Phys. Status Solidi A 206, No. 9, 2171–2176 (2009) / DOI 10.1002/pssa.200881794 p s sapplications and materials science

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Synthesis and characterization ofhost–guest materials obtained by insertingcoumarin into hydrotalcite layers for LED applicationsSumeet Kumar*, Marco Milanesio, Leonardo Marchese, and Enrico Boccaleri

Universita degli Studi del Piemonte Orientale and Nano-SiSTeMI Interdisciplinary Centre (DISTA), Via T. Michel 11, Alessandria15121, Italy

Received 15 October 2008, revised 1 April 2009, accepted 1 April 2009Published online 29 July 2009

PACS 61.43.−j, 78.55.Qr, 68.60.Dv

∗ Corresponding author: e-mail sumeet.kumar.iitk@gmail.com, Phone: +00 39 340 7547854, Fax: +00 39 0131 360250

The possible interactions between luminescent moieties withhydrotalcite (HTLC) host have been investigated. We presentthe work done in synthesizing luminescent materials containingcoumarin-3-carboxylic acid (3CCA). A hydrothermal synthesisis carried out to intercalate the 3CCA into the HTLC withempirical formula: [M(II)0.65M(III)0.35 (OH)2](X)0.35 · nH20],where MII = (Zn2+), MIII = Al3+, X = 3CCA− or NO−

3 . TheZnAl HTLC materials with [M(II)/M(III) ≈ 2.0] thus synthe-

sized were characterized by XRPD/FTIR/ICP-MS/TGA/SEMtechniques and solid-state luminescent measurements. Thethermal stability and the luminescent properties of obtainedhybrid materials are promising because of the observedphotoluminescence in the visible region and their high-temperature resistance indicated by the in situ XRPDmeasurements and TGA/DTG analysis.

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1 Introduction Recent studies on charge transfer (CT)effects on hole injection and device stability in an organicthin film transistor (OTFT) [1] and on the photo-physicalfeatures [2] and further organization of chromophores intolayered double hydrotalcite (HTLC) like materials [3]motivated us to follow a similar approach. In fact HTLC’shave evolved as an interesting group of materials because oftheir inorganic nature and capability to intercalate variousorganic molecules into the layered structure.

Owing to the known biological activities of 3-carboxylicacid (3CCA), the presently synthesized materials can alsohave interesting pharmacological features because of thepossible intercalation of 3CCA into the HTLC host [3, 4].Furthermore, the chelating nature of 3CCA− anion hasalso found its application into use as fluorescent probesfor the identification/inhibition of cancerous cells into thehuman body [4]. To further exploit the luminescent propertiesof 3CCA, we have intercalated the organic species inits anionic form into the HTLC layered structure. Theselection of sodium salt of 3CCA as antenna compound forAl3+ substituted HTLC’s should enhance the luminescenceproperties as reported [5]. However, together with the HTLC

phases, the formation of ZnO occurred but its evolutionand occurrence during hydrothermal synthesis might beuseful for the hybrid HTLC materials conductivity and itsluminescent properties [6].

2 Experimental and methods2.1 Hydrotalcite (HTLC) synthesis ZnAlHTLC in

its nitrate form was synthesized by standard methods. [5]ZnAlCoumarin HTLC was synthesized on modifying thisrecipe by adding the sodium salt of 3CCA to the hydrothermalsynthesis.

2.1.1 Preparation of ZnAlNitrate HTLC HTLCwith theoretical empirical composition Zn0.65Al0.35

(OH)2(NO3)0.35 · nH2O was synthesized by mixingZn(NO3)2 · 6H2O (Sigma–Aldrich with 98% purity)and Al(NO3)3 · 9H2O (Sigma–Aldrich with 98% purity)with a molar ratio of mZn2+ : mAl3+ = 2 into the reactionflask containing 100 mL of milli-Q water. All reagents wereused as such without further purification. The solution thusobtained was stirred for 3 h at room temperature (RT) withdrop-wise addition of a 1 molar aqueous solution of NaOH,

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until the pH of the solution stabilizes in the range of 7–7.5to obtain optimum crystallinity [2], and an opaque whitecomplex started to form. This milky white solution wasthen kept in a sealed autoclave until 12 h at 80 ◦C. The finalsolution mixture after the aforementioned hydrothermalsynthesis was filtered and washed with excess amount ofmilli-Q-deionized water to eliminate unreacted materialsand surplus ions. The filtration residue material was thenfurther dried at 60 ◦C (as suggested by Ref. [7]) for 2 h in aventilated furnace.

