J. Phys. Chem. B 2007, 111, 11157-11161
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Films of Novel Mesogenic Molecules at Air-Water and Air-Solid Interfaces Alpana Nayak, K. A. Suresh,* Santanu Kumar Pal, and Sandeep Kumar Raman Research Institute, SadashiVanagar, Bangalore - 560 080, India ReceiVed: April 25, 2007; In Final Form: July 16, 2007
Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.
1. Introduction The studies on discotic molecules at air-water (A-W) and air-solid (A-S) interfaces have drawn lot of attention.1,2 The thin films of discotic mesogens have technological applications in the fabrication of gas sensors,3 field-effect transistors,4 and photovoltaic diodes.5 As compared to conventional discotic liquid crystals, ionic discogens differ significantly as they combine the properties of liquid crystals and ionic liquids. This makes them promising candidates to design anisotropic ionconductive materials because of the anisotropic structural organization and the presence of ions as charge carriers.6 The long alkyl chains of the mesogenic molecule act as an insulating sheet for the ion-conductive channel. Yoshio et al. have reported one-dimensional (1D) ion transport in self-organized columnar ionic liquids.7 Discotic mesogens can self-assemble into 1D columnar superstructures due to the overlapping of the Π-orbitals of adjacent molecules and can arrange in a two-dimensional lattice (2D) at the A-W interface, thereby forming stable Langmuir monolayers.2,8 These thin films need to be transferred to solid supports for practical applications, and the preparation by means of the Langmuir-Blodgett (LB) technique is an efficient method to obtain highly ordered layers over large areas. LB films of discotic mesogens have been reported to have a surface morphology which are periodically modulated by columns that lie predominantly along the film deposition direction.1 Evidence has been presented for square-lattice discotic structures in the bilayer films obtained by LB technique.9 Ionov et al. have experimentally established smectic-like structures normal to the substrate, and the nematic-like in-plane arrangement of the discotic molecules in the LB films, resembling a discotic smectic-nematic phase.10 The possibilities to induce different structural arrangement in the LB films of discotic molecules allow us to undertake detailed studies of the intra- and interlayer molecular interactions, and especially of the steric and Π-Π interactions.10 These interactions also govern the structure and * Corresponding author. E-mail:
[email protected]. Telephone: +9180-28382924. Fax: +91-80-23610492.
properties of thin films of many biological and disk-shaped aromatic molecules.11 Although discotic molecules of alkoxytriphenylene derivatives have been reported to form stable Langmuir monolayers,12 there have been no reports to date on ionic alkoxytriphenylene derivative monolayers. In this contribution, we report our studies on Langmuir and LB films of novel cationic discotic molecules, that is, triphenylene-based pyridinium and imidazolium salts with bromine as counterion (Py-Tp and Im-Tp).13 We find that both the molecules exhibit a stable monolayer. The surface pressure-area per molecule (π-Am) isotherm is completely reversible. We find that the LB films of Py-Tp with two layers showed patterns with regular rectangular voids elongated in the film deposition direction, whereas the LB films of Im-Tp with two layers exhibited irregular shaped voids throughout the film. 2. Experimental Section The materials Py-Tp and Im-Tp were synthesized in our laboratory.13 The compounds were purified by repeated recrystallizations with diethyl ether and characterized by 1H NMR, 13C NMR, IR, UV spectroscopy, and elemental analysis which indicated high purity (99%) of the materials. The thermotropic liquid crystalline properties of these salts were investigated by polarizing optical microscopy and differential scanning calorimetry. Small-angle X-ray diffraction (XRD) was carried out on a powder sample using Cu KR (λ ) 0.154 nm) radiation from a Rigaku ultrax 18 rotating anode generator (4 kW) monochromated with a graphite crystal. The surface manometry experiments were carried out using a NIMA 611M trough. The subphase used was ultrapure deionized water (pH 5.8) obtained from Millipore Milli-Q system. The stock solution of 0.236 mM concentration was prepared using chloroform (HPLC grade). After spreading, the film was left for 20 min, allowing the solvent to evaporate. The π-Am isotherms were obtained by symmetric compression of the barriers with a constant compression rate of 0.103 nm2/ molecule/min. The surface pressure (π) was measured using the standard Wilhelmy plate technique. The equilibrium spreading pressure (ESP) was measured at a constant area of 100 cm2.
