969
Langmuir 1995,11, 969-976
"Crystal Engineering"Based on Two-Dimensional Molecular Assemblies. Relation between Chemical Structure and Molecular Orientation in Cast Bilayer Films Masatsugu Shimomura" and Satoshi Aiba Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan
Nobuyoshi Tajima, Noriko Inoue, and Kenji Okuyama Department of Biotechnology, Tokyo University of Agriculture & Technology, Koganei, Tokyo 184, Japan Received August 31, 1994. I n Final Form: December 1, 1994@
Optically transparent films of single chain ammonium amphiphiles having an azobenzene chromophore in the hydrophobicchain (CnAzoCmN+)were prepared by simple casting oftheir water or ethanol solutions. The UV-visible absorption spectrum ofthe azobenzene chromophore was strongly affected by the chemical structure of the amphiphile, especially on the chain length of the alkyl tail (n= 3- 14)and the spacer group (m = 4-12), and is classified into six groups: absorption maximum of group I, 300-305 nm; group 11, around 320 nm; groups I11 and IV,345-355 nm; group V, 360-370 nm; group VI, 375 nm. Structural requirement of the group I is found to be m - n 2 2. The amphiphiles with a long alkyl chain (n + m > 18) and structural relation of m - n 5 1 are comprised into the group 11. Amphiphiles with m = 8 and 7, except for the amphiphiles in groups I and 11, are classified in group 111. Groups IV and V consist of the amphiphiles with m = 4 and 6, respectively. Group VI consists of the series of amphiphiles with m = 5, except n = 3 and 4. X-ray diffraction experiments indicate that the cast films are composed of multiple stacked bilayers. Intensity distribution in the X-ray diffraction pattern of group V was very similar to group VI but different from groups I and 11. Supposing a tilt molecular packing with a head-to-tail chromophore orientation, the molecular axes inclines by 36" and 26" to the bilayer surface in groups V andVI, respectively. The molecules in groups I and I1 are supposed to be packed laterally in an antiparallel and mutually interdigitated fashion. Spectral difference of I and I1is ascribed to full and partial overlapping of the azobenzene chromophores, respectively. A peculiar small void space under the bromide counterion is expected to be formed in the group I films if the spacer chain is longer than the alkyl tail ( m - n > 2). Thermally induced structural transformation to V or VI was observed when the films of groups I and I1 were annealed above their phase transition temperatures. 1. Introduction Langmuir-Blodgett films are designed molecular assemblies providing novel molecular materials for molecular electronics and photonics devices, sensing devices, and so on.' Bilayer membranes are another sort of twodimensional molecular assemblies which are spontaneously formed from a large variety of amphiphilic molecules in ~ a t e r . ~ Since - ~ aqueous bilayer membranes were found to be immobilized as self-standing thin solid films by the solvent casting m e t h ~ d ,cast ~ , ~bilayer films have been used as new molecular material^^,^-'^ for chemical sensors, Abstract published in Advance A C S Abstracts, February 15, 1995. (1)Recent progress in Langmuir-Blodgett films presented at the 6th International Conference on Organized Molecular Films is published in the special issues of Thin Solid Films 1994,242,243,and 244. (2)Kunitake, T. Angew. Chem., Znt. Ed. Engl. 1992,31, 709. (3)Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Znt. Ed. Engl. 1988,27,113. (4)Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (5)Shimomura, M. Prog. Polym. Sci. 1993,18,295. (6)Kunitake, T.; Shimomura, M.; Kajiyama, T.; Harada, A.; Okuyama, K.; Takayanagi, M. Thin Solid Films 1984,121,L89. (7)Shimomura, M.; Kunitake, T. Polym. J . 1984,16,187. (8)Kuo, T.; O'Brien, D. F. J. Am. Chem. SOC.1988,110,7571. (9)Seki, T.; Ichimura, K. J . Phys. Chem. 1990,94, 3769. (10)Asakuma, S.;Okada, H.; Kunitake, T. J.Am. Chem. SOC.1991, 113,1749. (11)Hamachi, I.; Noda, S.; Kunitake, T. J . Am. Chem. SOC.1991, 113,9625. @
optical devices, and many other fields. Cast films have a pseudocrystalline nature of molecular ordering concurently with mechanical flexibility like polymer films. Structural characterizations of the immobilized bilayer assemblies are essential for the molecular design of the functional materials. On the bases of the systematic crystallographic investigation of single crystals of doublechain ammonium amphiphiles,17 Okuyama wrote a computer simulation program for the calculation of bilayer structures in cast bilayer films and bilayer thicknesses estimated from the repeating period in the X-ray diffraction data have been exclusively used for structural discussions. Bilayer formation and spectral properties of single-chain ammonium amphiphiles having an azobenzene chro~~
~
(12)Kunugi, S.;Nakaizumi, S.; Ikeda, K. Langmuir 1991,7, 1576. (13)Niwa, M.;Yamamoto, T.; Higashi, N. J . Chem. SOC.,Chem. Commun. 1991,444. (14)Okahata, Y.; Shimizu, 0. Langmuir 1987,3,1171. (15)Hayashi, K.; Yamafuji, K.; Toko, K.; Ozaki, N.; Yoshida, S.; Iiyama, S.; Nakashima, N. Sens. Actuators 1989,16,25. (16)Kunitake, T.; Kimizuka, N.; Shimomura, M.; Aoki, R. Synth. Met. 1987,18,861. (17)(a)Okuyama, K.;Soboi,Y.;Hirabayashi, IC;Harada, A,; Kumano, A.; Kajiyama, T.; Kunitake, T.; Takayanagi, M. Chem. Lett. 1984,2117. (b) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kaiivama. T. Bull. Chem. SOC.Jnn. 1988,61. 1485.(c) Okuvama, K.: Iiji-ma, N.; Hirabayashi, K.; Kunitake, T.; Kusunoki, M. B d l . Chem. SOC.Jpn. 1988,61,2337. (18)Harada, A.; Okuyama, K.; Kumano, A.; Kajiyama, T.; Takayanagi, M.; Kunitake, T. Polym. J . 1986,18,281.
