Complexation of guanine with dissolved oxygen in solution and study of excited-state lifetime

It has been shown that guanine forms a complex with the dissolved oxygen in a solution even without ultraviolet (UV) irradiation, although, if used, UV irradiation can accelerate the complexation due to the formation of singlet excited oxygen. Triplet-singlet transitions in guanine become allowed due to its complexation with oxygen. Electronic excitations of the complex cause jumping of the guanine molecule from the deeper well of the ground state potential surface to the shallower well of the same surface. The guanine-oxygen complex shows two new fluorescence peaks besides the 350-nm fluorescence from the first singlet excited state of guanine, at around 400 and 450 nm, which have been explained as originating from the second triplet and first triplet excited states of the molecule, respectively. On repeated irradiation of the guanine solution, the 450-nm fluorescence becomes prominent, while the other two are gradually suppressed. The 450-nm fluorescence has two components, one with a small lifetime (ca. 0.23 ns) and the other with a much longer lifetime (12.6 ns).