High-spin→low-spin relaxation in [Zn_1-xFe_x(6-mepy)_3-y(py) _ytren](PF₆)₂

The thermal spin transition in the diluted mixed crystal [Zn_1-xFe_x(6-mepy)₃tren](PF₆) ₂ (x=0.00025, (6-mepy)₃tren=tris4-[(6-methyl)-2-pyridyl]-3-aza-3-butenylamine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔH_HL⁰ and ΔS_HL⁰ of 1551(50) cm^-1 and 7.5(5) cm^-1/K and a molecular volume of reaction, ΔV_HL⁰, of 22(2) ų result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)₃tren](PF₆)₂ from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm^-1 and an interaction constant, Γ, of 109(15) cm^-1. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn_1-xFe_x(6-mepy)₃tren](PF₆) ₂ and the LS system [Zn_1-xFe_x(py)₃tren](PF₆)₂ in the pressure range between 1 bar and 1 kbar are presented. Above ≈100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in [Zn_1-xFe_x(py)₃tren](PF₆)₂ is more than three orders of magnitude larger than the one in [Zn_1-xFe_x(6-mepy)₃tren](PF₆) ₂. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.