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J-aggregate formation of amphiphilic merocyanine in Langmuir–Blodgett films
Journal of Luminescence 87}89 (2000) 800}802
J-aggregate formation of amphiphilic merocyanine in Langmuir}Blodgett "lms Hiroaki Tachibana *, Yasushi Yamanaka, Hideki Sakai, Masahiko Abe, Mutsuyoshi Matsumoto National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba 305-8565, Japan Science University of Tokyo, Noda 278-8510, Japan
Abstract A single monolayer of amphiphilic spiropyran was transferred on solid substrates using the Langmuir}Blodgett technique. Two types of circular domains were observed in the LB "lms: the larger domains with a diameter of 3}4 lm and the smaller ones with a diameter of 1 lm. The morphology changed depending on the concentration of the spreading solution, the subphase temperature and the spreading solvent. On irradiation with UV light, the spiropyran photoisomerized to merocyanine, followed by the J-aggregate formation of the merocyanine. Flower-like three-dimensional structures formed on the top of the larger domains accompanied by the J-aggregate formation. These structures developed until most of the molecules in the a!ected domains were transported to the structures. 2000 Elsevier Science B.V. All rights reserved. Keywords: J-aggregate; Merocyanine; Langmuir}Blodgett "lms; AFM
1. Introduction J-aggregate formation of dye molecules has been investigated extensively in the Langmuir}Blodgett (LB) "lms [1}9]. Typical features of J-aggregates are the red-shift of the absorption band with a small bandwidth and strong photoluminescence with a small Stokes shift. J-aggregate formation of dyes has been induced by the photoisomerization of azobenzene molecules existing in the same LB "lms [6}9]. Drastic morphological changes of the "lms were also noted. Spiropyran photoisomerizes to merocyanine on the irradiation with UV light. Merocyanine reverts to spiropyran when irradiated with 540 nm light. Further, J-aggregates of amphiphilic merocyanine formed when mixed LB "lms of the spiropyran and alkane were irradiated with UV light at temperatures above 353C [2] or when mixed LB "lms of the dye and azobenzene were irradiated alternately with UV and visible light at room temperature [9]. * Corresponding author. Fax: #81-298-54-4669. E-mail address: [email protected] (H. Tachibana)
In this paper, we will report on the J-aggregate formation in single-layer LB "lms of merocyanine (spiropyran) when irradiated with UV light at room temperature. The morphological change accompanied by the J-aggregate formation will be demonstrated. 2. Experiments 1 -Octadecyl-3 ,3 -dimethyl-6-nitro-8-[docosanoyloxymethyl]-spiro[2H-1-benzopyran-2,2 -indoline] (SP1822) was dissolved in an organic solvent such as chloroform, toluene and dichloromethane. The solution was spread onto pure water and the molecules were compressed on a Lauda trough. A single monolayer was transferred using the vertical dipping method at a surface pressure of 25 m Nm\ onto quartz and mica, for the measurements of UV-vis absorption spectra and the AFM observations, respectively. AFM images were recorded on a Seiko SPA300 operating in noncontact mode. The monochromatic light (334 nm) from a high-pressure mercury lamp was used as the source of UV light for the irradiation of the LB "lms.
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 1 6 - 0
H. Tachibana et al. / Journal of Luminescence 87}89 (2000) 800}802
801
Table 1 Preparation conditions of SP1822 LB "lms Part in Fig. 1
Solvent
Concentration (mM)
Subphase temp (3C)
(a) (b) (c) (d)
CHCl CHCl CHCl toluene
0.1 1.0 0.1 0.1
17 17 29 17
Fig. 2. Spectral change of a single-layer LB "lm of SP1822 on the irradiation with UV light (334 nm). The inset represents the change in absorbance of the J-band at 618 nm as a function of time.
Fig. 1. AFM images of single-layer SP1822 LB "lms prepared under various conditions. The details of the conditions are summarized in Table 1.
