A bifunctional chromogenic and fluorogenic probe for F� and Al3þ

based on azo-benzimidazole conjugate$

Murugan Iniya, Dharmaraj Jeyanthi, Karuppiah Krishnaveni, Duraisamy Chellappa n

School of Chemistry, Madurai Kamaraj University, Madurai 625021, Tamil Nadu, India

a r t i c l e i n f o

Article history:Received 3 February 2014Received in revised form8 August 2014Accepted 8 September 2014Available online 18 September 2014

Keywords:AluminumFluorideFluorescenceChromogenic sensorDFT

a b s t r a c t

A versatile bifunctional probe has been reported for selective individual detection of Al3þ and F�

through “turn-on” chromogenic and fluorogenic dual modes. The photonic behaviors of probe uponaddition of analyte have been determined using UV–vis absorption, fluorescence emission, quantumyield and fluorescence life-time measurement. Density functional theory calculations have beenperformed to establish the nature of interaction between probe and Al3þ/F� .

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The development of optical signaling systems based on organicscaffold for sensing and recognition of specific ions has been a recentarea of focus owing to their potential applications in environmentaldetection, molecular catalysis and monitoring biological processes[1–3]. Nevertheless efficient techniques are required to detectspecific metal ions or anions as often analytes tend to interfere witheach other. In recent years, Fluorogenic and chromogenic sensorshave been emerging as a valid alternative to conventional analyticalmethods [4–6] due to simplicity, high efficiency in detecting evenlow analyte concentration and application to bioimaging [7–9].Moreover, design of multifunctional sensors with varying responsetowards different analytes is cost effective and convenient for realapplications. Nevertheless, designing sensors with multiple analyterecognition capability is a challenging task.

Among common anions, fluoride ion received the most attentionfrom chemists because of its unique properties [10]. For instance,fluoride plays crucial roles in dental care [11] and the treatment ofosteoporosis [12]. Beyond recommended doses, fluoride has beenassociated with fluorosis [13] and urolithiasis [14]. On the other hand,Aluminum, the most abundant metallic element is extensively usedas additives in food, in drugs (e.g., antacids), in consumer products(e.g., cooking utensils and aluminum foil) and in the treatment of

drinking water (e.g., coagulants) [15–17]. However if the amountrequired is greatly exceeded by our incidental intake, it exertsadverse effects, for instance, neurological diseases such as Alzheimer'sdisease, Parkinson's dementia and also to some bone disorders suchas osteoporosis and osteomalacia [18–20]. Aluminum toxicity is amajor constraint to crop production in almost 67% of the total acidsoil area [21], as it influences agricultural production in soils ofpHr5.5 [22].

Azo compounds have been extensively utilized in electronicapplications such as photochromic, proton-responsive, photon-mode high density information storage, photo-switching devices,optical computing and optical data storage [23–25], due to theirgood stability and suitable absorption band. Being an opticalgroup, azobenzene has been exploited well as signaling unit ofchemosensor in literature [26–27]. In addition, imidazole grouphas also attracted interest as metal ion sensors [28–30] and can beusually found in active site of many metalloproteins [31].

Interestingly, various fluorescent sensors specific for Fluoride[32–40] or Aluminum [41–43] have been reported, but to the bestof our knowledge, a single molecular sensor that shows varieddual response to fluoride and aluminum is still unexplored. In ourprevious reports, we have utilized benzimidazole platform for theselective recognition of Al3þ ion [44]. In continuing our researchto develop new fluorescent chemosensors [45–50], we havedesigned an azophenol linked benzimidazole unit that not onlybehaves as chromogenic and fluorogenic chemosensor but alsoselectively recognizes Al3þ and F� individually with “turn-on”fluorescent response. Recently, azo based Schiff base has beenutilized for the selective recognition of oxalic acid via counterion

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jlumin

Journal of Luminescence

http://dx.doi.org/10.1016/j.jlumin.2014.09.0180022-2313/& 2014 Elsevier B.V. All rights reserved.

☆Electronic Supplementary Information (ESI) available: [NMR and MS Spectraldata, UV–vis data and computational details]. See DOI: 10.1039/b000000x/

n Corresponding author. Tel.: þ91 452 2456614; fax: þ91 452 2459181.E-mail address: dcmku123@gmail.com (D. Chellappa).

