Semiconductor Photocatalysis for Chemoselective Radical Coupling Reactions.

Photocatalysis at semiconductor surfaces is a growing field of general photocatalysis because of its importance for the chemical utilization of solar energy. By analogy with photoelectrochemistry the basic mechanism of semiconductor photocatalysis can be broken down into three steps: photogenerated formation of surface redox centers (electron-hole pairs), interfacial electron transfer from and to substrates (often coupled with proton-transfer), and conversion of primary redox intermediates into the products. Sun driven water cleavage and carbon dioxide fixation are still in the state of basic research whereas aerial degradation reactions of pollutants have reached practical application for the cleaning of air. In addition, a great variety of organic transformations (not syntheses) have been reported. They include cis-trans isomerizations, valence isomerizations, cycloaddition reactions, intramolecular or intermolecular C-N and C-C couplings, partial oxidations, and reductions. In all cases, well-known products were formed but very rarely also isolated. As compared to conventional homogeneous organic synthesis, the photocatalytic reaction mode is of no advantage, although the opposite is quite often claimed in the literature. It is also noted that a high quantum yield does not implicate a high product yield, since it is measured at very low substrate conversion in order to minimize secondary photoreactions. That is especially important in semiconductor photocatalysis since photocorrosion of the photocatalyst often prevents long-time irradiation, as is the case for colloidal metal sulfide semiconductors, which in general are photochemically too unstable to be used in synthesis. In this Account, we first classify the numerous organic photoreactions catalyzed by semiconductor powders. The classification is based on easily obtainable experimental facts, namely the nature of the light absorbing reaction component and the reaction stoichiometry. Next we discuss the problem of quantitative comparisons of photocatalytic activities or apparent quantum yields and propose a basic three-step mechanistic model. Finally, we address the question whether or not the unique photoredox properties of simple inorganic semiconductor powders may lead to previously unknown visible light induced organic syntheses. For that, we summarize novel radical C-C- and C-N- couplings photocatalyzed by self-prepared cadmium sulfide powders. Electron acceptor and donor substrates like imines or 1,2-diazenes, and cyclic olefins or unsaturated ethers, respectively, undergo a linear addition reaction. The hitherto unknown products have all been isolated in good to moderate yields and may be of pharmaceutical interest. In the first reaction step photogenerated electron-hole pairs produce through proton-coupled electron transfer the corresponding radicals. Their subsequent chemoselective heterocoupling affords the products, correlating with an insertion of the imine or 1,2-diazene into an allylic C(sp3)-H bond of the donor substrate. In the absence of an imine or 1,2-diazene, cyclic allyl/enol ethers are dehydrodimerized under concomitant hydrogen evolution. Even a visible light photosulfoxidation of alkanes is catalyzed by titania. In these heterogeneous photoredox reactions the role of the semiconductor photocatalyst is multifunctional. It induces favorable substrate preorientations in the surface-solvent layer, it catalyzes proton-coupled interfacial electron transfer to and from substrates generating intermediate radicals, and it enables their subsequent chemoselective coupling in the surface-solvent interface. Different from molecular photosensitizers, which enable only one one-electron transfer with one single substrate, photoexcited semiconductors induce two concerted one-electron transfer reactions with two substrates. This is because the light generated electron-hole pairs are trapped at distinct surface sites and undergo proton-coupled interfacial electron transfers with unsaturated donor and acceptor substrates. The radicals diffuse in a solid-solute-surface layer to undergo chemo- and stereoselective C-C and C-N bond formation. Thus, the semiconductor photocatalyst functions like an artificial leaf. Since several minerals are known to have semiconductor properties, solar photocatalysis may be also relevant for prebiotic and environmental chemistry.

Semiconductor photocatalysis--mechanistic and synthetic aspects.

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

Visible light mediated homo- and heterocoupling of benzyl alcohols and benzyl amines on polycrystalline cadmium sulfide.

The oxidative coupling of sp(3) hybridized carbon atoms by photocatalysis is a valuable synthetic method as stoichiometric oxidation reagents can be avoided and dihydrogen is the only byproduct of the reaction. Cadmium sulfide, a readily available semiconductor, was used as a visible light heterogeneous photocatalyst for the oxidative coupling of benzyl alcohols and benzyl amines by irradiation with blue light. Depending on the structure of the starting material, good to excellent yields of homocoupling products were obtained as mixtures of diastereomers. Cross-coupling between benzyl alcohols and benzyl amines gave product mixtures, but was selective for the coupling of tetrahydroisoquinolines to nitromethane. The results demonstrate that CdS is a suitable visible light photocatalyst for oxidative bond formation under anaerobic conditions.

