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J. Phys. Chem. B 1997, 101, 702-704
Photoinduced Self-Organization in Langmuir-Blodgett Films Mutsuyoshi Matsumoto,* Hiroaki Tachibana, Fumiyasu Sato, and Samuel Terrettaz National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan ReceiVed: September 23, 1996; In Final Form: NoVember 22, 1996X
In the mixed Langmuir-Blodgett films of a cyanine dye and an azobenzene derivative, J-aggregate formation of the cyanine proceeded with the reversible photoisomerization of the azobenzene. AFM observations revealed that the J-aggregate formation was accompanied by transformation of the two-dimensional film structure into a three-dimensional one with a number of cone-shaped structures protruding from the surface.
Photoisomerization of a retinal chromophore is used in living creatures to enable the conversion of light into nerve impulses in the case of rhodopsins or proton pumping across the membrane in the case of bacteriorhodopsins.1 The important aspect involved in these processes is that the conformational change of the chromophore caused by the photoisomerization induces the structural change of the proteins required for the subsequent biological activities. Photoisomerization has also been utilized to functionalize molecular materials such as Langmuir-Blodgett (LB) films.2-12 Among others, the conformational change accompanied by the photoisomerization has been used to control the electrical conductivity of the LB films2,3,7-9 and the alignment of liquid crystals lying on the monolayer of the photochromic molecule.4 Further, morphological changes of the film surface have been observed when the films were illuminated.11,12 We report here that the conformational change can be used as a trigger to induce selforganization of the LB films and transformation of the twodimensional film structure into a three-dimensional one. The molecules used in this study are shown in Figure 1. CY forms J-aggregates13,14 in LB films under appropriate conditions.15 In J-aggregates, the absorption band of the molecule is red-shifted, which is considered to be due to two-dimensional chromophore arrangement as in a brickwork pattern.16 APT photoisomerizes reversibly in LB films.8,17 Figure 2 shows the change in the absorption spectrum of a mixed LB film of CY and APT (molar ratio, 1:1) on alternate irradiation with UV and visible (vis) light.18 In the LB film before irradiation, the absorption band at ca. 580 nm due to J-aggregates of CY is accompanied by two broad absorption bands ranging from ca. 500 to 550 nm, which are assigned to a dimer and a monomer of CY, respectively.19 On the other hand, in pure films most CY molecules are arranged in J-aggregates. On irradiation with UV light, the trans-to-cis photoisomerization of APT molecules proceeds, which is accompanied by an increase in absorption due to J-aggregates of CY. This absorption also increases with the cis-to-trans isomerization of APT on irradiation with vis light. Figure 3 summarizes the change in absorbance at 340 nm due mostly to trans-azobenzene and also the change in absorbance at 579 nm due to J-aggregates of CY on alternate irradiation with UV and vis light. APT isomerizes almost reversibly with the base line shifting, suggesting the reorganization of APT molecules accompanied by J-aggregation of CY. It is also seen that with the reversible isomerization of APT, J-aggregate formation proceeds until it comes to a saturated state after a sufficient number of irradiation cycles. Photoirradiation X
Abstract published in AdVance ACS Abstracts, January 15, 1997.
S1089-5647(96)02909-4 CCC: $14.00
Figure 1. Chemical structures of CY and APT.
Figure 2. Change in absorption spectrum of CY and APT (molar ratio 1:1) mixed 20-layer LB film on alternate irradiation with UV and vis light. The spectra taken after every three cycles of alternate irradiation with UV and vis light are shown. The spectrum of the LB film before the illumination and the one after the final illumination are shown in bold. The arrows indicate the progression of selected bands with the increased number of film illuminations. The illumination time was 5 min each.
plays an important role in the J-aggregation since keeping the as-deposited mixed LB film in the dark caused no J-aggregation. The presence of APT is critical since no J-aggregation of CY was induced by the photoirradiation of mixed LB films when an inert matrix such as octadecanol was used. Hence, we consider that the conformational change of APT serves as a trigger to induce the self-organization of CY, changing the aggregate state of CY in the LB films. After a few cycles of alternate irradiation, this self-organization process proceeds thermally at room temperature in the dark. The subsequent photoirradiation serves as a promoter which makes the time necessary for the self-organization shorter. This self-organization process was seen to depend critically on the ratio of CY and APT. Under our conditions, this photoinduced dye aggregation has been obtained only for films with a molar ratio of CY:APT very close to 1:1. The precise two-dimensional © 1997 American Chemical Society
Letters
Figure 3. Changes in absorbance at 340 nm (upper figure) and at 579 nm due to J-aggregates of CY (lower figure) on alternate irradiation with UV and vis light. Odd and even numbers of film illuminations represent absorbance of the two bands after UV and vis light irradiation, respectively.
