Surface Pressure Driven Supramolecular Architectures from Mixed H

We demonstrate a soft chemical approach for the synthesis of dimensionally dictated functionalized mesostructures by continuous tuning of the surface ...
0 downloads 0 Views 773KB Size
Download PDF
4758

Langmuir 2006, 22, 4758-4765

Surface Pressure Driven Supramolecular Architectures from Mixed H-Aggregates of Dye-Capped Azobenzene Derivative B. Vijai Shankar and Archita Patnaik* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ReceiVed NoVember 24, 2005. In Final Form: February 9, 2006 We demonstrate a soft chemical approach for the synthesis of dimensionally dictated functionalized mesostructures by continuous tuning of the surface molecular density of a photoreceptable molecule (E)-1-(3-chloro-4-(octyloxy)phenyl)-2-phenyldiazene (compound 1) with Rhodamine B (Rh B). Highly oriented cylindrical microtubules with a hollow center running the entire distance of the assembly in a parallel-packed configuration were formed at the air-water interface. The surface tension driven self-organized structures were evidenced from electronic absorption and steady-state fluorescence spectroscopy in conjunction with optical, polarizing, and epifluorescence microscopy and microspectroscopy; the structural building blocks were identified to be mixed H-aggregates from compound 1 and Rh B of 1:1 stoichiometry, corroborated by a blue shift in the characteristic absorption features. The appearance of a crossover point (apparent isosbestic point) instead of a sharp defined isosbestic point in the absorption spectra signified the formation of mixed H-aggregates from trans-azobenzenes in ion-dipole interaction with the charged Rh B. Increasing the temperature induced an end-to-end self-assembly of the hollow tubules, and photoisomerization of compound 1 did not serve as a trigger to induce self-organization. A nonfluorescent planar crystalline morphology with irregular topology was observed for its isomer (E)-1-(4-chlorophenyl)-2-(4-(octyloxy)phenyl) diazene (compound 2).

Introduction Study of amphiphiles that form nonspheroidal assemblies is fundamental to understand the interaction between molecular structure and supramolecular morphology and is an opening to the preparation and utilization of nanoscale materials for applications in molecular electronics.1-3 Engineering organic crystals at a molecular level in the nano/mesoscale utilizes the wide diversity of intermolecular interactions comprising electrostatics, van der Waals forces, hydrogen bonding, and so forth, and the secondary interactions of all categories.4-7 Materials with directional protonic conductivity have been achieved with macroscopic alignment of molecules through soft interactions.8 The organization and formation of higher-order structures of such mesoscopic soft materials are becoming significant in molecular systems.9,10 Recent interest in supramolecular aggregates of organic compounds has focused on ordered structures, the J- and H- aggregates,11-13 ever since their discovery by Jelley and Scheibe.14,15 They exhibit coherent excitation phenomena, * Corresponding author. Phone: 91-44-22574217. Fax: 91-44-22574202. E-mail: [email protected]. (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (2) Bohr, M. T. IEEE Trans. Nanotechnol. 2002, 1, 56. (3) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (4) Fabian, J. Nakazumi, H.; Matsuoka, M. Chem. ReV. 1992, 92, 1197. (5) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1993, 115, 11521. (6) Faul, C. J.; Antonetti, M. Chem.sEur. J. 2002, 12, 2764. (7) Gregory, B. W.; Vaknin, D.; Gray, J. D.; Ocko, B. M.; Cotton, T. M.; Struve, W. S. J. Phys. Chem. B 1999, 103, 502. (8) Maki-Onto, R.; De-Moel, K.; Polushkin, E.; Ekenstein, A. V. G.; Brinkle, G.; Ikkala, O. AdV. Mater. 2002, 14, 357. (9) (a) Antonetti, M.; Thunimann, A. F. Curr. Opin. Colloid Interface Sci. 1996, 1, 667. (b) Yokoyama, T.; Yokoyama, S.; Mamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413 619. (c) Weiss, P. S. Nature 2001, 413, 585. (10) Vranken, N.; Foubert, M.; Kohn, F.; Gronheld, R.; Scheblykin, L.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2002, 18, 8407. (11) (a) Meadows, P. J.; Dujardin, E.; Hall, S. R.; Mann, S Chem. Commun. 2005, 3688. (b) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 11884. (12) Vranken, N.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2002, 18, 1641. (13) Yao, H.; Kagodhima, Y.; Kitamura, S.; Isohashi, T.; Ozawa, Y.; Kimura, K. Langmuir 2003, 19, 8882. (14) Jelley, E. E. Nature 1936, 138, 1009. (15) Scheibe, G. Angew. Chem. 1936, 49, 563.

