Structural Control of ¹A_2u-to‑³A_2u Intersystem Crossing in Diplatinum(II,II) Complexes

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per­(difluoroboro)­tetrakis­(pyrophosphito)­diplatinate­(II) ([Pt₂(μ-P₂O₅(BF₂)₂)₄]^4–, abbreviated Pt­(pop-BF₂)), yields a radiative decay rate (<i>k</i>_r = 1.7 × 10⁸ s^–1) an order of magnitude greater than that of the parent complex, Pt­(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt­(pop) [ISC activation energies: 2230 cm^–1 for Pt­(pop-BF₂); 1190 cm^–1 for Pt­(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin–orbit coupling between the ¹A_2u state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt­(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in E_a is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt₂(P-O-P)₄ core that would allow direct ¹A_2u–³A_2u spin–orbit coupling