# 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.

## Key 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.