0743-746319512411-0969$09.0010 0 1995 American Chemical Society
Shimomura et al.
970 Langmuir, Vol. 11, No. 3, 1995 mophore 1 (C,AzoC,N+) in aqueous solutions were extensively investigated by Shimomura and Kunitake.lg On the basis of systematic organic synthesis and spectral CH3
C H , - ( C H , ) . . , - O ~ N = N ~ O - ( C H ~ ) ~ N + - C H , C H ~ OBr' H CH3
1
(C,AZOC,N+)
investigation of 21 azobenzene amphiphiles, UV-visible absorption spectra of the aqueous azobenzene bilayer membranes were found to be strongly affected by the chemical structure and temperature. The spectral versatility was ascribed to Davydov splitting due to strong intermolecular interaction in the ordered molecular assemblies. Semiquantitative calculation based on Kasha's molecular exciton theory20 predicted that hypsochromic shift (blue shift) and bathochromic shift (red shift) in the absorption spectrum observed in the aqueous bilayer solutions were attributed to the side-by-side and the headto-tail orientation of the azobenzene dipole moments, respectively. Molecular and aggregation structure of some azobenzene amphiphiles in single crystals are systematically investigated by Okuyama et a1.21-26 The spectral prediction of the chromophore orientation in the bilayer assemblies was very consistent with the X-ray structural analyses of the single crystals. Owing to well-characterized spectral properties and crystalline structure, the azobenzene amphiphiles are often used as cast bilayer films16*27-33 and LangmuirBlodgett Structural diversity of the bilayer assemblies enables us to design novel molecular materials based on cast bilayer films applicable in many fields of science and technology. The cast films of azobenzene bilayer membranes must be one of the most suitable candidates of the designed molecular materials because of their structuralvariety. Clarification of the relationship between aggregation structure of the cast film and chemical structure of the azobenzene amphiphile is essential and effective for the molecular design of functional molecular materials. Figure 1 briefly summarizes crystal structures and absorption spectra of the azobenzene amphiphiles. Aque(19)Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsenges. Phys. Chem. 1983,87,1134. (20)Kasha, M. Radiat. Res. 1963,20,55. (21)Okuyama, K.; Watanabe, H.; Shimomura, M.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T.; Yasuoka, N. Bull. Chem. SOC.Jpn. 1986, .59. .. 3351. I
(22)Okuyama, K.; Mizuguchi, C.;Xu,G.; Shimomura,M. Bull. Chem. SOC.Jpn. 1989,62,3211. (23)Xu, G.; Okuvama, K.; Shimomura, M. Bull. Chem. SOC.J m . 1991,64, 248. (24)Xu,G.;Okuyama, K.; Shimomura, M. Mol. Cryst. Liquid. Cryst. 1992. , 213.105. - - ,(25)Xu, G.; Okuyama, K.; Shimomura, M. Bull. Chem. S O ~Jpn. . 1993,66,2182. (26)Xu, G.;Okuyama,K.; Ozawa, K.; Shimomura, M. Mol. Cryst. Liq. Cryst. 1993,237,207. (27)Kunitake, T.; Ishikawa, Y.; Shimomura, M.; Okawa, H. J . A m . Chem. SOC.1986,108,327. (28) Okuyama, K.; Ikeda, M.; Yokoyama, S.; Ochiai, Y.; Hamada, Y.; Shimomura, M. Chem. Lett. 1988,1013. (29)Shimomura, M.; Hamada,Y.; Tajima, N.;Okuyama, K. J.Chem. SOC.,Chem. Commun. 1989,232. (30)Shimomura, M.; Tajima, N.; Okuyama, K. Chemistry of Functional Dyes; Yoshida, Z., Kitao, T., Eds.; Mita Press: Tokyo, 1989;pp 252-254. (31)Shimomura, M.; Tajima, N.; Kasuga, K. J . Photopolym. Sci. Technol. 1991, 4 , 267. (32)Okahata,Y.;Fujita, S.;Iizuka, N . J . Chem. Soc., Chem. Commun. 1986,751. (33)Okahata, Y.; Ebato, H.; Ye, X . J . Chem. SOC.,Chem. Commun. 1988,1037. (34)Shimomura, M.; Kunitake, T. Thin Solid Films 1985,132,243. ~
200
300 400 Wavelength (nm)
500
a
Figure 1. Crystalline structures and absorption spectra of azobenzene amphiphiles.
