On the Aggregation Behavior of Pseudoisocyanine Chloride in

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J. Phys. Chem. B 2001, 105, 8268-8274

On the Aggregation Behavior of Pseudoisocyanine Chloride in Aqueous Solution as Probed by UV/vis Spectroscopy and Static Light Scattering Bernd Neumann* Surfactant & Colloid Group, Department of Chemistry, The UniVersity of Hull, Hull, HU6 7RX U.K. ReceiVed: March 26, 2001; In Final Form: June 20, 2001

In this work, the aggregation behavior of the cyanine dye, pseudoisocyanine chloride, in aqueous solutions both in the absence and presence of added electrolyte (0.01 M NaCl) was studied by a combination of UV/ vis spectroscopy and static light scattering (SLS). In the case of pure aqueous dyestuff solutions, an apparent aggregation number of Napp ∼ 3 was found. A correction of the apparent molecular weights, MW,app, for the amount of the still present residual monomers gave only slightly larger values (N ∼ 4). However, it could be shown that, in the presence of 0.01 M NaCl at low dyestuff concentration (0.75 × 10-3 mol L-1), the absorption spectrum shows the typical features of what is commonly believed to represent a dimer spectrum, whereas the corresponding SLS data unequivocally prove the existence of larger aggregates, consisting of at least 30 molecules. At higher dyestuff concentrations (∼2 × 10-3 mol L-1) and in the presence of salt, apparent aggregation numbers of ∼1000 (uncorrected) and ∼2000 (corrected for cM) were obtained. A particle form factor analysis of the SLS data by means of the Koyama approximation revealed typical wormlike aggregate structures for the higher concentration solutions. Additionally, the linear mass densities could be obtained at large scattering vectors and from those the number of molecules per unit length. They range from 1 to 2 nm-1 (uncorrected) and 2 to 6 nm-1 (corrected for cM) and are consistent with a bundle formation of single aggregated strands. This is in agreement with recent cryo-transmission electron microscopy results obtained for the same compound.

Introduction Dyestuff aggregation has become popular again since the discovery of lyotropic liquid crystalline phases,1-5 which are known as chromonics, and the spontaneous occurrence of chirality of some squarine,6 cyanine,7,8 and porphyrine9 dye aggregates, although made up by nonchiral molecules. Recently, more and more combined methods such as UV/vis spectroscopy7,10-12,14,15 with NMR,11,12 cryo-TEM,7,10 neutron scattering7,10 (SANS), and static11,12 and dynamic light scattering13,14 (SLS and DLS) have been applied to dyes and also combinations such as conductivity, DLS, and NMR to certain amphiphilic drugs and penicillins15-18 to gain new insight into the complex self-assembly of such molecules, the thermodynamics, and the corresponding aggregate structures in more dilute systems. In this respect, the drastic spectroscopic changes of aqueous pseudoisocyanine chloride (PIC) solutions accompanying its aggregation are still peculiar, first reported by Jelley and Scheibe some 60 years ago.19-23 Whereas at low concentrations (c < 1 × 10-5 mol L-1) in aqueous solution a monomer spectrum is obtained, in the interval 1 × 10-5 < c < 1 × 10-3 mol L-1, the absorption spectra suggest an equilibrium between two different species, monomers and H aggregates. The latter species show an increase of a blue-shifted peak (located at 482 nm), relative to the monomer’s absorption maximum at 523 nm, upon increasing concentration. For higher concentration solutions, c > 1 × 10-3 mol L-1, a very sharp band occurs (the J band), now red-shifted to the monomer’s peak position. Many contradictory results regarding especially the aggregation numbers of the corresponding J aggregates have been * To whom correspondence should be addressed. E-mail: B.Neumann@ chem.hull.ac.uk

published.24-29 Those contradictions have recently been recognized as apparent ones because, in the case of the J aggregates, the aggregation numbers related to the J-band intensity, either obtained by means of the law of mass action or from variations of the half width of the J band with temperature, reflect especially at higher concentrations the delocalization size of the Frenkel exciton states.11,30 This delocalization size is referred to as the number of molecules over which the excitation can migrate, Ndel. However, for more dilute solutions, those values are very close to the real aggregation numbers as has been mentioned previously.30 It is well-known that in dilute solutions of PIC, where the J band is absent, the absorption spectrum represents overwhelmingly monomers and H aggregates. It is commonly believed that those H aggregates represent dimers.31-33 However, it has been suggested alternatively that the basic aggregate structure in both H and J aggregates should be very similar for the following two reasons:30 (i) NMR results from Graves and Rose revealed only minor differences in the corresponding chemical shifts for both aggregate types.34 (ii) The H aggregates should be seen as precursors of the J aggregates, and the spectroscopic changes upon increasing concentration, lowering the temperature, or raising the hydrostatic pressure can also be explained by a change of the offset of the interacting molecules within the aggregates (expressed by a change of the tilt angle, R, to values smaller than 54.7°).30 The connecting link between both aggregate types is suggested from the herringbone-like arrangement of the interacting molecules (cromophores) within the J aggregates as has been found by SNOM measurements.35 Interestingly enough, for the H aggregates, also a twisted alignment of the neighboring

