High-spin -> low-spin relaxation in [Zn1-xFex(6-mepy)(3-y)(py)(y)tren](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-pyri dyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-d ependent absorption spectroscopy. From thermodynamic analysis of the high-s pin (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 Intera ct. 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 l aser 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) i n the pressure range between 1 bar and 1 kbar are presented. Above approxim ate to 100 K the HS --> LS (low-spin) relaxations behave classically, where as they become almost temperature independent below 50 K. At ambient pressu re, 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 th e low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphono n relaxation. (C) 1998 American Institute of Physics. [S0021-9606(98)70246- 6].