Since the fluorescence of HE shows a very short lifetime (~0.2 ns), significant changes in its decay are hardly detectable with our experimental setup. Therefore, for an estimation of the quenching constants, we monitored the decrease of the emission quantum yields in the steadystate photoluminescence spectra. The concentration of HE employed was 2.55 × 10 -3 M and the concentrations of the other reagents were the same as in the reaction conditions (0.2-0.4 M). In the case of PhCCBr, we were unable to estimate the quenching constant because its absorption spectrum overlaps with the fluorescence of HE, causing strong reabsorption effects.