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Clay induced aggregation of a tetra-cationic metalloporphyrin in Layer by Layer self assembled film
Author’s Accepted Manuscript Clay induced aggregation of a tetra-cationic metalloporphyrin in layer by layer self assembled film Soma Banik, J. Bhattacharjee, S.A. Hussain, D. Bhattacharjee www.elsevier.com/locate/jpcs
PII: DOI: Reference:
S0022-3697(15)30039-1 http://dx.doi.org/10.1016/j.jpcs.2015.08.008 PCS7611
To appear in: Journal of Physical and Chemistry of Solids Received date: 20 March 2015 Revised date: 10 July 2015 Accepted date: 11 August 2015 Cite this article as: Soma Banik, J. Bhattacharjee, S.A. Hussain and D. Bhattacharjee, Clay induced aggregation of a tetra-cationic metalloporphyrin in layer by layer self assembled film, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2015.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Clay induced aggregation of a tetra-cationic metalloporphyrin in Layer by Layer self assembled film Soma Banikb, J. Bhattacharjeea, S.A. Hussaina, D. Bhattacharjeea* a
Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar 799022, Tripura, India b Department of Physics, Women’s College, Agartala 799001, Tripura, India
*Corresponding author. E-mail address: [email protected] Tel.: +91 9436130608: Fax: +91 381 2374802
Abstract: Porphyrins have a general tendency to form aggregates in ultrathin films. Also electrostatic adsorption of cationic porphyrins onto anionic nano clay platelets results in the flattening of porphyrin moieties. The flattening is evidenced by the red shifting of Soret band with respect to the aqueous solution. In the present communication, we have studied the clay induced aggregation behaviour of a tetra-cationic metalloporphyrin 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 molecules onto nano clay platelets resulted in the flattening of the meso substituent groups of the dye chromophore. In Layer-by-Layer ultrathin film, the flattened porphyrin molecules tagged nano clay platelets were further associated to form porphyrin aggregates. This has been clearly demonstrated from the UV-vis absorption spectroscopic studies. Atomic Force Microscopic (AFM) studies gave visual evidence of the association of organo-clay hybrid molecules in the LbL film.
Graphical Abstract
AFM image of monolayer LbL film
MnTMPyP in solution
MnTMPyP-clay In LbL film
Keywords: A. Surfaces; A. Thin films; A. Multilayers; D. Optical properties; D. Surface properties.
1. Introduction: Organized molecular assemblies of macro cyclic chromophores have gained a wide spread interest owing to their versatile applicability in sensing [1-3], molecular electronics [4-6], artificial light harvesting systems [7,8] and pharmacology [9,10]. Molecular assembiles in thin film often lead to molecular aggregation [11-13]. Molecular aggregation causes remarkable changes in the photophysical and electronic properties of the molecules. It is well known that Jaggregation (side-by-side) is characterised by strong, narrow, red shifted absorption bands (Jband), whereas H-aggregation (face-to-face) results in the blue shifted (H-band) band with respect to the monomeric absorption band [11-13]. According to Kasha et. al. [14] strong intermolecular interactions between monomeric species and delocalized excitonic energy over the whole assembly of aggregates are the characteristics of such phenomena. Due to their rigid planar geometry, high stability, intense electronic absorption and small HOMO-LUMO energy gap, porphyrins serve as a significant class of synthetic building blocks for functional nano materials [15]. Owing to their excellent photophysical, photochemical, electrochemical and structural properties, they have potential applications in the field of nonlinear optics [16] and for investigations of artificial light harvesting systems that mimic natural photosynthetic receptors [17]. Under various conditions porphyrins form J-aggregates due to hydrophobic π-π stacking and the electrostatic interactions between the anionic and cationic
groups. Aggregation of pophyrin molecules in aqueous solution can be tuned depending on the porphyrin structure and concentration, pH, ionic strength, counterions of inorganic salts in the medium [18-20]. Even the medium itself such as micells [21], ionic liquids [22], nueclic acids [23], polypeptides [24], proteins [25] and carbon nanotubes [26] can influence porphyrin aggregation behaviour. Metalloporphyrins are an important class of porphyrins as they can be used for mimicing many biological processes [27], catalysts in chemical and photochemical reactions [28], photosensitizers including the field of nonlinear optics [29], molecular electronic devices [30], solar energy cells [31], artificial photosynthesis [32], photoinduced hydrogen production [33] etc. In some of these applications porphyrin molecules were used in solid phase but in most of the cases they were required to be embedded into solid surfaces. Layered porphyrins on solid surfaces are suitable for various practical applications in heterogeneous catalytic reactions, recognition systems and in optoelectrical molecular devices [34-36]. In metalloporphyrins, in addition to intermolecular interaction and coordination binding, axial co-ordination via central metal ion plays an important role in the formation of organized multilayers as suggested by Beletskaya and co-workers [37]. Such multilayered supramolecular assemblies are used as models for biological processes and catalysts for many organic reactions. Adsorption and aggregation behaviour of cationic porphyrin derivatives on Tungsten (VI) oxide (WO3) nano colloid particles have been studied in aqueous colloidal dispersion [38]. Formation of J-aggregates strongly depend on the presence and difference of peripheral substituents. In aqueous dispersion, metalloporphyrin Sb(V)TSPP intercalated in polyfluorinated surfactant / clay hybrid microenvironment and formed H and J-type dimers only in the presence of penetrated benzene molecules [39]. The interaction of ionic porphyrin with ionic surfactant was studied using various ionic pophyrin derivatives and ionic surfactants in aqueous environment [40]. In several cases miceller aggregates led to the formation of J-type and H-type aggregates [41]. Takagi et. al. studied clay-porphyrin interactions in aqueous clay dispersion. They showed that accomodation of guest porphyrin molecules onto nano clay platelets led to the flattening of dye chromophores induced by strong host-guest interactions [42-44.]. Spectral red shift of about 10 nm – 30 nm of the Soret band position is the characteristic feature of the flattening of dye chromophore on the clay platelets [42]. In general, clay minerals act as
inorganic host materials for the adsorbed guest molecules and the adsorbed molecules have a tendency to aggregate on the nanosheets or in the interlayer space between clay sheets, thus strengthening the guest-guest interaction in presence of host [45]. It may be mentioned in this context that inorganic nano clay platelets have shown a great promise for the construction of hybrid organic/inorganic nanomaterials due to their unique material properties, colloidal size, layered structure and nano-scale platelet shaped dimension. Anionic nano clay platelets exhibit a high affinity for metachromic cationic dyes because the negative charges of the clay are compensated by the positive charges of cationic organic materials by cation exchange reaction. Cation exchange capacity (CEC) is an important property of clay minerals which depends on available surface area, crystal size, pH and the type of exchangeable cation. It is usually expressed in micro-equivalents per gram (µeq g-1 ). In the present work detailed investigations have been carried out to study the effect of nano clay platelets on the spectral characteristics of a tetra cationic metalloporphyrin (Manganese (III) 5, 10, 15, 20-tetra (4 pyridyl)-21 H, 23 H-porphine chloride tetrakis (methochloride)) abbreviated as MnTMPyP in Layer by Layer (LbL) self assembled films. Our results showed that in the LbL film MnTMPyP molecules were adsorbed on the clay platelets and consequently became flattened. This flattening of pophyrin moieties resulted in the red shifting of the Soret band. Also the flattened porphyrin molecules formed J-type aggregate resulting in the development of a new band in the longer wavelength region. Factors affecting the J-aggregate formation have been studied in detail by changing various parameters. J-aggregated sites of porphyrins 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 (methochloride) (MnTMPyP) (MW= 909.01), (purity 99 %), Poly (acrylic acid) (PAA) (purity 99 %) and Poly (allylamine hydrochloride) (PAH), (purity 99 %) were purchased from Aldrich Chemical Co.and used as received. The clay mineral used in the present work was Laponite, obtained from Laponite Inorganics, UK, and used as received. The size of the Laponite is less than 0.05 µm, and its CEC is 0.74 meq g-1.
