Journal of Luminescence 178 (2016) 347–355

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Journal of Luminescence

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Full Length Article

Photophysical behavior of layer-by-layer electrostatic self-assembledfilm of azo dye Chromotrope-2R and a polycation

Chaitali Hansda a,b, Bipan Dutta c, Utsav Chakraborty a, Tanmoy Singha a,Syed Arshad Hussain d, Debajyoti Bhattacharjee d, Sharmistha Paul e, Pabitra Kumar Paul a,n

a Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, Indiab Department of Physics, The University of Burdwan, Golapbag, Burdwan 713104, Indiac Department of Physics, Sammilani Mahavidyalaya, Baghajatin Station, E.M. Bypass, Kolkata 700075, Indiad Department of Physics, Tripura University, Suryamaninagar 799022, Tripura West, Indiae West Bengal State Council of Science and Technology, Vigyan Chetana Bhavan, Sector-I, Salt Lake, Kolkata 700064, India

a r t i c l e i n f o

Article history:Received 11 March 2016Received in revised form15 May 2016Accepted 6 June 2016Available online 11 June 2016

Keywords:Azo dyeLayer-by-layer self-assemblyAdsorptionUV–vis absorption spectroscopyAtomic Force MicroscopyJ-aggregations

x.doi.org/10.1016/j.jlumin.2016.06.01713/& 2016 Elsevier B.V. All rights reserved.

esponding author. Tel.: þ91 9477631142; fax:ail addresses: pabitra_tu@yahoo.co.in, pkpaul@

a b s t r a c t

This communication reports the fabrication of layer-by-layer electrostatic self-assembled films of an azodye Chromotrope-2R (CH2R) and a Polycation poly(allylamine hydrochloride) (PAH) onto solid substrate.UV–vis absorption and steady state fluorescence emission spectroscopy successfully confirm the incor-poration of dye molecules onto the PAH coated quartz substrate. The adsorption behavior of CH2R ontoPAH backbone in LbL films highly depends upon the variation of the microenvironment namely pH of thedye solution from which the film was fabricated. PAH layer onto quartz substrate was able to swellsufficiently in the dye solution at very high pH. The Density functional theory was also utilized here toexplain the origin of various spectral transitions from the ground electronic states for both in neutral andanionic form of CH2R. In LbL films the more closure association of dye molecules causes their aggre-gations which are reflected in their absorption and steady state fluorescence emission spectra whencompared to those of pure dye solution. Atomic force microscopic images of LbL films assembled fromCH2R aqueous solution at different pH clearly reveal the change in the surface morphology of the filmsand different degree of association of dye molecules in LbL films deposited at various pH of CH2R.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

Organic thin films with confined structures for guest moleculesand properties have drawn much attention in recent years for theirpotential applications in various fields namely, linear and non-linearoptics, electronics, sensing, surface coating and in medical indus-tries [1–7]. Among the different methods for fabrication of theseultrathin organic films, layer-by-layer (LbL) electrostatic self-assembly (ESA) technique based on spontaneous electrostaticinteractions between the oppositely charged polyions or species hasbecome an elegant and versatile approach showing multifunctionalproperties [8]. This method was first developed by Decher and Co-workers to assemble oppositely charged polymers [9] and laterextended to many other interesting materials including nanocrys-tals and soft nanoarchitechtures [10,11], biological macromolecules[12–14], semiconductor nanoparticles [15], dyes [16] etc. The basicmechanism of this method is the sequential and repeated

þ91 3324138917.phys.jdvu.ac.in (P.K. Paul).

electrostatic adsorption of positively and negatively charged mole-cules in a nanometer scale onto a suitable solid substrate from theiraqueous solutions. This can create an arrangement of such materialsin a molecular level to tailor the cooperative electronic and opticalproperties. The most attractive feature of the assembly procedure isthe nanoscale control over their molecular organizations by varyingseveral parameters such as pH of the solvent, polymer functionality,temperature, deposition time, thickness etc. [17–20]. This may bemanifested in their physical and physicochemical propertiesshowing variety of applications viz. ion-selective membrane, drugdelivery systems etc. [21–23].

Although the LbL ESA method is based on electrostatic interac-tions between oppositely charged species, the primary driving forcetowards the formation of organized molecular assembly is theentropy, not enthalpy [24]. The entropy of the composite systemmaybe increased due to electrostatic complexations of polyions onto acharged surface which liberates undissociated low molar masscounterions. However, there may be an additional entropy gain dueto the liberation of solvent molecules from the salvation shell of thespecific ionic groups bound to the polymer. During each alternateadsorption step, the substrates are immersed into the aqueous

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355348

solution of oppositely charged polymers or any organic materialsfollowing rinsing the substrates between intermediate adsorption bydeionized water or ethanol to remove any surplus ions loosely boundto the assembly. These washing steps may have several consequencesespecially materials loss during multilayer built up.

