A donor–acceptor photosensitizer-catalyst dyad for light-driven nicotinamide hydrogenation

Using light energy to drive chemical transformations is of great relevance, with photosynthesis in nature as a grand example. In artificial light-driven catalysis, part of nature's complex supramolecular architecture can be mimicked through the so-called covalently linked photosensitizer-catalyst (PS-CAT) dyads. We herein report a dyad using an organic donor-acceptor PS, with dipyridophenazine as the acceptor and <i>tert</i>-butylcarbazole as the donor (2 ^<i>t</i> BuCzDPPZ), that contains a coordination site for a rhodium(iii)Cp* center as the 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 ^<i>t</i> BuCzDPPZRhCp* shows - with methoxy-substituted 1,3-dimethyl-2-phenyl-2,3-dihydro-1<i>H</i>-benzo[<i>d</i>]imidazole (BIH-OMe) as the sacrificial electron donor - photocatalytic activity in 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 on 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.