Studies On The Stimuli-Responsive Properties Of Dynamic Molecular Crystals Based On Indole Derivatives

Organic crystalline materials have recently garnered significant attention from researchers due to their low cost,easily tunable chemical structures,high charge carrier mobility,long-range structural order,and low defect rates.These materials are now widely used in the field of organic optoelectronics.However,the brittleness of traditional organic crystalline materials leads to fractures under external stress,hindering their normal optoelectronic functions.Overcoming the brittleness of organic crystalline materials is a major challenge in this field.Fortunately,significant progress has been made in recent years,with researchers successfully synthesizing numerous flexible organic crystals and extensively studying the relationship between crystal structure and mechanical properties.They have proposed potential mechanisms for elastic/plastic deformation of crystals.This milestone discovery has injected new vitality into the research of organic crystalline materials and propelled them to the next stage of development.With the emergence of flexible organic crystals,these materials have demonstrated high application value in traditional fields,such as flexible organic lasers,flexible organic resonators,and flexible organic light transmission,achieving remarkable results.However,the stimulus-responsive conditions for flexible organic crystals are currently limited,and the mechanisms of some reported stimulus response property are not sufficiently clear or comprehensive.Additionally,it is challenging to combine the stimulus-responsive properties of flexible organic crystals with their light transmission properties.Therefore,our research focuses on previously unexplored areas of organic crystalline materials,leveraging the known properties of these materials to further expand their applications.To this end,we have successfully designed and synthesized various flexible organic crystalline materials based on indole rings as theπ-conjugated structure,thoroughly investigated their stimulus-responsive properties and explored their potential applications in sensing and light transmission.This work not only provides new ideas and methods for the development of organic crystalline materials but also demonstrates their application prospects in emerging fields such as sensing and light transmission.The specific content is as follows:In Chapter 2,we synthesized an indole-based chalcone compound and prepared high-quality,centimeter-scale needle-like single crystals of MMIAB.This crystal exhibits both elastic bending and plastic twisting and shows a fluorescence blue-shift under tensile stress.By measuring the fluorescence emission spectra of the crystal under different tensile strains,a clear linear relationship between the maximum fluorescence emission wavelength and tensile strain was found.This phenomenon of tensile-induced fluorescence blue-shift is repeatable as long as the crystal is not fractured.The maximum tensile strain of the crystal is 3.56%,with a fluorescence blue-shift of 17 nm compared to the initial state.X-ray diffraction data and energy framework analysis indicate that intermolecularπ···πinteractions,interlayer C-H···O interactions,and C-H···πinteractions are crucial for the crystal’s elasticity,while slip planes perpendicular to the growth direction enable plastic twisting.Theoretical calculations show a significant increase in vertical emission energy in the stretched state compared to the initial state,which rationalizes the observed fluorescence blue-shift in the stretched crystal.This work provides new theoretical and experimental support for studying the tensile response of organic crystals,and pave the way for applications in tensile sensing.In Chapter 3,we synthesized an indole-based chalcone compound and prepared large needle-like crystals of BPIPO.These crystals exhibit excellent elastic bending capabilities at both room temperature and liquid nitrogen conditions,the crystals emit green fluorescence at room temperature and orange fluorescence in liquid nitrogen.Variable-temperature fluorescence emission spectra show a red-shift of approximately40 nm as the temperature decreases from 277 K to 77 K,with a linear relationship between changes in maximum emission wavelength,fluorescence intensity and temperature.Additionally,the crystals possess excellent light transmission capabilities at both room temperature and low temperature,with optical loss coefficients of 0.16d B/mm and 0.17 d B/mm,respectively.Combining the active optical waveguide properties of the crystal with its temperature-dependent fluorescence characteristics makes it a potential optical waveguide temperature-sensing medium.Local low-temperature optical waveguide experiments demonstrate that BPIPO crystals can transmit the temperature of excited position from one end to the other end without being affected by the temperature along the propagation path.TDDFT calculations and single-crystal X-ray diffraction data analysis reveal that lower temperatures favor S₁state transitions and reduce intermolecularπ···πinteraction distances,leading to fluorescence red-shift.The rich intermolecular interactions retained at low temperatures provide the structural basis for low-temperature elasticity.This work offers new ideas for the development of temperature-sensitive optical devices.In Chapter 4,we synthesized an indole-based acrylonitrile compound and prepared needle-like single crystals of MMTPA.These crystals exhibit plastic deformation when subjected to external forces exceeding their elastic limit.When the ambient temperature of the crystal rises to 343 K,some crystals undergo bending,rolling,or jumping.When the temperature drops from higher to 330 K,the crystals exhibit the same mechanical motion,indicating that MMTPA crystals have thermosalient properties.Statistical analysis of the thermal actuation response types of some crystals with different sizes shows that thicker and longer crystals are more prone to mechanical motion,and bending deformation is the most likely among the three types of mechanical motions.DSC curves and variable-temperature X-ray powder diffraction pattern demonstrate that the thermosalient properties is due to reversible phase transitions.Variable-temperature single-crystal X-ray diffraction data show anisotropic changes in crystal axis lengths near the phase transition temperature,accompanied by sharp increases inα,β,andγangles,indicating that significant unit cell deformation is the intrinsic mechanism for thermosalient property.In addition to the thermosalient properties,MMTPA crystals also exhibit excellent light transmission performance,which remain even after plastic deformation and phase transitions.Combining thermosalient property and light conversion functions,we developed a thermally-induced optical waveguide switch function for molecular crystals,providing a new material system for integrated circuit devices,particularly temperature alarm systems.In Chapter 5,we successfully synthesized an indole-based hydrazone compound,CMHP,and obtained two crystal phases,needle-like Cry-G and block-like Cry-O.These two phases show significant differences in shape and mechanical properties,with the needle-like phase exhibiting plasticity under stress beyond the elastic limit,while the block-like phase is brittle.X-ray single-crystal diffraction data reveal structural reasons for the different mechanical properties of the two crystal phases.The compound CMHP exhibits acid stimulus-responsive properties,with significant fluorescence red-shift in hydrochloric acid vapor.Protonation process after acid stimulation was confirmed by nuclear magnetic resonance（NMR）spectroscopy.Theoretical calculations explained the mechanism of spectral red-shift after acid stimulation.Finally,we designed several optical applications for each aggregation of the compound,including erasable information encryption storage,unidirectional information transmission and two-dimensional information storage,demonstrating the substantial potential of CMHP in information storage and transmission.In summary,we systematically designed and synthesized a series of indole derivatives with various stimulus-responsive properties by modifying theirπ-conjugated structures from both molecular and supramolecular levels.Additionally,we developed new applications for organic crystalline materials by integrating different stimulus-responsive properties with their inherent excellent optical properties.By deeply understanding the structures and properties of these crystals,we not only explored the relationship between the properties and structures of flexible organic crystalline materials but also demonstrated their application prospects in information storage,sensing,and light transmission.This series of studies provide new insights into the future research and innovative applications of organic crystalline materials.