ous bilayer solutions of C&oCloN+ showed a large hypsochromic shift (curve a in Figure 1, absorption maximum 300 nm) relative to the monomeric azobenzene chromophore (curve c in Figure 1, absorption maximum 355 nm).I9 C&oCloN+ molecules pack in a n antiparallel arrangement and the azobenzene chromophores are stacked in the side-by-side orientation in the single crystal.23 Similar crystal structures are found when the amphiphile has the relationship of m - n = 2. Recently, a n odd-even effect of the alkyl chain length was reported to slightly affect the aggregation structure of the hydrophilic region.26 Aqueous bilayer solutions of C,AzoC5N+ having a long alkyl tail ( n I7) showed a large bathochromic shift (curve b in Fig. 1, absorption maxima 375 and 390 nm) relative to the monomeric azobenzene chromophore.lg Okuyama found that single crystals of C,AzoC5N+ ( n = 6-12) amphiphiles belonged to the same crystal system.21)22The amphiphiles are packed laterally with tilting by about 26" to the bilayer surface. The azobenzene chromophores are arranged in the head-to-tail orientation. In the previous report,lgonly spectral results were used for structural estimation of the cast bilayer films. We could not predict antiparallel molecular packing of C&oCloN+ both in a cast film6and in a single crystal.23 Recent progress in successful X-ray structural analyses of the single crystals enable us to determine the most plausible aggregation structure of the azobenzene amphiphiles in cast films. In this paper, X-ray diffraction experiments of the solvent cast films of 43 azobenzene amphiphiles were systematically investigated as well as spectroscopicmeasurements. Structural characterization (35)Shimomura,M.; Hamada,Y.; Onosato, T. Thin Solid Films 1988, 160,287.
Two-Dimensional Molecular Assemblies
Langmuir, Vol. 11, No. ,3, 1995 971
Table 1 Spectral Classification and Absorption Maxima (nm)of Cast Films tail n
spacer m
3
5
4
6
7
IPVb v 300,'363b 362
6
V 367
I 304 I 304 I 303 10
I 304
11 12
9 1 0 1 1 1 2 1 3 1 4
Iv
4 5
8
0)
vIvI 375 372 V 362 I11 346 1 306 1 1 302 305
I v v
347 344 355 vIvIvIvIvIvIvI 372 373 375 375 374 374 373 v v V 365 364 360 I11 I11 345 344 I11 I11 I11 I1 340 342 336 325
I
I I I 1 1 1 1 1 300 301 301 305 302 317 317 I 303
a
.4
:
22 a Wavelength (nm) 0.4
0.8
(b
I1 325
I 305
.'\-/ -
Water-cast film. Ethanol-cast film.
of the cast bilayer films is discussed in comparison with aqueous solutions and single crystals. Some novel functional properties of the cast films are described, too. Relationship between aggregation structure in the cast films and chemical structure of the azobenzene amphiphiles is intensively discussed. We would like to emphasize that the two-dimensional molecular assemblies, cast films and crystals of bilayer-forming amphiphiles, are suitable candidates for "crystal engineering" because of their simple structures compared with usual three-dimensional molecular crystals.
I
0
I
200
300 400 Wavelength (nm)
500
Figure 2. UV-visible absorption spectra of cast films: (a) group I (C&oCsN+), group I1 (Ci&oCioN+), group V (C7(b) ) ;group I11 (C~AZOC~N+), AzoC~N+), group VI ( C & Z O C ~ N + group
IV (C&OC~N+). 1
.
5
7 1 .O
2. Experimental Section The azobenzene amphiphiles were systematically prepared by previously reported reactions.19~36Amphiphiles used in this experiment are summarized in Table 1. Clear water solutions were obtained after sonication of aqueous suspensions of amphiphiles and were annealed at room temperature after sonication a t least for 1 day to stabilize aggregation structure. Ethanol solutions were used without aging. Optically transparent films were prepared on quartz plates (1 x 4 cm2)by casting of 300 p L of water or ethanol solutions (1 mM) at room temperature. We have already found that if the film was heated above its phase transition temperature, a large spectral shift was induced and the thermally formed aggregation state was kept even a t room temperature as a metastable state a t low h ~ m i d i t y . In ~ ~order ,~~ to remove the metastable states, cast films were carefully dried in a desiccator and then in vacuo without heating. Absorption spectra of cast films and water solutions (0.5mM) were measured with a JASCO Ubest-30 spectrophotometer a t room temperature. The hot stage of a melting temperature measurement apparatus (Yanaco) was used for thermal treatment of cast films. In order to keep constant humidity during moisture induced phase transition a t room temperature, the thermally annealed cast film on a quartz plate was sealed with a cotton piece wetted with a saturated solution of adequate inorganic salts (KzS04, KC1, NaCl, NaBr, &co3,and LiC1)in a quartz cell. Relative humidity in the quartz cell was monitored by a small size (8 x 8 x 4 mm3) humidity sensor (Hayashi Denko). Small angle X-ray diffraction was measured by RAD-C (Rigaku Corp.) with monochromatized Cu Ka radiation a t room temperature. Differential scanning calorimetry (DSC) of cast films was described e l ~ e w h e r e .CPK ~ ~ , space ~ ~ filling molecular models were used for estimation of molecular length.