Journal of Luminescence 157 (2015) 383–389

displacement assay by Singh et al. [51]. It is interesting to observethat formation of quinazoline nucleus rather than Schiff base [51]opens a new channel for fluorogenic and chromogenic response,which demonstrated subtle structural changes of sensors, can havea remarkable effect on recognizing analytes. In addition, a singlemolecular sensor that can selectively detect and distinguish morethan one species has rarely been reported. To the best of ourknowledge, this is the first bifunctional fluorescent chemosensorthat allows the detection of Al3þ and F� .

2. Materials and methods

2.1. Chemicals and starting materials

All the chemicals used in the present study were of analyticalreagent grade and purchased from Sigma-Aldrich Chemical Com-pany. Solvents used were of HPLC grade unless otherwise stated.Binding property of metal ion with probe AZIM was evaluated byusing Chloride salts of metal ions such as Naþ , Kþ , Mg2þ , Ca2þ ,Cr3þ , Mn2þ , Fe2þ , Fe3þ , Co2þ , Ni2þ , Cu2þ , Zn2þ , Agþ , Cd2þ ,Hg2þ , Al3þ and Pb2þ ions. All anions ( F� , Cl� , Br� , I� , OAc� ,NO3

� , HSO4� , H3PO4

� and CN� ) were used in the form of tetra-butylammonium (TBA) salts, which were stored in a vacuumdesiccator containing self-indicating silica.

2.2. Physical measurements

UV–vis absorption spectra were determined on a JASCO V-550spectrophotometer. Fluorescence spectra measurements wereperformed on an F-4500 Hitachi fluorescence spectrophotometer.The excitation and emission slit width was kept constant at 5 nm.Elemental analyses were carried out with Perkin-Elmer 4100elemental analyzer. 1H and 13C NMR spectra were recorded on aBruker 300 MHz NMR instrument with DMSO-d6 as solvent andTMS as internal reference. FT-IR spectra were measured on aThermo Fischer Nicolet iS50 FT-IR spectrometer using KBr plates.Electro spray Ionization mass spectral (ESI-MS) analysis wasperformed in the positive ion mode on a liquid chromatography-ion trap mass spectrometer (LCQ Fleet, Thermo Fisher InstrumentsLimited, US).

2.3. Recognition studies

The recognition properties of AZIM towards cations and anionswere investigated in HEPES buffer (20 mM, pH 7.4) solution(DMSO/H2O¼2:8) and DMSO solution through UV–vis and Fluor-escence measurements. Metal solutions were prepared with Doubledistilled water. Solutions of analytes were added in portions toAZIM and absorption/fluorescent intensity changes were recordedat room temperature after each addition.

2.4. Quantum yield calculation

The fluorescence quantum yield Φs was estimated from theabsorption and fluorescence spectra of probe according to equa-tion, where the subscript s and r stand for the sample andreference (quinine sulfate, (ΦF¼0.51) in sulfuric acid), respec-tively.Φ is the quantum yields, A represents the absorbance at theexcitation wavelength, S refers to the integrated emission bandareas and nD is the solvent refractive index [52]. The fluorescencequantum yields (ΦF) were estimated with equation as follows:

φs ¼φrSSSR

Ar

AS

n2DS

n2Dr

2.5. Computational details

Density functional theory (DFT) calculations were performedwith 6-31/6-311Gn and LANL2DZ basis set using Gaussian 03program to understand the turn on fluorescence behavior of AZIMon complexation with Al3þ/F� . Initially the geometries of bothprobe and the AZ–Al3þ were optimized by the program DFT-B3LYPbut using 6-31G and LANL2DZ basis sets respectively. Similarlyoptimized geometry of AZ–F� was obtained by DFT-B3LYP/6-311Glevel calculations.

