research article detection of metal ions and protons with a new...

Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2013, Article ID 628946, 6 pageshttp://dx.doi.org/10.1155/2013/628946

Research ArticleDetection of Metal Ions and Protons with a New BlueFluorescent Bis(1,8-Naphthalimide)

Stanislava Yordanova,1 Stanimir Stoianov,1 Ivo Grabchev,2 and Ivan Petkov1

1 Sofia University “St. Kliment Ohridski,” Faculty of Chemistry and Pharmacy, 1 James Bourchier Bowerard, 1164 Sofia, Bulgaria2 Sofia University “St. Kliment Ohridski,” Faculty of Medicine, 1 Koziak Street, 1407 Sofia, Bulgaria

Correspondence should be addressed to Ivo Grabchev; [email protected]

Received 5 December 2012; Accepted 31 January 2013

Academic Editor: Daniel L. Reger

Copyright © 2013 Stanislava Yordanova et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The synthesis of a new blue fluorescent bis(1,8-naphthalimide) has been described and its basic photophysical characteristics havebeen investigated in organic solvents of different polarity. The detection of protons and different metal cations (Ag+, Cu2+, Co2+,Ni2+, Fe3+ and Zn2+) with the new compound has been investigated by the use fluorescence spectroscopy.

1. Introduction

Metal ions pollution in the environment has received signif-icant attention because of its toxicity and adverse biologicaleffects. In this respect, environmental monitoring is impor-tant to ensure ecosystem health and humanity. Of particularinterest are the optical fluorosensors, which are moleculardevices able to detect the presence of environmental pollu-tants via the changes in their fluorescence intensity [1, 2].In this sense, sensors are of great importance to chemistry,biology, and medicine because they allow rapid detection ofdifferent compounds in the living organisms and environ-ment.Most of the known fluorescent sensors are based on thephotoinduced electron transfer (PET) [3, 4]. The fluorescentPET sensors are of great interest because of their variousapplications. Under appropriated conditions, the fluorophoreemission is quenched by the distal amino group by meansof electron transfer from the substituent to the fluorophorering. If the PET process is “switched off” by, for example,protonation of the amino group or complexation with metalions, the emission of the fluorophores is restored. Due to theirexcellent photophysical properties, 1,8-napthalimide deriva-tives are unsurpassed as a signal fragment in the design offluorescent chemosensors [5–10]. Various other mechanismsand fluorophores are used in the design of molecular deviceswith sensory properties [11–14].

In this work, the study is focused on the synthesis andphotophysical investigation of a blue fluorescent compound(Bis2) having N,N-dimethylaminoethyl group in C-4 posi-tion at the 1,8-naphthalimide structure as a receptor for metalions and protons. The functional properties of Bis2 havebeen investigated in organic solvents of different polarity. Itsphotophysical and supramolecular properties have been alsostudied in the presence of some metal cations.

2. Experimental

2.1. Materials and Methods. UV-Vis spectrophotometricinvestigations were performed using “Thermo SpectronicUnicam UV 500” spectrophotometer. Emission spectra weretaken on a “Cary Eclipse” spectrofluorometer. All spectrawere recorded using 1 cm pathlength synthetic quartz glasscells (Hellma, Germany). All organic solvents (dimethyl sul-foxide, N,N-dimethylformamide, acetonitrile, dichlorome-thane, and chloroform) used in this study were of spectro-scopic grade. Fluorescence quantum yield was determinedon the basis of the absorption and fluorescence spectra,using anthracene as reference (Φst = 0.27 in ethanol [15]).The effect of metal cations upon the fluorescence intensitywas examined by adding a few microliters of the metalcations stock solution to a known volume of the dendrimersolution (3mL). The addition was limited to 0.08mL, so

2 International Journal of Inorganic Chemistry

that dilution remains insignificant [16]. Cu(NO3)2⋅3H2O,

Ni(NO3)2⋅6H2O, AgNO

3, Co(NO

3)2⋅6H2O, Fe(NO

3)3and

Zn(NO3)2⋅4H2O were used as source of metal cations. The

NMR spectra were obtained on a Bruker DRX-250 spec-trometer, operating at 250.13 and 62.90MHz for 1H and 13C,respectively, using a dual 5mm probe head. The measure-ments were carried out in CD

3Cl solution at ambient temper-

ature. The chemical shift was referenced to tetramethylsilane(TMS). Thin layer chromatographic (TLC) analysis of thedyeswas followed on silica gel (Fluka F

60254 20× 20; 0.2mm)

using the solvent systemn-heptane/acetone (1 : 1) as an eluent.

