Microwave-Assisted Solid State in Situ Polymerization and

Article pubs.acs.org/JPCC

Microwave-Assisted Solid State in Situ Polymerization and Intercalation of Poly(carbazole) between Bentonite Layers: Effect of Microwave Irradiation and Gallery Confinement on the Spectral, Fluorescent, and Morphological Properties Ufana Riaz* and S.M. Ashraf† Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi-110025, India ABSTRACT: Nanoscopic confinement of conducting polymers in the clay galleries significantly influences their physical and chemical properties. This calls for the optimization of the properties of the resultant nanocomposites. The amount of loading and distribution of the polymer within the galleries plays an important role in controlling the conformation of the chains which has a profound effect on its physical properties. In this paper, we have investigated, for the first time, the role of confinement of polycarbazole (PCz) in the clay galleries on its spectroscopic, fluorescent, and morphological characteristics when synthesized by microwave-assisted solid state intercalation. The layered structure of bentonite efficiently confines polycarbazole (PCz) at different loadings (0.25 wt %, 0.5 wt %, and 0.1 wt %) the later exhibits remarkably different properties at each loading. This behavior has been explained on the basis of variable orientations and discrete and overlying distribution of PCz chains inside the clay galleries. Results reveal that the composite with a fairly small amount of PCz showed remarkable spectroscopic and fluorescent properties and hence can be used as a fluorescent probe or in electronic devices.

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INTRODUCTION Organic−inorganic hybrid nanocomposites have stimulated widespread interest among scientists because of their potential in diverse technological applications.1 Smectite-type clays, the inorganic layered materials, have been extensively used in the synthesis of organic−inorganic, guest−host hybrid materials.2Conducting polymers (CPs), a class of typical organic materials, have found way into the layered smectite−CP hybrid nanocomposites. Conducting polymers such as polyacetylene (PAc), polyaniline (PANI), polyphenylene (PP), polythiophene (PTh), polyfluorene (PF), and polycarbazole (PCz) combine interesting optical, mechanical, and electronic properties of semiconducting materials with facile synthesis.3 Polycarbazole (PCz) has been investigated for its electronic and optical properties applicable in photoconducting and holetransport devices.4 PCz has many advantages as an organic material because it is cheap, and the nitrogen atom can be easily functionalized with a large variety of substituents to modulate the properties of carbazole without increasing the steric hindrance near the backbone.5 The carbazole units can be linked at different positions, and their fully aromatic configuration provides good chemical and environmental stability.6 For higher performance in polymeric light emitting diodes (PLEDs), organic field-effect transistors (OFETs), and photovoltaic cells (PVCs), derivatives of PCz such as poly(Nvinylcarbazole) (PNVc) and poly(2,7-substituted carbazole), and others, have been lately investigated.7 © 2012 American Chemical Society

Engineering of guest−host, organic−silicate nanohybrids is tedious, due to the lack of control over the organization of molecules and clay mineral particles. If one can control the ordering and organization of these molecules within the clay galleries by proper choice of experimental conditions, the resulting nanocomposites can exhibit remarkable electronic, optical, and electrochemical properties.8,9 Several conducting polymers such as polyaniline (PANI), polypyrrole (Ppy), and polythiophene (PTh) are known to oxidatively polymerize when intercalated into inorganic host materials.10,11 Several works have been reported in the literature on different synthetic strategies to prepare PANI between the layers of montmorillonite (MMT).12−19 The properties of a conducting polymer depend strongly on the arrangement of its chains.20 An important strategy is to confine it in nanoscale environments by intercalating in clays.21,22 In such systems, the polymeric chains are reasonably isolated from one another by the clay particles, providing an opportunity to explore spectroscopic properties of single chains intercalated in the lamellar clay structure.21 Bentonite is one of the most attractive layered silicate clays for the preparation of nanocomposites because of its high aspect ratio and natural abundance.23 This laminar mineral possesses layers with overall Received: March 30, 2012 Revised: May 8, 2012 Published: May 14, 2012 12366