2.6. Synthesis

2.6.1. Synthesis of 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde (AS)

The starting material 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde (AS) was prepared according to the reportedmethod. P-Nitroaniline (6.91 g, 0.050 mol) was dissolved in awarm mixture of 40 mL of concentrated hydrochloric acid and10 mL of water and was stirred in ice–water bath for 10 min. Afteraddition of 20 ml of 20% NaNO2 to it, the stirring was continued for1 h to obtain a bright yellow solution. Salicylaldehyde (4.28 g;0.035 mol) was dissolved in a solution comprising 18 g of Na2CO3

and 150 ml of H2O and the resulting solution was added drop wiseto the bright yellow colored solution over 1 h. After stirring for 4 h,the reaction mixture was neutralized with HCl and brown crudesolid obtained was filtered. The resulting precipitate was purifiedby column chromatography on silica gel (Petroleum ether/ethylacetate) to give the desired product.

2.6.2. Synthesis of 6-(2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde)-2-yl-5,6-dihydrobenzo [4,5]imidazo-[1,2–c]quinazoline (AZIM)

A mixture of 2-(2-aminophenyl)-1H-benzimidazole (1.0 g,1 mM) and 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde(0.77 g, 1 mM) in ethanol was treated with acetic acid (0.5 ml)and allowed to reflux for 6 h. After completion of the reaction, anorange color solid was precipitated out. The precipitate wascollected, recrystallized in ethanol to afford desired product asorange powder in 85% yield. m.p. 233–235 1C; IR (KBr, cm�1) ν(C¼N) 1619; ν (N–H) 3290; ν (O–H), 3395; ν (C–H) 2922; ν (C–O)1291; ν (N¼N) 1525; ν (NO2) 1500, 1341; 1H NMR (300 MHz,DMSO-d6): δ¼11.42 (s, 1H), 8.30 (d, 2H, J¼8.8 Hz), 8.01 (d, 1H,J¼7.7 Hz), 7.82–7.98 (m, 3H, ArH), 7.68 (d, 1H, J¼7.6 Hz), 7.40–7.42(m, 2H), 7.36 (S, 1H), 7.07–7.24 (m, 5H), 6.80–6.91(m, 2H); 13C NMR(75 MHz, DMSO-d6): δ¼159.8, 155.5, 148.2, 147.6, 145.3, 144.2,143.5, 133.1, 132.0, 127.5, 125.3, 125.0, 123.4, 122.5, 122.7, 119.0,118.4, 117.3, 115.1, 111.8, 110.5, 63.6; MS (ESI) m/z: 462 (AZþH)þ;Elemental analysis: Calculated: C, 67.53; H, 3.92; N, 18.17 andfound C, 67.27; H, 3.76; N, 18.20.

3. Results and discussion

We have synthesized chemosensor AZIM by condensing etha-nolic solution of 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzalde-hyde with 2-(2-aminophenyl)-1H-benzimidazole in 1:l mole ratio(Scheme 1). To monitor the recognition event, nitrylazoscaffold isappended with benzimidazole moiety through quinazolinenucleus. In AZIM, nitrylazobenzene acts as chromogenic unit whilebenzimidazole functions as fluorogenic unit. Hence AZIM mayundergo intramolecular charge Transfer (ICT) from benzimidazolemoiety to azo group when excited by light. It could be expectedthat the AZIM can alter ICT effect and show “turn on” pathwayupon the addition of an analyte. Amongst the different photo-physical processes, charge transfer mechanism [53–55] has been

M. Iniya et al. / Journal of Luminescence 157 (2015) 383–389384

proved to be an efficient way to increase fluorescent outputchannels because of its features like unique spectral shift andquantitative detection. In addition, azophenyl can serve as chro-mogenic unit and phenolic –OH may act as hydrogen-donor group.These two units may facilitate receptor–ion interaction specificallyupon interaction with anions of sufficient basicity such as

F� through hydrogen bonding. The structure of AZIM was wellcharacterized by Elemental analysis, 1H NMR, 13C NMR, FT-IR, andESI-MS analysis (Figs. S1–S6).

