Radical cation intermediates in the formation of schiff bases on irradiated semiconductor powders

An overview on recent development in visible light-mediated organic synthesis over heterogeneous photo-nanocatalysts

The implementation of heterogeneous photo-nanocatalysts in organic syntheses has been investigated greatly in the last decade as a result of the increasing demand to achieve the organic reactions via the use of green approaches and through the availability of visible light source. Herein, the presented results describe the basic concepts and state-of-the-art of fundamental insight into key features that influence the catalytic performance in organic reactions to investigate and optimize a broad range of catalyzed organic transformations, that benefit the researchers in academia and chemical industry fields.

Visible-Light-Mediated [4+2] Annulation of N-Cyclobutylanilines with Alkynes Catalyzed by Self-Doped Ti3+ @TiO2

We herein report a visible-light-mediated heterogeneous [4+2] annulation of N-cyclobutylanilines with alkynes catalyzed by self-doped Ti3+ @TiO2 . The self-doped Ti3+ @TiO2 is stable under photooxidation conditions, easy to recycle, and can be used multiple times without appreciable loss of activity. Extensive mechanistic studies suggest that the annulation reaction is mediated by singlet oxygen, which is generated through the photosensitization of oxygen in the air by the self-doped Ti3+ @TiO2 . In contrast, the homogeneous variant catalyzed by a far more expensive iridium complex proceeds under an inert atmosphere, which indicates a different mechanism. The substrate scopes of the two processes are comparable.

Selective aerobic oxidation mediated by TiO(2) photocatalysis

TiO2 is one of the most studied metal oxide photocatalysts and has unparal-leled efficiency and stability. This cheap, abundant, and non-toxic material has the potential to address future environmental and energy concerns. Understanding about the photoinduced interfacial redox events on TiO2 could have profound effect on the degradation of organic pollutants, splitting of H2O into H2 and O2, and selective redox organic transformations. Scientists traditionally accept that for a semiconductor photocatalyst such as TiO2 under the illumination of light with energy larger than its band gap, two photocarriers will be created to carry out their independent reduction and oxidation processes. However, our recent discoveries indicate that it is the concerted rather than independent effect of both photocarriers of valence band hole (hvb(+)) and conduction band electron (ecb(-)) that dictate the product formation during interfacial oxidation event mediated by TiO2 photocatalysis. In this Account, we describe our recent findings on the selective oxidation of organic substrates with O2 mediated by TiO2 photocatalysis. The transfer of O-atoms from O2 to the corresponding products dominates the selective oxidation of alcohols, amines, and alkanes mediated by TiO2 photocatalysis. We ascribe this to the concerted effect of both hvb(+) and ecb(-) of TiO2 in contribution to the oxidation products. These findings imply that O2 plays a unique role in its transfer into the products rather than independent role of ecb(-) scavenger. More importantly, ecb(-) plays a crucial role to ensure the high selectivity for the oxygenation of organic substrates. We can also use the half reactions such as those of the conduction band electron of TiO2 for efficient oxidation reactions with O2. To this end, efficient selective oxidation of organic substrates such as alcohols, amines, and aromatic alkanes with O2 mediated by TiO2 photocatalysis under visible light irradiation has been achieved. In summary, the concerted effect of hvb(+) and ecb(-) to implement one oxidation event could pave the way for selective oxofunctionalization of organic substrates with O2 by metal oxide photocatalysis. Furthermore, it could also deepen our understanding on the role of O2 and the elusive nature of oxygen species at the interface of TiO2, which, in turn, could shed new light on avant-garde photocatalytic selective redox processes in addressing the energy and environmental challenges of the future.

Effect of beta-Cyclodextrin on the Photoinduced Charge Transfer in Sodium 1-Anilino-8-naphthalene Sulfate (ANS)/CdS Colloidal System

The S-center radical (ANS(.)) of sodium 1-anilino-8-naphthalene sulfate (ANS) generated by photoinduced charge transfer in ANS/CdS and ANS/CdS/beta-cyclodextrin(beta-CD) systems has been studied by using spin trapping electron spin resonance techniques, UV-visible spectroscopic methods, and fluorescence spectroscopic methods. It was found that the S-centered radical (ANS(.)) was produced by the charge transfer reaction between the ground state ANS and the positive hole h(+)(CdS) from the valence band of CdS colloids, by the charge transfer from the excited singlet state (1)ANS* to the conduction band of CdS colloids, or by both in the ANS/CdS and ANS/CdS/beta-CD systems. The ESR signal intensity of the spin adduct (5,5'-dimethyl-1-pyrroline-N-oxide (DMPO)-ANS)(.), which is formed from ANS(.) trapped by DMPO, in the latter system is 15 times stronger than that in the former system. The apparent association constants between ANS and CdS colloids in the absence and presence of beta-CD determined from fluorescence quenching experiments are 1097 and 1606 M(-1), respectively. From ESR and fluorescence results, it is estimated that the efficiency of photoinduced charge transfer from ANS to CdS colloids in the ANS/CdS/beta-CD system is 12.5 times that in the ANS/CdS system. Copyright 2001 Academic Press.