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Effect of Alcohols on J-Aggregation of a Carbocyanine Dye H. von Berlepsch,*,† S. Kirstein,‡ and C. Bo¨ttcher† Forschungszentrum fu¨ r Elektronenmikroskopie der Freien Universita¨ t Berlin, Fabeckstrasse 36 a, D-14195 Berlin, Germany, and Institut fu¨ r Physik, Humboldt Universita¨ t zu Berlin, Invalidenstrasse 110, D-10115 Berlin, Germany Received April 15, 2002. In Final Form: June 21, 2002 The morphology of J-aggregates formed by 3,3′-bis(3-carboxypropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine dye (C8O3) in aqueous solution upon addition of alcohols was characterized by spectroscopic methods, optical microscopy, and cryo-transmission electron microscopy (cryo-TEM). With increasing concentration of methanol, ethylene glycol, 1-octanol, or 1-decanol, the well-known 3-fold split excitonic absorption spectrum of the superhelical J-aggregates obtained in the absence of alcohols is replaced by a 2-fold split spectrum (von Berlepsch et al. J. Phys. Chem. B 2000, 104, 5255), while the circular dichroism signal generally increases. The spectral changes indicate alcohol-induced modifications of the aggregates’ molecular packing. Cryo-TEM revealed that these novel J-aggregates are composed of individual bilayer-walled tubules having a diameter of 11 ( 1 nm, which are assembled to form ropelike superhelices of several micrometers in length. The superhelices’ diameter is markedly increased if compared with that in the alcohol-free system.
Introduction The self-aggregation of molecules into structurally welldefined assemblies which possess distinctive photophysical and optical properties has attracted strong interest over the past years due to the great opportunities for technical applications of such materials.1 By control of the noncovalent interactions, different extended supramolecular assemblies could be designed.2-5 However, beyond the field of material science, molecular aggregates also play a fundamental role in living matter, for example, in the processes of photosynthesis.6 Biological pigments are noncovalently bound to proteins, forming so-called pigment-protein complexes. Light harvesting in plants and in bacteria is carried out by stacked protein-bound chlorophyll-carotenoid aggregates. Such aggregates serve as light-harvesting antennas absorbing light energy, which is eventually transferred over many chlorophyll molecules toward the photosynthetic reaction center. To mimic lightharvesting complexes, supramolecular assemblies of synthetic dyes, such as the well-known J-aggregates, have become popular model systems.7,8 Organic dyes, in particular cyanine dyes, are known to form tightly bound molecular assemblies in concentrated aqueous solution, which are characterized by the appearance of a new, extremely narrow, and, with respect to the monomer * To whom correspondence should be addressed. E-mail:
[email protected]. † Forschungszentrum fu ¨ r Elektronenmikroskopie der Freien Universita¨t Berlin. ‡ Institut fu ¨ r Physik, Humboldt Universita¨t zu Berlin. (1) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: Chichester, 1991. (2) Kuhn, H. Pure Appl. Chem. 1971, 27, 421. (3) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (4) von Berlepsch, H.; Mo¨ller, S.; Da¨hne, L. J. Phys. Chem. B 2001, 105, 5689. (5) Wu¨rthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.s Eur. J. 2001, 7, 2245. (6) Hu, X.; Schulten, K. Phys. Today 1997, No. 8, 28. (7) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517. (8) van Amerongen, H.; Valkunas, L.; van Grondelle, R. Photosynthetic Excitons; World Scientific: Singapore, 2000.