2.1.2 Preparation of intercalated ZnAlCoumarinHTLC The ZnAlCoumarin HTLC was prepared following asimilar hydrothermal synthesis as described for ZnAlNitrateHTLC, in which the intercalation occurred in presence ofthe desired anionic species, i.e. 3CCA− anion (Fig. 1). Theemployed Na+3CCA− intercalating species was obtainedfrom Sigma–Aldrich in its acidic form (with 99% purityand used without further purification). The starting saltsmolar ratios (mZn2+ : mAl3+ = 2, mNa+ 3CCA− : mAl(NO3)3

= 1,mNaOH : m3CCA = 1) were chosen to obtain a theoreticallytotal exchange of the anionic nitrate with 3CCA−. The finalsolution mixture obtained after the hydrothermal synthesiscarried out at 80 ◦C for 12 h was washed and filtered withmilli-Q water until the colour of the filtrate turned from green(due to the residual not-intercalated 3CCA) to colourless.The filtration residue material was further dried at 60 ◦C (assuggested by Ref. [7]) for 2 h in a ventilated furnace.

2.2 Characterization techniques The RT X-raypowder diffraction (XRPD) experiments were performedon a ThermoARL diffractometer XTRA using theBragg/Brentano geometry with Cu Kα radiation. Thedetector adopted was a solid state Peltier detector. All powderdiffraction patterns were measured in continuous mode using

Figure 1 Schematic view of the 3CCA− anion with H, C andO atoms represented by small gray, large gray and black spheresrespectively.

the following conditions: angular range 2–60◦ 2θ with a stepsize of 0.02◦ 2θ. XRPD data were indexed and then refined bythe Topas Academic software [8] (version 4.1), employingthe Pawley refinement method.

SEM analysis has been carried out on a FEI PhilipsQuanta 200 instrument after sample metallization by goldcoating.

TGA and DTG analyses were performed on a SetaramSETSYS Evolution instrument under inert argon atmosphere(gas flow 20 mL/min), heating the samples from 50 to1000 ◦C at a rate of 10 ◦C/min.

FT-IR spectra on self-supporting pellets of the samples(ca. 5 mg cm−2) were recorded with a Bruker Equinox 55spectrometer at a resolution of 4 cm−1. The pellets wereobtained on uniformly mixing 1 mg of ZnAlCoumarin HTLCpowder into 100 mg of anhydrous potassium bromide (KBr)powder. The mixture was subjected to a pressure of about10 tonnes, to obtain the desired pellets.

The in situ XRPD data were collected with the standardBM1B setup (ESRF, Grenoble, France) employing thesetup described by Boccaleri et al. [9], using a wavelengthof 0.50038 ± 0.00001 A. A continuous set of diffractionpatterns was obtained as a function of temperature (from 50to 500 ◦C and then down to RT with a ramp rate of 2 ◦C/min).XRD scans covered the interval 2◦–22◦ in 2θ with step of0.002 and finally summed with 0.01 intervals.

The elemental analysis was performed with an ICP-OESSpectro Genesis instrument.

The photoluminescence measurements were carried outin a quartz tube at RT employing a Horiba Jobin-YvonSpectrofluorometer Fluoromax-4.

3 Results and discussions The obtaining of 3CCA−

(Fig. 1) intercalated hybrid inorganic–organic materialswith lamellar Zn–Al layered double hydroxide HTLC wasconfirmed by various characterization techniques. Typicallayered morphology of the synthesized ZnAlNitrate andZnAlCoumarin HTLC can be seen in the SEM micrographs(see Fig. 2) but with a significant difference in particlesize (ZnAlCoumarin HTLC showing the smaller particles).The layered structure was further confirmed by the typicalidentically spaced peaks in the XRPD pattern (see Fig. 3)that were indexed with the following cell parameters in thePawley refinements by TOPAS: a = 3.104 (1) A, c = 17.612(3) A, V = 146.99 (3) A3 with space group P63/mmc forZnAlNitrate HTLC, and a = 3.104 (1) A, c = 26.083 (1) A,V = 209.304 (7) A3 with space group P6 for ZnAlCoumarinHTLC. The lowering of the symmetry in the ZnAlCoumarinHTLC sample is due to the 3CCA molecule, which has alower symmetry (Cs) with respect to the nitrate anion (d3h)symmetry.

