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J. Phys. Chem. B 2000, 104, 6332-6335
Supramolecular Photophysics of Self-Assembled Block Copolymers Containing Luminescent Conjugated Polymers Samson A. Jenekhe* and X. Linda Chen Department of Chemical Engineering, UniVersity of Rochester, Rochester, New York 14627-0166 ReceiVed: March 7, 2000; In Final Form: May 10, 2000
Luminescent micellar aggregates and spherical vesicles self-assembled from polyquinoline-polystyrene diblock and polyquinoline-polystyrene-polyquinoline triblock copolymers were investigated by photoluminescence excitation and emission spectroscopies and fluorescence microscopy. In both the diblock micelles and triblock vesicles, new absorption and emission features characteristic of the supramolecular morphology in solution and the solid state were observed. J-like aggregation of the conjugated polymer blocks within the self-organized assemblies explain the observed supramolecular photophysics. The large red shift between aggregate luminescence in the solid state compared to solution is attributed to excimer emission. Self-assembling block copolymers containing conjugated polymer blocks thus constitute a promising route to functional supramolecular materials with tailorable optoelectronic and photonic properties.
Diverse nanostructures and microstructures are being created by the supramolecular self-assembly of molecular and macromolecular building blocks.1-4 Envisioned applications of selfassembled mesostructures include nanoscale devices, molecular electronics, drug delivery devices, optoelectronics, photonics, and sensors. One approach to endow self-assembled nanostructures and microstructures with electronic, optoelectronic, and photonic functions and properties is through electroactive and photoactive molecular or macromolecular building blocks.2-4 Self-assembling block copolymers containing π-conjugated polymer blocks recently became available.2-5 They are opening up the opportunity for investigating emergent or cooperative phenomena and properties of macromolecular nanostructures and microstructures.2-4 π-Conjugated polymers are known to have important semiconducting, optical, nonlinear optical, photoconductive, and electroluminescent properties which are currently being exploited in light emitting diodes,6 solar cells,7 electrophotographic imaging,8 and lasers.9 The incorporation of π-conjugated polymers into block copolymer architectures capable of selfassembly can thus be expected to facilitate the creation of diverse semiconducting, nonlinear optical,10 photoconductive, luminescent, and photonic nanostructures and microstructures.3 Although several block copolymers having a π-conjugated polymer block were recently reported,2-5 very few studies of their self-assembly and properties of their supramolecular assemblies are known.3,4 In particular, the photophysics of luminescent supramolecular block copolymer assemblies such as micelles and vesicles, which is yet to be understood, is of fundamental interest. Here, we report studies of the supramolecular photophysics of micelles of polyquinoline-polystyrene diblock (1, PPQPS) and vesicles of polyquinoline-polystyrene-polyquinoline triblock (2, QSQ) copolymers (Figure 1). The luminescence of the supramolecular assemblies, which is due to the emissive PPQ block,6c facilitated the investigation of the aggregates in solution and solid state by fluorescence microscopy and spectroscopy. The synthesis and nonlinear optical, photoconductive, electrochemical, and electroluminescent properties of the
π-conjugated rigid-rod homopolymer PPQ have previously been reported.6c,11,12 Synthesis of the rod-coil block copolymers 1 and 2 and initial studies of their supramolecular self-assembly and morphologies have also been reported.3a,4 The observed selfassembly of 1 and 2 into ordered mesostructures such as micelles and vesicles3,4 raises the questions of how the luminescent PPQ blocks are organized within the supramolecular assemblies and, ultimately, the nature of their supramolecular photophysics. Dispersions of micellar aggregates of diblocks 1 in various volume ratios of trifluoroacetic acid (TFA)/dichloromethane (DCM) mixtures were prepared as previously described.3 The concentration of each dispersion was in the range of 10-5 to 0.5 wt %. Dispersions of spherical vesicles of triblock 2 in TFA/ DCM were similarly prepared by dissolving samples of 2a or 2b in the mixed solvent at room temperature (25 °C) without sonication or mechanical stirring.4 Optical and fluorescence microscopy observations of the liquid dispersions were made on dilute dispersions (10-2 to 10-1 wt %) sealed inside 6 mm × 50 mm glass tubes. Solid micelles and vesicles for microscopic observations and photophysical studies were obtained by drying drops of the liquid dispersions on glass substrates at room temperature (25 °C) or at 95 °C. The resulting films of aggregates were further dried in a vacuum at 60 °C for 24 h. Photoluminescence emission (PL) and excitation (PLE) spectra of the liquid dispersion (“solutions”) in cuvette and films on glass substrates were obtained on a Spex Fluorolog-2 spectrofluorimeter at 25 °C. The PL and PLE spectra were measured by using the front face geometry in which samples were positioned such that the emission signal was detected at 22.5° from the incident radiation beam. Further details of our photophysical experiments are similar to those described earlier.2b,c The transmission optical micrograph in Figure 2a shows the typical morphologies of vesicles of triblock 2a in the form of liquid dispersions in TFA/DCM. Although there is a wide distribution of sizes, only about three shell thicknesses are observed as judged from the optical transparency of the spherical particles. The morphology of the solid vesicles of 2a, after drying, are shown in the fluorescence micrographs of Figures 2b and 2c.
