8792
J. Phys. Chem. B 2000, 104, 8792-8799
Structure of J-Aggregates of Pseudoisocyanine Dye in Aqueous Solution H. von Berlepsch,*,† C. Bo1 ttcher,‡ and L. Da1 hne*,† Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, D-14424 Potsdam, Germany, and Forschungszentrum fu¨ r Elektronenmikroskopie, Freie UniVersita¨ t Berlin, Fabeckstraβe 36a, D-14195 Berlin, Germany ReceiVed: January 6, 2000; In Final Form: March 21, 2000
Cryo-transmission electron microscopy (cryo-TEM), absorption spectroscopy with polarized light, and polarizedlight optical microscopy have been used to characterize the J-aggregates formed by the dye 1,1′-diethyl-2,2′cyanine chloride (pseudoisocyanine chloride) in aqueous solution. Cryo-TEM visualizes for the first time directly the rodlike morphology of the J-aggregates. A rod diameter of 2.3 ( 0.2 nm is estimated. Absorption spectroscopy shows that J-aggregation is a strong function of dye concentration and starts in dilute solution before the viscosity increases and mesophases appear. For a 12.5 × 10-3 mol solution, the length of the J-aggregates is on the order of 350 nm, which corresponds to aggregation numbers of ≈3000. Optical microscope textures reveal columnar nematic and hexagonal phases at higher dye concentrations. Structural alternatives for the molecular packing within the J-aggregate based on the estimated rod geometry are discussed. A quasi-two-dimensional superstructure is proposed which could better explain the optical properties of the J-aggregates than previous models.
I. Introduction Cyanine and merocyanine dyes are known to form in concentrated aqueous solution with bound molecular assemblies, exhibiting a strong spectral shift of their absorption band toward longer wavelengths with respect to the monomer absorption. In honor of the two researchers who first discovered this phenomenon,1,2 the assemblies have been named Jelly (J-) or Scheibe aggregates. In other cases the absorption band is shifted upon aggregation toward smaller wavelengths (hypsochromic shift) and the corresponding assemblies have been termed H-aggregates. J-aggregates were first observed in aqueous solutions of the dye 1,1′-diethyl-2,2′-cyanine chloride (pseudoisocyanine chloride; PIC).
Despite a multitude of studies and data available on the optical and spectroscopic properties of PIC and related dyes, which were obtained over the more than sixty years of research, only little is known about the supramolecular structure of the aggregates. Recently, we could elucidate the complex morphology of J-aggregates formed by a novel series of tetrachlorobenzimidacarbocyanine dyes having dialkyl substituents.3 These studies gave us the motivation to investigate once more the wellknown PIC. PIC is the most investigated cyanine dye forming J-aggregates both in homogeneous solution and at interfaces.4 Its peculiar spectroscopic behavior was attributed by Scheibe to a reversible polymerization of the chromophores due to intermolecular interactions. The molecular exciton theory supplied the explana† ‡
Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung. Freie Universita¨t Berlin.
tion for the observed absorption spectra.5 The first qualitative model was based on the simple point dipole approximation.6 The more improved extended dipole approximation used by Kuhn and co-workers7 in their brickwork model of J-aggregates gave results which are in quantitative agreement with the experiment. All theoretical models predict an arrangement of adjacent monomeric units with large lateral displacements within the aggregate. It has been known since the early years of investigation8,9 that the aggregate can be oriented in solution by flow, supporting the assumption of its threadlike morphology. Absorption measurements in dependence on light polarization on oriented samples prepared in this way, on J-aggregates dispersed in thin polymeric films and oriented by vertical spincoating,10 as well as on polyelectrolyte-bound J-aggregates11 showed that the J-band is polarized mainly parallel to the direction of alignment. PIC solutions above a critical concentration of about 10 × 10-3 mol/L exhibit strong viscoelasticity.12 The rheological properties, recently studied in detail by Rehage et al.,13 indicate unequivocally the presence of supermolecular network structures, or the shear-induced deformation and orientation of rod-shaped aggregates, respectively. These authors found that the dynamics of viscoelastic relaxation is similar to that of entangled polymer solutions with a broad distribution of relaxation times. Stegemeyer and Sto¨ckel14 observed by polarizing microscopy at low temperatures and at dye concentrations larger than 5.5 × 10-3 mol/L an optical texture that is characteristic for a nematic liquid crystal. The mesophase formation at such extremely low concentrations is surprising when compared with nematic surfactant systems,15 and it was speculated to arise from a pronounced stiffness of the J-aggregates. Higgins and Barbara16 performed near-field imaging experiments on polyelectrolyte-bound J-aggregates of PIC and observed fibers with lengths in the one-hundred micrometer range. While there exists in the literature now a general consensus that the J-aggregates of PIC form elongated supramolecular