dx.doi.org/10.1021/jp303060p | J. Phys. Chem. C 2012, 116, 12366−12374

The Journal of Physical Chemistry C

Article

complete removal of impurities, methanol, and water. The color of the nanocomposites was green. However, the intensity of the color was dependent upon the amount of carbazole initially taken. In the case when carbazole was taken in the ratio bentonite:carbazole, 1:0.25, the color was lighter as compared to the ratio bentonite:carbazole, 1:0.5. The nanocomposites were designated as bentonite:PCz 1:0.25, 1:0.5, and 1:1 depending upon the weight ratios of bentonite:carbazole initially taken, the % apparent loading of carbazole in the three systems being 20%, 33.3%, and 50%, respectively Polymer Extraction. PCz present in the bentonite galleries was extracted by refluxing the polymer/clay nanocomposite in methanol overnight while stirring. The PCz was separated from the solid clay nanoparticles by centrifugation. The percent conversion and molar mass were determined using elemental analysis (CHNS) and gel permeation chromatography (GPC), respectively. Characterization. GPC measurements were carried out on a Viscotek GPC Max AUTO sampler system consisting of a pump, a Viscotek UV detector, and a Viscotek differential refractive index (RI) detector. A ViscoGEL GPC column (G2000HHR) (7.8 mm internal diameter, 300 mm length) was used. The effective molecular weight range of the column used was 456−42 800. THF was used as an eluent at a flow rate of 1.0 mL/min at 30 °C. Both detectors were calibrated with polystyrene (PS) standard having narrow molecular weight distribution. The data were analyzed using Viscotek OmniSEC Omni-01 software. Molecular weights were calculated with the help of polystyrene standard. X-ray diffraction patterns of the nanocomposites were recorded on a Philips PW 3710 powder diffractometer (nickel-filtered Cu Kα radiation). Crystallite cell size was calculated according to the Scherer equation29

negative charge which is compensated by exchangeable inorganic cations in the galleries. However, the preparation of many of these hybrid nanocomposites often involves lengthy procedures with several steps, and more time is needed for their formation. It is, therefore, necessary to develop more efficient methods of synthesis involving simplified preparation procedures, completing in a short time; the use of microwave-assisted synthesis is especially important in this regard.24 Consequently, the use of microwave irradiation for the synthesis of organic materials has gained greater importance in recent years.25 The method offers several advantages over conventional routes, the most important of them being short reaction times and energy economy.26 Solvent-free microwave-assisted reactions obviously reduce pollution and bring down handling costs due to simplification of the experimental procedure and workup technique which are important aspects for industrial production. Most of the polymer/clay nanocomposites have been synthesized through the solution route. Only a few studies are reported on solid-state synthesis and that by using a mechano-chemical mixing technique.27 The host−guest interaction obtained by intimately mixing the components leads to an ordered structure. The orientational flexibility of guest molecules in the solid state provides excellent opportunities to achieve a high degree of ordered arrangement.27 In our earlier preliminary work, we have reported a comparative study of the effect of mechano-chemical and microwave-assisted techniques on the solid state in situ polymerization of PCz within montmorillonite (MMT) clay galleries.28We have observed that the microwave-assisted synthesis resulted in controlled morphology of the nanocomposite as compared to mechano-chemical synthesis. In the present study, we have investigated, for the first time, the effect of PCz confinement in the bentonite galleries on its spectral, fluorescent, and morphological properties when the former is synthesized via microwave-assisted solid-state intercalation and in situ polymerization in the absence of solvent. The results obtained using FT-IR, UV−visible, XRD, fluorescence spectroscopy, and fluorescence confocal microscopy were analyzed to delineate information about the effect the above conditions of synthesis on its spectroscopic and physical properties.

τ=

0.9λ β cos θ

where τ is the crystallite size; β is full width at half maxima (fwhm); and λ is the wavelength of X-ray used. Peak parameters were analyzed through Origin 6.1. FT-IR spectra of nanocomposites were taken in the form of KBR pellets on FT-IR spectrophotometer model Shimadzu IRA Affinity-1. The integrated absorption coefficient, ∫ adν̅, of the NH absorption peak between 3420 and 3350 cm−1 for pure PCz and the peak of the SiO stretching vibration at 1047/1042 cm−1 were determined using the IRA Affinity-1 software through Gaussian−Lorentzian curve fittings as shown in Table 2. UV−visible spectra were taken on a UV−visible spectrophotometer model Shimadzu UV-1800 using NMP as solvent. The oscillator strength was calculated using the equation30

