A combination of cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, time-dependent density functional theory (TD-DFT), and Z-scan measurements employing a modified optically transparent thin-layer electrochemical (OTTLE) cell has been used to identify and assign intense transitions of metal alkynyl complexes at technologically important wavelengths in the oxidized state and to utilize these transitions to demonstrate a facile electrochromic switching of optical nonlinearity. Cyclic voltammetric data for the ruthenium(II) complexes trans-[RuXY(dppe)(2)] [dppe = 1,2-bis(diphenylphosphino)ethane, X = Cl, Y = Cl (1), Ctbd1;CPh (2), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (3); X = Ctbd1;CPh, Y = Ctbd1;CPh (4), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (5)] show a quasi-reversible oxidation at 0.50-0.60 V (with respect to ferrocene/ferrocenium 0.56 V), which is assigned to the Ru(II/III) couple. The ruthenium(III) complex cations trans-[RuXY(dppe)(2)](+) were obtained by the in situ oxidation of complexes 1-5 using an OTTLE cell. The UV-vis-NIR optical spectra of 1(+)-5(+) contain a low-energy band in the near-IR region ( approximately 8000-16 000 cm(-)(1)), in contrast to 1-5, which are optically transparent at wavelengths < 22 000 cm(-)(1). TD-DFT calculations have been applied to model systems trans-[RuXY(PH(3))(4)] [X = Cl, Y = Cl, Ctbd1;CPh, or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh; X = Ctbd1;CPh, Y = Ctbd1;CPh or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh] to rationalize the optical spectra of 1-5 and 1(+)-5(+). The important low-energy bands in the electronic spectra of 1(+)-5(+) are assigned to the promotion of an electron from either a chloride p orbital or an ethynyl p orbital to the partially occupied HOMO. These absorption bands have been utilized to demonstrate a facile switching of cubic nonlinear optical (NLO) properties at 12 500 cm(-)(1) (corresponding to the wavelength of maximum transmission in biological materials such as tissue) using the OTTLE cell, the first electrochromic switching of molecular nonlinear refraction and absorption, and the first switching of optical nonlinearity using an electrochemical cell.

The syntheses of trans- [Ru(CdCHR) Cl(pp) 2 ]PF 6 (pp ) dppm, R ) 4-C 6 H 4 CtCPh, 4-C 6 H 4 -CHO, 4,4′-C 6 H 4 CtCC 6 H 4 NO 2 , (E)-4,4′-C 6 H 4 CHdCHC 6 H 4 NO 2 , 4,4′,4′′-CtCC 6 H 4 CtCC 6 H 4 -CtCC 6 H 4 NO 2 ; pp ) dppe, R ) 4-C 6 H 4 CHO, (E)-4,4′-C 6 H 4 CHdCHC 6 H 4 NO 2 ) and trans-[Ru-(CtCR)Cl(pp) 2 ] (pp ) dppm, R ) 4-C 6 H 4 CtCPh, 4-C 6 H 4 CHO, 4,4′-C 6 H 4 CtCC 6 H 4 NO 2 , (E)-4,4′-C 6 H 4 CHdCHC 6 H 4 NO 2 , 4,4′,4′′-CtCC 6 H 4 CtCC 6 H 4 CtCC 6 H 4 NO 2 ; pp ) dppe, R ) 4-C 6 H 4 -CHO, (E)-4,4′-C 6 H 4 CHdCHC 6 H 4 NO 2 ) are reported, together with X-ray structural studies of trans-[Ru(CtCR)Cl(pp) 2 ] (pp ) dppm, R ) 4-C 6 H 4 CtCPh; pp ) dppe, R ) 4-C 6 H 4 CHO, (E)-4,4′-C 6 H 4 CHdCHC 6 H 4 NO 2 ). 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 transOxidation 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 NO 2 ); the complexes with 4-CtCC 6 H 4 NO 2 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 NO 2 ), alkynyl chain lengthening (in the series [4-CtCC 6 H 4 ] n -4-NO 2 , 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 twophoton effects; nevertheless, a substantial increase in |γ| on proceeding to the largest molecule, trans-Ru(4,4′,4"-CtCC 6 H 4 CtCC 6 H 4 CtCC 6 H 4 NO 2 )Cl(dppm) 2 , is observed. An order of magnitude difference in γ imag values (and therefore two-photon absorption (TPA) cross sections σ 2 ) for vinylidene/alkynyl complex pairs suggest that they have potential as protically switchable TPA materials at 800 nm.

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A combination of cyclic voltammetry (CV), UV-vis-NIR spectroscopy and spectroelectrochemistry, hyper-Rayleigh scattering (HRS) [including depolarization studies], Z-scan and degenerate four-wave mixing (DFWM) [including studies employing an optically transparent thin-layer electrochemical (OTTLE) cell to effect electrochemical switching of nonlinearity], pump-probe, and electroabsorption (EA) measurements have been used to comprehensively investigate the electronic, linear optical, and nonlinear optical (NLO) properties of nanoscopic pi-delocalizable electron-rich alkynylruthenium dendrimers, their precursor dendrons, and their linear analogues. CV, UV-vis-NIR spectroscopy, and UV-vis-NIR spectroelectrochemistry reveal that the reversible metal-centered oxidation processes in these complexes are accompanied by strong linear optical changes, "switching on" low-energy absorption bands, the frequency of which is tunable by ligand replacement. HRS studies at 1064 nm employing nanosecond pulses reveal large nonlinearities for these formally octupolar dendrimers; depolarization measurements are consistent with lack of coplanarity upon pi-framework extension through the metal. EA studies at 350-800 nm in a poly(methyl methacrylate) matrix are consistent with the important transitions having a charge-transfer exciton character that increases markedly on introduction of peripheral polarizing substituent. Time-resolved pump-probe studies employing 55 ps, 527 nm pulses reveal absorption saturation, the longest excited-state lifetime being observed for the dendrimer. Z-scan studies at 800 nm employing femtosecond pulses reveal strong two-photon absorption that increases significantly on progression from linear complex to zero- and then first-generation dendrimer with no loss of optical transparency. Both refractive and absorptive nonlinearity for selected alkynylruthenium dendrimers have been reversibly "switched" by employing the Z-scan technique at 800 and 1180 nm and 100-150 fs pulses, together with a specially modified OTTLE cell, complementary femtosecond time-resolved DFWM and transient absorption studies at 800 nm suggesting that the NLO effects originate in picosecond time scale processes.