New Insights into the State Trapping of UV-Excited Thymine.

Ljiljana Stojanović,Shuming Bai,Hans Lischka,Rachel Crespo-Otero,Mario Barbatti,Artur F Izmaylov,Jayashree Nagesh

doi:10.3390/molecules21111603

Abstract

After UV excitation, gas phase thymine returns to a ground state in 5 to 7 ps, showing multiple time constants. There is no consensus on the assignment of these processes, with a dispute between models claiming that thymine is trapped either in the first (S1) or in the second (S2) excited states. In the present study, a nonadiabatic dynamics simulation of thymine is performed on the basis of ADC(2) surfaces, to understand the role of dynamic electron correlation on the deactivation pathways. The results show that trapping in S2 is strongly reduced in comparison to previous simulations considering only non-dynamic electron correlation on CASSCF surfaces. The reason for the difference is traced back to the energetic cost for formation of a CO π bond in S2.

Highlights

After UV excitation, gas phase thymine returns to the ground state within 5 to 7 ps [1]
After photoexcitation into the πN1 π* state (a), thymine relaxes within 100 fs within 100 fs to the minimum of the S2 surface holding a πO4π* character (b)
When computed by finite differences, time-derivative nonadiabatic couplings (TDNC) σmn can be conveniently written in terms of wave function overlaps between consecutive time steps

Summary

Introduction

After UV excitation, gas phase thymine returns to the ground state within 5 to 7 ps [1]. Taking the picosecond time constant as an indication of internal conversion to the ground state—which is the most common interpretation—leaves thymine with the longest excited state lifetime among the isolated nucleobases [7,11]. This fact is in itself puzzling, as thymine’s potential energy surfaces obtained from high-level computational simulations are very similar to those of other short-lived pyrimidines (uracil, for instance), to justify the time constant differences [12]

Methods

Results

Conclusion