Photochemical Reaction, Acidichromism, and Supramolecular

Acidichromism and Supramolecular Chirality of Tetrakis(4-sulfonatophenyl)porphyrin in Organized Molecular Films. Li Liu, Yuangang Li, and Minghua Liu...
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Langmuir 2004, 20, 8042-8048

Photochemical Reaction, Acidichromism, and Supramolecular Nanoarchitectures in the Langmuir-Blodgett Films of an Amphiphilic Styrylquinoxaline Derivative Meifang Yin,† Haofei Gong,† Baowen Zhang,‡ and Minghua Liu*,† CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China and Technical Institute of Physics and Chemistry, CAS, Beijing 100101, P. R. China Received April 12, 2004. In Final Form: July 5, 2004 An amphiphilic styrylquinoxaline derivative, 3-(4-(hexadecyloxy)styryl)quinoxalin-2(1H)-one (SQC16), was newly synthesized to investigate their photochemical and gas responsive properties in organized molecular films. It was observed that SQC16 can spread as a monolayer on the subphases with various pH values and be subsequently transferred onto solid substrates. While SQC16 showed predominantly reversible trans-cis photoisomerization in methanol solution, it showed both photoisomerization and photodimerization in Langmuir-Blodgett (LB) films. Photodimerization was only observed in the LB film due to the face-to-face arrangement of the functional headgroup in the LB film, and the process was irreversible. In addition, the LB film showed acidichromism, i.e., when the film was exposed to HCl gas its color changed from yellow to red, and the color could be recovered after exposure to NH3 gas. The process was reversible and could be repeated many times. An interesting surface morphology of the SQC16 LB film was revealed. It was observed that SQC16 can form nanowire architecture in the transferred one-layer LB film. This morphology can be changed upon photoirradiation or in gas reactions. Through the atomic force microscopy measurements it was suggested that the photodimerization predominantly occurred from the nanowire structures, while during the acidichromism the reaction occurred preferentially in the flat region. X-ray diffraction studies revealed that while layer distance showed a slight change for the LB film during acidichromism and photoreaction, the layer structure of SQC16 LB film was retained.

1. Introduction Supramolecular assemblies responding to external stimulus such as light and gas have drawn much attention due to their potential applications in the fields of photoelectron devices, photoswitching, optical data recording, and gas or photosensor systems.1-6 To date, photochemical reactions have been widely investigated in various organized systems such as Langmuir film,7-9 LangmuirBlodgett (LB) film,10-14 self-assembly films,15-19 bilayer * Corresponding author. Phone: +86-10-8261-2655. Fax: +8610-6256-9564. E-mail: [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Technical Institute of Physics and Chemistry. (1) Ohtani, O.; Kato, H.; Yui, T.; Takagi, K. J. Am. Chem. Soc. 2003, 125, 14465. (2) Yamauchi, S.; Terazima, M.; Hirota, N. J. Phys. Chem. 1985, 89, 4804. (3) (a) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1977, 99, 877. (b) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1975, 97, 1602. (4) Laschewky, A.; Ringsdorf, H. Macromolecules 1988, 21, 1936. (5) Yamamoto, M.; Wajima, T.; Kameyama, A.; Itoh, K. J. Phys. Chem. 1992, 96, 10365. (6) Xiang, H.-Q.; Tanaka, K.; Kajiyama, T. Langmuir 2002, 18, 9102. (7) Neagele, D.; Ringsdorf, H. In Polymerization of Organized Systems; Elias, H.-G., Ed.; Gordon and Breach: New York, 1977; pp 79-88. (8) Xia, Q.; Feng, X.; Mu, J.; Yang, K. Langmuir 1998, 14, 3333. (9) Chowdhury, D.; Paul, A.; Chattopadhyay, A. J. Phys. Chem. B 2002, 106, 4343. (10) Mooney, W. F.; Brown, P. E.; Russell, J. C.; Pedersen, L. G.; Whitten, D. G. J. Am. Chem. Soc. 1984, 106, 5659. (11) Zhao, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 10463. (12) Yamamoto, M.; Furuyama, N.; Itoh, K. J. Phys. Chem. 1996, 100, 18483. (13) Li, C.; Huang, J.; Liang, Y. Langmuir 2001, 17, 2228. (14) Zhao, J.; Akiyama, H.; Abe, K.; Liu, Z.; Nakanishi, F. Langmuir 2000, 16, 2275. (15) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955.

