1. Ultrafast Spectroscopy Of Methyl Viologen: Effects Of Zeolite Entrapment Joseph Henrich, Haoyu Zhang, Jeremy White, Prabir Dutta and Bern Kohler The Ohio State University Department of Chemistry Molecular Spectroscopy Symposium June 17, 2008

2. zeolite cavities Electron acceptor solid zeolite membrane h photosensitizer molecule fluid phase charge transport initial CS state Efficient and long-lived charge separated state of a Ru polypyridyl complex e- fs pump-probe spectroscopy of MV 2+ in zeolite cavities

3. Methyl Viologen Good electron acceptor E° (MV 2+ /MV+ ) = -0.45 V Highly efficient excited-state electron acceptor E° (MV 2+ */MV+ ) = 3.65 V Photoreduction in methanol takes less than 250 fs (Peon, JPCA 2001) Excellent probe molecule for electron transfer from surrounding environment

4. Framework Structure  Crystalline, well-defined nano-pore structure  Charged framework  Spatial arrangement  Extreme Thermal & Chemical Stability Zeolites Pores of Zeolite Y 7.4×13Å

5. Zeolite Entrapment Size Discrimination Ion Exchange Encapsulation

6. Ultrafast ET in a H-bonding donor solvent Forward ET is instrument- limited (t  250 fs) from femtosecond fluorescence upconversion Ultrafast decay due to back ET (t = 400 fs) Approximately 12% MV radial cation formation Probe 600 nm Probe 270 nm Pump 265 nm J. Peon et al., J. Phys. Chem. A, 2001, 105, 5768.

7. MV 2+ Fluoresces in Nitrile Solvents fluorescence quantum yield, f F = 0.03  f = 1.0 ns emissionexcited state absorption

8. Water is Important ps Pump 266 nm Probe 600 nm No radical formation

9. MV in Zeolites Reports of methyl viologen radical formation when entrapped inside of zeolites supercages In Zeolite Y (Scaiano, JCPB 1997) In Zeolite X (Ranjit, JPCB 2002)

10. 0 10 20 30 40ps 0 0.2 r(t) zeolite/acetonitrile acetonitrile zeolite/water water 250 350 450nm absorptionemission Effects of Zeolite Entrapment in Solvated Zeolites absorptionemission Normalized Absorption and Emission 500450400350300250 Wavelength (nm) 250 350 450 nm anisotropy

11. Time delay (ps) MV 2+ in H 2 OMV 2+ in Z/H 2 O Pump 266 nm Probe 600 nm t 1 = 3.3 ± 0.1 ps t 1 = 4.6 ± 1.3 ps t 2 = 37.7 ± 6.8 ps 0 50 100 150 200ps Transient absorption of entrapped methyl viologen shows a bulk solvent like decay as well as a 25-40 ps decay component Photodynamics of MV2+ in a NaY Zeolite MV 2+ in ACN MV 2+ in Z/ACN t 1 = 24.8 ± 2.35 ps t 2 = 414 ± 59 ps t 1 = 817 ± 18 ps 0 500 1000 1500 2000ps

12. Conclusions The methyl viologen excited state has distinct decay pathways that are solvent dependent. Nano-zeolites make it feasible to make traditional pump/probe transient absorption measurements on encapsulated molecules with high signal-to-niose. When entrapped in zeolites methyl viologen is rigidly held within the supercage. A new decay process is created due to electron transfer from the zeolite framework to excited-state methyl viologen.

13. Acknowledgments Dr. Bern Kohler Dr. Prabir Dutta Dr. Haoyu Zhang Jeremy White Kohler Group Funding Department of Energy