Photochromism of polymorphic 4,4′-methylenebis(N-salicylidene-2,6-diisopropylaniline) crystals

4,4′-Methylenebis(N-salicylidene-2,6-dialkylaniline) derivatives were prepared and their structures were determined by 1H NMR, IR, DSC and X-ray crystallographic analyses. The 2,6-diisopropylaniline derivative yielded definite polymorphic crystals: the space groups of the crystals were C2/c and P21/n, respectively. It was found that both polymorphs were similarly photochromic but the thermal stability of the photochrome was different due to the variance of the shape of molecule and cavity in each crystal.