Effect of intermolecular excited-state interaction on vibrationally resolved optical spectra in organic molecular aggregates

Abstract

The optical spectra are effective means to reveal the molecular interactions and the luminescent mechanism of the organic molecules in aggregates. Herein, we systematically investigate the crystalline state vibrationally resolved absorption and emission spectra for a series of AIEgens and non-AIEgens by considering intermolecular excited state interaction by using Frenkel-exciton model coupled with quantum mechanics and molecular mechanics (QM/MM) calculations. It is found that the competition between the intramolecular vibronic coupling (λ) and the intermolecular exciton coupling (J) governs the crystalline aggregate spectral characters. At room temperature, when J/λ value is larger than a critical value (ca. 0.17), the exciton coupling would have a large effect on the optical spectra. For face-to-face H-aggregates, only when both intermolecular electrostatic and excitonic couplings are considered, can one obtain calculated vibrationally resolved spectra and well reproduce the experimental results, namely, remarkable blue-shift in absorption but much less red-shift in emission when compared with the gas-phase. The optical spectra of the AIE-active aggregates are determined by the intramolecular vibronic coupling because the ratio J/λ is less than the critical value.