CN115260233B - Imidazole type fluorescent molecule containing triphenylphosphine oxide structure, preparation method thereof and application of imidazole type fluorescent molecule as fluorescent probe - Google Patents

Abstract

The invention discloses an imidazole type fluorescent molecule containing a triphenylphosphine oxide structure, a preparation method thereof and application of the imidazole type fluorescent molecule serving as a fluorescent probe. The structural formula of the fluorescent molecule is as follows: Meanwhile, the fluorescent molecule containing the triphenylphosphine oxide structure and the imidazole structure has good physical and chemical stability, and shows good fluorescent performance under visible light and ultraviolet light, and the fluorescent molecule can be combined with Fe ³⁺ with high selectivity and generate fluorescence quenching when being used as a fluorescent probe for detecting Fe ³⁺, is little interfered by metal ions such as Al ³⁺、Ca²⁺、Li⁺、Cu²⁺、Na⁺、In³⁺, can be used for fluorescence detection of Fe ³⁺, and has the advantages of high detection sensitivity, low detection lower limit and the like.

Description

An imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure, a preparation method thereof, and its application as a fluorescent probe

Technical Field

The invention relates to a fluorescent molecule, in particular to an imidazole fluorescent molecule containing a triphenylphosphine oxide structure, and also to a preparation method thereof and application as a fluorescent probe for detecting Fe ³⁺ , belonging to the field of sensor technology.

Background Art

Imidazole derivatives are a general term for a class of substances that contain one or more imidazole ring structural units in the compound structure. Imidazole is a five-membered ring containing two nitrogen atoms and belongs to the 1,3-azole class of compounds. It has stable chemical properties and is widely found in natural products and bioactive substances. Imidazole derivatives have been reported in drug synthesis, protease activity inhibition, curing agents, etc. due to their antibacterial and anti-inflammatory pharmacological properties. Due to the many reaction sites on the imidazole ring, the modified derivatives have also been studied in the fields of optoelectronic materials, sensors, and fluorescent probes.

In recent years, researchers have reported a large number of materials designed with imidazole as the basic unit, which are widely used in room temperature phosphorescence, deep blue luminescent materials, mechanochromism, fluorescence sensing and ion detection.

For example, Liu et al. successfully developed pyrene[4,5-d]imidazole derivatives with different excited states, namely PyPA, PyPPA, PyPPAC and PyPAC. Among them, PyPPA and PyPPAC have mixed local and charge transfer states, pure blue fluorescence and high quantum yield. The CIE coordinates of the non-doped OLED based on PyPPA are (0.14, 0.13) and the maximum EQE is 8.47%. The maximum brightness of the non-doped OLED based on PyPPAC is 50046cd m ^-2 , which is located in the blue region. The CIE coordinates are (0.15, 0.21), EQE = 6.74%, and the brightness exceeds 10000cd m ^-2 .

For example, Wang et al. designed and synthesized a new phenazine-imidazolyl Schiff base (PIS) fluorescent probe, which has been developed for ratiometric detection of Cd ²⁺ ions in aqueous media under physiological pH conditions.

Wang et al. synthesized three novel dual-targeted fluorescent probes, which were linked to one galactose (IM-Gal-1), two galactoses (IM-Gal-2), and three galactoses (IM-Gal-3) via imidazole. These probes showed good selectivity and sensitivity to Fe ³⁺ , with a stoichiometric ratio of 1:2 recognition mode.

At present, organic light-emitting molecules containing both imidazole and carbazole structures in their molecules have also been designed and synthesized one after another. Such as the literature (Tagare J, Dubey D K, Yadav R A K, et al. Triphenylamine-imidazole-based luminophores for deep-blue organic light-emitting diodes: experimental and theoretical investigations [J]. Materials Advances, 2020, 1 (4): 666-679.); (Devesing Girase J, Rani Nayak S, Tagare J, et al. Solution-processed deep -blue(y～0.06)fluorophores based on triphenylamine-imidazole(donor-acceptor)forOLEDs: computational and experimental exploration[J].Journal of InformationDisplay,2021:1-15.);(Bourouina A,Rekis M.Comparison in optoelectronicproperties of triphenylamine-imidazole or imidazole as donor for dyesensit-ized solar cell:theoretical approach[J] .Journal of Molecular Modeling, 2021, 27(8):1-9.) et al. reported a deep blue organic electroluminescent material connected with imidazole and triphenylamine. However, there are currently no reports on the use of organic luminescent molecules containing both imidazole and triphenylphosphine oxide structures as fluorescent probes for the detection of iron ions.

There are many methods for synthesizing imidazole derivatives in the prior art, such as the Debus-Radziszewski imidazole synthesis method, which is one of the most classic methods for synthesizing imidazole compounds. Diketone compounds and aldehyde compounds can be used to synthesize 2,4,5-triarylimidazoles, phenanthroimidazoles, bisimidazole derivatives, etc. in the presence of amines or ammonium salts.

Sundar and Rengan reported a three-component reaction of benzyl alcohol, 1,2-diketone and excess ammonium acetate as raw materials. The oxidation process was catalyzed by 0.25 mol% supported bis-Ru(II) in the presence of oxygen, and the only by-product was water.

