Theoretical Study toward Understanding Ultrafast Internal Conversion of Excited 9H-Adenine

Abstract

The CASPT2/CASSCF method with the 6-311G basis set and an active space up to (14, 11) was used to explore the ultrafast internal conversion mechanism for excited 9H-adenine. Three minima, two transition states, and seven conical intersections were obtained to build up the two deactivation pathways for the internal conversion mechanism. Special efforts were made to explore the excited-state potential energy surfaces near the Franck-Condon region and determine the various barriers in the processes of deactivation. The barrier required from the 1pipi (1La) state to deactivate nonradiatively is found to be lower than that required from the 1pipi (1Lb) state. On 250 nm excitation, the 1pipi (1La) state is populated, and the transition from 1pipi (1La) to the lowest 1npi state involves very low barriers, which may account for the observed short (<50 fs) lifetime of the 1pipi excited state. The deactivation of the lowest 1npi state is required to overcome a barrier of 3.15 kcal/mol, which should be responsible for the 750 fs lifetime of the npi excited state. On 267 nm excitation, the vibrationally active 1pipi (1Lb) state is populated. Excitation at 277 nm prepares the 1pipi (1Lb) state without much excessive vibrational energy, which may be responsible for the observed >2 ps lifetime.

New Insights into the Photophysics of DNA Nucleobases

“…4a. It should be noted that a similar scenario has been proposed and discussed for Cyt 32 and Ade 33 . However, we believe that this scenario should be valid for all DNA nucleobases since the only dark state common to all nucleobases is the nπ * state.…”

Proton/Hydrogen Transfer Mechanisms in the Guanine–Cytosine Base Pair: Photostability and Tautomerism

“…17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42 It has been demonstrated that the fast deactivation of NABs after UV light absorption is due to a barrierless decay of the bright spectroscopic ππ* singlet excited state towards the ground state. Femtosecond pump-probe experiments with the canonical NABs reported multiexponential decay channels in the femtosecond and picosecond timescales, a strong evidence for the stability of the natural NABs.…”

Primary processes underlying the photostability of isolated DNA bases: Adenine

“…Unfortunately, the *͞n * proximity effect explains neither the methylation nor solvation effects (for details, see ref. 1), and many theoretical studies have sought to explain the discrepancies (4,9,10,13,15,20,(23)(24)(25)(26). Interestingly, these time-resolved ion yield studies did not reveal differences between adenine and 9-methyl adenine (19,22), thus apparently disfavoring the * channel.…”

“…Studies of the UV absorption spectrum showed that the 252-nm band is predominantly a transition to an optically bright * state (6,7). However, very close in energy lie at least one other much darker * state, the optically dark n * state, and a * state that is discussed in more detail below (1,4,(8)(9)(10). A widely accepted mechanism for adenine electronic relaxation involves two sequential steps: *3 n * followed by n *3S 0 (ground state) internal conversion (1).…”