providing large optical nonlinearities,16,17 and the molecular arrangements in these aggregates in relation to their optical and/ or physical properties18-20 have been modeled. Azobenzene derivatives form enthalpically driven pre-micellar ordered J- and H-aggregates. The characteristic molecular organization, aggregation, and cis T trans isomerization of these derivatives in solution as well as in their hydrophobically confined monolayers have revealed changes in the Brewster angle reflectivity, molecular packing, and surface potential, implying the presence of both monomers and H-aggregates.21 Penner et al. reported the formation of a mixed J-aggregate of a cyanine dye at the air-water interface.22 Kobayashi et al. reported23 that the carbon parity determined from the even-odd effect of alkylene spacer length affected the packing state of azobenzene-urea monolayers. The carbon number altered the aggregate state attributed to the long axis of the azobenzene chromophore, and a change in the molecular orientation was predicted from the intensity ratios of the π-π* band. Light-sensitive micellar aggregates were reported from a side-chain liquid crystalline azobenzene containing an amphiphilic diblock copolymer.24 Photoinduced isomerization in Langmuir-Blodgett (LB) films of polyion complex amphiphiles with two azobenzene units led to the formation of three-dimensional (3D) cone-shaped structures.25 Composite LB films of cyanine dye with amphiphilic azobenzene formed irreversible J-aggregates, with circular domains changing to fractal ones. Panambur et al. reported26 Second harmonic generation (SHG) measurements of polyion (16) Spano, F. C.; Kuklinski, J. R.; Mukamel, S. J. Chem. Phys. 1991, 94, 7534. (17) Hannamura, E. Phys. ReV. B. 1988, 37, 1273. (18) Potma, E. O.; Wiersma, D. A. J. Chem. Phys. 1998, 108, 4897. (19) Scherer, P. O. J.; Fischer, S. F. Chem. Phys. 1984, 86, 269. (20) Knapp, E. W. Chem. Phys. 1984, 85, 73. (21) Penner, T. L.; Mobius, D. Thin Solid Films 1985, 132, 185. (22) Maack, J.; Ahuja, R. C.; Tachibana, H. J. Phys. Chem. 1995, 99, 9210. (23) Kobayashi, T.; Seki, T.; Ichimura, K. Chem. Commun. 2000, 1193. (24) Wang, H.; He, Y.; Tuo, X.; Wang, X. Macromolecules 2004, 37, 135. (25) Matsumoto, M.; Terrettaz, S.; Tachibana, H. AdV. Colloid Interface Sci. 2000, 87, 147. (26) Panambur, G.; Zhang, Y.; Yesayan, A.; Galstian, T.; Bazuin, C. G.; Ritcey, A. M. Langmuir 2004, 20, 3606.

10.1021/la053186u CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006

Surface-Pressure-DriVen Supramolecular Structures

Langmuir, Vol. 22, No. 10, 2006 4759

Figure 1. UV-vis absorption spectra of compound 1 in chloroform upon titration with Rh B, showing a decrease in intensity at 350 nm, characteristic of the π-π* feature of compound 1. Inset shows a crossover point instead of an isosbestic point near 334 nm, suggesting the formation of mixed aggregates.

