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.