Vibronic structures in the optical spectra of distyrylbenzene derivatives

Distyrylbenzene (DSB) derivatives with methoxy substituents at the central and/or the terminal phenyl rings were synthesized. Absorption, fluorescence and excitation spectra of the isolated DSB molecules in solutions as well as in frozen glasses were collected. Fluorescence spectra of the substituted DSBs in solutions exhibited well-resolved vibronic structures comparable to unsubstituted DSB in solution, while poorly resolved vibronic structures were observed in their absorption spectra. Both excitation and fluorescence spectra of the substituted DSB-doped frozen glasses, on the other hand, showed well-resolved vibronic peaks. By resolving each optical spectrum into several peaks using peak-fit analysis vibrational energies derived from energy differences between two vibronic bands, Huang–Rhys factors and zero-vibrational transition energies were determined. Changes of the vibrational energies upon photoexcitation are discussed in terms of the conformational relaxation toward the planar structure at the excited states.