complexed monolayers formed from quarternary ammonium amphiphiles with 4-nitro-4′-alkoxy azobenzene chromophore spread on an anionic polyelectrolyte aqueous subphase; a noncentrosymmetric out-of-plane chromophore ordering was confirmed. Hong et al.27 fabricated internally ordered multilayer assemblies using a layer-by-layer electrostatic deposition method from a polyelectrolyte containing rigid azobenzene chromophores separated by spacer chains, where H-aggregates were generated by excitonic interaction of trans-azobenzenes arranged in faceto-face dimers. Molecularly ordered and oriented mono/multilayers prepared by adopting the LB technique enables the investigation on the molecular arrangement of the interfacially confined, two-dimensional (2D) ordered aggregates that induce 3D crystal nucleation. Further, it is a versatile tool to achieve excellent control of the self-assembly process that converts the surfactants from the disordered state into a well-ordered solid state.28-32 The present work is a “one-shot approach” of a facile and cooperative 2D synthesis of a new functional, highly ordered hybrid assembly. Continuous tuning of the surface molecular density of compound 1 in association with the fluorescent dye at room temperature formed highly oriented cylindrical microtubules with a hollow center running the entire distance of the assembly. In contrast, a planar crystalline morphology with irregular topology was observed for compound 2. Further, such structured microtubules were absent in the photoinduced trans f cis isomerized (λirr 360 nm) compound 1 and compound 2. An association complex between compound 1 and Rh B, inferred to be a mixed H-aggregate as a structural building block, formed highly ordered tubular assemblies upon careful control of surface tension with concomitant enhanced surface molecular density induced hydrophobic interaction, π-stacking, and Coulombic interaction of the ion-dipole type. Controlling the dynamic morphology with a detailed understanding of the molecular arrangement in the mesostructure provides significant knowledge leading to structural hierarchy in synthesizing functionalized structures. (27) Hong, J. D.; Jung, B. D.; Kim. C. H.; Kim, K. Macromolecules 2000, 33, 7905. (28) Velez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (29) Wu, Z. K.; Wu, S. X.; Liang, Y. Q. Langmuir 2001, 17, 7627. (30) Guo, L.; Wu, Z.; Liang, Y. Chem. Commun. 2004, 1664. (31) Shinkai, S.; Kinda, H.; Manabe, O. J. Am. Chem. Soc. 1984, 120, 115. (32) Landau, E. M.; Wolf, S. G.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J. Am. Chem. Soc. 1989, 111, 1436.

Experimental Section Spray solutions for forming mixed H-aggregates were prepared from 1 × 10-2 M and 1 × 10-5 M chloroform stock solutions of compound 1 and Rh B, respectively. The mesostructures were obtained by spreading the chloroform (Uvasol, Merck) solution of compound 1 and the Rh B mixture with a molar ratio of 1:0.01 on the surface of ultrapure water, (Millipore-Academic) of resistivity 18.2 MΩ cm. For isotherm acquisition, 50 µL of the solution was spread in 2-min intervals with 10 µL aliquots, and the surface pressure was measured by a platinum sensor with an accuracy of 0.1 mN/m. A 30-min delay was allowed for the solvent to evaporate before compression of the film. The Langmuir films were compressed at a barrier speed of 5 mm/min at 25 ( 0.1 °C and were transferred onto hydrophilic quartz substrates, pretreated with piranha solution (1:1 H2SO4 and 30% H2O2 at 70 °C) at a controlled speed of 3 mm/min by vertical dipping. UV-visible (UV-vis) absorptions of the LB films were measured in a Carey UV-vis spectrophotometer. Steady-state fluorescence measurements were made in a Jobin Yvon spectrophotometer. Micro-Raman was performed in WiTech instruments (GmBH) with an argon ion excitation laser at 514.2 nm.

