Journal of Physics and Chemistry of Solids 87 (2015) 128–135

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Journal of Physics and Chemistry of Solids

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Clay induced aggregation of a tetra-cationic metalloporphyrin in Layerby Layer self assembled film

Soma Banik b, J. Bhattacharjee a, S.A. Hussain a, D. Bhattacharjee a,n

a Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar 799022, Tripura, Indiab Department of Physics, Women’s College, Agartala 799001, Tripura, India

a r t i c l e i n f o

Article history:Received 20 March 2015Received in revised form10 July 2015Accepted 11 August 2015Available online 14 August 2015

Keywords:SurfacesThin filmsMultilayersOptical propertiesSurface properties

x.doi.org/10.1016/j.jpcs.2015.08.00897/& 2015 Elsevier Ltd. All rights reserved.

esponding author. Fax: þ91 381 2374802.ail address: debu_bhat@hotmail.com (D. Bhat

a b s t r a c t

Porphyrins have a general tendency to form aggregates in ultrathin films. Also electrostatic adsorption ofcationic porphyrins onto anionic nano clay platelets results in the flattening of porphyrin moieties. Theflattening is evidenced by the red-shifting of Soret band with respect to the aqueous solution. In thepresent communication, we have studied the clay induced aggregation behaviour of a tetra-cationicmetalloporphyrin Manganese (III) 5, 10, 15, 20-tetra (4 pyridyl)-21 H, 23 H-porphine chloride tetrakis(methochloride) (MnTMPyP) in Layer-by-Layer (LbL) self assembled film. The adsorption of dye mole-cules onto nano clay platelets resulted in the flattening of the meso substituent groups of the dyechromophore. In Layer-by-Layer ultrathin film, the flattened porphyrin molecules tagged nano clayplatelets were further associated to form porphyrin aggregates. This has been clearly demonstrated fromthe UV–vis absorption spectroscopic studies. Atomic Force Microscopic (AFM) studies gave visual evi-dence of the association of organo-clay hybrid molecules in the LbL film.

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1. Introduction

Organised molecular assemblies of macro cyclic chromophoreshave gained a wide spread interest owing to their versatile ap-plicability in sensing [1–3], molecular electronics [4–6], artificiallight harvesting systems [7,8] and pharmacology [9,10]. Molecularassemblies in thin film often lead to molecular aggregation [11–13]. Molecular aggregation causes remarkable changes in thephotophysical and electronic properties of the molecules. It is wellknown that J-aggregation (side-by-side) is characterised by strong,narrow, red-shifted absorption bands (J-band), whereas H-ag-gregation (face-to-face) results in the blue-shifted (H-band) bandwith respect to the monomeric absorption band [11–13]. Accord-ing to Kasha et. al. [14] strong intermolecular interactions betweenmonomeric species and delocalised excitonic energy over thewhole assembly of aggregates are the characteristics of suchphenomena.

Due to their rigid planar geometry, high stability, intenseelectronic absorption and small HOMO–LUMO energy gap, por-phyrins serve as a significant class of synthetic building blocks forfunctional nano materials [15]. Owing to their excellent photo-physical, photochemical, electrochemical and structural proper-ties, they have potential applications in the field of non-linear

tacharjee).

optics [16] and for investigations of artificial light harvesting sys-tems that mimic natural photosynthetic receptors [17]. Undervarious conditions porphyrins form J-aggregates due to hydro-phobic π–π stacking and the electrostatic interactions between theanionic and cationic groups. Aggregation of pophyrin molecules inaqueous solution can be tuned depending on the porphyrinstructure and concentration, pH, ionic strength, counterions ofinorganic salts in the medium [18–20]. Even the medium itselfsuch as micelles [21], ionic liquids [22], nucleic acids [23], poly-peptides [24], proteins [25] and carbon nanotubes [26] can influ-ence porphyrin aggregation behaviour.

Metalloporphyrins are an important class of porphyrins as theycan be used for mimicing many biological processes [27], catalystsin chemical and photochemical reactions [28], photosensitizersincluding the field of nonlinear optics [29], molecular electronicdevices [30], solar energy cells [31], artificial photosynthesis [32],photoinduced hydrogen production [33] etc. In some of theseapplications porphyrin molecules were used in solid phase but inmost of the cases they were required to be embedded into solidsurfaces. Layered porphyrins on solid surfaces are suitable forvarious practical applications in heterogeneous catalytic reactions,recognition systems and in optoelectrical molecular devices [34–36]. In metalloporphyrins, in addition to intermolecular interac-tion and coordination binding, axial co-ordination via centralmetal ion plays an important role in the formation of organisedmultilayers as suggested by Beletskaya and co-workers [37]. Suchmultilayered supramolecular assemblies are used as models for

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135 129

biological processes and catalysts for many organic reactions.Adsorption and aggregation behaviour of cationic porphyrin

derivatives on Tungsten (VI) oxide (WO3) nano colloid particleshave been studied in aqueous colloidal dispersion [38]. Formationof J-aggregates strongly depend on the presence and difference ofperipheral substituents. In aqueous dispersion, metalloporphyrinSb(V)TSPP intercalated in polyfluorinated surfactant/clay hybridmicroenvironment and formed H and J-type dimers only in thepresence of penetrated benzene molecules [39]. The interaction ofionic porphyrin with ionic surfactant was studied using variousionic pophyrin derivatives and ionic surfactants in aqueous en-vironment [40]. In several cases miceller aggregates led to theformation of J-type and H-type aggregates [41].