To evaluate the sensitivity and selectivity of AZ (formed in situfrom AZIM) for Al3þ in the presence of various metal ions, wemeasured the absorption spectral changes of AZ in HEPES buffer(20 mM, pH 7.4) solution (DMSO/H2O¼2:8). As shown in Fig. 1,UV–vis spectrum of AZ displays absorption bands at 367 and564 nm. Upon addition of Al3þ , the peak at 564 nm disappeared(Fig. S7). At the same time, an apparent color change from pink toyellow in ambient light could be observed by the ‘naked eye’(Fig. 2). It is obvious that the presence of azo group leads tosignificant improvement in chromogenic ability of the receptor.Under identical conditions, metal ions such as Naþ , Kþ , Mg2þ ,Ca2þ , Cr3þ , Mn2þ , Fe2þ , Fe3þ , Co2þ , Ni2þ , Cu2þ , Zn2þ , Agþ ,Cd2þ , Hg2þ , and Pb2þ failed to affect the photonics of AZ like Al3þ

(Fig. 1). In other words, unique binding of AZIM offers an inter-esting opportunity to develop it as highly selective chemosensorfor Al3þ .

As shown in Fig. 3, addition of tetrabutylammonium fluoride tothe solution of AZIM leads to increase in intensity of the high-energy band with simultaneous decrease of the intensity of thelow-energy band. Furthermore, a blue-shift of the latter band isobserved with two isobestic point at 523 and 655 nm due tostrong interaction between receptor and anion. In AZIM, theelectron withdrawing nitro group at the end increases not onlythe degree of π-conjugation but also the hydrogen bond donorability of the phenolic OH. Upon addition of F� , deprotonation of

Scheme 1. Synthetic route of probe AZIM.

Fig. 1. UV–vis spectra of AZ (10 mM) with various cations (1�10�3 M) in HEPESbuffer (20 mM, pH 7.4) solution (DMSO/H2O¼2:8).

M. Iniya et al. / Journal of Luminescence 157 (2015) 383–389 385

azophenolic group occurs, which further leads to electron densityshift towards the nitrosubsitutent of AZ. The addition of otheranions such as Cl� , Br� , I� , OAc� , NO3

� , HSO4� , H3PO4

� and CN�

displayed an insignificant change in absorption spectra due totheir low affinity with AZIM (Fig. S8).

To gain insight into the binding affinity and chemosensingproperties of AZ (formed in situ from AZIM) towards various metalions, fluorescent titrations were carried out in HEPES buffer(20 mM, pH 7.4) solution (DMSO/H2O¼2:8). AZ displays a weakemission band (Φf¼0.08) at 431 nm when it was excited at320 nm. The addition of Al3þ to solution of AZ (Fig. 4) leads tonot only the bathochromic shift of 43 nm from 431 nm to 474 nm(Φf¼0.48) but also hyperchromism in the fluorescent intensity.Among various cations, only Al3þ ions induced a red-shift andsizable enhancement in fluorescence intensity of AZ as shown inFig. S9. These results suggest that AZIM could be used as apotential Al3þ selective chemosensor in a competing environ-ment. Upon introduction of Al3þ , AZIM undergoes solvent assisted1,5-sigmatropic shift [29] and leads to AZ, which in turn exhibitsfluorescent enhancement. This may be due to a ligand to metal

charge transfer process. To study the nature of emission band ofAZ–Al3þ regarding the possible charge transfer character, fluores-cence spectra of AZ were also recorded with the gradual additionof Al3þ in less polar THF solvent. As shown in Fig. S10, uponaddition of Al3þ , the emission band at 431 nm undergoes hyper-chromism without shift. Hence it is clear that the emission bandundergoes red-shift with the increase in polarity of the solvent,which in turn supports AZ to metal charge transfer (LMCT) natureof emission in AZ–Al3þ complex.

To examine the reversibility of AZIM, the interaction of Al3þ–AZcomplex with EDTA was studied. The addition of EDTA quenchesthe fluorescence of AZ–Al3þcomplex, indicating that AZIM actsas a reversible fluorescent chemosensor (Fig. S11). Addition of F�

(0–10eq) to AZIM in DMSO afforded fluorescence enhancement ofemission at 423 nm (Φf¼0.42) accompanied with color changefrom pink to violet (Fig. 2). The observed color change andfluorescence enhancement may be attributable to the hydrogenbond interaction between fluoride and receptor molecule (Fig. 5).To investigate the selectivity of probe AZIM, fluorescence titrationwas performed with various anions. The addition of other anionssuch as Cl� , Br� , I� , OAc� , NO3

� , HSO4� , H3PO4

� and CN� failed to

Fig. 2. The photograph shows the color change of AZIM in the presence of analyte.From left to right (1) F� (2) receptor (3) Al3þ .

