Structural control of 1A2u-to-3A2u intersystem crossing in diplatinum(II,II) complexes.

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt(2)(μ-P(2)O(5)(BF(2))(2))(4)](4-), abbreviated Pt(pop-BF(2))), yields a radiative decay rate (k(r) = 1.7 × 10(8) 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-BF(2)); 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 (1)A(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 Pt(2)(P-O-P)(4) core that would allow direct (1)A(2u)-(3)A(2u) spin-orbit coupling.