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The cross-dehydrogenative coupling (CDC) reaction of N-aryltetrahydroisoquinolines (THIQ) is one of the most exploited photocatalytic transformation and a test reaction for an exceptional variety of catalysts. However, its mechanism remained unclear concerning involved intermediates, reactive pathways of the amine radical cation and the influence of oxygen and the light source. Therefore, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and synthetic methods were combined to provide a comprehensive picture of the reaction mechanism using Ru(bpy)3Cl2 as a photocatalyst under aerobic and anaerobic conditions. The reaction profiles and involved intermediates were monitored and analyzed by NMR spectroscopy. Several intermediates contributing to product formation were identified, the iminium ion, the hydroperoxide and dimer of THIQ, and a new ring opened intermediate, cleaved at the benzylic C-N bond. Mechanistic evidence is given that under anaerobic conditions preferentially the α-amino radical is formed by deprotonation, in contrast to the formation of iminium ions via H(•)-abstraction in the presence of oxygen. Further, the light-induced background reaction in the absence of the catalyst was studied in detail, revealing that the product formation rate is correlated to the intensity and wavelength of the light source and that oxygen is essential for an efficient conversion. The reaction rate and efficiency is comparable to previously reported photocatalytic systems, performed under aerobic conditions in combination with intense blue light sources. Thus, the multitude of reaction parameters investigated reveals the preference for hydrogen atom or proton abstraction in photoreactions and allows to assess the influence of experimental conditions on the mechanistic pathways.

Mechanistic insights into chemical photocatalysis are mainly the domain of UV/Vis spectroscopy, because NMR spectroscopy has been limited by the type of illumination so far. An improved LED-based illumination device can be used to obtain NMR reaction profiles of photocatalytic reactions under synthetic conditions and perform both photo-CIDNP and intermediate studies. Flavin-catalyzed photooxidations of alcohols show the potential of this setup. After identical initial photoreaction steps the stabilization of a downstream intermediate is the key to the further reaction mechanism and the reactivity. As a chemical photocatalyst flavin can act either as a one- or a two-electron mediator when the stability of the zwitterionic radical pair is moldulated in different solvents. This demonstrates the importance of downstream intermediates and NMR-accessible complementary information in photocatalytic reactions and suggests the control of photoorganic reactions by solvent effects.

Blue light irradiation of heterogeneous photocatalysts PbBiO 2 X (X = Cl, Br) in the presence of triethanolamine as an electron donor leads to hydrogen evolution, and the selective, clean and complete reduction of nitrobenzene derivatives to their corresponding anilines.Sunlight is the only sustainable energy resource on earth, and photovoltaic systems for the conversion of solar energy into electrical power have evolved into a wide range of applications. and the dehalogenation of a-alkylated esters. 4 Heterogeneous photocatalysts are typically semiconductors based on modified or unmodified TiO 2 or CdS. Unmodified TiO 2 has been investigated for the reduction of nitrobenzenes to their corresponding anilines under UV light 5 irradiation, whereas modified and dyesensitized TiO 2 reduces these molecules or oxidizes alcohols with blue or green light.6 CdS quantum dots have been found to be suitable photocatalysts for the reduction of azides to anilines, 7 but these systems have disadvantages: UV light is only a small part of the solar spectrum, which diminishes the efficiency of unmodified TiO 2 in sunlight. Modified CdS and TiO 2 absorbing in the visible range require special preparation. Oxide halides, such as PbPnO 2 X (Pn = Bi, Sb; X = Br, Cl), have been structurally characterized and applied to the oxidative photodegradation of organic dyes, e.g. methylene blue and methyl orange.8 These materials are potential heterogeneous photocatalysts for organic chemical synthesis using visible light and may nicely complement

The switching behavior of a photochromic model system was investigated in detail via NMR spectroscopy in order to improve understanding of the compound itself and to provide ways to obtain insights into composition trends of a photo switchable (polymeric) material containing spiropyran/merocyanine units. In addition to the classical irradiation performed outside the magnet (ex-situ), a device for irradiation inside the NMR spectrometer (in-situ) was tested. Both setups are introduced, their advantages and disadvantages as well as their limits are described and the setup for future investigations of photochromic materials is suggested. The influence of different sample concentrations, irradiation procedures, and light intensities on the model system was examined as well as the dependence on solvent, temperature, and irradiation wavelengths. Using the recently published LED illumination device, it was even possible to record two-dimensional spectra on this model system with rather short half-life (7 min in DMSO). This way (13) C chemical shifts of the merocyanine form were obtained, which were unknown before. Copyright © 2016 John Wiley & Sons, Ltd.