10.1021/jp073196z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/31/2007
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High humidity of the order of 92 ( 5% was maintained during the experiment to prevent any evaporation. To determine the ESP value, we monitored the surface pressure with time until its value reached a saturation. A Brewster angle microscope (BAM), MiniBAM (NFT, Nanotech, Germany) was employed to observe the films at the A-W interface. LB technique was employed to transfer various layers of the films onto hydrophilic and hydrophobic substrates at different target surface pressures (πt) with a dipping speed of 2 mm/min. To obtain hydrophilic surfaces, we treated polished silicon wafers for about 5 min in hot piranha solution (mixture of concentrated H2SO4 and H2O2 in 3:1 ratio) which were then then rinsed in ultrapure deionized water and later dried. For hydrophobic surfaces, freshly prepared hydrophilic silicon substrates were dipped in hexamethyldisilazane (HMDS) for 12 h and then rinsed with HPLC grade chloroform. Horizontal transfer technique was used to obtain films on substrates at low π. The atomic force microscope (AFM) studies on these LB films were performed using model PicoPlus (Molecular Imaging). We used silicon tips with spring constant of 21 N/m and resonance frequency 250 kHz. The AFM images were procured using the AC mode in ambient condition. UV-visible absorption spectroscopy was also carried out for the LB films transferred on quartz plate. All of the experiments were carried out at room temperature (25 ( 0.5 °C). 3. Results and Discussion The material Py-Tp in the bulk exhibits the following phase sequence: solid (S) - columnar phase (Col); 83.7 °C, Col isotropic (I); 95 °C. On cooling, the columnar mesophase appeared at 92 °C and remained stable down to room temperature. The structure of the columnar mesophase obtained by small angle XRD reveals a simple rectangular lattice of parameter a ) 4.24 nm and b ) 3.64 nm. The material Im-Tp in the bulk exhibits similar phase sequence: S - Col; 67 °C, Col - I; 101 °C. The columnar phase has a simple rectangular lattice with parameters a ) 5.76 nm and b ) 4.40 nm. On cooling, the columnar mesophase appeared at 98 °C with the mesophase solidifying at 38 °C. 3.1. Surface Manometry. The π-Am isotherm for Py-Tp (Figure 1a) and Im-Tp (Figure 1b) show a gradual rise in π up to 9 mN/m followed by a sharp rise. Both the isotherms exhibit a collapse pressure of about 44 mN/m. The limiting area per molecule (Ao) values obtained by extrapolation to zero surface pressure were 1.15 and 1.40 nm2/molecule at the steep increasing region and 3.1 and 3.4 nm2/molecule at the gradual increasing region respectively. Figure 2 shows molecular structures with approximate dimensions according to standard bond lengths and angles. The computed surface area of the aromatic core is equal to 0.785 nm2. The surface area occupied by the molecule increases to 4.5 nm2 if the extended peripheral hydrocarbon tails are included. The experimentally observed Ao values for the first transformation suggest an expanded phase, whereas for the second transformation, the observed Ao values suggest a condensed phase. The rigidity of the films were calculated from the compressional elastic modulus, |E|, given by
|E| ) (Am)dπ/dAm
(1)
The |E| value characterizes the nature of a monolayer’s stable phases. Specifically, a maximum value of the |E| in the range of 12.5 mN/m to 50 mN/m is characteristic of liquid expanded phase, whereas a value between 100 and 250 mN/m is characteristic of the liquid condensed phase.14 The variation of
Figure 1. Surface pressure (π)-area per molecule (Am) isotherms of (a) pyridinium-based discotic (Py-Tp) and (b) imidazolium-based discotic (Im-Tp) molecules.
