Organometallic Complexes for Nonlinear Optics. 22. Quadratic and Cubic Hyperpolarizabilities of <i>trans-</i>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-C6H4CHO, 4,4‘-C6H4C⋮CC6H4NO2, (E)-4,4‘-C6H4CHCHC6H4NO2, 4,4‘,4‘ ‘-C⋮CC6H4C⋮CC6H4C⋮CC6H4NO2; pp = dppe, R = 4-C6H4CHO, (E)-4,4‘-C6H4CHCHC6H4NO2) and trans-[Ru(C⋮CR)Cl(pp)2] (pp = dppm, R = 4-C6H4C⋮CPh, 4-C6H4CHO, 4,4‘-C6H4C⋮CC6H4NO2, (E)-4,4‘-C6H4CHCHC6H4NO2, 4,4‘,4‘ ‘-C⋮CC6H4C⋮CC6H4C⋮CC6H4NO2; pp = dppe, R = 4-C6H4CHO, (E)-4,4‘-C6H4CHCHC6H4NO2) 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CHC6H4NO2). 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-C6H4NO2; pp = dppe, R = Ph, 4-C6H4NO2) and trans-[Ru(C⋮CR)Cl(pp)2] (pp = dppm, R = Ph,
4-C6H4NO2, (E)-4,4‘-C6H4CHCHC6H4NO2; pp = dppe, R = Ph, 4-C6H4NO2), are reported.
Oxidation potentials for the RuII/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 RuIII/IV and RuI/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⋮CC6H4C⋮CC6H4C⋮CC6H4NO2)Cl(dppm)2, is observed. An order
of magnitude difference in γimag 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.