Structural Conditions for Spontaneous Generation of Optical Activity in

The results expose an interesting model for studying and understanding enantioselective processes in the biosphere. ... Journal of the American Chemic...
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J. Phys. Chem. B 1997, 101, 5646-5651

Structural Conditions for Spontaneous Generation of Optical Activity in J-Aggregates Andreas Pawlik and Stefan Kirstein Max-Planck-Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489 Berlin, Germany

Umberto De Rossi and Siegfried Daehne* Federal Institute for Materials Research and Testing, Rudower Chaussee 5, D-12489 Berlin, Germany ReceiVed: March 4, 1997; In Final Form: May 8, 1997X

Achiral molecules of 5,5′,6,6′-tetrachlorobenzimidacarbocyanine dyes having 1,1′-di-n-alkyl substituents longer than hexyl combined with 3,3′-bis(2-acidoethyl) or 3,3′-bis(3-acido-n-propyl) substituents form J-aggregates whose absorption spectrum exhibits Davydov-split subbands that display strong circular dichroism, indicating the enantioselective formation of chiral J-aggregates. In this way, for the first time, in the liquid phase selforganization of achiral dye molecules to chiral supramolecular aggregates has been realized which can be controlled by the molecular structure of the monomeric precursors. A helix-like, cylindric structure of the chiral J-aggregates is suggested. The results expose an interesting model for studying and understanding enantioselective processes in the biosphere.

Introduction The origin of the diversity of enantiomerically pure compounds in the biosphere is still an open question. Usually without chiral auxiliaries or external chiral perturbations syntheses yield racemic mixtures consisting of equal amounts of both enantiomers. Hitherto only few examples of more or less asymmetric syntheses have been described.1-6 One of the best investigated cases is the chiral symmetry breakage in the crystallization of sodium chlorate, NaClO3.6 Stirring the solution enhances the formation of only one enantiomorph crystal type. This violation of parity happening in each single experiment is brought about through secondary nucleation processes induced by the first-formed crystal seed which may be levo or dextro handed by chance. In this way an asymmetrical autocatalysis takes place.7 Recently preferred enantioselectivity has been observed in the course of self-organization processes of supramolecular structures of conjugated organic molecules. For instance, when supramolecular “quadrates” based on cyclic tetranuclear metalorganic complexes are synthesized from partly chiral building blocks, only one of the possible six diastereomers is formed with high yield.8 Whereas the formation of optically active dye aggregates in the presence of chiral supports is well-known,9-11 asymmetric induction in dye aggregates without chiral auxiliaries has been first realized through stirring supersaturated aqueous solutions of certain achiral dyes, such as pseudoisocyanine 112 and other cyanine dyes,13 which form so-called J-aggregates having in respect to the monomeric molecules a red-shifted absorption band. Those aggregates are rodlike nanoclusters with unique physicochemical properties such as the ability of excitonic energy migration.14,15 The optical activity of J-aggregates had been explained either through the formation of helical structures from achiral planar dye molecules12 or through staircaselike aggregates consisting of intramolecularly twisted dye monomers which are capable to form chiral aggregates.9 Recently a pinwheel model consisting of four aggregated dye molecules * To whom the correspondence should be addressed. Fax: 49-3063925787. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, June 15, 1997.

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has been suggested in order to explain the optical activity of so-called H-aggregates which have in respect to the monomeric molecules a blue-shifted absorption band.11

More interesting, however, is the spontaneous generation of optically active J-aggregates without stirring or other external manipulations of the aqueous solutions.16 This happens with the 5,5′,6,6′-tetrachlorobenzimidacarbocyanine chromophore 2 having in the 1,1′ position n-octyl groups 2A(x)8) or n-dodecyl groups 2A(x)12). Whereas the visible spectrum of aqueous solutions of the well-known 1,1′-diethyl-3,3′-bis(4-sulfo-n-butyl) derivative 2C(x)2;y)4) (termed TDBC17) due to the formation of threadlike J-aggregates has only one red-shifted absorption band at 587, nm the J-aggregates of 2A(x)8) and 2A(x)12) possess two red-shifted bands near 573 and 605 nm which were explained by so-called Davydov splitting.16 According to the exciton model developed by McRae and Kasha18 the existence © 1997 American Chemical Society