Takagi et. al. studied clay–porphyrin interactions in aqueousclay dispersion. They showed that accommodation of guest por-phyrin molecules onto nano clay platelets led to the flattening ofdye chromophores induced by strong host–guest interactions [42–44]. Spectral red-shift of about 10–30 nm of the Soret band posi-tion is the characteristic feature of the flattening of dye chromo-phore on the clay platelets [42]. In general, clay minerals act asinorganic host materials for the adsorbed guest molecules and theadsorbed molecules have a tendency to aggregate on the na-nosheets or in the interlayer space between clay sheets, thusstrengthening the guest–guest interaction in presence of host [45].

It may be mentioned in this context that inorganic nano clayplatelets have shown a great promise for the construction of hy-brid organic/inorganic nanomaterials due to their unique materialproperties, colloidal size, layered structure and nano-scale plateletshaped dimension. Anionic nano clay platelets exhibit a high af-finity for metachromic cationic dyes because the negative chargesof the clay are compensated by the positive charges of cationicorganic materials by cation exchange reaction. Cation exchangecapacity (CEC) is an important property of clay minerals whichdepends on available surface area, crystal size, pH and the type ofexchangeable cation. It is usually expressed in micro-equivalentsper gram (meq g�1 ).

In the present work detailed investigations have been carriedout to study the effect of nano clay platelets on the spectralcharacteristics of a tetra cationic metalloporphyrin (Manganese(III) 5, 10, 15, 20-tetra (4 pyridyl)-21 H, 23 H-porphine chloridetetrakis (methochloride)) abbreviated as MnTMPyP in Layer byLayer (LbL) self assembled films. Our results showed that in theLbL film MnTMPyP molecules were adsorbed on the clay plateletsand consequently became flattened. This flattening of pophyrinmoieties resulted in the red-shifting of the Soret band. Also theflattened porphyrin molecules formed J-type aggregate resultingin the development of a new band in the longer wavelength re-gion. Factors affecting the J-aggregate formation have been studiedin detail by changing various parameters. J-aggregated sites ofporphyrins show non-linear optical effects [46].

2. Material and methods

The tetra cationic metalloporphyrin Manganese (III) 5, 10, 15,20-tetra (4 pyridyl)-21 H, 23 H-porphine chloride tetrakis (meth-ochloride) (MnTMPyP) (MW¼909.01), (purity 99%), Poly (acrylicacid) (PAA) (purity 99%) and Poly (allylamine hydrochloride)(PAH), (purity 99%) were purchased from Aldrich Chemical Co. andused as received. The clay mineral used in the present work wasLaponite, obtained from Laponite Inorganics, UK, and used as re-ceived. The size of the Laponite is less than 0.05 mm, and its CEC is0.74 meq g�1.

Electrolytic deposition bath of cationic dye MnTMPyP wasprepared with 2�10�5 mol L�1 aqueous solution using triple-distilled deionized (electrical resistivity 18.2 MΩ-cm) Millipore

water. The anionic electrolytic bath of PAA (pH 6.8) was also pre-pared with triple-distilled deionized Millipore water(0.5 mg mL�1). The quartz substrates used for film fabricationmust be very clean to enable the formation of thin films. The firststep was to clean the quartz slides with soap solution for removalof grease/dirt. Then the slides were treated with chromic acid for30 min and washed in de-ionised water. Further they werecleaned with acetone and stored in a dry oven. Thoroughly cleanedquartz slides were first dipped into anionic PAA solution for15 min, dried and then rinsed with water to remove surplus an-ions attached to the surface. These slides were then dipped intocationic bath solution of MnTMPyP for 30 min followed by similarrinsing in a separate set of water bath to remove unadhered ca-tions and thus resulted in one layer of MnTMPyP–PAA LbL film.The whole sequence of process was repeated to get desirednumber of layers of MnTMPyP–PAA LbL films. All the adsorptionprocedures were carried out at room temperature (25 °C). Theincorporation of clay laponite into the LbL film was done with thehelp of polycationic PAH (pH 6.8) aqueous solution (0.5 mg mL�1).The CEC (cation exchange capacity) of laponite is 0.74 meq g�1

