We introduce a method (FrD-LVC) based on a fragment diabatization (FrD) for the parametrization of a Linear Vibronic Coupling (LVC) model suitable for studying the photophysics of multichromophore systems. In combination with effective quantum dynamics (QD) propagations with multilayer multiconfigurational time-dependent Hartree (ML-MCTDH), the FrD-LVC approach gives access to the study of the competition between intra-chromophore decays, like those at conical intersections, and inter-chromophore processes, like exciton localization/delocalization and the involvement of charge transfer (CT) states. We used FrD-LVC parametrized with TD-DFT calculations, adopting either CAM-B3LYP or ωB97X-D functionals, to study the ultrafast photoexcited QD of a Guanine-Cytosine (GC) hydrogen bonded pair, within a Watson-Crick arrangement, considering up to 12 coupled diabatic electronic states and the effect of all the 99 vibrational coordinates. The bright excited states localized on C and, especially, on G are predicted to be strongly coupled to the G->C CT state which is efficiently and quickly populated after an excitation to any of the four lowest energy bright local excited states. Our QD simulations show that more than 80% of the excited population on G and ~50% of that on C decays to this CT state in less than 50 fs. We investigate the role of vibronic effects in the population of the CT state and show it depends mainly on its large reorganization energy so that it can occur even when it is significantly less stable than the bright states in the Franck-Condon region.At the same time, we document that the formation of the GC pair almost suppresses the involvement of dark nπ* excited states in the photoactivated dynamics. File list (2) download file view on ChemRxiv main_FrD-LVC.pdf (2.14 MiB) download file view on ChemRxiv SI_FrD-LVC.pdf (4.59 MiB)

We have recently proposed a protocol for Quantum Dynamics (QD) calculations, which is based on a parameterisation of Linear Vibronic Coupling (LVC) Hamiltonians with Time Dependent (TD) Density Functional Theory (TD-DFT), and exploits the latest developments in multiconfigurational TD-Hartree methods for an effective wave packet propagation. In this contribution we explore the potentialities of this approach to compute nonadiabatic vibronic spectra and ultrafast dynamics, by applying it to the five nucleobases present in DNA and RNA. For all of them we computed the absorption spectra and the dynamics of ultrafast internal conversion (100 fs timescale), fully coupling the first 2–3 bright states and all the close by dark states, for a total of 6–9 states, and including all the normal coordinates. We adopted two different functionals, CAM-B3LYP and PBE0, and tested the effect of the basis set. Computed spectra are in good agreement with the available experimental data, remarkably improving over pure electronic computations, but also with respect to vibronic spectra obtained neglecting inter-state couplings. Our QD simulations indicate an effective population transfer from the lowest energy bright excited states to the close-lying dark excited states for uracil, thymine and adenine. Dynamics from higher-energy states show an ultrafast depopulation toward the more stable ones. The proposed protocol is sufficiently general and automatic to promise to become useful for widespread applications.

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.

Paper published as part of the special topic on Quantum Dynamics with ab Initio Potentials ARTICLES YOU MAY BE INTERESTED IN Modern quantum chemistry with [Open]Molcas

We study the ultrafast dynamics of 1,5dimethyl-cytosine, a model for 5methyl-cytidine, after photoexcitation to the first two bright ππ* states, focusing on the possible population transfer to dark nπ* states....