The synthesis, characterization, and functional properties of two new polymerizable 1,8-naphthalimides (MDs) have been described. Their copolymers with styrene designed to act as a fluorescence PET chemosensor have been investigated. The study also reports the influence of different metal cations (Ag+, Mg2+, Cu2+, Sr2+, Co2+, Pb2+, and Fe3+) on the fluorescence intensity of both low and high molecular weight fluorophores.
In the recent years the development of molecular recognition and sensing systems for different analytes has received considerable attention. Fluorescent chemosensors utilised for detection of metal cations and protons in the environment were based on different molecular structures [
Recently, new polymerizable 1,8-naphthalimide molecules to sensitive protons or metal cations have been synthesized and their functional properties are investigated. Due to the presence of a polymerizable group they can react with some commercial monomers, allowing copolymers with a green or blue fluorescence to be obtained. It was demonstrated that the copolymers of these polymerizable sensors can be used as homogeneous and heterogeneous photoinduced electron transfer (PET) fluorescent sensors sensitive to metal cations [
In this paper we present the synthesis and functional properties of two new polymerizable 1,8-naphthalimides (MDs) and their copolymer with styrene designed to act as fluorescence chemosensors. The influence of different metal cations on the fluorescence intensity of both low and high molecular weight sensors has been investigated and discussed.
4-Amino-N-allyl-1,8-naphthalimide was synthesized according to the method described previously [
UV/vis spectrophotometric investigations were performed using “Thermo Spectronic Unicam UV 500” spectrophotometer. The fluorescence spectra were taken from a “Cary Eclipse” spectrophotometer. All spectra were recorded using 1 cm path length synthetic quartz glass cells. Fluorescence quantum yield was determined on the basis of the absorption and fluorescence spectra, using anthracene as reference (
The effect of the metal cations on fluorescence intensity has been measured by adding a few microliters of stock aqueous solution (
IR analysis of compounds was carried out using the infrared Fourier transform spectrometer (IRAffinity-1 “Shimadzu”) with the diffuse-reflectance attachment (MIRacle Attenuated Total Reflectance Attachment). The NMR spectra were obtained from a Bruker DRX-250 spectrometer, operating at 250.13 and 62.90 MHz for 1H and 13C, respectively, using a dual 5 mm probe head. Deuteriochloroform and tetramethylsilane were used as a solvent and an internal standard.
Thin layer chromatographic (TLC) analysis of the monomeric dyes was followed on silica gel (Fluka F60 254
The molecular characteristics of the poly(St-co-MD) were determined on a GPC Water 244 apparatus. The solvent was THF at a flow rate of 1.0 mL min−1 at 45°C. Both a differential refractive index and an UV/vis absorption detector (
Synthesis of monomeric MD1 and MD2.
4-Amino-N-allyl-1,8-naphthalimide (0.01 mol) was dissolved in 50 mL of acetic acid and at 50°C 2 mL chloroacetyl chloride was added dropwise. The mixture was stirred at this temperature and after 2 h the product was isolated by pouring it into water and filtrated. After that 0.01 mol of this product was dissolved in dioxane and 0.05 mol of amines was added and stirred at 80°C for 4 h. The final products have been isolated with high yields and purity.
Yield 89%. FTIR (KBr) cm−1: 3248, 2935, 2818, 1692, 1651, 1614, 1538, 1380, 1235, 781.
1H-NMR (CHCl3)
13C-NMR (CHCl3) Analysis: C19H19N3O3 (337.22 g mol−1): Calc. (%): C-67.67, H 5.63, N 12.46. Found (%): C-67.44, H 5.69, N 12.39.
Yield 92%. FTIR (KBr) cm−1: 3223, 2946, 2838, 1693, 1656, 1620, 1532, 1379, 1237, 777.
1H-NMR (CHCl3)
13C-NMR (CHCl3) Analysis: C21H21N3O4 (379.24 g mol−1): Calc. (%): C-66.50, H 5.54, N 11.08. Found (%): C-66.84, H 5.48, N 11.19.
Free radical copolymerization of styrene with MDs was carried out in bulk [
The photophysical properties of 1,8-naphthalimide are known to depend mainly on the polarization of its chromophore system. In this study the functional properties of MDs were investigated in organic solvents of different polarity. Tables
Photophysical characteristics of the MD1 in organic solvents of different polarities (see text).
Solvent |
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Dimethyl sulfoxide | 375 | 458 | 11100 | 4833 | 0.03 | 0.03 | 0.12 |
Dimethylformamide | 374 | 452 | 11900 | 4614 | 0.03 | 0.05 | 0.12 |
Acetonitrile | 371 | 452 | 13000 | 4830 | 0.07 | 0.09 | 0.14 |
Methanol | 366 | 463 | 11900 | 5724 | 0.03 | 0.03 | 0.15 |
Ethanol | 370 | 458 | 12000 | 5193 | 0.04 | 0.04 | 0.14 |
Acetone | 372 | 450 | 12700 | 4660 | 0.01 | 0.02 | 0.13 |
Dichloromethane | 374 | 445 | 13100 | 4266 | 0.05 | 0.09 | 0.12 |
Tetrahydrofuran | 373 | 442 | 11500 | 4185 | 0.10 | 0.18 | 0.10 |
Chloroform | 374 | 440 | 11700 | 4215 | 0.08 | 0.16 | 0.11 |
Photophysical characteristics of the MD2 in organic solvents of different polarities (see text).