The real Zn2+/Al3+ ratio in the ZnAlNitrate HTLC hasbeen found out to be 1.978 by the ICP-MS analysis. TheXRPD Pawley fit analysis for the ZnAlCoumarin HTLC(see Fig. 3b) shows two phases, one with pure ZnAlNitrateun-intercalated HTLC and other with 3CCA− intercalatedincluding a small amount of ZnO phase (dotted arrows

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Figure 2 SEM micrographs showing the morphology obtained bythe ZnAlNitrate HTLC (a) and ZnAlCoumarin HTLC (b) samples.

in Fig. 3b) inevitably evolved when 3CCA is used in thesynthesis. The net Zn2+/Al3+ ratio in the ZnAlCoumarinHTLC has been found out to be 2.2 on ICP-MS analysis.The higher value than the expected starting ratio of 2.0 isdue to the Zn oxide extra-phase.

The typical [002] peak of ZnAlNitrate HTLCcharacterized in its nitrate form is observed at d

spacing = 8.955 ± 0.001 A (see Fig. 3a). Conversely in the3CCA containing sample, the [001] peak with d spacing26.1 ± 0.1 A was observed as expected [2] (see Fig. 3b). Theyield in 3CCA-intercalated phase is not high and is estimatedto be less than 25% from the ratio between the intensity ofthe basal [001] peak of the intercalated phase and the [002]basal peak of non-intercalated phase (refer Fig. 3).

3.1 Thermal stability The synthesized materialswere analysed for their temperature resistance by theTGA/DTG techniques and the in situ XRPD measurements.

Figure 3 Refinement of ZnAlNitrate HTLC (a) and ofZnAlCoumarin HTLC (b) showing the peaks of the layered phase. In(b) the black solid line indicates the intercalated (bold indexes) andun-intercalated (bold-italic indexes) phase, respectively, togetherwith the ZnO phase impurity (dashed arrows). Note that [002] and[003] peaks of the two HTLC phases are superimposed.

3.1.1 The thermo gravimetric analysis (TGA)The TGA profile of the ZnAlCoumarin HTLC sample isdepicted in Fig. 4. DTG plot in the same figure shows a peakat 120 ◦C, indicating loss of intercalated water molecules.The subsequent weight loss in the temperature range of280–290 ◦C corresponds to the losses due to dehydroxylationand at 400–500 ◦C, loss of intercalated ionic nitrate moleculesfrom both the ZnAlNitrate and ZnAlCoumarin HTLCoccurs. Furthermore, the weight losses in the region330–400 ◦C corresponds to the elimination of the 3CCAligand molecules. In fact, at this temperature, the in situXRPD data indicated a decrease and shifting of the intensityof the peaks (see Fig. 5). From the percentage loss ofTGA data around 120 ◦C, we can also determine theamount of water molecules interacting with the HTLC layerscorresponding to ∼7 wt.%. The resulting material still keepsa significant amount of intercalated by-products, that areonly evolved when temperature reaches 750 ◦C and above.The thermal degradation leads to a residue quantitativelysimilar to the expected inorganic content of the HTLCmaterial. Finally, from the difference in the total weightlosses occurring in the range of 50–1000 ◦C, TGA/DTGof ZnAlCoumarin and ZnAlNitrate HTLC, the minimumcarbon content originating from the 3CCA moiety, can beinferred out to be around 20 wt.%.

3.1.2 The XRPD in situ data The thermal stabilitywas further investigated by carrying out a variabletemperature XRPD data collection of ZnAlCoumarin HTLC(see Fig. 5).

The sample degradation and loss of crystallinity of thelayered structure can be observed from 350 ◦C onwards, as

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Figure 4 TGA/DTG plot of ZnAlNitrate HTLC showingintercalated water loss at 120 ◦C, dehydroxylation occurring at280 ◦C and Nitrate loss into NO2 and O2 taking place around400 ◦C–500 ◦C with a 35% total weight loss until 1000 ◦C (a)while in case of TGA/DTG plot of ZnAlCoumarin HTLC sampleloss of intercalated water occurs at 120 ◦C, dehydroxylationand nitrate losses occurs at 290 ◦C and 400–500 ◦C temperaturerange respectively, elimination of 3CCA− organic intercalate(330–400 ◦C) pertaining to the loss of order (see in situ XRPD data),and finally a full decomposition and degradation of the structure at900 ◦C (b).

a result of the 3CCA degradation (observed in TGA data).This process is not reversible since no re-crystallization isobserved in the ramp down to RT (see Fig. 5).

