Abstract
We present an implementation of the Frenkel exciton model in the framework of the semiempirical floating occupation molecular orbitals-configuration interaction (FOMO-CI) electronic structure method, aimed at simulating the dynamics of multichromophoric systems, in which excitation energy transfer can occur, by a very efficient approach. The nonadiabatic molecular dynamics is here dealt with by the surface hopping method, but the implementation we proposed is compatible with other dynamical approaches. The exciton coupling is computed either exactly, within the semiempirical approximation considered, or by resorting to transition atomic charges. The validation of our implementation is carried out on the trans-azobenzeno-2S-phane (2S-TTABP), formed by two azobenzene units held together by sulfur bridges, taken as a minimal model of multichromophoric systems, in which both strong and weak exciton couplings are present.
Highlights
The electronic excitation energy transfer (EET) is a fundamental process, in which electronic excitation is transferred from a donor fragment to an acceptor
We present an implementation of the Frenkel exciton model, which employs the floating occupation molecular orbital-configuration interaction (FOMO-CI) method,[20] in a semiempirical framework, to evaluate the relevant electronic wavefunctions and site energies
The two excitonic approaches yield very similar PESs; in Figure 2, we show those obtained with the transition charges (TC) scheme
Summary
The electronic excitation energy transfer (EET) is a fundamental process, in which electronic excitation is transferred from a donor fragment to an acceptor. The study of EET and other aspects of nonadiabatic dynamics in multichromophoric systems calls for employing some sort of “divide and conquer” strategy In this respect, one of the most successful schemes is offered by the Frenkel exciton model, where the electronic excited states of the multichromophoric system are represented by linear combinations of localized excitations. Journal of Chemical Theory and Computation instance, those based on transition multipoles or transition atomic charges, fitting the transition density.[13,17,18] The main problem in this context is the accurate evaluation of the analytical gradient of the exciton coupling, needed to perform molecular dynamics simulations.[13,19] More challenging, in multichromophoric systems, is the choice of the quantum chemistry method for the computation of site energies and wavefunctions In this respect, time-dependent density-functional theory (TDDFT) is computationally affordable but suffers from the problems of single reference methods, preventing, for example, a proper description of the decay to the ground state. To make the comparison with the standard “supermolecule” approach easier, an analysis in terms of localized diabatic states is performed for the latter
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