Rhodamine 6G Structural Changes in Water/Ethanol Mixed Solvent

In water rhodamine 6G (Rh6G) tends to form aggregates at higher concentrations while in ethanol the aggregation is minimal. The extent of aggregation can be controlled by changing the water to ethanol ratio. In ethanol the absorption spectra have a low energy peak and a higher energy shoulder, which are assigned to the S₁ π-π* transition and vibronic side band, respectively, of Rh6G monomers. In water the same two peaks absorption peaks are observed at low concentrations but at higher concentrations a new peak grows in, which is assigned to an H-dimer. Emission spectra are in agreement with these assignments, but also develop a third peak at higher concentrations that is assigned to emission from excimer aggregates. For the first time, the monomer and dimer average diameters were measured by light scattering to be 1.4 ± 0.2 nm and 3.3 ± 0.6 nm, which form in the ground state, leading to the observed excited states. In a mixed solvent the extent of aggregation can be controlled by selecting the ethanol to water ratio, even at the highest concentrations. Graphical Abstract ᅟ.