Proof of Chirality of J-Aggregates Spontaneously and

Institute of Chemistry, Humboldt Universtity, Hessische Strasse 1-2, D-10115 Berlin, Germany. J. Phys. Chem .... ACS Applied Materials & Interfaces 0 ...
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8664

J. Phys. Chem. B 2000, 104, 8664-8669

Proof of Chirality of J-Aggregates Spontaneously and Enantioselectively Generated from Achiral Dyes Chistian Spitz* and Siegfried Da1 hne Federal Institute for Materials Research and Testing, Richard-Willsta¨ tter-Strasse 11, D-12489 Berlin, Germany

Andre´ Ouart and Hans-Werner Abraham Institute of Chemistry, Humboldt UniVerstity, Hessische Strasse 1-2, D-10115 Berlin, Germany ReceiVed: May 17, 2000

The recently published claim1,2 that the achiral 5,5’,6,6’- tetrachlorobenzimidacarbocyanine chromophore spontaneously and enantioselectively generates chiral J-aggregates when it is substituted in the 1,1’-position with n-alkyl groups longer than hexyl and in the 3,3’-position with ω-acido-ethyl or ω-acido-n-propyl groups has been proved by embedding the J-aggregates of such a dye (1a) in a polymeric PVA film and measuring the CD signal at defined observation angles. Dye 1b, which is assumed to give achiral J-aggregates, has been investigated for comparison as well. Possible contibutions of linear dichroism and birefringence of ordered molecules to the measured CD signal have been excluded by careful checks using both a plain PVA film and a film containing monomers of a dye (2) of known chirality.

Introduction Hitherto, the origin of enantiomerically pure substances in the biosphere is an open question because, in the absence of chiral auxiliaries, chemical reactions yield only racemic mixtures of possible enantiomers. Thus, publications describing reactions with chiral symmetry breakage always have attracted great interest. So far the only known examples are in the solid state, e.g., the spontaneous precipitation of enantiomorphically enriched sodium chlorate crystals3, chiral symmetry breakage in stirred crystallization of supercooled melt,4 and the formation of chiral domains in liquid crystals.5 Also in solution the spontaneous generation of enantiomerically enriched J-aggregates from achiral monomers through stirring the solution had been reported by Hada et al.6,7 However, the results have been disproved by Norden8 and Saeva et al.9 because the measured circular dichroism (CD) turned out to be due to birefringence of ordered J-aggregates in linearly dichroitic (LD) microcrystalline species. Recently, we published examples in which we claimed spontaneous and enantioselective generation of chiral J-aggregates from achiral dye monomers in solution.1,2 It was found that symmetry breakage in the formation of J-aggregates takes place with amphiphilic dye monomers of the J-aggregating 5,5’,6,6’-tetrachlorobenzimidacarbocyanine chromophore 1 (see inset of Figure 1) when its 1,1’-positions R are substituted with n-alkyl groups longer than hexyl and its 3,3’-positions R’ with ω-acido-ethyl or ω-acido-n-propyl groups. On the other hand, when the 3,3’-positions R’ are substituted with ω-carboxy-nbutyl or ω-carboxy-n-pentyl groups no CD signal could be observed.2 Hypothetically, we assumed that the optically active J-aggregates possess either a herringbone-like1 or a cylindrical micellar2,10-12 structure. Recently, the J-aggregates of dye 1 have been visualized in the Fuhrhop group by cryo transmission * Corresponding author. Current address: Institute of Physics, University of Potsdam, Am Neuen Pallais 10, 14415 Potsdam, Germany. Fax: +49 331 977 1134. E-mail: [email protected].

