Morphology and properties of a polyrotaxane based on γ-cyclodextrin and a polyfluorene copolymer

Chemical Physics Letters 465 (2008) 96–101

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Morphology and properties of a polyrotaxane based on c-cyclodextrin and a polyfluorene copolymer Aurica Farcas a,*, Indrajit Ghosh b, Nathalie Jarroux c, Valeria Harabagiu a, Philippe Guégan c, Werner M. Nau b a

‘P. Poni’ Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda Alley, Iasi-700487, Romania Jacobs University Bremen, School of Engineering and Science, Campus Ring 1, D-28759 Bremen, Germany c Laboratoire Matériaux Polymères aux Interfaces, Université d’Evry Val d’Essonne, France b

a r t i c l e

i n f o

Article history: Received 23 July 2008 In final form 22 September 2008 Available online 26 September 2008

a b s t r a c t The effect of complexation of a poly[2,7-(9,9-dioctylfluorene-alt-2,7-fluorene)] copolymer into the inner cavity of c-cyclodextrin on the morphology, the optical and the electrical properties was investigated. The fluorescence spectra of the rotaxane copolymer and of a reference, non complexed copolymer exhibit typical well resolved blue emission bands arising from the fluorene chromophore units. The complexation of the fluorene copolymer chain increases the fluorescence lifetime from 0.54 to 0.59 ns and decreases the activation energy of the electrical conduction from 2.41 to 1.49 eV. The rotaxane copolymer presents higher semiconducting properties as compared to the reference copolymer. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The production of multifunctional materials directed towards diverse applications requires an interdisciplinary approach to the phenomena governing their properties, achievable through a combination of chemistry, physics, and material science, as modern work instruments. The academic as well as the industrial interest in conjugated polymers is driven by their potential application in various optoelectronic devices [1]. Recent reports relate these materials in electrochemical light-emitting cells [2], components in molecular electronics [3], and polymer-based injection laser [4]. Among the conjugated polymers, particularly, a number of fluorene copolymers (PFs) have been attracting great interest as very promising candidates for blue light-emitting diodes due to their pure blue and efficient luminescence combined with high mobility, excellent thermal and chemical stability, good film-forming and hole-transporting properties [5–8]. The practical application of such polymers is limited by their wide emission band during operation, attributed to thermo- or oxidative degradation of the PFs backbone, to secondary reactions at the 9th position of fluorene units resulting in fluorenone groups, to aggregates and/or interchain excimers [5,9,10]. Such effects are unfavorable for lightemitting devices and, to diminish them, the control of polymer morphology at molecular level is need. Various strategies have been used to reduce the formation of aggregates or of keto defects in PFs. One of the most frequently used methods seems to be the copolymerization of substituted and non-substituted fluorene monomers [11]. * Corresponding author. Fax: +40 232211299. E-mail address: [email protected] (A. Farcas). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.09.058

The encapsulation of conjugated polymers into macrocycle cavities in order to improve their electro-optical properties and to allow high performance applications received much attention in the last years and complex structures of pseudo- or rotaxane type represent a very active topic of science [12–19]. Most of the polyrotaxanes already reported contain (a-, b- or c-) cyclodextrin as macrocyclic component on the main or side chains. They showed notable changes of solubility, of thermal and opto-electronic properties as compared to the simple linear backbone of the polymer. Preliminary investigations conducted on polyrotaxanes indicated multilateral and interesting perspectives for the utilization of these complex structures in high-performance fields [12]. The interest in this domain is justified by the observation that these architectures have clearly defined characteristics, thus offering an alternative to the production of new materials with specific properties for diverse applications. While the success of these assemblies has been developed, the exact mechanisms leading to the improvement in application are still unclear. In the last few years many authors demonstrated that insulation of conjugated polymer chains through the inclusion into macrocycle cavities has a strong effect on the optical and electronic properties [12,13,16–19]. The purpose of this work is to combine the structural advantages of rotaxanes and to analyze the influence of a partial protection provided by the c-cyclodextrin (cCD) to the rotaxane structure on the properties of fluorene copolymer, focusing our attention on their morphology and on their UV–vis, fluorescence, lifetime decay and semiconducting properties. To this purpose, a copolymer with multiply blocked rotaxane architecture on the main chain, poly[2, 7-(9,9-dioctylfluorene)-alt-2,7-fluorene/cCD)] (PF-cCD), was syn-