Fig. 3. Changes in absorption spectra of AZIM (5 mM) upon gradual addition of F�

(0–50 mM) in DMSO.

Fig. 4. Fluorescence emission spectra of AZ (10 mM) upon gradual addition of Al3þ

(0–10 mM) in HEPES buffer (20 mM, pH 7.4) solution (DMSO/H2O¼2:8). Excitationat 360 nm; Slit width is 5 nm.

Fig. 5. Fluorescence emission spectra of AZIM (5 mM) upon gradual addition of F�

(0–50 mM) in DMSO. Excitation at 360 nm; Slit width is 5 nm.

M. Iniya et al. / Journal of Luminescence 157 (2015) 383–389386

alter the photonics of probe due to lower binding affinity of theseanions with probe AZ (Fig. S12). These facts indicate that AZIM ishighly selective for F� over the common anions.

From the fluorescence titration measurement, the binding con-stant [56] of probe for Al3þ and F� (Fig. S13) was respectively foundto be 5.56�102 M�1 and 6.96�103 M�1. The obtained good linearrelationships (R2¼0.9970 and R2¼0.9947) also indicates 1:1 bind-ing stoichiometry between AZIM and Al3þ/F� . The detection limits[57] were calculated from titration results and respectively found tobe 3.41�10�7 M and 9.189�10�8 M for Al3þ and F� , whichsubstantiates potential applications of AZIM in biological systems.The detection limit was quite appreciable relative to those reportedwith other receptors for Al3þ/F� [58–60].

To test the practical applicability, a competitive binding experi-ment was carried out for the receptor in the presence of compet-ing analytes. No significant variation was observed in the presenceof other competitive ions in comparison to solution containingonly Al3þ/F� (Fig. S14). These results suggest that AZIM could beused as a potential chemosensor in a competing environment. Tounderstand the stoichiometry of Al3þ/F� with in situ formed AZ/AZIM, Job's plot analysis was performed. Based on Job's plotanalysis, AZ/AZIM forms a 1:1 stoichiometric complex with Al3þ/F� (Fig. S15). To provide further support to the efficient binding ofprobe with Al3þ/F� , ESI-MS studies were performed. The ESI-MSspectra, (Figs. S5 and S6), shows peaks at m/z¼278.24 or 460.55corresponding to [AZ-2HþþAl3þþ2H2OþCH3OH]2þ or [AZIM-2Hþ]� respectively, which also corroborates formation of 1:1complex between receptor and guest species.

The effect of pH on the probe was tested by recordingfluorescence spectra over different pH values (Fig. S16). The resultshows that at low pH range, the red-shift of the emission spectra(431–469 nm) was probably due to the weakened ICT (Intramole-cular Charge Transfer) effect caused by the protonation of theprobe. However in high pH range, fluorescence intensity of probeincreases due to the deprotonation of phenolic hydroxyl groupleading to extended conjugation. Fluorescence intensity remainsweak at intermediate pH (HEPES buffer (20 mM, pH 7.4). Thefluorescence lifetimes of probe and its complex AZ–Al3þ/AZ–F�

were determined by time resolved spectrum and the correspondingdata are presented in Table 1. According to the equations: kr¼Φf/

oτf4 , 1/oτf4¼krþknr, the radiative rate constant (kr), fluorescencequantum field (Φf), average fluorescence life-time (τf) and non-radiative rate constant (knr) of probe and probe-Al3þ/F� are calcu-lated. The average fluorescence life-time for AZ–Al3þ(τf¼5.82 ns) andAZIM-F� (τf¼5.40 ns) exceeds that of free sensor AZ (τf¼5 ns) due toincrease in stability of the complex (Fig. S17). The data suggest thatdecrease in non-radiative decay channel leads to increase in averagefluorescence life-time which is evident from increasing order ofcalculated non-radiative rate constant [61]. All these factors supportthe enhancement of fluorescent intensity of AZ–Al3þ and AZIM-F�

relative to AZ.Furthermore, the intermolecular interactions between probe

and Al3þ/F� were investigated by running proton NMR titrationsin the presence and in the absence of Al3þ/F� in DMSO-d6 (Fig. 6).Upon addition of Al3þ , the signals on the aromatic rings changedslightly and resonance signals corresponding to imidazole N–Hand phenolic O–H shifted downfield and eventually disappearupon increasing concentration of Al3þ ion. Whereas the additionof fluoride results in the disappearance of proton signal of O–Hand N–H group, which suggests the involvement of deprotonationprocess among F� , phenolic O–H and imidazolic N–H group.However no signal was observed for [HF2]� even up to δ20 ppm probably due to its instability in highly polar solventssuch as DMSO [62].

