Highly Emissive Diborylphenylene-Containing Bis(phenylethynyl)benzenes: Structure-Photophysical Property Correlations and Fluoride Ion Sensing

A series of 2,5-bis(dimesitylboryl)-1,4-bis(arylethynyl)benzenes 1-6 that contain various p-substituents on the terminal benzene rings, including NPh2 (1), OMe (2), Me (3), H (4), CF3 (5), and CN (6) groups, were synthesized, and the effects of the p-substituents on the absorption and fluorescence properties were investigated both in solution and in the solid state. Linear relationships were obtained not only between the Hammett sigma(+)(p) constants of the p-substituents and the absorption and fluorescence maxima, quantum yields, and excited-state dynamics parameters in solution, but also between the sigma(+)(p) constants and the fluorescence quantum yields in the solid state. An important finding extracted from these results is that the suppressed fluorescence quenching in the solid state is a common feature for the present laterally boryl-substituted pi-conjugated skeletons. Hence, the diborylphenylene can serve as a useful core unit to develop highly emissive organic solids. In fact, most of the derivatives showed more intense emission in the solid state than in solution. In addition to these studies, the titration experiment of 1 by the addition of nBu(4)NF was conducted, which showed the stepwise bindings of two fluoride ions with high association constants as well as a drastic change in the fluorescence spectra, while constantly maintaining high quantum yields (0.61-0.76), irrespective of the binding modes. This result also demonstrated the potential utility of the present molecules as an efficient fluorescent fluoride ion sensor.