membranes or vesicles,20-22 and micelles.23 Monolayer and Langmuir-Blodgett methods are among the most effective ways in controlling the molecular orientation and packing at the molecular level and are some of the powerful methods to fabricate functional ultrathin films.24 The photochemical reaction in Langmuir-Blodgett film may differ from that in solution in that organization of the molecules in the LB films may cause different reactions or morphological changes which could been utilized to functionalize LB films.25-29 In this sense, much effort has been devoted to monolayer or ultrathin films of photo(16) Zhang, J.; Whitesell, J. K.; Fox, M. A. J. Phys. Chem. B 2003, 107, 6051. (17) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33, 475. (18) Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256. (19) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater.1996, 8, 428. (20) (a) Song, X.; Geiger, C.; Farahat, M.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 12481. (b) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (21) (a) Park, J.; Zhang, W.; Nakatsuji, Y.; Majima, T.; Ikeda, I. Chem. Lett. 1999, 1309. (b) Park, J.; Zhang, W.; Nakatsuji, Y.; Majima, T.; Ikeda, I. Chem. Lett. 2000, 182. (22) Shimomura, M.; Hashimoto, H.; Kunitake, T. Langmuir 1989, 5, 174. (23) Hou, S.; Man, K. Y. K.; Chan, W. K. Langmuir 2003, 19, 2485. (24) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (25) (a) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (b) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Phys. Chem. B 2001, 105, 10282. (c) Velez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (26) (a) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Phys. Chem. B 2001, 105, 10282. (b) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2000, 16, 2975.

10.1021/la0490745 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/19/2004

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responsive azobenzene derivatives,25 spiropyran,26 styrene derivatives,27 anthracene derivatives,28 and diacetylenes.29 On the other hand, the structural and morphological changes in organized thin films upon photoreaction can provide basic clues to their applications, such as photorecording and optical data storage. Some groups have investigated the morphological changes of the photoresponsive film during photochemical reactions. For example, Matsumoto25a et al. reported the reversible 3D nanoscale hill-structure formation on the smooth 2D azobenzene thin films upon alternative irradiation of UV and visible light. They also reported the nanoscale coneshaped protrusion formation in the long alkyl chain substituted spiropyran thin film upon photoirradiation due to J-aggregation formation of the photomerocyanine.25b Shimomura30 et al. investigated the reversible photoisomerization of a gemini-type surfactant with stilbene headgroup at the air/water interface. They observed the “melting” and “recrystallization” process of the monolayer assemblies accompanied by the trans-cis photoisomerization through fluorescence microscopy. On the other hand, the color changes upon external vapors are also attracting increasing attention because of their utility in chemical sensing applications.31-33 Acidichromism is one of these phenomena which describes reversible color changes depending on the pH in solution or stimulus from HCl and NH3 gases.34-36 Both gases and photoirradiation can induce the spectral, structural, and morphological changes in the film. LB film with photofunctional group and acidichromism can serve as a combined sensor of light and gas. In this paper we designed a new amphiphilic styrylquinoxaline derivative with a hydroxyl group neighboring the CdN double bond of the quinoxaline group. Some interesting features in the photochemical reaction, acidichromism as well as surface morphologies of the LB film, were observed. All properties, including characterization of the LB film, were studied by UV-vis spectra, atomic force microscopy (AFM), and FT-IR spectra. Although many photochemical reactions have been investigated in the LB film, we provide a systematic investigation on the photochemical reaction (27) Karthaus, O.; Ueda, K.; Yamagishi, A.; Shimomura, M. J. Photochem. Photobiol., A: Chem. 1995, 92, 117. (28) Mitsuishi, M.; Tanuma, T.; Matsui, J.; Chen, J.; Miyashita, T. Langmuir 2001, 17, 7449. (29) Alekseev, A. S.; Viitala, T.; Domnin, I. N.; Koshkina, I. M.; Nikitenko, A. A.; Peltonen, J. Langmuir 2000, 16, 3337. (30) Karthaus, O.; Shimomura, M.; Hioki, M.; Tahara, R.; Nakamura, H. J. Am. Chem. Soc. 1996, 118, 9174. (31) Exstrom, C. L.; Sowa, J. R.; Daws, C. A.; Janzen, D.; Mann, K. R.; Moore, G. A.; Stewart, F. F. Chem. Mater. 1995, 7, 15. (32) Daws, C. A.; Exstrom, C. L.; Sowa, J. R., Jr.; Mann, K. R. Chem. Mater. 1997, 9, 363. (33) Drew, S. M.; Janzen, D. E.; Buss, C. E.; MacEwan, D. I.; Dublin, K. M.; Mann, K. R. J. Am. Chem. Soc. 2001, 123, 8414. (34) Sun, X. D.; Fan, M. G.; Meng, X. J.; Knobbe, E. T. J. Photochem. Photobiol., A: Chem. 1997, 102, 213. (35) Liang, Y.; Ming, Y.; Fan, M.; Sun, X.; Knobbe, E. T. Sci. China B 1997, 40, 535. (36) Fan, M.; Sun, X.; Liang, Y.; Zhao, Y.; Ming, Y.; Knobbe, E. T. Mol. Cryst. Liq. Cryst. 1997, 298, 29.

