Linking triptycene to silole: a fruitful association

Abstract

1,1′-Disubstituted-2,3,4,5-tetraphenylsiloles incorporating triptycene moieties Tp-DMTPS and Tp-MPTPS were prepared and characterized. Like other 2,3,4,5-tetraphenylsiloles, both compounds exhibit aggregation-induced emission (AIE) behavior. Unexpectedly, the solid state PL emission of Tp-DMTPS show dependence according to the degree of crystallinity of the powders. While precipitates display a blue-green emission, microcrystalline powders emit in the yellow-orange region. Moreover a rare reversible mechanohypsochromic phenomenom is observed: upon grinding, the emission of a micro-crystalline powder changes from yellow-orange to blue-green, the original emission being restored by annealing the sample. The mechanism of this blue-shift upon crystal grinding was investigated by X-ray diffraction, ¹³C CP-MAS NMR and DFT calculations. Under mechanical stress, strong intermolecular CH/π interactions are found to promote the less energetically stable population of conformers present in the crystals, leading to this blue-shift. Finally the influence of the triptycene moieties on the thermal and structural stability of Tp-DMTPS and Tp-MPTPS was studied. Compared with DMTPS and MPTPS parents, a general enhancement of the properties is noticed, especially when used as active layer in OLED fabrication. Our results suggest that the attachment of triptycene moieties to π-conjugated molecules is an interesting way to improve their thermal, structural and photophysical properties for sensing or optoelectronic applications.