15508
J. Phys. Chem. 1996, 100, 15508-15516
Spectroscopic Studies on Phase Transitions in Langmuir-Blodgett Films of an Azobenzene-Containing Long-Chain Fatty Acid: Dependence of Phase Transitions on the Number of Monolayers and Transition Cycles among H-, J-, and J′-Aggregates in Multilayer Films Koshiro Taniike, Toshiki Matsumoto, Takashi Sato, and Yukihiro Ozaki* Department of Chemistry, School of Science, Kwansei-Gakuin UniVersity, Uegahara, Nishinomiya 662, Japan
Kenichi Nakashima Department of Chemistry, College of Liberal Arts, UniVersity of Saga, Honjo-machi, Saga 840, Japan
Keiji Iriyama Institute of DNA Medicine, The Jikei UniVersity School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105, Japan ReceiVed: March 19, 1996; In Final Form: June 10, 1996X
Ultraviolet-visible (UV-vis), infrared transmission, reflection-absorption (RA), and fluorescence spectra have been measured for one-, three-, and nine-monolayer Langmuir-Blodgett (LB) films of an azobenzenecontaining long-chain fatty acid (4-dodecyloxy-4′-(3-carboxytrimethyleneoxy)azobenzene, abbreviated 12A3H) at various temperatures to investigate dependences of phase transitions and annealing effects on the number of monolayers. The UV-vis spectra of the LB films have revealed that the fatty acid investigated forms H-aggregates in the LB films irrespective of the number of monolayers at room temperature. With a temperature increase the H-aggregate in the one-monolayer LB film gradually breaks into the monomers, while that in the three- and nine-monolayer LB films abruptly changes into the J-aggregate near 90 °C. These observations show that the one-monolayer LB film does not have a clear order-disorder transition, while the three-and nine-monolayer films have a phase transition near 90 °C. The infrared study has also given unambiguous evidence that supports this conclusion. For example, the peak intensities of CH2 antisymmetric and symmetric stretching bands at 2920 and 2850 cm-1 in the transmission spectrum of the one-monolayer film gradually decrease with temperature, suggesting that the alkyl chain becomes tilted little by little with respect to the surface normal. In contrast to the one-monolayer film, the peak intensities of most of the infrared bands of the three- and nine-monolayer films undergo a marked change near 90 °C. Therefore, it seems that the tilt angles of both the alkyl chain and chromophobic part change largely concomitantly with the conversion from the H-aggregate to the J-aggregate. Cyclic thermal treatment experiments for the UVvis spectra of the LB films show that the conversion from the H-aggregate to monomers in the one-monolayer film is nearly reversible, while annealing of the three- and nine-monolayer films causes a transition from the J-aggregate to another J-aggregate. The latter J-aggregate is further converted to the original H-aggregate by leaving the LB films in the atmosphere or irradiating them with UV laser light.
Introduction Investigations on thermal behaviors of amphiphilic molecules in organic thin films such as Langmuir-Blodgett (LB) films are important from three major viewpoints.1-16 First, orderdisorder transitions and annealing effects in the thin films may demonstrate specific features different from those of bulk hydrocarbon materials; for example, the LB films have been thought to be an example of a system exhibiting twodimensional phase transitions and melting.1-3 Second, studies on the thermal behaviors have attracted much attention from a practical point of view because the thermal behaviors are directly related to thermal stability of the organic thin films. Third, the studies may offer a new insight into functions of the thin films because those of some films are directly linked with their phase transitions.17-19 * To whom correspondence should be addressed (Fax +81-798-51-0914, E-mail
[email protected]). X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)00835-0 CCC: $12.00
We have been investigating order-disorder transitions and annealing effects of LB and evaporated films of organic functional dyes by using infrared and ultraviolet-visible (UVvis) spectroscopies.12-16 Infrared spectroscopy is powerful in exploring thermal behaviors of organic thin films because temperature-induced changes in molecular structure, mobility, and orientation of both alkyl chain and the chromophobic part of dyes can be monitored by frequencies, bandwidths, and band areas of infrared bands, respectively. UV-vis spectroscopy is useful in investigating molecular aggregation of the chromophobic part. In our first paper of a series of spectroscopic studies on the thermal behaviors of organic thin films, order-disorder transition in LB films of 2-(4′-(dioctadecylamino)phenylazo)-Nmethylbenzothiazolium perchlorate was reported.12 In that study it was found that the transition of the alkyl tail occurs around 70 °C and that prior to the transition the alkyl chain mobility begins to increase.12 It was also suggested that the increase in the mobility of the hydrocarbon chain is linked with the © 1996 American Chemical Society
LB Films of an Azobenzene-Containing Long-Chain Fatty Acid
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Figure 1. (A) Temperature-dependent changes in the UV-vis absorption spectrum of a one-monolayer LB film of 12A3H. (B) Temperaturedependent changes in the UV-vis absorption spectrum of a three-monolayer LB film of 12A3H. (C) Temperature-dependent changes in the UVvis absorption spectrum of a nine-monolayer LB film of 12A3H.
