Evidence for competing electron and energy transfer for the fluorescence quenching of singlet oxygen sensitizers by molecular oxygen

The rate constant of fluorescence quenching by molecular oxygen k was measured for a series of singlet oxygen (1O2)‐sensitizers in toluene and adopts values between 3.5·1010 and 0.4·1010M−1 s−1. log(k ) shows a correlation with the free energy of electron transfer ΔGel as calculated according to Rehm and Weller. k decreases with increasing ΔGel in the endergonic region. The efficiency ϕ for 1O2‐formation accompanying the S1‐state quenching by energy transfer of the S1‐state of the sensitizer is substantially >0 only in the cases, where the S1‐T1‐energy gap exceeds the excitation energy of 1O2(1Δg), 94 kJ/mol, where electron transfer is energetically unfavourable and the T2‐state of the sensitizer is lying above the S1‐state. Furthermore the temperature dependence of k was measured in toluene for two sensitizers. The quenching process for dicyanoanthracene (DCA) is activationless in the high temperature region which could be explained either by activationless energy transfer, or by exciplex formation.