J. Phys. Chem. 1993,97, 5124-5121
5124
Reversible Transitions of Two-Dimensional Domain Patterns in a Photosensitive Monolayer Mitsuru Yoneyama,' Akiteru Fuju, Shigeaki Kasuya, Shuichi Maeda, and Tetsuo Murayama Mitsubishi Kasei Corporation, Research center, 1000 Kamoshida-cho, Midori- ku. Yokohama 227, Japan Received: November 19, 1992; In Final Form: February 8, 1993
Reversible domain pattern transitions have been observed under irradiation with UV and visible light in mixed monolayers of porphyrin and spiropyran derivatives at the air-water interface. Monolayer textures arevisualized using fluorescence microscopy with excitation of the porphyrin molecules by a focused beam. Before UV irradiation, the monolayer in the liquid-gas coexistence region is covered with gaseous domains such as large bubbles and thick stripes. Upon UV irradiation, the domains undergo a shape transition into more extended, thinner stripes. It is shown that these thinner stripes revert to the original domains with compact shapes under illumination with the exciting light. These features are discussed in terms of a reversible change in a balance between dipolar repulsions and line tension in the monolayer, which is triggered by the photoisomerization of the spiropyran molecules.
Introduction Domain shape transition at the air-water interface is one of the subjects of great interest in current monolayer research from both an experimental and a theoretical standpoint.' A number of theoretical studies have been made of the shapes and shape transitions of lipid domains based on a competitionbetween shortrange van der Waals attraction and long-range dipolar repuls i o n ~ For - ~ fluid monolayerswhere the average dipole moments are vertical with respect to the monolayer surface, simple but quite informative models on the instability of an isolated domain have been proposed by McConnell and co-worker~.~~~ According to their calculations, a circular domain undergoes a shape transition into an elliptical domain at the critical radius where X is the line tension of the domain boundary and p is the difference in dipole densities between neighboring phases. This expression clearly signifes the dependence of domain shapes on X and p in a competitive manner. Experimentally, Rice and McConnell have found that shape transitions to noncircular domains can be induced in lipid monolayers through the use of light pulses that lead to changes in the ratio X/F2 due to photochemical effects.IO Their systems, however, entail irreversible factors sincethe reversal of the domain shape was achieved by the diffusion of photochemical products away from the illuminated region.I0 In previouswork,' I we have reported the observationof bubbleto-stripe shape transitions of gaseous domains in porphyrin Langmuir monolayers using fluorescence microscopy. There, the critical bubble size characterizing the transition was found to be highly sensitiveto subphase pH that can affect p, supporting the appeal of the theoretical picture. The purpose of the present paper is to show how such transitions can be controlled by light irradiation when photochromic compounds are incorporated in the monolayer. We present evidence of a reversible change in two-dimensionaldomain pattern with alternate irradiation of UV and visible light.
SP-18
Figure 1. Chemical structures of the porphyrin @-C18PyTTP) and spiropyran (SP-18).
in spectroscopicgrade chloroform each with a concentration of 5.0 X 10-4 M. Mixed monolayerswere formed by spreading the solutiononto a water surface of a Teflon-coated aluminum trough (25 X 10 cm) placed on the stage of a fluorescencemicroscope. The water for the monolayer substrate was purified in a Milli-Q system and then buffered to pH 4.0-6.5 with HCl and KHCOj. The experimental setup and methods for surface pressure-area (TA) isotherm and absorption measurements of the monolayers were the same as those reported previously." Domain patterns in the monolayers were observed by an Olympus fluorescence microscope by exciting the porphyrin molecules with blue light (410490 nm) which was focused on the monolayer through an objective (Olympus MSPlan5O). The light sourcewas a 100-Wmercury lamp. The fluorescence image was separated from the exciting light with a dichroic mirror and viewed with a SIT television camera, and was recorded on videotape. To induce photoisomerization of SP-18 into its ring-open merocyanine form,I2the monolayers were irradiated with a UV lamp (useful wavelength 365 nm, Spectronics Co.). The reconversion from merocyanine to spiropyran was achieved in the microscope field through illumination with the excitinglight (410490 nm). Results
Experimental Section Figure 1 shows the chemical structures of the film-forming chromophores. The porphyrin (p-ClsPyTTP) was synthesized in our laboratory. The spiropyran (SP-18) was purchased from Nippon Kankoh-ShikisoKenkyusho Co. and used without further purification. A solution of a pc18PyTTPIsP-18mixture with a molar ratio of 1:l was prepared by dissolving the compounds 0022-3654/93/2091-5 124$04.00/0
*-A Isotherms and Absorption Spectra. Figure 2 shows the T-A isotherms ofp-CI8PyTTP/SP-18 monolayers on pure water (pH 5.6), both beforeUVirradiationandunder 365-nmirradiation
(1 mW/cm2), plotted as surface pressure against area of the p-CI8PyTTP molecule. The unirradiated monolayer exhibits several transitions, the most prominent one being present as the inflection point at about 15 mN/m. This behavior is likely to be 0 1993 American Chemical Society
Transitions of Two-Dimensional Domain Patterns
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5125
a u-
50 100 150 200 Area per Porphyrin Molecule (A2)
0"
Figure 2. r-A isotherms of pc18PymP/sP-18 (1:l) monolayers on pure water: (-) without UV irradiation and (---) during UV irradiation.