Figure 1. Identical absorption spectrum in methane solution of monomeric dyes 1a and 1b (dotted line) and absorption spectra of dye 1a J-aggregates (dashed line) and dye 1b J-aggregates (straight line) embedded in PVA film. All spectra were scaled arbitrarily.

electron microscopy and turned out to form helical superstructures.13 In distinction to the visualized torsion of single conglomerates in this paper the proof is given for symmetry breakage in chirality of the whole macroscopic solution. Self-assembled dye aggregates are highly ordered systems that can contain several thousand molecules. The coupling of their electronic states leading to an extremely high absorption cross section and in excitonic energy delocalization along many molecules makes them very interesting for research as well as for technological applications. Especially J-aggregates that are characterized by a lower collective transition energy with respect to the monomers and, therefore, a red- shifted absorption band are widely used as spectral sensitizers in photography and as charge generation materials in electrophotography.14,15 Their application offers, for example, improvements in optical data storage techniques16 and organic light-emitting diodes.17 Their extremely high nonlinear optical susceptibility also makes them promising candidates for optical switching devices. Cylindrical

10.1021/jp001805w CCC: $19.00 © 2000 American Chemical Society Published on Web 08/22/2000

Proof of Chirality of J-Aggregates

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SCHEME 1: Chiral Pentamethine Dye 2

micellar dye aggregates especially might even be candidates for the development of artificial-light-harvesting systems for solar energy applications. To distinguish between the intrinsic circular dichroism of chiral entities and linear dichroism and birefringence, orientation- dependent measurements of the CD signal must be performed as described by Saeva and Olin.9 To test the method, the optically active pentamethine dye 218 was examined (cf., Scheme 1). It does not form aggregates and possesses two genuine chiral centers in the 2- and 2’-positions. In a next step the J-aggregates in question, formed from the achiral carbocyanines dye 1a and 1b, were investigated. The J-aggregates of dye 1a (cf., inset of Figure 1; R ) C8H17, R′ ) C3H6COOH) are examples for aggregates with excitonically split spectra as shown by the dashed line in Figure 1, whereas the J-aggregates of dye 1b (R ) C8H17, R′ ) C4H8COOH) have only one single narrow red-shifted J-band shown in Figure 1 likewise by the straight line. In this manner definite proof could be given that the observed circular dichroism of J-aggregates is in fact caused by spontaneously and enantioselectively generated chirality of the J-aggregates. In addition, a more or less great portion of linear dichroism of ordered aggregates was found to contribute to the CD signal. Method In the determination of circular dichroism by differential absorption measurements with right- and left-handed polarized light, various effects contribute to the observed signal.19 In a common CD spectrometer, the measured ellipticity ΘOBS is mainly composed of three terms:

ΘOBS ) ΘCD + ΘLD+ ΘBr

(1)

The first term represents the intrinsic circular dichroism ΘCD caused by genuine chiral substances, which, in contrast to the following contributions, is independent of the observation angle. The second represents the linear dichroism ΘLD which is involved when the spatial extension of anisotropic domains in the sample with polarization P is comparable to that of the measuring light beam. ΘLD is dependent on the angle β between the polarization plane of the spectrometer and the axis of the sample anisotropy and should follow the simple cosine function:

ΘLD ) P × cos(2β) 20

(2)

Here β can be changed by rotation of the sample perpendicular to the optical axis as shown in Figure 2b, where it is related to the rotation angle γ of the sample by β ) γ + constant. On the condition that the observed ellipticity is exclusively governed by linear dichroism ΘLD, the amplitude of the measured signal over the observation angle must be symmetric along the baseline as given by the cosine function in eq 2. This is important to mention because the cosine function will be parallel shifted to the baseline when the observed signals are additionally modified by circular dichroitic contributions ΘCD of chiral substances.

Figure 2. Setup for two different ways of turning the sample in respect to the light path (arrow): turning the film about an axis perpendicular to the light path, turning the film about the light path.