10.1021/jp000085q CCC: $19.00 © 2000 American Chemical Society Published on Web 08/24/2000
Structure of J-Aggregates in Aqueous Solution structures in aqueous solution, the detailed molecular architecture discussed is controversial. This is partly due to the nonplanar structure of the monomer. The two quinoline rings are twisted around the central methine group at an angle of 50.6°.17 Suggested models of molecular packing reach from the (quasi-one-dimensional) linear polymer chain after Scheibe to Kuhn’s two-dimensional brickwork model.18 Many modifications of these limiting models have been proposed19-22 to account for the nonplanarity of the chromophore and the circular dichroism observed under certain experimental conditions. Recently, Potma and Wiersma23 favored higher-dimensional models due to the finding that the one-dimensional exciton model is unsuitable to explain quantitatively the measured temperature-dependent radiative decay. An important structural characteristic of the J-aggregate which, in particular, is still discussed and controversial is the aggregation number, that is, the averaged number of molecules forming a J-aggregate. The lowest numbers of typically four molecules for a “polymerlike“ aggregate have been estimated from mass action considerations of absorption spectra.24 Also using spectroscopic methods, Stegemeyer14 and Neumann et al.25 have recently obtained values on the order of 50, which is in agreement with the former estimate of Daltrozzo et al.21 Stegemeyer stressed that these numbers should not increase markedly by entering the birefringent state upon increasing dye concentration or decreasing temperature. Due to the ability of PIC to form liquidcrystalline phases, Harrison et al.26 concluded, on the other hand, that those J-aggregates should be composed of many thousands of molecules. The application of the simple mass action model to the monomer T aggregate equilibrium is, thus, very doubtful. A value of about one hundred was derived by Kopainsky and Kaiser27 and later confirmed by Fidder and Wiersma28 for the effective number of coupled electronic states forming the Frenkel-exciton band of PIC. The apparent contradiction between these numbers dissolves when the optically estimated aggregation numbers are considered as solely characterizing the electronic states (delocalization length of the Frenkel-exciton band) but not the geometrical structure of the aggregates. The aim of the present study was to get experimental data characterizing the geometrical structure of the J-aggregates of PIC in aqueous solution. Using cryo-TEM, we were able for the first time to provide direct images of J-aggregates in their native environment. It becomes indeed apparent that the typical aggregation number is much larger than the number derived from spectroscopic investigations. Additional investigations by polarizing microscopy, polarized microabsorption spectroscopy, X-ray diffraction, and reflection spectroscopy on single crystals reveal a novel picture of the PIC J-aggregates. II. Experimental Section PIC chloride was obtained as a gift from AGFA AG and was used without further purification. The solutions were prepared in Milli-Q water (conductivity < 1 µS/cm at 20 °C). The dyewater mixtures were placed for dissolution in tightly closed glass tubes of 1.5 cm diameter. For dye concentrations up to 10 × 10-3 mol/L stirring at room temperature for at least 24 h was enough to fully dissolve the dye. At higher concentrations, elevated temperatures were necessary. The 30 × 10-3 mol/L samples were prepared by stirring at 80 °C for 5 h. The density of PIC in dilute aqueous solution was determined by a Paar DMA 60 density meter (Paar, Graz, Austria), yielding a value of F ) 1.203 g/cm3 at 25 °C. The solutions used for the measurement were prepared by diluting an 8 × 10-3 mol/L stock solution. With the molecular mass M0 ) 362.9 g/mol of PIC, a
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Figure 1. Set of absorption spectra of aqueous PIC solutions at various concentrations, which were measured with unpolarized light at T ) 21 °C. Dye concentration (in mol/L): (‚‚‚) 2.35 × 10-5; (- - -) 2.82 × 10-3; (- - -) 8.20 × 10-3; (s) 12.5 × 10-3.
