Langmuir 2000, 16, 10569-10572
Photoresponsive Langmuir Monolayers from Azobenzene-Containing Dendrons A. Sidorenko,† C. Houphouet-Boigny,† O. Villavicencio,‡ M. Hashemzadeh,§ D. V. McGrath,‡ and V. V. Tsukruk*,† Department of Materials Science & Engineering, Iowa State University, Ames, Iowa 50011, Department of Chemistry, University of Arizona, P.O. Box 210041, Tucson, Arizona 85721, and Department of Chemistry, University of Connecticut, 215 Glenbrook Road, Storrs, Connecticut 06269 Received July 17, 2000. In Final Form: October 13, 2000
Introduction Dendrimers represent interesting and novel building blocks for organized molecular films.1-5 Several examples of spherical, hemispherical, and cylindrical dendrimers have been tested for their ability to form organized molecular films at the air-water interface and on solid surfaces.2 Isolated molecules, densely packed organized monolayers, and uniform homogeneous layers have all been observed for different types of dendrimers localized on interfaces. Photoresponsive dendrimers with peripheral and focal photochromic groups have been very recently demonstrated, and uniform Langmuir-Blodgett monolayers have been fabricated from these compounds.3,4 Photomechanical response caused by trans-cis switching of Langmuir monolayers of various azobenzene derivatives has been reported on the water-air interface.6,7 Langmuir-Blodgett monolayers deposited on a solid substrate demonstrated, under certain conditions, photoresponsive behavior as well.8 Low-molecular and polymeric photochromic compounds were found to be able to change the occupied area on the air-water interface and change their spectroscopic properties. We report an investigation of the monolayer-forming properties of four generations of azobenzene-crowncontaining dendrons (Chart 1). We focus on this type of photochromic dendron for monolayer formation because of the potential ability to suppress crystallization of the * To whom correspondence should be sent:
[email protected]. † Iowa State University. ‡ University of Arizona. § University of Connecticut. (1) Newkome, G. R., Moorefield, C. N., Vogtle, F., Eds. Dendritic Molecules; VCH: Weinheim, 1996. Tomalia, D. A. Adv. Mater. 1994, 6, 529. Frechet, J. M. Science 1994, 263, 1711. Schenning, A. P.; ElissenRoman, C.; Weener, C. J.-W.; Baars, M. W.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. Percec, V.; Chu, P.; Ungar G.; Zhou, J. J. Am. Chem. Soc. 1995, 117, 11441. (2) Collaud, M.; Lorenz, K.; Kressler, J.; Frey, H.; Mulhaupt, R. Macromolecules 1996, 29, 8069. Tsukruk, V. V. Adv. Mater. 1998, 10, 253. Bliznyuk, V. N.; Tsukruk, V. Polymer 1998, 39, 5249. Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988. Zhou, Y.; Bruening, M. L.; Berbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (3) Weener, J.-W.; Meijer, E. W. Adv. Mater. 2000, 12, 741. (4) Tsukruk, V. V.; Luzinov, I.; Larson, K.; Li S.; McGrath D. V. J. Mater. Sci. Lett. 2000, accepted. (5) Hashemzadeh, M.; McGrath, D. V. Am. Chem. Soc., Div. Polym. Chem., Prepr. 1998, 39, 338. (6) Seki, T.; Sekizawa, H.; Fukuda, R.; Tamaki, T.; Yokoi, M.; Ichimura, K. Polymer J. 1996, 28, 613. (7) Seki, T.; Sekizawa, H.; Fukuda, R.; Morino, S.; Ichimura, K. J. Phys. Chem. 1998, 102, 5313. (8) Seki, T.; Kojima, J.; Ichimura, K. Macromolecules, 2000, 33, 2709. Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479.
