12332 DOI: 10.1021/la901614p Langmuir 2009, 25(20), 12332–12339Published on Web 10/28/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Molecular Arrangement in Langmuir and Langmuir-Blodgett Films of a

Mesogenic Bent-Core Carboxylic Acid

Ignacio Giner,†,§ Ignacio Gasc�on,†,§ Jorge Vergara,‡, ) M. Carmen L�opez,†,§ M. Blanca Ros,*,‡, ) andF�elix M. Royo*,†,§

† �Area de Quımica Fısica and ‡ �Area de Quımica Org�anica Departamento de Quımica Org�anica-Quımica Fısica,Facultad de Ciencias and §Instituto de Nanociencia de Arag�on and )Instituto de Ciencia deMateriales de Arag�on,

Universidad de Zaragoza, 50009 Zaragoza, Spain

Received May 6, 2009. Revised Manuscript Received July 17, 2009

A different alternative to previous research on Langmuir and Langmuir-Blodgett (LB) films of bent-core liquidcrystals is reported in this work. A bent-shaped molecule wearing a terminal carboxylic group has been used to obtainmonomolecular films with their long molecular axis almost perpendicular to the aqueous surface. Langmuir films at theair-liquid interface (pH=9) have been characterized by a combination of surface pressure and surface potential versusarea per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. A condensed phase isreached at surface pressures up to 20 mN 3m

-1. In this condensed phase, molecules are packed forming H-aggregateswith a well-defined molecular orientation. Langmuir films have been transferred onto quartz and silicon substrates andcharacterized bymeans ofUV-vis spectroscopy andXRR. The transference is Z-type, with a constant deposition of themonolayers. The total LBmonolayer film thickness is evaluated to be about 5.8 nm, which is in good agreement with thededuced orientation at the air-liquid interface as well as with the lamellar order observed within the solid obtained bycooling the sample from the mesophase.

Introduction

Bent-core or banana-shaped liquid crystals have been an activefield of study on mesomorphic materials since the pioneeringresults by Niori et al.1 Bent-core molecules, incorporating a bent-shaped rigid core instead of a linear one, could induce polar orderbut also chiral superstructures in some of their liquid crystallinemesophases, despite the fact that constituent molecules of thesemesophases were not chiral.2-7 Furthermore, such new meso-phases show noticeable optical, ferroelectric, and antiferroel-electric responses to add to the widely known ease ofmanufacture and control of supramolecular order that liquidcrystals offer.8 Consequently, bent-core compounds are verypromising for development of new materials that afford addi-tional functional responses to the liquid crystal behavior.

The outstanding properties of this type of liquid crystals arederived from their compact molecular packing that dramaticallyhinders the molecular rotation around their long molecular axis.But this packing is also responsible for one important aspect thatrepresents a difficulty in the use of bent-core liquid crystals, thatis, the bulk mesophase alignment, since the procedures normallyused for classical liquid crystals are not applicable in thesesystems. Sample alignment is a goal not only for applicationsbut also for basic research as monodomain samples will allow

better characterization, well-founded research proposals, andimproved comparative studies.8 However, few ideas have proveduseful to date to provide and to stabilize the attractive compactpacking of bent-core molecules.

The fundamental understanding of the basic relations betweenmolecular design and the liquid crystalline phase structures orfurther supramolecular systems as well as with materials proper-ties is critical for the development of new applications.9 Most ofthe research onbent-coremolecules has been focused on the studyof their organization in the mesomorphic phases;7 however, littleeffort has been made to explore the possibilities of bent-coremolecules on alternative supramolecular materials.

Langmuir films constitute a molecular organized system pro-viding unique opportunities for developing new materials with acontrolled architecture and particularlywith a low dimensionalityand a well-defined molecular density. In addition, stable Lang-muir films, transferred to solid substrates, may form alignedLangmuir-Blodgett (LB) films of liquid crystal molecules. Thus,the study of Langmuir films can give insight into the molecularpacking within layers, especially in the presence of an interfaceand also can allow us to obtain monodomain samples. However,to our knowledge, only a few works on Langmuir films of bent-core molecules can be found in the literature. Some of thesestudies have been focused on the behavior of suchmolecules at theair-water interface10-15 although the molecular orientation hasnot been clearly elucidated in most of them. In addition, a few*To whom correspondence should be addressed. Tel: (M.B.R.) þ 34 976

762277; (F.M.R.)þ 34 976 761198. Fax: þ 34 976 76 12 02. E-mail: (M.B.R.)bros@unizar.es; (F.M.R.) femer@unizar.es,(1) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater.

Chem. 1996, 6, 1231–1233.(2) Pelzl, G.; Diele, S.; Weissflog, W. Adv. Mater. 1999, 11, 707.(3) Walba, D. M., Topics in Stereochemistry; Wiley: New York, 2003; Vol. 42.(4) Ros,M. B.; Serrano, J. L.; de la Fuente,M. R.; Folcia, C. L. J.Mater. Chem.

2005, 15, 5093.(5) Reddy, R. A.; Tschierske, C. J. Mater. Chem. 2006, 16, 907–961.(6) Weissflog, W.; Murthy, H. N. S.; Diele, S.; Pelzl, G. Phil. Trans. R. Soc. A

2006, 364, 2657–2679.(7) Takezoe, H.; Takanishi, Y. Jpn. J. Appl. Phys. 2006, 45(2A), 597–625.(8) Etxebarria, J.; Ros, M. B. J. Mater. Chem. 2008, 18, 2919–2926.

