Intersystem Crossing in Diplatinum Complexes

Abstract

Intersystem crossing (ISC) in solid [(C_4H_9)_4N]4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from τ_(ISC) ≈ 0.9 ns at 310 K to τ_(ISC) ≈ 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-BF_2) 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, ^1A_(2u) intersystem crosses directly into ^3A_(2u) 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 ^3A_(2u)), 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 ^1A_(2u). 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-BF_2): 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.