Solvated/desolvated homochiral Fe(ii) complexes showing distinct bidirectional photo-switching due to a hidden state

Abstract

Spin-crossover (SCO) complexes featuring hidden spin states have recently garnered significant attention. The hidden states are challenging to access via a routine cooling strategy due to the extremely slow dynamics of the SCO process, however, they are accessible via light irradiation. In this study, we prepared two homochiral mononuclear Fe^II complexes [(S(R)-L)Fe(NCBH₃)₂]·MeOH (S1 and R1, S/R-L = (S/R)-N¹,N²-dibenzyl-N¹,N²-bis(pyridin-2-ylmethyl)propane-1,2-diamine), as well as their desolvated counterparts, S1d and R1d, which can be interconverted through a single-crystal to single-crystal (SCSC) transformation (i.e., S1 ↔ S1d). All the complexes crystalize in the enantiomorphic polar space group of P2₁. The solvated complexes exhibit a thermally-induced spin crossover behavior at around 98 K and exhibit a common light-induced excited spin state trapping (LIESST) effect with a T_LIESST of 40 K. Meanwhile, the desolvated phases remain in the high-spin (HS) state above 2 K and show a rare reverse-LIESST effect, revealing a hidden low-spin (LS*) state. In addition, both the solvated and desolvated complexes exhibit bidirectional switching upon irradiation at 532 and 808 nm: one for LS ↔ HS* and the other for HS ↔ LS*. This work reveals a new finding regarding the influence of lattice solvents on light-induced magnetic bistability, providing a feasible approach for manipulating both thermally and photo-induced SCO events.