Electrolytic deposition bath of cationic dye MnTMPyP was prepared 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 prepared with triple-distilled deionized Millipore water (0.5 mg mL-1). The quartz substrates used for film fabrication must be very clean to enable the formation of thin films. The first step was to clean the quartz slides with soap solution for removal of grease/ dirt. Then the slides were treated with chromic acid for 30 minutes and washed in de-ionized water. Further they were cleaned with acetone and stored in a dry oven. Thoroughly cleaned quartz slides were first dipped into anionic PAA solution for 15 min, dried and then rinsed with water to remove surplus anions attached to the surface. These slides were then dipped into cationic bath solution of MnTMPyP for 30 min followed by similar rinsing in a separate set of water bath to remove unadhered cations and thus resulted in one layer of MnTMPyP - PAA LbL film. The whole sequence of process was repeated to get desired number of layers of MnTMPyP - PAA LbL films. All the adsorption procedures were carried out at room temperature (250C). The incorporation of clay laponite into the LbL film was done with the help 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 mL1
laponite clay contains 0.74×10-3 mol L-1 of negative charges. In the mixed solution of
MnTMPyP in clay dispersion, 10 % of the CEC of clay means only 10 % charges of clay are sufficient to neutralize the charges of all the MnTMPyP molecules. Aqueous dispersion of clay was stirred for 24 h and then kept 30 min for sonication before using it. Aqueous MnTMPyPclay mixed dispersion with different loading (8%, 10%, 20%, 30%, 40%, 60% and 80 % of the CEC of clay) were then prepared. The quartz slide was dipped into the electrolytic polycation (PAH) solution for 15 min followed by same rinsing in water bath for 2 min followed by drying of the slide. The slide thus prepared was dipped into the mixed solution of MnTMPyP-clay dispersion. Sufficient time was given for deposition and drying of the slide. Thus, monolayer MnTMPyP-Clay-PAH hybrid film was fabricated. The repetition of the said procedure resulted into multilayered film fabrication. To study the temperature effect on the aggregation behaviour, the temperature of the fabricated monolayer MnTMPyP-clay-PAH LbL film was increased in a temperature controlled enclosure. The temperature of the monolayer was first increased from 250C to 1000C in steps and then decreased in exactly the reverse order. Spectroscopic characterizations of the LbL films and aqueous solution were done using 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 commercial AFM system (Bruker Innova). The experiment was performed at room temperature (20ºC). Relative humidity was kept under 60% (non-condensing). The AFM images presented here were obtained in intermittent contact (tapping) mode. The Si wafer substrates were 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 microcrystal film and (iii) monolayer LbL film. The absorption spectrum in aqueous solution consists of three distincts bands. It may be mentioned in this context that the highly delocalised planar π-framework in the core structure of porphyrin is responsible for the most fascinating features in the characteristic UV-vis absorption spectrum. The absorption bands of porphyrins are ascribed to the in-plane π–π* transitions [47]. Gourterman [47] successully explained the electronic absorption spectrum of porphyrins by applying the “four orbital’’ model with well discussion on the effect of charge localization on electronic spectroscopic properties. Out of these four orbitals two are HOMO’s and two are LUMO’s. Electronic transitions between these orbitals result into two excited states. Due to overlapping of orbitals these excited states are splited up resulting into two distinct transition bands - one with higher oscillator strength, higher energy called the Soret band or B band, the other with less oscillator strength, lower energy called the Q bands. In metalloporphyrins, the higher D4h symmetry of the porphyrin macrocycle upon co-ordination with metals collapses the Q-bands in the visible region responsible for red to purple colour. The Soret band position is sensitive to substituent groups. It is also highly sensitive to microenvironment. There are also additional bands (N, L, M bands) in the UV range, but these are usually quite weak [48]. In the UV-vis absorption spectrum of MnTMPyP aqueous solution, the additional bands (N, L, M bands) are observed in the 300 nm – 450 nm range. In 450 nm - 480 nm region intense Soret band (or B band) with sharp peak at 462 nm is observed [49]. This is the most sensitive band which is affected mostly due to the changing of micro-environment. Other is the Q band ranging from 525 nm - 625 nm having a weak peak at 562 nm. With changes in the micro environment small shifting in the peak position of the Q band is observed.
The UV-vis absorption spectra of thin cast microcrystal film as well as the LbL film of MnTMPyP show similar spectral profile having a red shifted Soret peak at 466 nm with respect to the 462 nm Soret peak in solution absorption spectrum. It may be mentioned in this context that being tetra cationic, MnTMPyP molecules do not form aggregates in aqueous solution due to predominance of repulsive force. However, MnTMPyP molecules became immobilized and came into closer contact with each other in the solid state either in the restricted geometry of LbL film or in thin cast microcrystal film. The closely associated molecules when obliquely stacked to form J-aggregates, spectral red shift is observed. Thus red shifting of the Soret band in LbL film and thin cast microcrystal film is an indication of closer molecular association leading to J- aggregates in the solid state. AFM image of monolayer LbL film discussed in the later section gives visual evidence of the formation of molecular aggregates. In the later section, it has been shown how flattening of porphyrin chormophore onto inorganic nano clay platelets resulted in the development of new J-band in the longer wave length region. Fig. 2 shows the effect of inclusion of clay in the UV-vis absorption spectra of MnTMPyP in aqueous solution and in the monolayer LbL film. In aqueous solution (Fig. 2a), graph (i) represents the UV-vis absorption spectra of MnTMPyP without clay and graph (ii) MnTMPyP with clay. Fig. 2b represents the monolayer LbL film of MnTMPyP, without clay (graph (i)) and with clay (graph (ii)). The dashed lines show the deconvolution of the respective spectra with clay. In both the cases of aqueous solution with 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 peak position of the Soret band was red shifted to 471 nm and a weak hump was developed in the longer wavelength side at 485 nm. In LbL film with clay (Fig. 2b graph (ii)) a broad and intense band was developed in the 460 nm - 500 nm region. Deconvolution spectra shows that the broad band has two overlapping peaks at 477 nm and 491 nm. The remarkable changes in the Soret band of the UV-vis absorption spectra in aqueous solution and in the LbL film in presence of clay may be assigned as due to the changes in the conformation of MnTMPyP molecules while adsorbing on the surface of the nano clay platelets by electrostatic interaction. It may be mentioned in this context that different authors have studied the Soret band of different porphyrin derivatives by changing various parameters which affected largely the micro
environment of the porphyrin molecules. As mentioned earlier, Takagi et. al. [42-44] reported that the spectral red shift of the Soret band is due to an effect of conformation change owing to the flattening of porphyrin moities upon adsorption on the surface of the inorganic nano sheets. Shi et. al. [50] reported the flattening of TMPyP molecules upon adsorption on chemically converted graphene sheets (CCG) resulted in the red shifting of the Soret band. Therefore it may be concluded that the shifting of the Soret band to 471 nm in aqueous clay dispersion and 477 nm in LbL film may be due to the flattening of the meso substituent groups of porphyrin moieties while adsorbing onto the nano clay platelets. However, the origin of the weak hump at 485 nm in aqueous clay dispersion and 491 nm band in monolayer LbL film can not be ascribed as due to flattening of porphyrin moieties. It was reported earlier by several authors that the longer wavelength band in the 480 nm - 500 nm region in the UV-vis absorption spectra of porphyrins may be originated as due to the formation of J-aggregates under different conditions. Aggregation of porphyrins occurs depending on various factors namely strong host-guest interaction, strong guest-guest interaction in presence of host, changing pH, in presence of ionic surfactants, in the core of micelles etc [39,51]. Belfield et. al. reported [15] that a functional polymer could be templeted to construct a supramolecular assembly of porphyrin based dye to facilitate J-aggregation and referred the origin of J-band at 490 nm as due to self assembly of obliquely stacked hydrophobic porphyrin rings. Periasamy et. al. [52] reported the formation of J-aggregates of anionic porphyrin derivative TPPS with NH4Cl in aqueous solution at pH 3.0. Shi et. al. [51] investigated the pH induced J-aggregates of TPPS in the core of micelles and reported the J-band at 490 nm at pH below 2.5. Laponite clay induced J-aggregate formation in cyanine dyes has been reported by Whitten et. al. [53]. Therefore it may be concluded that the 485 nm band in solution and 491 nm band in LbL film is the J-band arising due to the J-type aggregation of porphyrin molecules while adsorbing on the nano clay platelets. It may be mentioned in this context that the characteristic features of J-band is the red shifting with respect to the monomer band as well as also the sharp peak. However in the present case due to overlapping of J-band with the high energy band, a broad band is observed in the 460 nm - 500 nm range. Deconvolution of this broad band gives two sharp bands with peaks at 471 nm and 485 nm in aqueous clay dispersion and at 477 nm and 491 nm in LbL film. The sharp band at 485 nm (491 nm in LbL film) shows the characteristic features of the J-band.
