Thin Solid Films 519 (2011) 8066–8073

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Comparative study of porphyrin derivatives in monolayers at the air–water interfaceand in Langmuir–Blodgett films

Amrita Ghosh a, Prasenjit Mahato a, Sipra Choudhury b,⁎, Amitava Das a,⁎a Centre for Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar 364002, Gujarat, Indiab Chemistry Division, Bhabha Atomic Research Center, Trombay, Mumbai, 400 085, India

⁎ Corresponding authors. Tel.: +91 2782567760; faxE-mail addresses: sipra@barc.gov.in (S. Choudhury),

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.02.082

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 July 2010Received in revised form 21 February 2011Accepted 22 February 2011Available online 2 March 2011

Keywords:PorphyrinVesiclesLangmuir–Blodgett filmsTransmission electron microscopyAtomic force microscopy

The orientation and aggregation of various porphyrin derivatives at the air–water interface and in Langmuir–Blodgett films were investigated. Monolayer properties of these molecules, where long alkyl chain(s) werecovalently bound through different functionality of varying hydrophilicity were studied by measuring surfacepressure area isotherms. Such derivatives, where ether functionality (functionalities) was (were) used forlinking long alkyl chain(s), did not form uniform monolayer; instead they were found to form multilayerclusters or aggregates on the water surface. On the other hand, porphyrin derivative functionalized at the fourperipheral phenyl rings with eight hexadecyl ether chains formed stable spherical vesicles when deposited onmica. Tetra N-alkyl pyridinium porphyrins with long alkyl chain were found to form various phases on thewater surface. Evidence of transition from horizontal orientation to vertical orientation of porphyrin rings ofporphyrin molecules having C14 chains was observed. This type of transition was lost with the porphyrinmolecule with a relatively smaller chain (C8).

: +91 2782567562.amitava@csmcri.org (A. Das).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In nature, solar energy conversion is triggered by the capturing ofsunlight by hundreds of chlorophyll arrays. By an enormously efficienttransfer of energy the excited energy is channelized to a reactioncenter and is converted to chemical potential in the form of a long-lived charge separated state. To mimic these natural light harvestingevents, numerous porphyrinoid arrays linked by covalent bonds havebeen synthesized by construction of a large-scale supramoleculararchitecture [1–3]. Porphyrin derivatives with long conjugatedπ-electron system have shown promise for potential application ingas sensors, molecular electronics, photovoltaic cells, photodynamictherapy and as photosensitizers [4–6]. Recently, using self assemblyprocess morphologies like porphyrin nano-sheets have been achievedfor their probable applications in electronic and optoelectronicnanodevices [7]. It has been demonstrated that a porphyrin mono-layer, covalently assembled on a silica surface, could be used for thefabrication of an optical acid pH meter [8]. Langmuir–Blodgett (LB)films of metal substituted porphyrin have also been used as gas sensorfor various organic vapors [9].

Due to diverse packing or aggregation of aromatic macrocycles,porphyrin derivatives exhibit different photophysical and photo-chemical properties, which are important in many technologicalapplications. Many nano aggregates are formed due to aggregation or

self assembly such as nano tubes [10], nano disk [11], nano rods [12],hexagonal nanoprisms [13], nanofiber bundles [14], nanosheets [7]etc. Porphyrins are also well known to aggregate in various shapes—even porphyrinwheels [15] could be achieved using covalent and non-covalent interactions. Porphyrin molecules aggregate under certainconditions due to their strong π–π stacking interaction. The aggrega-tion behavior is also controlled through appropriate substitution of theperipheral phenyl ring providing additional effect in ordering theaggregates. Tetraaryl porphyrins and their metalloporphyrins tend toaggregate in mixed solvents where relative ratios of different solventsdetermine the aggregation phenomena and eventually lead to variousnanostructures under ambient conditions [12]. It has been shownthat aggregation of organized nano structure of tetraaryl porphyrin ispromoted by lateral π–π interaction among the porphyrin rings andthe hydrophobic effect among the long alkane chains [16–21].

However, till date only limited attempts are there to make well-defined 2-D organic structures using the Langmuir–Blodgett tech-nique with porphyrin-based systems which have diverse photophy-sical and photochemical properties [22–26].

For this, it is essential to form a stable monolayer at the air–waterinterface prior to the LB film deposition. Amonomolecular film formedat the air–water interface and transferred to a solid substrate by LBtechnique forms amodel systems for the investigation of the nature ofinteractions between porphyrin molecules [22–26]. LB technique ismuch useful method for the fabrication of monolayer and multilayerfilms because it provides a precise control over the film thickness ofamphiphilic molecules and its molecular organization. Developmentof devices requires specific control in molecular orientation and

Scheme 1. Synthetic procedure adopted for P1, P2 and P3.

