Cobalt- bis(Dioxolene ) Valence Tautomers : Molecular Properties and Progress toward Devices

NC STATE UNIVERSITY UNC-Chapel Hill

Cobalt-bis(Dioxolene) Valence Tautomers: Molecular Properties and

Progress toward Devices

David A. Shultz,*1 Robert D. Schmidt,1 Will Rice,2 Frank Tsui,2 Marco Buongiorno-Nardelli3 and Arrigo Calzolari4

1Department of Chemistry, NC State University, Raleigh, NC

27695-82042Department of Physics, UNC-Chapel Hill

3Department of Physics, NC State University4Department of Physics, University of Modena

NSF CCI: Center for Molecular Spintronics (CMS)

The CMS is exploring the utility of switchable/bistable paramagnetic molecules and semiconducting oligomers (based on metal complexes of semiquinones) -- as new ways to understand and to control both spin injection and spin-polarized transport. Our approach involves a highly-integrated program consisting of:(1) synthesis and characterization of these molecules and oligomers,(2) detailed surface structure and bonding studies, (3) spin injection and transport measurements, and (4) computational modeling.

Collaborative Structure to Date

Shultz GroupMolecule and polymer synthesis

You GroupMonolayer synthesis

Tracy GroupNanoparticle synthesis

Tsui GroupValence Tautomer“crystal devices”

Dougherty GroupNanoparticle devices

Rowe GroupDevice characterization

Jones GroupEducation/Outreach

Buongiorno-Nardelli GroupTheory/Computation

New Valence Tautomersand New Studies of “Old” Valence

Tautomers

Using Electron Transfer to Affect “Long-Range Exchange” Coupling via Organic Double Exchange

One quartet, one doublet

-15J/4

-3J/4Very weak superexchange across 11 bonds

Intramolecular ET within the mixed-valent SQ-Cat dyad promotes ferromagnetic coupling: double exchange

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One quartet, one doublet

-15J/4

-3J/4 Very weak superexchange across 11 bonds

ET = f(Hab)Hab Hab

Using Electron Transfer to Affect“Long Range Exchange Coupling”

Temperature (K)

• C2h complex has p-overlap between metal/dyad that C2 complex lacks• J is a direct measure of mixed-valent wavefunction

J = +74 cm-1

(Doublet + Triplet model; 3J gap)

J ≈ +4 cm-1

(Doublet + Triplet model; 3J gap)

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> 2 nm

Phenyl ring torsions are ≈15° from SQ and NN ring planes

ORTEPs of C2h Complexes

SQ-NN torsion ≈ 15°

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> 1.4 nm

CoIIISQ(Cat) formulation with crystallographic inversion center at Co

cparaT=1.1253-uncorrelated spins

High Temp Limit

cparaT=1.8753-coupled spins

Low Temp Limit

Temperature (Kelvin)

a•T (e

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Magnetic Susceptibility for

• Only quartet state populated at all temperatures

cparaT=1.1253-uncorrelated spins

High Temp Limit

cparaT=1.8753-coupled spins

Low Temp Limit

Temperature (Kelvin)

a•T (e

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Magnetic Susceptibility for

• Quartet and doublet states populated

A more sophisticated HDvV fit of the magnetic data…

JA (5 bonds)JB (11 bonds)

Results of trimer fit support delocalization of SQ/Cat!

Temperature (K)

JA = JB = 234 cm-1

g = 2.0 (fixed)q = -2.06 K

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234 cm-1Q

JA + JB + JA2 + JB

2 − JA JB

JA + JB − JA2 + JB

2 − JA JB€

J = J0e−α r−r0( )

ˆ H Trimer = −2JA ( ˆ S i • ˆ S j ) − 2JB ( ˆ S j • ˆ S k ) − 2JC ( ˆ S i • ˆ S k )

JA ≠ JB ≠ 0; JC = 0

Matrix elements of interacting doublets formed from local SQPhNN singlet states

nmax = 2Hab

Cat SQ CT band in IR region!

Energy (cm-1)

Double Exchange/Spin-Dependent Delocalization Model…

Electronic Structure/Spin-Dependent Delocalization Approach…

Double exchange:Girerd, et al.. Chem. Rev. 1990, 90, 1359.

