Photochromism Emergence in N‐Salicylidene p‐Aminobenzenesulfonate Diallylammonium Salts

N‐Salicylidene p‐aminobenzenesulfonate salts were prepared by in situ condensation of p‐aminobenzenesulfonate diallylammonium salt and salicylaldehyde. Modulation of thermo‐ and photochromism was achieved by varying the alkyl chain length of the diallylammonium counter‐cation. A structural–optical properties investigation reveals that both crystal packing and dihedral angle between aromatic rings of the N‐salicylidene aniline switch are not sufficient to predict the occurrence of photochromism in the solid state. The available free space around the N‐salicylidene p‐aminobenzenesulfonate, in addition to the flexibility of the nearby environment, is shown to be of major importance for the cis→trans isomerisation to occur as well as for the stabilisation of the trans‐keto form. Emergence of photochromic properties was determined from the diallylhexylammonium cation within the series of investigated counter‐cations. High stability is observed for the trans‐keto form of one polymorph of N‐salicylidene p‐aminobenzenesulfonate diallylhexylammonium salt (k=2.4×10^?7 s^?1).