Figure 2. Molecular structure of Py-Tp and Im-Tp molecules with approximate dimensions according to standard bond lengths and angles. The triphenylene disc area shown in dashed circle is about 0.785 nm2.
|E| with Am for Py-Tp and Im-Tp are shown in Figure 3. The |E| value showed a maximum of 83 mN/m at Am of 0.871 nm2/ molecule for Py-Tp. The maximum value of |E| for Im-Tp was 53.9 mN/m at Am of 0.977 nm2/molecule. The maximum |E| value attained by Py-Tp is much higher than that attained by Im-Tp monolayer. This indicates that the packing of the molecules in the Py-Tp monolayer to be better than that of ImTp monolayer. This may be attributed to the steric hindrance caused by an extra methyl group attached to the imidazolium nitrogen atom. We find a sharp change in the values of |E| indicating a phase transition at an Am of 1.6 nm2/molecule for Py-Tp molecule and at an Am of 1.8 nm2/molecule for Im-Tp molecule. The phase below these Am has much higher value of |E| compared to the phase above these Am values. The |E| values calculated as a function of Am (Figure 3) indicate that in both
Mesogenic Molecules at A-W and A-S Interfaces
Figure 3. Variation of compressional elastic modulus (|E|) with area per molecule (Am) for Py-Tp and Im-Tp molecules.
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Figure 6. Brewster angle microscope images of Py-Tp. (a) Condensed phase, (b) coexistence of 3D crystals and condensed phase corresponding to the collapsed state. The scale bar in each image represents 500 µm.
Figure 4. Equilibrium spreading pressure of Py-Tp and Im-Tp molecules.
Figure 7. AFM topography of Im-Tp monolayer transferred onto hydrophilic silicon substrates at (a) πt ) 5 mN/m, (b) πt ) 35 mN/m. The respective height profiles corresponding to the white line on the images are shown below. All of the images are shown without any image processing except flattening.
Figure 5. The π-Am isotherm cycles of Py-Tp showing reversibility from the collapsed state to the monolayer state with negligible hysteresis.
Py-Tp and Im-Tp, the monolayer undergoes a phase transition from an expanded phase to a condensed phase. The ESPs of Py-Tp and Im-TP molecules were found to be around 9 mN/m (Figure 4). The time scale for the molecules to elute from the crystallites was of the order of a few seconds. An interesting feature of the present two systems is that the monolayer on expanding and compressing shows negligible hysteresis. Also, in both the systems, the collapsed state completely reverts back to its monolayer state (Figure 5). Such a system with a reversible collapse may act as an ideal model system to understand several disc-like biological systems for example porphyrins, vitamins which play a significant role in the living systems. 3.2. Brewster Angle Microscopy. The BAM images show that the brightness increases gradually upon compression from nearly zero surface pressure (Figure 6a). This transforms to 3D crystals (Figure 6b) at the collapsed state. On expansion, these crystalline domains disappeared, and the system reverted back to the uniform intensity region indicating a monolayer state. This observation confirms the reversibility of the film. Some alkoxytriphenylene derivatives have been reported12 to form stable Langmuir monolayers with Ao value in the range 0.7-1.0 nm2 but the Py-Tp and Im-Tp systems exhibit
Figure 8. Schematic representation of (a) lying down and (b) standing up conformation at the A-W interface.
comparatively higher Ao values. This may be because of the presence of ionic groups, pyridinium and imidazolium, which tend to expand the films due to the direct electrostatic repulsion.15 On spreading the ionic discotic molecules, the small Brcounterions dissolve into the subphase due to the dissociation of the ionic groups. The presence of these Br- counterions and the charged insoluble film form an ionic double layer at the surface. The rigid aromatic rings which form the core of the discotics contain delocalized Π electrons. At low area per molecule, these cores overlap due to the strong Π-Π stacking interaction between the cores, thereby forming a 2D analogue of the columnar phase. 3.3. Atomic Force Microscopy. The AFM topography of the Im-Tp monolayer transferred at expanded phase (5 mN/m) by horizontal method on a hydrophilic silicon substrate (Figure 7a) shows the height of the film to be about 0.7 nm with reference to the substrate. This corresponds to the thickness of the molecules lying parallel to the A-S interface. When the monolayer was transferred at the condensed phase (35 mN/m)
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Figure 9. AFM topography of LB films of Py-Tp with two layers on hydrophobic silicon substrate at (a) πt ) 30 mN/m, (b) πt ) 35 mN/m, and (c) πt ) 42 mN/m. The uniformly distributed rectangular voids at πt ) 35 mN/m are shown at scan ranges (d) 2 × 2 µm2 and (e) 1 × 1 µm2. The respective height profiles corresponding to the white line on the images are shown below. All the images are shown without any image processing except flattening.