Wang et al. used various disubstituted alkynes, aldehydes and amines as raw materials and synthesized tetrasubstituted imidazole derivatives simply and efficiently through a multicomponent reaction mediated by pivalic acid.

Naidoo and Jeena used a cheap iodine/dimethyl sulfoxide (DMSO) system to synthesize 2,4,5-trisubstituted imidazoles through a "one-pot, two-step" method. The reaction does not require acidic conditions and metals, and the yield is moderate to good. The first step is that the alkyne reacts under I ₂ /DMSO conditions to obtain an intermediate: a disubstituted benzil, and the second step is the in-situ reaction of aldehydes and ammonium acetate to generate trisubstituted imidazole compounds.

Grigg et al. synthesized imidazole compounds by homodimerization of isocyanate under the catalysis of silver salt. Under the condition of catalytic amount of AcOAg, methyl isocyanoacetate reacts with methyl acrylate to form pyrroline, while under the condition of stoichiometric AcOAg, the reaction will cause homodimerization of isocyanide to form 1,4-disubstituted imidazole.

Shao et al. reported a cyclization reaction based on amidoacetonitrile and phenylboronic acid to form disubstituted imidazole compounds.

Xie et al. invented a novel base-mediated [3+2] cyclization reaction of active methylene isocyanate with ketene imine. The reaction is regioselective, and ketene imine reacts with active methylene isocyanate under the condition of a strong base such as t-BuOK to obtain 1,4,5-trisubstituted imidazole derivatives.

There are many methods for synthesizing imidazoles in the prior art, each of which has advantages and disadvantages. So far, there has been no report on the construction of a small molecule fluorescent probe containing an imidazole ring of a triphenylphosphine oxide structure through the classic Suzuki coupling and Debus-Radziszewski reaction.

Summary of the invention

In view of the defects of the prior art, the first object of the present invention is to provide an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure, which has good physical and chemical stability and exhibits good fluorescence properties under visible light and ultraviolet light.

The second object of the present invention is to provide a method for preparing an imidazole-type fluorescent molecule having a triphenylphosphine oxide structure, wherein the raw materials are readily available, the synthesis steps are relatively simple, the yield is considerable, and it is conducive to expanding production.

The third object of the present invention is to provide an application of an imidazole-type fluorescent molecule with a triphenylphosphine structure, which is used as a fluorescent probe for detecting Fe ³⁺ . The fluorescent molecule can bind to Fe ³⁺ with high selectivity and produce fluorescence quenching, and is less interfered by metal ions such as Al ³⁺ , Ca ²⁺ , Li ⁺ , Cu ²⁺ , Na ⁺ , In ^3+, etc., and can be used for fluorescence detection of Fe ³⁺ , with the characteristics of high sensitivity and low detection limit.

In order to achieve the above technical objectives, the present invention provides an imidazole fluorescent molecule containing a triphenylphosphine oxide structure, which has a structure of Formula 1 or Formula 2:

Wherein, R is hydrogen or aryl.

As a preferred solution, the aryl group can be a phenyl group or a group derived from a phenyl group, such as a substituted phenyl group, and the phenyl ring also contains common substituent groups, such as C ₁ to C ₅ alkyl groups, C ₁ to C ₅ alkoxy groups, halogen substituents, etc. The present invention preferably uses the simplest benzene ring, which can extend the conjugated system of the imidazole ring.

The fluorescent molecule of the present invention is a typical D-π-A structure formed by coupling an imidazole structure and an oxygen phosphine group through a benzene ring, has a large conjugated system, is relatively rigid, and has a maximum energy gap (ET) between HOMO and LUMO of 3.71 to 3.85 eV. From the perspective of molecular configuration, the oxygen phosphine group is an electron donor, the benzene ring is a π bridge, and the imidazole is an electron acceptor. The oxygen phosphine group is connected to multiple phenyl groups, which increases the conjugation of the oxygen phosphine group and changes the hole transport performance of the molecule. In addition, the introduction of the oxygen phosphine group can also improve the solubility and thermal stability of the fluorescent molecule, such as the thermal stability reaches about 250° C., which is conducive to processing and widens its application range.

The present invention also provides a method for synthesizing an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure, the method comprising the following steps:

1) Diphenylphosphine oxide and p-formylphenylboronic acid are reacted by Suzuki coupling reaction to obtain 4-diphenylphosphonylbenzaldehyde;

2) 4-diphenylphosphonylbenzaldehyde reacts with benzil or 9,10-phenanthrenequinone via Debus-Radziszewski imidazole synthesis to obtain a compound of formula 1 or formula 2.

The invention synthesizes an imidazole-type fluorescent molecule containing a triphenoxyl structure through two-step classical reactions. First, diphenylphosphine oxide and p-formylphenylboronic acid are subjected to a Suzuki coupling reaction to synthesize 4-diphenylphosphinoylbenzaldehyde. Then, 4-diphenylphosphinoylbenzaldehyde is subjected to a classical Debus-Radziszewski imidazole synthesis reaction with a diketone compound and an aromatic amine (or ammonia) to obtain a fluorescent molecule having both a triphenoxyl structure and an imidazole structure.