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thesized and its properties were compared with the reference copolymer, poly[2,7-(9,9-dioctylfluorene)-alt-2,7-fluorene)] (PF). 2. Experimental 2.1. Materials

c-Cyclodextrin (cCD) was purchased from Wacker Chemie (Germany) and used after drying under vacuum for 48 h at 100 °C. 2,7-dibromofluorene 97%, 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) 97%, tetrakis(triphenylphosphine)palladium (0), 99% were purchased from Aldrich Chemical Co. and were used as received. Dimethylformamide (DMF) was purchased from Carlo Erba Reactifs-SDS and freshly distilled before use. All other solvents were of analytical grade and used without further purification. Poly[2,7-(9,9-dioctylfluorene)-alt-2,7-fluorene/cCD)] (PF-cCD) rotaxane copolymer and poly[2,7-(9,9-dioctylfluorene)-alt-2,7-fluorene (PF) reference copolymer were synthesized by Suzuki coupling [20], according to a procedure reported in detail elsewhere [17]. As apropriate comonomers, which would be able to build through polycondensation the mentionated rotaxane copolymer, a 1/1 2,7-dibromofluorene (DBF)/cCD inclusion complex precursor and 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) (BTB) were chosen. In order to compare the electro-optical and morphological properties induced by the rotaxane architectures when cCD is used as host macrocycle, a PF reference copolymer without rotaxane architecture was also synthesized by coupling BTB with DBF. Copolymers having stable phenyl groups with structures presented in Fig. 1 were obtained by reacting stoechiometric mixtures of borate and bromine functionalized comonomers, adding a slight excess of BTB at the end of polymerization to introduce ester groups at both polymer chain ends, and, finally, terminating the polycondensation with bromobenzene. The termination of the growing chains with chemically stable phenyl end groups plays a key role in obtaining processable copolymers [21,22]. PF-cCD sample: 1H NMR (DMSO-d6/C6D6 (1/1 v/v) (T = 25 °C): d = 7.64–7.91 (m, 12H, fluorene units), 6.05 and 6.08 (16H, OH2+3, cCD), 5.11 (d, 8H1, cCD), 4.73 (8H, OH6, cCD), 3.84 (s, 2H, fluorene unit), 3.51–3.95 (m, 48 H2-6, cCD and H2O), 2.11 (t, 4H, dioctyl)), 0.83–1.29 (m, 24H, dioctyl), 0.67–0.71 (t, 6H, dioctyl). IR (KBr, cm1 ): 3370 (H-bonded OH), 1627 (OH), 1368 (OH), 1243 (OH), 1156 (COC and OH), 1079 and 1032 (COC) characteristic for cCD, 2980 and 2890 (alkyl C–H), 1460 (fluorene ring) and 810 (aryl C–H). PF: 1H NMR (C6D6): d = 7.54–7.59 (m, 12H), 3.70 (s, 2H), 2.02 (t, 4H), 0.90–1.16 (m, 24H), 0.63–0.66 (t, 6H).