400
500 600 Wavelength(nm)
700
Figure 3. Absorption spectra of pCl~Py"TP/SP-18 (1:l) monolayers on pure water measured at normal incidence at 2 mN/m: (-) without UV irradiation and (- - -) after 6 min of UV irradiation.
due to the crystallization and/or the squeezing out of SP-18 molecules by compression of the film, arising from the lack of hydrophilicityof the spiropyran chromophore. Several authors'2-'6 have indeed reported that hydrophobic spiropyran compounds are unstable with respect to formation of crystals on the water surface and cannot form stable monolayers. On the other hand, the irradiated monolayer gives a rather monotonic change in surface pressure during compression. In this case, SP-18 molecules are converted to zwitterionic merocyanine with larger polarity,12-'6 allowing the formation of wellmiscible monolayers with p-clsmmpeven at higher pressures. However, it is noted that the initially rising part of the isotherm is unaltered by UV irradiation. This suggests that the nature of molecular packing in expandedstates (-0 mN/m) is determined largely byp-C18PyTTP. Each porphyrin molecule thus provides a space in which SP-18 can effectively photoisomerize without affecting the porphyrin arrangement. Similar features were observed in porphyrin/azobenzene mixtures.' I The photoisomerization of SP-18 on the water surface is confirmed by absorption measurements. As an example, the absorption spectra of the mixed monolayer at a coatant pressure of 2 mN/m are depicted in Figure 3. Although the main contribution to the spectra comes from p c 1 8 m m P , the a p pearance of a broad band around 540 nm is obvious under UV light; this band corresponds to the open merocyanine structure of SP-18. The absorption at 540 nm is found to increase rapidly after the start of UV irradiation, reaching its maximum in about 5 min, and to remain constant during irradiation (-10 min). After switching off the UV light, the absorption decreases gradually in the dark by thermal relaxation and returns to its original value in about 15 min. This reversion process is accelerated by visible irradiation. For instance, it takes less than 5 min for the absorption at 540 nm to relax when the monolayer is irradiated with 0.5 mW/cm2 blue light (410-490 nm). Transitionsof Domain Patterns. As demonstratedin previous work,II the growth of gaseous domains can be easily monitored in liquid monolayers under microscope observation since continuous illumination with the exciting light causes thermal expansion of the focused portion of the monolayer into the liquidgas coexistence region. The same method was employed in the
Figure 4. Fluorescence microscope images of p-C IBP~TTP/SP18 ( 1:1)
monolayersat pH 4.21: (a) without UV irradiation and (b) after 2 min of UV irradiation. The monolayer was expanded to an average molecular area of about 210 A2 before starting microscope observation. The microscope stage was slowly moved in a horizontal direction to scan the monolayer surface. The dark domains are two-dimensional gaseous domains. (Bar = 50 pm.)
present experimentsto observe domain shapetransitions. Figure 4 shows the typical fluorescence microscope images of the mixed monolayer at pH 4.21. Throughout the observations, the microscopestagewas slowly moved to scan the monolayer surface so as to illustrate the evolution of domain patterns in a single picture. Thus, in Figure 4 the monolayer moves upward at a constant speed, passing through the illuminated spot inside the octagonal area in 2-3 s. When the monolayer is not irradiated with UV light, gaseous bubbles grow in the bright microscope field and then elongate into stripesof width 1.5 pm, forminga two-dimensional domain pattern (Figure 4a). These structures are commonly observed in porphyrin-containing monolayers by the "local expansion technique".' 1 Upon UV irradiation, the startingdomains,bubbles or stripes, undergo a transition intosmalland/or extended domains so that it becomes difficult to visualize their exact shapes within our experimental resolution. Moreover, these domainsare quite unstable under microscope observation: illumination with the exciting light promotes the reversion of the domains to the original large domains with compact shapes. We have therefore not succeeded in obtaining clear, stable images of the extended domains generated by UV light in the microscope field. Instead, we have been able to track successive shape reversions from extended to compact domains induced by visible irradiation. Figure 4b illustrates such transitions. In taking this particular image, the microscope field was narrowed as much as possible to minimize
-
Yoneyama et al.
