Photoluminescent Amphiphilic 1,4-Diketo-3,6-diphenylpyrrolo

Photoluminescent Amphiphilic 1,4-Diketo-3,6-diphenylpyrrolo-[3,4-c]-pyrrole Derivative and Its Complexes with Polyelectrolytes .... photovoltaic mater...
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Langmuir 2002, 18, 3815-3821

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Photoluminescent Amphiphilic 1,4-Diketo-3,6-diphenylpyrrolo-[3,4-c]-pyrrole Derivative and Its Complexes with Polyelectrolytes Markus Behnke† and Bernd Tieke* Institut fu¨ r Physikalische Chemie der Universita¨ t zu Ko¨ ln, Luxemburgerstr. 116, D-50939 Ko¨ ln, Germany Received December 7, 2001. In Final Form: February 18, 2002 The new amphiphile 1,4-diketo-5-methyl-2-(11-undecyl sulfate sodium salt)-3-(4-chlor)phenyl-6-phenylpyrrolo-[3,4-c]-pyrrole (Na-1) is described, carrying the diketopyrrolopyrrole chromophore in the hydrophobic part. An aqueous solution of Na-1 exhibits an orange color with an absorption maximum at 475 nm and a red-orange fluorescence with a maximum at 529 nm. The critical micelle concentration in water is 4.1 × 10-5 mol/L. Highly concentrated aqueous solutions exhibit lyotropic phase behavior. Na-1 crystallizes into a lamellar structure. After heating to the melting point, the compound does not recrystallize upon cooling but forms a supercooled melt with a glass transition temperature. The combination of aqueous solutions of Na-1 and cationic polyelectrolytes in a stoichiometric ratio leads to the formation of polyelectrolyte-surfactant complexes with mesomorphous structure and a glass transition temperature depending on the structure of the polyelectrolyte. Complexes of Na-1 with poly(allylamine hydrochloride) and poly[(methylbutylimino)hexy-lene bromide] are characterized. The optical properties of the complexes are comparable with those of Na-1, while the solubility is clearly lower. Because of their convenient optical properties and their plasticity and resistance to solvents, the complexes represent interesting new materials with potential applications as coloring agents.

1. Introduction In our contribution, a new photoluminescent amphiphile containing the diketodiphenylpyrrolopyrrole (DPP) chromophore1 and its use for the preparation of polyelectrolyte-surfactant complexes are described. DPP derivatives are technically important compounds, some derivatives being used as commercial red pigments because of their brilliant red color and high photostability in the solid state.2

Recent studies have shown that upon the incorporation of DPP units in polymers3,4 and dendrimers5 novel highly photoluminescent3,5 and electroluminescent6 polymeric * To whom correspondence should be addressed. E-mail: tieke@ uni-ko¨ln.de. † Present address: The Ohio State University, Department of Physics, 174 West 18th Avenue, Columbus, OH 43210. E-mail: [email protected]. (1) Farnum, D. G.; Mehta, G.; Moore, G. G. I.; Siegel, F. P. Tetrahedron Lett. 1974, 29, 2549. (2) (a) Hao, Z.; Iqbal, A. Chem. Soc. Rev. 1997, 26, 203. (b) Iqbal, A.; Jost, M.; Kirchmayr, R.; Pfenniger, J.; Rochat, A.; Wallquist, O. Bull. Soc. Chim. Belg. 1988, 97, 615. (3) Lange, G.; Tieke, B. Macromol. Chem. Phys. 1999, 200, 106. (4) Beyerlein, Th.; Tieke, B. Macromol. Rapid Commun. 2000, 21, 182. (5) Hofkens, J.; Verheijen, W.; Shukla, R.; Dehaen, W.; De Schryver, F. C. Macromolecules 1998, 31, 4493. (6) Beyerlein, Th.; Tieke, B.; Forero-Lenger, St.; Bru¨tting, W. Synth. Met. 2002, accepted.