Photochemical synthesis of N-adamantylhomoallylamines through addition of cyclic olefins to imines catalyzed by alumina grafted cadmium sulfide.

Grafting cadmium sulfide onto alumina induces a small bandgap widening and a more significant lifetime variation of the light generated charge carriers from 0.76 microseconds measured for pristine CdS to 0.75, 0.86, and 1.20 microseconds found for CdS/Al(2)O(3) containing 30, 20, and 9% of CdS, respectively. The quasi-Fermi level of electrons of -0.42 V (NHE) is not significantly changed. These alumina grafted semiconductor photocatalysts enable the linear addition of cyclopentene, cyclohexene, and α-pinene to N-adamantylimines affording novel homoallyladamantylamines in isolated yields of 21-85% through a regioselective C-C heterocoupling of intermediate allyl and α-aminobenzyl radicals. As by-products hydrodimers of the imine are formed by C-C homocoupling of the benzylic radicals. Different from heterocoupling, the homocoupling is a stereospecific process directed by the nature of the olefin employed in the reaction.

Mechanism of aerobic visible light formic acid oxidation catalyzed by poly(tri-s-triazine) modified titania.

Visible light aerial oxidation of formic acid catalyzed by N/C-modified titania (TiO(2)-N,C) is investigated by wavelength-dependent photocatalytic and photoelectrochemical experiments in the presence of oxygen, tetranitromethane, and methylviologen as electron acceptors. The title reaction is shown to proceed both through oxidative and reductive primary processes. Contrary to the urea-derived (TiO(2)-N,C), so-called "N-doped" titania (TiO(2)-N) as prepared from ammonia is inactive. In accord with photocurrent action spectra of corresponding powder electrodes, this different activity of the two photocatalysts is traced back to the different chemical nature of the reactive holes localized at the modifier. Hole stabilization by delocalization within an extended poly(tri-s-triazine) network of TiO(2)-N,C is proposed to render recombination with conduction band electrons less probable than in TiO(2)-N.

The degradation of microcystin-LR using doped visible light absorbing photocatalysts.

Microcystins are one of the primary hepatotoxic cyanotoxins released from cyanobacteria. The presence of these compounds in water has resulted in the death of both humans and domestic and wild animals. Although microcystins are chemically stable titanium dioxide photocatalysis has proven to be an effective process for the removal of these compounds in water. One problem with this process is that it requires UV light and therefore in order to develop effective commercial reactor units that could be powered by solar light it is necessary to utilize a photocatalyst that is active with visible light. In this paper we report on the application of four visible light absorbing photocatalysts for the destruction of microcystin-LR in water. The rhodium doped material proved to be the most effective material followed by a carbon-modified titania. The commercially available materials were both relatively poor photocatalysts under visible radiation while the platinum doped catalyst also displayed a limited activity for toxin destruction.

On the mechanism of urea-induced titania modification.

The mechanism of surface modification of titania by calcination with urea at 400 degrees C was investigated by substituting urea by its thermal decomposition products. It was found that during the urea-induced process titania acts as a thermal catalyst for the conversion of intermediate isocyanic acid to cyanamide. Trimerization of the latter produces melamine followed by polycondensation to melem- and melon-based poly(aminotri-s-triazine) derivatives. Subsequently, amino groups of the latter finish the process by formation of Ti--N bonds through condensation with the OH-terminated titania surface. When the density of these groups is too low, like in substoichiometric titania, no corresponding modification occurs. The mechanistic role of the polytriazine component depends on its concentration. If present in only a small amount, it acts as a molecular photosensitizer. At higher amounts it forms a crystalline semiconducting organic layer, chemically bound to titania. In this case the system represents a unique example of a covalently coupled inorganic-organic semiconductor photocatalyst. Both types of material exhibit the quasi-Fermi level of electrons slightly anodically shifted relative to that of titania. They are all active in the visible-light mineralization of formic acid, whereas nitrogen-modified titania prepared from ammonia is inactive.

On the origin of visible light activity in carbon-modified titania.

