The Ohio State UniversityDepartment of Chemistry Ultrafast Vibrational Cooling Dynamics in 9­Methyladenine Observed with UV Pump/UV Probe Transient Absorption Spectroscopy Chris Middleton, Boiko Cohen and Bern Kohler

The Ohio State UniversityDepartment of Chemistry - The bases in DNA and RNA absorb damaging UV light - UV initiated photochemistry can lead to skin cancer - IC places large amount of energy into the vibrational modes of ground state (> 1000 K) INTRODUCTION: DNA PHOTOPHYSICS

Intramolecular Vibrational Redistribution (IVR) - 100’s of femtoseconds Vibrational Cooling (VC) - 2 – 20 picoseconds VIBRATIONAL RELAXATION IN SOLUTION Vibrational population decay times can be predicted by Landau- Teller formula if the vibrational friction spectrum (  ) is well-known Many mechanism for vibrational relaxation are available to polyatomic molecules Deak et al. J. Raman Spec. 2000, 31, 263 VC is well described by a two stage model in many systems

Visible probe monitors S 1 population decay UV probe monitors S 0 population recovery The Ohio State UniversityDepartment of Chemistry TRANSIENT ABSORPTION 255 nm 285 nm Ground State Absorption of 9MA:

The Ohio State UniversityDepartment of Chemistry / nm Time / ps Pecourt, et al. J. Am. Chem. Soc. 2000, 122, 9348 Pecourt, et al. J. Am. Chem. Soc. 2001, 131, Dynamics observed in UV are much slower than those observed in the visible  vibrational cooling? - VC is very fast (0.4 – 2 ps)  VC mediated by hydrogen bonds? UV PROBE TA OF ADENOSINE IN H 2 O

The Ohio State UniversityDepartment of Chemistry TRANSIENT ABSORPTION OF 9MA IN H2O

The Ohio State UniversityDepartment of Chemistry SOLVENT EFFECT ON UV PROBE SIGNALS Large effect for aprotic vs. protic solvent (factor of 7) H/D Kinetic Isotope Effect (KIE) ~ 2

The Ohio State UniversityDepartment of Chemistry Cohen, et al. J. Am. Chem. Soc. 2003, 125, Solvent9MA S1 Lifetime H2OH2O220 ± 20 fs D2OD2O CH 3 CN350 ± 20 fs * Internal conversion rate is independent of solvent * (LACK OF A) SOLVENT EFFECT ON S 1 LIFETIME 560 nm probe

The Ohio State UniversityDepartment of Chemistry MACROSCOPIC PROPERTIES SolventH2OH2OD2OD2OMeCN Vibrational Cooling Time (ps) Ratio to H 2 O Value Heat Capacity (J/mol*K) Ratio to H 2 O Value Thermal Conductivity (W/m*K) Ratio to H 2 O Value Lack of a strong correlation with macroscopic properties suggests a microscopic mechanism is important

The Ohio State UniversityDepartment of Chemistry KINETIC ISOTOPE EFFECT – FRICTION EFFECT? Vibrational friction spectrum from Chorny et al. J. Chem. Phys. 2002, 116, MA frequencies from Xie and Yan Int. J. Quant. Chem. 2000, 76, 686 9MA-d0 9MA-d2 Magnitude of vibration friction is relative unchanged for modes associated with hydrogen bonding amino group Large increase in D 2 O at 600 cm -1 – 1200 cm -1 Importance of high frequency region suggest IP theory regime

Vibrational population decay time is proportional to reduced mass Vibrational friction for amino group modes is the same in H 2 O and D 2 O Good agreement with KIE for 9MA = 1.72 ± 0.08 The Ohio State UniversityDepartment of Chemistry KINETIC ISOTOPE EFFECT – HYDROGEN BONDING?

The Ohio State UniversityDepartment of Chemistry EFFECT OF INITIAL TEMPERATURE 9MA: 3N – 6 = 48T IVR ~ 1800 K AMP: 3N – 6 = 81T IVR ~ 1200 K 9MA KIE = 1.72 ± 0.08AMP KIE = 1.4 ± 0.1

The Ohio State UniversityDepartment of Chemistry CONCLUSIONS Long dynamics observed in the UV are assigned to vibrational cooling of the hot ground state Large acceleration VC rate in protic solvents suggests the hydrogen bonds strongly mediate VC A KIE isotope effect for VC has been observed with two possible mechanisms QUESTIONS RAISED Solute-solvent vs. solvent-solvent hydrogens bonds? Importance of lower frequency vs. higher frequency modes?

The Ohio State UniversityDepartment of Chemistry Dr. Bern Kohler Dr. Boiko Cohen (Wasielewski Group, Northwestern University) Kohler Group Gustafson Group NIH International Symposium on Molecular Spectroscopy ACKNOWLEDGEMENTS