10.1021/jp0111259 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/07/2001

On the Aggregation Behavior of PIC

Figure 1. H aggregate spectrum of PIC obtained from resolution of several absorption spectra in the interval 10-5 < c < 10-3 mol L-1 in aqueous solution at 20 °C after ref 30. (‚‚‚) monomer spectrum, (s) H aggregate spectrum; the arrows indicate the corresponding low- (J) and high-energy (H) bands of the H aggregates. (- - -) represents the resolved H band. The inset represents the transition dipole moments, M1 and M2, of the interacting nearest neighbor chromophores within an H aggregate, including the twist angle, β.

Figure 2. Structural formula for PIC.

molecules has been deduced from analysis of the resolved H aggregate absorption spectrum30 (Figure 1), representing the basic “optical unit cell” of this type of aggregates. Briefly, exciton theory36,37 predicts two electronic transitions (here in Figure 1 the corresponding bands are denoted as J and H), if the interacting chromophores incline a twist angle, β (inset in Figure 1). For PIC, this angle was found to be 26°.30 It has therefore been the major motivation for this work to combine UV/vis and SLS measurements on more dilute PIC solutions (below the gel state), where the J band is absent or appears with low intensity, to determine the aggregation numbers of the H and J aggregates by monitoring the corresponding light scattering data at constant temperature (20 °C). It is aimed to explore whether the H aggregates are larger species than dimers and to develop a better understanding of their evolution into the J aggregates upon increasing concentration. Experimental Section Materials. PIC (Figure 2) was obtained from Sigma Aldrich as the iodide and was turned into the chloride by ion exchange chromatography. The purity was found to be >95%. Further details are given elsewhere.30 UV/vis Spectroscopy. The dyestuff solutions were made up by diluting a stock solution of 5 × 10-3 mol L-1 with distilled water. Absorption spectra were recorded with a Lambda 19 DM UV/vis/NIR spectrometer (Perkin/Elmer). OS-type cuvettes (Hellma) with 1 cm path length were used for the methanolic solution, whereas for the aqueous solutions, a specific circular

J. Phys. Chem. B, Vol. 105, No. 34, 2001 8269

Figure 3. Absorption spectra of PIC in methanol at 1 × 10-5 mol L-1 (- - -) and in water at 5 × 10-3 mol L-1 (s) and T ) 20 °C.

brass cell was designed that allows for variable adjustment of the path length. Here, mylar foils of 75, 120, 250, and 350 µm thickness were used to achieve the path lengths required between the circular suprasil (Haereus) glass disks inserted into the cell. In that case, the sample temperature was held constant at 20 °C by placing the cell into a self-built Peltier hot stage. Fiber optical cables (Perkin/Elmer) were necessary to perform the measurements within the hotstage. Recording speed was varied between 15 and 60 nm min-1, and the spectral bandwidth was chosen as 1 nm. The overall experimental error for the absorption spectra is estimated to be within 3%. Static Light Scattering. Measurements were performed with an ALV-1800 instrument (ALV Langen, Germany). A krypton ion laser (Spectra Physics) provided light of λ ) 647.1 nm, operating at a power of 250 mW. The scattered intensities were recorded simultaneously at 18 different scattering angles. The intensity values were related to a toluene standard via Rθ ) (i/ist)Rst, where Rθ is the Rayleigh ratio of the standard and i and ist denote the scattering intensity of the sample and the standard, respectively. Dust-free cleaned cuvettes (Hellma) with 2 cm inner diameter were used. Dye and salt solutions were filtered separately through 0.22 µm pore-size millex GV filters (Millipore) into the cuvettes to prevent any loss of dyestuff on the filter material because of enhanced aggregation in the presence of salt. Samples were thermostated at 20 °C within a refractive index-matching toluene bath. The refractive index increment, dn/dc, was measured at 20 °C with a Chromatix KMX-16 differential refractometer operating at λ ) 633 nm. The instrument was calibrated against different concentrated NaCl solutions and dn/dc was determined from four different concentrated salt-free dyestuff solutions as 0.615 g-1 cm3. This value was used to calculate all mass based data obtained in saltfree solutions and in 0.01 M NaCl. The light scattering data such as radii of gyration, RG, and all mass based data are assumed to be within 10-15% accuracy. Results and Discussion Results in Pure Aqueous Solution. Figure 3 depicts the absorption spectra of PIC in methanol at 1 × 10-5 mol L-1 (dashed line) and in pure water at 5 × 10-3 mol L-1 (solid

