Impact of the Dynamic Electron Correlation on the Unusually Long Excited-State Lifetime of Thymine

Abstract

Relaxation of the photoexcited thymine in the gas-phase shows an unusually long excited-state lifetime, previously attributed to trapping in the absorbing excited state (S 2 -trapping mechanism).Here, we investigate this mechanism using the non-adiabatic molecular dynamics (NAMD) simulations combined with the recently developed Mixed Reference Spin-Flip (MRSF)-TDDFT method. We show that the S 2 -trapping was an artifact caused by an insufficient account of electron correlation in the electronic structure methodologies used for NAMD. The current work predicts instead an S 1 -trapping mechanism with two lifetimes, τ 1 =30±1 fs and τ 2 =6.1±0.035 ps, quantitatively consistent with time-resolved experiments. Upon excitation to S 2 (ππ * ) state, thymine undergoes an ultrafast internal conversion from S 2 → S 1 (ca. 30 fs) and resides 1 around the minimum on the S 1 (n O π * ) surface slowly decaying to the ground state (ca. 6.1 ps). The S 2 →S 1 internal conversion is mediated by bond length alternation stretching, which repeatedly passes through a newly found planar conical intersection region. The subsequent S 1 →S 0 internal conversion occurs through several conical intersections involving slow puckering motions of the pyrimidine ring.

Common Defects Accelerate Charge Carrier Recombination in CsSnI3 without Creating Mid-Gap States

“…The nonradiative e–h recombination is governed by the electronic energy gap, the NAC, and quantum coherence between the initial and final states. − The CsSnI 3 bandgap obtained in the calculation, Figure , agrees with the experimental value of 1.3 eV. The defects introduce little change to the bandgap.…”

Density Functional Theory for Electronic Excited States

“…[20][21][22] LR-TDDFT is also becoming increasingly popular for photochemical applications, [23][24][25] despite some problems with the description of conical intersections. [26][27][28] In part, this popularity is due to a growing recognition that complete active-space (CAS-)SCF methods cannot be considered quantitative approaches for excited-state dynamics, [29][30][31][32] due to a lack of dynamical electron correlation effects. This chapter provides an overview of TDDFT and other DFT-based methods for computing excitation spectra, excited-state properties, and for simulating photochemical reactions, emphasizing theory rather than applications but with some molecular examples to motivate the discussion.…”

Spin-Flip TDDFT for Photochemistry

“…TSH simulations of thymine photodynamics have been reported using MRSF-TDDFT, 129 which support trapping on S 1 as the explanation for a long-lived (∼ 5 ps) decay component time-resolved experiments. This is consistent with multireference results that include dynamical electron correlation, 130 but it is at odds with CASSCF predictions that indicate trapping on S 2 .…”