1. Molecular Spectroscopy Symposium @ OSU June 2012 1 TRANSIENT ABSORPTION AND TIME-RESOLVED FLUORESCENCE STUDIES OF SOLVATED RUTHENIUM DI-BIPYRIDINE PSEUDO-HALIDE COMPLEXES R. Compton, 1 D. Weidinger, 2 D. J. Brown, 3 W. Dressick 4 and J. C. Owrutsky 5 1 NRL-NRC Postdoctoral Fellow 2 Schafer Corporation, Arlington, VA 3 United States Naval Academy, Annapolis, MD 4 Center for Biomolecular Science & Engineering, Naval Research Laboratory, Washington, DC 5 Chemistry Division, Naval Research Laboratory, Washington, DC International Symposium on Molecular Spectroscopy 19 - June - 2012

2. Molecular Spectroscopy Symposium @ OSU June 2012 2 Ru(bpy) 2 (X) 2 Complexes Optical studies of ruthenium complexes Background - ruthenium bipyridine (bpy) complexes Sensitizers for solar cells (DSSCs) i.e., Ru N3 and related dyes see Kalyanasundaram & Gratzel, Coord. Chem. Rev., 1998 NCS ligands increase & redshift absorption Operate at ~ 10% efficiency Investigate related ligands in Ru(bpy) 2 X 2, X = (CN, NCS, N 3 ) Approach Synthesize Ru(bpy) 2 X 2 (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, N 3

3. Molecular Spectroscopy Symposium @ OSU June 2012 3 Steady-State IR & Vis Spectra Solvent: DMSO IR absorption spectrum is consistent with previous studies Frequency depends on solvent Acceptor Number (Timpson et al., JPC 1996). Visible Excitation: Ground (singlet) state → 1 MLCT Rapid transition to low-lying triplet state (ICS): 1 MLCT → 3 MLCT (near unity quantum efficiency) Damrauer et al., Science 1997 IR AbsorptionVisible Absorption h N 3 ligand extends the Vis. Absorption toward the red (more than NCS) by 55 nm

4. Molecular Spectroscopy Symposium @ OSU June 2012 4 TA TB Ultrafast 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

5. Molecular Spectroscopy Symposium @ OSU June 2012 5 Vibrational Energy Relaxation Depends on Ligand More Than Solvent Ligand Dependence of T 1 0 (cm -1 ) T 1 Complex (ps) T 1 Anion (ps)* CN2043.953.3 ± 7.5 NCS2075.089.5 ± 12.477.0 ± 5.6 N3N3 2035.84.9 ± 0.610.7 ± 1.11 *Anions studies: Q. Zhong et al., Chem. Phys. Lett. 2004 K. Dahl et al., J. Chem. Phys. 2005 Transient Absorption Transient Bleach Solvent: DMSO Probe :bleach or absorption maximum Transient spectrum (CN Ligand) A strong ligand dependence is observed T 1 for ligand in complex follows same trend as solvated ion

6. Molecular Spectroscopy Symposium @ OSU June 2012 6 0 (cm -1 ) T 1 (ps) EtOH2043.942.3 ± 3.7 NMF2039.047.3 ± 5.9 DMSO2043.953.3 ± 7.5 Solvent Dependence of T 1 Surprisingly, no observable variation in T 1 due to solvent environment Vibrational Energy Relaxation Depends on Ligand More Than Solvent Ru(bpy) 2 (CN) 2 Decay measured in various solvents

7. Molecular Spectroscopy Symposium @ OSU June 2012 7 Absorption, emission spectra consistent with literature Emission energy increases with acceptor number (Timpson et al., JPC 1996) Electronic Properties Depend on Solvent Ru(bpy) 2 (CN) 2 Emission Spectra Demonstrates viability using environment to mediate emission behavior Emission (radiative) 3 MLCT → Ground state + h Competing Processes (nonradiative): 3 MLCT → Ground state + heat 3 MLCT ↔ dd (Ru) dd → Ground state/decomposition Caspar & Meyer, Inorg. Chem., 1983 E GS dd 3 MLCT EE

8. Molecular Spectroscopy Symposium @ OSU June 2012 8 Emission Lifetime Strongly Depends on Ligand Solvent: DMSO Measurements performed under ambient conditions Very weak emission for N 3 complex Emission Lifetimes N 3 (<5 ns) << NCS (39 ns) << CN (215 ns) N 3 lifetime should increase upon cooling (Nazerruddin et al., JACS 1993) Emission SpectraEmission Decay

9. Molecular Spectroscopy Symposium @ OSU June 2012 9 Summary & Future Prospects N 3 ligand increases and redshifts MLCT absorption Fast nonradiative relaxation – may not preclude effective charge transfer - Low temperature emission - Test e-injection into nanocrystalline TiO 2 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 N3N3 NCSCN SolventDMSO EtOHNMF Vis. Absorption max (nm) 568514503 Fluorescence max (nm) None708675 Fluorescence decay time (ns) < 539215 VER (ps)4.9 ± 0.689.5 ± 12.453.3 ± 7.542.3 ± 3.747.3 ± 5.9

10. Molecular Spectroscopy Symposium @ OSU June 2012 10 Funding: Office of Naval Research Sponsorship: National Research Council Acknowledgements