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cord Carl Zeiss Jena FT IR spectrophotometer. Size exclusion chromatography (SEC) was performed on a Waters equipment with UV (k = 345 nm) and refractive index detectors. Analytic WATO45810 column was used, and the flow rate of the eluent was 0.3 mL min1. Solutions containing analytic quantities (3 mg mL1) of copolymers or cCD in 1/1 v/v DMF/toluene were injected to limit the diffusion phenomena and the adsorption of cCD. DMF was used as eluent. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere at a heating rate of 10 °C min1 from room temperature to 480 °C in a dynamic nitrogen atmosphere using a TA Instruments Q50. DSC curves were obtained on a TA Instruments Q100 calorimeter calibrated with indium, under a nitrogen flow of 50 mL min1, at a heating rate of 10 °C min1. The data were collected during the first and the second heating runs. UV-Vis spectra were recorded on a Specord M42 instrument. Fluorescence spectra were obtained on a Perkin Elmer LS 55 spectrometer. The following protocol was used: 0.037 mg of the reference copolymer (PF) or 0.051 mg of the copolymer with cCD (PFcCD) was dissolved in 5 mL of DMF and the obtained solutions were sonicated for around 15 min. The solubility of both compounds in DMF is relatively low, and after sonication the saturated solution can be obtained. Fluorescence lifetimes of PF-cCD and PF solutions were measured by time-correlated single-photon-counting (TCSPC) on a FLS-920 (Edinburgh Instruments) using a H2-flash lamp as excitation source. Film resistance for PF-cCD was measured with a Keithley model 6517 digital electrometer in a coplanar configuration with silver electrodes (at an internal width of 2–3 mm and a length of 6– 8 mm) evaporated onto the substrate before the deposition of the polymeric films. The measurements were performed by applying static electrical fields of low intensity (E < 102 V/cm). As experimentally established, under these conditions, the non-ohmic effects were insignificant in the investigated electrode/polymeric film/electrode systems. The surface profiles of copolymers were evaluated by Atomic Force Microscopy (AFM) on a SPM Solver PRO-M device. The experiments were performed by using the semi-contact mode. Commercially available high reflective gold coating silicon cantilever (NSG 10, NT-MDT Russia) with spring constant of 11.5 N/m and tip radius of 10 nm was used. The value of the resonance frequency was 279.242 KHz. The copolymer films were prepared by spincoating (1000 rpm for 30 s) of copolymer solutions in DMF (ca.106 M) on glass substrates, and drying at 100 °C for 1 h under vacuum. 3. Results and discussion

2.2. Measurements 3.1. Synthesis and structure Polymer analysis was performed by NMR spectrometry (Bruker Advance 400 MHz, in DMSO/C6D6 1/1 v/v, at 25 °C). The IR spectra of polymer powders were measured on KBr pellets by using a Spe-

Previously we described the preparation of multiblocked polyrotaxanes based on poly[2,7-(9,9-dioctylfluorene)-alt-2,7-fluo-

Fig. 1. The chemical structures of PF-cCD and PF copolymer samples.

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rene/cCD] rotaxane copolymer by stoechimetric Suzuki polycondensation of a 1/1 2,7-dibromofluorene (DBF)/cCD inclusion complex precursor and 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) (BTB), followed by the termination of the growing chains with bromobenzene [17]. In these conditions, copolymers having one chain end terminated in bromofluorene reactive groups were obtained. As proved by DSC analysis, the copolymer undergoes chemical crosslinking under heating [17]. Based on this observation, to obtain stable and processable materials, a slightly different synthetic approach was used, previously proposed for the preparation of PF-bCD rotaxane copolymers [21] or of fluorene-bisthienylbenzothiadiazole copolymers [22]. In this approach, a small excess of BTB was added at the end of polymerization to introduce ester groups at both polymer chain ends, and, finally, bromobenzene was added as a monofunctional end-capping reagent to react the boronic ester end groups. Thus, a copolymer having stable phenyl groups at both ends was obtained (Fig. 1). The bulky n-octyl substituents in the 9-position of DBF were used to improve the solubility of PF reference copolymer and as blocking groups of PF-cCD rotaxane copolymer. The spatial distribution of octyl groups is larger enough than the diameter of the inner cavity of cCD indicating their blocking effect [17]. In order to compare the electro-optical and morphological properties induced by the rotaxane architectures when cCD is used as host macrocycle, a non complexed PF reference copolymer terminated in stable phenyl groups (Fig. 1) was also synthesized by coupling BTB with DBF. The structure of copolymers was proved by 1H NMR, and IR spectroscopy (see Experimental part). In PF-cCD sample, the 1H NMR chemical shifts corresponding to the fluorene copolymer chains were enlarged and down-field shifted by about 0.01– 0.14 ppm as compared to the reference PF, confirming the formation of a rotaxane structure. A ratio of cCD/fluorene of about 0.7/ 1 in the copolymer was determined by 1H-NMR in a DMSO-d6)/ (C6D61/1 v/v solvent mixture. The formation of the polyrotaxane structure was also confirmed by SEC using DMF as eluent (not shown). The chromatograms of cCD, PF and PF-cCD copolymers were expressed in PEO equivalents. The peak of free cCD was not present in the chromatogram of PF-cCD and this point clearly evidenced that the rotaxane sample is not a physical mixture between components. The SEC trace of PF shows, besides the large peak between 2.6–3.0 mL (Mn = 38 000 g mol1 and Mw/Mn = 2.21), a supplementary peak at higher molecular weight (1.8 mL). The peak at lower elution volume could be attributed to the aggregation of the macromolecular chains, which appears in DMF – a poor solvent for PFs. The curve of the rotaxane copolymer visualized at around 2.5 mL is bimodal, suggesting the separation on the chromatographic column of the different copolymer chains with lower and higher content of threaded cCD macrocycles, behaviour reported for other rotaxane polymers [23]. However, the curve of the rotaxane copolymer is situated at lower elution volumes as compared to that corresponding to the PF reference copolymer, denoting a higher molecular weight. Very roughly the molecular weight of the rotaxane copolymer PF-cCD at around 2.5 mL could be (Mn = 68 000 g mol1 and 21 000 g mol1, respectively and Mw/Mn = 2.5 for the whole curve). A correct estimation of the molecular weight could not be obtained by SEC analyses because the molecular weight calculation is based on the hydrodynamic volume of the copolymer and in our case a rod like structure is supposed. 3.2. Thermal properties The thermal stability of the copolymers was investigated by TGA. TGA data revealed that all polymers were stable up to about 300 °C. The PF copolymer started to decompose at about 340 °C,

