Functional fluorescent aramids: aromatic polyamides containing a dipicolinic acid derivative as...

PolymerChemistry

PAPER

Departamento de Quımica, Facultad de Ci

Misael Ba~nuelos s/n, 09001 Burgos, Spain.

es; Web: http://sites.google.com/site/grupod

+34 947 25 80 85

† Electronic supplementary information (and polymer NMR and FT-IR spectra. See

Cite this: Polym. Chem., 2013, 4, 4256

Received 19th April 2013Accepted 21st May 2013

DOI: 10.1039/c3py00503h

www.rsc.org/polymers

4256 | Polym. Chem., 2013, 4, 4256–

Functional fluorescent aramids: aromatic polyamidescontaining a dipicolinic acid derivative as luminescentconverters and sensory materials for the fluorescencedetection and quantification of Cr(VI), Fe(III) and Cu(II)†

Jose Luis Barrio-Manso, Patricia Calvo, Felix Clemente Garcıa, Jesus Luis Pablos,Tomas Torroba* and Jose Miguel Garcıa*

This work describes the preparation of luminescent, high-performance aromatic polyamides. The

fluorescent behaviour was achieved by preparing a diacid monomer containing a fluorescent dipicolinic

acid derivative and polymerising it with commercially available aromatic diacid and diamine monomers

to give polyisophthalamides. These materials can be structurally compared to commercialised meta-

aramid fibres [poly(m-phenylene isophthalamide), brand names: NOMEX� and Teijinconex�]. The

pendant fluorescent motif provided the material with a light green fluorescence and was found to be a

selective host unit for cations of environmental, biomedical and industrial significance [Cr(VI), Fe(III)

and Cu(III)].

Introduction

Aromatic polyamides, also known as aramids, are a class ofpolymers considered high-performance materials for theiroutstanding thermal resistance and mechanical properties.Their remarkable characteristics arise from their whollyaromatic structure and from the amide groups, which establishstrong and highly directional interchain hydrogen bonds. Thishydrogen bonding results in rigid rod-like structures withhighly effective crystalline microdomains and, consequently, ahigh cohesive energy and a high degree of intermolecularpacking, resulting in impressive macromolecular mechanicalproperties and a high char yield upon exposure to re.1,2

In this context, the commercial aromatic polyamides, poly-(p-phenylene terphthalamide), poly(m-phenylene iso-phthalamide), and poly(p-phenylene-co-3,40-oxydiphenylene ter-ephthalamide),3 have been transformed into bres4 with broadapplications in value-added products in the automotive andaerospace, oil and gas, protection and defence, and telecomindustries and in civil engineering products.1–4 Thus, the wet, dryor dry-jet wet spinning of aromatic polyamide solutions renders,aer cold and heat drawing, continuous and staple bres andogs that can be transformed into isolating papers, llers, yarnsand fabrics for apparel, electrical insulation materials, bullet-

encias, Universidad de Burgos, Plaza de

E-mail: [email protected]; jmiguel@ubu.

epolimeros; Fax: +34 947 25 88 31; Tel:

ESI) available: Intermediate, monomerDOI: 10.1039/c3py00503h

4264

proof body armour, protective clothing, high temperatureindustrial lters, and sports fabrics, among others.

Knowing the relationship between the structure of thearamids and their properties, the introduction of functionalityin the main polymer chain can be used to exploit the impressivethermal and mechanical properties for novel and advancedtechnological applications. With this in mind, we have preparedaromatic copolyamides with uorescent receptor moieties thatcould be used to prepare uorescent materials, e.g., bres andapparels, with sensory behaviour towards different targetmolecules.5 Another possibility is luminescent converter coat-ings (LUCO), which when combined with a primary pumpingsource, e.g., a blue light-emitting diode (LED), can give rise toLEDs with potential lighting and backlighting applications.6

Thus, we have synthesised a dipicolinic acid (2,6-pyr-idinedicarboxylic acid) derivative containing both a pendantpyridine ring and an alicyclic tertiary amine as an aromatic diacidmonomer for polyamides with uorescent behaviour and sensorycapabilities – pyridine moieties have previously been exploited forimparting interesting optical, electrochemical and electrochromicproperties to high performance polymers.7 Specically, thismonomer imparts both selective sensory behaviour to cations ofinterest from the industrial, environmental and biomedicalperspectives, such as Cr(VI), Fe(III) and Cu(II), and a luminescenceconversion of ultraviolet radiation into visible light.