In recent times, much emphasis have been given for the assem-bling of nanostructured LbL films as a function of pH of the solventused to prepare anionic or cationic solutions. There are many reports[25,26] showing the dramatic changes of the photophysical or thephysicochemical properties of multilayered LbL films of weak poly-electrolytes for slight changes in their solution pH. More precisely theadsorption mechanism is largely influenced by their local micro-environment [27]. pH sensitive multilayered films have the potentialapplications in the area of controlled release of drugs, sensors, pur-ification etc. [28–30]. Sukhorukov et al. [31] reported the formationof hollow microcapsules composed of at least one weak polyelec-trolyte poly(allylamine)/poly(styrene sulfonate)(PAH/PSS) multi-layered LbL films and can be made to be porous in nature dependingon the local pH of the polyelectrolyte solutions. Open and closedconformations of the pores as a result of slight change in pH canfacilitate loading and release of macromolecules from the capsulecore. Rubner and Co-workers also showed the loading and release ofsmall anionic molecules from multilayered polymeric films using pHdependent film swelling and local hydrophobicity in the films [32].Apart from the change in local microenvironment, the multilayeredgrowth of LbL film formations of organic molecules are not onlyfound to be as linear [33] but also exponential [34] in many differentcases and the growth mechanism of LbL films has been studied bythe quartz crystal microbalance (QCM) technique [35,36]. Addition-ally for weak polyelectrolyte the acid-base dissociation behavior ofthe polymer films is found to be a major factor controlling the overallproperties of the LbL films.

In this present communication, we have addressed the preparationof LbL self-assembled films of an azo dye Chromotrope-2R abbreviatedas CH2R and a polycation poly(allylamine hydrochloride) abbreviatedas PAH onto solid substrates via electrostatic adsorption mechanism.The chemical structures of both the dye and polycation are shown inFig. 1a and b respectively. The film growth kinetics and pH dependentadsorption properties of CH2R onto PAH layer and their detail pho-tophysical properties have been studied in the light of UV–visabsorption and steady-state fluorescence emission techniquesaided by Density functional theory (DFT). CH2R is a class of monoazo(–N¼N–) disulfonated organic hydrophilic dye which has potentialapplications in industry such as textiles, papers, gasoline etc. [37–40].CH2R can be applied to for plasma staining and counterstaining inanimal histology [41]. The most interesting observation in this work isthat, adsorption of CH2R molecules to the cationic backbone of PAHlayer assembled onto quartz substrate is largely influenced by the pHof dye solution from which it was assembled. Photophysical investi-gations of this LbL films also reveal the interaction and aggregationbehaviors of CH2R dye molecules in LbL films. The morphology ofPAH/CH2R LbL films were closed packed and homogeneous whichhave been confirmed by Atomic Force Microscopy (AFM).

Fig. 1. Molecular structure of (a) CH2R (b) PAH.

2. Materials and methods

2.1. Materials

The anionic dye Chromotrope-2R (CH2R) (MW¼468.37 g/mol, dyecontent �75%) and cationic polyelectrolyte poly(allylamine hydro-chloride) (PAH) (MW¼70,000 g/mol), purity499%, were purchasedfrom Aldrich Chemical Company, USA and were used without furtherpurification. The solubility of CH2R in the studied aqueous phases wasfound to be 1 mg/ml. For the fabrication of LbL films, aqueous solutionof PAH was prepared using triple distilled deionized water (Milli-Q,resistivity of 18.2 MΩ cm, Millipore) and the polyelectrolyte depositionbath was made using aqueous solution of PAH having concentration of1 mM (based on monomer molecular weight). Aqueous solution ofCH2R was also prepared for a concentration of 10 mM using tripledistilled deionized Milli-Q water and the solution pH was adjustedwith HCl and NaOH. Fluorescence grade quartz slide, Fisher brandmicroscopic glass slide and polished silicon wafers (100) were used assubstrates to deposit LbL films for various characterizations. Thesesubstrates were cleaned in a 3:1 mixture of H2SO4/H2O2 for 30 min.followed by subsequent rinsing three times with Milli-Q water andthen dried in Nitrogen air prior to use [42,43]. This type of cleaninghas the advantage to remove very small amount of organic con-taminants from the substrates and render them hydrophilic (by addingOH-groups) in a highly oxidative acid bath. These hydrophilic sub-strates are then suitable to adsorb the PAH from its aqueous solutiononto their surface.

2.2. Fabrication of PAH/CH2R LbL self-assembled films

LbL ESA film fabrication by dipping the above suitable substratesalternately in CH2R and PAH aqueous solutions with the aid ofcommercially available automatic dip coater (Model: SDC-2007C,Make: Apex Instruments Co Ltd, India). This method of film fabri-cation utilizes both van der Walls interaction between PAH andsubstrates and electrostatic interaction between oppositely chargedPAH and CH2R moieties. PAH was used here to fix and organize thesmall hydrophilic dye molecules (CH2R) onto solid substrates.Thoroughly cleaned substrates (quartz, glass, Si wafer etc.) asalready described above were then immersed into PAH solution for15 min followed by rinsing with deionized water and dried in anitrogen air flow for 1 h. The Purpose of rinsing basically is to washout the surplus cations attached to the substrate. The dried sub-strate containing one layer of PAH was then immersed into CH2Rsolution having desired concentration for 15 min. followed by samerinsing procedure and finally dried in Nitrogen air flow for 1 h. PAH/CH2R LbL self-assembled film was thus prepared and the multi-layered growth was monitored by recording the optical absorbancespectrophotometrically after each cycle of deposition. This processof film fabrication was carried out in a cyclic manner until thedesired number of PAH/CH2R layer is deposited. The materials lossif any due to subsequent rinsing of LbL films with deionized waterduring multilayer build up was checked by monitoring the changein optical absorbance of the LbL film spectrophotometrically beforeand after each rinsing steps followed by drying the film in N2 air.

