A conformationally constrained hapten is presented that is capable of catalyzing the first antibody-mediated photo-Fries rearrangement. In this reaction, absorption of light energy by a diphenyl ether substrate results in homolytic C-O bond cleavage followed by recombination to yield biphenyl-derived products. The most proficient antibody studied converts 4-phenoxyaniline 15 into 2-hydroxy-5-aminobiphenyl 16 under high-intensity irradiation at a rate of 8.6 microM/min. These results support a recent hypothesis stating that immunization with conformationally constrained haptens provides higher titers for the acquisition of simple binding antibodies; however, in this case, conformational constraint does not ensure the development of more efficient catalysts. Using the obtained antibodies, the presence of products resulting from escape of free radicals from the solvent cage can be suppressed, altering the excited state energy surface such that free radicals are funneled into the formation of the desired biphenyl product. However, studies also show the inactivation of the antibodies as a result of photodecay of the biphenyl product. Using an isocyanate scavenging resin, the photodecay product could be removed and the inactivation of the antibody drastically reduced. Furthermore, despite the observed photodecay, turnover of the antibody was present; this represents the first case in which true turnover of a photochemical reaction using a catalytic antibody could be observed.