conversion from H-aggregate to monomers in the chromophobic part of the azo dye.12 The second paper dealt with orderdisorder transitions in LB films of tetracyanoquinodimethane (TCNQ) with an alkyl chain of different length (2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-TCNQ; abbreviated as dodecyl-, pentadecyl-, and octadecyl-TCNQ, respectively).13 Dependence of the order-disorder transition upon the length of the alkyl chain bonded was investigated in that study. Our latest two papers reported dependences of thermal behaviors on the number of layers in the LB films of octadecyl-TCNQ.15,16 In those papers order-disorder transitions in the films and annealing effects on them were discussed from both morphological and molecular structural aspects. The main purpose of the present study is to investigate dependences of thermal behaviors on the number of monolayers in LB films of an azobenzene-containing long-chain fatty acid (Figure 1; abbreviated 12A3H). Studies of the dependence of the thermal behaviors of LB films on the number of monolayers are important because the nature of the order-disorder transition and annealing effects of a one-monolayer LB film may be different from that of corresponding multi-monolayer films and
the thermal stability may change with the number of monolayers. The number-of-monolayer dependence of the order-disorder transition was investigated not only for the LB films of octadecyl-TCNQ15,16 but also for those of stearic acid.11 However, the thermal behaviors of the LB films of 12A3H may be more complicated than those of the LB films of octadecylTCNQ and stearic acid because not only molecular structure and orientation but also molecular aggregation are expected to show temperature-dependent changes in the films of 12A3H. LB films and spread monolayers of azobenzene-containing long-chain fatty acids such as 12A3H and their salts have been studied extensively from various points of view,17-32 and it is well-known for bilayer films that they undergo conversion from H- to J-aggregates with temperature.17,18 Therefore, temperature-dependent changes in the molecular structure and orientation of the LB films of 12A3H may also be linked with the conversion from H- to J-aggregate. The present study may provide new insight into functions of thin films of amphiphilic compounds containing an azo group because it has been pointed out that “molecular memory” devices may be constructed on the basis of temperature-induced
15510 J. Phys. Chem., Vol. 100, No. 38, 1996 conversion between H- and J-aggregates of CnAzoCmN+B- in an immobilized bilayer film.17-19 Experimental Section Sample Preparation. The azobenzene-containing long-chain fatty acid 12A3H was commercially supplied by Dojin Chemical Co. Ltd. (Kumamoto, Japan) and used without further purification. It gave a single spot in a thin-layer chromatogram. A Kyowa Kaimen Kagaku Model HBM-AP Langmuir trough with a Wilhelmy balance was employed for the π-A isotherm measurements as well as LB film preparations. A chloroform solution (1.0 × 10-3 M) of 12A3H was placed onto an aqueous subphase of water doubly distilled from deionized water (pH 6.2). The temperature of the water subphase was kept at 20 °C. After the chloroform was permitted to evaporate, the monolayer was compressed at a constant rate of 30 cm2 min-1 up to the surface pressure of 20 mN m-1. The π-A isotherm of the spread monolayer of 12A3H showed that the monolayer was a solid-condensed film at this surface pressure. The monolayers were transferred by the vertical dipping method onto quartz plates (for UV-vis and fluorescence measurements), CaF2 plates (for UV-vis and infrared transmission measurements), or gold-evaporated glass slides (for infrared reflection-absorption [RA] measurements) at the given pressure. The substrates used had been subjected to ultrasonifications in chloroform and then in distilled water. After drying they had been cleaned by a homemade UV-ozone cleaner. The transfer ratio was found to be nearly unity throughout the experiments. Spectroscopy. Infrared spectra of the LB films were measured on a Nicolet Magna 550 FT-IR spectrometer equipped with a MCT detector. The spectra were taken at a 4 cm-1 resolution, and typically, 256 interferograms were coadded to yield a spectra of high signal-to-noise ratio. For the infrared RA measurements, a reflection attachment (JEOL IR-RSC 110) was employed at the incident angle of 78°, together with a JEOL IROPT02 polarizer. UV-vis absorption spectra were recorded on a Shimadzu UV-3101 PC spectrophotometer. Fluorescence spectra were measured with a Hitachi F-4000 spectrofluorometer with a cellholder in the front-face configuration. To measure both UV-vis and infrared spectra at elevated temperatures, the quartz plates or CaF2 substrates on which the LB films had been deposited were inserted into a sample holder in a copper block that contained a ceramic