Approaches to Calculation of Exciton Interaction Energies for a Molecular Dimer

Abstract

For two model systems, a dimer of N,N‘-dimethyl thiacarbocyanine and a dimer of C.I. Pigment Yellow 12, we compare results of several approaches to calculation of the exciton interaction energy. The sum over Coulombic interactions between atomic transition charges is compared to the point-dipole and extended-dipole approximations and to the direct evaluation of the Coulomb interaction integral over transition charge densities, for a range of dimer configurations. Calculations are carried out at semiempirical, ab initio Hartree−Fock, and ab initio configuration interaction−singles levels. Finally, we discuss the relation of these interaction energies to those calculated using a supermolecular approach. We conclude that for the materials studied, semiempirical methods are adequate to describe the excitonic shift.

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…In the simplest case, only two point charges may be used to represent the transition dipole moment for a given transition, thus giving rise to the so-called extended-dipole approximation. [245,246] In contrast to the Fçrster approximation, which uses point dipoles, the spatial extent of the transition density can be modeled by the positions at which the point charges are placed. Methods based on transition monopoles derive partial charges ("transition charges") for all atoms, which should represent the full transition energy.…”

“…A number of tests and comparisons of the different methods have been carried out over the past few years. [246,259,260,263,264,272] Even for dimers of small molecules, non-Fçrster-dipole effects can become significant at distances shorter than about 25 bohr (Na 2 ) and 16 bohr (benzaldehyde dimer). [273] In photosynthetic systems, the distances are typically smaller, so that more sophisticated approaches are necessary for quantitative calculations.…”

Chromophore-specific theoretical spectroscopy: From subsystem density functional theory to mode-specific vibrational spectroscopy

“…The orbital transitions mentioned mix considerably in both π → π * transitions. One important test concerns the distance-dependence of the excitation-energy couplings, since discrete models of the transition-density coupling, in particular couplings calculated from transition dipoles, may already show significant errors at distances smaller than 15 Å [401,403]. Excitation energies have been calculated in Ref.…”

A Mixed Quantum–Classical Description of Excitation Energy Transfer in Supramolecular Complexes: Förster Theory and beyond

“…While the first problem has been tackled in the last decade with high precision mainly due to advances in molecular electronic structure calculations (see refs. [12][13][14][15]) a satisfactory ab initio computation of what is known from Fçrster's theory as the frequency overlap (here represented by D EET ) is missing.…”