and a less studied acidichromism for a new amphiphilic styrylquinoxaline. Furthermore, while fewer studies on morphological changes during the photochemical or gas reaction in LB films have been reported, we present some interesting morphologies formed by the amphiphile and give clear morphological changes during photochemical reaction and acidichromism through the in situ or ex situ AFM observations. 2. Experimental Section Materials. The amphiphile 3-(4-(hexadecyloxy)styryl)quinoxalin-2(1H)-one (SQC16) was newly synthesized in the route shown in Scheme 1. The starting material p-hydroxylbenzaldehyde was refluxed with equimolar 1-bromohexadecane to give p-hexadecyloxybenzaldehyde. This compound was then condensed with compound 4 to give the target compound. The compound was purified through silica gel (H60) column chromatography. The SQC16 was obtained as a yellow crystal: mp 176.5-177.0 °C. 1H NMR (CDCl3), δ, ppm: 0.87 (t, 3H), 1.251.56 (m, 28H, (CH2)14); 4.0 (t, J ) 6.53 Hz, 2H, -OCH2); 6.9 (d, J ) 8.7 Hz, 2H), 7.11 (d, J ) 7.96 Hz, 1H), 7.34 (t, J ) 7.7 Hz, 1H), 7.45 (t, J ) 7.7 Hz, 1H), 7.63 (d, J ) 8.7 Hz, 2H), 7.9 (d, J ) 8.0 Hz, 1H), 8.2 (s, 1H), 8.4 (s, 1H), 9.27 (s, 1H). EI-MS: m/e 488 (100%). Anal. Calcd for C32H44N2O2‚0.5H2O: C, 77.22; H, 9.11; N, 5.63. Found: C, 77.32; H, 8.93; N, 5.99. Chloroform was distilled before use. Hydrochloride acid and sodium hydroxide were analytical reagents and used without further purification. Milli-Q purified water (18 MΩ cm) was used in all cases. Procedure. Measurements of surface pressure-area (π-A) isotherms and film depositions were carried out on a KSV minitrough (Helsinki, Finland). Langmuir film of SQC16 was formed by spreading its chloroform solution (2 × 10-3 mol L-1) onto the aqueous subphases with different pH values adjusted by adding hydrochloric acid or sodium hydroxide. After evaporation of the solvent for 10 min, the π-A isotherms of the spreading films were recorded at 20 ( 0.8 °C by compressing the Langmuir film at a barrier speed of 10 mm/min. Quartz substrates were cleaned by mixed chromic acid and washed with water thoroughly. The substrates were made to be hydrophobic before deposition by rubbing the substrates with ferric stearate. The film was transferred onto solid substrates by the vertical lifting method. The dipping speed was 10 mm/min both downward and upward. To investigate the acidichromism, the LB film was exposed to HCl and NH3 gas separately for 1 min. The films were purged with N2 gas before spectral measurements. Two kinds of UV lamps (20 W) centered at 365 and 254 nm were used as the source of UV light. For in situ AFM measurements the sample was not moved from the scanner in order to follow the morphological change on photoirradiation in the same zone of the sample. During UV irradiation the lamp was placed over the sample at a distance of 10 cm while the scanner paused momentarily. From time to time irradiation was cut off and in situ images of the sample were acquired. All measurements were carried out at room temperature in air. Instruments. To characterize the LB film, 40, 50, and 60 layers of SQC16 were transferred onto quartz, glass, and CaF2 substrates for UV-vis spectra, X-ray diffraction (XRD), and FTIR spectra measurements, respectively. One layer of SQC16 LB film was deposited onto freshly cleaved mica for AFM measurements. X-ray diffractions were obtained using a Hitachi Natural D/Max-RB X-ray diffractometer (Japan) with Cu KR radiation

