Synthesis, characterization and luminescentproperties of new copolymers ofdimethacrylate derivatives of naphthalene-2,7-diolBeata Podkościelnaa*, Agnieszka Lipkeb, Barbara Gawdzika andMarek Majdanb

Synthesis and characterization of new type photoluminescent, bulk copolymers of 2,7-(2-hydroxy-3-methacryloyloxypropoxy)naphthalene (2,7-NAF.DM) with different vinyl comonomers (methyl methacrylate, 2-hydroxyethyl methacrylate, butyl acrylate, divinylbenzene, styrene and N-vinyl-2-pyrrolidone) are presented. Thechemical structure of 2,7-NAF.DM was confirmed by NMR, GC-MS and elemental analysis. The copolymers were char-acterized by ATR and thermal (differential scanning calorimetry) analyses. Their luminescent properties were studiedin terms of quantum efficiency (Φabs), which was shown to change in the range of 50–90% depending on the type ofcomonomer. It was found that the green-emitting species in these polymers can be excited directly by low-energy(400–520nm) photoirradiation. These materials can be applied in the liquid or solid states in the form of powders,films or monoliths. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: bulk copolymers; dimethacrylate; DSC; naphthalene-2,7-diol; NMR; photoluminescence

INTRODUCTION

Naphthalene derivatives, due to their unique luminescent prop-erties, have been investigated and used as fluorophores ofluminescent materials,[1–3] polymer light-emitting diodes,[4,5]

photo-voltaic and optical devices,[6–8] etc. Polynaphthalenes withdifferent linkages in 1,4-, 1,5-, 2,6- and 2,7-positions have beenused as conducting polymers.[9]

Hohloch et al. reported the synthesis and electroluminescenceproperties of poly(2,6-naphthylenevinylene) derivatives withyellow–green emission.[8] They initiated work on the synthesisof several naphthalene containing polymers which were usedas luminophores in optical devices.[10–12] Yamamoto and Leealso reported the preparation and chemical properties ofπ-conjugated poly(naphthalene-2,6-diyl)-type polymers with Natoms at all of the four o-positions.[13] Premachandran et al.stressed that many fluorescent dyes contained segments ob-tained from 2-naphthol units. Easily processable fluorescentpolymers containing naphthol chromophores could have poten-tial applications in the production of high laser resistance mate-rials, laser dyes and fiber optic sensors.[14] Luminescentcomposite materials based on semiconductor nanocrystals withan ionic liquid-based polymer which possesses an acrylate groupwere prepared by Nakashima et al.[15]

The focus of the present study is on new compounds derivedfrom 2,7-(2-hydroxy-3-methacryloyloxypropoxy)-naphthalene(2,7-NAF.DM).[16] The copolymers were synthesized using differ-ent comonomers: methyl methacrylate (MMA), 2-hydroxyethylmethacrylate (HEMA), butyl acrylate (BA), divinylbenzene (DVB),styrene (ST) and N-vinyl-2-pyrrolidone (NVP). These comonomerswere chosen because of their different chemical character, which

depended on the presence of aromatic or aliphatic substituentsin their structure. The choice of the comonomers was also dic-tated by the fact that they are active solvents, which makes po-lymerization easier. Moreover, their spatial distribution in thestructure enables modification of the luminescent properties ofpolymers and, most importantly, prevents interaction betweenpolymer chains that would otherwise result in agglomeration,which, in turn, would worsen luminescence.

To summarize, we present new polymeric naphthalene deriva-tives, which in future have a chance to find application in thepreparation of different luminophores. We also pay attention tothe thermal properties of the investigated copolymers, taking intoaccount their potential application as thermally resistant coatings.

EXPERIMENTAL

Preparation of compositions and their polymerization

Detailed information about the synthesis of 2,7-NAF.DM was pre-sented elsewhere.[16] Briefly, this monomer was obtained from

* Correspondence to: Beata Podkościelna, Department of Polymer Chemistry,Maria Curie-Skłodowska University, M. Curie Skłodowska Sq. 5, 20-031 Lublin,Poland.E-mail: beatapod@umcs.pl

a B. Podkościelna, B. GawdzikDepartment of Polymer Chemistry, Maria Curie-Skłodowska University, M.Curie Skłodowska Sq. 5, 20-031, Lublin, Poland

b A. Lipke, M. MajdanDepartment of Inorganic Chemistry, Faculty of Chemistry, Maria Curie-Skłodowska University, M. Curie Skłodowska Sq. 5, 20-031, Lublin, Poland

Research article

Received: 8 September 2014, Revised: 13 November 2014, Accepted: 23 November 2014, Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI: 10.1002/pat.3447

Polym. Adv. Technol. 2014 Copyright © 2014 John Wiley & Sons, Ltd.

naphthalene-2,3-diol in two-step synthesis. First, the reaction ofphenolic group with 2-(chloromethyl)oxirane in a two-phaseliquid/liquid system including organic and aqueous phases wasperformed. Then, esterification of the obtained diglycidyl etherwas carried out with the use of methacrylic acid.