• Hab describes delocalization of the mixed-valent dyad one set of quartets and two sets of doublets• HDvV (Heitler-London)Double exchange (Zener)single J only: JB = 0

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ST =32

=−J Hab

Hab −J

ST = 12

−J Hab2 0 0

Hab2 −J 0 0

0 0 J −Hab2

0 0 −Hab2 J

Cat SQ

NN NNJ

SQ Cat

NN NNJHab

• vibronic structure• ∆n1/2 less than Hush prediction

• weak solvent dependence• Class II/III or Class III mixed valent

(DELOCALIZED)

nmax = 2Hab

Energy (cm-1)

Bersuker, et al., Adv. Chem. Phys. 1992, 81, 703.

Double Exchange/Spin-Dependent Delocalization Model…

Electronic Structure/Spin-Dependent Delocalization Approach…

Double exchange:Girerd, et al.. Chem. Rev. 1990, 90, 1359;

Off-diagonal blocks allow for interaction of doublet states six state SDD model

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ST =32

=−J Hab

Hab −J

ST = 12

−J Hab2 0 3

Hab2 −J 3

2 Hab 0

0 32 Hab J −Hab

2 Hab 0 −Hab2 J

nmax = 2Hab

Energy (cm-1)

3500 cm-1

Electronic Structure/Spin-Dependent Delocalization Approach…NC STATE UNIVERSITY

J + Hab − J 2 + Hab2 + JHab

J + Hab − J 2 + Hab2 − JHab

J + Hab + J 2 + Hab2 − JHab

J + Hab + J 2 + Hab2 + JHab

JCKirk, M. L.; Shultz, D. A.; Schmidt, R. D.; Rodriguez, D. H.; Lee, H.; Lee, J. J. Am. Chem. Soc. 2009, 131. 18304.

0 50 100 150 200 250 300

HDVVDouble Exchange

c paraT

Temperature

CoIII(py)2(SQPhNN)(CatPhNN)

6-State Double ExchangeErrorValue

0.0756-2q289465J

1.44e+058.57e+030.005161.99g

3-State HDVVErrorValue

0.0713-2.06q9.74e+05466J9.73e+05466J'0.0005012g

3-State HDvV

JA 234cm-1

JB 234cm-1

g 1.99q -2.06 K

-J+HAB

2J+Hab/2

2J-Hab/2

-Hab/2

-J-Hab

Classical Double ExchangeHDvV

JSQ-Ph-NN

= 100 cm-1

Thermally Accessible Excited

Singlet State

) ) )ˆ ˆ ˆ ˆ ˆ ˆˆ 2 2 2

, 0; 0

Trimer A i j B j k C i k

H J S S J S S J S S

0 50 100 150 200 250 300

HDVVDouble Exchange

c paraT

Temperature

CoIII(py)2(SQPhNN)(CatPhNN)

6-State Double ExchangeErrorValue

0.0756-2q289465J

1.44e+058.57e+030.005161.99g

3-State HDVVErrorValue

0.0713-2.06q9.74e+05466J9.73e+05466J'0.0005012g

6-State Double Exchange

J 465 cm-1

Hab 8570 cm-1

g 1.99q -2.00 K

3-State HDvV

JA 234cm-1

JB 234cm-1

g 1.99q -2.06 K

2J+Hab/2

2J-Hab/2

-Hab/2

-J+Hab

-J-Hab

S 2J D

2J DB*

JSQ-Ph-NN

= 100 cm-1

Classical Double ExchangeHDvV

Singlet State

−J + Hab

−J − Hab€

− J 2 + Hab2 + JHab

− J 2 + Hab2 − JHab

J 2 + Hab2 − JHab

Spin-Dependent Delocalization

J 2 + Hab2 + JHab

Kirk, M. L.; Shultz, D. A.; Schmidt, R. D.; Rodriguez, D. H.; Lee, H.; Lee, J. J. Am. Chem. Soc. 2009, 131. 18304.

