Synthesis and Fluorescent Property Study of Novel 1,8-Naphthalimide-Based Chemosensors

A series of novel mono- and di-substituted N-n-butyl-1,8-naphthalimide derivatives were synthesized simultaneously via a three-step reaction. The single crystal structure of N-n-butyl-4-[N′,N′-bis(2′,4′-dichlorobenzoyl)ethylamino]-1,8-naphthalimide (3f) was determined. The UV-vis and fluorescence properties of compound 3f were investigated. The 3f showed highly selective and sensitive fluorescence changes response towards Pb2+. A titration of monomer with Pb2+ ion was performed. When Pb2+ ion concentration increased from 0 to 10 eq., the fluorescent intensity of 3f decreased from 199.97 to 48.21. The pH effect on 3f showed that it is stable in a wide range of pH. The results indicated that 3f might be a probe molecule for Pb2+.


Introduction
Determining the contents of metal cations in environmental objects and biological systems is an important practical task for industry, medicine and ecology and for chemical and biochemical studies [1][2][3][4][5]. Among the extensive modern physicochemical methods of analysis, optical electron spectroscopy has gained great popularity due to the relative simplicity of experimental procedures combined with its high sensitivity toward analytes. In particular, fluorescent chemosensors have been proven to be highly valuable tools for sensing chemical and biological species such as metal ions, anions and amino acids, because of their high sensitivity, selectivity, rapidity, portability and the availability of a wide range of indicator dyes, etc. [6]. There is no doubt that fluorescent chemosensors with differential responses toward multiple components are economic and highly desirable for practical applications.
In recent years, 1,8-naphthalimide derivatives have been widely applied as fluorescent dyes, metal sensors and organic light emitting materials because of their excellent fluorescence properties, high absorption coefficients, good fluorescence and quantum yields, large stokes shifts, good stability and easy modification. The considerable interest in 1,8-naphthalimide derivatives as the photoactive component of optical chemosensors is caused, on the one hand, by relative simplicity of their synthesis, on the other hand, by the great diversity of their photo-physical properties [7][8][9]. Naphthalimide-based probes have been developed to detect H + , Hg 2+ , Zn 2+ , Cu 2+ , Ag + , Cd 2+ , Pd 2+ , Cr 3+ , Al 3+ , Fe 3+ and F − via chromogenic and fluorometric analyses [10][11][12][13][14][15]. Therefore, the naphthalimide derivatives are potential carriers that could be used in the preparation of new optical chemosensors. Though extensive excellent work has been reported on the 1,8-naphthalimide derivatives with an O- [16,17], N- [18], S-substituted group [19], or five-member heterocycle [20,21] in the C-4 position as an electron donating group, it is still meaningful to extend the research of such materials, including introducing different electron-donating groups, such as hydroxyl or ester group to explore their fluorescence property. materials, including introducing different electron-donating groups, such as hydroxyl or ester group to explore their fluorescence property. However, few studies have focused on fluorescent 1,8-naphthalimide esters containing mono-or di-substituted ester at the C-4 position.
In this study, we designed and synthesized a series of N-n-butyl-1,8-naphthalimide fluorescent sensors with a diethyl amino link at the C-4 position. As shown in Scheme 1, a functional group containing a nitrogen atom and a carbonyl group was introduced to 1,8-naphthalimide; these groups are commonly used as acceptor units in probes designed for metal ions and anions [22,23]. The biological importance of the combination of the carbonyl groups with naphthalimide prompted us to explore a new class of fluorescent probe for selective recognition of metal ions in aqueous media. This is the first attempt to synthesize mono-and di-substituted 1,8-naphthalimide esters simultaneously at the C-4 position. In addition, a single crystal was prepared to characterize its spatial configuration and to propose its possible complex with metal ions. The spectroscopic changes that result from structural changes signal the existence of different metal ions.