The chemical structure of 2,7-NAF.DM was confirmed by 1H,13C-NMR and elemental analysis.

1H-NMR (2,7-NAF.DM): (―CH2), δ= 4.09–4.19 and δ= 4.30–4.45; (―CH3), δ= 1.96–2.16 ppm; (―OH), δ= 2.82 ppm;(―CH¼CH2), δ= 5.60–5.63 and δ=6.16–6.82 ppm.

13C-NMR (2,7-NAF.DM): CH2¼CH(CH3)―, 126.4 ppm and135.9 ppm; CH3―, 18.4 ppm; C¼O, 167.6 ppm; CH2―, 65.7,68.9 ppm; C―C(CH3)―OH, 68.6 ppm; naphthalene ring, 106.5,116.4, 124.8, 129.4, 135.7, and 157.1 ppm.

Elemental analysis (2,7-NAF.DM): C 65.08% (cal. 64.85%); H6.32% (cal. 6.35%).

The compositions obtained in this study consisted of 2,7-NAF.DM, one of the vinyl monomers (MMA, HEMA, BA, DVB, ST orNVP) and a polymerization initiator (α,α′-azoiso-bis-butyronitrile).For all compositions, a constant weight ratio of 2,7-NAF.DM toactive diluent (1:10 w/w) was applied. After the monomers haddissolved, the initiator (0.5%) was added, and the compositionwas placed in glass forms. The forms were moved into a waterbath for 12 hrs at 60°C and then into a heating chamber for4 hrs at 100°C in order to complete the crosslinkingreaction.[17,18]

Spectral characterization of the samples

ATR-IR spectra were obtained on a Bruker FTIR spectrophotome-ter TENSOR 27. NMR spectra were recorded on a Bruker 300 MSLInstrument (Bruker, Germany).

GC-MS was performed on a Thermo-Finnigan DSQ spectrome-ter (Finnigan, USA) hyphenated with a gas chromatograph TraceGC-Ultra equipped with a fused-silica RTX-5 capillary column(20m×0.18mm I.D.; film thickness, 0.20μm). The conditionswere as follows: injector PTV-split 1:20; temperature program

35–320°C at a heating rate 20°C min�1; MS electron ionizationat 70 eV; temp. of ion volume 220°C.Differential scanning calorimetry (DSC) thermograms were ob-

tained with the use of a DSC Netzsch 204 calorimeter (Netzsch,Günzbung, Germany). All DSC measurements were carried outin aluminum pans with a pierced lid with a sample weight of~5–10mg under nitrogen atmosphere (30ml/min). Dynamicscans were performed at a heating rate of 10 K/min in the tem-perature range of 25–500°C. Curing characteristics, such as thetemperature of the peak maximum and final curing temperaturewere determined. The decomposition temperatures (T1, T2), thefinal decomposition temperature (Td) and enthalpy of decompo-sition (ΔHd) were also evaluated.Room temperature UV–Vis absorption spectra were recorded

using a V-660 JASCO spectrophotometer, and emission spectrawere obtained with a Photon Technology International spectro-fluorimeter equipped with a continuous wave Xe-arc lamp as alight source. Spectral resolution was maintained at 1 nm. Liquidsamples were measured in a quartz cuvette with a 2-mm opticalpath and solid samples—in appropriate adapters. UV–Vis reflec-tance spectra were registered using a Horizontal Sampling Inte-grating Sphere (Model PIV-756) connected to the V-660spectrophotometer. The decay curves of polymer samples wereexcited at 450 nm by an Nd:YAG pulse laser (Opolette System,Quantel, USA) connected to the Photon Technology Interna-tional spectrofluorimeter. The absolute quantum yields of thefluorescence (Φabs fl) of powdered polymers were determinedusing a K-Sphere “Petite” integrating sphere (Photon TechnologyInternational, Inc.).

RESULTS AND DISCUSSION

The chemical structures of the photoluminescent monomer (2,7-NAF.DM) and the active diluents (comonomers) used in copoly-merization reactions are shown in Fig. 1. The copolymers ob-tained were characterized by different chemical structures

Figure 1. Chemical structure of the compounds used for copolymerization.