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photochromic fragments. The treelike structure of dendron shells and the high intralayer molecular mobility of polar crown groups facilitate this suppression. For comparison purposes, an azobenzene compound AH with a carboxylic acid polar group is used (Chart 1). Kinetic behavior during photochromic isomerization at the air-water interface is discussed for these photochromic dendrimers. Detailed discussion of the microstructure of these monolayers deposited on solid substrates will be published elsewhere.9 Experimental Section Compounds AD0, AD1, AD2, and AD3 were prepared as previously described.5 Compound AH was prepared as follows. A slurry of ethyl 4-(4′-hydroxyphenylazo)benzoate (1.44 g, 5.32 mmol), 3,4,5-tris(dodecyloxy)benzyl chloride (3.74 g, 5.50 mmol), K2CO3 (2.03 g, 14.8 mmol), and dry DMF (100 mL) was kept at reflux under nitrogen for 18 h.10 After thin-layer chromatography (TLC) (SiO2, 70:30 hexanes-ethyl acetate) indicated that the reaction was complete, the solvent was removed in vacuo. The resulting solid residue was partitioned between H2O (25 mL) and CH2Cl2 (25 mL). The organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (3 × 25 mL). The combined organic layers were washed with water and brine, dried (Na2SO4), and concentrated. Flash chromatography of the residue (SiO2, 70:30 hexanes-ethyl acetate dryloaded in CH2Cl2) gave ethyl 4-(4′-[3′′,4′′,5′′-tris{dodecyloxy}benzyloxy]phenylazo)benzoate (4.71 g, 97%) as an orange solid: mp 56-57 °C; 1H NMR (250 MHz, CDCl3) δ 8.19 and 7.91 (AA′BB′ pattern, J ) 8.5 Hz, 4H), 7.97 and 7.11 (AA′BB′ pattern, J ) 8.9 Hz, 4H), 6.65 (s, 2H), 5.06 (s, 2H), 4.41 (q, J ) 7.1 Hz, 2H), 3.98 (q, J ) 6.5 Hz, 6H), 1.271.83 (overlapped, 63H), 0.89 (t, J ) 5.94 Hz, 9H). A mixture of ethyl 4-(4′-[3′′, 4′′, 5′′-tris{dodecyloxy}benzyloxy]phenylazo)benzoate (0.20 g, 0.22 mmol), LiOH (0.05 g, 2.0 mmol), THF (15 mL), and H2O (5 mL) was kept at reflux under nitrogen for 18 h. After removal of THF by rotary evaporation, 10 mL of CH2Cl2 was added to the mixture. With vigorous stirring, concentrated HCl was added dropwise until the pH was acidic. The organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (3 × 10 mL). The combined organic layers were washed with water and brine, dried (Na2SO4), and concentrated to give the benzoic acid (0.09 g, 47%) as a yellow solid: mp 86-88 °C; 1H NMR (250 MHz, d -acetone) δ 8.21 (m, 2H), 7.93 (m, 4H), 7.05 6 (m, 2H), 6.73 (s, 2H), 4.62 (s, 2H), 4.00 (t, J ) 6.3 Hz, 6H), 1.291.67 (overlapped, 60H), 0.88 (t, J ) 6.7 Hz, 9H). The substrates were one side polished silicon wafers (Semiconductor Processing Co.) of the {100} orientation, pretreated according to the standard procedure described elsewhere.11 The solid substrate was set in the bath before the spreading of the dendrimer solution and gently pooled out at 100 µm/s after compression at the desired pressure. Monomolecular films of dendritic compounds were prepared by the Langmuir technique on an RK-1 trough (Riegel & Kirstein GmbH) at room temperature. The compounds were dissolved in chloroform (Fisher, reagent grade) to obtain the following concentrations: 1 mmol/L for AD0, AD1, AH, and stearic acid (StA), and 0.1 mmol/L for AD2 and AD3. The desired amount (approximately 3 × 10-5 mol) of the dendrimer solution was spread over the water (NanoPure, >18 MΩ cm) subphase. The surface pressure-area (π-A) isotherms were obtained for nonilluminated as well as illuminated Langmuir monolayers. The films were compressed after 30 min of relaxation up to the minimal area of the bath while the π-A isotherms were recorded. To obtain the isotherms of the illuminated compounds, the Langmuir monolayers were preilluminated for 15 min and then compressed under the instant illumination of Blak-Ray ultraviolet lamp (UVP, model B-100 (9) Sidorenko, A.; Houphouet-Boigny, C.; Villavicencio, O.; McGrath, D. V.; Tsukruk V. V. Submitted. (10) Corvazier, L.; Zhao, Y. Macromolecules 1999, 32, 3195. (11) Tsukruk, V. V.; Bliznyuk, V. N.; Hazel J.; Visser D.; Everson M. P. Langmuir 1996, 12, 4840.
10.1021/la001013t CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/2000
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Langmuir, Vol. 16, No. 26, 2000
Notes
Chart 1. Compounds Used for Monolayer Fabricationa
a
ADN corresponds to azo-dendron of N generation, AH is used as a reference sample.
