Langmuir 2009, 25, 839-844
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Novel Mesogenic Azobenzene Dimer at Air-Water and Air-Solid Interfaces Bharat Kumar,† A. K. Prajapati,‡ M. C. Varia,‡ and K. A. Suresh*,§ Raman Research Institute, SadashiVanagar, Bangalore-560 080, Applied Chemistry Department, The M. S. UniVersity of Baroda, KalabhaVan, Vadodara-390 001, and Centre for Liquid Crystal Research, P.B.No:1329, Jalahalli, Bangalore-560 013, India ReceiVed March 10, 2008. ReVised Manuscript ReceiVed NoVember 7, 2008 We have synthesized a novel mesogenic azobenzene molecule and studied its monolayer film properties at air-water interface (Langmuir film) and air-solid interface (Langmuir-Blodgett film). The material, H-shaped dimer bis[5(4′-n-dodecyloxy benzoyloxy)-2-(4′′-methylphenylazo)phenyl] adipate (12D1H) exhibits a smectic C phase between 51 and 48 °C on cooling. Surface manometry studies showed the formation of a stable monolayer at the air-water interface. Brewster angle microscopy (BAM) showed that liquid domains coexisting with the gas region at large area transformed to a uniform liquid phase with increasing surface density and finally to a collapsed state. We have carried out atomic force microscope (AFM) studies on Langmuir-Blodgett (LB) films transferred onto freshly cleaved hydrophilic mica substrate. The AFM images showed domains of height of about 3.8 nm, which corresponds to the estimated height of the molecule confirming the formation of monomolecular film. On a hydrophobic silicon substrate, the LB transfer yields a bilayer film, which dewets to form uniform nanodroplets of diameter of about 100 nm and height in the range 10-50 nm. Our analysis indicated that the mechanism involved in the formation of nanodroplets can be attributed to spinodal dewetting. The 12D1H molecule containing an azobenzene group undergoes a trans to cis transformation in the presence of ultraviolet light. Our surface manometry studies showed that the monolayer in the presence of ultraviolet light was more stable with a collapse pressure three times that of the monolayer in the dark.
1. Introduction The study of monolayer films of molecules with different shapes at air-water interface (Langmuir monolayer) is of interest to understand the packing of molecules based on their structure, intermolecular interactions, and interactions with the subphase.1,2 These films can be transferred onto solid substrates for studying the wetting behavior, which is important for some device applications. Stable and defect-free films are required for many technological applications, while controlled dewetting processes are important for producing thin film microstructuring for microelectronics, optical devices, and biochip technology.3 Wetting of films on a substrate depends on factors like the surface treatment given to the substrate and the nature of the material of the film. Molecules with different architecture have been used to study such properties. Among them, azobenzene molecules are of special interest, as the photoisomerization reaction can bring in situ changes in the molecular architecture, resulting in interesting behavior.4 Additionally, the molecules with an azobenzene group find application in photomechanical devices.5 Liquid crystals containing the azobenzene group are promising materials for devices involving photomechanics,6 optical switching, and image storing because of their high resolution and sensitivity.7 * Corresponding author. E-mail:
[email protected], Tel: +91-80-2838 2924, Fax: +91-80-2838 2044. † Raman Research Institute. ‡ The M. S. University of Baroda. § Centre for Liquid Crystal Research.
(1) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. ReV. Mod. Phys. 1999, 71, 779. (2) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York; 1990. (3) Rezende, C. A.; Lee, L. T.; Galembeck, F. Microsc. Microanal. 2005, 11, 110. (4) Ignes-Mullol, J.; Claret, J.; Albalat, R.; Crusats, J.; Reigada, R.; Romera, T. M.; Sagues, F. Langmuir 2005, 21, 2948. (5) Barrett, C. J.; Mamiya, J.-i.; Yager, K. G.; Ikeda, T. Soft Matter 2007, 3, 1249. (6) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145.