Synthesis and Characterization
As shown in Scheme 2, the mono-and di-substituted 1,8-naphthalimide esters were synthesized via three steps. Although many efforts have been made to prepare 1,8-naphthalimide derivatives with different substitutions at position 4 [1,9,10,[24][25][26][27][28], this is the first attempt to synthesize mono-and di-substituted 1,8-naphthalimide esters simultaneously at position 4. N-n-Butyl-4-bromo-1,8-naphthalimide was prepared by the convenient substitution of 4-bromo-1,8-naphtalic anhydride with n-butyl amine. The intermediate N-n-butyl-4-(N ,Ndihydroxyethyl)amino-1,8-naphthalene imide (BNI) was obtained through nucleophilic substitution with diethanolamine. The mono-and di-substituted products were obtained simultaneously in fair to moderate yields via the acylation reaction (Scheme 2). To determine the optimal reaction conditions for the efficient preparation of the target molecules, the reaction temperature was varied. The results indicated that room temperature is crucial for preventing the facile generation of anhydride during the addition of aroyl chloride. The target molecular structure depended on the molar ratio of compound BNI and aroyl chloride. When the molar ratio of compound BNI and aroyl chloride was less than 1:1.5, the product was primarily the mono-substitution product 4. However, when the molar ratio was 1:2.2-1:2.5, both monoand di-substituted products were generated. The yields of compounds 3 and 4 indicated that the substituents on the benzene ring exerted negligible effects. All of the 1,8-naphthalimide derivatives were confirmed via 1 H-NMR, 13 C-NMR, MS, elemental analysis and the data are reported in the experimental section.
To further confirm the structure of the synthesized compounds, a single-crystal structure of compound 3f was obtained by dissolving the crystal in ethanol followed by slow evaporation of the solvent at room temperature over approximately 3 days. The molecular structure of compound 3f is shown in Figure 1. A packing diagram is shown in Figure 2. In Figure 2 showed that all of the atoms of the rings were almost in the same plane. The 3D net consists of moderate π-π stacking interactions with a centroid-to-centroid distance of 3.6548 Å and the shortest distance (X1A to C2, C3) between the core planes was 3.490 Å (Figure 3).

Absorption Spectra and Fluorescence
The photo-physical properties of 1,8-naphthalimides mainly originate from the polarization of the naphthalimide moiety due to the electron donor-acceptor interaction that occurs between the substituents at C-4 (electron donor) and the carbonyl groups from the imide structure (electron acceptor) of the molecule [29]. The normalized UV-vis spectra of compounds 3a-3f and 4a-4f in

Absorption Spectra and Fluorescence
The photo-physical properties of 1,8-naphthalimides mainly originate from the polarization of the naphthalimide moiety due to the electron donor-acceptor interaction that occurs between the substituents at C-4 (electron donor) and the carbonyl groups from the imide structure (electron acceptor) of the molecule [29]. The normalized UV-vis spectra of compounds 3a-3f and 4a-4f in

Absorption Spectra and Fluorescence
The photo-physical properties of 1,8-naphthalimides mainly originate from the polarization of the naphthalimide moiety due to the electron donor-acceptor interaction that occurs between the substituents at C-4 (electron donor) and the carbonyl groups from the imide structure (electron acceptor) of the molecule [29]. The normalized UV-vis spectra of compounds 3a-3f and 4a-4f in

