The effect of solvent, concentration and temperature on the spectra of a highly polar merocyanine

Abstract

AbstractSpectroscopic studies of a polar merocyanine dye subjected to a variety of conditions have been made. Extensive evidence is presented to substantiate the conclusion that in protonic solvents, the acid‐base reaction dye + solvent ⇄ protonated dye + solvent anion is dominant in determining the state of the dye molecule. Conversely, in non‐protonic solvents, the reaction n dye ⇄ (dye)n (n = 2.3…) leading to the formation of dimers and eventually higher aggregates dominates. The temperature effect on the aggregation reaction is illustrated and evidence is given that indicates aggregation may also be possible in alcohols, especially at low temperatures. An unusual temperature effect, whereby all visible absorption maxima decrease in intensity with increasing temperature, is observed in benzene and other aromatic solvents of low polarity. It is possible that at higher temperature these solvents are less able to solvate the polar dye. As a result, the reaction merocyanine ⇄ spiropyrane shifts to the right with increasing temperature rather than to the left as is the normal behavior.