materials become accessible. Moreover, DPP-based bolaamphiphiles have been prepared7 and used for building deeply colored self-assembled multilayers with oppositely charged polyelectrolytes.7 Up to now, several examples of luminescent surfactants8-10 and polyelectrolyte-surfactant complexes11,12 were reported containing xanthene11 and cyanine dyes,10,12 the viologen moiety,9 and others8 as the luminescent chromophore. However, in none of these studies was a technically important chromophore with a high fluorescence quantum yield such as diketopyrrolopyrrole used. Therefore, we prepared the DPP-containing anionic amphiphile 1 (sodium salt, Na-1) (Scheme 1) and studied its characteristic properties. In this article, the aggregation behavior of Na-1 in the aqueous phase, the optical and thermal properties, and the structure of solid Na-1 derived from small- and wide-angle X-ray studies are reported. If ionic surfactants are combined with oppositely charged polyelectrolytes, so-called polyelectrolyte-surfactant complexes13-15 are obtained. Complexes with a 1:1 molar ratio of the two components are known to display mesomorphous phase behavior with formation of lamellar and cylindrical phases13 and to show interesting materials properties, if appropriate surfactants and polyelectrolytes (7) Saremi, F.; Lange, G.; Tieke, B. Adv. Mater. 1996, 8, 923. (8) (a) Rehfeld, S. J. J. Colloid Interface Sci. 1970, 34, 518. (b) Schore, N. E.; Turro, N. J. J. Am. Chem. Soc. 1974, 96, 306. (9) Takuma, K.; Sakamoto, T.; Nagamura, T.; Matsuo, T. J. Phys. Chem. 1981, 85, 619. (10) Barni, E.; Savarino, P.; Pelizzetti, E.; Rothenberger, G. Helv. Chim. Acta 1981, 64, 1943. (11) Koizumi, M.; Mataga, N. J. Am. Chem. Soc. 1954, 76, 614. (12) (a) Horng, M.-L.; Quitevis, E. L. J. Phys. Chem. 1993, 97, 12408. (b) Rousseau, E.; van der Auweraer, M.; De Schryver, F. C. Langmuir 2000, 16, 8865. (c) Liao, X.; Higgins, D. A. Langmuir 2001, 17, 6051. (d) Bourbon, S.; Gao, M.; Kirstein, S. Synth. Met. 1999, 101, 152. (13) (a) Antonietti, M.; Burger, C.; Conrad, J.; Kaul, A. Macromol. Symp. 1996, 106, 1. (b) M Antonietti, Thu¨nemann, A. F. Curr. Opin. Colloid Interface Sci. 1996, 1, 667. (14) Zhou, S.; Chu, B. Adv. Mater. 2000, 12, 545. (15) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17.

10.1021/la011773j CCC: $22.00 © 2002 American Chemical Society Published on Web 04/09/2002

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Langmuir, Vol. 18, No. 10, 2002 Scheme 1. Preparation of Na-1

