J. Phys. Chem. B 2007, 111, 9301-9308
9301
Regulation of Aggregation and Morphology of Cyanine Dyes on Monolayers via Gemini Amphiphiles Guocheng Zhang, Xiaodong Zhai, Minghua Liu,* Yalin Tang, and Yazhou Zhang Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Science, 100080, Beijing, People’s Republic of China ReceiVed: February 23, 2007; In Final Form: June 3, 2007
The aggregation of dyes is of considerable importance in relating to their functions and applications. In this paper, we have investigated the regulation on the aggregation and morphology of two cyanine dyes, 3,3′disulfopropyl-4,5,4′,5′-dibenzo-9-methylthiacarbocyanine triethylammonium salt (MTC) and 3,3′-disulfopropyl4,5,4′,5′-dibenzo-9-phenylthiacarbocyanine triethylammonium salt (PTC), using a series of gemini amphiphiles (bis(2′-heptadecyl-3′-ethylimidazolium)-1,n-alkane dibromide, abbreviated as Gn, n ) 2, 4, 6, 8, 10). It has been found that both of the dyes could be adsorbed onto the monolayers of the gemini amphiphiles through the electrostatic and π-π interaction and stacked into H- or J-aggregate. The spacer of the gemini amphiphile showed good control over the aggregation of MTC: H-aggregate was favored when gemini amphiphiles with short spacer were applied, while J-aggregation was preferred in the case of longer spacer. Only J-aggregate was observed for PTC on gemini monolayer, regardless of the structure of the gemini amphiphiles. Interesting morphologies were observed for all the gemini/dye complex monolayers. Network structure and nanofibers were formed for the gemini/MTC films transferred below the plateau surface pressure and close to the collapse pressure, respectively. The ability of the complex monolayers to form nanofibers strongly depended on the component amphiphiles, G2 > G4 > G6, and no nanofibers were observed for G8/MTC and G10/MTC after the collapse. Only squared domains were observed for gemini/PTC monolayers. When both G2 and G10 were mixed, an individual control of the gemini amphiphiles over the aggregation of MTC in the complex monolayers was observed. The relationship among the spacer, dye structure, and aggregation was revealed.
Introduction Supramolecular assemblies based on various noncovalent interactions, such as electrostatic, hydrogen-bond, hydrophobic, and π-π stacking interactions, and coordination have been drawing considerable interests from both fundamental and applied viewpoints.1-5 Among various building blocks for the supramolecular assemblies, organic dyes, whose π-conjugated systems usually exhibit novel functionalities in various fields, are one of the most attractive building blocks.6-9 Besides diverse functions, many dyes could show interesting aggregation behaviors, which are greatly related to the functions of the dyes as well. Typical aggregates include the J-aggregate and Haggregate. J-aggregate is characteristic of the sharp and intense absorption band in the longer wavelength in comparison with the monomer and is suggested to be due to the arrangement of the transition dipole of the molecules in a “head-to-tail” way.10-13 H-aggregate is characterized by a blue shift of the absorption band compared with the monomer.14-16 Both of these aggregates showed some special properties. For example, J-aggregate can be used as spectral sensitizers in photography due to its optical characteristics.17-20 H-aggregates can possibly be used as antenna molecules to harvest light energy whereby the aggregation of the molecules played an important role.15 With the aggregation of dyes addressed, the control of the aggregation of the dyes is very important besides the design of the dyes. The supramolecular method is another effective way to control the aggregation of the dye, which utilized the * Corresponding author. E-mail:
[email protected].