Absorption Spectra and Fluorescence
The photo-physical properties of 1,8-naphthalimides mainly originate from the polarization of the naphthalimide moiety due to the electron donor-acceptor interaction that occurs between the substituents at C-4 (electron donor) and the carbonyl groups from the imide structure (electron acceptor) of the molecule [29]. The normalized UV-vis spectra of compounds 3a-3f and 4a-4f in EtOH/H 2 O (v/v = 4:1) with a concentration of 1 × 10 −5 M are listed in Table 1. As shown in Table 1, the main absorption band of these dyes is centered between 414 and 440 nm. The main absorption band of mono-substituted compounds was centered at 440 nm and that of the di-substituted compounds was centered at 420 nm because the substituents at benzene affected the molecular polarities of compounds 3 and 4.  The emission was detected at 521-529 nm. Compared with the emission maximum of compound 3a (λ em = 521 nm), those of the other di-substituted compounds, namely 3b-3f, were all slightly red-shifted, which might be the result of the electron-withdrawing and electron-donating capabilities of the substituents of the benzene. In addition, the emission maxima of the mono-substituted compounds were almost equal [30]. However, the mono-substituted compounds exhibited greater fluorescence quantum yield (Φ F ) at approximately 329-398 nm, almost twofold greater than that of the di-substituted compounds. Di-substitution with hydroxyl groups, which exerted a steric effect, resulted in a bathochromic shift in the emission maximum. Because of this spatial steric effect, the fluorescence emissions of all the mono-substituted compounds exceeded those of the di-substituted compounds. This result indicated that the mono-substituted compounds might be better probe molecules due to their greater fluorescence quantum yield and the large space between the hydroxyl and carbonyl.