Behnke and Tieke Scheme 2. Preparation of PAH-1

Scheme 3. Preparation of Ionene-1

are combined. Relevant complexes with low surface energy,16 photo- and electroluminescent properties,17 and the ability to immobilize or release drugs18 and proteins19 have already been described. By mixing aqueous solutions of Na-1 and poly(allylamine hydrochloride) (PAH) or poly[(methylbutylimino)hexylene bromide] (PMHB), deeply colored and fluorescing polyelectrolyte-surfactant complexes could be prepared, which are denoted as PAH-1 and PMHB-1, respectively. In this article, the characteristic properties of these complexes are reported. 2. Experimental Section Materials. PAH (molecular weight, 5-6 × 104) was obtained from Aldrich, and the ionene PMHB was kindly supplied by M. Dreja, who prepared the polymer according to the literature.20 The amphiphilic sodium salt Na-1 (1,4-diketo-5-methyl-2-(11undecyl sulfate sodium salt)-3-(4-chlor)phenyl-6-phenylpyrrolo[3,4-c]-pyrrole) was prepared from 1,4-diketo-5-methyl-3-(4chlor)phenyl-6-phenylpyrrolo-[3,4-c]-pyrrole 2,21 which was kindly supplied by Dr. S. H. Eldin, Ciba Specialty Chemicals, Basel. The synthesis is outlined in Scheme 1. In a first step, 2 was reacted with 11-bromo-1-undecanol and potassium carbonate in dimethyl formamide to yield 1,4-diketo-5-methyl-2-(11-hydroxyundecyl)-3-(4-chlor)phenyl-6-phenylpyrrolo-[3,4-c]-pyrrole 3.21 Subsequently, 3 was converted into Na-1 according to the following procedure: Preparation of Na-1 (Scheme 1). A solution of 4.80 g of pyridine-SO3 complex (Fluka) in 30 mL of dimethyl formamide (DMF) is added dropwise under nitrogen at 60 °C within 1 h to (16) (a) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1998, 14, 4898. (b) Thu¨nemann, A. F.; Schno¨ller, U.; Nuyken, O.; Voit, B. Macromolecules 1999, 32, 7414. (17) (a) Thu¨nemann, A. F. Adv. Mater. 1999, 11, 127. (b) Thu¨nemann, A. F.; Ruppelt, D. Langmuir 2001, 17, 5098. (18) (a) Thu¨nemann, A. F. Langmuir 1997, 13, 6040. (b) Thu¨nemann, A. F.; Beyermann, J. Macromolecules 2000, 33, 6878. (19) (a) Ponomarenko, E. A.; Tirrell, D. A.; Macknight, W. J. Macromolecules 1998, 31, 1584. (b) Wang, L.; Yoshida, J.; Ogata, N.; Sasaki, S.; Kajiyama, T. Chem. Mater. 2001, 13, 1273. (20) (a) Sonessa, J.; Cullen, W.; Ander, P. Macromolecules 1980, 13, 196. (b) Knapick, E. G.; Hirsch, J. A.; Ander, P. Macromolecules 1985, 18, 1015. (c) Knapick, E. G.; Ander, P.; Hirsch, J. A. Synthesis 1985, 58. (21) Ciba Specialty Chemicals (Inv. S. H. Eldin and A. Iqbal); Eur. Pat. 0787731, 1997.