by LB technique (Figure 7b), the topography shows a uniform and homogeneous film with a height of about 2 nm with reference to the substrate. This corresponds to the thickness of the molecules lying normal to the A-S interface. The Π-Am isotherms and the calculated |E| values indicate a phase transformation from an expanded phase to a condensed phase. The Ao values obtained in the expanded phase were 3.1 and 3.4 nm2/molecule and in the condensed phase were 1.15 and 1.40 nm2/molecule for Py-Tp and Im-Tp, respectively. Also, the AFM topography images showed a height of about 0.7 nm for the film transferred at expanded phase and a height of about 2 nm for the film transferred at condensed phase. Comparing all of these results with the molecular dimensions (Figure 2), we infer the molecules to have a face-on structure at the expanded phase and an edge-on structure at the condensed phase. This also indicates that the structure of the Langmuir films was maintained throughout the transfer process from the A-W interface to the A-S interface. On the basis of these results, we schematically represent the face-on (lying down) and edge-on (standing up) configuration at the A-W interface as shown in Figure 8. In face-on configuration, the core lies flat on the water surface and the hydrocarbon tails (partially interdigited) extend away from the interface. Such a configuration may be preferred if the core of the molecule is capable of hydrogen bonding. It also maximizes
the configuration space available to the aliphatic tails. In the edge-on configuration the core lies normal to the interface with the polar end submerged in the water and bottom-most tails extending roughly parallel and slightly away from the interface. Such a conformation allows the polar oxygen to be in contact with water, while minimizing water contact with the hydrophobic alkyl tails.12 This maximizes the Π-Π interactions between the conjugated cores with the molecules arranged into columns parallel to the water surface, forming the 2D analogue of a columnar phase. Josefowicz et al.1 have reported the edgeon columnar structure in the LB films of triphenylene-based discotic molecules with the column alignment predominantly along the film deposition direction. However, they did not report face-on arrangement based on AFM.1 In our system, the presence of highly polar pyridinium and imidazolium moieties together with six oxygen atoms at the periphery of the triphenylene core have enhanced the hydrophilicity of the molecules, thereby facilitating the face-on configuration at large Am values at the A-W interface. Figure 9 shows AFM images of the LB films of Py-Tp with two layers transferred onto hydrophobic silicon substrates at various πt. For the films transferred at πt below 30 mN/m, irregular-shaped domains with significantly low surface coverage (45%) were observed (Figure 9a). However, the films transferred at πt of 35 mN/m (Figure 9b) and 42 mN/m (Figure 9c) revealed
Mesogenic Molecules at A-W and A-S Interfaces a much higher surface coverage (75%) with narrow rectangular voids elongated in the dipping direction of the LB transfer process. The height profile yields a value of about 4.0 ( 0.2 nm. This corresponds to the value of two layers of the molecules in the edge-on configuration. Panels d and e of Figure 9 show different scan ranges for the pattern with narrow rectangular voids (width 80-100 nm, length 250-400 nm) uniformly distributed over the entire region. We find consistency in the scaling of the void shapes and sizes with different scan ranges. The corresponding phase images show uniform polarity of the film, indicating good packing of the discotic core as expected due to Π-Π stacking. We also find that the number of voids decreased and the film became more compact with increase in π. However, full coverage could not be obtained although these films showed smooth top surfaces (Figure 9). The topography image of the LB films of Im-Tp with two layers on hydrophobic