heater. Temperature control was achieved by using an Omron E5T temperature controller. The temperature was monitored with a thermocouple connected with the sample holder and was raised by 1 °C min-1. Peak intensities of infrared bands were calculated by a computer system for the infrared measurements and analyzed on a JEOL JIR-6500 FT-IR spectrometer. The precision and reproducibility of measuring temperature-induced changes in infrared frequencies were better than (0.2 cm-1. Overlapped CH2 antisymmetric and symmetric stretching bands due to the alkyl tail were separated with the curve-fitting technique by using the mixed Gaussian-Lorentzian shape. The 355 nm line from a Nd:YAG laser (Spectra Physics 3800S) was employed for the conversion from J- to H-aggregate in the LB films. Results UV-Vis Absorption Spectra of One-, Three-, and NineMonolayer LB Films of 12A3H. Figure 1 illustrates the temperature dependence of the UV-vis spectrum of a onemonolayer LB film of 12A3H. An absorption band at 302 nm in the spectrum measured at 30 °C is assigned to a π-π*
Taniike et al. transition of the chromophore with the transition moment parallel to its long axis.33 Of note is that the 302 nm band in the spectrum at 30 °C shows a large blue shift by about 55 nm compared with the corresponding band (359 nm) of the solution. This shift may be due to the formation of a strong H-aggregate in the LB film of 12A3H.34 The spectrum of the one-monolayer film remains unchanged up to about 50 °C. However, above 50 °C the band at 302 nm shows a gradual red shift with considerable intensity increase, and the spectrum obtained at 130 °C is close to that of the solution. This result suggests that the H-aggregate breaks into monomers slowly with temperature. Figure 1B,C depicts temperature-dependent changes in UVvis spectra of three- and nine-monolayer LB films of 12A3H, respectively. The spectra measured at 30 °C are almost identical with that of the one-monolayer LB film obtained at the same temperature. The temperature-dependent changes in the UVvis spectra of multi-monolayer LB films are markedly different in two points from those in the spectrum of the one-monolayer film (Figure 1A,B,C). The first point is that the one-monolayer film shows gradual spectral changes, while the multi-monolayer films give an abrupt change near 90 °C. This point suggests that the multi-monolayer LB films of 12A3H show a phase transition near 90 °C, while the one-monolayer LB film does not show a clear phase transition. The second different point is that above 90 °C the absorption maximum is shifted to a longer wavelength side by about 10 nm in the spectra of the multi-monolayer LB films compared with that in the spectrum of the solution. The shift may be due to the formation of J-aggregate in the multi-monolayer LB films.17,18 The band in the 350-400 nm region is broad and has a shoulder near 385 nm, indicating the existence of at least two kinds of J-aggregates (hereafter the J-aggregates formed at elevated temperatures are referred to as J′-aggregates.17,18). Similar heat-induced conversion from H- to J′-aggregate was reported for cast films of CnAzoCmN+Br-.17,18 The temperature-dependent spectral changes in the three- and nine-monolayer LB films are similar to each other. However, it should be noted that the conversion occurs in a wider temperature range (85-97 °C) in the three-monolayer film than in the nine-monolayer film (91-94 °C). Above 97 or 94 °C, the bands in the 350-400 nm region become weak for both films. Of note is that the relative intensity of two bands near 370 and 385 changes with temperature in the spectra of the nine-monolayer LB films. Thus, it seems likely that the J′-aggregate with the absorption maximum near 370 nm changes into another J′-aggregate with a maximum near 385 nm. Figure 2A,B shows annealing effects on the UV-vis spectra of the three- and nine-monolayer LB films of 12A3H, respectively. For the experiments of the annealing effects, first, the spectrum was measured at 30 °C, and then the temperature of the sample was raised to 100, 130, or 160 °C. After the spectrum was measured at the elevated temperature, the temperature was lowered to 30 °C and the subsequent spectrum recorded. It can be seen from Figure 2A,B that the annealing always causes a red shift for the bands in the 350-400 nm region. These observations indicate that the molecular structure and orientation undergo significant changes in the LB films upon annealing, resulting in the formation of different J-aggregates (hereafter, the J-aggregates formed at room temperature are referred to as J-aggregates). For the one-monolayer film the annealing below 130 °C brings about reversible spectral changes; the H-aggregate is formed again after annealing. Infrared Spectra of a One-Monolayer LB Film of 12A3H. Figure 3 depicts infrared transmission spectra of a onemonolayer LB film of 12A3H measured at 30, 70, and 110 °C.