5126 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
light-inducedthermal expansion. Thus, thedomain shapechanges seen in Figure 4b can be interpreted as mainly due to the isomerization of SP-18 from merocyanine to spiropyran as discussed later. It is evident from Figure 4b that, initially, thin stripes appear in the lower part of the bright field. Exposed to the intense exciting light these stripes rapidly shrink and thicken until they turn into the original circular structures, the structures that are observed in the UV-unirradiated monolayer (see Figure 4a). The effect of UV and visible irradiation on the domain pattern is more clearly demonstrated in Figure 5 where the pattern evolution of a UV-treated monolayer is followedunder microscope observationin a wider field. The images present transient features observed immediately after widening the microscope field from its smallest to its largest size. In Figure 5a, there is a marked difference in domain pattern between inside and outside of the region scanned previously by the focused exciting light: large, compact domains are found in the scanned region while the other section is covered with faint patterns composed of thin, extended stripes. The contrast disappears upon visible illumination: a random array of thick stripesof width 1.5 pm are formed across the entire field (Figure 5c). Fluorescence studies have been carried out for samples with p-CI 8PyTTP/SP-18 ratios other than 1:1. Essentially the same pattern transition of gaseous domains is alwaysobserved, but the domain shape becomes much more compact as the spiropyran ratio increases. At a mixing ratio of 15, for example, only the gaseous bubbles appear in the monolayer before UV irradiation; these bubbles begin to interact with each other before elongating into stripes and form a foamlikestructure. Upon UV irradiation, these bubbles undergo a transition to thin stripes,the exact width of which is again difficult to obtain with our experimental accuracy.
P--
-
-
r
-
1
-
Discussion Theoretically, the sizes and shapes of lipid domains on the water surface are determined by the ratio X/p2.2*3The photochromic reaction in the present photosensitive monolayer is expected to bring about a significant change in this ratio through a change in p or A, or both. Assuming that the isomerization from spiropyran to merocyanine produces a net decrease in this ratio, we can givea qualitativeexplanationfor the domain pattern transitions seen in Figures 4 and 5. For convenience, we resort to the theoretical treatment for a shape transition from a square to rectangular domain.3 In such a hypothetical system, thedomain shapes can be analytically calculated both at the transition point and far from the transition point, and their dimensionalchanges can be most explicitly represented by a plot of width vs area of the domain depending on X/p2. Figure 6 shows the schematic width-area diagram for our photosensitive monolayer based on such calculations. Curves I and I1 correspond to the monolayers containing SP-18 in its spiropyran form and in its more polar merocyanine form, respectively. Without UV irradiation, a gaseous bubble converts to a stripe at a large critical size As on monolayer expansion along a path indicated by arrow 1 in Figure 6;the patterns seen in Figure 4a and in the prescanned area of Figure 5 belong to this type of transition. In contrast, the monolayer is driven to the other state characterized by curve I1 upon UV irradiation;the domain pattern consists of smaller and/ or less compact domains with the bubble-stripe transition occurring at a smaller size A,. The microscope observation then induces a reversible transition of the starting thin stripes with areas below As either along path 2 or along path 3 in Figure 6. Path 2 describes a transition to large bubbles as experimentally observed when light-induced expansion is negligible (Figure 4b), whereas path 3 correspondstoa transition to thickstripesproduced by a combination of merocyanine-to-spiropyranreconversion and light-induced expansion of the monolayer (outside of the prescanned region of Figure 5).
Figure 5, Shape transitions of gaseous domains induced by visible irradiation for a pC18PyTTP/SP-18(1:l) monolayer at pH 4.15. The monolayer was expanded to an average molecular area of about 210 A2 and irradiated for 4 min by UV light. The monolayer surface was then scanned by the exciting light through an objective, keeping the microscope field at its smallest size. The images were taken immediately after opening the microscope field to its largest size. The time elapsed between a and c is about 700 ms. (Bar = 50 pm.)
Recently, Yamaguchi et a1.16 have investigated the effect of
UV irradiationon the surfacepotential of a pure SP-18monolayer and found that the open merocyanine form has a larger dipole moment comparedwith theclosed spiropyran form. This implies that the ratio X/p2 in the SP-18 monolayer can be reduced by UV irradiation through an increase in p, supporting the assumption
Transitions of Two-Dimensional Domain Patterns
T
Q
I
k
A9
Area
Figure 6. Schematic width-area diagram for gaseous domains. Curves I and I1 represent shape changes in spiropyran- and merwyaninecontaining monolayers, respectively. At As (A,,,), the domain undergoes and w indicate typical a bubble-stripe shape transition. e, -, domain shapes of the phases. The arrows labeled 1, 2, and 3 show the paths followed in experiments described in the text.
