Characterizing Counterion-Dependent Aggregation of Rhodamine B by Classical Molecular Dynamics Simulations

The aggregation in a solution of charged dyes such as Rhodamine B (RB) is significantly affected by the type of counterion, which can determine the self-assembled structure that in turn modulates the optical properties. RB aggregation can be boosted by hydrophobic and bulky fluorinated tetraphenylborate counterions, such as F5TPB, with the formation of nanoparticles whose fluorescence quantum yield (FQY) is affected by the degree of fluorination. Here, we developed a classical force field (FF) based on the standard generalized Amber parameters that allows modeling the self-assembling process of RB/F5TPB systems in water, consistent with experimental evidence. Namely, the classical MD simulations employing the re-parametrized FF reproduce the formation of nanoparticles in the RB/F5TPB system, while in the presence of iodide counterions, only RB dimeric species can be formed. Within the large, self-assembled RB/F5TPB aggregates, the occurrence of an H-type RB-RB dimer can be observed, a species that is expected to quench RB fluorescence, in agreement with the experimental data of FQY. The outcome provides atomistic details on the role of the bulky F5TPB counterion as a spacer, with the developed classical FF representing a step towards reliable modeling of dye aggregation in RB-based materials.

Automated summary

The aggregation of charged dyes in a solution, such as Rhodamine B (RB), is affected by the type of counterion, which determines the self-assembled structure and optical properties. Hydrophobic and bulky fluorinated tetraphenylborate counterions, like F5TPB, can enhance RB aggregation and form nanoparticles, affecting the fluorescence quantum yield (FQY) depending on the degree of fluorination. A classical force field (FF) based on the generalized Amber parameters was developed to model the self-assembling process of RB/F5TPB systems, consistent with experimental evidence. The simulations demonstrated the formation of nanoparticles in the RB/F5TPB system and only RB dimeric species in the presence of iodide counterions. This research provides atomistic details on the role of F5TPB counterions as spacers and represents progress towards reliable modeling of dye aggregation in RB-based materials.