LB Films of an Azobenzene-Containing Long-Chain Fatty Acid
J. Phys. Chem., Vol. 100, No. 38, 1996 15511
Figure 2. Annealing effects on the UV-vis absorption spectra of three- (A) and nine-(B) monolayer LB films of 12A3H.
Figure 3. Infrared transmission spectra of a one-monolayer LB film of 12A3H measured at 30, 70 , and 110 °C and an infrared spectrum of 12A3H in the solid state.
An infrared spectrum of 12A3H in a solid state is also shown at the bottom of Figure 3 for comparison. Infrared spectra of LB films of azobenzene-containing long-chain fatty acids and their salts have been studied well, and band assignments have been proposed for most of their major bands.26,32 Bands at 2918
and 2850 cm-1 are assigned to CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chain of 12A3H, respectively. The frequencies of the CH2 stretching bands are sensitive to the conformation of a hydrocarbon chain; low frequencies (2920 and 2850 cm-1) of the bands are characteristic of a highly ordered (trans-zigzag) alkyl tail, while their upward shifts are indicative of the increase in conformational disorder, i.e. gauche conformers, in the hydrocarbon chain.35-37 The CH2 stretching bands appear at 2918 and 2850 cm-1 in the infrared spectrum measured at 30 °C (Figure 3), suggesting that the hydrocarbon chain of 12A3H is ordered, i.e. trans-zigzag, in the one-monolayer film. Bands at 1603, 1500, and 1251 cm-1 are assigned to phenyl ring stretching ν8a, phenyl ring stretching ν19a, and phenyl ring-O-C antisymmetric stretching modes, respectively. Of note in Figure 3 is that the intensities of the bands in the 1750-1000 cm-1 region are weak in the spectrum measured at 30 °C compared with those in the spectrum of the solid state. Another notable point in Figure 3 is gradual intensity decreases, shifts, and broadening of the two CH2 stretching bands with temperature. The temperature-dependent shift of the CH2 antisymmetric stretching band of the one-monolayer LB film of 12A3H is shown in Figure 4, where the corresponding shifts for three- and nine-monolayer films are also given (see below). It can be clearly seen from Figure 4 that the band shifts upward little by little with temperature for the onemonolayer film. Figure 5A,B illustrates temperature dependences of the peak intensities of the CH2 antisymmetric and symmetric stretching bands, the phenyl ring stretching band at 1602 cm-1 and the phenyl ring-O-C antisymmetric band at 1251 cm-1, respectively. The intensities of all the bands decrease as temperature increases. Infrared Spectra of Three- and Nine-Monolayer LB Films of 12A3H. Figure 6 shows infrared transmission spectra of a nine-monolayer LB film of 12A3H measured at room and elevated temperatures. The corresponding spectra were measured also for the three-monolayer film, but the spectra are not shown here. It is noted in the spectrum measured at 30 °C that three bands are observed in the 1750-1700 cm-1 region. The band at 1736 cm-1 is due to a CdO stretching mode of the
15512 J. Phys. Chem., Vol. 100, No. 38, 1996
Taniike et al.
Figure 4. Temperature dependences of the frequency of the CH2 antisymmetric stretching band for the one-, three-, and nine-monolayer LB films of 12A3H.
Figure 6. Infrared transmission spectra of a nine-monolayer LB film of 12A3H measured at 30, 85, and 91 °C.
