A multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission in excited-state molecules and its preparation method thereof.

Abstract

This invention belongs to the field of organic fluorescent materials and discloses a multifunctional luminescent molecule exhibiting excited-state intramolecular proton transfer (ESIPT) and aggregation-induced fluorescence emission effects, as well as its preparation method. This invention prepares a fluorescent molecule with ESIPT and aggregation-induced fluorescence emission effects by introducing 2-hydroxybenzothiazole (HBT) at the 2,7 positions of the carbazole backbone and covalently linking a p-aniline group to the N atom of the carbazole. The hydroxyl and amine groups in the molecule are sensitive to solvent polarity and pH. The aniline group in the molecule undergoes an aldehyde-amine condensation reaction with formaldehyde to achieve a fluorescent response to formaldehyde. The ESIPT molecule provided by this invention exhibits aggregation-induced emission characteristics, a large Stokes shift, and fluorescence responses to acids, bases, and formaldehyde.

Description

Technical Field

[0001] This invention belongs to the field of organic fluorescent materials, and more specifically relates to a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state and its preparation method. Background Technology

[0002] Excited-state intramolecular proton transfer (ESIPT) is an important photophysical process. It involves the transfer of protons between neighboring proton donors and acceptors within a molecule after photoexcitation, accompanied by dual fluorescence emission. Short-wavelength and long-wavelength fluorescence originate from the normal excited state and the tautomer generated during proton transfer, respectively. The necessary conditions for ESIPT are the simultaneous presence of proton donor groups (such as -OH and -NH2) and proton acceptor groups (such as =N- and -C=O) within the molecule, with intramolecular hydrogen bonds between the donor and acceptor groups. Based on its unique four-level system, ESIPT fluorescent molecules possess advantages such as large Stokes shift, two-photon emission, high quantum yield, high proton transfer rate, and high sensitivity. ESIPT fluorescent molecules are easily influenced by the surrounding microenvironment; their emission spectra depend on hydrogen bonds, rotational isomerism, the surrounding medium, pH, and the types of donor and acceptor units. ESIPT, as a core sensing mechanism for fluorescent molecules, has been widely used in fields such as fluorescent probes, bioimaging, biodetection, disease diagnosis, biology, molecular logic devices, environmental science, and organic optoelectronics, thus attracting increasing interest from scientists.

[0003] There are currently many reports on the design, synthesis and application of ESIPT fluorescent molecules, such as (1) fluorescent probe development: using 2-(2'-hydroxyphenyl)benzoxazole (HBO) as the fluorophore, the acryloyl group of the recognition site is grafted onto the proton donor -OH of HBO by condensation reaction to inhibit the occurrence of ESIPT process. Electron-withdrawing substituents of different strengths (-F, -CHO, -H, -CH3) are introduced at the para position of the recognition site to design and synthesize four ESIPT-based fluorescent probes. Based on the property of KMnO4 to oxidize unsaturated alkenes, KMnO4 can break the carbon-carbon double bond in the ESIPT-based fluorescent probe to form an ortho-dihydroxy group. Subsequently, the ester bond breaks to release the proton donor, the ESIPT process is excited, and the fluorescence detection of KMnO4 is realized. Its detection limit can reach 0.96 nM (Anal. Chem. 2023, 95, 23, 9014-9024). (2) ESIPT-induced photoisomerization of two-dimensional covalent organic frameworks (2DCOFs) materials—revealing the molecular mechanism of efficient photocatalysis: Taking ketene-amine COFs with ESIPT effect as the research object, the donor-acceptor reconstruction mechanism of excited-state COF photocatalytic materials and the influence of photoexcitation-induced symmetry breaking on the electronic structure of COF photocatalysts were revealed (JACS Au 2023, 3, 12, 3391–3399; Small, 2024, 20, 2307138). (3) ESIPT materials can be applied to environmental monitoring, water quality analysis and other fields. For example, by introducing aliphatic amine "tail" structures into the fluorescent core, under excited-state conditions, the protons of the amide group can be precisely transferred to the "tail" acceptor, thereby forming a stable ESIPT dual emission state. This mechanism remains stable in a variety of solvents ranging from weakly polar to strongly polar, and can intelligently identify spontaneous shutdown in aqueous environments, thereby improving the probe's sensitivity to changes in non-aqueous microenvironments (J. Am. Chem. Soc. 2025, 147, 18, 15602–15613). (4) Novel laser material: A zwitterionic ESIPT laser material achieves low-threshold amplified spontaneous emission, exhibiting large Stokes shift, high radiative transition rate and strong optical stability, and is expected to become a high-performance material for realizing continuous light lasers and electrically pumped lasers (J. Am. Chem. Soc. 2022, 144, 30, 13499–13510).

