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Layered Nanoarchitecture of a Fluorescent Polyelectrolyte Complex Andreas F. Thu¨nemann* and Dirk Ruppelt Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg, 14476 Golm, Germany Received October 12, 1999. In Final Form: December 17, 1999 Multilayer structures of an optically active ionic polymer complex, which are formed by poly(1,4phenyleneethynylenecarboxylic acid) and dimethylditetradecylammonium bromide, were prepared. The complex as bulk material was investigated by small-angle X-ray scattering and found to be lamellar in structure with a repeat unit of 3.2 nm. This mesomorphous structure consists of alternating ionic layers (1.3 nm) and nonionic layers (1.9 nm) with periodic undulations of a hexagonal symmetry and a maximumto-maximum distance of 7.5 nm. The positions of the undulations in adjacent layers are not correlated; this results in typical asymmetric random-layer peak profiles. By contrast, the lamellae of thin complex films (the film thicknesses were in the range of 14-140 nm) show no undulations. X-ray reflectivity and atomic force microscopy (AFM) were used for the film characterization. As revealed by AFM the multilayer films show dewetting instabilities at elevated temperatures. Symmetric complex droplets are the final stage of the dewetting process. The absorptive and emissive behaviors of complex solutions and complex films were examined by UV-vis and fluorescence spectroscopy. It was shown that the supramolecular structuring of the complex has a substantial influence on the optical properties.
Introduction Linear rigid rod polymers with a conjugated π-electron system1,2 and polyelectrolyte-surfactant complexes in the solid state3,4, have attracted considerable interest in materials science. In the past decade, conjugated organic polymers have found applications as emitting layers in light-emitting diodes (LEDs),5,6 “plastic” lasers,7 and lightemitting electrochemical cells,8 just to mention a few examples. In contrast to the strictly crystalline inorganic semiconductors with their well-defined long-range order, conjugated polymers, as organic semiconductors, display a wide variety of different morphologies and solid-state structures with changes in their optical properties and band gaps, even when the same backbone is used. Recently Bunz et al.9 demonstrated the aggregate formation for dialkylpoly(p-phenyleneethynylenes) in solution by fluorescence investigations. A recently developed synthetic method makes the synthesis of poly(phenyleneethynylene) possible without diine defects.10 On the basis of such welldefined polymers, Schnablegger et al. found a three-step hierarchy of structures.11 For example, potassium poly(1,4-phenyleneethynylene carboxylate) forms aggregates similar to cylinders in the first step. In the second step the cylinders form fibrils of a higher order, and in the third step bundles of fibrils associate into globular clusters. The polymer can perform a structure transition toward (1) Grell, M.; Bradly D. C. Adv. Mater. 1999, 11, 895. (2) Fiesel, R.; Halkyard, C. E.; Rampey M. E.; Kloppenburg, L.; StuderMartinez, S. L.; Scherf, U.; Bunz, U. H. F. Macromol. Rapid Commun. 1999, 20, 107. (3) Ober, C.; Wegner, G. Macromolecules 1997, 9, 117. (4) Thu¨nemann A. F. Polym. Int., in press. (5) Neher, D. Adv. Mater. 1995, 7, 691. (6) Kraft, A,; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 37, 403. (7) Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Acc. Chem. Res. 1997, 30, 430. (8) Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (9) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; StuderMartinez, S. L.; Bunz, H. F. Macromolecules 1998, 31, 8655. (10) Ha¨ger, H.; Heitz, W. Macromol. Chem. Phys. 1998, 199, 1821. (11) Schnablegger, H.; Antonietti, M.; Go¨ltner, C.; Hartmann, J.; Co¨lfen, H.; Samori, P.; Rabe, J. P.; Ha¨ger, H.; Heitz, W. J. Colloid Interface Sci. 1999, 212, 24.
