Molecular Spectroscopy OSU June 2012 1 TRANSIENT ABSORPTION AND TIME-RESOLVED FLUORESCENCE STUDIES...
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Transcript of Molecular Spectroscopy OSU June 2012 1 TRANSIENT ABSORPTION AND TIME-RESOLVED FLUORESCENCE STUDIES...
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
Molecular Spectroscopy Symposium @ OSU June 2012 3
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
Molecular Spectroscopy Symposium @ OSU June 2012 5
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
Molecular Spectroscopy Symposium @ OSU June 2012 6
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
Molecular Spectroscopy Symposium @ OSU June 2012 7
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
Molecular Spectroscopy Symposium @ OSU June 2012 8
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
Molecular Spectroscopy Symposium @ OSU June 2012 9
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
Molecular Spectroscopy Symposium @ OSU June 2012 10
• Funding: Office of Naval Research
• Sponsorship: National Research Council
AcknowledgementsAcknowledgements