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 ( a f up to 0.8) excited complex with dibenzoylmethanatoboron difluoride (DBMBF2). The rate constant k,, for the complex formation varies between 3.5 x lo9 and 2.6 x loL0 M-I s-I 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-I 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 DBMBF? and the exciplex, on one hand, and between the latter and the triplex are 56.6 and 0.68 M-', respectively, when the concentration of p-xylene in cyclohexane is 3.3 M. Can. J. Chem. 73: 1902Chem. 73: -1913Chem. 73: (1995

Exciplexes or ground state complexes of (dibenzoylmethanato)boron difluoride and benzene derivatives? A study of their optical properties revisited via liquid state investigations and structure calculations

“…We can note, similarly to the literature data [2,3], a red shift of the maximum and broadening of the fluorescence band of DBMBF2 with increasing benzene concentration (see Fig.3). Similarly to the spectral variations in absorption, the red shift of the fluorescence band becomes significant when the molar fraction of benzene becomes higher than 0.08 (see Fig.4).…”

“…Because they did not observe the presence of any isobestic point in the absorption bands of DBMBF2 in the binary mixtures, to collect the fluorescence, Chow et al [2,3] have excited DBMBF2 either at 389 [3] or 366 nm [4], two wavelengths close to the position of the maximum of the lowest transition of the DBMBF2. As it can be seen from Figure 1, the intensity of this transition continuously varies upon increasing amount of benzene in the mixtures and therefore the number of photons absorbed by the solutions was not kept constant.…”

“…Thus, (dibenzoylmethanato)boron difluoride (DBMBF2) appeared to be a good candidate, since previous works reported its ability to form highly fluorescent exciplexes with monocyclic aromatic compounds in cyclohexane [2][3][4] or acetonitrile [5] solutions. Moreover, these exciplexes were found to display fluorescence spectra which shift to the red with increasing number of methyl groups on the benzene ring [2,3]. One could make use of these particularly interesting properties to obtain a good selectivity for a sensor.…”

“…In particular, we noted that, in previous works, very high concentrations of the aromatic compounds were needed to be able to detect the exciplexes. In cyclohexane solutions, the minimum amount of benzene compound to be added to the solution in order to detect the exciplex fluorescence is 0.094 mol.L -1 and corresponds to a molar ratio aromatic:DBMBF2 of 9400 [3]. With such concentrations, the aromatic compound becomes a co-solvent of cyclohexane and the properties of DBMBF2 correspond to those of DBMBF2 in binary mixtures.…”

“…Chemicals (Dibenzoylmethanato)boron difluoride was prepared according to the procedure described by Brown and Bladon [8] and re-crystallized three times from ether giving a yellow powder: mp: 194-196 °C. The compound was analyzed via proton NMR and 19 F NMR in CDCl 3 . All chemicals are of spectroscopic or analytical grade and are used without further purification.…”