Excitation‐Wavelength‐Dependent Structure in the Fluorescence Spectra of Bu States of Diphenylpolyenes

Experiments have used a tunable picosecond laser to study evolving fluorescencespectra of 1,8-diphenylocta-l,3,5,7-tetraene (DPO) in hexane solution at room temperture.The spectra recorded synchronously (±20 ps) with the irradiation event areanalysed in terms of the time-integrated spectra, spectra obtained through a delayedgate, resonance Raman signals and fluorescence from the second excited singlet state.Data are presented to show the dependence of the S 2 → S 0 spectrum on the excitation wavelength.The spectra clearly show that vibrational relaxation is incomplete on thetimescale of the S 2 lifetime. On the other hand, the spectral bands show little tendencyat room temperature towards line-narrowing effects. This indicates that the rate ofmolecular site relaxation is at least comparable to the lifetime of the S 2 state underthese conditions.