Molecular insights into solid-state photochromism in bulk and confined N-salicylidenes

N-Salicylidenes are well known to exhibit solid-state photochromism, but the precise structure-property relationships are not fully understood and it is difficult to control or impart specific photochromic properties through molecular design alone. In this study, we use solid-state NMR and DFT calculations to investigate the link between the solid-state structure and the photochromic properties. We show that the photochromic properties are highly dependent on the molecular geometry within the crystal structure, which itself is an indicator of the amount of free space available for light-induced tautomerisation. Specifically, 13C solid-state NMR experiments and DFT calculations reveal that the imine chemical shift of the ground-state enol isomer is highly dependent on the molecular geometry, and this helps to rationalise the known empirical torsion angle dependence of the photochromic properties. Upon inclusion within a metal-organic framework, the N-salicylidene molecules are found to adopt geometries close to their ground-state energy minima and this is coupled with the emergence of photochromism for molecules that are not photochromic in the bulk crystalline state. This work highlights that controlling the amount of available steric freedom is key to unlocking the photoactive conformation in the solid state.