PHOTOCHEMISTRY ON SURFACES: TRIPLET‐TRIPLET ENERGY TRANSFER ON MICROCRYSTALLINE CELLULOSE STUDIED BY DIFFUSE REFLECTANCE TRANSIENT ABSORPTION and EMISSION SPECTROSCOPY*

Abstract

Abstract Triplet‐triplet energy transfer has been studied between benzophenone and an oxazine dye (2,7‐bis(diethyl‐amino)‐phenazoxonium chloride) co‐adsorbed on the surface of microcrystalline cellulose. Ground state absorption and fluorescence measurements provide evidence for dimer formation of the oxazine dye when adsorbed on cellulose in contrast to the behaviour in ethanol solution where no dimerization is observed. The equilibrium constant for dimerization, which is found to be (1.0 × 0.1) × 10 6 mol −1 g (2560 × 250 dm 3 mol −1 ) for oxazine alone on cellulose decreases in the presence of co‐adsorbed benzophenone. Fluorescence is detected from excited monomeric but not from excited dimeric oxazine. The absorption spectrum of the triplet state of oxazine adsorbed on cellulose was obtained and its extinction coefficient evaluated relative to that of triplet benzophenone which was used as a sensitizer. The lifetime of adsorbed triplet oxazine is 4.3 ms which is 300 times longer than that in acetonitrile solution. The efficiency of energy transfer from triplet benzophenone to oxazine on cellulose was studied using both time resolved sensitized absorption and phosphorescence intensity measurements as a function of oxazine concentration. Lifetime measurements show that the energy transfer process involves static quenching since the benzophenone lifetime is independent of oxazine loading at the surface. A mechanism is proposed to explain the results in which one oxazine molecule is suggested as being able to quench phosphorescence from a “pool” consisting of 2 to 3 benzophenone molecules.