Supramolecular control of spin-crossover phenomena in lipophilic Fe(II)-1,2,4-triazole complexes

The spin-crossover properties of lipophilic, supramolecular Fe(II) complexes bridged by 4-(3-dodecyloxy)propyl-1,2,4-triazole [Fe(II)(1) ₃Cl₂] were investigated in chloroform and cast films. A purple low-spin (LS) complex in a powdery form was transformed into pale yellow high-spin (HS) polymers by dissolution in chloroform. The formation of lipophilic molecular wires in chloroform was observed with transmission electron microscopy. The casting of chloroform solutions onto solid supports produced purple, transparent films (LS state). The cast films exhibited sluggish spin-crossover (LS ⇌ HS) behavior without thermal hysteresis. On the other hand, the cocasting of equimolar dodecanol or tetradecanol with Fe(II)(1)₃Cl₂ produced composite films in which alcohol molecules were bound to the complex by ionic hydrogen bonding (ROH⋯Cl^-) and van der Waals interactions. At room temperature, the cast films exhibited regular lamellar structures before and after alcohol doping; this was confirmed by wide-angle X-ray diffraction measurements. Interestingly, the Fe(II)(1)₃Cl₂/C_nOH (n = 12 or 14) ternary films showed a reversible abrupt spin crossover accompanied by thermal hysteresis. The observed bistability was related to dynamic structural transformations between lamellar and hexagonal structures. This study provides a novel supramolecular approach to designing spin-crossover polymer films with controlled thermal bistability.