New conjugated polyenes with 1,3-dialkyl-2-thiobarbituric acid moiety as materials for nonlinear optics: theoretical calculations, synthesis and spectral properties

In this work we report the results of our study on electronic and spectral properties of conjugated polyenes with electron-accepting 1,3-dialkyl-2-thi obarbituric acid moiety. In model calculations, we examine the effect of th e conjugated polyene length on infrared (IR) and Raman spectra of the polye nes by means of ab initio HF/3-21G*. Nonlinear properties were also studied by AMI method in frames of the sum-over-states (SOS) and finite-field form alism. It was concluded that in well-resolved IR and Raman spectra the freq uencies and band intensifies can provide valuable information relating to C =C bond lengths in polyene chain and relative polarizabilities. Near-linear correlation between polarizability and integral IR band intensity, corresp onding to all C=C stretching modes, and the rather nonlinear relationship o f polarizability with integral Raman activity, was found. In our calculatio n we predict that polarizability and the first hyperpolarizability increase s with elongation of polyene chain while the second hyperpolarizability inc reases smoothly in a quadratic way. In contrast to the linear relationship between polarizability and polyene chain length the dipole moment versus ch ain length is predicted to be nonlinear. A good agreement was found between experimental and calculated Raman spectra of one newly synthesized compoun d studied. (C) 2000 Elsevier Science Ltd. All rights reserved.