8270 J. Phys. Chem. B, Vol. 105, No. 34, 2001

Neumann TABLE 1: Initial and Approximate Concentration of Aggregated Species, cM and c - cMa c/ (c - cM)/ RG / MW,app/ MW*/ 10-3 mol L-1 10-3 mol L-1 nm g mol-1 g mol-1 b

Figure 4. Scattering data, (0) Kc/∆Rθ (uncorrected) and (9) K(c cM)/∆Rθ (corrected for cM) versus q2 for PIC in pure aqueous solution at 5 × 10-3 mol L-1 and 20 °C.

line). In aqueous solution, a typical H aggregate spectrum is shown with an absorption maximum at ∼482 nm, which is blueshifted to that of the monomer spectrum at ∼523 nm (0,0 transition) obtained in methanol. The H aggregate spectrum is believed to correspond to dimers31-33 as has been already mentioned. Because it could be demonstrated for the azo dye, Acid Red 266, that “apparent dimers” can be fairly large species, consisting of several hundred molecules,11 it was hence of interest to investigate whether in the case of relatively dilute PIC solutions similar findings could be made. For this, the absorption spectra were related to the data of the SLS measurements performed at the same concentrations. The scattering data were evaluated according to Zimm:38

Kc/∆Rθ ) 1/MW,app[1 + (RG2/3)q2] + 2A2c + ...

(1)

Here, ∆Rθ is the solvent-corrected Rayleigh ratio, representing the excess intensity of the scattered light, MW,app denotes the apparent molecular weight because extrapolation to c ) 0 was not performed because MW most probably depends on the concentration, c. RG is the radius of gyration, and K ) 4πn2(dn/dc)2/λ4NA is an optical constant with n as the refractive index of the solution and λ as the wavelength of the laser light used. q ) (4πn/λ) sin(θ/2) is the magnitude of the scattering vector, and A2 is the second virial coefficient, which was neglected because the dyestuff concentrations are low and therefore the A2 value is assumed to be small, too.11 In Figure 4, the scattering data obtained for aqueous PIC solutions at 5 × 10-3 mol L-1 show practically no angle dependence, pointing to very small aggregates. The data displayed in the same figure, which are ascribed as “corrected”, will be discussed later. The apparent molecular weights obtained from salt-free dyestuff solutions at three different concentrations are compiled in Table 1. Interestingly, MW,app is constant (∼1000 g mol-1; monomer mass of the cation, 362.5 g mol-1) over the concentration range investigated, implying two important results: (i) The influence of the A2 value is indeed negligible even without additional electrolyte because MW,app is not affected significantly as might be expected from eq 1. (ii) The aggregation numbers for c e 5 × 10-3 mol L-1 in pure aqueous solution are in fact very low (Napp ∼ 3) and therefore consistent with very small aggregates. It should be stressed that these N values are not very accurate according to the low scattering intensity and small slope in the corresponding Zimm plot, as shown in Figure 4. Note that this concentration regime is still below the critical concentration of 1 × 10-2 mol L-1, where gelation sets in and

2.00 2.25 5.00

0.76 0.83 1.52

0 M NaCl 18 890 16 930 13 1000

1500 1400 1400

0.75 1.75 2.00 2.25

0.33 0.68 0.76 0.83

0.01 M NaCl 385 11000 220 230000 204 420000 216 490000

20000 390000 680000 780000

Napp 2.5 2.6 2.8 30 650 1200 1400

N* b 4.1 3.9 3.9 55 1100 1900 2200

a Corrected and uncorrected mass based data, M W,app, MW, Napp, and N, and radii of gyration, RG, obtained from SLS for aqueous solutions of PIC in the absence and presence of 0.01 M NaCl at 20 ˚C. b All symbols marked with an asterisk do still represent apparent quantities because extrapolation to c ) 0 was not performed.

the J band is clearly developed. However, it has to be taken into account that, at such low dyestuff concentrations, the residual monomer concentration, cM, can be higher than 50%, as has been demonstrated recently for aqueous solutions of the azo dye, Acid Red 266.12 Consequently, a correction for this residual monomer concentration will give lower intercepts in the corresponding Zimm plots (Figures 4 and 6) and therefore higher molecular weights and aggregation numbers (Table 1). Such a correction becomes easily possible in the absence of electrolyte because the “apparent dimerization constant” is known. This value (KD ) 210 ( 20 L mol-1)30 was determined previously and is in perfect agreement with Scheibe’s value of 200 L mol-1.32 From the law of mass action for this apparent dimerization, one obtains

