Fluorescence quantum yield determination of molecules in liquids by thermally driven conical diffraction

We report on a method to determine fluorescence quantum yield (η) of molecules in solution based on the observation of conical diffraction (CD) patterns induced by thermally driven self-phase modulation (SPM). In this approach, the central part of a laser beam is acquired as a function of its power after passing through the sample. If the thermal properties of the solvent are known, one can directly obtain η by measuring the sample optical absorption and the rings produced by CD. Otherwise, if the thermal properties of the sample are unknown, an additional measurement with the same solvent containing nonfluorescent molecules (blue-black ink, for instance) for thermal SPM generation is also needed. In this case, besides determination of η, one can also find the thermal conductivity (K) of the unknown solvent provided that the thermo-optic coefficient (dn/dT) is independently measured. K values obtained with this approach are in good agreement with the literature. CD technique was applied in rhodamine 6 G and a chromophore derivative of vitamin B6 in different solvents. Therefore, the present method may serve as a low-cost optical approach for η determination in liquid samples and for K measurements in unknown solutions.