(9) Tschierske, C. J. Mater. Chem. 2008, 18, 2869–2871.(10) Zou, L.; Wang, J.; Beleva, V. J.; Kooijman, E. E.; Primak, S. V.; Risse, J.;

Weissflog, W.; J€akli, A.; Mann, E. K. Langmuir 2004, 20, 2772.(11) Wang, J.; Zou, L.; J€akli, A.;Weissflog,W.;Mann, E. K.Langmuir 2006, 22,

3198.(12) Yamamoto, T.; Oguchi, S.; Manaka, T.; Iwamoto, M. Thin Solid Films

2006, 499, 242.(13) Yamamoto, T.; Manaka, T.; Iwamoto, M. Colloids and Surfaces A:

Physicochem. Eng. Aspects 2006, 284-285, 154.(14) Duff, N.; Wang, L.; Mann, E. K.; Lacks, D. J. Langmuir 2006, 22, 9082.(15) Duff, N.; Mann, E. K.; Lacks, D. J. Langmuir 2008, 24, 4456.

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promising results have been obtained in the study of the opticaland electrical properties of the LB films of these molecules.16-21

In this work, we study the Langmuir films of a bent-core mole-cule, herein called compound A (see Scheme 1). This bent-coremolecule is characterized by its unsymmetrical structure, with twodifferent terminal tails attached to the rigid aromatic core. Thereare different from both the length and, interestingly, from theirhydrophilic andhydrophobic nature.Namely, oneof them is ended

on a polar carboxylic headgroup. This concept, previously devel-oped by Tschierske et al.,22 facilitates the spreading of themoleculeon the aqueous subphase and its anchoring onto this surface. Thus,a different alternative to previous research on films based on bent-core molecules is reported in this work. Films characterization bydifferent methods have confirmed their monomolecular character.We also have investigated the monolayer transfer to form LB filmsconfirming that well-organized films with a defined molecularalignment can be obtained using the LB technique.

This is the first report of a series of systematic studies we arecarrying out using different bent-core molecules to providesignificant insight into the influence of the molecular structure(terminal groups, number of aromatic rings, linkage groups,length of the hydrophilic chains, etc.) on the molecular orienta-tion at different surfaces.

Experimental Section

Synthesis. Compound A was synthesized following thesynthetic route described in Scheme 1. Compounds 1 and 4 wereprepared according to the synthetic procedures described

Scheme 1. Synthesis of the Bent-Core Carboxylic Acid A

(16) Kinoshita, Y.; Park, B.; Takezoe, H.; Niori, T.; Watanabe, J. Langmuir1998, 14, 6256.(17) Ashwell, G. J.; Amiri, M. A. J. Mater. Chem. 2002, 12, 2181.(18) Baldwin, J. W.; Amaresh, R. R.; Peterson, I. R.; Shumate, W. J.; Cava, M.

P.; Amiri, M. A.; Hamilton, R.; Ashwell, G. J.; Metzger, R. M. J. Phys. Chem. B2002, 106, 12158–12164.(19) Blinov, L. M.; Geivandov, A. R.; Lazarev, V. V.; Palto, S. P.; Yudin, S. G.;

Pelzl, G.; Weissflog, W. Appl. Phys. Lett. 2005, 87, 241913.(20) Geivandov, A. R.; Palto, S. P.; Yudin, S. G.; Blinov, L. M.; Pelzl, G.;

Weissflog, W. Ferroelectrics 2006, 344, 3.(21) Garcıa-V�azquez, P.; Morales-Saavedra, O. G.; Pelzl, G.; Guadalupe

Ba~nuelos, J.; Carre�on-Castro, M. P. Thin Solid Films 2009, 517, 1770–1777.(22) Kardas, D.; Prehm, M.; Baumeister, U.; Pociecha, D.; Reddy, R. A.; Mehl,

G. H.; Tschierske, C. J. Mater. Chem. 2005, 15, 1722–1733.

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previously.22 Esterification of compound 1 with p-hydroxyben-zaldehyde and the subsequent oxidation of the compound 2allowed the preparation of compound 3. The intermediate 3 wasused for the esterification with compound 4 to prepare the benzylester 5 which led to the bent-core carboxylic acid A by hydro-genolytic debenzylation.

Experimental data of compounds 1 and 4were defined accord-ing to their expected chemical structure.

