Molecular Spectroscopy Symposium @ OSU June 2012 1

TRANSIENT ABSORPTION AND TIME-RESOLVED FLUORESCENCE TRANSIENT ABSORPTION AND TIME-RESOLVED FLUORESCENCE STUDIES OF SOLVATED RUTHENIUMSTUDIES OF SOLVATED RUTHENIUM

DI-BIPYRIDINE PSEUDO-HALIDE COMPLEXESDI-BIPYRIDINE PSEUDO-HALIDE COMPLEXES

R. Compton,1 D. Weidinger,2 D. J. Brown,3 W. Dressick4 and J. C. Owrutsky5

1 NRL-NRC Postdoctoral Fellow2 Schafer Corporation, Arlington, VA 3 United States Naval Academy, Annapolis, MD4 Center for Biomolecular Science & Engineering, Naval Research Laboratory, Washington, DC5 Chemistry Division, Naval Research Laboratory, Washington, DC

International Symposium on Molecular Spectroscopy19 - June - 2012

Molecular Spectroscopy Symposium @ OSU June 2012 2

Ru(bpy)2(X)2 Complexes

Optical studies of ruthenium complexesOptical studies of ruthenium complexes

Background - ruthenium bipyridine (bpy) complexes• Sensitizers for solar cells (DSSCs)

i.e., Ru N3 and related dyessee Kalyanasundaram & Gratzel, Coord. Chem. Rev., 1998

• NCS ligands increase & redshift absorption Operate at ~ 10% efficiency

• Investigate related ligands inRu(bpy)2X2 , X = (CN, NCS, N3)

Approach• Synthesize Ru(bpy)2X2

(D. Brown & W. Dressick) Structure: NMR and elemental analysis

• Steady state and transient measurements:-vibrational spectra & relaxation dynamics-absorption, emission spectra and emission lifetimes-solvent & ligand effects

• Compare/correlate with free anions in solution

Ligands (X): CN, NCS, N3

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Steady-State IR & Vis SpectraSteady-State IR & Vis Spectra

Frequency (cm-1)

2000 2050 2100 2150

Nor

mal

ized

Abs

orba

nce

0.0

0.2

0.4

0.6

0.8

1.0N3NCS CN

Solvent: DMSO

IR absorption spectrum is consistent withprevious studies

Frequency depends on solvent AcceptorNumber (Timpson et al., JPC 1996).

Wavelength (nm)400 500 600 700 800 900

Nor

mal

ized

Abs

orba

nce

(a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0CNNCSN3

Visible Excitation:Ground (singlet) state → 1MLCT

Rapid transition to low-lying triplet state (ICS):1MLCT → 3MLCT (near unity quantum efficiency)

Damrauer et al., Science 1997

IR Absorption Visible Absorption

h

N3 ligand extends the Vis. Absorption toward the red (more than NCS) by 55 nm

Molecular Spectroscopy Symposium @ OSU June 2012 4

TA

Frequency

Abs

orpt

ion TB

Ultrafast Experimental SetupUltrafast Experimental Setup

Ultrafast IR pump / IR probe

- 300 fs IR pulses, near 2000 cm-1

- Tunable around IR excitation frequency (~150 cm-1 range)

- Transients measured by:Absorption: decay of v=2←1 transition Bleach: recovery of v=1←0 transition

Emission Lifetime Measurements

- 400 nm pump

- Signal detected in a PMT

V = 0

1

2

TB

TA

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Vibrational Energy Relaxation Vibrational Energy Relaxation Depends on Ligand More Than SolventDepends on Ligand More Than Solvent

Time (ps)0 50 100 150N

orm

aliz

ed A

bsor

banc

e C

hang

e,

A

0.0

0.4

0.8

1.2

0 5 10 15-1.0

-0.5

0.0

NCS

CN

N3

Ligand Dependence of T1

0 (cm-1) T1 Complex (ps)