that is aqueous dispersion of 1 g L�1 or 1 mg mL�1 laponite claycontains 0.74�10�3 mol L�1 of negative charges. In the mixedsolution of MnTMPyP in clay dispersion, 10% of the CEC of claymeans only 10% charges of clay are sufficient to neutralise thecharges of all the MnTMPyP molecules. Aqueous dispersion of claywas stirred for 24 h and then kept 30 min for sonication beforeusing it. Aqueous MnTMPyP–clay mixed dispersion with differentloading (8%, 10%, 20%, 30%, 40%, 60% and 80% of the CEC of clay)were then prepared. The quartz slide was dipped into the elec-trolytic polycation (PAH) solution for 15 min followed by samerinsing in water bath for 2 min followed by drying of the slide. Theslide thus prepared was dipped into the mixed solution ofMnTMPyP–clay dispersion. Sufficient time was given for deposi-tion and drying of the slide. Thus, monolayer MnTMPyP–Clay–PAHhybrid film was fabricated. The repetition of the said procedureresulted into multilayered film fabrication. To study the tempera-ture effect on the aggregation behaviour, the temperature of thefabricated monolayer MnTMPyP–clay–PAH LbL film was increasedin a temperature controlled enclosure. The temperature of themonolayer was first increased from 25 °C to 100 °C in steps andthen decreased in exactly the reverse order. Spectroscopic char-acterizations of the LbL films and aqueous solution were doneusing UV–vis absorption (Lambda-25, UV–vis spectrophotometer,Perkin-Elmer) spectroscopy. Atomic force microscopic (AFM)images of monolayer LbL film were taken in air with commercialAFM system (Bruker Innova). The experiment was performed atroom temperature (20 °C). Relative humidity was kept under 60%(non-condensing). The AFM images presented here were obtainedin intermittent contact (tapping) mode. The Si wafer substrateswere used for the AFM measurement.

3. Results and discussion

Fig. 1 shows the UV–vis absorption spectra of MnTMPyP in(i) aqueous solution (2�10�5 mol L�1), (ii) thin cast microcrystalfilm and (iii) monolayer LbL film. The absorption spectrum inaqueous solution consists of three distinct bands. It may bementioned in this context that the highly delocalised planar π-framework in the core structure of porphyrin is responsible for themost fascinating features in the characteristic UV–vis absorptionspectrum. The absorption bands of porphyrins are ascribed to thein-plane π–π* transitions [47]. Gouterman [47] successfully ex-plained the electronic absorption spectrum of porphyrins by ap-plying the “four orbital” model with well discussion on the effectof charge localisation on electronic spectroscopic properties. Out

Fig. 1. Normalised UV–vis absorption spectra of aqueous solution of (i) MnTMPyP[2�10�5 mol L�1], (ii) thin cast microcrystal film of MnTMPyP and (iii) monolayerLbL film of MnTMPyP without clay.

Fig. 2. (a) Normalised UV–vis absorption spectra of MnTMPyP in (i) pure aqueoussolution (2�10�5 mol L�1), (ii) aqueous solution (2�10�5 mol L�1) at dye loadingof 8% of the CEC of clay. Dashed (——) lines show deconvoluted spectra.(b) Normalised UV–vis absorption spectra of monolayer LbL film of MnTMPyP in(i) absence of clay and (ii) in presence of clay at dye loading of 8% of the CEC of clay.The dashed (——) lines show deconvoluted spectra.

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135130

of these four orbitals two are HOMO's and two are LUMO's. Elec-tronic transitions between these orbitals result into two excitedstates. Due to overlapping of orbitals these excited states are splitup resulting into two distinct transition bands-one with higheroscillator strength, higher energy called the Soret band or B band,the other with less oscillator strength, lower energy called the Qbands. In metalloporphyrins, the higher D4h symmetry of theporphyrin macrocycle upon co-ordination with metals collapsesthe Q-bands in the visible region responsible for red to purplecolour. The Soret band position is sensitive to substituent groups.It is also highly sensitive to microenvironment. There are alsoadditional bands (N, L, M bands) in the UV range, but these areusually quite weak [48].

In the UV–vis absorption spectrum of MnTMPyP aqueous so-lution, the additional bands (N, L, M bands) are observed in the300–450 nm range. In 450–480 nm region intense Soret band (or Bband) with sharp peak at 462 nm is observed [49]. This is the mostsensitive band which is affected mostly due to the changing ofmicro-environment. Other is the Q band ranging from 525–625 nm having a weak peak at 562 nm. With changes in the microenvironment small shifting in the peak position of the Q band isobserved.

The UV–vis absorption spectra of thin cast microcrystal film aswell as the LbL film of MnTMPyP show similar spectral profilehaving a red-shifted Soret peak at 466 nm with respect to the462 nm Soret peak in solution absorption spectrum. It may bementioned in this context that being tetra cationic, MnTMPyPmolecules do not form aggregates in aqueous solution due topredominance of repulsive force. However, MnTMPyP moleculesbecame immobilized and came into closer contact with each otherin the solid state either in the restricted geometry of LbL film or inthin cast microcrystal film. The closely associated molecules whenobliquely stacked to form J-aggregates, spectral red-shift is ob-served. Thus red-shifting of the Soret band in LbL film and thincast microcrystal film is an indication of closer molecular asso-ciation leading to J-aggregates in the solid state. AFM image ofmonolayer LbL film discussed in the later section gives visual

evidence of the formation of molecular aggregates. In the latersection, it has been shown how flattening of porphyrin chormo-phore onto inorganic nano clay platelets resulted in the develop-ment of new J-band in the longer wave length region.

Fig. 2 shows the effect of inclusion of clay in the UV–vis ab-sorption spectra of MnTMPyP in aqueous solution and in themonolayer LbL film. In aqueous solution (Fig. 2a), graph(i) represents the UV–vis absorption spectra of MnTMPyP withoutclay and graph (ii) MnTMPyP with clay. Fig. 2b represents themonolayer LbL film of MnTMPyP, without clay (graph (i)) and withclay (graph (ii)). The dashed lines show the deconvolution of therespective spectra with clay. In both the cases of aqueous solutionwith clay and LbL film with clay, the loading of the sample was 8%of the CEC of clay.

