Forcing aggregation of cyanine dyes with salts: a fine line between dimers and higher ordered aggregates

It is uncommon to read about cyanine dyes in the literature and not have their aggregation discussed. They are of high interest considering their propensity to undergo self-organization in aqueous solution, leading to interesting photophysical properties resulting from the formation of their dimers and higher ordered aggregates. Currently, the study of their aggregation is in high demand due to their diverse application range including dye-sensitized solar cells. However, their aggregation in high salt solutions is under studied, and the effect on aggregation in congruence with high ionic strength is often overlooked. In a previous study, our group established the role of specific ion effects and in particular the necessity of matching water affinity to induce aggregation of a cationic cyanine dye, thiazole orange. In order to advance the understanding of this topic, we present in this article the diverse aggregation of cyanine dyes, as a single monovalent salt can cause different aggregation responses in a variety of these dyes. We established via absorption spectroscopy combined with chemometric analyses that the inherent monomer-dimer equilibrium of a dye depends on its geometry. More interestingly, experimental data coupled with DFT calculations reveal that not only the geometry of a dye but also its charge location plays a role in the aggregate morphology formed by the interaction of a cationic cyanine dye and an anion. It is thought that contact ion pair formation and effective charge screening generated within that ion pair are responsible for aggregates with a greater order.

Size does matter: how to control organization of organic dyes in aqueous environment using specific ion effects

Understanding the role played by external factors on the organization of molecules has the potential to contribute greatly to fundamental research and applications in fields as diverse as nanotechnology, medicine, material chemistry, etc. Countless studies involve the organization of small organic molecules in environments rich in ionic species, yet their participation in molecular organization is often overlooked. Herein, we critically assess the organization in aqueous solution of the cationic cyanine dye, thiazole orange, in the presence of different monovalent sodium salts. Our findings clearly indicate that not all ions are identical with regards to the organization of thiazole orange molecules and specific ions effects are at play. The conventional Debye and Hückel model is not sufficient to explain our results, and the participation of ionic species in molecular organization is explained in terms of the recent theory of water matching affinity. Herein, by choosing the right counterion with the appropriate size, we have shown that it is possible to either induce a simple shift in the monomer-dimer equilibrium of thiazole orange or to turn on the formation of larger organized structures.