Intersystem Crossing in Diplatinum Complexes.

Abstract

Intersystem crossing (ISC) in solid [(C4H9)4N]4[Pt2(μ-P2O5(BF2)2)4], abbreviated Pt(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from τISC ≈ 0.9 ns at 310 K to τISC ≈ 29 ns below 100 K. A classical model based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt(pop-BF2) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, 1A2u intersystem crosses directly into 3A2u using a high energy B-F or P-O vibration as accepting mode, resulting in a temperature-independent ISC rate. In channel 2, ISC occurs via a deactivating state of triplet character (which then rapidly decays to 3A2u), using Pt-Pt stretching (160 cm-1) as a distorting mode to provide the energy needed. Fitting indicates that the deactivating state, 3X, is moderately displaced (S = 0.5-3) and blue-shifted (ΔE = 1420-2550 cm-1) from 1A2u. Our model accounts for the experimental observation that ISC in both temperature independent and thermally activated channels is faster for Pt(pop) than for Pt(pop-BF2): in the temperature independent channel because O-H modes in the former more effectively accept than B-F modes in the latter, and in the thermally activated pathway because the energy gap to 3X is larger in the latter complex.