On the use of 9,10-dimethylanthracene as chemical rate constant actinometer in singlet molecular oxygen reactions

Abstract

Time resolved near IR luminescence detection of singlet oxygen, O-2((1)Delta (g)), and steady-state photolysis experiments were performed to study in detail limitations and approaches involved when 9,10-dimethylanthracene (DMA) is used as actinometer to measure the chemical rate constants, k(r), for the reaction between excited oxygen and a given substrate. Our results show that in solvents in which singlet oxygen lifetime is long, the actinometer opens an additional pathway to the singlet oxygen disappearance at a rate of similar magnitude to the decay rate constant of O-2((1)Delta (g)), k(Delta). This reactive pathway decreases singlet oxygen concentration. In this case erroneous values of the chemical reaction constant, k(r)(M) for the reaction between singlet oxygen and a given substrate M will be obtained. Additionally, we have found that not in all the solvents, can the total rate constant, k(T)(DMA), for reaction between singlet oxygen and 9,10-dimethylanthracene obtained from time resolved experiments be taken as the "reactive" rate constant, k(r)(DMA), when DMA is employed as an actinometer, The chemical reaction constant, k(r)(M), for the reaction between singlet oxygen and a given substrate M obtained in these conditions will be smaller than the true values. Then, to employ DMA as actinometer, k(T)(DMA) and k(r)(DMA) must be previously evaluated. If k(T)(DMA) and k(t)(DMA) values are very close, nearly ideal conditions to employ DMA as actinometer are fulfilled. Moreover, if k(T)(DMA) and k(r)(DMA) differ in a greater extent, further corrections must be applied to improve k(r)(M) values.