ARTICLE pubs.acs.org/Langmuir
Fluorescence Properties of Photonic Crystals Doped with Perylenediimide Aurel Diacon,†,‡ Edina Rusen,*,† Alexandra Mocanu,† Pietrick Hudhomme,‡ and Corneliu Cincu† † ‡
Department of Polymer Science, University Politehnica of Bucharest, 149 Calea Victoriei, RO-010072 Bucharest, Romania Laboratoire MOLTECH-Anjou, Universite d'Angers, CNRS UMR 6200, 2 Boulevard Lavoisier, 49045 Angers, France
bS Supporting Information ABSTRACT: This study aims to present the fabrication of colloidal photonic crystals (PC) with increased fluorescence properties. The use of a highly fluorescent perylenediimide derivate (PDI) during the soap-free emulsion polymerization of styreneacrylic acid resulted in monodisperse coreshell particles which allowed the fabrication of PC films. The properties of the hybrid material were studied in comparison with hybrid materials obtained by impregnation of films with chromophore solutions. In both cases an increase of the fluorescence response was observed in addition to a blue shift for the PDI core particles, proving the incorporation of the dye inside the copolymer particles.
1. INTRODUCTION Photonic crystals (PCs) constitute a fascinating class of materials as promising candidates for nanoscaled optoelectronic devices for the next generation information technology.13 These are generally characterized by artificial structures with a periodic dielectric arrangement which does not allow propagation of light in all directions for a given frequency range. This phenomenon induces the opening of photonic stop-bands or band gaps due to Bragg diffraction.1,2,4,5 The stop-band of the PCs represents the narrow range of specific wavelengths in which the propagation of light is prohibited. On this topic, synthesis of monodisperse colloidal spheres with submicronic diameters has lately attracted alot of interest from various researchers because of the self-assembling properties of such systems, which give crystalline structures of synthetic opal after the removal of the dispersion medium. Crystalline lattices of inorganic or polymer particles have a highly ordered structure that leads to photonic crystal (PC) properties. From the several known methods for obtaining monodisperse colloidal polymer particles, the most promising seems to be soap-free emulsion polymerization.611 Various methods for obtaining films of colloidal particles, such as gravitational sedimentation, centrifugation, vertical deposition, physical confinement, and interfacial or electric field induced selfassembly, have been described in the literature.8,1218 Organic dyes, such as synthetic opals, are commonly used because of their high fluorescence quantum yields for investigating stimulated emissions from photonic structures with stopbands.19,20 From a general point of view, the localization of light emitters in PC-based architectures allows the investigation of the r 2011 American Chemical Society
stop-band influence on the photoluminescence properties.2125 Near the stop-band of PCs, light propagates at reduced group velocities owing to resonant Bragg scattering, which can enhance luminescence because of stimulated emission and can amplify the absorption of incident light. Hybrid materials have been fabricated by incorporation of dye molecules into PCs through an emulsion polymerization process in the case of water-soluble chromophores.21,25 Another strategy consists of incorporating dyes by a swelling and deswelling process of the already synthesized colloids.25,26 Perylenediimide (PDI) dyes represent a well-recognized class of functional chromophores and fluorophores27 which have been extensively used as organic n-type semiconducting materials for a wide range of applications including organic field effect transistors,28,29 organic light-emitting diodes,30,31 and organic solar cells.32,33 These building-blocks have also been incorporated in architectures to obtain artificial photosynthetic systems,34,35 logic gates,36 molecular optical switches,37,38 sensors,39 and photosensitizers.40,41 In this study, hybrid materials composed of PDI and PCs have been obtained by two different methods: • Soap-free emulsion polymerization of styrene and acrylic acid in the presence of PDI, despite poor solubility of the dye in water. The substitution on the bay area and the imide functionalization27,42 of PDI enhances the solubility Received: March 9, 2011 Revised: May 15, 2011 Published: May 23, 2011 7464
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Langmuir in organic solvents, such as styrene, thus making possible the polymerization process. Consequently, we have chosen a dissymmetrical PDI derivative for which the substitution at the bay region allows solubility in organic solvent and the presence of the alcohol functionality increases the hydrophilic character. • Impregnation of PCs with a PDI solution. The characteristics of solubility of this PDI derivative appeared particularly adapted, making possible the impregnation method without disturbing the crystal arrangement of the PCs. The hybrid materials obtained through both methods have been investigated by photoluminescence spectroscopy, and the results presented here describe the influence of the PC structure on the PDI fluorescence emission.