10.1021/jp000896u CCC: $19.00 © 2000 American Chemical Society Published on Web 06/20/2000
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J. Phys. Chem. B, Vol. 104, No. 27, 2000 6333
Figure 1. Molecular structures of the rod-coil diblock (a) and triblock (b) copolymers. Also shown are schematic illustrations of the supramolecular structures of the diblock (c) and triblock (d) assemblies.
The yellow color of the hollow spheres results from the intrinsic yellow fluorescence of the spherical vesicles in the solid state. The relatively narrow size distribution of the vesicles in the solid state (Figure 2b,c), compared to the liquid dispersion (Figure 2a), is a result of size-selective self-ordering of the spherical particles during drying. The results of optical and fluorescence microscopy observations of triblock 2b were very similar to those of triblock 2a. In contrast, the optical/fluorescence microscopy images of micellar aggregates of the diblock copolymers (1a, 1b) showed both spherical and nonspherical (cylinders, disks) assemblies in accord with prior observations.3a Figure 3a shows the photoluminescence excitation (PLE) and emission (PL) spectra of dilute solutions of 2a and of liquid
dispersions of triblock 2a vesicles. In very dilute solution (10-5 wt %), where there is no aggregation, a broad absorption band centered at 388 nm with full-width-at-half-maximum (fwhm) of 77 nm was observed. At higher concentration (10-3 wt %) the absorption band seen in PLE is split into two Davydov components with peaks at 340 and 420 nm. At still higher concentration (10-2 wt %) where the vesicles are clearly visible in optical microscopy as shown in the micrograph of Figure 2a, a more intense and sharp (fwhm ) 21 nm) absorption band at 433 nm emerges. The emission band of the same triblock in dilute solution (10-5 wt %) has a peak at 460 nm which can be taken as the isolated chromophores (nonaggregated PPQ blocks). Suprisingly, there is only a slight change in the emission band
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Figure 2. Optical (a) and fluorescence (b, c, 340-nm excitation) micrographs of QSQ-1 triblock copolymer vesicles in TFA/dichloromethane (a) and after they were dried (b, c).
as the concentration of 2a in solution was increased to 10-2 wt %, where clear evidence of aggregation is seen by optical microscopy (Figure 2a). Identical aggregate emission and excitation spectra were obtained for triblock 2b. These results, particularly the absorption characteristics seen in the PLE spectra, are reminiscent of J-aggregation of dyes.13 The solid-state PLE spectra of 2a and 2b vesicles both have a broad absorption band with a sharp peak at 423 nm and a shoulder at 463 nm, which are significantly red-shifted from the corresponding isolated chromophore which has a peak at 390 nm (Figure 3b). This large spectral red shift in excitation spectra is evidence of J-like chromophore aggregation within the vesicle shells.13 The corresponding PL spectra of vesicles of 2a and 2b have structureless emission bands centered at 598608 nm, regardless of the excitation wavelength, which are significantly red-shifted from the isolated 2a or 2b chromophore which emits at 460 nm (Figure 3b). Similar to the solid-state PPQ homopolymer which emits at 589 nm,6c we can interpret the solid state PL emission spectra of these self-organized assemblies of PPQ blocks as due to excimers.14 The PLE and PL spectra of micellar aggregates of diblocks 1 in solution and the solid state were more complex than those of the triblock 2 vesicles. Figure 4a shows the PLE and PL spectra of 1b at different concentrations in the 10-4 to 10-1 wt % range, where various micellar aggregates have been observed. At the highest concentration one narrow band (fwhm ) 45 nm) at 435 nm was seen in the excitation spectrum. Structured absorption bands with a sharp peak at 425 nm were observed in the PLE spectra of 10-4 to 10-2 wt % samples. The rather broad PL spectra of all the 1b solutions varied significantly with concentration. The emission maxima decreased from 582 nm in the 10-4 wt % sample to 515-520 nm in the two highest concentrations. In addition to the main emission band, a small peak at 455-460 nm was observed in all the PL spectra. Similar
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Figure 3. (a) PLE and PL spectra of QSQ-2 solutions in 6:4 TFA/ DCM at 25 °C. PLE spectra (1, 2, 3) correspond to 540-nm emission and PL spectra (4, 5) are for excitations from 330 to 430 nm. (b) PLE (monitored at 600 nm) and PL (excited at 460 nm) spectra of the solidstate QSQ vesicles shown in Figure 2b. Also, shown are the solid film PLE (monitored at 480 nm) and PL (390-nm excitation) spectra of the isolated QSQ-2 copolymer dispersed in poly(ethylene oxide)(PEO) at 0.1 wt %.
