Symmetry breaking and light-induced spin-state trapping in a mononuclear Fe^II complex with the two-step thermal conversion

Marylise Buron-Le Cointe (1) , Nawell Ould-Moussa (1, 2) , E. Trzop (1) , Alain Moréac (1) , Gábor Molnár (2) , Loïc Toupet (1) , Azzedine Bousseksou (2) , Jean-François Letard (3) , Galina Matouzenko (4)

1 IPR - Institut de Physique de Rennes
2 LCC - Laboratoire de chimie de coordination
3 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux
4 LC - Laboratoire de Chimie - UMR5182

Crystallographic, magnetic, and Raman investigations of the mononuclear [Fe^II(Hpy-DAPP)](BF₄)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 (0k0) k odd Bragg peaks. Symmetries of the high-temperature high-spin state and of the low-temperature low-spin state are both monoclinic P2₁/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 [Fe^II(Hpy-DAPP)] 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 [Fe(Hpy-DAPP)](BF₄)₂ compound offers the opportunity to discuss the interplay between spin conversion, molecular conformational change, and ordering processes.

Domaines: Matériaux; Physique [physics]

Marylise Buron-Le Cointe, Nawell Ould-Moussa, E. Trzop, Alain Moréac, Gábor Molnár, et al.. Symmetry breaking and light-induced spin-state trapping in a mononuclear Fe^II complex with the two-step thermal conversion. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2010, 82 (21), 214106 (11 p.). ⟨10.1103/PhysRevB.82.214106⟩. ⟨hal-00569998⟩