Theoretical study of the conjugation length effect on the electronic and second‐order nonlinear optical properties of amino–nitro diphenylacetylenes

We presnet a theoretical study of the effect of the conjugation length on the electronic properties and second‐order molecular polarizabilities β in p ‐amino‐ p ′‐nitrodiphenylacetylene molecules where the number of triple bonds in the conjugated segment varies from 1 to 4. The β values are calculated via an intermediate neglect of differential overlap/single configuration interaction ( INDO / SCI ) sum‐over‐states ( SOS ) approach. We test the convergence of the SOS method and the validity of the two‐state model to describe the β response. The results indicate that increasing the conjugation length results in a decrease of the charge transfer within the molecule. The two‐state model is shown to break down as the conjugated segment extends to four triple bonds; this is due to the appearance of several low‐lying nearly isoenergetic excited states that significantly contribute to the β response. The theoretical results are in excellent agreement with recent experimental data. © 1994 John Wiley & Sons, Inc.