  In this work, the aggregation behaviour of RhB has been studied with the
change in the concentration of the RhB dye in water, ethanol and propanol using
absorption and laser induced fluorescence spectroscopy. The dimer and monomer
fluorescence emissions were observed simultaneously in all the solvents. The
monomer to dimer fluorescence intensity ratio has been calculated and found
that at a certain concentration of RhB the magnitude of the ratio is different
in different solvents. The ratio becomes equivalent to one for 1.5 gm per l, 3
gm per l and 4.2 gm per l, concentration of RhB in water, ethanol and propanol
respectively. This concentration of RhB at which the monomer to dimer ratio
becomes unity is proposed as critical concentration. Above this critical
concentration, the dimeric fluorescence dominates and below which the monomeric
fluorescence dominates. Moreover, in the solvent for which the magnitude of
critical concentration is less the fluorophore molecule aggregates more easily
and vice versa. So, water is found to be the most suitable solvent for RhB
molecules for self-aggregation than ethanol and propanol but with less dimeric
quantum yield. It is also observed that the critical concentration varies
inversely with the dielectric constant of the solvents. Altogether, it can be
said that the proposed ratio and critical concentration quantify the
aggregation process of the fluorophore in the solvent. Hence, the critical
concentration value of the fluorophore could be assumed as the characteristic
parameter to study the aggregation behaviour of the fluorophore in solvents.
The described method has the merit of use in the study of lasing ability and
the wavelength tunability of the dye laser gain media as well as in the heavy
metal sensing technology in water. This method may also be extended for other
fluorophores and solvents.