As a preferred solution, diphenylphosphine oxide and p-formylphenylboronic acid are reacted in the presence of 1,4-bis(diphenylphosphino)butane, palladium acetate and potassium carbonate at a temperature of 85 to 95° C. for 20 to 24 hours.

As a more preferred solution, the molar ratio of diphenylphosphine oxide to p-formylphenylboronic acid is 1:1.5-1.6.

As a more preferred solution, the molar amount of 1,4-bis(diphenylphosphino)butane is 4-6% of the molar amount of diphenylphosphine oxide.

As a more preferred solution, the molar amount of palladium acetate is 4-6% of the molar amount of diphenylphosphine oxide.

As a more preferred solution, the molar amount of potassium carbonate is 0.4 to 0.6 times the molar amount of diphenylphosphine oxide.

As a preferred solution, 4-diphenylphosphonylbenzaldehyde reacts with benzil or 9,10-phenanthrenequinone in the presence of ammonium acetate and glacial acetic acid at a temperature of 115 to 125°C for 15 to 20 hours; or, 4-diphenylphosphonylbenzaldehyde reacts with benzil or 9,10-phenanthrenequinone and aromatic amine in the presence of ammonium acetate and glacial acetic acid at a temperature of 115 to 125°C for 15 to 20 hours.

As a more preferred solution, the molar ratio of 4-diphenylphosphonylbenzaldehyde to benzil or 9,10-phenanthrenequinone is equal.

As a more preferred solution, the molar ratio of ammonium acetate to 4-diphenylphosphonylbenzaldehyde is 2 to 3:1.

As a more preferred solution, the molar ratio of ammonium acetate, arylamine and 4-diphenylphosphonylbenzaldehyde is 1 to 1.5:1 to 1.5:1.

The synthetic route of the imidazole fluorescent molecule containing a triphenylphosphine oxide structure of the present invention is as follows (the four target fluorescent molecule target products are C ₁ , C ₂ , C ₃ and C ₄ ):

The present invention also provides an application of an imidazole type fluorescent molecule containing a triphenylphosphine structure, which is used as a fluorescent probe for detecting Fe ³⁺ . The imidazole type fluorescent molecule containing a triphenylphosphine structure of the present invention not only has good fluorescence performance, but also can be highly selectively coordinated with Fe ³⁺ , and Fe ³⁺ can quench its fluorescence. Utilizing this feature, fluorescence detection of Fe ³⁺ can be achieved, and has the characteristics of high detection sensitivity, low lower limit, etc.

As a preferred solution, the imidazole fluorescent molecule containing a triphenylphosphine oxide structure is applied to the fluorescence detection of Fe ³⁺ in a system where at least one of Al ³⁺ , Ca ²⁺ , Li ⁺ , Cu ²⁺ , Na ⁺ , and In ³⁺ coexists with Fe ³⁺ . The imidazole fluorescent molecule containing a carbazole structure of the present invention has high selectivity for Fe ³⁺ , and various metal ions have little interference with iron ions.

Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

The fluorescent molecules of the present invention all have good thermal stability, for example, the thermal stability of the fluorescent molecules C ₁ , C ₂ , C ₃ and C ₄ all reaches about 250°C.

The fluorescent molecules of the present invention have good fluorescence properties. The fluorescent molecules C1 and C2 are pure white under visible light and show blue fluorescence under 365nm ultraviolet light. Among them, C3 and C4 are light yellow under visible light and show blue-green fluorescence under 365nm ultraviolet light. C2 has the highest fluorescence quantum yield (56.35%) and C4 has the longest fluorescence lifetime (5.1147ns).

The fluorescent molecules of the present invention have both triphenylphosphine structure and imidazole structure. The fluorescent molecules C1 to C4 can all react with metal Fe ³⁺ to form a complex, so that the fluorescence of the molecules is suppressed and they show high selectivity for iron ions. The fluorescence intensity in the solvent is significantly weakened or even quenched with the addition of iron ions, so that they can be used as fluorescent probes for trivalent iron ion detection. Among them, C3 shows the best selectivity and can be used for quantitative detection of Fe ³⁺ . The calculation results show that there is a certain linear relationship between the fluorescence intensity of the four molecules and the iron ion concentration, and the detection limits LOD are 9.61× ^10-5 M, 7.98× ^10-5 M, 6.75× ^10-5 M and 8.02× ^10-5 M, respectively.

The fluorescent molecule synthesis method and steps of the present invention are simple, and can be synthesized through classic Suzuki coupling and Debus-Radziszewski imidazole synthesis reaction, etc., and the raw materials are easily available, the yield is considerable, and it is conducive to expanding production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is the ¹ H NMR spectrum of the fluorescent molecule C1.

FIG2 is the ¹³ C NMR spectrum of the fluorescent molecule C1.

FIG3 is the HRMS spectrum of the fluorescent molecule C1.

FIG4 shows the FTIR spectra of fluorescent molecules C1 to C4.

FIG5 is the normalized UV-visible absorption spectra of fluorescent molecules C1 to C4.

FIG6 (a) is the solid-state fluorescence emission spectra of fluorescent molecules C1 to C4; FIG6 (b) is the solid-state fluorescence emission spectra of fluorescent molecules C1 to C4 after normalization.