Thus flattening of porphyrin moieties occurred upon adsorption of MnTMPyP molecules onto the nano clay platelets and under certain conditons these flattened MnTMPyP molecules were obliquely stacked to form J-aggregated sites. Intense J - band may be obtained by changing various parameters.
3.1. Enhancement of J - band: 3.1.1. By Changing the sample loading percentage of the CEC of clay: Fig. 3a shows the normalised UV-vis absorption spectra of MnTMPyP in aqueous clay dispersion at different sample loading percentage of the CEC of clay. The loading of MnTMPyP was varied from 80% to 8%.of the CEC of clay. Concentration of clay was 0.1 mg mL-1 in aqueous clay dispersion. From the Fig. it is observed that at 80% loading of MnTMPyP of the CEC of clay, the peak of the Soret band remained at 462 nm and almost coincided with the peak of the Soret band of MnTMPyP in aqueous solution without clay. However, a weak longer wavelength hump at 485 nm was developed. With decreasing loading percentage of the sample the longer wavelength band at 485 nm increased in intensity while 462 nm Soret peak was gradually red shifted. At 8% loading of the sample, the Soret band was red shifted to 471 nm and a strong hump was developed at 485 nm. As discussed earlier the origin of this longer wavelength band at 485 nm was due to predominence of J-aggregated sites of MnTMPyP molecules adsorbed on the nano clay plalets. Fig. 3b shows the normalised UV-vis absorption spectra of monolayer LbL film of MnTMPyP - clay hybrid molecules at different loading percentage of the sample with respect to the CEC of clay. The loading of MnTMPyP was varied from 80% to 8% of the CEC of clay. From the Fig. it is observed that at 80% loading of the sample with respect to the CEC of clay, the Soret band remained at 466 nm and coincided with the monolayer LbL film of MnTMPyP without clay. However, a longer wavelength hump was developed at 491 nm. With decreasing loading percentage of the sample, the longer wavelength hump at 491 nm became gradually intense and the 466 nm Soret peak was red shifted. At 8% loading of the sample the Soret band was red shifted to 477 nm and an intense peak was developed at 491 nm. These two peaks were overlapped and a broad band was formed in the 460 nm - 500 nm range. The longer wavelength band with peak at 491 nm has been assigned as the J-band originating due to the predominance of J-aggregated sites in the LbL film.
MnTMPyP, a tetra cationic water soluble dye, got adsorbed onto the anionic nano clay platelets through electrostatic interaction. At lower sample loading, due to the availability of large number of anionic nano clay platelets, all the four meso substitutents of MnTMPyP molecules were attached onto the clay platelets by electrostatic interactions and thus the molecules became completely flattened. These flattened MnTMPyP molecules tagged nano clay platelets formed the organo-clay hybrid molecules. The 477 nm peak was attributed to the flattening of the porphyrin moieties owing to the adsorption onto the clay platelets. Further these completely flattened MnTMPyP molecules tagged nano clay platelets were obliquely stacked in the LbL film resulting in the formation of J-aggregates (491 nm band). This has been discussed schematically in Fig. 7. Thus, both flattening (477 nm band) and consequent formation of longer wavelength J-band (491 nm) became prominent especially at lower sample loading. Thus the aggregation in this case was highly induced by the available clay platelets. At higher sample loading, the intense peak at 466 nm was observed which was at the same position as that observed in case of the LbL film without clay. It might be that at higher sample loading, due to the non availability of sufficient number of nano clay platelets and the presence of large number of MnTMPyP molecules, flattening of the molecules was hindered to a larger extent. Accordingly, the 477 nm band arising due to flattening and the 491 nm J-band reduced to weak humps at higher sample loading.
3.1.2. By increasing the number of layers of the LbL films: Fig. 4 shows (a) UV-vis absorption spectra of different layered LbL films of MnTMPyP molecules without clay, (b-c) the normalised UV-vis absorption spectra of different layered LbL films of MnTMPyP - clay hybrid molecules. The loading of the sample was 80% of the CEC of clay in Fig. 4b and 8% of the CEC of clay in Fig. 4c. In both the cases of Fig. 4b and 4c, inset of the figures show the ratio of the absorbance values at 491 nm and 466 nm respectively as a function of the number of deposited layers. With increasing number of layers, the UV-vis absorption spectra of MnTMPyP LbL film without clay show an intense 466 nm Soret peak. No changes in peak position or development of new band were observed. This indicates that molecular aggregate pattern does not change with increasing layer number, in absence of clay.
At 80% loading, in monolayer LbL film, Soret peak was observed at 466 nm along with a weak hump at 491 nm. With increasing layer number 491 nm weak hump gradually increased in intensity and at higher layer number Soret peak reduced to a weak hump and 491 nm J-band became intense as shown in Fig. 4b. It might be that with increasing layer number, greater number of MnTMPyP tagged clay platelets became available for preffered orientation to form increased J-aggregated sites resulting in the intense J-band at 491 nm. Similarly at 8% loading, Jband became more intense with increasing layer number as shown in Fig. 4c. In this case also availability of large number of MnTMPyP tagged nano clay platelets with increasing layer number resulted in increased intensity of the J-band at 491 nm. The overlapping of different bands in both the Fig. 4b and 4c is an indication of the existence of different pattern of molecular association in LbL film. In both the cases, the insets show that the ratio of the absorbance values tends to become almost constant after 15 layers.