2.2. Synthesis of Bis(4-Bromo-N-ethyl-1,8-naphthalimide)Amine (Bis1). 0.01M diethylenetriamine was added tosolution of 0.02M4-bromo-1,8-naphtalic anhydride in 50mLof absolute ethanol and heated in reflux for 60min. Aftercooling to room temperature, the precipitate was filtered,washed with diethyl ether, dried, and recrystallized withethanol. Yield was 84%.

FTIR (cm−1): 3067, 2960, 2831, 1701, 1660, 1557, 1438, 1345,1233, 779.1HNMR (CDCl

3, 𝛿, ppm): 8.52 (d, 2H, HAr), 8.38 (d, 2H,

HAr), 8.11 (d, 2H HAr), 7.69 (m, 4H HAr), 7.26 (s, 1H, NH),4.34–4.27 (m, 4H,–CH

2–), 3.10 (t, 4H,–CH

2–).

13C-NMR (CDCl3, 𝛿, ppm): 163.7, 163.4, 132.9, 131.7, 130.9,

129.9, 128.8, 127.9, 125.4, 122.9, 121.1, 104.6, 47.0, 39.6.Analysis: C

28H19N3O4Br2(620.9 g mol−1).

Calculated (%): C 54.11, H 3.06, N 6.76.Found (%): C 54.39, H 3.10, N 6.89.

2.3. Synthesis of Bis (4-N,N-dimetylaminoethoxy-N-ethyl-1,8-naphthalimidyl) Amine (Bis2). A solution of 0.01mol of Bis1in 50mL 2-(dimethylamino)ethanol was refluxed in thepresence of 0.03M KOH for 6 hours. The process wascontrolled by thin-layer chromatography. After cooling toroom temperature, the liquor was poured into water and theresulting precipitate was washed with water, and then driedin vacuum at 40∘C. Yield was 98%.

FTIR (cm−1): 3064, 2946, 2822, 1698, 1657, 1590, 1439,1385, 1349, 1268, 1236, 1170, 1031, 779.1H NMR (CDCl

3, 𝛿, ppm): 8.49 (dd, 𝐽 = 1.0, 8.4Hz, 2H,

HAr), 8.37 (dd, 𝐽 = 1.0, 7.2Hz, 2H, HAr), 8.16 (d, 𝐽 = 8.3Hz,2H HAr), 7.65 (m, 4H HAr), 6.96 (1H, NH), 4.32 (m, 8H, –CH2–), 3.2–2.8 (m, 8H, –CH

2–), 2.43 (s, 12H, CH3).

13C-NMR (CDCl3, 𝛿, ppm): 164.7, 164.4, 159.8, 133.7, 133.4,

131.5, 131.1, 129.4, 128.6, 128.1, 126.8, 125.8, 122.6, 105.8, 67.4,57.9, 47.4, 46.1, 39.7.

Analysis: C36H39N3O6(609.1 g mol−1).

Calculated (%): C 70.92, H 6.40, N 6.90.Found (%): C 70.74, H 6.59, N 6.92.

3. Results and Discussion

3.1. Synthesis of Bis2. 4-Bromo-1,8-naphthalic anhydride hasbeen used as starting material for Bis1 synthesis. Bis1 wassynthesized by the condensation of diethylentriamine and 4-bromo-1,8-naphthalic anhydride in boiling ethanol solution[17].

Table 1: Photophysical characteristics of Bis2.

