Abstract. Fluorescence and photo-Fries reaction of N-aryl-urethanes can be quenched by diffusion-controlled energy transfer to biacetyl, acetone and other aliphatic ketones. Quenching of the photo reaction and fluorescence obeys the Stern-Volmer Equation. The efficiency of quenching may by influenced by solvents and substituents changing the S, energies of donors and acceptors. The identical rate constant3 determined by ...