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A fluorescent perylene-assembled polyvinylpyrrolidone film: synthesis, morphology and nanostructure† Mengmeng Sun, Yong He, Wantai Yang and Meizhen Yin* A fluorescent polyvinylpyrrolidone (PVP) film with assembled nanostructures is successfully prepared in one pot by using perylene-3,4,9,10-tetracarboxylic acid dianhydride (PDA) as the fluorophore. The reactive anhydride groups in PDA play an important role in the covalent bonding of fluorescent molecules in the fluorescent PVP film, which has been demonstrated by a small-molecule model reaction. Interestingly, rod-like structures are found on the surface of the fluorescent film, which is attributed to the p–p
Received 5th September 2013 Accepted 10th February 2014
interaction of PDA that occurred in the crosslinked PVP film. Further polarized optical microscopy (POM), X-ray diffraction (XRD) and optical analyses demonstrate the stable existence of p-conjugated nanostructures in the fluorescent PDA-based PVP film. The reactive anhydrides and the p–p interaction of
DOI: 10.1039/c3sm52350k
perylene molecules are essential for the fabrication of the fluorescent perylene-assembled PVP film. This
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method could be extended to the preparation of the fluorescent PVP film with self-assembled nanostructures.
Introduction Polymer uorescent lms have attracted increasing attention during the last decades owing to their wide and potential applications in the elds of chemical sensors,1,2 molecular recognization3 and biomedicine.4,5 Polymer uorescent lms are usually prepared via spin coating, casting, layer by layer or other physical methods. Perylene derivatives have been widely applied in chemical sensors,6–8 biomedicine9–11 and solar cells12,13 because of their outstanding properties such as high thermal and photochemical stability, broad colour range, and high uorescence quantum yield. Moreover, the self-assembly of perylene derivatives also has attracted considerable attention over the past decade due to the intrinsic p–p stacking interaction between perylene backbones.14–16 The p-conjugated structures can be tuned by introducing different reactive groups onto the bay region or end positions in the perylene, which lead to different morphologies and functions of the material.17–20 Polyvinylpyrrolidone (PVP) is an important linear polymer with excellent solubility, biocompatibility, chemical stability and is physiologically inert, thus it has been widely used as a drug additive, paint and adhesive. In addition, PVP has been focused on the formation of the novel PVP-based composites21,22
State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, 100029 Beijing, China. E-mail:
[email protected] † Electronic supplementary 10.1039/c3sm52350k
information
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available.
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and used as a protecting agent in the synthesis of monodisperse metal nanoparticles.23–25 A facile synthesis of a self-crosslinked blank PVP lm has been currently reported as a communication by our group.26 The reaction mechanism of ring-opening and self-crosslinking of linear PVP was proposed and well interpreted (Scheme S1†). In this study, in order to explore the potential role of the reactive group substituted in the scaffold of perylene, two kinds of dyes have been selected for the preparation of uorescent PVP lms. The structures of the dyes are given in Fig. 1, i.e. perylene3,4,9,10-tetracarboxylic acid dianhydride (PDA) and N,N0 -bis(2,6-diisopropylphenyl)-perylene-3,4,9,10-tetra-carboxylic diimide (PDI). The small molecular simulation experiment demonstrated that the PDA dye was incorporated into the PVP network via chemical bonds, thus leading to the stable uorescent lm. In general, the existence of an active anhydride group in perylene derivatives is essential for the covalent bonding of uorescent molecules in the ring-opened and crosslinked PVP lm. Simultaneously, the perylene species is found to have an effect on the morphology of the uorescent PVP lm. Rod-like structures are observed on the surface of the uorescent PDA-based PVP lm, and this is attributed to the p– p interaction of perylene molecules that occurred during the fabrication of the crosslinked PVP lm. The nature of the nanostructure on the PVP lm was investigated and discussed.
Experimental Materials Perylene-3,4,9,10-tetracarboxylic acid dianhydride (PDA, 98%) and N,N0 -bis(2,6-diisopropylphenyl)-3,4,9,10-perylene tetracarboxylic
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incorporated in the PVP lm was 18 mg, which was calculated by the formula below.
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PDAfilm ¼ film (PVPoriginal PVPresidue)
Reaction of PDA and nipecotic acid
Fig. 1
Structures of PDA and PDI.
diimide (PDI, 98%) were purchased from Beijing Wenhaiyang Perylene Series Chemical Company and used as received. Polyvinylpyrrolidone (PVP, M ¼ 1 300 000 g mol1), acetone, ethanol and tetrahydrofuran (THF) were purchased from Beijing Chemical Plant and used directly without further purication.