double layers when adsorbed on surfaces. Recently we reported on a polyelectrolyte-surfactant complex of dihexadecyldimethylammonium-poly(1,4-phenyleneethynylene carboxylate) which forms an undisturbed lamellar mesophase in the solid state and can be used for a blue LED with a low turn-on point.12 In this paper we report on the first example of a polyelectrolyte complex with a random layer structure formed by the fluorescent dimethylditetradecylammonium-poly(1,4-phenyleneethynylene carboxylate). The complex formation is schematically given in Figure 1. Experimental Section Materials. Poly(2-ethylhexyloxycarbonyl-1,4-phenyleneethynylene) without diine defects was synthesized by the method described by Heitz and Ha¨ger.10 The molecular weigh numbers, as determined by gel permeation chromatography, were Mw ) 101 500 g/mol and Mn ) 12 400 g/mol. Poly(1,4-phenyleneethynylene carboxylate) was prepared from poly(2-ethylhexyleoxycarbonyl-1,4-phenyleneethynylene) by hydrolysis as described elsewere.10,11 The amount of hydrolyzed monomers was 83% (determined by 1H NMR spectroscopy). The surfactant dimethylditetradecylammonium bromide (98%, Aldrich) and the solvent tetrahydrofurane (HPLC grade, Aldrich) were used as received. Complex Preparation. For the complex preparation 1 mmol of the polymer and 1 mmol of the surfactant was soluted in a 1% aqueous NaOH solution heated to 80 °C and treated in a ultrasound bath for 10 min. Both solutions were heated to 80 °C again, and the polymer solution was added slowly to the surfactant solution, which was strongly stirred. The brownish precipitate was centrifuged and washed several times with hot water to remove compounds of low molecular weight. The yield was 80%. For thin film preparation a 1% solution of the complex in chloroform was spin cast onto a silicon wafer at a speed of 3000 rpm. Methods. Wide-angle X-ray scattering (WAXS) measurements were carried out with a Nonius PDS120 powder diffractometer by transmission geometry. A FR590 generator was used as the source for Cu KR radiation, monochromatization of the primary beam was achieved by means of a curved Ge crystal, and the scattered radiation was measured with a Nonius CPS120 position-sensitive detector, which has a resolution of 0.018° in 2θ. Small-angle X-ray scattering (SAXS) measurements were (12) Thu¨nemann, A. F. Adv. Mater. 1999, 11, 127.
10.1021/la9913396 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/11/2000
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Figure 1. Sketch of complex formation. Poly(2-ethylhexyloxycarbonyl-1,4-phenyleneethynylene) (1) was saponified and then complexed with dihexadecyldimethylammonium bromide to dimethylditetradecylammonium poly(1,4-phenyleneethynylene carboxylate) (2).
Figure 2. Wide-angle X-ray scattering diagram of dimethylditetradecylammonium-poly(1,4-phenyleneethynylene carboxylate) (a) and dimethylditetradecylammonium bromide (b). carried out with an X-ray vacuum camera with pinhole collimation (Anton Paar, Austria, model A-8054) equipped with image plates (type BAS III, Fuji). The image plates were read with a MAC Science Dip-Scanner IPR-420 and IP reader DIPR-420. X-ray reflectivity was performed with a θ/2θ instrument (Sot/Superman DF4, U ) 40 kV, I ) 30 mA, λ ) 0.154 nm). The beam divergence of the incoming beam was 0.1°, the resolution in 2θ was 0.05°. A secondary monochromator selected the Cu KR lines; a scintillation counter served as detector. Differential scanning calorimetry measurements were performed on a Netsch DSC 200. The samples were examined at a scanning rate of 10 K min-1 by applying two heating scans and one cooling scan. AFM measurements were carried out at ambient conditions with a Multi Mode from Digital Instruments, operating in tapping mode at a resonance frequency of about 300 Hz. The Si tip had a spring constant of 30-50 mN/m. UV-vis spectra were recorded on a UVICON spectrophotometer 931 (Kontron Instruments). The luminescence of complex films (0.2 mm thickness) was analyzed using a Perkin-Elmer LS-50B luminescence spectrometer with front surface accessory. Excitation spectra were collected within the range of 300-470 nm. The emission wavelength was 550 nm, and the slits were 10/10 mm. Emission spectra were collected in the range of 400-700 nm. The excitation wavelength was 380 nm, and the slits were 10/5 mm.