a stirred solution of 6.60 g of 3 in 150 mL of DMF. After further stirring at 60 °C for 48 h, 100 mL of DMF is removed in a vacuum and the residue is poured in a concentrated aqueous NaCl solution. An orange precipitate is obtained, which is filtered off and suspended in 500 mL of water. After 3-fold extraction with 250 mL of ether, the product is isolated from the aqueous phase by evacuating the water at 60 °C. An orange polycrystalline powder (5.98 g, 75.5% of theory) of Na-1 is obtained. Melting point: 115 °C. 1H NMR (D2O, 80 °C): δ (ppm) ) 1.0-1.6 (m, 16 H, -(CH2)8-), 1.67 (m, 2H, β-CH2), 3.22 (s, 3 H, >N-CH3), 3.71 (t, 2 H, -CH2OSO3-), 4.10 (m, 2 H, >N-CH2-), 7.3-7.9 (m, 9 H, aromatic H) (see also Figure 2). FTIR (KBr): 2924 cm-1 (νas CH2), 2853 cm-1 (νs CH2), 1678 cm-1 (ν CdO), 1612 cm-1 (ν CdCarom), 1220 cm-1 (νas -OSO3-), 1091 cm-1 (νs -OSO3-) (see also Figure 10). Preparation of the Polyelectrolyte Complex PAH-1 (Scheme 2). Na-1 (2.0 g) is dissolved in 175 mL of water under stirring and heating to 60 °C. Subsequently, a solution of 0.31 g of PAH in 25 mL of water is added dropwise. The fine precipitate is filtered off, washed with water several times, and finally dried in a vacuum oven at 60 °C. Complex PAH-1 (1.89 g, 97.2% of theory) is obtained as an orange powder. FTIR (KBr): 31002800 cm-1 (νas, νs N-H), 2923 cm-1 (νas CH2), 2852 cm-1 (νs CH2), 1677 cm-1 (ν CdO), 1611 cm-1 (ν CdCarom), 1205 cm-1 (νas -OSO3-), 1090 cm-1 (νs -OSO3-) (see also Figure 10). Preparation of the Polyelectrolyte Complex PMHB-1 (Scheme 3). The complex was prepared according to the procedure described for PAH-1, that is, stoichiometric amounts of Na-1 and PMHB in aqueous solutions were mixed at 60 °C under stirring. The complex formed as a fine precipitate, which was filtered off, washed with water several times, and finally dried in a vacuum oven at 60 °C. Complex PMHB-1 (2.31 g, 95% of theory) is obtained as an orange powder. FTIR (KBr): 31002800 cm-1 (νas, νs N-H), 2926.6 cm-1 (νas CH2), 2853.1 cm-1 (νs CH2), 1711.0 cm-1 (ν CdO), 1611 cm-1 (ν CdCarom), 1451.2 cm-1 (δs CH2, δas CH3), 1211.1 cm-1 (νas -OSO3-), 1090.7 cm-1 (νs -OSO3-). Methods. The 1H NMR spectra were recorded on an 80 MHz spectrometer (Brucker AC 80). Tetramethylsilane was used as the standard. The UV/vis spectra were recorded on a Perkin-

Photoluminescent Amphiphilic Pyrrole Derivative

Figure 1. UV/vis absorption and fluorescence excitation and emission spectra of Na-1 in dimethyl sulfoxide. The Stokes shift ∆λ is also indicated. Fluorescence was excited at 475 nm, and the excitation spectrum was monitored at 529 nm. Elmer Lambda 14 spectrometer, the fluorescence spectra were measured on a Perkin-Elmer LS 50 B spectrometer, and for the Fourier transform infrared (FTIR) spectra a Perkin-Elmer Spektrum 1000 spectrometer was used. Samples were measured as KBr pellets. Differential scanning calorimetry was carried out using a Perkin-Elmer DSC 2 apparatus. The heating and cooling rate was 10 K min-1. For calibration, indium and tin were used. All measurements were carried out in closed aluminum pans, and the weight of the samples was 4-10 mg. Wide-angle X-ray diffraction studies were carried out on a Philips PW 1050/25 powder diffractometer. Small-angle X-ray measurements were carried out on samples in fine glass capillary tubes (diameter, 1 mm) using a Kratky camera (Anton Paar) with block collimation. The acceleration voltage was 40 kV at the anode current of 30 mA. Reflexes were monitored in continuous scan mode. The scattering curves were corrected for slit-smearing effects by a computational desmearing procedure.22 For all X-ray studies, Ni-filtered Cu KR radiation was used. Polarizing micrographs were taken on a standard Zeiss microscope equipped with hot stage and camera (Zeiss MC 80). Static surface tension measurements of the aqueous surfactant solutions were performed using the Wilhelmy plate technique (Kru¨ss digital tensiometer K 10 ST; temperature, 25 ( 0.1 °C).

