Reactions of Spiropyrans with Inorganic Reagents

6-Nitro-8-methoxyBIPS in acetone solution in the dark forms a chelate with copper(II) chloride ( .= 400 nm). In tetrahydrofuran, the copper salt has poor 

The merocyanine form of numerous BIPS compounds in solution complex with many transition and rare-earth metal ions. The complexation between 6-nitro-8-methoxyBIPS and several ions was studied by spectrophotometric, luminescent, stopped-flow, and nanosecond laser flash photolysis techniques. The absorption maximum of the dye, 580 nm, is shifted to the 480-500 nm region, and the relatively weak fluorescence shows a similar hypsochromic shift. The kinetics of the complexation involved a fast reaction between the components, followed by a slow equilibrium of the complex to its most stable isomer. The photoreactions of the complexes include formation of a short-lived triplet state (lifetime about 2 x 10 5 s, 

The 6-nitroBIPS having a 4-butylsulfonic acid salt group in the l -position, when intercalated into the interlayers of an Li-Al-layered double hydroxide, showed reversible photochromism in both the presence and absence of cointercalated 4-toluenesulfonic acid. When the acid was present, the absorption peak (X, ca. 520 nm) was sharpened and split into two peaks, and the fluorescence intensity was increased.179 

Two methods of bonding a spiropyran to the surface of silica gel have already been mentioned the reaction of silica-bound salicylaldehyde with Fischer s base, and the reaction of 6-lithioBIPS with silica. Silica-bonded spiropyrans exhibited properties (reverse photochromism, visible light photobleaching, and color changes upon acid treatment due to salt formation) very similar to those of spiropyrans merely adsorbed onto silica. The differences in behavior were attributed to the different extent of absorption and desorption of atmospheric water caused by the difference in the number of free SiOH groups available in the two cases.86 

Upon treatment with acid (or on contact with an acidic surface), many spiropyrans give the salt of the open form or the open form itself, depending upon the relative base strengths of the spiro and open forms. Thus, treatment of several BIPS (7-diethylamino, 6-nitro, and 5 -nitro) with trifluoroacetic acid in the nonprotic solvents acetonitrile and chloroform gave the protonated merocyanine form, which upon neutralization with base gave the open colored form.180 This sequence of operations causes coloration by a non-thermal, non-photochemical route the adsorption coloration was utilized in the early applications of spiropyrans in carbonless (pressure-sensitive) copy papers. In this application, dialkylamino-substituted spirodi(benzopyrans) were preferred paper containing BIPS compounds turned pink on storage. 

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