The substituent guest effect on four-step spin-crossover behavior

Abstract

Until now, the synthesis of new multi-step spin-crossover (SCO) materials is still a great challenge although they have attracted significant attention due to their potential applications in high-order data storage and multi-switches. Here, based on the four-step SCO complex [Fe(dpoda){Ag(CN)₂}₂]·1.5naph (1·1.5naph, dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole, naph = naphthalene), the strategy of incorporating a substituent on an aromatic guest is applied. Since the replacement of H with an F atom can be regarded as isosteric substitution, the introduction of the 2-fluoronaphthalene (Fnaph) guest does not give rise to large structural changes and the four-step SCO behavior is maintained in 1·1.5Fnaph; however, fine-tuning of the framework is indeed achieved by the fluoro substituent. The spin transition temperatures of 1·1.5Fnaph shift to the low-temperature region, which should be due to the contributions of the size effect and the decrease in the Fe–NC angle. Meanwhile, the thermal hysteresis loop disappears, which results from the competitive contributions of the π⋯π and F⋯Ag interactions. Therefore, the F substituent on the guest can effectively modify the four-step SCO behavior, which provides a simple solution to synthesize more multi-step SCO materials.