3.1.3. Temperature effect on J-Band Fig. 5 shows the normalised UV-vis absorption spectra of monolayer LbL film of MnTMPyP - clay hybrid molecules, at different temperatures. Fig. 5a shows the spectra during increase of temperature and Fig. 5b shows the spectra during decrease of temperature of the same film. The loading of the sample used was 80% of the CEC of clay. The process of heating was carried on continuously from room temperature (250C) upto 1000C. With increasing temperature the J-band at 491 nm band increased gradually in intensity and at 1000C it became most prominent. The film was then slowly cooled in the reverse order in proper steps until the room temperature was attained again. With decreasing temperature, J-band again reduced to a weak hump. The increase and decrease of the intensity of J-band was almost reversible with temperature. Although the initial spectrum was reverted after a waiting time about twelve hour. The experiment was repeated five times using the same film and identical result was obtained. It may be that at higher temperature due to thermal agitation, the sample tagged clay plaelets might orient in a preferred direction and thus created a favourable condition for increased number of J-aggregated sites.
3.2. AFM study:
The surface structure and morphology of the monolayer LbL films on the Siwafer were studied by Atomic Force Microscope (AFM). Fig. 6 shows the 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% of the CEC of clay laponite. The height profiles and the roughness analyses for all scanned area are also provided. Corresponding values of average height and the RMS roughness of the scanned area are given in Table 1. Large values of average height and RMS roughness are observed in monolayer MnTMPyP LbL film without clay. This is due to the formation of large scale molecular aggregates in the LbL film in absence of clay. In the restricted geometry of LbL film, MnTMPyP molecules came into closer proximity with each other resulting in the formation of molecular aggregates. The broadening and red shifting of the peak position of the Soret band in the UV-vis absorption spectrum of this film discussed in the previous section also supported this. In monolayer LbL film of MnTMPyP tagged nano clay platelets (Fig.6 (B-D)), the AFM images at different scales show densely packed nano clay platelets. The surface coverage is also high (more than 90%). The clay platelets are overlapped on each other and formed tilted orgnisation. The dye molecules adsorbed on the clay platelets are not distinguishable since the dimension of the molecules are beyond the resolution of the AFM system. The average height and RMS roughness are much smaller (Table 1) than that of the monolayer LbL film of MnTMPyP without clay. It confirms that upon adsorption of MnTMPyP molecules onto nano clay platelets, it became flattened reducing the average height of the film, moreover, adsorption onto nano clay platelets led to the increased molecular organization, preventing the formation of large scale molecular association. The tilted and overlapping organisations of MnTMPyP tagged nano-clay platelets might result in the preferred orientation of sample tagged nano clay platelets with respect to each other enhancing the formation of J-aggregated sites in the LbL film. This has been shown schematically in Fig. 7 and discussed in the next section. 3.3. Schematic representation: Fig. 7a shows one MnTMPyP molecule with central Manganese ion and four meso substituents each oriented in arbitrary plane as referred by several researchers [42-44]. Upon adsorption onto nano clay platelets, the four meso substituents of MnTMPyP are restricted to orient themselves on the plane of the nano clay sheet resulting into a flattened molecule [42-44].
In Fig. 7b the adsorbed and flattened MnTMPyP molecule onto nano sheet has been shown. Fig. 7c shows the schematic representation of a multiple array of flattened molecules on the surface of laponite. Fig. 7d shows the schematic representation of an overlapped and tilted organisation of MnTMPyP tagged nano clay platelets in the monolayer LbL film. This has been designed in conformity with the AFM images as shown in Fig. 6. The tilted organisation of clay platelets led to the preferred orientation of MnTMPyP molecules adsorbed onto clay platelets. This resulted in the increased number of J-aggregated sites in the LbL film. Origin of J-band as shown in UV-vis absorption spectra (Fig. 4) was due to the formation of J-aggregated sites of MnTMPyP tagged nano-clay platelet laponite in the LbL film. With increasing layer number, overlapping of large number of clay platelets occured resulting in the intense J-band in the UV-vis absorption spectra.
4. Conclusion: Our results showed that tetra cationic MnTMPyP molecules were adsorbed onto anionic nano clay platelets in aqueous clay dispersion and consequently formed Layer by Layer (LbL) self assembled films. In the process of adsorption porphyrin moieties became flattened onto clay surface as evidenced from the red shifting of the Soret band. Under certain conditions longer wavelength J-band was developed as shown in the UV-vis absorption spectra. With increasing layer number in the LbL film, intense J-band was observed. It indicates the predominance of Jaggregated sites with increasing layer number. By changing temperature, reversible J-band was observed. AFM studies clearly demonstrated that MnTMPyP tagged clay platelets got overlapped and oriented in preferrable associations for the formation of J-aggregated sites in the LbL film.
Acknowledgements The author SB is grateful to UGC- NERO for financial support to carry out this research work through MRP (Ref. No. F. 5-139/2012-13/MRP/NERO/541). The author JB is grateful to DST for financial support to carry out this research work through Women Scientist Project (Ref. No. SR/WOS-A/PS-47/2013). The author SAH is grateful to DST and CSIR for financial support
to carry out this work through DST Fast-Track Project Ref. No. SE/FTP/PS-54/2007, CSIR project Ref. 03(1146)/09/EMR-II.
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Figure caption: Fig. 1: Normalized 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) monolayer LbL film of MnTMPyP without clay.
Fig. 2: (a) Normalised UV-vis absorption spectra of MnTMPyP in (i) pure aqueous solution (2×10-5 mol L-1) , (ii) aqueous solution (2×10-5 mol L-1) at dye loading of 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.
Fig. 3: Normalised UV-vis absorption spectra of (a) MnTMPyP in aqueous clay dispersion and (b) MnTMPyP-clay hybrid monolayer LbL film. In both the cases the dye loading percentage varies from 80% to 8% of the CEC of clay.
Fig. 4: (a) UV-vis absorption spectra of different layered LbL films of MnTMPyP molecules without clay, (b-c) the normalised UV-vis absorption spectra of different layered LbL films of MnTMPyP - clay hybrid molecules. The loading of the dye was (b) 80% and (c) 8% of the CEC
of clay. The inset graphs (in both (b) and (c)) show the ratio of the absorbance values at 491 nm and 466 nm as a function of the number of deposited layers.
Fig. 5: Normalised UV-vis absorption spectra of MnTMPyP-clay hybrid monolayer LbL film during the process of (a) heating and (b) cooling. The loading of the dye was 80% of the CEC of clay.
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% of the CEC of clay. Fig. 7: Schematic representation of (a) one MnTMPyP molecule before adsorption onto nano clay platelet Laponite, (b) adsorption followed by flattening of one MnTMPyP molecule onto laponite clay platelet, (c) multiple array of flattened MnTMPyP molecules onto a clay sheet, (d) overlapping and association of a number of MnTMPyP tagged clay platelets onto monolayer LbL film at high clay density. Table caption: Table 1: The values of RMS roughness and average height of the scanned area of different AFM images.
Figures:
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
A
D B
C
B
Fig. 6
Table 1 Scan area RMS Roughness Average Height (µm2) (nm) (nm) MnTMPyP monolayer without clay 2.8847 7.6775 10×10 LbL film of
MnTMPyP - clay hybrid monolayer at different scales
6×6
1.2822
3.9848
2×2
1.2068
3.3696
1×1
0.9712
3.6382
(a) MnTMPyP molecule
(b) Flattened MnTMPyP molecule on Laponite
( c) Multiple array of flattened molecules
(d) Stacking of MnTMPyP tagged clay platelets in LbL film
Fig. 7
Highlights 1. Interaction of tetra cationic metalloporphyrin MnTMPyP with anionic nano clay platelets laponite. 2. Flattening of MnTMPyP molecules upon adsorption onto nano clay platelets. 3. Overlapping and tilted organization of MnTMPyP tagged nano clay platelets onto LbL film. 4. Formation of J-aggregated sites in LbL film due to preferred organization of MnTMPyP tagged nano clay platelets.
