Organic schiff base crystal material with adjustable luminescence at low temperature and preparation method thereof

Abstract

The application belongs to the technical field of optical functional materials, and provides a low-temperature organic Schiff base crystal material with adjustable light emission and a preparation method thereof. The application adopts a naphthyl Schiff base core structure and a halogen substituent group precise regulation strategy, realizes wide-range and reversible spectral regulation from blue shift (-30 nm) to red shift (+92 nm). On this basis, the prepared crystal has excellent mechanical flexibility and elasticity in a deep low-temperature environment, and the composite film also exhibits stable light emission performance, effectively solving the core material problem of flexible low-temperature photoelectric devices. Moreover, the crystal preparation route is simple, does not need complex catalysts or inert protection, and is low in cost, and has a significant industrialization prospect. Finally, with the unique low-temperature adjustable light emission and flexible integration performance, the series of crystal materials prepared by the application have broad application potential in the fields of flexible low-temperature optical sensing, intelligent identification and display, optical anti-counterfeiting and encryption and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical functional materials, and particularly relates to a low-temperature organic Schiff base crystal material capable of adjustable light emission and a preparation method thereof. BACKGROUND

[0002] Low-temperature optical functional materials are an important research direction in current material science and optoelectronic technology, and are widely used in fields such as cryogenic sensing, quantum computing, space exploration, low-temperature optoelectronic devices and information storage. Unlike normal temperature environments, the molecular motion is weakened and the non-radiative process is increased in a low-temperature environment, which leads to the performance degradation or even failure of most organic light-emitting materials. Therefore, the development of an organic crystal material that can maintain stable structure and excellent optical performance under low-temperature conditions is of great significance to promote the development of low-temperature optoelectronics.

[0003] Existing organic light-emitting crystal materials mainly include polycyclic conjugated dyes, conjugated polymers and part of small molecule self-assembly systems. Among them, polycyclic conjugated dyes (such as chlorins, thiophenes, indole compounds) realize near-infrared or even low-temperature light emission by extending the molecular conjugated chain, and have high degree of π electron delocalization, and the emission wavelength can be adjusted to the interval of 700-900 nm, but there are obvious defects: the reduced energy gap leads to the increased non-radiative transition channel, and the light-emitting efficiency is low; the molecular weight is large, the synthesis route is complex, and it is difficult to realize high-efficiency large-scale preparation; more importantly, such molecules are prone to π-π stacking in a solid state or high-concentration environment, which increases the intermolecular energy migration and non-radiative relaxation channel, and causes aggregation-caused quenching (ACQ effect), further reducing the light-emitting efficiency and stability at low temperature. At the same time, due to the dependence on large π conjugated structure or polycyclic aromatic hydrocarbon skeleton, such materials also have the problems of low polarity and poor solubility, and need a complex multi-step synthesis and purification process.

[0004] Conjugated polymers (such as MEH-PPV, PFO and derivatives thereof) are another common organic light-emitting crystal material, which forms a highly conjugated system through repeating units of molecular chains, and can maintain light-emitting performance to a certain extent. However, the defects are also prominent: the large molecular weight and wide structure distribution lead to poor batch consistency and difficulty in ensuring repeatability, which is not conducive to realizing controllable low-temperature light emission behavior; the crystallinity in a solid state is limited, and often presents a non-uniform amorphous stacking state, leading to large fluctuations in optical performance; there are differences in flexibility between chain segments, which makes the local amorphous region prone to chain segment freezing or contraction at low temperature, causing microphase separation and energy level inhomogeneity, eventually leading to wavelength shift and intensity fluctuation of light emission, and the overall structure and performance stability are poor.

[0005] In recent years, researchers have attempted to construct organic luminescent crystals by self-assembly of small molecules or crystal engineering. For example, small molecule dyes are used as waveguides or laser cavities by evaporation crystallization or interfacial self-assembly to obtain single crystals or microcrystals. Such materials exhibit high crystallinity and directionality at room temperature, which helps to suppress partial non-radiative relaxation processes. However, the intermolecular packing mode of such materials is uncontrollable, and most of them lack flexibility at low temperatures. During the cooling process, internal thermal stress may be generated due to the anisotropy of the thermal expansion coefficient or the uneven intermolecular forces. At low temperatures, the molecular vibration and displacement ability is limited, and the stress cannot be released through molecular motion, ultimately leading to lattice fracture or phase transition, causing structural brittleness. This mechanical instability not only destroys the integrity of the crystal, but also causes discontinuity or even complete failure of the luminescence signal, making it difficult to meet the dual demands of stability and adjustability in a cryogenic environment.

