Influence of the nature of sol–gel matrices on absorption and fine-structure fluorescence spectra of hypericin

Absorption and fluorescence spectra of hypericin dispersed in an inorganic or a hybrid organo–inorganic have been studied at room or liquid-nitrogen temperature. Changes observed in the absorption spectra and the absence of fluorescence in the inorganic host are assigned to hydrogen bonding between hypericin and hydroxyl groups covering the surface of the pore in the matrix. Analysis of relative intensities and frequency changes in the fine-structure fluorescence and excitation spectra at 4.2 K indicate an increase of the strength of hydrogen bonds under photoexcitation of hypericin. The character of the guest–host interaction in the hybrid xerogel is discussed.