PII: DOI: Reference:
S0022-3697(15)30039-1 http://dx.doi.org/10.1016/j.jpcs.2015.08.008 PCS7611
To appear in: Journal of Physical and Chemistry of Solids Received date: 20 March 2015 Revised date: 10 July 2015 Accepted date: 11 August 2015 Cite this article as: Soma Banik, J. Bhattacharjee, S.A. Hussain and D. Bhattacharjee, Clay induced aggregation of a tetra-cationic metalloporphyrin in layer by layer self assembled film, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2015.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Clay induced aggregation of a tetra-cationic metalloporphyrin in Layer by Layer self assembled film Soma Banikb, J. Bhattacharjeea, S.A. Hussaina, D. Bhattacharjeea* a
Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar 799022, Tripura, India b Department of Physics, Women’s College, Agartala 799001, Tripura, India
*Corresponding author. E-mail address: [email protected] Tel.: +91 9436130608: Fax: +91 381 2374802
Abstract: Porphyrins have a general tendency to form aggregates in ultrathin films. Also electrostatic adsorption of cationic porphyrins onto anionic nano clay platelets results in the flattening of porphyrin moieties. The flattening is evidenced by the red shifting of Soret band with respect to the aqueous solution. In the present communication, we have studied the clay induced aggregation behaviour of a tetra-cationic metalloporphyrin 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 molecules onto nano clay platelets resulted in the flattening of the meso substituent groups of the dye chromophore. In Layer-by-Layer ultrathin film, the flattened porphyrin molecules tagged nano clay platelets were further associated to form porphyrin aggregates. This has been clearly demonstrated from the UV-vis absorption spectroscopic studies. Atomic Force Microscopic (AFM) studies gave visual evidence of the association of organo-clay hybrid molecules in the LbL film.
Graphical Abstract
AFM image of monolayer LbL film
MnTMPyP in solution
MnTMPyP-clay In LbL film
Keywords: A. Surfaces; A. Thin films; A. Multilayers; D. Optical properties; D. Surface properties.
1. Introduction: Organized molecular assemblies of macro cyclic chromophores have gained a wide spread interest owing to their versatile applicability in sensing [1-3], molecular electronics [4-6], artificial light harvesting systems [7,8] and pharmacology [9,10]. Molecular assembiles in thin film often lead to molecular aggregation [11-13]. Molecular aggregation causes remarkable changes in the photophysical and electronic properties of the molecules. It is well known that Jaggregation (side-by-side) is characterised by strong, narrow, red shifted absorption bands (Jband), whereas H-aggregation (face-to-face) results in the blue shifted (H-band) band with respect to the monomeric absorption band [11-13]. According to Kasha et. al. [14] strong intermolecular interactions between monomeric species and delocalized excitonic energy over the whole assembly of aggregates are the characteristics of such phenomena. Due to their rigid planar geometry, high stability, intense electronic absorption and small HOMO-LUMO energy gap, porphyrins serve as a significant class of synthetic building blocks for functional nano materials [15]. Owing to their excellent photophysical, photochemical, electrochemical and structural properties, they have potential applications in the field of nonlinear optics [16] and for investigations of artificial light harvesting systems that mimic natural photosynthetic receptors [17]. Under various conditions porphyrins form J-aggregates due to hydrophobic π-π stacking and the electrostatic interactions between the anionic and cationic
groups. Aggregation of pophyrin molecules in aqueous solution can be tuned depending on the porphyrin structure and concentration, pH, ionic strength, counterions of inorganic salts in the medium [18-20]. Even the medium itself such as micells [21], ionic liquids [22], nueclic acids [23], polypeptides [24], proteins [25] and carbon nanotubes [26] can influence porphyrin aggregation behaviour. Metalloporphyrins are an important class of porphyrins as they can be used for mimicing many biological processes [27], catalysts in chemical and photochemical reactions [28], photosensitizers including the field of nonlinear optics [29], molecular electronic devices [30], solar energy cells [31], artificial photosynthesis [32], photoinduced hydrogen production [33] etc. In some of these applications porphyrin molecules were used in solid phase but in most of the cases they were required to be embedded into solid surfaces. Layered porphyrins on solid surfaces are suitable for various practical applications in heterogeneous catalytic reactions, recognition systems and in optoelectrical molecular devices [34-36]. In metalloporphyrins, in addition to intermolecular interaction and coordination binding, axial co-ordination via central metal ion plays an important role in the formation of organized multilayers as suggested by Beletskaya and co-workers [37]. Such multilayered supramolecular assemblies are used as models for biological processes and catalysts for many organic reactions. Adsorption and aggregation behaviour of cationic porphyrin derivatives on Tungsten (VI) oxide (WO3) nano colloid particles have been studied in aqueous colloidal dispersion [38]. Formation of J-aggregates strongly depend on the presence and difference of peripheral substituents. In aqueous dispersion, metalloporphyrin Sb(V)TSPP intercalated in polyfluorinated surfactant / clay hybrid microenvironment and formed H and J-type dimers only in the presence of penetrated benzene molecules [39]. The interaction of ionic porphyrin with ionic surfactant was studied using various ionic pophyrin derivatives and ionic surfactants in aqueous environment [40]. In several cases miceller aggregates led to the formation of J-type and H-type aggregates [41]. Takagi et. al. studied clay-porphyrin interactions in aqueous clay dispersion. They showed that accomodation of guest porphyrin molecules onto nano clay platelets led to the flattening of dye chromophores induced by strong host-guest interactions [42-44.]. Spectral red shift of about 10 nm – 30 nm of the Soret band position is the characteristic feature of the flattening of dye chromophore on the clay platelets [42]. In general, clay minerals act as
inorganic host materials for the adsorbed guest molecules and the adsorbed molecules have a tendency to aggregate on the nanosheets or in the interlayer space between clay sheets, thus strengthening the guest-guest interaction in presence of host [45]. It may be mentioned in this context that inorganic nano clay platelets have shown a great promise for the construction of hybrid organic/inorganic nanomaterials due to their unique material properties, colloidal size, layered structure and nano-scale platelet shaped dimension. Anionic nano clay platelets exhibit a high affinity for metachromic cationic dyes because the negative charges of the clay are compensated by the positive charges of cationic organic materials by cation exchange reaction. Cation exchange capacity (CEC) is an important property of clay minerals which depends on available surface area, crystal size, pH and the type of exchangeable cation. It is usually expressed in micro-equivalents per gram (µeq g-1 ). In the present work detailed investigations have been carried out to study the effect of nano clay platelets on the spectral characteristics of a tetra cationic metalloporphyrin (Manganese (III) 5, 10, 15, 20-tetra (4 pyridyl)-21 H, 23 H-porphine chloride tetrakis (methochloride)) abbreviated as MnTMPyP in Layer by Layer (LbL) self assembled films. Our results showed that in the LbL film MnTMPyP molecules were adsorbed on the clay platelets and consequently became flattened. This flattening of pophyrin moieties resulted in the red shifting of the Soret band. Also the flattened porphyrin molecules formed J-type aggregate resulting in the development of a new band in the longer wavelength region. Factors affecting the J-aggregate formation have been studied in detail by changing various parameters. J-aggregated sites of porphyrins 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 (methochloride) (MnTMPyP) (MW= 909.01), (purity 99 %), Poly (acrylic acid) (PAA) (purity 99 %) and Poly (allylamine hydrochloride) (PAH), (purity 99 %) were purchased from Aldrich Chemical Co.and used as received. The clay mineral used in the present work was Laponite, obtained from Laponite Inorganics, UK, and used as received. The size of the Laponite is less than 0.05 µm, and its CEC is 0.74 meq g-1.