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caused by differences in the structure of the monomers used intheir synthesis. The most dense polymer network was achievedfor the 2,7-NAF.DM-DVB copolymer synthesized from twotetrafunctional monomers. All copolymers contained aromaticnaphthalene rings coming from 2,7-NAF.DM. A copolymer withHEMA possessed the highest number of hydroxyl groups in itschemical structure, which made it the most hydrophilic of thecopolymers. Similar properties were observed for the 2,7-NAF.DM-NVP copolymer containing the strongly water-absorbingmonomer, NVP.

ATR-FTIR analysis

The data obtained from the ATR-IR spectra of the 2,7-NAF.DM co-polymers are presented in Table 1. An analysis of these data con-firms the differences in the structure of the monomers used inthe copolymerization reactions. For all copolymers, absorption

of aromatic naphthalene (C―H) was observed in the range of830–841 cm�1. For the 2,7-NAF.DM-DVB and 2,7-NAF.DM-ST co-polymers, aromatic skeletal absorption was observed at 1602and 1600 cm�1, respectively. A band at 3402 cm�1 was causedby stretching vibrations of O―H bonds in the 2,7-NAF.DM-HEMAcopolymer. A C¼O signal produced coming from carboxylicgroups was detected at 1690–1734 cm�1.

GC-MS

The chemical structure of 2,7-NAF.DM was also confirmed by GC-MS analysis. As 2,7-NAF.DM spectra were not available in the li-brary, identification was achieved by analyzing molecular andfragmentary ions. The molecular ions corresponding to the cal-culated molecular weight of 2,7-(2-hydroxy-3-methacryloyloxypropoxy)naphthalene (444) and a number offragmentary ions are well visible in the spectra shown in Fig. 2.

Table 1. FTIR spectra of obtained copolymers

Copolymer /cm�1

2,7-NAF.DM-MMA – 830 – 1734 29242,7-NAF.DM-BA – 841 – 1723 29372,7-NAF.DM-HEMA – 846 3402 1718 29442,7-NAF.DM-ST 1600 831 – 1720 29182,7-NAF.DM-DVB 1602 830 – 1725 29232,7-NAF.DM-NVP – 839 – 1690 2918

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Figure 2. GC-MS spectra of 2,7-NAF.DM. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

GREEN LUMINESCENCE OF THE NEW NAPHTHALENE COPOLYMERS

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Thermal properties

The DSC curves of cross-linked copolymers are presented in Fig. 3and Table 2. The DSC analysis shows some differences in thethermal behavior of the copolymers studied. In the case of 2,7-NAF.DM-DVB, only one exothermic peak is visible, at 149.5°C.This exothermic effect may be related to the additionalcrosslinking of unreacted double bonds. In the case of 2,7-NAF.DM-HEMA and 2,7-NAF.DM-NVP, single endothermic peaks arevisible at 90.1 and 95.4°C, respectively. These effects are associ-ated with the removal of water. The endothermic peaks at391.5–450.6°C, with ΔHd from 166.82 to 730.8 J g�1, are associ-ated with the final degradation of the copolymers. In the caseof 2,7-NAF.DM-BA and 2,7-NAF.DM-ST, only one endothermicpeak (at 405.6 and 424.9°C, respectively) each is visible (Fig. 1).Therefore, it can be concluded that the compositions obtainedin this study are homogeneous.

Differences in the enthalpy of decomposition (ΔHd) result fromthe different chemical structure of the comonomers used. Thehigher the decomposition temperature and ΔHd, the more ther-mally stable the polymeric structure is. In other words, the factthat more heat must be supplied to the decomposition of a poly-mer means that the polymer is thermally more stable. In the caseof heterocyclic or aromatic monomers (ST, DVB and NVP), the co-polymers obtained are characterized by higher thermal resis-tance and higher ΔHd values. On the other hand, aliphatic(meth)acrylates, such as MMA, BA and HEMA have a tendencyto degrade at lower temperatures. This phenomenon is the

consequence of the cracking of linear C―C bonds, which makesdecomposition easier compared with aromatic compounds. Theresults indicate that all of the bulk copolymers studied are char-acterized by good thermal behavior and may have potential ap-plication as thermally resistant materials.

Photoluminescent properties

UV–Vis absorption spectra of the polymers in their powder formare given in Fig. 4. The strong absorption in the UV region, up to350 nm, obviously originates from naphthalene.[13] The band450 nm probably originates from the π conjugation in thenaphthalene-diepoxymethacrylate moiety. The copolymerizationdoes not change either the position or the shape of this band.This means that there is no conjugation along the chain of copol-ymers of 2,7-NAF.DM.It is interesting to note that the 450-nm absorption band is ab-

sent in the spectra of the naphthalene-2,3-dioldiepoxymethacrylate derivative (2,3-NAF.DM) and naphthalene-2,7-diol (2,7-NAF), which demonstrates the important role ofsymmetry in the structure of the polymers and delocalizationof electrons as well as pointing to the crucial role of thediepoxymethacrylate subunit in the absorption of photons. It isworth mentioning that there are no differences in the shape of

Figure 3. DSC curves of the copolymers in N2.