3. Results 3.1. Spectral Properties of Cast Films. Typical absorption spectra of the cast films observed in this (36) Series of m = 11 were not prepared because 1,ll-dibromoundecane was not commercially available. (37) Okuyama, K. Formation of Bimolecular Films and Crystal Structure. In Reactivity in Molecular Crystals; Ohashi,Y., Kodansha, Eds.; VCH: Tokyo, 1993; p 299.
Wavelength (nm)
Figure 3. UV-visible absorption spectra of water solutions (0.5 mM) of azobenzene amphiphiles classified in group I.
experiment are shown in Figure 2. Two strong absorption bands attributed to x-x* electronic transition are observed in the UV-visible region. The absorption band located around 250 nm is attributed to a transition dipole moment along a short axis of the azobenzene chrom ~ p h o r e .The ~ ~ long axis transition a t lower photoexcitation energy was found to be strongly affected by the chemical structure of the amphiphile in cast films as well as in aqueous bilayer s o l ~ t i o n s . 'Absorption ~ spectra of the cast films are classified into following six groups (see Table 1). Group I: Casting from ethanol solutions of amphiphiles with the structural relation of m - n 2 2 shows a sharp absorption at ca. 300 nm attributed to a hypsochromic shift typical of the side-by-side chromophore orientation. A small red shift with sharp double peaks in the short axis transition is characteristic of this group. Some amphiphiles showed a remarkable solvent effect on spectral shape. As shown in Figure 3 an aqueous solution of C&oCloN+ has two absorption maxima of the long axis transition a t 310 and 355 nm. A slightly red-shifted absorption spectrum (360 nm) with some sharp shoulders attributed to vibronic transitions, which are not clearly observed in ethanol (see curve c in Figure l),is found in a n aqueous solution of C&oCloN+ and C&zoC12N+. The spectral shape of water-cast films of these amphiphiles is (38) Beveridge, D.; Jaff6, H. J.An. Chem. SOC.1966,88,1948.
972 Langmuir, Vol. 11, No. 3, 1995
Shimomura et al.
similar to that ofwater solutions, while absorption maxima of their ethanol-cast films locate a t 300 nm. Group 11: Absorption maximum of the group I1 locates around 320 nm. Significant solvent effect on spectral shape was not found. Spectral red shift and two sharp shoulders in the short axis transition are found in the U V region. Structural requirements of the group I1 are m n 5 1and the total alkyl chain length ( m n ) > 18, except m = 5 series included in group VI (see Table 1). Group I11 and Group Tv: Spectral shape and absorption maxima in the visible region of these two groups are almost identical. Four vibronic structures are included in the visible absorption band (ca. 345, 355, 370, and 410 nm). The second vibronic transition a t 355 nm is in some cases more intense than the first transition a t 345 nm (e.g. ClzAzoC4N+). Discrimination criterion of these two groups is the spectral shape in the ultraviolet region (Figure 2b). Group I11 shows sharp shoulders attributed to the vibronic transitions, while group IV has a broad band without splitting. Six amphiphiles listed in the middle part of Table 1are classified in group I11 and three amphiphiles with m = 4 are in group IV. Group V: All amphiphiles with m = 6 and C&oC5N+ belong to group V. A n ethanol-cast film of the shortest alkyl chain derivative, CsAzoC5N+, which cannot form stable bilayer assembly in water and its spectral shape of water solution is very similar to that of a n ethanol solution (see Figure 31, is classified in this group, too. Interestingly, a water-cast film of C&oC5NC shows a typical group I spectrum (see Discussion). Group VI: A typical bathochromic shift attributed to the head-to-tail chromophore orientation shown in Figure 1 is found in cast films of most amphiphiles of m = 5 (except n = 3 and 4). Crystallographic investigations on single crystals of the m = 5 series has been systematically studied by Okuyama et al. and the amphiphiles are packed laterally with tilting by about 26" to the bilayer surface. The calculated orientational angle of the azobenzene chromophore estimated from Kasha's equation (eq 2) based on spectral Davydov splitting20 was very consistent with that of the single crystal. 3.2. X-ray Diffractions of Cast Films. X-ray diffractions from cast films provide useful information of bilayer structure. Periodic peaks in small- and middleangle diffraction from cast films on glass plates are attributed to the reflections from (h,O,O)planes of the multiple lamella structure. The spacing of higher order reflections ( h > 1)satisfies with numerical relation of llh of the long period calculated from the first-order reflection ( h= 11,which is equivalent to the bilayer thickness. Every cast film measured in this experiment showed more than six reflection peaks. The diffraction profiles of cast films are roughly grouped into two types. Every cast film of groups I and I1 showed a diffraction pattern with the second-order reflection as the strongest peak (Figure 4a). The films of groups IV, V, and VI showed the strongest first-order reflection with weaker higher order reflections (Figure 4b). The bilayer thicknesses estimated as the long period of the cast films are summarized in Table 2. Since the length of the longest axis in the unit cell of a single crystal reported by Okuyama et al. is very similar to the long period of the cast film, aggregation structure in the cast film is assumed to be almost the same as that in the single crystal. Figure 5 shows relation between the long period ( d ) and the total number of methylene units in the chain (n + m). Two linear relations are found in Figure 5a where d values of the cast films in groups V and VI are plotted against the total chain length. Bilayer thicknesses ofthese cast films simply increase linearly with increasing total
la
C5AZ0C8N+ I
2
+
1.5
10 28 (deg.)