absorption, red-shifted absorption band, enabling a fast and efficient excitation energy migration over many molecules.9,10 In honor of the two researchers who first discovered this phenomenon,11,12 these molecular assemblies have been named Scheibe or Jelly (J-) aggregates. Despite a multitude of studies and data available on optical and spectroscopic properties of cyanine dyes,13 the knowledge of the supramolecular structure of the aggregates is only fragmentary14 and still an object of scientific activity.15-21 Using electron microscopy of vitrified samples (cryo-transmission electron microscopy, cryoTEM), we had previously visualized for the first time the complex morphologies of J-aggregates of different cyanine dyes.22-26 In particular, extended structural and spectroscopic studies were performed on a series of dye (9) de Boer, S.; Vink, K. J.; Wiersma, D. A. Chem. Phys. Lett. 1987, 137, 99. (10) Moll, J.; Da¨hne, S.; Durrant, J. R.; Wiersma, D. A. J. Chem. Phys. 1995, 102, 6362. (11) Scheibe, G. Angew. Chem. 1936, 49, 563; 1937, 50, 51. (12) Jelly, E. E. Nature 1936, 138, 1009; 1937, 139, 631. (13) J-Aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996. (14) Mo¨bius, D. Adv. Mater. 1995, 7, 437. (15) Wolthaus, L.; Schaper, A.; Mo¨bius, D. Chem. Phys. Lett. 1994, 225, 322. (16) Higgins, D. A.; Kerimo, J.; Vanden Bout, D. A.; Barbara, P. F. J. Am. Chem. Soc. 1996, 118, 4049. (17) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310. (18) Collings, P. J.; Gibbs, E. J.; Starr, T. E.; Vafek, O.; Yee, C.; Pomerance, L. A.; Pasternack, R. F. J. Phys. Chem. B 1999, 103, 8474. (19) Neumann, B.; Huber, K.; Pollmann, P. Phys. Chem. Chem. Phys. 2000, 2, 3687. (20) Owens, R. W.; Smith, D. A. Langmuir 2000, 16, 562. (21) Janssens, G.; Touhari, F.; Gerritsen, J. W.; van Kampen, H.; Callant, P.; Deroover, G.; Vandenbroucke, D. Chem. Phys. Lett. 2001, 344, 1. (22) von Berlepsch, H.; Bo¨ttcher, C.; Da¨hne, L. J. Phys. Chem. B 2000, 104, 8792. (23) von Berlepsch, H.; Bo¨ttcher, C. J. Phys. Chem. B 2002, 106, 3146. (24) von Berlepsch, H.; Bo¨ttcher, C.; Ouart, A.; Burger, C.; Da¨hne, S.; Kirstein, S. J. Phys. Chem. B 2000, 104, 5255. (25) von Berlepsch, H.; Bo¨ttcher, C.; Ouart, A.; Regenbrecht, M.; Akari, S.; Keiderling, U.; Schnablegger, H.; Da¨hne, S.; Kirstein, S. Langmuir 2000, 16, 5908. (26) von Berlepsch, H.; Regenbrecht, M.; Da¨hne, S.; Kirstein, S.; Bo¨ttcher, C. Langmuir 2002, 18, 2901.
10.1021/la0203640 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002
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molecules of the 5,5′,6,6′-tetrachlorobenzimidacarbocyanine chromophore having 1,1′-dialkyl substituents combined with 3,3′-bis(4-sulfobutyl), 3,3′-bis(4-carboxybutyl), or 3,3′-bis(3-carboxypropyl) substituents. For the 1,1′dioctyl-3,3′-bis(3-carboxypropyl)-substituted chromophore (C8O3), we found the most complex architecture, consisting
of ropelike assemblies built up by several helically twisted tubular strands of ∼10 nm cross-sectional diameter and a wall thickness of ∼4 nm. The wall thickness value is typical for a packing of the dye in a molecular bilayer arrangement. The chromophores are stacked within each layer in a plane-to-plane orientation, whereas the attached octyl chains are intercalated between the layers to avoid contact with the surrounding aqueous medium due to the hydrophobic effect. Bilayer formation is thus the result of the amphiphilic nature of the alkyl-substituted carbocyanine dye.24 The formation of the helical superstructure correlates with the observation of optical activity of the J-aggregate, although the single C8O3 molecule is achiral and adopts a planar conformation in solution.27,28 Another interesting feature of this derivative25 is observed when ionic surfactants are added to the Jaggregates. Single-walled tubules of ∼15 nm crosssectional diameter, ∼4 nm wall thickness, and typical lengths in the order of hundreds of nanometers are formed when the anionic surfactant sodium dodecyl sulfate (SDS) is added. These particles are not stable but completely transform into multilamellar tubules after a few days. In contrast, the addition of the cationic surfactant trimethyltetradecylammonium bromide (TTAB) yields spheroidal mono- and multilamellar vesicles, which after a few weeks again completely transform into tubular structures of ∼10 nm cross-sectional diameter and micrometer length. These morphological transformations are thought to be driven by the incorporation of the amphiphilic molecules into the bilayers of the J-aggregates by hydrophobic forces and in the case of the cationic surfactants presumably also by additional electrostatic forces.25,26 It is obvious that the accompanying changes in the optical spectra and the modified geometries indicate alterations in the internal molecular packing of the J-aggregates. The strong influence of ionic surfactants on J-aggregation raises the question of whether there are other surfaceactive additives by which the structure of aggregates could be varied continuously and controllably (“tuned”) toward new morphologies. Among a large number of possible additives, alcohols are of special interest.29 Medium-chain n-alkanols are common “cosurfactants” added to oil/water/ surfactant systems to facilitate the formation of microemulsions by partitioning of the alcohol molecules into the interfacial films.30 Short-chain alcohols such as methanol or ethanol on the other hand are often used to (27) Pawlik, A.; Kirstein, S.; De Rossi, U.; Da¨hne, S. J. Phys. Chem. B 1997, 101, 5646. (28) De Rossi, U.; Da¨hne, S.; Meskers, S. C. J.; Deckers, H. P. J. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 760. (29) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (30) Kahlweit, M. J. Phys. Chem. 1995, 99, 1281.