We have synthesized and studied the monolayer properties of a novel H-shaped mesogenic azobenzene molecule bis[5-(4′n-dodecyloxybenzoyloxy)-2-(4′′-methylphenylazo)phenyl] adipate (12D1H) at air-water and air-solid interfaces. The monolayer at the air-water interface was studied using the techniques of surface manometry and Brewster angle microscopy (BAM). The film was transferred onto hydrophilic and hydrophobic solid substrates by Langmuir-Blodgett (LB) technique, and the wetting behavior was studied using atomic force microscopy (AFM). The film transferred onto the hydrophobic silicon substrate dewets to yield nanodroplets. Our analysis of AFM images indicated that the dewetting of the film to form nanodroplets was through the mechanism of spinodal dewetting. These azobenzene molecules are photosensitive and, by shining ultraviolet (UV) light, can be switched from the trans isomer state (trans-12D1H) to the cis isomer state (cis-12D1H). We have carried out surface manometry studies of the monolayer in the dark and in the presence of UV light of the wavelength 365 nm. We find that the monolayer in the presence of UV light (cis-12D1H) is even more stable with a collapse pressure about three times higher than that of the monolayer in the dark (trans12D1H).
2. Experimental Section 2.1. Synthesis and Characterization. The material, bis[5-(4′n-dodecyloxy benzoyloxy)-2-(4′′-methylphenylazo)phenyl] adipate (12D1H) was synthesized in our laboratory. The starting materials, 4-hydroxybenzoic acid, the appropriate n-alkyl bromides (BDH), KOH, resorcinol, p-toluidine, NaNO2, thionyl chloride (Sisco Chem.), adipic acid, 4-N,N-dimethylaminopyridine (DMAP), and N,N′dicyclohexylcarbodiimide (DCC) were used as received. All the solvents were dried and distilled prior to use. The H-shaped compound (12D1H) was prepared by following the pathway shown in the scheme (Figure 1). 4-Methyl-2′,4′-dihydroxy azobenzene (A), 4-n-alkoxy(7) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873.
10.1021/la8030733 CCC: $40.75 2009 American Chemical Society Published on Web 12/22/2008
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Figure 1. Scheme for synthesis of the material 12D1H. On bottom left, energy-minimized structure of 12D1H is shown.
benzoic acids (B), 4-n-alkoxybenzoyl chlorides (C), and 4-methyl2′-hydroxy-4′-(4-n-dodecyloxy) azobenzene (D) were synthesized following the methods described earlier.8 To a solution of 4-methyl2′-hydroxy-4′-(4-n-alkoxybenzoyloxy) azobenzene (1.6 mmol), adipic acid (0.69 mmol), and 4-N,N-dimethylaminopyridene (0.25 mmol) in dry tetrahydrofuran (15 mL), N,N′-dicyclohexylcarbodiimide (1.6 mmol) was added, and the resulting solution was stirred at room temperature for about 120 h. After filtration to remove the precipitated materials, the solvent was evaporated under reduced pressure. The product was purified by column chromatography (petroleum ether 60-80 °C/ethyl acetate, 98/2, v/v) over silica gel and recrystallized from ethanol. Microanalysis of the compound was carried out on a Coleman carbon-hydrogen analyzer, and the values obtained were in close agreement with the calculated values. IR spectra were determined in KBr pellets, using a Shimadzu IR-8400 spectrophotometer (Supporting Information, Figure S1). 1H NMR spectra were obtained (8) Vora, R. A.; Prajapati, A. K.; Kevat, J. B.; Raina, K. K. Liq.Cryst. 2001, 28(7), 983.
with a Perkin-Elmer R-32 spectrometer using tetramethylsilane (TMS) as the internal reference standard. The chemical shifts are quoted as δ (parts per million) downfield from the reference. CDCl3 was used as a solvent. The details of IR and NMR studies are given in the Appendix. Liquid crystalline properties were investigated using a Leitz Laborlux 12 POL microscope equipped with a Mettler FP82HT hot stage. The material 12D1H is photosensitive. We have carried out UV-visible spectroscopy studies of the material in chloroform solution. The spectrum shows peaks corresponding to the absorption, which causes the transition from trans to cis state, and vice versa (Supporting Information, Figure S2). 2.2. Film Characterization. Films of 12D1H were studied at air-water and air-solid interfaces using techniques of surface manometry, BAM, and AFM. The surface manometry studies were carried out using a Nima trough (model 611M). The trough was enclosed in an aluminum box to prevent ambient light, air drag, and contaminants. Ultrapure deionized water (Millipore water, MilliQ) was used as the subphase. A dilute solution of 12D1H prepared in HPLC-grade chloroform was spread drop by drop at the air-water
Mesogenic Azobenzene Dimer at Interfaces
Figure 2. Surface pressure (π) - area per molecule (Am) isotherm for the monolayer in dark (curve with circles) and for the monolayer in the presence of UV light of wavelength 365 nm (curve with squares).
