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Langmuir 1988, 4 , 1068-1069
Stable J-Aggregate Formation of Photoinduced Merocyanine in Bilayer Membrane Takahiro Seki* and Kunihiro Ichimura Research Institute for Polymers and Textiles, 1-1 -4 Higashi, Tsukuba, Ibaraki 305, Japan
Eiji Ando Central Research Laboratories, Matsushita Electric Industrial Co. Ltd., Moriguchi, Osaka 570, Japan Received February 19, 1988. I n Final Form: April 20, 1988
A spiropyran compound having two long alkyl chains was incorporated into ammonium-type bilayer membranes. UV-induced merocyanine of this spiropyran formed a very stable J-aggregate within the bilayer matrices, which was disrupted by neither the fluidity change of the membrane nor the prolonged mechanical sonication. Introduction Photoisomerization and accompanying thermal isomerization of chromophores (photochromism) in bilayer membranes have been the subject of extensive studies in relation to photobiological processes such as vision.' In such bilayer matrices, the kinetics and rates of photochromic reactions are often controlled by the change in membrane f l ~ i d i t y . ~ -It~ has been shown earlier that thermal isomerization kinetics of spiropyrans (UV-induced merocyanine (PMC) starting spiropyran (SP)) is altered by the crystal to liquid crystal phase transition (the temperature, T,) of an ammonium-type bilayer membrane.4 On the other hand, dye aggregation in bilayer membranes has attracted attention as a useful model for understanding the arrangement or organization of biological pigrnents.'+'O We report in this paper that PMC compounds of a spiropyran having two long alkyl chains (3) form a very stable J-aggregate (J-PMC) in bilayer membranes regardless of the physical state of the matrices. Once the J-aggregate was formed, the back reaction to SP virtually did not take place either in the dark below 50 O C or under visible light exposure. Formation of J-PMC of 3 and its marked stability has already been reported in the system of Langmuir-Blodgett (LB) film composed of the mixture of 3 and ~ c t a d e c a n e .This ~ paper demonstrates that J-PMC can be dispersed in water by means of being supported by the bilayer matrix. -+
(1) (a) Wald, G. Nature (London) 1969,219, 800. (b) O'Brien, D. F. Photochem. Photobiol. 1979,29,679. (c) Kano, K.; Tanaka, Y.; Shimomura, M.; Okahata, Y.; Kunitake, K. Chem. Lett. 1980, 421. (2) Suddaby, B. R.; Brown, P. E.; Russell, J. C.; Whitten, D. G. J.Am. Chem. SOC.1985, 107, 5609. (3) Shimomura, M.; Kunitake, T. J. Am. Chem. SOC. 1987,109,3228. (4) (a) Seki, T.; Ichimura, K. J. Chem. SOC.,Chem. Commun. 1987, 1187. (b) Seki, T.; Ichimura, K. J . Colliod Interface Sci., submitted. (5) (a) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985,133,21. (b) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Int. Symp. Future Electron Deuices 1985, 11-5, 47. (6) Kurihara, K.; Toyoshima, T.; Sukigara, M. J.Phys. Chem. 1977, 81, 1883. (7) Lee, A. G. Biochemistry 1975, 14, 4337. (8) Nakashima, N.; Ando, R.; Fukushima, H.; Kunitake, T. J. Chem. SOC.,Chem. Commun. 1982, 707. (9) Nakashima, N.; Fukushima, H.; Kunitake, T. Chem. Lett. 1981, 1555. (10) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134.
0743-7463/88/2404-l068$01.50/0
n.18
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Results and Discussion A cast film was prepared from chloroform solution with dissolved 3 and dioctadecyldimethylammonium bromide (4)or ditetradecyldimethylammonium bromide (5)." This film was subsequently exposed to UV light (A < 366 nm with a 500-W super-high-pressure mercury lamp and a glass filter) to achieve photoisomerization of SP to PMC. On UV irradiation above 30 "C and a t a 3/4 molar ratio ( R ) of less than 0.006, only monomeric PMC was observed (the absorption maximum (Am,) = 575 nm and the halfwidth (LAlls = 108 nm) (Figure la). In contrast to this, when R was higher than 0.01 a red-shifted sharp absorption band appeared (A, = 618 nm, AAl/2 = 45 nm) in the same procedure (Figure lb). This characteristic absorption band is assignable to that of J-aggregated PMC, which is also confirmed by the fluorescence spectro~copy.~ J-Aggregation was completed within 10 min. Under the same condition no J-aggregation was observed with spiropyrans having no (1) or one (2) long alkyl chainall After J-PMC was formed, the solid film was then dispersed in distilled water with a probe-type sonicator a t 50 "C, resulting in a slightly turbid blue suspension. Even after hydration of the bilayer, the absorption spectrum of J-PMC was kept to 615 essentially unchanged with a slight blue shift of ,A, nm. The spectral shape of J-PMC in bilayer 4 did not (11) For the syntheses of spiropyran compounds, see ref 4b and 5. Bilayer-forming amphiphiles were purchased from Sago Pharmaceutical
co.