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prepare aqueous solutions of poorly soluble dyes. Keeping in mind that additives could have a strong influence on aggregation, common sample preparation techniques have to be examined carefully. If, for example, an alcoholic solution of the dye C8O3 is titrated into an aqueous 10-2 M sodium hydroxide solution (“alcoholic route”), a much larger circular dichroism signal is obtained,28,31 as compared to preparations in which the dye was directly dissolved in sodium hydroxide solution (“direct route”). Here the question arises of whether the formed Jaggregates are identical when prepared by the different routes, and if not, at which amount of alcohol do alterations in the molecular organization occur? The influence of polyvalent alcohols, that is, ethylene glycol or glycerol, on micellization of surfactants has been the object of many studies, and the observed effects are still not fully explained.29 Ethylene glycol (EG) is also an important additive in the spectroscopy of dyes, because mixed water/EG solvents are commonly used for optical measurements at low temperature in the glassy state,32 and an effect on aggregation cannot be excluded a priori, as spectroscopic studies on aggregated solutions of 9-ethylthiacarbocyanines revealed.33,34 The aim of the present investigations was to clarify the role of alcohols in the aggregation behavior of the C8O3 dye and the final structure of the J-aggregates. In particular, the effect of alcohol concentration and chemical structure has been addressed. Besides absorption and circular dichroism (CD) spectroscopy, the structure and morphology of the aggregates has been characterized by means of optical microscopy and cryo-TEM. Experimental Section Materials and Sample Preparation. The dye C8O3 was supplied by FEW Chemicals (Wolfen, Germany) and has been used as received. High-performance liquid chromatography (HPLC) showed no impurities. According to differential thermoanalysis, it consists of 54% dye betain (cf. formula) and 46% dye bromide. The effective molecular mass was taken to be 846 g/mol. The molar extinction coefficient in dimethyl sulfoxide is 1.67 × 105 L/(mol cm). To get homogeneous solutions with maximum formation of J-aggregates, 10-2 M NaOH was used as the solvent.24 Stock solutions of 2.8 × 10-4 M dye were prepared through stirring at room temperature for at least 24 h. For spectroscopic measurements, the solutions were diluted. Within the experimental limits of error (the scattering in absorbance is not larger than (10%), the spectra from different stock solutions were reproducible. Also, the signature of the circular dichroism signal was not influenced by the solution preparation conditions.35 All measurements were performed at 21 ( 1 °C. Methanol, ethanol, and acetone (p.a. quality) were purchased from Merck; 1-octanol and 1-decanol of 99% chemical purity were purchased from Sigma-Aldrich. EG with purity of g99.5% was obtained from Fluka. The long-chain alcohols (1-octanol and 1-decanol) were first dissolved in ethanol (22 × 10-2 M) and then added to the dye solutions up to the desired concentration using a Hamilton syringe. Methods. The absorption spectra were measured with a Lambda 9 spectrophotometer (Perkin-Elmer), and the circular dichroism spectra with a J-715 spectropolarimeter from Jasco Corp. (31) Spitz, C.; Da¨hne, S.; Ouart, A.; Abraham, H.-W. J. Phys. Chem. B 2000, 104, 8664. (32) Renge, I.; Wild, U. P. J. Phys. Chem. A 1997, 101, 7977. (33) Drobizhev, M. A.; Sapozhnikov, M. M.; Scheblykin, I. G.; Van der Auweraer, M.; Varnavsky, O. P.; Vitukhnovsky, A. G. Chem. Phys. 1996, 211, 455. (34) Scheblykin, I. G.; Drobizhev, M. A.; Varnavsky, O. P.; Van der Auweraer, M.; Vitukhnovsky, A. G. Chem. Phys. Lett. 1996, 261, 181. (35) Ouart, A. Ph.D. Thesis, Humboldt-Universita¨t, Berlin, 2000. http://dochost.rz.hu-berlin.de/dissertationen/ouart-andre-2000-09-28/ PDF/Ouart.pdf.
