Photosensitized water reduction

Sacrificial photosensitized water reduction has been extensively investigated both in homogeneous solutions and in the presence of organized assemblies (micelles, microemulsions, polyelectrolytes, polymers, vesicles and... 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

Photosensitized Dissociation of Water using Dispersed Suspensions of n-Type Semiconductors Focus of study on Ti02 and SrTi03 using EDTA as an electron donor and Fe3+ as acceptor for tests of water reduction and oxidation activity respectively (c.f. Figs, lb and lc). 251... 

Metal phthalocyanines and porphyrins as photosensitizers for reduction of water to hydrogen. 

Darwent, J.R., P. Douglas, A. Harriman, G. Porter, and M.-C. Richoux (1982). Metal phthalocyanines and porphyrins as photosensitizers for reduction of water to hydrogen. Coord. Chem. Rev. 44, 83-126. 

Fig. 4. (a) Ir turnover numbers (TON) (b) Rh turnover numbers and (c) kinetic and rate trace at 0.075 mM iridium photosensitizer (PS) and rhodium water reduction catalyst (WRC) concentration, from evaluating the effect of catalyst concentration on performance with [Ir(f- mppy)2(dtbbpy)](PF6) and [RhldtbbpylaKPFela in photosynthetic-H2 reactions (0.5-1.5 Atmol of PS and WRC in 10 mL of 0.6 M TEA in 80% THF-H2O, 460 mn, 500 mW, 22 h). Reprinted from Ref 60, copyright 2008, with kind permission from the American Chemical Society. 

Luo S, Mejia E, Friedrich A, Pazidis A, Junge H, Surkus A, Jackstell R, Demirra S, Gladiali S, Lochbnmner S, Beller M (2013) Photocatalytic water reduction with copper-based photosensitizers a noble-metal-free system. Angew Chem Int Ed 52 419-423... 

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

Another method that may have less carcinogenic potential is to topically deliver the photosensitizer (methoxsalen) to the skin by adding it to bath water (bath PUVA) or as a topical cream (PUVA cream) instead of through systemic administration. Advantages of this approach include minimal risk of systemic effects, overall reduction of PUVA dose to one-... 

Figure 7.8 Photosensitized cleavage of water based on bimolecular photoredox reaction between an electron donor D and an acceptor A. Cat. 1 and cat. 2 are charge storage catalysts for reduction and oxidation of water, respectively.

---