European Recon-
struction and Development Fund project LIAA Nr.2010/0308/
2DP/2.1.1.1.0/10/APIA/VIAA/051; ‘‘Development of polymer
based electro-optical modulator prototype’’.
J. Mater. Chem., 2012, 22, 11268–11276 | 11275
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