partial molecular volume of V0 ) 0.500 nm3/molecule results. This value has been taken as an estimate of the size of the solvated PIC molecule in the J-aggregate. The molecular volume is slightly larger than that in the crystalline phase, amounting to 0.464 nm3/molecule.17 The absorption spectra were measured with a Varian Cary 4 spectrophotometer. A microspectrophotometer (UMSP 80, Zeiss) was used for texture observation between crossed polarizers and the measurement of the dichroic absorption spectra with polarized light. The diameter of the circular measuring area was between 3.1 and 10 µm. Rectangular quartz cells (Hellma) of 0.1 cm optical path length were used for dilute samples. The cuvettes utilized for concentrated samples were sealed, disk-shaped, quartz plates with optical path lengths ranging from 2.6 to 100 µm. All spectroscopic measurements were carried out at room temperature (21 °C). The samples for cryo-TEM were prepared at room temperature by placing a droplet (5 µL) of the solution on hydrophilized perforated carbon filmed grids (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 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 in order to avoid 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 lowdose protocol at a primary magnification of 58 300×. The defocus was chosen in all cases to be 0.9 µm, corresponding to a first zero of the phase contrast transfer function at 1.8 nm. Diffraction patterns were taken at a calibrated primary camera length of 481 mm. III. Results and Discussion Absorption Spectra. Figure 1 depicts a set of absorption spectra of aqueous PIC solutions at various concentrations, which were measured with unpolarized light. The evolution of the absorption spectra with increasing concentration reflects the progressive build-up of aggregates.12,21 At a highly dilute concentration (cD ) 2.35 × 10-5 mol/L), the monomer peaks at 490 and 525 nm are found. With increasing concentration, a maximum peak around 482 nm appears, which has been ascribed to dimers.29 Formation of J-aggregates is indicated by the
8794 J. Phys. Chem. B, Vol. 104, No. 37, 2000 appearance of a new absorption band within the wavelength region from λ ) 572 to 577 nm. A band of width (hwhm) ∆ν ∼ 425 cm-1, with the maximum at 577 nm, was already visible at cD ) 7.0 × 10-4 mol/L. Absorption spectra obtained in that range of concentration are known to depend, to a certain extent, on the experimental conditions. One possible reason could be12,30 surface-induced J-aggregation of PIC molecules at the cuvettesolution interface. Upon further increase of the dye concentration, the considered band becomes narrower (∆ν ∼ 177 cm-1, hwhm) and shifts to 572 nm at cD ) 8.2 × 10-3 mol/L. Note that, at this concentration, the sample is still fluid at 21 °C, but the J-band is already fully developed, as the comparison with corresponding spectra obtained in the viscoelastic state above ≈10 × 10-3 mol/L shows. Simultaneously with the appearance of the J-band at 572 nm upon increasing aggregation, the absorption within the 450-550 nm wavelength region is reduced. This originates from a decrease in absorption due to monomers and dimers in favor of the J-band. However, absorption does not completely disappear, even up to very large concentrations. An explanation that was given for that finding is10,21 that two additional absorption bands ascribed to the J-aggregate at 498 and 533 nm appear, which superpose with those of residual monomers and dimers. From the absorption spectra, one has to conclude that J-aggregation in the bulk starts already below the critical dye concentration12 cD ≈ 10 × 10-3 mol/L (21 °C), where the solution’s viscosity drastically increases.13 The behavior is similar to that of surfactant solutions showing a transition from small (usually spheroidal) to rodlike micelles upon increasing surfactant concentration, due to self-aggregation.31 For these systems, it is well documented that viscoelasticity and gelation arise when the growing rodlike particles form entanglements or permanent cross-linking points, after reaching a critical size. The rather narrow range of concentration from the first appearance of J-aggregates up to gelation could indicate a faster growth as compared with that of ordinary surfactant micelles. Cryo-Transmission Electron Microscopy. Figure 2 presents a cryo-TEM micrograph of a 12.5 × 10-3 mol/L PIC solution. A homogeneous and closely packed network of long rodlike particles can be seen. The few dark spots also visible are frost artifacts. The image visualizes directly, for the first time, J-aggregates of PIC in solution. It is not possible to trace the individual aggregates from the beginning to the end, but total average lengths 〈L〉 of at least 350 nm may be safely estimated. Equally, it is not possible to identify cross-linking points between individual aggregates. The formation of a network is the reason for the strong viscoelasticity observed in the solutions. Generally, the aggregates are only slightly bent. A persistence length of at least 150 nm can be estimated from the pictures, which is large compared with those of typical surfactant micelles.31 The rod diameter, D, obtained from the micrographs is about 2.3 ( 0.2 nm. Assuming tight packing of the molecules within the aggregate and utilizing the measured partial molecular volume of V0 ) 0.500 nm3 gives, on average, 8.3 molecules within a 1 nm length segment. The estimated aggregation number of a J-aggregate (at cD ) 12.5 × 10-3 mol/L) is then at least on the order of 3000 and, as expected, much larger than the value determined by spectroscopic methods. As already discussed above, one should consider, however, that the selfaggregation of dye molecules is a strong function of dye concentration and smaller J-aggregates are supposed to exist below 10 × 10-3 mol/L. Due to experimental difficulties connected with the explosive growth of aggregates within the
von Berlepsch et al.