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EXPERIMENTAL SECTION Materials. Bentonite (Sigma Aldrich, USA), carbazole (Sigma Aldrich, USA), ferric chloride (Merck, India), Nmethyl-2-pyrolidone (NMP) (Merck, India), and methanol (Merck, India) were used without further purification. Synthesis of Bentonite−PCz Nanocomposites. The microwave irradiation was carried out for 180 s in a Ladd Research Laboratory microwave oven (MW frequency, 2500 MHz; power source, 230 V ∼ 50 Hz; energy output, 800 W; input power, 1200 W). Prior to the synthesis, the microwave oven time was set to 180 s, temperature to 35 °C, ramp rate to 15.0, the cycle time to 4.0, and proportional band to 10.0. Bentonite clay (1 g) was mixed with different loadings of carbazole (0.25, 0.5, and 1 g). The mole ratio of oxidant (ferric chloride):carbazole was taken to be 1:1. During microwave synthesis, the color of bentonite clay changed from cream to green which indicated the polymerization of carbazole within the bentonite galleries. The nanocomposites obtained were then washed with distilled water and methanol several times and dried in a vacuum oven at 70 °C for 72 h to ensure

f = 4.32 × 10−9

∫ εdν ̅ = 4.32 × 10−9 ∫ aMdν ̅

where ε is the molar absorption coefficient and a and M are, respectively, the absorption coefficient at the wavelength in question and molarity of the PCz solution. In precision analytical determinations, the integrated absorption coefficient is used in place of the absorption coefficient at the maximum wavelength, λmax. ∫ adν̅ was obtained from the area under the transition peak when the spectra were plotted as an absorption coefficient vs wavenumber. The area under the peak was obtained using Origin 6.1 which was standardized to the Gaussian−Lorentzian shape. For the determination of oscillator 12367



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strength of the UV−vis peaks of each nanocomposite, the molar mass, Mw, of extracted PCz from the corresponding nanocomposite was used. Fluorescence Spectroscopy. Fluorescence spectroscopic studies were performed on a fluorescence spectrophotometer Varian Cary Eclipse by preparing solutions of the nanocomposites and PCz in NMP. The quantum yield Φ was calculated using the following equation,31 taking anthracene as reference material Φsample =

n2 sampleA ref Isampleϕref n2 sampleA sampleIref

where nsample and nref are the refractive indices of samples and reference; Asample and Aref are the optical absorbance values of sample and reference of extremely dilute solution at the excitation maximum; Isample and Iref are the emission intensity maximum of sample and reference; and Φsample and Φref are the quantum yield of sample and reference Confocal Microscopy. Fluorescence images were obtained with a 100× objective at room temperature using a Laser Confocal Microscope with Fluorescence Correlation Spectroscopy (FCS) - Olympus FluoView FV1000 equipped with a He−Ne laser and oil immersion objective. λmax for laser excitation was 410 nm. The nanocomposites were placed on a glass slide covered with a coverslip. TEM Analysis. Transmission electron micrographs (TEMs) were taken on Morgagni 268-D TEM, FEI, USA. The samples were prepared by placing an aqueous drop on a carbon-coated copper grid and subsequently drying in air before transferring it to the microscope, operated at an accelerated voltage of 120 kV.

Figure 1. XRD spectra of bentonite/PCz nanocomposites.

Table 1. XRD Analysis of Bentonite and Bentonite/PCz Nanocomposites sample bentonite 1:1 bentonite:PCz 1:0.5 bentonite:PCz 1:0.25 bentonite:PCz

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RESULTS AND DISCUSSION Solid state intercalation and polymerization of carbazole into the clay galleries of the bentonite were carried out by microwave-assisted irradiation as mentioned in the Experimental Section. Ferric chloride goes into the clay galleries through the exchange of mono- and divalent cations present therein. Microwave irradiation provides abundant thermal energy to carbazole molecules to diffuse into the basal spacing of the bentonite, and polymerization occurs under solid state conditions. The molar masses of extracted PCz from nanocomposites, bentonite:PCz, 1:1, 1:0.5, and 1:0.25, were found to be, respectively, 3124 ± 8, 3294 ± 8, and 3440 ± 10. CHN analysis showed the actual loading of PCz as 40%, 26%, and 17.1%, respectively, for the above nanocomposites against the apparent loading of 50%, 33.3%, and 20%. The percent conversion of carbazole into PCz was obtained as 80%, 78.8%, and 85.5% respectively. XRD Analysis. The XRD analysis of bentonite:PCz nanocomposites establishes both intercalation and polymerization of carbazole within the clay galleries. XRD diffractograms of bentonite and bentonite/PCz nanocomposites are shown in Figure 1. The peak analysis of these diffractograms was carried out using Origin 6.1 software, and the values of the calculated parameters are given in Table 1. It reveals that the height of the clay gallery increases as carbazole polymerizes inside the former under microwave irradiation. Bentonite:PCz, 1:1, 1:0.5, and 1:0.25, showed an increase in the gallery height by 0.5, 0.85, and 1.2 Å as compared to pure bentonite. The small increase in the gallery height of 0.5 Å in the case of bentonite:PCz, 1:1, results from lateral orientation of PCz molecules along the

peak position (2θ)

peak area (au)

peak width (Å)

crystallite size (Å)

gallery height (Å)

