Comparative theoretical evaluation of hyperpolarizabilities of push-pull polyenes and polyynes. The important role of configuration mixing in the excited states

Abstract

The potential use of push-pull polyenes and polyynes for second harmonic generation has been compared on the basis of hyperpolarizabilities computed for a large number of model systems of the type D(HC=CH)_nA and D(C≡C)_nA (n=2-8, and D=NH₂, Br, or I; A=NO₂ or CN). Geometries optimized at the AM1 level were used in a CI calculation including single and pair excitations involving up to 193 configurations. The sum-over-states expression was used to obtain the β values. While the trends in the computed β are the same for the two series of compounds, the values are generally smaller and increase less rapidly with increasing n for the polyynes. The results are interpreted in terms of the nature of the lowest excited states which contribute to β in these systems. Mixing of in-plane and out-of-plane π- π^∗ excited configurations reduces the contribution of the lower excited states to β. Push-pull polyynes with higher β values than the corresponding polyenes can be designed by the combined use of donor and acceptor groups such as I (or Br) and CN which can interact with both sets of orthogonal π Mos.