Solid-state fluorescence studies of some polymorphs of diflunisal*.
The solid-state luminescence spectroscopy of organic molecules is strongly affected by the effects of excited state energy transfer, with the fluorescence of solids often differing significantly from the fluorescence of the molecule dissolved in a solution phase. Because the magnitude of these solid-state effects is determined by the crystallography of the system, solid-state fluorescence studies can be used to gain insight into the polymorphism of the system. To this end, the spectroscopic properties of four polymorphs of diflunisal have been obtained, and compared to the properties of the molecule in the solution phase. Fluorescence excitation and emission spectra were obtained on four polymorphic forms of diflunisal, and on the compound dissolved in water. It was found that exciton effects dominate the excitation spectra of diflunisal in the four studied polymorphic forms. These phenomena lead to a decrease in the energy of the excitation bands relative to that observed for the free molecule in fluid solution, and in a splitting of the excitation peak into two Davydov components. The trends in the excitation and emission spectra led to the grouping of diflunisal Forms I, II, and III into one category, and diflunisal Form IV into a separate category. Because other work has established that Form IV is characterized by the highest crystal density and consequent degree of intermolecular interaction, the magnitude of the exciton coupling can be used to estimate the degree of face-to-face overlap of the salicylate-type fluorophores.