The third term represents the birefringence ΘBR which occurs by the polarization-dependent reflections at the surface of the sample even in the absence of birefringence of the substance. This term is proportional to the ratio of the reflected light polarized either parallel or perpendicular to the reflection plane. This reflectivity is given by the Fresnel’ s law of reflection. Taking into account a certain value δ for the nonideality of the surface and in order to avoid singularities in the equation, due to surface reflections is described by

[ [

] ]

sin(φ - φ′) 2 +δ R⊥ sin(φ + φ′) ΘBR ≈ ≈ R| tan(φ - φ′) 2 +δ tan(φ + φ′)

(3)

depending on the reflection angle R that determines φ and φ’ by R ) φ + 90° and (sin φ’)/(sin φ) ) n/n’. It should be noted that, on one hand, ΘBR is dependent on the reflection angle R that is determined by the angle of the sample surface in respect to the optical axis and can be changed by rotating the sample as shown in Figure 2a. On the other hand, ΘBR is dependent on the refraction index n(λ) that can vary with wavelength. The linear birefringence as a bulk property of the sample itself contributes to the observed circular dichroism in the same angle dependence as the linear dichroism ΘLD. Both quantities will not be distinguished in this paper. One should bear in mind that the ΘLD and ΘBR effects can be 3 orders of magnitude larger than that of the intrinsic ΘCD.21 To determine the different terms of the observed CD signal by their angle-dependent behavior, the J-aggregates that were investigated in the liquid phase in refs 1 and 2 must be fixed to a solid state. This was done by embedding them in a polymeric poly(vinyl alcohol) (PVA) film which could be oriented in the spectrometer to control the angles R and γ as shown in Figure 2a,b. Experimental Section The aggregated solutions of dyes 1a and 1b22 were prepared from 10-4 M solutions in ethanol. These monomeric solutions were titrated in 10 µL steps into water containing 10-2 M NaOH until the total dye concentration amounted to 10-5 mol/L. Preparing the J-aggregate solutions in this manner is known to yield maximum circular dichroism.1 The solutions contained neither crystallites observable with an optical microscope nor birefringence of liquid crystals between crossed polarizers that can be found in solutions which are 3 orders of magnitude higher in concentration.

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Figure 3. Amplitude of the CD signal measured while the plain PVA film was turned about the optical axis (cf., Figure 2b). The inset shows the spectral dependence of the CD signal scaled arbitrarily.

To fix the J-aggregates in a film, each solution was mixed 1:1 with an aqueous PVA solution (6 g PVA-72000 per 100 mL) and vaporized on a horizontal plane support. The nonaggregated PVA film of dye 2 was prepared by dissolving at once the entire dye in water, resulting in 10-5 mol/L concentration. The preparation of the film was the same as with the aggregated solutions. The resulting films were taken off the support and mounted between two glass plates. This sandwich ensemble was placed vertically into the circular dichroism spectrometer with a horizontal beam path allowing either a defined rotation about a vertical axis (Figure 2a) or a defined rotation within the sample plane (Figure 2b). The spectrometer (Jasco J-710 CD Spectropolarimeter) was calibrated with a 0.06% (w/v) aqueous solution of ammonium d-10-camphor sulfonate. Results and Discussion Methodical Test. To test the angle dependence of ΘLD given by eq 2 in the first step, a plain PVA film without any dye was prepared. When the sample is placed exactly perpendicular to the light path and the film is turned about the optical axis as shown in Figure 2b neither ΘCD nor ΘBR should contribute to the observed circular dichroism ΘOBS of the pure PVA film. The only remaining portion must be linear dichroism, which may eventually be caused by partially ordered regions of the PVA resulting from flow direction or by anisotropies of the evaporation front during preparation of the film. As expected, no spectral dependence of ΘOBS was found in the wavelength region where the film does not absorb (see inlet of Figure 3), while the angle dependence of the amplitude of ΘOBS is in very good agreement with the cosine(2β) function predicted in eq 2 as shown by the squares in Figure 3. To ensure that a sample having exclusively intrinsic circular dichroism actually showed an angle-independent dichroism ΘOBS, in the next step a film with the chiral pentamethine dye 218 was prepared. According to the visible absorption spectrum (λmax ) 488 nm) the dye was dissolved as monomeric molecules which are isotropically oriented in the film and, therefore, no linear dichroism was to be expected. By placing the film exactly perpendicular to the light path to avoid ΘBR and turning the film about the optical axis of the spectrometer as shown in Figure 2b, 36 CD spectra in steps of ∆γ ) 10° were measured. All spectra had the same wavelength-dependent shape of the CD couplet with nearly constant amplitude as shown in the inlet of Figure 4. The high value of the CD signal was caused by

Spitz et al.

