Photoinduced Nonadiabatic Dynamics of Pyrimidine Nucleobases: On-the-Fly Surface-Hopping Study with Semiempirical Methods

Abstract

The photoinduced relaxation dynamics of pyrimidine nucleobases (uracil, thymine, and cytosine) was studied using the surface-hopping approach at the semiempirical OM2/MRCI level of theory. The relevant potential energy surfaces were characterized by performing geometry optimizations of the energy minima of the lowest electronic states and of the most important conical intersections and by computing excitation energies at each configuration. Surface-hopping molecular dynamics simulations were performed to describe the nonadiabatic dynamics after excitation into the optically active state. In each of the molecules, the two lowest excited singlet states are involved in the dynamics, and there are competing relaxation paths. The dynamics is dominated by a two-step relaxation mechanism in uracil and thymine, while the direct decay to the ground state is most important in cytosine. For all three molecules, the simulations yield ultrafast S(2)-S(1) deexcitation within 50 fs and internal conversion to the ground state in less than 1 ps, consistent with recent experimental results from time-resolved photoelectron spectroscopy.

Photophysics of cytosine tautomers: new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

“…The CASSCF method is occasionally not accurate even qualitatively due to the lack of dynamic electron correlation as seen in the present study, and also the TD-DFT method has an intrinsic limitation to describe the potential energy surfaces around CIs. Recent on-the-fly excited-state dynamical simulations [34][35][36][37][38] for keto cytosine show consistency in predicting the decay times of the subpicosecond range, but they proposed different deactivation pathways as the main channel primarily due to the different potential energy surfaces employed in the simulations. In the present work, our simulations at the CASSCF level predict seemingly consistent excited-state lifetimes with experimental findings for the keto and enol tautomers (Figure 10), but the underlying deactivation mechanisms of these forms are completely different from the mechanisms predicted by the MS-CASPT2 calculations.…”

“…Most of the previous computational studies have focused on the keto form of cytosine and its derivatives such as 5-fluorocytosine because of its biological significance. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] For the keto tautomer, three types of CIs between the S 0 and S 1 states are well defined, denoted as ( 1 ππ*/gs) CI , ( 1 π N3 π*/gs) CI , and ( 1 n O π*/gs) CI in this work (details of these CIs are explained in Section III). Potential energy profiles for the decay pathways connecting the Franck-Condon (FC) region to these CIs have been investigated at the various levels of theory.…”

“…On-the-fly excited-state dynamical simulations including the nonadiabatic transitions have been also performed in recent years to elucidate the deactivation mechanisms from a dynamical point of view. [34][35][36][37][38] However, theoretical studies on the imino and enol tautomers of cytosine are still very scarce. The vertical excitation energies of cytosine tautomers have been calculated by several groups 16,22,39,40 , but to the best of our knowledge only one literature is available which explored the excited-state potential energy profiles of the imino and enol tautomers of cytosine.…”

Perspective: Nonadiabatic dynamics theory

“…Significant progress has already been achieved for both Ehrenfest and surface hopping MQC dynamics. [71][72][73][74][75][76][77] MQC dynamics approaches-Ehrenfest and surface hopping-have been the methods of choice for simulating nonadiabatic chemical processes for the past 20 years or more. This must certainly change; even when treating nuclear motion on a single PES, neglect of quantum effects such as tunneling, zero-point motion, and quantized energy levels can seriously degrade accuracy.…”

Characterizing the dark state in thymine and uracil by double resonant spectroscopy and quantum computation

“…Figure 5 In agreement with previously reported studies on thymine and uracil, the minima of the S 1 states of both species are of n → π* character with almost planar structure, similar to the ground state minima. The minima of S 2 are characterized by a distortion from planarity with pyramidalization at the C6 atom 10,[55][56][57][58] . The triplet state minima are of π →π* character and they are also characterized by pyramidalization of the pyrimidine ring at the C6 atom.…”