3. Results and Discussion 3.1. Preparation of Amphiphile 1. The dye-containing anionic amphiphile 1 was prepared from the Nmonosubstituted 1,4-diketo-2-methyl-3(4-chlor)phenyl-6phenylpyrrolo-[3,4-c]-pyrrole 2.21 2 was converted into the corresponding long-chain alcohol derivative 3 upon substitution of the lactam hydrogen with 11-bromo-1-undecanol. Finally, 3 was converted into the sodium salt of 1 (Na-1) upon reaction of the alcohol group with the pyridine-sulfur trioxide complex. The complete reaction pathway is outlined in Scheme 1. The total yield of the two reaction steps was only 33% of the theory mainly because of side reactions in the first step, which made it necessary to purify compound 3 using column chromatography. While 3 was obtained only as a viscous oil, the final compound Na-1 could be precipitated as a polycrystalline solid. In aqueous solution, 1 showed extensive foam formation, a clear indication of the surfactant properties of the compound. 3.2. Characteristic Properties of Na-1 in Solution. The sodium salt of the DPP-based amphiphile, Na-1, exhibits an orange color and shows a strong red-orange fluorescence in solution. In Figure 1, the UV/vis absorption, (22) (a) Kops, A. Dissertation, Universita¨t zu Ko¨ln, Shaker, Aachen, Germany, 1995. (b) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley: New York, 1955.

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Figure 2. 1H NMR spectrum of Na-1 in deuterium oxide at 80 °C. At room temperature, the surfactant aggregates into micelles and the spectrum is poorly resolved.

excitation, and photoluminescence spectra of the amphiphile in dimethyl sulfoxide are shown. The maxima of the absorption and fluorescence occur at 475 and 529 nm, respectively, and the Stokes shift is 54 nm. The extinction coefficient of Na-1 at the absorption maximum is 17 040 (1000 cm2 mol-1). Similar high values were also reported for other N-substituted DPP-derivatives.23 The tangent method described by Langhals24 offers a better description of overlap of absorption and fluorescence spectra than the commonly used Stokes shift. According to this method, a spectrum separation of 24 nm is obtained. For a detailed characterization, proton NMR spectra of Na-1 in deuterium oxide were measured. At room temperature, the compound strongly aggregates in solution and the spectrum is only poorly resolved. However, heating of the sample to 80 °C, that is, to a temperature above the clearing point, produced a spectrum with much better resolution. As shown in Figure 2, the hyperfine structure of all peaks is recognizable except for the expected triplet of the CH2 group in the R position to the sulfate group. The integration of the peaks is in good agreement with their assignment. For a more detailed study of the aggregation behavior, surface tension measurements of Na-1 in aqueous solution were carried out. In Figure 3, the surface tension γ is plotted versus the logarithm of the surfactant concentration in weight percent (wt %). The curve shows the expected behavior with a strongly increasing surface tension at concentrations below the critical micelle concentration, cmc. The cmc was 2.5 × 10-3 wt %, which corresponds to 4.1 × 10-5 mol/L. From the slope of (dγ/d ln c) at concentrations below the cmc, the surface excess concentration can be calculated using the Gibbs equation,

Γ)-

dγ 1 nRT d ln c

(

)

T′

(1)

where n depends on the number of molecular species (with n being 2 for a 1:1 ionic surfactant in a solution of low concentration and at complete dissociation25 as in the case of R-OSO3-Na+). Since the slope was 9 × 10-3 N m-1 at 25 °C, a Γ value of 1.815 × 10-6 mol m-2 could be calculated. (23) (a) Potrawa, T.; Langhals, H. Chem. Ber. 1987, 120, 1075. (b) Mizuguchi, J.; Wooden, G. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1264. (24) Langhals, H. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 730. (25) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1988; pp 64-69.

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Figure 3. Plot of surface tension γ versus logarithm of surfactant concentration of Na-1 in water at room temperature.

Figure 4. Molecular model of Na-1 (blue, nitrogen; red, oxygen; green, chlorine; yellow, sulfur; purple, sodium; black and white, carbon and hydrogen).