Figure 5. Temperature dependences of the peak intensities of the CH2 antisymmetric and symmetric stretching bands (A) and those of the phenyl ring stretching (ν8a) and phenyl-O-C antisymmetric stretching bands (B) for the one-monolayer LB film of 12A3H.
free carboxyl group, while those at 1722 and 1703 cm-1 may be ascibed to the pair split by the factor group in the cis isomer of the ring dimer.38-40 Infrared RA spectra were also measured for a nine-monolayer film of 12A3H (the spectra are not shown here). Comparison between the transmission and RA spectra of the nine-monolayer films measured at 30 °C reveals that the bands in the 17501000 cm-1 region are weak in the former but strong in the latter, while the bands due to the CH2 antisymmetric and symmetric stretching modes and that at 845 cm-1 assigned to an out-of-
plane deformation mode of the phenyl ring are strong in the former but weak in the latter. The three-monolayer films show temperature-dependent changes in the infrared transmission and RA spectra similar to the nine-monolayer films. In Figure 4 the frequencies of the CH2 antisymmetric stretching band are plotted as a function of temperature for the transmission spectra of the three- and ninemonolayer films. It should be pointed out in Figure 4 that the nine-monolayer film shows a sharp upward shift in the narrow temperature range near 90 °C while the three-monolayer film gives a rather gradual shift at the higher temperature range near 95 °C. Figure 7A,B illustrates temperature dependences of the peak intensities of the CH2 antisymmetric and symmetric bands and the bands arising from the phenyl ring mode (ν8a) and phenylO-C antisymmetric stretching mode, respectively, for the infrared transmission spectra of the three-monolayer film. The peak intensities of all the bands undergo abrupt changes near 90 °C. Annealing Effects on the Infrared Transmission Spectra of One-, Three-, and Nine-Monolayer LB Films of 12A3H. Figure 8A,B shows annealing effects on the frequency of the CH2 symmetric stretching band and the peak intensities of the CH2 antisymmetric and symmetric stretching bands, respectively, for the infrared transmission spectra of the one-monolayer LB film. The annealing effects were investigated as follows. First, an infrared spectrum was measured at 30 °C, and then the temperature of the sample was raised to 60 °C and the spectrum obtained. Next, the temperature was lowered to 30 °C, and the subsequent spectrum recorded. The temperature was raised again. The results in Figure 8A,B elucidate that the frequency of the CH2 symmetric stretching band and the peak intensities of the two bands recover considerably even after
LB Films of an Azobenzene-Containing Long-Chain Fatty Acid
Figure 7. Temperature dependences of the peak intensities of the CH2 antisymmetric and symmetric stretching bands (A) and those of the phenyl ring stretching (ν8a) and phenyl-O-C antisymmetric stretching bands (B) for the nine-monolayer LB film of 12A3H.
raising the temperature to 160 °C, which is above the transition point. Similar recovery was observed for the peak intensity of the phenyl-O-C antisymmetric stretching band. In Figure 9A are plotted the frequencies of the CH2 symmetric stretching band of the three- and nine-monolayer LB films of 12A3H subjected to the cyclic thermal treatments. Figure 9B presents plots of the peak intensities of the phenyl ring stretching (ν8a) and phenyl-O-C antisymmetric stretching bands for the nine-monolayer LB film subjected to the cyclic thermal treatment. Results similar to those in Figure 9B were obtained for the three-monolayer film. In contrast to the one-monolayer film, the peak intensities of the two bands do not recover after the annealing above 90 °C for the three- and nine-monolayer films, although the frequencies of the two CH2 bands recover to some extent. The peak intensities of the CH2 stretching bands also do not recover, although their frequencies recover to some extent. Fluorescence Spectra of Nine-Monolayer LB Films of 12A3H. Fluorescence spectra were measured for the H- and J-aggregates formed in the nine-monolayer LB films of 12A3H. For both aggregates fluorescence was too weak to be detected. It is well-known that azo compounds do not give strong fluorescence. Therefore, even J-aggregates, which are usually characterized by a sharp strong fluorescence peak, do not give fluorescence. Discussion Molecular Aggregation, Orientation, and Structure in the LB Films of 12A3H. To discuss phase transition in the LB
J. Phys. Chem., Vol. 100, No. 38, 1996 15513
Figure 8. (A) Changes in the frequency of the CH2 symmetric stretching band of the one-monolayer LB film of 12A3H subjected to cyclic thermal treatments. (B) Changes in the peak intensities of the CH2 antisymmetric and symmetric stretching bands of the same film as A.
