| In recent years,organic light-emitting materials have become the focus of attention and research focus of scientists due to their wide application and huge development potential in human production and life.Among them,some organic light-emitting materials are very sensitive to external mechanical stimulation.Under the action of stretching,grinding or hydrostatic pressure,their absorption and light-emitting color will change significantly.Therefore,they have broad application prospects in pressure sensors,information storage,light-emitting devices and other fields.However,under high pressure,due to the dense packing effect between molecular aggregates and the increasing ?-? interaction between aromatic rings,the emission intensity of most organic light-emitting materials is gradually weakened,thus losing the original piezochromism performance.Therefore,avoiding the fluorescence quenching effect under high pressure is the key to obtain good pressure-response organic light-emitting materials.In previous studies,scientists have observed a novel phenomenon of pressure-induced emission enhancement(PIEE): some organic light-emitting materials can emit stronger fluorescence under high pressure than at ambient conditions.But so far,only a few literatures have reported the PIEE phenomenon of organic light-emitting materials.Therefore,a more in-depth study of the PIEE phenomenon and further exploration of its mechanism are not only helpful for understanding the basic photophysical processes,but also provide a reference for the design and synthesis of highly efficient luminescent materials.In this paper,we use the diamond anvil cell(DAC)technology combined with in-situ high-pressure measurements methods to explore the PIEE characteristics and piezochromic behavior of triphenylethylene(Tri PE)under high pressure.Pressure is a basic physical parameter,increasing pressure can effectively shorten the distance between adjacent atoms,the degree of orbital overlap of adjacent electrons has increased,and have discovered many new materials and laws that have never existed under atmospheric pressure,new theories have been formed,which can greatly expand the research space of material science.In this paper,we systematically study the evolution of the optical properties under high pressure of triphenylethylene crystals by in-situ high-pressure fluorescence spectra,in-situ high-pressure UV-visible absorption spectra,in-situ high-pressure angle-dispersive X-ray diffraction(ADXRD)experiments,in-situ high-pressure Raman spectroscopy and in-situ high-pressure infrared(IR)spectroscopy.The experimental results show that triphenylethylene crystals show obvious emission enhancement in the pressure range of 0.0-0.8 GPa,the fluorescence spectrum and absorption spectrum show red shift,and there is piezochromic behavior upon compression.In-situ high pressure IR spectroscopy and ADXRD experimental analysis demonstrated that the PIEE of Tri PE was attributed to the enhancement of intermolecular C-H···? and C-H···C hydrogen bonds under high pressure,suppressed the rotational vibration of the benzene ring,thereby reducing the energy loss of non-radiative transitions and improving the fluorescence emission efficiency.When the pressure exceeds 0.8 GPa,the fluorescence intensity decreases and the crystallinity gradually decreases.The pressure-induced conformational planarization was the main cause of the red shift phenomena of the absorption emission peak.It is worth noting that infrared spectroscopy analysis shows that when the pressure exceeds 14.2 GPa,the benzene ring in the triphenylethylene molecule will undergo a ring-opening reaction,resulting in irreversible piezochromic behavior.Our research reveals the optical response mechanism of triphenylethylene crystals under high pressure,and the mechanism of fluorescence enhancement and piezochromism is described from the molecular interaction and molecular microscopic movements,which provides guidance for the design and synthesis of high-performance PIEE materials in the future. |
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