Instruments Fourier transform infrared (FTIR) spectra were recorded with a Nicolet-50 DXC FTIR spectrophotometer. Dry samples were prepared as KBr pellets at room temperature. Matrix-assisted laser-desorption ionization time-of-ight mass spectrometry (MALDI-TOF MS) was performed on a Bruker Daltonics Inc. BIFLEX III MALDI-TOF (Mw < 3000) and a AXIMA-CFR plus MALDI-TOF (Mw > 3000) mass spectrometer. X-ray diffraction (XRD) patterns were recorded on a D/max2500 VB2+/PC X-ray diffractometer (Rigaku) using Cu Ka radiation in the 2q range 5– 90 (l ¼ 0.154 nm). High resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM2100 high resolution transmission electron microscope operating at an acceleration voltage of 200 kV. Polarized optical micrographs (POM) were obtained with an Axioskop 40A Pol Optical Microscope (Carl Zeiss). Fluorescence measurements were recorded on a FluoroMax-4 spectrouorometer (Horiba Jobin Yvon, France). The samples were observed using a Hitachi S-4700 scanning electron microscope (SEM). All specimens were coated with gold before examination.
Nipecotic acid (100 mg, 0.774 mmol) and PDA (30 mg, 0.076 mmol) were dissolved in acetone (50 mL) under ultrasonication and mechanical stirring for 30 min. Aer degassing by nitrogen the mixture was put into a Teon-lined stainless steel autoclave with a capacity of 60 mL and maintained at 160 C for 30 h. Then the autoclave was cooled to room temperature naturally. The product mixture was characterized by thin layer chromatography (TLC) and MALDI-TOF MS.
Results and discussion Previously, we have successfully proposed and demonstrated the reaction mechanism of ring-opening and self-crosslinking of linear PVP.26 In order to investigate whether the reactive anhydride group located in the perylene scaffold plays an important role in the fabrication of the stable uorescent PVP lm, two kinds of perylene derivatives (PDA and PDI) were used as the uorophore. On the basis of the PVP ring-opening and self-crosslinking mechanism (Scheme 1), two crosslinked PVP lms were obtained by using linear PVP as the starting material and the above-mentioned perylenes as the dye. As shown in Fig. 1, PDA contains the active-site of two anhydride end groups, while PDI has no such active group. These two dyes were separately dissolved together with linear PVP in the mixed solvent of ethanol and acetone. The reactions were performed at 160 C in an autoclave for 30 h, resulting in two PVP lms with different colors (Fig. 2). The PVP lms were washed with ethanol and THF until no dye could be washed out from the lm, which was detected by the UV-vis absorbance spectrum. The nal color
Preparation of a uorescent PDA-based PVP lm PVP (500 mg) and the perylene derivative (PDA or PDI) (30 mg) were dissolved in the mixture of ethanol (10 mL) and acetone (40 mL) under ultrasonication and mechanical stirring for 30 min (weight ratio of WPVP/WPDA ¼ 16.7/1). Subsequently the mixture was put into a Teon-lined stainless steel autoclave with a capacity of 60 mL and maintained at 160 C for 30 h. Then the autoclave was cooled to room temperature naturally. The obtained lm was 230 mg aer being washed with ethanol ve times and dried under vacuum. The residue PVP that did not involve in the formation of the PDA-based PVP lm was 288 mg, which was obtained by ltering the reaction mixture and drying the ltrate under vacuum. The amount of PDA This journal is © The Royal Society of Chemistry 2014
Scheme 1 Schematic representation of the formation of the fluorescent PDA-assembled PVP film.
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Fig. 2
film.