Knowing Γ, the headgroup area A of the surfactant can be obtained according to

A ) (NAΓ )-1

(2)

Using this equation, an A value of 0.91 nm2 is found for a single anion 1. This value is significantly higher than for the dodecylsulfate anion of SDS (A ) 0.53 nm2),26 for example. It indicates that the interfacial aggregation of 1 is not merely determined by the packing of the hydrophilic headgroups but also depends on the rather voluminous chromophoric DPP group. The space-filling model shown in Figure 4 gives a good impression of the high steric demand of the DPP group. In a highly concentrated aqueous solution, the amphiphile forms a lyotropic mesophase. Between crossed polarizers, a 60 wt % solution of Na-1 exhibits an oily streaklike texture which we ascribe to the presence of a lamellar phase (LR) (Figure 5). 3.3. Characteristic Properties of Na-1 in the Bulk. The thermal properties of bulk Na-1 were investigated (26) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. J. Phys. Chem. 1986, 90, 2413.

using differential scanning calorimetry (DSC). The DSC trace of a pristine sample crystallized from solution indicates two endothermic phase transitions at 340 and 391 K (Figure 6, left). While the first, broad peak can be attributed to a transition from a crystalline to a liquid crystalline phase due to melting of the alkyl chains of 1, the second, sharp peak indicates the transition into the isotropic melt. If the sample is cooled and heated again, the original DSC trace is no longer observed; that is, the molten paraffinic chains do not recrystallize upon cooling. Thus, in the second heating run only a single, broad transition with a maximum at 348 K is found (Figure 6, right). In further heating runs, the same behavior as in the second run is found. Structure and structural changes upon the heat treatment of Na-1 were studied using wide-angle X-ray diffraction. As shown in the X-ray powder patterns of Figure 7, the solution-crystallized sample exhibits several sharp diffraction peaks, while a sample heated to the melting point and recooled to room temperature shows a rather featureless diffraction pattern. From the equidistant positions of the low-angle X-ray peaks, a layered structure of the solution-precipitated Na-1 is evident. If the peaks are indexed as 00l reflections, a layer spacing of 4.35 ((0.1) nm can be calculated. Once molten, the samples do not recrystallize upon cooling and instead form a supercooled amorphous melt. Therefore, the DSC peak at 348 K obtained in the second heating run (Figure 6, right) may also be interpreted as a glass transition of the compound. Additional small-angle X-ray scattering (SAXS) measurements (Figure 8, lower curve) indicate a narrow peak at a 1/d value of 0.244 nm-1, which corresponds to a layer spacing of 4.10 ((0.1) nm, in fairly good agreement with the value obtained from the wide-angle powder pattern. From the X-ray results, the structure model shown in Figure 9 has been deduced. It is based on the assumption that the individual molecules exhibit a maximum length l/2 of 2.34 nm and that the molecules attain a noninterdigitated bilayer-type arrangement. The 2.34 nm value was calculated using standard bond lengths and angles. It is only a rough estimation because the exact position of the sodium counterion is unknown. Since the layer spacing d determined by X-ray diffraction is considerably lower than the calculated length of two molecules, l ) 4.68 nm, it has been concluded that the amphiphiles attain a tilted arrangement. Assuming a d value of 4.10 nm in agreement with the SAXS measurement, a tilting angle β with regard to the layer plane of 61.2° is obtained. Alternatively, an interdigitated bilayer arrangement with a close packing of the DPP units in the middle of the bilayer can be considered. However, roughly estimated, the layer spacing of such a structure would be about 3.5 nm, which is much shorter than the experimental value. For this reason, we prefer the structure proposed in Figure 9. 3.4. Polyelectrolyte-Surfactant Complexes. Polyelectrolyte-surfactant complexes of 1 were prepared by mixing equimolar amounts of Na-1 and the corresponding polyelectrolyte, PAH or PMHB, in aqueous solutions at elevated temperature. First, we discuss the complex PAH1, whose formation is outlined in Scheme 2. When the solutions of the two components were mixed, PAH-1 precipitated as a fine orange solid, which was separated from the aqueous phase by filtration. The complex was only poorly soluble in common solvents. For purification, it was therefore necessary to wash the complex several times with distilled water. Due to the poor solubility, a structural characterization by solution NMR spectroscopy was not possible. The infrared spectrum of PAH-1 is

Photoluminescent Amphiphilic Pyrrole Derivative

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Figure 5. Polarizing micrograph of a concentrated aqueous solution of Na-1 at room temperature showing a lyotropic mesophase (concentration, 60 wt %).