Characterization of a commercial carbon-modified titania visible light photocatalyst (VLP) reveals a quasi-Fermi level of -0.50 V at pH 7 and characteristic C1s binding energies of 284.8 eV and 286.3 eV as measured by XPS. Treatment with sodium hydroxide affords a soluble brown extract SENSex exhibiting in the IR spectrum intense peaks at 1420 cm(-1) and 1580 cm(-1), tentatively assigned to an arylcarboxylate group. Both the residue and the solution SENSex do not induce significant visible light mineralization of 4-chlorophenol. However, after heating them together in suspension, followed by calcination at 200 degrees C the resulting powder VLPreas exhibits the same quasi-Fermi level and C1s binding energies as the original VLP. Furthermore, within experimental error its visible light activity is identical with that of VLP. These results clearly indicate that, at least for VLP but probably also for other "carbon-doped" titania materials, an aromatic carbon compound and not substitutional or interstitial carbon is the origin of visible light activity.

Visible light photo-oxidations in the presence of alpha-Bi(2)O(3).

alpha-Bismuth oxides of specific surface areas of 1-3 m(2) g(-1) were prepared by three different methods and their visible light activity was tested in the photodegradation (lambda>or= 420 nm) of 4-chlorophenol. In method A, which led to powders of poor to moderate photoactivity, the starting materials BiONO(3), Bi(NO(3))(3) x 5H(2)O, (BiO)(2)CO(3), and BiOCl were annealed at 500 degrees C without any pretreatment. In method B the salt (BiO)(2)CO(3) was washed with water and subsequently calcined at 450 degrees C affording a very active powder. In method C the salts BiONO(3), Bi(NO(3))(3) x 5H(2)O and (BiO)(2)CO(3) were dissolved in nitric acid and Bi(OH)(3) was precipitated by addition of sodium hydroxide. After annealing at 500 degrees C the resulting oxides exhibited moderate activity in the case of the (BiO)(2)CO(3) precursor whereas highly active powders were obtained from BiONO(3) and Bi(NO(3))(3) x 5H(2)O inducing almost complete photomineralization of 4-chlorophenol. XRD analysis indicated the presence of 40-140 nm large crystallites of alpha-Bi(2)O(3). From diffuse reflectance spectroscopy bandgaps of 2.80 +/- 0.05 eV and 2.93 +/- 0.05 eV were obtained, assuming an indirect or direct semiconductor, respectively. The quasi-Fermi potential of electrons at pH 7 was determined as -0.08 +/- 0.05 V (vs. NHE) through pH dependent photovoltage measurements. Repeated use of the presumed catalyst powder revealed that the mineralization is not a catalytic but a bismuth oxide assisted photo-oxidation. This result shed a critical light on previous reports on the photocatalytic action of binary and ternary bismuth oxides.

Tuning the optical and photoelectrochemical properties of surface-modified TiO2.

Surface-modification of TiO(2) is found to be a powerful tool for manipulating the fundamental optical and photoelectrochemical properties of TiO(2). High surface area nanocrystalline TiO(2) was modified by urea pyrolysis products at different temperatures between 300 degrees C and 500 degrees C. Modification occurs through incorporation of nitrogen species containing carbon into the surface structure of titania. The N1s XPS binding energies are 399-400 eV and decrease with increasing modification temperature whereby the Ti2p(3/2) peak is also shifted to lower binding energies by about 0.5 eV. With increasing modification temperature the optical bandgap of surface-modified TiO(2) continuously decreases down to approximately 2.1 eV and the quasi-Fermi level of electrons at pH 7 is gradually shifted from -0.6 V to -0.3 V vs. NHE. The surface-modified materials show enhanced sub-bandgap absorption (Urbach tail) and exhibit photocurrents in the visible down to 750 nm. The maximum incident photon-to-current efficiency (IPCE) was observed for the materials modified at 350 degrees C and 400 degrees C (IPCE approximately 14% at 400 nm, and IPCE approximately 1% at 550 nm, respectively). The efficiency of photocurrent generation is limited by surface recombination, which leads to a significant decrease in IPCE values and significantly changes the shape of the IPCE spectra in dependence on the optical bandgap.

Photodynamic activity of platinum(IV) chloride surface-modified TiO2 irradiated with visible light.

The visible light-induced phototoxicity of titanium dioxide modified with platinum(IV) chloride complexes, [TiO2/PtCl4], was tested. In vitro experiments with the mouse melanoma cells (S-91) have demonstrated phototoxicity of the [TiO2/PtCl4] material. Detection of efficiently generated various reactive oxygen species (.OH, O2. -, H2O2, 1O2) and also reactive chlorine species has proven the photodynamic activity of the tested material, induced by visible light (lambda>455 nm). The cellular death (recognized as a necrosis) is a result of the cell membrane peroxidation.