Results and Discussion Molecular and Supramolecular Structures. Information concerning the molecular packing of the complex 2 material in bulk form was obtained by wide-angle X-ray scattering. It can be seen in Figure 2 (curve a) that only a broad maximum is present in the wide-angle region; this position corresponds to a Bragg spacing of 0.45 nm. This value is typical for long alkyl chain polyelectrolytesurfactant complexes13 and is slightly larger than that found for dihexadecyldimethylammonium-poly(1,4-phen-
Thu¨ nemann and Ruppelt
Figure 3. Small-angle X-ray scattering diagram of (2) (solid line) and a fit (dotted line) which is a superimposition of two random layer profiles according to eq 3 with the indices (100) and (110) and three equidistant Lorentzian peak profiles indexed as (001), (002), (003).
yleneethynylene carboxylate) (0.43 nm).12 By contrast, numerous sharp reflections are present in the pattern of the pristine crystalline surfactant (Figure 2, curve b). This proves that 2 contains neither side-chain crystalline regions nor crystalline impurities of noncomplexed surfactant molecules or sodium bromide. The absence of crystallinity was verified by differential calorimetry measurements. Within a temperature range of -100 to 250 °C, a glass transition was found in the thermograms at -47 °C, but there were no melt transitions. This is in contrast to a homologue complex with longer alkyl chains, dihexadecyldimethylammonium-poly(1,4-phenylene-ethynylene carboxylate), which shows, in addition to a glass transition (Tg ) -41 °C), a side-chain crystallinity. About 10% of the alkyl chains are crystalline at temperatures below -16 °C. We conclude that the transition from a noncrystalline to a side-chain crystalline complex lies between a chain length of C14 and C16. The small-angle scattering pattern of 2 in bulk material is characteristic (see Figure 3). In addition to three sharp symmetric reflections at higher angles with relative positions of 1:2:3, two asymmetric reflections at lower angles are present with the relative positions of 1:31/2. The latter show a steep decrease in direction of the lowangle side and a slow decrease toward the high-angle side. Such a scattering pattern is typical of stacks of random layers. We ascribe this X-ray pattern to stacks consisting of alternating ionic layers, which are formed by the polyelectrolyte and the ionic headgroups, and nonionic layers, which contain the alkylated surfactant tails. The long period was determined to be 3.20 nm from the positions of the (001), (002), and (003) reflections, and from their width the correlation length was given to be in the range 300-500 nm. A fit of the lamellar reflections with Lorentzian profiles is shown in Figure 3. The large value of the correlation length must result from a good stacking order and, to our knowledge, is the highest value observed to date for polyelectrolyte-surfactant complexes. For the determination of the layer thicknesses, the “stacking model” is used14,15 in which the statistics of the lamellar stacks are determined by the distribution h1(d1) and h2(d2) of the thicknesses of the two lamellae d1 and d2. These distributions are considered to be statistically (13) Antonietti, M.; Conrad, J.; Thu¨nemann, A. F. Macromolecules 1994, 27, 6007. (14) Wolff, T.; Burger, C.; Ruland, W. Macromolecules 1994, 27, 3301. (15) In Hosemann, R.; Bagchi, S. N. Direct Analysis of Diffraction by Matter; North-Holland Publishing Co.: Amsterdam, The Netherlands, 1962.