Compound 2. 4-Hydroxybenzaldehyde (1.45 g, 12 mmol), 1(4.81 g, 12 mmol), and DMAP (37 mg, 3 mmol) were dissolved in250 mL of dry CH2Cl2. A solution of DCC (3.71 g, 18 mmol) indry CH2Cl2 (10 mL) was added dropwise, and the mixture wasstirred at room temperature for 24 h. The white precipitate of N,N0-dicyclohexylurea was filtered from the solution. The solventwas evaporated, and the resulting product was purified by crystal-lization with ethanol which gave 2 as a white solid (5.6 g, 90%yield),mp 73 �C. 1HNMR(400MHz,CDCl3):δ=1.30 (m, 10H),1.47 (m, 2H), 1.65 (m, 2H), 1.78 (m, 42H), 2.36 (t, J=7.6 Hz, 2H,-OCOCH2), 4.025 (t, J = 6.6 Hz, 2H, -OCH2), 5.12 (s, 2H,-OCH2Ar), 6.98 (d,J=8.8Hz, 2H,Ar), 7.35 (m, 5H,Ar), 7.04 (d,J=8.4Hz, 2H, Ar), 7.96 (d, J=8.4Hz, 2H, Ar), 8.13 (d, J=8.8Hz, 2H, Ar), 10.02(s, 1H, CHO). 13C NMR (400 MHz, CDCl3):δ = 24.8, 25.8, 28.9, 29.0, 29.1, 29.2, 29.3, 34.2, 65.9, 68.2, 114.3,120.7, 122.4, 128.0, 131.1, 132.3, 133.7, 136.0, 155.8, 163.7, 164.1,173.1, 190.8. FTIR (KBr, ν = cm-1): 2916, 2850 (C-H), 2724(CHO), 1728 (CdO), 1597 (CdC, Ar), 1511 (CdC, Ar), 1212(C-O). Mass: MALDI-MS m/z= 539.2 [M þ Na]þ.

Compound 3.Compound 2 (5.4 g, 12.6mmol) was dissolved inacetic acid (90%, 40mL), and a solution of CrO3 (2.0 g, 21mmol)in acetic acid (60%, 18mL) was added. The resulting solutionwasstirred under solvent reflux for 5 h, 20mLofwaterwas added, andthe solution was allowed to cool to room temperature and wasstored overnight in a refrigerator (0-4 �C). The solid formed wasfiltered off, washed with water, and crystallized from ethanolleading to 3 as a white solid (3.6 g, 65% yield), mp crystal 95 �C,SmA 135 �C, N 160 �C, isotropic liquid. 1H NMR (400 MHz,CDCl3): δ=1.30 (m, 10H), 1.47 (m, 2H), 1.65 (m, 2H), 1.82 (m,2H), 2.38 (t, J=7.3Hz, 2H,-OCOCH2), 4.05 (t, J=6.4Hz, 2H,-OCH2), 5.12 (s, 2H, -OCH2Ar), 6.98 (d, J= 8.8 Hz, 2H, Ar),7.35 (m, 7H, Ar), 8.14 (d, J=8.4Hz 2H, Ar), 8.19 (d, J=8.0Hz2H,Ar); 13CNMR (400MHz, CDCl3): δ=24.4, 25.8, 28.9, 29.0,29.1, 29.2, 29.4, 34.5, 65.9, 68.3, 114.3, 120.9, 122.4, 128.1,131.1, 132.5, 133.7, 136.0, 155.8, 163.8, 164.1, 173.1; FTIR(KBr, ν =cm-1): 3428-2663 (-OH), 2923, 2847 (C-H), 1730(CdO), 1657 (CdC,Ar), 1555 (CdC,Ar), 1252 (C-O).MALDI-MS: m/z=555.2 [M þ Na]þ.

Compound 5. A mixture of 4 (5.3 g, 8.5 mmol), 3 (4.53 g,8.5 mmol), and DMAP (26 mg, 2.1 mmol) was dissolved in250 mL of dry CH2Cl2. A solution of DCC (2.1 g, 10 mmol) indry CH2Cl2 (10 mL) was added dropwise, and the mixture wasstirred at room temperature for 24 h. The white solid of N,N0-dicyclohexylurea was filtered from the solution. The solvent wasevaporated, and the resulting product was purified by crystal-lizationwith ethanol that gave 5 as awhite solid (6.8 g, 70%yield),mp crystal 88 �C, SmCP 106 �C, isotropic liquid. 1H NMR(400 MHz, CDCl3): δ = 0.88 (t, J = 6.4 Hz, 3H, -CH3)1.48-1.27 (m, 32H), 1.48 (m, 4H), 1.65 (m, 2H), 1.83 (m, 4H),2.36 (t, J=7.6 Hz, 2H, -OCOCH2), 4.06 (t, J=6.6 Hz, 4H,-OCH2), 5.12 (s, 2H, -OCH2Ar), 6.99(d, J=8.8 Hz, 4H, Ar),7.24 (m, 1H, Ar), 7.31 (d, J=8.4 Hz, 2H, Ar), 7.35 (m, 5H, Ar),7.38 (d, J=9.2Hz, 2H,Ar), 7.39 (d, J=8.8Hz, 2H,Ar), 7.46 (m,1H,Ar), 7.52 (d, J=4.8Hz, 2H,Ar), 7.67 (d, J=8.8Hz, 2H,Ar),8.16 (d, J=8.8Hz, 4H,Ar), 8.30 (d, J=8.8Hz, 2H,Ar), 8.31 (d,J=8.4 Hz, 2H, Ar). 13C NMR (400 MHz, CDCl3): δ=14.2, 22.7,24.9, 25. 9, 26.0, 29.1, 29.2, 29.3, 29.3, 29.3, 29.4, 29.5, 29.6, 29.6,29.7, 29.7, 31.9, 34.3, 66.1, 68.4, 68.4, 114.3, 11.4, 114.5, 120.4,120.6, 120.9, 121.9, 122.1, 122.1, 122.2, 124.7, 126.8, 126.8, 128.1,128.25, 128.2, 128.3, 128.5, 129.9, 131.8, 132.3, 132.4, 136.1, 138.0,142.1, 150.6, 151.3, 155.4, 163.8, 164.35, 164.4, 164.5, 173.7.FTIR(KBr, ν = cm-1): 2915, 2849 (C-H), 1731 (CdO), 1600 (CdC,

Ar), 1583 (CdC, Ar), 1260 (C-O). Mass: MALDI-MS m/z=1159.7 [MþNa]þ. Anal. Calcd for C72H80O12: C, 76.03; H, 7.09;Found: C, 75.50; H, 7.13.