20
Figure 4. X-ray diffractions of cast films: (a) C&oC8Nf; (b) CI&OC~N+.
chain length of the azobenzene amphiphile. Assuming that the molecule extends linearly with the tilt chromophore orientation in the cast film as well as in the single crystal (Figure l),the tilt angle to the bilayer surface a is roughly estimated from eq 1
d = 21 sin a
(1)
where 1 is molecular length calculated from the CPK molecular model. Estimating the molecular length as 3.59 and 3.97 nm for C,AzoC7N+ and C&zoC5N+,the tilt angles of groups V and VI are calculated to be 36" and 26", respectively. The later value is just same as that of the single crystal. Spectral shift (in wavenumber, Av) of the azobenzene chromophore caused by intermolecular interaction can be estimated by using Kasha's equation (21, which is a function of the transition moment (u), distance between dipoles (r),and number of interacting molecules in bilayer assemblies (A9
Equation 2 indicates that an increase of the tilt angle from 26"to 36"results in a small blue shifi ofthe absorption maximum (in wavelength) in the visible region. Spectral observation of groups VI (375 nm) and V (360 nm) is very consistent with the structural estimation from the X-ray diffraction experiments. The d values for group IV amphiphiles are plotted in Figure 5a, too. The correlation of the group IV is very similar to the group VI. Taking the spectral difference between groups VI and IV into account, the aggregation structure in group IV must be considerably different from those in group VI (see Discussion). Since the molecular length of C&oCloN+ estimated from the CPK model (4.10 nm) is almost the same as the long period, the C&oCloN+ molecule is assumed to be packed laterally and interdigitated in cast films as well as in single crystals shown in Figure 1. Plots of long period versus total chain length of group I are shown in Figure 5b. These points can be classified into two groups. The
Two-Dimensional Molecular Assemblies spacer m
Langmuir, Vol. 11, No. 3, 1995 973
Table 2. Long Periods (nm)of Cast Films Calculated from the X-ray Diffraction Experiment tail n 3
5
4
6
4
5
VI
1,"VIb 2.6,a 3.2b (2.5866Y
3.9 I 2.9 (2.9500)
7
I
8
9 10
(2.8570)
v
I
I
3.5
3.5
3.4
I
I 4.0
I
3.9
9
10
Iv
Iv
2.8
3.0
11
12
13
14
Iv 3.3
VI
VI
VI
VI
VI
(3.0098)
3.1 (3.1125)
3.2 (3.233)
3.3 (3.3136)
3.4 (3.4243)
V I V I 3.7 3.7
4.1 I11 4.3 I 3.4 (3.404) I
3.3
a
VI
(2.7446)
v
6
8
7
Water-cast film. Ethanol-cast film.
()
3.9
I 3.9 (3.881)
I1 4.7
I1 4.8
length of the longest axis in the unit cell of single crystals.37 1 .o
a
aJ
0 C
gm 0.5 v)
P
a
0 2.5
200
i "a, mz4)
7
9
300
400
500
Wavelength (nm)
11 13 1 5 17 19 21 n+m
I b
-
0.0
1 0
5
10 15 Time (min)
20
25
Figure 5. Plots of total alkyl chain length (n m) and long period of cast film: (a) groups IV,V, and VI; (b) group I.
Figure 6. Spectral change of the annealed film by moisture treatment. (a)C&oCloN+ film was sealed in a quartz cell with 62%humidity after annealing. The type V spectrum (broken line) immediately moved t o the type VI spectrum and then shifted to the type I absorption. (b) Humidity effect on time courses ofthe spectral change.Ao andAt are absorbance at 370 nm immediately and t min after being stored in 62%humidity, respectively.