Figure 4. Difference from the maximum value to the minimum value (squares) and background (crosses) of the CD spectra measured while dye 2 embedded in a PVA film was turned about the optical axis (cf., Figure 2b). The nearly angle-independent CD spectrum is shown in the inset.

the chirality of the chromophore 2.18 The angle dependence of the amplitude, determined by the difference between the minimum and the maximum of the signal at 493 and 500 nm, respectively, gave a single cosine(γ + constant) function shown by squares in Figure 4. The deviation from a constant value can be explained by a small inhomogenity in optical density of the film in the case that the center of rotation does not exactly match the optical axis. Apart from this small deviation, no angle dependence occurs at this sample. This is true especially for the cosine(2β) function that is given by eq 2 for linear dichroism. The spectrally unspecific background, quantified by the values measured spectrally far away from the CD couplet at 400 and 600 nm, respectively, exhibits no pronounced angle dependence as shown by crosses in Figure 4. In the next step, a PVA film was prepared with dye 1a J-aggregates which exhibit an excitonically split absorption spectrum and were assumed to consist of intrinsic chiral entities.1 First the influence of birefringence ΘBR due to reflections on the surfaces was investigated. To control the reflections a film of the aggregated dye 1a was turned about the vertical axis as shown in Figure 2a and 36 CD spectra were measured in steps of ∆R ) 10° We found the spectra to be composed of three components: The first component was a shift parallel to the abscissa which was nearly independent of the wavelength. The second component was equal with the characteristically split absorption spectrum of the J-aggregates of dye 1a embedded in the polymeric film. This spectral feature was reproduced by subtracting any pair of spectra which were 180° tilted against each other. The third component looked like a typical circular dichroitic couplet of the J-aggregates which had an excitonically split absorption spectrum and could either be reproduced by adding any of two spectra that were measured 180° tilted to each other or by adding up all 36 CD spectra measured. Each of the 36 spectra measured was graphically decomposed into the three components mentioned before. An example is given in Figure 5 for a film which was 65° tilted against the optical axis. As it is to be expected, in dependence on the rotation angle R, the three components contribute with different amplitudes to the observed ellipticity ΘOBS. This is shown in Figure 6 for components 1 and 2. These components follow eq 3 representing Fresnel’s law of birefringence by reflection on surfaces as shown by the straight line. Hence both are due to ΘBR and must be identified by their wavelength dependence. Thereby component 1, which is nearly independent of wavelength, can be assigned to reflections at the interfaces between

Proof of Chirality of J-Aggregates

Figure 5. CD spectrum (straight line) of the PVA-embedded Jaggregates of dye 1a measured when the film was tilted by R ) 65° in respect to the optical axis. The sum of components 1 to 3 (dashed lines) matches the measured spectrum when the components are weighted by their appropriate amplitudes.

Figure 6. Amplitude of components 1 (crosses) and 2 (squares) shown in Figure 5. The straight line which represents eq 3 with n/n’ ) 1,5 and δ ) 0.01.

Figure 7. Amplitude of component 3 shown in Figure 5.

air and the supporting glass plates of the sample and component 2, which follows the absorption spectrum of the J-aggregates embedded in the polymeric film, can be assigned to reflections on the surface of the film. In the latter case the spectral behavior is determined by the wavelength dependence of the refraction index of the aggregate solution. Both kinds of reflection happen twice: once in the front and once in the rear part of the sample. Therefore eq 3 is fulfilled twice by each component in Figure 6. The amplitude of component 3 that is given in Figure 7 follows approximately a cosine function and must be due to ΘCD and ΘLD which cannot be distinguished in the experimental setup shown in Figure 2a.

