Structural Control of 1A2u-to‑3A2u Intersystem Crossing in Diplatinum(II,II) Complexes
journal contributionposted on 20.02.2016, 13:17 by Alec C. Durrell, Gretchen E. Keller, Yan-Choi Lam, Jan Sýkora, Antonín Vlček, Harry B. Gray
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 (kr = 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 1A2u 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 Ea 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 1A2u–3A2u spin–orbit coupling.