Time-resolved determinations of deuterium isotope effects on O2(1.DELTA.g) lifetimes in solution

Abstract

Results of experiments applying a time-resolved substrate bleaching method to studies of determination of the natural lifetime (tau/sub ..delta../) of the lowest excited state of molecular oxygen, O/sub 2/(/sup 1/..delta../sub g/) as a function of the solvent and its isotopic condition are described. Fully protiated and fully deuterated species of benzene, acetonitrile, acetone, and chloroform were used as solvents, and diphenylisobenzofuran (DPBF) or rubene (R) were used as monitoring substances (M) for directly monitoring the loss of optical absorbance of M owing to the elementary process O/sub 2/(/sup 1/..delta../sub g/) + M ..-->.. (k/sub m/) -M. Values of tau/sub ..delta..//sup D//tau/sub ..delta..//sup H/ obtained from these experiments do not resolve the discrepancy of previously reported values obtained by different workers. However, it is concluded that in solvents where large tau/sub ..delta../ values (approx. 1000 ..mu..s) are exhibited, direct luminescence measurements technique is to be preferred over indirect methods. Contrarily, for tau/sub ..delta../ values of less than approx. 200 ..mu..s, it is contended that use of the time-resolved substrate bleaching method following flash excitation gives very reliable results. (BLM)