Forcing Aggregation of Cyanine Dyes with Salts: A Fine Line between Dimers and Higher Ordered Aggregates

Abstract

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.

H-Aggregates of an Indocyanine Cy5 Dye: Transition from Strong to Weak Molecular Coupling

“…37,38,46 Thereby the type of salt often plays a role. 32,47,48 Here we find for the first time that the type of H-aggregate depends on the presence of salt. The effect of salt on aggregation is usually explained by charge screening due to increased ionic strength.…”

Tight ultrafiltration membrane: Preparation and characterization of thermally resistant carboxylated cardo poly (arylene ether ketone)s (PAEK-COOH) tight ultrafiltration membrane for dye removal

“…The introduction of background electrolytes in dye solution can promote dye aggregation, because anionic dye would aggregate under the influence of mental ions. This is because the presence of counterions in the aggregates would offset the Coulombic repulsive forces, so the effect, as well as the dye hydrophobicity is accelerated by the addition of electrolytes to dye solution [32,33]. BBR has one negative charge and a molecular weight of 840Da with a triangle molecular configuration, so the charge-to-mass ratio is very lower; dye aggregation effect of dye molecules induced by salts is notable.…”

An Ultrafast Molecular‐Rotor‐Based Fluorescent Turn‐On Sensor for the Perrhenate Anion in Aqueous Solution

“…One of them is the increased ionic strength effect, which has already been discarded in the present system. Another important way, in which this repulsive interaction is minimized, is through the formation of contact ion pairs . Now, it is important to gain an understanding of contact ion pair formation of oppositely charged ions in accordance with the well‐known theory of Collins, which highlights the crucial role of the free hydration energy (the energy representing the capability of ions to attach to the water molecules in their own hydration sphere).…”

“…The decay traces were fitted with the multi‐exponential function given in Equation ; The mean fluorescence lifetime was calculated according to Equation , in which A i = α i τ i / ; and non‐radiative rate constants were calculated by using Equation , in which Φ f represents the emission quantum yield and τ f represents the fluorescence lifetime. …”