In homogenous photocatalysis, broadly, PCs can be divided into two main families: transition metal complexes and organic photocatalysts. The initial explosion of interest in photocatalytic reactions over the past 15 years or so has been accompanied by the frequent use of organometallic Ru(II) and Ir(III) complexes as photocatalysts (Figure ), which interact with substrates exclusively from their triplet excited state. Despite their ability to photocatalyze a wide range of reactions, the platinoid metals are scarce and costly and the toxicity profile is suboptimal, particularly if late-stage photocatalysis is to be used in pharmaceutical synthesis. The most prominent organic PCs are either natural organic dyes, like Eosin Y, Rose Bengal, fluorescein, and acridinium salts, or donor-acceptor (D-A) thermally activated delayed fluorescent (TADF) emitters originally designed for use in organic lightemitting diodes. Unlike fluorescent dyes, the singlet (S1) and triplet (T1) excited states of TADF compounds are both populated, facilitated by a small energy gap (∆EST) between the two that permits exciton cycling by a combination of intersystem crossing (ISC) and reverse intersystem crossing (RISC). The small ∆EST results from the small exchange integral between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). In donoracceptor TADF emitters, this separation is most commonly achieved when the donor and acceptor adopt a strongly twisted conformation such that electronic communication between the two is very weak. Luo et al. first demonstrated that prototypical TADF carbazolyl dicyanobenzenes (CDCBs) like 4CzIPN (Figure , Figure ) can be highly efficient visiblelight PCs. Due to its remarkably similar photophysical properties to the widely used photocatalyst [Ir(dF(CF3)-ppy)2(dtbbpy)]PF6, 4CzIPN has been repeatedly shown to be a highly versatile and competitive photocatalyst across a broad range of transformations. Modulating the structure of 4CzIPN and its derivatives results in the facile tuning of their ground and excited state redox potentials, making this class of PCs much more versatile than the naturally occurring organic dyes historically used for photocatalysis. Beyond 4CzIPN and structurally related PCs, a small number of other D-A TADF compounds have been explored as PCs. Recently, we reported that pDTCz-DPmS (Figure ) was a versatile PC across a range of photocatalytic reactions. Bouzrati-Zerelli et al. used a similar D-A PC containing electron-deficient diphenyl sulfone to initiate a free radical polymerisation of methacrylates. Hojo et al. showed that a group of imidazophenothiazine (IPTZ)-based TADF emitters (Figure ) can serve as efficient photocatalysts in five different PEnT reactions. This work built upon related imidazoacridine-based TADF compounds that were demonstrated to photocatalyse [2+2] cycloadditions in a PEnT mechanism. ), matching or even outperforming D-A TADF and organometallic PCs across a mechanistically broad range of reactionsError! Reference source not found.. MR-TADF emitters are typically n-and p-doped nanographene compounds that have short-range charge transfer (SRCT) excited states that lead to relatively small ΔEST. Importantly, the SRCT stare is less strongly influenced by the polarity of the solvent than long-range CT (LRCT) states of D-A TADF compounds, meaning that less energy is lost in these compounds in polar solvents. As well, due to their rigid structure, there is less geometric reorganization in the excited state, leading to bright, narrowband emission and intense low-energy absorption bands The high molar absorptivity in the visible region of these bands is beneficial in photocatalysis as it means that the concentration of PC* is higher and the rate of the reaction is generally faster.