π-Extended Heterocycle/Carbene Hybrids as Geometrically Constrained Dyes for TADF Energy and Electron Transfer Photocatalysis

Abstract

We present an organic redox system derived from the combination of N-heterocyclic carbenes with azadibenzo[e,l]pyrene. It features three stable oxidation states, which could be isolated and structurally characterized and are supported by nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) spectroscopy, and X-ray analysis. Due to the rigid π-extended heterocyclic framework, geometrical changes during the redox cycling are reduced to a minimum, resulting exclusively in the rotation of the central C–C bond and a potential expansion in the cyclic voltammogram. The photoactive chromophore, which shows thermally activated delayed fluorescence (TADF) behavior, was employed in the dicationic oxidation state as a photosensitizer for C–N-coupling via the direct oxidation of benzene and biphenyl derivatives, intramolecular [2 + 2] cycloadditions of olefins, and isomerization of activated olefins mediated by energy transfer, which represents a promising alternative to metal-based systems. The versatile photocatalyst has a similar triplet state energy (∼2.3 eV) and a much longer triplet state lifetime (64 μs) compared to well-established metal-based sensitizers. Mechanistic experiments using time-resolved emission and transient absorption spectroscopy demonstrate the highly oxidizing excited state, the remarkable lifetime of the high-energy triplet state, and they support key mechanistic steps and intermediates.