Effect of Alcohols on J-Aggregation The optical microscope images were obtained by using a color video camera (model TK-1070E, JVC) set on an optical microscope (model BH-2, Olympus) equipped with a phase contrast attachment (model BH2-PC). The samples were prepared by placing a droplet of the solution on a microscope slide and protected against drying by a cover glass. The samples for cryo-TEM were prepared at room temperature by placing a droplet (5 µL) of the solution on a hydrophilized perforated carbon-filmed grid (60 s plasma treatment at 8 W using a BALTEC MED 020 device). The excess fluid was blotted off to create an ultrathin layer (typical thickness of 100 nm) of the solution spanning the holes of the carbon film. The grids were immediately vitrified in liquid ethane at its freezing point (-184 °C) using a standard plunging device. Ultrafast cooling is necessary for an artifact-free thermal fixation (vitrification) of the aqueous solution avoiding crystallization of the solvent or rearrangement of the assemblies. The vitrified samples were transferred under liquid nitrogen into a Philips CM12 transmission electron microscope using the Gatan cryoholder and -stage (model 626). Microscopy was carried out at -175 °C sample temperature using the microscope’s low-dose protocol at a primary magnification of 58 300×. The defocus was chosen in all cases to be 1.2 µm corresponding to a first zero of the phase contrast transfer function at 1.8 nm.
Results and Discussion Spectroscopy. The J-band of the pure C8O3 dye is composed of three single components24 peaked at 561, 582, and 600 nm, which are all due to the band structure of the extended molecular excitons. A full quantitative description of the aggregate spectrum does not exist at present, but qualitative arguments can be given. Thus, splitting of the J-band into two components has been interpreted under the assumption of molecular packing in a cylindrical geometry.36,37 On this basis, the 3-fold split spectrum of the bilayer-walled tubular J-aggregates can be qualitatively explained under the assumption that the inner and outer chromophore layers have different absorption energies.38 The spectra plotted in Figure 1 reveal the strong changes occurring upon titration with methanol. Thus, with increasing methanol concentration the longest wavelength component (600 nm) shifted to an even longer wavelength, while the two other components (561 and 582 nm) were replaced by a single band with maximum absorption at ∼575 nm. At the highest applied concentration (34.4 vol %), the J-band disappeared completely and the spectrum of the monomer, peaked at ∼519 nm, was obtained, indicating dissolution of the J-aggregates into dye monomers. The elapsed time between each titration step was about 5 min. Qualitatively the same changes in the absorption spectra were observed when other shortchained alkanols, such as ethanol or 1-propanol, the polyalcohol EG, or acetone was added. The effectiveness of producing spectroscopic changes is, however, a strong function of the additive’s concentration. In general, for increasing alcohol carbon numbers decreasing amounts are necessary, a behavior that is obviously related to the increasing hydrophobic character of the alcohol. For an initial dye concentration of 5.75 × 10-5 M, the 2-fold split spectrum was obtained at a methanol content of 12.6 vol % and the monomer spectrum at about 34 vol %. For 1-propanol, the respective concentrations were ∼2 and ∼15 vol %. To understand the effect of alcohols on J-aggregation, a comparison with the effect on other self-aggregating systems is helpful. Thus, the effect of alcohols on the (36) Bednarz, M.; Knoester, J. J. Phys. Chem. B 2001, 105, 12913. (37) Didraga, C.; Knoester, J. Chem. Phys. 2002, 275, 307. (38) Spitz, C.; Knoester, J.; Ouart, A.; Da¨hne, S. Chem. Phys. 2002, 275, 271.
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Figure 1. Change of absorption spectra of an aqueous C8O3 solution (containing 10-2 M NaOH) as a function of added methanol. The plotted extinction, E, is normalized by the respective dye concentration, cD. Initial dye concentration: 5.75 × 10-5 M. Methanol concentration (in vol %): 0, 1.65, 3.25, 5.50, 7.70, 12.6, 14.4, 17.3, 20.1, 24.0, 27.4, 30.6, and 34.4. Arrows indicate the effect of increasing methanol concentration.