Solvent 𝜆𝐴𝜆𝐹𝜈𝐴− 𝜈𝐹

𝜀Φ𝐹nm nm cm−1 Lmol−1 cm−1

Dimethyl sulfoxide 351 443 5917 20090 0.004N,N-dimethylformamide 349 438 5822 20169 0.008

Acetonitrile 347 435 5670 19175 0.008Ethanol 346 435 5913 20773 0.002Acetone 347 430 5563 20773 0.09Dichloromethane 350 425 5042 21199 0.19Chloroform 351 423 4849 23684 0.29

The final product Bis2 has been obtained in high yieldsand purity by nucleophilic substitution of the bromine atomin Bis1 withN,N-dimethylaminoethyl group. In this case, theelectron accepting carbonyl groups of the 1,8-naphthalimidemolecule favors the reaction of nucleophilic substitutionwherein the bromine atom is replaced by the alkoxy group. Itis well known that this substituent is widely used in the designofmolecular sensor devices which are able to coordinate withmetal ions and protons [18–20].

The route employed for the synthesis, according to themethod described is presented in Scheme 1.

3.2. Photophysical Properties of Bis2. Thephotophysical prop-erties of the 1,8-naphthalimides depend basically on thepolarization of naphthalimide molecule due to the elec-tron donor-acceptor interaction occurring between the sub-stituents at C-4 and the carbonyl groups from the imidestructure of the chromophoric system. Table 1 presents thespectral characteristics of Bis2 in seven organic solvents withdifferent polarity: the absorption (𝜆

𝐴) and fluorescence (𝜆

𝐹)

maxima, the extinction coefficient (𝜀), Stokes shift (𝜈𝐴− 𝜈𝐹),

and quantum yield of fluorescence (Φ𝐹).

The solvent polarity was characterized by the dielectricconstant. As can be seen from the data in Table 1, the polarityof organic solvents play a significant role on the photo-physical properties of Bis2. In all organic solvents, the newcompound absorbs in the ultraviolet region with maxima inthe near UV range of 346–351 nm and emitting blue flu-orescence with maxima in the range of 423–443 nm. Themolar extinction coefficients at 𝜆

𝐴maximum are at the range

of 𝜀 = 19175–23684 lmol−1 cm−1, indicating that the longwavelength band in the spectrum is a band of charge transfer,due to n→ 𝜋∗ electron transfer at S

0→S1transition. The

extinction coefficients for a monomeric 1,8-naphthalimidehaving the same substituent at C-4 determined in our pre-vious studies are 12000–14000mol L−1 cm−1 [21, 22]. As canbe seen from the data presented in Table 1, the molar extinc-tion coefficient for the new bis-chromophoric compound isapproximately 2-fold higher than that of the monomeric 1,8-naphthalimide derivative.That suggests a lack of ground stateinteraction between the 1,8-naphthalimide units [23].

Figure 1 plots the fluorescence maxima of Bis2 in differ-ent media. As it can be seen there is correlation between

International Journal of Inorganic Chemistry 3

OO ON

HN N

O

O

O

O

N NH N

O

O

O

O

BrBr Br

KOH2

Bis1

Bis2

EtOH

NH2CH2CH2NHCH2CH2NH2 OHCH2CH2N(CH3)2

OCH2CH2N(CH3)2(H3C)2NH2CH2CO

Scheme 1: Synthesis of Bis2.

0 10 20 30 40 50420

425

430

435

440

Fluo

resc

ence

(nm

)

445

Dielectric constant

12

3

45

6

7

Figure 1: Dependence of fluorescencemaxima of Bis2 on the dielec-tric constant: (1) chloroform, (2) dichloromethane, (3) acetone, (4)ethanol, (5) acetonitrile, (6) N,N-dimethylformamide, (7) dimethylsulfoxide.

the media polarity and fluorescence maxima (Δ𝜆𝐹= 20 nm).

Moreover it is seen that Bis2 has a positive solvatochromism.Figure 2 presents an example of absorption and fluo-

rescence spectra of Bis2 in DMF solution. It is seen thatthe fluorescence spectrum has an emission band with asingle maximum, without vibrational structure. The overlapbetween absorption and fluorescence spectra is low and anaggregation effect for the concentration at about 10−5mol L−1has not been observed.

