Photochromism Control of Salicylideneaniline Derivatives by Acid–Base Co-Crystallization

Acid–base co-crystallization has been used to control the photochromic reactivities of salicylideneaniline derivatives in co-crystals. The series of co-crystals <i>N</i>-salicylidene-3-carboxyaniline (<b>1</b>) with 2-aminopyridine (<b>a</b>), guanylthiourea (<b>b</b>), cytosine (<b>c</b>), 4,4′-bipyridyl (<b>d</b>), piperazine anhydrous (<b>e</b>), 1,3-di-<i>o</i>-tolylguanidine (<b>f</b>), and dibenzylamine (<b>g</b>) and <i>N</i>-salicylidene-4-carboxyaniline (<b>2</b>) with 4,4′-bipyridine (<b>d</b>) and <i>N</i>,<i>N</i>-dibenzylamine (<b>g</b>) have been synthesized. The weak photochromic compound <b>1</b> becomes nonphotochromic or strongly photochromic in the co-crystals and the nonphotochromic compound <b>2</b> becomes photochromic in the co-crystal <b>2g</b>. The photochromic properties of compounds <b>1</b> and <b>2</b> change because of the conformational changes induced in the salicylideneaniline moieties in the crystal structure. The lifetimes of the colored species formed in the photochromic reaction are also affected by the changes in the environment around the molecule in the crystal. As shown in this study, acid–base type co-crystallization may be a promising method to control the photochromic reactivities of salicylideneaniline derivatives