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Figure 1. Surface pressure-area isotherms of SQC16 spreading on different pH subphases at 20 °C: (a) 1.6, (b) 3.1, (c) 6.8 (pure water), and (d) 11.2.

Figure 2. UV-vis spectra of SQC16: (a) in CHCl3 solution (1 × 10-5 mol L-1) and (b) in CH3OH solution (2 × 10-5 mol L-1). (c) LB film transferred at 20 mN/m. (d) LB film transferred at 5 mN/m. (λ ) 0.154 nm). UV-vis spectra were measured with a JASCO UV-530 spectrophotometer. AFM measurements were performed using a tapping-mode atomic force microscope (Nanoscope IIIa, Digital Instruments). TOF-MS was carried out on the BIFLEXIII. FT-IR spectra were recorded using a JASCO FT-IR 660 spectrometer. 1H NMR spectra were recorded on a dm × 300 (Bruker) instrument. Elemental analysis was performed on a Carlo-Erba1106 instrument.

3. Results and Discussion 3.1. Aggregation and Surface Morphology of SQC16 in Monolayer and LB Films. Monolayer Formation: Surface Pressure-Area Isotherms. Figure 1 shows the π-A isotherms of SQC16 on subphases with different pH values. On the pure water surface (pH ) 6.8) the isotherm shows an onset of surface pressure at ca. 0.6 nm2/molecule. On the acidic subphase (pH ) 1.6 and 3.1) the isotherm exhibits a condensed-type monolayer. On the alkaline subphase the isotherm is rather expanded. These changes in the isotherms of the SQC16 monolayers on the subphase with different pH values can be attributed to protonation and/or orientational changes of the molecules. On the other hand, the limiting area of each isotherm falls into the range of 0.3-0.4 nm2/molecule. On the basis of the CPK molecular model, a vertical orientation of the aromatic ring of SQC16 will at least occupy an area of 0.25 nm2/molecule while a flat orientation of the aromatic ring will take about 0.78 nm2/molecule. The experimental limiting areas indicate that the aromatic ring of SQC16 in all monolayers take an inclined orientation to the normal direction of the water surface. UV-Vis Spectral Study. The spreading monolayer from chloroform solution on Milli-Q pure water surface (pH ) 6.8) was deposited onto solid substrates, and the UV-vis spectra were measured. Figure 2 shows the UVvis spectra of SQC16 in solutions and LB films. The compound shows its absorption maxima at 398 and 397 nm in chloroform and methanol solution, respectively. This