Compound A. Compound 5 (3 g, 2.6 mmol) was dissolved in50mLof dry THF, and then Pd/C (10%) (0.3 g) was added. Afterthe solution was flushed with hydrogen, the hydrogenation wascarried out at 1 atmwith shaking (Parr hydrogenation apparatus)for 5 h.Then the solutionwas filtered, THFwas evaporatedunderpressure, and the product crude was purified by column chroma-tography (silica gel, eluyent CH2Cl2/ethyl acetate 10:0.5) giving toA as a white solid (1.9 g, 70% yield): mp crystal 147 �C (46.8 kJmol-1), isotropic liquid/isotropic liquid 145 �C (18.4 kJ mol-1),mesophase 131 �C (25.8 kJ mol-1), crystal. 1H NMR (400 MHz,CDCl3): δ=0.88 (t, J=6.8Hz, 3H,-CH3) 1.32-1.27 (m, 30H),1.48 (m, 4H), 1.64 (m, 2H), 1.83 (m, 4H), 2.36 (t, J=7.6 Hz, 2H,-OCOCH2), 4.05 (t, J = 6.6 Hz, 4H, -OCH2), 7.10 (d, J=8.8 Hz, 4H, Ar), 7.23 (m, 1H, Ar), 7.31 (d, J = 8.8 Hz, 2H, Ar),7.38 (d, J=8.8Hz, 2H,Ar), 7.39 (d, J=8.8Hz, 2H,Ar), 7.46 (m,1H,Ar), 7.51 (d, J=4.8Hz, 2H,Ar), 7.67 (d, J=8.4Hz, 2H,Ar),8.15 (d, J=8.8 Hz, 4H, Ar), 8.30 (d, J=8.8 Hz, 2H, Ar), 8.31 (d,J=8.4 Hz, 2H, Ar). 13CNMR (400MHz, CDCl3): δ=14.1, 22.7,24.7, 25.9, 29.0, 29.1, 29.2, 29.3, 29.4, 29.5, 29.6, 29.7, 31.9, 33.8,68.3, 68.4, 114.4, 120.5, 121.0, 122.1, 122.1, 124.7, 126.8, 126.9,128.3, 129.9, 131.9, 132,4; 138.0, 142.1, 150.6, 151.3, 155.4,163.8, 164.4, 164.5; FTIR (KBr, ν=cm-1): 3435-2665 (-OH),2920, 2843 (C-H), 1731 (CdO), 1600 (CdC, Ar), 1512 (CdC,Ar), 1249 (C-O). Mass: MALDI-MS m/z=1069.7 [M þ Na]þ.Anal. Calcd for C65H74O12: C, 74.54; H, 7.12; Found: C, 74.37;H, 7.20.

Materials Characterization. NMR experiments were per-formed on Bruker ARX-300 MHz and Bruker AV-400 MHzAvance spectrometer. Infrared spectra for all the compoundswereobtained using either a Mattson Genesis II FTIR or a NicoletAvatar 360 (FTIR) spectrophotometer in the 400-4000 cm-1

spectral range. Microanalyses were performed with a Perkin-Elmer 2400 microanalyser. Mass spectrometry was performedwith a Microflex (MALDI-ToF). Mesomorphic properties werestudied by optical microscopy using anOlympus BH2microscopewith crossed polarizers. The microscope was connected to aLinkam THMS 600 hot stage and an Olympus DP-12 camera.Transition temperatures were determined by differential scanningcalorimetry (DSC) using either a TA2910 differential calorimeteror a DSC Q20 or Q2000 calorimeter of TA Instruments. Eachapparatus was calibrated with indium (156.6 �C, 28.44 J/g) and tin(232.1 �C, 60.5 J/g) using a scanning rate of 10 �C/min in mostcases. X-ray diagrams on nonoriented samples were carried outwith a powder diffractometer equipped with a high-temperatureattachment. Measurements were performed in Debye-Scherreroperation mode using Lindemann capillaries of diameter 0.6 mm.The materials were introduced in the isotropic phase into thecapillaries. A linear position-sensitive detector, with an angularresolution better than 0.01�, was employed to detect the diffractedintensity in the 2θ interval 0-30� (θ is the diffraction angle).Monochromatic Cu KR radiation (λ=1.5406 A) was used.

Films Fabrication and Characterization. Two differenttroughs were used in this work: a Nima trough with dimensions720 � 100 mm2 was used to record simultaneously π-A andΔV-A isotherms and also to carry out UV-vis reflection experi-ments,while ahomemadeTeflon troughwith dimensionsof 460�210mm2 has been used to record BAM images and to prepare LBfilms. Both were housed in a constant temperature (20 ( 1 �C)clean room.