noncovalent interaction between the dyes or dye and matrix molecules, due to its nondestructive and reversible merit. One of the most important elements in such control is to find appropriate control agents or condition. So far, various compounds such as biopolymers,21,22 polyelectrolytes,23,24 inorganic salts,25 and amphiphiles26-30 were used to control or switch the aggregation of some dyes. Here, we applied a series of gemini amphiphiles to control the dye aggregation in the organized molecular films. Air/water interface provided a good platform to control the aggregation of the dyes. Only a small amount of dye is necessary in order to get dye aggregates using the technique.31-33 In addition, many amphiphilic matrix molecules can be applied to control the aggregation. Although many conventional amphiphiles, which have one head group and one or two hydrophobic tails, have been wildly used in this sense,25-29 gemini amphiphiles have been less investigated. Gemini amphiphiles have two long hydrophobic chains and two hydrophilic headgroups, connected by a rigid or flexible spacer.35-38 They have a wealth of parameters, such as the length of the hydrophobic chains, the size of the hydrophilic groups, the type of counterions, and the flexibility and polarity of the linker group (spacer), which can provide a good modulation on the interfacial properties. Among these parameters, the spacer is particularly interesting. Previously, we have found that the spacer of the gemini amphiphiles had a certain control on the morphologies of the cyanine dye in the organized molecular films but is not obvious on their aggregation.39 In this paper, we selected another two cyanine dyes, with different 9-substitutents and aromatic
10.1021/jp071515g CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007
9302 J. Phys. Chem. B, Vol. 111, No. 31, 2007 SCHEME 1: Chemical Structures and the Abbreviations of the Compounds Used in the Work
Zhang et al. To measure the AFM of the transferred films, freshly cleaved mica was used and AFM was recorded on a Digital Instruments Nanoscope IIIa (Santa, Barbara, CA) with a silicon cantilever, using the tapping mode. AFM images were shown in the height mode without any image processing except flattening. Results
ring conjugations, and investigated their aggregation on the monolayers of gemini amphiphiles. We have found that the spacer of the gemini amphiphile could show very good control on both the aggregation and morphologies of the dyes, and thus clearly revealed the relationship among the spacer, dye structure, and aggregation. A supramolecular control over the dye aggregation through the spacer of the gemini amphiphiles was realized. The aggregation of the dye on the monolayers was studied by the surface pressure-area isotherms, and the transferred multilayer films were investigated using UV-vis spectroscopy and atomic force microscopy (AFM) measurement. Experimental Details 2.1. Materials. Scheme 1 shows the chemical structure of the amphiphiles and the cyanine dyes used in this work. The synthesis and the fundamental interfacial properties of the gemini amphiphiles Gn (n ) 2, 4, 6, 8, 10) were reported previously.40 The synthesis and some basic properties of the dyes, 3,3′disulfopropyl-4,5,4′,5′-dibenzo-9-methylthiacarbocyanine triethylammonium salt, MTC, and 3,3′-disulfopropyl-4,5,4′,5′dibenzo-9-phenylthiacarbocyanine triethylammonium salt, PTC, were reported previously.41 2.2. Procedures. The adsorption and aggregation of the dyes onto the spreading monolayers of the gemini amphiphiles were performed on a KSV trough (KSV 1100, Helsinki, Finland). Certain amounts of the amphiphiles in chloroform solutions (ca. 0.1 mM) were spread on the aqueous subphase containing 2 × 10-6 M cyanine dyes. In this dye concentration MTC existed in a dimer as well as H-aggregate without any J-aggregation. PTC showed predominantly as a monomer with a slight J-aggregation. Further dilution of this concentration did not cause the disappearance of both the dimer and J-band in MTC and PTC, respectively. However, it took a long time for the dyes to adsorb on the spreading monolayers. Therefore, we used the subphase with a dye concentration of 2 × 10-6 M. Twenty minutes after the spreading, which was sufficient for the evaporation of the solvent and the adsorption of the dye, the surface pressure-molecular area (π-A) isotherms were recorded by compressing the barriers from two directions at a constant speed of 7.5 cm2/min. The subphase was kept at a constant temperature of 20 °C with water circulation. The in situ formed complex monolayers of the amphiphiles/dyes were transferred onto hydrophobic quartz substrates, which were precoated with a monolayer of iron(III) stearate, by a horizontal lifting method for characterization. In the film transfer, the substrates were put parallel to the film surface. After the substrates were gently attached to the film and then lifted up, a uniform transfer of the film with a transfer ratio near 2 was realized. Generally, 25 layers of the film were transferred onto quartz plates for UVvis spectral measurements. UV-vis spectra of the transferred multilayer films were measured on a JASCO UV-560 spectrometer.