Optical Activity in J-Aggregates

Figure 1. Wavelength positions in nanometers of the J-aggregates’ absorption band maxima of the 3,3′-bis(3-carboxy-n-propyl) groups containing chromophore 2A in dependence on the length of the 1,1′di-n-alkyl substituents. Aqueous solutions at temperature 25 ( 2 °C (if not otherwise denoted) and at concentrations of about 10-5 mol/L. Optical activity is indicated by the circular dichroitic signal CD. The diagram illustrates that only Davydov-split J-bands are optically active and hence point to chiral J-aggregates.

of Davydov components indicates a herringbonelike structure of the aggregates. In addition such solutions exhibit circular dichroism (CD) indicating chirality with rather high optical purity of one enantiomer.16 The effect was interpreted as an intermolecular twist of the dye molecules within one herringbone strand against the dye molecules in the opposite strand. Dye 2A(x)12) even produces both types of J-aggregates, the threadlike one which is achiral having only one absorption band, and the herringbonelike one which is chiral and displays two red-shifted Davydov components of the J-band. By moderate temperature changes both types can be reversibly converted in one another.16 In all experiments the parity of the generation of chirality is preserved as there is an equal chance of getting either levo or dextro turning CD signals. Therefore, it is likely to assume that spontaneous generation of chirality in supramolecular J-aggregates follows a similar autocatalytic pattern of primary seeding and secondary nucleation processes as it was observed with sodium chlorate.7 Obviously the structure of the Jaggregates is controlled by a delicate balance between various intermolecular forces: dispersion forces due to the strongly delocalized π-electron systems, hydrophobic interactions of the dyes’ n-alkyl substituents, electrostatic interaction of the dyes’ ω-acido-n-alkyl substituents, and sterical prerequisites of the

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Figure 2. Wavelength positions in nanometers of the J-aggregates’ absorption band maxima of the 1,1′-di-n-octyl groups containing chromophore 2B in dependence on the length of the 3,3′-bis(ω-carboxyn-alkyl) substituents. Aqueous solutions at temperature 25 ( 2 °C and at concentrations of about 10-5 mol/L. The vertical dotted line indicates the absorption maximum of the monomeric dyes in methanol solution. Optical activity is indicated by the circular dichroitic signal CD. The diagram shows that only the 3,3′-bis(2-carboxyethyl) and the 3,3′-bis(3-carboxy-n-propyl) substituted chromophores 2B gives Davydov-split J-bands which are optically active, indicating chiral Jaggregates.

dyes’ molecular structure, to name a few. However, unlike the enantioselective formation of macroscopic enantiomorph crystals of sodium chlorate and some other substances,4,7 for the first time the results obtained with chromophore 2 show that spontaneous generation of chirality can be performed even in dilute solutions19 and that it seems to be not restricted to few individual substances. Therefore, it is important to ascertain the structural preconditions for symmetry breakage in order to get supramolecular J-aggregates having high CD amplitudes from achiral dye molecules. Results To begin with, in this article the influence of the substituents in the 1,1′ and 3,3′ positions of chromophore 2 on the J-aggregates’ UV/vis spectral behavior is systematically investigated and conclusions concerning the presumable structure of chiral supramolecular J-aggregates are made. The syntheses of the new dyes followed known routes17 and will be described elsewhere.22 All monomeric dyes in methanolic solution absorb nearly at the same wavelength of about 520 ( 3 nm. If not stated otherwise, the experiments are performed at room temperature. In Figure 1 the position of the J-aggregate absorption bands in aqueous solution of 2 is shown when the length of the 1,1′di-n-alkyl groups is systematically varied and the 3,3′-bis(3carboxypropyl) substituents are kept constant. The chromophores with short alkyl groups up to hexyl [2A(x)2-6)] exhibit only one single J-band without optical activity, whereas derivatives with heptyl and longer alkyl groups [2A(x)7-12)] produce both Davydov splitting and optical activity which is

5648 J. Phys. Chem. B, Vol. 101, No. 29, 1997

Figure 3. Absorption spectra (part A) and CD spectrum (part B) of the 1,1′-bis(3-carboxy-n-propyl)-3,3′-di-n-octyl-5,5′,6,6′-tetrachlorobenzimidacarbocyanine 2B(y)2) in aqueous solution at 25 ( 2 °C. Part A, full line: spectrum of a freshly prepared solution, c ) 1.5 × 10-4 mol/L having Davydov-split absorption bands at 572 and 601 nm which are optically active; dotted line: spectrum of the same solution after storing in darkness for 1 h displaying only one J-band at 587 nm which is opically inactive; broken line: spectrum of the stored solution after diluting 1:10 with water containing 10% methanol. The spectrum shows only a blue-shifted H-band at 498 nm which is optically inactive likewise. Part B: CD spectrum of the freshly prepared solution whose absorption spectrum is shown in part A (full line). The couplet which coincides with the Davydov components in the absorption spectrum corroborates the excitonic coupling of both transitions.The other two solutions of part A do not give any CD signal.