0 1988 American Chemical Society
Letters
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Langmuir, Vol. 4 , No. 4, 1988 1069 T /"C a
03t
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70 60
12 min
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I
50
40
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I
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Figure 1. Spectral changes of cast films consisting of the mixture of 3 and 4 upon W irradiation. For spectroscopic measurements, thin films prepared by means of spin coating (2000 rpm for 30 s) were used. Essentially the same results were obtained with the films prepared by spin coating and simple casting methods. The film was exposed to UV light (A > 366 nm, 500-W super-high pressure mercury lamp) at 30 "C and at R (3/4 molar ratio) = 0.006 (a) and R = 0.03 (b). Irradiation times are indicated in the
Figure.
change in the temperature region examined, from 25 to 70 "C, which indicates that the aggregate is unperturbed by the fluidization of the membrane ( T , = 45 0C).12 The insensitivity of J-PMC to the change in membrane fluidity is worthy of remark, because aggregation and dissociation of dyes in bilayer membranes reported so far are sensitive to the physical state of the membrane, and thus the spectral change could be a good indicator of the phase transition.&'O In the hydrated bilayer matrix, photoisomerization between PMC and SP of the monomeric 3 proceeded many times, but reversible formation of J-PMC was not fully achieved because of a very slow aggregation rate that competed with irreversible fading of the chromophore upon exposure to UV light. Figure 2 shows Arrhenius plots for the first-order rate constants, k, of thermal decoloration (isomerization from PMC to SP) of monomeric and J-aggregated PMC in bilayer 4. k was determined by monitoring the absorbances at,,A with time. Data in this figure were obtained in the system of R = 0.01; under this condition monomeric and J-aggregated PMC coexisted in the bilayer matrix, and thus proper comparisons of reaction rates could be made. The fluidity change of the bilayer influenced the kinetics of monomeric PMC; i.e., the Arrhenius plot gave an inflection a t T , as was observed in the systems with other spiropyrans, 1 and Z4Whether or not the decoloration of J-PMC is affected by the phase transition could not be elucidated in this experiment due to too slow reaction below 50 "C. However, the kinetics of J-PMC decoloration seems to be unaffected by the fluidity change judging from the following observation. J-PMC in the two bilayer matrices of different T,, 4 and 5 ( T , = 16 "C),I2 were further sonicated at 30 "C for 2 h. Nearly all of the J-PMC (>95%) was preserved in both bilayer matrices even after this prolonged sonication. This fact indicates that J-PMC is very stable to the mechanical stimulus regardless of the physical state of the bilayer, namely, in the crystalline state (in bilayer 4) or in the liquid crystalline state (in bilayer 5).
There is a large difference in activation parameters of this thermal reaction. Activation energy (E,) and entropy (12)Okahata, Y.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1981,85, 789.
T-'/ 16' K-' Figure 2. Arrhenius plot for the first-order rate constants of thermal decoloration of monomeric (a) and J-aggregated (b) PMCs incorporated into bilayer 4 at R = 0.01. Concentration of 4 was 5.0 X mol (dm3)-'.
(AS') for monomeric PMC were 17.5 kcal mol-' and -11.6 cal mol-' K-l, respectively, above T, and 32.1 kcal/mol-' and 34.3 cal mol-' K-', respectively, below T,. These values are in good agreement with those obtained in other spiropyran system^.^ E , and AS* for J-PMC are noticeably higher, 102 kcal mol-l and 231 cal mol-l K-l, respectively. Due to the large difference in E,, the rate difference between the two species becomes more pronounced a t lower temperatures. At 60 "C the decoloration of J-PMC (k = 1.5 X s-', a close value obtained in the LB film5b)was 440 times slower than that of monomeric one (k = 6.6 X s-I), and a t 25 "C it would be 3.7 X lo8 times slower (k(monomeric) = 5.5 X lo4 s-l, k(J-PMC) would be 1.5 X s-I), which is evaluated from the extrapolation to 25 "C of the Arrhenius plot on an assumption that J-PMC decaying is insensitive to the membrane fluidity. A high activation energy and a highly positive activation entropy are often observed in entropy-driven processes such as the thermal denaturation (loosening) of globular proteins (frequently E , > 100 kcal mol-l, AS* > 200 cal mol-' K-1).13J4 On the basis of the activation parameters, for J-PMC decoloration, the dissociation process of the chromophore, which accompanies a large increase of entropy is likely to be the rate-determining step followed by the fast decoloration of individual PMC. J-PMC was also very stable to visible light. After the suspension was allowed to stand in a lighted room for 2 months, decrease of J-PMC was suppressed within 4% of the initial amount. Under the same condition monomeric PMC was completely bleached within 1 h. In conclusion, a very stable J-aggregate of PMC is formed in the matrix of bilayer membranes as well as in LB film. Applicability of J-aggregation phenomenon of PMC to liquid (water) systems and facility in preparation may find technological importance. Registry No. 3, 114789-59-2; 4, 3700-67-2; 5, 68105-02-2. (13)Glasstone, S.;Laidler, K. J.; Eyring, H. The Theory of Rate Processes, McGraw-Hill: New York, 1941;p 442-447. (14)Lauffer, M. A. Entropy-Driuen Processes in Biology, SpringerVerlag: Berlin, 1975;Chapter 9.