The amount of ZnO phase is found to increase with thedestruction of the layered matrix. No new phases have beenobserved during the in situ XRPD experiment, and thus Al-containing phase is supposed to be present in its amorphousform(s).

3.2 FT-IR spectroscopic data analyses The FTIRexperiments were carried out at RT and showed the presenceof the –C O signal (1650 cm−1) in ZnAlCoumarin together

Figure 5 XRPD data collected during the in situ thermal treatmentfrom RT to 500 ◦C and back to RT of the ZnAlCoumarin HTLCsample.

with a signal of NO−3 (1386 cm−1, shown as broken black

line, see Fig. 6). At the same position (1386 cm−1, shown inblack continuous) we have a sharp signal of NO−

3 present inZnAlNitrate form of HTLC. However, both the ZnAlNitrateand ZnAlCoumarin HTLC’s have weak peak at 835 cm−1

(corresponding to the ν2 vibration mode of the NO−3 group).

Thus, the presence of the anionic 3CCA into the layeredstructure of ZnAlCoumarin HTLC can be confirmed, whilethe presence of NO−

3 can be seen into both ZnAlNitrate HTLCand ZnAlCoumarin HTLC (see Fig. 6) confirming the partialintercalation of 3CCA− into the ZnAlCoumarin HTLCsample. Quite noteworthy are the large bands at 3480 cm−1

which correspond to the vibration of hydroxyl groups, ν(OH).Conversely, the bands observed in the low-frequency regionof the spectrum correspond to the lattice vibration modesand can be attributed to M–O (841 and 647 cm−1) and

Figure 6 FTIR data collected at RT of ZnAlCoumarin HTLC(broken line) and ZnAlNitrate HTLC (continuous line).

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Figure 7 The photoluminescence emission spectra observed around 500 nm for the ZnAlNitrate HTLC in solid state (a), deprotonated3CCA− in aqueous NaOH (b) and ZnAlCoumarin HTLC in solid state (c) carried out with an excitation wavelength (λexcitation) of 394 nm.The emission band positioned at 500 nm for ZnAlCoumarin HTLC is observed also when the excitation wavelength is changed from 390to 400 nm (increment of 1 nm), thus showing its persistent luminescence (d).

O–M–O (435 cm−1) vibrations, where M = Metal atom intothe HTLC framework [10]. Both the lower frequency regionsdid not reveal significant differences, thus indicating a similarstructure of the inorganic layers in the two samples.

3.3 Luminescence properties The luminescencespectra are reported, concentrating on the 500 nm emissionband with excitation at 394 nm wavelength as observedby 3CCA in basic medium of aqueous NaOH, shown inFig. 7b. ZnAlNitrate HTLC sample does not show such

emission band in this region (as shown in Fig. 7a) whereasthe ZnAlCoumarin HTLC sample showed a clear emissionband (see Fig. 7c) similar to that of 3CCA at 500 nm. Thisluminescence emission band was persistent on changing theexcitation wavelength in the 390–400 nm range as reportedin the 3D photo luminescence curve shown in Fig. 7d.

4 Conclusions HTLC based materials were synthe-sized by intercalating 3CCA into its layers. The yieldwas not high (about 25%) but the synthesis method rather

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simpler (one pot synthesis) than that proposed in theliterature [2]. The ZnAlCoumarin hybrid material was foundto be luminescent in the visible, conserving the typicalemission of 3CCA but with increased stability (due to theencapsulation of 3CCA by the inorganic host) with respectto 3CCA alone. High-temperature resistance due to theintercalation of 3CCA into the inorganic HTLC host was alsodemonstrated by the results of TGA and variable temperatureXRPD measurements. We can therefore conclude thatZnAlCoumarin HTLC appears to be a promising material forapplications into future display materials, not only becauseof its luminescence but also its thermal stability.

Acknowledgements SK acknowledges Drs. L. Palin, G.Croce, W. van Beek, F. Carniato, G. Gatti and F. Ghigliotti(Universit‘a del Piemonte Orientale, Alessandria, Italy) for helpfulsuggestions during the experiments. SNBL and ESRF are alsoacknowledged for the beam time. Finally we would like to thank thefinancial contribution by Regione Piemonte (NANOLED project)and MIUR (project ‘Sviluppo di nanocompositi ibridi “host–guest”per il rilascio modificato di farmaci mediante approcci innovatividi caratterizzazione sperimentale a livello molecolare’ funded asPRIN in 2007).

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