while its polyrotaxane homologue showed the beginning of the decomposition at about 305 °C, not much higher than the decomposition temperature of cCD (290 °C), which provides an indication that the rotaxane architecture slightly increased the stability of the macrocycle. Both copolymers exhibit good thermal stabilities, loosing less than 10% of their weight on heating to about 380 °C. The DSC curves of both PF and PF-cCD samples showed that no glass or melting transitions appears between 20 and 250 °C. The same results were obtained by using b-CD as macrocycle [21]. 3.3. Solution optical properties The optical properties of PF and PF-cCD were studied in DMF solution with sonicated or unsonicated samples. The UV–vis spectra of PF and PF-cCD showed a maximum at 368 nm for unsonicated samples and a maximum at ca. 374 nm for sonicated samples, corresponding to the p–p* transition of the polymer backbone (Fig. 2). For the unsonicated samples, the absorption intensity of PF-cCD is lower than that of PF (Fig. 2a). After sonication of the samples the absorption intensity of PF-cCD became stronger than that of PF and the absorption remained then stable with time (Fig. 2b). We therefore attribute the increase in absorption intensity to an accelerated solubilization upon sonication. The stronger absorption intensity in the rotaxane sample could be indicative of a constructive excitonic coupling among the polymer chain caused by protection of the macrocycle. Excitation spectra were taken before and after the sonication for both the samples. In both cases, the excitation spectra were the same suggesting that major structural changes have not occurred. The excitation spectra have been normalized to get a better comparison between the two sonicated samples. Both sonicated as well as unsonicated samples show the same excitation spectra. The normalized excitation spectrum for unsonicated samples it is not shown in the Fig. 3, because the spectra would be difficult to differentiate. In the fluorescence emission spectra of the unsonicated samples, the emission peak intensities at ca. 413 and 437 nm were higher for the rotaxane structures. In contrast, in the fluorescence spectra of the sonicated samples (Fig. 3), there was no significant difference in the electronic absorption and emission spectra of the reference and the rotaxane copolymer, consistent with previously reported results for acetylene dye rotaxane [19]. The similarity between the absorption and fluorescence excitation spectra revealed for each case that the emissions arose from the actual compounds and not from impurities. The fluorescence spectra of both copolymers exhibited typical well-resolved blue emission bands arisen from the fluorene chromophore units. The optical properties of the PF-cCD polymer in solution were very similar to those of PF. This suggests that the electronic transitions are not affected by the macrocycle, which in turn could lead to welldefined vibronic structures in the emission spectra, indicating that the polymers have a rigid and well-defined backbone structure [24]. The fluorescence lifetimes of PF and PF-cCD were measured by TCSPC. Fig. 4 shows the fluorescence intensity decay profiles and a clear difference was observed between PF and PF-cCD samples. The lifetimes of PF and PF-cCD are 0.54 ns and 0.59 ns, respectively. There is an increase in the protection of the rotaxane copolymer molecules from the solvent and we presume that solventinduced deactivation is slightly reduced in the cyclodextrin-encapsulated chromophores. We also performed induced circular dichroism (ICD) measurements, because cyclodextrin-encapsulated chromophores, in this case fluorene, are expected to exhibit an ICD signal [25,26]. Unfortunately, we were unable to detect a signal, which is presumably

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Fig. 2. UV–vis spectra of PF-cCD and PF copolymers in DMF solutions: (a) unsonicated samples and (b) sonicated samples.