stability of surfactant micelles and lamellar phases has been the object of various studies.29,39-43 These studies have shown that the additives can control the micelles’ stability, in general, by changing the dielectric and hydrophobic properties of the solvent or by incorporating into the micelles through direct hydrophobic interactions. It appeared difficult, however, to correlate the complex experimental findings quantitatively with the physicochemical properties of the considered additives. On the contrary, a simple two-state model for the solubilization of surface-active additives which discriminates adsorbed from incorporated additives is well suited to explain a number of experimental findings in micellar systems.44 By adoption of this model, it may analogously be assumed that the highly water-soluble additives such as methanol or EG do not considerably penetrate into the aggregate’s interior but are preferentially solubilized in the aggregateto-solvent interface layer, comprising the dye headgroups, the counterions, and water. Medium- and long-chain alcohols may be solubilized in the interface too but will additionally penetrate into the hydrophobic interior of the aggregate when they are present in sufficient concentration. Examples of absorption spectra obtained for different additives are given in Figure 2. The shapes of the spectra are very similar. The amount of the water-soluble additive EG or acetone which is necessary to produce 2-fold split absorption bands is in the order of 30 and 10 vol %, which corresponds to molar additive/dye ratios of 2.6 × 104 and 1.8 × 105, respectively. A quite different situation was expected for typical long-chained alkanols. Because of their low solubility in water, the penetration effect should dominate here and an impact on the molecular packing should be observed at close to equimolar alcohol/dye ratios. For checking this, we added 1-octanol and 1-decanol and obtained the displayed 2-fold split absorption spectra for (39) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 3320. (40) Luo, H.; Boens, N.; Van der Auweraer, M.; De Schryver, F. C.; Malliaris, A. J. Phys. Chem. 1989, 93, 3244. (41) Reekmans, S.; Luo, H.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1990, 6, 628. (42) Jonstro¨mer, M.; Strey, R. J. Phys. Chem. 1992, 96, 5993. (43) Penfold, J.; Staples, E.; Tucker, I.; Cummins, P. J. Colloid Interface Sci. 1997, 185, 424. (44) Mukerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 153.
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Figure 2. Set of normalized absorption spectra of aqueous C8O3 solutions containing various additives. (s) EG: cD ) 2.0 × 10-4 M; [EG] ) 30.7 vol %. (- - -) Acetone: cD ) 7.6 × 10-6 M; [acetone] ) 10.3 vol %. (- - -) 1-Decanol: cD ) 3.4 × 10-4 M; molar ratio: [1-decanol]/[dye] ) 1:1. (- - -) 1-Octanol: cD ) 3.4 × 10-4 M; molar ratio: [1-octanol]/[dye] ) 5:1. The shortwavelength shoulder that appeared for EG (at ∼530 nm) indicates an appreciable contribution from dye monomers.
Figure 3. Change of circular dichroism spectra of an aqueous C8O3 solution (containing 10-2 M NaOH) prepared by the direct route as a function of added methanol. Initial dye concentration: 7.0 × 10-5 M. Methanol concentration (in vol %): 0, 7.0, 10.0, 13.0, 15.8, 18.4, 22.0, 25.2, 28.3, 32.0, and 36.8. The CD signal first increases (solid lines), reaches maximum intensity at 18.4 vol %, and then again decreases (broken lines) with methanol concentration.
molar alcohol/dye ratios of 5:1 or 1:1, respectively (the usage of molar ratios instead of vol % is appropriate here). To summarize, we can state in analogy to the recently investigated effect of ionic surfactants25 that nonionic additives of different hydrophobic character induce modified molecular packings inside the J-aggregates, which are reflected in the appearance of a 2-fold split absorption spectrum. Owing to the influence on molecular packing, the additives can be expected to affect also the CD spectra of the C8O3 J-aggregates. The CD spectrum of the pure J-aggregate is characterized by three bands with extrema at around 568, 582, and 605 nm but alternating signs of the subbands (two maxima (+) and a minimum (-) between).24 This type of CD spectrum, in particular showing a subband pattern of (+, -, +), was measured in about 98% of all freshly prepared solutions.35 Therefore, we concluded that nearly all samples contain an excess of aggregates of the same helicity. The absolute handedness on the molecular level is unknown at present. However, the helical superstructure of the assembled J-aggregates exhibits both right- and left-handed twist (the pitch is in the order of several hundreds of nanometers but generally varies) whereas the amount of left-handed helices dominates.24,45 If such an aqueous solution is titrated by methanol, the total CD signal starts to increase, reaches maximum intensity, and decreases again. A corresponding set of spectra is reproduced in Figure 3. The maximum CD signal was measured for an amount of 18.4 vol % methanol, that is, at a concentration where the 2-fold absorption spectrum is well developed, and monomers are still absent (cf. Figure 1). At 36.8 vol %, that is, when the J-aggregates are dissolved and only monomers are present (as can be deduced from the absorption spectra), the CD signal disappears as expected for the achiral C8O3 molecule. The signs of the subbands were not altered by the addition of methanol. Obviously, the excess of those aggregates, whose handedness is prevailing prior to the addition of methanol, strongly increased. Because the molecule itself is achiral, the measured chirality is a “supramolecular chirality”,46 due to long-