[0004] Currently, the design, synthesis, and multifunctional expansion of multifunctional luminescent molecules with the ESIPT effect are challenging due to the varying requirements for molecular structure in different application fields. Furthermore, existing ESIPT luminescent materials suffer from drawbacks such as cumbersome molecular design, complex synthesis, low yields, or the need for stringent reaction conditions.

[0005] ESIPT-effect multifunctional luminescent molecules possess multi-scenario application capabilities, can provide materials with more functions, and have broader application value in practical applications. Therefore, how to provide an ESIPT-effect multifunctional luminescent molecule is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] To overcome the shortcomings and deficiencies of the prior art, this invention provides a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state and a method for its preparation.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state is shown in the following structural formula:

[0009] .

[0010] This invention also provides a method for preparing a multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, comprising the following steps:

[0011] (1) Under nitrogen protection, 2,7-dibromocarbazole and p-nitrofluorobenzene were added to the reaction vessel, N,N-dimethylamide solvent was added, and then alkali was added. The mixture was heated and reacted for 3-4 h. The solvent was removed and purified by silica gel column chromatography to obtain intermediate 1.

[0012] (2) Add intermediate 1 and reducing agent to alcohol solvent, heat the reaction for 1-2 h, then cool the mixture to room temperature, pour it into water, extract with dichloromethane and wash with brine, combine the organic layers, dry with anhydrous magnesium sulfate, filter, remove solvent, purify by silica gel column chromatography to obtain intermediate 2;

[0013] (3) Add intermediate 2, boron reagent, potassium acetate and palladium catalyst to tetrahydrofuran-ethanol mixed solvent, heat the reaction for 5-8 h, then cool the mixture to room temperature, pour it into water, extract with chloroform and wash with brine, combine the organic layers, dry with anhydrous magnesium sulfate, filter, remove solvent, purify by silica gel column chromatography to obtain intermediate 3;

[0014] (4) The intermediate 3,4-bromo-2-hydroxybenzothiazole and the base were added to toluene, and then palladium catalyst was added. The mixture was stirred and heated to reflux for 2-3 h. The solvent was removed, and the mixture was purified by silica gel column chromatography to obtain a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state.

[0015] The synthetic route for the multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state is shown below:

[0016] .

[0017] Preferably, the alkali in step (1) is one of potassium carbonate, sodium carbonate, sodium bicarbonate and potassium acetate.

[0018] More preferably, the alkali in step (1) is potassium carbonate.

[0019] Preferably, in step (1), the molar ratio of 2,7-dibromocarbazole, p-nitrofluorobenzene and base is 1:1:0.2-1.6.

[0020] Preferably, the reducing agent in step (2) is one of zinc powder, stannous chloride, sodium borohydride and palladium-carbon.

[0021] More preferably, the reducing agent in step (2) is zinc powder.

[0022] Preferably, the molar ratio of intermediate 2, boron reagent and palladium catalyst in step (3) is 1:1.5:0.05-0.2.

[0023] Preferably, the palladium catalyst in step (3) is selected from one of PdCl2, Pd(dppf)Cl2 and Pd(PPh3)4.

[0024] More preferably, the palladium catalyst in step (3) is Pd(dppf)Cl2.

[0025] Preferably, the alkali mentioned in step (4) is selected from one of Na2CO3, K2CO3, KOH, and NaOH.

[0026] More preferably, the alkali in step (4) is K2CO3.

[0027] Preferably, the palladium catalyst in step (4) is selected from one of PdCl2, Pd(dppf)Cl2 and Pd(PPh3)4.

[0028] More preferably, the palladium catalyst in step (4) is selected as Pd(PPh3)4.

