Spin Crossover Properties of the [Fe(PM-BiA)2(NCS)2] Complex — Phases I and II

In the present review, we reexamine the photomagnetic properties of the [Fe(PMBiA)₂(NCS)₂], cis-bis(thiocyanato)-bis[(N-2′-pyridylmethylene)-4-(aminobiphenyl)]iron(II),compound which exhibits, depending on the synthetic method, an exceptionally abrupt spin transition (phase I) with a very narrow hysteresis (T_½ ↓ = 168K and T_½ ↑ = 173 K) or a gradual spin conversion (phase II) occurring at 190 K. In both cases, light irradiation in the tail of the 1MLCT-LS absorption band, at 830 nm, results in the population of the high-spin state according to the light-induced excited spin-state trapping (LIESST) effect. The capacity of a compound to retain the light-induced HS information, estimated through the T(LIESST) experiment, is determined for both phases. Interestingly, the shape of the T(LIESST) curve is more gradual for the phase II than for the phase I and the T(LIESST) value is found considerably lower in the case of the phase II. The kinetics parameters involved in the photo-induced high-spin → low-spin relaxation process are estimated for both phases. From these data, the experimental T(LIESST) curves are simulated and the particular influence of the cooperativity as well as of the parameters involved in the thermally activated and tunneling regions are discussed. The Light-Induced Thermal Hysteresis (LITH), originally described for the strongly cooperative phase I, is also reinvestigated. The quasi-static LITH loop is determined by recording the photostationary points in the warming and cooling branches.