Visible light inactivation of bacteria and fungi by modified titanium dioxide.

Visible light induced photocatalytic inactivation of bacteria (Escherichia coli, Staphylococcus aureus, Enterococcus faecalis) and fungi (Candida albicans, Aspergillus niger) was tested. Carbon-doped titanium dioxide and TiO2 modified with platinum(IV) chloride complexes were used as suspension or immobilised at the surface of plastic plates. A biocidal effect was observed under visible light irradiation in the case of E. coli in the presence of both photocatalysts. The platinum(IV) modified titania exhibited a higher inactivation effect, also in the absence of light. The mechanism of visible light induced photoinactivation is briefly discussed. The observed detrimental effect of photocatalysts on various microorganism groups decreases in the order: E. coli > S. aureus approximately E. faecalis>>C. albicans approximately A. niger. This sequence results most probably from differences in cell wall or cell membrane structures in these microorganisms and is not related to the ability of catalase production.

Support-controlled chemoselective olefin-imine addition photocatalyzed by cadmium sulfide on a zinc sulfide carrier.

The semiconductor catalyzed photoaddition of cyclopentene or cyclohexene to various novel electron-poor imines of type p-XC(6)H(4)(CN)C[double bond, length as m-dash]N(COPh) (X = H, F, Cl, Br, Me, MeO) was investigated as a function of the nature of the cadmium sulfide photocatalyst. Irradiation (lambda>/= 350 nm) of silica supported cadmium sulfide surprisingly did not afford the expected olefin-imine adducts but an imine hydrocyanation product via an unprecedented dark reaction. However, when silica was replaced by zinc sulfide as the support for cadmium sulfide, the expected homoallylic N-benzoyl-alpha-amino cyanides were isolated in yields of 65-84%. Thus, chemoselectivity is introduced through replacing an insulating by a semiconducting support, a hitherto unknown effect in semiconductor photocatalysis. From the sign of the time resolved photovoltage it is found that the mixed metal sulfide interface CdS/ZnS increases the lifetime of photogenerated electron-hole pairs by about one order of magnitude as compared to the SiO(2)/CdS system. The reaction rate increases with increasing imine sigma-Hammett constants and decreasing stability of intermediate benzyl radicals.

On the mechanism of nitrogen photofixation at nanostructured iron titanate films.

The photofixation of dinitrogen to ammonia at a nanostructured iron titanate thin film, prepared from iron(III) chloride and titanium tetraisopropylate, was established by isotopic labeling employing (15,15)N(2). It is found that traces of iron chloride in the film are required to observe significant amounts of ammonia. It is therefore proposed that the photogenerated hole oxidizes chloride to an adsorbed chlorine atom and the latter subsequently oxidizes ethanol, the reducing agent necessary for ammonia formation. However, thin films obtained from a chloride-free precursor like iron tris-acetylacetonate are also active. Upon prolonged irradiation ammonia is oxidized to nitrate by traces of oxygen. It is found that this final reaction step does not require photoexcitation of the iron titanate thin film but occurs thermally. Titania films exhibit about the same catalytic activity in ammonia oxidation whereas iron oxide films are much less active. Contrary to this thermal reaction step, the reduction of intermediate hydrazine by ethanol occurs only photochemically.

Bandgap widening of titania through semiconductor support interactions.

Silica-supported titania powders with 50, 36, 13 and 4 wt% of TiO2 (TiO2-50/SiO2, TiO2-36/SiO2, TiO2-13/SiO2 and TiO2-4/SiO2) were prepared by hydrolysis of TiCl4 in the presence of silica, followed by calcination at 500 degrees C. The formation of Ti-O-Si linkages was confirmed by diffuse reflectance infrared Fourier transform spectroscopy. Atomic force microscopy indicated the presence of titania crystals larger than 15 nm. All supported materials exhibited a blue-shift of the TiO2 absorption edge, which was attributed to an electronic semiconductor support interaction. Bandgap energies of TiO2-50/SiO2, TiO2-36/SiO2, TiO2-13/SiO2 and TiO(2)4/SiO2 were measured to be 3.28, 3.36, 3.40 and 3.42 eV, respectively, as compared to 3.15 eV for unsupported TiO2. From these values, and from the quasi-Fermi level of electrons, a high anodic shift of both the valence and the conduction band was estimated. X-ray photoelectron spectroscopy (XPS) measurements of oxygen 1s- and titanium 2p-binding energies confirmed the anodic shift of the band edges.