Katy A. Green

Switching the Cubic Nonlinear Optical Properties of an Electro-, Halo-, and Photochromic Ruthenium Alkynyl Complex Across Six States

The impending breakdown in Moore s law has prompted the search for molecule-based information-processing components such as molecular switches and logic gates. Bistable molecules that afford states possessing distinct linear optical properties (absorbance, fluorescence) and that can be interconverted by chemical, redox, magnetic, or photonic stimuli have attracted considerable interest for Boolean logic operations. In principle, molecular computing could exploit ternary or higher-order digit representations, which would permit smaller device components. “Switchable” molecular properties are also of intense interest for sensor applications. Despite this interest, molecules that can exist in more than two stable and independently addressable states, which could be employed for complex and higher-order logic functions, have been explored significantly less than two-state molecules. One potentially very important procedure to exploit molecular switches is to utilize their cubic nonlinear optical (NLO) properties, and in particular their nonlinear absorption. However, this avenue is also poorly explored and is to date restricted to switching between three states at most, although such switching offers 1) the prospect of broadening the available spectral range (and in particular utilizing telecommunications-relevant wavelengths) and 2) the possibility of enhanced spatial control (compared to linear optical switching) because of the higher-order dependence on the intensity of the incident light. Herein we show that a specific binuclear metal alkynyl complex incorporating a functionalized 5,5’-dithienylperfluorocyclopentene (DTE) bridge can afford six stable and switchable states that possess distinct cubic NLO properties. The complex is comprised of independently addressable modules that respond orthogonally to protic (alkynyl ligandQvinylidene ligand), electrochemical (metal-centered redox: RuQRu), and photochemical (DTE ring-openingQring-closing) stimuli. The six states are interconverted along seven pathways, all of which result in distinct changes to cubic nonlinearity for specific regions of the spectrum. Our results demonstrate that complexes of this type have the potential to be used, among other things, in the construction of multi-input logic gates responding to diverse stimuli across a broad spectral range. The synthesis of the dinuclear ruthenium alkynyl complex oa(II) is depicted in Figure S1 in the Supporting Information. The DTE unit is obtained in the “open” form as its 5,5’diethynyl derivative; the open DTE is thermally stable, and this open form persists through the preparative steps that ultimately afford oa(II). Complete synthetic and characterization details are given in the Supporting Information. Complex oa(II) can be reversibly protonated to the di(vinylidene) complex ov(II), reversibly oxidized to the Ru complex dication oa(III), and photoisomerized with UV light to the closed alkynyl complex ca(II). Complex ca(II) can be reversibly protonated to cv(II) and reversibly oxidized to ca(III) and undergoes photoreversion to oa(II) on irradiation with red light. The vinylidene forms and the oxidized forms undergo reversible photoisomerization under analogous irradiation conditions. Note that oxidation of ruthenium vinylidene complexes with these coligands is an irreversible process, as assessed by cyclic voltammetry, so there is a maximum of six switchable states for this complex. The interconversions of the six stable states of the complex are depicted in Scheme 1, and key spectral data are tabulated in the Supporting Information (Table S1). Cyclic voltammograms and UV/Vis/NIR spectral progressions for the redox processes, the latter acquired with an optically transparent thin-layer spectroelectrochemical (OTTLE) cell and demonstrating stable isosbestic points, are given in the Supporting Information, together with NMR spectra demonstrating spectroscopically complete 1) photoisomerization between the open and closed forms and 2) protonation/deprotonation to afford the vinylidene and alkynyl complex forms, and IR spectra demonstrating spectroscopically complete oxidation from the Ru to the Ru form. Several organometallic alkyne-functionalized DTE complexes have been reported, and the effect of oxidation state on photochromic behavior has been probed, although there was no assessment of the effect of protonation/deprotonation on photochromism. [CpFe(h-C5H4C C DTE C C-hC5H4)FeCp] [11] and [(dppe)CpFe(C C DTE C C)FeCp[*] K. A. Green, Dr. M. P. Cifuentes, T. C. Corkery, Prof. M. G. Humphrey Research School of Chemistry, Australian National University Canberra, ACT 0200 (Australia) Fax: (+ 61)2-6125-0760 E-mail: mark.humphrey@anu.edu.au