Solvent |
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Dimethyl sulfoxide | 375 | 451 | 11400 | 4494 | 0.04 | 0.02 | 0.11 |
Dimethylformamide | 374 | 448 | 12500 | 4417 | 0.06 | 0.03 | 0.12 |
Acetonitrile | 372 | 434 | 13200 | 3840 | 0.11 | 0.05 | 0.11 |
Methanol | 366 | 458 | 12300 | 5488 | 0.04 | 0.03 | 0.15 |
Ethanol | 370 | 450 | 11500 | 4805 | 0.05 | 0.03 | 0.12 |
Acetone | 372 | 434 | 12400 | 3840 | 0.02 | 0.01 | 0.10 |
Dichloromethane | 375 | 451 | 13400 | 4494 | 0.10 | 0.04 | 0.13 |
Tetrahydrofuran | 373 | 434 | 11500 | 3768 | 0.20 | 0.09 | 0.09 |
Chloroform | 375 | 445 | 12400 | 4195 | 0.19 | 0.06 | 0.11 |
In all organic solvents under study, the monomeric 1,8-naphthalimides are colourless absorbing in the ultraviolet region at 366–375 nm and emit blue fluorescence at 434–463 nm. The results show that the substituents in C-4 position do not affect the maxima positions significantly.
The Stokes shift is a parameter, which indicates the difference in the properties and structure of the fluorophore between the ground state
The Stokes shift values clearly depend strongly on the solvent polarity
The ability of the MD molecules to emit absorbed light energy is characterized quantitatively by the quantum yield of fluorescence ΦF. The quantum fluorescence yield of MDs in all organic solvents was calculated on the basis of the absorption and fluorescence spectra using
As seen from the data in Tables
An important characteristic of MDs is the oscillator strength (
The values obtained for
The spectral characteristics of poly(St-co-MD) in the solid state are of interest in order to evaluate this copolymer as a heterogeneous fluorescent sensor. The copolymer was transparent and colourless, emitting blue fluorescence. The absorption and fluorescence spectra of poly(St-co-MD) in the solid state as a thin polymer film (
Absorption and fluorescence spectra of poly(St-co-MD2) in solid state as a thin polymer film (
The amount of MDs incorporated into the polymer macromolecules has been determined spectrophotometrically by using a standard curve [
The molecular weight characteristics of the copolymers obtained are collected in Table
Molecular weight and spectral characteristics in tetrahydrofuran solution of the copolymers (see text).
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( |
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Poly(St-co-MD1) | 1.11 | 0.68 | 1.63 | 375 | 421 | 2914 |
Poly(St-co-MD2) | 1.34 | 0.75 | 1.78 | 373 | 428 | 3446 |
The photophysical properties of MDs in the presence of different metal cations (Ag+, Mg2+, Cu2+, Sr2+, Co2+, Pb2+, and Fe3+) have been investigated in the view of their its potential sensor application. The ability of both MDs to detect metal cations has been tested in acetonitrile solution by monitoring the changes in their absorption and fluorescence spectra in the presence of various metal cations. Acetonitrile was chosen as solvent for all spectral measurements as it guarantee good solubility of the metal salts, ligands, and the respective complexes. Its ability to mix well with water allows direct usage of aqueous solutions of the metal salts in this study.
In the presence of the guest metal cations an enhancement or quenching of fluorescence intensity was observed. The enhancement (FE) and the quenching (FQ) of the fluorescence emission have been used as a qualitative parameter for the detection of metal cations. FE =
Figures
Effect of the metal cations at concentration
Fluorescence quenching factor of MD2 in the presence of different metal cations (
The highest enhancement of the monomer fluorescence intensity is observed in the presence of Fe3+ cations for compound MD1 (FE = 16.42) as can be seen from Figure
Figure
The presence of the guest metal cations (Cu2+, Sr2+, Co2+, Ni2+, and Fe3+) in poly(St-co-MD) solution in this work is signaled by the quenching of the fluorescence intensity. The changes of the fluorescence intensity of poly(St-co-MD1) in aqueous solution, induced by the metal cations, have been investigated and the respective fluorescence responses of a polymer chromophore are presented in Figure
Effect of the metal cations at concentration
The typical change in the fluorescence intensity of the copolymer induced by Fe3+ cations has been presented in Figure
The influence of Fe3+ cations on the fluorescence intensity of the poly(St-co-MD1) at different concentrations of Fe3+ in aqueous solution.
In this paper we discuss the synthesis and some functional properties of two new blue fluorescent polymerizable 1,8-naphthalimides and their copolymers with styrene. The photophysical characteristics of both low and high molecular weight fluorophores have been investigated. The influence of various metal cations (Ag+, Mg2+, Cu2+, Sr2+, Co2+, Pb2+, and Fe3+) on the fluorescence intensity of the new 1,8-naphthalimides has been studied with regard to their potential application as fluorescent sensors for metal ions. It has been shown that the fluorescence intensity depends strongly on the nature of metal cations. The new monomer compounds can detect Pb2+ and Fe3+. It has been shown that the blue fluorescent polymer films possess properties allowing their use as sensors for Fe3+ cations. On the basis of the present investigation it can be assumed that the new monomers and fluorescent copolymers are suitable for the selective detection of Fe3+cations.
The authors declare that there is no conflict of interests regarding the publication of this paper.