Figure 3. AFM images of one-layer LB film on mica in the dark under (a) 5, (b) 20, (c) 30, and (d) 50 mN/m. The scan area is 1 µm × 1 µm for part a and 5 µm × 5 µm for parts b-d.

band can be attributed to the charge-transfer band of SQC16. In the LB films deposited at 5 and 20 mN/m, the charge-transfer band shows a blue shift in comparison with the solution. In addition, comparing the spectrum of the film deposited at 20 mN/m with that at 5 mN/m, a slight further blue shift was observed. This blue shift of the absorption spectra in the LB films might be ascribed to the face-to-face arrangement of the functional group in the film. This is in agreement with the π-A isotherm measurements where the functional aromatic rings are suggested to have a vertical arrangement.37 AFM Measurements. To reveal the surface morphology, one layer of each monolayer at various conditions was deposited on the mica surface and their AFM images were measured, as shown in Figures3 and 4. Apart from the flat region covered with SQC16 molecules, wire-like nanostructures are observed for the film deposited at 5 mN/m. These wire-like structures have a width of about 25 nm and a length of several hundreds of nanometers. From the height profiles it is found that the wires are about 1 nm higher. On increasing the surface pressure, some larger domains with a height of ca. 2.4 nm are observed for the film deposited at a surface pressure of 20 mN/m. In addition, the nanowire structure can still be seen in that film. At surface pressures of 30 and 50 mN/m, the domain size increases but their heights are still below 3 nm. Interestingly, these aggregates are aligned along the wire formed in the lower surface pressure. On the basis of the CPK model, the length of SQC16 is estimated to be 3.3 nm. It seems that those nanowires are formed through squeezing of the molecules from the monolayer (37) Chen, L.; Geiger, C.; Perlstein, J.; Whitten, D. G. J. Phys. Chem. B 1999, 103, 9161.

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Figure 4. AFM images of one-layer-deposited LB film on mica from the subphases of different pH: (a) 1.6, (b) 3.1, and (c) 11.2. The transfer pressure was kept at 5 mN/m in all cases. The scan area is 2 µm × 2 µm.

Figure 5. (a) Reversible photoreaction of SQC16 in CH3OH solution (2 × 10-5 mol L-1) under UV 365 nm (solid line) and UV 254 nm (dash line) photoirradiation. (b) UV-vis spectral changes in LB films under 365 nm UV light irradiation for different time intervals.

according to these AFM observations and their height profiles. Due to the fact that wire structures are observed even at lower surface pressure, it is suggested that the film can be easily compressed to aggregation, which could be due to the strong interaction between the aromatic conjugate systems. The AFM images change as a function of surface pressure, which further suggests that once wire structure is formed, the other molecules will easily gather around the wire to form larger blocks. At higher surface pressures, larger regular shape domains can be seen around the wire. This indicates that more molecules are squeezed at these stages. However, the height of the domain is about 3 nm. This indicates that the squeeze is mainly limited in a two-dimensional way. On the other hand, the pH of the subphase not only affects the π-A isotherms but also can greatly affect the surface morphology of the film. Figure 4 shows the AFM images of the transferred one-layer LB film at different pH subphases. Unlike the pure water subphase (see Figure 3a), the film shows some small close-packed domain in the subphase pH of 1.6. With a subphase pH of 3.1, besides the flat regions, some short nanowire structures are observed. On the alkaline subphase, the film shows closely packed nanorod aggregates. These different morphologies are suggested to be due to the subtle packing or orientation differences of the aromatic rings in the subphases with various pH values. 3.2. Photochemical Reaction in LB Films. UV-vis Spectra. Stilbene derivatives are reported to show photodimerization and trans-cis isomerization.38 To investigate whether SQC16 can show photochemical activity and further study the effect of organization on the photochemical reaction, we measured the photoinduced spectral changes of SQC16 both in methanol solution and LB films. In methanol solution typical trans-cis isomerization is observed as shown in Figure 5a. Upon irradiating the methanol solution with 365 nm light for (38) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502.