Digital photographs of the (A) PDA-based PVP film and (B) PDI
of the PDA-based PVP lm is red (Fig. 2A). As shown in Fig. 2B, the color of the PDI lm is light and consistent with that of the blank PVP lm (Fig. S1†) aer washing with solvents. This indicates that the PDI dye is incorporated into the lm via noncovalent molecular interactions and can be easily disassociated from the lm. TLC results also veried the stability of PDI in the reaction system because the dot of disassociated PDI from the PVP crosslinked lm remained unchanged by comparison with that of the free PDI (Fig. S2†). Banik27 reported that cyclic anhydride could react with the secondary amine in dichloromethane under reux temperature and Bao28 used bifunctional anhydrides as cross-linkers for the lm processing at a high temperature of 100–120 C. Since PDA contains two anhydrides, we assume that PDA might react with the ring-opened PVP under higher temperature and pressure (Scheme 1). Fig. 3 shows the fourier transform infrared (FTIR) spectra of all the materials. The characteristic peaks of C]O stretching vibration can be observed at 1660 cm1 in the FTIR spectrum of PVP, and a strong peak at 1774 cm1 can be assigned to the two carbonyl antisymmetric stretching vibrations in the FTIR spectrum of PDA. The absorptions at 1660 cm1 and 1774 cm1 are derived from PVP and PDA, respectively, indicating the successful incorporation of PDA into the PVP lm. We assumed that PDA incorporated into the PVP lm via chemical bonds (C–O of anhydride group and C–N of amide group), but they could not be distinguished by FTIR spectroscopy due to the inherent existence of the anhydride group and the amide group in PDA or PVP.
Fig. 3
FTIR spectra of PVP, PVP film, PDA-based PVP film and pure PDA.
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To further test the hypothesis, a reaction of nipecotic acid and PDA was carried out under the same reaction conditions as those for preparing uorescent lms (Scheme S2†). Nipecotic acid has a carboxyl group and an amine group, thus it is close to the ring-opened PVP chain. As shown in Fig. S3 and S4,† both the TLC and the mass spectrum of the product mixtures demonstrated that the dye was used up and some new products were generated simultaneously. Although we have not yet determined the exact chemical structures of these products because of the hard isolation of the products, this should not hinder the establishment of the above hypothesis. It is convincible that the PDA covalently bonded to the PVP lm by chemical reactions. Finally the PDA dye became the component parts and labeled the lm with uorescence. By contrast, PDI has no such active-site because the anhydride has been protected by 2,6-diisopropylaniline. Thus PDI did not react with any molecule and can be washed out completely from the PVP lm. In short, the active-site of anhydride located in the end of perylene is essential for the covalent bonding of uorescent molecules with the ring-opened PVP and the nal achievement of the stable uorescent lm. The surface morphology of the crosslinked blank PVP lm was reported to be composed of many microspheres with a diameter around 3–4 mm.24 It was explained that the self-intertwining of PVP led to the formation of PVP microspheres during the progress of ring-opening and crosslinking of PVP. In order to investigate the morphology and nanostructure of the uorescent PDA-based PVP lm, scanning electron microscopy (SEM) was performed. Interestingly, rod-like structures (1– 2 mm) were found on the surface of the uorescent PDA lm (Fig. 4, weight ratio of WPVP/WPDA ¼ 16.7/1). In order to investigate whether the rod lengths can be tuned by varying the crosslinking density of PVP, another PDA-based PVP lm was prepared by decreasing the weight ratio of WPVP/WPDA (10/1). As shown in Fig. S5,† the lengths of the rod-like structures are around 0.5–1.5 mm. Therefore, the rod-like structures become shorter by decreasing the amount of PVP in the reaction system. This phenomenon indicates that the rod lengths can be tuned by varying the crosslinking density of PVP. High resolution transmission electron microscopy (HRTEM) was performed to investigate the rod-like structure of the PDA lm in the reaction system. As shown in Fig. 5, the lattice structure can be easily
Fig. 4 SEM image of the fluorescent PDA-based PVP film (weight ratio of WPVP/WPDA ¼ 16.7/1).
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(A) HRTEM of the rod-like structures in the reaction system (inset: magnified image of a section in (A) of rod-like structure). (B) A section of the inset showing the assembly of PDA molecules (inset: selected area electron diffraction pattern).
