Matrix effects on the high-spin→low-spin relaxation in [M1-xFex(bpy)3](PF6)2 (M = Cd, Mn and Zn, bpy = 2,2′-bipyridine)

Temperature-dependent laser flash photolysis experiments on the low-spin iron(II) systems [M1-xFex(bpy)(3)](PF6)(2) (M = Cd, Mn and Zn, x approximate to 0.01, bpy = 2,2'-bipyridine) under external pressure are presented. Below 50 K the high-spin --> low-spin relaxation is an almost temperature-independent tunnelling process. Above that temperature it tends towards a thermally activated behaviour. A change of the host from cadmium to zinc results in an increase of the low-temperature tunnelling rate constant by two orders of magnitude. An external pressure of 1 kbar accelerates the low-temperature tunnelling process by a factor of 2. [Mn1-xFex(bpy)(3)](PF6)(2) and [Zn1-xFex(bpy)(3)](PF6)(2) show a phase transition at approximate to 1.1 kbar, which increases the tunnelling rate by a factor of about 6. (C) 1998 Elsevier Science B.V. All rights reserved.