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packing in thin films. However, pure unsubstituted tetraphenylpor-phyrin or its metallated form does not form stable monolayer at theair/water interface [27]. Long-chain amphiphilic substituents on thephenyl groups seem to be necessary for the formation of stablemonolayers [28,29]. Functionalized ionic porphyrins containing longhydrophobic chains at the periphery of the conjugated π-electronsystem have been studied with regard to Langmuir monolayer at theair/water interface and LB film deposition onto a solid surface [30,31].Surfactant carboxyporphyrins have also been studied in Langmuirmonolayers as well as LB films deposited on solid substrates [32–34].Barring few studies, no reports are available in the literature on atomicforce microscopy (AFM) studies for revealing the surface morphologyof these monolayer/multilayer films developed by LB technique [35].

In this paper, we have studied the possibility of the monolayerformation for P1 and P2 (Scheme 1) [12] at the air–water interface.Both of these complexes were found to form clusters or aggregates ofthe molecules at the interface. In order to examine the effect ofenhanced hydrophobicity on the film formation or aggregationbehavior at the air–water interface, we have used an already published[36] synthesized porphyrin derivative P3 (Scheme 2; 5,10,15,20-(octakis-3,4-bisalkoxy aryl) porphyrin) for studies. Hydrophobicity ofthe molecule was expected to enhance with the increase in thenumber of long chain alkyl substituents on the peripheral phenyl ringand thus, expecting a higher degree of order in the monolayer

Scheme 2. Synthetic p

formation [37,38]. However, P3 was found to form spherical vesicleswithin the monolayer when spread on the water surface and wasconfirmed by AFM images.

Ester functionality is known to be more polar compared to thecorresponding ether functionality and thus porphyrin derivative withester functionality (P4, 5,10,15,20-(octakis-3,4-alkylester aryl) por-phyrin) was synthesized. P4, having more hydrophilic head group fora favorable interaction with the water surface, was expected to form abetter monolayer. In order to further enhance the hydrophilicity ofthe head group with the water surface, cationic pyridinium groups(P5 and P6) are introduced in the four peripheral rings of theporphyrin backbone (Scheme 3). We have presented a comparativestudy, on monolayer formation, aggregation behaviors and surfacemorphologies of various porphyrin films developed from differentderivatives (P1–P6) in this article.

2. Experimental details

2.1. Instrumentation

Electrospray ionization-mass spectra (ESI-MS) measurementswere carried out on Waters QTof-Micro instrument. Microanalysis(C, H, N) were performed using a Perkin-Elmer 4100 elementalanalyzer. Fourier transform infrared spectroscopy (FTIR) spectra were

rocedure for P4.

Scheme 3. Synthetic methodologies adopted for P5 and P6.

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recorded as KBr pellets using Perkin Elmer Spectra GX 2000spectrometer. 1H NMR (nuclear magnetic resonance) spectra wererecorded on Bruker 200 MHz FT NMR (model: Advance-DPX 200)spectrometer at room temperature (25 °C). Electronic spectra wererecorded with Shimadzu UV-3101 PC/Varian Cary 500 Scan Uv–vis-NIR spectrophotometer. AFM studies were carried out under ambientconditions using scanning probe microscope SPM Solver P47, incontact mode using rectangular cantilever of Si3N4. The absorptionspectra of LB films were recorded on a Chemito UV 2600spectrophotometer.

2.2. Deposition of Langmuir–Blodgett film

Aryl porphyrin derivatives were dissolved in chloroform. N-pyridinium porphyrin derivatives were first dissolved in methanoland then were diluted with chloroform (methanol–chloroform, 1:10(v/v)). Final concentration of the individual porphyrin derivative inthe solution was adjusted to ~1 mg/mL. This solution was allowed tospread on the water surface with a Hamilton micro syringe. Waitingtime of minimum 20 min was given for complete evaporation ofsolvents and to equilibrate molecules on the water surface. Pressurearea isotherm measurements were carried out by using computercontrolled KSV 5000 Langmuir double barrier Teflon trough. Surfacepressure was measured with platinum Wilhelmy plate microbalance.The compression rate (the barrier speed) used was 5 mm/min. Usingthe conventional dipping method, LB films were transferred ontoquartz, freshly cleaved mica and glass.

Monolayers andmultilayers of porphyrin molecules were depositedon various substrates such as glass, quartz, and mica. P1was depositedat a surface pressure of 30 mN/mand transfer ratiowas unity at upwardmovement of slide while negligible amount was transferred atdownward movement. Transfer ratio for P2 at 30 mN/m was found tobe very small on glass substrate. P3was deposited at a surface pressureof 30 mN/m and transfer ratiowas varied from0.6 to 0.8. For P4 transferratio of P4was ~0.6 at a surface pressure of 30 mN/m. In both cases no

deposition took place when the substrate moved downward. P5 wasdeposited at different surface pressures of 20 mN/m and 40 mN/m.Transfer ratiowas nearly in unity and depositionmodewas Y-type in allpressure. Z-type deposition was observed in case of P6molecule with atransfer ratio of more than one at a surface pressure of 20 mN/m [36].The observed transfer ratio is more than 1, which could be due to thebilayer formation while depositing on the substrate. Monolayersdeposited on mica surface were used only for AFM studies.

