Photosensitized H Evolution and NADPH Formation by Photosensitizer/Carbon Nitride Hybrid Nanoparticles.

The broadband CN semiconductor absorbs in the UV region, λ = 330-380 nm, a feature limiting its application for light-to-energy conversion. The unique surface adsorption properties of CN allow, however, the binding of a photosensitizer, operating in the visible-solar spectrum to the surface of CN. Coupling of the energy levels of the photosensitizer with the energy levels of CN allows effective photoinduced electron-transfer quenching and subsequent charge separation in the hybrid structures. Two methods to adsorb a photosensitizer on the CN nanoparticles are described. One is exemplified by the adsorption of Zn(II)-protoporphyrin IX on CN using π-π interactions. The second method utilizes the specific binding interactions of single-stranded nucleic acids on CN and involves the binding of a Ru(II)-tris-bipyridine-modified nucleic acid on the CN nanoparticles. Effective electron-transfer quenching of the photoexcited photosensitizers by CN proceeds in the two hybrid systems. The two hybrid photosystems induce the effective photosensitized reduction of ,'-dimethyl-4,4'-bipyridinium, MV, to MV, in the presence of NaEDTA as a sacrificial electron donor. The generation of MV is ca. 5-fold higher as compared to the formation of MV in the presence of the photosensitizer alone (in the absence of CN). The effective generation of MV in the photosystems is attributed to the efficient quenching of the photosensitizers, followed by effective charge separation of the electrons in the conduction band of CN and the holes in the oxidized photosensitizer. The subsequent transfer of the conduction-band electrons to MV and the oxidation of NaEDTA by the oxidized photosensitizers lead to the effective formation of MV. The photogenerated MV by the two hybrid photosystems is used to catalyze H evolution in the presence of Pt nanoparticle catalysts and to mediate the reduction of NADP to NADPH, in the presence of ferredoxin-NADP reductase, FNR. The ability to couple the photogenerated NADPH to drive NADP-dependent biocatalytic transformations is demonstrated.