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 N-salicylidene-3-carboxyaniline
(1) with 2-aminopyridine (a), guanylthiourea
(b), cytosine (c), 4,4′-bipyridyl
(d), piperazine anhydrous (e), 1,3-di-o-tolylguanidine (f), and dibenzylamine (g) and N-salicylidene-4-carboxyaniline (2) with 4,4′-bipyridine (d) and N,N-dibenzylamine (g) have
been synthesized. The weak photochromic compound 1 becomes
nonphotochromic or strongly photochromic in the co-crystals and the
nonphotochromic compound 2 becomes photochromic in the
co-crystal 2g. The photochromic properties of compounds 1 and 2 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.