interface using a microsyringe (Hamilton) to form a monolayer. The temperature of 24.0 ( 0.1 °C was maintained by circulating water in the trough. The monolayer was compressed at the rate of about 14 (Å2/molecule) min-1. The Wilhelmy method was used to measure the surface pressure. BAM studies were carried out using a MiniBAM (NFT, Nanotech). For studies on the monolayer of 12D1H molecules, which are photosensitive, care was taken to minimize the effect of ambient light. Since it is known that UV light induces a trans to cis transition in the azobenzene group, the solution of 12D1H was stored in the dark for more than 12 h,9 before forming the monolayer. The experiments were carried out by placing the Nima trough in the aluminum box in the dark room to study the trans-12D1H monolayer. To study the effect of UV light on the monolayer, a mercury lamp was used as the UV source along with the appropriate filter to illuminate the monolayer with radiation of the wavelength 365 nm. LB films of 12D1H were prepared by transferring the monolayer on two different substrates, freshly cleaved mica and 1,1,1,3,3,3 hexamethyldisilazane (HMDS) coated silicon.10 The transfer of the monolayer of 12D1H in the trans state was carried out by LB technique at a surface pressure of 1 mN/m with a dipping speed of 2 mm/min. The mica sheet being a hydrophilic substrate gets coated with one layer of the film in one dipping cycle (consisting of one downstroke and one upstroke). The transfer of the film occurred during the upstroke. For HMDS-coated silicon, which is a hydrophobic substrate, two layers of the film get coated in one dipping cycle. The transfer occurred during both the downstroke and the upstroke of the substrate. It was found that, both on freshly cleaved mica and on HMDScoated silicon, the transfer ratio was better than 0.85, which was sufficient for our AFM studies. The films were characterized by AFM (model PicoPlus, Molecular Imaging) in the acoustic ac mode (tapping mode). The spring constant of the cantilever used was 31 N/m. AFM images of bare substrates of mica and hydrophobic silicon are given in the Supporting Information (Figure S3 and Figure S4).
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Figure 3. Compressional modulus (|E|) of the monolayer as a function of area per molecule (Am) for the monolayer in the dark (curve with circles) and for the monolayer in the presence of UV light of wavelength 365 nm (curve with squares).
forms a stable monolayer at the air-water interface. The surface pressure (π) - area per molecule (Am) isotherm of the monolayer in the dark (trans-12D1H) is shown in Figure 2. At large area per molecule (Am), the isotherm shows zero surface pressure. At Am of around 2.50 nm2, the surface pressure starts increasing. Around 2.10 nm2, the isotherm shows a steep increase in surface pressure. The monolayer collapses at Am of 1.84 nm2 with a collapse pressure of 1.8 mN/m. The small collapse pressure can be attributed to the absence of a strong polar group and weak chain-chain interactions. The limiting area measured from the isotherm yields a value of 2.01 nm2. After collapse, there was a gradual increase in the surface pressure. At still lower Am, the isotherm shows a small plateau after which the surface pressure again gradually increases. The compressional modulus |E| was calculated from the π-Am isotherm using the equation11
( )
|E| ) Am
dπ dAm
(1)
The material 12D1H was studied under a polarizing microscope. On cooling from the isotropic state, the sample transformed to a mesophase with schlieren texture characteristic of smectic C (SmC) phase. Differential scanning calorimetry studies were also carried out to confirm the transition temperatures (see Supporting Information, Figure S5). Surface manometry studies of 12D1H at the air-water interface showed that the material
Here, (dπ/dAm) is the change in surface pressure with area per molecule. The compressional modulus as a function of Am for the monolayer in the dark is shown in the Figure 3. |E| takes a maximum value of 20 mN/m at 1.87 nm2 for the monolayer in the dark. According to the criteria given in the literature,11,12 the monolayer with |E| values between 12.5 and 50 mN/m corresponds to the liquid expanded phase. Figure 4 shows the BAM images of the monolayer in the dark for different values of Am. Figure 4a shows the coexistence of dark and bright regions (with smooth phase boundary) at a large Am. This on compression exhibited a uniform phase. On further compression, in the collapsed region, the BAM images showed three-dimensional (3D) crystallites (Figure 4b). Hence, from the π-Am isotherm, compressional modulus, and BAM images, we infer that, above Am of 2.10 nm2, the monolayer exhibits coexistence of gas and liquid expanded phases, and between 2.10 nm2 and 1.84 nm2, it exhibits a uniform liquid expanded phase. The limiting area of 2.01 nm2 corresponds to the estimated area per molecule (using Chemdraw 3D) where
(9) Crusats, J.; Albalat, R.; Claret, J.; Ignes-Mullol, J.; Sagues, F. Langmuir 2004, 20, 8668. (10) Kumar Gupta, R.; Suresh, K. A. Eur. Phys. J. E 2003, 14, 35.