indicative of chiral J-aggregates. In the case of very long alkyl groups such as undecyl [2A(x)11)] and dodecyl [2A(x)12)], the Davydov-split and optical active J-aggregates are reversibly converted into achiral ones when the temperature is lowered from 25 to 2 °C. Obviously, there is preferred formation of chiral J-aggregates with alkyl groups having chain length between heptyl and decyl. No difference in the formation tendency could be detected so far between dyes having either even-numbered or oddnumbered alkyl substituents. This is important to mention because usually the adsorption energy, the minimum surface area demand, and the phase transition temperatures of dyes substituted with even-numbered alkyl groups are different from those of dyes having odd-numbered alkyl substituents.23,24 But nevertheless the hydrophobic character of the 1,1′-di-n-alkyl substituents of chromophore 2 must play an important part for the formation of chiral supramolecular J-aggregates because the hydrophobicity is higher the longer the alkyl chain.25 In Figure 2 the result is shown when the length of the 3,3′bis(ω-carboxy-n-alkyl) groups is varied and the 1,1′-di-n-octyl substituents are kept constant. Only dyes with 3,3′-bis(2carboxyethyl) groups [2B(y)2)] and 3,3′-bis(3-carboxy-npropyl) groups [2B(y)3)], respectively, give Davydov-split, optically active J-bands. Dyes with longer 3,3′-bis(ω-carboxyn-alkyl) groups such as 2B(y)4) and 2B(y)5) behave like that having 1,1′-dialkyl groups of shorter chain length up to hexyl shown in Figure 1. The dye 2B(y)1) with 3,3′-bis(carboxymethyl) substituents displays besides one red-shifted J-band at 559 nm a blue-shifted H-band at 472 nm which both do not exhibit CD signals. In freshly prepared solutions of dye 2B(y)2) preferentially chiral J-aggregates absorbing at 572 and

Pawlik et al.

Figure 4. Wavelength positions in nanometers of the J-aggregates’ absorption band maxima of the 3,3′-bis(ω-sulfo-n-alkyl) groups containing chromophores 2C having 1,1′-di-n-alkyl substituents of different lengths. Aqueous solutions at temperature 25 ( 2 °C (if not otherwise denoted) and at concentrations of about 10-5 mol/L. Optical activity is indicated by the circular dichroitic signal CD. The diagram illustrates that only dye 2C(x)8;y)3) at 25 °C possess a Davydov-split J-band which is optically active due to chiral J-aggregates.

601 nm are formed which are converted to achiral ones absorbing at 587 nm on storage the solution. On dilution or further storage of the samples, both types are converted into an H-aggregate which absorbs at 498 nm indicating a certain instability of the J-aggregates. As an example, characteristic UV/vis spectra of the three aggregate types are shown in Figure 3A. Exclusively the Davydov-split J-aggregates give a CD couplet which is shown in Figure 3B. Interesting for clarifying the structural conditions for breaking symmetry in supramolecular J-aggregates is also the influence of acidity of the nitrogen substituents. Therefore, in dyes 2C the much higher acidic sulfo groups were substituted for the carboxy groups in dyes 2A and 2B. Only dye 2C(x)8;y)3) gives Davydov-split, optically active J-aggregates which are rather instable, however, and are converted to achiral ones when the temperature is slightly lowered, similar to dyes 2A(x)11) and 2A(x)12). Figure 4 shows the position of the J-aggregates’ absorption bands of some 3,3′-bis(ω-sulfo-n-alkyl) substituted derivatives 2C. Comparison with the 3,3′-bis(ω-carboxy-nalkyl) entities of Figures 1 and 2 shows that Davydov-split and hence optically active absorption bands exhibit only the 3,3′bis(3-acido-n-propyl) derivatives 2B(y)3) and 2C(x)8;y)3) having 1,1′-di-n-octyl groups whereas the related 3,3′-bis(4acido-n-butyl)-substituted dyes 2B(y)4) and 2C(x)8;y)4) do not. Also dyes having longer 3,3′-bis(ω-acido-n-alkyl) substituents, such as 2B(y)5), do not bring about optically active J-aggregates. The result proves that the higher acidity of sulfo groups as compared to carboxy groups does not have strong influence on the formation of chiral J-aggregates. To clarify further the influence of the acidic substituents on the aggregation behavior also dye 2 substituted with four 3-carboxy-n-propyl groups in 1,1′,3,3′-position has been synthesized. However, this dye does not aggregate. Obviously,

