| The cyano group is formed by the interconnection of a carbon atom and a nitrogen atom through a triple bond.It is a strong electron-withdrawing group,which can participate in not only the formation of hydrogen bonds and the preassembly of supramolecular systems,but also the formation of intramolecular charge-transfer(ICT)inducing dual fluorescence.Moreover,the research of polymerization of cyano groups has attracted much attention,and polymers of nitriles have potential applications in conductive materials,superhard materials and other areas.The increase of the pressure can effectively improve the density of the system,change the intermolecular interaction,affect the overlap of electronic clouds,improve the energy of the system and reduce the stability of the system,leading to changes in the molecular conformation,which can make the abrupt structural changes and chemical reactions become possible.Therefore,the structural and chemical properties of organic-molecule crystals especially in systems containing cyano groups under high pressure are likely to change significantly.Based on the effects of the polymerization,preassembly,molecular conformational stability and ICT involved in cyano groups,cyanamide,4-aminobenzonitrile,succinonitrile and 4-(N,N-dimethylamino)benzonitrile(DMABN)were selected to be studied in this paper.We investigated the high-pressure chemical reactions,structural stability,molecular conformation and fluorescent properties of these samples,and obtained the following innovative results:We studied the chemical reaction of the cyanamide crystal under high pressure by a diamond anvil cell(DAC)combining with in situ high-pressure Raman and synchrotron angle-dispersive X-ray diffraction(ADXRD)up to 21.7 GPa.The abrupt changes in the Raman spectra provided convincing evidence for the occurrence of oligomerization at about 13.3 GPa.In addition,the chemical reaction broke the original arrangement of hydrogen bonds and changed the crystal of cyanamide into a disordered state,which is consistent with the discontinuity of ADXRD spectra under pressure.The Raman,NMR,and UV-visible absorption results of the high-pressure products illustrated that C=N bonds formed exactly,and the oligomerization was irreversible.Moreover,the solubility of the high-pressure products confirmed that the recovered samples were oligomers.The subsequent discussions of reaction kinetics indicated that elevating pressure increased the reaction rate.The observed changes of the activation volume values with pressure are attributed to the effects of the steric hindrance on the system.In addition,based on the Raman spectra of the products and the ADXRD results,we studied the reactive sites under different pressures.From 4.5 GPa to 11.8 GPa,the C-C and C-N bonds coexisted in the products and the formation of C-N bonds occupied the main part of the reactions below 7.9 GPa,because the close C···N distance made C and N atoms of adjacent C?N groups act as the most likely reactive sites.When the value of pressure was elevated,the amount of C-C bonds increased and the amount of C-N bonds gradually decreased in the products.Meanwhile the C···C distance of adjacent C?N groups is more compressible and it has a tendency for these C atoms to become the closest reactive sites above 7.9 GPa.These results indicate that the application of pressure can control the reaction rate and reaction sites,which can affect the process of the reaction.So we believe that pressure is an important factor in tuning polymerization reactions and even other chemical reactions in the future.Then we studied the crystal structure,the effect of N-H···N and the molecular arrangement of 4-aminobenzonitrile under high pressure.A DAC combining the in situ high-pressure Raman and ADXRD experimental apparatus were used.It was observed that both the crystal and the molecular structure of 4-aminobenzonitrile shrank during the compression process and the N-H···N hydrogen bonds between molecules were distorted.When the pressure was higher than 0.2 GPa,the crystal structure of 4-aminobenzonitrile underwent a structural transformation,the crystal symmetry decreased,the molecular arrangement changed slightly and the molecular groups shrank discontinuously.The N-H···N hydrogen bonds and amino groups were suddenly distorted at the same time.Because of the existence of the benzene ring and the geometry of the molecular arrangement,the increased steric hindrance led to the absence of cyano polymerization in the pressure range below 20.3 GPa.In addition,when the pressure was released,we found that this high pressure phase transition was reversible.These results indicate that studying the changes of the crystal structure and the hydrogen-bonded networks of 4-aminobenzonitrile crystals under high pressure is important to explore the nature of hydrogen bonds and understand the role of hydrogen bonds in organic crystal high-pressure phase transitions.In addition,we tried to explore the effect of high pressure on the conformational equilibrium and chemical stability of a non-preassembled organic molecular plastic material—succinonitrile within 24 GPa.High-pressure Raman and ADXRD spectra have shown two phase transitions.The plastic phase(phase I)changed to be a crystal phase(phase II)when the pressure exceeded 0.7 GPa.The phase transition corresponded to an orientational disorder-order transition.The original equilibrium of the molecular conformation was broken.The initial coexistence of the gauche and trans conformers transferred to the existence of only trans conformers in phase II.This phenomenon is exactly opposite to the reported low-temperature phase transition of succinonitrile,because phase II had centrosymmetric achiral conformers(trans conformers)to accommodate the higher density.Phase II then changed into another crystal phase(phase III)at 2.9 GPa.The succinonitrile molecules were rearranged and changed to denser trans conformers in the crystal structure of phase III.All the observed transitions were completely reversible upon release to ambient conditions.Moreover,phase III was stable up to 24 GPa without the occurrence of polymerization.This result demonstrates that unsaturated molecules that have not been preassembled are difficult to react under compression.Moreover,pressure is supposed to be an efficient factor in obtaining sole conformation from the conformational mixture.The molecular behaviors of succinonitrile under compression may elucidate the conformational equilibrium of organic molecules.Finally,we studied the molecular conformation and dual fluorescence of the typical molecule—DMABN in the crystal state under high pressure.The quenching of the locally excited(LE)emission band and the enhancement of the intramolecular charge-transfer(ICT)emission band under external pressure stimuli were observed.These variations are related to the decrease of the dihedral angle between the dimethylamino(NMe2)and phenyl moieties(i.e.the planarity of the molecular conformation).Meanwhile the slight rotation of NMe2 can have similar effects on the ICT emission band.The calculated results indicate that S1 state dominates ICT emission and S2 state dominates LE emission.The excitation energy of S1 state and S2 state reduced in the more planar conformation,which is corresponding with the redshifting emission under high pressure.More importantly,the increased oscillator strength of ICT state(S1)is also responsible for increased radiative transition rate of the ICT emission band.The reduction of the HOMO-LUMO band gap is also consistent with the redshifting UV-visible absorption edge of DMABN.We anticipate that the conformational planarity and the rotation of donor groups can switch the excited state,which will inspire the development of a new class of piezochromic luminescent materials. |
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