[0006] Schiff base compounds are a class of molecules obtained by condensation of aldehyde and amine, with the advantages of simple structure, efficient synthesis and good potential for molecular engineering regulation. Literature has reported its application in photoluminescence, molecular recognition and catalysis, but its research in the field of low-temperature optical crystal materials is still limited. In particular, how to use the structural adjustability of Schiff base to realize the synergistic relationship between molecular conformation, crystal packing and low-temperature luminescence performance has not been systematically reported.

[0007] In addition, there are also deficiencies in the study of low-temperature luminescence regulation mechanism. Current studies attempt to adjust the emission wavelength by introducing substituents or external field regulation, but most of them are limited to qualitative observation at room temperature or liquid nitrogen temperature, lacking systematic research on the relationship between molecular conformation changes, crystal packing mode and low-temperature spectral response, which makes the emission regulation lack of predictability and universality, and it is difficult to achieve precise control from blue shift to red shift.

[0008] In summary, although significant progress has been made in the research of low-temperature organic luminescent crystal materials in recent years, there are still many deficiencies in the existing technology, which makes it impossible to balance stable luminescence and structural integrity in a cryogenic environment, severely limiting its application in low-temperature optoelectronic devices and sensing fields. Therefore, the present application proposes an organic Schiff base crystal material with adjustable luminescence at low temperature and a preparation method thereof. SUMMARY

[0009] The purpose of the present application is to provide an organic Schiff base crystal material with adjustable luminescence at low temperature and a preparation method thereof, aiming to solve the problems raised in the background art.

[0010] The purpose of the present application is achieved by the following technical solutions:

[0011] An organic Schiff base crystal material capable of adjustable luminescence at low temperature, wherein the molecule of the organic Schiff base crystal material has an NSB core skeleton of (E)-1-(((substituted phenyl) imino) methyl) naphthalen-2-ol; the substituent group in the substituted phenyl is a halogen atom selected from at least one of F, Cl, Br and I, and the number of substituents on the phenyl is single substitution, double substitution or triple substitution, and the substitution sites are ortho, meta or para; the crystal material can realize reversible blue shift or red shift of luminescence under a low temperature environment of 77K.

[0012] Further, the organic Schiff base crystal material is specifically selected from at least one of (E)-1-(((3,5-difluorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((4-chlorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((2,3-dichlorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((3,4-dichlorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((3,4,5-trichlorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((2,4,5-trichlorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((4-bromophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((2-fluoro, 5-chlorophenyl) imino) methyl) naphthalen-2-ol, (E)-1-(((3-iodophenyl) imino) methyl) naphthalen-2-ol.

[0013] A preparation method of the organic Schiff base crystal material capable of adjustable luminescence at low temperature, comprising the following steps:

[0014] refluxing naphthalene salicylaldehyde and halogenated aromatic amine in anhydrous ethanol;

[0015] cooling to room temperature after the reaction is completed, precipitating solid product, collecting by suction filtration, washing with cold ethanol, and then purifying by recrystallization or column chromatography to obtain a Schiff base compound;

[0016] dissolving the Schiff base compound in a good solvent, slowly adding a poor solvent, sealing and placing in a constant-temperature environment without vibration for slow evaporation to obtain the organic Schiff base crystal.

[0017] Further, the recrystallization adopts an ethanol / water mixed solvent, and the column chromatography adopts petroleum ether / ethyl acetate gradient elution.

[0018] Further, the good solvent is selected from dichloromethane or ethyl acetate, and the poor solvent is selected from n-hexane or petroleum ether.

[0019] A low-temperature optical sensor comprising a sensing element, which is the organic Schiff base crystal material capable of adjustable luminescence at low temperature.

[0020] Further, the sensing element is a single crystal or a polymer composite film formed by doping 5% of an organic Schiff base crystal material in polydimethylsiloxane.

