title: Interplay between kinetically slow thermal spin-crossover and metastable high-spin state relaxation in an iron(II) complex with similar T1/2 and T(LIESST).
abstract: This paper describes the first material to show the well-known light-induced excited spin-state trapping (LIESST) effect, the metastable excited state of which relaxes at a temperature approaching its thermal spin-crossover. Cooling polycrystalline [FeL(2)][BF(4)](2).x H(2)O (L=2,6-bis[3-methylpyrazol-1-yl]pyridine; x=0-1/3) at 1 K min(-1) leads to a cooperative spin transition, taking place in two steps centered at 147 and 105 K, that is only 54 % complete by magnetic susceptibility. Annealing the sample at 100 K for 2 h results in a slow decrease in chi(M)T to zero, showing that the remainder of the spin-crossover can proceed, but is kinetically slow. The crystalline high- and fully low-spin phases of [FeL(2)][BF(4)](2).x H(2)O are isostructural (C2/c, Z=8), but the spin-crossover proceeds via a mixed-spin intermediate phase that has a triple unit cell (C2/c, Z=24). The water content of the crystals is slowly lost on exposure to air without causing decomposition. However, the high-spin/mixed-spin transition in the crystal proceeds at 110+/-20 K when x=1/3 and 155+/-5 K when x=0, which correspond to the two spin-crossover steps seen in the bulk material. The high-spin state of the compound is generated quantitatively by irradiation of the low-spin or the mixed-spin phase at 10 K, and in approximately 70 % yield by rapidly quenching the sample to 10 K. This metastable high-spin state relaxes back to the low-spin ground state at 87+/-1 K in one, not two, steps, and without passing through the intermediate phase. This implies that thermal spin-crossover and thermally activated high-spin-low-spin relaxation in this material become decoupled, thus avoiding the physical impossibility of T(LIESST) being greater than T(1/2).
authors.affiliation: Department of Chemistry, University of Durham, South Road, Durham DH1 3 LE, UK.
authors.forename: Victoria A
authors.initials: VA
authors.lastname: Money
authors.forename: Chiara
authors.initials: C
authors.lastname: Carbonera
authors.forename: Jérôme
authors.initials: J
authors.lastname: Elhaïk
authors.forename: Malcolm A
authors.initials: MA
authors.lastname: Halcrow
authors.forename: Judith A K
authors.initials: JA
authors.lastname: Howard
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Létard