Optical Activity in J-Aggregates

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Figure 5. (A) Molecular structure of the dye 2A(x)11) as obtained from molecular mechanics geometry optimization procedure (hydrogen, white; carbon, light-blue; nitrogen, deep blue; chlorine, green; oxygen, red). (B) Sketch of the proposed structure of chiral J-aggregates of dye 2A(x)11). The blue cylinder represents the hydrophobic 1,1′-di-n-alkyl chains in their disordered fluid phase. The chromophores together with their 3,3′-bis(ω-acido-n-alkyl) groups are assumed to be located within the red shell along the screw lines. The structure of two molecules is drawn as an example. The shift between the molecules’planes explains the red-shift of the J-band (see text). The circular arrangement of the molecular planes is reponsible for the Davydov splitting and the helix-like screw causes optical activity. Assuming a length of the n-alkyl chains of about 15 Å and a height of the chromophores (i.e., the length of the short axis of the chromophores’ plane) of about 10 Å, the diameter of the micelle is in the order of 50 Å.

the presence of at least two hydrophobic, long-chain alkyl groups is essential for the spontaneous generation of chirality in supramolecular J-aggregates. The question remains to be answered whether four long-chain 1,1′,3,3′-n-alkyl substituents would be sufficient to create chirality through aggregation. Obviously, this is not the case likewise because it had been already shown by Chen et al.26 that none of the 1,1′,3,3′-tetra-n-alkyl (CnH2n+1 with n ) 2, 3, 5, 8) substituted dyes give Davydov-split J-aggregates. Only one J-band or even H-bands are formed. Therefore, one has to assume that the presence of 3,3′-bis(ω-acidoalkyl) groups is a second precondition for spontaneous generation of chirality in supramolecular J-aggregates. Presumably these groups effect sufficient solubility as well as sufficient lipophilicity of the dyes. However, as neither dye 2B(y)1) with the very short 3,3′bis(carboxymethyl) substituents nor the dyes 2B(y)4), 2B(y)5), and 2C(x)8;y)4) with rather long ω-acidoalkyl groups produce Davydov-split, optically active J-aggregates the 3,3′-bis(ωacidoalkyl) substituents must additionally fulfill other requirements. To ensure good sticking of the chromophores, probably they must be flexible enough to avoid sterical hindrances or mutual Coulomb repulsion, as is the case with the rather unstable

chiral J-aggregates of dye 2B(y)2) which are easily converted into achiral J- or even H-aggregates (cf. Figure 3). For the same reason dye 2B(y)1) even does not form any Davydovsplit, optically active J-aggregate but two coexisting blue- and red-shifted absorption bands which cannot be explained yet. On the other side the 3,3′-bis(ω-acidoalkyl) groups must not be longer than propyl in order to produce optically active J-aggregates. It may be that longer 3,3′-bis(ω-carboxyalkyl) groups reduce the shielding of the dyes’ positive charge through the negatively charged acido substituents or they bring about again stronger sterical hindrance due to their longer polymethylene chain. Finally, by means of insertion of trisdecafluorinated 1,1′-din-octyl substituents (n-C6F13C2H4) into dye 2A the hydrophobicity of alkyl groups was strongly enhanced with concomitant increasing of their volume being required. However, the aggregates of this dye exhibit exclusively a blue-shifted H-band at 484 nm giving no CD signal. Discussion Although, up to now, structural analyses of optically active J-aggregates are not available, several models are conceivable

