SATURATION OF THE 2ND HYPERPOLARIZABILITY FOR POLYACETYLENES

Polyacetylene (PA) polymers -(-CH=CH-)(N)- lead to large second hyperp olarizabilities (gamma) that increase with N. For small N the increase is quite rapid, eventually saturating (becoming linear in N) for N gr eater than or equal to N-sat(gamma). Both experimental and theoretical attempts have been made to estimate N-sat(gamma). In 1994 this led to two Science papers, one obtaining N-sat(gamma) = 20 (semiempirical th eory) and the other obtaining N-sat(gamma) = 125 (experiment, but on a substituted disordered form of PA!). We report here ab initio calcula tions up through N = 49 (Cg(98)H(100)) and show that N-sat(gamma) = 45 +/- 5. The much larger experimental value, N-sat(gamma) = 125, is exp lained in terms of the structural defects and disorder introduced by t he synthetic method.