CONJUGATION LENGTHS IN LONG POLYENE CHAINS

Since the low energy absorption spectrum of a linear polyene is domina ted by the strongly allowed 1(1)A(g) --> 1(1)B(u) transition (S-0 --> S-2), it can be reconstructed for any chain length if the 0-0 band exc itation energy, the band profile, and intensity are known. This makes it possible to use information developed in high resolution experiment s on well-defined linear polyenes to extract the distribution of effec tive conjugation lengths in an ensemble of long molecular chains from solution absorption spectra. A previously published analysis of absorp tion 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 stat istical model which is reviewed here and used to calculate ensemble av erages for powers of conjugation length.