Figure 2. Cryo-TEM image of a 12.5 × 10-3 mol/L aqueous PIC solution quenched from 21 °C into liquid ethane. Bar ) 50 nm. The image shows a homogeneous and closely packed network of long rodlike particles of diameter 2.3 ( 0.2 nm.
narrow concentration range from 8 to 10 × 10-3 mol/L at 21 °C, we did not try to check this prediction quantitatively. A second important question was to look at what happens at higher concentrations. The formation of liquid crystalline phases has been described,14,26,32 but the question should also be answered whether changes in the morphology of single aggregates occur. We started with a 30.0 × 10-3 mol/L sample. The preparation of a liquid film that is thin enough for cryoTEM was difficult due to the high viscosity of the solutions but, lately, has been successful. A typical micrograph is reproduced in Figure 3a. The same rodlike J-aggregates as those observed for the more diluted sample are again visible, but the aggregates appear generally much more closely packed. Further aggregation into larger structures, for example the ribbonlike lamellar structures as supposed by Rehage et al.13 for concentrations exceeding ≈20 × 10-3 mol/L, cannot be fully excluded. Yet, to our surprise, at higher magnification, the dense regions sometimes show patterns of extremely fine parallel lines (cf. Figure 3b). The size of these structured domains is about 130 × 130 nm2. Fourier transform (Figure 3c) of the input images yields a line repetition period of 1.38 nm (i.e., a dimension that is smaller than that of a single rodlike J-aggregate). Because of the high order of the line patterns and since individual J-aggregates should be ruled out due to their larger size, we suppose that the patterns are views of nanometer-sized PIC crystals. The lines may be interpreted as density modulations reflecting molecular packing within the crystals. The following experimental findings support this assumption. First, it was observed that stock solutions which were fully transparent after preparation in the glass tubes became slightly turbid after some days. This could indicate a precipitation of nanocrystals. Obviously, at that concentration, the samples were in a twophase state of coexisting J-aggregates and crystals. More direct
Structure of J-Aggregates in Aqueous Solution
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Figure 3. (a) Cryo-TEM image of a 30.0 × 10-3 mol/L aqueous PIC solution. The fibrous aggregates are still much more closely packed than in Figure 2, but the fiber diameter is again 2.3 nm. Bar ) 50 nm. (b) Region of a cryo-TEM image of the 30.0 × 10-3 mol/L aqueous PIC solution showing a pattern of fine parallel lines. (c) The Fourier transform taken from the line pattern shown in Figure 3b gives a line repetition period of 13.8 Å.
Figure 4. Typical electron diffraction pattern (inverted contrast) taken from a domain showing a pattern of fine lines in cryo-TEM (cf. Figure 3b).