Experimental

All materials and solvents were commercially available and usedas received, unless otherwise indicated. N-Methyl-2-pyrrolidone

This journal is ª The Royal Society of Chemistry 2013

Paper Polymer Chemistry

(NMP, Aldrich) was vacuum distilled twice over phosphoruspentoxide and then stored over 4 A molecular sieves. Lithiumchloride (Aldrich) was dried at 400 �C for 12 hours prior to use.Triphenylphosphite (TPP, Aldrich) was vacuum distilled twiceover calcium hydride and then stored over 4 A molecular sieves.Pyridine (Merck) was dried at reux temperature over sodiumhydroxide for 24 hours, then distilled and stored over 4 Amolecular sieves. m-Phenylenediamine (MPD) (Aldrich, >99%)was puried by double vacuum sublimation. 6-(Piperidin-1-yl)pyridine-3-boronic acid pinacol ester (BAPE) was purchasedfrom Aldrich (97%) and used without further purication. Iso-phthalic acid (IA; Aldrich) was puried by crystallisation (3 g in1.2 L of boiling water).

Intermediates and monomers

The aromatic diacid monomer 6-(piperidin-1-yl)-[3,40-bipyr-idine]-20,60-dicarboxylic acid (DPAD) was synthesised accordingto the reaction conditions described in Scheme 1. Dimethyl 4-bromopyridine-2,6-dicarboxylate (DPABr) was prepared andpuried according to a previously described procedure.8

Monomer synthesis, 6-(piperidin-1-yl)-[3,40-bipyridine]-20,60-dicarboxylic acid (DPAD). Pd(PPh3)4 (58 mg, 0.05 mmol) wasadded under nitrogen to a solution of dimethyl 4-bromopyr-idine-2,6-dicarboxylate (DPABr) (822 mg, 3.0 mmol) in tetra-hydrofuran (120 mL), and the mixture was stirred for 30minutes at room temperature. Solid 6-(piperidin-1-yl)pyridine-3-boronic acid pinacol ester (BAPE) (865 mg, 3.0 mmol) andcesium carbonate (1.47 g, 4.5 mmol) dissolved in water (15 mL)were successively added and the mixture was heated at reux

Scheme 1 Monomer synthesis.


for 24 h. The solvent was then evaporated under reducedpressure, and the residue was suspended in water (10 mL). Asolution of NaOH (5% w/v in H2O) was added dropwise untilthe pH was 10, and the mixture was dissolved in tetrahydro-furan (60 mL) and heated at reux for 1 hour until completehydrolysis of the diester and monoester mixture was achieved.The mixture was then chilled in an ice bath, and concentrateHCl was added dropwise until the pH reached 1. The solventwas then evaporated under reduced pressure, and the solidresidue was dried under a vacuum. The solid residue wassuspended in anhydrous dichloromethane (125 mL) underinert nitrogen gas, oxalyl chloride (3.00 g, 24 mmol) andanhydrous dimethylformamide (0.2 mL) were added, and themixture was stirred overnight. Subsequently, the solvent andthe excess oxalyl chloride were evaporated under reducedpressure, and anhydrous methanol (5 mL, 124 mmol) indichloromethane (60 mL) was added under nitrogen and themixture was stirred overnight. The solvent was evaporatedunder reduced pressure and the residue was puried by ashchromatography (silica, 3 � 50 cm), using CH2Cl2 to CH2Cl2/MeOH of 9 : 1 v/v, to yield dimethyl 6-(piperidin-1-yl)-[3,40-bipyridine]-20,60-dicarboxylate (DPAMe) as a light yellow solid(522 mg, 49%). M.p. 146–148 �C, Rf (DCM/MeOH, 50 : 3): 0.83;1H NMR (CDCl3, 399.92 MHz): d ¼ 8.62 (s, J ¼ 2.7 Hz, 1H, ArH);8.46 (s, 2H, ArH); 7.84 (dd, J1 ¼ 9.0 Hz, J2 ¼ 2.7 Hz, 1H, ArH);6.73 (d, J ¼ 9.0 Hz, 1H, ArH); 4.04 (s, 6H, 2 � CH3); 3.67 (t, J ¼5.7 Hz, 4H, 2 � CH2); 1.68 (m, 6H, 3 � CH2);

