Synchronous Reconfiguring Triplet Energy and Excitonic Redox Charge-Transfer States in Dual-Chromogen-Embedded...

Donor-acceptor (D-A) conjugated polymers (CPs) exhibit electron- and energy-transfer photocatalysis due to their accessible hybrid local and charge-transfer (HLCT) states, strong visible-light absorption, efficient charge separation, and notable photostability. Herein, we report that alkyl-chain-substituted phenothiazine- and benzodithiophenedione-based solution-processable CPs (PTZ-BDD and PTZSO-BDD), among which PTZSO-BDD shows efficient photocatalytic trans-to-cis isomerization of alkenes (up to 84% yield) within 2 h via triplet energy transfer from the photocatalyst to trans-alkenes. Effective recovery of these homogeneous catalysts via a reverse-dissolution process, achieved by heterogenization through solvent alteration, creates a platform that offers distinct advantages of both homogeneous and heterogeneous photocatalysts. Further, altering the chromogens in these CPs modulates the excitonic redox potential and frontier orbital alignment, enabling access to CT states that allow diverse photoredox catalysis such as hydrodebromination of phenacyl bromide, pinacol-type photochemical coupling of benzaldehyde, decarboxylative Giese-type conjugate addition, and dual metallophotoredox catalysis reactions, via oxidative and reductive quenching pathways. Femtosecond transient absorption spectroscopy revealed a prolonged absorption signal persisting up to 400 ps for PTZ-BDD, indicating a longer exitonic lifetime that enables efficient electron transfer from excited charge-transfer states. High chromophoric density in the polymeric backbone enables lower catalyst loading (up to 5 ppm) for the atom-transfer radical photopolymerization of methacrylates with high molecular weights. This work represents the first demonstration of accessing HLCT states by fine-tuning the donor and acceptor motifs of a solution-processable polymeric photocatalyst toward diverse and synthetically valuable photoredox organic transformations at exceptionally low catalyst loadings. Therefore, these metal-free photocatalysts not only demonstrated straightforward recycling but also meet crucial advantages of both homogeneous and heterogeneous catalysts, with low catalyst loading at the ppm level, improving economic sustainability.