Organometallic complexes for nonlinear optics. 30. Electrochromic linear and nonlinear optical properties of alkynylbis(diphosphine)ruthenium complexes

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), C≡CPH (2), 4-C≡CC₆H₄C≡CPh (3); X = C≡CPh, Y = C≡CPh (4), 4-C≡CC₆H₄C≡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 (∼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(PH₃)₄] [X = Cl, Y = Cl, C≡CPh, or 4-C≡CC₆H₄C≡CPh; X = C≡CPh, Y = C≡CPh or 4-C≡CC₆H₄C≡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.