Saturation of the second hyperpolarizability for polyacetylenes

Polyacetylene (PA) polymers −(−CHCH−)N− lead to large second hyperpolarizabilities (γ) that increase with N. For small N the increase is quite rapid, eventually saturating (becoming linear in N) for N ⩾ Nsatγ. Both experimental and theoretical attempts have been made to estimate Nsatγ. In 1994 this led to two Science papers, one obtaining Nsatγ = 20 (semiempirical theory) and the other obtaining Nsatγ = 125 (experiment, but on a substituted disordered form of PA!). We report here ab initio calculations up through N = 49 (C98H100) and show that Nsatγ = 45 ± 5. The much larger experimental value, Nsatγ = 125, is explained in terms of the structural defects and disorder introduced by the synthetic method.