First hyperpolarizabilities of 1,3,5-tricyanobenzene derivatives: Origin of larger β values for the octupoles than for the dipoles

A series of donor-acceptor substituted stilbene and diphenylacetylene derivatives and their octupolar analogues have been synthesized and the linear and nonlinear optical properties (β) studied by both experiments and theoretical calculation. The λ_max of the dipoles increases with the conjugation length and is always larger when the C=C bond is used, instead of the C≡C bond, as the conjugation bridge. Although the λ_max values of the octupoles show no clear trend, they are much larger than those of the dipoles. The β(0) values of the dipoles increase with conjugation length and as the conjugation bridge is changed from the C≡C to C=C bond. This increase is accompanied by an increase in either λ_max or the oscillator strength. Similarly, the β(0) values of the octupoles increase with the conjugation length and with a change in the donor in the order: NEt₂ < N(i-amyl)Ph < NPh ₂. Moreover, β_yyy/β_zzz ratios are in the range of 1.6-3.9 and decrease with the conjugation length. β values calculated by the finite-field and sum-over-states methods are in good agreement with the experimental data. Also, there is a parallel relationship between the calculated β values and bond length alternation (BLA). From these results, the origin of the larger β values for octupoles than for dipoles is assessed.