5.84 5.64

1150 190

1.98 2.17

42.17 36.6

15.1 15.6

5.54

205

2.14

37.16

15.95

5.42

266

1.48

53.7

16.30

planes of the siloxane layers. The increase in the gallery height by 1.2 Å for bentonite:PCz, 1:0.25, a relatively large increase, indicated transverse orientation of the PCz molecules in the clay galleries The composite bentonite/PCz, 1:0.5, showed an increase of 0.85 Å in the gallery height, lower than the increase in bentonite/PCz, 1:0.25, and higher than the increase in bentonite/PCz, 1:1, which results from mixed orientation of PCz molecules in the basal spacings of the clay. The area under the (001) diffraction peak was found to be the highest, 1150, for pure bentonite. This area decreases in the order bentonite:PCz 1:1 < 1:0.5 < 1:0.25 (Table 1). The progressive decrease in the area with the increasing loading of carbazole in these nanocomposites confirms intercalation as well as polymerization of carbazole in the clay galleries. The decrease in the area under the (001) peak may be considered proportional to the amount of the PCz intercalated in the clay samples. The presence of PCz in the basal spacing of the clay galleries obstructs the X-ray reflection from the (001) plane and reduces the area under the peak. This explains the relatively larger area under the peak in the case of bentonite:PCz, 1:0.25. This also explains that the amount of PCz formed is proportional to the original amount of the carbazole monomer taken for the purpose. The crystallite size calculated for pure bentonite was found to be 42.17 Å and for bentonite:PCz 1:0.25 was found to be 53.7 Å, which shows that the crystallite size was larger for the latter. This is caused by the increase in the gallery height which is brought about by the transverse orientation of PCz in the bentonite galleries. The crystallite size for bentonite:PCz 1:1 was found to be 36.6 Å, which was observed to be smaller than that of pure bentonite by 5.5 Å. The lowering of crystallite size in this case is 12368



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groups. As the former is prone to error, an integrated absorption coefficient (∫ adν)̅ can be used as a qualitative estimate of the interaction of the interacting groups (Table 2a).32 In pristine PCz, ∫ adν̅ for the NH vibration peak at 3419 cm−1 is the highest. Upon hydrogen bonding, the absorption intensity of the peak will decrease, and so will the ∫ adν̅ (Table 2a). The integrated absorption coefficient is also dependent on the concentration of the absorbing species apart from the interaction effect. Because of the concentration effect, the ∫ adν̅ for the nanocomposite bentonite:PCz 1:1 should be the highest and for bentonite:PCz 1:0.25 should be the lowest as the NH group will be the highest in the first nanocomposite and lowest in the second nanocomposite. Only a small difference is noticed in the ∫ adν̅ values of the two nanocomposites, though the concentration difference is quite large between the two. This can only be explained by considering the higher value of the extinction coefficient, ε, of the latter nanocomposite. The extinction coefficient, ε, is related to the transition dipole moment. Since the PCz chains are distributed discretely and are oriented transversely in the bentonite:PCz 1:0.25 nanocomposite, the transition dipole moment from different chains in the clay galleries will vectorially add up, hence the net extinction coefficient of the system will be large and so will the integrated absorption coefficient. In the bentonite:PCz 1:1 nanocomposite, the molecules are longitudinally oriented in the galleries and are also entangled because of higher loading; some of the individual transition dipole moments may, therefore, vectorially cancel each other, which results in a smaller net transition dipole moment. This annuls the effect of the higher concentration of the NH group in this case and causes only a small increase in the integrated absorption coefficient values compared to the nanocomposites bentonite:PCz 1:0.25. Due to the large amount of NH groups in bentonite:PCz 1:1 nanocomposites, the hydrogen bonding will also be large, and this will also cause the integrated absorption coefficient values to decrease. The ∫ adν̅ value for bentonite:PCz 1:0.5 nanocomposites can also be explained in a similar way. The SiO vibrational peak for bentonite and bentonite:PCz nanocomposites was observed as shown in Table 2b. Five vibrational peaks of Si−O were observed. Other authors have reported four vibrational peaks.33 It was observed that for all the SiO peaks in bentonite the corresponding peak in bentonite:PCz nanocomposites showed a decrease of a few cm−1 toward lower energy except the vibrational peaks at 1100 and 1113 cm−1 .The last peak showed a small blue shift of a few cm−1 in the case of the three nanocomposites. The small decrease in the peaks at 1026, 1046, and 1086 cm−1 is due to hydrogen bond formation, NH···O− Si. Apical oxygen will not interact with the NH of PCz as it is buried into the packing layers of Si−O and Al−O polyhedra. This peak therefore does not show any shift in the low or high energy side. The shifting of the 1113 cm−1 peak toward the high energy side in the three nanocomposites indicates that oxygen in the SiO group is carrying a negative charge SiO−. The attractive force between NH and SiO− will increase the bond length of Si···O−, which will increase its dipole moment, causing a shift of absorption peak to the higher energy side. We have chosen the peak at 1047 cm−1 for ∫ adν̅ analysis, and the latter for pure bentonite and bentonite:PCz nanocomposites 1:1, 1:0.5, and 1:0.25 was found, respectively, as 560.74, 53.87, 103.9, and 175.6 cm−2. Compared to pure bentonite, the ∫ adν̅ for the nanocomposites was found to be low, which shows an

