Synthesis, properties and photochromism of novel charge transfer compounds with Keggin anions and protonated 2,2′-biquinoline

Several novel compounds with protonated 2,2'-biquinoline (biqui) and Keggin polyoxoanions (alpha-isomers), having the general formula (Hbiqui)(m)[XM12O40](.)nsolv, X = P (m = 3), Si (m = 4); M = Mo, W; n =0, 3; solv = H2O, N,N-dimethylformamide (dmf), were synthesized and characterized by analytical, spectroscopic and X-ray diffraction techniques. Electronic spectroscopy (visible/ UV) indicated the presence of intermolecular charge transfer between the organic and inorganic moieties in the solid state. Evidence for the existence of intermolecular electronic interaction in solution was found for compounds with the [SiW12O40](4-) anion, a fact quite uncommon for charge transfer compounds based on Keggin anions. A single crystal X-ray diffraction study could be performed on (Hbiqui)(4)[SiW12O40](.)3dmf crystals, but the refinements revealed the presence of highly disordered 2,2'-biquinoline molecules and alpha-[SiW12O40](4-) anions. Photosensitivity to sunlight and to W-lamp visible light was assessed for all compounds. Photochromic properties were found for solids with [XMo12O40](m-) Keggin anions. Reduction of these anions upon irradiation was observed by diffuse reflectance and EPR spectroscopy. After a minimum of 5 h under sunlight, the extent of anion reduction followed the order (Hbiqui)(3)[PMo12O40](.)3dmf>(Hbiqui)(4)[SiMo12O40](.)3dmf>(Hbiqui)(3)[PMo(12)o(40)]>(Hbiqui)(4)[SiMo12O40](.)4H(2)O. The Kurtz powder test was used to evaluate the second-order non-linear optical properties of the prepared compounds. (Hbiqui)(4)[SiW12O40](.)3H(2)O originated a second harmonic generation signal with intensity about 15% that of urea for 1064 nm radiation. (C) 2002 Elsevier Science Ltd. All rights reserved.