d value increases with increasing alkyl chain length when the experimental data of the homologous series of m - n = 2 are connected. An odd-even effect on the long period was recently found by Xu in crystalline samples of these amphiphiles.26 Another correlation is found from connecting the points of the same spacer series, e.g. m = 10. The long period of this series is almost identical. Two types of aggregation structure are proposed for group I films (see Discussion). 3.3. Thermal Transition of Cast Films. We have already reported that a water-cast film of C&oCloN+ showed two endothermic peaks in differential scanning calorimetry (DSC) a t 115 and 177 "C,corresponding to a solid-solid and a solid-isotropic phase transition, respectively.28 On cooling a third solid state was formed. "he X-ray diffraction studies suggested that this third solid phase in the annealed film is a metastable state at
room temperature and is transformed to the high temperature solid state on heating to 60 "C. In addition, the absorption spectrum of the cast film was strongly affected by the peculiar solid-solid phase transition, too.2s,29A n as-cast film, i.e., the initial solid state, showed a typical group I spectrum having absorption maximum a t 302 nm. Absorption maximum shifted to 360 nm upon heating above the first phase transition and moved to 370 nm on cooling to room temperature. Spectral shapes of the high temperature solid state and the third solid state are very similar to groups V and VI, respectively. Every cast film of groups I and I1 showed similar spectral transition after annealing above their phase transition t e m ~ e r a t u r e . ~ ~ An interesting finding was a reversible spectral change coupled with moistening ofthe cast film.28Figure 6a shows spectral change of the annealed film of C&oCloN+ kept in 62%humidity at room temperature. Type VI absorption
2.0
7
9
11
13 1 5 n+m
17
19
+
Shimomura et al.
974 Langmuir, Vol. 11, No. 3, 1995
4. Discussions
Wavelength (nm)
Figure 7. Spectral change of the annealed film (C&oC*N+) by moisture treatment. The broken line spectrum of the annealed film was stable in a dry condition but immediately shifted to the type I11 spectrum within 30 s in a humid atmosphere. 5.0 /iype
III
4.5
-E
4.0
'D
3.5
Y
3.0 2.5 11
13
15
17 19 n+m
21
23
+
Figure 8. Plots of the total alkyl chain length (n m )and long period of annealed cast film (types I11 and VI) of groups I and 11. An as-cast film of C,AzoC7N+ of group I11 is plotted as a full
circle.
gradually shifts to type I spectrum that is identical to the as-cast film. As shown in Figure 6b, a transition rate of the isothermal spectral change strongly depends on the relative humidity. Spectral transition is found to be accelerated in a moist atmosphere. This is concluded that the type VI state is a metastable state in the annealed film in low humidity. Another spectral change was found when a cast film of C&zoCBN+ was annealed and then moistened (Figure 7). The type I spectrum of the as-cast film changed to the type V spectrum (the broken-line spectrum in Figure 7) after heating above its phase transition (115 0C37)and then immediately shifted to the type I11 spectrum within 30 s in a 75%humidity condition a t room temperature. The type I11 state seemed to be another metastable state in the annealed film because the moisture induced isothermal transition from type I11 to type I required a long period (e.g., 13 h even a t 75% humidity). Every cast film classified in groups I and I1 shows one of these two types of spectral change with annealing and moistening. Two metastable spectral states in the annealed films, Le., types VI and 111, were stable enough during the X-ray diffraction measurement in our experiments. As shown in Figure 8, two correlations are given from plots of the total chain length and long period of the annealed films. These correlations consist of annealed films showing types I11 and VI spectra, respectively. The correlation line of group VI shown in Figure 5a is plotted again in this Figure. The aggregation structure of the metastable state VI thermally formed in the annealed film is assumed t o be very similar to that in the as-cast film of group VI. The d value of the as-cast film of C7AzoC7N+classified in group I11 fits on the correlation curve of type I11 films thermally formed from groups I and 11.
4.1. Spectral Polymorphism. The azobenzene chromophore was found to be a good spectral probe for bilayer formation because its absorption spectrum is strongly affected by chromophore interaction resulting from molecular aggregation. As summarized in Figure l, the molecularly dispersed azobenzene chromophore has a n absorption maximum a t 355 nm and aggregated chromophores show large spectral shifts induced by strong intermolecular coupling in the ground state. Spectral splitting (A, = 310 and 355 nm) of an aqueous solution of C3AzoCloN+ shown in Figure 3 is ascribed to a monomer-aggregate equilibrium in water. Kunitake et al. noted that short alkyl tail ( n 5 6 ) of the single-chain amphiphile having a rigid aromatic segment was not sufficient for forming a stable bilayer membrane in water.39 Shimomura et al. judged stable bilayer formation of the azobenzene amphiphiles by using some criteria based on spectral properties and thermal behaviors (e.g. crystalliquid crystal phase t r a n ~ i t i o n ) . 