Year: 2026
Type: preprint
Abstract: This study derives expressions for the photophysics of a molecule exhibiting the Herzberg–Teller (HT) effect. The derivation is based on Fermi’s golden rule, and the Longuet-Higgins representation, where the electron–photon and vibronic couplings are treated as perturbations, is used. All the vibrational modes are included, thus enabling the assignment of vibronic progressions in the spectrum originating from Franck–Condon factors. A forbidden transition is regarded as the photon absorption or emission accompanied by phonon absorption or emission, clarifying the contribution of vibronic structures to the transition. The derived expressions are applied to the absorption from S0 to S1 and fluorescence from S2 to S0 for azulene, a molecule known to exhibit the HT effect. Using the S4 state as an intermediate state, the calculated fluorescence spectrum and rate constant agree well with the experimental results. The in-plane stretching vibration of the five-membered ring contributes to phonon emission, whereas that at the junction of the five-and seven-membered rings contributes to the acceptance of the excess excitation energy. Furthermore, the vibronic coupling density analyses reveal that the vibronic couplings originate from the five-membered ring. This study presents a theory that allows for a detailed assignment of vibronic structures involved in the HT effect, providing design guidelines for HT molecules.
Source: ChemRxiv
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220658473