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-visibl e (UV-Vis) absorptions of the phenyl,groups and conjugated main chain for c is- and trans-poly(phenylacetylene)s (PPAs) in solution were investigated o n the basis of CNDO/S calculation for the cis-transoidal and +/-tau degrees -deflected trans-transoidal structures. The two kinds of the UV-Vis absorpt ion peaks were observed at 263 and 390 nm for the cis-transoidal PPA and at 227 and 248 nm for 90 degrees-deflected trans-transoidal PPA, which were d ue to the pi-pi* transitions of phenyl groups and the conjugated bonds in t he main chain. After cis-trans isomerization, the larger blue shift of the pi-pi* transition absorption in the UV spectrum was confirmed from the CNDO /S calculation for the +/-90 degrees-deflected trans-transoidal structures.