Squaraine dye based molecular wires containing flexible oxyethylene chains as sensors.: Enhanced fluorescence response on Li+ recognition

Two new pyrrole-derived squaraine molecular wires 4a and 4b containing flexible oxyethylene side chains have been prepared and characterized. GPC analysis of 4a and 4b showed molecular weights of nearly 1800 g/mol. The UV-vis absorption maxima of these molecular wires were broad and red-shifted (11-12 nm) when compared to that of a model squaraine dye 7a. Nevertheless, the absorption maxima of the molecular wires 4a and 4b were much lower than the expected values despite their extended conjugation which could be due to the reduced charge-transfer interactions in these systems. Addition of alkali metal ions such as Li+, Na+, and K+ could not produce considerable changes in the absorption properties of the molecular wires except a marginal change in the case of 4a on addition of Li+. However, significant changes in the emission characteristics could be noticed in the case of 4a upon addition of Li+, whereas addition of K+ and Na+ did not produce any measurable response. Interestingly, the absorption and emission characteristics of the monomeric squaraine dyes 7a and 7b showed only marginal response upon the addition of Li+, Na+, and K+. The enhanced response of the molecular wire 4a over the monomeric squaraine dye 7a has been explained on the basis of the optimum chain length of the flexible binding site and due to a collective system response of the molecular wire, which is associated with a flexible to a rigid conformational change upon the alkali metal ion binding.