With the inclusion of clay in the aqueous solution the peakposition of the Soret band was red-shifted to 471 nm and a weakhump was developed in the longer wavelength side at 485 nm. InLbL film with clay (Fig. 2b graph (ii)) a broad and intense band wasdeveloped in the 460–500 nm region. Deconvolution spectrashows that the broad band has two overlapping peaks at 477 nmand 491 nm. The remarkable changes in the Soret band of the UV–vis absorption spectra in aqueous solution and in the LbL film inpresence of clay may be assigned as due to the changes in theconformation of MnTMPyP molecules while adsorbing on thesurface of the nano clay platelets by electrostatic interaction.

It may be mentioned in this context that different authors havestudied the Soret band of different porphyrin derivatives bychanging various parameters which affected largely the microenvironment of the porphyrin molecules. As mentioned earlier,Takagi et. al. [42–44] reported that the spectral red-shift of theSoret band is due to an effect of conformation change owing to theflattening of porphyrin moieties upon adsorption on the surface ofthe inorganic nano sheets. Shi et. al. [50] reported the flattening ofTMPyP molecules upon adsorption on chemically converted gra-phene sheets (CCG) resulted in the red-shifting of the Soret band.Therefore it may be concluded that the shifting of the Soret bandto 471 nm in aqueous clay dispersion and 477 nm in LbL film maybe due to the flattening of the meso substituent groups of por-phyrin moieties while adsorbing onto the nano clay platelets.

However, the origin of the weak hump at 485 nm in aqueous

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135 131

clay dispersion and 491 nm band in monolayer LbL film can not beascribed as due to flattening of porphyrin moieties. It was reportedearlier by several authors that the longer wavelength band in the480–500 nm region in the UV–vis absorption spectra of porphyr-ins may be originated as due to the formation of J-aggregatesunder different conditions. Aggregation of porphyrins occurs de-pending on various factors namely strong host–guest interaction,strong guest–guest interaction in presence of host, changing pH, inpresence of ionic surfactants, in the core of micelles etc [39,51].Belfield et. al. reported [15] that a functional polymer could betemplated to construct a supramolecular assembly of porphyrinbased dye to facilitate J-aggregation and referred the origin ofJ-band at 490 nm as due to self assembly of obliquely stackedhydrophobic porphyrin rings. Periasamy et. al. [52] reported theformation of J-aggregates of anionic porphyrin derivative TPPSwith NH4Cl in aqueous solution at pH 3.0. Shi et. al. [51] in-vestigated the pH induced J-aggregates of TPPS in the core ofmicelles and reported the J-band at 490 nm at pH below 2.5. La-ponite clay induced J-aggregate formation in cyanine dyes hasbeen reported by Whitten et. al. [53]. Therefore it may be con-cluded that the 485 nm band in solution and 491 nm band in LbLfilm is the J-band arising due to the J-type aggregation of por-phyrin molecules while adsorbing on the nano clay platelets.

It may be mentioned in this context that the characteristicfeatures of J-band is the red-shifting with respect to the monomerband as well as also the sharp peak. However in the present casedue to overlapping of J-band with the high energy band, a broadband is observed in the 460–500 nm range. Deconvolution of thisbroad band gives two sharp bands with peaks at 471 nm and485 nm in aqueous clay dispersion and at 477 nm and 491 nm inLbL film. The sharp band at 485 nm (491 nm in LbL film) shows thecharacteristic features of the J-band.

Thus flattening of porphyrin moieties occurred upon adsorp-tion of MnTMPyP molecules onto the nano clay platelets and un-der certain conditions these flattened MnTMPyP molecules wereobliquely stacked to form J-aggregated sites. Intense J-band maybe obtained by changing various parameters.

Fig. 3. Normalised UV–vis absorption spectra of (a) MnTMPyP in aqueous clay dispersloading percentage varies from 80% to 8% of the CEC of clay.

3.1. Enhancement of J-band

3.1.1. By changing the sample loading percentage of the CEC of clayFig. 3a shows the normalised UV–vis absorption spectra of

MnTMPyP in aqueous clay dispersion at different sample loadingpercentage of the CEC of clay. The loading of MnTMPyP was variedfrom 80% to 8% of the CEC of clay. Concentration of clay was0.1 mg mL�1 in aqueous clay dispersion. From the figure it is ob-served that at 80% loading of MnTMPyP of the CEC of clay, the peakof the Soret band remained at 462 nm and almost coincided withthe peak of the Soret band of MnTMPyP in aqueous solutionwithout clay. However, a weak longer wavelength hump at485 nmwas developed. With decreasing loading percentage of thesample the longer wavelength band at 485 nm increased in in-tensity while 462 nm Soret peak was gradually red-shifted. At 8%loading of the sample, the Soret band was red-shifted to 471 nmand a strong hump was developed at 485 nm. As discussed earlierthe origin of this longer wavelength band at 485 nm was due topredominance of J-aggregated sites of MnTMPyP molecules ad-sorbed on the nano clay platelets.

