Elementary Steps in Gold Photocatalysis

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Despite great progress, many key steps in photocatalysis involving dinuclear gold(I) complexes or mononuclear gold(III) complexes remain poorly understood. A detailed understanding of elementary reactions and key intermediates is therefore crucial to conceptually design new synthetic routes. As such, the formation of free “alkynyl radicals” in the reaction of dinuclear gold complexes with iodo-substituted alkynes remains highly doubtable. Therefore, a central question involves the origin of such “alkynyl radical”-derived products and other present reactive species. In this regard the formation of gold-alkynyl species could be feasible. Thus, the exact mechanistic steps need to be elucidated.Gold-catalyzed electron transfer mechanisms were already proposed for dinuclear gold complexes as well as gold(III) complexes. In case of the dinuclear complex which does not absorb the light of blue LEDs, a base is necessary in order to form a gold-base aggregate so that the use of highly energetic UVA light can be avoided. The nature of this intermediate still needs to be clarified. For this, a combined experimental/computational approach is required. Experimental mechanistic studies consist of in situ spectroscopy, stoichiometric control experiments, fluorescence quenching experiments and isolation of key intermediates. In this regard, isolation of key intermediates could be achieved by modification of the substrates or metal complex guided by a theoretical approach. With a theoretical description of the photocatalytic behavior of dinuclear gold complexes being rare in comparison to their experimental application, a comprehensive computational protocol will first be developed. This not only enables the investigation of the energetics of such reaction mechanisms but also the calculation of excited state properties. This will be employed for the identification of photoactive intermediates and for the thorough evaluation of the subsequent catalytic steps within dinuclear Au-photocatalysis. Key features within this examination of the catalytic cycle will be the prediction of spectroscopic signatures of suspected intermediates for experimental verification as well as the computation of the energetic profile of the mechanism. Furthermore the protocol will be instrumental for the systematic theoretical elucidation of differences between SET and EnT mechanisms in dinuclear gold photocatalysis.