3.1. Adsorption of the Dyes onto the Monolayer at the Air-Water Interface. Figure 1 shows the π-A isotherms of gemini amphiphiles spread on aqueous subphase containing 2 × 10-6 M MTC or PTC at 20 °C, respectively. The spreading behaviors of the individual gemini amphiphiles were reported in a previous paper, in which isotherms without any transition were observed.41 When dyes exist in the subphase, great changes are observed for the isotherms in their limiting molecular areas and shapes. Table 1 lists the liftoff and limiting molecular areas of the gemini amphiphiles spreading on water, MTC, and PTC, respectively. The liftoff molecular areas are the first point of the isotherm where a monolayer shows a detectable surface pressure, while the limiting molecular areas are obtained by extrapolating the linear part of the isotherm below the transition point to zero surface pressure. Since the monolayers on the subphase of the dyes have transition points, the limiting molecular areas are obtained for the isotherm below the transition points. It is obvious that the monolayers spread on the dyes subphase have small liftoff molecular areas in comparison with those of the corresponding gemini amphiphiles on plain water surface. Since the amphiphiles used are positively charged, it is obvious that strong repulsion existed for gemini monolayers on the water surface, resulting in the more expanded isotherm. When negatively charged dyes existed in the subphase, an electrostatic interaction between the amphiphiles and dyes occurred, which reduced the repulsion between the gemini amphiphile and resulted in the condensation of the complex monolayers. In comparison with the gemini monolayers on the plain water surface, those on MTC show slightly increased limiting molecular areas, while very close values or even smaller limiting molecular areas are observed for the gemini monolayers on the PTC subphase. The differences between the molecular areas of the monolayers on dyes and plain water surface are more obvious for those gemini amphiphiles with short spacer length (G2 and G4). This indicates that when the spacer length is shorter, a partial amount of the dyes penetrates into the monolayer, while such penetration is suppressed when the spacer length becomes longer due to the localization of the positive charges within the gemini amphiphiles. The monolayers of G0, which has one headgroup, is different from those of the gemini amphiphile. G0 could not form a stable monolayer on plain water due to partial dissolution in the subphase. When dyes were spread on the subphase, strong electrostatic interaction causes the complex formation, which stabilizes the monolayer. The complex monolayer of G0/dye has a larger liftoff and limiting molecular area than those of all the gemini/MTC monolayers and is very close to those of the gemini/PTC monolayers with longer spacer. Since G0 has only one headgroup and the geminis have two headgroups, the larger molecular area of G0 monolayer than those of the Gn/MTC monolayers implied that more dye molecules penetrate into the G0 monolayers. Depending on the spacer length, gemini amphiphile showed different behaviors on the dye subphase. In the case of gemini monolayers on MTC solution, all the isotherms exhibited plateau regions, which were suggested to be related to the spacer of
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Figure 1. Surface pressure-molecular area isotherms of Gn (a, n ) 2; b, n ) 4; c, n ) 6; d, n ) 8; e, n ) 10; f, n ) 0) monolayers spread on aqueous solution containing 2 × 10-6 M MTC (A) and PTC (B), respectively.
TABLE 1: Liftoff Molecular Area (A1) and Limiting, Molecular Area (A2) of the Gemini Monolayers, Spread on Various Subphases at 20 °C, Respectively (nm2/molecule) water
MTC
PTC
subphase
A1
A2
A1
A2
A1
A2
G2 G4 G6 G8 G10 G0
5.15 5.87 5.87 6.02 7.61 1.48
1.71 1.97 2.23 2.33 2.46
2.58 2.93 4.01 4.01 4.96 6.13
2.37 2.48 2.49 2.54 2.73 3.01
3.30 4.03 4.59 4.89 6.02 5.05
1.91 2.12 2.16 2.21 2.46 2.28
the gemini amphiphiles. For the G2/MTC monolayer, an ambiguous inflection point is observed around 12 mN/m. When the spacer length of the gemini increased, the inflection become clear and the plateau region longer. In the case of the G4/MTC and G6/MTC monolayers, the surface pressures where the plateau region started are observed at 18 and 24 mN/m, respectively. When the spacer length of the gemini amphiphiles increases to G8/MTC and G10/MTC, a mount was observed in the isotherm. The surface pressure increased rapidly after the plateau region and the monolayers collapse above 40 mN/m. The collapse pressure did not linearly change with the increment of the space length. It shows a minimum at G6. Previously, others and we reported that the interfacial properties of the gemini amphiphiles with hydrophobic spacer could show a turning point at G6-G10, which was due to the conformational change of the spacer from a parallel to water surface to upward curved.42,43 The experimental data here indicated that similar conformational changes might occur for the dye/gemini monolayers upon compression. In the case of PTC, only vague inflection points are observed around 25 mN/m. It is interesting to note that with an increasing number of the methylene groups in the spacer of gemini amphiphiles, the isotherms are only slightly shifted to a larger molecular area. In addition, both the shape and limiting molecular areas are very close. These results imply that the interaction between the PTC may be very strong and the gemini amphiphile could not show good control on the aggregation of PTC. 3.2. Aggregation of the Dyes in the Multilayer Films: UV-Vis Spectral Investigation. All the complex monolayers were transferred onto the solid substrate by a horizontal lifting method. Figure 2 shows the UV-vis spectra of the transferred complex multilayer films of the geminis/dyes in comparison with those of the corresponding solution. In aqueous solution, MTC shows two strong bands at 515 and 534 nm, with a shoulder peak around 574 nm, respectively, which could be assigned to the H-aggregate, dimer, and monomer, respectively.41 PTC aqueous solution shows three bands at 545, 624,