Electrolytic deposition bath of cationic dye MnTMPyP was prepared 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 prepared with triple-distilled deionized Millipore water (0.5 mg mL-1). The quartz substrates used for film fabrication must be very clean to enable the formation of thin films. The first step was to clean the quartz slides with soap solution for removal of grease/ dirt. Then the slides were treated with chromic acid for 30 minutes and washed in de-ionized water. Further they were cleaned with acetone and stored in a dry oven. Thoroughly cleaned quartz slides were first dipped into anionic PAA solution for 15 min, dried and then rinsed with water to remove surplus anions attached to the surface. These slides were then dipped into cationic bath solution of MnTMPyP for 30 min followed by similar rinsing in a separate set of water bath to remove unadhered cations and thus resulted in one layer of MnTMPyP - PAA LbL film. The whole sequence of process was repeated to get desired number of layers of MnTMPyP - PAA LbL films. All the adsorption procedures were carried out at room temperature (250C). The incorporation of clay laponite into the LbL film was done with the help 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 mL1
laponite clay contains 0.74×10-3 mol L-1 of negative charges. In the mixed solution of
MnTMPyP in clay dispersion, 10 % of the CEC of clay means only 10 % charges of clay are sufficient to neutralize the charges of all the MnTMPyP molecules. Aqueous dispersion of clay was stirred for 24 h and then kept 30 min for sonication before using it. Aqueous MnTMPyPclay mixed dispersion with different loading (8%, 10%, 20%, 30%, 40%, 60% and 80 % of the CEC of clay) were then prepared. The quartz slide was dipped into the electrolytic polycation (PAH) solution for 15 min followed by same rinsing in water bath for 2 min followed by drying of the slide. The slide thus prepared was dipped into the mixed solution of MnTMPyP-clay dispersion. Sufficient time was given for deposition and drying of the slide. Thus, monolayer MnTMPyP-Clay-PAH hybrid film was fabricated. The repetition of the said procedure resulted into multilayered film fabrication. To study the temperature effect on the aggregation behaviour, the temperature of the fabricated monolayer MnTMPyP-clay-PAH LbL film was increased in a temperature controlled enclosure. The temperature of the monolayer was first increased from 250C to 1000C in steps and then decreased in exactly the reverse order. Spectroscopic characterizations of the LbL films and aqueous solution were done using 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 commercial AFM system (Bruker Innova). The experiment was performed at room temperature (20ºC). Relative humidity was kept under 60% (non-condensing). The AFM images presented here were obtained in intermittent contact (tapping) mode. The Si wafer substrates were 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 microcrystal film and (iii) monolayer LbL film. The absorption spectrum in aqueous solution consists of three distincts bands. It may be mentioned in this context that the highly delocalised planar π-framework in the core structure of porphyrin is responsible for the most fascinating features in the characteristic UV-vis absorption spectrum. The absorption bands of porphyrins are ascribed to the in-plane π–π* transitions [47]. Gourterman [47] successully explained the electronic absorption spectrum of porphyrins by applying the “four orbital’’ model with well discussion on the effect of charge localization on electronic spectroscopic properties. Out of these four orbitals two are HOMO’s and two are LUMO’s. Electronic transitions between these orbitals result into two excited states. Due to overlapping of orbitals these excited states are splited up resulting into two distinct transition bands - one with higher oscillator strength, higher energy called the Soret band or B band, the other with less oscillator strength, lower energy called the Q bands. In metalloporphyrins, the higher D4h symmetry of the porphyrin macrocycle upon co-ordination with metals collapses the Q-bands in the visible region responsible for red to purple colour. The Soret band position is sensitive to substituent groups. It is also highly sensitive to microenvironment. There are also additional bands (N, L, M bands) in the UV range, but these are usually quite weak [48]. In the UV-vis absorption spectrum of MnTMPyP aqueous solution, the additional bands (N, L, M bands) are observed in the 300 nm – 450 nm range. In 450 nm - 480 nm region intense Soret band (or B band) with sharp peak at 462 nm is observed [49]. This is the most sensitive band which is affected mostly due to the changing of micro-environment. Other is the Q band ranging from 525 nm - 625 nm having a weak peak at 562 nm. With changes in the micro environment small shifting in the peak position of the Q band is observed.
The UV-vis absorption spectra of thin cast microcrystal film as well as the LbL film of MnTMPyP show similar spectral profile having a red shifted Soret peak at 466 nm with respect to the 462 nm Soret peak in solution absorption spectrum. It may be mentioned in this context that being tetra cationic, MnTMPyP molecules do not form aggregates in aqueous solution due to predominance of repulsive force. However, MnTMPyP molecules became immobilized and came into closer contact with each other in the solid state either in the restricted geometry of LbL film or in thin cast microcrystal film. The closely associated molecules when obliquely stacked to form J-aggregates, spectral red shift is observed. Thus red shifting of the Soret band in LbL film and thin cast microcrystal film is an indication of closer molecular association leading to J- aggregates in the solid state. AFM image of monolayer LbL film discussed in the later section gives visual evidence of the formation of molecular aggregates. In the later section, it has been shown how flattening of porphyrin chormophore onto inorganic nano clay platelets resulted in the development of new J-band in the longer wave length region. Fig. 2 shows the effect of inclusion of clay in the UV-vis absorption spectra of MnTMPyP in aqueous solution and in the monolayer LbL film. In aqueous solution (Fig. 2a), graph (i) represents the UV-vis absorption spectra of MnTMPyP without clay and graph (ii) MnTMPyP with clay. Fig. 2b represents the monolayer LbL film of MnTMPyP, without clay (graph (i)) and with clay (graph (ii)). The dashed lines show the deconvolution of the respective spectra with clay. In both the cases of aqueous solution with 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 peak position of the Soret band was red shifted to 471 nm and a weak hump was developed in the longer wavelength side at 485 nm. In LbL film with clay (Fig. 2b graph (ii)) a broad and intense band was developed in the 460 nm - 500 nm region. Deconvolution spectra shows that the broad band has two overlapping peaks at 477 nm and 491 nm. The remarkable changes in the Soret band of the UV-vis absorption spectra in aqueous solution and in the LbL film in presence of clay may be assigned as due to the changes in the conformation of MnTMPyP molecules while adsorbing on the surface of the nano clay platelets by electrostatic interaction. It may be mentioned in this context that different authors have studied the Soret band of different porphyrin derivatives by changing various parameters which affected largely the micro
environment of the porphyrin molecules. As mentioned earlier, Takagi et. al. [42-44] reported that the spectral red shift of the Soret band is due to an effect of conformation change owing to the flattening of porphyrin moities upon adsorption on the surface of the inorganic nano sheets. Shi et. al. [50] reported the flattening of TMPyP molecules upon adsorption on chemically converted graphene sheets (CCG) resulted in the red shifting of the Soret band. Therefore it may be concluded that the shifting of the Soret band to 471 nm in aqueous clay dispersion and 477 nm in LbL film may be due to the flattening of the meso substituent groups of porphyrin moieties while adsorbing onto the nano clay platelets. However, the origin of the weak hump at 485 nm in aqueous clay dispersion and 491 nm band in monolayer LbL film can not be ascribed as due to flattening of porphyrin moieties. It was reported earlier by several authors that the longer wavelength band in the 480 nm - 500 nm region in the UV-vis absorption spectra of porphyrins may be originated as due to the formation of J-aggregates under different conditions. Aggregation of porphyrins occurs depending on various factors namely strong host-guest interaction, strong guest-guest interaction in presence of host, changing pH, in presence of ionic surfactants, in the core of micelles etc [39,51]. Belfield et. al. reported [15] that a functional polymer could be templeted to construct a supramolecular assembly of porphyrin based dye to facilitate J-aggregation and referred the origin of J-band at 490 nm as due to self assembly of obliquely stacked hydrophobic porphyrin rings. Periasamy et. al. [52] reported the formation of J-aggregates of anionic porphyrin derivative TPPS with NH4Cl in aqueous solution at pH 3.0. Shi et. al. [51] investigated the pH induced J-aggregates of TPPS in the core of micelles and reported the J-band at 490 nm at pH below 2.5. Laponite clay induced J-aggregate formation in cyanine dyes has been reported by Whitten et. al. [53]. Therefore it may be concluded that the 485 nm band in solution and 491 nm band in LbL film is the J-band arising due to the J-type aggregation of porphyrin molecules while adsorbing on the nano clay platelets. It may be mentioned in this context that the characteristic features of J-band is the red shifting with respect to the monomer band as well as also the sharp peak. However in the present case due to overlapping of J-band with the high energy band, a broad band is observed in the 460 nm - 500 nm range. Deconvolution of this broad band gives two sharp bands with peaks at 471 nm and 485 nm in aqueous clay dispersion and at 477 nm and 491 nm in LbL film. The sharp band at 485 nm (491 nm in LbL film) shows the characteristic features of the J-band.
