PHOTOPHYSICAL STUDY OF THE DECOMPOSITION OF WATER USING VISIBLE-LIGHTAND TUNGSTEN TRIS(DITHIOLENES) AS PHOTOSENSITIZERS-CATALYSTS

Excitation of four unsymmetrically substituted tungsten tris(dithiolen es), in various solvents with 355 nm laser pulses produce weak but eas ily measurable transient absorptions. The four complexes used are char acterized as photocatalyst-catalysts (PC-C),for the photodecomposition of water and their names are l)-2-phenyl-1,2-ethylenodithiolenic-S,S' ]tungsten, tris-[1-(4-methoxyphenyl)-2-phenyl-1 ,2-ethylenodithiolenic -S,S']tungsten, hoxy-phenyl)-1,2-ethylenodithiolenic-S,S']tungsten and tris-[1-(phenyl)-1 ,2-ethylenodithiolenic-S,S']tungsten. The lifetime s of the transient absorptions observed are long, persisting into the millisecond time domain. In acetone-water, and in the presence of MV2 there are at least two transients, one corresponding closely to the k nown spectrum of MV+, assigned to the hydrogen precursor, the other li ving much longer and assigned to the O-2 precursor. The results indica te that the electron is quickly removed from the excited state of the photocatalyst-catalyst and is temporarily stored in the electron accep tor, in a way that is unfavorable to recombination. The first stages o f the process can be schematically represented as: MV2+-(W-L)-OH2 -->( hv) MV2+-(W--L+-OH2-->MV.+-(W-L)-+OH2-->H-2+(W-L)+O-2. The dotted line s connecting the reagents with the PC-C (W-L) were put in this scheme in order to emphasize the supramolecular nature of the interactions. T he opposite charges created by light absorption are quickly transforme d into repulsive positive charges due to the presence of the electron acceptor MV2+ and two centers are created, two molecular diameters apa rt, one the precursor of the H-2, the other the precursor of O-2. The combination of a small space separation charges and of the electrostat ic factor (transformation of these charges into repulsive ones) are pe rhaps crucial factors in the separation of the reduction from the oxid ation. However, there is also a kinetic factor. The reduction is quick , irreversibly leading to H-2 not to recombination. The splitting of w ater is time resolved. The reactions at the reduction end of the supra molecular complex (presumably because the involve fast proton and elec tron transfers) lead fast and irreversibly to H-2 formation. (C) 1998 Elsevier Science S.A. All rights reserved.