Organometallic complexes for nonlinear optics. 42. Syntheses, linear, and nonlinear optical properties of ligated metal-functionalized oligo(p-phenyleneethynylene)s

A combination of UV-vis-NIR spectroscopy, femtosecond Z-scan measurements, and time-dependent density functional theory (TD-DFT) calculations have been used to comprehensively investigate the linear optical and nonlinear optical (NLO) properties of π-delocalizable metal-functionalized oligo(phenyleneethynylene)s. A range of unsymmetrically or symmetrically end-functionalized mono-, di-, tri-, penta-, hepta-, and nona(phenyleneethynylene)s were synthesized, with larger examples bearing varying numbers of 2,5-di(hexyloxy)phenyl groups to ensure sufficient solubility of the metal complex derivatives. The effect of Incorporating varying numbers of solubillzing substituents In the OPE bridge, peripheral group modification, OPE lengthening, coligand variation, and metal location In the OPE on the linear optical properties has been established, with the first three molecular modifications resulting In significant changes In the optical absorption maxima. TD-DFT calculations reveal that the most intense transition In the linear optical spectra is localized on the OPE bridge and involves excitation from acetylenic to cumulenlc molecular orbltals that are not greatly spatially separated from one another. The nonlinear optical properties are dominated by two-photon absorption, which for all but 1,4-{trans-[RuCI(dppm) ₂]C=C}₂C₆H₄ appears as a band around 11400 cm^-1 and a sharp increase of nonlinear absorption at frequencies >17000 cm^-1. Surprisingly, there is relatively little influence of the length of the OPE bridge on the magnitude of the two-photon absorption cross sections, which are In the range 300-1000 GM.