Theory of excited states in polyphenylenes

Exact and high order configuration interaction calculations are done on oligomers of poly(paraphenylene) within a molecular exciton basis to investigate high energy excited states relevant in linear and nonlinear optical absorptions. Two different sets of calculations are done. In the first of these, the exciton basis is constructed out of both delocalized and localized highest bonding and lowest antibonding molecular orbitals of each benzene unit. In the second, only the delocalized frontier orbitals of benzene are retained. Fully pictorial descriptions of all correlated eigenstates are obtained. Comparison of the two sets of results indicates that while the lowest energy eigenstates of polyphenylenes can be obtained within effective linear chain models with very large bond alternation, this may not be true for high energy states of the real materials with finite chain lengths. Low energy photoinduced absorption within the model is to an even parity charge-transfer exciton the analog of which occurs also in linear chain polymers. High energy photoinduced absorption, at least in the short oligomers, is to a new kind of two-exciton state that is absent in linear chain systems.