Conjugation lengths in long polyene chains

Since the low energy absorption spectrum of a linear polyene is dominated by the strongly allowed 11Ag → 1 1Bu transition (S0 → S2), it can be reconstructed for any chain length if the 0-0 band excitation energy, the band profile, and intensity are known. This makes it possible to use information developed in high resolution experiments on well-defined linear polyenes to extract the distribution of effective conjugation lengths in an ensemble of long molecular chains from solution absorption spectra. A previously published analysis of absorption spectra of solutions of long polyenes showed that the longer the molecular chain, the more short conjugation lengths contribute to the conjugation length distribution. This is consistent with a simple statistical model which is reviewed here and used to calculate ensemble averages for powers of conjugation length.