(11) Dervichian, D. J. J. Chem. Phys. 1939, 7, 932. (12) Broniatowski, M.; Macho, Sandez., I.; Minones, J., Jr.; DynarowiczLatka, J. Phys. Chem. B 2004, 108, 13403.
3. Results and Discussion
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Figure 4. BAM images of the monolayer in the dark. (a) Coexistence of gas and liquid expanded phases at Am ) 2.60 nm2. (b) Collapsed state with 3D crystallites at Am ) 0.74 nm2.
Figure 5. AFM image of 12D1H monolayer on hydrophilic mica sheet. Here, the film was transferred at a target pressure of 1.0 mN/m. The line profile yields an average height of 3.8 nm.
the two ester groups of the molecule are in contact with water. Here, the molecules have the freedom to orient their aliphatic chains in different directions. This is similar to the monolayers of fatty acids with carbon-carbon double bonds in the aliphatic chain. The carbon-carbon double bond will disturb the packing of the molecules, which leads to a more expanded phase.2 The surface manometry studies were repeated at different temperatures of the subphase, in the dark. We find that the limiting area per molecule determined from the π-Am isotherm of the monolayer increased with temperature. AFM images of 12D1H film on hydrophilic mica prepared by LB method showed domains of height of 3.8 nm (Figure 5). This value corresponds to the estimated height of the molecule when two of the ester groups are in contact with the water surface and the alkyl chains and azobenzene group protrude into the air (Figure 6). The limiting area per molecule calculated from the isotherm also indicated similar conformation of the molecules at the interface. Such changes in the orientation of aliphatic chains in the molecules at the air-water interface as compared to their bulk conformation have been observed in the monolayers of other non-trivially shaped molecules like bent core molecules.13
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In the case of HMDS-coated silicon, which behaves as a hydrophobic silicon substrate, two layers of the film get coated in one dipping cycle, that is, when the substrate was dipped and taken out of the subphase. The AFM image of the film on the hydrophobic silicon substrate showed a bilayer domain coexisting with small uniform droplets (Figure 7a). Figure 7b shows the small droplets and Figure 7c shows a large domain at higher magnification. The height of the large domain was 7.6 nm, which corresponds to the bilayer thickness. The height of the small droplets varied between 10 and 50 nm with a size of about 100 nm. Figure 8 shows the 3D view of the AFM image of the small droplets on the hydrophobic silicon substrate and the corresponding Abbot curve. The Abbot curve gives the height distribution of the droplets on the surface of the hydrophobic silicon. It can be seen that some of the droplets have a height up to 50 nm. From these images, we conclude that the bilayer film, transferred on to the hydrophobic silicon substrate, dewetted to form smaller droplets of larger height. The direction in which the droplets are aligned appears to be along the small groovelike features, which are usually present on the hydrophobic silicon substrates (Supporting Information, Figure S4). Formation of such small droplets was not observed in the case of film on mica substrates. Recently, Hashimoto and Karthaus have reported micrometer-sized droplets formed by dewetting.14 They demonstrated that the diameter and height of the droplets formed can be controlled. We attribute the formation of nanodroplets to the post-transfer reorganization of the film. For a thin liquid film on a nonwettable solid substrate, the film will be unstable,15 and it may rupture. So, the formation of droplets is likely due to the dewetting of the bilayer film of 12D1H on hydrophobic silicon. The unstable film on a rough solid substrate can rupture with two possible mechanisms: (i) spinodal dewetting and (ii) dewetting due to substrate roughness. In the case of spinodal dewetting, the unstable film on a solid substrate develops surface fluctuations of various wavelengths. Fluctuations with wavelengths greater than the critical wavelength (λc) are amplified in the field of repulsive interaction between the film and the nonwettable surface at the expense of the fluctuations with wavelengths smaller than the critical wavelength. This leads to the rupture of the film. The critical wavelength (λc) is called the critical spinodal wavelength. In the case of nonwettable smooth substrate, the critical spinodal wavelength λcis determined by the surface tension of the fluid. The dominant wavelength (λm) of the spinodal dewetting is the wavelength of the fastest amplified fluctuation and is related to the critical spinodal wavelength (λc) as λm ) λc2. The developed fluctuations with dominant spinodal wavelength determines the structure of the dewetted films. The dominant spinodal wavelength can be estimated from the direct measurement of the average distance between the patterns of the dewetted film. Volodin et al. have studied the criteria for the two possible mechanisms, based on substrate-film interactions.16 We have analyzed the AFM images of the bare hydrophobic silicon substrate and the film transferred on this substrate using the software WSxM 4.0 DeVelop 12. From the roughness analysis, we find that the roughness of the bare substrate is around 0.18 nm (Supporting Information, Figure S4). The power spectral density (PSD) function of the AFM data can be used for the calculation of the periodicity of the surface structure of the substrate. The plot of PSD as a function of wavenumber for the bare substrate showed a maximum at 0.614 µm-1, which (13) Duff, N.; Wang, J.; Mann, E. K.; Lacks, D. J. Langmuir 2006, 22, 9082. (14) Hashimoto, Y.; Karthaus, O. J. Colloid Interface Sci. 2007, 311, 289. (15) Kumar, S.; Matar, O. K. J. Colloid Interface Sci. 2004, 273, 581. (16) Volodin, P.; Kondyurin, A. J. Phys. D: Appl. Phys. 2008, 41, 065306.