3 min, the trans-isomer absorption band at ca. 400 nm decreases while the cis-isomer band at ca. 260 nm increases. If we subsequently irradiate the solution with 254 nm UV light, the absorption band at 260 nm decreases while the band at 400 nm increases. This indicates that a reversible trans-cis photoisomerization takes place in the methanol solution of SQC16. For the LB films great differences are observed. Figure 5b shows the UV-vis spectral changes of SQC16 LB film transferred at 20 mN/m on pure water surface upon photoirradiation. Different from that in solution, the broad charge-transfer band at 382 nm decreases and a new band centered at 281 nm appears upon irradiation with 365 nm light. From the similar spectral changes reported for the styrylpyridinium system,39 both the decrease at 382 nm and the appearance of a new peak at 281 nm can be mainly ascribed to a [2 + 2] photodimerization. In addition, three isosbestic points are observed in these spectra, indicating that SQC16 in the LB film might undergo trans-cis isomerization in addition to major photodimerization.39 The same photoinduced spectral change is observed in the LB film deposited at 5 mN/m, which indicates that photodimerization is due to the face-toface arrangement of the functional groups in the LB films. To further confirm that the photodimerization really occurs in the LB film, the product was examined by TOFMS after washing the film with CHCl3, where a peak at 976 corresponding to the dimmer of SQC16 was observed. It is well known that the face-to-face aggregation is topologically allowed for photodimerization. We propose that the nanowire structure is composed of face-to-face aggregation of SQC16; thus, photodimerization may take place within the nanowires. How the nanowire morphology will be changed during photodimerization is very interesting. Comparing the photoreaction in methanol solution, where reversible trans-cis isomerization occurred, it is (39) Lee, S. W.; Chang, T.; Ree, M. Macromol. Rapid Commun. 2001, 22, 941.

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Figure 7. UV-vis spectral changes of SQC16 LB film upon exposure to HCl and NH3 gases. (Insert) Plot of absorbance at 500 nm as a function of repeated number of the exposure to HCl and NH3 gases.

Figure 6. In situ AFM observation of one-layer SQC16 film on mica from pure water surface at 5mN/m with UV irradiation (365 nm) for (a) 0, (b) 20, (c) 40, and (d) 80 min. The scan area is 3 µm × 3 µm. Scheme 2. Schematic Illustration of Photoinduced Structural Change in the LB Film

noted that photodimerization occurs predominantly in the LB film when irradiated with 365 nm UV light. Upon irradiation with 254 nm light, only a slight recovery of the spectra was observed. This indicated that the photodimerization in the LB films was irreversible. However, in methanol solution photodimerization cannot occur due to the disordered dispersing of the molecules in solution. In Situ AFM Observation for the Photochemical Reaction in the LB Film. To reveal the morphological change, in situ AFM observation for one-layer SQC16 LB film on mica was performed, as shown in Figure 6. Nanowire morphology is clearly seen before photoirradiation. Upon irradiation with 365 nm UV light for 20 min, dotted protrusions with a vertical distance ca. 1.4 nm higher than the flat surface appear in the nanowire, while no obvious morphological change is observed in the flat region. More protrusions appear and are denser in the film with increasing irradiation time, while there is no obvious change in the vertical distance. However, this morphological change is not reversible under irradiation of 254 nm UV light. As we know, when photodimerization occurs, it draws two molecules close together since the covalent bonds have a much stronger affinity than intermolecular interactions. However, a more extended structure may form with dimerization due to partial loss of conjugation when two olefinic groups change to the cyclobutane ring.40,41 Expansion of area with photodimerization may exert a force on the other molecules and squeeze them out of the nanowire (40) Savion, Z.; Wernick, D. L. J. Org. Chem. 1993, 58, 2424. (41) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647.