AP, 100 W) equipped with a 365 nm filter from a distance of 40 cm. The variation of the molecular area for repeated measurements of the π-A isotherms did not exceed 3-5%, which allowsdetailed kinetics studies for the photoinduced variation of the surface area in the range of 10-20% for some dendrimers. Kinetic measurements were carried out under illumination at a constant surface pressure while area changes were recorded as a function of the illumination time. The geometrical parameters of dendritic molecules were estimated by molecular modeling with the Cerius2 program using Dreiding force field on a SGI workstation.
Results and Discussion Surface Behavior. All dendritic compounds selected for this study can be spread on an air-water interface to form a stable monolayer. The lateral compression of all monolayers results in a gradual increase of the surface pressure that resembles classic amphiphilic behavior (compare data for dendrons and stearic acid, Figure 1). As expected, an increase in the generation number causes a significant shift in the π-A isotherm to higher crosssectional area per molecule, A0. This value increases from 0.45 nm2 for zeroth generation AD0 with 2 alkyl tails to 4.0 nm2 for third generation AD3 with 16 alkyl tails (Figure 1). The A0 for a reference compound with one alkyl chain, stearic acid StA, is about 0.2 nm2, as expected for a virtually straight alkyl chain.12 Azobenzene compound AH, lacking the crown-ether headgroup but possessing three alkyl tails, demonstrates the cross-sectional area of 0.6 nm2. Simple comparison shows that for conventional azobenzene amphiphilic molecules with a carboxyl polar head, dense surface packing is predetermined by the A0 of a single alkyl chain multiplied by a number of alkyl tails in a molecule. However, a very different situation is (12) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991.
Figure 1. Pressure-area isotherms for Langmuir monolayers from dendrimers AD0, AD1, AD2, and AD3, azobenzene compound AH with three tails, and stearic acid (StA). Note the x axis is a log2 scale.
observed for the dendrimer compounds. The systematic increase in the cross-sectional area per molecule with increasing number of alkyl tails in the outer shell of dendrons is obvious from the plot of A0 versus the number of peripheral alkyl chains (Figure 2). Data for the two compounds with one (StA) and three (AH) alkyl tails fit nicely to the trend observed for the dendrons. The fact that the theoretical and experimental values of A0 values are virtually identical for all molecules studied here with one, two, three, and four alkyl tails indicates that dense packing of vertically oriented tails in monolayers fabricated from these compounds is predominant. However, for highest generation dendrimers (2 and 3), the A0 values
Notes
Figure 2. Variation of cross-sectional area per molecule of the monolayers as a function of the number of peripheral alkyl tails in the dendron shell: (a) experimentally measured for original (b) and illuminated (O) monolayers and calculated for a given number of alkyl tails in vertical orientation (--); (b) 2D elastic modulus found for “solid” phases of nonilluminated (filled circles) and illuminated (hollow circles) monolayers. Hollow circle for StA compound is E for liquid monolayer state.
are 30% higher than theoretical values estimated from the A0 of a single, vertically aligned alkyl tail (about 0.2 nm2) multiplied by the number of alkyl tails (Figure 2). This deviation suggests that steric limitations in the outer shell of higher generation dendrons cause the peripheral chains to be oriented more along the radial direction from the focal point. In this arrangement, effectively tilted tails will occupy a larger area than vertically aligned tails. Surface Behavior under UV Illumination. The next step is the elucidation of surface behavior of azo-containing dendrimers illuminated with UV light that can initiate a photoisomerization reaction. As is known, this reaction leads to a significant change of the cross-sectional area at the air-water interface because of geometrical changes of azobenzene moiety shape.6 Indeed, we found an increase of A0 values by 10-20% for AD1 and AD2 dendrons after illumination (Figure 2). Although the increase is not very pronounced, it is 2-3 times higher than statistical variations of the isotherms measurements (3-5% for A0). Higher (AD3) and lower (AD0) generations demonstrate minor changes (5-10%) after UV illumination, which is too close to the statistical variation to be analyzed. The principal feature in the monolayer surface behavior of photochromic dendrons with crown-ether polar heads
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as compared to conventional azobenzene amphiphiles was the very different lateral compliant properties of these monolayers at the air-water interface. The 2D elastic modulus was used to characterize these properties and structure of amphiphile monolayers in “solid” state.13-15 We estimated the 2D elastic modulus, E, from the known relationship: E ) -A(dπ/dA) on the basis of the π-A isotherms at the surface pressures below apparent monolayer collapse (