range positional and long-range orientational order of molecules. The molecular mechanism by which an additive-induced modified packing enhances the CD signal is not clear, however. Due to chirality transfer46 from the level of the molecular packing onto the next structural level of superhelical tubule bundles which is a characteristic feature of the pure C8O3 system,24 the hereindescribed CD spectra also strongly suggest the formation of twisted bundles of individual strands for the alcoholmodified J-aggregates. The cryo-TEM micrographs will give the final experimental proof. Strongly enhanced CD signals have also been observed for the penetrating alcohols 1-octanol and 1-decanol, but in these cases the spectra are often not reproducible and show anomalous steps, pointing to large contributions from linear dichroism to the measured total signal. These effects suggest that domains with high local order are present.47 A further striking effect was the finding28 that aggregated C8O3 solutions prepared by the alcoholic route exhibit usually a much stronger circular dichroism than solutions prepared by directly dissolving the crystalline material in 10-2 M NaOH. Figure 4 shows corresponding absorption and CD spectra of a 6.2 × 10-5 M C8O3 sample prepared by the alcoholic route. Here an aqueous stock solution containing 2.3 × 10-3 M dye and 2.7 vol % methanol was first prepared by adding a methanolic C8O3 solution into 10-2 M NaOH for aggregation. This stock was diluted by 10-2 M NaOH after 1 day of storage for the spectroscopic measurement. The CD signal is enhanced by a factor of about 40, as compared to samples prepared by the direct route, that is, in absence of alcohol (cf. the curve plotted in Figure 3 for 0 vol % methanol), while the absorption spectrum is not affected. Note that the total methanol concentration after dilution was 0.073 vol % and thus, according to Figure 1, should be much too small to modify the 3-fold split absorption spectrum of already aggregated solutions. This finding indicates that alcohols can effectively control the self-organization of dye molecules during aggregation in such a way that the excess of aggregates with the (+, -, +) circular dichroism signature increases, even if the respective amount of alcohol is very small and the molecules do not appreciably
(45) von Berlepsch, H.; Kirstein, S.; Bo¨ttcher, C. Paper in preparation. (46) Kuball, H.-G. In Liquid Crystals Today; Taylor & Francis Ltd.: London, 1999; Vol. 9, p 1.
(47) Norden, B. J. Phys. Chem. 1977, 81, 151.
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Figure 4. Absorption (left ordinate) and CD (right ordinate) spectra of an aqueous 6.2 × 10-5 M C8O3 solution (containing 10-2 M NaOH) prepared by the alcoholic route from a monomer solution in methanol. Total methanol content: 7.3 × 10-2 vol %.
penetrate into the aggregate. For that reason, a comparison of results obtained for J-aggregates prepared by the different dissolution routes has to be considered with caution. Precipitation of Alcohol-Saturated J-Aggregates. The absorption spectra plotted in Figure 1 indicate that upon titratition of aggregated solutions with methanol increasing amounts of monomers are formed. Above a definite methanol concentration (≈34 vol % for the 5.75 × 10-5 M dye solution of Figure 1), the J-aggregates are completely dissolved, yielding stable solutions of dye monomers. However, up to that concentration J-aggregates and monomers coexist and kinetic effects of molecular reorganization can be expected, because of the enormous geometrical size of the aggregates24 and the slowness of involved dynamical processes of cooperative nature. For studying kinetic effects, we prepared a set of samples starting from an aggregated 3 × 10-4 M C8O3 stock solution, to which different amounts of EG (in steps of 10 wt %) have been added. The absorption spectra of all these samples were measured immediately after mixing and later on weekly up to 1 month. In analogy to the methanol titration experiment (Figure 1), we expected that the increased dye solubility in mixed solvent should depress aggregation and finally lead to smaller typical sizes of the J-aggregates. However, we were extremely surprised to find a totally different behavior. The minimum EG concentration that was necessary to produce stable solutions of monomers was found to be about 60 wt %. All absorption spectra in the concentration range from 8 up to 50 wt % EG (where J-aggregates and monomers coexist) revealed pronounced time dependence of the absorbance and the shape of spectra. The same qualitative behavior has been observed in long-time measurements for 10-2 M NaOH/methanol, /ethanol, and /acetone mixed solvents as well as in the presence of the long-chain alcohols, suggesting a common origin of the spectral changes, while the amount of additive necessary to produce similar spectra depended again on its type and chain length because of differences in hydrophobic character (cf. the discussion of Figure 2). The spectra of a sample containing 20 vol % methanol measured 1, 10, and 20 days after mixing are given in Figure 5. The characteristic J-bands around 575 and 605 nm prevail after storage, but a shoulder at around 595 nm evolved and the line width of the main J-band strongly increased.