and 678 nm, which could be assigned to the H-aggregate, monomer, and J-aggregate, respectively.41 Significant changes are observed in the multilayer films. In the case of G2/MTC, a strong absorption is observed at 486 nm with a shoulder peak at 515 nm. The shoulder peak at 515 nm can be assigned to the H-band, which is also observed in the solution. The significant blue-shifted band at 486 nm could be ascribed to the formation of another H-aggregate in the film. We tentatively assigned the band around 515 and 486 nm to be H1 and H2 bands, respectively. A similar result is observed for the G4/MTC complex film. However, significant spectral change is observed from G6/MTC multilayer films. Two strong absorption bands are observed at 517 and 663 nm, with the disappearance of the band at 486 nm. When the spacer length of the gemini is increased to G8 and G10, only an absorption at 663 nm is observed for the complex films. The band at 663 nm can be assigned to the J-aggregate of MTC. All these results indicate that MTC could easily be adsorbed on the gemini monolayers, and the spacer length of the gemini amphiphiles has a good control on the aggregation of the dye. The shorter spacer is favored for the formation of the H-aggregate, while the longer one prefers the J-aggregate. The G0/MTC film shows a spectrum containing both H1-aggregate and J-aggregate bands. This could be due to the loose stacking and relative uniform charge distribution of the G0 monolayer. Either attachment of the dye to the G0 headgroup is possible. It is interesting to note that the spectra of G6/MTC and G0/MTC are very similar. It seems that when the spacer length becomes C6, their control on the aggregation is similar to the one headed G0 monolayer. In the case of geminis/PTC multilayer films, as shown in Figure 2b, a strong absorption band is observed at 692 nm regardless of the amphiphile structure. The band at 692 nm can be assigned to the J-aggregate of PTC. The results indicate that PTC has a strong tendency to aggregate itself and the gemini amphiphile could hardly control the aggregation of PTC in the complex films. 3.3. AFM Investigation of the Complex Films. AFM is a useful technique to get the information on surface topography of the films, particularly for the flat LB films. We have transferred one layer of the complex films onto mica surfaces and measured their AFM pictures. Since the monolayers of the gemini/MTC showed obvious plateau regions, we transferred the films at a surface pressure below and above the plateau region, respectively. In addition, the films were further compressed to collapse, and their surface morphologies were also investigated by AFM. Figure 3 shows the AFM images of the gemini/MTC monolayers transferred under different conditions. In comparison with the uniform flat films formed by the gemini amphiphiles on plain water surface, as reported in a previous
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Figure 2. UV-vis spectra of 25-layer complex films of gemini/MTC (a) and geminis/PTC (b) transferred at 15 mN/m, respectively.
Figure 3. AFM images of complex films deposited from 2 × 10-6 M MTC aqueous subphase at different surface pressures. G2 (a1-a3, 5, 15, and 45 mN/m); G6 (b1-b4, 5, 15, 30, and 42 mN/m); G4 (c1 and c2, 15 and 45 mN/m); G8 (d1 and d2, 15 and 44 mNm); G10 (e1 and e2, 15 and 45 mNm); G0 (f1 and f2, 15 and 37 mN/m). All of the other images are 5 µm × 5 µm, while the inset images are the amplified images of the corresponding complex films, with size of 500 nm × 500 nm.
paper,39 various interesting morphologies of the complex films are observed. First, a network structure is observed for all the complex monolayers transferred at a lower surface pressure below the plateau region. Depending on the spacer length, the grid of the network shows slight differences. The network is composed of the thick and thinner grid for the G2/MTC monolayer, whereas it is composed of a uniform grid for the other complex monolayers. In addition, the network tends to align when the spacer goes longer. Second, when compressed over the plateau region to collapse, aligned nanofibers are observed for the films with shorter spacer (G2-G6/MTC), while only uniform or flat morphologies with some hole defects are seen for the G8/MTC and G10/MTC films, respectively. The monolayer of G0/MTC shows similar morphological changes to those of the G6/MTC monolayer. At low surface pressure,
network morphology is observed. After collapse, a nanofiber is observed. This is in accordance with the UV-vis spectra, in which both G6/MTC and G0/MTC films show very similar absorption band. Figure 4 shows the AFM pictures of the gemini/PTC monolayers. Since the monolayers showed no significant transition region in the isotherms, the film was transferred at 15 mN/m. Short nanorods are essentially found for the complex films with short spacer. For the film with long spacer, rectangular domains are found. When compressed, the domains become closely packed, but the basic shape of the domains do not change (Figure 4g). In some cases, both the rectangular and square domains coexist in the film. Enlargement of the rectangular domain reveals that there existed some holes in each domain (Figure 4h). It is obvious that the surface morphologies
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Figure 4. AFM images of complex films deposited from 2 × 10-6 M PTC aqueous subphase for G2 (a), G4 (b), G6 (c), G8 (d), G10 (e), and G0 (f) at 15 mN/m and G10 at 25 mN/m (g), respectively. Those images are 5 µm × 5 µm in size. The image of h, which is the amplified image of a nanorectangle, and the inset image of a is 500 nm × 500 nm in size.