[0021] Further, when the sensing element is (E)-1-(((3-iodophenyl)imino)methyl)naphthalen-2-ol crystal, the sensor is a nonlinear threshold temperature alarm sensor in the temperature range of 130 K~150 K; when the sensing element is (E)-1-(((3,4-dichlorophenyl)imino)methyl)naphthalen-2-ol crystal, the sensor is a linear colorimetric sensor in the temperature range of 77 K~298 K.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] Low-temperature active regulation of luminescence performance is achieved: by adopting a naphthyl Schiff base core structure and combining a halogen substituent precise regulation strategy, the present application successfully breaks the traditional cognition that low temperature leads to fluorescence solidification, and realizes wide-range and reversible spectral regulation from blue shift (-30 nm) to red shift (+92 nm), thereby providing a new material platform for low-temperature optical applications.

[0024] Both low-temperature flexibility and luminescence stability are taken into account: based on the mechanical flexibility provided by the naphthyl Schiff base core structure and the unique molecular packing mode formed by the halogen substituent regulation, the crystal still maintains excellent flexibility and elasticity at low temperature, and its composite film (such as a PDMS doped film) also has low-temperature flexibility and stable luminescence performance, thereby solving the key material bottleneck of flexible low-temperature optoelectronic devices.

[0025] The synthesis and preparation process is simple and efficient: the crystal can be prepared on a large scale by one-pot Schiff base condensation and slow solvent evaporation, and the route is simple, does not require complex catalysts or inert protection, and is low in cost, thereby having excellent industrial production prospects.

[0026] The application prospect is broad: based on the low-temperature adjustable luminescence and flexible integration performance realized by halogen substituent precise regulation, the series of materials of the present application can be immediately applied to flexible low-temperature optical sensors, intelligent identification and display in deep low-temperature environments, optical anti-counterfeiting and encryption requiring low-temperature activation, and other frontier fields. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The molecular formula of the target compound.

[0028] Figure 2 The synthesis route map of the target compound.

[0029] Figure 3 The nuclear magnetic resonance hydrogen spectrum of 35F-NSB, 4Cl-NSB and 23Cl-NSB.

[0030] Figure 4 NMR spectra of hydrogen of 34Cl-NSB, 345Cl-NSB and 245Cl-NSB.

[0031] Figure 5 NMR spectra of hydrogen of 4Br-NSB, 2F5Cl-NSB and 3I-NSB.

[0032] Figure 6 The reversible bending photos and stress-strain curves of crystals at 298 K and 77 K; wherein: a is the reversible bending photo of 3I-NSB crystal at 298 K and 77 K; b is the reversible bending photo of 4Cl-NSB crystal at 298 K and 77 K; c is the stress-strain curve of 3I-NSB crystal; d is the stress-strain curve of 4Cl-NSB crystal.

[0033] Figure 7 The packing structure of the crystal bending surface and narrow surface of the Schiff base compound.

[0034] Figure 8 The fluorescence change schematic diagram and fluorescence spectrum of five kinds of Schiff base crystals at 298 K and 77 K; wherein: a is the fluorescence change schematic diagram of five kinds of Schiff base crystals at 298 K and 77 K; b is the fluorescence spectrum of 3I-NSB and 345Cl-NSB at 298 K and 77 K; c is the fluorescence spectrum of 4Cl-NSB, 35F-NSB and 34Cl-NSB at 298 K and 77 K.

[0035] Figure 9 The single crystal structure comparison of 3I-NSB, 345Cl-NSB, 4Cl-NSB, 35F-NSB and 34Cl-NSB at 298 K and 77 K.

[0036] Figure 10 The fluorescence photos and spectra of 3I-NSB crystal and PDMS composite film at different temperatures; wherein: a is the fluorescence photo and fluorescence spectrum of 3I-NSB crystal at different temperatures; b is the fluorescence photo and fluorescence spectrum of 3I-NSB PDMS composite film at different temperatures.

[0037] Figure 11 The fluorescence photos and spectra of 34Cl-NSB crystal and PDMS composite film at different temperatures; wherein: a is the fluorescence photo and fluorescence spectrum of 34Cl-NSB crystal at different temperatures; b is the fluorescence photo and fluorescence spectrum of 34Cl-NSB PDMS composite film at different temperatures.

