ENHANCED NONLINEAR-OPTICAL PROPERTIES AND THERMAL-STABILITY OF DONOR-ACCEPTOR-SUBSTITUTED OLIGOTHIOPHENES

Linear and nonlinear optical properties of a series of novel donor-acc eptor substituted alpha-oligothiophenes were investigated by means of electrooptical absorption measurements (EOAM) and electric field induc ed second harmonic generation (EFISH). The second-order polarizabiliti es beta(-2 omega; omega, omega) were related to dipole changes Delta m u(ag) and transition dipoles mu(ag) associated with low-lying charge-t ransfer (CT) excitations by using the perturbational two-level approxi mation, Systematic variation of the donor and acceptor groups led to c ompounds with exceptional nonlinearity and thermal stability. Too stro ng donor/acceptor pairs, however, yielded structures in the charge-res onance (CR) limit with small or vanishing Delta mu(ag) and beta. (C) 1 997 Elsevier Science B.V.