13C NMR (CDCl3,100.57 MHz): d ¼ 165.6 (C]O), 159.8 (CAr), 148.8 (CHAr), 147.2(CHAr), 135.6 (CHAr), 128.4 (CAr), 123.8 (CHAr), 119.3 (CAr), 106.6(CHAr), 53.4 (CH3), 46.1 (CH2), 25.7 (CH2), 25.0, 24.8 (CH2); MS(EI) m/z (%): 355 (M+, 100), 326 (55), 312 (30), 299 (20), 272 (32),84 (28); HRMS (EI): calcd for C19H21N3O4: 355.1532; found:355.1535. Diester DPAMe (512 mg, 1.44 mmol) was dissolved ina mixture of tetrahydrofuran (10 mL) and water (1 mL). Asolution of NaOH (5% w/v in H2O) was added dropwise untilthe pH reached 10, and the mixture was heated at reux for 1hour until complete hydrolysis of the diester was achieved. Themixture was then chilled in an ice bath, and a solution ofconcentrate HCl was added dropwise until the pH reached 1.The solvent was evaporated under reduced pressure, and thesolid residue was extracted several times with 1 : 1 dichloro-methane/methanol. Evaporation of the solvent under reducedpressure afforded pure 6-(piperidin-1-yl)-[3,40-bipyridine]-20,60-dicarboxylic acid (DPAD) as a light yellow solid (448 mg, 95%from DPAMe, 47% from DPABr). M.p. 140–142 �C (decompo-sition); 1H NMR (CD3OD, 399.92 MHz): d ¼ 8.54 (s, 2H, ArH);8.32 (s, 1H, ArH); 8.17 (d, J ¼ 9.3 Hz, 1H, ArH); 7.13 (d, J ¼ 9.3Hz, 1H, ArH); 3.72 (m, 4H, 2 � CH2); 1.73 (m, 6H, 3 � CH2). IR(KBr, cm�1) ¼ 3207 (OH), 2933, 2855, 1745 and 1718 (C]O),1649, 1599, 1392, 1293, 775; MS (EI) m/z (%): 327 (M+, 75), 283(92), 239 (45), 157 (41), 84 (100). HRMS (EI): calcd forC17H17N3O4: 327.1219; found: 327.1216.

Polymer synthesis

The polymer synthesis and codes are depicted in Scheme 2. Thepolymerisation reactions were carried out as follows: in a 50 mL

Polym. Chem., 2013, 4, 4256–4264 | 4257

Scheme 2 Copolymer synthesis and associated codes.