3. Results and discussion The morphology of single-layer LB "lms of SP1822 was investigated using AFM. The preparation conditions of the LB "lms are summarized in Table 1. Two types of circular domains are seen in Fig. 1(a): the larger ones with a diameter of 3}4 lm and the smaller ones with a diameter of 1 lm. The larger domains have irregularly shaped structures on the top while the smaller ones are free from such defects. This image is compared with those of the LB "lms fabricated under di!erent preparation conditions. When the concentration of the spreading solution is ten times higher, only the smaller domains are formed (Fig. 1(b)). Some domains have lower regions in the center. This suggests that the number of nuclei for the domain formation on the water surface is larger for spreading solutions with higher concentrations. When
the subphase temperature is high, large domains with a diameter of over 10 lm are observed (Fig. 1(c)). The large domains are formed probably because of the larger mobility of the molecules or domains on the subphase at higher temperatures. There are very small domains between the large ones. These may be formed by the quick evaporation of the solvent at higher temperatures. The usage of di!erent solvents gives complicated effects on the domains. In the case of toluene (Fig. 1(d)), the domains are larger than the ones in Fig. 1(a) but with less-de"ned shape. Some irregularly shaped structures are also seen on the top of the domains. When dichloromethane was used as the solvent, two types of domains were observed: one with a diameter of a few lm and the other with a diameter of sub-lm (data not shown). No irregular structures were seen on the top of the domains. These results should be related to the di!erence in the vapor pressure and the spreading pressure of the solvents. The LB "lms prepared under the same conditions as in Fig. 1(a) were irradiated with UV light and the subsequent photoreactions were monitored using UV-vis absorption spectroscopy. Fig. 2 shows the change in the absorption spectra. Before irradiation, the absorption bands of SP1822 are seen at ca. 240, 270 and 340 nm. In the initial regime of the UV irradiation, the main photoreaction is the isomerization of SP1822 to PMC. This explains the decreases in intensities of the absorption bands at ca. 240 and 270 nm and the concomitant increases in intensities of the bands at ca. 380 and 560 nm. The formation of a small amount of J-aggregate is con"rmed by the presence of a shoulder band at ca.
802
H. Tachibana et al. / Journal of Luminescence 87}89 (2000) 800}802
Fig. 4. AFM images of a single-layer SP1822 LB "lm shown in Fig. 1(a) after the irradiation with UV light. The irradiation time is (a) 30 min and (b) 90 min.
Fig. 3. Photoreactions of SP1822 in the LB "lms on the irradiation with UV light.
620 nm. Further irradiation strengthens the above tendency but with an increasing intensity of a narrow J-band at 618 nm. In this regime, the J-aggregate formation follows the photoisomerization of SP1822 into PMC. Finally, the J-aggregate formation comes to a saturated state. The inset shows the course of the change in absorbance of the J-band. The saturating behavior is clear. The J-aggregates were stable at room temperature even after one month. These results are summarized in Fig. 3. In our case, the J-aggregate formation of PMC was observed when the "lms were irradiated at room temperature. It was reported that the J-aggregates were formed when 6-layer SP1822 LB "lms were irradiated with UV light at temperatures above 353C but not at room temperature [2]. The inconsistency may be due to the di!erence in layer number (we used single-layer LB "lms) and/or the instability of SP1822 monolayers noted in the literature. Morphological change of the LB "lms with the Jaggregate formation was investigated by AFM observations. Fig. 4 shows the morphological change of the LB "lm shown in Fig. 1(a) caused by the UV irradiation. A drastic change of the domains occurred with the Jaggregate formation. Flower- or leaf-like three-dimensional structures formed on the top of the larger domains (Fig. 4(a)). This morphological change is considered to be due to the J-aggregate formation [6}9]. The three-dimensional structures is considered to consist of a number of J-aggregates. The shape of the three-dimensional structures is not so di!erent from the one observed when the J-aggregate of SP1822 was formed in the mixed LB "lms of SP1822 and an azobenzene on the alternate irradiation with UV and visible light [9]. Close observa-
tion of the #ower-like structures reveals that most of the structures are accompanied by the adjacent lower regions, probably due to the transport of the molecules. The absence of the morphological change in the smaller domains suggests that the irregularly shaped structures present on the top of the larger domains before the irradiation may serve as the nucleation sites for the morphological change, which should be closely related with the J-aggregate formation. This morphological change continued until most of the molecules in the a!ected domains have been transported to the #ower-like structures (Fig. 4(b)).
4. Conclusions This paper demonstrates that the size and the shape of the domains can be controlled by the preparation conditions of SP1822 LB "lms. Moreover, the J-aggregate formation of SP1822 was observed on the UV irradiation of the "lms at room temperature. A large morphological change was noted and #ower-like three-dimensional structures appeared from the larger domains but not from the smaller domains. This suggests that the J-aggregate formation and the morphological change are a!ected by the domain size.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
V. Czikklely et al., Chem. Phys. Lett. 6 (1970) 207. E. Ando et al., Thin Solid Films 133 (1985) 21. K. Saito et al., J. Appl. Phys. 71 (1992) 1401. S. Kuroda, Colloids Surf. A. 71 (1993) 127. D. MoK bius, Adv. Mater. 7 (1995) 437. M. Matsumoto et al., J. Phys. Chem. B 101 (1997) 702. H. Tachibana et al., Thin Solid Films 327}329 (1998) 813. M. Matsumoto et al., Molec. Cryst. Liq. Cryst. 316 (1998) 113. S. Terrettaz et al., Langmuir 14 (1998) 7511.