[0029] Preferably, the molar ratio of intermediate 3,4-bromo-2-hydroxybenzothiazole and palladium catalyst in step (4) is 1:1:0.02-0.2.

[0030] As can be seen from the above technical solutions, compared with the prior art, the present invention provides a multifunctional luminescent molecule with ESIPT and aggregation-induced fluorescence emission effect and its preparation method, which has the following beneficial effects:

[0031] 1. This invention prepares a fluorescent molecule exhibiting ESIPT and aggregation-induced fluorescence emission effects by introducing 2-hydroxybenzothiazole (HBT) at the 2,7 positions of the carbazole backbone and covalently linking a p-aniline group to the N atom of the carbazole. The hydroxyl and amino groups in the molecule are sensitive to solvent polarity and pH, while the amino group in the aniline group undergoes an aldehyde-amine condensation reaction with formaldehyde to achieve fluorescent detection of formaldehyde. The ESIPT-effect luminescent molecule provided by this invention exhibits aggregation-induced emission characteristics and a large Stokes shift.

[0032] 2. The synthesis process of this invention is simple, the reaction conditions are mild, the yield is high, it is soluble in a variety of organic solvents, and it has good solution processability.

[0033] 3. The ESIPT and the aggregation-induced fluorescence emission effect of the present invention have broad application prospects in many fields such as acid and alkaline gas detection and formaldehyde gas detection. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0035] Figure 1 The UV-Vis absorption and fluorescence emission spectra of the multifunctional luminescent molecule with excited-state intramolecular proton transfer and aggregation-induced fluorescence emission effects in different solvents in Example 1 are shown, where a is the UV-Vis absorption spectrum and b is the fluorescence emission spectrum.

[0036] Figure 2 The fluorescence emission spectrum of the multifunctional luminescent molecule with excited-state intramolecular proton transfer and aggregation-induced fluorescence emission effect in Example 1 is shown in a tetrahydrofuran / water mixed solvent, and the fluorescence intensity changes under different water contents are shown in Figure a. a is the fluorescence emission spectrum in the tetrahydrofuran / water mixed solvent, and b is the fluorescence intensity change under different water contents.

[0037] Figure 3 The fluorescence emission spectrum of the multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in Example 1, is shown in powder form.

[0038] Figure 4 The fluorescence emission diagrams of the multifunctional luminescent molecule of Example 1, which exhibits intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, are shown under different pH conditions.

[0039] Figure 5 The fluorescence emission diagrams of ESIPT and the multifunctional luminescent molecule with aggregation-induced fluorescence emission effect in Example 1 after the addition of formaldehyde are shown. Detailed Implementation

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0041] A method for preparing a multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state includes the following steps:

[0042] (1) Preparation of intermediate 1

[0043] Under nitrogen protection, 2,7-dibromocarbazole (325.0 mg, 1 mmol) and p-nitrofluorobenzene (141.0 mg, 1 mmol) were added to a dry three-necked flask, along with appropriate amounts of N,N-dimethylamide and potassium carbonate (138 mg, 1 mmol). The mixture was heated to 85-90 °C and reacted for 3.5 h. The solvent was then evaporated under reduced pressure. The residue was purified by silica gel column chromatography using a mixture of ethanol and ethyl acetate (v / v = 1:1) as the eluent to give a pale yellow intermediate 1 in 70% yield. MS (ESI): m / z [M] + calcd for C 18 H 10 Br2N2O2: 446.104, found: 445.983. Elemental Analysis calcd: C, 48.42; H, 2.24; Br, 35.43; N, 6.28; O, 7.17, found: C, 48.61; 1 H NMR (400 MHz, CDCl3): δ 8.26-8.27 (m, 2H), 8.13 (d,2H), 8.04-8.05 (m, 2H), 7.63 (m, 2H), 7.51 (m, 2H).

