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Ultrafast Decay of Electronically Excited Singlet Cytosine via a π,π* to nO,π* State Switch

Ismail, Nina, Blancafort, Lluís, Olivucci, Massimo, Kohler, Bern, Robb, Michael A.·2002·Journal of the American Chemical Society

Abstract

Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump-probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistribution (IVR) to populate a carbon pyramidalization mode, (2) a state switch between the pi,pi* and nO,pi* (excitation from oxygen lone pair) excited states, and (3) decay to the ground state through a conical intersention. A second decay path through the nN,pi* state (excitation from the nitrogen lone pair), with a higher barrier, involves out-of-plane bending of the amino substituent.

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Authors
Ismail, N., Blancafort, L., Olivucci, M., Kohler, B., & Robb, M. A.
Venue
Journal of the American Chemical Society
Published
2002-05-25
Volume
124
Issue
24
Pages
6818-6819
ISSN
  • 0002-7863 (ISSN-L)
  • 1520-5126
OpenAlex
W2064526317
Cited by
315
References
0
FWCI
11.97
Type
journal article
Language
en
Open access
closed

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