Photophysics at solid surfaces. Evidence of dimer formation and polarization of monomer and excimer fluorescences of pyrene in the adsorbed state on silica-gel surfaces

Abstract

The fluorescence, fluorescence excitation and fluorescence-polarization spectra of pyrene adsorbed on a silica gel surface activated at 473 K have been observed as a function of the amount of adsorbed pyrene, as well as the ratio of fluorescence quantum yields of excimer-like fluorescence to monomer-like fluorescence. Kinetic considerations reveal that a part of the adsorbed pyrene forms a ground-state dimer. The excited dimer rearranges its conformation during the fluorescence lifetime from which excimer-like fluorescence is emitted. The degree of polarization, P, has values other than zero. These findings indicate that the excimer formation can be accomplished by slight displacement which does not depolarize the emission on the silicagel surface during the formation of the excimer from the ground-state dimer. It is also shown that the polarization of excimer emission is along the a axis of the pyrene molecule, and the excimer is favourably formed by b-axis excitation rather than a-axis excitation.