Effects of substitution position with different charge characteristics groups on mechanofluorochromic behaviors of the pyridine-based difluoroboron compounds

Abstract

Two donor-acceptor type pyridine-based difluoroboron compounds (3F-Py-TPA BF2 and TPA-Py-3F BF2) were designed and successfully synthesized, which including triphenylamine unit acted as the electron donor and difluoroboron moiety and 3,4,5-trifluorophenyl group served as the electron acceptors. Combined with the results of the photophysical properties, it was found that compound 3F-Py-TPA BF2 with 4-position of pyridine skeleton substituted by 3,4,5-trifluorophenyl unit and triphenylamine unit location at the end of the bridged phenyl displayed more excellent intramolecular charge transfer character and solvatochromic effect. Moreover, the solid emission spectrum of 3F-Py-TPA BF2 was red-shifted from 550 to 594 nm, that of TPA-Py-3F BF2 was red-shifted from 526 to 546 nm under external mechanical stimulation, which caused by phase transformation between the ordered crystalline and disordered amorphous states according to the powder X-ray diffraction measurements. Based on the results of theoretical calculations and X-ray single-crystal diffraction analysis, we speculated that TPA-Py-3F BF2 with 4-position of pyridine skeleton substituted by triphenylamine unit and 3,4,5-trifluorophenyl unit location at the end of the bridged phenyl had higher molecular polarity and too loose molecular packing mode, resulting in the mechanofluorochromic behavior was worser than that of 3F-Py-TPA BF2. This work presented a strategy for regulating mechanofluorochromic behaviors of the pyridine-based difluoroboron molecular materials.