J-aggregate formation of amphiphilic merocyanine in Langmuir}Blodgett "lms Hiroaki Tachibana *, Yasushi Yamanaka, Hideki Sakai, Masahiko Abe, Mutsuyoshi Matsumoto National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba 305-8565, Japan Science University of Tokyo, Noda 278-8510, Japan
Abstract A single monolayer of amphiphilic spiropyran was transferred on solid substrates using the Langmuir}Blodgett technique. Two types of circular domains were observed in the LB "lms: the larger domains with a diameter of 3}4 lm and the smaller ones with a diameter of 1 lm. The morphology changed depending on the concentration of the spreading solution, the subphase temperature and the spreading solvent. On irradiation with UV light, the spiropyran photoisomerized to merocyanine, followed by the J-aggregate formation of the merocyanine. Flower-like three-dimensional structures formed on the top of the larger domains accompanied by the J-aggregate formation. These structures developed until most of the molecules in the a!ected domains were transported to the structures. 2000 Elsevier Science B.V. All rights reserved. Keywords: J-aggregate; Merocyanine; Langmuir}Blodgett "lms; AFM
1. Introduction J-aggregate formation of dye molecules has been investigated extensively in the Langmuir}Blodgett (LB) "lms [1}9]. Typical features of J-aggregates are the red-shift of the absorption band with a small bandwidth and strong photoluminescence with a small Stokes shift. J-aggregate formation of dyes has been induced by the photoisomerization of azobenzene molecules existing in the same LB "lms [6}9]. Drastic morphological changes of the "lms were also noted. Spiropyran photoisomerizes to merocyanine on the irradiation with UV light. Merocyanine reverts to spiropyran when irradiated with 540 nm light. Further, J-aggregates of amphiphilic merocyanine formed when mixed LB "lms of the spiropyran and alkane were irradiated with UV light at temperatures above 353C [2] or when mixed LB "lms of the dye and azobenzene were irradiated alternately with UV and visible light at room temperature [9]. * Corresponding author. Fax: #81-298-54-4669. E-mail address: [email protected] (H. Tachibana)
In this paper, we will report on the J-aggregate formation in single-layer LB "lms of merocyanine (spiropyran) when irradiated with UV light at room temperature. The morphological change accompanied by the J-aggregate formation will be demonstrated. 2. Experiments 1 -Octadecyl-3 ,3 -dimethyl-6-nitro-8-[docosanoyloxymethyl]-spiro[2H-1-benzopyran-2,2 -indoline] (SP1822) was dissolved in an organic solvent such as chloroform, toluene and dichloromethane. The solution was spread onto pure water and the molecules were compressed on a Lauda trough. A single monolayer was transferred using the vertical dipping method at a surface pressure of 25 m Nm\ onto quartz and mica, for the measurements of UV-vis absorption spectra and the AFM observations, respectively. AFM images were recorded on a Seiko SPA300 operating in noncontact mode. The monochromatic light (334 nm) from a high-pressure mercury lamp was used as the source of UV light for the irradiation of the LB "lms.
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 1 6 - 0
H. Tachibana et al. / Journal of Luminescence 87}89 (2000) 800}802
801
Table 1 Preparation conditions of SP1822 LB "lms Part in Fig. 1
Solvent
Concentration (mM)
Subphase temp (3C)
(a) (b) (c) (d)
CHCl CHCl CHCl toluene
0.1 1.0 0.1 0.1
17 17 29 17
Fig. 2. Spectral change of a single-layer LB "lm of SP1822 on the irradiation with UV light (334 nm). The inset represents the change in absorbance of the J-band at 618 nm as a function of time.
Fig. 1. AFM images of single-layer SP1822 LB "lms prepared under various conditions. The details of the conditions are summarized in Table 1.
