A Theoretical Analysis of the Excited State of Oligoacene Aggregates: Local Excitation vs. Charge‐Transfer Transition

Abstract

A new approach to evaluate quantitatively the weights of local excitation (LE) and charge‐transfer (CT) transition in the excited state of molecular aggregates has been proposed. It can be achieved by the combination of the molecular orbital fragmentation and natural transition orbital analysis. Armed with this method, we investigated the singlet excited state of acene molecules for representative dimer configurations, i.e., face‐to‐face and herringbone configurations of anthracene, tetracene, and pentacene at scaled opposite spin (SOS)–configuration interaction single (CIS(D)) level of theory. In the case of the face‐to‐face configuration, we found excited state wavefunctions significantly delocalized, giving rise to equal contributions from LE and CT. In the herringbone configuration, however, we found CT character of the excited state marginal. It turned out, however, that, when dimers get closer via intermolecular stretching vibration, CT admixture can be enhanced. Furthermore, in the case of heptamer, CT admixture was substantially magnified, consisting with previous report by Spano and coworkers (J. Chem. Phys. 2011, 134, 204703). Hence, the excited states of molecular aggregates can be quantum chemically analyzed via our approach and important insights into them can be obtained.