5650 J. Phys. Chem. B, Vol. 101, No. 29, 1997 to understand their spontaneously generated chiral structure (e.g.,refs 16, 27, and 28). Taking into account the aforementioned results that the presence of long n-alkyl chains is one of the preconditions for chirality it is likely to assume that the optically active J-aggregates have structural features similar to micelles formed by surfactants above a critical concentration in aqueous solutions.24 The low solubility of the hydrophobic groups in polar solvents such as water effects the spontanous formation of aggregates where the chains are surrounded and covered by the hydrophilic part of amphiphilic molecules. The preferred geometry of those aggregates can be deduced from the structure of the molecule by means of a surfactant parameter Ns which is defined as Ns ) V/la0, where V is the volume of the hydrophobic part of the amphiphilic molecule, l is the length of the hydrocarbon chain, and a0 describes an effective area of the hydrophilic headgroup which is occupied by the molecules to stack close together shaping the micelle’s surface. Its theoretical value amounts to 0.33 for spherical micelles, 0.5 for infinite cylinders, and 1 for planar bilayers.24 In case of the long-chain molecules 2, the chromophore including its 3,3′-bis-ω-acido-n-alkyl substituents forms the hydrophilic headgroup positioned above two parallel oriented 1,1′-di-n-alkyl chains (see Figure 5a). Taking the total volume of the two undecyl groups V ) 0.65 nm3, the length of each undecyl group l ) 1.55 nm, and the effective area of the headgroup, i.e., the area of its long narrow edge, a0 ) 0.76 nm2 (calculated from the thickness of the chromophore of 0.40 nm times it’s length of 1.90 nm) the surfactant parameter of dye 2A(x)11) amounts to Ns ) 0.55. This is close to the optimal condition for the formation of cylindrical micelles, the possible structure of which is sketched in Figure 5b. Such a structure is indeed capable of describing both Davydov splitting and optical activity of the J-bands. The 1,1′-di-n-alkyl chains of dye 2 are confined within the inner core of the micelle with a diameter not larger than 2 times the length of a single n-alkyl chain. Due to the free movability of alkyl chains they are assumed to be in a disordered fluid phase which is demonstrated in Figure 5 by the blue column. The helical arrangement of the chromophores that is necessary for optical activity is indicated by the screw lines within the orange-red shell around the hydrophobic part. Neighboring dyes are shifted against each other and rotated by an angle thus roughly explaining the occurrence of J-bands by the shift as well as the Davydov splitting by the rotation angle. The optical activity follows immediately from the handiness of the helix. It should be emphasized that using the data published in ref 16, H. Kuhn and C. Kuhn have deduced a similar structure model based on calculations of the J-aggregates’ spectral properties.28 Their results strongly support the model presented in Figure 5. Also light-scattering measurements performed in our group are indicating the cylindrical nature of the micelles. Very recently, also in the liquid-crystalline phase of highly concentrated, aqueous J-aggregate solutions of a benzoxacarbocyanine dye the existence of cylindrical (but not helical!) micelles has been experimentally proved by combination of NMR spectroscopy, polarized-light optical microscopy, and synchroton X-ray diffraction measurements.21 However, it is not clear yet how the structure of dyes 2A(x)11), 2A(x)12), and 2B(y)2) may change on storage the solution or lowering the temperature. A plausible assumption seems to be that a transformation of the cylindrical micelles toward planar double layers takes place by ordering the n-alkyl chains as is well-known with the behavior of other lipids.24 This could explain both the vanishing of the Davidov splitting and the disappearance of the optical activity.

