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 International Symposium on Molecular Spectroscopy 23 June 2005

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 = τ = c Gas phase 6 τ = τ = 1.1 τ = τ = 5.12 τ = τ = Crespo-Hernandez, et al. Chem. Rev. 2004, 104, Cohen, et al. J. Am. Chem. Soc. 2003, 125, Kang, et al. J. Chem. Phys. 2003, 118, Kang, et al. J. Am. Chem. Soc. 2002, 124, He, et al. J. Phys. Chem. A 2004, 108, Canuel, et al. J. Chem. Phys. 2005, 122, 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

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

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)

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

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

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

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

SolventDielectric constant  Singlet (ps)  2 (ps)  Trip (ns) pH 7 buffer * ± ± 42.5 ± 5 acetonitrile ± ± 15 ± 1 methanol * ± ± ± 20 n-butanol ± ± 2 n-pentanol ± ± 1 chloroform ± ± 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 * ± ± 42.5 ± 5 acetonitrile ± ± 15 ± 1 methanol * ± ± ± 20 n-butanol ± ± 2 n-pentanol ± ± 1 chloroform ± ± 705 ± 2

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. Solution Methanol Acetonitrile Dielectric Constant Φ ISC (exp.) 1CHU Φ ISC (lit.) 1 Uracil Aq. Solution Methanol Acetonitrile

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

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

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

Obligatory Laser System Slide 1

Obligatory Laser System Slide 2

Results: Relative Amplitudes in Acetonitrile

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