THE TEMPERATURE DEPENDENT PHOTOCHEMISTRY OF ARYL AZIDES AND ARYL DIAZO COMPOUNDS

The photochemistry of phenyl, 1-naphthyl and 2,6-difluorophenyl azides was studied. The sequence of intermediates generated by photolysis of these azides was determined by a combination of techniques including ESR spectroscopy, laser flash photolysis and product studies. The prevailing reaction pathway of aryl azides at ambient temperature is different from that at 77 K. The change in mechanism is traced to a temperature-dependent branching ratio from the primary photoproduct singlet aryl nitrene. At low temperatures, triplet aryl nitrene chemistry is highly favored. It was also determined that triplet aryl nitrenes generated in matrix are very susceptible to secondary photolysis. In a rigid matrix, triplet 2,6-difluoro phenyl nitrene abstract hydrogen from frozen toluene to give anilino-radical pairs which subsequently combine to give formal CH insertion products. ESR spectroscopy and product studies demonstrate that these radical pairs are generated by secondary photolysis of triplet 2,6-difluorophenyl nitrene at 77 K.;The photochemistry of aryl diazo compounds was also studied. It was observed that matrix conditions enhance the yield of triplet C-H insertion products. This enhancement is probably due to secondary photolysis in the case of long-lived aryl carbenes such as triplet diphenyl carbene (DPC) in alcoholic matrix. It was also observed that the atom (H or D) abstraction of dibenzocycloheptadienylidene (DBC) or DPC with toluene proceeds by a quantum mechanical tunneling mechanism in solution. This explains the low pre-exponential factors previously measured by laser flash photolysis for this reaction. The chemistry of fluorenylidene (Fl), DPC and DBC in matrix is dominated by the triplet state carbene giving high yields of C-H insertion products with H donor solvents. The considerable lower yields of triplet insertion products obtained in deuterated matrices indicate a change in mechanism in the triplet carbene decay between protic and deuterated matrices. This change explains the low k(,H)/k(,D) values obtained by ESR. The lifetime of ('3)Fl in low temperature glasses determined by ESR was several orders of magnitude smaller than the one obtained by laser flash photolysis. This indicates that ESR focuses only on the carbenes formed in very reactive sites.