Mesoporous assemblies of p-CuS/n-CdS nanocrystal junctions with high surface areas and uniform pores demonstrate a high performance and stability in photocatalytic hydrogen generation from water using visible light.
We investigate a biomimetic model of a Tyr /His pair, a hydrogen-bonded phenol/imidazole covalently attached to a porphyrin sensitizer. Laser flash photolysis in the presence of an external electron acceptor reveals the need for water molecules to unlock the light-induced oxidation of the phenol through an intramolecular pathway. Kinetics monitoring encompasses two fast phases with distinct spectral properties. The first phase is related to a one-electron transfer from the phenol to the porphyrin radical cation coupled with a domino two-proton transfer leading to the ejection of a proton from the imidazole-phenol pair. The second phase concerns conveying the released proton to the porphyrin N coordinating cavity. Our study provides an unprecedented example of a light-induced electron-transfer process in a Tyr /His model of photosystem II, evidencing the movement of both the phenol and imidazole protons along an isoenergetic pathway.
A new zinc phthalocyanine-zinc porphyrin dyad (ZnPc-ZnP) fused through a pyrazine ring has been synthesized as a receptor for imidazole-substituted C (CIm) electron acceptor. Self-assembly via metal-ligand axial coordination and the pertinent association constants in solution were determined by H-NMR, UV-Vis and fluorescence titration experiments at room temperature. The designed host was able to bind up to two CIm electron acceptor guest molecules to yield CIm:ZnPc-ZnP:ImC donor-acceptor supramolecular complex. The spectral data showed that the two binding sites behave independently with binding constants similar in magnitude. Steady-state fluorescence studies were indicative of an efficient singlet-singlet energy transfer from zinc porphyrin to zinc phthalocyanine within the fused dyad. Accordingly, the transient absorption studies covering a wide timescale of femto-to-milli seconds revealed ultrafast energy transfer from ZnP* to ZnPc (k ∼ 10 s) in the fused dyad. Further, a photo induced electron transfer was observed in the supramolecularly assembled CIm:ZnPc-ZnP:ImC donor-acceptor complex leading to charge separated states, which persisted for about 200 ns.
A supramolecular triad composed of a fused zinc phthalocyanine‐free‐base porphyrin dyad (ZnPc‐H2P) coordinated to phenylimidazole functionalized C60 via metal‐ligand axial coordination was assembled, as a photosynthetic antenna‐reaction centre mimic. The process of self‐assembly resulting into the formation of C60Im:ZnPc‐H2P supramolecular triad was probed by proton NMR, UV‐Visible and fluorescence experiments at ambient temperature. The geometry and electronic structures were deduced from DFT calculations performed at the B3LYP/6‐31G(dp) level. Electrochemical studies revealed ZnPc to be a better electron donor compared to H2P, and C60 to be the terminal electron acceptor. Fluorescence studies of the ZnPc‐H2P dyad revealed excitation energy transfer from 1H2P* to ZnPc within the fused dyad and was confirmed by femtosecond transient absorption studies. Similar to that reported earlier for the fused ZnPc‐ZnP dyad, the energy transfer rate constant, kENT was in the order of 1012 s−1 in the ZnPc‐H2P dyad indicating an efficient process as a consequence of direct fusion of the two π‐systems. In the presence of C60Im bound to ZnPc, photoinduced electron transfer leading to H2P‐ZnPc.+:ImC60.− charge separated state was observed either by selective excitation of ZnPc or H2P. The latter excitation involved an energy transfer followed by electron transfer mechanism. Nanosecond transient absorption studies revealed that the lifetime of charge separated state persists for about 120 ns indicating charge stabilization in the triad.
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