Toluidine blue: aggregation properties and structural aspects

The toluidine blue (TB) aggregation phenomenon in water solutions was investigated by UV-Vis absorption spectroscopy in a wide range of concentrations, from 10(-9) to 10(-2) M. The experimental data were fitted by assuming that the spectrum was due to the overlapping of the bands of six different aggregation species simultaneously present in solution. The model and the peak centres of the different species were hypothesized on the basis of exciton theory. All the spectra obtained were simultaneously processed thus allowing the derivation of the K constant, which was assumed to be the same for all the equilibria and which was found equal to 10 500. The epsilon spectra of the different aggregation species were also obtained. The overall TB spectrum may be mainly attributed to the H-type aggregation, although some of the species also show the J type bands. The structural features of TB and its dimeric aggregate were also investigated by means of molecular mechanics semi-empirical potential energy calculations. The results obtained for the dimer confirm the H type aggregation and show that the lowest energy model of the four possible is the one obtained by a mirror plane of symmetry containing the aromatic rings, in agreement with the x-ray diffraction data which will be presented in a forthcoming paper.