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Langmuir 1997, 13, 870-872
Atomic Force Microscopy Studies of Photoisomerization of an Azobenzene Derivative on Langmuir-Blodgett Monolayers M. Ve´lez,*,† S. Mukhopadhyay,‡ I. Muzikante,§ G. Matisova,| and S. Vieira† Laboratorio de Bajas Temperaturas, Departamento Fı´sica de la Materia Condensada, C-III Universidad Auto´ noma de Madrid, 28049 Madrid, Spain, South Bank University, School of EEIE, 103 Borough Road, London SE1 OAA, U.K., Institute of Physical Energetics, 21 Aizkraules Strasse, Riga LV 1006, Latvia, and Latvian Institute of Organic Synthesis, 21 Aizkraules Strasse, Riga LV 1006, Latvia Received June 5, 1996. In Final Form: October 29, 1996
Materials which undergo a reversible change in their optical transmission characteristics when irradiated (the photochromics) offer potentially exciting applications for optical data storage or as molecular switching devices.1-3 The azobenzenes belong to this family of photochromic materials. They undergo a reversible cis/trans photoisomerization upon UV irradiation in solution. For the incorporation of these materials in functional molecular devices the organic molecules can be deposited on solid substrates as thin films using the Langmuir-Blodgett technique.4 However, once the organic molecules are incorporated into a rigid matrix, the question arises as to whether the cis/trans isomerization process occurs and, if so, to what extent.5 Extensive photochemical studies of azobenzene chromophores in polymer solids have addressed the influence of free volume distribution on the extent of trans/cis isomerization.6,7 It is also of interest to study the effect of spatial restrictions on the isomerization process in Langmuir-Blodgett films.8,9 The azobenzene derivative studied in this work is the 4′-carboxy-4-(methylpalmitoylamino)azobenzene (A4):
where R ) C15H31. The synthesis of this and similar derivatives was described by E. Markava and coauthors.10 The reversible photoisomerization has been observed in chloroform solution,10 but when the monolayer at the air/water interface was irradiated, at least at the pressure of 10 * Corresponding author. E-mail:
[email protected]. † C-III Universidad Auto ´ noma de Madrid. ‡ South Bank University. § Institute of Physical Energetics. | Latvian Institute of Organic Synthesis. (1) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658660. (2) Roberts, G., Langmuir-Blodgett Films; Plenum Press: New York, London, 1990. (3) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press, Inc.: New York, 1991. (4) Sienicki, K., Ed. Molecular Electronics and Molecular Electronic Devices; CRC Press: Boca Raton, FL, 1994; Vol. III. (5) Liu, Z. F.; Loo, B. H.; Baba, T. R.; Fujishima, A. Chem. Lett. 1990, 1023-1026. (6) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558-563. (7) Naito, T.; Horie, K.; Mita, I. Polymer 1993, 34 (19), 4140-4145. (8) Maack, J.; Ahuja, R. C.; Mo¨bius, D.; Tachibana, H.; Matsumoto, M. Thin Solid Films 1994, 242, 122-126. (9) Sasaki, Y. J.; Suzuki, Y.; Tomioka, Y.; Ishibashi, T.; Takahashi, M.; Satoh, I. Langmuir 1996, 12, 4173-4175. (10) Freimanis, J.; Markava, E.; Matisova, G.; Gerca, L.; Muzikante, I.; Rutkis, M.; Silinsh, E. Langmuir 1994, 10, 3311-3314.
S0743-7463(96)00554-9 CCC: $14.00
Figure 1. Monolayer of an azobenzene deposited on glass before irradiation with ultraviolet light. (A) Topographic image of a 1 µm2 area. The different gray tones reflect different heights, white being highest and black lowest (total height range is 5 nm). (B) Profile of the white line shown in (part A). (C) Friction image of the same area shown in (part A). The lighter regions represent areas that have a stronger interaction with the scanning tip. (D) Small area scanned on top of the monolayer. The inset is the Fourier transform of the image.
