Spectroscopy and photochemistry of fluorene at a silica gel/air interface

The emission spectroscopy of fluorene has been studied at a silica gel/air interface at surface loadings from 1.5% to 77.1% of a monolayer. Both monomer and excimer-like emissions are observed on the surface. The excimer-like emission, centered at 350 nm, arises from excitation of ground-state van der Waals pairs of fluorene molecules. Photolysis into the van der Waals dimers at room temperature does not lead to dissociation into constituent monomers; however, irradiation of fluorene monomers at a silica gel/air interface leads to production of 9-fluorenone as the only identifiable photoproduct in 72% yield. Fluorene triplet and cation radical are both observed at the silica gel/air interface by transient diffuse reflectance spectroscopy. Sensitization of singlet molecular oxygen by silica-sorbed methylene blue, in the presence of co-sorbed fluorene, does not lead to fluorene oxidation, thus suggesting that oxidation by direct photolysis does not involve an energy transfer mechanism. An electron transfer oxidation mechanism is proposed to account for the observed photochemical oxidation of fluorene at the silica gel/air interface. (C) 1997 Published by Elsevier Science S.A.