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Figure 8. CD spectra of the PVA-embedded J-aggregates of dye 1a when turning the film about the optical axis (Figure 2b) by γ ) 20° and γ )110° (dashed lines). The mean spectrum of all 36 spectra, measured in steps of ∆γ ) 10°, is shown by the solid line.

From this experience we are able to recognize the typical spectral behavior of ΘBR due to reflections which follows the optical absorption spectrum of the sample (see component 2 in Figure 5). ΘBR can be avoided by placing the sample exactly perpendicular to the optical axis which was carefully done in the following experiments and controlled by the wavelength dependence of the observed circular dichroism ΘOBS. By the measurements described above the applicability of eqs 1 and 2 was tested and it was demonstrated that intrinsic circular dichroism can be identified by the angle-independent portion of the measured ellipticity when polarization-dependent reflections are avoided by placing the sample exactly perpendicular to the light beam. Proof of Chirality. In the following, the PVA film of the optically active J-aggregates prepared from the achiral dye 1a was placed perpendicular to the optical axis of the spectrometer. By turning the film about the optical axis of the spectrometer in steps of ∆γ ) 10° as shown in Figure 2b, again 36 CD spectra were measured. The spectra were composed of two components. The first component looked like a typical circular dichroitic couplet of the J-aggregates which had an excitonically split absorption spectrum as already published in ref 1. This was modified with wavelength by the second component which had a shift nearly parallel to the abscissa and led to an either rising or falling tendency of the spectrum. Two examples are given in Figure 8 by the CD spectra measured at 20° and 110°, respectively, (dotted lines) together with the mean signal of all 36 spectra (straight line). In dependence on the rotation angle, the components contributed with different amplitudes to the observed ellipticity. The amount of component 1 was determined by the difference between the maximum value at 611 nm and the minimum value at 580 nm while component 2 was quantified by the difference between the signal at the lower wavelength edge at 500 nm and the upper edge at 650 nm. The results are shown in Figure 9. Both amplitudes can be approximated by a cosine(2β) function as it is to be expected when it is caused by linear dichroism. However, the cosine function of component 1 (squares) is not positioned around the zero line but shifted to positive values up to 25 millidegrees. This is a clear indication that besides linear dichroism ΘLD a second term which is independent of the observation angle contributes to the observed ellipticity ΘOBS. Hence, this term is ΘCD due to the chirality of J-aggregates. As the shift of the cosine function of the upper component in Figure 9 to positive values is by a factor of 4 greater than its amplitude, one can conclude that the intrinsic circular dichroism

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Figure 9. Angle-dependent amplitudes of the two components extracted from the CD spectra partly shown in Figure 8. The second component exactly matches a cos(2β) function as shown by the solid line.

ΘCD is about four times greater than the observed linear dichroism ΘLD at the measured spot of the sample. When the circular dichroism of the film was measured at different spots several centimeters apart from each other no difference in the value of ΘCD could be observed except from deviations due to the differing optical density of the film. In contrast to that the portion of the linear dichroism ΘLD varied over a wide range as well as its orientation, which is given by the γ-β shift of the cosine(2β) function in eq 2, had completely different values at different spots of measurement. Because the J-aggregates have extensions in micrometer dimensions the break of symmetry of the CD signal cannot be brought forth from measuring single aggregates. This could be additionally excluded by the fact that the measured CD signal in solution fulfils the Lambert-Beer’s law, i.e., the signal is proportional to both sample thickness and concentration when diluting the solution. Moreover, the appearance by chance of left- or righthanded chirality in independently aggregated samples1 excludes the possibility that any chiral impurity present in the environment may act as a nucleation center and, therefore, be responsible for the observed effect. In conclusion, our measurements clearly prove the presence of some heredity mechanism from a seeding aggregate nucleus which transfers its chirality to the majority of the later-formed aggregates. The last experiment was concerned with a PVA film which contained dye 1b J-aggregates having only one single narrow red-shifted J-band (cf., Figure 1). It was supposed that these J-aggregates are achiral.2 This test was necessary because in contrast to our former publication also these J-aggregates eventually gave a CD couplet (cf., Figure 10) when extra purified dye samples were used.23 Therefore the origin of this signal has to be clarified. The PVA flim containing J-aggregates of dye 1b was mounted vertically in the spectrometer as shown in Figure 2b and turned about the optical axis. In analogy to the results of dye 1a represented in Figure 9 the spectra were composed of two components: one couplet and a spectral unspecific background. But in constrast to the results of dye 1a the angle dependence of both components follow the cosine(2β) function given in eq 2 symmetric to the baseline without any shift at all investigated positions of the film as shown in Figure 11. Therefore the observed CD signal of the dye 1b J-aggregates must be assigned to linear dichroism ΘLD caused by a certain ordering of the J-aggregates in the film.