[0038] Figure 12Schematic diagram of low-temperature luminescence regulation based on molecular packing engineering.

[0039] Figure 13 Fig. 3 is a normalized fluorescence spectrum, temperature and maximum emission wavelength relationship and CIE color coordinate change of 3I-NSB crystals at 77 K~298 K; wherein: a is the normalized fluorescence spectrum of 3I-NSB crystals between 77 K~298 K; b is the temperature and maximum emission wavelength relationship of 3I-NSB crystals between 77 K~298 K; c is the CIE color coordinate change of 3I-NSB crystals between 77 K~298 K.

[0040] Figure 14 Fig. 4 is a normalized fluorescence spectrum, temperature and maximum emission wavelength relationship and CIE color coordinate change of 34Cl-NSB crystals at 77 K~298 K; wherein: a is the normalized fluorescence spectrum of 34Cl-NSB crystals between 77 K~298 K; b is the temperature and maximum emission wavelength relationship of 34Cl-NSB crystals between 77 K~298 K; c is the CIE color coordinate change of 34Cl-NSB crystals between 77 K~298 K. DETAILED DESCRIPTION

[0041] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present application, the technical solutions of the present application will be described in detail below, but it should not be understood as limiting the scope of the present application. In the present application, the materials, reagents or instruments used are not specified by the manufacturer, and are conventional products that can be purchased on the market.

[0042] The specific implementation of the present application will be described in detail below in combination with specific embodiments.

[0043] The present application provides an organic Schiff base crystal material with low-temperature adjustable luminescence performance, which can realize reversible luminescence blue shift or red shift at low temperature environment (such as 77 K), and has flexibility and mechanical strength, and is suitable for low-temperature optoelectronic devices and optical sensors.

[0044] Example 1: molecular design, controllable synthesis and crystal growth

[0045] 1. Molecular structure design

[0046] The core of this invention lies in the active control of crystal packing patterns and low-temperature luminescence behavior through precise molecular design. The target compound is a Schiff base compound formed by a one-pot condensation reaction of naphthyl salicylaldehyde and a haloaromatic amine, with the general chemical structure (E)-1-(((substituted phenyl)imino)methyl)naphth-2-ol (hereinafter referred to as the NSB core skeleton). By systematically adjusting the type (F, Cl, Br, I), number (monosubstituted, disubstituted, trisubstituted), and position (ortho, meta, para) of halogen substituents on the aniline ring, the molecular conformation, intramolecular / intermolecular hydrogen bonds (such as OH···N), and weak interactions such as halogen bonds can be effectively controlled, thereby achieving precise "programming" of the crystal packing pattern. The target compound includes, but is not limited to, (…). Figure 1 : (E)-1-(((3,5-difluorophenyl)imino)methyl)naphthalene-2-ol (35F-NSB), (E)-1-(((4-chlorophenyl)imino)methyl)naphthalene-2-ol (4Cl-NSB), (E)-1-(((2,3-dichlorophenyl)imino)methyl)naphthalene-2-ol (23Cl-NSB), (E)-1-(((3,4-dichlorophenyl)imino)methyl)naphthalene-2-ol (34Cl-NSB), (E)-1-(((3,4,5-trichlorophenyl)imino)methyl)naphthalene-2-ol (34Cl-NSB), (E)-1-(((3,4,5-trichlorophenyl)imino)methyl)naphthalene-2-ol (35F-NSB), (E)-1-(((3,5-difluorophenyl)imino)methyl)naphthalene-2-ol (35F-NSB), (E)-1-(((3,4,5-trichloro ... (E)-1-(((2,4,5-trichlorophenyl)imino)methyl)naphthyl-2-ol (345Cl-NSB), (E)-1-(((4-bromophenyl)imino)methyl)naphthyl-2-ol (4Br-NSB), (E)-1-(((2-fluoro,5-chlorophenyl)imino)methyl)naphthyl-2-ol (2F5Cl-NSB), (E)-1-(((3-iodophenyl)imino)methyl)naphthyl-2-ol (3I-NSB).

