Optical properties of dye molecules as a function of the surrounding dielectric medium

We perform an optical study of sulforhodamine B and sulforhodamine 101 molecules solved in water droplets (reversed micelles) that are stabilized by a surfactant and suspended in various mixtures of heptane and toluene. While the dielectric permittivity of the surrounding medium varies with the proportions of heptane and toluene, the chemical environment of the dye molecules always remains the same. The modifications of the optical properties of the dye molecules can then be solely attributed to the effect of surrounding dielectric medium on the electromagnetic field. As predicted by the theory, the amplitude of the luminescence spectrum of a given type of dye molecules increases with the refractive index of the microemulsion while the amplitude of the absorption spectrum remains almost unchanged. The various experimental results are also used to evaluate the radiative lifetimes of the dye molecules in the various microemulsions using three different approaches: (i) from the luminescence decay times using a Fermi’s golden rule description of the spontaneous emission process taking into account the variations with the surrounding dielectric medium of both the local electromagnetic field and the density of available photon modes, (ii) from the variation of the intensity of luminescence as a function of the luminescence decay time, and (iii) from the absorption spectra using a modified Einstein relation between absorption and emission. An excellent agreement between the three methods is found for sulforhodamine 101. For sulforhodamine B, there are discrepancies between the values obtained from the luminescence and absorption measurements.