proof is given by electron diffraction. A typical electron diffraction pattern taken from the domain is presented in Figure 4. The large number of streaks which are arranged on concentric rings reflects a highly-ordered sample. Up to 12 more or less sharp rings have been detected. To get more information on the structure, we tried to correlate the electron diffraction pattern with the single-crystal X-ray structural data. PIC chloride crystallizes in the monoclinic space group P21/n, with the lattice parameters a ) 13.706 Å, b ) 10.495 Å, c ) 13.584 Å, and γ ) 90.75°, and four molecules per unit cell.17,33 Three of the
four largest d-values estimated from electron diffraction are compatible with the X-ray structure data within the error range set by the experiment (2%). All other experimental d-values are placed within the quasi-continuum of wide-angle powder reflections. However, due to the small number of reflections and the strong texture (Figure 3b), an unambiguous indexing of the diffraction pattern is not possible. Note that the repetition period of the fine lines in the corresponding cryo-TEM image of 1.38 nm corresponds roughly to the lattice constant a or c. Summarizing, we conclude that cryo-TEM and electron diffraction indicate the coexistence of J-aggregates and nanocrystals at high dye concentration in water. However, experimental data on the internal molecular structure of the J-aggregates could not been obtained by these methods. Polarizing Microscopy. The elucidation of the liquidcrystalline phases formed in aqueous PIC solutions was beyond the scope of this study. We used polarizing microscopy as a simple qualitative method to characterize the mesogenic properties, because phase investigations can give additional structural information. All viscoelastic samples within the concentration range from 10 to 30 × 10-3 mol/L appeared in the glass tubes used for sample preparation as optically isotropic between crossed polarizers (21 °C). They exhibit, however, strong birefringence when gently pushed from one side. Birefringence was always found immediately after putting a small drop of these solutions between thin quartz slides (spacing between 2.6 and 50 µm) for polarizing microscopy. Then, a typical Schlieren texture, as represented in a 30 × 10-3 mol/L sample in Figure 5a, was observed. These experiments demonstrate the strong shear sensitivity of the liquid-crystalline structure. Upon heating and subsequent cooling, with a slow cooling rate, sealed samples
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Figure 6. Origin of the striated texture by a thermomechanical instability. Upon concentrating up by evaporation of water, the nematic phase (left) transforms into a hexagonal phase (middle). Upon cooling, strain is produced, which is relieved by the formation of multiple fault lines (right), giving the striated appearance.
Figure 5. (a) Typical nematic Schlieren texture of a 30 × 10-3 mol/L sample observed between crossed polarizers at 21 °C. Sample thickness: 19 µm. Magnification: 60×. (b) Striated texture of the hexagonal phase (M) obtained for a 30 × 10-3 mol/L sample between crossed polarizers at 21 °C after concentrating up by peripheral evaporation. Sample thickness: 19 µm. Magnification: 360×.
of low concentration (12.5 × 10-3 mol/L) became, at first, optically isotropic but showed birefringence again after 2 days. The occurrence of a typical Schlieren texture gives evidence for nematic ordering of the J-aggregates, as previously suggested by Stegemeyer and Sto¨ckel.14 Besides, it should be noted that the cryo-TEM pictures often exhibit a preferential orientation of the rodlike aggregates, supporting this conclusion. The volume fraction for the isotropic-to-nematic (I f Ν) transition of rodlike particles may be calculated from the estimated size of the aggregates by the relation15 Φ ∝ D/〈L〉 ) (2.3 nm)/(350 nm) ) 6.6 × 10-3, giving a molar concentration of about 20 × 10-3 mol/L, that is indeed in the expected range. In marked contrast, samples contained between quartz slides which were not tightly sealed during heating exhibit a striated texture after being cooled to 21 °C. An example is shown in Figure 5b. Similar textures have already been observed for other cyanine dyes or, more commonly, chromonic systems,32 but also for conventional amphiphiles.34,35 The changed optical texture indicates the transition to the more concentrated hexagonal (M) phase by evaporation of water. It may be assumed that, upon concentrating up, the columns of dye molecules grow in length,
pack closer, and form a hexagonal phase. The effect of creating striations can be explained by a thermomechanical instability32,36 by which the columns show undulations or, as outlined in the idealized scheme of Figure 6, zigzag conformations. During cooling, the hexagonal lattice parameter decreases. Since the sample dimensions are kept fixed, strain is produced. This strain leads to the creation of multiple fault lines, giving the striated texture. Supported by this explanation, we can expect that the rodlike J-aggregates are still present at high concentration, whereas individual aggregates are difficult to resolve by cryoTEM due to their high packing density. Beyond the structural implications just discussed, this finding should open the possibility to prepare samples with highly-oriented J-aggregates simply by thermal treatment. Dichroic Absorption Spectra. From experiments on floworiented aggregates, Scheibe already concluded8 