3RD-ORDER NONLINEAR-OPTICAL PROPERTIES OF OLIGOMERS OF THIENYLENEETHYNYLENES AND THIENYLENEVINYLENES

Third-order optical nonlinear properties of four thienyleneethynylenes and two thienylenevinylenes containing from two to five thiophene units were measured. Third-order nonlinear susceptibilities chi((3))(-3 omega;omega,omega,omega) at the fundamental wavelength of 1064 nm were measured by third harmonic generation (THG) for thin films of the oligomers in poly(methyl methacrylate) (PMMA) using the Maker fringe technique. The data were reduced to the equivalent molecular second hyperpolarizabilities gamma(-3 omega;omega,omega,omega). Theoretical calculations of the static (zero-frequency) [gamma] values were performed at the semiempirical Austin model one/finite field (AM1/EF) and intermediate neglect of differential overlap/single and double configuration interaction (INDO/SDCI) sum-over-states (SOS) levels and at the nonempirical valence effective Hamiltonian (VEH)-SOS level. The evolution of gamma as a function of the torsion angle between the thiophene rings was investigated via the AM1/FF method. With the INDO-SOS technique, the frequency dependence of gamma(-3 omega;omega,omega,omega) was also analyzed. For the ethynylenes, the experimental THG values of gamma lie between 63 x 10(-36) and 2300 x 10(-36) esu and increase with the number of heterocycle units of the oligomers. These values are affected by three-photon resonance. The resonant THG gamma-values of the ethynylenes lie within 10% of the corresponding values of the vinylenes. After a simple correction for three-photon resonance, the dependence of gamma-values on the number of pi-electrons is fitted to power laws and compared with the theoretical calculations. The theoretical static gamma-values indicate a higher response in the vinylene derivatives than in the corresponding ethynylene derivatives. However, the experimental THG results provide the reverse trend: this can be explained by a stronger resonance enhancement of the THG gamma-values in the ethynylene compounds, as is demonstrated by the results of the frequency-dependent calculations.