films of 12A3H, we, first, have to discuss their molecular aggregation, orientation, and structure at room temperature. As described in the Introduction, structural characterization of LB films of azobenzene-containing long-chain fatty acids and their salts has been studied in some detail by using UV-vis and infrared spectroscopy.25,26,32 As for 12A3H, a UV-vis study by Kawai et al.25 revealed that it forms strong H-aggregates in the LB films. Moreover, a tilt angle of the π-π* transition moment around the surface normal in an 11-monolayer LB film of 12A3H was calculated to be 29°.25 In the following paper,26 they reported a tilt angle of the hydrocarbon chain (29°) calculated from the measurement of the infrared spectrum of an 11-monolayer LB film of 12A3H. Our present results for the UV-vis spectra confirm that 12A3H forms strong H-aggregates in the LB films and that the tilt angle of the π-π* transition moment around the surface normal is fairly small. In infrared spectroscopy, a comparison of infrared transmission and RA spectra of LB films enables one to discuss the molecular orientation in them.41-43 The comparison of the infrared transmission (Figure 6) and RA spectra of the nine-monolayer LB films of 12A3H leads us to conclude that both the hydrocarbon chain and long axis of the azobenzene moiety are nearly perpendicular to the surface normal in the films at 30 °C. This conclusion is in good agreement with that obtained by Kawai et al.25,26 The same conclusion may be reached for the one-monolayer LB film because the relative intensities of the bands in the 1750-1000 cm-1 region are weak in the transmission spectrum of the onemonolayer film compared with those in the spectrum of the solid state (Figure 3).
15514 J. Phys. Chem., Vol. 100, No. 38, 1996
Figure 9. (A) Changes in the frequencies of the CH2 symmetric stretching band of the three- and nine-monolayer LB films of 12A3H subjected to cyclic thermal treatments. (B) Changes in the peak intensities of the phenyl ring stretching (ν8a) and the phenyl-O-C antisymmetric stretching bands of the nine-monolayer LB films of 12A3H subjected to cyclic thermal treatments.
Phase Transitions in One-, Three-, and Nine-Monolayer LB Films of 12A3H. The present UV-vis and infrared results reveal that the one-monolayer LB film does not show a distinct phase transition. The UV-vis spectrum remains almost unchanged until 50 °C and above 50 °C alters gradually with temperature. The changes in the infrared spectra are also gradual with temperature. However, it should be noted that the frequencies of the CH2 antisymmetric and symmetric stretching bands start to shift upward and their peak intensities become weak just above room temperature (Figures 4 and 5A). Therefore, even near room temperature the hydrocarbon chain becomes partially disordered and starts to tilt with respect to the surface normal. It may be considered that when the tilt angle and the content of gauche conformers become large enough, the H-aggregates start to decompose into the monomers. In other words, the changes in the molecular orientation and conformation of the alkyl tail trigger the decomposition of the H-aggregates. Figure 5B suggests that the molecular orientation of the chromophoric part also undergoes a predecomposition change below 50 °C. The temperature-dependent changes in the UV-vis and infrared spectra of the three- and nine-monolayer LB films of 12A3H are completely different from those of the onemonolayer film. The UV-vis spectra of the multilayer films are almost unchanged up to near 90 °C, and then they show a drastic change (Figure 1B,C). This drastic spectral change is ascribed to the conversion from the H- to J′-aggregate.17,18 Therefore, we can conclude that the multilayer LB films of
Taniike et al. 12A3H show a clear phase transition near 90 °C, while the onemonolayer film does not show it. This difference in the thermal behavior between the one- and multi-monolayer LB films may be attributed to the following two reasons. One is that the interaction between the substrate and the first monolayer affects largely the thermal behavior for the one-monolayer LB film, while its effect are small for the multi-monolayer LB films. Another is that there is no longitudinal intermonolayer interaction in the one-monolayer film, but the intermonolayer interaction such as the formation of the ring dimer structure of the carboxylic group plays a very important role in the multimonolayer LB films. In any case the one-monolayer film cannot show the phase transition as a LB film, while the multimonolayer LB films give the peculiar phase transition. This phase transition may be similar to that observed for the cast film of CnAzoCmN+ because the temperature-dependent UVvis spectral variations are almost identical to each other except for the phase transition temperature.22,23 If we compare the phase transition between the three- and nine-monolayer LB films, it becomes clear that the phase transition occurs in the narrower temperature range in the ninemonolayer LB film than in the three-monolayer LB film (see Figures 1B,C and 4). This is probably because the effect of the interaction between the substrate and the first monolayer is negligible and that of the intermonolayer interaction is strong enough for the nine-monolayer film. In both cases, however, probably melting of the alkyl groups causes the transition in the packing of the chromophore because the pretransitional disordering of the chains starts even just above room temperature (Figure 4). Molecular Orientation and Structure in the J′-aggregates of 12A3H. The molecular orientation in the J′-aggregate existing in the LB films of 12A3H can be investigated from the relative intensities of the bands in the infrared transmission spectra