| Mechanically Responsive Luminescence?MRL?materials,as an intelligent functional material,can change the color and intensity of their luminescence when exposed to external mechanical forces such as shear,grinding or hydrostatic pressure.This kind of materials have attracted widespread attention due to their potential appalication in mechanical sensors,memory,chips and security inks.However,conventional MRL materials are generally weakly luminescent at external pressure due to pressure-induced non-radiative transition enhancement such as intersystem crossing or internal conversion.Therefore,avoiding such a difficult pressure-induced quenching?PCQ?process is the key to obtaining an MRL material having excellent optical properties.In the past few years,pressure-induced emission enhancement?PIEE?phenomenon has been observed:some MRL materials emitted stronger under high pressure than under ambient conditions.This will greatly enhance the practical application value and possibility of such materials.But so far,only a few literature reports on the PIEE phenomenon,so the study of its mechanism is not comprehensive.In this paper,we used in-situ high pressure experiments to explore the anomalous pressure-responsive optical properties of planar carbazole crystals and propeller type1,2,3,4-tetraphenyl-1,3-cyclopentadiene crystals,2,3,4 5-tetraphenylthiophene crystals and 2,3,4,5-tetraphenylfuran crystals under hydrostatic pressure.The following innovative results have been achieved:First,we explored the pressure-responsive behavior of planar carbazole molecule.In-situ fluorescence spectroscopy data indicated that carbazole exhibited significant fluorescence enhancement in the high pressure range of 0.0-1.0 GPa,and its fluorescence intensity gradually decreased when the pressure exceeded 1.0 GPa.The in-situ high-pressure infrared data showed that the C-H stretching vibration band moved to the high frequency direction as the pressure increased throughout the compression process.An increasing of the C-H stretching vibration frequency meant that the non-radiative transition process was enhanced,that is,the radiation transition process was weakened.However,when the pressure exceeded 1.0 GPa,the shape of the??C-H?band changed significantly.This was due to a change of the complex C-H···?interaction,which means that the intermolecular interactions have changed.In-situ high-pressure synchrotron radiation data combined with Hirshfeld surface theory calculations showed that when compressed,molecules spontaneously adjusted their position to accommodate smaller volume changes.The close motion of molecules eventually led to an increase in intermolecular interactions.From crystallographic point of view,the molecules in the herringbone conformation became closer and the intermolecular interactions became stronger,which meant that the molecules rotated to a more parallel state.This state effectively promoted the intermolecular?...?stacking.Therefore,within 1.0 GPa,the?...?interaction was relatively weak,and when it exceeds 1.0 GPa,the?...?effect was significantly enhanced.However,the N-H stretching vibration appeared to move toward the low frequency and then to the high frequency.This suggested that the N-H bond was shortened as the pressure increased.Below 1.0 GPa,the N-H bond was elongated,indicating that the N-H···?hydrogen bond increased as the pressure increased.Stretching vibration of the N-H bond was inhibited by the N-H···?hydrogen bond,thereby suppressing the non-radiative vibration transition process.The increase in the radiation transition process ultimately led to the promotion of fluorescence emission of the carbazole crystal within 1.0 GPa.Over 1.0 GPa,a significant?...?stacking effect,eventually leading to fluorescence quenching.And along the change of intensity,the fluorescence peak position also showed a significant red shift.The in-situ synchrotron radiation data indicates that the rotation of the molecular plane and the decrease of the intermolecular distance are the main reasons for the red shift of the fluorescence peak position.Secondly,using in-situ fluorescence spectroscopy,we found that propeller-shaped1,2,3,4-tetraphenyl-1,3-cyclopentadiene molecule exhibited significant pressure-induced fluorescence enhancement and multi-color luminescence under high pressure.Significant fluorescence enhancement was observed over a pressure range of 1.0-10.3GPa,while in the other two pressure ranges,fluorescence emission decreased with increasing pressure.The maximum fluorescence emission intensity at 10.3 GPa was about 3.5 times higher than the maximum emission