Ultrapure Milli-Q water (F=18.2 MΩ 3 cm) has been used todissolve NaOH to obtain a subphase with pH=9. The surfacepressure (π) was measured by a Wilhelmy paper plate pressuresensor. The spreading solutions were prepared in chloroform/ethanol in proportion 4:1 (v/v). The ethanol (99.9%) and chloro-form (99.9%) were provided by Aldrich and Panreac, respec-tively. The solution was spread drop to drop on the aqueoussurface using a microsyringe held very close to the aqueous

DOI: 10.1021/la901614p 12335Langmuir 2009, 25(20), 12332–12339

Giner et al. Article

surface, and then the solvent was allowed to completely eva-porate over a period of at least 15 min before compression ofthe monolayer at a constant sweeping speed of 0.02 nm2

3molecule-1

3min-1. Each compression isotherm was registeredat least three times to ensure the reproducibility of the results. TheΔV-A measurements were carried out using a Kelvin probeprovided by Nanofilm Technologie GmbH, Germany. A mini-Brewster angle microscope (mini-BAM), also from NanofilmTechnologie, was employed for direct visualization of the mono-layers at the air-liquid interface, and a commercial UV-visreflection spectrophotometer was used to obtain the reflectionspectra of the Langmuir films upon the compression process.

The monolayers were deposited at a constant surface pressureby the vertical dipping method, and the dipping speed was0.6 cm 3min-1. The solid substrates used to support the LB filmswere quartz for the ultraviolet measurements and Si (100) for theX-ray reflectivity measurements. More details concerning thecleaning procedure of the substrates and the preparation condi-tions have been reported before.23

Ultraviolet (UV) spectra were acquired on a Varian Cary50 spectrophotometer. X-ray reflectivity experiments were per-formed with a Bruker D8 Advance with Cu KR radiation. Con-tinuous scans along 2θ/ω (2θ - ω) were obtained. The reflectedbeam intensity was recorded as a function of the wave vectortransfer along the substrate normal. Thewave vector transfer (Qz)is directly related to the incident angle (θinc),Qz=sin θinc� 4π/λ.No off-specular/background scattering has been subtracted, andthe intensities are given in arbitrary units, since the curves havebeen rescaled. Simulations were carried out usingLeptos softwaresuite, were a layered sample model was constructed to generate asimulated reflectivity curve, given the initial mass density, thick-ness and interface roughness parameters. Initial parameters wererefined to minimize the deviation between the experimental andsimulated reflectivity curves. The simulated annealing algorithmwas used in the trial-and-error process.

Results and Discussion

The ability of this molecule to induce well-packed supramole-cular arrangements was initially confirmed as compound A

exhibits liquid crystal properties. This compound was studiedby polarized optical microscopy (POM) and calorimetry (DSC),and both textures and enthalpic transition changes are accordingto the formation of a bent-core mesophase (Figure 1a,b). Due tothe short-range of the monotropic mesophases, the samplecrystallizes during the X-ray diffraction experiments at variabletemperature. Interestingly, twopeaks at small angles, correspond-ing to (001) and (002) reflections, can be observed that lead us topropose that in the crystallized material a lamellar packing with alayer spacing around 6.5 nm exists.

Solutions of compound A in chloroform/ethanol 4:1 v/v ofseveral concentrations (ranging from 1� 10-6 to 5� 10-5 M)have been studied with UV-vis light using a 1 mm cuvetteto verify that the Lambert-Beer law is followed for solutionsof concentration lower than 5 � 10-5 M, as shown in Figure 2.A 2.5� 10-5 M solution has been employed to fabricate theLangmuir films.Langmuir Films. Previous studies with liquid crystal (LC)

compounds suggest that the LC inherent order has an impact onwater surface organization of thesemolecules.24 Ordinary smectic

LCs form monolayers at the air-water interface, as well asmultilayers when compressed,25-27 while bent-core moleculesusually form multilayers.10,11 Moreover, carboxylic acids usuallyform dimers at the air-water interface. It was therefore necessaryto prepare the films onto an aqueous basic subphase (pH= 9), inwhich the carboxylic groups can be expected to be fully ionized,

Figure 1. (a) Microphotograph of the texture observed for com-pound A at the liquid crystalline phase on cooling. (b) DSC tracesof the second heating and cooling runs of compound A at10� 3min-1.

Figure 2. UV-vis absorption spectra of compound A in chloro-form/ethanol solution 4:1 (v/v) at several molar concentrationsobtained using a 1 mm cuvette.

(23) Cea, P.; Lafuente, C.; Urieta, J. S.; L�opez, M. C.; Royo, F. M. Langmuir1996, 12, 5881.(24) Suresh, K. A.; Blumstein, A.; Rondelez, F. J. Phys. (Orsay. Fr.) 1985, 46,

453.(25) Rapp, B.; Gruler, H. Phys. Rev. A 1990, 42, 2215.(26) de Mul, M. N. G.; Mann, J. A. Langmuir 1994, 10, 2311.(27) Wang, L.; Tian, Y.; Xi, S.; Ren, Y. J. Phys. Chem. B 1998, 102(43), 8353–

8256.

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allowing a more expanded monolayer due to the repulsiveCoulombic forces between the negatively charged head groupswhile also serving to eliminate formation of aggregates within theLangmuir film.