FIG7 (a) is the chromaticity coordinates of fluorescent molecules C1-C4; (b) is a photograph of fluorescent molecules C1-C4 under natural light (i) and 365 nm ultraviolet light (ii).

Figure 8 (A) shows the PL spectra of C1 in five solvents with different polarities (1×10 ^–5 mol/L); (B) shows the PL spectra of C2 in five solvents with different polarities (1×10 ^–5 mol/L).

Figure 9 (C) shows the PL spectra of C3 in five solvents with different polarities (1×10 ^–5 mol/L); (D) shows the PL spectra of C4 in five solvents with different polarities (1×10 ^–5 mol/L).

FIG10 shows the fluorescence emission spectra (1a, 2a, 3a, 4a) and histograms (1b, 2b, 3b, 4b) of fluorescent molecules C1 to C4 in different ionic solutions.

FIG. 11 is a fluorescence image of fluorescent molecules C1 to C4 under 365 nm ultraviolet irradiation; the sample concentration in THF/H ₂ O (f _W = 20%) solution is 1×10 ^–5 mol/L.

Figure 12 (a) shows the fluorescence spectra of C1 in solutions with different Fe ³⁺ concentrations; (b) shows the fluorescence emission intensity of C1 changing with the Fe ³⁺ concentration; the sample concentration in THF/H ₂ O (f _W ＝20％) solution is 1×10 ^–5 mol/L; the Fe ³⁺ concentration is 0～1×10 ^–2 mol/L.

FIG. 13 shows the CV curves of fluorescent molecules C1 to C4.

DETAILED DESCRIPTION

The following examples are intended to further illustrate the present invention in detail, rather than to limit the scope of protection of the claims.

The detection methods and instruments involved in the following examples are:

Bruker-Avance 400 MHz NMR instrument was used to measure ¹ H (400 MHz) and ¹³ C (100 MHz) NMR spectra, with CDCl ₃ or DMSO-d ₆ as solvent and tetramethylsilane (TMS) as internal standard. Chemical shifts were expressed in ppm and all coupling constants (J values) were expressed in Hertz (Hz).

Infrared spectroscopy was performed using a Perkin-Elmer SP One infrared spectrophotometer using potassium bromide (KBr) pellets.

The melting points of the prepared compounds were determined using a WC-1 micro melting point apparatus, and the temperatures were not corrected.

The UV-visible absorption spectra (wavelength: 200–800 nm) were measured using a Hitachi U-3100 spectrophotometer.

The fluorescence spectrum was measured using a Hamamatsu steady-state transient fluorescence spectrometer (Edinburgh Instruments, FLS980) under HeCd laser excitation at 325 nm.

Cyclic voltammetry (CV) measurements were performed on an AV1301A electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.) with a scan rate of 50 mV/s. The three electrodes consisted of a platinum wire working electrode, a platinum wire counter electrode, and a silver/silver chloride reference electrode.

The chemical agents involved in the following examples, unless otherwise specified, are conventional commercially available drugs.

Example 1

(1) Synthesis of 4-diphenylphosphonylbenzaldehyde

Under nitrogen atmosphere, diphenylphosphine oxide (5mmol), 4-formylphenylboronic acid (7.5mmol), 1,4-bis(diphenylphosphino)butane (0.25mmol), palladium acetate (0.25mmol), potassium carbonate (2.5mmol) and 15mL dioxane were added to a 50mL flask with a polytetrafluoroethylene branch pipe mouth in sequence, and reacted at 90℃ for 24h. After the reaction, it was filtered with diatomaceous earth while hot, extracted with saturated salt water and dichloromethane, and dried to obtain a crude product, which was separated and purified by column chromatography (eluent: petroleum ether/dichloromethane, volume ratio: 10:1) to obtain a white solid with a yield of about 71%. ¹ H NMR (400MHz, CDCl ₃ ): δ(ppm)＝9.88(d,J＝15.4Hz,1H),7.78(s,2H),7.36–7.14(m,12H).

(2) Synthesis of 2-(4-diphenylphosphonylphenyl)-4,5-diphenyl-1H-imidazole (C1)

In a nitrogen environment, benzil (5mmol), ammonium acetate (10mmol), 4-diphenylphosphonylbenzaldehyde (5mmol) and glacial acetic acid (20mL) were added to a 150mL three-necked flask and reacted at 120℃ for 18h. After the reaction, the mixture was allowed to stand and cool to room temperature, and an appropriate amount of water was added to precipitate a gray solid. The crude product was obtained by suction filtration, washing with water and drying. The crude product was separated and purified by column chromatography (eluent was petroleum ether/ethyl acetate, volume ratio was 10:1) to obtain a white solid with a yield of 74%; the melting point was 239–240℃. ¹ H NMR (400MHz, DMSO-d ₆ ): δ (ppm) = 12.94 (s, 1H), 8.27–8.20 (m, 2H), 7.79–7.62 (m, 8H), 7.62–7.49 (m, 8H), 7.46 (t, J = 7.3Hz, 2H), 7.40 (d, J = 7.1Hz, 1H), 7.31(t,J＝7.4Hz,2H),7.23(t,J＝7.2Hz,1H); ¹³ C NMR (100MHz, CDCl ₃ ): δ (ppm) = 145.43, 137.90, 137.11, 137.00, 135.55, 134.13, 133.91, 133.72, 131.49, 131.32, 129.54, 129.33, 129.26, 129.15, 129.09, 128.98, 12 8.66,128.33,127.55,127.04,125.88,125.81; HRMS Calculated for C ₃₃ H ₂₆ N ₂ OP[M+H] ⁺ m/z 497.1777, found 497.1770.