Results and Discussion A. Solution Phase Formation of Mixed H-Aggregates of Compound 1 and Rh B. Pristine compound 1 showed absorption maxima at 350 and 432 nm for π f π* and n f π* transitions, respectively, while Rh B has a characteristic prominent absorption at 550 nm. Upon addition of Rh B solution to compound 1, a new peak centered at 319 nm grew, as shown in Figure 1, with concomitant decrease in the absorption of the monomer band at 350 nm. The trans isomer of azobenzene is a planar structure in solution; because of the interaction between the π orbitals of the chromophore and the phenyl rings, the transition is regarded as being localized in the phenyl rings. Upon addition of the cationic dye, the appearance of the new feature, blue-shifted with respect to the π f π* band, evidences the formation of mixed H-aggregates. The mixed aggregates are association complexes between compound 1 and Rh B formed upon ion-dipole interaction. These unit mixed aggregates effectively give rise to an H-aggregate from a face-to-face orientation and favorable van der Waals interaction, as evidenced from the observed blue shift. Binding occurs between compound 1 and the cationic dye Rh B; because of the more apolar neighborhood of the bound aggregate, λmax shifts to a lower wavelength. Azobenzenes and

4760 Langmuir, Vol. 22, No. 10, 2006

Shankar and Patnaik

their derivatives are known to independently form H-aggregates.33-37 However, the nature of the mixed aggregates has not been explored. To justify the formation and the nature of the aggregate formed, the following considerations have been worked out: (i) Rh B could induce H-aggregation in compound 1, leading to a dimeric or higher aggregate form. (ii) Rh B could form mixed H-aggregates via electrostatic interaction with compound 1. To explore the first case, Benisi-Hildebrand kinetics was employed in terms of the following equilibrium:

aggregate S n (monomer) Thus,

K ) [monomer]n/[aggregate]

(1)

ln K ) n ln[monomer] - ln[aggregate]

(2)

where K is the equilibrium constant of the above process. The above equation demonstrates the relationship between the monomer concentrations and that of the aggregate in solution using “n”, the aggregation number. Applying Benisi-Hildebrand kinetics in terms of Aagg and Amono as the absorbances of the aggregates and the monomer, respectively, at equilibrium we have

ln(Aagg) ) n ln(Amono) + B

(3)

where B is a constant. For an equilibrium of the type shown by eq 1, the plot of ln(Aagg) versus ln(Amono) in Figure 2b should yield a straight line with a slope of n. Deviation from linearity in Figure 2b discards the above equilibrium and the formation of any homo H-aggregates of compound 1 induced by Rh B. This inference is supported by the absence of an isosbestic point near 334 nm, as depicted in the inset of Figure 1. The possibility of formation of any induced homo H-aggregate of compound 1 alone is ruled out by the appearance of a crossover point instead of an isosbestic point. Koti et al.38 reported formation of mixed aggregates from a 1:4 stoichiometric meso-tetrakis(4-sulfonatophenyl)phorphine dianion and cationic 3,3′-dihexyloxacarbocyanine iodide upon electrostatic binding, with a binding constant of 7.7 × 1022 M-1. On the other hand 3,3′-diethyloxadicarbocynine iodide was found to induce J-aggregation in the tetrakis(4-sulfonatophenyl)phorphine dianion. In Figure 2a, the aggregate absorbance versus the mole ratio of Rh B/compound 1 indicates the saturation threshold arrives at 1:1.09. These titration studies thus confirm the formation of an ∼1:1 stoichiometrically mixed aggregate in solution where the following equilibrium sets in: Kb

compound 1 + Rh B {\} compound 1 - Rh B Thus

Kb ) [compound 1 - Rh B]/[compound 1][Rh B] with Kb as the binding constant of the mixed aggregate formed, and [compound 1- Rh B], [compound 1], and [Rh B] as the (33) Schonhoff, M.; Mertresdorf, M.; Losche, M. J. Phys. Chem. 1996, 100, 7558. (34) Hafiz, H. R.; Nakanishi, F. Nanotechnology 2003, 14, 649. (35) Nanguo, L.; Chen, Z.; Dunphy, D. R.; Jiang, Y. B.; Assink, R. A.; Brinker, C. J. Angew. Chem. Int. Ed. 2003, 42, 1731. (36) Kim, I.; Rabolt, J. F.; Sytroeve, P. Colloids Surf., A 2000, 171, 167. (37) Velez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (38) Koti, A. S. R.; Periasamy, N. J. Mater. Chem. 2002, 12 (8), 2312.

