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.

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.

Studies involving metal enhancement effects have gained popularity, and enhancement of fluorescence due to the close proximity of a dye molecule to a metal nanoparticle is well documented. Although enhancement of singlet oxygen production by metal has been reported, studies are relatively scarce and so far only stationary silver island films have been proven to be adequate to do so. Herein, we describe the synthesis and characterization of core-shell nanoparticles on which a photosensitizer acting as source of singlet oxygen has been covalently attached to the nanoparticle surface. As a proof of concept, silver nanoparticles with a diameter around 68 nm were chosen as the metallic core, and were coated by a silica shell of about 22 nm in thickness. The silica shell plays a dual role as a spacer and a medium onto which the photosensitizer, rose bengal (RB), has been covalently attached. These novel core-shell nanoparticles allow for the amplification of singlet oxygen production by 3.8 times, which is similar to the amplification found for RB in proximity of silver island films.

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