The nature of the styrylindolium dye: transformations among its monomer, aggregates and water adducts

Abstract

The optical properties of styrylindolium dyes are strongly influenced by the external environment. After a detailed investigation by electronic absorption spectroscopy, we found that five states of 2-methoxylstyryl-indolium dye viz. monomer, D-aggregate, H-aggregate, J-aggregate and water adducts can be observed under different conditions. The spectroscopic studies revealed that the conversion rate of the dye among its monomer and the other aggregates is slow due to the steric hindrance of the 3,3-dimethyls. Meanwhile, it could form water adducts by the reaction of the water residue in solvent with the charged electron-deficient CN double bond of the dye. The formation of water adducts is also an important factor that influences the optical properties of the dye because the water addition reaction breaks the dye's π-conjugate. Generally, the dye exists mainly in the monomeric form in freshly prepared solutions. A small portion of the D-aggregate, H-aggregate and J-aggregate can be observed in some cases. In equilibrium solutions, however, significant changes are observed in most solvents with different polarities. The J-aggregate is an unstable state which is irreversibly converted to other species. The monomer converts reversibly among the D-aggregate, H-aggregate and water adducts in the equilibrium state. At the same time, the water adducts are sensitive to the external environment and their content can be modulated by temperature and pH conditions. The dye's prominent optical signal changes arising from pH- and thermal dependence of water addition or the disassociation of the D-aggregate led to potential applications in the field of thermoresponsive probes under given conditions.