measured above 90 °C (Figure 6). In contrast to the spectra obtained at 30 °C, the relative intensities of the two CH2 stretching bands and of the band due to the out-of-plane deformation mode of the phenyl ring are weak, while those of the bands in the 1720-1100 cm-1 region are strong in the transmission spectra measured above 90 °C. Therefore, it is very likely that both the hydrocarbon chain and the long axis of the azobenzene moiety are tilted considerably in the J′aggregate. X-ray analyses of H- and J′-aggregates formed in the cast films of CnAzoCmN+Br- showed that the hydrocarbon chain and the long axis of the azobenzene moiety are nearly perpendicular to the substrate surface in the H-aggregate, while they are inclined largely from the substrate normal in the J′aggregate.17,18,44,45 Thus, the present results for the H- and J′aggregates found in the LB films of 12A3H may be similar to those for CnAzoCmN+Br- in the cast films. The infrared spectra of the LB films measured above 90 °C give interesting information also about the molecular structure in the J′-aggregate. It is noted that the bands at 1736 and 1722 cm-1 are very weak in both infrared transmission and RA spectra, but that the band at 1701 cm-1 arising from the CdO stretching mode of the trans isomer of the ring dimer is very intense in the transmission spectra (the assignment of the trans isomer is supported by the appearance of the band at 1300 cm-1).38-40 These observations lead us to conclude that most of the carboxyl groups of 12A3H are involved in the trans isomer of the ring dimer in the J′-aggregate. Annealing Effects and Phase Transition Cycle in the LB Films. It was reported for the cast film of CnAzoCmN+Brthat the annealing of J′-aggregates produces another kind of
LB Films of an Azobenzene-Containing Long-Chain Fatty Acid J-aggregate and that the J-aggregate is further transformed to the original H-aggregate by keeping the films in the atmosphere without any desiccant or by irradiating them with ultraviolet light.17,18,24 Therefore, it is interesting to compare the annealing effects between the LB films of 12A3H and the cast films of CnAzoCmN+Br-. As for the one-monolayer LB film, it may be concluded that the conversion between the H-aggregates and monomers is nearly reversible. This conclusion can be reached based on the UV-vis and infrared spectroscopic experiments of the cyclic temperature treatments of the one-monolayer LB films (the results of the UV-vis spectra are not shown here, but those of the infrared spectra are presented in Figure 8). The annealing cycle of the multi-monolayer LB films of 12A3H seems to be similar to that of the cast film of CnAzoCmN+Br-. As in the case of the latter,17,18,24 the J′-aggregates of the former show a red shift by 5-10 nm in the UV-vis spectra by cooling down the film from above 100 to 30 °C (Figure 2). This observation strongly suggests that the J′-aggregate is transformed to the J-aggregate after annealing. It was also confirmed by the UV-vis experiments that the J-aggregate is converted again to the J′-aggregate when the temperature is increased. On the other hand, the H-aggregate recovers when the J-aggregate is kept in the atmosphere without any desiccant. The laser irradiation of 355 nm of the J-aggregate formed in the multi-monolayer LB films of 12A3H also brings about the formation of the original H-aggregate. Thus, it has become clear that there is a reversible transition cycle among the H-, J′-, and J-aggregates in the LB films. The infrared study on the annealing effects gives us interesting information about differences in the molecular orientation and structure between the J′- and J-aggregates. The frequencies of the CH2 symmetric stretching band are lower for the J-aggregate than for the J′-aggregate (Figure 9A), indicating that the alkyl chain is more ordered in the J-aggregate. Figure 9B shows that the peak intensities of the ν8a and phenyl-O-C antisymmetric stretching bands, which have their transition moment in the direction of the molecular axis, change a little between the Jand J′-aggregates. This result suggests that the tilt angle of the chromophoric part of 12A3H does not change significantly upon the phase transition between the two kinds of J-aggregates. It is of particular interest to compare the phase transition cycle of the LB films of 12A3H with that of the cast films of CnAzoCmN+Br- (Figure 10). There may be significant differences in the structure of J- and J′-aggregates between the two kinds of films. In the LB film the tilt angle of the azo group does not undergo an appreciable change upon going from one J-aggregate to another, while it changes considerably in the cast film.17-19 However, the magnitude of the shift of the absorption maximum upon going from one J-aggregate to another is similar between the two kinds of the films. The magnitude of the shift can be expressed as follows:46
∆E ) 2{(N - 1)/N}(µ2/r3)(1 - 3 cos2 θ) here, N, µ, r, and θ represent the number of molecules involved in one particular aggregate, the transition moment, the distance between the neighboring molecules, and the tilt angle of chromophoric part from the surface normal, respectively. The change in θ between the J- and J′-aggregates is much larger for the cast film than for the LB film, so that changes in N and/or r should be more significant for the LB films. The number of molecules, N, and the distance of the neighboring molecules, r, should be related to each other because the longer the distance is, the smaller the number is. We infer that the increase in the number of molecules engaged in one aggregate upon going from
J. Phys. Chem., Vol. 100, No. 38, 1996 15515
Figure 10. Transition cycles observed for the cast films of CnAzoCmN+Br-17-19 and LB films of 12A3H.
