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

TL;DR

The paper introduces a new light-driven catalyst system that uses a donor-acceptor photosensitizer to reduce NAD+ efficiently.

Contribution

A novel donor–acceptor photosensitizer-catalyst dyad is designed for light-driven nicotinamide hydrogenation.

Findings

The PS-CAT dyad 2tBuCzDPPZRhCp* achieves a TON of 3.2 in NAD+ reduction after 4 hours.

Photophysical properties depend strongly on solvent polarity due to the donor–acceptor structure.

Femtosecond and Raman studies reveal insights into the excited state dynamics of the dyad.

Abstract

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 tert-butylcarbazole as the donor (2tBuCzDPPZ), 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 2tBuCzDPPZRhCp* shows – with methoxy-substituted 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]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.

This work presents a novel photosensitizer-catalyst dyad containing a noble metal-free donor acceptor photosensitizer, which can be employed in light-driven nicotinamide hydrogenation.

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