Robert Stranger

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.

This work considers the prospects of developing a commercially-feasible water electrolyser with 95–100% energy efficiency (relative to the Higher Heating Value, HHV, of hydrogen) at the cells in the near future.

The syntheses of trans-[Ru{4,4'-C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (19), trans-[Ru{4,4',4''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (20), trans-[Ru{4,4',4'',4'''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppe)(2)] (21), trans-[Ru{4,4',4'',4'''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (22), trans-[Ru{4,4',4'',4'''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (23), and trans-[Ru{4,4',4'',4''',4''''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (24) are reported, together with those of precursor alkynes, complexes with the donor-pi-bridge-acceptor formulation that affords efficient quadratic and cubic NLO compounds; the identity of 19 was confirmed by a structural study. The electrochemical properties of 19-24 and related complexes with shorter pi-bridge ligands were assessed by cyclic voltammetry, and the linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties were assayed by UV-vis-NIR spectroscopy, hyper-Rayleigh scattering studies at 1064 and 1300 nm, and broad spectral range femtosecond Z-scan studies, respectively. The Ru(II/III) oxidation potentials and wavelengths of the optical absorption maxima decrease on pi-bridge lengthening, until the tri(phenyleneethynylene) complex is reached, further chain lengthening leaving these parameters invariant; theoretical studies employing time-dependent density functional theory have shed light on this behavior. The quadratic nonlinearity beta(1064) and two-photon absorption cross-section reach maximal values at this same pi-bridge length, a similar saturation behavior that may reflect a common importance of ruthenium-to-alkynyl ligand charge transfer in electronic and optical behavior in these molecules.

The quadrupolar ruthenium alkynyl cruciform complex is reversibly converted to tetravinylidene complex or tetraruthenium(III) tetracationic complex forms by protonation or oxidation, respectively; linear and nonlinear proto- and electrochromism studies reveal independent switching of NLO properties by the protic and electrochemical stimuli, respectively, aspects of which have been rationalized by theoretical studies.