(3) Synthesis of 2-(4-diphenylphosphonylphenyl)-1,4,5-triphenyl-1H-imidazole (C2)

In a nitrogen environment, benzil (5mmol), ammonium acetate (5mmol), aniline (5mmol), 4-diphenylphosphonylbenzaldehyde (5mmol) and glacial acetic acid (20mL) were added to a 150mL three-necked flask and reacted at 120℃ for 20h. After the reaction, the mixture was allowed to stand and cool to room temperature, and an appropriate amount of water was added to precipitate a gray solid. The crude product was obtained by suction filtration, washing with water and drying. The crude product was separated and purified by column chromatography (eluent was petroleum ether/ethyl acetate, volume ratio was 10:1) to obtain a white solid with a yield of about 78%; the melting point was 223–225℃. ¹ H NMR (400MHz, CDCl ₃ ): δ (ppm) = 7.60 (ddd, J = 31.2, 17.8, 8.1Hz, 1H), 7.45 (t, J = 6.3Hz, 1H), 7.33–7.27 (m, 1H), 7.25–7.15 (m, 1H), 7.12 (d, J = 6.9Hz, 1H), 7.0 4(d, J=7.1Hz, 1H); ¹³ C NMR (100MHz, CDCl ₃ ): δ (ppm) = 145.56, 138.71, 136.68, 134.02, 133.83, 132.78, 132.71, 132.15, 132.05, 132.02, 131.92, 131.72, 131.08, 130.22, 129.37, 128.71, 12 8.60,128.48,128.44,128.30,128.23,127.37,126.88; HRMS Calculated for C ₃₉ H ₃₀ N ₂ OP[M+H] ⁺ m/z 573.2090, found 573.2100.

(4) Synthesis of 2-(4-diphenylphosphonylphenyl)-1H-phenanthro[9,10-d]imidazole (C3)

Similar to the synthesis method of C1, light yellow solid, yield 58%; melting point 247-249°C. ¹ H NMR (400 MHz, CDCl ₃ ): δ (ppm) = 8.70 (t, J = 8.4 Hz, 1H), 8.58 (d, J = 7.6 Hz, 1H), 8.43 (d, J = 6.6 Hz, 1H), 8.28 (d, J = 7.4 Hz, 1H), 7.88 (dd, J = 11.1, 8.2 Hz, 1H), 7.83-7.40 (m, 7H); ¹³ C NMR (100 MHz, CDCl ₃ ): δ (ppm) = 160.84, 145.24, 135.62, 132.73, 132.63, 132.26, 132.20, 132.10, 131.59, 130.63, 129.55, 128.96, 128.77, 128.65, 127.53, 127.36, 12 6.99,126.87,126.77,126.37,126.00,123.76,123.44,122.92,120.95,120.82.HRMS Calculated for C ₃₃ H ₂₄ N ₂ OP[M+H] ⁺ m/z 495.1621,found 495.1602.

(5) Synthesis of 1-phenyl-2-(4-diphenylphosphorylphenyl)-1H-phenanthro[9,10-d]imidazole (C4)

Similar to the synthesis method of C2, pale white solid, yield 55%; melting point 242-243°C. ¹ H NMR (400 MHz, DMSO-d ₆ ): δ (ppm) = 8.90 (dd, J = 20.8, 8.3 Hz, 2H), 8.68 (d, J = 7.8 Hz, 1H), 7.88-7.48 (m, 19H), 7.34 (t, J = 7.6 Hz, 1H), 7.07 (d, J = 8.2 Hz, 1H), 5.95 (s, 1H); ¹³ C NMR (100 MHz, CDCl ₃ ):δ(ppm)＝149.45,138.35,137.60,132.64,132.27,132.14,132.11,132.08,132.05,131.95,131.59,130.39,130.16,129.50,129.24,129.12,128.93,128.64,128.56,128.52,128.41,127.39,127.07,126.40,125.86,125.24,124.14,123.17,122.89,122.70,120.95；HRMS Calculated forC ₃₉ H ₂₈ N ₂ OP[M+H] ⁺ m/z 571.1934,found.571.1932.