intriguing. It appears that the distribution of the PCz molecules in the gallery spacing occurs in such a way that some compression in the b-dimension of the siloxane unit cells of bentonite takes place which causes the decrease in the crystallite size. PCz chains parallel to the rows of the oxygen atoms of siloxane layers enable the interaction of the NH group of PCz with the oxygen atoms of SiO chains that causes a compression in the b-dimension of the siloxane unit cell. The above observations clearly show that at higher loading the orientation of PCz chains is lateral and parallel to the siloxane layer, while at lower loading the orientation of the chains is transverse which leads to the difference in the physical properties of the nanocomposites. FT-IR Analysis. The FT-IR spectra of PCz and bentonite:PCz nanocomposites are shown in Figure 2, and

Figure 2. FT-IR spectra of bentonite and bentonite/PCz nanocomposites (inset: second derivative of the peaks of the SiO vibrations).

other peak information is given in Table 2a and 2b. The second derivative plot of the peaks at 1020 cm−1 (SiO stretching vibration) and 3400 cm−1 (NH stretching vibration) was obtained through the built-in software to explore the actual number of peaks lying in the above two t ranges. The NH stretching vibration of PCz is obtained at 3419 and 3393 cm −1, and five SiO stretching peaks are observed between 1020 and 1120 cm−1. Some authors have reported four stretching peaks of SiO in the above region in the case of octahedral smectites.32 It is expected that the NH group of PCz will form a hydrogen bond with the Si−O group. Since PCz chains orient longitudinally or transversely, the probability of hydrogen bond formation will be larger in the former case and smaller in the latter case. The intensity of the absorption peak can be used to assess the strength of interaction between the two interacting 12369



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Table 2. (a) Integrated Absorption Coefficient Values of Nanocomposites at 3420−3350 cm−1 and (b) FT-IR Peaks of Nanocomposites between 1020 and 1120 cm−1 (a) sample

∫ adν̅ (integrated absorption coefficient), cm−2

pure PCz 1:1 bentonite:PCz 1:0.5 bentonite:PCz 1:0.25 bentonite:PCz (b)

133.4 52.60 56.37 51.89

bentonite (cm−1)

1:1 bentonite:PCz (cm−1)

1:0.5 bentonite:PCz (cm−1)

1:0.25 bentonite:PCz (cm−1)

functional group

1026 1047 1086 1100 1113

1022 1041 1080 1100 1120

1018 1042 1080 1100 1117

1020 1042 1084 1100 1118

SiO stretching SiO stretching SiO bend SiO stretching (Apical) SiO− stretching

Table 3. Oscillator Strength and λmax of PCZ and PCz:Bentonite Nanocomposites

interaction between the NH of PCz with the SiO of clay through hydrogen bonding. SiO content in the bentonite:PCz nanocomposite 1:0.25 is the highest, while it is the lowest for bentonite:PCz 1:1. The ∫ adν̅ values of these nanocomposites match with the above trend. The difference in the ∫ adν̅ values of the nanocomposites, bentonite:PCz 1:1 and 1:0.25, is far larger than the difference of the SiO content of the two composites. The presence of the NH group of PCz in the proximity of the SiO group and their relative orientations significantly affects the net transition dipole moment and consequently the extinction coefficient of SiO; the latter contributes to the large difference in the integrated absorption coefficient values. The integrated absorption coefficient analysis of NH and SiO vibrational peaks establishes that the interaction of NH of PCz and SiO of bentonite varies with the extent of loading of PCz. UV−Visible Spectroscopy. The UV−visible spectral investigations of pure PCz and bentonite:PCz nanocomposites are shown in Figure 3, while their peak parameters are given in

