Crystallographic, magnetic, and Raman investigations of the mononuclear $\left[{\text{Fe}}^{\text{II}}\left(\text{Hpy-DAPP}\right)\right]{\left({\text{BF}}_4\right)}_2$ complex are presented. Its particular feature is a two-step thermal spin conversion in spite of a unique symmetry-independent iron site per unit cell. The plateau around 140 K is associated with a symmetry breaking visible by the appearance of weak $\left(0k0\right)$ $k$ odd Bragg peaks. Symmetries of the high-temperature high-spin state and of the low-temperature low-spin state are both monoclinic $P{2}_1/c$, so that the symmetry breaking on the plateau is associated with a reentrant phase transition. It is discussed in relation with Ising-type microscopic models. At the plateau level, the two symmetry-independent molecules differ both by their spin state and the conformation (chair versus twist-boat) of one metallocycle. At low-temperature photoinduced phenomena have been investigated: a partial phototransformation [light-induced excited spin-state trapping (LIESST) effect] is observed under visible red irradiation. Raman spectroscopy shows that the molecular photoinduced state is the high-spin one. Nevertheless, as no macroscopic symmetry breaking is observed, the unique average cationic $\left[{\text{Fe}}^{\text{II}}\left(\text{Hpy-DAPP}\right)\right]$ state of the unit cell is intermediate between pure low-spin and high-spin states and presents a conformational disorder for one metallocycle. Reverse-LIESST has also been evidenced using near infrared excitation. Thus, the mononuclear $\left[\text{Fe}\left(\text{Hpy-DAPP}\right)\right]{\left({\text{BF}}_4\right)}_2$ compound offers the opportunity to discuss the interplay between spin conversion, molecular conformational change, and ordering processes.

• Received 7 September 2010

DOI:https://doi.org/10.1103/PhysRevB.82.214106