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independent; i.e., there is no correlation between the two variances σ12 and σ22. The period d is given by d ) d1 + d2. For a randomly oriented two-phase system with sharp boundaries and normalized distributions h1(d1) and h2(d2), the scattering intensity for the stacking model is given by
I(s) )
[
]
(1 - H1(s))(1 - H2(s)) k Re 3 4 1 - H1H2(s) 4π s
(1)
where Hn is the one-dimensional Fourier transform of hn which is exp(-2πiσis2 - 2πidis) and Re stands for the “real part”. A fit of eq 1 to the scattering curve results in d1 ) 1.30 ( 0.05 nm and d2 ) 1.9 ( 0.05 nm (not shown). From the estimated volume fractions of polyelectrolyte and surfactant we can assign d1 to the thickness of the ionic layers and d2 to the nonionic layers. Interesting details of the scattering curve are the random layer profiles, which may be caused by hexagonal undulations or perforations. Both, periodic undulations and periodic perforations within the layers represent two-dimensional lattices, which lead to additional reflections. If the undulations of different layers are not correlated, then the peak profiles are asymmetric. In the event of truly random orientations of the two-dimensional layers, no (00l) reflections could be produced, but in the great majority of cases the layers tend to stack together into parallel-layer groups as is to be expected.16 These structures have no interlayer order, other than the parallelism and degree of separation of the layers. Therefore 00l and hk0 but no mixed hkl reflections are present. The scattering of random-layer (turbostratic) structures was described in detail by Ruland.17,18 An exact solution for a Lorentzian intensity distribution {Ih} ) L/(1 + π2L2s2) normal to an hk reciprocal lattice rod was given as
I(s) )
( ) [ (
1 L 4s πs
1/2
F
)]
1 πL 2 s - s2h - 2 2 2s πL
(2)
where
F(z) )
(
)
(z2 - 1)1/2 + z z2 + 1
1/2
(3)
and L is defined as the integral width of {Ih}. Peak profiles of eq 2 are shown in Figure 3. From the peak positions of the (100) and (110) reflections, given by eq 2, we calculated the distance between the perforations or undulations of 7.5 nm. It must be stressed here that it is not possible, on the basis of the scattering data, to distinguish between an undulated and a perforated structure. This distinction has to be made by further experiments (e.g., by gas permeation measurements). Therefore, we tentatively propose a model with hexagonal undulated ionic layers as shown in Figure 4. Regular undulations of a polyelectrolyte surfactant complex were first proposed by Antonietti et al. and called the “mattress” phase.13 In that dimethylhexadecylammonium-polystyrenesulfonate complex the “mattress” ripples of alternating layers are correlated. Recently quadratically undulated lamellar bilayers, stacked in a rotational disordered way were proposed for the structure of surfactant bilayers after polyelectrolyte addition.19 In contrast to the water-free structure described (16) In Klug, H. P.; Alexander, L. E. X-ray diffraction procedures for polycrystalline and amorphous materials, 2nd ed.; Wiley-Interscience Publications: New York, 1974. (17) Ruland, W. Acta Crystallogr. 1967, 22, 615. (18) Ruland, W.; Tompa, H. Acta Crystallogr. 1968, 24, 93.
Figure 4. Model proposed for the interpretation of the random layer profiles of the small-angle scattering curves of (2) with hexagonal undulations of the ionic layers. Alkyl chains in an amorphous state fill the space between adjacent layers.
