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Excitation of four unsymmetrically substituted tungsten tris(dithiolenes), in various solvents with 355 nm laser pulses produce weak but easily measurable transient absorptions. The four complexes used are characterized as photocatalyst-catalysts (PC-C). for the photodecom-position of water and their names are tris-[ 1-(4-dimethylaminophenyl)-2-phenyl-l,2-ethylenodithiolenic-S,S′]tungsten,tris-[ 1-(4-methox-yphenyl)-2-phenyl-1, 2-ethylenodithiolenic-S,S′] tungsten, tris-[1 -(4-methoxy-phenyl)-1,2-ethylenodithiolenic-.S,S′] tungsten and tris-[ 1 -(phenyl)-1,2-ethylenodithiolenic-S,S′]tungsten. The lifetimes 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 known spectrum of MV , assigned to the hydrogen precursor, the other living much longer and assigned to the O2 precursor. The results indicate that the electron is quickly removed from the excited state of the photocatalyst-catalyst and is temporarily stored in the electron acceptor, in a way that is unfavorable to recombination. The first stages of the process can be schematically represented as: In-MV2+-(W-L)--OH2→MV2+ --(W−-L+)--OH2→ MV --(W-L)- +OH2→H2 + (W-L) +O2. The dotted lines connecting the reagents with the PC-C (W-L) were put in this scheme in order to emphasize the supramolecular nature of the interactions. The opposite charges created by light absorption are quickly transformed into repulsive positive charges due to the presence of the electron acceptor MV2+ and two centers are created, two molecular diameters apart, one the precursor of the H2, the other the precursor of O2. The combination of a small space separation charges and of the electrostatic factor (transformation of these charges into repulsive ones) are perhaps crucial factors in the separation of the reduction from the oxidation. However, there is also a kinetic factor. The reduction is quick, irreversibly leading to H2 not to recombination. The splitting of water is time resolved. The reactions at the reduction end of the supramolecular complex (presumably because the involve fast proton and electron transfers) lead fast and irreversibly to H2 formation. |
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