Solvent Effect
Compound 3f was selected for the fluorescence intensity experiment because its single crystal structure was determined by X-ray analysis, which was able to explain the ion-recognition performance based on the molecular configuration. As shown in Figure 4, the fluorescence emission of compound 3f is more dependent on the solvent. As the solvent polarity increased, the emission wavelength λ em was red-shifted and the fluorescence intensity decreased. The fluorescence maximum shifts to longer wavelengths from nonpolar to polar solvents are indicative of the charge transfer nature of the emitting state. As shown in Figure 4, the λ em for 3f shifted from 495 nm to 529 nm as the solvent was changed from dichloromethane to dimethylformamide. Examination of the chemical structure of entitled dyes suggested that the occurrence of an intramolecular charge transfer (ICT) process might be responsible for their solvent sensitivity. The absorption of light by these molecules is accompanied by electron density shift from the donor to the acceptor and most often, results in an increase in the dipole moment on going to the excited state. Depending on the mode of receptor attachment, binding of the cation would induce either a bathochromic or hypochromic shift of the absorption maximum with a simultaneous change in the intensity [31]. Therefore, EtOH and H 2 O were selected as the solvents to reduce the use of organic solvents and allow the dissolution of metal ions. hypochromic shift of the absorption maximum with a simultaneous change in the intensity [31]. Therefore, EtOH and H2O were selected as the solvents to reduce the use of organic solvents and allow the dissolution of metal ions.  Figure 5 shows the fluorescence emission spectra of compound 3f in EtOH/H2O (v/v = 4:1) solution (1 × 10 −5 M) that was excited at its absorption maximum. As shown in Figure 5, upon addition of 3 eq. of each cation, only Pb 2+ induced a distinct spectrum change while other metal ions (Cu 2+ , Ag + , Na + , K + , Ba 2+ , Ca 2+ , Co 2+ , Mg 2+ , Mn 2+ , Ni 2+ , Sn 2+ , Zn 2+ , Al 3+ , Hg 2+ and Fe 3+ ) showed either no or slight changes in the fluorescence spectra relative to the free compound 3f, which indicated that compound 3f might be a probe molecule for Pb 2+ . The fluorescence intensity difference might be attributable to the formation of a compound 3f-Pb 2+ complex. The fluorescence intensity difference might be attributable to the formation of a 3f-Pb 2+ complex. The fluorescence intensity of 3f is strong in original, after Pb 2+ added, the N, carbonyl O and OH are likely to coordinate with it, resulting in fluorescence decrease. On the basis of the aforementioned fact, a plausible binding mode of the complex was proposed, as shown in Figure 6.    Figure 5 shows the fluorescence emission spectra of compound 3f in EtOH/H 2 O (v/v = 4:1) solution (1 × 10 −5 M) that was excited at its absorption maximum. As shown in Figure 5, upon addition of 3 eq. of each cation, only Pb 2+ induced a distinct spectrum change while other metal ions (Cu 2+ , Ag + , Na + , K + , Ba 2+ , Ca 2+ , Co 2+ , Mg 2+ , Mn 2+ , Ni 2+ , Sn 2+ , Zn 2+ , Al 3+ , Hg 2+ and Fe 3+ ) showed either no or slight changes in the fluorescence spectra relative to the free compound 3f, which indicated that compound 3f might be a probe molecule for Pb 2+ . The fluorescence intensity difference might be attributable to the formation of a compound 3f-Pb 2+ complex. The fluorescence intensity difference might be attributable to the formation of a 3f-Pb 2+ complex. The fluorescence intensity of 3f is strong in original, after Pb 2+ added, the N, carbonyl O and OH are likely to coordinate with it, resulting in fluorescence decrease. On the basis of the aforementioned fact, a plausible binding mode of the complex was proposed, as shown in Figure 6. hypochromic shift of the absorption maximum with a simultaneous change in the intensity [31]. Therefore, EtOH and H2O were selected as the solvents to reduce the use of organic solvents and allow the dissolution of metal ions.  Figure 5 shows the fluorescence emission spectra of compound 3f in EtOH/H2O (v/v = 4:1) solution (1 × 10 −5 M) that was excited at its absorption maximum. As shown in Figure 5, upon addition of 3 eq. of each cation, only Pb 2+ induced a distinct spectrum change while other metal ions (Cu 2+ , Ag + , Na + , K + , Ba 2+ , Ca 2+ , Co 2+ , Mg 2+ , Mn 2+ , Ni 2+ , Sn 2+ , Zn 2+ , Al 3+ , Hg 2+ and Fe 3+ ) showed either no or slight changes in the fluorescence spectra relative to the free compound 3f, which indicated that compound 3f might be a probe molecule for Pb 2+ . The fluorescence intensity difference might be attributable to the formation of a compound 3f-Pb 2+ complex. The fluorescence intensity difference might be attributable to the formation of a 3f-Pb 2+ complex. The fluorescence intensity of 3f is strong in original, after Pb 2+ added, the N, carbonyl O and OH are likely to coordinate with it, resulting in fluorescence decrease. On the basis of the aforementioned fact, a plausible binding mode of the complex was proposed, as shown in Figure 6.   hypochromic shift of the absorption maximum with a simultaneous change in the intensity [31]. Therefore, EtOH and H2O were selected as the solvents to reduce the use of organic solvents and allow the dissolution of metal ions.  Figure 5 shows the fluorescence emission spectra of compound 3f in EtOH/H2O (v/v = 4:1) solution (1 × 10 −5 M) that was excited at its absorption maximum. As shown in Figure 5, upon addition of 3 eq. of each cation, only Pb 2+ induced a distinct spectrum change while other metal ions (Cu 2+ , Ag + , Na + , K + , Ba 2+ , Ca 2+ , Co 2+ , Mg 2+ , Mn 2+ , Ni 2+ , Sn 2+ , Zn 2+ , Al 3+ , Hg 2+ and Fe 3+ ) showed either no or slight changes in the fluorescence spectra relative to the free compound 3f, which indicated that compound 3f might be a probe molecule for Pb 2+ . The fluorescence intensity difference might be attributable to the formation of a compound 3f-Pb 2+ complex. The fluorescence intensity difference might be attributable to the formation of a 3f-Pb 2+ complex. The fluorescence intensity of 3f is strong in original, after Pb 2+ added, the N, carbonyl O and OH are likely to coordinate with it, resulting in fluorescence decrease. On the basis of the aforementioned fact, a plausible binding mode of the complex was proposed, as shown in Figure 6.