) ) )ˆ ˆ ˆ ˆ ˆ ˆˆ 2 2 2

, 0; 0

Trimer A i j B j k C i k

H J S S J S S J S S

12,0, 1

32,1, 1

12,1, 1

State Energies calculated with J = 465 cm-1

HAB/cm-1

DoubletDoublet

Quartet

Energy (cm-1)

Spectroscopic Hab

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Evaluating Hab by bothTheory and Spectroscopy

“IVCT” nmax = 2Hab

For Hab > ~1000 cm-1, ∆EQD is constant

NC STATE UNIVERSITY UNC-Chapel HillElectronic Structure/Spin-Dependent Delocalization Approach…

3500 cm-1

229 cm-1

Temperature (K)

J = 580 cm-1

Hab = 1750 cm-1 (fixed)g = 1.99q = -2.06 K

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J + Hab − J 2 + Hab2 + JHab

J + Hab − J 2 + Hab2 − JHab

J + Hab + J 2 + Hab2 − JHab

J + Hab + J 2 + Hab2 + JHab

DA*S 2J D

Singlet State

JSQ-NN

≈ 500 cm-1

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• This gap should be larger

• Doublet thermally inaccessible

J = 580 cm-1

JSQNN = 100 cm-1

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Strong SQNN exchange in SDD molecule(s) can be explained using VBCI model

Kirk, M. L.; Shultz, D. A.; Schmidt, R. D.; Rodriguez, D. H.; Lee, H.; Lee, J. J. Am. Chem. Soc. 2009, 131. 18304.

New Valence Tautomersand New Studies of “Old” Valence

Tautomers

OCoIII

X = HX = OMe

X = CN

X = Br X = NO2

Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102, 4951-4957.Schmidt, R. D.; Shultz, D. A.; Martin, J. D. Inorg. Chem. 2010, 49, 3162-3168.

Schmidt, R. D.; Shultz, D. A.; Martin, J. D.; Boyle, P. D. J. Am. Chem. Soc. 2010, 132, 6261-6273.

X = Me

NC STATE UNIVERSITY UNC-Chapel HillNC STATE UNIVERSITY UNC-Chapel Hill

High temperature values:Non-active – 0.375 → CoIII(Cat)(SQ)

Low temperature values – 0.375 – Appropriate for CoIII(Cat)(SQ)

Schmidt, R. D.; Shultz, D. A.; Martin, J. D. Inorg. Chem. 2010, 49, 3162-3168; J. Am Chem. Soc. 2010, 132, 6261-6273.

Active – 2.85 – 4.75 → hs-CoII(SQ)(SQ)

Accessibility of metastable state via thermal cycling or irradiation

Thermal Conversion Photoinduced Conversion

0 2 4 6 8 10Cycle

c para

·T (e

0 5000 10000 15000

Decay % 10KDecay % 20KDecay % 30KDecay % 40KDecay % 50KDecay % 55KDecay % 60KDecay% 80K

Time (s)

c para

Long-lived kinetic stability

• Anisotropic lattice distortions implicate C-H•••N and C-H•••O interactions• Totally symmetric Co-L stretching coupled to lattice modes cooperativity ?

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Variable Resistor Circuits:Constant applied bias – variable current flow

Photoinduced Thermal-induced

Photoinducedor

Thermal-induced

SiO2 baseelectrode 2electrode 1

VT single crystal or polymer filmCurrent Measurement Devices

Field Effect Structures for StudyingElectronic Properties of Valence Tautomer

• SiOx/n-Si/Au back gate• Gate capacitance ~0.7 nF

(forward bias)• Crystalline Valence Tautomer

Au contacts without sample Au contacts with sample mounted

• Making contacts via solution• 10 m spacing between contacts

(Van der Pauw)• Forward gate bias to access

HOMO states

100 m 100 m

L1 (+ Gate)- to C3 on separate sample mount

(contacts back Au)

L8 (- Crystal)

L4 (+ Gate)

L3L2 (+ Crystal)

Electronic Transport of Tautomer source current vs. gate and source voltages

at room temperature

Source Voltage Vs (V

Gate Voltage Vg (V)

0-12 A

• High threshold voltage (HOMO edge): 7.1 V• Low mobility (hole): 10-6 cm2/V-s

A More Conductive Valence Tautomer is needed…

Polymeric Pyrazine-BridgedCobalt Semiquinonate-Catecholate Complex

• Photomechanical polymers that show light-induced transition between different spin states – different geometries