Figure 5. Surface pressure-molecular area (π-A) isotherms of G2 (a), G10 (e), and their mixtures with different ratios (b, 4:1; c, 1:1; d. 1:4) spread on aqueous solution containing 2 × 10-6 M MTC at 20 °C. The dashed line shows the π-A isotherm of G2/G10 of 1:1 ratio spread on plain water at 20 °C.
of gemini/PTC monolayers are completely different from those of gemini/MTC. Such differences could be due to the different structure. The phenyl group in the 9-position causes a strong π-π interaction and results in the J-aggregation of PTC and the relatively unchangeable morphologies. 3.4. Control of the Aggregation through Mixing of the Gemini Amphiphiles. It has been clearly shown from the above that the aggregation of MTC is controlled by the gemini amphiphiles with different spacers. Particularly, both H-aggregation and J-aggregation could be selectively formed when G2 and G10 are employed, respectively. To further clarify this, we have studied the adsorption and aggregation of the MTC on the mixed monolayers of G2 and G10 with various ratios. Figure 5 shows the π-A isotherms of mixed G2/G10 monolayer on the MTC subphase in comparison with that on the plain water surface. On the water surface, no transition is observed, while obvious transition is observed for the mixed monolayer on the MTC subphase. The isotherms are essentially the combination of both G2 and G10 isotherms on MTC with different molar ratios. The mixed complex monolayers were also transferred to solid substrate, and their aggregation as well as the morphologies was investigated by UV-vis spectra and AFM, respectively. Figure 6 shows the UV-vis spectra of the transferred multilayer film on quartz plates at 5 and 15 mN/m, respectively. Three basic absorption bands are observed at 486, 515, and 630
nm in these spectra, which could be assigned to H2, H1aggregate, and J-aggregate, respectively. A control over the aggregation by the gemini amphiphile is seen clearly from the spectra. When G2 is excess in the mixed films, H-aggregate is predominantly formed. When the ratio of G2/G10 is 1:1, a shoulder H2-band, H1-aggregate, and J-aggregate band coexist. When G10 is in excess, the J-band predominates and the H-band disappears. The surface pressure has also an effect on the aggregation. Although, for the G2/MTC film, the H2-band still existed at a higher surface pressure, the band disappeared when the surface pressure was increased after mixing with G10. In addition, the H1 band obviously appeared in any of the mixed films. The mixing effect is also clear in the AFM, as shown in Figure 7. When the molar fraction of G2 is higher, the AFM image is similar to that of G2, while the AFM image is similar to that of G10 when its molar fraction is high. When the molar ratio of G2/G10 is 1:1, we can see clearly the domains belong to different aggregates. That is, both network domains and ribbon domains coexist in the films. When the film was compressed to collapse, nanofiber structure is observed for all the films. In all these cases, the nanofibers are larger than those of the individual gemini amphiphile and MTC. Discussion The adsorption and aggregation of dyes on the Langmuir monolayers is an interesting topic and has been investigated previously by several groups.25-29 However, most of the investigations are limited to certain conventional amphiphiles or surfactants. In all these cases, the aggregation control of the dye was limited to the concentration or different matrix molecules. In our case, the gemini amphiphiles are used and a good control over the aggregation through the hydrophobic spacer is shown. In addition, many interesting features such as the spacer-modulated aggregation and formation of network structure or the nanofibers are observed. These different features are considered to be strongly related to the dye structure and their interaction with the gemini amphiphiles. Dye structure is a very important factor in the aggregation. It has been reported that the meso-substitution in the cyanine dyes plays an important role in their aggregation. While 9-mesoethylthiacarbocyanine dye favors J-aggregation, the 9-mesomethyl one prefers H-aggregation.26,44 In the present case, the steric hindrance by the phenyl group in the meso-substitutent
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Figure 6. UV-vis spectra of 10-layer films of G2(a), G10(e), and their mixture with different ratios (b, 4:1; c, 1:1; d. 1:4) deposited from aqueous subphase containing MTC at 5 (A) and 15 mN/m (B), respectively.
Figure 7. AFM images of complex films of G2/G10 ) 4:1 (a-c), G2/G10 ) 1:1 (d-f), G2/G10 ) 1:4 (g-i) transferred at 5, 15, and 47 mN/m, respectively. The images are 5 µm × 5 µm in size, while the inset images are 500 nm × 500 nm in size, respectively.