here, the quadratically undulated lamellar phase described earlier exists as a highly diluted phase in water with repeat units of 200 nm. In previous studies we described polyelectrolyte-lipid complexes which show highly, but nonperiodic, undulations20 which were regarded as “plastic membranes” as well as periodic superstructures of a quadratic order, and of egg carton shape.21 In both cases the fine tuning of the structure is given by a variation of the ionic headgroups. The period of the undulations of complex 2 (7.5 nm) is very close to that calculated in terms of bending elasticity by Goetz and Helfrich for periodically curved membrane superstructures (ca. 7 nm).22 Probably the undulations are preliminary stages of regular pores within the complex layers such as simulated by Mu¨ller and Schick for polymeric bilayers, which originate from peristaltic fluctuations of the bilayer thickness.23 In addition, undulation on length scales only somewhat larger than the bilayer thickness play a role in the interpretation of the physical properties of lipid membranes.24 Complex Films at Silicon Surfaces. Thin complex films on silicon wafers were prepared by the spin-coating technique from complex solutions. The film structure was then investigated by X-ray reflectivity. Typical examples are shown in Figure 5. To our surprise well-defined doublelayer stacks in the range of 14 nm to about 140 nm developed within some seconds simply as the result of the deposition of droplets of the complex solution. In Figure 5 the presence of Kiessig fringes indicates a smooth complex film. A thickness of 14 ( 0.5 nm was calculated from the angular positions of the fringes.25 The intensity of the fringe around 0.3 nm-1 is significantly enhanced compared to that expected from a single layer film. This can be interpreted as a multilayer structure consisting of (19) Berlepsch, H.; Burger, C.; Dautzenberg, H. Phys. Rev., E 1998, 58, 7549. (20) Antonietti, M.; Kaul, A.; Thu¨nemann, A. F. Langmuir 1995, 11, 2638. (21) Antonietti, M.; Wenzel, A.; Thu¨nemann, A. F. Langmuir 1996, 12, 2111. (22) Goetz R.; Helfrich, W. J. Phys. II 1996, 6, 215. (23) Mu¨ller M.; Schick, M. J. Chem. Phys. 1996, 105, 8282. (24) Goetz R.; Gompper, G.; Lipowsky, R. Phys. Rev. Lett. 1999, 82, 221. (25) Holy, V.; Pietsch U.; Baumbach, T. In Springer Tracts in Modern Physics, Vol. 149: High-Resolution X-ray Scattering from Thin Films and Multilayers; Springer: Berlin, 1999.
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Figure 5. X-ray reflectivity curves of thin complex films on silicon wafers. Curve a results from a complex film with a thickness of 14 nm consisting of four double layers plus one ionic layer at the wafer/complex interface. The thickness was determined from the position of angular positions of the Kiessig fringes. Curve b results from a film of about 140 nm in thickness (determined by a surface profiler). The peaks correspond to the first and second Bragg reflection of the lamellar stacking.
alternating ionic and nonionic layers with the same repeat unit as in the material in bulk form. Assuming this, the reflectivity curve can be explained as resulting from four double layers (ionic-nonionic) plus an ionic layer at the complex/wafer interface. For a film of about 140 ( 20 nm thicknesssdetermined by a surface profilerstwo sharp Bragg reflections with equidistant positions are found (Figure 5b). A lamellar spacing of 3.18 ( 0.02 nm was calculated from their positions. Within the experimental error this value is identical to that determined for the complex material in bulk form (3.2 nm). From the integral width of the reflections, a correlation length of about 140 nm was determined. This is of the same order of magnitude as the film thickness. Therefore we conclude that the complex layers are oriented almost perfectly parallel to the wafer surface. The strong depression of the asymmetric reflectionssonly a weak shoulder is present at about 0.28 nm-1sis probably due to boundary effects of the underlying wafer surface, which prevent undulations of the lamellae. To prove this assumption, visualization of the complex surfaces was achieved by atomic force microscopy (AFM). The film surface is smooth at a micrometer scale, whereas defects at the edges of the wafer can be found at a nanometer level (Figure 6a). These defects have the form of plate islands consisting of terraces of differing height. Each terrace has a height of about 3 nm, which is in good agreement with the lamellar repeat unit. As expected from the reflectivity measurements in addition to defects no regular undulations on the surface were found. At elevated temperatures (40 °C) self-dewetting of the complex films sets in quickly and finally results in symmetric complex droplets with diameter sizes in the range of 50-500 nm and heights in the range of about 30-300 nm (see Figure 6b). Therefore we conclude that the flat lamellar complex structures are metastable when oriented macroscopically onto a wafer surface. Such instabilities of thin films have not yet been reported on polyelectrolyte-surfactant complexes, but they are wellknown for typical polymers. For example Reiter et al. showed that even dispersion forces of less than 1 Pa are capable of destroying 100 nm thick polymer films of poly(dimethylsiloxane).26 Vandenbrouck et al. have shown that liquid crystal films show a dewetting of thin films (26) Reiter G.; Sharma A.; Casoli, A.; David, M.-O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551.