Figure 3 shows the surface pressure-area (π-A) and surfacepotential-area (ΔV-A) isotherms obtained simultaneously ontoan aqueous subphase (pH=9). The surface pressure starts toincrease slightly at ca. 0.90 nm2

3molecule-1, although π remainsas small as 1 mN 3m

-1 until an area of 0.65 nm23molecule-1 is

reached. From this point, surface pressure increases continuouslywhen the available area per molecule is reduced until the collapseof the monolayer, which is observed at ca. 45 mN 3m

-1. Theobserved areas per molecule in the condensed phase regionindicate that a closely packed interfacial arrangement of themolecules is obtained.

Measurements of the surface potential, ΔV, provide comple-mentary information about molecular organization at the air--liquid interface. A sharply surface potential increase of 370 mVcan be observed when the available area is reduced until valuessmaller than 1.10 nm2

3molecule-1 are attained.Then a slight dropin ΔV takes place, followed by a new increase in surface potentialuntil a value of 525 mV is attained at 0.65 nm2

3molecule-1. Thesurface potential slope varies and ΔV increases in the 0.65-0.48nm2

3molecule-1 region, reaching a value of 640mV. After that, anew change to a more pronounced ΔV slope is observed, until asudden and pronounced drop occurs at 0.32 nm2

3molecule-1.That areapermolecule is bigger than themonolayer collapse in theπ-A isotherm at ca. 0.29 nm2

3molecule-1. This could be due tothe appearance of some local collapses yielding 3D structures.Such 2D-3D transformations often occur with condensed andcrystalline phase monolayers at surface pressures below thecollapse surface pressure,28 and they are more easily detected inΔV-A isotherms.

The inset in Figure 3 shows the product (ΔV � A) versus thearea per molecule. A quantitative relationship between thisproduct and the normal component of the dipole momentof the molecules (μn) has been established by means of modelsbased on the Helmholtz equationΔV ¼ μn

Aεrε0þ Ψ0 where εr and

ε0 are the relative dielectric constant and the permittivity ofvacuum, respectively, and Ψ0 is the double-layer contribution.The main limitations to use this equation are that we do notknow the values of Ψ0 and εr for molecule A; consequently,we have limited our discussion to a qualitative interpretation.After the sharp increase of surface potential that takes place at ca.

1.10 nm2, (ΔV � A) oscillates without changing significantlyits value between 1.00 and 0.75 nm2, which is the same regionwhere surface pressure takes off. From this point, (ΔV � A)values continuously diminish. A more marked decrease isobserved when the collapse of the monolayer takes place at ca.0.32 nm2

3molecule-1.BAM images were also recorded at different stages of compres-

sion (Figure 4). Uncovered subphase areas together with abranched morphology are observed at high areas per molecule.When the compression continues the structure becomes morecompact forming a brighter layer with several holes in theirstructure. These holes are successively covered, forming a com-pact and brighter monolayer until a value of ca. 20 mN 3m

-1

(0.42 nm23molecule-1) is reached, when almost the whole surface

is covered by the monolayer. Further compression makes theimages brighter, suggesting a tilt of the molecules to a morevertical position.

To complete the air-liquid film characterization UV-visreflection spectra were obtained during compression. Figure 5shows the UV-vis normalized reflection spectra of A upondifferent compression states. The intensity of the normalizedspectra, ΔRn = ΔR � A, increases when surface pressure raises,which is indicative of a gradual augment of the angle formed bythe normal to the surface and the transition dipole moment of themolecules.29 In addition to the relative variation in reflection

Figure 3. π-A (black line) and ΔV-A (blue line) isotherms ofcompound A at the air-liquid interface. Inset: ΔV � A versusmolecular area.

Figure 4. BAM images of compoundA at the air-liquid interfaceduring the compression process.

(28) Angelova, A.; Ionov, R. Langmuir 1999, 15, 7199.(29) Pedrosa, J. M.; Martın-Romero, M. T.; Camacho, L. J. Phys. Chem. B

2002, 106, 2583.

DOI: 10.1021/la901614p 12337Langmuir 2009, 25(20), 12332–12339

Giner et al. Article

intensity, there is a blue shift in themaximumwavelength fromaninitial value of 265 to 254 nm at high surface pressures. Thisshift could be indicative of the formation of H-aggregates atthe air-liquid interface, when the molecules reach a compactpacking.

To interpret the intensity increase of the normalized reflection,we should take into account the geometry of the bent-shapedmolecule and his transition dipole moment direction. DFTcalculations30 have recently shown the influence of the orienta-tion of the ester linkage groups on the structural and elec-tronic properties of bent-shaped molecules with a structuresimilar to that of compound A (molecules with a central1,3-phenylene unit).

We have estimated the total dipolar moment direction ofmolecule A by means of the MOPAC200931 software packageusing the semiempirical methods PM632 and RM1.33 Thesemethods can obtain results close to those obtained by DFT inthe calculation of quantummechanical descriptors such as dipolemoment.34

As we can see in Scheme 2 the polar axis of the molecule A isalmost perpendicular to the long molecular axis. Consequently,the increase of the angle formed by the normal to the surface andthe transition dipole moment of the molecules implies a morevertical position of the molecules (i.e., dipole transition momentparallel to the aqueous subphase, maximum reflection). We willsee shortly that a quantitative analysis of these spectra confirmsour hypothesis.