silicon substrates shows increased roughness with irregular voids throughout the film. This suggests that the molecular organization is sensitive to the particular head group-substrate interaction. With discotic mesogens, the effects of Π-Π and steric interactions on the molecular organization of the LB films are stronger as compared to the case of the classical rodlike molecules. UV-visible absorption spectra taken for the LB film with two layers of Py-Tp yield peaks at 258, 275, 305, and 344 nm. Similar absorption peaks were also observed for the ImTp films. These are particularly significant, indicating Π-Π stacking interaction between the conjugated triphenylene cores. We suggest that the molecular organization of the ionic discotic mesogens is driven by the interaction between the substrates and the polar head (pyridinium, imidazolium) groups, the steric intermolecular interactions, and the strong Π-Π stacking interaction between the triphenylene cores. In addition, the molecular packing is also affected by natural dewetting of the substrate while draining or evaporation of entrapped water within the films.16,17 4. Conclusions The novel mesogenic molecules exhibited stable Langmuir monolayer which show negligible hysteresis on expanding and compressing. Also, the collapsed state completely reverts to the monolayer state. As compared to the monolayers of nonionic triphenylene derivatives reported so far, these cationic discotics showed higher limiting area per molecule due to the direct
J. Phys. Chem. B, Vol. 111, No. 38, 2007 11161 electrostatic repulsion between the molecules within the film. We find that the molecules Py-Tp and Im-Tp were arranged into columns in the bulk as seen by our X-ray studies, and twodimensional columns at A-W and A-S interfaces as seen by our surface manometry and AFM studies. The presence of a cationic moiety in the discotic molecule opens the scope for studying electrostatic interaction between such a molecule and a negatively charged species such as DNA at interfaces. Our system of ionic discotic mesogens can act as a model to understand several disk-shaped biological systems such as porphyrins and vitamins. Supporting Information Available: X-ray diffraction patterns obtained for Py-Tp and Im-Tp molecules in the columnar phase at 85 °C; AFM topography images of LB films with two layers of Im-Tp molecules; AFM phase images of LB films with two layers of Py-Tp molecules. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Josefowicz, J. Y.; Maliszewskyj, N. C.; Idziak, S. H. J.; Heiney, P. A.; McCauley, J. P.; Smith, A. B. Science 1993, 260, 323. (2) Gidalevitz, D.; Mindyuk, O. Y.; Heiney, P. A.; Ocko, B. M.; Henderson, P.; Ringsdorf, H.; Boden, N.; Bushby, R. J.; Martin, P. S.; Strzalka, J.; McCauley, J. P.; Smith, A. B. J. Phys. Chem. B 1997, 101, 10870. (3) Dunbar, A. D. F.; Richardson, T. M.; McNaughton, A. J.; Hutchinson, J.; Hunter, C. A. J. Phys. Chem. B 2006, 110, 16646. (4) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mullen, K. AdV. Mater. 2005, 17, 684. (5) Schmidtke, J. P.; Friend, R. H. J. Chem. Phys. 2006, 124, 174704. (6) Binnemans, K. Chem. ReV. 2005, 105, 4148. (7) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2004, 126, 994. (8) Maliszewskyj, N. C.; Heiney, P. A.; Blasie, K. J.; McCauley, J. P.; Smith, A. B. J. Phys. II France 1992, 2, 75. (9) Maliszewskyj, N. C.; Heiney, P. A.; Josefowicz, J. Y.; McCauley, J. P.; Smith, A. B. Science 1994, 264, 77. (10) Ionov, R.; Angelova, A. Phys. ReV. E. 1995, 52, 21(R). (11) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (12) Mindyuk, O. Y.; Heiney, P. A. AdV. Mater. 1999, 11, 341. (13) Kumar, S.; Pal, S. K. Tetrahedron Lett. 2005, 46, 2607; 4127. (14) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961. (15) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: Boston, 1996. (16) Sikes, H. D.; Woodward, J. T., IV; Schwartz, D. K. J. Phys. Chem. 1996, 100, 9093. (17) Reiter, G. Phys. ReV. Lett. 1992, 68, 75.