3. Materials

All chemicals were obtained from commercial sources and used asreceived, unless mentioned otherwise. Cetyl alchohol obtained fromMerck (India) and 4-hydroxybenzaldehyde, 3,4-dihydroxybezaldehydeand 4-pyridinecarboxaldehyde were obtained from Sigma-Aldrich Che-micals. Thionyl chloride, pyrrole, propionic acid and Cu(CH3COO)2.2H2Owereobtained fromS.DFineChemicals (India). Solventsused for reactionsand various other studieswere of A.R. grade (Merck, India) andwere usedas received. Pyrrole was distilled under reduced pressure prior to use.

3.1. Experimental procedure

3.1.1. SynthesisHexadecyl chloride (1), 4-hexadecyloxybenzaldehyde (2), [39]

3,4-bis(hexadecyloxy)benzaldehyde (3) [36], 5,10,15,20-tetrakis-(4-hexadecyloxy-phenyl)-porphyrin (P1) [12], 5,10,15,20-tetrakis(4-(hexadecyloxy)phenyl)-copper(II)porphyrin (P2) [12] and4,5,9,10,14,15,19,20-octakis-(8-hexadecyloxyphenyl)porphyrin (P3)[36] were synthesized following known procedure [12,39]. Analyticaldata matched well with values reported earlier and the proposedstructure for the respective compounds.

3.1.1.1. Hexadecyl chloride (1). Yield: 72%; 1H NMR (200 MHz, CDCl3, δ(ppm)): δ 3.52 (t, 2H, J=6.7 Hz, H[―CH2Cl]), 1.77 (p, 2H, J=6.5 Hz, H[―CH2−]), 1.2 (b, 30H, H[aliphatic chain]), 0.88 (t, 3H, J=6.2 Hz, H

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[―CH3]). FTIR (KBr;υ/cm−1): 2925, 654; ESI-Ms (m/z): 284 (M++Na+;10%), 261 (M+; ~5%).

3.1.1.2. 4-hexadecyloxybenzaldehyde (2). Yield: 74%; 1H NMR(200 MHz, CD2Cl2; δ (ppm)): δ 9.8 (s, 1H, H[―CHO]), 7.8 (d, 2H,J=9 Hz, H[phenyl]2,6), 6.99 (d, 2H, J=8.6 Hz, H[phenyl]3,5), 4.04 (t,2H, J=6.5 Hz, H[―OCH2]), 1.8–1.2 (b, 37H, H[aliphatic chain]), 0.87(t, 3H, J=6.0 Hz, H[―CH3]); FT IR (KBr; υ/cm−1): 2919, 1690, 1602,1509, 1165, 832; ESI-MS (m/z): 347 (M++H, ~15%); 369 (M++Na+,75%). Elemental analysis: calculated for C23H38O2: C 79.76, H 11.05%;found: C 79.6, H 11.5%.

3.1.1.3. 3,4-bis(hexadecyloxy)benzaldehyde (3). Yield : 3.75 g (45%);1HNMR (200 MHz, CD2Cl2, δ (ppm)): δ 9.8 (s, 1H, H[―CHO]), 7.38 (d,1H, J=5.8 Hz, H[phenyl]2), 7.37 (d, 1H, J=5.8 Hz, H[phenyl]3), 6.96(d, 1H, 8.2 Hz, H[phenyl]6), 4.06 (t, 4H, J=6.4 Hz, H[―OCH2]), 1.2 (b,60H, H[aliphatic chain]), 0.87 (t, 6H, J=5.8 Hz, H[―CH3]); FTIR (KBr;υ/cm−1) 2919, 1688, 1587, 1511, 1134, 808; ESI-MS (m/z): 587 (M+

~17%); 610 (M++Na+, 45%). Elemental analysis: calculated forC39H70O3: C, 79.80; H, 12.02%; found: C, 80.0; H, 12.1%.

3.1.1.4. 5,10,15,20-tetrakis-(4-hexadecyloxyphenyl)porphyrin (P1).Yield: 49%; 1H NMR (200 MHz, CD2Cl2; δ (ppm)): δ 8.86 (s, 8H, βpyrrole), 8.064 (d, 8H, J=8.6 Hz, H[phenyl]2,6), 7.22 (d, 8H, J=8.4 Hz,H[phenyl]3,5), 4.18 (t, 8H, J=6.4 Hz, H[―OCH2]); 1.8–1.2 (b, 125H, H[aliphatic chain]), 0.87 (t, 12H, J=5.8 Hz, H[―CH3]), −2.7 (b, 2H,NH); UV–vis (CH2Cl2; λ/nm; (log ε/M−1 cm−1)): 421 (5.49), 518(4.24), 556 (4.12), 595 (3.86), 696 (4.00); FTIR (KBr; υ/cm−1): 2921,1604, 1507, 1507, 1173, 802; elemental analysis: calculated forC108H158N4O4: C 82.28, H 10.1, N 3.55%; found: C 81.5, H 10.1, N 3.4%.