Fig. 3b shows the normalised UV–vis absorption spectra ofmonolayer LbL film of MnTMPyP–clay hybrid molecules at differ-ent loading percentage of the sample with respect to the CEC ofclay. The loading of MnTMPyP was varied from 80% to 8% of theCEC of clay. From the figure it is observed that at 80% loading of thesample with respect to the CEC of clay, the Soret band remained at466 nm and coincided with the monolayer LbL film of MnTMPyPwithout clay. However, a longer wavelength hump was developedat 491 nm. With decreasing loading percentage of the sample, thelonger wavelength hump at 491 nm became gradually intense andthe 466 nm Soret peak was red-shifted. At 8% loading of thesample the Soret band was red-shifted to 477 nm and an intensepeak was developed at 491 nm. These two peaks were overlappedand a broad band was formed in the 460–500 nm range. Thelonger wavelength band with peak at 491 nm has been assigned asthe J-band originating due to the predominance of J-aggregatedsites in the LbL film.

MnTMPyP, a tetra cationic water soluble dye, got adsorbed ontothe anionic nano clay platelets through electrostatic interaction. Atlower sample loading, due to the availability of large number of

ion and (b) MnTMPyP–clay hybrid monolayer LbL film. In both the cases the dye

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135132

anionic nano clay platelets, all the four meso substituents ofMnTMPyP molecules were attached onto the clay platelets byelectrostatic interactions and thus the molecules became com-pletely flattened. These flattened MnTMPyP molecules taggednano clay platelets formed the organo-clay hybrid molecules. The477 nm peak was attributed to the flattening of the porphyrinmoieties owing to the adsorption onto the clay platelets. Furtherthese completely flattened MnTMPyP molecules tagged nano clayplatelets were obliquely stacked in the LbL film resulting in theformation of J-aggregates (491 nm band). This has been discussedschematically in Fig. 7. Thus, both flattening (477 nm band) andconsequent formation of longer wavelength J-band (491 nm) be-came prominent especially at lower sample loading. Thus the ag-gregation in this case was highly induced by the available clayplatelets.

At higher sample loading, the intense peak at 466 nm wasobserved which was at the same position as that observed in caseof the LbL film without clay. It might be that at higher sample

Fig. 4. (a) UV–vis absorption spectra of different layered LbL films of MnTMPyP moleculLbL films of MnTMPyP–clay hybrid molecules. The loading of the dye was (b) 80% and (absorbance values at 491 nm and 466 nm as a function of the number of deposited lay

loading, due to the non availability of sufficient number of nanoclay platelets and the presence of large number of MnTMPyPmolecules, flattening of the molecules was hindered to a largerextent. Accordingly, the 477 nm band arising due to flattening andthe 491 nm J-band reduced to weak humps at higher sampleloading.

3.1.2. By increasing the number of layers of the LbL filmsFig. 4 shows (a) UV–vis absorption spectra of different layered

LbL films of MnTMPyP molecules without clay, (b,c) the normal-ised UV–vis absorption spectra of different layered LbL films ofMnTMPyP–clay hybrid molecules. The loading of the sample was80% of the CEC of clay in Fig. 4b and 8% of the CEC of clay in Fig. 4c.In both the cases of Figs. 4b and c, inset of the figures show theratio of the absorbance values at 491 nm and 466 nm respectivelyas a function of the number of deposited layers.

With increasing number of layers, the UV–vis absorptionspectra of MnTMPyP LbL film without clay show an intense

es without clay, (b,c) the normalised UV–vis absorption spectra of different layeredc) 8% of the CEC of clay. The inset graphs (in both (b) and (c)) show the ratio of theers.

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135 133

466 nm Soret peak. No changes in peak position or development ofnew band were observed. This indicates that molecular aggregatepattern does not change with increasing layer number, in absenceof clay.

At 80% loading, in monolayer LbL film, Soret peak was observedat 466 nm along with a weak hump at 491 nm. With increasinglayer number 491 nm weak hump gradually increased in intensityand at higher layer number Soret peak reduced to a weak humpand 491 nm J-band became intense as shown in Fig. 4b. It might bethat with increasing layer number, greater number of MnTMPyPtagged clay platelets became available for preferred orientation toform increased J-aggregated sites resulting in the intense J-band at491 nm. Similarly at 8% loading, J-band became more intense withincreasing layer number as shown in Fig. 4c. In this case alsoavailability of large number of MnTMPyP tagged nano clay plate-lets with increasing layer number resulted in increased intensity ofthe J-band at 491 nm. The overlapping of different bands in boththe Figs. 4b and 4c is an indication of the existence of differentpattern of molecular association in LbL film. In both the cases, theinsets show that the ratio of the absorbance values tends to be-come almost constant after 15 layers.

3.1.3. Temperature effect on J-bandFig. 5 shows the normalised UV–vis absorption spectra of

monolayer LbL film of MnTMPyP–clay hybrid molecules, at dif-ferent temperatures. Fig. 5a shows the spectra during increase oftemperature and Fig. 5b shows the spectra during decrease oftemperature of the same film. The loading of the sample used was80% of the CEC of clay. The process of heating was carried oncontinuously from room temperature (25 °C) upto 100 °C. Withincreasing temperature the J-band at 491 nm band increasedgradually in intensity and at 100 °C it became most prominent. Thefilm was then slowly cooled in the reverse order in proper stepsuntil the room temperature was attained again. With decreasingtemperature, J-band again reduced to a weak hump. The increaseand decrease of the intensity of J-band was almost reversible withtemperature. Although the initial spectrum was reverted after awaiting time about twelve hour. The experiment was repeated fivetimes using the same film and identical result was obtained.

