Incorporation of amphiphilic ruthenium(II) ammine complexes into langmuir-blodgett thin films with switchable quadratic nonlinear optical behavior

Leila Boubekeur-Lecaque, Benjamin J. Coe, James A. Harris, Madeleine Helliwell, Inge Asselberghs, Koen Clays, Stijn Foerier, Thierry Verbiest

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    Nine nonlinear optical (NLO) chromophores with pyridinium electron acceptors have been synthesized by complexing new proligands with {Ru II(NH 3) 5} 2+ electrondonor centers. The presence of long alkyl/fluoroalkyl chain substituents imparts amphiphilic properties, and these cationic complexes have been characterized as their PF 6 - salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Each complex shows three reversible/quasireversible redox processes; a Ru III/II oxidation and two ligand-based reductions. The energies of the intense visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions correlate to some extent with the ligand reduction potentials. 1H NMR spectroscopy also provides insights into the relative electron-withdrawing strengths of the new ligands. Single crystal X-ray structures have been determined for two of the proligand salts and one complex salt, [Ru II(NH 3) 5(4-C 16H 33PhQ +)]Cl 3•3.25H 2O (PhQ + = N-phenyl-4,4′-bipyridinium), showing centrosymmetric packing structures in each case. The PF 6 - analogue of the latter complex has been used to deposit reproducibly high-quality, multilayered Langmuir-Blodgett (LB) thin films. These films show a strong second harmonic generation (SHG) response from a 1064 nm laser; their MLCT absorbance increases linearly with the number of layers (N) and I 2ω/I ω 2 (I 2ω = intensity at 532 nm; I ω = intensity at 1064 nm) scales quadratically with N, consistent with homogeneous deposition. LB films on indium tin oxide (ITO)-coated glass show electrochemically induced switching of the SHG response, with a decrease in activity of about 50% on Ru II → Ru III oxidation. This effect is reversible, but reproducible over only a few cycles before the signal from the RuII species diminishes. This work extrapolates our original solution studies (Coe, B. J. et al. Angew. Chem., Int. Ed. 1999, 38, 366) to the first demonstration of redoxswitching of NLO activity in a molecular material. © 2011 American Chemical Society.
    Original languageEnglish
    Pages (from-to)12886-12899
    Number of pages13
    JournalInorganic Chemistry: including bioinorganic chemistry
    Issue number24
    Publication statusPublished - 19 Dec 2011


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