'It ~ was concluded that a stable bilayer membrane could be hardly formed from some azobenzene amphiphiles if the absorption spectrum of water solution was different from that of the cast film. The casting solvent is a determining factor of the spectral shape of the cast film. The amphiphiles sufficient for stable bilayer formation in water never show significant solvent effect on spectral shape of cast film. But for some amphiphiles a remarkable solvent effect on the absorption spectrum was found. Water-cast films of C&ZOC~ON+, C&oCloN+, and C&oC12N+ showed similar absorption spectra of their water solutions shown in Figure 3, while ethanol-cast films of these amphiphiles gave a large blue shift to 300 nm. The aggregation structure formed in water is assumed to be kept in the water-cast films. The amphiphiles molecularly dispersed in ethanol aggregate to form the most stable two-dimensional crystalline assemblies during casting process. A peculiar solvent effect was found for the amphiphile C&zoC6N+ which satisfies structural requirements of group I ( m - n > 2) and VI ( m = 5). The water- and ethanol-cast films of C3AzoC5N+showed groups I and V spectra, respectively. Thus group V is defined as a n analogue of group VI (see Figure 9). 4.2. Aggregation Structure in Cast Films. X-ray diffraction studies strongly indicate that the films of the azobenzene amphiphiles simply cast on solid substrates are composed of highly oriented multiple stacked bilayer structures parallel to the substrate surface. A repeating period consistent with the bilayer thickness in cast film is found to be almost same as that of the single crystals whose bilayer structures have been completely determined. On the basis of spectral observation and X-ray diffraction experiments, we propose structural models of the six spectral groups (Figure 9). Taking the balance of the cross sectional area of the hydrophilic head and hydrophobic part in consideration, Okuyama predicted molecular packing in the twodimensional bilayer assembly.37 Since the cross sectional area of the hydrophilic head (AI) is about twice as large as that of the aromatic moiety (AZ),two types of the most densely molecular packing models are proposed as plausible structural models of bilayer assembly: (1) tilt molecular orientation with the tilting angle a satisfying A2 = A1 sin a (model VI in Figure 9) and (2) interdigitated orientation of two molecules to balance A1 = 2 A 2 (model I in Figure 9). (39) Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.; Takarabe, K. J.Am. Chem. SOC.1981,103,5401.
Langmuir, Vol. 11, No. 3, 1995 975
Two-Dimensional Molecular Assemblies
I’
I
V
I1
III,IV
VI ~~
300
350 Wavelength (nm)
~
400
Figure 9. Schematic models of bilayer structures of azobenzene amphiphiles in cast films.
The homologous series of m = 5 (except n = 3 and 4) forms bilayer structure VI with the tilted molecular orientation in cast films as well as in single crystals21z22 (see Figure 1). Structural model V is described as an analogous bilayer structure of model VI and formed from amphiphiles with very similar chemical structure (m = 4 and 6) as the amphiphiles of m = 5. A striking example for the application of these results is the formation of a n intermolecular CT complex in a bilayer membrane. On the basis of the molecular design of the tilted molecular orientation, a viologen amphiphile having a hydrophobic biphenyl moiety, 2 (CnBphCmV+),was synthesized in
2
(C,BphC,V*+)
order to form an intermolecular charge transfer complex between the biphenyl donor and the viologen acceptor in the bilayer membrane. The hydrophilic viologen head and the hydrophobic aromatic segment of the adjacent molecule were expected to be closely packed in the tilted molecular orientation. A broad absorption band a t 460 nm attributed to a CT complex was observed in the aqueous bilayer solution of ClzBphC5V2+,whereas a water solution of CsBphCloV2+had no absorption in visible region. A long lived viologen radical was also found on visible light irradiation of the CT complex.40 The balance of the chain length between the spacer group (m) and the alkyl tail (n)is another determining factor of aggregation structure. Structural analysis of the single crystals indicates that hydrophilic ammonium groups tightly bind with bromide ions to form twodimensional charge network a t the bilayer surface. A free space larger than the van der Waals volume of bromide anion (closed circle in structural models of Figure 9) is required between ammonium ions. If the amphiphiles with the structural relation of m - n = 2 are packed as the structural model I, the distance between the terminal methyl group of the alkyl tail and the bromide anion of the adjacent molecule (0.41 nm) is very close to their van der Waals distance (0.39 nm).25,26In the case of the packing model I, molecules are densely packed with full n-electron overlap of the azobenzene chromophores. If the difference of the chain length ( m - n) is smaller than 2, the adjacent molecules must slide out to secure enough spaces of the bromide ions (structural model I1 in Figure 9). Then only partial overlapping of azobenzene chro(40) Shimomura, M.; Aiba, S.; Oguma, S.; Oguchi, M.; Matsute, M.; Shimada, H.; Kajiwara, R.; Emori, H.; Yoshiwara, K.; Okuyama, K.; Miyashita, T.; Watanabe, A.; Matsuda, M. Supramol. Sci. 1994,1,33.