[0047] This type of molecule systematically regulates molecular conformation and crystal packing patterns by introducing different types, numbers, and positions of halogen substituents onto the aniline ring. Specifically: fluorinated series: by introducing highly electronegative fluorine atoms, molecular polarity and intermolecular hydrogen bonding are adjusted; chlorinated series: utilizing the moderate size and polarizability of chlorine atoms, steric hindrance and intermolecular interactions are balanced; brominated and iodinated series: through the heavy atom effect and larger atomic radius, molecular packing density and intermolecular interactions are significantly altered. This systematic substituent design enables this invention to establish a complete structure-property relationship map, providing a sufficient compound basis for achieving predictable low-temperature luminescence regulation.

[0048] 2. Synthesis method;

[0049] The target Schiff base compound was prepared by one-pot condensation reaction using naphthalene salicylaldehyde and the corresponding halogenated aromatic amines as raw materials. Figure 2 ).

[0050] (1) Reaction raw materials:

[0051] Aldehyde component: naphthalene salicylaldehyde (2-hydroxy-1-naphthaldehyde);

[0052] Amine component: halogenated aromatic amines (including 3,5-difluoroaniline, 4-chloroaniline, 2,3-dichloroaniline, 3,4-dichloroaniline, 3,4,5-trichloroaniline, 2,4,5-trichloroaniline, 4-bromoaniline, 5-chloro-2-fluoroaniline, and 3-iodoaniline).

[0053] (2) Reaction operation:

[0054] Dissolve naphthalene salicylaldehyde (1.0 equivalent) and the corresponding halogenated aromatic amine (1.0-1.2 equivalent) in anhydrous ethanol; add a small amount of anhydrous acetic acid (1-2 drops) as a catalyst, or react without adding a catalyst; reflux at 79°C for 3-4 hours, monitor the reaction progress by thin layer chromatography (TLC); after the reaction is completed, cool to room temperature, and the solid product is precipitated; collect the solid product by suction filtration, and wash with cold ethanol to remove unreacted raw materials; purify the crude product by recrystallization (ethanol / water mixed solvent) or column chromatography (petroleum ether / ethyl acetate gradient elution) to obtain high-purity Schiff base compounds.

[0055] This method has significant advantages: simple operation, mild reaction conditions, no need for inert gas protection and complex catalysts, high yield (usually up to 85%-95%), and is suitable for large-scale production. By changing the type of halogenated aromatic amine, nine different substitution mode Schiff base compounds were systematically obtained, and all products were structurally confirmed by nuclear magnetic resonance hydrogen spectrum (H NMR). 1 H NMR). Figure 3 、 Figure 4 and Figure 5 ).

[0056] 3. Crystal growth;

[0057] After obtaining high-purity Schiff base compounds, high-quality single crystals suitable for low-temperature optical research are prepared by crystal engineering means (using slow solvent evaporation method):

[0058] Crystal growth: dissolve high-purity Schiff base compounds in a good solvent (such as dichloromethane or ethyl acetate) to prepare a solution with a concentration of 5-15 mmol / L. Then, slowly add an appropriate amount of a poor solvent (such as n-hexane or petroleum ether) to the solution, and seal it in a constant-temperature environment without shaking (temperature controlled at 15-25°C).

[0059] Post-processing: after 3-7 days of slow evaporation and molecular self-assembly, millimeter-sized, regularly-shaped, and excellent optical quality Schiff base single crystals can be obtained.

[0060] By controlling the solvent ratio, temperature, and evaporation rate, high-quality crystals suitable for performance testing and device integration can be obtained reproducibly.

[0061] Example 2: Low-temperature flexibility and molecular packing mode;

[0062] The Schiff base crystals of this invention exhibit excellent mechanical flexibility on a macroscopic scale, with single crystals capable of large-scale reversible elastic bending at the centimeter scale. Represented by 3I-NSB and 4Cl-NSB, their crystals can withstand repeated bending without fracture at both room temperature (298 K) and low temperature (77 K), demonstrating good environmental adaptability. Figure 6 (a and c). To quantitatively characterize the mechanical properties of the crystals, three-point bending tests were performed. The results show that the stress-strain curves of both 3I-NSB and 4Cl-NSB crystals exhibit typical linear elastic characteristics, with fracture strains reaching 1.5% and 1.3%, respectively. Figure 6 The high content (b and d) of these materials, exceeding that of many known organic functional crystals (often around 1%), confirms their excellent flexibility. Furthermore, their fracture strengths are as high as 163 ± 19 MPa and 86 ± 25 MPa, respectively, with corresponding elastic moduli of 10.9 ± 0.7 Gpa and 6.6 ± 0.4 Gpa, indicating that they both possess good mechanical strength.

