Enhanced Intersystem Crossing, Yet Still Fluorescence Upon Introduction of Intermediate Charge-Transfer States in...

Photoactive zinc(II) complexes typically undergo fluorescence from the singlet excited state as the dominant radiative pathway, as the operative spin-orbit coupling is usually very small and phosphorescence from the triplet state is strongly forbidden. Although dicationic zinc(II) tris(bipyridine) strictly follows this scheme with fluorescence at λ_em = 326 nm, constructing the ligand sphere as a hemicage was reported to lead to quantitative intersystem crossing (ISC) and subsequent fast phosphorescence with λ_em = 485 and a short radiative lifetime of ca. 1 μs. Surprised by this finding, we reinvestigated [Zn(bpy)₃]²⁺ and its hemicage derivative in great detail, including variable temperature and time-resolved photophysical measurements in solution and solid state as well as high-level theoretical calculations to resolve their excited state behavior. Our investigations suggest that both compounds undergo fluorescence at room temperature with significantly different radiative rate constants of k_r = 2 × 10⁸ and 1.2 × 10⁶ s^-1, respectively, and only weak phosphorescence on the millisecond time scale at low temperatures. The major difference is the occurrence of additional charge-transfer states within the ligand scaffold of the hemicage, which accelerate the ISC to the ³LC(bpy) state from 350 s down to 82 ns and reduce the fluorescence rate constant.