Titration Experiments
To further assess its utility as a Pb 2+ -selective probe, its UV-vis spectrum response to Pb 2+ as demonstrated in Figure 7, in the concentration range of 0 and 10 eq. the absorbance of 3f decreased from 0.118 to 0.054. The results led us to conclude that 3f could be an effective colorimetric probe for Pb 2+ .

Titration Experiments
To further assess its utility as a Pb 2+ -selective probe, its UV-vis spectrum response to Pb 2+ as demonstrated in Figure 7, in the concentration range of 0 and 10 eq. the absorbance of 3f decreased from 0.118 to 0.054. The results led us to conclude that 3f could be an effective colorimetric probe for Pb 2+ . As shown in Figure 8, in the concentration range of 0 and 10 eq., emission intensity decreased significantly with increasing Pb 2+ concentration, implying that Pb 2+ can be quantitatively detected in a wider concentration range. These results will support the design of novel fluorescent materials based on 1,8-naphthalimide derivatives.

Effect of pH
The pH effect on 3f was investigated in EtOH-H2O solution (v/v = 4:1) in a pH value 2-12 ( Figure 9). No obvious fluorescence emission change of probe 3f was observed with the pH changes and this indicated that the free probe 3f was stable at a wide pH range. Meanwhile, an obvious fluorescence decrease was observed after mixing 3f with Pb 2+ in the range pH 2-12. These suggested that compound 3f could act as a fluorescent probe for Pb 2+ under physiological pH conditions and also be applicable at complex environment conditions. As shown in Figure 8, in the concentration range of 0 and 10 eq., emission intensity decreased significantly with increasing Pb 2+ concentration, implying that Pb 2+ can be quantitatively detected in a wider concentration range. These results will support the design of novel fluorescent materials based on 1,8-naphthalimide derivatives.

Titration Experiments
To further assess its utility as a Pb 2+ -selective probe, its UV-vis spectrum response to Pb 2+ as demonstrated in Figure 7, in the concentration range of 0 and 10 eq. the absorbance of 3f decreased from 0.118 to 0.054. The results led us to conclude that 3f could be an effective colorimetric probe for Pb 2+ . As shown in Figure 8, in the concentration range of 0 and 10 eq., emission intensity decreased significantly with increasing Pb 2+ concentration, implying that Pb 2+ can be quantitatively detected in a wider concentration range. These results will support the design of novel fluorescent materials based on 1,8-naphthalimide derivatives.

Effect of pH
The pH effect on 3f was investigated in EtOH-H2O solution (v/v = 4:1) in a pH value 2-12 ( Figure 9). No obvious fluorescence emission change of probe 3f was observed with the pH changes and this indicated that the free probe 3f was stable at a wide pH range. Meanwhile, an obvious fluorescence decrease was observed after mixing 3f with Pb 2+ in the range pH 2-12. These suggested that compound 3f could act as a fluorescent probe for Pb 2+ under physiological pH conditions and also be applicable at complex environment conditions.

Effect of pH
The pH effect on 3f was investigated in EtOH-H 2 O solution (v/v = 4:1) in a pH value 2-12 ( Figure 9). No obvious fluorescence emission change of probe 3f was observed with the pH changes and this indicated that the free probe 3f was stable at a wide pH range. Meanwhile, an obvious fluorescence decrease was observed after mixing 3f with Pb 2+ in the range pH 2-12. These suggested that compound 3f could act as a fluorescent probe for Pb 2+ under physiological pH conditions and also be applicable at complex environment conditions.