In solution, the orientation of the molecules is random, andtherefore, the absorption must be proportional to a factor of 2/3given that only two out of the three components of the transitionmoment are interacting with the incident unpolarized light.Nevertheless, at the air-liquid interface, there is a preferential

orientation of the molecules. For a general case, and with astatistical distribution of the transition moments around thesurface normal, the orientation factor is given by:

forient ¼ 3

2sin2 φ ð1Þ

where φ is the angle formed by the normal to the surface andthe transition dipole moment of A (Scheme 2). This equationis applicable only if there is a homogeneous distribution ofthe transition moments around the surface normal. As shownpreviously in BAM images, surface areas free of monolayerare present below a surface pressure of 20 mN 3m

-1. In thissituation, φ represents the average polar angle formed by thenormal to the surface and the transition dipole moment ofthe molecules. In the equations below, the calculation to obtainthis angle is presented.

An integration of the absorption bandofA in solution yields anoscillator strength of f = 1.8830 according to the equation35

f ¼ 4ε02:303mec0

NAe2

Zband

εdυ ¼ 1:44� 10-19

Zband

εdυ ð2Þ

where ε0 is the permittivity of vacuum,me the electronmass, e theelectron charge, c0 the light speed in a vacuum, and NA theAvogadro constant. The factor 1.44 � 10-19 is expressed inmol 3L

-13 cm 3 s.

The comparison of the oscillator strength obtained in solutionwith the apparent oscillator strength, fapp, calculated from thenormalized reflection spectra, allows the determination of theorientation factor defined as

forient ¼ fapp

fð3Þ

where fapp is given by

fapp ¼ 2:6� 10-12

Zband

ΔRndυ ð4Þ

The numeric factor 2.6 � 10-12 is expressed in nm-23 s.

Substitution of the values obtained in eq 2 (oscillator strength)and eq 4 (apparent oscillator strength) in eq 3 gives the orientation

Figure 5. Normalized UV-reflection spectra of compound A, atthe air-liquid interface, during the compression process.

Scheme 2. Orientation of CompoundA at the Air-liquid Interface: φ

Is the Average Angle between the DipoleMoment of theMolecule and

the Normal to the Aqueous Subphase

(30) Krishnan, S. A. R.; Weissflog, W.; Pelzl, G.; Diele, S.; Kresse, H.;Vakhovskayab, Z.; Friedemann, R. Phys. Chem. Chem. Phys. 2006, 8, 1170–1177.(31) Stewart, J. J. P. Stewart Computational Chemistry, Colorado Springs, 2008.(32) Stewart, J. J. P. J. Mol. Model 2007, 13, 1173–1213.(33) Rocha,G. B.; Freire, R. O.; Simas, A.M.; Stewart, J. J. P. J. Comput. Chem.

2006, 27, 1101–1111.(34) Puzyn, T.; Suzuki, N.; Haranczyck, M.; Rak, J. J. Chem. Inf. Model 2008,

48, 1174–1180.(35) Kuhn, H.; F€orsterling, H. D.Principles of Physical Chemistry; JohnWiley &

Sons: New York, 1999.

12338 DOI: 10.1021/la901614p Langmuir 2009, 25(20), 12332–12339

Article Giner et al.

factor, which is used in eq 1 to obtain the angle φ formed by thenormal to the surface and the transition dipole moment of A.

In Figure 6 we have plotted (ΔV�A) values together withφ expressed in degrees. It should be noted that spectra recorded atbig areas per molecule show a high dispersion in the angle values(not shown), probably due to the presence of nonuniformdomains that produce significant fluctuations in the signal36 fromonemeasurement to another. For areas permolecule smaller than1.40 nm2 the spectra follow a systematic evolution and φ increasesfrom a value of ca. 20� at 1.40 nm2 to 43� at 1.00 nm2, indicatingan average evolution of the molecules to a more vertical position.After that, we can distinguish three different regions undercompression before the collapse of the monolayer: φ slightlyincreases between 1.00 nm2 and 0.55 nm2. A marked φ increase isthen observed until a value of ca. 58� is achieved when thecondensed phase is reached at about 0.42 nm2 (π=20mN 3m

-1).φ does not change significantly in the condensed phase, and itsmaximum value is ca. 60�. Finally, φ diminishes at small areas permolecule, indicating the collapse of the monolayer.

Figure 6 also allows interpretation of the (ΔV�A) dependencywith the normal component of the dipole moment of the mole-cules; the sharp increment at ca. 1.10 nm2 could be mainlyattributed to the reorientation of water molecules and ions inthe subphase. For smaller areas, molecules are closer and surfacepressure starts to increase; the total dipole moment becomesmoreparallel to the liquid surfacewhenmolecules adopt amore verticalposition, and consequently, μn and (ΔV�A) diminishes when theφ angle increases.Langmuir-Blodgett Films. Langmuir monolayers were

transferred onto solid substrates (quartz or silicon) by the ver-tical dipping method at 25 mN 3m

-1. The hydrophilic substrateswere initially immersed in the subphase, and the depositionprocess took place during the upstroke of the substrate (Z-typedeposition). The deposition ratio is very close to unity on each ofthe substrates assayed, indicating good transference of the mole-cules. Figure 7 shows the UV-vis spectra of the LB films up toeight layers. There is a blue shift in the maximum wavelengthwhich is placed at 245 nm in the LB films, while it is located at254 nm at the air-liquid interface and 269 nm in solution. Thisshift could be indicative of an increase of the formation of two-dimensional H-aggregates, already present at the air-liquidinterface. The relationship between the absorbance and numberof layers was found to be linear up to eight LB layers. The linear

relationship confirms a constant transfer ratio during the LBfabrication.

