Intercalation and photochromism of spiropyrans on clay interlayers

Abstract

1′,3′,3′-Trimethylspiro[2H-1-benzopyran-2,2′-indoline](abbreviated to H–SP) and its derivatives, 6-nitro (NO₂–SP) and 6-nitro-8-(pyridinium)methyl spiropyrans (Py⁺–SP), have been intercalated in the interlayers of Montmorillonite. The intercalation was highly dependent on the substituent groups on the SPs. Charged Py⁺–SP was quantitatively adsorbed on the clay as an equilibrium mixture with the corresponding merocyanine (Py⁺–MC) in aqueous colloidal solution, the ratio Py⁺–SP:Py⁺–MC being 35:65 and exhibiting reversed photochromism. In contrast, a preferential adsorption as Py⁺–SP was observed for the hydrophobic clay layers coadsorbing cetyltrimethylammonium bromide (CTAB), which displays normal photochromism. The thermal isomerization of Py⁺–SP intercalated in aqueous colloidal clay exhibited a linear combination of two components of the first-order reactions, while that of the Py⁺–SP–CTAB coadsorbed system satisfied a single first-order kinetics. The two-component kinetics are explicable in terms of the intervention of at least two different kinds of microscopic environment, e.g. non-polar and polar phases, in the clay interlayers. We also show that the photochemical decoloration of Py⁺–MC was significantly retarded by the intercalation. All of these results are discussed on the basis of the polarity of the intercalation.