THEORETICAL EVOLUTION OF THE 3RD-ORDER MOLECULAR POLARIZABILITIES AS A FUNCTION OF CHAIN-LENGTH IN THIOPHENE AND PYRROLE OLIGOMERS

On the basis of INDO (intermediate neglect of differential overlap)/MRD-CI (multireference determinant-configuration interaction) calculations, the SOS (sum-over-states) formalism is applied to calculate the third-order polarizabilities, gamma, in thiophene and pyrrole oligomers. For both types of oligomers, the chain-length dependence of gamma can be divided into three regimes: Describing gamma as a power law of the number of rings (N), we observe that the power value first strongly increases with N, than reaches a nearly constant value (power regime), and, finally, decreases toward one, indicating the appearance of a saturation regime. Very good agreement with the experimental evolution has been found in the case of the oligothiophenes. The comparison of the results obtained for the two types of oligomers indicates a lower polarizability in the oligopyrroles. (C) 1994 John Wiley & Sons, Inc.