Low temperature laser flash photolysis and spectral studies of Methyl Red

Abstract

The low temperature flash photolysis and spectral studies of Methyl Red {2-[4-(dimethylamino)phenylazo]benzoic acid} show the presence of four conformers in equilibrium. Besides the common cis and trans conformers, there are two types of quinoid zwitterions. The absorption intensity at room temperature is mostly contributed by quinoid zwitterions. In methyl cyclohexane the absorption maximum of the trans conformer is at 420 nm and those of two zwitterions are at 475 and 455 nm, respectively. From flash excitation spectra, the cis conformer appears to show an absorption maximum at 350 nm. The distribution of the species changes on flash excitation and the normal distribution is restored via one or more equilibrium steps as indicated by the negative activation energy of the process.