COMPARATIVE THEORETICAL EVALUATION OF HYPERPOLARIZABILITIES OF PUSH-PULL POLYENES AND POLYYNES - THE IMPORTANT ROLE OF CONFIGURATION MIXINGIN THE EXCITED-STATES

The potential use of push-pull polyenes and polyynes for second harmon ic generation has been compared on the basis of hyperpolarizabilities computed for a large number of model systems of the type D(HC=CH)(n)A and D(C=C)(n)A (n = 2-8, and D = NH2, Br, or IX; A = NO2 or CN). Geome tries optimized at the AM1 level were used in a CI calculation includi ng single and pair excitations involving up to 193 configurations. The sum-over-states expression was used to obtain the beta values. While the trends in the computed beta are the same for the two series of com pounds, the values are generally smaller and increase less rapidly wit h increasing n for the polyynes. The results are interpreted in terms of the nature of the lowest excited states which contribute to beta in these systems. Mixing of in-plane and out-of-plane pi-pi excited con figurations reduces the contribution of the lower excited states to be ta. Push-pull polyynes with higher beta values than the corresponding polyenes can be designed by the combined use of donor and acceptor gro ups such as IX (or Br) and CN which can interact with both sets of ort hogonal pi MOs.