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Figure 6. Schematic representation of the arrangement of molecules at the air-water interface. This is based on the limiting area of 2.01 nm2 obtained from π-Am isotherm and the height of 3.8 nm obtained from the AFM image of the monolayer film on the hydrophilic mica sheet. Here, the ester groups of the molecule are in contact with the water surface and the alkyl chains protrude into the air.
Figure 7. AFM images of 12D1H transferred at 1.0 mN/m onto hydrophobic silicon substrate. (a) 10 µm scan range image showing the coexistence of droplets of uniform size and a bilayer domain (top right). (b) 5 µm scan range image showing droplets of about 100 nm size with height distribution of 10 to 50 nm. (c) 500 nm scan range image showing bilayer film with a height of 7.6 nm.
yields a value of 608 nm for the wavelength of the surface roughness. The dominant spinodal wavelength (λm) of the film was calculated from the average distance measured between the neighboring droplets, and it was found to be about 350 nm. The critical spinodal wavelength (λc ) λm/2) was found to be about 247 nm. According to the criteria given by Volodin et al.,17 if the wavelength of the substrate roughness is much larger than the critical spinodal wavelength (λc) of the film, then spinodal dewetting of the film can be observed. In our case, we find that the wavelength of the surface roughness (608 nm) to be about three times larger than λc (247 nm). Hence, the mechanism of dewetting of the 12D1H bilayer film on a hydrophobic silicon can be attributed to the spinodal nature. The techniques like electrowetting-based microactuation for manipulation of droplets ranging from nanoliters to microliters in volume have been shown by Pollack et al.18 Similar nanodewetting structures were observed in ultrathin films of polystyrene polymer on silicon substrates with different oxide thicknesses.19 They observed nanoscale dimples coexisting with mesoscopic drops. Different structures can be observed during dewetting, depending on the mechanism and kinetics of the rupture (17) Volodin, P.; Kondyurin, A. J. Phys. D: Appl. Phys. 2008, 41, 065307. (18) Pollack, M. G.; Shenderov, A. D.; Fair, R. B. Lab Chip 2002, 2, 96. (19) Muller-Buschbaum, P.; Vanhoorne, P.; Scheumann, V.; Stamm, M. Europhys. Lett. 1997, 40, 655.
of the film.20 The nanoscale droplets of the material with azobenzene can also be micromanipulated by trans-cis photoisomerization. 3.1. Effect of Ultraviolet Light. The material 12D1H containing the azobenzene group was studied in the presence of UV light of wavelength 365 nm. The monolayer formed in the UV light exhibited significant changes in the π-Am isotherm. The collapse pressure increased to a value three times that of the monolayer in the dark. This indicated that the 12D1H molecules had undergone a transition from trans to cis conformation. This is shown in Figure 1. The isotherm showed zero surface pressure for an Am greater than 2.50 nm2. At an Am of 2.30 nm2, the π increased sharply. The monolayer collapsed with a π of 6.6 mN/m at an Am of 1.71 nm2. The limiting area was 2.10 nm2. The compressional modulus study of the monolayer in the presence of UV light yielded a maximum value of |E| ) 34 mN/m at 1.79 nm2 (Figure 2). The BAM images of the monolayer showed the coexistence of gas and liquid phases at Am greater than 2.50 nm2 (Figure 9a). In between Am of 2.30 nm2 and 1.71 nm2, the BAM image showed a uniform phase. Since the maximum value of |E| was 34 mN/m, this uniform phase between 2.30 nm2 and 1.71 nm2 can be considered a liquid expanded phase. Below an Am of 1.71 nm2, BAM images showed a collapsed state of the monolayer (Figure 9b). (20) Reiter, G. Phys. ReV. Lett. 1992, 68, 75.
