title: Biphasic behavior of the high-spin-->low-spin relaxation of [Fe(btpa)](PF6)2 in solution (btpa = N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'- bipyridine).
abstract: The light-induced high-spin-->low-spin relaxation for the Fe(II) spin-crossover compounds [Fe(btpa)](PF6)2 and [Fe(b(bdpa))](PF6)2 in solution, where btpa is the potentially octadentate ligand N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine and b(bdpa) is the analogous hexadentate ligand N,N'-bis(benzyl)-N,N'-bis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'- bipyridine, respectively, has been studied by temperature-dependent laser flash photolysis. [Fe(b(bdpa))](PF6)2 shows single-exponential 5T2-->1A1 relaxation kinetics, whereas [Fe(btpa)](PF6)2 exhibits solvent-independent biphasic relaxation kinetics. The fast process of [Fe(btpa)](PF6)2 with a rate constant, kHL, of 2.5 x 10(7) s-1 at 295 K and an activation energy, Ea, of 1294(26) cm-1 in methanol can be assigned to the 5T2-->1A1 relaxation as well. The slow process with a kHL(295 K) of 3.7 x 10(5) s-1 and a Ea of 2297(32) cm-1 in methanol--which is the slowest light-induced relaxation process observed so far for an Fe(II) spin-crossover complex in solution--is assigned to a coupling of the 5T2-->1A1 relaxation process to a geometrical rearrangement within the pendent pyridyl arms.
authors.affiliation: Kemisk Institut, Syddansk Universitet, Campusvej 55, 5230 Odense M, Denmark.
authors.forename: S
authors.initials: S
authors.lastname: Schenker
authors.forename: P C
authors.initials: PC
authors.lastname: Stein
authors.forename: J A
authors.initials: JA
authors.lastname: Wolny
authors.forename: C
authors.initials: C
authors.lastname: Brady
authors.forename: J J
authors.initials: JJ
authors.lastname: McGarvey
authors.forename: H
authors.initials: H
authors.lastname: Toftlund
authors.forename: A
authors.initials: A
authors.lastname: Hauser