| Organic pressure-induced emission enhancement（PIEE）materials have valuable potential applications in various fields such as sensors,security inks,and anti-counterfeiting materials due to their unique pressure sensitivity,rich colors,flexible molecular design,and diversity.At present,most of the research on organic PIEE materials are based on the aggregation-induced emission（AIE）and their restriction of intramolecular motion（RIM）mechanism,which have not yet formed a complete research system,and need to be further investigated.In the early exploration of this thesis,we found that there is no corresponding relationship between AIE material and PIEE material,that is,an AIE material does not necessarily have a PIEE phenomenon;also,a PIEE material may have no AIE phenomenon.Meanwhile,researchers are also actively seeking ways to preserve the enhanced emission down to ambient pressure.Therefore,new organic PIEE chromophore and new PIEE mechanisms need to be further explored and discovered urgently,which can not only greatly improve the possibility and the practical application value of new PIEE materials,but also provide new ideas for the development of new organic luminescent materials.The fundamental factor affecting the fluorescence properties of materials is chemical bonds and intermolecular interactions,which determine various properties such as molecular conformation and crystal structure.The chemical bonds are determined at the beginning of the material design and synthesis.Therefore,it is crucial to explore the influence of pressure-regulated intermolecular interactions on materials’fluorescence property.In this way,we can explore new organic PIEE materials and mechanisms,and find ways to preserve the excellent high pressure fluorescence property to ambient pressure.Herein,the PIEE and mechanisms of salicylic acid（SA）,dibenzo[b,d]thiophene 5,5-dioxide（DBTS）,1,2,3,4-tetra（phenyl）naphthalene（TPN）and 1,6-diphenyl-1,3,5-hexatriene（DPH）were investigated in order of the strength of the intermolecular interactions by adopting a high pressure regulation strategy,and the following results were achieved:Firstly,the emission enhancement caused by the pressure-induced isomerization of SA crystals was achieved by regulating intramolecular and intermolecular O-H...O hydrogen bonds under high pressure.Before 4 GPa,the rotation process of carboxyl group induced energy dissipation,leading to the emission intensity decreased.At the pressure range of 4-12 GPa,the Rotamer II of SA resulted in the inhibition of the excited state intramolecular proton transfer（ESIPT）process and the keto-emission gradually disappeared.At the same time,the strengthened intermolecular O-H...O hydrogen bonds led to the restriction of O-H vibration that attenuated the non-radiative decay,which resulted in enol-emission enhancement.Enol-emission also had a red shift,which is attributed to the strong intermolecular coupling that promotes the formation of resonance dimers and reduces the energy of the lowest excited state.After 12 GPa,the shorter intermolecular distance increased the degree of electron orbital overlap that induced increased non-radiative energy transfer,leading to fluorescence quenching.Further,through pressure regulated C-H...O=S weak hydrogen bonds experiments in DBTS crystals,it was found that lower-strength intermolecular hydrogen bonds can also have a significant impact on the structure and fluorescence properties of materials under high pressure,and a new PIEE mechanism of"pressure-restricted chemical bond vibration"was proposed.For DBTS chromophore,the fluorescence emission intensity gradually increased after applying pressure to 6 GPa by DAC.The emission enhancement was mainly caused by the restriction of the deformation vibrations of the C-H bonds through the intermolecular C-H...O=S hydrogen bonds interactions.After applying pressure more than 6 GPa,the enhancedπ-πinteraction and non-radiative vibration together dominate the fluorescence quenching.Throughout the compression process,the emission of DBTS crystals undergoes a continuous red-shift,which is mainly caused by the reduction of the energy of the lowest excited state caused byπ-πinteraction.In addition,the emission wavelength of the DBTS crystal changed linearly with the pressure under high pressure.The cyclic stability tests proved that the DBST crystal have practical value as a pressure sensor.Taking the TPN as research object,the preservation of enhanced emission was achieved for the first time in organic materials by high pressure regulation of intermolecular interactions.The crystal with parallel-packed structure（TPN-P）produced a 50-time emission enhancement at 4 GPa,and a new white fluorescence with18-time enhancement after decompressed from 18 GPa.The crystal with orthogonal-packed structure（TPN-C）had 6 times emission enhancement at 3 GPa and the bright blue fluorescence keep 3 times enhancement after decompressed from 10 GPa.Research showed that:TPN-P had a loose parallel packing mode,during 3-4 GPa,a large number of new generated C-H...C interactions that restricted the non-radiative motion of phenyl rings A and D were the key to the 50-time emission enhancement of TPN-P crystals.After decompression,the strongπ-πinteractions generated by phenyl rings under high pressure make the molecules maintained a stable planar conformation and molecular rigidity,which was the main reason of the 18-time enhanced white light.Since TPN-C had tight orthogonal packing mode,the gain effect on the restriction of the motion of the phenyl rings was limited,thus only producing 6-time emission enhancement within 0-3 GPa.After decompression from 10 GPa,the crystal structure of TPN-C is similar to that at 1 GPa,resulting in emission intensity improved 3 times compared to the initial.Continuing to reduce the strength of intermolecular interactions and regulated the C...H/H...C and H...H interactions by high pressure,a new PIEE mechanism of"pressure-induced allowed transition"was proposed,also confirmed that weak interactions in organic materials at high pressure can lead to novel mechanical responses in crystalline structure.Under anisotropic grinding,the crystallinity of DPH decreased,which made a fluorescence blue-shift,similar to that of single-molecule state.Under hydrostatic pressure,when the hydrostatic pressure was relatively low,DPH was subjected to different intermolecular interactions along different lattice axes,resulted in molecular symmetry of DPH decreased,which made the probability of the S0→S1transition caused by the asymmetric vibration gradually increasing.As a result,DPH produced a 13 times emission enhancement at hydrostatic pressure of 0-4 GPa.Red-shift and reduced emission intensity were mainly caused byπ-πinteractions and the energy dissipation by nonradiative vibrations,respectively.With increased pressure,high-pressure-stiffened H...C/C...H and H...H interactions,guided by the intra-layer W-shape packing mode,enable DPH molecules to generate negative linear compressibility（NLC）mechanical responses along the b-axis in the range of 9-15 GPa.These findings provide important reference for the development of new luminescent materials and NLC materials. |
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