3.4. Film morphology and electrical properties

Fig. 3. Normalized excitation (kem = 415 nm) and emission (kex = 374 nm) fluorescence spectra of the PF and PF-cCD copolymers in DMF solutions.

For opto-electronic application, flat films with good mechanical properties are required. The surface morphology of PF and PF-cCD copolymer films was studied by atomic force microscopy. Fig. 5 shows the bidimensional images as well as the height profiles (generated along the straight lines). It was not possible to obtain a good quality of film of 1/1 physical mixture of PF and cCD for AFM studies. The film of the PF copolymer was almost flat, while the PF-cCD copolymer film presented very clear globular formations over the same scanning scale area. The PF-cCD copolymer showed a distinctive texture, which indicated that the rotaxane architecture changed the morphology of the PF copolymer. Moreover, the height of the protuberances detected in PF-cCD is significantly higher than those of PF. Furthermore, the average roughness determined for PF-cCD and PF is 38.1 nm and 3.9 nm, respectively. The electrical properties of the PF-cCD copolymer film were followed by studying the temperature dependence of the electrical resistance. It was experimentally established that samples with stable structure, can be obtained by subjecting them, after preparation, to a heat treatment consisting of several successive heating/ cooling cycles within a certain temperature range, DT, characteristic for each polymer. After heat treatment, the temperature dependence of the electrical conductivity became reversible. This showed that the rotaxane sample became stable in the respective temperature range. Moreover, the characteristic lnR = f(103/T) dependencies could not be approximated by a single activation energy within the investigated temperature range. The resistance curve was characterized by two distinct parts: a part with a larger slope (within the higher temperature range) and a part with a smaller slope (within the lower temperature range). These results are similar to extrinsic and intrinsic temperature ranges observed in inorganic semiconductors. The temperature dependence of the electrical conductivity, r, can be expressed according to equation 1 [27,28].

r ¼ r0 expðDE=2kTÞ

Fig. 4. Time-resolved fluorescence decay profiles of (1), PF and (2), PF-cCD copolymers.

due to the high dilution of the chromophores and the comparably low sensitivity of ICD measurements.

ð1Þ

where DE denotes the thermal activation energy of electrical conduction, r0 is a parameter depending on the polymer nature, and k is Boltzmann’s constant. The activation energy calculated with Eq. (1) is lower for PFcCD rotaxane (1.49 eV) as compared to PF reference copolymer (2.41 eV). The values of the electrical conductivities (r) are 6.4  107 and 8.2  108 S cm1 for PF-cCD and PF films, respectively, indicating semiconducting properties. The higher semicon-

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Fig. 5. AFM 2D images and height profiles of (a), PF-cCD and (b), PF copolymer films.

ductivity of the studied rotaxane copolymer is presumably due to its molecular structure, which affords extended conjugation of the electrons in the copolymer chain and reduces aggregation by partial encapsulation of single polymer chains. 4. Conclusions The electro-optical properties and the morphology of a cCDpolyrotaxane based on fluorene copolymer were investigated and compared with the reference copolymer. As expected, the physical and spectroscopic properties were influenced by the threading of cCD onto copolymer chain. The fluorescence spectra of both copolymers exhibited the typical blue emission bands arising from the fluorene chromophore. The rotaxane copolymer displaced also a longer fluorescence lifetime, induced by a partial protection of the macrocycle. The rotaxane architecture could positively affect the aggregation in the solid state through interpolymer interactions, the crystallinity, and the molecular packing, while potentially leaving the desirable electronic properties related to pconjugation unaltered. We conclude that this approach will offer a broad range of applications. Experiments are in progress towards exploring the behaviour of these new materials in electroluminescent light-emitting diodes.

of Dr. Elena Hitruc of ‘Petru Poni’ Institute of Macromolecular Chemistry for AFM characterization is also acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

[16] [17]

Acknowledgements One of the authors, A.F., is grateful to the University Evry Val d’Essonne, France, and to RAINS EC project (INCO-CT-2005017142) for financial support of this work. The useful cooperation

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