Multimetastability in a spin-crossover compound leading to different high-spin-to-low-spin relaxation dynamics

The relaxation kinetics of both the thermally trapped and photoinduced high-spin (HS) states of the spin-crossover compound [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO (1) were measured and found to differ significantly. Calorimetry measurements then demonstrated that relaxation of the thermally trapped phase was concurrent with two separate processes, not previously detected as such. Determination of the photogenerated HS structure revealed a new metastable HS state of the system, much closer structurally to the low-spin phase than the thermally trapped one. This difference is proposed as the root of the disparate kinetic behavior, which is proposed to require two processes in the case of the structurally more complex thermally trapped state. Therefore, light irradiation is shown as a mechanism to decouple effectively the structural and magnetic phase transitions that occur in 1 during the course of its spin crossover.