  The advent of two-dimensional (2D) crystals has led to numerous scientific
breakthroughs. Conventional 2D systems have in-plane covalent bonds and a weak
out-of-plane van-der-Waals bond. Here we report a new type of 2D material
composed of discrete magnetic molecules, where anisotropic van-der-Waals
interactions bond the molecules into a 2D packing. Through mechanical
exfoliation, we can obtain single-crystalline molecular monolayers, which can
be readily integrated into other 2D systems. Optical spectroscopy suggests the
few-layered molecules preserve the temperature-induced spin-crossover switching
observed in the bulk form but show a drastic increase in thermal hysteresis
unique to these thin 2D molecule assemblies. The trapping of spin bistability
with decreasing layer number can arise from domain wall dynamics in reduced
dimensions. Our results establish molecular solids with strong anisotropy of
intermolecular interactions as precursors to a novel class of 2D materials,
affording new possibilities to control functionalities through substrate and
interlayer interactions.
