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-sta te absorption spectra of all-trans-1,4-diphenylbutadiene (DPB) in hydr ocarbon solvents vary with excitation wavelength when exciting on the extreme red edge of the ground-state absorption spectrum. This contras ts with the wavelength independence observed for the excited singlet-s tate 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 ana logs of DPB, respectively. The spectral changes in DPB can be explaine d in terms of an excitation wavelength-dependent production of s-cis a nd s-trans rotamer populations in the excited state. The DPB fluoresce nce emission spectrum was resolved into s-cis and s-trans components. The vibronic structure of the s-cis fluorescence spectrum is similar t o that of s-trans, but the band origin is red-shifted and there is a s lightly larger amplitude on the red edge. The excited-state absorption spectrum of s-cis DPB appears to be red-shifted relative to that of s -trans DPB as well.