2. MATERIALS AND METHODS 2.1. Materials. Latex spheres have been synthesized according to a single-stage polymerization process based on the formation and growth of polymeric nuclei dispersed in an emulsion consisting of water, styrene (ST), acrylic acid (AA), and potassium persulfate (KPS). Styrene (ST) (Merck) and acrylic acid (AA) (Merck) have been purified through vacuum distillation. Potassium persulfate (KPS) (Merck) has been recrystallized from an ethanol/water mixture and then vacuum-dried. Perylenediimide (PDI)43,44 and Zn-phthalocyanine45 (ZnPc) were obtained according to previous methods. The structure of the chromophore is presented in Scheme 1.
Scheme 1. Structure of the Chromophore N-20 -Hydroxyethyl-N0 -pentyl-1,6,7,12-tetrakis(p-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide)
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2.2. Soap-Free Emulsion Polymerization. 2.2.1. Preparation of the STAA Colloidal Dispersion. A 6.5 mL amount of ST and 2 mL of AA were added to 100 mL of distilled water containing 62.5 mg of KPS. The reaction mixture was purged with nitrogen and then maintained for 8 h at 75 °C under continuous stirring. The final dispersion was dialyzed in distilled water for 7 days, using cellulose dialysis membranes (molecular weight cutoff: 12 00014 000), in order to remove the unreacted monomers and initiator. 2.2.2. Preparation of the STAAPDI Colloidal Dispersion. A mixture of 6.5 mL of ST and 25 mg of PDI was added to 100 mL of distilled water containing 62.5 mg of KPS and 2 mL of AA. The reaction mixture was purged with nitrogen and then maintained for 8 h at 75 °C under continuous stirring. The final dispersion was dialyzed in distilled water for 7 days, using cellulose dialysis membranes (molecular weight cutoff: 12 00014 000), in order to remove the unreacted monomers and initiator. 2.2.3. Preparation of the PDI Film. A 102 mol 3 L1 solution of PDI in carbon tetrachloride was prepared, and the PDI was deposited on a glass substrate by a dip-coating method. 2.2.4. Preparation of STAAPDI on Surface Film. STAA film with synthetic opal properties was obtained by gravitational sedimentation on a glass substrate and dried at 70 °C for 1 h. A PDI solution in carbon tetrachloride (103 mol 3 L1) was deposited on STAA surface by a dip-coating method. 2.3. Characterization. The morphologies of polymer particles were investigated through an XL-30-ESEM TMP scanning electron microscope (SEM). The samples were sputtered with a thin layer of gold prior to imaging. The particle size measurement, through dynamic light scattering (DLS), and the Z potential were obtained with a Nani ZS device (redbadge). The infrared absorption spectra were recorded at room temperature with a Nicolet 6700 FTIR spectrometer in the range of 4000400 cm1. Microphotographs were obtained using an optical microscope (Olympus, BX-41) equipped with a CCD camera. The UVvis spectra were recorded using a V-500 Able Jasco spectrophotometer. The 3D fluorescence spectra were registered using a FP6500 Able Jasco spectrofluorimeter.
3. RESULTS AND DISCUSSION First, the latexes and the contained particles were characterized by DLS and SEM techniques. The SEM analysis confirms the presence of ordered structures, in both cases, having a cubic close packing structure (ccp) presenting photonic crystal characteristics (Figure 1). The major difference concerns the particles, namely a decrease in particle size from 210 nm (STAA) to 150 nm (STAAPDI). DLS was employed to sustain the dimensional uniformity of the obtained particles (Figure 2). The
Figure 1. SEM images for particles from STAAPDI (a) and STAA (b) latexes. 7465
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Figure 2. DLS analysis for latexes: STAA (a) and STAAPDI (b).
Scheme 2. (a) Dye Molecules Incorporated in Polymer Particles in Our Study and (b) Dye Molecules on the Surface of Polymer Particles
Figure 3. UVvis spectra for the obtained films: (STAA) and (STAAPDI).