2.3. Characterizations

Various analytical techniques have been employed to char-acterize the CH2R aqueous solution and PAH/CH2R LbL self-assembled films deposited onto solid substrates. UV–vis absorp-tion spectroscopy (Lambda-25, Perkin Elmer, USA), steady statefluorescence emission spectroscopic (LS-55, Perkin Elmer, USA)methods were used to understand the photophysical properties ofPAH/CH2R self-assembled LbL films deposited onto quartz sub-strates. The detailed surface morphology of the films deposited onto

Fig. 2. The optimized molecular structure of (a) neutral and (b) anionic forms ofthe CH2R molecule in electronic ground (S0) state as obtained from the B3LYP/6-311 g(d,p) level of theory.

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355 349

smooth Si substrates was characterized by a commercial AtomicForce Microscope (Inova, Bruker, Germany) in intermittent contact(tapping) mode. The typical scan area of the images as obtained inthe present work was 2�2 μm2.

3. Theoretical calculation

The theoretical calculations were carried out using Gaussion-09suite of software [44]. Various forms of the CH2R molecule havebeen optimized by Density functional theory (DFT). Becke's threeparameter hybrid exchange (B3) [45] and Lee–Yang–Parr correla-tion functional (LYP) [46] were used for the DFT calculations. Theoptimized geometries and their respective SCF energies wereobtained from the B3LYP/6-311 g(d,p) level of theory. Theoreticalabsorption spectrum along with the oscillator strengths (f) of thevarious forms of the molecule at the ground state was also esti-mated at TDDFT/ B3LYP/ 6-311g(d,p) level of theory. The solventeffects on the structure of the molecule were incorporated throughthe integral equation formalism polarized continuum model(IEFPCM) [47]. During the process of geometry optimization for thefully relaxed method, the absence of imaginary values in the wavenumbers along with the convergence of all of the calculationsconfirmed the attainment of local minima on the potential energysurface. The pKa value and the variations in the population ofvarious forms of the CH2R molecular species with pH were envi-saged using the Marvin 5.1.0 software [48].

4. Results and discussions

The optimized structure of the neutral and the anionic forms ofthe CH2R molecule in the electronic ground (S0) state is shown inFig. 2a and b respectively. The molecular structure belongs to C1point group symmetry. Fig. 3a and b show the experimentallyobserved UV–vis absorption and steady state normalized fluores-cence emission spectra of pure CH2R solution (concentration of10 mM), one bilayer PAH/CH2R LbL self-assembled film, PAH/CH2Rmixed aqueous solution (1:1 volume ratio) and CH2R bulk micro-crystal respectively. For fluorescence emission, the excitations of the

samples were done at wavelength of 490 nm. The CH2R solutionabsorption spectrum shows distinct and intense band system in theregion of 200–600 nm having a broad structured band having itsmaximum located at around 508 nm along with a vibronic shouldercentered at around 540 nm. Two bands observed in the ultravioletregion centered at around 305 nm and 368 nmwhich are due to thebenzene and naphthalene rings of CH2R and are assigned to a π–π*transition and partly forbidden n–π*transition respectively [49]. Themore intense and broad band observed at 508 nm may be attrib-uted to the presence of two forms of the dye molecules in solutionand stems from an allowed π–π* transition. In fact, CH2R moleculeis subjected to intramolecular hydrogen bonding tautomeric inter-action (N17–N29–H19…O18¼O11–C2) between the C¼O group of thenaphthyl ring and the hydrogen of the hydrazone linkage. However,the hydrazone form is normally bathochromic compared to the azoform (–N¼N–) and usually has a higher tinctorial strength (i.e. highcolour depth). Therefore, the band observed at lower energy sidecorresponds to the hydrazone form of CH2R molecules [49].

The theoretically simulated absorption spectrum of neutral CH2Rmolecule in aqueous background is shown in Fig. 4a. The oscillatorstrengths of the respective electronic transitions are also marked inthe same Figure. Table 1 shows the experimentally observed andtheoretically predicted low lying electronic transitions along with theiroscillator strengths (f). These energy level transitions have somewhatdeviations from the actual experimental observations because theo-retical calculations refer to isolated molecule where as experimentaldata are obtained in more condensed phases. The electronic absorp-tion peaks at �508 nm (calculated at 490 nm, f¼0.001) and 540 nm(calculated at 560 nm, f¼0.3739) correspond to HOMO-LUMOþ1,and HOMO�4-LUMO transitions associated with S0-S6 and S0-S5states respectively. Close inspections of the related frontier molecularorbitals are shown in Fig. 4b.

