Spin crossover meets diarylethenes: efficient photoswitching of magnetic properties in solution at room temperature.

A photoisomerizable diarylethene-derived ligand, phen*, has been successfully introduced into a spin-crossover iron(II) complex, [Fe(H2B(pz)2)2phen*] (1; pz =1-pyrazolyl). A ligand-based photocyclization (photocycloreversion) in 1 modifies the ligand field, which, in turn, results in a highly efficient paramagnetic high-spin → diamagnetic low-spin (low-spin → high-spin) transition at the coordinated FeII ion. The reversible photoswitching of the spin states, and thus the associated magnetic properties, has been performed in solution at room temperature and has been directly monitored by measuring the magnetic susceptibility via the Evans method. The observed spin-state photoconversion in 1 exceeds 40%, which is the highest value for spin-crossover molecular switches in solution at room temperature reported to date. The photoexcited state is extraordinarily thermally stable, showing a half-time of about 18 days in solution at room temperature. Because of the outstanding photophysical properties of diarylet...