Figure 6. DSC traces of Na-1: First heating of a solutioncrystallized sample (left), recooling (middle), and second heating (right). Once molten, the samples do not recrystallize upon cooling and form a supercooled melt.

Figure 8. Small-angle X-ray patterns of Na-1, ionene-1, and PAH-1.

Figure 7. X-ray powder patterns of Na-1 in the pristine, solution-crystallized state (upper curve) and after heating to 120 °C and recooling to room temperature (lower curve).

similar to that of Na-1, except that the asymmetric S-O stretching mode occurring in Na-1 at 1220 cm-1 is shifted to 1205 cm-1 (Figure 10). This indicates the salt formation with the ammonium group of PAH causing a weakening of the S-O bond. UV/vis absorption spectra of the PAH-1 complex in dimethyl sulfoxide (DMSO) are nearly identical with those of Na-1; the maximum also occurs at 475 nm (not shown).

Figure 9. Proposed structure of Na-1 showing a tilted bilayertype arrangement of amphiphilic anions 1. The sodium ions are not indicated.

The extinction coefficient at the absorption maximum is 17 600 (1000 cm2 mol-1), in good agreement with the value found for Na-1 (see above). The maximum fluorescence of the complex in DMSO occurs at 529 nm, again in agreement with Na-1. Since the concentration of PAH-1 in DMSO was only 1 mg/L, a complete dissociation of the

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Figure 12. DSC traces of PAH-1: First heating of a solutionprecipitated sample (left), recooling (middle), and second heating (right). After first heating and recooling, a supercooled melt is formed, which exhibits a glass transition in the second heating run. Figure 10. FTIR spectra of PAH, Na-1, and the PAH-1 complex recorded from KBr pellets.

Figure 13. DSC traces of ionene-1: First heating of a solutionprecipitated sample (left), recooling (middle), and second heating (right). The thermal behavior resembles that of PAH-1 (see also Figure 12).

Figure 11. UV/vis absorption spectra of PAH-1 and Na-1 monitored from solution-cast thin films and from solutions in dimethyl sulfoxide.

complex is possible. This could explain the identical spectra of Na-1 and PAH-1. In solution-cast thin films, however, the maximum absorption of Na-1 is bathochromically shifted to 500 nm (Figure 11). Bathochromic shifts of the absorption on going from the solution to the solid state are a well-known phenomenon of DPP derivatives and are ascribed to charge-transfer interactions between the chromophores in the solid state.23b,27 Thin films of PAH-1 only exhibit a shift to 496 nm, perhaps due to a somewhat different packing of the chromophores causing less intense interactions. Alternatively, the lower shift may originate from disorder in the mutual arrangement of the chromophores induced by the atactic polyallylammonium counterions. In fact, wide- and small-angle X-ray diagrams indicate rather disordered structures. The wide-angle X-ray scattering (WAXS) diagram shows only a broad halo (not shown), while the SAXS pattern shows a single peak with a 1/d value of 0.429 (Figure 8, upper curve). SAXS patterns with only a single reflection are frequently found for polyelectrolyte-surfactant complexes16b,17a,18b and indicate a mesomorphous structure. If the peak is indexed as the 001 reflection, it can be ascribed to the layer spacing of a lamellar structure. Because of the small d value of 2.33 nm, a head-tail arrangement of the amphiphiles is most likely. The thermal behavior of PAH-1 is similar to that of Na-1. DSC traces are shown in Figure 12. The first scan of a pristine, solution-precipitated sample exhibits two endothermic transitions at 344 and 427 K. The first peak (27) (a) Mizuguchi, J.; Homma, S. J. Appl. Phys. 1989, 66, 3104. (b) Mizuguchi, J.; Rihs, G. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 597.

