A Donor-Acceptor Photosensitizer-catalyst Dyad for Light-Driven Nicotinamide Hydrogenation

Abstract

Using light energy to drive chemical transformations is of great relevance, with nature’s photosynthesis as grand example. In artificial light-driven catalysis, part of nature’s complex supramolecular architecture is mimicked through so-called covalently linked photosensitizer-catalyst (PS-CAT) dyads. We herein report on a dyad using an organic donor-acceptor PS, with dipyridophenazine as acceptor and tert-butylcarbazole as the donor (2^tBuCzDPPZ), that contains a coordination site for a rhodium(III)Cp* center as catalyst. The organic PS shows a charge-transfer transition upon visible-light irradiation and has redox properties similar to typically used ruthenium-based PSs. The resulting PS-CAT dyad 2^tBuCzDPPZRhCp* shows – with methoxy-substituted 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH-OMe) as sacrificial electron donor – photocatalytic activity in the light-driven NAD⁺ reduction with a TON of 3.2 (after 4 h). Femtosecond transient absorption and resonance Raman spectroscopy, as well as time-dependent density functional theory (TDDFT) calculations, shed light into the photophysical properties of the PS and PS-CAT dyad and reveal a high dependency of the photoluminescence quantum yield and excited state properties on solvent polarity – in line with its donor-acceptor structure. This work presents a new design concept for PS-CAT dyads in artificial light-driven catalysis and provides important insight into the interplay between solvation dynamics of organic donor-acceptor systems and their photophysics, paving the way for future design strategies.