Structural characterization:

The structures of the four imidazole molecules C1 to C4 were characterized by nuclear magnetic resonance hydrogen spectrum, carbon spectrum and Fourier transform infrared spectrum. The hydrogen spectrum of 4-diphenylphosphonylbenzaldehyde showed a characteristic single peak of hydrogen at the 1st position of the aldehyde group near the chemical shift value of 10.0ppm, with a number of 1, and the total number of hydrogen atoms was the same as the total integral fraction in the spectrum, indicating that the aldehyde was successfully prepared. The hydrogen spectrum of C1 is shown in Figure 1. The hydrogen single peak of the N-H bond in the imidazole ring appeared near the chemical shift value of 13.0ppm, and the double peak of hydrogen on the 2nd position of the benzene ring in the triphenylphosphine oxide structure appeared near 8.25ppm; the remaining hydrogens were all near the aromatic region, and the total number of hydrogen atoms was equal to the remaining integral fraction in the hydrogen spectrum. Comprehensive carbon spectrum Figure 2 and mass spectrum Figure 3 of C1: the peak at 145.430ppm is the characteristic peak of imidazole carbon No. 2, and the total number of carbon peaks is less than or equal to the total number of carbons in the same chemical environment in C1; the theoretical value of the molecule after hydrogenation in the mass spectrum is 497.1777, and the actual value is 497.1770, which is within the standard molecular weight error range of the mass spectrometry, indicating that C1 was successfully synthesized.

The Fourier infrared spectra of C1 to C4 are shown in Figure 4. The vibration absorption peak of the imidazole N-H bond in the C1 and C3 structures is at 3440 cm ^-1 . Since C1 to C4 all contain structures such as benzene rings, phosphine oxide double bonds, and imidazole ring carbon-nitrogen double bonds, they have the same absorption peaks, such as: 3051 cm ^-1 is the =C-H stretching vibration absorption peak of the benzene ring, 1603 cm ^-1 is the characteristic absorption peak of C=N in the imidazole ring, 1580 cm ^-1 and 1600 cm ^-1 are the absorption peaks of C=C of the benzene ring, and 1190 cm ^-1 is the characteristic absorption peak of P=O.

UV spectrum:

C1 to C4 were dissolved in THF solution to prepare the test solution with a concentration of 1×10 ^–5 mol/L, and then UV-visible absorption spectrum test was performed. After normalization, it is shown in Figure 5. From Figure 5, it can be seen that C1 and C2 have two obvious absorption bands, approximately around 240nm and 330nm; compounds C3 and C4 have five relatively obvious absorption bands, approximately around 250nm, 260nm, 335nm, 345nm and 365nm. The absorption bands at 240nm, 250nm and 260nm should be caused by the π-π* transition of the benzene ring group, and the absorption band near 330nm belongs to the P-P* transition of imidazole. Compared with C1, the absorption band of C2 around 330nm has blue-shifted by about 13nm, which may be due to the higher degree of conjugation of the molecule after the N-H bond of imidazole is replaced by a benzene ring; compared with C3, there is no obvious change in the positions of the three absorption peaks of C4 above 330nm, which may be related to their very similar structures.

Solid-state fluorescence analysis:

The fluorescence properties of C1-C4 in the solid state were tested by a steady-state transient fluorescence spectrometer. Figure 6(a) and Figure 6(b) are the fluorescence emission spectra of C1-C4 in the solid state and their normalized graphs, respectively. Figure 7(a) and Figure 7(b) are their color coordinate diagrams and solid photos under natural light or 365nm ultraviolet light, respectively. Table 1 shows the excitation wavelength, emission wavelength, fluorescence lifetime and fluorescence quantum yield of compounds C1-C4 in the solid state. It can be seen that except for C4, which is a light yellow solid under visible light, the rest are white solids; under 365nm ultraviolet light, C1-C4 are roughly blue-green fluorescence, which is consistent with the color of the light area where the C1-C4 coordinates are located in the color coordinate diagram, and the emission wavelength is about 400nm. Among them, C1 has the largest solid-state fluorescence emission intensity, which is 2.76×10 ⁵ (au), C4 has the longest fluorescence lifetime, which is 5.1147ns, and C2 has the highest fluorescence quantum yield, which is 56.35%. Since C1 and C2 have similar structures, C2 adds a benzene ring on the basis of C1. Similarly, C4 introduces one more benzene ring than C3. By comparing their fluorescence lifetimes and fluorescence quantum yields, it can be seen that the introduction of benzene rings can improve the fluorescence lifetimes and fluorescence quantum yields of the corresponding compounds (for example, the fluorescence quantum yield of C2 is 56.35%, that of C1 is 17.62%; the fluorescence lifetime of C4 is 5.1147ns, and that of C3 is 0.4778ns). This may be because the introduction of benzene rings makes the electrons of the entire molecule more averaged in the entire conjugated system, which not only increases the degree of conjugation, but also regulates the donor-acceptor capacity in the molecule to a certain extent. This provides a good strategy for the design and regulation of luminescent groups with better performance in the future.

Table 1. Fluorescence data of fluorescent molecules C1 to C4

^aThe maximum excitation wavelength of solid powder.

^b Maximum emission wavelength of solid powder.

^c Fluorescence intensity of solid powder.

^d Fluorescence lifetime of solid powder.

^e Fluorescence quantum yield of solid powder.

Solvation Effects:

Table 2. Maximum emission wavelengths ^a of C1-C4 in five solvents

^aSolution concentration: 1×10 ^–5 mol/L.