Fig. 5. Normalised UV–vis absorption spectra of MnTMPyP–clay hybrid monolayer LbL fi

of the CEC of clay.

It may be that at higher temperature due to thermal agitation,the sample tagged clay platelets might orient in a preferred di-rection and thus created a favourable condition for increasednumber of J-aggregated sites.

3.2. AFM study

The surface structure and morphology of the monolayer LbLfilms on the Si-wafer were studied by Atomic Force Microscope(AFM). Fig. 6 shows the AFM images of (A) monolayer LbL film ofMnTMPyP without clay, (B–D) MnTMPyP–clay hybrid monolayerLbL film at different scales. The loading of the dye was 8% of theCEC of clay laponite. The height profiles and the roughness ana-lyses for all scanned area are also provided. Corresponding valuesof average height and the RMS roughness of the scanned area aregiven in Table 1.

Large values of average height and RMS roughness are ob-served in monolayer MnTMPyP LbL film without clay. This is dueto the formation of large scale molecular aggregates in the LbL filmin absence of clay. In the restricted geometry of LbL film,MnTMPyP molecules came into closer proximity with each otherresulting in the formation of molecular aggregates. The broad-ening and red-shifting of the peak position of the Soret band in theUV–vis absorption spectrum of this film discussed in the previoussection also supported this.

In monolayer LbL film of MnTMPyP tagged nano clay platelets(Fig.6 (B–D)), the AFM images at different scales show denselypacked nano clay platelets. The surface coverage is also high (morethan 90%). The clay platelets are overlapped on each other andformed tilted organisation. The dye molecules adsorbed on theclay platelets are not distinguishable since the dimension of themolecules are beyond the resolution of the AFM system. Theaverage height and RMS roughness are much smaller (Table 1)than that of the monolayer LbL film of MnTMPyP without clay. Itconfirms that upon adsorption of MnTMPyP molecules onto nanoclay platelets, it became flattened reducing the average height ofthe film, moreover, adsorption onto nano clay platelets led to theincreased molecular organisation, preventing the formation oflarge scale molecular association. The tilted and overlapping

lm during the process of (a) heating and (b) cooling. The loading of the dye was 80%

Fig. 6. AFM images of (A) monolayer LbL film of MnTMPyP without clay, (B–D) MnTMPyP–clay hybrid monolayer LbL film at different scales. The loading of the dye was 8% ofthe CEC of clay.

Table 1The values of RMS roughness and average height of the scanned area of differentAFM images.

LbL film of Scan area(mm2)

RMS roughness(nm)

Average height(nm)

MnTMPyP monolayer with-out clay

10�10 2.8847 7.6775

MnTMPyP–clay hybridmonolayer at differentscales

6�6 1.2822 3.98482�2 1.2068 3.36961�1 0.9712 3.6382

MnTMPyPmolecule

Flattened MnTMPyPmolecule on Laponite

Multiple array offlattened molecules

Stacking of MnTMPyP tagged clay platelets in LbL film

Fig. 7. Schematic representation of (a) one MnTMPyP molecule before adsorptiononto nano clay platelet Laponite, (b) adsorption followed by flattening of oneMnTMPyP molecule onto laponite clay platelet, (c) multiple array of flattenedMnTMPyP molecules onto a clay sheet and (d) overlapping and association of anumber of MnTMPyP tagged clay platelets onto monolayer LbL film at high claydensity.

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135134

organisations of MnTMPyP tagged nano-clay platelets might resultin the preferred orientation of sample tagged nano clay plateletswith respect to each other enhancing the formation of J-ag-gregated sites in the LbL film. This has been shown schematicallyin Fig. 7 and discussed in the next section.

3.3. Schematic representation

Fig. 7a shows one MnTMPyP molecule with central Manganeseion and four meso substituents each oriented in arbitrary plane asreferred by several researchers [42–44]. Upon adsorption ontonano clay platelets, the four meso substituents of MnTMPyP arerestricted to orient themselves on the plane of the nano clay sheetresulting into a flattened molecule [42–44]. In Fig. 7b the adsorbedand flattened MnTMPyP molecule onto nano sheet has beenshown. Fig. 7c shows the schematic representation of a multiplearray of flattened molecules on the surface of laponite. Fig. 7d

shows the schematic representation of an overlapped and tiltedorganisation of MnTMPyP tagged nano clay platelets in themonolayer LbL film. This has been designed in conformity with theAFM images as shown in Fig. 6. The tilted organisation of clayplatelets led to the preferred orientation of MnTMPyP moleculesadsorbed onto clay platelets. This resulted in the increased num-ber of J-aggregated sites in the LbL film. Origin of J-band as shown

S. Banik et al. / Journal of Physics and Chemistry of Solids 87 (2015) 128–135 135

in UV–vis absorption spectra (Fig. 4) was due to the formation ofJ-aggregated sites of MnTMPyP tagged nano-clay platelet laponitein the LbL film. With increasing layer number, overlapping of largenumber of clay platelets occurred resulting in the intense J-band inthe UV–vis absorption spectra.