[0063] The macroscopic flexibility of crystals stems from their unique molecular packing patterns. For example... Figure 7 As shown, this series of compounds generally employs a highly slip-based, "fishbone-like" layered stacking structure. In this structure: along the curved surfaces of the crystal, the molecular layers are connected by abundant weak interactions (such as C–H···π, halogen bonds, π···π interactions), forming a tough interface that facilitates interlayer slip while maintaining structural integrity. Along the narrow faces of the crystal, the molecules are connected in a certain direction by hydrogen bonds or tight halogen-halogen interactions, forming a framework that maintains the long-range order of the crystal. When external forces are applied to the crystal, the weakly interacting layers can effectively dissipate energy through reversible slip and reconstruction, thus allowing the crystal to undergo large elastic deformation without cleavage or fracture.

[0064] Example 3: Low-temperature luminescence properties and reversible changes;

[0065] 1. Low-temperature fluorescence redshift and blueshift characteristics;

[0066] One of the most significant features of the Schiff base crystals of this invention is their highly tunable and completely reversible low-temperature luminescence behavior. For example... Figure 8As shown in a, through systematic studies of representative crystals (3I-NSB, 345Cl-NSB, 4Cl-NSB, 35F-NSB, 34Cl-NSB), it is observed that their photoluminescence spectra exhibit distinct variation trends at low temperature (77 K), covering a continuous tuning from blue to red shift. Specifically (a) Blue shift type: Take 3I-NSB and 345Cl-NSB as examples, their maximum emission wavelengths at room temperature (298 K) are located at 535 nm (yellow-green light). At 77 K, the emission spectra are blue-shifted and exhibit a double-peak structure (505 nm and 526 nm), and the luminescence color changes to green light. Figure 8 (b) and (c) in a:

[0067] Blue shift type: Take 3I-NSB and 345Cl-NSB as examples, their maximum emission wavelengths at room temperature (298 K) are located at 535 nm (yellow-green light). At 77 K, the emission spectra are blue-shifted and exhibit a double-peak structure (505 nm and 526 nm), and the luminescence color changes to green light.

[0068] Weak red shift type: Take 4Cl-NSB as an example, its room temperature emission peak is located at 539 nm (yellow-green light), and a weak red shift of about 10 nm occurs at 77 K, and the luminescence color is still yellow.

[0069] Significant red shift type: Take 35F-NSB and 34Cl-NSB as examples, their room temperature emissions are located at 579 nm and 555 nm (orange / yellow light), respectively, and are significantly red-shifted to 644 nm and 647 nm at 77 K, respectively, entering the red light region.

[0070] Importantly, all fluorescence changes are completely reversible and respond quickly during temperature cycling (298 K and 77 K), demonstrating excellent stability and reusability of the materials.

[0071] 2. The structure-activity relationship between luminescence behavior and molecular packing;

[0072] To reveal the microscopic origin of the above-mentioned different luminescence phenomena, the single crystal structures of the five crystals are compared (a) Figure 9 It is found that the luminescence color and the spectral shift trend at low temperature are closely related to the intermolecular π-π stacking overlap area and its change at low temperature, following the evolution rule from monomer-dominated luminescence to aggregate (dimer / exciton) -dominated luminescence. The intrinsic structure-activity relationship can be summarized as follows:

[0073] Blue shift mechanism (monomer luminescence): As in 3I-NSB and 345Cl-NSB, they maintain a very small π-π stacking overlap area (close to zero) at room temperature and low temperature, and the molecules exist in the form of monomers. The low-temperature blue shift is mainly due to the freezing of molecular vibration and the increase in structural rigidity, which reduces the structural relaxation of the excited state, resulting in an increase in emission energy (blue shift). The double peak at low temperature may be due to the emission of different vibrational energy levels.