Table 2. DSC data of thermal behavior of obtainedcopolymers

Copolymer /°C J g�1

T1 T2 Td ΔHd

2,7-NAF.DM-MMA – 293.5 391.5/450.6 166.822,7-NAF.DM-BA – – 405.6 508.62,7-NAF.DM-HEMA 90.1 249.5 390.1 331.42,7-NAF.DM-ST – – 424.9 730.82,7-NAF.DM-DVB – – 450.8 401.32,7-NAF.DM-NVP 95.4 – 427.1 593.6

Wavelength (nm)300 350 400 450 500 550 600

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450

(B) Copolymers:

Figure 4. Absorption spectra of 2,7-NAF.DM in solutions (A) and copol-ymers (B) (2,7-NAF and 2,3-NAF.DM given for comparative purposes).

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the spectrum between monomer solutions and the correspond-ing copolymers, except for the lowering of band intensity for thesolid samples. The lack of the red shift of the 450-nm band, char-acteristic of aggregated polymers,[4] probably attests to the ab-sence of any strong interactions among the chains of thepolymer and the absence of a π conjugation along the copoly-mer backbone. The presence of bulky substituents in the struc-ture of the copolymers precludes interaction among thestructural chains.

The excitation and luminescence spectra of the 2,7-NAF.DM-DVB copolymer in its powder form and the corresponding solu-tion are shown in Fig. 5. We found that they did not differ qual-itatively from the spectra of copolymer monoliths. The shape ofthe bands for the solution and the solid state is very similar.The intensity of luminescence is strengthened as an effect of po-lymerization, which is another proof for the lack of any strong in-teractions among polymer chains. It should be emphasized thatthe excitation is in the visible region of the spectrum, a fact thatis very important from the practical viewpoint of convenientluminophore properties. Similarly to UV–Vis absorption spectra,there is no essential difference between the luminescence ofthe solid and liquid samples, i.e. the shape and the position ofthe bands are nearly the same (Fig. 6). The small differencesamong the intensities of the 490–500 nm band are a result ofthe different homogeneity of the samples and, thus, their trans-parency, i.e. some liquid samples are foggy and some solid sam-ples are rough.

The quantum efficiency of the luminescence (Φabs fl) of the in-vestigated copolymers is given in Table 3. The values of Φabs fl

are very high and range from 51% for the NVP-copolymer to90% for the ST- and DVB-copolymers. This can be rationalizedpartially by the high hydrophilicity of the NVP and HEMA sol-vents. It is commonly known that absorbed water moleculesquench luminescence, a fact that should be taken into accountin the future design of luminophores based on 2,7-NAF.DM de-rivatives. To summarize, the small shift of about 0.27 eV betweenthe absorbance and luminescence spectra (Stokes shift) providesevidence for the lack of nonradiational electronic transitionswhen the electrons return from the excited to the ground state.What is more, this fact suggests a lack of 2,7-NAF.DM

Figure 5. Excitation and emission spectra of 2,7-NAF.DM.

Figure 6. Emission spectra of 2,7-NAF.DM in solutions (A) and copoly-mers (B) (2,7-NAF and 2,3-NAF.DM given for comparative purposes).

Table 3. Quantum efficiency data of obtained copolymers

Copolymer Φabs (%) Std. dev. 95% conf.

2,7-NAF.DM-MMA 73.2 10.3 25.72,7-NAF.DM-HEMA 65.2 5.3 13.22,7-NAF.DM-ST 88.3 1.6 3.92,7-NAF.DM-NVP 51.9 4.3 4.52,7-NAF.DM-DVB 90.8 3.7 9.1

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Figure 7. Fluorescence decay curves of the copolymers.

GREEN LUMINESCENCE OF THE NEW NAPHTHALENE COPOLYMERS

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aggregation. The fluorescence lifetimes, based on the decaycurves (Fig. 7), for the system with the highest and the lowestΦabs fl are very similar (ca. 5.8 ns). This additionally confirms theobservation that the type of solvent has no effect on fluorescentmolecules.

CONCLUSIONS

1. The copolymers obtained in the present study can be used inthe design of luminophores which show luminescence uponirradiation in the visible region of the spectrum and can beapplied in the liquid or solid states.

2. The 2,7-NAF.DM copolymers are characterized by good ther-mal behavior and may be used in the synthesis of thermallyresistant materials.

3. The form of the copolymers studied can be accommodated tothe demands of the industry, i.e. they can be synthesized asfilms or monoliths, and then powdered. The crosslinking oftheir structure improves their mechanical resistance.

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