Figure 8. FT-IR spectral change of SQC16 LB film upon exposure to HCl and NH3 gas.

as shown in Scheme 2. The same photocycloaddition reaction-induced area expansion has been observed previously in Langmuir monolayer, which induced the surface pressure increasing or area expansion in the monolayer.42 In contrast, the area expansion in LB films cannot be released by increasing the surface pressure or expansion of the barrier but squeeze other molecules out of the twodimensional surface and form three-dimensional protrusions in the film. These observations indicate that the photochemistry in LB films may have the character of that in both fluid monolayer at the air/water interface and solid crystalline. The two-dimensional structure of LB film may change to a three-dimensional one when there is no longer free volume in the film, as reported by Matsumoto.25a On the other hand, the photoreactions are topochemical controlled, which are characteristic of solid photochemistry. 3.3. Acidichromism of the SQC16 LB Film. UVvis and FT-IR Spectra. The SQC16 LB film is sensitive to acid gas; it shows obvious color changes upon reaction with acidic gas. A yellow film of SQC16 turned red immediately with exposure of the LB film to HCl gas, but it changed to the original color reversibly when exposing it to basic gas such as NH3. Figure 7 shows the UV-vis spectral change of the acidichromism.43 A new absorption band at ca. 500 nm appears when exposing the film to HCl gas, while the absorption band at ca. 380 nm decreases. The new absorption band can be attributed to a complex of SQC16 with HCl. The absorption spectrum can almost return to the original one when exposing the film to NH3 gas, and this kind of color change can be repeated many times. The insert in Figure 7 shows the absorption intensity change at 500 nm as a function of the number of repeated exposures to HCl and NH3 gases. We (42) Zhao, J.; Abe, K.; Akiyama, H.; Liu, Z.; Nakanishi, F. Langmuir 1999, 15, 2543. (43) Similar spectral changes were observed for one-layer and multilayer LB films upon photoirradiation and gas exposing.

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Figure 9. Ex situ AFM images of one-layer SQC16 LB film: (a) as prepared, (b) after exposure to HCl gas, and (c) subsequent exposure to NH3 gas. The scan area is 1 µm × 1 µm. Scheme 3. Acidichromism in the SQC16 LB Film

can find that this kind of reaction can be repeated over 20 times without obviously changing their absorption. An isosbestic point is observed in these spectra, indicating the transformation of two species in the film. To clarify the mechanism of acidichromism in the SQC16 LB film, FT-IR spectra were investigated as shown in Figure 8. The symmetric and asymmetric vibrational bands of CH2 are observed at 2852 and 2919 cm-1, and they do not change their position during the gas reaction. This indicates that the gas can penetrate the alkyl chain to react with the headgroup of the layer and the reaction does not change the packing and orientation of the alkyl chain. A new absorption band is observed at 1564 cm-1 after the film is exposed to HCl gas, which can be assigned to the bending mode of N-H.44 This suggests that upon exposure to HCl gas, the quinoxaline nitrogen in the SQC16 becomes N-H after changing the neighboring double bond. The carbonyl absorption band centered at 1664 cm-1 is weakened after reaction with HCl gas. On the other hand, the band centered at 1625 cm-1, which can be ascribed to the conjugated cyclic -CdN- vibrational band, is weakened after exposure to HCl gas. All spectral changes are reversible when the film is exposed to NH3 gas, as indicated by the UV-vis spectra. These spectral changes clearly indicate that with the gas penetration and reaction, the conjugation structure of SQC16 is changed and a new protonated SQC16 is formed in the film as shown in Scheme 3. It is also noted that with the increasing of the repeated exposure of the SQC16 film to HCl and NH3 gases, some new bands at 1409, 3050, and 3148 cm-1 appear and increase in intensity. These bands can be assigned to the vibrations of NH4Cl in the film, which indicates that NH4Cl can be formed in the film during the acidichromism. Nevertheless, this salt can be wiped off by dipping the film in water. AFM Study. We have further measured the morphological changes in the film upon exposure to HCl and NH3 gases using AFM, as shown in Figure 9. Upon exposure to HCl gas, the image does not obviously change except that the surface roughness of the film becomes larger. The film changed to a mosaic shape when exposed to NH3 gas. A more rough film was formed after one reaction cycle. It was noticed that the nanowire structure has no change during the gas reaction, which implies that the nanowire is composed of closely-packed face-to-face aggregation of SQC16. On the other hand, most of the morphological changes take place in the flat part of the (44) Bolte, M.; Ku¨hl, C.; Lu¨ning, U. Acta Crystallogr. 2001, E57, o502.

