Aggregation behavior of a model carbocyanine dye: Polar organic solvent versus ionic liquid mixture

Abstract

Dye aggregation within ionic liquids based solutions is a topic of immense importance due to the wide-spread application potential of the dyes and these environmentally benign designer solvents. In this manuscript, we have presented a detailed comparative investigation on the aggregation behavior of a cyanine dye 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4-sulfobutyl)- benzimidazolo carbocyanine sodium salt (TDBC) in mixed ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluorobarate ([bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) as well as in some common organic solvents like ethanol, dimethylsulfoxide (DMSO) and acetonitrile. We have demonstrated the competition between two ILs [bmim][BF4] and [bmim][PF6] in controlling the aggregation behavior of the cyanine dye TDBC in IL mixtures. It was observed that addition of 3 wt% of 1 M aqueous NaOH in TDBC dissolved in mixture of ILs ([bmim][BF4]) + ([bmim][PF6]) can trigger dye to form H-aggregates in [bmim][BF4] rich milieu and J-aggregates in [bmim][PF6] rich milieu. Furthermore, addition of 4 wt% of 1 M aqueous NaOH, in TDBC dissolved in acetonitrile, resulted into the formation of both J-aggregates as well as H-aggregates in a time dependent fashion, whereas, only H-aggregates are formed on addition of 4 wt% of 1 M aqueous NaOH in TDBC dissolved in DMSO and similar addition of 1 M NaOH in TDBC dissolved in ethanol resulted J-aggregation of the dye to very small extent. The present study could contribute in many technological applications in which different forms of a dye molecule are required and it will also further enhance the potential applications of ionic liquids as well. The results presented in this work lead to a better understanding of carbocyanine dye's behavior in ionic liquid based media as well as different organic solvents based media.