KD ) cD/cM2 ) (c - cM)/2cM2

(2)

where cD, cM, and c denote the apparent dimer, monomer, and initial concentration, respectively. Here, cD represents the concentration of all aggregated species, regardless of their real aggregation number.12 Rearrangement of eq 2 leads to an expression for the residual monomer concentration:

cM ) -(4KD)-1 + [(4KD)-2 + (2KD)-1c]1/2

(3)

Now, the initial concentration in eq 1 can be replaced by an approximate concentration of scattering particles, c - cM, in analogy to micellar systems, where c is commonly corrected for the critical micelle concentration, CMC.39 However, the aggregation behavior of dyes in general is different from that of typical surfactants because the driving force for aggregation is more enthalpic rather than entropic.4,5 The correction eventually leads to N ) 4. Results in the Presence of 0.01 M NaCl. In Figure 5, the absorption spectra of PIC in saline solution are shown. At the lowest dyestuff concentration, 0.75 × 10-3 mol L-1, the spectrum (dotted line) clearly resembles a typical monomer/ dimer equilibrium, apart from the very low intensity at ∼574 nm, where commonly the J band occurs at sufficiently high concentrations (here, a shoulder of very low intensity can be noticed). It is worthwhile to mention that the monomer peak at 523 nm and that of the H aggregates at 482 nm are of comparable intensity, pointing to a higher amount of monomers when compared to the spectrum of PIC in pure aqueous solution at c ) 5 × 10-3 mol L-1 (Figure 3). To evaluate the corresponding SLS intensity data (Kc/∆Rθ), which exhibit a curvature when plotted versus q2 (Figure 6), a

On the Aggregation Behavior of PIC

J. Phys. Chem. B, Vol. 105, No. 34, 2001 8271

Figure 5. Absorption spectra of PIC in the presence of 0.01 M NaCl at three different dyestuff concentrations. The symbols denote (‚‚‚‚) 0.75, (- - -) 1.75, and (s) 2.25 × 10-3 mol L-1 at 20 °C.

Figure 6. Scattering data, Kc/∆Rθ (uncorrected) and K(c - cM)/∆Rθ (corrected for cM), versus q2 for PIC in 0.01 M NaCl at 20 °C. The symbols correspond to (0) 1.75, (O) 2.00, and (4) 2.25 × 10-3 mol L-1 and corrected (9) 1.75, (b) 2.00, and (2) 2.25 × 10-3 mol L-1.

modified Zimm expression was used, accounting for this curvature by an additional term, Bq4:11,40

Kc/∆Rθ ) 1/MW,app[1 + (RG2/3)q2 + Bq4]

(4)

In this case, data evaluation was based on the first eight data points, corresponding to 0 < q2 < 2.5 × 10-10 cm-2 (solid lines in Figure 6). A justification of this equation is given in ref 40. Interestingly, the corresponding SLS data revealed an aggregation number of 30 for c ) 0.75 × 10-3 mol L-1. A correction of the scattering data for cM with the same KD as in pure aqueous solution seems not to be appropriate at first sight because at least three different species are present: monomers and H and J aggregates. However, formally, a two state model is still sufficient to describe the physical situation as an equilibrium between monomers and aggregates, regardless of the corresponding aggregate type. Moreover, it has been shown for some squarine dyes that the aggregation numbers for J and H aggregates do not differ in magnitude, although the corresponding spectra are quite different.14

