COOPERATIVE EFFECTS IN THE [FE(MTZ)(6)](BF4)(2) SPIN-CROSSOVER SYSTEM- FINE-TUNING THE ENERGY-GAP

In the [Fe(mtz)(6)] (BF4)(2) spin-crossover system, the Fe(II) complex es occupy two nonequivalent lattice sites. Complexes on lattice site A show a thermal high-spin (HS) --> low-spin (LS) transition at 74 K, w hereas complexes on site B remain in the high-spin state down to 10 K. Complexes on both sites exhibit light-induced spin-state conversions at 20 K: LS --> HS on site A after irradiation at 514.5 nm; HS --> LS on site B after irradiation at 830 nm. The subsequent relaxation proce sses are strongly influenced by cooperative effects of elastic origin, leading to characteristic deviations from first-order kinetics. This is attributed to a buildup of an internal pressure during the relaxati on process, giving rise to a shift in both the horizontal (bond length difference Delta r(HL)) and the vertical (energy gap Delta E(HL)(O)) displacements of the potential wells of the two states relative to eac h other. The shifts can be estimated from the observed shift of the d- d absorption bands on the order of 100 cm(-1). For site B complexes, t his may lead to a change in the true ground state.