Thus flattening of porphyrin moieties occurred upon adsorption of MnTMPyP molecules onto the nano clay platelets and under certain conditons these flattened MnTMPyP molecules were obliquely stacked to form J-aggregated sites. Intense J - band may be obtained by changing various parameters.
3.1. Enhancement of J - band: 3.1.1. By Changing the sample loading percentage of the CEC of clay: Fig. 3a shows the normalised UV-vis absorption spectra of MnTMPyP in aqueous clay dispersion at different sample loading percentage of the CEC of clay. The loading of MnTMPyP was varied from 80% to 8%.of the CEC of clay. Concentration of clay was 0.1 mg mL-1 in aqueous clay dispersion. From the Fig. it is observed that at 80% loading of MnTMPyP of the CEC of clay, the peak of the Soret band remained at 462 nm and almost coincided with the peak of the Soret band of MnTMPyP in aqueous solution without clay. However, a weak longer wavelength hump at 485 nm was developed. With decreasing loading percentage of the sample the longer wavelength band at 485 nm increased in intensity while 462 nm Soret peak was gradually red shifted. At 8% loading of the sample, the Soret band was red shifted to 471 nm and a strong hump was developed at 485 nm. As discussed earlier the origin of this longer wavelength band at 485 nm was due to predominence of J-aggregated sites of MnTMPyP molecules adsorbed on the nano clay plalets. Fig. 3b shows the normalised UV-vis absorption spectra of monolayer LbL film of MnTMPyP - clay hybrid molecules at different loading percentage of the sample with respect to the CEC of clay. The loading of MnTMPyP was varied from 80% to 8% of the CEC of clay. From the Fig. it is observed that at 80% loading of the sample with respect to the CEC of clay, the Soret band remained at 466 nm and coincided with the monolayer LbL film of MnTMPyP without clay. However, a longer wavelength hump was developed at 491 nm. With decreasing loading percentage of the sample, the longer wavelength hump at 491 nm became gradually intense and the 466 nm Soret peak was red shifted. At 8% loading of the sample the Soret band was red shifted to 477 nm and an intense peak was developed at 491 nm. These two peaks were overlapped and a broad band was formed in the 460 nm - 500 nm range. The longer wavelength band with peak at 491 nm has been assigned as the J-band originating due to the predominance of J-aggregated sites in the LbL film.
MnTMPyP, a tetra cationic water soluble dye, got adsorbed onto the anionic nano clay platelets through electrostatic interaction. At lower sample loading, due to the availability of large number of anionic nano clay platelets, all the four meso substitutents of MnTMPyP molecules were attached onto the clay platelets by electrostatic interactions and thus the molecules became completely flattened. These flattened MnTMPyP molecules tagged nano clay platelets formed the organo-clay hybrid molecules. The 477 nm peak was attributed to the flattening of the porphyrin moieties owing to the adsorption onto the clay platelets. Further these completely flattened MnTMPyP molecules tagged nano clay platelets were obliquely stacked in the LbL film resulting in the formation of J-aggregates (491 nm band). This has been discussed schematically in Fig. 7. Thus, both flattening (477 nm band) and consequent formation of longer wavelength J-band (491 nm) became prominent especially at lower sample loading. Thus the aggregation in this case was highly induced by the available clay platelets. At higher sample loading, the intense peak at 466 nm was observed which was at the same position as that observed in case of the LbL film without clay. It might be that at higher sample loading, due to the non availability of sufficient number of nano clay platelets and the presence of large number of MnTMPyP molecules, flattening of the molecules was hindered to a larger extent. Accordingly, the 477 nm band arising due to flattening and the 491 nm J-band reduced to weak humps at higher sample loading.
3.1.2. By increasing the number of layers of the LbL films: Fig. 4 shows (a) UV-vis absorption spectra of different layered LbL films of MnTMPyP molecules without clay, (b-c) the normalised UV-vis absorption spectra of different layered LbL films of MnTMPyP - clay hybrid molecules. The loading of the sample was 80% of the CEC of clay in Fig. 4b and 8% of the CEC of clay in Fig. 4c. In both the cases of Fig. 4b and 4c, inset of the figures show the ratio of the absorbance values at 491 nm and 466 nm respectively as a function of the number of deposited layers. With increasing number of layers, the UV-vis absorption spectra of MnTMPyP LbL film without clay show an intense 466 nm Soret peak. No changes in peak position or development of new band were observed. This indicates that molecular aggregate pattern does not change with increasing layer number, in absence of clay.
At 80% loading, in monolayer LbL film, Soret peak was observed at 466 nm along with a weak hump at 491 nm. With increasing layer number 491 nm weak hump gradually increased in intensity and at higher layer number Soret peak reduced to a weak hump and 491 nm J-band became intense as shown in Fig. 4b. It might be that with increasing layer number, greater number of MnTMPyP tagged clay platelets became available for preffered orientation to form increased J-aggregated sites resulting in the intense J-band at 491 nm. Similarly at 8% loading, Jband became more intense with increasing layer number as shown in Fig. 4c. In this case also availability of large number of MnTMPyP tagged nano clay platelets with increasing layer number resulted in increased intensity of the J-band at 491 nm. The overlapping of different bands in both the Fig. 4b and 4c is an indication of the existence of different pattern of molecular association in LbL film. In both the cases, the insets show that the ratio of the absorbance values tends to become almost constant after 15 layers.
