Mechanistic Implications of Pressure-Dependent Photochemical and Photophysical Parameters of Chromium(III) Complexes

Abstract

Abstract Mechanistic interpretation of observed pressure-dependent photochemical Φ(X) and photophysical (τ) parameters can be very complicated when more than one photoactive electronic state must be considered. The 4T2g and 2Eg states of Cr(NH3)3+ 6 differ by (4–5) × 103 cm−1 and are both photoactive. Proposed models either postulate a single channel for deactivation of the 2Eg state or several competing channels. Systematic approaches for the mechanistic evolution of pressure-dependent quantum yields and lifetimes are presented. The implications of the published pressure dependencies depend on the mechanistic model proposed. For example, the single channel, back intersystem crossing model (4T2g ←2Eg) requires a volume of activation for NH3 aquation from the 4T2g state, δV‡ NH3 ≃ −13 ± 2 cm3 mol−1, while a model assuming multiple competing channels for doublet decay results in δV‡ NH3= −4 cm3 mol−1. Both of these models imply a strongly associative pathway for (4T2g) Cr(NH3)3+ 6 equation. Other models are...