Research On Emission Features Of Tetra(4-Nitrophenyl) Ethylene And Di(P-Methoxylphenyl) Dibenzofulvene Under High Pressure

Solid-state luminescent materials have gained extensive attention because of their promising applications in anti-counterfeiting technology,fluorescent probes,sensors,and biological science.In this paper,by conducting a series of high-pressure photophysical experiments such as high-pressure synchrotron X-ray diffraction,infrared spectrum,ultraviolet-visible absorption spectrum,and fluorescence spectrum,we have explored the pressure-induced evolution of the aggregation microenvironment,which in turn can affect the molecular conformation,stacking mode,and intermolecular interaction of solid-state luminescent materials C₂₆H₁₆N₄O₈（TPE-4N）and C₂₈H₂₂O₂（FTPE）.The results are as follows:1.By conducting the high-pressure synchrotron X-ray diffraction,infrared spectrum,ultraviolet-visible absorption,and fluorescence spectrum,we have studied the regulatory effects of pressure on the molecular conformation,structure,stacking mode,intermolecular interaction,and emission behavior of TPE-4N.In-situ high-pressure angle-dispersive X-ray diffraction and infrared spectrum studies indicated that pressure induced reduction in the unit cell of TPE-4N between 0 GPa and 6.3 GPa(0 GPa≈1.01×10^-4 GPa).The diffraction peaks became wider and shifted to high angles,which indicated that pressure induced the collapse in the unit cell volume and change of crystallinity.At pressures larger than 6.3 GPa,the diffraction peaks continued to became wider until the they overlap with each other,suggesting that the amorphous phase was dominant.The results of in-situ infrared spectrum indicated that pressure has damaged the porous structure of hydrogen-bonded organic frameworks of TPE-4N,resulting in the reduction of the hydrogen bonds.In contrast,the weakening of hydrogen bond resulted in the enhancement of the C-H stretching vibration.Simultaneously,the refinement results suggested that the compression of TPE-4N was anisotropic,with the compression ratio of a-and b-axis higher than c-axis.Combined with the molecular stacking of TPE-4N,we inferred that the pressure parallelized the molecular conformation,so that,the UV-vis absorption spectrum showed a red shift under pressure stimulation.In-situ high-pressure fluorescence spectrum shown that the intensity increased with pressure below 6.8 GPa,which was due to the pressure-induced collapse in the unit cell volume restricted the molecular movement.So that the energy was released in the form of radiative transition,resulting in the increase in the emission intensity.However,the intensity decreased with pressure above 6.8 GPa,perhaps due to the π–π interactions,the formation of excimers and exciplexes,or an amorphous phase.2.By conducting the high-pressure synchrotron X-ray diffraction,infrared spectrum,and ultraviolet-visible absorption,we have studied the changes of the molecular conformation,stacking mode,and intermolecular interaction of FTPE under high pressure.Besides,under multiple excitation channels,three distinctemission behaviors were observed corresponding to the pressure-induced structural evolution.In-situ high-pressure angle-dispersive X-ray diffraction,infrared spectrum,and ultraviolet-visible absorption studies indicated that there was an isostructural phase transition of FTPE about 5 GPa.The molecular conformation did not change obviously when the pressure was lower than 5 GPa.The applied pressure mainly caused the shortening of the intermolecular distance,leading to a rapid cell volume shrinkage.When the pressure was higher than 5 GPa,FTPE crystal was compressed to a certain extent,there was insufficient space for the molecules to maintain the prevailing molecular conformation.As a result,the molecular conformation changed largely,inducing a planar adjustment of the crystalline structure and tight molecular stacking,thereby leading to the enhancement of intermolecular interactions and even the occurrence of the isostructural phase transformation.At pressures larger than 13.0 GPa,the diffraction peaks became wider owing to diffuse scattering,suggesting that the amorphous phase was dominant.In-situ high pressure fluorescence measurement results of FTPE with three different laser excitation wavelengths（355 nm,532 nm,and633 nm）as excitation sources showed that three distinct emission behaviors under high pressure were observed by selecting three different laser excitation wavelengths.With increasing pressure,the emission obtained under 355 nm laser excitation decreased in intensity,and there was a red shift of the emission maximum.Further increase in pressure led to a planar conformation,resulting in an increase in intramolecular conjugation.The intensity of the emission under 532 nm laser excitation increased slowly during low pressure range.When the pressure was higher than 5 GPa（the isostructural phase transition）,there was a significant enhancement of emission intensity.Combined with the structure evolution,it might be inferred that the molecules that were gradually turning planar might have been excited by the 532 nm laser.There was no emission or weak emission with a constant pressure-dependent intensity below5.3 GPa under 633 nm laser excitation.After the isostructural phase transition,the intensity of the emission increased gradually.And,there was a higher rate of the emission intensity increase after 12.9 GPa.Combined with the pressure–structure data,it could be inferred that the fluorescence observed under the 633 nm laser excitation might be due to the amorphous phase generated under the compression force.