SUBSTITUENT EFFECTS ON THE PHOTOCHEMISTRY OF DIPHENYLKETYL RADICALS - ELUCIDATION OF THE RATE CONSTANTS AND QUANTUM YIELDS OF RELAXATION PROCESSES FROM THE EXCITED RADICALS

Two-color, two-laser flash photolysis has been employed to carry out a structure-reactivity investigation on excited diphenylketyl radicals. The effects of varying substitution at the 4 and 4' positions on the probabilities of radiative, nonradiative, and photobleaching processes have been elucidated in a quantitative manner. Rate constants and quantum yields have been measured for all of the above processes, allowing the complete relaxation schemes for these radicals to be obtained. Substituent effects have a large effect on observed fluorescence parameters such as lifetime and quantum yield. The relative invariability of radiative rate constant (k(rad)) demonstrates that this is due to the much larger substituent effects on both the photobleaching (k(bl)) and internal conversion (k(ic)) rate constants. The strong correlation observed between the latter rate constants suggests that the O-H bond of the ketyl radical is important for both k(bl) and k(ic); as a result, bleaching quantum yields show only modest dependence on substitution. The photophysical and photochemical properties of the excited radicals show a dependence on the electron-withdrawing character of the substituent, with disubstitution enhancing the monosubstitution effect.