1. The Ohio State UniversityDepartment of Chemistry Femtosecond Mid-infrared Spectroscopy Of Pyrimidine DNA Bases: Intramolecular Vibrational Redistribution, Vibrational Cooling And Excited State Photophysics Chris T. Middleton, Patrick M. Hare †, Kristin I. Mertel and Bern Kohler † Current address: University of Notre Dame Radiation Laboratory, Notre Dame, Indiana, 46556

2. Nucleic acid bases UV initiated photochemistry can lead to mutations and skin cancer (photocarcinogenesis)

3. Pyrimidine Photophysics in Solution

4. Hare, et al., JPCB 110 18641 (2006) AcetonitrileH2OH2O UV/Visible Transient Absorption of 1CHU 1-cyclohexyluracil (1CHU) - both the lifetime of 1 nπ* state and the triplet quantum yield are dramatically reduced in protic solvents - broad electronic absorption spectra of 1 nπ* and triplet strongly overlap

5. Transient mid-IR Spectra of 1CHU *

6. Kinetics of 1 nπ* band a Hare, et al., JPCB 110 18641 (2006) - solvent dependence of lifetime confirms assignment to 1 nπ* - no ground state modes between 1750 cm -1 and ~3000 cm -1 - excited state vibrational spectra provide information about the electronic structure, gauge of theoretical calculations

7. Thymine Triplet Dynamics excellent agreement between short and long time spectra confirms that triplet is formed rapidly (< 10 ps)

8. vibrationalcooling Vibrational cooling T ≈ 2000 K T ≈ 1800 K

9. Solvent Isotope Effects on Vibrational Cooling

10. Comparison of UV and IR Probing of Vibrational Cooling

11. Conclusions - 1 nπ* state of 1CHU characterized by a unusually blue-shifted vibrational band - Triplet band lies at 1600 cm -1 and appears within 10 ps, providing further support for the current model of ISC in pyrimidine bases - Solvent isotope effects for vibrational cooling suggest the energy transfer between high frequency solute and solvent modes is important - Differences between vibrational cooling rate measured by UV and mid- IR probing suggests that IVR may take place significantly slower than expected

12. The Ohio State UniversityDepartment of Chemistry Acknowledgements Bern Kohler Patrick Hare – UV/visible measurements on 1 nπ* and triplet, ns-TRIR Kristin Mertel – ns-TRIR Terry Gustafson, Matt Platz Gotard Burdzinski, Jacek Kubicki Kohler Group & Gustafson Group Funding: NIH Instrumentation: