Spectroscopic Anomalies in the 4-Aryl-2,2-Difluoro-6-Methyl-1,3,2-Dioxaborine Series

The spectroscopic properties of 4-methyl-6-phenyl-2,2-difluoro-1,3,2-dioxaborine 1 and of several of its derivatives 2–5 substituted in para position on the phenyl ring or bearing an alkylene chain between their carbo-cyclic and heterocyclic rings have been studied in solution of different polarities and in the solid state. In solution, an important intramolecular charge transfer (ICT) process is characterized in the excited state. This charge transfer occurs in a planar configuration of the molecules, as demonstrated by comparison of the data obtained for 1 and 3 with those of the rigid molecules 6 and 7, respectively. The singlet excited state of molecules 1–5 is typical of species having a free rotating substituent and for which the small fluorescence quantum yield and lifetime increase when a planar configuration can be stabilized by electron-donor substitution. The low fluorescence quantum yield found for molecules 1 and 2, with respect to molecules 3–5, has been attributed to an important deactivation channel resulting from the rapid free rotation of the phenyl moiety of the molecule and not to an efficient intersystem crossing to the triplet excited state, as this process has a quantum yield lower than 0.01. Theoretical calculations lead to a low oscillator strength for molecules 1 and 2 which are more stable with a planar configuration, while a significant higher oscillator strength is calculated for compounds 3 and 4 substituted with electron-donating groups. The singlet excited state of compounds 4 and 5 might be stabilized by the formation of a twisted species. In the solid state, the molecules are shown by X-ray analysis to be planar and packed in such a way that two molecules lie head-to-tail in two parallel planes separated from each other by ca. 360 pm. As a result, this proximity enables the development of a ground state charge transfer. This charge transfer species has been confirmed by increasing the concentration of compound 2 in dichloromethane solution, giving rise to a new fluorescence band in the long wavelength region.