A Triple-Acceptor Motif for Chalcogen-Bond-Directed Circular Assemblies that Enhance Singlet Oxygen Generation.

Herein, we report a triple-acceptor donor-acceptor (D-A) molecule designed with three consecutively linked acceptors to confer substantial conformational flexibility. This conformational flexibility translates into distinct crystalline packing modes, which yield strikingly different photosensitizing efficiencies. Notably, a unique crystal structure is formed by the self-assembly of six distinct conformers into a circular architecture via intermolecular chalcogen bonding, an arrangement unambiguously confirmed by single-crystal X-ray diffraction. The cyclic assembly delivers an impressive solid-state 1O2 quantum yield of 80%, a value far higher than that of the solution-dispersed monomer (62%) and a non-cyclically packed polymorph with both chalcogen bonds and π-interactions between A groups (43%). This enhancement stems from the unique circular packing motif that increases the energy mismatch between the singlet excited state (S1) and singlet ground state (S0) to facilitate intersystem crossing (ISC), while concurrently suppressing π-π stacking-induced quenching. The generality of this strategy is further demonstrated by a heavy-atom-containing analogue, which achieves a similarly high 1O2 quantum yield of 84% in its chalcogen-bond-directed circular assembly (comprising three conformers), markedly outperforming a counterpart polymorph (45%) with both chalcogen bonds and π-interactions between A groups and non-circular packing. These findings establish a versatile and robust approach for developing high-performance crystalline photosensitizers.