Pawlik et al. For molecules having shorter 1,1′-n-alkyl chains such as 2A(x)2,4,6) the J-aggregates’ structure is dominated by the interaction forces between the chromophores themselves which as usual favor threadlike or planar structures. Additionally, the formation of cylindrical micelles is unlikely in the latter case, because the volume per length required by the n-alkyl chains is small as compared to the volume enclosed by the chromophores covering the surface. On the other hand, if the 1,1′n-alkyl groups are too long, as in 2A(x)11,12), the cylindric micelles become unstable and conversion to planar layers may take place. Conclusions The results expose an interesting new model system for studying enantioselective processes in the biosphere. Moreover dye aggregates may serve as interesting new systems to investigate the laws of micelle formation in dilute solutions. In the future, they may also be important in the application of chiroptical effects in nonlinear optics because, as compared to monomeric molecules, J-aggregates have strongly enhanced third-order optical susceptibilities due to their excitonic energy delocalization.29 Investigations concerning the thermodynamics and kinetics of spontaneous generation of chiral supramolecular J-aggregates as well as the enantioselective behavior of Jaggregate formation of other chromophores having lipid character are in progress. Acknowledgment. The research described in this report was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 337, and DFG Projekt Ki 544/1-1. Helpful discussions with H. von Berlepsch, C. Spitz, and R. Wagner are gratefully acknowledged. References and Notes (1) Penzien, K.; Schmidt, G. M. J. Angew. Chem. 1969, 81, 628; Angew. Chem., Int. Ed. Engl. 1969, 8, 608. (2) Moradpour, A.; Nicoud, J. F.; Balavoine, G.; Kagan, H.; Tsoucaris, G. J. Am. Chem. Soc. 1971, 93, 2353. (3) Bernstein, W. J.; Calvin, M.; Buchardt, O. J. Am. Chem. Soc. 1972, 94, 494; 1973, 95, 527. (4) Pincock, R. E.; Wilson, K. R. J. Am. Chem. Soc. 1971, 93, 1291. Wilson, K. R.; Pincock, R. E. Ibid. 1975, 97, 1474. Wilson, K. R.; Pincock, R. E. Can. J. Chem. 1977, 55, 889. (5) Gericke, P. Naturwissenschaften 1975, 62, 38. (6) Kondepudi, D. K.; Kaufman, R.; Singh, N. Science 1990, 250, 975. (7) Kondepudi, D. K.; Bullock, K. V.; Digits, J. A.; Hall, J. K.; Miller, J. M. J. Am. Chem. Soc. 1993, 115, 10211. (8) Stang, P. J.; Olenyuk, B. Angew. Chem. 1996, 108, 798; Angew. Chem., Int. Ed. Engl. 1996, 35, 732. (9) Stryer, L.; Blout, E. R. J. Am. Chem. Soc. 1961, 83, 1411. (10) Scheibe, G.; Haimerl, F.; Hoppe, W. Tetrahedron Lett. 1970, 3067. (11) Chen, H; Farahat, M. S.; Law, K.-Y.; Whitten, D. G. J. Am. Chem. Soc. 1996, 118, 2584. (12) Honda, C.; Hada, H. Tetrahedron Lett. 1976, 177. (13) Honda, C.; Hada, H. Photogr. Sci. Eng. 1977, 21, 91, 97. (14) Fidder, H.; Wiersma, D. A. Phys. Status Solidi B 1995, 188, 285. (15) Daehne, S.; De Rossi, U.; Moll, J. J. Soc. Phot. Sci. Technol. Jpn. 1996, 59, 250. (16) De Rossi, U.; Daehne, S.; Meskers, S. C. J.; Dekkers, H. P. J. M. Angew. Chem. 1996, 108, 827; Angew. Chem., Int. Ed. Engl. 1996, 35, 760. (17) De Rossi, U.; Moll, J.; Spieles, M.; Bach, G.; Daehne, S.; Kriwanek, J.; Lisk, M. J. Prakt. Chem. 1995, 337, 203. (18) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (19) Recently birefringence has been observed in aqueous J-aggregate solutions of pseudoisocyanine20 and other cyanine dyes21 at concentrations higher than 0.1 wt % due to the formation of nematic liquid crystals. Although due to their birefringence those solutions gave CD signals likewise such effects can be ruled out here because with dye 2A(x)8) and 2A(x)12), the optical activity is already observed in diluted solutions of less than 0.001wt % where liquid-crystalline phases are rather improbable to exist. (20) Stegemeyer, H.; Stoeckel, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 9.

Optical Activity in J-Aggregates (21) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310. (22) Pawlik, A.; Kirstein, S.; Daehne, S., to be published. (23) . (23) Lunkenheimer, K.; Laschewski, A. Prog. Colloid. Polym. Sci. 1992, 89, 239. (24) Evans, D. F.; Wennerstroem, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet; VCH Publishers: New York, 1994. (25) Raikhina, R. D.; Lifshits, E. B.; Eventova, I. I. Ukr. Khim. Zh. 1991, 57, 534.

J. Phys. Chem. B, Vol. 101, No. 29, 1997 5651 (26) Chen, B.; Li, B.-F.; Li, J.-R.; Jiang, L. Sci. China 1993, 36B, 927. (27) De Rossi, U. Steuerung von J-Aggregateigenschaften: Einfluss der Substituenten eines Benzimidacarbocyanins auf die Excitonendynamik und die Aggregatstruktur. Thesis, Free University, Berlin, 1996. (28) Kuhn, H.; Kuhn, C. In J-Aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996; pp 1-40. (29) Kobayashi, S.; Sasaki, F. Nonlinear Opt. 1993, 4, 305.