Correction with KD ) 210 L mol-1 then yields N ∼ 60 (Table 1). It should be mentioned that the apparent equilibrium constant in 0.01 M NaCl will not be drastically different from that obtained in water because the electrolyte concentration added is relatively low. For the azo dye, Acid Red 266, KD in the presence of 0.05 M NaCl is twice as large as that value obtained for pure water as solvent.12 Additionally, for some other azo dyes, similar increasing variations of KD upon adding electrolyte were reported.14 Hence, the corrected aggregation numbers obtained in 0.01 M NaCl should be seen as lower limits. Note that aggregation numbers in the range of 30-60 at 2 × 10-3 e c e 25 × 10-3 mol L-1 in the absence of salt are still in agreement with those values obtained from absorption spectra for weakly J-aggregated solutions at sufficiently high temperatures by applying the law of mass action, which has been reported previously (N ∼ 20-100).30 The radius of gyration obtained for the aggregates at 0.75 × 10-3 mol L-1 is found to be larger than the RG values for the higher concentration solutions (Table 1), whereas the opposite should be expected. Similar findings have also been made on very dilute aqueous solutions of Acid Red 266,42 however further conclusions based on these observations cannot be made at present. To this point, it is interesting to mention some results of recently conducted calculations on H and J aggregates of a merocyanine dye.43 In that case, at low aggregation numbers of N > 20 (for H aggregates) and N > 12 (for J aggregates), rapid convergence of the upper and lower aggregates’ energy levels, respectively, was revealed. It could also be shown that for the molecules at terminal positions within an aggregate only small energy differences exist compared to those located at central positions.43 Those results along with the various size distributions of the aggregates in solution provide a further explanation of the apparent discrepancies between spectroscopically inferred aggregation numbers and those obtained from light scattering or other techniques; smaller energy differences in the corresponding aggregate spectra are blurred out in the spectra recorded because of the polydispersed aggregate sizes. At higher dyestuff concentrations, the absorption spectra exhibit the typical J band, occurring as a sharp signal at 574 nm (Figure 5). For the 1.75 × 10-3 mol L-1 solution in 0.01 M NaCl (dashed line), however, the J band intensity is still quite low, and the spectrum resembles more that of monomers and H aggregates rather than that of J aggregates apart from the J band because the typical additional aggregate bands at ∼535 and ∼494 nm are not clearly developed; only a broadening of the aforementioned maxima and a shoulder around 494 nm can be observed at c ) 2.25 × 10-3 mol L-1 (solid line). Nevertheless, N is now 650 (uncorrected) and 1100 when corrected for cM. Accordingly, RG was found to be 220 nm, clearly demonstrating the presence of much larger species than monomers and dimers only, which are still present. In this case, N values obtained from optical spectroscopy by applying the law of mass action do not correspond to the physical aggregate size anymore. They now characterize the size over which the Frenkel excitons are delocalized. Particle Form Factors. To give an idea about the corresponding aggregate structures, the particle form factors were determined from the SLS data according to the following expression:

P(q) ≈ [K(c - cM)/∆R0]/[K(c - cM)/∆Rθ] ) ∆Rθ/∆R0

(5)

where ∆Rθ and ∆R0 correspond to the excess scattering intensity at the scattering angle, θ and θ ) 0, respectively.

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Neumann

Figure 7. Casassa-Holtzer plot of normalized scattering data, P(q)qRG of PIC in 0.01 M NaCl at 20 °C versus the normalized scattering vector, qRG, for various dyestuff concentrations. The symbols denote (9) 1.75, (b) 2.00, and (2) 2.25 × 10-3 mol L-1. The lines ascribed as 1 and 2 represent P(q)qRG for monodisperse (MW/MN ) 1) and polydisperse (MW/MN ) 2) rigid rods, respectively. (3) denotes the curve obtained from a Koyama calculation using L ) 650 nm, a ) 400 nm, and MW/MN ) 2.

The experimentally obtained scattering factors, P(q), are plotted in normalized form in Figure 7 (Casassa-Holtzer plot44,45) as P(q)qRG versus the product of q and RG to allow for a size-independent comparison with theoretical form factor curves for monodisperse (MW/MN ) 1) and polydisperse (MW/ MN ) 2) rigid rods46 (curves 1 and 2, respectively). Additionally, a curve is delineated (3), matching the experimental data quite well. That one was calculated by means of the Koyama approximation,47 which combines the two borderline cases of rigid rods and random coils, to allow a description of scattering particles representing structures within both limits. Although this hybrid function has been critizised47-49 to be inaccurate especially in cases where the persistence length, a, becomes close to the contour length, L, it still provides useful qualitative information on the corresponding structures to be investigated. Here, L ) 650 nm, a ) 400 nm, and MW/MN ) 2 were used. These results are in agreement with recent cryo-TEM data, which gave estimates of L ∼ 350 nm and a ∼ 150 nm for aqueous salt-free PIC solutions at c ) 12.5 × 10-3 mol L-1,10 confirming fairly rigid aggregate structures at relatively low concentrations (c < 0.5% (w/w)). Linear Mass Densities. It is well-known that for rigid particles at large scattering vectors where P(q) ∝ q-1 the particle form factor approaches a constant plateau value when multiplied by the scattering vector, from which the mass per unit length, ML,app, can be determined:44,45,50

q[K(c - cM)/∆Rθ]-1 f πML,app

(6)

Those values provide additional information on the aggregate structure as will be shown in the following. The scattering data for PIC obtained at different concentrations in 0.01 M NaCl are presented in a Casassa-Holtzer bending-rod plot (Figure 8) according to eq 6. The asymptotic, constant behavior of qP(q) at large q values is clearly demonstrated. For evaluation of ML,app, only the last six values of each data set were considered because below q < 0.021 nm-1, no plateau is reached. Also in that figure, the corrected and uncorrected data are shown for comparison. The differences between the data, resulting from a correction for cM, are significant. Very similar findings have been made in the case of Acid Red 266.12 The corresponding linear mass densities for PIC are given in Table 2. From these values also

Figure 8. Casassa-Holtzer bending-rod plot for different concentrated PIC solutions in 0.01 M NaCl at T ) 20 °C. The symbols correspond to (0) 1.75, (O) 2.00, and (4) 2.25 × 10-3 mol L-1 uncorrected and corrected for the residual monomer concentration (9) 1.75, (b) 2.00, and (2) 2.25 × 10-3 mol L-1. Solid lines refer to the last six data points in each series.