Polymer Chemistry Paper

three-necked ask tted with a mechanical stirrer, 10 mmol ofdiamine (MPD), 10 mmol of aromatic diacid or a mixture ofdiacids [10 mmol of diacid (IA) for the synthesis of homopoly-mer I, 9 mmol of diacid (IA) and 1 mmol of diacid DPA for thesynthesis of copolymer II, and 9mmol of diacid (IA) and 1mmolof diacid DPAD for the synthesis of copolymer III], and 1.4 g oflithium chloride were dissolved in a mixture of 6 mL of pyri-dine, 22 mmol of TPP and 20 mL of NMP under inert nitrogengas. The solution was heated to 110 �C and stirred undernitrogen for 4 hours. The system was then cooled to roomtemperature, and the solution was poured into 300 mL ofmethanol to yield a swollen, brous precipitate. The obtainedpolymer was ltered and washed with methanol and distilledwater, aer which it was subjected to Soxhlet extractionwith acetone for 24 h and dried in a vacuum oven overnight at120 �C. The yields were quantitative for all of the synthesisedpolymers.

Measurements. 1H and 13C NMR spectra were recorded witha Varian Inova 400 spectrometer operating at 399.92 and100.57 MHz, respectively, in deuterated chloroform (CDCl3),deuterated methanol (CH3OD) or deuterated dimethylsulph-oxide (DMSO-d6). Infrared spectra (FT-IR) were recorded on aNicolet Impact spectrometer. Inherent viscosities weremeasured with an Ubbelohde viscometer at 30 � 0.1 �C withsulphuric acid (98%) as the solvent at a concentration of 0.5g dL�1. Thermogravimetric analysis (TGA) of 5 mg samples

4258 | Polym. Chem., 2013, 4, 4256–4264

under a nitrogen or oxygen atmosphere were performed on aTA Instrument Q50 TGA analyser at a scan rate of 10 �C min�1.Polymer solubility was determined by mixing 10 mg of thepolymer with 1 mL of the solvent followed by stirring for 24 hat room temperature. The uorescence spectra were recordedusing a Hitachi F-7000 uorescence spectrophotometer. Milli-pore-Q water was used to prepare the organic–inorganic solu-tions (DMSO/water, 1/1, v/v). For the titration experiments,increasing concentrations of each cation in the quartzmeasuring cuvette were achieved by adding aliquots of aconcentrated stock solution of the appropriate cation.Quantum yields were determined using quinine sulphate in0.05 M H2SO4 as a reference (f ¼ 0.53 � 0.02) at an excitationwavelength of 366 nm.9

Results and discussion

This report describes the synthesis and properties of aromaticcopolyamides featuring a dipicolinic acid (2,6-pyr-idinedicarboxylic acid) derivative (DPAD, Scheme 1) in the mainchain. In addition to the two acid moieties, this monomer has abipyridine unit with an attached alicyclic tertiary amine. Thisstructure is prone to different interactions with target speciesdue to the sp2 and sp3 nitrogens and the amide groups of thepolymer backbone.



Monomer and polymer synthesis and characterisation

Simple, well-known reactions were used to obtain the diacidmonomers from widely available, inexpensive chemicals ingood yield and purity. The preparation and characterisation ofthe precursor dimethyl 4-bromopyridine-2,6-dicarboxylate(DPABr) were accomplished according to the proceduresdescribed by Lamture et al.8 CrudeDPADwas then prepared by aSuzuki coupling of DPABr with 2-(1-piperidinyl)-5-(4,4,5,5-tet-ramethyl-1,3,2-dioxaborolan-2-yl)pyridine in a conventionalmanner.10 However, the purication was not straightforwardand was accomplished by an esterication/column chroma-tography/hydrolysis procedure, yielding a monomer of highpurity, as demonstrated by the NMR spectra. The intermediatesand monomers were characterised by IR, 1H and 13C NMRspectroscopy and mass spectrometry, unambiguously conrm-ing the chemical structure of all the products (Fig. 1).

Fig. 1 1H NMR spectra of themonomerDPAD, homopolymerMPDI, and copolymealso observed for monomer DPAD corresponding to the acid groups).


