Polyene Spectroscopy. Photoelectron Spectra of the Diphenylpolyenes

Photoelectron spectra in the region from 6 to 20 eV have been obtained for the diphenylpolyenes stilbene, diphenylbutadiene, diphenylhexatriene, diphenyloctatetraene, diphenyldecapentaene, diphenyldodecahexaene, and diphenylhexadecaoctaene, [formula ommited], n= 1,2, 3, 4, 5, 6, and 8. The similarity of these spectra allows an empirical classification of the ionization peaks to be made, namely, those which are independent of chain length and those which move to lower energy as the chain length increases. The peaks which are independent of chain length are at 9.1, 11.4, and 14.2 eV and are identified as phenyl group ionizations. The peaks which change energy with chain length are identified as predominantly polyene chain ionizations from orbitals with small coefficients for the phenyl rings. The lowest energy ionizations occur at 7.94, 7.56, 7.33, 7.19, 7.05, 7.07, and about 7.2 eV for the polyenes with 1, 2, 3, 4, 5, 6, and 8 double bonds, indicating a convergence to a constant value at long chain length. This behavior is reminiscent of the convergence of the excitation energies of the dimethylpolyenes. A Hückel molecular orbital treatment of the entire series was performed, and the ionization peaks were identified with filled MO energy levels. Adjustment of the two parameters α and β₀ to fit 22 assigned transitions gave values of |α| = 6.32 ± 0.04 eV and |β₀| = 2.80 ± 0.04 eV with a standard deviation between calculated and observed points of 0.07 eV which is approximately equal to the experimental error. The agreement is equally good at high and low energies.