is so strong that PTC molecules are impossible to arrange in a face-to-face mode. Consequently, only J-aggregate was obtained either in the solution or in the adsorbed films. MTC is flexible due to its weak steric hindrance, and various kinds of aggregates could be formed under certain conditions. When dyes interact with the amphiphiles, there is a competence between the dye/ dye and dye/amphiphiles interactions. When the interaction between the dyes is stronger, the effect of the amphiphile will be weak. This is the case of PTC. It always formed J-aggregation in the film, escaping from the control of the amphiphiles. It seems that the interaction between MTC molecules, given the lack of a benzene group, is relatively weak and the amphiphiles show a control on its aggregation. Therefore, we observe both H- and J-aggregate in the film when different gemini amphiphiles are applied. During the interaction of the gemini amphiphiles with MTC, the spacer of the gemini plays a very important role. There exists a match between the size of the dye and the distance between the charge centers of the gemini amphiphiles. According to the
CPK model, the distances between the two positively charged ammonium groups are estimated to be 0.38, 0.63, 0.89, 1.14, and 1.40 nm for G2, G4, G6, G8, and G10, respectively, assuming that the alkyl spacer is parallel to the water surface. On the other hand, the dye is a rigid molecule and can be regarded as a brick. The net charge of MTC is negative. The distance of the two SO3- end groups, which have the largest negative charge density, is about 0.65 nm. When G2 and G4 were used, the two charge centers are so short that the two ends of the MTC dye cannot match the spacer. In this case, the dye prefers to adsorb on the monolayer using one end of its headgroup, as shown in Scheme 2a (left). When the spacer length is larger than C6, both ends of one dye molecule can directly attach to the two positive charge centers of the gemini amphiphile monolayer, resulting in the J-aggregation, as shown in Scheme 2b (right). In the intermediate spacer length, both of these modes could happen and thus we obtain the mixed H1aggregate, and J-aggregate. Previously, we have found that gemini amphiphiles have an effect on the aggregation of a
Regulation of Cyanine Dyes on Monolayers SCHEME 2: Illustration of the Spacer-Controlled Dye Aggregation
selenacarbocyanine dye, 3,3-disulfopropyl-9-methylselenacarbocyanine, SeCy.39 However, in that case, we could only get the J-aggregate although the morphologies were different for the gemini amphiphiles with different spacers. In addition, such control was only obvious for the case of G2. In the present case, a continuous change of the aggregates as well as the morphologies as a function of the spacer of the gemini amphiphiles is observed. Particularly, H-aggregates could be formed. This may be explained by the larger aromatic conjugate system in MTC than SeCy, which has a stronger π-π interaction and favors H-aggregation. As for the case of G0, since the charge distribution of G0 is relative uniform, both of the modes are possible, therefore, we could obtain both H1-aggregate and J-aggregate in the film. The control of the aggregation is not limited to the individual gemini amphiphiles, when the two extremes of the gemini amphiphiles with shortest and longest spacer are mixed, they still show a control on the dye aggregation individually. Because in the gemini amphiphiles the positive charges are localized in the inner part of the spacer, it is suggested that the dye will first adsorb at these sites. Although the two gemini amphiphiles are mixed, the charge centers will not be mixed. Thus, the MTC will absorb on the different sites of the geminis. The MTC molecules adsorbing under the G2 molecules prefer to Haggregate, while those interacting with G10 to J-aggregate. Thus, the whole mixed system will be like the mixture of two kinds of aggregates. Yamaguchi et al. have investigated systematically the mixing effect of different cyanine dyes and revealed the characteristics of various types of mixed aggregates.33a According to the spectra and the AFM images in our case, it seems that mosaic type aggregates (M-aggregate) are formed when the mixture of G2 and G10 are applied to the MTC dye. It is interesting that only using the same dye molecule, we could get different types of aggregates. In addition, a lot of highly oriented straight nanofibers are observed after the film is compressed to collapse. The nanofibers are considered to be formed by the curl of the monolayers during the collapse, similar to those results showing the formation of one-dimensional nanostructure in the LB films.45,46 It is interesting to note that the ability to form this kind of nanofiber shows a dependence on the amphiphile structure: nanofibers are observed for the collapsed films composed of shorter-spacered gemini, but not for the longer spacered one. From the results on the π-A isotherms and the aggregation, it is supposed that the alkyl spacer is stretched to parallel to water but could curve upward from G6. Thus, for the complex monolayers with shorter gemini spacers, the spacer is parallel to the water and the mixed film could be curled upon compression. For the complex film with longer spacer, it seemed that the curved spacer could prevent the film from rolling upon compression and no nanofiber is formed. Therefore, we observe nanofiber structure for G2/ MTC and G4/MTC films and less fiber structure for G6/MTC film, while no naofibers for G8-G10/MTC films.
