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)/MR D-CI (multireference determinant-configuration interaction) calculatio ns, the SOS (sum-over-states) formalism is applied to calculate the th ird-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 i ncreases with N, than reaches a nearly constant value (power regime), and, finally, decreases toward one, indicating the appearance of a sat uration regime. Very good agreement with the experimental evolution ha s been found in the case of the oligothiophenes. The comparison of the results obtained for the two types of oligomers indicates a lower pol arizability in the oligopyrroles. (C) 1994 John Wiley & Sons, Inc.