T1 Anion (ps)*

CN 2043.9 53.3 ± 7.5

NCS 2075.0 89.5 ± 12.4 77.0 ± 5.6

N3 2035.8 4.9 ± 0.6 10.7 ± 1.11

*Anions studies: Q. Zhong et al., Chem. Phys. Lett. 2004 K. Dahl et al., J. Chem. Phys. 2005

Frequency (cm-1)

2000 2020 2040 2060 2080 2100

Abs

orba

nce

Cha

nge

(mO

D)

-30

-20

-10

0

10

20

30

40

Transient Absorption

Transient Bleach

• Solvent: DMSO

• Probe : bleach or absorption maximum

Transient spectrum (CN Ligand)

• A strong ligand dependence is observed

• T1 for ligand in complex follows same trend as solvated ion

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0 (cm-1) T1 (ps)

EtOH 2043.9 42.3 ± 3.7

NMF 2039.0 47.3 ± 5.9

DMSO 2043.9 53.3 ± 7.5

Time (ps)0 50 100 150N

orm

aliz

ed A

bsor

banc

e C

hang

e,

A

0.0

0.5

1.0

1.5

2.0

2.5

3.0EtOH

NMF

DMSO

Solvent Dependence of T1

• Surprisingly, no observable variation in T1 due to solvent environment

Vibrational Energy Relaxation Vibrational Energy Relaxation Depends on Ligand More Than SolventDepends on Ligand More Than Solvent

• Ru(bpy)2(CN)2

• Decay measured in various solvents

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Wavelength (nm)550 600 650 700 750 800 850

Fluo

resc

ence

Inte

nsity

(a. u

.)

0.0

0.2

0.4

0.6

0.8

1.0 DMSO EtOH NMF AcCN

• Absorption, emission spectra consistent with literature •Emission energy increases with acceptor number (Timpson et al., JPC 1996)

Electronic Properties Depend on Solvent Electronic Properties Depend on Solvent

Ru(bpy)2(CN)2 Emission Spectra

Demonstrates viability using environment to mediate emission behavior

Emission (radiative)3MLCT → Ground state + h

Competing Processes (nonradiative):3MLCT → Ground state + heat

3MLCT ↔ dd (Ru)

dd → Ground state/decompositionCaspar & Meyer, Inorg. Chem., 1983

E

GS

dd

3MLCT

E

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Time (ns)0 200 400 600

Nor

mal

ized

Inte

nsity

(a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0CNNCS

Emission Lifetime Strongly Depends on LigandEmission Lifetime Strongly Depends on Ligand

Wavelength (nm)600 650 700 750 800 850

Nor

mal

ized

Inte

nsity

(a. u

.)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CN NCS N3

• Solvent: DMSO

• Measurements performed under ambient conditions

• Very weak emission for N3 complex

• Emission LifetimesN3 (<5 ns) << NCS (39 ns) << CN (215 ns)

• N3 lifetime should increase upon cooling (Nazerruddin et al., JACS 1993)

Emission Spectra Emission Decay

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Summary & Future ProspectsSummary & Future Prospects

• N3 ligand increases and redshifts MLCT absorption

• Fast nonradiative relaxation – may not preclude effective charge transfer - Low temperature emission

- Test e-injection into nanocrystalline TiO2 films (J. Pietron)

• VER rates are a potential probe for the charge transfer states - Vibrational dynamics open the door for further ligand effects

following visible excitation

N3 NCS CN

Solvent DMSO DMSO DMSO EtOH NMF

Vis. Absorption max (nm)

568 514 503

Fluorescence max

(nm)None 708 675

Fluorescence decay time (ns)

< 5 39 215

VER (ps) 4.9 ± 0.6 89.5 ± 12.4 53.3 ± 7.5 42.3 ± 3.7 47.3 ± 5.9

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• Funding: Office of Naval Research

• Sponsorship: National Research Council

AcknowledgementsAcknowledgements