The polyamides were synthesised by combining m-phenyl-enediamine with the diacid monomers DPA, IA, and DPAD. Thepolymers were synthesised according to the method describedby Yamazaki et al.11 Their structures are shown in Scheme 2,and the inherent viscosities are given in Table 1. Fig. 1 showsthe 1H NMR spectra of the copolymers. As expected from thequantitative yield of the polymerisations, the copolymercompositions correspond to the monomer feed ratio and couldbe calculated from the integration of nuclei m1 and m2 and thedifferent amide signals in the 1H NMR spectra (Fig. 1). More-over, the amide signals point to a random copolymerisation, asexpected; this is shown for polyamide I in Fig. 2. The mostabundant nuclei (proton 1, 70%) correspond to blocks of oli-go(m-phenylene isophthalamide), while the probability of anisophthalic residue having a vicinal DPA moiety is representedby the area of protons 2 and 3 (20%). The probability of having

rs I, II and III (solvent: DMSO-d6, *¼ solvent signals; a broad signal at 13.2 ppmwas

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Table 1 Inherent viscosities and thermal DSC and TGA data of the polymers (N2

atmosphere)

Polymerhinh(dL g�1)

DSC TGA

Tg(�C)

Tda

(�C)T5

a

(�C)T10

a

(�C)Charyieldb (%) LOIc

MPDI 1.82 275 415 424 444 53 39I 0.86 nd 371 377 412 59 41II 0.74 nd 376 396 434 65 43III 0.41 nd 382 384 401 53 39

a Onset temperature of initial decomposition (Td), 5% weight loss (T5),10% weight loss (T10).

b At 800 �C. c Limiting oxygen index, calculatedfrom TGA data14 (LOI ¼ 17.5 + 0.4 CR, where CR is the char yield in %weight at 800 �C).

Fig. 2 Expansion of the 1H NMR amide region (solvent: DMSO-d6) with theassignments of the protons to the observed signals.

Fig. 3 TGA of MPDI and copolyamides I and II under a nitrogen atmosphere(discontinuous lines: first derivatives).


two vicinal DPA residues is approximately 10%, as calculatedfrom the area ratio of protons 5 and 6 to proton 4. This result isalso in full agreement with random copolymerisation. Theinherent viscosity and the 1H NMR data were consistent with ahigh degree of polycondensation.

Properties of the polymers

Although the polyamides of interest are the uorescent poly-mers II and III, the homopolymer MPDI was synthesised withthe constitution of a commercialised meta-aramid for compar-ative purposes, i.e., to obtain structure–property relationships.For the same reason, copolymer I was prepared to clarify theinuence of the dipicolinic substitution (DPAD vs. DPA) on theuorescent behaviour of the materials and also to elucidate thebinding mechanism of metal cations.

Thermal properties. The thermal behaviour of the polyamideswas evaluated by DSC and TGA. The reference polyamide MPDIshowed a glass transition temperature (Tg) of 275 �C. The Tg ofcopolyamides I, II, and III could not be determined, most likelybecause of the initial degradation step observed between 250 and

4260 | Polym. Chem., 2013, 4, 4256–4264

300 �C in the derivative of the TGA curves (a weight loss ofapproximately 0.5%) that was not observed forMPDI (Fig. 3). Thisfact agrees with the data reported for the homopolyamidesprepared from DPA and MPD.12

Regarding the thermal stability, the data depicted in Table 1show slightly lower decomposition temperatures for the copol-ymers containing small quantities of DPA (I) or DPAD (II andIII), compared with the reference homopolymer MPDI.However, the char yield at a given temperature is an even moreindicative parameter than the decomposition temperatures forevaluating the thermal resistance of high-performance mate-rials. The char yield at 800 �C is higher for copolymers I and IIthan for MPDI. This result could be attributed to the formationof less volatile residues of the copolymers due to the reactivity ofthe electron decient pyridine ring and/or to the hydrogenbonding between these moieties and the amide groups, whichis responsible for the helicity of these types of polymers.13 Thechar yield could be used to estimate the limiting oxygen index(LOI) of the polymers (see Table 1).14 Although this correlationoverestimates the LOI values for aromatic polyamides, thecomparison of the polyamides with MPDI, calculated in thesame manner, is sufficient to show that the materials are self-extinguishing polymers. (LOI is dened as the minimum frac-tion of oxygen in a mixture of oxygen and nitrogen that willsupport combustion aer ignition. As air contains approxi-mately 21% oxygen, anymaterial with an LOI higher than 21 willmost likely not burn in an open air atmosphere.)