that the transition of the J-band (572 nm) is polarized parallel to the axis of the rodlike aggregate. Figure 7a shows the dichroic absorption spectra measured on a 12.5 × 10-3 mol/L sample exhibiting the Schlieren texture. The measurement was done on an area 10 µm in diameter, in a homogeneous domain. The exact absorption direction has been determined by rotating the polarizer until the relevant absorption has reached its maximum. The direction of maximum absorption of incident light is assumed a priori to be parallel to the aggregates. E| and E⊥ are the absorbances for light polarized parallel and perpendicular to the axis of alignment. Also shown as a dashed line is the dichroism, defined by ∆E ) E| - E⊥. The long wavelength J-band (572 nm) shows positive dichroism, having a dichroic ratio of about 2, while the two bands at 497 and 535 nm show negative dichroism. The dichroic spectra of the sample showing a striated texture are not shown here because they are very similar to those obtained for the more dilute sample. However, it is important to realize that the alignment direction of the aggregates is known in that case. Indeed, maximum absorption is observed perpendicular to the stripes of the texture. This finding subsequently provides evidence for the explanation of the texture given above. The spectra show nearly the same dichroic ratio. This obviously means that the supposed higher order within the hexagonal phase does not necessarily lead to a better alignment of the aggregates. The dichroic spectra of Figure 7a are similar to those obtained recently by Misawa et al.10 for oriented J-aggregates dispersed in polyvinyl alcohol films. Following their reasoning, it should be possible to distinguish between large aggregates, residual monomers, and dimers. If the latter are still present, then their absorption should be independent of polarization and dichroism should be negligible. Because of the observed high and concentration independent dichroism, it can be concluded that the absorption peaks at 497 and 535 nm arise exclusively from the J-aggregate.37,38 Even at a lower concentration (12.5 × 10-3 mol/L), almost no dimers or monomers should be present.
Structure of J-Aggregates in Aqueous Solution
Figure 7. (a) Dichroic absorption spectra measured on a 12.5 × 10-3 mol/L sample exhibiting a Schlieren texture. The two absorption spectra (left ordinate) were measured with light polarized parallel (|) and perpendicular (⊥) to the direction of main absorption. The orientation was determined by rotating the polarizer until the relevant absorption reached the maximum. The calculated dichroism (difference spectrum) is plotted as a dashed line (right ordinate). The spectra were taken from a domain of homogeneous texture with a circular measuring area of L ) 10 µm. T ) 21 °C. Sample thickness: 19 µm. (b) Dichroic absorption spectra of a PIC single crystal calculated from measured polarized reflection spectra using the Kramers-Kronig relation.33 The imaginary part of the dielectric constant, Im , is plotted versus wavelength.
The dichroic absorption spectra of the oriented aggregates in solution will now be compared with that of the single crystals obtained by polarized reflection spectroscopy, given in Figure 7b. For the measurement such crystal faces ((0-11) and (10-1) faces after the notation of ref 39) were selected, for which the excitonic transition dipole moments are parallel to the surface. The reflection spectra were transformed into the absorption spectra using the Kramers-Kronig relation.33,40 The single crystal shows a strong absorption band at 572 nm, which is polarized along the stacks of dye molecules (parallel to the crystallographic x-axis). A weaker band with its maximum at 569 nm was measured perpendicular to the stack axis. This optical behavior is consistent with the herringbone stacking structure deduced from X-ray structural analysis, that will be described below (cf. Figure 8a). The herringbone-like arrangement of dye molecules gives rise to an excitonic Davydov splitting.5 The small difference in the peak positions of the Davydov components of only 3 nm points to weak interactions between the two stacks of molecules. Hence, the strong absorption at approximately 572 nm arises mainly from transition-dipole interactions between the molecules within one molecular stack. The absorption on the short wavelength side of this J-band is much smaller for the crystal than for solutions.40 The reason for that behavior is not completely explained, but it evidently reflects variations in the fine structure of the aggregate.38,41,42 Small changes in the dislocation angle between
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Figure 8. (a) Threadlike arrangement of the PIC molecules derived from the X-ray analysis of PIC chloride single crystals. Both stacks are symmetric to each other, but their inclinations with respect to the paper plane differ. The single stack was often used as a structural model for a one-dimensional PIC J-aggregate.21 The chinoline rings are sketched as rectangles, and the positions of the C-atoms were taken from the X-ray analysis. Crystal water and counterions were omitted. (b) Top view along the x-axis. Taking the hydrogen atoms and the van der Waals radii into account, one stack can be modeled by a thread with an elliptical cross section. (c, d) Two possible models for the PIC J-aggregate: a starlike and a tubular arrangement of six stacks within a cylinder 2.3 nm diameter.