Si (100) substrates with one monolayer transferred at25 mN 3m

-1 were analyzed with the XRR technique. The experi-mental data are shown in Figure 8 expressed by reflectivity vsmomentum transfer (QZ) together with the fitting model. Theresults confirm that a uniformmonolayer can be transferred ontothe silicon substrate. The fitting vary the free parameters in orderto achieve a match between the measured and the calculatedreflectivity. A layer of SiO2 on top of the Si(100) has been added

Figure 6. ΔV�A isotherm (black line) and calculated values fromnormalizedUV-reflection spectra of the average angle (φ) betweenthe dipole moment of the molecule and the normal to the aqueoussubphase (blue circles).

Figure 7. UV-vis absorption spectra of LB films of com-pound A transferred onto quartz substrate at a surface pre-ssure of 25 mN 3m

-1. Inset: linear dependence of absorbancewith the number of transferred layers at the maximum absorp-tion peak.

Figure 8. XRR of a single LB monolayer of A transferred ontoSi(100) substrate at a surfacepressureof 25mN 3m

-1: experimental(red circles) and simulated data (continuous black line).

(36) Gil, A.; Arıstegui, I.; Su�arez, A.; S�andez, I.; M€obius, D. Langmuir 2002, 8(22), 8527–8534.

DOI: 10.1021/la901614p 12339Langmuir 2009, 25(20), 12332–12339

Giner et al. Article

in order to obtain a better model.37 The fitting indicates that theoverall thickness of the monolayer is ca. 5.8 nm, while the lengthof the molecule in this configuration has been estimated to be6.6 nm using ACD/Laboratories 11.0 software package. Further-more, this data is in the range of the layer thickness measured inthe solid phase (around 6.5 nm), suggesting that the molecularpacking at the films are very close to the molecular arrangementobtained by cooling the compound from the bulk liquid crystalstate.

In Scheme 3 we have shown a schematic representation of themolecular alignment in the LB films taking into account all theexperimental data. As can be seen, the aliphatic chain functiona-lized with the acid carboxylic group is almost perpendicular tosubstrate surface, which leads to an angle between the dipolemoment and the perpendicular to the surface of about 60�, thesame value that has been obtained from the normalized reflectionspectra of the molecule at the air-liquid interface.

Conclusions

Stable Langmuir films of compoundA have been prepared andcharacterized onto an aqueous subphase (pH=9). Monolayer

formation has been investigated using π-A, ΔV-A isotherms,and BAM images, showing that a condensed phase is achieved at20 mN 3m

-1. UV-vis reflection experiments have shown thatmolecules are tilted up during compression, reaching a maximumvalue of the angle between the dipole moment and the normal tothe surface of about 60� when the monolayer is in the condensedphase. Furthermore, a blue-shift in the UV-vis reflection spectrais observed during compression, indicating the formation ofH-aggregates in the monolayer. X-ray reflection measurementsconfirm the formation of homogeneous monolayer film; the totalfilm thickness of this monolayer is about 5.8 nm, from this value asimilar alignment of themolecules in the LB films to that observedat the air-liquid interface can be deduced. Transference of severalmonolayers onto a quartz substrate shows a constant depositionof the monolayers. The transference is Z-type, resulting in forma-tionofnoncentrosymmetric LB layers. Interestingly our results arein very good agreement with those recently reported by Marceliset al.38who, by at first, covalently attached bent-coremesogen to asilicon surface in order to achieve monolayers. Thus, our resultscan be considered as an easier alternative to the covalent attach-ment and point out that LB films open very attractive potentialsfor bent-core molecules and bent-core based materials to developnew functional materials with order-dependent response.

Acknowledgment. We are grateful for the Spanish and Eur-opean financial assistance (MEC and FEDER) in the frameworkof the project CTQ2006-05236 and MAT2006-13571-CO2-01.We are also indebted to Arag�on Government for financialsupport. I.G. and J.V. gratefully acknowledge their predoctoralfellowships from the Arag�on Government and the BS-UZ pro-gram, respectively. We are grateful to Luis Morell�on and LauraCasado (Instituto de Nanociencia de Arag�on) for their help withthe XRR experiments.

Note Added after Print Publication. The original web andprint versions of this article did not contain “Instituto deCiencia deMateriales de Arag�on” in the author address line.This error was corrected, and the article was reposted to theweb on October 28, 2009. An Addition and Correction alsoappears in the December 1, 2009 issue (Vol. 25, No. 23).

Scheme 3. Representation of the Orientation of Compound A on LB

Films Transferred onto Solid Substrates at 25 mN 3m-1

(37) Xia, W. M., B. A.; Armstrong, M. D. Langmuir 2004, 20, 7998–8005.(38) Scheres, L.; Achten, R.; Giesbers, M.; de Smet, L. C. P. M.; Arafat, A.;

Sudh€olter, E. J. R.; Marcelis, A. T. M.; Zuilhof, H. Langmuir 2009, 25, 1529–1533.