J′- to J-aggregates is much more significant for the LB films. The structure of the hydrophilic part of 12A3H is quite different from that of CnAzoCmN+Br-; the hydrophilic part of the former can easily form hydrogen bonds. This difference in the structure of the hydrophilic part may make a large difference in the increase of N. Conclusion The present study has demonstrated that the phase transition in the LB films of 12A3H depends upon the number of monolayers and that there is a transition cycle in the multilayer films. The following conclusions can be reached from the present study. (i) The one-monolayer LB film of 12A3H does not show a clear phase transition, while the three- and nine-monolayer films do show a phase transition near 90 °C. Probably, the existence of the interaction between the substrate and the first monolayer and the lack of the longitudinal interactions between monolayers prevent the one-monolayer LB film from having its own phase transition. However, the multi-monolayer films, where the effect of the interaction between the substrate and the first monolayer is negligible while that of the longitudinal interactions is strong, can show the phase transition peculiar to them. The nine-monolayer film gives a sharper phase transition than the three-monolayer film. (ii) In the one-monolayer film the hydrocarbon chain starts tilting and falling into partial disorder even just above room temperature. It seems that the changes in the molecular orientation and structure of the alkyl tail become a trigger for the decomposition of the H-aggregate in the one-monolayer LB
15516 J. Phys. Chem., Vol. 100, No. 38, 1996 film. It is also likely that the melting of the alkyl chains brings about the transition in the packing of the chromophore. (iii) Both the hydrocarbon chain and the chromophoric part are nearly perpendicular to the surface at room temperature in the multi-monolayer LB films, but they become inclined little by little with respect to the surface normal with temperature. Concomitant with the conversion of the H-aggregate to the J′aggregate, both the hydrocarbon chain and the chromophoric part tilt largely and the disorder of the chain proceeds abruptly. (iv) The conversion between the H-aggregate and monomers takes place nearly reversibly in the one-monolayer LB film. The cyclic temperature treatment experiments performed by using the nine-monolayer LB film of 12A3H elucidate that there is a transition cycle among the H-aggregate, J-aggregate, and the high-temperature J′-aggregate. (v) The transition cycle in the LB films of 12A3H is similar to but significantly different in the following point from that in the cast films of CnAzoCmN+Br- (Figure 10). The number of molecules participating in the J-aggregate increases largely upon going from the J′- to the J-aggregate in the former, while the tilt angle of the azo group changes dominantly in the latter. References and Notes (1) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (2) Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. Thin Solid Films 1985, 134, 173. (3) Dierker, S. B.; Murray, C. A.; Legrange, J. D.;Schlotter, N. E. Chem. Phys. Lett. 1987, 137, 453. (4) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3045. (5) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1987, 86, 1601. (6) Barbaczy, E.; Dodge, F.; Rabolt, J. F. Appl. Spectrosc. 1987, 41, 176. (7) Rabe, J. P.; Novotny, V.; Swalen, J. D.; Rabolt, J. F. Thin Solid Films 1988, 159, 359. (8) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (9) Kobayashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1989, 178, 453. (10) Hasegawa, T.; Kamata, T.; Umemura, J.; Takenaka, T. Chem. Lett. 1990, 1543. (11) Umemura, J.; Takeda, S.; Hasegawa, T. J. Mol. Struct. 1993, 297, 57. (12) Katayama, N.; Enomoto, S.; Sato, T.; Ozaki, Y.; Kuramoto, N. J. Phys. Chem. 1993, 97, 6880. (13) Terashita, S.; Ozaki, Y.; Iriyama, K. J. Phys. Chem. 1993, 97, 10445. (14) Nakagoshi, A.; Wang,Y.; Ozaki,Y.; Iriyama, K. Langmuir 1995, 11, 3610.
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