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Figure 5. Absorption spectra of a 4.6 × 10-4 M C8O3 solution prepared in 10-2 M NaOH mixed with 20 vol % methanol as a function of storage time after mixing: 1 day (solid line), 10 days (long dashes), and 20 days (short dashes).
Figure 6. The fibrous structure of aggregates of an aqueous 4.6 × 10-4 M C8O3 solution containing 20 vol % methanol (cf. Figure 5) as a function of storage time after mixing, visualized by optical microscopy: (a) after 10 days and (b) after 20 days. The flexible fibers show increasing diameters and larger stiffness with growing storage time. Bar ) 10 µm.
After a period of about 3 weeks, in all solutions containing the different additives precipitation started, suggesting a dramatic growth of the suspended particles. Particle growth has been followed by optical microscopy on the micrometer scale. Figure 6 shows optical micro-
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Figure 9. Cryo-TEM image of a 3.4 × 10-4 M C8O3 solution in 10-2 M NaOH 1 day after adding 1-decanol. Equimolar mixing ratio. Bar ) 50 nm. Figure 7. Cryo-TEM image of a 4 × 10-4 M C8O3 solution in 10-2 M NaOH 10 days after adding 20 vol % methanol. Bar ) 50 nm.
graphs of the 20 vol % methanol containing solutions after 10 and 20 days of storage. After 10 days (Figure 6a), a network of flexible fibers can be noticed, whereas the latter appear polydisperse in diameter. After 20 days (Figure 6b), thin fibers became scarce, while they are replaced by thicker ones reaching diameters in the order of 600 nm and lengths of several tens of micrometers. This tendency continues after longer storage times. Even the thickest fibers bent upon movement in the solvent indicating inherent flexibility. Crystallization (demonstrated for the 3,3′-bis(4-carboxybutyl) derivative (C8O4) recently26 by optical microscopy) has never been observed, which led us to the conclusion that the precipitate consists of massive alcohol-saturated agglomerates of aggregates, which are formed during the long-time storage of these dye solutions. Due to the structural anisotropy of these “giant” aggregates, the solutions exhibit strong birefringence when viewed between crossed polarizers. The large contribution from linear dichroism to the CD signal, in particular observed in the presence of the long-chain alcohols, agrees with this finding. Morphology of 2-Fold Split J-Aggregates. Typical cryo-TEM micrographs of C8O3 solutions containing different types of alcohols, but all exhibiting 2-fold split absorption spectra, are shown in Figures 7-10. The aggregates’ morphology resembles that found for the alcohol-free system.24 Generally, the aggregates are
Figure 10. Cryo-TEM image of a single tubule in the C8O3/ 1-decanol system (cf. Figure 9). Raw image data were bandpass-filtered (low-frequency cutoff ) 0.04, high-frequency cutoff ) 0.3). Bar ) 25 nm.
composed of single strands, which are twisted to form multiple superhelices. The morphology appears to be independent of the type of alcohol. Figure 7 represents aggregates obtained from a 4 × 10-4 M sample containing 20 vol % methanol 10 days after mixing (cf. Figure 5 for an absorption spectrum). Other samples contained 1-octanol (molar 1-octanol/C8O3 ratio, 5:1; Figure 8) and 1-decanol (molar ratio, 1:1; Figures 9 and 10) and were prepared 1 day after the addition of the respective alcohol. Due to problems in obtaining properly vitrified samples in water/EG mixtures, good quality micrographs could not be obtained. Similar morphologies became visible, but micrographs are not presented here. In addition to the ropelike aggregates, bundles of untwisted strands (e.g., at the bottom of Figure 9) and also isolated strands (Figures 7, 9, and 10) have been found. As a proof of its
Figure 8. Cryo-TEM image of a ropelike J-aggregate of a 3.4 × 10-4 M C8O3 solution in 10-2 M NaOH 1 day after adding 1-octanol. Molar mixing ratio: [1-octanol]/[C8O3] ) 5.4:1. Raw image data were band-pass-filtered (low-frequency cutoff ) 0.04, high-frequency cutoff ) 0.3) in order to remove unwanted high-frequency noise and low-frequency brightness gradients, which obscure fine image details. Bar ) 50 nm.