J. Phys. Chem. B, Vol. 111, No. 31, 2007 9307 Conclusion The regulation on the aggregation and morphology of two kinds of cyanine dyes via gemini amphiphiles is revealed. In the case of MTC, a significant dependence of the aggregation on the spacer length of the gemini amphiphile was found. Amphiphiles G2 or G4 with short spacer acted as the bridge to combine neighboring MTC molecules to stack face-to-face and form H-aggregates. When the spacer is as long as G8 or G10, two neighboring MTC molecules have to tilt greatly to match with the geometry of the amphiphiles and only J-aggregates were observed. In the case of G6, the spacer length is moderate so that two interaction modes are possible; hence, H1-aggregate and J-aggregate were observed. The mixture of G2/G10 induced both H1-aggregate and J-aggregate, and the intensity of H1aggregate or J-aggregate is affected by mixed molar ratio and surface pressure. It is suggested that the G2 and G10 form mosaic type mixture domains in the complex film and H1aggregate and J-aggregates were induced in separate domains. Network structure and nanofibers were obtained for the gemini/ MTC films transferred below the plateau surface pressure and close to the collapse pressure, respectively. Collapse of the amphiphile/MTC complex film induced the formation of nanotubes when the spacer of the gemini amphiphiles are short, while no such structure was obtained when gemini amphiphiles with longer spacer were applied. In the case of PTC, because of larger steric hindrance in 9-position, PTC molecules tend to stack into J-aggregate and show no dependence on the amphiphiles used. The present work presented an easy way to regulate the aggregation as well as the morphology via gemini amphiphiles at the interface. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20533050 and 50673095), the Basic Research Development Program (Grant 2007CB808005), and the Fund of the Chinese Academy of Sciences. References and Notes (1) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (2) Claessens, C. G.; Stoddart, J. F. J. Phys. Org. Chem. 1997, 10, 254. (3) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148. (4) Moore, J. S. Curr. Opin. Colloid Interface Sci. 1999, 4, 108. (5) Crego-Calama, M.; Reinhoudt, D. N.; ten Cate, M. G. J. Templates Chem. II 2005, 249, 285. (6) Ishi-i, T.; Shinkai, S. Supermolecular Dye Chemistry; SpringerVerlag: Berlin, 2005; Vol. 258, p 119. (7) Tokuhisa, H.; Hammond, P. T. AdV. Funct. Mater. 2003, 13, 831. (8) Wang, L. L.; Yoshida, J.; Ogata, N. Chem. Mater. 2001, 13, 1273. (9) Era, M.; Adachi, C.; Tsutsui, T.; Saito, S. Chem. Phys. Lett. 1991, 178, 488. (10) (a) Jelly, E. E. Nature (London) 1936, 138, 1009; 1937, 139, 631. (b) Scheibe, G. Angew. Chem. 1936, 49, 563; 1937, 50, 212. (11) Wiltberger, M.; Sharma, R.; Penner, T. L. Langmuir 1992, 8, 2639. (12) Tani, T.; Suzumoto, T.; Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1992, 96, 2778. (13) Muenter, A. A.; Brumbaugh, D. V.; Apolito, J.; Horn, L. A.; Spano, F. C.; Mukamel, S. J. Phys. Chem. 1992, 96, 2783. (14) Nuesch, F.; Moser, J. E.; Shklover, V.; Gratzel, M. J. Am. Chem. Soc. 1996, 118, 5420. (15) Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 209. (16) Kashida, H.; Asanuma, H.; Komiyama, M. Supramol. Chem. 2004, 16, 459. (17) Daltrozzo, E.; Gschwind, K.; Haimerl, F. Photogr. Sci. Eng. 1974, 18, 441. (18) James, T. H., Ed. The Theory of the Photographic Process, 4th ed.; Macmillan: New York, 1977. (19) Rubtsov, I. V.; Ebina, K.; Satou, F.; Oh, J. W.; Kumazaki, S.; Suzumoto, T.; Tani, T.; Yoshihara, K. J. Phys. Chem. A 2002, 106, 2795.
