Expanding the TIESST Toolbox: Inner Metal Ion Effects and Doping Influence in 2D Hofmann Spin Crossover Systems

Temperature-induced excited spin state trapping (TIESST) remains a promising strategy for manipulating spin states in spin crossover (SCO) materials. However, the scarcity of compounds exhibiting the TIESST phenomenon limits our understanding to engineer such materials. In this work, we investigate the TIESST effect in a series of Hofmann-type 2D coordination polymers {Fe(4- vinylpyridine)2[M(CN)4]} [M = Ni (1-Ni), Pd (1-Pd), Pt (1-Pt)], which exhibit hysteretic SCO behaviors. Notably, the complexes 1-Ni and 1- Pd demonstrate an unprecedented scan rate-dependent spin state trapping, while 1-Pt maintains a consistent SCO profile, due to the strong Fe–NC bond and its contribution to the elastic interaction. However, dilution of 1-Pt with 5% Zn instills the TIESST effect. Through metal ion dilution studies, we demonstrate that in-plane elastic interaction has a major influence on the realization of the metastable HS state by thermal quenching. This work expands upon the limited reports of TIESST in Hofmann-type frameworks and highlights the tunability of this phenomenon through judicious choice of the inner metal ions as well as doping with non-SCO metal ion. It further emphasizes the need for investigation of the underlying kinetics of the TIESST phenomenon to advance the design and exploit the potential of these materials.