TABLE 2: Corrected and Uncorrected Linear Mass Densities, ML,app and ML, Numbers of Molecules Per Unit Length, NL,app and NL, and Contour Lengths, Lapp and L, for PIC in 0.01 M NaCl at Different Concentrations and T ) 20˚ C c/ ML,app/g mol-1 M*L a/g mol-1 NL,app/ N*L a/ Lapp/ L*a/ 10-3 mol L-1 nm-1 nm-1 nm nm nm-1 nm-1 1.75 2.00 2.25

350 720 800

910 1900 2200

1.0 2.0 2.2

2.5 5.2 6.1

656 430 587 361 612 361

a All symbols marked with an asterisk do still represent apparent quantities because extrapolation to c ) 0 was not performed.

the number of molecules per unit length, NL,app, can be calculated. Interestingly, they increase from 1 to 2 nm-1 (uncorrected) and from 2.5 to 6 nm-1 when corrected for cM. For Acid Red 266, corrected NL values were found to be ∼9 nm-1 at 1 × 10-4 mol L-1 in 0.05 M NaCl.12 Still aware of the crudeness of this correction, the larger NL values are more consistent with the observed stiffness of the aggregates and, additionally, infer a quite reasonable aggregation mode, which is compatible with a bundle formation of isolated aggregate strands. This finding is in line with a proposed aggregate structure by von Berlepsch et al., where six “unit strands” of PIC are supposed to fit to a J-aggregate diameter of 2.3 nm as determined from cryo-TEM images.10 Because the extraction of quantitative information by means of Koyama’s approximation, such as the contour length, is without sufficient accuracy in transition regimes, where L is close to a, the contour lengths were evaluated independently from this model by relating the molecular weights to the linear mass densities:

Lapp ) MW,app/ML,app

(7)

Lapp and L values obtained from this equation were found to be ∼600 nm (uncorrected) and ∼400 nm when corrected for cM (Table 2). The latter value is smaller than L ) 650 nm used for the Koyama calculation to obtain P(q) in Figure 7. Alternatively, a value of L ∼ 400 nm can also be calculated from RG ∼ 200 nm, as in the limit of polydisperse (MW/MN ) 2) rigid rods, L ) 2RG.51 Besides, with respect to the inaccuracies introduced by calculating the approximate residual monomer concentration for PIC in the presence of 0.01 M NaCl by an equilibrium constant obtained for salt-free solutions, the order of magnitude

On the Aggregation Behavior of PIC is acceptable and also in fair agreement with (visual) estimates of L from cryo-TEM images.10 Although further experimental work is still demanded to elucidate the aggregation process of PIC in more detail, especially in the concentration regime 1 × 10-3 e c e 1 × 10-2 mol L-1, the present work has already demonstrated that, although at low dyestuff concentrations in 0.01 M NaCl, where practically no J band is present, the H aggregates of this dye are larger than dimers. According to the drastic spectroscopic changes upon increasing concentration, one can speculate that probably still growing H aggregates may assemble further into more rigid structures. Those might then appear spectroscopically as J aggregates after reorganization of their overall structure in parts or in total, consequently leading to changes in the specific alignment of the interacting chromophores by increasing the offset (tilt angle, R) between nearest neighbors (R > 54.7 ° f R < 54.7 °).30 It still remains an unanswered question whether a bundle formation can induce a change of the offset or whether this takes place in the individual aggregates before any further association. A possible explanation for the sudden occurrence of the J band and the increasing aggregation numbers within a relatively narrow concentration regime might be that the primary aggregation and/or the secondary aggregation (bundle formation) is further promoted in terms of a cooperative process (hydrogenbonding might be possible but seems to be more unlikely for this dye). Such a process appears to be reasonable because, from a topological point of view, it is striking that the (physical) aggregation process of small units such as dye molecules into larger and more rigid structures, which finally leads to a stiff gellike state upon increasing concentration, is a common feature in biological relevant systems such as proteins and peptides.52-54 In this context, bundle formation could be accomplished by a helix formation, where the single strands are wound around each other. This would give a reasonable explanation for a change of the specific alignment of the molecules within the aggregates toward a larger offset (R < 54.7 °). Recent cryo-TEM images in addition to SANS data for other cyanine dye aggregates have revealed such arrangements. They also provide an explanation for the occurrence of induced31 and intrinsic55 chirality in J aggregates of PIC. Conclusions UV/vis spectroscopic and static light scattering experiments were performed on pure aqueous solutions of PIC and on those in the presence of 0.01 M NaCl. In the absence of salt, only small apparent aggregation numbers (Napp ∼ 3) were obtained, which are consistent with a predominance of monomers and dimers at c e 5 × 10-3 mol L-1. The apparent molecular weights, MW,app, and correspondingly the aggregation numbers of the aggregates were corrected for the residual monomer concentration, cM. This was accomplished by treating the spectroscopic data in terms of a simple dimerization, based on the law of mass action, which summarizes all aggregated species as “apparent dimers”. For pure aqueous solutions, this correction leads only to N ) 4, nevertheless indicating the presence of some larger aggregates. At c ) 0.75 × 10-3 mol L-1 and 0.01 M NaCl, the absorption spectrum exhibits practically no J band and resembles to a typical monomer/dimer equilibrium. However, the corresponding SLS data revealed aggregation numbers of 30 (uncorrected) and 60 when corrected for the residual monomer concentration. This finding clearly shows that also for PIC in the presence of additional electrolyte, the H aggregates can be much larger than dimers, as has been observed

