J. Phys. Chem. 1996, 100, 5949-5955
5949
Aggregation of Amphiphilic Squaraines at the Air-Water Interface and in Langmuir-Blodgett Films Huijuan Chen,† Kock-Yee Law,*,‡ and David G. Whitten*,† NSF Center for Photoinduced Charge Transfer, Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Wilson Center for Research and Technology, Xerox Corporation, 800 Phillips Road, 114-39D, Webster, New York 14580 ReceiVed: December 18, 1995X
A series of amphiphiles incorporating the squaraine chromophore (1-3) has been synthesized and these amphiphiles have been studied as films at the air-water interface and supported Langmuir-Blodgett (LB) films on glass. For the spread films at the air-water interface, aggregate formation is observable even at very low surface pressures and in relatively diluted mixtures; in certain cases the type of aggregate formed is sensitive to the surface pressure. The most frequently encountered spectrally blue-shifted or H-aggregate (λmax ) 530 nm, compared to the monomer, λmax ) 630 nm), is attributed to a “unit aggregate” which we have shown previously to be a cyclic, chiral tetramer. The extended aggregate in compressed films is thus a mosaic of these unit aggregates which exist even before compression. For certain squaraine amphiphiles and mixtures we obtain evidence for a second species which has spectral characteristics consistent with the red-shifted or J-aggregate, and they form only under compression. This species is metastable in several cases and can be converted to the H-aggregate under a variety of conditions. The relationships between amphiphile structure, microenvironment, aggregates formed, and aggregate stability are discussed.
Introduction Aggregation has been observed as a general phenomenon for a wide variety of aromatic compounds and dyes.1 It is frequently encountered in crystals and in organized media such as Langmuir-Blodgett (LB) films where the close packing of chromophores with certain favored orientations leads to strong coupling of the interacting dipoles and distinct changes in spectral and photophysical behavior.2-4 There are two limiting types of aggregates based on the orientation of transition dipoles and their spectral characteristics. The J-aggregate, with “headto-tail” transition dipole arrangements, is characterized by a sharp, intense, and red-shifted absorption compared to the monomer electronic transition;5-7 the second type is the H-aggregate, which is characterized by “head-to-head” transition dipole arrangements and shows a blue shift of the prominent transition compared with the monomer.8 The correlation between spectral features and the aggregation structure has been developed through the exciton models developed by Kasha and Hochstrasser,9,10 and by Kuhn and co-workers;11,12 the latter generally gives better agreement with experimental results. Typical compounds forming “J” (red-shifted) aggregates are cyanines and derivatives and have the transition moments along the long molecular axis and substituents which force the molecule to pack in a “head-to-tail”, “brickstone”, or “herringbone” fashion.13,14 In contrast, amphiphilic trans-stilbenes,15 trans-azobenzenes,16 and hemicyanine derivatives,17,18 in which the chromophore has the transition dipoles arranged more or less along the amphiphile backbone, tend to form blue-shifted H-aggregates.19 An interesting case is the aggregation of porphyrin derivatives, which is complicated by the two transition dipoles of the Soret band that are in the porphyrin plane but perpendicular to one another. Blue-shifted, or red-shifted transitions, or both can be observed depending on the orientation of the porphyrin chromophores and the alignment of transition dipoles.20 †
University of Rochester. Xerox Corp. X Abstract published in AdVance ACS Abstracts, March 15, 1996. ‡
0022-3654/96/20100-5949$12.00/0
