Structural Control of ^1A_(2u)-to-^3A_(2u) Intersystem Crossing in Diplatinum(II,II) Complexes

Abstract

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt_2(μ-P_2O_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 ^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 Pt_2(P-O-P)_4 core that would allow direct ^1A_(2u)–^3A_(2u) spin–orbit coupling.