mophores is allowed in the molecular packing of model 11. The difference between the chromophore overlapping in models I and I1 is assumed to be reflected in spectral difference of these groups (I, Am= = 300 nm; 11, Aman = 320 nm). Amphiphiles of m - n > 2 are classified in group I, too. A large hypsochromic shift to 300 nm in the absorption spectrum indicates side-by-side molecular packing with full overlapping of the azobenzene chromophores. The long period of these cast films is independent of the total chain length (see m = 10 series in Figure 5b). A modified molecular packing model proposed as the structural model I’ is consistent with experimental results of X-ray diffraction and spectroscopy. A peculiar void space under bromide counterion is characteristic for this structural model. Spectral experiments on incorporation of sodium alkylsulfonates a s guest molecules into the host bilayer assembly strongly suggest formation of the void space expected in the structural model I’. Cast films of a 1:l mixture of C&oCloN+ and sodium n-butylsulfonate (Amm = 303 nm) or sodium propylsulfonate (Aman = 302 nm) showed a similar absorption spectrum of pure C&oCloN+ cast film (a,,,= = 301 nm). Longer guest molecules, sodium = 326 nm), n-hexylsulfonate (A, n-pentylsulfonate (aman = 329 nm), and n-heptylsulfonate (Aman = 337 nm), disrupted the original spectrum of the host bilayer assembly. Systematic investigations on the spectral probing of “host-guest interaction” by using the peculiar void space in the bilayer assemblies formed from the group I’ amphiphiles ( m - n > 2) are now p r ~ g r e s s i n g . ~ ~ 4.3. Structural Polymorphism of Cast Films. According to Okuyama’s prediction of the molecular packing in the bilayer assembly, every azobenzene amphiphile used in this experiment is expected to form both the tilted bilayer structure (models VI and V) and the interdigitated structure (modelsI and 11). The amphiphile C3AzoC5N+has two structural characteristics of m = 5 (group VI) and m - n = 2 (group I). Recently Okuyama and co-workers have reported that aggregation structure of single crystalline C3AzoC5N+ is classified into group I structure.25 In this experiment we found a peculiar solvent effect on absorption spectrum and X-ray diffraction of the cast film. Judging from a large hypsochromic shift and X-ray diffraction pattern, model I structure is assumed to be formed in the water-cast film. On the other hand, the ethanol-cast film forms the tilted bilayer structure of model V. Structural polymorphism has been already reported as a peculiar solid-solid phase transition with a large spectral shift in the cast film of C & Z O C ~ O N +The . ~ ~type ~ ~ ~I spectrum was thermally transformed to type VI spectrum and then backed to type I by the isothermal moisture treatment. The reversible spectral change between the types I and VI is good experimental evidence of Okuyama’s prediction on the molecular packing. Since the type VI state is assumed to be a metastable state, the isothermal phase transition to the type I state is expected to be induced by some external stimuli. Water molecules adsorbed to cast bilayer films might act as an accelerator of the phase transition. Photochemical isomerization reaction of the azobenzene chromophore is well-known to trigger phase transitions of liquid ~ r y s t a l s . ~Recently ~ - ~ we have found (41) Okuyama, K.; Shimomura, M. In New Developments in Construction and Functions oforganic Thin Films; Kajiyama, K., Aizawa, M., Eds.; Elsevier Science: Amsterdam, in press. (42)Sackmann, E. J.Am. Chem. Soc. 1971,93,7088. (43)Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoi, A,; Aoki, K. Langmuir 1988,4, 1214. (44) Ikeda, T.;Sasaki, T.; Ichimura, K. Nature 1993,361,428.
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976 Langmuir, Vol. 11, No. 3, 1995
the isothermal phase transition from state VI to state I of the cast film of C&oCloN+ induced by photoirradiat i ~ n . ~Since l state VI was very stable in a dry condition, spectral change of the annealed cast film of C&oCloN+ was not observed in dark. A peculiar spectral change was induced by U V light irradiation. The absorption band a t 450 nm, attributed to a n-z* transition of the photochemically formed cis azobenzene isomer, increased at the first stage of irradiation and then decreased gradually concomitant with the increase of absorption a t 300 nm. Formation of cis isomer in the metastable state VI might accelerate the isothermal phase transition to the stable state I. At the present stage, we suppose that the aggregation structuresofgroups I11and IV (modelsI11and W i n Figure 9) are intermediate states between the tilt bilayer model (models V and VI) and the interdigitated antiparallel packing model (models I, 1’, and 11), because a similar aggregation structure is found as a metastable transient state in the thermally annealed films of the groups I and I1 (Figures 7 and 8).
Conclusion We systematically investigate spectral properties and aggregation structure of solvent cast films of 43 azobenzene amphiphiles. It can be concluded that the alkyl chain
length ofthe amphiphile is a definitive determiningfactor for the molecular orientation in the cast bilayer film. Our findings are one of successful examples of structural prediction and design of two-dimensional pseudocrystals, the so-called “crystal engineering”. On the basis of the “crystal engineering” of the two-dimensional molecular assemblies, we are developing cast bilayer films as novel molecular materials potentially used as charge transfer, molecular recognition, and optical memory devices. Tailored charge transfer complexes in the viologen bilayer assembly effectively produce long lived viologen radicals on visible light irradiation. The peculiar void space formed in the aggregation structure of model I’ could recognize size and shape of guest molecules. Taking advantages of the reversible spectral switching by heating and photoirradiation and a large spectral bistability due to the structural polymorphism, the cast bilayer films of the azobenzene amphiphiles are applicable as an optical memory device.
Acknowledgment. This work was partly supported by Grant-in-Aid for Scientific Research No. 05804035 and Priority-Area-Research “Photoreaction Dynamics” (No. 06239203) from the Ministry of Education, Science and Culture, Japan. LA940683U
