Detection of Dipolar Contribution to the Mechanism of the Triplet-Triplet Energy Transfer Process in Molecular Solids

Abstract

It is commonly accepted that triplet-triplet excitation transfer between aromatic molecules takes place via an electron exchange mechanism. Since the probability of this mechanism falls much more rapidly (exponentially) with distance than the weak dipolar mechanism, it is possible that a distance can be found for which the dipolar mechanism begins to contribute to the observed transfer. The technique of laser luminescence line narrowing is used at 4.2 K to study the spectral diffusion in l-bromo-4-chloronaphthalene (BCN), a one-dimensional orientationally disordered solid. The 0,0 band of its singlet-triplet transition is inhomogeneously broadened. It is shown that this technique can be used to determine the donor decay, and thus the mechanism of triplet-triplet energy transfer, as the donor-acceptor distance is continuously changed by simply changing the wavelength of the exciting laser. At long wavelength, this technique is particularly convenient for use in disordered solids, i.e., solids with large inhomogeneous absorption line width. At short distances, the donor phosphorescence decay is found to fit the expected one-dimensional electron exchange type mechanism. At distances longer than 10Å, a deviation from one-dimensional exchange fit is observed, which could be accounted for by either a change in the dimensionality or a change in the transfer mechanism from exchange to dipolar. A microwave-phosphorescence double resonance experiment is used to determine the energy distribution of molecules in the different spin levels as a function of time. The results suggest that at long donor-acceptor separation, molecules in radiative spin level’s (i.e., with transition dipole moments) transfer with higher probability than those in dark spin levels. This suggests that at long distances, the dipolar mechanism contributes to the triplet energy transfer in this solid.