1. Solvent Effects on the Excited State Dynamics of 1-cyclohexyluracil Patrick M. Hare Bern Kohler The Ohio State University Department of Chemistry 100 West 18 th Avenue Columbus, OH 43210 International Symposium on Molecular Spectroscopy 23 June 2005

2. Environment9H-AdenineThymineUracil Aqueous solution 1 τ = 0.18 τ = 0.54 a τ = 0.21 a Acetonitrile 2 τ = 0.44 Gas phase 3,4 τ = 1.0 τ = 6.4 τ = >100 b Gas phase 5 τ = 22000 τ = 12000 c Gas phase 6 τ = 0.100 τ = 1.1 τ = 0.105 τ = 5.12 τ = 0.130 τ = 1.05 1 Crespo-Hernandez, et al. Chem. Rev. 2004, 104, 1977. 2 Cohen, et al. J. Am. Chem. Soc. 2003, 125, 13594. 3 Kang, et al. J. Chem. Phys. 2003, 118, 6717. 4 Kang, et al. J. Am. Chem. Soc. 2002, 124, 12958. 5 He, et al. J. Phys. Chem. A 2004, 108, 943. 6 Canuel, et al. J. Chem. Phys. 2005, 122, 074316 a ribonucleoside b assigned to 3 Thy c Thy(H 2 O) 1 Base S 1 Lifetimes in ps Introduction: Nucleic Acid Photophysics Thymine Uracil Adenine Cytosine

3. nn Introduction: Uracil Photophysics   S0S0 T0T0 -Highest triplet yield of the bases -Φ ISC reported in solvents with dielectric constants of 80 (H 2 O) – 25 (EtOH) -10-fold increase in Φ ISC in less polar solvents -μs lifetime quenched by O 2, self- quenching -Peaks at ~370 nm Our Tasks:  Investigate the excited state dynamics, especially in low polarity solvents and aprotic solvents  Determine the triplet yields  Investigate pump wavelength dependence The Triplet State in Uracil The Singlet State in Uracil -~200 fs lifetime in aqueous solution -ESA peaks at ~600 nm nn Gas phase / nonpolar solution nn Aqueous Solution

4. Tools: Transient Absorption, 1-cyclohexyluracil Uridine (Urd) 1-cyclohexyluracil (1CHU) Pump 266 nm I.C. S1S1 S0S0 SNSN Probe (visible) I.S.C. T0T0 Probe (visible) T1T1 Probe (UV)

5. Results: Transient Absorption Acetonitrile 1 Cohen, et al. Faraday Disc. 2004, 127, 137 Uridine in aqueous solution: τ Singlet = 0.21 ± 0.03 ps 1 τ Singlet = 0.27 ps τ Therm. = 4.5 ps τ Trip = 5 ns

6. From Wood, P. D. and R. W. Redmond, JACS, (1996) 118, 4256 Results: Transient Triplet Spectrum

7. Results: UV Probe τ VC = 12 ps Acetonitrile τ Trip = 5 ns τ Singlet = 0.27 ps τ Therm. = 4.5 ps τ Trip = 5 ns

8. Results: Oxygen Quenching in Acetonitrile 400 nm probe 250 nm probe

9. SolventDielectric constant  Singlet (ps)  2 (ps)  Trip (ns) pH 7 buffer * 80.100.3 ± 0.117 ± 42.5 ± 5 acetonitrile36.640.27 ± 0.084.5 ± 15 ± 1 methanol * 33.00.2 ± 0.1200 ± 10010 ± 20 n-butanol17.840.20 ± 0.014 ± 2 n-pentanol15.130.22 ± 0.014 ± 1 chloroform4.810.21 ± 0.02110 ± 705 ± 2 * Single wavelength fits Protic solvents Results: Lifetimes from Global Fits of Visible Probe Transients SolventDielectric constant  Singlet (ps)  2 (ps)  Trip. (ns) pH 7 buffer * 80.100.3 ± 0.117 ± 42.5 ± 5 acetonitrile36.640.27 ± 0.084.5 ± 15 ± 1 methanol * 33.00.2 ± 0.1200 ± 10010 ± 20 n-butanol17.840.20 ± 0.014 ± 2 n-pentanol15.130.22 ± 0.014 ± 1 chloroform4.810.21 ± 0.02110 ± 705 ± 2

10. Results: Relative Quantum Yields / H-Bonding 1 Salet, C., R. Bensasson, et al. Photochem. Photobio. 1979, 30, 325. Dielectric Constant Φ ISC (exp.) 1CHU Φ ISC (lit.) 1 Uracil Aq. Solution80.100.040.02 Methanol33.00.0750.06 Acetonitrile36.640.250.2 Dielectric Constant Φ ISC (exp.) 1CHU Φ ISC (lit.) 1 Uracil Aq. Solution80.100.040.02 Methanol33.00.0750.06 Acetonitrile36.640.250.2

11. Results: Variable Pump Wavelength No observable changes in dynamics with variable pump wavelength Probe: 400 nm τ Therm. = 5.0 ± 1.2 ps τ Trip = 5 ± 1 ns

12. Conclusions 1CHU is soluble in solvents with ε >5 ns lifetime observed in 1CHU is the triplet state Triplet is formed very rapidly Excited state lifetime shows no appreciable solvent effects Hydrogen bonding is important for decreasing Φ ISC No change in lifetimes with varying pump wavelength Future work Determine Φ ISC in more solvents Further investigation of the variation of Φ ISC with pump wavelength

13. Acknowledgements Dr. Bern Kohler Funding National Institutes of Health Dr. Terry Gustafson Kohler Group

14. Obligatory Laser System Slide 1

15. Obligatory Laser System Slide 2

16. Results: Relative Amplitudes in Acetonitrile

17. 266 nm τ IC = 0.2 ps S1S1 S0S0 SNSN τ ISC ~ 0.2 ps T0T0 T1T1 ~600 nm τ ISC ~ 5000 ps ~370 nm τ Therm ~ 5 ps τ VC = 12 ps Results: Summary of Dynamics in Acetonitrile