3.1.1.5. 5,10,15,20-tetrakis(4-(hexadecyloxy)phenyl)-copper(II)porphyrin(P2).Yield: 90%; UV–vis (CH2Cl2; λ/nm (log ε/M−1 cm−1)): 418 (5.48),541 (4.19), 580 (2.5); FTIR (KBr; υ/cm−1) 2921, 1605, 1504, 1174, 802;ESI-Ms (m/z): 1636 (M+, ~18%); elemental analysis: calculated forC108H156N4O4Cu: C 79.19, H 9.6, N 3.4%, found: C 78.6, H 9.5, N 3.3%.

3.1.1.6. 4,5,9,10,14,15,19,20-octakis-(8-hexadecyloxyphenyl)porphyrin(P3). Yield: 0.59 mg (~12%); 1H NMR (200 MHz, CD2Cl2, δ (ppm)): δ8.89 (s, 8H, H [β pyrroles]), 7.7 (d, 4H, J=8 Hz, H[phenyl]6), 7.22 (d, 8H,J=5.6 Hz, H[phenyl]2,3), 4.29 (t, 8H, J=6.2 Hz, H[―OCH2]); 4.09 (t, 8H,J=5.2 Hz, H[―OCH2]); 1.25 (b, 150H, H [aliphatic chain]), 0.85 (t, 24H,J=5.2 Hz, H[―CH3]),−2.75 (b, 2H,―NH); FTIR (KBr; υ/cm−1): 2919,1742, 1468, 1135, 798; elemental analysis: calculated for C172H286N4O8:C 81.39, H 11.36, N 2.21%, found: C 81.88, H 11.7, N 2.01%.

3.1.1.7. 1,5,10,15-tetrakis-(4-hydroxy phenyl) porphyrin (4). This wassynthesized following known procedure by reacting 4-hydroxy benz-aldehyde and freshly distilled pyrrole (used in 1:1 mole equivalent).Propionic acid was used as solvent for the reaction [40–42]. Purifiedcompound, following column chromatography, was characterized bystandard analytical and spectroscopic techniques. Various analytical andspectroscopic data matched well with the reported values. Yield: 0.75 g(~12%). 1H NMR (200 MHz, CD2Cl2, δ (ppm)): δ 8.84 (s, 8H, H [βpyrroles]), 7.98 (d, 8H, J=8.4 Hz, H[phenyl]3,5), 7.21 (d, 8H, J=8.4 Hz,H[phenyl]2,4); FTIR (KBr; υ/cm−1): 3323, 1606, 1509, 1225, 1169, 802;ESI-Ms (m/z): 679 (M++H+, 100%); elemental analysis: calculated forC44H30N4O4: C, 77.86; H, 4.46; N, 8.25%, found: C 76.9, H 4.3, N 8.2%.

3.1.1.8. 1,5,10,15-tetrakis-(4-carboxylato pentadecylphenyl) porphyrin(P4). Compound 4 (0.04 g, 0.059 mM) was dissolved in dry THF(30 mL) under dinitrogen atmosphere. Excess triethylamine (0.5 mL,predried over CaH2) was added to this solution. To this palmitoylchloride (R2Cl) (Scheme 2) (0.07 mL, 0.236 mM) was added underinert atmosphere. The reaction mixture was refluxed for 5 h at 70 °Cand then it was kept overnight at room temperature. After that THF

was removed under reduced pressure and the desired compound wascollected in non-aqueous layer following solvent extraction withCH2Cl2–water mixture. Solvent was removed to get the crude solidproduct. Column chromatography was performed in silica gel columnwith chloroform/n-hexane (1:1, v/v) mixture as eluent. The firstfraction was collected. Yield: 0.09 g (~92%). 1H NMR (200 MHz,CD2Cl2, δ (ppm)): δ 8.87 (s, 8H, H[β pyrroles]), 8.2 (d, 8H, J=8.4 Hz, H[phenyl]2,6), 7.49 (d, 8H, J=8.4 Hz, H[phenyl]3,5), 3.66 (t, 8H,J=6.2 Hz, H[―OCH2]); 1.2 (b, 120H, H[aliphatic chain]), 0.88(t, 12H, J=5.4 Hz, H[―CH3]), FTIR (KBr; υ/cm−1): 2920, 1759,1500, 1162, 966; ESI-Ms (m/z): 1633 (M+, ~15%); elemental analysis:calculated for C108H150N4O8: C 79.46, H 9.26, N 3.43%, found: C 79.7,H 9.4, N 3.7%.

