Photo‐Induced Spin Transition of Iron(III) Compounds with π–π Intermolecular Interactions

Photo‐switching: The LIESST effect could be observed by introducing π–π intermolecular interaction even for iron(III) spin‐crossover (SCO) compounds being the relatively small ΔrHL. Introduction of strong intermolecular interactions can be widely used in the design of a variety of optically switchable molecules (see figure). Iron(III) spin‐crossover compounds [Fe(pap)2]ClO4 (1), [Fe(pap)2]BF4 (2), [Fe(pap)2]PF6 (3), [Fe(qsal)2]NCS (4), and [Fe(qsal)2]NCSe (5) (Hpap=2‐(2‐pyridylmethyleneamino)phenol and Hqsal=2‐[(8‐quinolinylimino)methyl]phenol) were prepared and their spin‐transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X‐ray analysis, and structures in high‐spin (HS) and low‐spin (LS) states were revealed. Light‐induced excited‐spin‐state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1–3 and at 800 nm for 4 and 5. The activation energy Ea and the low‐temperature tunneling rate kHL(T→0) of iron(III) LIESST compound 1 were estimated to be 1079 cm−1 and 2.4×10−8 s−1, respectively, by HS→LS relaxation experiments. The Huang–Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as π–π stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex. Photo‐switching: The LIESST effect could be observed by introducing π–π intermolecular interaction even for iron(III) spin‐crossover (SCO) compounds being the relatively small ΔrHL. Introduction of strong intermolecular interactions can be widely used in the design of a variety of optically switchable molecules (see figure).