sample PCz

1:1 bentonite:PCz

1:0.5 bentonite:PCz

1:0.25 bentonite:PCz

λmax (nm)

oscillator strength

950 850 600 960 650 550 960 650 550 960 650 550

0.04 0.03 0.09 0.16 0.17 0.15 0.20 0.18 0.15 0.23 0.19 0.17

peak positions in the visible range compared with those of pure PCz and are observed at 550 and 700 nm. The peaks with λmax at 950 nm show a very small red shift of 10 cm−1, while the 375 nm peak does not show any blue or red shift. PCz synthesized by other methods showed peaks only in the UV range which corresponded to interband transitions and defect states formed with wide energy gaps.33−36 It has been observed that PCz extends to limited conjugation length as the same ends at the nitrogen atom37 that produce a large band gap between HOMO and LUMO and other energy bands in the UV region. PCz has been substituted by a variety of substituents to improve its electronic properties. A soluble hybrid copolymer of 3,6-bis(2-(3,4-ethylenedioxy)thienyl)-N-eicosylcarbazole (BEDOT-NC(20)Cz) revealed transition peaks at 1.0, 2.0, and 2.9 eV in the UV and visible ranges under cyclic voltammetry (CV). 3,6-Bis(pyrrol-2-yl)-9ethylcarbazole (BP-NECz) under similar conditions showed oxidation peaks at 0.8, 1.4, 2.2, and 3.0 eV.38 This shows polaronic energy band formation in these substituted copolymers within the HOMO−LUMO band gap. Leclerc and co-workers38 synthesized three substituted copolymers of PCz−poly(2,7-substituted carbazole) designated as PCDT, PCDTB, and PCDTBT [poly-N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]. PCDT, PCDTB, and PCDTBT copolymer films showed UV absorption peaks at 465, 450, 400, and 575 nm, respectively.37,38 Under CV, these copolymers showed oxidation peaks at 440, 630, 440, 640, 420, 600, and 790 nm. In the case of simple UV spectroscopy, these copolymers showed one or

Figure 3. UV−visible spectra of PCz and bentonite:PCz nanocomposites.

Table 3. The spectral transitions for pure PCz are obtained at 375, 600, 850, and 950 nm. The band gap energy calculated from the onset of the absorption edge was found to be 2.48 eV. The above peaks that fall in the visible range can be assigned to polaronic transitions. The polaron bands form in the energy gap of HOMO and LUMO. The bentonite:PCz nanocomposites 1:1, 1:0.5, and 1:0.25 show a blue shift in the 12370



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bentonite:PCz 1:0.25 or PCz in bentonite can be detected spectrophotometrically. Fluorescence Spectroscopy. PCz and bentonite:PCz nanocomposites were investigated for excitation and emission spectra, fluorescence decay time, and quantum yield. Excitation and emission spectra of these materials indicated the same excitation and same emission wavelengths, λexc = 305 and 350 nm and λemis = 410 nm (Figure 4). Intercalation of PCz in