3.1.1.9. Synthesis of 5,15,20-25-tetrakis-(4-pyridyl)porphyrin (5). Com-pound 5 was synthesized by following standard procedures [40–42].Analytical and spectroscopic data matched well for reported com-pound. Yield: 1.2 g (~5%). 1H NMR (200 M Hz, CD2Cl2, δ (ppm)): δ9.063 (d, 8H, J=6.0 Hz, H[pyridine]2,6), 8.872 (s, 8H, H[β-pyrrole]),8.162 (d, 8H, J=6.0 Hz, H[phenyl]3,5); FTIR (KBr; υ/cm−1): 1590,1404, 969, 797; ESI-Ms (m/z): 618 (M+, 100%); elemental analysis:calculated for C40H26N8: C, 77.65; H, 4.24; N, 18.11%, found: C 78.0,H 4.2, N 17.9%.

3.1.1.10. 5,15,20-25-tetrakis-(4-N-tetradecylpyridyl)porphyrin (P5).Compound 5 (0.1 mg, 0.162 mM) was dissolved in chloroform–

methanol mixture and was stirred at ~50 °C for 15 min in a roundbottom flask. Then excess amount of alkyl bromide (R3Br) (Scheme 3),dissolved in chloroform was added to this. The whole reactionmixture was refluxed for 2 days. Then the solvent was evaporated andthe desired compoundwas purified by column chromatography usingsilica gel as stationary phase. Initially chloroform was used as theeluent and gradually polarity of the eluent was enhanced by addingincreasing proportions of methanol and finally chloroform–methanolmixture (10:1; v/v) was used as eluent. Yield : 0.04 mg (~14%). 1H NMR(200MHz, CD3OD, δ (ppm)): δ 9.52 (d, 8H, J=6.2 Hz, H[pyridine]2,6),9.1 (s, 8H, H[β pyrroles]), 9.0 (d, 8H, J=6.2 Hz, H[pyridine]3,5), 2.41(t, 8H, J=4.5 Hz, H[―OCH2−]); 1.27 (b, 98H, H[aliphatic chain]), 0.87(t, 12H, J=5.0 Hz, H[―CH3]); FTIR (KBr; υ/cm−1): 2922, 1637, 1464,801; elemental analysis: calculated for C96H142Br4N8: C, 66.73; H, 8.28;N, 6.49% found: C, 66.9; H, 8.0; N, 6.3%.

3.1.1.11. 5,15,20-25-tetrakis-(4-N-octylpyridyl) porphyrin (P6). Com-pound P6 was synthesized by following similar route as that wasadopted forP5; except compound5was reactedwith R4Br (Scheme3),instead of R3Br. Yield : 0.035 mg (~13.3%). 1H NMR (200 MHz, CD3OD,δ (ppm)): δ 9.5 (d, 8H, J=6.2 Hz, H[pyridine]2,6]), 8.9 (s, 8H, H[βpyrroles]), 8.2 (d, 8H, J=6.2 Hz, H[pyridine]3,5), 2.42 (t, 8H, J=4.2 Hz,H[―OCH2]); 1.3 (b, 60H, H[aliphatic chain]), 0.9 (t, 12H, J=4.6 Hz, H[―CH3]); FTIR (KBr; υ/cm−1): 2921, 1667, 1567, 821; elementalanalysis: calculated for C72H94Br4N8: C, 62.16, H, 6.81, N, 8.05% found:C, 62.71, H, 6.98, N, 8.02%.

4. Results

4.1. Surface pressure area isotherm

Surface pressure area isotherms at room temperature (25 °C) forporphyrin amphiphiles at the air–water interface are shown in Fig. 1.To determine the areas at zero surface pressure, the mean molecularareas of the amphiphiles were obtained by extrapolating the firstvertical region of the isotherms. The area per molecule for P1 wasfound to be much smaller (38 Å2) than the area (90 Å2) of aperpendicularly oriented tetra phenyl porphyrin. This indicates theformation of multilayer or overlapping [43] of P1 molecules on thewater surface. It was expected that incorporation of Cu(II)-ion into the

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Fig. 3. UV–visible spectra of the porphyrin derivatives (P1–P6) in chloroform solution(black curve) and in LB films (red curve). Intensity is normalized by peak maxima.

0 100 200 300

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Fig. 1. Surface pressure-area isotherms of different porphyrin derivatives (P1 to P6) onwater at 25 °C.