the PIC molecules in the double strand could lead to the different spectra on the short wavelength side of the main J-band. Summarizing, we conclude that the similarity of the spectra of single crystals and J-aggregates in solution suggests almost the same arrangement of adjacent chromophores. In particular, the stack found in single crystals could also be the unit structure element of the J-aggregate. However, from the dichroic aggregate spectra in solution it cannot be deduced if a herringbonelike arrangement exists or if the “unit strands” forming the composed aggregate are arranged along the aggregate axis in a disordered state. IV. Structure Model for the J-Aggregate As outlined in the Introduction, several structures for the PIC J-aggregate have been proposed. Perhaps the most common structure model is that of Daltrozzo et al.,21 assuming a quasione-dimensional threadlike aggregate composed of two adjacent monomers per unit length, in which the quinoline rings of either monomers are sandwich-like arranged, but with large displacement within (cf. Figure 8a). This model was derived from the stacking of the molecules in single crystals. On the basis of our previous crystallographic data,33 the following interpretation of the crystalline packing can be given. The PIC crystal is built up from stacks of single strands, with an averaged diameter on the order of 0.7-0.8 nm. Adjacent molecules forming one strand are arranged along the x-axis of the crystal, with a tilt angle 24.8°. Two such single strands with oppositely oriented molecules form a double strand (Figure 8a) with herringbonelike architecture. The three-dimensional periodicity of the crystal is obtained by the repetition of the “herringbones” into the y and z direction of the space. The cryo-TEM micrographs directly demonstrate the rodlike morphology of the J-aggregate and prove that its structure is more complex than a single sandwich-like thread. The following model accounting for the geometrical size obtained by the EM images may be proposed. Assuming a mass density as in the
8798 J. Phys. Chem. B, Vol. 104, No. 37, 2000 single crystal, the volume of a cylindrical particle with crosssectional diameter 2.3 nm can be filled by approximately six “unit strands”. Note that the experimental density of PIC in aqueous solution differs only slightly from that of crystals. Taking into account the van der Waals radii of the dye molecules, the cross section of one strand may be approximated by an ellipse, as schematically shown in Figure 8b. Because of steric reasons, it is not possible to arrange six strands in a cylindrical rod of diameter 2.3 nm in the same manner as in the single crystal. But, by allowing rotation of the strands around their axis, different packing structures are possible. Two highly symmetrical ones are shown in Figure 8c and d. A certain degree of disorder should be allowed for because of the amphiphilic nature of the dye molecules and the surrounding solvent. PIC, as well as other cyanine dyes, belongs to the group of chromonic amphiphiles,32 which are characterized by an aromatic ring architecture. The solubilizing groups are arranged around the periphery of the molecules or, as in the case of PIC, in the methin chain. It is well established that self-aggregation in amphiphilic systems is driven by entropy changes in the aqueous phase, called the “hydrophobic effect.” 43 The balance of enthalpically driven interactions between the delocalized π-electron systems of the dye and the hydrophobic interactions should ultimately determine the architecture of the dye assembly. Because the dye monomers are ionized in the aqueous medium, strong electrostatic repulsion forces will result between them. To obtain stable molecular assemblies, the attractive forces must be stronger than the repulsive ones. Presumably, the electrostatic forces are at least partially screened out by incorporated counterions. The dye assembly manages the interaction problem by creating a packing of single strands, which is characterized by a large interface to the solvent. The optimal packing, in the case of the PIC J-aggregate, leads to the apparently monodisperse cross-sectional diameter and prevents further growth in thickness. In addition, an aggregate composed of several “unit strands” should be characterized by a high stiffness or large persistence length, respectively, as indeed found by cryo-TEM. The two proposed structures, sketched in Figure 8c and d, may no longer be considered as one-dimensional. Figure 8d presents a tubular structure in which the chromophores are arranged in a strongly bent quasi-two-dimensional brickwork array. Similar models have recently been favored by Potma and Wiersma23 for the explanation of the temperature-dependent radiative