J. Phys. Chem. B, Vol. 105, No. 34, 2001 8273 recently for a few other dyes.11-14 Furthermore, the particle form factors were determined for higher concentration solutions in the presence of NaCl, being consistent with wormlike aggregate structures. Interpretation of the scattering data in terms of Koyama’s approximation for wormlike chains, allowed a rough estimation of the contour length, L ) 650 nm, and the persistence length, a ) 400 nm, at aggregation numbers of ∼1000, in agreement with recent estimates from cryo-TEM images (L ∼ 350 nm, a ∼ 150 nm, and N ∼ 3000). Independently from this model, the contour lengths obtained from the linear mass densities range from 400 nm (corrected for cM) to 600 nm (uncorrected). Finally, a possible transition mode from H to J aggregates has been proposed, based on the linear mass densities obtained, which imply an increase of the aggregates’ cross section upon increasing dyestuff concentration from 1 to 2 nm-1 (uncorrected) and 2 to 6 nm-1 when corrected for cM. Acknowledgment. B.N. wishes to thank Prof. P. Pollmann (Universita¨t Paderborn) for permission of the UV/vis measurements, the CIBA AG (Grenzach-Wyhlen) for giving access to the light scattering facilities, and D. Jacobi (Universita¨t Essen) for performing the dn/dc measurements. Valuable discussions with Dr. J. H. Clint and proofreading by L. Boocock (both University of Hull) are gratefully acknowledged. References and Notes (1) Tiddy, G. J.; Mateer, D. L.; Ormerod, A. P.; Harrison, W, J.; Edwards, D. J. Langmuir 1995, 11, 390. (2) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. B 1996, 100, 2310. (3) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. Faraday Discuss. 1996, 104, 139. (4) Lydon, J. E. Curr. Opin. Colloid Interface Sci. 1998, 3, 458. (5) Lydon, J. E. In Handbook of Liquid Crystals, Vol. 2B.; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W, Vill, V., Eds.; Wiley VCH: Weinheim, 1998; pp 981-1007. (6) Kock-Yee Law, H. C.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257. (7) von Berlepsch, H.; Bo¨ttcher, C.; Ouart, A.; Burger, C.; Da¨hne, S.; Kirstein, S. J. Phys. Chem. B 2000, 104, 5255. (8) Spitz, C.; Da¨hne, S.; Ouart, A.; Abraham, H.-W. J. Phys. Chem. B 2000, 104, 8664. (9) Rubires, R.; Farrera, J.-A.; Ribo, J. M. Chem. Eur. J. 2001, 7, 436. (10) von Berlepsch, H.; Bo¨ttcher, C.; Da¨hne, L. J. Phys. Chem. B 2000, 104, 8792. (11) Neumann, B.; Huber, K.; Pollmann, P. Phys. Chem. Chem. Phys. 2000, 2, 3687. (12) Neumann, B. Langmuir 2001, 17, 2675. (13) Reeves, R. L.; Maggio, M. S.; Harkaway, S. A. J. Phys. Chem. 1979, 83, 2359. (14) Wojtyk, J.; McKerrow, A.; Kazmaier, P.; Buncel, E. Can. J. Chem. 1999, 77, 903. (15) Taboada, P.; Atwood, D.; Ruso, J. M.; Sarmiento, F.; Mosquera, V. Langmuir 1999, 15, 2022. (16) Ruso, J. M.; Attwood, D.; Rey, C.; Taboada, P.; Mosquera, V.; Sarmiento, F. J. Phys. Chem. B 1999, 103, 7092. (17) Ruso, J. M.; Attwood, D.; Taboada, P.; Mosquera, V.; Sarmiento, F. Langmuir 2000, 16, 1620. (18) Taboada, P.; Attwood, D.; Ruso, J. M.; Sarmiento, F.; Mosquera, V. Langmuir 2000, 16, 3175. (19) Jelley, E. E Nature 1936, 138, 1009. (20) Jelley, E. E. Nature 1937, 139, 631. (21) Scheibe, G. Angew. Chem. 1937, 50, 212. (22) Scheibe, G.; Kandler, L.; Ecker, H. Naturwissenschaften 1937, 5, 75. (23) Scheibe, G.; Mareis, A.; Ecker, H. Naturwissenschaften 1937, 29, 475. (24) West, W.; Carrol, B. H. J. Chem. Phys. 1951, 19, 417. (25) Zimmermann, H.; Scheibe, G. Z. Elektrochem. 1956, 60, 566. (26) Scheibe, G. Kolloid-Z. 1938, 82, 1. (27) Scheibe, G.; Scho¨ntag, A.; Katheder, F. Naturwissenschaften 1939, 29, 499. (28) Stegemeyer, H.; Sto¨ckel, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 9.