Five solvents with good solubility were selected: DMF, acetone, dichloromethane, tetrahydrofuran and toluene, and the solvation effect of C1~C4 was analyzed respectively. The maximum absorption wavelength data of the five solvents are shown in Table 2. The test solution concentration is 1× ^10-5 mol/L. Figure 8 (A), (B) and Figure 9 (C), (D) correspond to the normalized spectra and liquid fluorescence photos of C1~C4 in five different solvents. Analysis shows that: C1~C4 have no obvious solvation color change effect, and the liquid fluorescence is blue light. Among them, the absorption wavelength of C3 has a gradually increasing change with the increase of solvent polarity, that is, red shift; the reason for the lack of solvation effect may be that C3 is not obvious in these five solvents, but from the perspective of liquid fluorescence, it is unlikely that C3 has a solvation effect.

Metal ion detection performance analysis:

(1) Selective detection of metal ions:

Select inorganic salts with chloride ions as anions and different metal cations to eliminate the interference of anions. Seven kinds of inorganic salts containing chloride ions: AlCl ₃ , CuCl ₂ , LiCl, NaCl, CaCl ₂ , InCl ₃ and FeCl ₃ , a mixed solution of THF/H ₂ O (f _w = 30%) was selected as the solvent; taking C1 as an example, 3 mL of a mixed solution of tetrahydrofuran/water (f _w = 30%) containing C1 and having a sample concentration of 1×10 ^-5 mol/L was first prepared, and then a certain amount of inorganic salts were weighed and added to the mixed solution to make the metal ion concentration 1×10 ^-2 mol/L, and then gently shaken and placed in an ultrasonic cleaning instrument, and the ultrasonic power was set to 100% and the time was set to 30 min. After the metal ions were fully complexed with the test solution containing the sample, the liquid fluorescence emission test was immediately performed after standing for 24 hours to ensure that the test solutions containing different metal ions were tested under the same voltage, the same excitation wavelength and the same batch. The same method was used for C2 to C3.

The fluorescence emission curves of C1 to C4 are shown in Figure 10. Combined with their liquid fluorescence photos under 365nm ultraviolet light, Figure 11, it can be seen that their maximum fluorescence emission intensities in Al ³⁺ , Li ⁺ , Na ⁺ , Ca ²⁺ , and In ³⁺ solutions remain at a high level, and some are even slightly enhanced. There is a certain fluorescence inhibition in Cu ²⁺ solution, which is not particularly obvious, but there is a strong fluorescence inhibition phenomenon in Fe ³⁺ solution, and the fluorescence almost disappears. This may be because C1 and C3 containing imidazole structures have unsubstituted N-H bonds, and the lone pair of electrons on their N can combine with Fe ³⁺ to form a complex, destroying the conjugated system of the molecule and the aggregation mode between molecules, thereby weakening the fluorescence of C1 and C3 in tetrahydrofuran aqueous solution. Through preliminary ion screening, it was found that C1 and C3 can selectively sense Fe ³⁺ . The specific reasons for C2 and C4 are unknown, but both have the potential to be Fe ³⁺ fluorescent probes.

(2) Metal ion sensitivity detection:

Through the metal ion selectivity analysis in the previous step, it was found that Fe ³⁺ has a strong inhibitory effect on the liquid fluorescence intensity of C1~C4, and C1~C4 can be considered to be applied to Fe ³⁺ detection. Taking C1 as an example, within a certain concentration range, sample solutions with Fe ³⁺ concentrations of 0, 0.005× ^10-2 M, 0.0125× ^10-2 M, 0.02 ^{× 10-2} M, 0.0275× ^10-2 M, 0.035× ^10-2 M, 0.1× ^10-2 M and 1× ^10-2 M were prepared. The sample solution was a tetrahydrofuran/water solution ( _fw = 30%) containing C1 at a concentration of 1× ^10-5 M. After shaking evenly, it was placed in an ultrasonic cleaner for 30 minutes, with the power set to 100%. After the ultrasonication was completed, it was left to age for one day, and then the liquid fluorescence emission test of different concentrations of Fe ³⁺ was immediately carried out.

According to the Stern–Volmer equation: I ₀ /I＝1+K _SV [Fe ³⁺ ], their linear relationship was fitted, where I ₀ and I are the fluorescence emission intensities of C1 without Fe ³⁺ and with Fe ³⁺ at the highest fluorescence emission wavelength, [Fe ³⁺ ] represents the change in iron ion concentration, and K _SV is the Stern–Volmer constant. Then the formula LOD＝3σ/K was used for calculation, where LOD represents the ion detection limit, σ is the standard deviation of the fluorescence emission intensity of blank solvent measured 15 times when C1 does not add Fe ³⁺ , and K represents the slope of the fitted linear function, which is the K _SV value.

As shown in Figure 12, the fluorescence quenching response of C1 to different concentrations of Fe ³⁺ has regular changes, and its fluorescence emission intensity decreases linearly with the increase of ion concentration. According to the Stern–Volmer equation, we calculated that the LOD of C1 is 9.61×10 ^{- 5} M, and its linear equation is I ₀ /I＝1.6417+7770[Fe ³⁺ ](R ² ＝0.9965), where R ² reaches about 0.99, which can be considered to have the accuracy standard as a fluorescent probe. The linear equations of the remaining C2 to C4 are similar to the above method.