Squaraines have been the subject of many investigations due to their unusual electronic structures as well as their properties as semiconductive and photogeneration materials in electrophotographic applications.21 Studies by us as well as by others show that squaraines can form different aggregates in solution,22,23 in microheterogeneous media,24,25 in LB films,26-28 and also in microcrystals.29,30 In terms of aggregate structures, X-ray structural data indicate that bis(4-methoxyphenyl)squaraine crystal exhibits a translational layer structure in which squaraine chromophores are in a near “card-pack” array, resulting in a blue-shifted solid state absorption relative to the monomer absorption.31 It was shown that bis(2-methyl-4-(N,Ndimethylamino)phenyl)squaraine adopts a “slipped stack” arrangement of chromophores which causes the solid state absorption to be broad and red-shifted relative to solution absorption.32 In order to gain a fundamental understanding of the aggregation of squaraines and the subsequent effects on the optical, photophysical, and photoelectrochemical properties, we have structurally modified the squaraine chromophore such that it can form different types of aggregates in solution and in organized media such as LB films.24,27,33 Since LB techniques are particularly suited for studying the structural and functional behavior of dye aggregates as they provide highly ordered environments for amphiphilic molecules,34 we focus on the aggregation behavior of different functional squaraines at the air-water interface and in LB films in the present paper. The squaraine structures used in the present study are shown in Scheme 1. One type of squaraines is squaraine acids (1a-1d and 2a-2b), in which the squaraine chromophore is embedded in an amphiphilic structure with a carboxylic acid head group. A second type includes alkyl substituted squaraines (3a-3b). These squaraines have been found to form different types of aggregates in LB films depending on the specific individual structure and the conditions used. This systematic study has led to a better understanding of the tendency of the squaraine chromophores to interact and aggregate. © 1996 American Chemical Society
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SCHEME 1: Structures of Squaraines
Experimental Section Materials. The synthesis of squaraines 1, 2, and 3 has been reported elsewhere.25,27 Synthetic reagents were generally purchased from Aldrich and used as received. Cadmium chloride (99.99%) and sodium bicarbonate (99%) were purchased from Aldrich. The spreading solvent for monolayer work was pentene-stabilized HPLC Grade chloroform from Fisher. Suitable water was obtained by passing purified in-house distilled water through a Millipore-RO/UF water purification system. General Techniques. Squaraine monolayers were obtained by spreading chloroform solutions of squaraines (ca. 10-4 M) onto an aqueous subphase, which contained cadmium chloride (2.5 × 10-4 M) and sodium bicarbonate (3 × 10-5 M) in a KSV 5000 film balance at room temperature (ca. 20 °C). Precleaned35 hydrophilic glass substrates were used for the deposition of monolayers. Absorption spectra for monolayers at the air-water interface were recorded in situ with a SD 1000 fiber optics spectrometer (Ocean Optics, Inc.) equipped with optical fibers, a LS-1 miniature tungsten halogen lamp, and a CCD detector. Absorption spectra for both solution and LB films were obtained on a Hewlett-Packard 8452A diode array spectrophotometer. Results Squaraine Acids (1 and 2). Squaraines 1a-d and 2a-b all form stable monolayers at the air-water interface when spread from evaporating chloroform solutions over water at pH 6; the collapse pressures of these squaraine monolayers are 4045 mN/m while limiting areas of 38-45 Å2/molecule are observed. The surface pressure-area isotherm and the hysteresis behavior for a typical squaraine, such as 1a, has been reported elsewhere.26 Generally, the monolayers of most of these squaraines at the air-water interface show absorption dominated by transitions near 530-540 nm that are blue-shifted compared to the monomer absorption (∼630-635 nm in CHCl3), and no changes in shape and absorption maxima with changes of surface pressure. Figure 1 shows the absorption spectra of monolayers of 2a at the air-water interface at various surface pressures. It is noteworthy that the blue-shifted spectrum is observed even at very low surface pressures (2 mN/m) where little “forced” packing of the chromophores is anticipated. Mixed monolayers of these squaraines (such as 2a) and stearic acid can also be prepared. However, dilution of the squaraine monolayer of 2a with stearic acid (up to 1:50 dye:stearic acid) shows no obvious effects on the shape and absorption maxima of the blue-shifted aggregates at the air-water interface. An exception to these aggregation behaviors is observed for squaraine 1d. Surprisingly, the surface absorption spectrum of
Figure 1. Absorption spectra of monolayers of squaraine 2a at the air-water interface: (a) 2 mN/m; (b) 20 mN/m; and (c) 30 mN/m.
Figure 2. Absorption spectra of monolayers of squaraine 1d at the air-water interface at various surface pressures.