two peaks in the visible range. Obviously, the copolymers under CV are in the oxidized or electrochemically doped state, and so the number of transition peaks increases, and some of them fall in the visible region. In our case, PCz synthesized under microwave irradiation was found to produce a polaronic transition state which gives rise to peaks for interband transition falling at the bottom of the visible range, with the other three peaks in the visible range (600, 850, and 950 nm). This behavior has not been shown by chemically synthesized or complexedly substituted PCzs. The microwave synthesized PCz has therefore the advantage that its various electronic states can be accessed by visible light. As noted earlier, the PCz synthesized in bentonite galleries did not show a blue shift for 375 and 950 nm peaks, while the blue shift occurred in 600 and 850 nm peaks. This happens because of the lowering of polaronic energy states, while the valence and conduction band levels and the energy gap between polaronic levels remain almost unchanged. The comparison of oscillator strength of the nanocomposites of the three peaks in the visible region and that of pure PCz is expected to throw more light on how the electronic and polaronic transitions are influenced by the polymerization under microwave irradiation and how the loading of PCz in the clay galleries influences the transition characteristics of these peaks. It can be observed from Table 3 that the oscillator strength of pure PCz spans over 0.039−0.09 for the three peaks in the visible region; for the three nanocomposites, it spans over 0.15−0.23 (Table 3). Bentonite:PCz nanocomposites thus show higher oscillator strength than pure PCz. This is a consequence of PCz being confined in narrow clay galleries and in the negatively charged environment of siloxane layers. The higher values of oscillator strength of bentonite:PCz nanocomposites emanate from the higher transition dipole moment in these cases. In the confined environment of clay galleries, molecular orbitals appear to be more symmetrically oriented which produce a higher transition dipole moment. It was also observed that except for the 960 nm peak in the three nanocomposites the oscillator strength for the two remaining peaks increases slightly over the three nanocomposites in the order bentonite:PCz 1:0.25 > bentonite:PCz 1:0.5 > bentonite:PCz 1:1, although the loading of PCz is the lowest in the first and highest in the last nanocomposite. The oscillator strength for the peak at 960 nm was found to increase significantly from bentonite:PCz 1:1 to bentonite:PCz 1:0.25, their values being 0.16 and 0.32, respectively (Table 3). How does it happen that the oscillator strength is the highest when the loading of PCz in the nanocomposite is the lowest? Because of the lower loading in bentonite:PCz 1:0.25 (17.1% PCz), the PCz chains are oriented transversely and are distributed discretely in the clay galleries. Under this condition, the net transition dipole moment will vectorially add up, and the oscillator strength will be higher. At higher loading, bentonite:PCz 1:1 (40% PCz), the PCz molecules will be entangled with each other and will be oriented longitudinally in the narrow space of the clay galleries. Some of the transition dipole moments will vectorially cancel each other, resulting in the lower value of oscillator strength than the one in the previous case. It is thus found that with significantly low loading of PCz in bentonite higher oscillator strength was produced, the latter being far higher than pure PCz. We have checked the absorbance value of a 0.1 ppm solution of bentonite:PCz 1:0.25 (17.1% PCz) in NMP at λmax 375 and 950 nm and found it to be, respectively, 1 and 0.15. Thus, an extremely small amount of the composite

Figure 4. Fluorescence emission spectra of bentonite:PCz nanocomposites.

bentonite and interaction of the NH group of PCz with SiO of clay do not bring about any change in the excitation and emission maxima, which may be due to weak interaction between the reacting groups. It is, however, worth noticing that the emission intensity of bentonite:PCz 1:0.25 is much higher (375) than that of bentonite:PCz 1:1 (170) and pure PCz (185). The emission spectra appear to be well formed in all the cases, indicating that the decay of the excited species is smooth. The highest emission intensity in the nanocomposite with the lowest loading of carbazole can be attributed to the discrete presence of the transversely oriented PCz chains. It is also observed that the trend of variation of maximum emission intensity between the three nanocomposites matches with the variation of the absorbance intensity at λmax in the UV−visible spectra. Because of the large concentration of PCz in bentonite:PCz 1:1 nanocomposites, the decay by nonradiative process will be large which will lower the emission intensity. In bentonite:PCz 1:0.25, the nonradiative decay process will be small because of the discrete presence of the PCz chains, which will enhance the emission intensity. The above observation of highest emission intensity for bentonite:PCz 1:0.25 and the lowest for bentonite:PCz 1:1 was also corroborated by quantum yield (Φ) and decay time (τ) analyses. The values of Φ for pure PCz and the nanocomposites were, respectively, found to be 0.049, 0.03, 0.09, and 0.20. This trend was identical to the emission intensity of these nanocomposites. The quantum yield of the nanocomposites bentonite:PCz 1:0.25 is the highest among the three nanocomposites which matches with the highest oscillator strength value of the former. It shows that the electronic properties of the PCz molecules are profoundly influenced by the nature of its distribution and orientation in clay galleries. 12371