[0074] Weak redshift mechanism (weakly coupled aggregated state): For example, in 4Cl-NSB, the π-π stacking overlap area increases slightly (about 1.34%) at low temperatures, forming a weakly coupled aggregated state. At this time, the low-energy emission caused by the aggregated state begins to appear, but it is similar to the emission energy of the individual particles, and they jointly contribute to the spectrum, resulting in a weak redshift of the overall spectrum.

[0075] Significant redshift mechanism (strongly coupled aggregated states): Examples include 35F-NSB and 34Cl-NSB, which possess a large π-π stacking overlap area at room temperature (35F-NSB reaches 18.88%~26.93%). At low temperatures, lattice contraction further enhances the face-to-face π-π interactions, resulting in highly delocalized excited states throughout the dimer / aggregate, significantly reducing the band gap, and thus exhibiting a significant redshift exceeding 65 nm, with luminescence entirely dominated by the aggregated states.

[0076] In summary, this invention, through precise molecular design and crystal engineering, has successfully achieved the ability to programmatically customize the luminescence color and displacement direction of crystals at low temperatures by controlling the key structural parameter of π-π stacking overlap area.

[0077] Example 4: Comparison of fluorescence in various morphologies (thin films, single crystals);

[0078] To comprehensively evaluate the material properties of the present invention and its application potential in flexible optoelectronic devices, the luminescence behavior of the target molecules 3I-NSB (representing the low-temperature blue-shift type) and 34Cl-NSB (representing the low-temperature red-shift type) in different forms, such as single crystal and polymer composite thin film (doped in polydimethylsiloxane (PDMS) at a mass ratio of 5%), was systematically compared.

[0079] For 3I-NSB: its single crystal ( Figure 10 a) and PDMS composite film ( Figure 10 (b) When the temperature drops from 298 K to 77 K, the emission spectrum shows a significant blue shift. This consistent behavior demonstrates that 3I-NSB molecules exhibit a monomer-dominated luminescence mechanism, whether in the long-range ordered crystalline state or the amorphous dispersed state. In the PDMS composite film, the molecules are effectively isolated by the polymer matrix and cannot form effective π-π stacking. Therefore, its luminescence characteristics are consistent with the monomer luminescence behavior in single crystals, exhibiting a blue shift at low temperatures due to molecular vibrational freezing and increased rigidity.

[0080] For 34Cl-NSB: its single crystal ( Figure 11 a) exhibits a significant redshift (over 90 nm) at low temperatures, while its performance in PDMS composite films ( Figure 11In b), the emission peak position remains essentially unchanged with decreasing temperature, exhibiting only spectral narrowing. This stark contrast strongly demonstrates that the significant redshift characteristic of 34Cl-NSB crystals originates entirely from their close, face-to-face molecular packing structure. When the molecules are diluted and isolated in the PDMS matrix, this strong intermolecular interaction is disrupted, preventing the formation of redshifted emission states. Therefore, its thin-film behavior reverts to monomeric luminescence characteristics, exhibiting only spectral narrowing at low temperatures due to weakened nonradiative transitions, without peak shift.

[0081] Multimorphic comparison experiments provided strong evidence that although the low-temperature luminescence behavior of the material is related to the molecular structure, it is mainly dominated by the molecular packing mode. This confirms that the core of the low-temperature luminescence regulation strategy of this invention lies in the precise control of molecular packing, rather than the inherent properties of the molecules themselves. This provides clear guidance for subsequent molecular and crystal engineering design.

[0082] In summary, this invention achieves programmed control over the π-π stacking overlap area of ​​molecules in Schiff base crystals by precisely controlling the type, number, and position of halogen substituents, thereby actively regulating their fluorescence emission color and red-shift / blue-shift behavior at low temperatures. Figure 12 The establishment of this "structure-performance" relationship provides a new design strategy and a solid material platform for developing next-generation smart low-temperature optical materials with predictable and customizable luminescence properties, and has broad application prospects in fields such as flexible optoelectronic devices, optical sensing, and anti-counterfeiting encryption.

[0083] Example 5: Low-temperature optical sensor based on Schiff base crystals with significant blue or red shift at low temperatures;

[0084] Single crystals of 3I-NSB (representing the low-temperature blue-shift type) and 34Cl-NSB (representing the low-temperature red-shift type) were used as sensing elements and fixed on the sample stage of a cryostat. Excitation light (e.g., 365 nm ultraviolet light) was guided by an optical fiber, and its fluorescence signal was collected and connected to a spectrometer for real-time monitoring. The temperature was controlled by a program between 77 K and 298 K, and the fluorescence spectra were recorded. Figure 13 a and Figure 14 a) Changes in color coordinates.

