Comparative Computational Study of Electronic Excitationsof Neutral and Charged Small Oligothiophenes and Their ExtrapolationsBased on Simple Models

摘要

This work reports electronic excitation energies of neutral and charged oligothiophenes (OTn) with repeat unit n = 2–6 computed by routinely used semiempirical and time-dependent density functional theory (TD-DFT) methods. More specifically, for OTn, OTn⁺, and OTn^–, we calculated vertical transition energies for electronic absorption spectroscopy employing the Zerner’s version of intermediate neglect differential overlap method for structures optimized by the PM6 semiempirical method and the TD-DFT method with three different functionals, B3LYP, BVP86, and M06-2X, for structures optimized by the ground-state DFT method employing the same functionals. We also calculated vertical transition energies for the emission spectroscopy from the lowest singlet excited states by employing the TD-DFT method for the structures optimized for the lowest singlet excited states. In addition to computational results in vacuum, solution phase data calculated at the level of polarizable continuum model are reported and compared with availableexperimental data. Most of the data are fitted reasonably well bytwo simple model functions, one based on a Frenkel exciton theoryand the other based on the model of independent electrons in a boxwith sinusoidal modulation of potential. Despite similar levels offitting performance, the two models produce distinctively differentasymptotic values of excitation energies. Comparison of these withavailable experimental and computational data suggests that the valuesbased on the exciton model, while seemingly overestimating, are closerto true values than those based on the other model. This assessmentis confirmed by additional calculations for a larger oligomer. Thefitting parameters offer new means to understand the relationshipbetween electronic excitations of OTs and their sizes and suggestthe feasibility of constructing simple coarse-grained exciton-bathmodels applicable for aggregates of OTs.