Proximal Oblique‐Packing of Heptamethine Cyanines through Spiro‐Connection Boosts Triplet State Generation in Near‐Infrared

Near‐infrared (NIR) triplet dyes are the cornerstones of cutting‐edge biomedical and material applications. The difficulty in rational development of triplet dyes increases exponentially as the absorption wavelength shifts deeper into the NIR range. Although classical H‐/J‐typed packing of NIR dyes has the potential to enhance intersystem crossing (ISC) compared with that in single‐chromophore dyes, the triplet state quantum yields remain limited in such strategy. Herein, proximal oblique‐packed (V‐shaped) heptamethine cyanines (SZ780) through spiro‐connection were achieved. Multi‐channel ultrafast ISC were direct observed in SZ780 and a record high ISC rate constant (up to ~1011 s‐1) is registered among all the reported NIR triplet dyes. SZ780 exhibits a triplet state quantum yield of 18.9% upon excitation at 750 nm, which is almost an order of magnitude higher than that of the monomer (IR780, 2.1%) and nearly threefold increase compared to that of the H‐packed dimer (SC780) (6.7%). Moreover, SZ780 efficiently generates singlet oxygen under 808 nm light irradiation, inducing cancer cell apoptosis in vivo. These findings demonstrate that constructing V‐aggregated dyes system by spiro‐connection offers a powerful approach for the design of high‐performance NIR triplet sensitizers.