[0044] (2) Preparation of intermediate 2

[0045] Accurately weigh intermediate 1 (892.2 mg, 2 mmol) and zinc powder (32.5 mg, 0.5 mmol) into a dry three-necked flask, add ethanol, heat to reflux, and react for 1.5 h. Cool the mixture to room temperature, filter, extract the filtrate with dichloromethane and wash with brine, combine the organic layers, dry with anhydrous magnesium sulfate, filter, evaporate to remove solvent, and purify by silica gel column chromatography to give white intermediate 2 in 75% yield; MS (ESI): m / z [M] + calcd for C 18 H 12 Br2N2: 416.120, found: 416.082. Elemental Analysis calcd: C, 51.96; H, 2.91; Br, 38.40; N, 6.73, found: C, 51.78; H, 2.92; Br, 38.55; N, 6.75. 1 H NMR (400 MHz, CDCl3): δ8.15(m, 2H), 8.04(m, 2H), 7.49-7.53(m, 2H), 7.12-7.14(m, 2H), 6.70-6.72 (m,2H), 4.08 (s, 2H).

[0046] (3) Preparation of intermediate 3

[0047] Intermediate 2 (832.2 mg, 2 mmol), bis(pinacol)diboron (761.8 mg, 3 mmol), potassium acetate, and palladium catalyst Pd(dppf)Cl2 (292.7 mg, 0.4 mmol) were added to a tetrahydrofuran-ethanol mixed solvent and heated for 5–8 h. The mixture was then cooled to room temperature, poured into water, extracted with chloroform, and washed with brine. The organic layers were combined, dried over anhydrous magnesium sulfate, filtered, and the solvent was evaporated. The mixture was purified by silica gel column chromatography to give an off-white intermediate 3. MS (ESI): m / z [M] + calcd for C 30 H 36 B2N2O4: 510.250, found: 510.292. Elemental Analysis calcd: C, 70.62; H, 7.11; B, 4.24; N, 5.49; O, 12.54, found: C, 70.60; 1H NMR (400 MHz, CDCl3): δ 8.35(m, 2H), 8.04-8.13(m, 2H), 7.68-7.70(m, 2H), 7.40-7.42(m, 2H), 6.70-6.73(m, 2H), 4.08 (s, 2H), 1.23(s,24H).

[0048] (4) Preparation of ESIPT and aggregation-induced fluorescence emission molecules

[0049] Intermediate 3 (1020.5 mg, 2 mmol), 4-bromo-2-hydroxybenzothiazole (612.45 mg, 2 mmol), and a base were added to toluene, followed by tetrakis(triphenylphosphine) (462.2 mg, 0.4 mmol) palladium catalyst. The mixture was heated to reflux and reacted for 2.5–3 h. The solvent was evaporated under reduced pressure, and the mixture was purified by silica gel column chromatography to obtain ESIPT and a multifunctional luminescent molecule exhibiting aggregation-induced fluorescence emission. MS (ESI): m / z [M] + calcd for C 44 H 28 N4O2S2: 708.85, found: 708.82. ElementalAnalysis calcd: C, 74.55; H, 3.98; N, 7.90; O, 4.51; S, 9.05, found: C, 74.53; H, 3.99; N, 7.91; 1 H NMR (400 MHz, CDCl3): δ 8.95(s, 2H), 8.11-8.17(m, 2H), 7.88-7.99(m, 6H), 7.79-7.83(m, 2H), 7.65-7.80(m, 2H), 7.42-7.52(m, 4H), 7.35-7.39(m, 2H), 7.13-7.16(m, 2H), 7.04-7.07(m, 2H), 6.70-6.73(m, 2H), 4.08(s, 2H).

[0050] To investigate the effect of solvent polarity on the UV-Vis absorption and fluorescence emission of ESIPT and the aggregation-induced fluorescence emission multifunctional luminescent molecule obtained in Example 1, different polar solvents were selected, including toluene, tetrahydrofuran, dichloromethane, acetone, ethanol, N,N-dimethylformamide, and dimethyl sulfoxide. The maximum absorption peaks of ESIPT and the aggregation-induced fluorescence emission multifunctional luminescent molecule in different solvents were around 330 nm, showing no solvent dependence. However, their fluorescence emission spectra exhibited strong solvent dependence and ESIPT characteristics: an emission peak at 568 nm in acetone, a double emission peak at 484 / 545 nm in toluene, and a double emission peak at 453 / 570 nm in dimethyl sulfoxide. The UV-Vis absorption and fluorescence emission spectra are shown in the appendix. Figure 1 .

