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Center for Nonlinear Studies |
The ultra-fast radiationless decay of the photoexcited DNA and RNA nucleobases is a behavior that is vital to life itself, since it helps to keep UV absorption, say from the sun, from leading to damaging photochemical reactions. In order to understand the decay mechanisms theoretically, it is necessary to map out the potential energy surfaces (PESs) of the ground and excited states of these bases, and locate minima, barriers and conical intersections, where different surfaces intersect and radiationless, nonadiabatic transitions are promoted. By analyzing these energy landscapes, pathways that can lead to either radiationless decay or fluorescence can be elucidated. Such explorations require high-level ab initio electronic structure methods, such as multi-reference configuration interaction (MRCI). Focusing on cytosine, a comprehensive picture of MRCI PESs will be presented, including minima, transition states, and conical intersections. Comparisons to some cytosine analogs, which are structurally similar to cytosine, but can display very different photophysical behaviors, will be made. One fluorescent analog in particular, 5M2P, will be emphasized. It will be shown that there are indeed many energetic and conformational similarities between cytosine and 5M2P in their respective excited state landscapes, and the reasons for radiationless decay as opposed to fluorescence can be attributed to fairly subtle energetic differences between these two bases. Theoretical studies of the influence of water solvent on the photophysical properties of uracil and cytosine will also be presented.
Host: Sergei Tretiak, T-1: PHYSICS AND CHEMISTRY OF MATERIALS, 667-8351