particle size variation may be due to the scavenger capability of PDI during the soap-free polymerization process. In our previous studies, this effect was observed during the polymerization reaction in the presence of fullerene C60.10 Thus, by homogeneous nucleation, a higher number of particles with smaller dimensions are formed, due to the inhibition in the water phase of the propagating radicals by the PDI molecules transported by the ST from the organic phase. In agreement with our previous investigations proving that the majority of hydrophilic groups from the monomer are arranged by homogeneous nucleation at the surface,911 the PDI moieties should be reasonably arranged inside the copolymer particles. In order to study the presence of dye and the optical properties of modified PCs, we have employed UVvis and fluorescence
spectroscopy. For the UVvis characterization, a reflection integration sphere was used on the basis of the PC properties. The analysis of the spectra in Figure 3 shows a hypsochromic shift of the signal in the case of the PDI-containing film from 475 to 450 nm, which is consistent with the decrease in particle size.8,14,17,46 For the STAAPDI film (Figure 3), a decrease in the reflection response in the absorption domain of PDI is not observed. This can be explained by the incorporation of PDI (scavenger) in the particles and the overlay of the absorption domain and the stop-band of the PC. Consequently, the influence of the adsorption band corresponding to free dye is not observed on the basis of the coreshell structure of the hybrid material obtained. All the dye molecules have been incorporated in the polymer particles. Scheme 2 presents a comparison of the structure of the hybrid material obtained by us and the possible 7466
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Figure 4. 3D fluorescence spectra (emission intensity versus emission wavelength and excitation wavelength) for (a) STAA and(b) STAAPDI.
structure where the dye might have influence on the optical properties of the material. In order to study the influence of the dye on the optical properties of the PCs relative to its insolubility in water, studies have been carried out for comparison with the dye ZnPc (tetracarboxy zinc phthalocyanine), as its potassium salt. We have confirmed the PC characteristics for the STAAZnPc film with the conservation of the particle size and the lack of aggregation (see Supporting Information). For more information, the STAAPDI films have been further characterized by fluorescence spectroscopy. Figure 4 presents the 3D fluorescence spectra which consist of emission intensity versus emission wavelength and excitation wavelength. It was previously reported that the PCs may act as a Bragg mirror and can effectively increase fluorescence intensity of organic dyes because they enhance excitation and/or emission light.47 A condition for fluorescence enhancement is that the excitation wavelength should be in the stop-band of the PCs. The excitation light reflected by a Bragg mirror can stimulate more dye molecules, which is also in favor of fluorescence enhancement. In our case, the excitation wavelength is higher than the maximum reflection value of the stopband. However, in Figure 4, the response of the STAAPDI shows new signals that require a more detailed analysis.
Figure 5 presents the response of the films at an excitation wavelength of 550 nm, in the stop-band of the photonic crystal. The emission spectrum of the PDI film is characterized by a large band centered at around 660 nm. By comparison with the emission spectra of the STAA and STAAPDI, a new but less intense band centered at around 600 nm is observed for the PC-containing dye. The blue shift of the emission maximum can be explained by light confinement in defects of the PC lattice and by the optical path up to the PDI core particle.48,49 The lack of fluorescence enhancement for the STAAPDI system may be due to weak interaction between the excitation wavelength of PDI and the stop-band of the colloidal crystal (weak reflection of the excitation wavelength). However, the decrease in the intensity compared to the PDI film is explained by the incorporation of the dye inside the polymer particles which could lead to a weaker excitation of the molecules. Because of the substitution on the bay area which induces a twist of the molecule,38 the formation of excimers appears unfavorable for the PDI derivative used in this study. However, in the solid film, the excimer-like emission is present. The emission spectrum of PDI in solution (106 M (CH2Cl2), λex = 550 nm) is shown in the inset of Figure 5. The emission takes place at 614 nm for monomeric species.43 For 7467
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Figure 5. Fluorescence spectra of the films at an excitation wavelength of 550 nm: STAA; STAAPDI; PDI. Concentration of PDI solution (106 M in CH2Cl2).
Figure 6. Fluorescence spectra of the films at an excitation wavelength of 250 nm: STAA; STAAPDI; PDI.
the STAAPDI film (the dye inside the polymer particles), the emission is situated to the same band (614 nm). This behavior can be explained by the low concentration of PDI in the polymerization systems, which induces a small amount of PDI in the polymer particle and thus a weak interaction between the PDI molecules inside the polymer particles. The behavior of PDI inside of the polymer particle is the same as that in solution (monomeric species). As a result of geometric relaxation in the excited state, the emission takes place from an excimer-like species, i.e., an electronic intermediate, at 660 nm as a strongly red-shifted band emission (Figure 5) for the PDI film.50 The fluorescent nature of PDI in the solid state (film) may be related to its crystal structure.51 The 3D fluorescence spectra shows a high intensity emission of STAAPDI for an excitation wavelength of 250 nm (Figure 6). The STAAPDI system presents a high increase of fluorescence at 280360 nm, which is not characteristic for PDI. In order to find an explanation for this behavior, the next step was the impregnation of a STAA film with a PDI solution in carbon tetrachloride (103 mol 3 L1). Figure 7a and 7b illustrates the results related to Figure 6 explaining the increase of the fluorescence noticed in the case of STAAPDI
Figure 7. Fluorescence spectra of the films at an excitation wavelength of 550 nm: (a) STAA; STAA PDI on a surface and PDI; (b) STAA; STAA PDI on a surface, PDI, and STAAPDI; (c) fluorescence spectra of the STAA PDI film at an excitation wavelength of 250 nm on a surface and PDI.