Figure 6. AFM images of a complex 2 surface coated on a silicon wafer. Defects were found as terraces of different heights (a). The final stage of the self-dewetting process (2 weeks at 40 °C) is symmetric complex droplets (b). The lower figure (c) shows the height profile of the terraces from a section of (a, straight line). The height of the steps is about 3 nm.
while thick ones remain stable. He interpreted this behavior as a competition between elasticity and van der Waals forces.27 Further, Mo¨ller et al. investigated the self-dewetting of smectic A side-chain liquid crystalline polymers and discussed it in terms of progressive disordering of the side chains with increasing distance from the substrate.28 As for these polymers, the dewetting phenomena of complex 2 films have an important influence on any possible thin film applications of such complexes. Optical Properties. Neher29 pointed out that morphology plays a crucial role in the manipulation of the optical properties of conjugated polymers, and recently Bunz et al. gave evidence of lamellar, nematic, and sanidic crystalline structures of alkylated poly(p-phenyleneethynylene)s which were proposed to allow an improved design of optical devices based on these organic semiconductors to be developed.30 As Bunz showed for the side-chain crystalline polymers, we expect a strong influence of the mesomorphous complex structure on its optical properties. To prove this assumption, we performed UV-vis and fluorescence measurements on the complex in solution and in the solid state. The absorption spectra of complex solutions in tetrahydrofuran, methanol, methanol/chloroform (1:1) mixture, and chloroform are shown in Figure 7a. It can be seen there that complex 2 is a strong solvatochromic compound. In chloroform three transitions are present at 277, 296, and 315 nm. In a 1:1 mixture of methanol and chloroform the higher energetic transition is reduced, and in methanol clear maxima at 294 and 311 nm are present. The maximum at 277 nm is absent. In (27) Vandenbrouck F.; Valignat M. P.; Cazabat, A. M. Phys. Rev. Lett. 1999, 82, 2693. (28) Sheiko, S.; Lermann, E.; Mo¨ller, M. Langmuir 1996, 12, 4015. (29) Neher, D Adv. Mater. 1995, 7, 691. (30) Kloppenburg, L.; Jones, D.; Claridge, J. B.; Loye, H.-C.; Bunz U. H. F. Macromolecules 1999, 32, 4460.
Multilayer Structures
Figure 7. Absorption spectra of complex 2 in different solvents (10-3%) (a) and in the solid state (b, solid line). The dotted line of (b) is the absorption spectrum of poly(2-ethylhexyloxycarbonyl-1,4-phenyleneethynylene).
Figure 8. Emission spectra of poly(2-ethylhexyloxycarbonyl1,4-phenyleneethynylene) (a) and the complex 2 in tetrahydrofuran (b) at different concentrations (the excitation wavelength is 380 nm).
a solution of tetrahydrofuran a strong band at 278 nm dominates the spectrum. Only a weak maximum is present at 315 nm. We assume that the complex in all the solutions has at least three different possible transitions in the UVvis region (277, 296, and 315 nm). The solvent may have an influence on the conformational state of 2 which influences its absorption spectrum. In Figure 7b it can be seen that the absorption spectrum of the complex in the solid state with a strong transition at 268 and a weak transition at 326 nm resembles that of a complex solution in tetrahydrofuran but shows a stronger absorbance at lower wavelengths. By contrast, the absorbance of 1 is dominated by a first maximum at 413 nm and a second at 444 nm. The latter is similar to that observed by Bunz for aggregates of alkylated poly(1,4-phenyleneethynylene).9 He interpreted the first maximum as the absorbance of the undisturbed polymer and the second maximum results from aggregates, which form excimers. From the similarity of the UV-vis spectrum of the aggregates observed by Bunz in solution and that of 2 as a thin film, we conclude that the molecular order within aggregates and within the complex films is probably similar. Further Bunz showed that the fluorescence emission spectra depend on the polarity of the solvent: the higher the amount of methanol in a chloroform/ methanol mixture, the higher the excimer intensity was. It can be seen in Figure 8a that the emission spectrum of 1 in tetrahydrofuran also depends significantly on the polymer concentration. The emission maximum at 428 nm shifts from 428 nm at a concentration of 10-4% (w/w)
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Figure 9. Fluorescence excitation and emission spectra of 1 and 2 in solutions of 0.1% (w/w) tetrahydrofuran (a) and as solid-state materials (b). The dashed lines correspond to 1 and the solid lines to 2. The excitation wavelength is 380 nm; the emission wavelength is 550 nm.