J. Phys. Chem. B, Vol. 101, No. 5, 1997 703 is a few nanometers, which shows that this film is twodimensional although there are defects in the film. Photoirradiation changes the morphology of the film drastically. The most striking feature is the development of a number of coneshaped structures.22 The height of these structures is ca. 10 nm, and the diameter of the base is ca. 100 nm.23,24 Repeated scans of the same regions produced essentially identical images. We consider that the photoisomerization of APT has induced the formation of these three-dimensional structures which should be closely associated with J-aggregation. We further assume that these structures consist of J-aggregates of CY since the morphology of the LB films consisting uniquely of APT did not change significantly on alternate irradiation with UV and vis light. In this sense, the morphological change observed in the mixed LB films should have a different mechanism compared to the cases in the single-component LB films.11,12 After a sufficient number of alternate irradiation cycles have been applied, J-aggregate formation comes to a saturated state as is shown in Figure 3. In the saturated state, the threedimensional cone-shaped structures have developed further, and, for large cones, the height is ca. 30 nm and the diameter of the base ca. 200 nm. This fact supports our assumption that the cone-shaped structures consist of J-aggregates of CY. The number of CYs in these large cones can be obtained by estimating the size of CY in the structures. The length of CY was estimated to be ca. 3 nm using molecular modeling and the cross-sectional area 0.5 nm2 from the value in the condensed monolayer region in the surface pressure-area isotherm. By assuming that, for simplicity’s sake, the cones have layered structures as in usual LB films, the number of CY molecules is calculated to be ca. 2 × 105 in each structure. The overall picture is that molecules which are forced to mix with each other in an as-deposited state are driven to selforganize by the stimulus produced by the photoisomerization of APT. The prominent feature of this self-organization process is that CY, which forms three-dimensional structures, does not photoisomerize and that the energy necessary for this selforganization should be provided by APT. In this sense, this self-organization method should be applied to other molecules, and it may be possible to obtain unique molecular arrangement and morphology. This method may find applications to optical storage, confinement of excitons, and nanolithography. References and Notes
Figure 4. Atomic force microscope images of CY and APT (1:1) mixed one-layer LB film on mica before (upper figure) and after (lower figure) three alternate irradiation cycles with UV and vis light. Before irradiation the film is considered to be two-dimensional, which transforms, by the photoirradiation of APT, into a three-dimensional structure with a number of cone-shaped features.
structure of LB films in terms of mixing, domain size, and so on necessary for the described effect is still unknown. Figure 4 shows the AFM images of a one-layer LB film of CY and APT on mica before and after three cycles of alternate photoirradiation.20,21 Before irradiation, the surface roughness
(1) Birge, R. R. Annu. ReV. Phys. Chem. 1990, 41, 683-733. (2) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 111, 30803081. (3) Tachibana, H.; Goto, A.; Nakamura, T.; Matsumoto, M.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. Thin Solid Films 1989, 179, 207213. (4) Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K. Macromolecules 1989, 22, 3505-3506. (5) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658660. (6) Iwamoto, M.; Majima, Y.; Naruse, H.; Noguchi, T.; Fuwa, H. Nature 1991, 353, 645-647. (7) Tachibana, H.; Azumi, R.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Chem. Lett. 1992, 173-176. (8) Tachibana, H.; Manda, E.; Azumi, R.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Appl. Phys. Lett. 1992, 61, 2420-2421. (9) Tachibana, H.; Matsumoto, M.; Manda, E. Mol. Cryst. Liq. Cryst. 1995, 267, 341-346. (10) Miyasaka, T.; Koyama, K.; Itoh, I. Science 1992, 255, 342-344. (11) Geue, T.; Stumpe, J.; Pietsch, U.; Haak, M.; Kaupp, G. Mol. Cryst. Liq. Cryst. 1995, 262, 157-166. (12) Scho¨nhoff, M.; Chi, L. F.; Fuchs, H.; Lo¨sche, M. Langmuir 1995, 11, 163-168.
704 J. Phys. Chem. B, Vol. 101, No. 5, 1997 (13) Jelly, E. Nature 1936, 138, 1009-1010. (14) Scheibe, G. Angew. Chem. 1937, 50, 51. (15) Penner, D. L.; Mo¨bius, D. J. Am. Chem. Soc. 1982, 104, 74077413. (16) Czikklely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207-210. (17) APT was used in this study since the azobenzene derivative without TCNQ did not photoisomerize in the LB films. (18) The sample was a 20-layer mixed LB film of CY and APT (mixed in the same monolayers at molar ratio of 1:1) transferred using a horizontal lifting method. The film illuminations were done using monochromatic UV (365 nm) and vis (436 nm) light from a 500-W high-pressure mercury lamp. (19) Steiger, R.; Kitzing, R.; Junod, P. J. Photogr. Sci. 1973, 21, 107116. (20) The AFM image was taken on a Seiko SPA 300 with an SPI 3700 probe station using noncontact mode (dynamic force mode) at 27 kHz.
Letters Commercially available Si3N4 cantilevers with a force constant of 1.5 N/m were used. (21) For the use of scanning probe microscopes for organic thin films, see: Bottomley, L. A.; Coury, J. E.; First, P. N. Anal. Chem. 1996, 68, 185R-230R and references therein. (22) The three-dimensional structures formed on the film surface may also be called “hills” since the height is about an order of magnitude smaller than the diameter of the base. (23) Assuming that CY molecules leave holes after they have moved to make three-dimensional cone-shaped structures, we estimate that the increment of the area of the holes should be less than 1% of the total area. (24) Generally the shape of the images is influenced by the shape of the AFM tip; see: Louder, D. R.; Parkinson, B. A. Anal. Chem. 1995, 67, 297A-303A. Hence, the detailed analysis on the actual shape of the threedimensional cone-shaped structures was not done in this Letter.