Effect of Alcohols on J-Aggregation
tubular nature, an isolated single strand is presented at larger magnification in Figure 10. The wall of the tubule showing a thickness in the order of 4 ( 0.5 nm is clearly perceptible. The mean total diameter is about 11 ( 1 nm. Thus, the present aggregates seem to be insignificantly swollen as compared to alcohol-free aggregates for which a diameter of about 10 nm has been reported.24 The number of dye molecules forming the tubular strand in that case has been estimated to be in the order of about 70 molecules within a rod segment of 1 nm length. Compared with the cylinder model of the J-aggregate advanced by Spitz et al.,38 our structural study thus yields a number of molecules per circumference that is roughly 10 times larger. This experimental finding has to be taken into account in future modeling of optical properties of these J-aggregates. A second noticeable effect of alcohols is the apparently higher multiplicity of the composed ropelike aggregates. This result is in line with the pronounced thickness growth of aggregates during the long-term storage of the solutions that has been revealed by optical microscopy on a much larger length scale. As an example, Figure 8 shows a section of such a thick ropelike bundle, which is composed of at least 30 individual tubules. Earlier computer modeling24,48 showed that the tubules are assembled on an equilateral triangular lattice and twisted around the helix axis. The projection Moire´ pattern of tens of twisted tubules is much more complicated but can in principle be understood in the same way. The pattern of ultrafine lines (spacing, 4.1 nm), which is visible in the vicinity of knots and displayed in Figure 8, obviously reproduces the single molecular layers constituting the assemblies. The thickness growth of the superhelical aggregates due to added alcohols requires a physical interpretation. The dye chromophores are functionalized by carboxyl groups, which are at least partially ionized in the presence of sodium hydroxide, enabling the dye’s solubility in water. The formation of supramolecular aggregates is a complex effect of different intermolecular interactions, in general, and so far not fully explained.49 Thus, in addition to attractive stacking forces due to the highly delocalized π-electron systems of the chromophores, attractive hydrophobic interactions arising from the attached octyl side chains and also electrostatic repulsion forces are effective. We have argued earlier24 that under the assumption of partial deprotonization and screening of the strong repulsive ion-ion interactions by the counterions, the formation of hydrogen bonds between neighboring tubules could stabilize their arrangement in a complex superstructure. One should expect that the effective surface (48) Kirstein, S.; von Berlepsch, H.; Bo¨ttcher, C.; Burger, C.; Ouart, A.; Reck, G.; Da¨hne, S. Chem. Phys. Chem. 2000, 1, 146. (49) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565.
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charge density at the aggregate-solvent interface will be reduced by the adsorption or incorporation of uncharged molecules, eventually decreasing their solubility and/or enforcing further aggregation. However, an opposite effect due to the penetration of alcohol molecules into the assemblies is the decrease in the dielectric constant, leading again to increased repulsions between the headgroups and resultant dissociation of additional headgroups, as to reduce these repulsions.29 Apart from the complexity of these electrostatic effects, however, the increased number of hydroxyl groups at the interface upon alcohol partition could facilitate the cooperative formation of hydrogen-bonded networks, thus enabling higher stability of aggregates and inducing thickness growth. Summary and Conclusions The addition of alcohols into aggregated aqueous solutions of the dye C8O3 induces changes in the absorption spectra. With increasing concentration of alcohol, the wellknown 3-fold split spectrum of the J-aggregate is replaced, in general, first by a 2-fold split spectrum and later by the monomer spectrum. The amount of alcohol required to produce these effects decreases with the alcohol’s chainlength, indicating the importance of hydrophobic interactions in alcohol partition. In addition, the novel Jaggregates with 2-fold split absorption spectra show usually enhanced circular dichroism signals. The spectral changes arise from alcohol-induced modifications of the aggregates’ molecular packing. This is directly evident for the penetrating long-chain alkanols but appears also plausible for short-chain alcohols, which preferentially enter the aggregate-to-solvent interface. Cryo-TEM revealed that the novel alcohol-modified J-aggregates are characterized, however, by a similar morphology. They are composed of individual bilayer-walled tubules and usually assembled to form ropelike superhelices. The tubules having a cross-sectional diameter of 11 ( 1 nm are insignificantly swollen as compared to those forming the J-aggregates in the absence of alcohol with a diameter of about 10 nm. In contrast, the total size of the assembled ropelike J-aggregates is increased by an order of magnitude in the alcohol-affected case, which can be attributed to the interfacial hydroxyl groups involved in the formation of additional stabilizing hydrogen bonds. An alcoholinduced structural transformation into cylindrical aggregates with micellar arrangement of the dye molecules can be safely excluded, which will have consequences for future theoretical modeling of the spectral properties of these J-aggregates.38 Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 448 “Mesoskopisch strukturierte Verbundsysteme”) is gratefully acknowledged. LA0203640
