High-spin→low-spin relaxation in [Zn1-xFex(6-mepy)3-y(py)ytren](PF6)2

The thermal spin transition in the diluted mixed crystal [Zn1 - xFex(6-mepy)(3)tren](PF6)(2) (x = 0.00025, (6-mepy)(3)tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for Delta H-HL(0) and Delta S-HL(0) of 1551(50) cm(-1) and 7.5(5) cm(-1)/K and a molecular volume of reaction, Delta V-HL(0), of 22(2) Angstrom(3) result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)(3)tren](PF6)(2) from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Delta, of 208(15) cm(-1) and an interaction constant, Gamma, of 109(15) cm(-1). Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn1 - xFex(6-mepy)(3)tren](PF6)(2) and the LS system [Zn1 - xFex(py)(3)tren](PF6)(2) in the pressure range between 1 bar and 1 kbar are presented. Above approximate to 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 [Zn1 - xFex(py)(3)tren](PF6)(2) is more than three orders of magnitude larger than the one in [Zn1 - xFex(6-mepy)(3)tren](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. (C) 1998 American Institute of Physics. [S0021-9606(98)70246-6].