9308 J. Phys. Chem. B, Vol. 111, No. 31, 2007 (20) Lanzafame, J. M.; Muenter, A. A.; Brumbaugh, D. V. Chem. Phys. 1996, 210, 79. (21) (a) Wang, M. M.; Dilek, I.; Armitage, B. A. Langmuir 2003, 19, 6449. (b) Garoff, R. A.; Litzinger, E. A.; Connor, R. E.; Fishman, I.; Armitage, B. A. Langmuir 2002, 18, 6330. (c) Wang, M. M.; Silva, G. L.; Armitage, B. A. J. Am. Chem. Soc. 2000, 122, 9977. (22) Urbanova, M.; Setnicka, V.; Kral, V.; Volka, K. Biopolymers 2001, 60, 307. (23) Horng, M.; Quitevis, E. L. J. Phys. Chem. 1993, 97, 12408. (24) Liu, M. H.; Kira, A.; Nakahara, H. J. Phys. Chem. 1996, 100, 20138. (25) Fukushima, Y. Bull. Chem. Soc. Jpn. 1996, 69, 1719. (26) (a) Saito, K.; Ikegami, K.; Kuroda, S.; Saito, M.; Tabe, Y.; Sugi, M. J. Appl. Phys. 1990, 68, 1968. (b) Saito, K.; Ikegami, K.; Kuroda, S.; Saito, M.; Tabe, Y.; Sugi, M. J. Appl. Phys. 1991, 69, 8291. (c) Saito, K.; Ikegami, K.; Kuroda, S.; Saito, M.; Tabe, Y. J. Appl. Phys. 1992, 71, 1401. (27) (a) Bliznyuk, V. N.; Kirstein, S.; Moehwald, H. J. Phys. Chem. 1993, 97, 569. (b) Kirstein, K.; Steitz, R.; Garbella, R.; Moehwald, H. J. Chem. Phys. 1995, 103, 818. (28) (a) Vranken, N.; Van der Auweraer, M.; De Schryver, F. C.; Lavoie, H.; Be’langer, P.; Salesse, C. Langmuir 2000, 16, 9518. (b) Vranken, N.; Van der Auweraer, M.; De Schryver, F. C.; Lavoie, H.; Salesse, C. Langmuir 2002, 18, 1641. (c) Vranken, N.; Foubert, P.; Koehn, F.; Gronheid, R.; Scheblybin, I.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2002, 18, 8407. (a) Tian, C. H.; Zoriniants, G.; Gronheid, R.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2003, 19, 9831. (b) Tian, C. H.; Liu, D. J.; Gronheid, R.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2004, 20, 11569. (29) Ono, S.; Yao, H.; Matsuoka, O.; Kawabata, R.; Kitamura, N.; Yamamoto, S. J. Phys. Chem. B 1999, 103, 6909. (30) Ray, K.; Nakahara, H. J. Phys. Chem. B 2002, 106, 92. (31) (a) Penner, T. L.; Moebius, D. J. Am. Chem. Soc. 1982, 104, 7407. (b) Vaidyanathan, S.; Patterson, L. K.; Moebius, D.; Gruniger, H. R. J. Phys. Chem. 1985, 89, 491. (32) (a) Nakahara, H.; Moebius, D. J. Colloid Interface Sci. 1986, 114, 363. (b) Nakahara, H.; Fukuda, K.; Moebius, D.; Kuhn, H. J. Phys. Chem. 1986, 90, 6144.
Zhang et al. (33) (a) Yamaguchi, A.; Kometani, N.; Yonezawa, Y. J. Phys. Chem. B 2005, 109, 1408. (b) Hada, H.; Hanawa, R.; Haraguchi, A.; Yonezawa, Y. J. Phys. Chem. 1985, 89, 560. (34) (a) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (b) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (c) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (35) (a) Zana, R. AdV. Colloid Interface Sci.2002, 97, 205. (b) Zana, R. J. Colloid Interface Sci. 2002, 248, 203. (c) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (d) Zana, R.; Talmon, Y. Nature (London) 1993, 362, 228. (e) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Karthauser, J.; Van Os, N. M.; Zana, R. Science (Washington, D. C.) 1994, 266, 254. (36) (a) Oda, R.; Huc, I.; Candau, S. Chem. Commun. (Cambridge) 1997, 2105. (b) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. 1998, 37, 2689. (c) Faure, D.; Gravier, J.; Labrot, T.; Desbat, B.; Oda, R.; Bassani, D. M. Chem. Commun. (Cambridge) 2005, 1167. (37) Kirby, A. J.; Camilleri, P.; Engberts, J. B. F. N.; Feiters, M. C.; Nolte, R. J. M.; Soderman, O.; Bergsma, M.; Bell, P. C.; Fielden, M. L.; Rodriguez, C. L. G.; Guedat, P.; Kremer, A.; McGregor, C.; Perrin, C.; Ronsin, G.; van Eijk, M. C. P. Angew. Chem., Int. Ed. 2003, 42, 1448. (38) Muller, P. U.; Akpo, C. C.; Stockelhuber, K. W.; Weber, E. AdV. Colloid Interface Sci. 2005, 114, 291. (39) Zhang, G. C.; Zhai, X. D.; Liu, M. H. J. Phys. Chem. B 2006, 110, 10455. (40) Zhai, X. D.; Zhang, L.; Liu, M. H. J. Phys. Chem. B 2004, 108, 7180. (41) Zhang, Y. Z.; Xiang, J. F.; Tang, Y. L.; Xu, G. Z.; Yan, W. P. Dyes Pigm., in press. (42) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (43) Chen, X.; Wang, J.; Shen, N.; Luo, Y.; Li, L.; Liu, M.; Thomas, R. K. Langmuir 2002, 18, 6222. (44) Sturmer, D. M. In Special Topics in Heterocyclic Chemistry; Weissberger, A., Taylor, E. C., Eds.; Wiley: New York, 1977; Chapter 8. (45) Guo, L.; Wu, Z. K.; Liang, Y. Q. Chem. Commun. (Cambridge) 2004, 1664. (46) Gao, P.; Liu, M. H. Langmuir 2006, 22, 6727.
