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

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), is investigated. The formation of the J-type dimer aggregates are obtained by comparing the differences in the triplet excited state properties of the neutral and aggregated forms of the dyes by using time-resolved spectroscopy.