[0085] Nonlinear temperature sensor based on 3I-NSB crystal;

[0086] like Figure 13 As shown in Figure b, the fluorescence behavior of the 3I-NSB crystal exhibits a three-stage nonlinear response during cooling, demonstrating extremely high temperature sensitivity.

[0087] Stage I (298 K~200 K): The maximum emission wavelength slowly blue-shifts with decreasing temperature; this stage corresponds to a gradual decrease in the molecular thermal vibrational energy. Stage II (200 K~150 K): The emission wavelength enters a plateau region with minimal change, indicating that the molecular conformation is in a metastable state. Stage III (150 K~130 K): Within a very narrow temperature range (20 K), the emission wavelength undergoes a dramatic blue-shift (e.g., from 525 nm to 505 nm). Stage IV (130 K~77 K): The emission wavelength again enters a plateau region.

[0088] Correspondingly, its CIE 1931 color coordinates change non-linearly from (0.31, 0.65) yellow-green at room temperature (298 K) to (0.20, 0.70) blue-green at 77 K. Figure 13 (c). This nonlinear wavelength-temperature relationship with a steep transition region makes the 3I-NSB crystal particularly suitable for manufacturing highly sensitive threshold temperature alarm sensors or precise calibration sensors for specific temperature ranges (130 K~150 K).

[0089] A wide-range linear colorimetric sensor based on 34Cl-NSB crystal;

[0090] like Figure 14 As shown in Figure b, the fluorescence behavior of the 34Cl-NSB crystal exhibits different characteristics: its maximum emission wavelength gradually redshifts almost monotonically from 298 K to 77 K as the temperature decreases, and although the redshift rate is nonlinear, the overall change is smooth and continuous. Particularly noteworthy is that its CIE 1931 chromaticity coordinates change during cooling, from (0.64, 0.35) orange-yellow at room temperature (298 K) to (0.47, 0.51) red at 77 K, showing a highly linear migration trajectory on the chromaticity diagram. Figure 14 (c)

[0091] This excellent linear relationship between color coordinates and temperature allows for intuitive and rapid temperature readings through simple RGB color recognition or colorimeter measurement. Therefore, 34Cl-NSB crystals are ideally suited for constructing colorimetric or ratiometric optical temperature sensors for wide temperature ranges (77 K to 298 K) without the need for complex spectrometers, significantly reducing the complexity and cost of the equipment.

[0092] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent.

Claims

1. An application of an organic Schiff base crystal material as a tunable luminescent material at low temperatures, characterized in that: The organic Schiff base crystal material is selected from: (E)-1-(((3,5-difluorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((4-chlorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((2,3-dichlorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((3,4-dichlorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((3,4-dichlorophenyl)imino)methyl)naphth-2-ol, (E)-1-((( 3,4,5-Trichlorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((2,4,5-Trichlorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((4-bromophenyl)imino)methyl)naphth-2-ol, (E)-1-(((2-fluoro,5-chlorophenyl)imino)methyl)naphth-2-ol, (E)-1-(((3-iodophenyl)imino)methyl)naphth-2-ol.

2. The application according to claim 1, characterized in that, The organic Schiff base crystal material is used to prepare sensing elements in low-temperature optical sensors.

3. The application according to claim 2, characterized in that, The sensing element is a single crystal or a polymer composite film, wherein the polymer composite film is formed by doping the organic Schiff base crystal material in polydimethylsiloxane at a mass ratio of 5%.

4. The application according to claim 2 or 3, characterized in that, When the sensing element is (E)-1-(((3-iodophenyl)imino)methyl)naphthalene-2-ol crystal, the sensor is a nonlinear threshold temperature alarm sensor in the temperature range of 130 K to 150 K. When the sensing element is (E)-1-(((3,4-dichlorophenyl)imino)methyl)naphthalene-2-ol crystal, the sensor is a linear colorimetric sensor in the temperature range of 77 K to 298 K.

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