4. Conclusion

Our results showed that tetra cationic MnTMPyP moleculeswere adsorbed onto anionic nano clay platelets in aqueous claydispersion and consequently formed Layer by Layer (LbL) self as-sembled films. In the process of adsorption porphyrin moietiesbecame flattened onto clay surface as evidenced from the red-shifting of the Soret band. Under certain conditions longer wave-length J-band was developed as shown in the UV–vis absorptionspectra. With increasing layer number in the LbL film, intenseJ-band was observed. It indicates the predominance of J-ag-gregated sites with increasing layer number. By changing tem-perature, reversible J-band was observed. AFM studies clearlydemonstrated that MnTMPyP tagged clay platelets got overlappedand oriented in preferable associations for the formation of J-ag-gregated sites in the LbL film.

Acknowledgements

The author SB is grateful to UGC–NERO for financial support tocarry out this research work through MRP (Ref. no. F. 5-139/2012-13/MRP/NERO/541). The author JB is grateful to DST for financialsupport to carry out this research work through Women ScientistProject (Ref. no. SR/WOS-A/PS-47/2013). The author SAH is gratefulto DST, Govt. of India and CSIR, Govt. of India for financial supportto carry out this work through DST Fast-Track Project Ref. no. SE/FTP/PS-54/2007, CSIR project Ref. 03(1146)/09/EMR-II.

References

[1] P. Guo, G. Zhao, P. Chen, B. Lei, L. Jiang, H. Zhang, W. Hu, M. Liu, ACS Nano 8(2014) 3402–3411.

[2] B.E. Murphy, S.A. Krasnikov, N.N. Sergeeva, A.A. Cafolla, A.B. Preobrajenski, A.N. Chaika, O. Lubben, I.V. Shvets, ACS Nano 8 (2014) 5190–5198.

[3] A.D.F. Dunbar, S. Brittle, T.H. Richardson, J. Hutchinson, C.A. Hunter, J. Phys.Chem. B 114 (2010) 11697–11702.

[4] H. Masai, J. Terao, S. Seki, S. Nakashima, M. Kiguchi, K. Okoshi, T. Fujihara,Y. Tsuji, J. Am. Chem. Soc. 136 (2014) 1742–1745.

[5] D. Boer, T. Habets, M.J.J. Coenen, M. Maas, T.P.J. Peters, M.J. Crossley, T. Khoury,A.E. Rowan, R.J.M. Nolte, S. Speller, J.A.A.W. Elemans, Langmuir 27 (2011)2644–2651.

[6] D. Conklin, S. Nanayakkara, T. Park, M.F. Lagadec, J.T. Stecher, M.J. Therien, D.A. Bonnell, Nano Lett. 12 (2012) 2414–2419.

[7] Y. Ishida, D. Masui, H. Tachibana, H. Inoue, T. Shimada, S. Takagi, ACS Appl.Mater. Interfaces 4 (2012) 811–816.

[8] Y. Ishida, T. Shimada, D. Masui, H. Tachibana, H. Inoue, S. Takagi, J. Am. Chem.Soc. 133 (2011) 14280–14286.

[9] G.A.M. Sáfar, R.N. Gontijo, C. Fantini, D.C. Martins, Y.M. Idemori, M.V.B. Pinheiro, K. Krambrock, J. Phys. Chem. C. 119 (2015) 4344–4350.

[10] K. Hirakawa, H. Umemoto, R. Kikuchi, H. Yamaguchi, Y. Nishimura, T. Arai,

S. Okazaki, H. Segawa, Chem. Res. Toxicol. 28 (2015) 262–267.[11] S. Chakraborty, D. Bhattacharjee, H. Soda, M. Tominaga, Y. Suzuki, J. Kawamata,

S.A. Hussain, Appl. Clay Sci. 104 (2015) 245–251.[12] J. Bhattacharjee, S.A. Hussain, D. Bhattacharjee, Appl. Phys. A 116 (2014)

1669–1676.[13] S. Chakraborty, P. Debnath, D. Dey, D. Bhattacharjee, S.A. Hussain, J. Photo-

chem. Photobiol. A 293 (2014) 57–64.[14] M. Kasha, H.R. Rawls, M.A. El-Bayoumi, Pure Appl. Chem. 11 (1965) 371–392.[15] S. Biswas, H.Y. Ahn, M.V. Bondar, K.D. Belfield, Langmuir 28 (2012) 1515–1522.[16] Z. Liu, Y.F. Xu, X.Y. Zhang, X.L. Zhang, Y.S. Chen, J.G. Tian, J. Phys. Chem. B 113

(2009) 9681–9686.[17] E. Maligaspe, N.V. Tkachenko, N.K. Subbaiyan, R. Chitta, M.E. Zandler,

H. Lemmetyinen, F. D'Souza, J. Phys. Chem. A 113 (2009) 8478–8489.[18] Y. Arai, H. Segawa, J. Phys. Chem. B 115 (2011) 7773–7780.[19] J.V. Hollingsworth, A.J. Richard, M.G.H. Vicente, P.S. Russo, Biomacromolecules

13 (2012) 60–72.[20] K. Siskova, B. Vlckova, P. Mojzes, J. Mol. Struct. 744–747 (2005) 265–272.[21] X. Li, D. Li, W. Zeng, G. Zou, Z. Chen, J. Phys. Chem. B 111 (2007) 1502–1506.[22] P. Dutta, R. Rai, S. Pandey, J. Phys. Chem. B 115 (2011) 3578–3587.[23] M. Lee, B. Jin, H.M. Lee, M.J. Jung, S.K. Kim, J. Kim, Bull. Korean Chem. Soc. 29

(2008) 1533–1538.[24] G. Mező, L. Herényi, J. Habdas, Z. Majer, B.M. Kurdziel, K. Tóth, G. Csík, Biophys.