3.1.3. Temperature effect on J-Band Fig. 5 shows the normalised UV-vis absorption spectra of monolayer LbL film of MnTMPyP - clay hybrid molecules, at different temperatures. Fig. 5a shows the spectra during increase of temperature and Fig. 5b shows the spectra during decrease of temperature of the same film. The loading of the sample used was 80% of the CEC of clay. The process of heating was carried on continuously from room temperature (250C) upto 1000C. With increasing temperature the J-band at 491 nm band increased gradually in intensity and at 1000C it became most prominent. The film was then slowly cooled in the reverse order in proper steps until the room temperature was attained again. With decreasing temperature, J-band again reduced to a weak hump. The increase and decrease of the intensity of J-band was almost reversible with temperature. Although the initial spectrum was reverted after a waiting time about twelve hour. The experiment was repeated five times using the same film and identical result was obtained. It may be that at higher temperature due to thermal agitation, the sample tagged clay plaelets might orient in a preferred direction and thus created a favourable condition for increased number of J-aggregated sites.
3.2. AFM study:
The surface structure and morphology of the monolayer LbL films on the Siwafer were studied by Atomic Force Microscope (AFM). Fig. 6 shows the 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% of the CEC of clay laponite. The height profiles and the roughness analyses for all scanned area are also provided. Corresponding values of average height and the RMS roughness of the scanned area are given in Table 1. Large values of average height and RMS roughness are observed in monolayer MnTMPyP LbL film without clay. This is due to the formation of large scale molecular aggregates in the LbL film in absence of clay. In the restricted geometry of LbL film, MnTMPyP molecules came into closer proximity with each other resulting in the formation of molecular aggregates. The broadening and red shifting of the peak position of the Soret band in the UV-vis absorption spectrum of this film discussed in the previous section also supported this. In monolayer LbL film of MnTMPyP tagged nano clay platelets (Fig.6 (B-D)), the AFM images at different scales show densely packed nano clay platelets. The surface coverage is also high (more than 90%). The clay platelets are overlapped on each other and formed tilted orgnisation. The dye molecules adsorbed on the clay platelets are not distinguishable since the dimension of the molecules are beyond the resolution of the AFM system. The average height and RMS roughness are much smaller (Table 1) than that of the monolayer LbL film of MnTMPyP without clay. It confirms that upon adsorption of MnTMPyP molecules onto nano clay platelets, it became flattened reducing the average height of the film, moreover, adsorption onto nano clay platelets led to the increased molecular organization, preventing the formation of large scale molecular association. The tilted and overlapping organisations of MnTMPyP tagged nano-clay platelets might result in the preferred orientation of sample tagged nano clay platelets with respect to each other enhancing the formation of J-aggregated sites in the LbL film. This has been shown schematically in Fig. 7 and discussed in the next section. 3.3. Schematic representation: Fig. 7a shows one MnTMPyP molecule with central Manganese ion and four meso substituents each oriented in arbitrary plane as referred by several researchers [42-44]. Upon adsorption onto nano clay platelets, the four meso substituents of MnTMPyP are restricted to orient themselves on the plane of the nano clay sheet resulting into a flattened molecule [42-44].
In Fig. 7b the adsorbed and flattened MnTMPyP molecule onto nano sheet has been shown. Fig. 7c shows the schematic representation of a multiple array of flattened molecules on the surface of laponite. Fig. 7d shows the schematic representation of an overlapped and tilted organisation of MnTMPyP tagged nano clay platelets in the monolayer LbL film. This has been designed in conformity with the AFM images as shown in Fig. 6. The tilted organisation of clay platelets led to the preferred orientation of MnTMPyP molecules adsorbed onto clay platelets. This resulted in the increased number of J-aggregated sites in the LbL film. Origin of J-band as shown in UV-vis absorption spectra (Fig. 4) was due to the formation of J-aggregated sites of MnTMPyP tagged nano-clay platelet laponite in the LbL film. With increasing layer number, overlapping of large number of clay platelets occured resulting in the intense J-band in the UV-vis absorption spectra.
4. Conclusion: Our results showed that tetra cationic MnTMPyP molecules were adsorbed onto anionic nano clay platelets in aqueous clay dispersion and consequently formed Layer by Layer (LbL) self assembled films. In the process of adsorption porphyrin moieties became flattened onto clay surface as evidenced from the red shifting of the Soret band. Under certain conditions longer wavelength J-band was developed as shown in the UV-vis absorption spectra. With increasing layer number in the LbL film, intense J-band was observed. It indicates the predominance of Jaggregated sites with increasing layer number. By changing temperature, reversible J-band was observed. AFM studies clearly demonstrated that MnTMPyP tagged clay platelets got overlapped and oriented in preferrable associations for the formation of J-aggregated sites in the LbL film.
Acknowledgements The author SB is grateful to UGC- NERO for financial support to carry out this research work through MRP (Ref. No. F. 5-139/2012-13/MRP/NERO/541). The author JB is grateful to DST for financial support to carry out this research work through Women Scientist Project (Ref. No. SR/WOS-A/PS-47/2013). The author SAH is grateful to DST and CSIR for financial support
to carry out this work through DST Fast-Track Project Ref. No. SE/FTP/PS-54/2007, CSIR project Ref. 03(1146)/09/EMR-II.
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Figure caption: Fig. 1: Normalized 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) monolayer LbL film of MnTMPyP without clay.
Fig. 2: (a) Normalised UV-vis absorption spectra of MnTMPyP in (i) pure aqueous solution (2×10-5 mol L-1) , (ii) aqueous solution (2×10-5 mol L-1) at dye loading of 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.
Fig. 3: Normalised UV-vis absorption spectra of (a) MnTMPyP in aqueous clay dispersion and (b) MnTMPyP-clay hybrid monolayer LbL film. In both the cases the dye loading percentage varies from 80% to 8% of the CEC of clay.
Fig. 4: (a) UV-vis absorption spectra of different layered LbL films of MnTMPyP molecules without clay, (b-c) the normalised UV-vis absorption spectra of different layered LbL films of MnTMPyP - clay hybrid molecules. The loading of the dye was (b) 80% and (c) 8% of the CEC
of clay. The inset graphs (in both (b) and (c)) show the ratio of the absorbance values at 491 nm and 466 nm as a function of the number of deposited layers.
Fig. 5: Normalised UV-vis absorption spectra of MnTMPyP-clay hybrid monolayer LbL film during the process of (a) heating and (b) cooling. The loading of the dye was 80% of the CEC of clay.
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% of the CEC of clay. Fig. 7: Schematic representation of (a) one MnTMPyP molecule before adsorption onto nano clay platelet Laponite, (b) adsorption followed by flattening of one MnTMPyP molecule onto laponite clay platelet, (c) multiple array of flattened MnTMPyP molecules onto a clay sheet, (d) overlapping and association of a number of MnTMPyP tagged clay platelets onto monolayer LbL film at high clay density. Table caption: Table 1: The values of RMS roughness and average height of the scanned area of different AFM images.
Figures:
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
A
D B
C
B
Fig. 6
Table 1 Scan area RMS Roughness Average Height (µm2) (nm) (nm) MnTMPyP monolayer without clay 2.8847 7.6775 10×10 LbL film of
MnTMPyP - clay hybrid monolayer at different scales
6×6
1.2822
3.9848
2×2
1.2068
3.3696
1×1
0.9712
3.6382
(a) MnTMPyP molecule
(b) Flattened MnTMPyP molecule on Laponite
( c) Multiple array of flattened molecules
(d) Stacking of MnTMPyP tagged clay platelets in LbL film
Fig. 7
Highlights 1. Interaction of tetra cationic metalloporphyrin MnTMPyP with anionic nano clay platelets laponite. 2. Flattening of MnTMPyP molecules upon adsorption onto nano clay platelets. 3. Overlapping and tilted organization of MnTMPyP tagged nano clay platelets onto LbL film. 4. Formation of J-aggregated sites in LbL film due to preferred organization of MnTMPyP tagged nano clay platelets.