Charge-Transfer Exciton Formation by Designing Donor-Acceptor Motifs for Polymeric-Semiconductor-Based Photocatalytic Singlet Oxygen Production.

Metal-free polymeric semiconductors exhibit substantial potential in attaining photocatalytic solar energy conversion via exciton-based energy transfer, whereas their inefficient triplet-exciton yields caused by faint spin-orbit coupling set limits to the relevant applications. Here, we present an alternative pathway for harvesting long-lived triplet excitons in polymeric photocatalysts, where a charge-transfer exciton (CTE), rather than an intrinsic Frenkel exciton, hosted in donor-acceptor motifs is employed for triggering energy-transfer-mediated photocatalysis. By taking polymeric carbon nitride (PCN) as a prototype, we used an aromatic heterocyclic compound, 2,3-diaminopyridine (DAP), to modify the relevant excitonic properties. Spectroscopic and theoretical analyses confirm that DAP bonding brings about a subtle difference in electronic density distributions between adjacent tri-s-triazine units. Such a difference enables the formation of donor-acceptor motifs for accommodating a long-lived triplet CTE, without introducing additional undesired exciton-dissociation driving forces. Triplet CTE harvesting in DAP-modified PCN enables high-efficiency energy-transfer-mediated molecular oxygen activation for triggering selective naphthalene-derivative oxidation.