3. Results and discussion The morphology of single-layer LB "lms of SP1822 was investigated using AFM. The preparation conditions of the LB "lms are summarized in Table 1. Two types of circular domains are seen in Fig. 1(a): the larger ones with a diameter of 3}4 lm and the smaller ones with a diameter of 1 lm. The larger domains have irregularly shaped structures on the top while the smaller ones are free from such defects. This image is compared with those of the LB "lms fabricated under di!erent preparation conditions. When the concentration of the spreading solution is ten times higher, only the smaller domains are formed (Fig. 1(b)). Some domains have lower regions in the center. This suggests that the number of nuclei for the domain formation on the water surface is larger for spreading solutions with higher concentrations. When
the subphase temperature is high, large domains with a diameter of over 10 lm are observed (Fig. 1(c)). The large domains are formed probably because of the larger mobility of the molecules or domains on the subphase at higher temperatures. There are very small domains between the large ones. These may be formed by the quick evaporation of the solvent at higher temperatures. The usage of di!erent solvents gives complicated effects on the domains. In the case of toluene (Fig. 1(d)), the domains are larger than the ones in Fig. 1(a) but with less-de"ned shape. Some irregularly shaped structures are also seen on the top of the domains. When dichloromethane was used as the solvent, two types of domains were observed: one with a diameter of a few lm and the other with a diameter of sub-lm (data not shown). No irregular structures were seen on the top of the domains. These results should be related to the di!erence in the vapor pressure and the spreading pressure of the solvents. The LB "lms prepared under the same conditions as in Fig. 1(a) were irradiated with UV light and the subsequent photoreactions were monitored using UV-vis absorption spectroscopy. Fig. 2 shows the change in the absorption spectra. Before irradiation, the absorption bands of SP1822 are seen at ca. 240, 270 and 340 nm. In the initial regime of the UV irradiation, the main photoreaction is the isomerization of SP1822 to PMC. This explains the decreases in intensities of the absorption bands at ca. 240 and 270 nm and the concomitant increases in intensities of the bands at ca. 380 and 560 nm. The formation of a small amount of J-aggregate is con"rmed by the presence of a shoulder band at ca.
802
H. Tachibana et al. / Journal of Luminescence 87}89 (2000) 800}802
Fig. 4. AFM images of a single-layer SP1822 LB "lm shown in Fig. 1(a) after the irradiation with UV light. The irradiation time is (a) 30 min and (b) 90 min.
Fig. 3. Photoreactions of SP1822 in the LB "lms on the irradiation with UV light.
620 nm. Further irradiation strengthens the above tendency but with an increasing intensity of a narrow J-band at 618 nm. In this regime, the J-aggregate formation follows the photoisomerization of SP1822 into PMC. Finally, the J-aggregate formation comes to a saturated state. The inset shows the course of the change in absorbance of the J-band. The saturating behavior is clear. The J-aggregates were stable at room temperature even after one month. These results are summarized in Fig. 3. In our case, the J-aggregate formation of PMC was observed when the "lms were irradiated at room temperature. It was reported that the J-aggregates were formed when 6-layer SP1822 LB "lms were irradiated with UV light at temperatures above 353C but not at room temperature [2]. The inconsistency may be due to the di!erence in layer number (we used single-layer LB "lms) and/or the instability of SP1822 monolayers noted in the literature. Morphological change of the LB "lms with the Jaggregate formation was investigated by AFM observations. Fig. 4 shows the morphological change of the LB "lm shown in Fig. 1(a) caused by the UV irradiation. A drastic change of the domains occurred with the Jaggregate formation. Flower- or leaf-like three-dimensional structures formed on the top of the larger domains (Fig. 4(a)). This morphological change is considered to be due to the J-aggregate formation [6}9]. The three-dimensional structures is considered to consist of a number of J-aggregates. The shape of the three-dimensional structures is not so di!erent from the one observed when the J-aggregate of SP1822 was formed in the mixed LB "lms of SP1822 and an azobenzene on the alternate irradiation with UV and visible light [9]. Close observa-
tion of the #ower-like structures reveals that most of the structures are accompanied by the adjacent lower regions, probably due to the transport of the molecules. The absence of the morphological change in the smaller domains suggests that the irregularly shaped structures present on the top of the larger domains before the irradiation may serve as the nucleation sites for the morphological change, which should be closely related with the J-aggregate formation. This morphological change continued until most of the molecules in the a!ected domains have been transported to the #ower-like structures (Fig. 4(b)).
4. Conclusions This paper demonstrates that the size and the shape of the domains can be controlled by the preparation conditions of SP1822 LB "lms. Moreover, the J-aggregate formation of SP1822 was observed on the UV irradiation of the "lms at room temperature. A large morphological change was noted and #ower-like three-dimensional structures appeared from the larger domains but not from the smaller domains. This suggests that the J-aggregate formation and the morphological change are a!ected by the domain size.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
V. Czikklely et al., Chem. Phys. Lett. 6 (1970) 207. E. Ando et al., Thin Solid Films 133 (1985) 21. K. Saito et al., J. Appl. Phys. 71 (1992) 1401. S. Kuroda, Colloids Surf. A. 71 (1993) 127. D. MoK bius, Adv. Mater. 7 (1995) 437. M. Matsumoto et al., J. Phys. Chem. B 101 (1997) 702. H. Tachibana et al., Thin Solid Films 327}329 (1998) 813. M. Matsumoto et al., Molec. Cryst. Liq. Cryst. 316 (1998) 113. S. Terrettaz et al., Langmuir 14 (1998) 7511.