Solubility. The polymer solubilities are shown in Table 2. Thecopolyamides (I–II) were soluble in aprotic polar solvents,exhibiting solubility similar to that of the reference poly-isophthalamide, MPDI. Usually, both copolymerisation and theintroduction of bulky pendant groups enhance the solubility.However, such an enhancement was not observed, most likelybecause of the polar nature of the bulky structure of the DPADresidue, i.e., the extra aromatic sp2 and sp3 nitrogens cancontribute to the overall cohesive energy by establishing newhydrogen bonds and dipole–dipole interactions. This contri-bution counteracts the effect of the random copolymerisation indiminishing the overall effectiveness of the interchain hydrogenbonds and of the bulky group in lowering the cohesive energydue to their contribution to the increase in the mean interchain


Table 2 Solubilitya of the polymer and copolymers

Polymer NMP DMA DMF DMSOH2SO4,conc. THF, CH

MPDI + ++ + ++ ++ �I + + + + ++ �II ++ ++ ++ ++ ++ �III + + + + ++ �a ++ ¼ soluble at room temperature, + ¼ soluble on heating, +� ¼partially soluble, � ¼ insoluble, NMP ¼ N-methylpyrrolidone, DMA ¼N,N-dimethylacetamide, DMF ¼ N,N-dimethylformamide, THF ¼tetrahydrofuran, CH ¼ cyclohexanone.


distance. Nevertheless, the solubility in aprotic polar solventswas enough to prepare cellulose bre coatings, membranes bythe phase inversion technique and solutions to evaluate thesensory performance of the materials in the detection andquantication of cations.

Fluorescence characteristics. We previously mentioned thatuorescence is a key parameter for certain advanced applica-tions, such as LUCO and sensory materials.5,6 Copolymers II andIII are uorescent materials, showing emission maxima of 500and 497 nm, respectively, in organic–aqueous solutions uponexcitation at 370 nm (Fig. 4). These polymers exhibit similaruorescence behaviour because they have the same uorescentDPAD residue. The small difference in the uorescence maximaand intensities under identical conditions could be ascribed tointer- and intrachain interactions; II has amain chain of poly(m-phenylene isophthalamide) and III of poly(m-phenylene 2,6-pyridinedicaboxamide). Both the reference homopolymerMPDIand copolymer I showed negligible uorescence under the sameconditions, thus conrming that the uorescence properties aresolely a result of the DPAD moiety in copolymers II and III.

The uorescence spectra can be correlated with the nakedeye perception using a chromaticity diagram. The human eyehas three types of colour sensitive cones, and their response isdescribed in terms of three tristimulus values. However, a

Fig. 4 Left: fluorescence spectra of copolyamides II and III in solution. Right: CIE 1www.commons.wikimedia.org/wiki/File:CIE1931xy_blank.svg) along with the obsersolutions, lex. ¼ 370 nm, concentration of the fluorescent DPAD residues ¼ 1 � 10


diagram mapping the human colour perception in terms of twoCIE15 chromacity coordinates, x and y, can be drawn. Theseparameters are obtained from the normalised values of