On the other hand, the absorption spectrum (experimental) ofPAH/CH2R LbL self-assembled film deposited onto quartz substrateis slightly red shifted when compared to the absorption spectrum ofpure dye solution. The maximum absorbance is observed at 510 nm.Actually the sulfonic groups (SO3

– –) of dye molecules wereattached electrostatically to the positive binding sites (NH2

þ) ofPAH chains in the LbL films. As a result dye molecules are arrangedin one dimension to achieve a parallel orientation of their transitionmoments with an angle of zero between the transition momentsand the line joining the center of the molecules [50]. Therefore, theshift of the absorption curve of PAH/CH2R LbL films towards higherwavelength side (compared to CH2R solution) clearly reveals theformation of J-aggregates that is the J-type aggregation of aromaticdyes, such as the ones used in this study, results in a side-by-sidealignment of the aromatic rings on neighboring molecules [51]. Thistype of aggregation is favorable for these dyes because of thepotential for hydrogen bonding between the substituents of thearomatic rings such as the hydroxyl groups of CH2R and thehydroxyl and amino group of J-aggregation. It has been previouslyobserved in polyelectrolyte multilayer systems containing a poly-electrolyte and azo chromophores electrostatically attached to thepolymer backbone [52]. The absorption spectrum of CH2R/PAHmixed solution as shown in Fig. 3a gives its maximum at around500 nm and a very weak shoulder centered at around 555 nm alongwith all other bands which are also present in the case of pure CH2Rsolution absorption spectrum. The change of shape of this absorp-tion spectrum along with peak shifting when compared to theabsorption spectrum of pure CH2R solution essentially due to theinteraction of dye molecules with the cationic backbone of PAH inaqueous medium. The weak vibronic band (at 555 nm) is possiblydue to the different degrees of aggregation (side by side) of CH2Rmolecules while interacting with positively charged group (NH2

þ)of PAH. If we look into the microcrystal (bulk CH2R sample ontoquartz substrate) spectrum of CH2R, the spectrum is also red shifted

Fig. 3. (a) UV–vis absorption spectra of CH2R aqueous solution (10�5M), CH2R microcrystal, PAH/CH2R mixed aqueous solution (1:1 volume ratio), one bilayered PAH/CH2RLbL self-assembled film. (b) Steady-state normalized fluorescence emission spectra of CH2R aqueous solution (concentration of 10�5M and 10�4M), PAH/CH2R mixedsolution (1:1 volume ratio), one bilayered PAH/CH2R LbL film and CH2R microcrystal. (λex¼490 nm).

Fig. 4. (a) Theoretically simulated absorption spectrum of the neutral form of CH2Rmolecule as estimated from B3LYP/6-311 g(d,p) level of theory. (b) Frontier mole-cular orbitals associated with the transitions of S0-S6 and S0-S5 states for neutralform of the CH2R molecule.

Table 1Theoretically simulated and experimentally observed absorption bands at twodifferent pH of CH2R molecule as estimated from B3LYP/6-311 g (d,p) level oftheory.

pH of themedium

Absorption

Theoretical Experimental λ innm

λ in nm Oscillatorstrength (f)

Transition

7 622 0.4223 HOMO�1-LUMO

–

560 0.3739 HOMO�4-LUMO

540 (s)

490 0.0010 HOMO-LUMOþ1

508 (s)

12 510 0.6546 HOMO�1-LUMO

512 (s)

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with its maximum at around 534 nm and another broad band ataround 579 nm with respect to its solution absorption spectrum.Actually in the solid state, dye molecules are close enough forintermolecular dipole interactions and certainly they form aggre-gates which are reflected as the absorption band at 579 nm. How-ever, it should be noted that the difference of the aggregation of dyemolecules in the CH2R/PAH mixed solution as well as in micro-crystal is definitely due to the different microenvironment in boththe cases as experienced by the dye molecules. Dye molecules are inmuch more restricted geometry in the solid state when comparedto its pure solution. The emergence of new band at higher

wavelengths both in CH2R/PAH mixed solution as well as in LbLfilms is due to the side by side stacking geometries of dye moleculesresulting their molecular J-aggregates. According to the excitoncoupling theory this corresponds to a lower electronic energy statewhich is manifested as the red shift of the absorption spectrum ofCH2R bulk microcrystal, PAH/CH2R LbL films and PAH/CH2R mixedsolution when compared to that of pure dye in aqueous solution.

The steady state normalized fluorescence spectrum of pureCH2R solution as shown in Fig. 3b gives distinct bands systemswithin the region of 500–675 nm. With increase in concentrationof CH2R in aqueous solution, the fluorescence intensity of the 0–0band centered at 567 nm increases. However, the spectrumbecomes diffused and is sufficiently broadened at the concentra-tion of 10�4 M of CH2R. This may possibly be due to the aggre-gation of dye molecules and thereby overlapping of severalvibrational bands of the electronic states at such very high molarconcentration [53,54]. Interestingly, fluorescence spectrum ofCH2R/PAH mixed aqueous solution shows the intense 0–0 band ofCH2R at around 536 nm along with a new broad structurelessband in the longer wavelength region with peak centered ataround 626 nm. This shifting of 0–0 band of CH2R as well as theformation of higher wavelength broad structureless band clearlyindicate the interactions of CH2R molecules with PAH backbone insolution and as a consequence the formation of dye aggregates. Onthe other hand, in case of PAH/CH2R LbL films, there is shift of the

Fig. 5. UV–vis absorption spectra of (a) different bi-layered PAH/CH2R LbL film deposited onto quartz substrates. Deposition time for both PAH and CH2R were 15 min. Insetshows the absorbance of 510 nm band of LbL film with layer number (b) one bi-layered PAH/CH2R LbL film deposited onto quartz substrates at various deposition time forCH2R. Inset shows the absorbance of 510 nm band of LbL film with deposition time of dye.