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Fig. 5
observed. The orientation of the rod-like structures was further investigated by polarized optical microscopy (POM). Under crossed polarizers, these rod-like structures display a birefringence with an optical extinction upon rotation (45 ). This observation indicates that the self-assembled PDA molecules are locally oriented and ordered in the uorescent PVP lm (Fig. 6). In order to determine the molecular packing of the PDA dye in the uorescent lm, the powder X-ray diffraction (XRD) pattern was recorded. As shown in Fig. 7 and S6,† four strong peaks at 0.32, 0.36, 0.72 and 0.93 nm were observed both in the XRD spectrum of the PDA-based PVP lm and pure PDA. It was reported that the p–p plane distance ranged from 0.334 to 0.355 nm.29–31 Since 0.32 and 0.36 nm are very close, the two peaks could be assigned to the distance of the single-layer of p-stacked perylene. The data of 0.72 nm are about two times that of 0.36 nm and 0.93 nm are about three times that of 0.32 nm. Therefore, the peak at 0.72 nm and 0.93 nm could be assigned to the distance of double-layer and three-layer of p-stacked perylene, respectively. The peak intensities of the PDA-assembled PVP lm are different from those of pure PDA. When PDA was incorporated into the PVP lm, the intensities of peaks at 0.32 nm and 0.36 nm decreased. This phenomenon indicates that PDA tends to form double-layer and three-layer of p–p stacked aggregates during the fabrication of the crosslinked PVP lm. In addition, a big broad peak was found in the XRD spectrum of the PDA-based PVP lm, which was attributed to the amorphous PVP polymer. Thereby, the XRD result demonstrated that the p-conjugated structures existed in the PDA-based PVP lm. In this case, we assumed that PVP not only self-intertwined but also intertwined and combined with
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Fig. 6 (A) Polarized optical micrographs of rod-like structures in the reaction system. (B) Extinction occurs upon rotation (45 ) between crossed polarizers.
Fig. 7 XRD pattern of the fluorescent PDA-assembled PVP film.
PDA-aggregates to form the rod-like composites in the lm (Scheme 1). These p–p stacked aggregates were stably bonded to the uorescent lms because the dye could not be extracted by solvents. The produced PVP lm using PDI as the chromophore exhibited the similar morphology as that in the blank PVP lm,26 but the microspheres on the PDI lm were smaller than those on the PVP lm (Fig. S7†). Although PDI could not reacted with ring-opened PVP, it still involved in the process of PVP self-intertwining and precipitation, thus leading to the small microspheres. In short, owing to the strong intermolecular p–p interaction and the intertwining of PVP, the PDA dye tends to form double-layer and three-layer of p–p stacked aggregates during the preparation of the crosslinked PVP lm, Soft Matter, 2014, 10, 3426–3431 | 3429
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thus leading to the formation of rod-like nanostructures with long-range ordering. The emission spectra of the obtained uorescent PDAassembled PVP lm and its original free PDA dye are shown in Fig. 8. One can clearly see that a signicant red shi (110 nm) of the uorescence emission peak compared with free PDA dye (Fig. 8). Since the chemical modication at the imide positions of perylenes has little effect on the molecular-level electronic and optical properties,32–34 the signicant red shi of the emission must have been resulted from the aggregation of PDA molecules. In order to demonstrate this assumption, aggregation dependent uorescence spectra of PDA were recorded. As shown in Fig. 9, at the lower concentrations of PDA (1.0 107 to 1.0 104 mol L1), the maximum emission peak of PDA was observed at 528 nm. With the increasing concentration of PDA (1.0 104 to 1.0 102 mol L1), the maximum emission peak of PDA shows a signicant red shi (130 nm). Such signicant red shi in emission together with the rod-like nanostructures demonstrated that PDA molecules existed as p– p stacked aggregates in the lm.35,36 Since the PDA dye was covalently bound to the PVP lms by chemical reactions, the uorescence properties of perylene, such as excellent chemical
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and photo stability were still maintained. In one word, the optical property of the uorescent PDA-assembled PVP lm was in agreement with the above SEM and XRD results.
Conclusions In summary, a uorescent perylene-assembled PVP lm has been successfully prepared in one pot. A small-molecule model experiment demonstrated that the PDA dye was incorporated into the lm network via covalent bonds, thus the reactive group of anhydride in perylene played an important role in the preparation of the stable uorescent lm. Moreover, rod-like aggregates were observed in the uorescent PDA-assembled PVP lm due to the p–p stacking of the perylene molecules, which was conrmed by POM and XRD and supported by a signicant red shi of the maximum uorescence emission peak. The facile and versatile synthesis approach could be extended to the preparation of PVP uorescent lms with selfassembled nanostructures by using the uorophore that usually has an active anhydride group.
Acknowledgements This work was nancially supported by the National Science Foundation of China (21174012, 51103008 and 51221002), the New Century Excellent Talents Award Program from the Ministry of Education of China (NCET-10-0215) and the Doctoral Program of Higher Education Research Fund (20120010110008) and Chinese Universities Scientic Fund (ZZ1208).
Notes and references
Fig. 8 Emission spectra of the fluorescent PDA-assembled PVP film in ethanol (lex ¼ 480 nm).
Aggregation dependent fluorescence spectra of PDA in dimethylformamide (concentration: 1.0 107 to 1.0 102 mol L1, lex ¼ 480 nm). Fig. 9
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