and 431 nm at 10-3% (w/w) to 435 nm at 10-2% (w/w). Within this line the amount of excimer emission (455 nm) increases considerably. Obviously the aggregate formation depends strongly on the polymer concentration and sets in already at very low concentrations. This observation is consistent with the work of Schnablegger et al.11 who found a complicated aggregation behavior in 1. By contrast, no concentration dependence of the emission spectrum was found in the same range of concentration for the complex 2. It can be seen in Figure 8b that the relative intensities of the emission maxima at 430 and 453 nm do not depend on the concentration within the same range of concentration. From this we conclude that aggregates are present in the solution of 1 and 2, but the aggregates of 2 are constant in their composition over a large range of concentrations of at least 3 decades while the aggregate composition of 1 depends strongly on its concentration. A direct comparison of the fluorescence properties of 1 and 2 at a higher concentration is shown in Figure 9. In a 0.1% solution of tetrahydrofuran the excitation of the complex (Figure 9a solid line) and that of the alkylated polymer (Figure 9a dashed line) have a maximum at 393 nm. The spectra differ slightly in the regions of their lower wavelengths. In the emission of the complex (Figure 9a, solid line) a strong maximum is found at 429 nm and a weak maximum at 455 nm (excimer). Compared to that the emission spectrum of 1, it shows a slight red-shift of the intense maximum (435 nm) and an increased intensity of the second maximum, which indicates a higher amount of excimers. Apart from these details the emission in solutions of 1 and 2 show the same characteristics. The excitation spectrum of 1 in the solid state has maxima at 400 and 439 nm, that of 2 at 408 and 445 nm (see Figure 9b). The band at a higher wavelength is more intense for 2 than for 1. The emission spectra of both compounds differ strongly. That of 1 is structureless with a broad and intense maximum at 498 nm and a weak maximum at 420 nm, whereas the emission spectrum of 2 is highly structured with maxima at 425, 458, 486, 496, 520, and 575 nm (see Figure 9b). This was not expected, and the reason is not yet clear. For a first interpretation we assume that the structure of the emission results from the regularity of the undulations of the lamellar structure, which may lead to a number of definite emission bands. But more detailed work has to be done in order to clarify the relation between the mesomorphous ordering and the optical properties of fluorescent polyelectrolyte surfactant complexes.
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Conclusion We have demonstrated that the optical properties of the highly fluorescent alkylated polymer 1 can be changed to a considerable degree by its conversation to the ionic polymer complex 2. The effect of complexation is pronounced in the solid state where the emission properties of 1 are structureless, while those of 2 show a detailed structuring. The solid-state structure of 2 is lamellar with a hexagonal in-plane order of undulations but with no plane-to-plane correlation. The complex can be prepared as a highly ordered multilayer film in a single-step
Thu¨ nemann and Ruppelt
procedure, which makes it, in principle, superior because it is quicker to produce compared to established methods for the formation of thin multilayer films. Acknowledgment. The authors thank Professor W. Heitz for providing samples of poly(2-ethylhexyleoxycarbonyl-1,4-phenyleneethynylene), Dr. C. Burger for a discussion of the X-ray data, S. Kubowicz for X-ray reflectivity, E. Klok-Lermann for the AFM measurements, and the Max Planck Society for their financial support. LA9913396
