Ultrafast Dynamics of the Two Lowest Bright Excited States of Cytosine and 1-Methylcytosine: A Quantum Dynamical Study

Abstract

The nonadiabatic quantum dynamics (QD) of cytosine and 1-methylcytosine in gas phase is simulated for 250 fs after a photoexcitation to one of the first two bright states. The nuclear wavepacket is propagated on the coupled diabatic potential energy surfaces of the lowest seven excited states, including ππ * , nπ * and Rydberg states along all the vibrational degrees of freedom. We focus in particular on the interplay between the bright and the dark nπ * states, not considering the decay to the ground electronic state. To run these simulations we implemented an automatic general procedure to parametrize linear vibronic coupling (LVC) models with time-dependent density functional theory (DFT) computations, and interfaced it with Gaussian package. The wavepacket was propagated with the multilayer version of the multiconfigurational time dependent Hartree method. Two different density functionals, PBE0 and CAM-B3LYP, which provide a different description of the relative stability of the lowest energy dark states, were used to parametrize the LVC Hamiltonian. Part of the photoexcited population on lowest HOMO-LUMO transition (π H π * L ) decays within less than 100 fs to a nπ * state which mainly involves a promotion of an electron from the oxygen lone pair to the LUMO (n O π * L ). The population of the second ππ * state decays almost completely, in < 100 fs, not only to π H π * L and to n O π * L state, but also to another nπ * L states involving the nitrogen lone pair. The efficiency of the adopted protocol allowed us to check the accuracy of the predictions by repeating the QD simulations with different LVC Hamiltonians parametrized either at the ground state minimum or at stationary structures of different relevant excited states.

Excited state absorption of DNA bases in the gas phase and in chloroform solution: a comparative quantum mechanical study

“…A detailed description of the lowest energy excited states of cytosine can be found in ref. 42 and 52. The S 1 state is a ππ* transition, with predominant HOMO → LUMO character, while the S 2 nπ* state involves the excitation from the lone pair of the nitrogen in position 3 towards the LUMO.…”

The Ultrafast Quantum Dynamics of Photoexcited Adenine–Thymine Basepair Investigated with a Fragment-based Diabatization and a Linear Vibronic Coupling Model

“…We consider 3 different choices of reference states in order to examine their effect on the LVC model, and resultant dynamics. We set the reference states as equivalent to the adiabatic states of the MC at the reference geometry and label this as the standard LVC (St-LVC) approach. This is a typical choice adopted for LVC models, and we have used this method previously in the study of individual chromophores. ,− We use our recently proposed fragment diabatization (FrD) technique to define the reference states . In this approach, the reference states are the adiabatic states of the isolated fragments of the MC (for LEs), or one electron transitions between orbitals on different fragments (for CT states).…”

“…We set the reference states as equivalent to the adiabatic states of the MC at the reference geometry and label this as the standard LVC (St-LVC) approach. This is a typical choice adopted for LVC models, and we have used this method previously in the study of individual chromophores. ,− …”

“…As previously done for DNA nucleobases and G-quadruplexes, ,− , we express the reference states and overlap matrix within the framework of TD-DFT, and further details can be found in these papers. Similarities and differences of our approach with the work of Tamura and Burghardt on conjugated polymers and fullerene systems − have also been analyzed in ref .…”

“…The diabatization was performed with an in-house code interfaced with Gaussian 16 that is freely available upon request. LVC models for individual 9-methyladenine and 1-methylthymine have also been parametrized at the same level of theory following the protocol for individual bases we have recently used. ,, Since in the diabatization procedure we compute the projection of the diabatic states onto the adiabatic states of the AT dimer, this information can be employed to precisely measure the similarity among diabatic and adiabatic states. In more detail, for each adiabatic state m we can compute the square of the elements of the overlap matrix S im = ⟨ R i | a m MC ⟩ for each of the N reference states i .…”