Selected solid-state behaviour of three di-tert-butyl-substituted N-salicylideneaniline derivatives: temperature-induced phase transitions and chromic behaviour

The synthesis, single-crystal structures and chromic behaviour of three related Schiff bases, namely, (E)-2,4-di-tert-butyl-6-{[(4-fluoro­phen­yl)imino]­meth­yl}phenol, C(21)H(26)FNO, 1, (E)-2,4-di-tert-butyl-6-{[(4-chloro­phen­yl)imino]­meth­yl}phenol, C(21)H(26)ClNO, 2, and (E)-6-{[(4-bromo­phen­yl)imino]­meth­yl}-2,4-di-tert-butyl­phenol, C(21)H(26)BrNO, 3, are reported. Two polymorphs of 1 were obtained, which were found to have different photochromic properties. Schiff bases 2 and 3 were found to be isostructural and underwent a phase transition upon cooling which was attributed to the dynamic disorder in one of the tert-butyl groups resolving at low temperature. All of the structures were found to exist in the enol rather than the keto form based on the C—O(H) and imine C=N bond lengths, and contained an intra­molecular O—H⋯N hydrogen bond alongside weaker inter­molecular C—H⋯O contacts.