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.