Photophysics of the π,π* and n,π* States of Thymine: MS-CASPT2 Minimum-Energy Paths and CASSCF on-the-Fly Dynamics

Abstract

The photodynamics along the main decay paths of thymine after excitation to the lowest pi,pi* state have been studied with MS-CASPT2 calculations and semiclassical CASSCF dynamics calculations including a surface hopping algorithm. The static calculations show that there are two decay paths from the Franck-Condon structure that lead to a conical intersection with the ground state. The first path goes directly to the intersection, while the second one is indirect and involves a minimum of the pi,pi* state, a small barrier, and a crossing between the pi,pi* and n,pi* states. From the static calculations, both paths have similar slopes. The dynamics calculations along the indirect path show that, after the barrier, part of the trajectories are funneled to the intersection with the ground state, where they are efficiently quenched to the ground state. The remaining trajectories populate the n,pi* state. They are also quenched to the ground state in less than 1 ps, but the static calculations show that the decay rate of the n,pi* state is largely overestimated at the CASSCF level used for the dynamics. Overall, these results suggest that both direct and indirect paths contribute to the subpicosecond decay components found experimentally. The indirect path also provides a way for fast population of the n,pi* state, which will account for the experimental picosecond decay component.

Ultrafast X-ray Auger probing of photoexcited molecular dynamics

“…They all showed a very short, B100 fs, decay constant followed by additional 5-7 ps decay [15][16][17][18] . The time constants are generally consistent with calculations of the potential energy surface [19][20][21][22][23] . However, the assignment of the transients to electronic or nuclear relaxation varies among theoretical approaches, as it depends strongly on the reaction path and reaction barrier in the pp* state, which is particularly sensitive to the chosen quantum chemical methodology.…”

“…This suggests an interpretation of the fastest time constant as electronic relaxation. A second, so-called indirect path 23 was suggested to compete with the direct ultrafast electronic relaxation to the ground state. The full dynamics simulation 19 predicts an initial ultrafast nuclear relaxation into the pp* state minimum involving the C(4)-O(8) stretch.…”

“…A return of photoexcited population to the vibrationally hot ground state as well as population of the dark np* state is possible and was controversially discussed in the theoretical literature [19][20][21][22][23] . The simulated np* spectrum (red line, Fig.…”

Probing the excited state relaxation dynamics of pyrimidine nucleosides in chloroform solution

“…[15][16][17] A host of theoretical calculations 28,37,[44][45][46][47][48][49][50][51][52] on the isolated thymine nucleobase led experimentalists to propose the above interpretation for the extracted time constants. However, transient vibrational absorption spectroscopy (TVAS) measurements by Kohler and co-workers on T in acetonitrile-d 3 indicate a significant fraction of excited state flux also undergoes intersystem crossing (ISC) into a longlived T 1 ( 3 ππ*) triplet state (with lifetime τ ≈ 0.6 µs).…”

Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2

“…Both static [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and dynamic [16,17,[35][36][37][38][39][40][41] theoretical studies have been performed over the last decade, the former usually employing more accurate and correlated methods due to their lower computational cost. Multiconfigurational methods are usually the preferred choice, particularly the complete active space self-consistent field (CASSCF) [42] and its second-order perturbation theory extension (CASPT2) [43].…”