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porphyrin ring (P2) would have introduced more hydrophiliccharacter and hence imparted more stability towards the monolayerformation. But on the contrary, Fig. 1 reveals that the area permolecule (25 Å2) shrinks while going from metal free amphiphiles(P1) to the corresponding Cu(II)-complexed one (P2). A similarobservation has been reported for cryptand based amphiphiles also[44], where shrinkage in area per molecule was explained based onthe probable inclined orientation of the metallated amphiphiles at theair–water interface. The apparent anomaly could be explained basedon the fact that on metallation, porphyrin core of P2 became morehydrophilic in character, which would cause a stronger interactionwith water molecules. This was expected to improve solubility ofthese molecules to some extent in the water subphase. The area permolecule was found to be 80 Å2 for P3, larger than that for P1 and P2.Molecular area of P3 being 80 Å2, which is very close to the literaturevalue of perpendicularly oriented phorphyrin backbone (90 Å2).Slight decrease in molecular area could be due to aggregate formationto some extent resulting vesicles at compressed state (30 mN/m) atthe air–water interface. Ester functionality is expected to be morehydrophilic as compared to the ether one and thus P4was expected toform an improved monolayer at the air–water interface. The meanmolecular area of P4was found to be ~90 Å2 (Fig. 1). This suggests thepresence of perpendicularly oriented porphyrin ring with respect towater surface. Significant increase in surface pressure was observedonly after complete monolayer coverage was attained and this

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Fig. 2. Surface pressure-area isotherms of P5 molecule at various subphase pH=3.8(violet), pH=4.8 (green), pH=5.8 (red) and pH=9.9 (blue) on water at 25 °C.

monolayer was relatively incompressible, as indicated by the sharpincrease in π upon further compression. Pressure area isotherm(Fig. 1) shows that area per molecule for P5 is 300 Å2, which is veryclose to the area of a flat tetra(4-N-methylpyridyl)porphyrin (TMPyP)molecule of 320 Å2 [45]. This suggests that P5 molecules float on thewater surface due to the strong interactions between the C14-tailgroups. Replacement of the phenyl groups in P1–P4 by cationicpyridinium functionality was expected to enhance the hydrophilicityof the porphyrin ring, thereby to favor interactions with water surfaceand the monolayer formation [46]. Repulsion of the positive chargeson the porphyrin ring in P5 lowered the possibility of self aggregationamong the P5 molecules [47]. Presumably this accounted for thehigher area per molecule observed in the case of P5 (Fig. 1). The longtransition at π=25 mN/m between the expanded and condensedphase could be assigned to a change in orientation of porphyrinmolecules from parallel to oblique or tilted one relative to the watersurface. For P6, the mean molecular area was 105 Å2. Significantlylower area per molecule for P6 compared to P5, revealed thatporphyrin core was not parallel to water surface—rather it might haveadopted a tilted orientation. The low collapse pressure of 32 mN/m

Table 1Absorption band of the Soret band for the respective porphyrin derivative in solutionand when deposited as LB film on quartz.

Porphyrin derivative λmax (nm) in solution λmax (nm) in LB films

P1 423 400, 440P2 419 403,434P3 426 438P4 419 424,445P5 426 430a,435b

P6 421 434

a λmax of LB film deposited at 40 mN/m.b λmax of LB film deposited at 20 mN/m.

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Fig. 4. UV–visible spectra of P5 molecule: seven layers of LB films deposited at surfacepressure of 40 mN/m (- - - -) and 11 layers of LB films deposited at surface pressure of20 mN/m (• • • • •) at 26 °C. Solution spectrum is represented by a solid curve (——). Insetshows the plot of absorbance vs. number of layers of LB films deposited at differentsurface pressure at 25 °C.

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further indicated that the monolayer was unstable due to thepresence of short chain (C8). These results are comparable withprevious reports [47–48], where Gonclaves et al. have studied thepressure area isotherm for an analogous C18 derivative of N-alkylatedpyridinium porphyrin.

Effect of variation of subphase pH on the LB film formation wasalso studied. A small increase in area per molecule P3with decrease inpH has been observed without any change in the nature of isotherm[49–51]. This could be due to increase in positive charge on theporphyrin ring lowering the possibility of aggregation. Fig. 2 showsthe surface pressure area isotherm of P5 molecule at differentsubphase pH. The change in area per molecule and the transitionpressure in case of P5 were not significant with the large variation ofpH (pH=3.8 to 9.9; Fig. 2). This was understandable if one consideredthat the presence of the cationic pyridinium units in P5 did not favorfurther protonation of the pyrrole N-atoms and thus accounted for theinsignificant change with change in subphase pH. Thus, for P5 therewas apparently no influence of the subphase pH on the hydrophilicityof the porphyrin ring. Similar transfer ratio was observedwhen quartzwas used as the solid substrate instead of glass. It is mentioned earlierthat P6 molecules, having shorter side chains, formed unstablemonolayer on the water surface. Thus, the observed transfer ratio ofmore than 1, while transferring on solid, could be due to the bilayerformation. Monolayers deposited on mica surface were used only forAFM studies.

