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Conical intersections in thymine.

Serhiy Perun¹, Andrzej L Sobolewski, Wolfgang Domcke

¹Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland.

The Journal of Physical Chemistry. A

|December 8, 2006

Summary

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Radiationless deactivation in thymine is explained by conical intersections. The lowest energy conical intersection provides a fast pathway for excited state quenching, explaining its diffuse absorption spectrum.

Area of Science:

Photochemistry
Quantum Chemistry
Molecular Spectroscopy

Background:

Thymine's excited states play a crucial role in photochemistry and photobiology.
Understanding radiationless deactivation pathways is key to explaining thymine's spectral properties.

Purpose of the Study:

Investigate the mechanisms of radiationless deactivation for thymine's excited singlet states.
Characterize conical intersections between excited and ground electronic states.

Main Methods:

Utilized multireference ab initio methods, including Complete-Active-Space Self-Consistent-Field (CASSCF) and CASSCF second-order perturbation theory (CASPT2).
Employed the CC2 (approximated singles and doubles coupled-cluster) method.
Calculated vertical and adiabatic excitation energies, equilibrium geometries, and conical intersections.

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Main Results:

Located three conical intersections (CI) between the S1 and S0 energy surfaces.
Identified the lowest-energy conical intersection (CI1) as a barrierless pathway from the 1pipi* state to the ground state.
Found that CI1 involves strongly out-of-plane distorted geometries.
Determined that optically dark 1npi* states lie below CI1.

Conclusions:

The direct and fast accessibility of CI1 explains the diffuse absorption spectrum of thymine.
The energetic positioning of dark 1npi* states below CI1 accounts for the long-lived population of dark excited states observed experimentally.