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

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)ₙ A (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.