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

Abstract

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.

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

“…This was predicted theoretically for conjugated oligomer model systems. 1 We have also obtained experimental evidence that charged polarons are delocalized over several chains for intermediate and high MW, while for low MWs the degree of polaron delocalization is significantly lower as evidenced by a much reduced intensity of the charge transfer chargeinduced transitions in the mid infrared. This provides a microscopic explanation for the observed rapid increase of mobility from low to intermediate MWs.…”

“…When the conjugation length is longer than the intermolecular separation, as is generally the case in polymers, quantumchemical calculations have shown that the exciton bandwidth decreases as the conjugation length increases. 1,46,47 This is because for long conjugation lengths the point dipole approximation breaks down and there is a tendency for positive and negative contributions to the atomic transition densities to cancel each other, leading to a reduction of the interaction energy/exciton bandwidth. Within this model we can now interpret the subtle variations in the absorption spectra as a function of MW in terms of variations of the intrachain conjugation length.…”

“…A particularly interesting question is how interchain interaction effects depend on the molecular weight of the polymer. Many studies on model oligomeric systems have been devoted to studying the dependence of interchain interactions on the molecular, 1 but in polymers such effects are much more challenging to investigate because they require a detailed understanding of the complex microstructure of the polymer. On the other hand a better physical understanding of interchain interaction effects in polymers is essential as these materials are becoming an attractive class of materials for realizing field-effect transistor ͑FET͒ circuits on low-cost flexible substrates by solution processing and direct-write printing techniques.…”

Reflection and extinction of light by self-assembled monolayers of a quinque-thiophene derivative: A coherent scattering approach

“…This range is consistent with advanced quantum chemical calculations of exciton energies in a layer of oligothiophene molecules. 45 In addition also the gas-to-crystal shift of the excited state energy of the individual molecule which arises from the more polarizable environment of the molecule in the crystal is a factor that can contribute to the exact position of the exciton levels. In brief, we treat the energy of the k = 0 exciton level as an adjustable parameter.…”

Effect of synthesis temperature and alkoxy side chain length on molecular structure and photoelectrochemical properties of terthiophenes

“…[15][16][17][18] Theoretical studies have been devoted to understanding interchain interactions and their effect on the optical properties of thiophene oligomers. [19][20][21][22][23] The effect of both the planar and the interchain distances on the gap energy (E gap ) of oligothiophene systems are well known, and so are the effect of chain torsions on π-conjugation length. 24 In all the previous cases, differences presumably due to the preparation method and conditions and the resulting structures have observed between the various materials.…”