system at an excitation wavelength of 250 nm. There are only small modifications (signal overlap at 300325 nm) for the surface-adsorbed PDI film, compared to the STAA film. Thus, a reason for the increase of fluorescence for a 250 nm excitation wavelength in the case of the STAAPDI film could be the difference in diameter and a better compaction of the particles. An emission around 300 nm (Figure 7a) is present in the case of STAA and the PDI film. In the case of the STAA film, the 7468
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Figure 8. Optical microscopy images: latex STAA (a, b, c); latex STAAPDI (d, e, f); latex STAA PDI on a surface (g, h, i).
signal is due to the polystyrene structure; up-converted emission is believed to result from the second singlet excited state through a multiphoton process.52 Many perylene derivatives have been demonstrated to aggregate in solid films where head-to-tail interaction and face-to-face packing occurs.27 The peak at 300 nm could be due to the Bragg diffraction of dye molecule aggregates. Moreover, this behavior could be assigned to the resonance effect between the incident electromagnetic radiation and the electron’s polarization, which leads to the coupling of electrons in PDI films to the oscillating electric field. A fluorescence enhancement in the case of the PDI-impregnated PCs is noticed (Figure 7c), although the excitation wavelength of 550 nm is not in the maximum reflection domain of the PC stop-band. To obtain a fluorescence enhancement for the STAAPDI system, the maximum reflection value of the stop-band should be around 550 nm, which would be characteristic for colloidal particles with an increased diameter, according to formula 1:24 λð111Þ ¼ neff 1:632d
ð1Þ
where the effective refractive index is given as neff = fnSTAA þ (1 f)nair, the refractive index of STAA, which can be estimated as the refractive index of polyST, is given as nPST = 1.55, nair = 1, and the filling factor for ideal face-centered cubic packing is f = 0.74. According to formula 1, the ideal diameter should be 240 nm, which will be a topic of future work. In general, taking into account the excitation and/or the emission wavelength of the dye, respectively, the composition and the size of the polymer particles that allows control of the stop-band domain of the PC, fluorescence enhancement can be achieved. The comparative macroscopic characterization of the films (STAA, STAAPDI, STAA PDI on the surface) was studied. These were obtained by deposition on glass plates having an area of 4 cm2 and the volume of 0.3 mL for the used latexes. This characterization has been performed in order to gain
information on the quality of the obtained systems. Figure 8 presents images from optical microscopy at different magnifications. The images in Figure 8 exhibit a better film quality in the case of STAAPDI. The improvement is explained by the lower density of cracks and the higher distance between them, sustaining a higher continuity of the film.
