IRON(II)-1,2,4-TRIAZOLE SPIN TRANSITION MOLECULAR MATERIALS

The phenomenon of spin transition is probably one of the most spectacu lar examples of molecular bistability. Our main goal along this line c oncerns the design of molecular materials exhibiting abrupt spin trans itions accompanied by thermochromic and large thermal hysteresis effec ts. The ideal situation, in terms of possible application of these com pounds as active elements of memory devices, is realized when room tem perature falls in the middle of the thermal hysteresis loop. In this c ontext, we review our work concerning iron(II)-1,2,4-triazole compound s. We first introduce the idea that the cooperativity should be more p ronounced in polymeric than in mononuclear compounds. Then we present some results concerning trinuclear iron(II)-1,2,4-triazole species. Th e heart of the paper is devoted to the polymeric compounds of formulae [Fe(Htrz)(2)(trz)] (BF4) and [Fe(Htrz)(3)](BF4)(2) . H2O. We report f irst on the magnetic, optical and calorimetric, then on the structural properties of these compounds. Afterwards, we introduce the concept o f spin transition molecular alloy, and emphasize that it is possible t o fine tune the spin transition regime of these alloys through their c hemical composition. For some alloys, room temperature falls within th e thermal hysteresis loop. In the conclusion, the mechanism of coopera tivity in the spin transition polymeric compounds is briefly discussed .