Aggregation properties of heavy atom substituted squaraine dyes: evidence for the formation of J-type dimer aggregates in aprotic solvents

The squaraine dye bis(2,4,6-trihydroxyphenyl)squaraine (SqH) was earlier reported to form J-type dimer aggregates in acetonitrile solutions at higher concentrations. Subsequent studies have suggested that concentration-dependent changes in the absorption spectrum of SqH in acetonitrile could be attributed to shifts in the acid-base equilibrium due to the presence of water as an impurity. In this work, we describe our studies on the effect of varying acid and dye concentration on the absorption spectra of the bromo and iodo substituted dyes, bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (SqBr) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (SqI). Analysis of the changes in the absorption spectra as a function of dye concentration and the nature of the solvent composition confirmed the formation of J-type dimer aggregates in aprotic solvents in this class of dyes. Further confirmation for the formation of the J-type dimer aggregates could be obtained by comparing the differences in the triplet excited state properties of the neutral and aggregated forms of the dyes using time-resolved spectroscopy.