Fig. 5. Atomic force microscopy (AFM) images of P1 on m

4.2. Spectrophotometric studies

UV–visible absorption spectra of the monomeric porphyrinmolecule in solution showed an intense Soret band in the wavelength region of ~419–426 nm and weaker Q-bands in the~500–670 nm region as shown in Fig. 2. These bands arose from theporphyrin based allowed (Soret band) and quasi allowed (Q bands)π–π* transitions. On aggregation of the porphyrin molecules, positionand intensity of both B and Q bands are known to be affected. Twotypes of aggregates are generally observed in porphyrin molecules,namely, J- and H-aggregates. Depending on the nature of theaggregation, the absorption bands shifts to higher or lower wave-length compared to the monomer and the effect is more prominentfor the intense Soret band. In general, phenomena like broadening,hypochromicity and splitting of the Soret band suggest the aggrega-tion of porphyrins. In general, absorption spectra of the LB films of P1,P2, P3, P4, P5 and P6 deposited on quartz showed distinct red shifts(434–440 nm) as compared with that recorded for solution of therespective porphyrin derivatives (Fig. 3) at the Soret band. Thepositions of absorption maxima in solution and in LB films arecompared in Table 1.

Based on the molecular excitation model [52], a red shiftedabsorption band could be attributed either to the in-line alignment ofthe transition dipole moments in porphyrin derivatives or to the co-planarly inclined transition dipoles (J-aggregation), where the anglebetween the center-to-center vector and the transitionmoment is lessthan 54.7°. Porphyrin molecules stack in the head-to-tail fashion forJ-aggregates and result a red-shifted absorption band. Relativelysmaller red shift (Table 1) observed for P1–P6 in the present study,suggests the presence of loose structure composed of J-aggregates andthis accounts for the higher full width at half maximum of the Soretband of the respective derivatives in LB films. A similar model wasadopted and reported by earlier researchers to explain the spectro-scopic behavior of cyanine dyes [53].

Electronic spectra recorded for LB films of P1 and P2 revealed anadditional band at ~400 nm, which was blue shifted as compared tothe Soret-band of the corresponding derivative in solution. Blueshifted additional absorption band at ~400 nm suggests formation ofsmall amount of H-aggregates. Four Q-bands were observed in LB filmof P1 whereas there were two Q-bands in case of P2. These werecharacteristics of the non-metallated porphyrin with D2h symmetryand metallated porphyrin with D4h symmetry. LB film of P3 moleculeexhibited the Soret band at 438 nm which was red shifted by 12 nmfrom the corresponding chloroform solution. The width of the band inLB filmwas very broad due to decrease in order of themolecules in thefilm [54]. As compared to P1, the shift of the Soret band was lesswhich was indicative of a weak interaction between the porphyrinrings in LB film of P3. LB film of P4 showed a maximum of 445 nm

ica (a) LB film deposited at 30 mN/m (b) cast film.

Fig. 6. AFM images of LB monolayers of P3 deposited on mica at 30 mN/m pressure and25 °C.

8072 A. Ghosh et al. / Thin Solid Films 519 (2011) 8066–8073

with an asymmetric band. The red shift of the Soret band by 26 nm ascompared to solution spectrum (419 nm) was due to the formation ofJ-aggregates in LB films. The larger shift in Soret band signified thestronger interaction between the porphyrin rings. A shoulder at424 nm in the absorption band was presumably due to the electronictransitions between the energy levels of monomers and theaggregates indicating the equilibrium between the monomer andthe aggregation. For P5 and P6 the shifting in the higher wavelengthregion of the films than the solution phase is due to the sameJ-aggregation. Fig. 4 shows the absorption spectrum of a LB film ofexpanded monolayer of P5 deposited at π=20 mN/m. Soret bandappeared at 435 nm, while the spectra recorded for solution of P5 inmethanol showed Soret band at 426 nm. The red shift (~9 nm) of theSoret band relative to the solution spectrum could suggest a J-typeaggregation. Relatively smaller red shift revealed the presence of aloose structure composed of J-aggregates. This further supports ourobservation for the pressure area isotherm recorded for P5. TheSoret band of a LB film of condensed monolayer of P5 deposited atπ=40 mN/mwhich appeared at 430 nmwas also shown in Fig. 4. Theblue shift, relative to the expanded region, was due to the presence ofH-type aggregates of P5 molecule [54].