Chem. 155 (2011) 36–44.[25] C. Brunet, R. Antoine, J. Lemoine, P. Dugourd, J. Phys. Chem. Lett. 3 (2012)

698–702.[26] T. Hasobe, S. Fukuzumi, P.V. Kamat, J. Am. Chem. Soc. 127 (2005) 11884–11885.[27] S. Asayama, K. Mizushima, S. Nagaoka, H. Kawakami, Bioconjugate Chem. 15

(2004) 1360–1363.[28] S. Afzal, W.A. Daoud, S.J. Langford, A.C.S. Appl. Mater. Interfaces 5 (2013)

4753–4759.[29] M. Zawadzka, J. Wang, W.J. Blau, M.O. Senge, J. Phys. Chem. A 117 (2013)

15–26.[30] J.R. Sommer, R.T. Farley, K.R. Graham, Y. Yang, J.R. Reynolds, J. Xue, K.

S. Schanze, ACS Appl. Mater. Interfaces 2 (2009) 274–278.[31] M. Urbani, M. Gratzel, M.K. Nazeeruddin, T. Torres, Chem. Rev. 114 (2014)

12330–12396.[32] G. Bottari, O. Trukhina, M. Ince, T. Torres, Coord. Chem. Rev. 256 (2012)

2453–2477.[33] A.S.D. Sandanayaka, R. Chitta, N.K. Subbaiyan, L. D'Souza, O. Ito, F. D'Souza, J.

Phys. Chem. C 113 (2009) 13425–13432.[34] P. Ma, Y. Chen, Y. Bian, J. Jiang, Langmuir 26 (2010) 3678–3684.[35] M.A. Mezour, R. Cornut, E.M. Hussien, M. Morin, J. Mauzeroll, Langmuir 26

(2010) 13000–13006.[36] S. Takagi, M. Eguchi, D.A. Tryk, H. Inoue, Langmuir 22 (2006) 1406–1408.[37] I. Beletskaya, V.S. Tyurin, A.Y. Tsivadze, R. Guilard, C. Stern, Supramolecular

chemistry of metalloporphyrins, Chem. Rev. 109 (2009) 1659–1713.[38] N. Kanetada, C. Matsumura, S. Yamazaki, K. Adachi, ACS Appl. Mater. Interfaces

5 (2013) 12991–12999.[39] L.A. Lucia, T. Yui, R. Sasai, S. Takagi, K. Takagi, H. Yoshida, D.G. Whitten,

H. Inoue, J. Phys. Chem. B 107 (2003) 3789–3797.[40] O. Yaffe, E. Korin, A. Bettelheim, Langmuir 24 (2008) 11514–11517.[41] L.M. Scolaro, C. Donato, M. Castriciano, A. Romeoa, R. Romeo, Inorg. Chim. Acta

300–302 (2000) 978–986.[42] Y. Ishida, D. Masui, T. Shimada, H. Tachibana, H. Inoue, S. Takagi, J. Phys. Chem.

C 116 (2012) 7879–7885.[43] T. Egawa, H. Watanabe, T. Fujimura, Y. Ishida, M. Yamato, D. Masui, T. Shimada,

H. Tachibana, H. Yoshida, H. Inoue, S. Takagi, Langmuir 27 (2011)10722–10729.

[44] T. Tsukamoto, T. Shimada, S. Takagi, J. Phys. Chem. A 117 (2013) 7823–7832.[45] T. Shichi, K. Takagi, J. Photochem. Photobiol. C 1 (2000) 113–130.[46] K. Fujiwara, H. Monjushiro, H. Watarai, Chem. Phys. Lett. 394 (2004) 349–353.[47] M. Gouterman, G.H. Wagniere, J. Mol. Spectrosc. 11 (1962) 108–127.[48] X. Huang, K. Nakanishi, A. Berova, Chirality 12 (2000) 237–255.[49] H.L. Wang, Q. Sun, M. Chen, J. Miyake, D.J. Qian, Langmuir 27 (2011)

9880–9889.[50] Y. Xu, L. Zhao, H. Bai, W. Hong, C. Li, G. Shi, J. Am. Chem. Soc. 131 (2009)

13490–13497.[51] L. Zhao, R. Ma, J. Li, Y. Li, Y. An, L. Shi, Biomacromolecules 9 (2008) 2601–2608.[52] A.S.R. Koti, J. Taneja, N. Periasamy, Chem. Phys. Lett. 375 (2003) 171–176.[53] L. Lu, R.M. Jones, D. McBranch, D. Whitten, Langmuir 18 (2002) 7706–7713.