Spin-Transition Polymers: From Molecular Materials Toward Memory Devices

TLDR

Some 3d transition‑metal compounds exhibit a cooperative low‑spin to high‑spin transition that is abrupt and shows thermal hysteresis, giving the system a memory effect. The authors aim to design a compound whose room‑temperature lies in the middle of the hysteresis loop. Cooperativity is enhanced in polymeric Fe(II) triazole compounds where Fe²⁺ ions are triply bridged, and transition temperatures can be tuned by a molecular‑alloy approach. These materials change color from violet in the low‑spin state to white in the high‑spin state and could serve as temperature sensors, display elements, and information‑storage devices.

Abstract

Some 3d n (4 ≤ n ≤ 7) transition metal compounds exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state. This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite interactions and thus the cooperativity are magnified in polymeric compounds such as [Fe(Rtrz) 3 ]A 2 ·nH 2 O in which the Fe 2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole molecules. Moreover, in these compounds, the spin transition is accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition temperatures of these materials can be fine tuned, using an approach based on the concept of a molecular alloy. In particular, it is possible to design a compound for which room temperature falls in the middle of the thermal hysteresis loop. These materials have many potential applications, for example, as temperature sensors, as active elements of various types of displays, and in information storage and retrieval.