UV PHOTOELECTRON-SPECTROSCOPY OF SUBSTITUTED POLYACETYLENES

Ionization threshold energies of 26 substituted polyacetylenes were de termined by ultraviolet photoelectron spectroscopy. Examination of the data obtained, along with data for other polymers, and the energy of the lowest energy electronic excitation revealed that the threshold en ergies correspond to the photoemission from the highest occupied orbit als in the pi-conjugated main chains. For the majority of the polymers , the main origin of the significant variation of threshold energy and electronic excitation energy is ascribed to the change of the degree of pi-conjugation due to steric hindrance of substituents. Calculation s with Huckel theory gave semiquantitative agreement with the observed results. Polymers with Cl substituents at the main chain also showed a large inductive effect by the electron-withdrawing Cl atom. The HOMO of these polymers was found to be derived from the pi-conjugated main chain.