Chemicals and Instruments
All of the solvents and reactants are commercially available and were used without purification. The melting points were determined using a Beijing Taike melting point apparatus (X-4) and were uncorrected. The 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker AVANCE 300 MHz or 400 MHz nuclear magnetic resonance spectrometer using CDCl3 or DMSO-d6 as the solvent and TMS as the internal standard. The mass spectra were recorded on a Waters XevoTQ mass spectrometer. The elemental analysis was performed on FLASH EA1112 elemental analyzer. Absorption spectra were collected using a PERSEE TU-1900 ultraviolet spectrophotometer. Fluorescence emission spectra were obtained on a Perkin Elmer LS55 fluorospectrophotometer. The X-ray data were collected on a Bruker AXSII CCD area-detector diffractometer using graphite monochromated Mo Ka radiation (λ = 0.71073 Å) at 293(2) K. All of the calculations were performed with the SHELX-97 program package [32,33].

Testing Methods
Compound 3f was dissolved in EtOH to form a 10 −4 M stock solution. Deionized water was used throughout the experiment and the solutions of metal ions were prepared from CuCl2·2H2O, AgNO3, NaCl, KCl, BaCl2, CaCl2, CoCl2, MgCl2, MnCl2, NiCl2, Pb(NO3)2, SnCl2, ZnCl2, AlCl3, Hg(OAc)2 and FeCl3, respectively. All the cations were dissolved in deionized water to obtain 10 −2 M stock solutions. For fluorescent measurement of Pb 2+ , the mixed stock solutions were diluted to 10 mL with EtOH and H2O to form EtOH/H2O (v/v = 4:1) solutions. For the titration experiments, 3f stock solution (10 μL) was mixed with a certain amount of Pb 2+ stock solution and diluted to 10 mL with EtOH and H2O to form EtOH/H2O (v/v = 4:1) solutions. The wide pH range solutions were prepared by adjustment of 0.05 mol·L −1 Tris-HCl solution with HCl or NaOH solution.

Chemicals and Instruments
All of the solvents and reactants are commercially available and were used without purification. The melting points were determined using a Beijing Taike melting point apparatus (X-4) and were uncorrected. The 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker AVANCE 300 MHz or 400 MHz nuclear magnetic resonance spectrometer using CDCl 3 or DMSO-d 6 as the solvent and TMS as the internal standard. The mass spectra were recorded on a Waters XevoTQ mass spectrometer. The elemental analysis was performed on FLASH EA1112 elemental analyzer. Absorption spectra were collected using a PERSEE TU-1900 ultraviolet spectrophotometer. Fluorescence emission spectra were obtained on a Perkin Elmer LS55 fluorospectrophotometer. The X-ray data were collected on a Bruker AXSII CCD area-detector diffractometer using graphite monochromated Mo Ka radiation (λ = 0.71073 Å) at 293(2) K. All of the calculations were performed with the SHELX-97 program package [32,33].

General Procedure for the Synthesis of N-n-Butyl-4-[N ,N -dihydroxyethyl]-1,8-naphthalene Imide
A mixture of BNI (1.0 g, 2.81 mmol) and TEA (0.63 g, 6.25 mmol) solution in dichloromethane (40 mL) was stirred and substituted benzoyl chloride (6.25 mmol) was then slowly added over 30 min and allowed to react for 8 h at 25 • C. The product was neutralized by saturated Na 2 CO 3 (aq) and purified by column chromatography on a silica gel column using cyclohexane-EtOAc (v/v = 1:3) as the eluent.      The spectra of synthesized compounds are available in supplementary material.

Conclusions
A series of novel N-n-butyl-1,8-naphthalimide derivatives with mono-and di-substitution at position 4 were synthesized via direct arylation. Probe 3f exhibited an obvious quenched fluorescence in the presence of Pb 2+ over a range of metal cations. A titration of monomer with Pb 2+ ion was performed. When Pb 2+ ion concentration increased from 0 to 10 eq., the fluorescent intensity of 3f decreased from 199.97 to 48.21 and the absorbance of 3f decreased from 0.118 to 0.054. The results led us to conclude that 3f could be an effective colorimetric and fluorescent probe for Pb 2+ . In addition, the pH effect on 3f showed that this sensor should be valuable for Pb 2+ analysis in environmental samples and the biological systems.