Rational Design of TADF‐Based Organic Photoredox Catalysts: Insights from DFT‐Based Structure–Property Relationships

Abstract

Organic photoredox catalysts (OPCs) are being developed as more sustainable options for use in visible‐light mediated transformations. In this study, using three donors (phenothiazine (PTZ), carbazole (CCz), and N‐substituted carbazole (NCz)), four diphenyl sulfone‐derived (DPS) acceptors, and two π‐bridges (phenyl (Ph) and pyrimidine (Pm)), 36 donor‐acceptor‐donor (D–A–D) structured OPCs are designed. Density functional theory (DFT) calculations are used to predict photophysical and redox properties, including highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps, ionization potential, electron affinity, absorption energies, and excited‐state redox potentials, of these OPCs. Molecules characterized by large singlet–triplet gaps (ΔES1–Tn > 0.20 eV) are fluorescent OPCs and therefore could be classified as suitable for singlet‐state mediated electron transfer. CCz_DPS and CCz_Ph_DPS are potent photoreductants ( < −1.70 V), and CCz_Pm_DPmS and NCz_Pm_DPyS are good photooxidants ( > +1.30 V). In contrast, OPCs with smaller ΔES1–Tn (< 0.20 eV) is classified into intersystem crossing‐dominant (triplet‐mediated) and reverse intersystem crossing‐dominant (singlet‐mediated). PTZ_DPS is identified as a strong triplet‐state photoreductant ( (T1) = −1.73 V). Furthermore, comparison between newly designed OPCs with reported OPCs (4CzIPN and NCz‐DPS) reveals that former have improved excited‐state redox potentials. Overall, these findings establish essential structure–property correlations and highlight a design rationale for OPCs structured with customizable redox activities for targeted photocatalytic functions.