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355 351

0–0 band (c.a. 573 nm) of about 6 nm only with very low intensitywhen compared to the fluorescence spectrum of pure CH2Rsolution. Moreover the lower energy broad band at around 616 nmin LbL films also may be due to the formation of dye aggregatesand the intermolecular vibrational coupling between neighboringCH2R molecules adsorbed within PAH chains in LbL films. Thisvibrational coupling can produce some deformations of the elec-tronic states of dye molecules confined in LbL films therebycausing a change in their dipole–dipole interactions [55] whichmay facilitate their edge-on orientation (J-aggregates) [56,57]. Theformation of dye aggregates and the intermolecular vibrationalcoupling may also correspond to some intersystem crossing whichin turn facilitates the reduction of the intensity of 0–0 band. Thespectral profile is also similar to the fluorescence spectrum ofCH2R bulk microcrystal deposited onto quartz substrate as shownin Fig. 3b. That is in the solid state CH2R molecules come closureenough so that there is some overlapping of several vibrationalstates of each electronic energy state of the excited CH2R mole-cules. Therefore the shifting of the 0–0 band along with the gen-eration of new higher wavelength band in the fluorescence spectraof both PAH/CH2R mixed solution and PAH/CH2R LbL films isdefinitely owing to the closer association of dye chromophoreswhile binding with the positively charged PAH chain and therebysubsequent formation of molecular aggregates.

Fig. 5a shows the UV–vis absorption spectra of different bilayered(1-14 bilayers) PAH/CH2R LbL films deposited onto quartz substrate.In each layer of deposition the emersion time of the substrate to PAHor CH2R solutionwas 15 min. pH of both the solutions were set at 7.0.There was no significant materials loss due to rinsing the LbL films bydeionized water during multilayer build up as observed from the UV–vis absorption spectra (Figure not shown) of LbL film monitoredbefore and after each rinsing steps. It is interesting to observe fromFig. 5a that the absorption spectral profile after deposition of eachbilayer (PAH/CH2R) in LbL film shows almost similar pattern exceptthe sequential increase in absorption intensity. The multilayer growthin the LbL films was almost linear up to ten bilayers as is evidencedfrom the plot of absorbance intensity at 510 nm band with layernumber (inset of Fig. 5a). This linear increase of absorbance is due tothe surface charge overcompensation after each layer of depositiononto solid substrate. The distribution of CH2R molecules in this waymight be homogeneous throughout the LbL film. More precisely, thisincrease in absorption intensity with increase in layer number defi-nitely indicates the growth of PAH/CH2R complex molecular

assemblies with increase in film thickness and also confirms thesuccessful charge alternation after each layer of deposition in LbLfilms. However, at higher layer numbers i.e. 410 bilayers, the changein absorption intensity at 510 nmwas not linear rather zigzag pattern.This is because of the fact that roughness of the film might haveincreased at such higher layer number and therefore the electrostaticinteraction between PAH and CH2R does not work as well causing theinhomogeneous distribution of CH2Rmolecules in LbL films. This typeof observation was also reported in our earlier works [19].

In the present work, we have also examined the effects ofdeposition time on the adsorption behavior of dye molecules ontothe PAH layer in LbL film. Fig. 5b shows the UV–vis absorptionspectra of one bilayer PAH/CH2R LbL films for various dye deposi-tion time. The deposition time for PAH in all the cases was 15 min.whereas CH2R deposition time was varied from 1 to 60 min. Fromthe figure it is observed that the absorbance at 510 nm of PAH/CH2RLbL films increases almost linearly up to 15 min. and then decreasesabruptly and continue to irregular or zigzag pattern as shown ininset of Fig. 5b. This observation reveals that the electrostaticinteraction between PAH and CH2R was completed up to 15 min.However, after this time the rapid decrease in the intensity of theabsorption band (510 nm) may be due to some enhanced dye-dyerepulsive interaction between CH2R molecules in their aqueoussolution and that present in LbL films. Also the PAH layer depositedonto quartz substrate became sufficiently hydrated for prolongedexposure to CH2R aqueous solution. This can cause certain releaseof dye from the already adsorbed LbL films due to enhanced of dye-dye interaction between film and the solution. Furthermore, therewere no available free cationic binding sites of PAH in LbL films toadsorb further CH2R molecules after the saturation time of 15 min.That is why deposition time of 15 min. was considered for the CH2Rdeposition throughout the experiment.

4.1. Effect of pH on dye loading in the LbL films

The effective loading behavior of the small molecules azobenzenederivative dye CH2R onto the PAH layer in LbL films may also dependupon variety of factors such as hydrophilic/hydrophobic balance ofPAH, pKa, charge density of dyes, size, local electrostatic environmentetc. In the present work efforts are also given to explore the pHdependence of the adsorption of CH2R molecules onto PAH coatedquartz substrate from a fundamental point of view. The theoreticallyestimated populations of neutral form (pHr7.0) of the CH2R

Fig. 6. The variation in populations with pH for neutral and the anion form of theCH2R molecule.

