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, γ 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 γ calc differences between the symmetrically and asymmetrically disubstituted OPVs are relatively minor. The real part (γ real ) and the imaginary part (γ 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 γ real from positive to negative appears earlier in chloroform than in THF. The γ 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 |γ| than the symmetrically disubstituted series. A very large |γ| value, up to 2.5 × 10 - 32 esu, was obtained in the symmetrically disubstituted OPVs.