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

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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 considered, and photo substitution in the most plausible of these also appears to be associative. Thus prevailing theories, based on dissociative photoreaction pathways, are probably inconsistent with the published pressure effects.

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