Fig. 7. (a) UV–vis Absorption spectra of CH2R aqueous solution at different pH.(b) Theoretically simulated absorption spectrum of the anionic form of CH2Rmolecule as estimated from B3LYP/6-311 g(d,p) level of theory.

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355352

molecule and its anionic form with pH (pH49.5) in aqueous solutionare shown in Fig. 6. The pKa value for the O16–H functional group ofthe molecule has been estimated to be 7.56. Thus, depending upon thepH of the medium, molecule exists in neutral and in anionic forms.

The population of the neutral form of the molecule is pre-ponderant at acidic pH of the medium, decreases with increasingpH and vanishes at alkaline pH. The population of the anionicspecies of the CH2R molecule is almost zero at lower pH butmonotonically increases with increase in pH value and maximumat the pH value �9.43. The anionic form of the molecule exhibitsmaximum population at alkaline pH medium.

The UV–vis absorption spectra of CH2R molecule in aqueoussolution at various pH of the medium are shown in Fig. 7a. Theabsorption band maxima along with the shape of the absorptioncurves are almost same for pH of 3, 5 and 7. The absorption max-imumwas observed at 504 nm for all these pH values. However, onfurther increase in pH of the medium from 7 i.e. at pH of 9.5, 11 and12, the band centered at 368 nm and 540 nm have been dis-appeared. Furthermore, the absorption band observed at � 510 nmis considerably broadened, red shifted and structureless as well asdiffused pattern when compared to the absorption spectrum ofCH2R solution at pH higher than 7 as described. The appearance ofred shifted band at �510 nm in the absorption spectra may pri-marily signify the preferential existence of the anionic form of theCH2R molecule at alkaline pH of the medium. Fig. 7b shows thetheoretically simulated absorption spectrum of the modeled anionicform of the molecule which is in harmony with the experimentalobservation. The corresponding absorption parameters of CH2R atsome higher pH namely, 12 are summarized in Table 1. The simu-lated absorption spectrum at this pH (as obtained from DFT study)is presented here for comparison and to explore the origin of thenew red-shifted band observed at 510 nm of CH2R experimentalsolution absorption spectrum at such high alkaline pH when com-pared to that at pH of 5. Additionally at this high pH, the populationof the neutral form of the dye molecules vanishes (as predictedfrom Fig. 6) and the optical absorption profile of CH2R moleculesassembled onto the PAH backbone in LbL film fabricated at suchhigh pH was significantly different than that fabricated at pH 5 asdiscussed in the next paragraph.

Fig. 8a represents the UV–vis absorption spectra of 1 bilayeredPAH/CH2R LbL films assembled onto quartz substrate from CH2Raqueous solution at various pH namely, 3, 5, 7, 9.5, 11 and 12. Fromthe figure it is observed that the overall absorption intensityincreases along with red shifting of the main absorption peak for

the LbL films fabricated from the dye solution at lower pH (namelyat 3) when compared to that assembled at neutral pH (i.e. 7). Thisincrease in absorption intensity is basically due to the fact thatCH2R molecules which contain two sulfonated groups prefer to beadsorbed within the PAH matrix at lower pH because of strongelectrostatic attractive forces between fully charged free aminegroups of PAH chains and dye molecules. More precisely, surfaces ofthe PAH layer onto quartz substrate are highly protonated whenexposed to CH2R solution at low pH namely 3 due to the presenceof excess Hþ ions. Interestingly, at pH of 5, the absorbance (at510 nm) is maximum and on further decrease in pH of dye solution,the intensity of the main absorption band decreases along with redshift and spectral broadening. As the electrostatic repulsive forcesbetween positive charges of PAH chain are greatest at lower pH, thisin turn allows less number of CH2R molecules adsorbed ontopositive ionic sites of PAH in LbL film rather it enhances more directinteraction between CH2R molecules in LbL film. This essentiallyresults more closer association of dye molecules which can avoidany ionic cross-linking between dye and PAH in LbL films therebyforming their molecular aggregates. The red shift accompanied withspectral broadening as well as the development of new band athigher wavelength (at 620 nm) may be due to the formation of J-type aggregates of dye molecules as this is consistent with theexciton splitting theory [58]. In this case the CH2R molecules arearranged in one side-by-side due to the arrangement of the aro-matic rings on neighboring molecules [51]. So, in the lower pHregion i.e. 3 and 5, the red shift of the absorption band is basically

Fig. 8. (a) UV–vis absorption spectra of one bi-layered PAH/CH2R LbL film depos-ited onto quartz substrate from CH2R solution at various pH values. (b) Schematicpresentation of the adsorption scheme for the formation of PAH/CH2R LbL filmsfabricated at pH of 5 and 11 of dye solution at ambient temperature.

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355 353

due to the formation of aggregate but not just due to the incor-poration of CH2R molecules in the LbL films.

