EVIDENCE FOR ENHANCED INTERSYSTEM CROSSING ON PYRENE FLUORESCENCE QUENCHING WITH STABLE FREE-RADICALS

The quenching mechanism for pyrene fluorescence by TEMPO and galvinoxy l radicals has been studied by measuring the time evolution of the tra nsient absorption. The quenching rate constants by both radicals were determined to be 1.6 X 10(10) mol(-1) dm(3) s(-1) for TEMPO and 1.2 X 10(10) mol(-1) dm(3) s(-1) for galvinoxyl. It is demonstrated from an analysis of the dual exponential time profiles of the transient absorp tion due to S-m <-- S-1 and T-n --> T-1 transitions that radicals larg ely enhance the intersystem crossing from the S-1 state to triplet man ifolds. The detailed mechanism is discussed.