material with selective absorptive and emissive responses to methanol and water. The yellow anhydrous form converts reversibly on a subsecond timescale to a red hydrate in the presence of parts-per-thousand levels of atmospheric water
vapour. Exposure to methanol induces a similarly-rapid and reversible colour change to a blue methanol solvate. Stable “smart coatings” on glass demonstrate robust switching over 104 cycles, and flexible microporous polymer membranes
incorporating microcrystals of the complex show identical vapochromic behaviour. The rapid vapochromic response can be rationalized from the crystal structure, and in combination with quantum-chemical modelling we provide a complete
microscopic picture of the switching mechanism. We discuss how this multiscale design approach can be used to obtain new compounds with tailored VOC selectivity and spectral responses.