Inset of Fig. 8a also reveals that the absorbance at 510 nm band ofLbL films deposited at lower pH initially increases and reachesmaximum at pH of 5 because PAH and CH2R became fully charged atthis pH and this corresponds to the highest ionic cross-link betweenpolymer and dyes in LbL film. On further increase in pH, the absor-bance at 510 nm rapidly decreases and became minimum at pH of 11.In fact the concentration of CH2R in LbL films rapidly decreases whenthe pH of CH2R solution is going towards more basic condition (i.e.11) because of reduced charged density of the free amine (NH2

þ)group of PAH (as the pKa value of PAH�10.8) when exposed to anaqueous environment of CH2R molecules at pH of 11. At this condi-tion, the increased repulsion between free SO3

� � groups of CH2Ralso responsible for their reduced adsorption onto the LbL film.Additionally at very high pH namely, 11, the relative population ofanionic form of CH2R is much higher compared to that at neutral pH,thereby causing the minimum absorbance at 510 nm. It is wellknown that PAH layer assembled onto a substrate starts to swellwhen exposed very high basic medium [59] above its pKa value. As aresult some of the dye molecules might have directly occupied thefree space in between the polyelectrolyte chain under this circum-stance. The adsorption scheme at pH 5 and 11 is schematically shownin Fig. 8b. Also, the attractions of counterions by PAH layer in LbL filmmay increase leaving very few positive binding sites of PAH onto thesubstrate for sufficient adsorption of CH2R molecules. Hence, theabsorbance at 510 nm band is less for LbL film fabricated from verylow pH of CH2R solution.

However, the most interesting observation is that the number ofCH2R molecules in LbL films is unexpectedly increased on furtherincrease of pH of dye solution after 11. This is reflected as the increasedabsorbance (at 510 nm) as shown in the inset of Fig. 8a. Furthermore,the spectrum becomes sufficiently broadened along with the higherwavelength band centered at 620 nm. This spectral broadening and the

observed lower energy band are due to the aggregation of dye mole-cules in LbL films as also discussed earlier. The increase in absorbanceof 510 nm band along with the emergence of aggregation band pos-sibly due to the increased dye molecular density in LbL films fabricatedat higher pH namely, 12. One possible reason is that when the PAHcoated quartz substrate exposed to dye solution at high pH (i.e.12), thePAH layer loose most of the positive charges and therefore increases itsswelling ability [59,60] due to unbalanced electric charges whileimmersed into the dye solution; that is the polyelectrolyte layer in LbLfilms become sufficiently hydrated. As a consequence, this increasedswelling ability creates more free space to accommodate the dyemolecules between the polymer chains onto the quartz substrate butnot above the PAH layer. However, the swelling ability may dependupon many factors like local electrostatic environment of the hydratedfilms, hydrophilic/hydrophilic balance of the polymer chain etc. Someauthors [25,61] suggest that polyelectrolyte multilayers films also ableto swell when exposed to very high pH aqueous environment becauseof the variation in charge density of the polymer chain onto solidsubstrate as influenced by the change in pH of the medium in whichthey are emerged for dye adsorption. Additionally, the increased freespace in the LbL films along with enhanced hydrophobic interactionsbetween polycation and dye essentially provides more mobility to thedye molecules to overcome any undesirable electrostatic forces. As aresult dyemolecules find suitable sites for more favorable van derWaalor hydrogen bonding interactions. However, the rate of swelling of PAHlayer at higher pH should be much faster than the time required toadsorb the dye molecules to the LbL films. The swelling behavior ofpolyelectrolytic multilayers films has been rigorously studied byTenchak and Barrett [62]. Their mobility and increasing number of dyemolecules in LbL films fabricated at very high pH also favour theiraggregation which is already reflected as the appearance of 620 nmband in their UV–vis absorption spectra.

The detailed surface morphology of PAH/CH2R LbL self-assembled films deposited onto smooth Si substrate from two dif-ferent pH of CH2R solution has been studied by tapping modeAtomic Force Microscopy (AFM). Fig. 9 shows the AFM topographicimages along with height profile and histogram analysis for PAH/CH2R LbL films deposited from CH2R solution at pH of 5 and 11respectively. The dye molecules as well as their aggregates areclearly observed in AFM images which also confirm the formationof nano-dimensional dye/polycation self-assembled LbL filmdeposited onto solid substrate for the dye solution at two differentpH namely, 5 and 11. However, if we further look at the heightprofile of the images, we immediately observe that the averageheight as well as the RMS roughness of the PAH/CH2R assemblyonto solid substrate deposited at pH of 5 is much higher than thatfabricated at higher pH namely 11. The average surface roughness ofthe LbL films deposited at pH 5 and 11 are found to be 6.2567 nmand 2.8505 nm respectively. The other important morphologicalparameters of LbL films are illustrated in Table 2. The decrease inaverage height as well as average surface roughness of LbL filmsonto the solid substrate assembled at higher pH (11) definitelyconfirms that the dye molecules did not attached onto the top ofPAH rather dye molecules eventually occupied in the free spacebetween the PAH chain onto the substrate at such higher pH. Fur-thermore, PAH layer loose the charge density of NH2

þ groups at pHof 11 causing very less number of CH2R molecules adsorbed in thebackbone of PAH in LbL film because of unfavorable electrostaticinteraction between PAH and CH2R molecules. This is also con-sistent with the lowest absorbance value at 510 nm band of LbL filmas observed from UV–vis absorption spectroscopic results (Inset ofFig. 8a). It is already mentioned earlier that when PAH layerassembled onto the substrate exposed to CH2R solution at lower pHnamely, 5, the PAH layer becomes highly protonated causing strongelectrostatic interaction between fully charged NH2

þ groups ofpolymer chain and SO3

� � group of CH2R. As a result more number

Fig. 9. AFM images of one bi-layered PAH/CH2R LbL film deposited onto smooth Si substrate for (a) pH of 5 and (b) pH of 11. Concentration of CH2R was 10�5 M, depositiontime for CH2R was 15 min.