4. CONCLUSIONS Photonic crystals modified with fluorescent chromophores have been obtained by employing soap-free emulsion polymerization in the presence of PDI, a physical mixture of an aqueous solution of the potassium salt of ZnPc and STAA latex, and the impregnation of the STAA film with a PDI solution. This study demonstrated that the PDI influence on the soap-free polymerization of STAA led to a decrease in particle size by its scavenger effect. The PCs have been characterized by UVvis and fluorescence spectroscopies, and the shift in the emission response observed in the case of the STAAPDI system can be explained by light confinement in defects of the PC lattice and by the optical path up to the PDI core particle. The lack of fluorescence enhancement for the STAAPDI system may be due to weak interaction between the excitation wavelength of PDI and the stop-band of the colloidal crystal corresponding to a weak reflection of the excitation wavelength. It was also demonstrated that the decrease in the intensity compared to the PDI film resulted from the incorporation of the dye in the polymer particles which led to a weaker excitation of the molecules. Moreover, the impregnation of the PC with PDI resulted in a fluorescence enhancement, although the excitation wavelength of 550 nm is not in the maximum reflection domain of the PC stop-band. In order to obtain a fluorescence enhancement in the case of the STAAPDI system, the ideal particle dimension should be 240 nm, which will be a topic of future work. 7469
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’ ASSOCIATED CONTENT
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Supporting Information. Experiments involving PC impregnated with tetracarboxyphthalocyanine zinc(II) (ZnPc), UVvis spectra, optical characterization of films. This material is available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT The work has been funded by the Sectoral Operational Programme Human Resources Development 20072013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreement POSDRU/6/1.5/S/16. The authors also recognize financial support from the European Social Fund through POSDRU/89/1.5/S/54785 project: “Postdoctoral Program for Advanced Research in the field of nanomaterials”. ’ REFERENCES (1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58 (20), 2059. (2) John, S. Phys. Rev. Lett. 1987, 58 (23), 2486. (3) Galisteo-Lopez, J. F.; Ibisate, M.; Sapienza, R.; Froufe-Perez, .; Lopez, C. Adv. Mater. 2011, 23 (1), 30–69. L. S.; Blanco, A (4) Megens, M.; Wijnhoven, J. E. G. J.; Lagendijk, A.; Vos, W. L. J. Opt. Soc. Am. B 1999, 16 (9), 1403–1408. (5) Lin, S.-Y.; Chow, E.; Hietala, V.; Villeneuve, P. R.; Joannopoulos, J. D. Science 1998, 282 (5387), 274–276. (6) Herzog Cardoso, A.; Leite, C. A. P.; Zaniquelli, M. E. D.; Galembeck, F. Colloids Surf., A 1998, 144 (13), 207–217. (7) Qin, D.; Lian, G.; Qin, S.; Ford, W. T. Langmuir 2009, 26 (9), 6256–6261. (8) Waterhouse, G. I. N.; Waterland, M. R. Polyhedron 2007, 26 (2), 356–368. (9) Rusen, E.; Mocanu, A.; Marculescu, B. Colloid Polym. Sci. 2010, 288 (7), 769–776. (10) Rusen, E.; Mocanu, A.; Corobea, C.; Marculescu, B. Colloids Surf., A 2010, 355 (13), 23–28. (11) Preda, N.; Matei, E.; Enculescu, M.; Rusen, E.; Mocanu, A.; Marculescu, B.; Enculescu, I. J. Polym. Res. 2011, 18 (1), 25–30. (12) Zhang, J.; Sun, Z.; Yang, B. Curr. Opin. Colloid Interface Sci. 2009, 14 (2), 103–114. (13) Zhang, L.; Xiong, Y. J. Colloid Interface Sci. 2007, 306 (2), 428–432. (14) Ge, H.; Song, Y.; Jiang, L.; Zhu, D. Thin Solid Films 2006, 515 (4), 1539–1543. (15) Chiappini, A.; Armellini, C.; Chiasera, A.; Ferrari, M.; Fortes, L.; Clara Gonc- alves, M.; Guider, R.; Jestin, Y.; Retoux, R.; Nunzi Conti, G.; Pelli, S.; Almeida, R. M.; Righini, G. C. J. Non-Cryst. Solids 2009, 355 (1821), 1167–1170. (16) He, Y. Q.; Wang, X. D.; Wang, J. Y.; Feng, Y.; Zhao, Y. Q.; You, X. D. Chin. Chem. Lett. 2007, 18 (11), 1395–1398. (17) Wang, X.; Husson, S. M.; Qian, X.; Wickramasinghe, S. R. Mater. Lett. 2009, 63 (23), 1981–1983. (18) Dushkin, C. D.; Nagayama, K.; Miwa, T.; Kralchevsky, P. A. Langmuir 1993, 9 (12), 3695–3701. (19) Yang, Z.; Zhou, J.; Huang, X.; Yang, G.; Xie, Q.; Sun, L.; Li, B.; Li, L. Chem. Phys. Lett. 2008, 455 (13), 55–58. (20) Withnall, R.; et al. J. Opt. A: Pure Appl. Opt 2003, 5 (4), S81. (21) Romanov, S. G.; Maka, T.; Torres, C. M. S.; Muller, M.; Zentel, R. Appl. Phys. Lett. 1999, 75 (8), 1057–1059. (22) Petrov, E. P.; Bogomolov, V. N.; Kalosha, I. I.; Gaponenko, S. V. Phys. Rev. Lett. 1998, 81 (1), 77. (23) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10 (13), 1028– 1032. (24) Muller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Sotomayor Torres, C. M. Chem. Mater. 2000, 12 (8), 2508–2512.
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