[0051] In addition, the fluorescence emission spectra of ESIPT and the aggregation-induced fluorescence emission effect (AIE) multifunctional luminescent molecules were investigated under different ratios of tetrahydrofuran / water mixed solvents and under different water contents. With the addition of water, a poor solvent, the fluorescence intensity of ESIPT and the AIE multifunctional luminescent molecules continuously increased. When the water content was 70%, the fluorescence increased 17 times, exhibiting aggregation-induced emission characteristics. The results are shown in the appendix. Figure 2 .

[0052] The powder of the ESIPT-effect multifunctional luminescent molecule emitted strong fluorescence, with an emission peak at 565 nm, exhibiting aggregation-induced emission properties. (See attached figure for results.) Figure 3 .

[0053] Using dioxane as a solvent and adjusting the pH with alkali, the fluorescence changes of ESIPT and aggregation-induced fluorescence emission (AIE) multifunctional luminescent molecules under alkaline conditions were investigated. As the pH increased from 7 to 8, the emission peak intensity at 583 nm continuously decreased, while the emission peak intensity at 502 nm increased. This indicates that under alkaline conditions, the hydroxyl hydrogen atoms of ESIPT and AIE multifunctional luminescent molecules are alkalized, disrupting the ESIPT structure and exhibiting alkali-induced color-changing properties. Results are attached. Figure 4 .

[0054] Using dioxane as a solvent, the fluorescence spectrum changes of ESIPT and aggregation-induced fluorescence emission multifunctional luminescent molecules in the presence of formaldehyde were studied. The addition of formaldehyde enhanced the fluorescence signal, with increases in the emission peak intensities at 445 and 570 nm. The results are shown in the appendix. Figure 5 .

[0055] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, characterized in that, The structural formula is as follows: 。 2. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 1, is characterized in that... Includes the following steps: (1) Under nitrogen protection, 2,7-dibromocarbazole and p-nitrofluorobenzene were added to the reaction vessel, N,N-dimethylamide solvent was added, and then alkali was added. The mixture was heated and reacted for 3-4 h. The solvent was removed and purified by silica gel column chromatography to obtain intermediate 1. (2) Add intermediate 1 and reducing agent to alcohol solvent, heat the reaction for 1-2 h, then cool the mixture to room temperature, pour it into water, extract with dichloromethane and wash with brine, combine the organic layers, dry with anhydrous magnesium sulfate, filter, remove solvent, purify by silica gel column chromatography to obtain intermediate 2; (3) Add intermediate 2, boron reagent, potassium acetate and palladium catalyst to tetrahydrofuran-ethanol mixed solvent, heat the reaction for 5-8 h, then cool the mixture to room temperature, pour it into water, extract with chloroform and wash with brine, combine the organic layers, dry with anhydrous magnesium sulfate, filter, remove solvent, purify by silica gel column chromatography to obtain intermediate 3; (4) The intermediate 3,4-bromo-2-hydroxybenzothiazole and the base were added to toluene, and then palladium catalyst was added. The mixture was stirred and heated to reflux for 2-3 h. The solvent was removed, and the mixture was purified by silica gel column chromatography to obtain a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state. The synthetic route for the multifunctional luminescent molecule exhibiting intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state is shown below: 。 3. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that... In step (1), the molar ratio of 2,7-dibromocarbazole, p-nitrofluorobenzene and base is 1:1:0.2-1.

6.

4. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that... The reducing agent mentioned in step (2) is one of zinc powder, stannous chloride, sodium borohydride and palladium-carbon.

5. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that... The molar ratio of intermediate 2, boron reagent and palladium catalyst in step (3) is 1:1.5:0.05-0.

2.

6. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that... The palladium catalyst mentioned in step (3) is selected from one of PdCl2, Pd(dppf)Cl2 and Pd(PPh3)4.

7. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that... The alkali mentioned in step (4) is selected from one of Na2CO3, K2CO3, KOH, and NaOH.

8. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that, The palladium catalyst mentioned in step (4) is selected from one of PdCl2, Pd(dppf)Cl2 and Pd(PPh3)4.

9. The method for preparing a multifunctional luminescent molecule with intramolecular proton transfer and aggregation-induced fluorescence emission effects in the excited state, as described in claim 2, is characterized in that... The molar ratio of intermediate 3,4-bromo-2-hydroxybenzothiazole and palladium catalyst in step (4) is 1:1:0.02-0.2.

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