mN/m, no surface pressure changes were appreciated, meaning that the trans/cis photoisomerization is probably blocked by the close packing of the azobenzene groups. However, reversible cis/trans isomerization in 14 layer Langmuir-Blodgett (LB) films of 4-octyl-4′-(5-carboxylpentamethyleneoxy)azobenzene on glass substrates has been observed by Z. F. Liu and coauthors5 provided the LB films were pretreated with UV light. The authors suggested that the first irradiation produced a gradual expansion of the film and perhaps a partial rearrangement of the molecules, probably occurring in the “hole” or “defect” regions of the film. We therefore explored the structure of the monolayer after transfer to a solid substrate before and after sufficient UV irradiation to induce isomerization in solution. We have characterized the structure using an atomic force microscope, a powerful surface probe technique that allows studying surfaces with molecular resolution. We present here images of the overall morphology of the film before irradiation, as well as images of the molecular packing of the bidimensional crystal. Thermal relaxation of the irradiated monolayer was observed to produce significant morphological modifications in the frontier region between crystalline domains of different orientations and in the edges of the discontinuous monolayer. To prepare the LB films, the molecules were dissolved in chloroform at a concentration of 2 × 10-3 M and the monolayer was formed on a subphase of distilled water (18 MΩ resistance). The trough used was a KSV 5000 (KSV Instruments LTD, Helsinki, Finland). All experiments were carried out at room temperature (20-22 °C). The monolayer was transferred to a clean glass substrate at a surface pressure of 18 mN/m. The AFM images were taken with an in-house built microscope under atmospheric conditions using commercial Nanoscope cantilevers (Digital Instruments, Santa Barbara, CA). A 1 µm2 area of the surface of the monolayer deposited on glass is seen in Figure 1A. It is observed that the © 1997 American Chemical Society
Notes
monolayer forms irregularly shaped slabs that do not completely cover the substrate. Two small higher patches are observed (arrow heads in Figure 1). The smaller one (open arrowhead) has a height that corresponds to two monolayers, probably because two small areas of adjacent slabs have overlapped during transfer, whereas the larger patch (black arrowhead) has a height of about 2.5 nm that could correspond to a Z type deposition (see below) in which the second layer has a different thickness, due either to a larger inclination angle or to a partial interdigitation into the bottom monolayer. Further studies would be required in order to distinguish between these possibilities. Figure 1C corresponds to an image of the same area taken in the friction mode. In this mode, the lighter regions represent hydrophilic areas that have a stronger interaction with the hydrophilic scanning tip. We can therefore say that the lower regions correspond to the bare glass whereas the higher parts correspond to the upper surface of the hydrophobic tails of the azobenzene derivatives. The surface of the two small higher patches has the same friction as the rest of the monolayer, meaning that the molecules in them are also oriented with their aliphatic tails up (Z type deposition). The height of the slabs (Figure 1B) is 4.0 ( 0.5 nm, which corresponds well with the height of 3.6 nm for the extended molecule calculated from the CPK model. The small height difference could be accounted for by a molecule of water staying between the glass substrate and the carboxyl group of the derivative. When a smaller region of the surface of the slabs is scanned, individual molecules can be resolved. Each hump corresponds to the last methyl group of the acyl chain of the azobenzene derivative. A highly ordered array of molecules is observed (Figure 1D). Domains in the size range of a few 100 nm2 with different molecular orientations are observed before and after UV irradiation of the monolayer (see Figure 4). The area per molecule measured directly from the images is 0.23 ( 0.02 nm2, which is the same as the area for the trans isomer obtained from the isotherms of the monolayers.10 The molecules are packed in an hexagonal closed-packed crystalline array. The Fourier transform of the images shows that, before irradiation, the hexagons present a slight distortion with one of the angles being 70° instead of 60° and the distance between molecules being 0.6 ( 0.1 nm. The lower frequency spots reveal the presence of a longer range periodicity in one of the directions. The picture is then that of a compact bidimensional crystal with the azobenzene molecules standing vertically with the carboxyl group in touch with the glass hydrophilic surface and the hydrophobic tails standing away from it, as would be expected for a Langmuir-Blodgett monolayer of this amphipathic material deposited on a hydrophilic substrate. The packing of the azobenzene groups imposes some restrictions to the packing of the aliphatic chains in an hexagonal closed-packed array. The monolayers were irradiated with a 100 W mercury lamp (with a maximum emission peak of 365 nm) for 3060 min. Under these conditions, the A4 compound dissolved in chloroform underwent reversible isomerization cycles that could be followed spectrophotometrically by a decrease of the UV absorption band at 340 nm (trans isomer) along with the increase of the 450 nm absorption band (cis isomer) after UV irradiation (see Figure 2). We observed no changes in absorbance in bands responsible for the trans (340 nm) and cis isomers (450 nm) in LB multilayers (10-16 dips). An AFM analysis of the irradiated monolayer revealed no appreciable change in the slab height (Figure 3A and B). Areas on the surface of the slabs with different frictions can be observed (Figure 3C). Changes in the orientation of the molecules can affect
Langmuir, Vol. 13, No. 4, 1997 871
Figure 2. Absorption spectra of A4 solution in chloroform: (curve 1) before irradiation; (curve 2) after 30 min of irradiation with UV light at 360 nm; (curve 3) after 50 min of irradiation with visible light at 452 nm.