adopted above. Of course, the possibility of the line tension X contributing to changes in X/p* cannot be ruled out. It is not clear at this stage which factor plays a more dominant role. The observed dependence of the domain shape on the p-Cl!jPyTPP/ SP- 18ratio, at least for theUV-unirradiated monolayers, indicates that the critical bubble size determined by Alp2 increases with increasing SP-18 ratio. Sincethe spiropyran form has a negligible dipole moment perpendicular to the water surface,I6 this dependence is likely to be explained by changes in A. Detailed information on Xis therefore needed to discuss thedomain pattern transition systematically. As shown in previous work,l' domain shape transitions in porphyrin/fatty acid monolayers are strongly dependent on subphase pH. We have also investigated the effect of subphase pH on the present domain pattern. However, the dependence is found to be very weak over the pH range that we have studied (pH 4.0-6.5): the critical bubble size is almost constant, with only a slightly increasing trend observed at lower pH. The weak dependenceis reasonable because unlike fatty acids, no noticeable protonation or deprotonation is thought to be involved in the present film-forming molecules at the air-water interface. We utilized the exciting light as a visible light source for converting the merocyanine form of SP-18 back to its spiropyran form. The use of the exciting light affords a means to easily observe domain pattern changes inside the microscope field, but makes it difficult to see equilibrated domain structures under UV irradiation. This is an inherent drawback to fluorescence microscopyin obtaining quantitativeinformation on domain shape transitions. We cannot therefore make definitive comparisons between experiment and theory on the observed domain pattern transition at the present stage. Moreover, because of this drawback the film morphologies that can be detected are quite limited. For example, it is predicted theoretically4that circular domains will distort to various noncircular domains with higher harmonic symmetrieswhen a sudden decreasein X/r2 is produced. A transition of this kind should take place in our monolayer upon
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5127 irradiation of UV light that is intense enough to bring about rapid photoisomerization of SP-18; the mode into which the circular domain (ground state) is driven could be altered in a controlled manner by adjusting the light intensity. However, unambiguous observation of such transitions were impossible under the present experimental conditions. This limitation may be removed by employing direct visualizationtechniques such as 3rewster angle ~pectroscopy.~~J* A combination of such tools with fluorescence microscopy will uncover a variety of interesting phenomena that help in understandingfurther the natureof phases and pattern formation in monolayer systems. In conclusion, the observation of the domain pattern changes under irradiationwith UV and visible light provides experimental evidence of a reversible domain pattern transition induced by the photoisomerization in the photosensitive monolayers which were studied. A key requirement for such a transition to be clearly identifiedis a large change in molecular structure and/or polarity that changes the value of X/p2. It is thus anticipated that the light-induced pattern transition is universal in photochromic monolayers. From this point of view, we will extend our experiment to other photochromic monolayers as well as perform surface potential measurements to clarify the quantitative dependence of the domain shape on A/p2.
Acknowledgment. This work was performed under the management of FED (the R&D Association for Future Electron Devices) as a part of the RBD of Basic Technology for Future Industries supported by NED0 (New Energy and Industrial Technology Development Organization).
References and Notes (1) McConnell, H. M. Annu. Rev. Phys. Chem. 1991,42, 171. (2) Keller, D. J.; Korb, J. P.;McConnell, H. M. J . Phys. Chem. 1987, 91, 6417. (3) McConnell, H. M.; Moy, V. T. J. Phys. Chem. 1988, 92, 4520. (4) McConnell, H. M. J. Phys. Chem. 1990, 94,4728. ( 5 ) Andelman, D.; Brqhard, F.; Joanny, J. F. J . Chem. Phys. 1987,86, 3673. (6) Vanderlick, T. K.; M6hwald, H. J . Phys. Chem. 1990, 94, 886. (7) Hurley, M. M.; Singer, S.J. J. Phys. Chem. 1992, 96, 1938. (8) Hurley, M. M.; Singer, S. J. J . Phys. Chem. 1992, 96, 1951. (9) Deutch, J. M.; Low, F. E.J . Phys. Chem. 1992, 96, 7097. (10) Rice, P. A,; McConnell, H. M. Proc. Narl. Acad. Sci. U.S.A.1989, 86, 6445. (1 1) Yoneyama, M.; Fujii, A.; Maeda, S.; Murayama, T. J . Phys. Chem. 1992, 96, 8982. (12) Polymeropoulos, E. E.; Mdbius, D. Ber. Bunsen-Ges. Phys. Chem. 1979,83, 1215. (13) Holden, D. A.; Ringsdorf, H.; Deblauwe, V.; Smets, G. J . Phys. Chem. 1984,88, 716. (141 McArdle. C. B.: Blair. H. S. Colloid Polvm. Sci. 1984. 262. 481. (15) Ando, E.: MiyaAki, J:; Morimoto, K.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 21. (16) Yamaguchi, T.: Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J . Appl. Phys. 1992,31, 1160. (17) Honig, D.; Mdbius, D. J . Phys. Chem. 1991, 95, 4590. (18) Henon, S.;Meunier, J. Rev. Sci. Instrum. 1991, 62, 936.