Figure 2. (a) Plot of aggregate absorbance at 319 nm against a mole ratio showing 1:1 stoichiometry between Rh B and compound 1. (b) Variation in the monomeric absorbance of compound 1 vs aggregate absorbance at 319 nm.

equilibrium concentrations of the corresponding species in solution. From the titration experiments, Kb was found to be 7.56 × 107 M-1. B. Surface Pressure Driven Supramolecular Assemblies at the Air-Water Interface. The pressure-area (π-A) isotherm of compound 1 mixed with Rh B in a 1:0.01 molar ratio at 25 °C is shown in Figure 3, exhibiting a distinct gas phase and a liquid condensed phase coexisting with a condensed phase. The isotherm does not form a well-defined collapse, suggesting the formation of domains/multilayers instead of an ideal monolayer. Ideal monolayer formation also could not be observed from onecomponent solutions of compound 1 and compound 2 (structures shown in Chart 1) in CHCl3. The probable orientations of the trans-azobenzene moiety can be discussed from the molecular models of compound 1 as shown in Figure 3A-C. The experimental molecular dimensions of trans-azobenzene were obtained from X-ray diffraction analysis as 13.6 Å long, 7.0 Å wide, and 3.6 Å thick.39 These values lead to three basic cross-sectional areas, namely, 25.2 Å2 along the short axis, 49.0 Å2 along the long axis, and 95.2 Å2 in the plane of the ring system. The formation of a condensed phase together with the existence of a liquid condensed phase arises at a molecular area

Surface-Pressure-DriVen Supramolecular Structures

Figure 3. Pressure-area isotherms of (a) compound 1 with Rh B, (b) compound 1 with Rh B, (c) compound 2, and (d) compound 3. The arrow indicates the pressure at which the film was transferred. (A-C) Possible orientations of compound 1 at the air-water interface.

threshold of 20 Å2, which is close to a normal orientation (Figure 3A). However, this conformation is improbable because of the

Langmuir, Vol. 22, No. 10, 2006 4761

absence of a polar group on the molecular skeleton. The flat-lie (Figure 3B) and sideways (Figure 3C) orientations are equally improbable, having much larger areas than the experimentally observed area. An insight into the probable orientation was obtained from the ab initio (HF 6-31+G*) geometry optimized structure of compound 1 as depicted in Chart 1. The calculated dipole moment is maximum along the Y axis, and, accordingly, the electron density contour cross-section shows maximum electron density around the chlorine and oxygen atoms, thus predicting the probable 2D orientation of compound 1 at the air-water interface. Long-chain derivatives of azobenzene used in this work are shown in Chart 1. C. Polarizing and Fluorescence Microscopy of the Supported LB Films: Formation of Tubules. Examination of the transferred films on quartz substrates under optical microscope revealed fiberlike tubular aggregates with a hollow center running the entire distance of the assembly for compound 1, while compound 2 formed planar crystallites with irregular morphology. The images are shown in Figure 4. The tubes are of uniform width of 2-3 µm, smooth and unflocculated, and stacked along the long axis of the tube. The tubes thus formed varied in length from 10 to 100 µm. Epifluorescence micrographs showed fluorescent tubular structures at λex ) 510 nm for compound 1(see Figure 4c), while the planar crystallite assemblies formed from compound 2 were nonfluorescent (Figure 4e). Polarized light microscopy is a useful tool to examine the regular alignment of molecules in an aggregate that exhibit strong birefringence. Figure 4d shows a typical polarized light microscope image under crossed polarizers. The isotropic region appears dark with the tubular mesostructure showing excellent birefringence. The mesostructures showed a straight extinction, that is, the image became dark when the long axis of the tubes were oriented parallel to the extinction direction. This characteristic feature indicates that the tubes possess uniaxial crystalline structure. D. UV-Vis Absorption Characteristics of the 2D Mesostructures. Figure 5 shows the UV-vis absorption profiles of the mesostructures on quartz substrates. The films show the characteristic absorption at 319 nm, similar to the one in bulk solution, implying the existence of mixed H-aggregates. Linear dichroic measurements produce polarizability tensors related to tilt angles, defining molecular orientation. The tilt angle direction of the azobenzene chromophores in the multilayer mixed

