Unusually strong emission from an exciplex formed between benzenoid solvents and dibenzoylmethanatoboron difluoride. Formation of a triplex

Abstract

Benzene derivatives give, in cyclohexane solution, a strongly emitting (Φf up to 0.8) excited complex with dibenzoylmethanatoboron difluoride (DBMBF2). The rate constant kex for the complex formation varies between 3.5 × 109 and 2.6 × 1010 M−1 s−1 depending on the benzene derivative, and the complex emission can already be detected at very low concentrations of the aromatics. This behaviour is different from that found in acetonitrile where the DBMBF2 emission disappeared completely before the new emission of the complex could be detected. The position of the complex emission and its lifetime depend upon the redox potential of the aromatics: the lower the latter, the longer the lifetime and the more the emission is red-shifted. The monotonous increase of the lifetime and of the fluorescence quantum yield of the emission with the concentration of the aromatics has been interpreted on the basis of an equilibrium between the exciplex and excited DBMBF2. The equilibrium constant varies with the electron-donating property of the aromatics, from 2.7 for benzene to 560 M−1 for isodurene. A decrease in the excited complex lifetime or in its fluorescence quantum yield is observed in some cases when the aromatics concentration is close to that of the pure solvent. Such behaviour is believed to arise from a new equilibrium resulting from the interaction of the exciplex with a second molecule of the aromatics to form triplex species. A detailed kinetic study has been undertaken in case of p-xylene, and the rate constants of all the processes involved in the double equilibrium have been determined. No basic difference is observed in the rate constant for exciplex formation when a single or double equilibrium is assumed; on the other hand, large discrepancies are found in the exciplex deactivation processes, specially for the back reaction from the exciplex to excited DBMBF2. The equilibrium constants between DBMBF2 and the exciplex, on one hand, and between the latter and the triplex are 56.6 and 0.68 M−1, respectively, when the concentration of p-xylene in cyclohexane is 3.3 M. Keywords: fluorescence, exciplex, triplex, dibenzoylmethanatoboron difluoride, kinetics.