8274 J. Phys. Chem. B, Vol. 105, No. 34, 2001 (29) Neumann, B.; Pollmann, P. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 15. (30) Neumann, B.; Pollmann, P. Phys. Chem. Chem. Phys. 2000, 2, 4784. (31) Daltrozzo, E.; Scheibe, G.; Gschwind, K.; Haimerl, F. Photogr. Sci. Eng. 1974, 18, 44. (32) Scheibe, G. Z. Elektrochem. 1948, 52, 283. (33) Kopainsky, B.; Kaiser, W. Chem. Phys. Lett. 1982, 88, 375. (34) Graves, R. E.; Rose, P. I. J. Phys. Chem. 1975, 79, 746. (35) Higgins, D. A.; Reid, P. J.; Barbara, P. F. J. Phys. Chem. 1996, 100, 1174. (36) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 371. (37) McRae, E. G. Aust. J. Chem. 1961, 14, 229; 344; 354. (38) Zimm, B. H. J. Chem. Phys. 1948, 16, 1093. (39) Evans, F. D.; Wennerstro¨m, H., The Colloidal Domain, Where Physics, Chemistry, Biology, and Technology meet; VCH: New York, 1994; pp 147. (40) Ingles, S. E.; Katzenstein, A.; Schlenker, W.; Huber, K. Langmuir 2000, 16, 3010. (41) Hamada, K.; Take, S.; Iijima, T.; Amiya, S. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3141.

Neumann (42) Neumann, B. Unpublished results. (43) Millie, P.; Momicchioli, F.; Vanossi, D. J. Phys. Chem. B 2000, 104, 9621. (44) Casassa, E. F. J. Chem. Phys. 1955, 23, 596. (45) Holtzer, A. J. Polym. Sci. 1955, 17, 432. (46) Burchard, W. Static and Dynamic Light Scattering from Branched Polymers and Biopolymers. AdVances in Polymer Sciences; SpringerVerlag: New York, 1983; Vol. 48, p 58. (47) Koyama, R. J. Phys. Soc. Jpn. 1973, 34, 1029. (48) Pedersen, J. S.; Schurtenberger, P. Macromolecules 1996, 29, 7602. (49) Po¨tschke, D.; Hickl, P.; Ballauf, M.; Åstrand, P.-O.; Pedersen, J. S. Macromol. Theory Simul. 2000, 9, 345. (50) Neugebauer, T. Ann. Phys. 1943, 42, 509. (51) Oberthu¨r, R. C. Makromol. Chem. 1978, 179, 2693. (52) Lashuel, H. A.; LaBrenz, S. R.; Woo, L.; Serpell, L. C.; Kelly, J. W. J. Am. Chem. Soc. 2000, 122, 5262. (53) Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; McLeish, T. C. B.; Pitkeathly, M.; Radford, S. E. Nature 1997, 386, 259. (54) Agelli, A.; Bell, M.; Boden, N.; Keen, J. N.; McLeish, T. C. B.; Nyrkova, I.; Radford, S. E.; Semenov, A. J. Mater. Chem. 1997, 7, 1135. (55) Norden, B. J. Phys. Chem. 1977, 81, 151.