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be attributed to the improved packing of the molecules in the cis state. Here, the increase in the dipole moment of the molecule after isomerization21 results in strong anchoring of the molecules on a water surface increasing the stability of the monolayer in the cis state. AFM images of the film transferred in the presence of UV light showed similar features to those of the films transferred in the dark.
4. Conclusions We have synthesized a novel H-shaped molecule bis[5-(4′n-dodecyloxy benzoyloxy)-2-(4′′-methyl phenylazo)phenyl] adipate (12D1H). The compound exhibited monotropic SmC phase while cooling. We find that the material is amphiphilic and forms a stable monolayer at the air-water interface. AFM studies of the LB film on a hydrophobic silicon substrate indicated that the 12D1H bilayer film transferred onto the substrate, dewetted to form uniform nanodroplets of size about 100 nm. Our analysis of the AFM images indicated that the dewetting process is through spinodal dewetting. Controlled dewetting of the films of azobenzene compounds can have potential applications in optical devices, biochip technology, and microelectronics. We have demonstrated that the molecules organized in the cis state exhibit a 3-fold increase in the collapse pressure compared to those in the trans state at the air-water interface. This 3-fold increase in the collapse pressure is attributed to the increase in the dipole moment of the molecule after isomerization, which results in strong anchoring of the molecules on a water surface.21
Appendix
Figure 8. (a) 3D view of AFM image of small droplets on hydrophobic silicon substrate (X, Y, and Z scales are in nm). (b) Abbot curve showing the surface height as a function of percentage bearing area.
Figure 9. BAM images of the monolayer in the presence of UV light of wavelength 365 nm. (a) Coexistence of gas and liquid expanded phases at Am ) 2.60 nm2. (b) Collapsed state of the monolayer at Am ) 0.70 nm2.
The increase of the collapse pressure in the presence of UV light to about three times that of the monolayer in the dark can
In our synthesis, the sample yield was 40%. Elemental analysis showed C 73.42%, H 7.98%, N 4.57%; whereas C70H86N4O10 requires C 73.56%, H 7.53%, and N 4.9%. The IR spectrum of the compound showed the -COO- stretching vibrations at 1760 cm-1 (-CH2COOAr) and 1730 cm-1 (ArCOOAr). Other peaks observed were at 3041, 2922, 1640 (-NdN-), 1580, 1510, 1470, 1325, 1270 (Ar-O-R), 1230, 1180, 1150, 860, 825 cm-1 (Supporting Information, Figure S1). 1H NMR spectrum (200 MHz) showed: δ 0.90 (t, 6H, 2 × -CH3), 1.25-1.50 (m, 40H, 2 × Ar-O-C-C-(CH2-)9 and -OOC-C-(CH2)2)), 1.757.95 (m, 4H, 2 × Ar-O-C-CH2-), 2.40 (s, 6H, 2 × Ar-CH3), 2.70 (t, 4H, 2 × -OOC-CH2-), 4.05 (t, 4H, 2 × Ar-O-CH2-), 6.95 (d, J ) 9 Hz, 4H, 2 × ArH at C-3′′ and C-5′′), 7.15-7.35 (m, 6H, 2 × ArH at C-3, C-4, and C-6), 7.70 (d, J ) 9 Hz, 4H, 2 × ArH at C-2′′ and C-6′′), 7.85 (d, J ) 9 Hz, 4H, 2 × ArH at C-3′ and C-5′), 8.10 (d, J ) 9 Hz, 4H, 2 × ArH at C-2′ and C-6′). Supporting Information Available: Infrared spectrogram, UV-visible spectrogram of 12D1H, AFM image of bare hydrophilic mica substrate, AFM image of bare hydrophobic silicon substrate, and plot of temperature versus heat flow obtained from differential scanning calorimeter. This material is available free of charge via the Internet at http://pubs.acs.org. LA8030733 (21) Ichimura, K.; Oh, S.; Nakagawa, M. Science 2000, 288, 1624.