Chart 1. (A) Azobenzene Molecules under Study. (B) The Electron Density Contour Cross-Section for the Probable Orientation of Compound 1 at the Air-Water Interface Showing Localized Electron Density over Chlorine and Oxygen Atoms. (C) Compound 1 and Compound 2, Geometry Optimized with Ab Initio HF Calculations with the 6-31+G* Level of Theory.

4762 Langmuir, Vol. 22, No. 10, 2006

Shankar and Patnaik

Figure 4. Optical and epifluorescence micrographs of compound 1 and compound 2 showing (a) tubular/cylindrical bundles, (b) an expanded single tube showing an opening at the end, (c) a fluorescent tube of compound 1, (d) a tube under crossed polarizers, (e) nonfluorescent crystallites of compound 2, and (f) crystallites of compound 2.

Figure 5. UV-vis absorption characteristics of the formed mesostructures on quartz substrates. Inset explains the enhanced absorbance with respect to the perpendicular dipping direction, where the electric vector component of the radiation interacts with the more tilted alignment in the direction to the surface normal.

aggregate can be visualized assuming that π-π* is directed along the molecular axis of the compound 1. When the dipping direction was rotated by 90° for parallel and perpendicular configurations, the films exhibited linear dichroism, implying a (39) Brown, C. J. Acta Crystallogr. 1996, 21, 146.

regular and periodic arrangement of the constituent molecules. An enhanced intensity in the figure signifies a more tilted configuration toward the surface normal when the B E the electric vector component of the radiation interacts with the film in parallel dipping direction. For comparison, the absorption profiles of the spin-coated films are also shown in Figure 5. The spray solution was spin-coated onto pretreated hydrophilic quartz plates at 2500 rpm, and the absorption profiles showed only the presence of pristine components at 355 nm. This observation uniquely shows the 2D surface pressure to be the driving force for the formation of self-assembled structures in Langmuir films. A recent report by Acharya et al.40 substantiated the prevalent 2D surface pressure to be the driving force for the coalescence of ZnS nanorods into nanowires near room temperature. E. Microepifluorescence Spectroscopy of the 2D Mesostructures. Figure 6A shows the microfluorescence spectrum of the tubes excited by 514 nm argon ion laser with emissions collected in the reflection mode. The tubes appeared yellowish red when excited with the laser, as shown in the inset of Figure 6A. The spectrum shows a distinct Rh B emission band at 582 nm. Rh B in the isotropic phase, when excited at 510 nm, shows an intense fluorescent band at 550 nm, while pristine compound 1 does not show any fluorescence at that excitation wavelength (Figure 6B). On the other hand, compound 1 shows weak fluorescence emission at 412 nm, 437 nm, and a shoulder at 465 nm when excited at 350 nm. Thus, from the 582 nm emission (40) Acharya, S.; Efrima, S. J. Am. Chem. Soc. 2005, 127, 3486.

Surface-Pressure-DriVen Supramolecular Structures

Langmuir, Vol. 22, No. 10, 2006 4763

Figure 7. TEM micrographs showing open tubules at 5 mN/m (a,c,d) and at 2 mN/m (b). The tubules formed at 5 mN/m have a thicker wall than the one formed at 2 mN/m.