1d is relatively sharp and the absorption maximum is shifted further to the blue as the surface pressure increases (Figure 2). The compressed films for squaraines 1 and 2 at the air-water interface can be readily transferred to solid supports such as glass, quartz, or optically transparent tin oxide electrodes to form supported monolayers or multilayers. Transfer ratios are close to unity, and multilayer assemblies containing up to 11 layers of pure squaraines 1a, 1b, 2a, and 2b can easily be prepared with good reproducibility, while monolayers of both 1c and 1d can also be transferred to solid supports successfully. Most of the transferred squaraine monolayers except for that of 1d show relatively sharp absorption at 520-530 nm and no detectable fluorescence. Similar to the surface monolayer, the mixed monolayer of squaraine and stearic acid (dilution to a 1:10 or 1:20 1a/stearic acid molar ratio) shows only a small decrease in the absorption at 530 nm and little increase in the monomer absorption with a weak fluorescence (∼660 nm) corresponding to the monomer. The blue-shifted aggregates observed for the transferred films are similar to those found at the air-water interface for most of 1a-c and 2a-b squaraines. Contrasting results are obtained for 1d when comparing the spectral properties of this molecule at the air-water interface and in the LB film (Figure 3). A broad red-shifted spectrum with a dominant peak at 660 nm is observed for 1d in the LB film, which is in contrast to the sharp blue-shifted absorption (506 nm) observed at the air-water interface. As mentioned above, squaraines 1 and 2 except 1d all form blue-shifted aggregates in the LB films. These blue-shifted aggregates can be converted to the red-shifted ones by physical treatment. For example, we have previously shown that the blue-shifted aggregate (530 nm) of 1a can be converted to a red-shifted aggregate (655 nm) upon heating at 110 °C over a period of 2 h.26 The blue- to red-shifted aggregate conversion is evidently quite clean as indicated by the presence of an
Aggregation of Amphiphilic Squaraines
J. Phys. Chem., Vol. 100, No. 14, 1996 5951
Figure 3. Absorption spectra of monolayers of squaraine 1d (a) at the air-water interface and (b) on a glass substrate.
isosbestic point during the spectral changes.26 This red-shifted aggregate can be partially converted back to the blue-shifted aggregate at 530 nm when it is treated with steam. Similar phenomena are observed for squaraines 1b and 1c. Compared with the previous results on 3a,28 which showed an almost full conversion of the blue-shifted aggregate (530 nm) in the LB film of 3a to a red-shifted aggregate (660 nm) or a J-aggregate (690 nm) by thermal treatments and the reversal of the redshifted and J-aggregate upon steam treatment,28 the conversion for squaraines 1 and 2 is much slower and far less complete even under stronger conditions. In addition, the J-aggregate at 690 nm reported for 3a is not observed for 1a-c. For squaraines 2a and 2b, this process is even more difficult; only a very small amount of the blue-shifted aggregate is converted to the red-shifted one and this is followed by thermal decomposition of the squaraine chromophore. As discussed earlier, both 1b and 1c form blue-shifted aggregates in the pure squaraine monolayer, either at the airwater interface or on solid supports. These two squaraines have very similar structures except for their different chromophore positions in the hydrophobic chain. Presumably, a 1:1 mixture of 1b and 1c should permit an offset arrangement of squaraine chromophores for at least some of the molecules in films if the packing is random. The surface pressure-area isotherms for the pure and 1:1 mixed monolayers of 1b and 1c are shown in Figure 4, part 1. The 1:1 mixed monolayer has a relatively lower collapse surface pressure and somewhat larger mean molecular area than those of the pure squaraine monolayers. Surface absorption spectroscopic studies on the mixed monolayers clearly show the formation of the red-shifted aggregate with maxima at 612 and 670 nm (Figure 4, part 2). The redshifted absorption is not present for either 1b or 1c individually. As the 1:1 mixture of 1b and 1c is compressed on the water surface, the intensity of the red-shifted absorption increases as the surface pressure increases, while the shapes of these absorptions remain almost the same (Figure 5, a,b and d,e). We attribute the red-shifted absorption to an “offset” dimer or aggregate since it absorbs in the same wavelength as the heat-induced red-shifted aggregate and those reported in the literature and shows no fluorescence.26-28 As shown in Figure 5 c, f, and g, the red-shifted transitions slowly disappear from the film during a hysteresis cycle of compressiondecompression-recompression, and the frequently observed blue-shifted aggregate transition (530 nm) grows in and remains stable. The 1:1 mixed monolayer of 1b/1c at the air-water interface can also be transferred to solid substrates. The absorption spectrum of the mixed LB film (Figure 6) shows the coexistence of the blue-shifted (530 nm) and the red-shifted (670 nm) aggregates in comparison with pure 1b or 1c LB film where only the blue-shifted aggregate at 530 nm exists. It is observed
Figure 4. (1) Surface pressure-area isotherms of squaraine monolayers (a) pure 1b, (b) pure 1c, and (c) 1b/1c 1:1 mixture; (2) absorption spectra of squaraine monolayers at various surface pressure, (a) to (d) 1b/1c (1:1) monolayers (a, upon spread; b, 2 mN/m; c, 10 mN/m; and d, 26 mN/m); and (e) monolayer of 1b at 30 mN/m.
