Organometallic complexes for nonlinear optics. 22.¹ quadratic and cubic hyperpolarizabilities of trans-bis(bidentate phosphine)ruthenium σ-arylvinylidene and σ-arylalkynyl complexes

The syntheses of trans-[Ru(C=CHR)Cl(pp)₂]PF₆ (pp = dppm, R = 4-C₆H₄C≡CPh, 4-C₆H₄-CHO, 4,4′-C₆H₄C≡CC₆H₄ NO₂, (E)-4,4′-C₆H₄ CH=CHC₆H₄NO₂, 4,4′,4″-C≡CC₆H₄C≡CC₆ H₄- C≡CC₆H₄NO₂; pp = dppe, R = 4-C₆H₄CHO, (E)-4,4′-C₆H₄CH=CHC₆H₄ NO₂) and trans-[Ru-(C≡CR)Cl(pp)₂] (pp = dppm, R = 4-C₆H₄C≡CPh, 4-C₆H₄CHO, 4,4′-C₆H₄C≡CC₆ H₄NO₂, (E)-4,4′-C₆H₄CH=CHC₆H₄ NO₂, 4,4′,4″-C≡ CC₆H₄C≡CC₆H₄ C≡CC₆H₄NO₂; pp = dppe, R = 4-C₆H₄-CHO, (E)-4,4′-C₆H₄CH=CHC₆H₄ NO₂) are reported, together with X-ray structural studies of trans-[Ru(C≡CR)Cl(pp)₂] (pp = dppm, R = 4-C₆H₄C≡CPh; pp = dppe, R = 4-C₆H₄CHO, (E)-4,4′-C₆H₄CH=CHC₆H₄ NO₂). Cyclic voltammetric, linear optical, and quadratic and cubic nonlinear optical response data for these new complexes, together with the corresponding data for the previously reported trans-[Ru(C=CHR)Cl(pp)₂]PF₆ (pp = dppm, R = Ph, 4-C₆H₄-NO₂; pp = dppe, R = Ph, 4-C₆H₄NO₂) and trans-[Ru(C≡CR)Cl(pp)₂] (pp = dppm, R = Ph, 4-C₆H₄NO₂, (E)-4,4′-C₆H₄CH=CHC₆H₄ NO₂; pp = dppe, R = Ph, 4-C₆H₄NO₂), are reported. Oxidation potentials for the Ru^II/III couple increase on proceeding from the neutral alkynyl complex to the analogous cationic vinylidene complex and on introduction of an acceptor group (CHO or NO₂); the complexes with 4-C≡CC₆H₄NO₂ ligands are the most difficult to oxidize. In some instances, the Ru^III/IV and Ru^I/II processes have been identified together with, where relevant, nitro-centered reduction processes. The oxidized and reduced vinylidene complexes are shown to transform electrochemically into the corresponding alkynyl complexes. Optical absorption maxima undergo a red shift upon increase of acceptor strength, replacement of the coligand dppm with dppe, and replacement of the alkynyl ligand yne linkage with an ene linkage. Proceeding from the vinylidene complex to an analogous alkynyl complex results in a small red shift in absorption maximum and a significant increase in extinction coefficient. Quadratic molecular nonlinearities by hyper-Rayleigh scattering measurements at 1064 nm increase upon introduction of ligated metal (proceeding from precursor alkyne to alkynyl or vinylidene complex), an increase in acceptor strength (introduction of CHO or NO₂), alkynyl chain lengthening (in the series [4-C≡CC₆H₄]_n-4-NO₂ proceeding from n = 1 and 2 to 3), and replacing the yne linkage with an ene linkage. Significant differences in β value for two vinylidene/alkynyl complex pairs suggest that they could function as precursors to protically switchable quadratic NLO materials at 1064 nm. Cubic molecular nonlinearities by Z-scan measurements at 800 nm are in many cases characterized by negative real and significant imaginary components, indicative of two-photon effects; nevertheless, a substantial increase in |γ| on proceeding to the largest molecule, trans-Ru(4,4′,4″-C≡CC₆H₄ C≡CC₆H₄C≡ CC₆H₄NO₂)Cl(dppm)₂, is observed. An order of magnitude difference in γ_imag values (and therefore two-photon absorption (TPA) cross sections σ₂) for vinylidene/alkynyl complex pairs suggest that they have potential as protically switchable TPA materials at 800 nm.