film. This indicates that the flat area is composed of loosely arranged molecules and the gas can easily penetrate into this part. The morphological change is not reversible, although it is reversible in the spectra. Compared with the above morphological changes in photoreaction, it can be suggested that photodimerization predominantly occurs in the nanowire region where molecules are supposed to arrange in a face-to-face aggregate, while gas reaction prefers to occur at the flat places where the molecules seem to be more loosely packed. 3.4. XRD Study. As discussed, both photoirradiation and acidichromism lead to morphological changes in the film. To further reveal the reaction nature in the LB films, we measured the X-ray diffraction of the LB films before and after photoirradiation or acidichromism. One diffraction peak was observed in all cases, indicating that the lamellar structure was maintained during either the photoreaction or the gas reaction. The 2θ values of the diffraction peaks were observed at 2.28°, 2.24°, and 2.10° for the as-prepared, photoirradiated, and HCl-exposed SQC16 films, which correspond to layer distances of 3.87, 3.94, and 4.20 nm, respectively, according to Bragg’s equation. The molecular length of SQC16 is estimated to be 3.3 nm from the CPK model. Therefore, it is suggested that SQC16 formed a Y-type LB film with the alkyl chain inclined to the film plane. After photoirradiation for 1 h, the long spacing of the lamellar structure increased a little compared with that before irradiation. This result suggests that the photoreactions took place within each layer, and the photoreaction does not change the periodical structure of the layer, although the morphology has significant changes during photoreaction. In addition, the photodimerization changed the trans-conformation of SQC16 to be more vertical way like cyclobutane. Therefore, a slight increase was observed. On the contrary, the film after HCl exposure has an increased spacing up to 4.20 nm, which suggests that the gas can penetrate into each layer of the multilayer film and the film keeps its lamellar structure after reaction. Because the alkyl chain does not change in the gas reaction process, the increased spacing may be attributed to the orientation change of the headgroup of SQC16 due to the change of its conjugation structure in each layer during the gas reaction. From X-ray diffraction of the LB films, it can be concluded that both reactions took place mainly within the layers. 4. Conclusion We have demonstrated that a novel styrylquinoxaline derivative SQC16 can form face-to-face aggregates in LB films. Such aggregation can lead to nanowire morphology in the LB films. While SQC16 mainly undergoes isomerization upon photoirradiation in methanol solution, dimerization can occur in the LB films due to face-to-face aggregation of the functional groups, which was verified by time-of-flight mass spectrometry spectra. In the LB

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film, the trans-cis photoisomerization is reversible while the photodimerization is irreversible. In addition, the SQC16 film showed an acidichromism, i.e., when the film was exposed to HCl and NH3 alternatively, the film can reversibly changes its color between yellow and red. AFM measurement indicated that photochemical reaction predominantly occurs in the nanowire structure of the LB film while the acidichromism prefers to occur in the flat part of the film. In addition, although obvious morphological changes are observed during photoreaction or acidichromism, the lamellar structure of the LB film is kept unchanged during these processes. This indicates that both the photoreaction and gas reaction mainly take

Yin et al.

place within the layer. These results give a systematic understanding of the nanoarchitecture formation, photochemical reaction, and acidichromism in the organized LB films. Acknowledgment. This work was supported by the Major State Basic Research Development Program 973 (No. G2000078103), Outstanding Youth Fund (No. 20025312), National Natural Science Foundation (No. 90306002), and the Fund of the Chinese Academy of Sciences. LA0490745