Interchain interactions in conjugated materials: The exciton model versus the supermolecular approach

Correlated quantum-chemical techniques are applied to the description of electronic excitations in interacting conjugated chains. The focus is on the magnitude and conjugation-length dependence of the splitting of the lowest optically allowed excitonic state, which is induced by interchain interactions. We first examine cofacial dimers formed by linear polyene chains of various lengths and use two strategies to compute the exciton coupling energy. One is based on molecular exciton theory, which assumes that the excited-state wave functions of the isolated chains remain unperturbed by the intermolecular forces; in the other, the supermolecular approach, the wave functions are obtained from molecular orbital calculations performed for the whole system and are therefore not constrained to a single chain. We find that the two techniques lead to consistent results, provided an appropriate form for the interchain Coulomb interactions is adopted in the excitonic model. In particular, both formalisms indicate a peak behavior for the evolution of the exciton splitting energy with the length of the interacting conjugated chains. As an illustration, the chain-length dependence of the Davydov splitting is evaluated in the case of oligothiophenes on the basis of the experimental x-ray crystal structures; the results are compared to recent polarized absorption data.