X ¼ð780

380

IðlÞxðlÞdl

Y ¼ð780

380

IðlÞyðlÞdl

Z ¼ð780

380

IðlÞzðlÞdl

where I(l) is the spectral power distribution, l is the wavelengthof the equivalence monochromatic light in nm, and �x(l), �y(l),and �z(l) are colour matching functions, i.e., the numericaldescription of the chromatic response of the human eye, wherex ¼ X/(X + Y + Z) and y ¼ Y/(X + Y + Z). According to the CIE 1931colour matching functions, the coordinates for copolymer II arex ¼ 0.20 and y ¼ 0.42; for III, the coordinates are x ¼ 0.19 andy ¼ 0.39. However, the quantum yield of copolymer II (fII ¼0.07, fDPAD ¼ 0.02) should be improved for practical applica-tions, i.e., using these materials as LUCO converters for theobserved colours. At this point, it is important to consider thepreparation of uorescent copolymers not only to obtain high-performance materials that are luminescent but also to increasethe quantum yield of the existing uorophores. This effect canbe observed by the 4-fold increase in quantum efficiency goingfrom the diacid monomer (DPAD) to the polymer. The higherefficiency of the polymer may be because of the fact that theuorophore moieties cannot self-interact as a result of themovement restrictions imposed by the polymer backbone; thusthe polymer partially avoids the radiationless self-deactivationof the excited state. Nevertheless, for obtaining white light,

931 xy chromaticity diagram (Wikipedia commons, public domain image, http://ved colours of copolymer solutions II and III. Conditions: DMSO/water (99/1, v/v)�4 equivalents per L.

Polym. Chem., 2013, 4, 4256–4264 | 4261

Fig. 5 Left: fluorescence spectra of copolyamide II in a DMSO/water (99/1, v/v) solution (lex. ¼ 370 nm) upon adding Cu(II), Fe(III) and Cr(VI). Right: quenchingefficiency of target cations, (F0/F) � 1. The concentration of the DPAD residues was 1 � 10�4 equivalents per L and the concentration of the cations was 1 � 10�3 M.


further tuning of the uorescent monomer is needed, althoughthe visual perception of the solid material irradiated at 365 nmis not far away (Fig. 4).

Sensing of cations. Aromatic homopolyamides preparedfrom DPA and MPD have been previously reported as hostmaterials for cations, specically Cu(II). Hassan et al. proposed amechanism of the interaction of Cu(II) involving both thenitrogen of the pyridine ring of the DPA residue and the vicinalcarbonyl groups of the main chain amide linkages.12 Althoughthis is a possibility, we did not observe it while studying poly-amide I because the uorescence of this copolymer was negli-gible compared with II and III. Furthermore, the interaction ofCu(II) with copolyamides II and III indicates a complex stoi-chiometry of 1 : 1 Cu(II) : DPAD. This suggests complexation ofthe cation through interactions with both the sp2 and sp3

nitrogens of the pendant dipicolinic acid derivative. Of course,the interaction proposed by Hassan et al.12 between Cu(II) andthe main chain pyridine ring of the DPAD derivative could beweaker and undetectable at the lower concentrations used inthe uorescence experiments.

Fig. 6 Left: fluorescence spectra of copolyamide II in DMSO/water (99/1, v/v) (lex.¼quenching efficiency vs. cation concentration) and Job's plot (bottom, change influorescence quenching experiments of copolyamide II in DMSO/water (99/1, v/v) (lof Cr(VI).

4262 | Polym. Chem., 2013, 4, 4256–4264

The interaction of the copolyamides with cations wasstudied for the range of metal ions shown in Fig. 5. The uo-rescence of solutions of copolymers II and III were quenchedselectively by Cr(VI), Fe(III) and Cu(II), with 26-, 20- and 4-foldquenching of the uorescence maxima, respectively. Noresponse was induced by the other cations.

