Polymorphic triphenylamine derivative showing aggregation-induced emission and high-contrast mechanical force-induced blue shift

Abstract

Mechanical force-induced, high-contrast blue-shift luminescent mechanochromic (MC) molecules have rarely been reported. Herein, we designed and synthesized an AIE luminogens (AIEgens) triphenylamine derivative N, N-diphenyl-4-(6-phenyl-[2,2′-bipyridin]-4-yl)aniline (DPTPA). Three different emission wavelengths from blue crystals (B-DPTPA), sky-blue crystals (S-DPTPA), and green crystals (G-DPTPA) were obtained by culturing the crystals under different crystallization conditions. All three crystal polymorphs showed high solid luminescent efficiency, each of which was sensitive to external pressure and was accompanied by significant fluorescence blue shift. Under a mechanical force, B-DPTPA shows a 12 nm blue shift, S-DPTPA fluorescence color changed from sky-blue to blue with an increase emission and G-DPTPA changed its color from green (510 nm) to sky-blue (428 nm) with an 82 nm blue shift. Such obvious color change and large blue shift were rarely been reported. Characterization of the photophysical properties, theoretical calculations, hydrostatic pressure, powder X-ray diffraction and X-ray single crystal structures showed that three crystal polymorphs of DPTPA originated from single molecules, space interactions, molecular pairs (dimers), and the blue shift of B-DPTPA, S-DPTPA, G-DPTPA were ascribed to the solid-state morphology transition from crystal to amorphous due to the destruction of space interactions and dimers. This study provides the relationship between the molecular packings and the blue shift emission of AIEgens materials under mechanical forces is understood in the molecular level.