Spitz et al.

Figure 10. Above: Absorption spectra of aggregated dye 1b in solution (solid line) and embedded in PVA film (dashed line). Below: CD spectra of aggregated dye 1b in solution (straight line) and in PVA film (dashed line). All values are arbitrarily scaled.

Figure 11. Difference from the maximum value to the minimum value (crosses) and background (squares) of the CD spectra measured while dye 1b embedded in a PVA film was turned about the optical axis (cf., Figure 2b).

Conclusion From the results given above follows doubtlessly that the achiral molecules of dye 1a spontaneously and enantioselectively generate supramolecular chiral J-aggregates. Therefore, apart from the self-reproducing systems in nature, this is the first example of breaking symmetry in the formation of chiral matter in the liquid state. To explain this fundamental effect, the hypothesis of a helical structure of dye 1a J-aggregates similar to the observed superstructure13 seems to be very likely because it is in agreement with their behavior under high pressure,10 with their molecular dimensions,2,11 and with a plausible mechanism of their spontaneous break of symmetry in handedness since onedimensional columnar structures which can only grow in one direction will certainly not grow to infinite length. More certainly after reaching a certain length they will break into smaller parts and each part continues to grow separately. By this mechanism the handedness of one starting aggregate is instantly transferred onto a magnitude of subsequently generated aggregates. After the first aggregate nucleus is formed, the generation of new aggregates from independent monomers is suppressed by the high hydrophobic energy in the micelle formation process which can be estimated to be 50% of the aggregation enthalpy of dye 1a.11 This gain of hydrophobic energy only appears with its full amount when already-existing cylindrical micelles are continuing to grow, but it is not effective during the primary nucleation process. For that reason the dye

Proof of Chirality of J-Aggregates concentration needed for growing existing aggregates is significantly lower than that for formation of new aggregates. Analogous to the effect that leads to the spontaneous precipitation of enantiomorphically enriched sodium chlorate crystals,3 the first formed aggregate nucleus will quickly lower the concentration of dye monomers below the amount that is necessary to form new independent aggregates. The combination of screwlike handedness and micellar character makes helical cylindrical micelles ideal candidates for autocatalytic amplification of handedness causing the observed breakage in symmetry. It should be noted that by this mechanism no predetermined handedness of the chirality is given. Therefore, in a large number of independently aggregated samples a 50% chance of either chirality happens as already published in our previous paper1. Referring to Norden’s classification,24 the herein reported symmetry breakage upon generation of helical dye aggregates represents an example of microscopic selection as opposed to the macroscopic handedness in nature. From the fact that the J-aggregates of dye 1b are achiral, although they give a CD signal in solution and in films which fulfils Lambert-Beer’s law, it must be concluded that this dye has a strong tendency to form liquid crystals even at low concentrations, as it has been observed to do with some other dyes.25,26 In such cases the observed CD signal is due to linear dichroism. Therefore, one has to assume that the J-aggregates of dye 1b do not form cylindrical micelles. Acknowledgment. We thank Prof. Dr. Christian Reichardt, University of Marburg, Germany, for providing us with dye 2. This work was supported by the Deutsche Forschungsgemeinschaft (SFB 337, AB 74/5 -1,-2,-3). References and Notes (1) De Rossi, U.; Daehne, S.; Meskers, S. C. J.; Deckers, H. P. J. M. Angew. Chem. 1996, 108, 827-830; Angew. Chem., Int. Ed. Engl. 1996, 35, 760-763. (2) Pawlik, A.; Kirstein, S.; De Rossi, U.; Da¨hne, S. J. Phys. Chem. B 1997, 101, 5646-5651.

