A new photo-functional material constituted by a spirooxazine supported on a zirconium diphosphonate fluoride

Abstract

The photochromic behaviour of a spirooxazine (1,3,3-trimethylspiro[indoline-2,3′-[3H]naphth[2,1-b][1,4]oxazine]) was investigated in a solid matrix at room temperature. The solid matrix belongs to the class of layered zirconium phosphonates and was obtained by slowly decomposing zirconium fluoro complexes in a water–methanol solution containing dissolved CH₃(CH₂)₇N(CH₂PO₃H₂)₂ diphosphonic acid. The white powder consists of spherical particles (average diameter 50 µm) made up of lamellar microcrystals of formula Zr[O₃PCH₂NHCH₂PO₃(CH₂)₇CH₃]F·0.2H₂O. The lamellar structure arises from the packing of layers, each constituted by an inorganic bidimensional array with pendant alkyl chains. The spirooxazine, adsorbed onto the solid surface, maintains the thermoreversible photochromic behaviour exhibited in solution (photocolouration under UV irradiation and thermal bleaching in the dark). The microenvironment, generated by the solid phase, influences both the chromaticity and decolouration dynamics of the system. Two absorption bands were observed in the visible: one at about 600 nm, due to the coloured photomerocyanine and the other, shifted to the red (∼700 nm), was assigned to J-aggregates of the photomerocyanines, based on temperature effect and spectral position. Both visible absorptions increased under UV irradiation and partially bleached in the dark. The relaxation times, different in the two spectral regions, were longer than in a fluid solution. Biexponential decay kinetics, observed at 610 nm, were explained by assuming that the detachment of the photomerocyanine from the stacks was the rate-determining step of the bleaching reaction for the molecules adsorbed onto the surface of the aggregated lamellae.