Stokes shift is an important parameter of the fluorescentcompound indicating the difference in the properties andstructure of the fluorophore between the ground state 𝑆

0,

300 400 500 6000

0.2

0.4

0.6

0.8

1

Abso

rban

ce/fl

uore

scen

ce

Wavelength (nm)

A F

Figure 2: Normalized absorption (A) and fluorescence (F) spectraof Bis2 in DMF solution.

and the first exited state 𝑆1. The Stokes shift is found by the

following equation:

(𝜈𝐴− 𝜈𝐹) = (1

𝜆𝐴

−1

𝜆𝐹

) × 10−7

. (1)

The Stokes shift values range obtained in this work is 𝜈𝐴−𝜈𝐹=

4849–5917 cm−1. They depend on the polarity of the organicsolvents used. It is seen that in the nonpolar media the valuesof Stokes shift are lower, if compared to those obtained inpolar media (Table 1 and Figure 3).

The molecules ability to emit the absorbed light energyis characterized quantitatively by the fluorescence quantumyield.The fluorescence quantum yield has been calculated on


0 10 20 30 40 50

4800

5000

5200

5400

5600

5800

6000

Dielectric constant

1

2

3

45 6

7

Stok

es sh

ifts (

cm−1)

Figure 3: Dependence of Stokes shift of Bis2 on the dielectric con-stant: (1) chloroform, (2) dichloromethane, (3) acetone, (4) ethanol,(5) acetonitrile, (6) N,N-dimethylformamide, (7) dimethyl sulfox-ide.

the basis of the absorption and fluorescence spectra by thefollowing equation:

Φ𝐹= Φst𝑆𝑢

𝑆st

𝐴 st𝐴𝑢

𝑛2

𝐷𝑢

𝑛2

𝐷st, (2)

where the Φ𝐹is the emission quantum yield of the sample,

Φst is the emission quantum yield of the standard, 𝐴 st and𝐴𝑢represent the absorbance of the standard and sample at

the excited wavelength, respectively, while 𝑆st and 𝑆𝑢 are theintegrated emission band areas of the standard and samplerespectively, 𝑛

𝐷st and 𝑛𝐷𝑢 are the solvent refractive index ofthe standard and sample, and 𝑢 and st refer to the unknownand standard, respectively.

The calculated Φ𝐹is in the region 0.002–0.29. As it

can be seen from Table 1 the fluorescence quantum yieldvalues depend strongly on the solvent polarity. The lowestΦ𝐹has been observed in ethanol (Φ

𝐹= 0.002) and its value

increases more than 145 times in chloroform solution (Φ𝐹=

0.29). This great difference in the quantum yield values isdue to the photoinduced electron transfer that is quenchedin nonpolar media. In this case, the quenching leads torestored fluorescence emission of the fluorophore. Suchbehavior has also been exhibited by similar monomeric 4-N,N-dimetylaminoethoxy-N-allyl-1,8-naphthalimide havinga smallΦ

𝐹in polar organic solvents and higher in non-polar

solvents [18, 19].

3.3. Effect of Protons andMetal Cations on the Spectral Proper-ties of the Bis2. In the presence of protons, the absorption andfluorescence maxima of Bis2 do not change their position.However, the fluorescence intensity in an ethanol/water (v/v1 : 4) solution is pH dependent, as can be seen from Figure 4.This correlation has been investigated in the 3.5–11.0 pHvalue range and gives evidence that Bis2 responds to pHchanges due to its high sensitivity to proton concentration.The constant value of the fluorescence intensity decreases

4 5 6 7 8 9 10 110

200

400

600

800

Fluo

resc

ence

inte

nsity

pH

pKa = 7.61

Figure 4: The influence of pH upon fluorescence intensity of Bis2in ethanol-water solution (1 : 4, v/v).

0

10

20

30

40

50

60

70

None

FE

Co2+Ni2+ Zn2+ Ag+Cu2+Fe3+

Figure 5: Fluorescence enhancement factor (FE) of Bis2 in acetoni-trile solutions (𝑐 = 10−5mol L−1) in presence of metal cations (𝑐 =8.10−5mol L−1) at excitation wavelength 350 nm.

after reaching pH 5.0–5.5 and at values higher than pH 9.5 thecurve forms also a plateau. A 9-fold fluorescence quenchingis observed for the pH range investigated.

