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The spectrum of Rose Bengal, C-2' ethyl ester, C-6 sodium salt dimer, in 2% ethanol has been observed in the presence of KN03, and equilibrium constants for its formation have been determined. It is proposed that aggregation occurs in two steps: (a) formation of a neutral complex between the anion of the dye and potassium ion and (b) reaction between the neutral complex and a second dye anion to produce the dimeric species.

An excimer (excited dimer) is formed by the diffusioncontrolled reaction between a molecule in the excited state and another in the ground state and is characterized by a broad, structureless emission red-shifted with respect to the fluorescence emission of the monomer.12 Provided that only two excited states are involved, the ratio of pyrene monomer to excimer emission is a linear function of the medium viscosity.