Building a photocatalyst library of MR-TADF compounds with tunable excited-state redox potentials

Epitomized by 4CzIPN, donor-acceptor (D-A) thermally activated delayed fluorescence (TADF) compounds based on the carbazoyl di- cyanobenzene have now become widely used, as they are sustainable photocatalyst alternatives to organometallic complexes owing to their similar optoelectronic properties compared with many of the iridium-based photocatalysts. Multiresonant TADF (MR-TADF) compounds offer distinct advantages over D-A TADF compounds, as they have more intense low-energy absorption bands, offering faster reaction kinetics, and are less sensitive to the polarity of the environment, mitigating undesired energy loss that typically accompanies the charge transfer (CT) excited states of the photocatalysts. Here, we report the assessment of strongly photoreducing boron- and nitrogen-doped MR-TADF compounds DABNA-1, tDABNA, CzBN, and DtBuCzB across a range of benchmark photochemical reactions. The structural differences between the members of this library of photocatalysts enable modulation of their ground- and excited-state redox potentials. These photocatalysts performed competitively compared to the literature-known 4CzIPN, Ph-PTZ, and fac-Ir(ppy)₃.