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The fluorescence decay time τ is inversely proportional to the Φ, i.e., the lower the decay time the higher the quantum yield. The decay curves for pure PCz and bentonite:PCz nanocomposites are shown in Figure 4 .The decay time was determined from the curves by reading off the time at 0.337 of the maximum fluorescence intensity. The value of τ for pure PCz and bentonite:PCz nanocomposites was found, respectively, as 9.6, 9.3, 8.5, and 8.25 ns.The fluorescence lifetime also shows that the nanocomposite bentonite:PCz 1:0.25 has a different pattern of distribution and orientation than bentonite:PCz 1:1, which causes this difference in the fluorescence decay time. Confocal Microscopy Analysis. This technique gives a visual evidence of the intercalation and polymerization of PCz in clay galleries. Bentonite:PCz 1:1 (Figure 5(a)) nanocomposites revealed blue colored densely distributed particles of varying sizes, while bentonite:PCz 1:0.25 (Figure 5(b)) showed very fine bright blue particles uniformly distributed in the clay matrix. A noticeable difference in the distribution of the PCz nanoparticles in the two cases is visible, which has already been inferred from our previous observations and analysis TEM Analysis. The TEM micrographs of bentonite:PCz composites show aggregate formation by composite particles (Figures 6(a), (b), (c)). Since in the bentonite:PCz composites the clay has the dominant part while the PCz particles occupy the interlamellar space of 1.5 nm width, the TEM micrographs of the composites, therefore, do not reveal the microstructure of PCz grown in them. In bentonite:PCz 1:0.25 (Figure 6(a)), the distorted tactoidal clay particles join each other and form close chain aggregates. The bentonite:PCz 1:0.5 and 1:1 composites show more or less spherical clay particles joining each other and forming open chains which closely pack themselves and form large aggregates (Figure 6(b),(c)). The difference in the shapes and arrangements of the composite particles in the bentonite:PCz 1:0.25 and bentonite:PCz 1:0.5 and bentonite:PCz 1:1 can be clearly seen in Figures 6(a), (b), and (c). For observing the microstructure of PCz formed in the interlamellar space of bentonite, it was extracted as described in the Experimental Section. The TEM micrographs of PCz extracted from the composite bentonite:PCz 1:0.5 and 1:1 are shown in Figures 6(f) and (g); those extracted from bentonite:PCz 1:0.25 are shown in Figure 6(d),(e). The PCz extracted from bentonite:PCz 1:0.5 and 1:1 shows a wellformed structure of rods of large length and small diameters (Figures 6(f),(g)). The diameter of the rods varies in the range 140−180 nm, while the full length of the rods appears to be several micrometers. The rods also overlie one above the other (Figure 6(f),(g)). It can be argued that if PCz molecules can organize themselves to form large thin rods in the extracts then they can also organize themselves in the interlamellar space to form similar miniscule rods (thin elongated particles). We have already concluded from different experimental observations that in the case of bentonite:PCz 1:0.5 and 1:1 the PCz particles arrange themselves longitudinally in the x−y plane of the gallery space of the clay, which also overlie one above the other. The microstructure of PCz particles in this case matches with our observation about their structure in the interlamellar space. The TEM micrographs of PCz extracted from the composite benotnite:PCz 1:0.25 show cucumber-shaped particles (Figure 6 (d),(e)). The enlarged figure of the cucumber given in the inset PCz (Figure 6(e)) shows fibers around and within the cucumber. These PCz fibers organize themselves to form the

Figure 5. Confocal images of (a) bentonite:PCz 1:0.25 and (b) bentonite:PCz 1:1.

large cucumber-shaped structures. The diamter of these cucumber varies between 400 and 700 nm. The regions where these fibers are densely packed appear black, while the regions where they are sparsely arranged appear almost transparent. These PCz fibers can similarly organize themselves inside clay galleries to form miniscule cucumbers which orient transversely in the clay galleries and lead to the increase in the height of the gallery. The above conclusion matches with our earlier observation that in the composite bentonite:PCz 1:0.25 the PCz chains orient transversely in the interlamellar space of the clay particles. 12372



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form produces relatively higher oscillator strength and quantum yield in the latter. Thus, a small quantity of PCz or its composite with bentonite can be used as a fluorescent probe or electronic material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. † Retired.

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ACKNOWLEDGMENTS U.R. wishes to acknowledge the Department of Science and Technology (DST) - Science and Engineering Research Council (SERC), India vide sanction no. SR/FT/CS-012/ 2008 for granting the Young Scientist fellowship under the “Fast Track Scheme for Young Scientists”. The authors also wish to acknowledge the Sophisticated Analytical Instrumentation Facility (S.A.I.F.) at All India Institute of Medical Sciences (A.I.I.M.S.), New Delhi, India for providing the TEM facility.

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REFERENCES

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Figure 6. TEM micrographs of (a) bentonite:PCz 1:0.25, (b) bentonite:PCz 1:0.5, (c) bentonite:PCz 1:1, (d) PCz extracted from composite bentonite:PCz 1:0.25, (e) PCz extracted from composite bentonite:PCz 1:0.25, (f) PCz extracted from composite bentonite:PCz 1:0.5, and (g) PCz extracted from composite bentonite:PCz 1:1.

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CONCLUSIONS Microwave irradiation provides an easy and fast method for polymerization and intercalation of PCz in the solid state. Microwave irradiation also produces a polaronic state in the PCz molecules as does the cyclic voltammetry. The amount of PCz in the clay galleries determines the distribution and orientation of its molecules. At a lower amount (17%), the distribution of PCz is discrete, and orientation is transverse in the clay galleries. The confinement in the basal spacings in this 12373



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Microwave-Assisted Solid State in Situ Polymerization and

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