Evidence for enhanced intersystem crossing on pyrene fluorescence quenching with stable free radicals

The quenching mechanism for pyrene fluorescence by TEMPO and galvinoxyl radicals has been studied by measuring the time evolution of the transient absorption. The quenching rate constants by both radicals were determined to be 1.6 × 1010 mol−1 dm3 s−1 for TEMPO and 1.2 × 1010 mol−1 dm3 s−1 for galvinoxyl. It is demonstrated from an analysis of the dual exponential time profiles of the transient absorption due to Sm ← S1 and Tn → T1 transitions that radicals largely enhance the intersystem crossing from the S1 state to triplet manifolds. The detailed mechanism is discussed.