Figure 5. Absorption spectra of mixed monolayer of 1b/1c (1:1) at the air-water interface (a) upon spread; (b) to (c) 26 mN/m, t ) 0, and t ) 45 min, respectively; (d) monolayer in (c) relaxed to 2 mN/m; (e) to (g) second compression to 26 mN/m, t ) 0 min, t ) 15 min, and t ) 60 min, respectively.
that the blue-shifted aggregate of the mixed film can be converted to the red-shifted one (670 nm) easily under mild heating (70 °C, 5 min), which is in contrast to the difficult heat induced conversion of the blue-shifted aggregate to the redshifted one for either of the pure squaraine 1b or 1c LB films.
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Figure 6. Absorption spectra of squaraine monolayers on glass (a) 1b as prepared; (b) 1b/1c (1:1) as prepared; (c) sample in (b) heated at 70 °C for 5 min.
Here again there is substantial change in the spectra obtained at the air-water interface and in the transferred, supported layers. Alkyl-Substituted Squaraines (3a and 3b). Structurally, squaraines 3a and 3b both have two octadecyl groups as hydrophobic chains but differ in the substitution pattern, which presumably can cause a change in the chromophore orientation. Previous studies showed that both 3a and 3b form stable monolayers at the air-water interface with mean molecular areas of 52 and 60 Å2, respectively.27 The monolayers for both 3a and 3b can be transferred to solid substrates successfully with transfer ratios close to unity. The transferred film of 3a has a blue-shifted absorption with absorption maximum at 530 nm relative to the monomer (∼634 nm in CHCl3), while the absorption spectrum for the LB film of 3b is broad with a dominant red-shifted peak at 660 nm.27 As mentioned above, previous studies showed that the conversion of the blue-shifted aggregate (530 nm) of squaraine 3a to a red-shifted aggregate (660 nm) and a J-aggregate (690 nm) can be achieved by thermal treatment; this process can be reversed by steam treatment of the heated film.28 In the present study, the absorption spectra of monolayers of 3a and 3b at the air-water interface are examined. The monolayer of squaraine 3a at the air-water interface shows a blue-shifted absorption (534 nm) similar to that of the transferred film, even at very low surface pressure. In contrast, an interesting phenomenon is observed for squaraine 3b. Figure 7, a and b, shows the pressure-area isotherm and surface absorption spectrum of squaraine 3b, respectively. At low surface pressures (10 nm at shorter wavelength as compared to other blue-shifted aggregates. When this blue-shifted aggregate is transferred to solid support, it rearranges to the red-shifted aggregate. The occurrence of the rearrangement is probably due to the so-called free area effect and the drier environment in the LB assembly on the solid support. The most exciting demonstration of the architectural and structural effects on the aggregation is observed from the 1:1 mixture of 1b/1c. These two compounds form blue-shifted aggregates as pure monolayers. The formation of the red-shifted aggregate in the 1:1 mixed monolayer at the air-water interface presumably results from a slipped stack arrangement enabled by the “offset” location of the two chromophores in the mixture. While the red-shifted species is the stable aggregate at the airwater interface, rearrangement occurs during the film transfer where a mixture of red- and blue-shifted aggregates are obtained in the supported LB film. Upon mild heating, a pure red-shifted aggregate results. The observation is consistent with an explanation that as water is driven away thermally, intermolecular charge transfer becomes the controlling force and redshifted aggregates are formed. Acknowledgment. The authors thank the National Science Foundation for the financial support of this work as part of the NSF Center for Photoinduced Charge Transfer (CHE-9120001). References and Notes (1) See, for example: Yuzhakov, V. I. Russ. Chem. ReV. (Engl. Transl.) 1979, 48, 1076. Rabinowitch, E. J. Am. Chem. Soc. 1941, 63, 69. Mataga,
