Photochromic behaviour of surfactant spiro [2H-1-benzopyran-2,2′-[2,3]-dihydroindole]s (spiropyrans) adsorbed into clay interlayers

Abstract

Spiro[2H-1-benzopyran-2,2′-[2,3]-dihydroindole]s (abbreviated as spiropyrans) containing long ω-pyridinoalkyl chains have been intercalated into clay interlayers and studied to observe the photochromic characteristics of the resulting molecular assemblies. 1′-Octadecyl-3′,3′-dimethyl-6-nitro-8-pyridinomethyl (SP1801⁺), 1′-(10-pyridinodecyl)-3′,3′-dimethyl-6-nitro-(SP10⁺00) and 1′-(10-pyridinodecyl)-3′,3′-dimethyl-6-nitro-8-icosylcarbonyloxymethyl-spiropyrans (SP10⁺ 22) were all quantitatively intercalated into the clay interlayer and exhibit normal photochromism in aqueous clay dispersions. Their thermal decolourations obey simple first-order kinetics and are about one-tenth as slow in clay dispersions and about one-hundredth as slow in clay films as that in bulk solution without clay at 50 °C. However, 1′,3′,3′-trimethyl-6-nitro-8-pyridinomethylspiropyran (SP0101⁺), a homologue without a long alkyl chain, exhibits inverse photochromism and its thermal decolouration undergoes two kinds of first-order kinetics in aqueous clay dispersions. These results are explicable in terms of the restricted conformational motions of the intercalated spiropyrans.