A laser flash photolysis study of some simple para-substituted derivativesof singlet phenyl nitrene

The photochemistry of several para-substituted aryl azides was studied by l aser flash photolysis (LFP) techniques in pentane. Para F, Cl, Br, I, CH3, CF3, COCH3, OCH3, and NO2 substituted aryl azides were studied. LFP of aryl azides generates singlet aryl nitrenes which were detected in absorption w ith maxima between 300 and 365 nm in pentane. The nitrene decay can be fit to an exponential function and analyzed to yield observed rate constant, k( obs), which was measured as a function of temperature. The value of k(obs) = k(R) + k(ISC) where k(R) and k(ISC) are the rate constants of nitrene rea rrangement to azirine intermediates and intersystem crossing to the lower e nergy triplet states, respectively. Values of k(obs) decrease as the temper ature is reduced and reach a limiting value attributed to k(ISC). Assuming that k(ISC) is temperature independent allows values of k(R) to be deduced along with the Arrhenius parameters to cyclization for para CH3, CF3, COCH3 , F, Cl, and Br singlet phenyl nitrenes. The Arrhenius parameters to rearra ngement closely resemble those of parent singlet phenyl nitrene with the ex ception of the para bromo compound. For para I and OCH3, k(ISC) is large an d prevents measurement of k(R) The increased rate of intersystem crossing i n these two singlet nitrenes is due to heavy-atom and to electron-donation effects, respectively. No transient absorptions were detected upon LFP of p -nitrophenyl azide.