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 diarylethenes, including
single-crystalline photochromism, molecular switch 1 may
offer a promising platform for controlling the magnetic properties
in the solid state and ultimately at the single-molecule level with
light at room temperature.