Synthesis and third-order nonlinear optical properties of end-functionalized oligo-phenylenevinylenes

Novel homologues series of the highly soluble and coplanar oligo-phenylenevinylenes (OPV), containing up to eight phenyl rings symmetrically end-functionalized with multiple polyalkyleneoxy substituents (OPV(n)-OR), and containing up to six phenyl rings, asymmetrically end-functionalized with polyalkyleneoxy and alkylsulfonyl substituents (OPV(n)-ORSO), were synthesized for structure-third-order optical nonlinearities investigation. The theoretical molecular nonlinearities, gamma(calc) at zero frequency calculated by the Finite Field method in MOPAC6 and the time-dependent coupled perturbed Hartree-Fock method within the MOPAC93 package showed a similar trend of the increase with an extension of chain length. However, the predicted gamma(calc) differences between the symmetrically and asymmetrically disubstituted OPVs are relatively minor. The real part (gamma(real)) and the imaginary part (gamma(imag)) of the molecular nonlinearities of the newly synthesized OPVs were determined by the closed- and open-aperture Z-scan technique at 800 nm with 100-fs laser pulses in two different solvents. It is evidenced that solvent effect plays an important role in determining third-order nonlinearities of molecules in solution. Although the absorption behavior of these OPVs is very similar in chloroform and tetrahydrofuran (THF), the sign reversal of gamma(real) from positive to negative appears earlier in chloroform than in THF. The gamma enhancement due to asymmetric substitution in lower homologues of these oligomers is significant; however, such contribution becomes less pronounced in the higher homologues. In addition, the asymmetrically disubstituted OPVs show a faster convergence of \gamma\ than the symmetrically disubstituted series. A very large \gamma\ value, up to 2.5 x 10(-32) esu, was obtained in the symmetrically disubstituted OPVs.