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UV-vis Absorption Study of cis- and trans Poly(phenylacetylene)s Based on CNDO/S Calculation

Before and after the thermal isomerization, the observed ultraviolet-visible (UV-vis) absorptions of the phenyl groups and conjugated main chain for cis- and trans-poly(phenylacetylene)s (PPAs) in solution were investigated on the basis of CNDO/S calculation for the cis-transoidal and ±τ°-deflected trans-transoidal structures. The two kinds of the UV-vis absorption peaks were observed at 263 and 390 nm for the cis-transoidal PPA and at 227 and 248 nm for 90°-deflected trans-transoidal PPA, which were due to the π–π* transitions of phenyl groups and the conjugated bonds in the main chain. After cis–trans isomerization, the larger blue shift of the π–π* transition absorption in the UV spectrum was confirmed from the CNDO/S calculation for the ±90°-deflected trans-transoidal structures.