Excited-state s-cis rotamers produced by extreme red edge excitation of all-trans-1,4-diphenyl-1,3-butadiene

The shapes of the fluorescence emission and lowest excited singlet-state absorption spectra of all-trans-1,4- diphenylbutadiene (DPB) in hydrocarbon solvents vary with excitation wavelength when exciting on the extreme red edge of the ground-state absorption spectrum. This contrasts with the wavelength independence observed for the excited singlet-state absorption and fluorescence emission spectra of 1,5-diphenyl-2,3,4,6,7,8-hexahydronaphthalene and for the fluorescence emission spectra of 1,4-diphenyl-1,3-cyclopentadiene, s-trans and s-cis structural analogs of DPB, respectively. The spectral changes in DPB can be explained in terms of an excitation wavelength-dependent production of s-cis and s-trans rotamer populations in the excited state