Electrochemical performance analysis:

Take C1 as an example: tetrabutylammonium hexafluorophosphate is used as the electrolyte and deoxygenated THF is used as the solvent. A certain mass of C1 is weighed to prepare a test solution with a concentration of 1 mg/mL. The test solution is deoxygenated and immediately subjected to CV testing. Ag/AgCl is selected as the reference electrode, and the redox potential is calibrated using ferrocene-ferrocene (Fc–Fc ⁺ ) redox pair as the external standard; platinum wire is used as the auxiliary electrode and the glassy carbon electrode is used as the working electrode for testing. First, turn on the computer host and cyclic voltammetry test software, connect the electrochemical workstation, and set the parameters after the connection is stable: the scan rate is set to 6.25mV/s, the number of turns is set to 4, and the voltage range is continuously optimized after testing.

Table 3. Electrochemical properties of fluorescent molecules C1 to C4

^a Theonset of oxidation potentials relative to Fc–Fc ⁺ couple.

^b Estimated from absorption onset.

^c Determined from E _{onset OX} .

^d Estimated from E _{onset OX} and E _g.

After such a reciprocating process, the cyclic voltammetry test curve of C1 to C4 is obtained in Figure 13. According to the figure and calculation formula, the oxidation and reduction potentials and electrochemical data are listed in Table 3. The highest molecular orbital (HOMO) energy level and the lowest molecular orbital (LUMO) energy level are calculated using the formulas E _g =(1240/λ _start )(eV), E _LUMO =(E _HOMO +E _g )(eV). λ _start can be estimated from the onset wavelength of the UV-visible absorption spectrum.

Through the above calculation formula, we can get the HOMO/LUMO results of C1, C2, C3, and C4 as -4.74/-1.03eV, -5.32/-1.47eV, -5.25/-1.56eV, and -5.29/-1.58eV, respectively. Similarly, the electrochemical energy gaps of C1 to C4 are calculated to be 3.71eV, 3.85eV, 3.69eV, and 3.71eV, respectively. The difference is not big, and by comparing C1 and C3, and C2 and C4, respectively, we can see that the introduction of benzene rings can increase the energy gap of molecules with similar structures.

Claims (8)

1. An imidazole fluorescent molecule containing a triphenylphosphine oxide structure, characterized in that it has the following structure: , or . 2. The method for synthesizing an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure according to claim 1, characterized in that it comprises the following steps: 1) Diphenylphosphine oxide and p-formylphenylboronic acid are reacted by Suzuki coupling reaction to obtain 4-diphenylphosphinoylbenzaldehyde; 2) 4-Diphenylphosphonylbenzaldehyde reacts with benzil or 9,10-phenanthrenequinone through the Debus-Radziszewski imidazole synthesis reaction to obtain an imidazole-type fluorescent molecule containing a triphenoxyphosphine structure. 3. The method for synthesizing an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure according to claim 2, characterized in that diphenylphosphine oxide and p-formylphenylboronic acid are reacted in the presence of 1,4-bis(diphenylphosphino)butane, palladium acetate and potassium carbonate at a temperature of 85 to 95 ^° C for 20 to 24 hours. 4. The method for synthesizing an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure according to claim 3, characterized in that: The molar ratio of diphenylphosphine oxide to p-formylphenylboronic acid is 1:1.5-1.6; The molar amount of 1,4-bis(diphenylphosphino)butane is 4 to 6% of the molar amount of diphenylphosphine oxide; The molar amount of palladium acetate is 4 to 6% of the molar amount of diphenylphosphine oxide; The molar amount of potassium carbonate is 0.4 to 0.6 times the molar amount of diphenylphosphine oxide. 5. The method for synthesizing an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure according to claim 2, characterized in that: 4-diphenylphosphonylbenzaldehyde reacts with benzil or 9,10-phenanthrenequinone in the presence of ammonium acetate and glacial acetic acid at 115-125 ^° C for 15-20 hours; or, 4-Diphenylphosphonylbenzaldehyde reacts with benzil and aniline in the presence of ammonium acetate and glacial acetic acid at 115 ~ 125 ^o C for 15 ~ 20 hours. 6. The method for synthesizing an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure according to claim 5, characterized in that: 4-diphenylphosphonylbenzaldehyde and benzil or 9,10-phenanthrenequinone in equal molar ratios; The molar ratio of ammonium acetate to 4-diphenylphosphonylbenzaldehyde is 2-3:1, or the molar ratio of ammonium acetate, aniline and 4-diphenylphosphonylbenzaldehyde is 1-1.5:1 to 1.5:1. 7. The use of an imidazole-type fluorescent molecule containing a triphenylphosphine oxide structure as claimed in claim 1, characterized in that it is used as a fluorescent probe for detecting Fe ³⁺ . 8. The use of an imidazole-type fluorescent molecule containing a triphenylphosphine structure according to claim 7, characterized in that it is applied to the fluorescence detection of Fe ³⁺ in a system where Fe ³⁺ coexists with at least one of Al ³⁺ , Ca ²⁺ , Li ⁺ , Cu ²⁺ , Na ⁺ , and In ³⁺ .