Figure 6. (A) Microepifluorescence spectrum of the 2D tubular mesostructures excited at 514 nm. Inset shows the corresponding microepifluorescene image mapping (40 × 40 µm) of the tubules. (B) Steady-state fluorescence spectrum of compound 1 in chloroform excited at 350 nm. Inset shows the fluorescence emission of the same excited at 514 nm.

band, it can be inferred that the walls of the tubes are bilayered with outer Rh B. F. Transmission Electron Microscopy (TEM) of the 2D Self-Organized Structures. The TEM images of the tubes shown in Figure 7 clearly illustrate the formation of hollow tubes. The tubes transferred at 2 mN/m are 19 nm wide with a smaller wall thickness of ∼15 nm, while the tubes transferred at 5 mN/m are 170 nm wide with a wall thickness 45 nm. From geometry optimization, the width of the bilayer turns out to be 1.68 nm with width contributions of 1.2 nm from Rh B and 0.48 nm from compound 1. This suggests that, during compression, the 2D sheet made of mixed aggregate arrays of Rh B and compound 1 roll up to form multiwalled tubules. At lower compression and higher areas, the 2D sheets with fewer numbers of building units roll up, forming tubules of small wall thickness (Figure 7b). At higher compression and smaller areas, the 2D sheets having larger building units roll up to form thick-walled tubes. From the micrographs, we found that some of the tubules were twisted so that the open mouth is seen clearly. (Figure 7b,d).

Figure 8. (a) Optical micrographs of compound 1 at 40 °C showing end to end self-assembly at the air-water interface. (b) Epifluorescence micrograph of the end-to-end assembled tubes.

G. Temperature Effect on 2D Supramolecular Geometry. It has been observed that domain shapes in assembled structures change with temperature. Anomalous temperature dependence was observed in the shape of condensed-phase domains in the Langmuir monolayers of monoglycerol ether using Brewster angle microscopy.41 Shape transitions as a result of an enhanced in-plane dipole moment at low temperatures were revealed by (41) Nandi, N.; Volhardt, D. J. Phys. Chem. B 2004, 108, 18793 and references there in.

4764 Langmuir, Vol. 22, No. 10, 2006

Shankar and Patnaik

Scheme 1. Mechanism of Surface-Driven Self-Assembly Leading to Two-Dimensional Tubular Aggregates of Compound 1 and Rh B

quantum mechanical calculations. The process of self-assembly depends critically on thermal energy, where the resultant motion allows structures to attain the optimum positions for highly ordered structures with thermally induced self-assembly, typically requiring repulsive or only very weakly attractive interactions.42 A compact shape would be favored with decreasing temperature, whereas elongated shapes will be favored with increasing temperature. Experiments done at 40 °C formed only isolated rods against parallel stacking due to the increase in the line tension between the condensed phase and the fluidic phase.43 Surprisingly, the microstructures assembled in end-to-end fashion against parallel stacking at room temperature and are shown in Figure 8. Steady-state fluorescence measurements of solutions of Rh B mixed with compound 1 showed quenching of the 550 nm emission band of Rh B, while compound 2 did not (vide Figure 9), indicating the absence of any interaction of the latter with Rh

Figure 9. Fluorescence quenching of Rh B and (A) compound 1 and (B) compound 2, with varying concentrations of Rh B: (a) Rh B 100%, (b-e) 2-8% v/v of azobenzene derivative (excitation wavelength ) 510 nm).

B. The formation of a coulombically associated complex between Rh B and compound 1 is attributed to the fact that the lone pair of electrons on the alkoxy oxygen facilitates coulombic binding of the ion-dipole type, with a net positive charge existing on the quaternary nitrogen of Rh B. For compound 2, the electronegativity of the chloro group situated para to the -Nd N- decreased the coordinating ability of the alkoxy oxygen. (42) Ramos, L.; Lubensky, T. C.; Dan, N.; Nelson, P.; Weitz, D. A. Science 1999, 286, 2325. (43) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171.