At low Cr(VI), Fe(III) and Cu(II) concentrations, the quenchingresponse of polyamides II and III could be described by theStern–Volmer equation (F0/F ¼ 1 + KSV[Q], where [Q] is theconcentration of the quencher, F0 is the initial uorescentintensity, F is the uorescent intensity of the copolymer at agiven concentration [Q], and KSV is the Stern–Volmer quenchingconstant). Thus, plotting the quenching efficiency, (F0/F)� 1, vs.the concentration of cations gives the titration curve, which is astraight line at low cation concentrations with a slope of KSV (asan illustrative example, see the data for Cr(VI) depicted inFig. 6).16 Although the quenching process may be dynamic orstatic, the deviation from linearity at higher cation concentra-tions points to a mixed nature of the quenching phenomenon;static quenching is predominant at lower concentrations, with a

370 nm) upon adding increasing quantities of Cr(VI). Right: titration of Cr(VI) (top,the fluorescence vs. molar fraction of fluorophore, cDPAD) corresponding to the

ex.¼ 370 nm, fluorescence intensity at 500 nm) upon adding increasing quantities


Fig. 7 Digital picture of paper strips (5 � 5 mm) coated with II after dipping inwater containing Cr(VI). As a reference, the right strip marked as a paper is theuncoated cellulose material.


complex stoichiometry of 1 : 1. The binding stoichiometry canbe observed in Job's plot for the complexation of Cr(VI), wherethe maximum is observed at a DPAD molar fraction of 0.5(Fig. 6). The calculated KSV values are 11 600 � 1300, 11 700 �800 and 10 300 � 700 M�1 for copolymer II with Cr(VI), Fe(III)and Cu(II), respectively. In terms of selectivity and sensitivity,the differences between copolymers II and III were negligible,again suggesting the exclusive interaction of the DPAD residueswith the target cations and the minor inuence of the copol-ymer chain.

To extend the sensing capabilities to aqueous environments,a lm from copolymer II was prepared by casting from DMA viathe solution evaporationmethod.17 However, the densematerialshowed no uorescent response toward the target cations.Despite the fact that the aramid is hydrophilic, the water uptakeis not sufficient to swell the dense material and permit thesolvated cations to enter into the lm and interact with thesensory motifs.18 Our second attempt to achieve sensing inwater involved increasing the surface area of the sensorymaterial to provoke a response from the interaction of the targetcations with the surface. However, the preparation of asym-metric membranes by the phase inversion technique was notsuccessful.17 Our third attempt followed the same direction, andwe coated the surface of a natural cellulose substrate to increasethe surface exposed to water and the target cations. Commerciallter paper was used as a cellulose substance because itpossesses porosity and a large surface area. To fabricate acellulose-based sensory material, the lter paper was immersedin a DMF solution of II (1.6 � 10�4 M DPAD). The coated paperwas situated over a UV lamp (l¼ 365 nm) and exhibited a stronguorescence, while the uncoated paper showed no uorescence(Fig. 7). The sensing experiments were carried out by exposingthe cut cellulose strips to aqueous solutions of Cr(VI), Fe(III) andCu(II) at different concentrations for 15 min. Depending on theconcentration, uorescence quenching could be clearlyobserved, as depicted in Fig. 7 for Cr(VI). A strong quenching wasapparent to the naked eye at millimolar concentrations of Cr(VI)and Fe(III), while the same effect for Cu(II) could only beobserved at concentrations two orders of magnitude higher.

Conclusion

This work describes the synthesis and characterisation of green-blue uorescent aromatic copolyamides bearing pendant uo-rescent moieties. These polyamides showed a reasonably goodthermal behaviour and solubility suitable for solution trans-formation, e.g., for coatings or spinning into bres. Moreover,


the dipicolinic acid derivative was found to be a receptor, orhost, unit for cations of environmental, biomedical andindustrial signicance, proving to be effective uorescent che-mosensors for Cr(VI), Fe(III) and Cu(II) in organic–aqueoussolutions.

Acknowledgements

The authors gratefully acknowledge the nancial supportprovided by the Spanish Ministerio de Economıa y Com-petitividad – Feder (MAT2011-22544 and CTQ2012-31611), andby the Consejerıa de Educacion - Junta de Castilla y Leon(BU246A12-1).

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