Figure 3. Monolayer after UV irradiation. (A) Topographic image of a 1.5 µm2 area. The different gray tones reflect different heights, white being highest and black lowest (total height range is 5 nm). (B) Profile of the white line shown in part A. (C) Friction image of the same area shown in part A. The lighter regions represent areas that have a stronger interaction with the scanning tip. (D) Small area scanned on top of the monolayer. The inset is the Fourier transform of the image.
their interaction with the scanning tip, giving rise to the different frictions observed in the image.11 A higher magnification image does indeed show that the highly ordered molecules of the monolayer have experienced a rearrangement and that the crystalline packing has been altered (Figure 3D). However, no significant changes in the area occupied by each molecule or the total height of the slabs was observed. When the structure of the monolayer was observed for some time after switching off the UV irradiation, the relaxation of the strain produced by the rearrangement of the molecules induced the formation of holes in boundary areas between differently oriented domains of the monolayer. Figure 4 shows topographic and the corresponding friction images of the same area taken after 2 (A) and 3 (B) h, respectively, after turning off the UV irradiation. The molecular rearrangement observed is very slow, probably because, as the (11) Overney, R. M.; Takano, H.; Fujihira, M.; Overney, G.; Paulus, W.; Ringsdorf, H. In Forces in Scanning Probe Methods; Gu¨ntherodt, J., et al., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; pp 307-312.
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Notes
Figure 5. (A) Topographical image of the monolayer taken 2 h after switching off the UV irradiation. (B) Topographical image of the same area 1 h after image A was taken (total height range in the topographic image is 5 nm).
Figure 4. Monolayer after UV irradiation. (A) Topographic (left) and friction (right) images of a 200 nm area (total height range in the topographic image is 1 nm) of the monolayer 2 h after the UV irradiation was switched off. (B) Topographic (left) and friction (right) images of the same area taken 1 h after image A was taken (total height range in the topographic image is 5 nm).
images were taken in the dark, the observed changes can only be due to a thermal relaxation of the UV-induced excitation. The images illustrate how an irradiated area with two clearly distinguishable domains breaks open after relaxation. The higher friction observed in the bottom of the hole and its 4.0 ( 0.5 nm height indicate that it is as deep as the monolayer and that the scanning tip is reaching the glass substrate. The overall shape of the slabs also changes after thermal relaxation (Figure 5). The area per molecule for the cis isomer was estimated from the dependence of the limiting area per molecule on the cis/trans isomer ratio, and the value is of the order of 0.60 ( 0.05 nm2.10 This value corresponds to an almost horizontal orientation of the azobenzene derivative. The distance between molecules of about 5-6 nm is too small to provide enough space for the full isomerization from trans to cis of all the molecules in the monolayer. However, the AFM images show that, in spite of the lack of absorbance changes on the irradiated multilayers, some extent of isomerization, enough to modify in some regions the crystallinity of the monolayer, is taking place. Since the AFM images can only detect alterations of the surface
or the total height of the slab, we cannot describe in detail neither the extent nor the exact changes in molecular orientation responsible for the surface structural rearrangement observed. We nonetheless detect significant morphological modifications of the monolayer in the domain boundary between areas with different crystalline orientations and in the edges of the slabs, places where the free volume available for molecular rearrangement is likely to be larger. In summary, we report in this note a structural characterization of an LB monolayer of an azobenzene derivative and a study of the UV irradiation effect on the film. The irradiation alters the crystalline packing in some regions, and the following thermal relaxation induced significant morphological modifications in the frontier region between crystalline domains of different orientations and in the edges of the discontinuous monolayer. Our observations provide experimental support to the suggestion put forward by Liu and collaborators5 that a pretreatment of an LB film with UV can induce a rearrangement of the molecules in the hole or defect regions of the film. This rearrangement could provide sufficient free volume to allow the smooth trans to cis transitions they observed in subsequent irradiations of their layers. More extensive structural studies of the surface of monolayers and multilayers after subsequent irradiations are under way. Acknowledgment. This work was supported in part by Grant PB93-0278 (to S.V.) from the Direccio´n General de Polı´tica Cientı´fica. We also acknowledge the British Council and the Spanish Ministry of Science and Education for the Integrated Action HB93-047 given to S.M.’s and S.V’s groups. LA9605549
