Dual phosphorescence and excited state dynamics of p-bromotoluene

Excited state dynamics of dual phosphorescent p-bromotoluene (PBT) was investigated in low temperature matrix between 77 K and 91 K. Dual phosphorescence was ascribed to two low-lying triplet states, 3ππ* and 3πσ*. The phosphorescence decay rate constants were divided into temperature dependent and independent terms. The temperature independent decay rate constants of 3ππ* state was 180 times larger than that of p-chlorotoluene (PCT). The decay rate constant of chlorobenzene (CB) and bromobenzene (BB) were determined in our previous study, and the ratio of the decay rate constants were 13, which has been successfully explained by increase of spin–orbit interaction due to internal heavy atom effect. In the case of p-bromotoluene, the large decay rate constant is partly explained by taking into account of non-radiative decay process due to spin–vibronic interaction through promoting vibrational modes possible even at 77 K. Moreover, homolysis reaction to produce bromine atom was also suggested to induce non-radiative process within triplet manifold.