intensity at 1.0 GPa.And when compressed to 19.1 GPa,the fluorescence emission maximum exhibited a continuous and remarkable red shift of about 120 nm,and its crystal luminescence color gradually changed from blue to yellow.The in-situ high-pressure synchrotron radiation data combined with theoretical calculations showed that when the pressure reached 19.4GPa,the crystal structure experienced a large volume collapse of about 32%,indicating that the initial structure was relatively loose.In the initial stage of compression,the intermolecular distance was too large to form a new hydrogen bond.Therefore,the structure of the initial stage of compression was more relaxed than any other stage.As a result,a loose molecular arrangement would allow the phenyl ring of each molecule to rotate easily during the initial stages of compression.Since the rotation of the benzene ring increased the non-radiative vibration process,the fluorescence emission was reduced at the beginning of compression.When the pressure exceeded 1.0 GPa,the molecules became more and more dense so that the formation of new hydrogen bonds inhibited the rotation of the phenyl ring.That resulted in a weakening of the non-radiative transition,thereby enhancing the fluorescence intensity.Through the data of the change of unit cell parameters with pressure,we also found that the crystal exhibits anisotropic compression behavior,in which the b-axis was the most easily compressed.This indicated that twisted molecules minght tend to be flattened.This trend was conducive to the redshift.What was even more exciting was that when the pressure exceeds 24.7 GPa,a new peak appears at 2920 cm-1 in the infrared spectrum of the sample after releasing pressure,which indicated that the phenyl ring has experienced a ring opening reaction.And as the highest pressure increased,this peak became stronger and stronger,which meant that the reaction degree was promoted,eventually leading to irreversible color change of the sample,and the color of the sample released from 35.9GPa was redder than the color of the sample released from 24.7 GPa.In-situ high-pressure synchrotron radiation data,high-resolution transmission electron microscopy images combined with 1H-NMR results confirmed that the black sample released from35.9 GPa was an amorphous product composed of unsaturated alkenes and saturated linear alkanes.Finally,we studied the high-pressure optical properties of 2,3,4,5-tetraphenylthiophene and 2,3,4,5-tetraphenylfuran which were samilar to the propeller-shaped TPC molecule.The experimental results suggested that the fluorescence intensity of 2,3,4,5-tetraphenylthiophene crystals decreased with increasing pressure in the pressure range of 0.00 GPa to 0.98 GPa.A significant PIEE phenomenon was exhibited at the pressure range of 0.98 GPa to 5.71 GPa.The emission intensity gradually decreased when pressure was beyond 5.71 GPa.Theoretical calculations showed that in the initial stage of compression,the molecular arrangement was so loose that the phenyl ring was easier to rotat.That resulted in enhancement of non-radiative transition,and then reduced fluorescence emission.When further compressed,the molecules was closely packed and new hydrogen bonds were formed between the molecules.The formation of new hydrogen bonds limited the rotation of the phenyl ring,which promoted the fluorescence emission.When the pressure increased,the intermolecular?...?interaction was enhanced,resulting in a decrease in fluorescence.However,the fluorescence intensity of 2,3,4,5-tetraphenylfuran crystals decreased with increasing pressure,and no PIEE phenomenon occurred.The in-situ high-pressure synchrotron radiation data combined with theoretical calculations showed that the?...?stacking in the 2,3,4,5-tetraphenylfuran crystals remarkably increased under high pressure,resulting in weaker emission.By comparing the molecular arrangement of the two crystals,we found that the arrangement of the molecular pairs was closely related to its electroluminescence behavior.The in-situ high-pressure infrared data indicated that when further compression,a new peak appears at 2920 cm-1 in the infrared spectrum of the sample after depressurization,which showed that the phenyl ring underwent a ring-opening reaction.That eventually led to an irreversible color change of sample.This study not only gained deeply understanding of the relationship between optical properties and structural evolution,but also proposed strategies for designing PIEE materials from the perspective of molecular arrangement. |
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