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Photochromic spiropyran was covalently immobilized using alkyldiamine couplers with varying chain length (2, 4, 6, 8 methylene groups) onto an optically transparent polymeric surface. The relatively nonpolar and colourless spiropyran form can be reversibly switched to a charged, highly coloured, zwitterionic merocyanine isomer using UV and visible light, respectively. The merocyanine form complexes with metal ions, and this results in a shift in the absorbance spectrum, and a corresponding colour change is observed. Upon irradiation of the complex with visible light, the cation is released and the merocyanine (active) isomer reverts back to the spiropyran (passive) form. Optical switching and complexation behaviour of the polymer-bound spiropyran–merocyanine system is highly dependent on the tether length of the spiropyran from the polymeric surface. The effect is demonstrated using cobalt(II) chloride in solution.
Byrne et al. (Sun,) studied this question.