• Potential for functional molecular spintronic devices

• Synthesized (Pierpont, JACS, 1994) but not characterized so far

CoIII(N-N)-(DBSQ)(DBCat)Three spin configurations: Paramagnetic Ferromagnetic Antiferromagnetic

Eg(Γ)up = 0.54 eVEg(Γ)dw = 0.25 eV

Metal Semiconductor Semiconductor

Eg(Γ)up = 0.30 eVEg(Γ)dw = 0.30 eV

Dispersive bands indicate possibility for band transport along the polymer axis

Marco Buongiorno-Nardelli and Arrigo Calzolari

Buongiorno-Nardelli

New Magnetic Data

Conclusions• Electroactive Donor-Bridge-Acceptor biradicals can be used to prepare double exchanged systems that show “long-range electron correlation.”

• This is a general molecular architecture: mixed-valent organic dyad covalently attached to a pair of localized spins.

• Molecular analog of dilute magnetic semiconductors?

• Ongoing studies of valence tautomer-oriented devices

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Acknowledgements

$$$National Science Foundation (CHE-0910585 and -0943975)

NC STATE UNIVERSITY

Rob Schmidt Ubie Sullivan

Geoff LewisCooper Dawson Professor Marty Kirk

Diana Habel-Rodriguez Department of Chemistry, University of New Mexico

Professor Marco Buongiorno-NardelliDepartment of Physics, NC State University

Professor Arrigo Calzolari, Trieste

Professor Frank Tsui Will RiceDepartment of Physics, UNC-Chapel Hill

Ground Configuration

Quartet

Excited Configuration A

Quartet

Excited Configuration B

Quartet

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VBCI model used to explain ferromagnetic exchange in biradical ligand complex suggests stronger ferromagnetic exchange in SDD molecule(s) due to more superexchange pathways

SiO2 baseelectrode 2electrode 1

VT single crystal

Current Measurement Designs

Self Assembled Monolayers of VT and Devices

SHAu film

OCoIII

Wei YouCoII

Preliminary Results1.SAM of Pyridine thiol made;2.Ligand exchange with VT 1 was

attempted; it works!

3.XPS of surfaces were carried out; it seems that pyridine thiol was not making satisfactory SAM; new molecule was proposed and to be synthesized

CMS Concept Chart (Phase I)

Valence Tautomer PolymerSynthesis

Voltage, Temperature

TemperaturePressure

Cat SQ SQSQCoIII CoII

SemiQuinone Conjugated/Conducting Polymer Synthesis

Purpose: Investigation into carrier/spin transport mechanisms and spin valve properties of magnetic polymers when placed between source and drain.Note: The current backbone is the ortho-polymer, but future work could focus on other topologies (i.e., para and/or meta)

SemiQuinone Conjugated/Conducting Polymer Synthesis(pending)

Self Assembled Monolayers of VT and Devices

SHAu film

OCoIII

Wei You

FerromagnetX X X X X X X X

PEDOT:PSSTop Electrode

Ferromagnetic nanoparticle

VT molecules

Bottom Electrode

Future & Ultimate Goal

Electronic and Magnetic Properties of Molecular Materials

Will RiceDavid Harris and Michelle Labrecque (Undergraduates)

Prof. Frank TsuiPhysics UNC-Chapel Hill

• Electronic states and transport of valence tautomer− Collaborators: Prof. David Shultz and Rob Schmidt

(Chemistry NCSU)• Tunneling and spin dependent transport through SAMs of

organic molecules (e.g. alkanedithiols) − Collaborators: Prof. Wei You and Jeremy Niskala

(Chemistry UNC-CH)

• Overcome challenge of attaching 4 terminals to the crystals for measurements in Van der Pauw geometry

• Explore temperature and field dependent transport parameters

• Investigate electronic states, including band gap, mobility, density of states

• Examine effects of light exposure

What’s Next

Image of layered valence tautomer crystal

CoIII(SQ)(Cat) CoII(SQ)(SQ)

D. A. Shultz, et al., J. Am. Chem. Soc. 2005, 127, 5328

CoIICat SQ

CoIIICat SQ

Cobalt- bis(Dioxolene ) Valence Tautomers : Molecular Properties and Progress toward Devices

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