This type of aggregation was further supported by the low area permolecule at π=40 mN/m. The face-to-face aggregates correspond toa stacked arrangement of the macrocycles with a tilted alignment ofthe transition dipole moments relative to the line of molecularcenters. Further, for P5 the intensity of the Soret band deposited athigher pressure was larger than that deposited at lower pressure(inset of Fig. 4). This was due to increase in surface concentration orpacking density at higher pressure as found in aggregates of otherporphyrins [55]. Also, a narrow spectrum in case of H-aggregates,unlike J-aggregates was observed [56]. The linear dependence ofabsorption intensity on the number of LB layers deposited at both thesurface pressures is shown as inset in Fig. 4. This indicates the

Fig. 7. (a) AFM image of P5 deposited on mica at 25 mN/m pressur

reproducible transfer of monolayer from the interface to the substratein every dipping cycle. The absorption band for LB film of P6 appearedat 434 nm (Fig. 2) which was red shifted compared to solutionspectrum and suggested formation of the J-aggregates.

4.3. AFM studies

AFM image on a monolayer of P1 deposited on mica showed theformation of clusters (cluster sizes were of 10–30 nm) and wasfurther supported by very low observed area per molecule (35 Å2)(Fig. 1). AFM studies revealed that these clusters were formed bymultilayers of P1 on the water surface and are shown in Fig. 5. Theaverage height from the top of the clusters to the mica surface was~60 nm (Fig. 5a), indicating the formation of amultilayer of moleculeswithin the cluster as shown in Fig. 5a. AFM image of cast film of P1 didnot show any cluster formation, instead a surface roughness of 300 Åwas observed (Fig. 5b). Transfer of monolayer of P2 was difficult asthese molecules interacted strongly with water and formed moremultilayer cluster structure at the interface.

An AFM image of themonolayer of LB film of P3, deposited onmicais shown in Fig. 6, which clearly reveals spherical vesicle formationwithin the monolayer. Reports on vesicle formation from porphyrinderivatives are not common in the literature [16–21,57]. Thedisordered assembly of the long chains in the vesicular formation ispresumed to be responsible for the observed broadening of absorptionspectra (Fig. 3c). AFM images showed a distribution of vesicles ofvarying diameter ranging from 100 to 700 nm while heights for thesevesicles were ranging from 10 to 30 nm. These multi lamellar [35]vesicles were found to be stable even in the dry state as deposited onmica. Despite several attempts, no proper AFM image for P4 could beobtained. P5monolayers deposited on the surface of mica at 20 mN/mappear to be densely packed and homogeneous, indicating good filmforming property of P5 molecules. P5 showed distinct phasetransition at the air–water interface, as observed in pressure-areaisotherm (Fig. 1). Different phases, that were present in thetransferred film at transition pressure (25 mN/m), were observed inthe AFM image of a monolayer LB film deposited onmica. AFM imageson monolayers of P5, deposited at surface pressure of 25 mN/m,showed bright and dark region within the monolayer film as shown inFig. 7a. The bright region was 3 nm higher in topographical heightthan dark region as indicated by the section profile of the AFM image(Fig. 7b). This 3 nm height difference was due to the perpendicularstanding of porphyrin moiety [35,57] within flat lying porphyrinmolecules at the air–water interface. At the transition pressure, theporphyrin rings started to orient vertically from flat like orientation.The films deposited at surface pressure of 40 mN/m show uniformsmooth surface indicating single phases where all the porphyrinmolecules are oriented vertically with the mica substrate. This type ofphase transition was not observed in case of N-alkylated

e at 25 °C; (b) section profile of the line indicated in figure (a).

Fig. 8. AFM image of P6 monolayer on mica deposited at 20 mN/m surface pressure.

8073A. Ghosh et al. / Thin Solid Films 519 (2011) 8066–8073

tetrapyridinium porphyrin derivatives having shorter chains (P6molecule). The low area per molecule (105 Å) corresponds to the tiltedarrangement of the porphyrin ring. P6 showed red shift of theabsorptionmaxima indicating the J-aggregates. AFMshowed the surfaceroughness of 2.5 nm (Fig. 8). This demonstrates that J-aggregatesdeposited on mica surface form a bilayer instead of multilayer.

5. Conclusions

A comparative study has been carried out on the aggregationbehavior of the porphyrin derivatives at the air–water interface and inLB films. Aryl porphyrin derivatives, with four long aliphatic chainswere found to be form multilayer aggregates at the air–waterinterface; whereas derivative with eight aliphatic chains formedvesicles. Different phases were observed at the air water interface forsubstituted porphyrin derivative with N-alkyl pyridinium moietywith longer aliphatic chain length (C14); while these different phaseswere absent for analogous derivative with shorter chain length (C8).The difference in packing and aggregation were observed by usingUV–visible spectroscopy and AFM studies. The difference in aggrega-tion behavior is evident in the AFM studies and is also reflected in thespectral behavior.

Acknowledgment

The Department of Science and Technology, Government of India,supports this work. AG and PM wish to acknowledge CSIR for SRFfellowship, respectively. AD wishes to thank Dr. P. K. Ghosh (CSMCRI)for his keen interest in this work.

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