Table 2Morphological parameters of PAH/CH2R LbL films as obtained by Atomic Force Microscopy.

Systems RMS roughness(nm)

Peak to peak height(nm)

Average roughness(nm)

Average height(nm)

PAH/CH2R LbL films fabricated from dye solution at pH of 5 1.8337 20.8374 1.4290 6.2567PAH/CH2R LbL films fabricated from dye solution at pH of 11 1.2492 13.2380 0.8917 2.8505

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355354

of CH2R molecules adsorbed onto the top of highly charged PAHlayer in the solid substrate. Therefore the average height, peak-to-peak height as well as RMS roughness of the LbL film fabricated atpH of 5 were much higher than prepared at higher pH i.e. 11.Additionally, strong electrostatic interaction between PAH andCH2R at pH of 5 facilitates more closer association of dye moleculesin LbL films thereby forming large number of dye aggregates ontothe top of PAH moieties and this is also responsible for theincreased RMS roughness as observed from AFM images and cor-responding height profile analysis (Fig. 9a)., Therefore, from theAFM analysis, it is also concluded that the loading of CH2R mole-cules and the degree of their aggregations in LbL films also dependupon the pH of dye solution from which they were fabricated.

5. Conclusions

In summary, our experimental observations reveal that theabsorption spectrum of pure CH2R solution comprises the bandsystem in the region of 200–600 nm with a maximum at around at504 nm and is responsible for an allowed π–π* transition. DFTstudies successfully predicted the origin of these electronic spectraltransitions of CH2R molecules both in neutral and anionic statesand are consistent with the experimental results. In the case of PAH/CH2R LbL films deposited onto quartz substrate the absorptionspectrum is slightly red shifted when compared to pure solutionabsorption spectrum of CH2R. The shift of the absorption curve of

PAH/CH2R LbL films towards higher wavelengths side clearlyreveals the formation of J-type aggregates. In fact, the sulfonicgroups (SO3

� �) of CH2R were attached electrostatically to thepositive binding sites (NH2

þ) of PAH chains in the LbL films and thisfacilitates the parallel orientation of their transition moments.Absorption spectrum of CH2R microcrystal is further red shifted.Therefore, the degree of interactions between dye molecules in allcases is different because of different microenvironments asexperienced by dye molecules. Steady state fluorescence spectro-scopy clearly confirms the shifting of 0–0 band of CH2R along withthe generation of a lower energy band in LbL films when comparedto that of pure CH2R solution and these definitely confirm the for-mation of dye aggregates in LbL films. The multilayer growth ofPAH/CH2R LbL films was almost linear up to 10 bilayers and thenbecomes non-uniform. The adsorption behavior of CH2R onto PAHbackbone in LbL films was found to be time dependent. The mostinterestingly, the adsorption of CH2R molecules onto LbL filmsdepends upon pH of CH2R solution fromwhich it was assembled. Atlower pH value namely 3 the red shift and spectral broadening wasdue to the formation of dye aggregates because of much closerassociation of dye molecules in the LbL films at this pH. However, atpH of 5, the absorption intensity at 510 nm band was maximumsuggesting more favorable electrostatic environment between dyeand PAH in the LbL films. At higher pH namely, 11, the adsorption ofCH2R onto LbL films was minimum because of reduced chargedensity of the polyelectrolyte (PAH) at this pH and PAH layer wasjust able to swell within the dye solution. On further increase of pH

C. Hansda et al. / Journal of Luminescence 178 (2016) 347–355 355

absorption intensity of CH2R increases because of larger availablefree space to dye solution due to increased swelling of PAH layerwhen exposed to CH2R solution at very high pH. Atomic ForceMicroscopy confirms the visual evidence of the surface morphologyof LbL films deposited at pH 5 and 11. The surface roughness of thefilm was much higher when fabricated at low pH namely, 5 thanthat fabricated at higher pH namely, 11. This also reveals that theadsorption and association of dye molecules in the LbL films ishighly depends on the microenvironment of dye solution fromwhich it was assembled.

Acknowledgements

Author P. K. Paul is grateful to Science and EngineeringResearch Board (SERB) of DST, Govt. of India for financial assis-tance (Project Ref. no. SB/EMEQ-142/2014) for this work. S. Pauland P. K. Paul are also thankful to West Bengal State Council ofScience and Technology (WBSCST) (Project Ref. no. 493/WBSCST/F/0545/15(Pt-I)) for financial assistance and Jadavpur University forproviding excellent infrastructural and equipment facilities toperform this work.

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