PLE and PL spectra were observed in solutions of diblock 1a. These results, which show the variation of diblock aggregate emission and excitation spectra with concentration, can also be explained by the J-aggregation of the luminescent PPQ blocks within the micelles and the fact that different aggregate morphologies (spheres, cylinders, flat lamellar disks)3a exist in solution. Through a variation and careful control of the self-assembly conditions (e.g., TFA/DCM ratio, concentration, temperature, and solvent evaporation rate), diblock micellar aggregates of one predominant supramolecular morphology (spheres, cylinders, or flat lamellar disks) in the solid state have been achieved.3a Initial PL spectroscopy of the different solid-state micellar aggregates of 1a showed that the aggregate emission properties varied with the surpramolecular morphology,3a as shown in Figure 4b. The emission band maximum varied from ∼454 to 460 nm in the spheres and 576 nm in the lamellae to 594 nm in the cylinders (Figure 4b). Variation in the excitation spectra is also observed with aggregate geometry. Similar to the triblock 2 results, these PLE and PL spectra of 1a can now be interpreted in terms of the H-like or J-like aggregation of the PPQ blocks. The key molecular packing parameter that can explain these results is the tilt angle R in the schematic of Figure 1c, d. Although the actual values of the tilt angle and the details of the supramolecular structures are not known, its variation would make the PPQ assemblies analogous to H- and Jaggregates of dye molecules. Thus the variation of the tilt angle of the PPQ blocks can explain the observed variation of photophysical properties with aggregate morphology (Figure 4b).
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J. Phys. Chem. B, Vol. 104, No. 27, 2000 6335 properties that relate to the supramolecular morphology. The aggregate luminescence in solution and the solid state are explained by J-aggregation and excimer formation in the selfassembled π-conjugated polymer blocks of the micelles and vesicles. Block copolymers containing luminescent conjugated polymers such as 1 and 2 represent a novel class of functional supramolecular materials and are model systems for exploring supramolecular photophysics. Acknowledgment. This research was supported by the Office of Naval Research. We thank X. Zhang for technical assistance and discussion. References and Notes
Figure 4. (a) PLE (monitored at 500-560 nm) and PL (excited at 380-390 nm) spectra of 1b solutions in TFA/dichloromethane (1:1 vol). (b) PLE (emission maxima) and PL (380-nm excitation) spectra of 1a solid films of aggregates of different morphologies.
Although H- and J-aggregates of dyes have been studied extensively and are known to have novel cooperative optical and nonlinear optical properties,13 very little is currently known about H- and J- aggregates of π-conjugated polymers.15 The present results provide evidence of J-aggregation of π-conjugated polymers in well-defined, ordered supramolecular assemblies such as micelles and vesicles. Block copolymers containing conjugated blocks are thus a promising route to preparing well-defined, self-organized, macromolecular J-aggregates which represent new forms of conjugated polymers that might exhibit novel phenomena and properties. For example, the self-assembled, highly luminescent and robust solid hollow spheres of 2 are ideal candidates for developing whispering gallery mode lasers where the dominant modes could be easily controlled by the vesicle wall thickness and diameter.16 In summary, luminescent micellar aggregates and vesicles formed by polyquinoline-polystyrene diblock and triblock copolymers have been observed to have novel photophysical
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