To throw light on receptor–guest interaction mechanism, DFTstudies were performed. The optimized structure of AZIM/AZ

Table 1Fluorescence lifetime of free sensor and AZ–Al3þ/F� in DMSO/H2O solvent.

Sample φf τf (ns) Kr (109 s�1) Knr (109 s�1) χ2

AZ 0.08 5 0.016 0.184 1.19AZ–Al3þ 0.48 5.82 0.082 0.089 1.04AZ–F� 0.42 5.40 0.077 0.108 1.18

Fig. 6. 1H NMR spectra of AZIM with Al3þ and F� in DMSO-d6. From bottom to top: AZIM, AZIMþ0.5eq of Al3þ , AZIMþ1eq of Al3þ , and AZIMþ1eq of F� .

M. Iniya et al. / Journal of Luminescence 157 (2015) 383–389 387

(formed in situ) and the complex formed by coordination of F�/Al3þ

with probe was obtained using DFT/B3LYP-6-31G and B3LYP/LanL2DZ basis set [63] respectively (Fig. S18). To better understandthe fluorescence enhancement of receptor after binding, TD–DFTcalculations were performed using λ/B3LYP-6-31G basis set. From theoptimized structure of receptor, the bond length of O–H, N–H andCH¼N is calculated to be 0.976 Å, 1.012 Å and 1.294 Å, respectively.Upon introduction of F� , the bondlength of O–H and N–H elongatedto 1.05 Å and 1.136 Å respectively, confirms the binding of fluorideion to receptor AZ.

As shown in Fig. 7, in AZ/AZIM, plotting of HOMO and LUMOshows that HOMO spreads over aminophenylbenzimidazole groupand LUMO is localized on azophenol group. Upon addition of Al3þ ,while aminophenylbenzimidazole group retains its HOMO char-acter, LUMO is distributed over metal center. Hence it is clear thatupon Al3þ binding, a substantial charge transfer takes place fromaminophenylbenzimidazole part to aluminum when molecule isexcited. This could be the underlying factor for the fluorescenceenhancement of AZIM after the addition of Al3þ . The large red-shift of emission band with increase in solvent polarity experi-mentally confirms the charge transfer nature. After the appendageof F� to probe, azophenol part behaves as HOMO whereas LUMOspreads over aminophenylbenzimidazole unit with little contribu-tion from fluoride ion. Hence deprotonation facilitates a reversecharge transfer process from deprotonated azophenol group toaminophenylbenzimidazole unit [64].

4. Conclusion

In summary, we have designed a new azo based chromogenicand “turn on” fluorogenic chemosensor for selective recognition ofAl3þ and F� over other competing ions. To the best of ourknowledge, this is the first bifunctional chromogenic and fluoro-genic chemosensor that allows the detection of Al3þ and F� withvaried responses. In other words, the probe exhibits remarkablyenhanced fluorescent intensity and significant color change. Thecoordination of AZ with Al3þ was found to be reversible anddisplayed little interference from the other common metal ions.The nature of fluorescence behavior of receptor upon addition ofAl3þ/F� has been obtained from TD–DFT calculations. It isexpected that this new fluorescent probe may find potential bio-medical and environmental remediation applications.

Acknowledgment

M.I., D.J and K.K thank UGC-BSR (F4-1/2006(BSR)/7-119/2007(BSR)) for research fellowship. M.I., D.J., K.K. and D.C. also acknowl-edge DST-IRHPA, FIST and PURSE for instrumental facilities. Theauthors gratefully acknowledge DBT-IPLS, School of BiologicalSciences, MKU for providing instrumentation facilities.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jlumin.2014.09.018.

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