dynamics in PIC. The extraordinary optical properties of J-aggregates, such as large domains of coherent coupling,28 the phenomenon of superradiance,44 and the extremely narrow absorption bands37,38 require small disorder in the aggregate’s structure.45 The effect of disorder is expected to be larger in one-dimensional than in two-dimensional systems. Thus, the proposed structures would much better explain the low degree of disorder in the aggregates which is necessary to fit the optical data. The complex architecture of the J-aggregate of PIC is obviously connected with the nonplanar structure of the monomer. Note that the PIC dye, after aggregation, shows always nearly the same red-shifted absorption band (572 nm), independent of the surrounding medium (aqueous solution, at a surface, dispersed in polymer film, single crystal) and experimental conditions. Probably, the distortion of the PIC molecule promotes, on the molecular scale, this specific stacking structure. In contrast, there are some examples of planar dye molecules, which can form different aggregate structures. We showed that the simple streptocyanine dye 1,7-bis(dimethyl-
von Berlepsch et al. amine)heptamethine can adopt, in single crystals, more than five different stacking structures.33,46 Certain substituted tetrachlorobenzimidacarbocyanine dyes3 form J-aggregates in which the molecules are arranged in monolayers and bilayers, respectively. The proposed structure model for the PIC J-aggregate shows some similarities to the hollow brickwork chimney model recently suggested for other cyanine dyes.32 However, the alignment of the dye molecules perpendicular to the aggregate axis assumed in this model can be excluded for PIC, because the polarization of the J-band is directed along the aggregate axis. Note that the transition dipole moment and, thus, the polarization of the PIC molecule are directed along the molecular axis. V. Conclusions J-aggregates of PIC in aqueous solution have been directly visualized by cryo-TEM. The aggregates possess a rodlike morphology, with a rod diameter of 2.3 ( 0.2 nm. At cD ) 12.5 × 10-3 mol/L, the J-aggregates have total lengths on the order of 350 nm, corresponding to aggregation numbers of about 3000. The results modify the present view of the aggregate structure. On the basis of cryo-TEM results and additional investigations by linearly polarized absorption spectroscopy on the micrometer scale, polarized-light optical microscopy, and X-ray structure analysis and reflection spectroscopy on single crystals, we proposed a novel structure model for the PIC J-aggregate. The spectroscopic data show that the dye molecules are assembled on the molecular scale in “unit stacks”, composed of two adjacent monomers per unit length, like in single crystals. These quasi-one-dimensional stacks do not represent the J-aggregate, as has been often assumed. Instead, presumably six of the stacks form a cylindrical rod of 2.3 nm thickness. In contrast to previous structure models, the superstructure could better explain the optical properties of the J-aggregates. It must be emphasized, however, that, besides the estimated size of the J-aggregate and the spectroscopically deduced short-range order within the J-aggregate, the exact details of molecular packing remain open and demand further investigations. Acknowledgment. We thank G. Reck for helpful discussions concerning the crystallographic data and E. Biller for the optical texture pictures and the dichroic UV/VIS spectroscopy. The work was supported by the Deutsche Forschungsgemeinschaft. References and Notes (1) Scheibe, G. Angew. Chem. 1936, 49, 563; Angew. Chem. 1937, 50, 51. (2) Jelly, E. E. Nature 1936, 138, 1009; Nature 1937, 139, 631. (3) von Berlepsch, H.; Bo¨ttcher, C.; Ouart, A.; Burger, C.; Da¨hne, S.; Kirstein, S. J. Phys. Chem. B 2000, 104, 5255. (4) Mo¨bius, D. AdV. Mater. 1995, 7, 437. (5) Davydov, A. S. Theory of Molecular Excitons; Plenum Press: New York, 1971. (6) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (7) Czikkely, V.; Dreizler, G.; Fo¨rsterling, H. D.; Kuhn, H.; Sondermann, P.; Tillmann, P.; Wiegand, J. Z. Naturforsch. 1969, 24a, 1821. (8) Scheibe, G.; Kandler, L. Naturwissenschaften 1938, 26, 412. (9) Scheibe, G. In Optische Anregung Organischer Systeme; Foerst, W., Ed.; Verlag Chemie: Weinheim, 1966; p 109. (10) Misawa, K.; Ono, H.; Minoshima, K.; Kobayashi, T. Appl. Phys. Lett. 1993, 63, 577. (11) Horng, M.-L.; Quitevis, E. L. J. Phys. Chem. 1993, 97, 12408. (12) Scheibe, G. Kolloid-Z. 1938, 82, 1. (13) Rehage, H.; Platz, G.; Struller, B.; Thunig, C. Tenside, Surfactants, Deterg. 1996, 33, 6. (14) Stegemeyer, H.; Sto¨ckel, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 9.
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