can be ascribed to a phase change in the mesomorphous structure, while the second peak indicates the transition into the isotropic melt. Upon cooling, the original phases are not restored, but a supercooled melt is formed. Thus, in the second (and all further) heating runs only a weak transition at 362 K is observed, which can be interpreted as the glass transition of the complex. The corresponding complex PMHB-1 was obtained upon mixing stoichiometric amounts of Na-1 and the ionene in aqueous solution. The formation is outlined in Scheme 3. A fine orange precipitate was formed, which was filtered off and purified upon washing with water. Like PAH-1, the PMHB-1 complex was only poorly soluble in common solvents. For a structural characterization, wide- and small-angle X-ray measurements of samples precipitated from solution were carried out. Wide-angle measurements indicated only a broad halo with a maximum at a 2Θ value of about 22° typical for a disordered system (not shown). The SAXS pattern (Figure 8, middle curve) shows three peaks with 1/d values of 0.217, 0.437, and 0.658 nm-1, which according to their 1:2:3 ratio indicate Bragg peaks of a lamellar mesophase structure. If the peaks are indexed as 001, 002, and 003 reflections, a layer spacing of 4.60 nm can be calculated, which is about 0.5 nm larger than for Na-1 determined by SAXS. The difference can be ascribed to the additional presence of the polyelectrolyte chains. While the pure ionene exhibits a glass transition at 277 K, the PMHB-1 complex does not show this transition (Figure 13). Instead, the solution-precipitated sample exhibits a weak endothermic transition at 321 K followed by an intense peak at 361 K. While the first peak can be ascribed to a phase change in the mesomorphous structure, the second peak indicates the transition into the isotropic melt. Upon cooling of the sample, the original structure is not restored. As in the case of Na-1 or PAH-1, a

Photoluminescent Amphiphilic Pyrrole Derivative

supercooled melt is formed. Therefore, in the second heating only a single transition at 321 K occurs indicating the glass transition of the complex. 4. Conclusions In our study, a new functional amphiphile containing the diketodiphenylpyrrolopyrrole chromophore is described. In polar solvents, deep orange solutions with a bright red-orange fluorescence are formed. Aqueous solutions exhibit a cmc of 4.1 × 10-5 mol/L; at a high surfactant concentration such as 60 wt %, lyotropic mesophases are formed. Stoichiometric complexes with polyelectrolytes can be easily prepared. They represent mesomorphous materials with a glass transition temperature depending on the nature of the polyelectrolyte, while the pure surfactant precipitates from solution as a brittle crystalline solid. A quantitative study of the photoluminescence of the surfactant and its complexes in solution, in films, and in micellar aggregates is the subject of a forthcoming, more detailed investigation. The complexes represent comb polymers with electrostatic binding of the DPP-containing side group to the polymer

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backbone. Compared with conventional DPP-containing side chain polymers in which the chromophoric unit is covalently bound to the main chain via an alkylene spacer,28 the complexes exhibit a much lower solubility and a higher thermal stability. Owing to their convenient optical properties and their plasticity and resistance to solvents, the complexes represent interesting new materials with potential applications as coloring agents. Acknowledgment. The authors acknowledge helpful discussions with Drs. A. Iqbal and S. H. Eldin from Ciba Speciality Chemicals, Basel (Switzerland), as well as financial support from Ciba Specialty Chemicals. Professor G. Trafara helped us with his X-ray and DSC equipment. D. Pawlowski and A. Haibel (II. Physikalisches Institut der Universita¨t Ko¨ln) are thanked for the SAXS measurements. M. Dreja is thanked for preparation of the ionene. LA011773J (28) Kluge, K. Diplomarbeit, University zu Ko¨ln, Ko¨ln, Germany, 1996.