The pH dependence of fluorescence intensity has beenanalyzed using the following equation:

pH − pKa = log((𝐼𝐹max − 𝐼𝐹)

(𝐼𝐹− 𝐼𝐹min)) . (3)

The calculated pKa value for Bis2 is 7.61. This value is smallerthan that of the monomer fluorophore, having the samesubstituents at C-4 position (pKa = 8.40) [18].

The investigation of photophysical properties of Bis2 asa ligand in the presence of different metal cations has beenof particular interest. Its properties signaling the presence oftransitionmetal cations have been investigated in acetonitrilewith regard to potential applications as a PET sensor.Acetoni-trile has been chosen as the solvent for all the measurementssince it guarantees a good solubility of the used metal salts,ligand, and the respective complexes.

International Journal of Inorganic Chemistry 5

N NHN

O

O O

O

O

O

347 nm

347 nm(CH3)2N

N(CH3)2

e−

e−

M𝑛+M𝑛+

Scheme 2: Proposed mechanism of photoinduced electron transfer of Bis2.

N N

O

O O

O

O

O

347 nm

347 nm

(CH3)2N

435 nm

N(CH3)2e−

e−

M𝑛+

M𝑛+

HN

Scheme 3: Proposed mechanism of photoinduced electron transfer of Bis2.

In acetonitrile, Bis2 has a veryweak fluorescence emissionas expected for a good PET fluorescence switch. A dramati-cally enhancement in the fluorescence intensity in presence ofthe different metal cations has been observed. The influenceof the metal cations on the fluorescence enhancement (FE) ispresented in Figure 5. The FE = 𝐼/𝐼

𝑜has been determined

from the ratio of maximum fluorescence intensity 𝐼 (afteraddition of metal cations) and minimum fluorescence inten-sity 𝐼𝑜(before metal cations addition). Upon the addition of

metal cations the enhancements of the fluorescence emissionis determined by the nature of the cations added.The highestvalues have been observed in the presence of Zn2+ cations(FE = 59) and a rank can be given as follows:

Zn2+ > Ni2+ > Cu2+ > Co2+ > Fe3+ > Ag+. (4)

The 1,8-naphthalimide under study is subjected to a PETproceeding from the distal amino groups of N,N-dimethyl-aminoethyl oxy moieties at C-4 position to the 1,8-naphthali-mide units. The interaction between the 1,8-naphthalimideas a fluorophore and the N,N-dimethylamino group as areceptor provoking PET leads to a quenching of the flu-orescence emission (Scheme 2). The presence of transitionmetal cations in the solution changes the photophysicalproperties of Bis2 since in this case the system fluorescesintensively (Scheme 3). The enhancement of fluorescenceintensity confirms the existence of coordination interactionbetween the metal cations and oxygen at C-4 position of thenaphthalene ring and the N,N-dimethylamino group [19].

4. Conclusion

The synthesis and the photophysical characteristics of anew bis-1,8-naphtahlimide have been described. The strongdependence of the fluorescence intensity on the solventpolarity has been observed and was explained by meansof possible photoinduced electron transfer. In the presenceof protons and metal cations, the fluorescence intensity ofthe bis-1,8-naphtahlmide is higher than that in acetonitrilesolution free of metal cations. The relative affinity of the bis-1,8-naphtahlmide to form metal complexes increases in therange Zn2+ > Ni2+ > Cu2+ > Co2+ > Fe3+ > Ag+. On thebasis of the present investigation, it can be assumed that thenew bis-1,8-naphtahlmide is suitable for detecting of metalcations and protons based on the quenching of photoinducedelectron transfer processes at concentration ranges from 0 to8.10−5mol L−1.

Acknowledgments

Financial support from the National Science Fund of Bul-garia, project DCVP 02/2—2009 UNION, and BeyondEver-est, project FP7-REGPOT-2011-1, is greatly appreciated.

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