The specificity of the chlorine moiety on the molecular skeleton of compound 1 plays a vital role in the structure formation: the absence of chlorine in compound 1 as well as in compound 2 did not form such ordered structures. A probable explanation could be attributed to the electron density localization by the electronegative chlorine substituent, facilitating the ion-dipole interaction between compound 1 and Rh B. The above mechanism of self-assembly suggests that the linear configuration of the azobenzene moiety (trans isomer) alone favors such self-assembly. To prove this further, when the spray solution was irradiated with UV light (λ ) 350 nm, t ) 5 min, 500 W mercury lamp; Oriel Corporation, Stratford, CT) and sprayed onto the airwater interface, no ordered structural assemblies were observed. H. Proposed Model for the Self-Assembled 2D Tubular Assembly. From the results of microscopic and spectroscopic investigations, we propose a model for the tubular assembly of the dimension-specific H-aggregates. The general aspects of tubule formation in the amphiphilic systems based on our investigation thus reveal that the aggregates exist in different morphologies depending on the substitution pattern of the azobenzene chromophores in the presence of the Rh B dye. This indicates the sensitivity of the aggregate morphology to the slightest chemical modifications in the molecular framework. Tubule formation has been observed and modeled by many workers.44-47 It has been suggested that the anisotropy of films due to symmetry breaking in the molecular packing could lead to a tubular structure. The minimization of packing and interface energy results in structural complexity with high order. In addition to this, the energy contribution from the stacking of the rigid π-system of Rh B and the electronic interaction account for the observed shape. Further, the hydrophobic tails attached to the chromophores favor an aggregation of the building blocks (the associated complex between compound 1 and Rh B) into bilayer sheets, which is rolled into a tubular assembly with the variation in surface tension as the driving force. With the direction of the molecular alignment in the rod aggregate evaluated from the polarization excitation under the 530 nm retardation plate, the formation mechanism for tubular assembly is shown in Scheme 1.

Conclusion This work illustrated a facile and cooperative 2D synthesis of a new functional and stoichiometric highly ordered assembly with parallel-stacked unit mixed H-aggregates as building blocks, (44) Von Berlepsch, H.; Bottcher, C.; Quart, A.; Regenbrecht, M.; Akari, S.; Keidrling, U.; Schnablegger, H.; Dahne, S.; Kirstein, S. Langmuir 2000, 16, 5908. (45) Fuhrhop, J. H.; Helfrich, W. Chem. ReV. 1993, 93, 1565. (46) Schnur, J. M. Science 1993, 262, 1669. (47) Orr, G. W.; Barbour, L. J.; Atwood, J. L. Science 1999, 285, 1049.

Surface-Pressure-DriVen Supramolecular Structures

upon the neat addition of charged fluorescent units of Rh B to compound 1. For these integrated assemblies, molecular order is encoded in the shape and the chemical functionality and is governed by the strength and directionality of secondary interactions. Here, we showed the predominant impact of electrostatic interactions, the distribution of cohesive energy along the charged objects, and the amphiphilic association in structure building. The solution-phase H-aggregates required long-range orientational order; transformation into a more stable tubular architecture with controlled two-dimensional molecular alignment was energetically favorable, solely on account of surface induced phenomena. This noncovalent soft chemical approach with concomitant control of surface molecular density resulted in obtaining desired shapes of functionalized self-assembles; Rh B influenced the π-π interaction and the anisotropic polarizability in addition to the electronic interactions, while the diazene derivative supported coulombic binding. The structure of the self-assembles could be fine-tuned by adopting to conditions

Langmuir, Vol. 22, No. 10, 2006 4765

such as incorporating an appropriate polar moiety and/or increasing the alkyl chain length in the molecular framework and by controlling the temperature at which the self-assembly occurs. The overall process represents a significant advance toward controlled molecular self-assembly with predictable shapes by controlling the surface molecular density and bulk-phase chemical structure. The results expose an interesting new model system for dye aggregates from which laws of cluster formation could be deduced, and the ability to organize the building blocks in a specified direction could provide insight for nanoelectronic design. Acknowledgment. The present research work is financially supported by the Government of India (No. ERIP/ER/0100164/ M/01). B.V.S. acknowledges research fellowship from CSIRIndia. Our sincere thanks to Prof. T. Pradeep for making available the micro-Raman spectral facility. LA053186U