Role of internal mechanisms in energy transfer processes in organic liquid scintillators

Electronic excitation energy transfer from 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole(PBD) (Donor, 10−2 M) to 2,5-Di-(5-tert-butyl-2-benzoxazoly)-thiophene (BBOT) (acceptor, 1.5 × 10−5 M to 10−3 M) are carried out using both steady state and time resolved fluorescence methods. The observed donor peak intensities of the steady state fluorescence are corrected for radiative transfer and direct excitation of the acceptor. The bimolecular quenching rate parameters ‘kq’, determined according to Stern-Volmer equation using both corrected fluorescence intensities and decay times, are in good agreement, but larger than the theoretical value of ‘kq’, the translational diffusion rate parameter determined according to Umberger-Lamer equation, indicating that the diffusion process alone does not play a part in energy transfer process. The critical transfer distances ‘R0’, determined from both Förster equation and half quenching concentration, are close to one another and are of the order of 46.3 Å indicating that the Förster mechanism plays an important role in overall energy transfer of the donor. The mean diffusion length ‘d1’ is found to be less than R0, supporting the dominance of long-range interaction. Further, from the nature of the decay curves, it may be concluded that long-range interaction of the Förster type is more predominant than energy migration and diffusion processes.