Aggregation of Amphiphilic Squaraines N. Bull. Chem. Soc. Jpn. 1957, 30, 375. West, W.; Pearce, S. J. Phys. Chem. 1965, 69, 1894 and references therein. (2) Kuhn, H. In Light-Induced Charge Separation in Biology and Chemistry; Geischer, H., Katz, J. J., Eds.; Dahlem Konferenzen: West Berlin, 1979; p 151. (3) Gilman, P. B. In Photographic SensitiVity; Cox, R. J., Ed.; Academic Press: London, 1973; p 187. (4) Sturmer, D. M. In Special Topic in Heterocyclic Chemistry; Weissberger, A., Taylor, E. C., Eds.; Wiley: New York, 1977; p 540. (5) Vaidyanathan, S.; Patterson, L. K.; Mobius, D.; Gruniger, H. R. J. Phys. Chem. 1985, 89, 491. (6) Nakahara, H.; Fukuda, K.; Mobius, D.; Kuhn, H. J. Phys. Chem. 1986, 90, 6144. (7) Duschl, C.; Kemper, D.; Frey, W.; Meller, P.; Ringsdorf, H.; Knoll, W. J. Phys. Chem. 1989, 93, 4587. (8) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502. (9) Kasha, M.; El-Bayoumi, M. A.; Rhodes, W. J. Chim. Phys. 1961, 58, 916. (10) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317. (11) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (12) Czikkely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (13) Kuhn, H.; Mo¨bius, D. Angew. Chem., Int. Ed. Engl. 1972, 10, 620. (14) Mo¨bius, D.; Kuhn, H. J. Appl. Phys. 1988, 64, 5138. Kirstein, S.; Moehwald, H. Chem. Phys. Lett. 1992, 189, 408. (15) Furman, I.; Geiger, H. C.; Whitten, D. G.; Penner, T. L.; Ulman, A. Langmuir 1994, 10, 837. (16) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7816. (17) Heeseman, J. J. Am. Chem. Soc. 1980, 102, 2167. (18) Evans, C. E.; Song, Q.; Bohn, P. W. J. Phys. Chem. 1993, 97, 12302. (19) Chen, H.; Farahat, C. W.; Farahat, M.; Geiger, H. C.; Leihos, U. W.; Liang, K.; Song, X.; Penner, T. L.; Ulman, A.; Perlstein, J.; Law, K. Y.; Whitten, D. G. MRS Bull. 1995, XX(6), 39.
J. Phys. Chem., Vol. 100, No. 14, 1996 5955 (20) Cox, G. S. Ph.D. Thesis, University of North Carolina at Chapel Hill, 1982. (21) Law, K. Y. Chem. ReV. 1993, 93, 449. (22) Buncel, E.; McKerrow, A.; Kazmaier, P. M. J. Chem. Soc., Chem. Commun. 1992, 1242. (23) Das, S.; Thanulingam, T. L.; Thomas, K. G.; Kamat, P. V.; George, M. V. J. Phys. Chem. 1993, 97, 13620. (24) Chen, H.; Law, K. Y.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257. (25) Chen, H.; Farahat, M.; Law, K. Y.; Whitten, D. G. J. Am. Chem. Soc., submitted for publication. (26) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 5138. (27) Law, K. Y.; Chen, C. C. J. Phys. Chem. 1989, 93, 2533. (28) Liang, K.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 13379. (29) Bernstein, J.; Goldstein, E. Mol. Cryst. Liq. Cryst. 1988, 164, 213. (30) Tristani-Kendra, M.; Eckhardt, C. J. J. Chem. Phys. 1984, 81, 1160. (31) Law, K. Y. J. Phys. Chem. 1988, 92, 4226. (32) Wingard, R. E. IEEE Ind. Appl. 1982, 1251. (33) Liang, K.; Perlstein, J.; Law, K. Y.; Whitten, D. G., manuscript in preparation. (34) Kuhn, H.; Moebius, D.; Buecher, H. in Physical Methods of Chemistry; Weissberger, A., Ed.; Interscience: New York, 1972; Part IIIB, Chapter VII, p 577. (35) Quina, F.; Zhou, M., unpublished results. (36) Song, X.; Geiger, H. C.; Leinhos, U.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 10340. (37) Chen, H.; Liang, K.; Samha, H.; Song, X.; Law, K. Y.; Perlstein, J.; Penner, T. L.; Whitten, D. G. In Micelles, Microemulsion and Monolayers: Science and Technology; Shah, D., Eds.; Marcel Dekker, Inc.: New York, in press. (38) No emission can be detected for these J-aggregates formed at the air-water interface.
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