J. Phys. Chem. B, Vol. 104, No. 36, 2000 8669 (3) Kondepudi, D. K.; Bullock, K. L.; Digits, J. A.; Hall, J. K.; Miller, J. M. J. Am. Chem. Soc. 1993, 115, 10211-10216. (4) Kondepudi, D. K.; Laudadio, J.; Asakura, K. J. Am. Chem. Soc. 1999, 121, 1448-1451. (5) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Koerblova, E.; Walba, D. M. Science 1997, 278, 1924-1927. (6) Honda, C.; Hada, H. Tetahedron Lett. 1976, 177-180. (7) Honda, C.; Hada, H. Photogr. Sci. Eng. 1977, 21, 91-96. (8) Norden, B. J. Phys. Chem. 1977, 81, 151-159. (9) Saeva, F. D.; Olin, G. R. J. Am. Chem. Soc. 1977, 99, 4848-4850. (10) Spitz, C.; Daehne, S. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 738-744. (11) Spitz, C. Ph.D. Thesis, Freie Universita¨t, Berlin, Germany, 1999, http://www.diss.fu-berlin.de/1999/15. (12) Kuhn, H.; Kuhn, C. J-Aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, New York, New Jersey, London, Hong Kong, 199; pp 1-40. (13) von Berlepsch, H.; Boettcher, C.; Ouart, A.; Burger, C.; Daehne, S.; Kirstein, S. J. Phys. Chem. B, in press. (14) Bach, G.; Daehne, S. Cyanine Dyes and Related Compounds in RODD’s Chemistry of Carbon, 2nd Suppl. Of 2nd Ed.; Sainsbury, M., Ed.; Elsevier Science, Amsterdam, 1997; pp 383 ff. (15) Borsenberger, P. M.; Hoesterey, D. C. J. Appl. Phys. 1980, 51, 4248-4251. (16) Ishimoto, C.; Tomimuro, H.; Seto, J. Appl. Phys. Lett. 1986, 49, 1677-1679. (17) Era, M.; Adachi, C.; Tsutsui, T.; Saito, S. Chem. Phys. Lett. 1986, 178, 1677-1679. (18) Reichardt, C.; Harms, K.; Kinzel, M.; Schaefer, G.; Stein, J.; Wocadlo, S. Liebigs Ann. 1995, 317-327. (19) (a) Rodger, A.; Norden, B. Circular Dichroism and Linear Dichroism; Oxford University Press: Oxford, New York, Tokyo, 1997. (b) Davidsson, A.; Norden, B. Spectrochim. Acta 1976, 32A, 717-722. (c) Davidsson, A.; Norden, B. Chem. Scr. 1976, 9, 49-53. (20) Tunis Schneider, J. B.; Maestre, M. F. J. Mol. Biol. 1970, 52, 521. (21) Kuball, H.-G.; Karstens, T.; Schoenhofer, A. Chem. Phys. 1976, 12, 1-13. (22) Dyes 1a and 1b were purchased from FEW Forschungs- und Entwicklungsgesellschaft Wolfen mbH, Industriepark Wolfen-Thalheim, P.O. Box 1340, D-06756 Wolfen, Germany. (23) Private communication from Dr. H. von Berlepsch, Max Planck Institute fu¨r Kolloid und Grenzfla¨chenforschung, Golm, Germany. (24) Norden, B. J. Mol. EVolution 1978, 11, 313-332. (25) Stegemeyer, H.; Stoeckel, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 9-14. (26) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. Faraday Discuss. 1996, 104, 139-154.