Synthesis of Metallo-conjugated Copolymer with Electron Deficient Main Chain and the Formation of Polymer–Carbon Nanotube Hybrids
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We report the synthesis of a conjugated polymer incorporated with electron-deficient dipyrridophenanzine units and pendant ruthenium complexes. The electronic transition and photophysical properties of the polymer were thoroughly studied. Due to the presence of an extended conjugated system, polymer main chain was able to interact strongly with carbon nanotube (CNT) surface by π–π interaction, resulting in the formation of a dispersion in organic solvents. The polymer–nanotube hybrid was characterized by different microscopic and spectroscopic experiments. Raman spectroscopic results suggested that the electron density of the CNT decreased after the formation of polymer/nanotube hybrids, which suggests there is an electronic interaction between the polymer main chain and CNT.
KeywordsConjugated polymer Carbon nanotube Hybrid materials Computation
The functionalization of carbon nanotubes (CNTs) by polymeric materials has been a very interest research topic in the past decade because of the promising application potentials in a variety of areas such as sensing, optoelectronics, and biomedical applications [1, 2, 3]. In general, CNT can be functionalized by covalent or non-covalent approaches [4, 5]. In the first approach, the functional units are linked to the CNT surface via covalent bonds . However, the π-conjugated system on CNT surface will be disrupted, resulting to changes in their electronic properties. The second approach relies on the non-covalent interaction (e.g. π–π stacking) between the functional units and CNT surface , and is a preferable method because the π-electron system will not be affected as a result. In the past few years, our research group has been working on the synthesis of functional polymers for the formation of polymer–CNT hybrids and studying of their physical properties. Block copolymers and conjugated polymers functionalized with metal complexes have been synthesized [7, 8]. They were able to disperse both multiwalled and single walled CNTs (SWCNTs) and form stable dispersions in organic solvents. It has been demonstrated that the presence of metal complexes on the polymer pendant group or main chain could strongly perturb the photophysical properties of the resulting hybrid materials. In one of the examples, it was shown that the ruthenium complex attached was able to enhance the photosensitivity of the polymer–CNT hybrid, which was confirmed by photoconductivity AFM experiments . In other examples, the behavior of the transient species formed and the photoinduced electron transfer processes were found to be strongly affected by the introduction of metal complexes to the polymers [8, 10, 11]. Recently, it was found that the direction of photoinduced electron transfer in a polymer–CNT hybrid material is dependent on the relative energy level of the two entities . CNT can function as either electron acceptor or donor after photoexcitation. This means that electron- or hole-carrying CNTs can be generated if a proper functional polymer is chosen. The resulting materials will have great application potentials in opto-electronic devices.
Based on the previous works, we have synthesized a metalloconjugated copolymer functionalized with electron deficient dipyrridophenanzine units on the main chain and ruthenium terpyridine complexes at the pendant chain. It is envisaged that the electron deficient N-heterocyclic units and the sensitizing ruthenium complexes have significant effect to the physical properties to the hybrid formed. The polymer was able to disperse SWCNTs, and the interaction between the polymer and CNTs was studied by various computation and spectroscopic techniques.
2 Experimental Sections
2.1 Reagents and Materials
2,7-Dibromofluorene, pyridine, tetra-n-butylammonium bromide, tetra-n-butylammonium hydroxide and ruthenium(III) chloride hydrate were purchased from Aldrich Chemical Co. 5,5′-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′-bithiophene (7) was purchased from TCI Chemicals. 2,7-Dibromo-9,9-bis(6′-bromohexyl)fluorene (1) [12, 13, 14, 15], 4′-(4-hydroxyphenyl)-2,2′:6′,2″-terpyridine (2) procedure , 5,6-diamino-1,10-phenanthroline [17, 18] (4), and 2,2′:6′,2″-terpyridine ruthenium trichloride (9) were synthesized according to literature procedures .
1H and 13C NMR spectra were recorded on Bruker DPX 300 NMR spectrometer (300 MHz and 75 MHz respectively) or Bruker Avance 400 NMR spectrometer (400 MHz and 100 MHz respectively). CDCl3, DMSO-d0, CD3CN-d3 and DMF-d7 were purchased from Cambridge Isotope Laboratories, Inc.; 1,1,2,2-tetrachloroethane-d2 was purchased from Acros Organics. For 1H NMR spectra, the signals were referenced to 0 ppm for tetramethylsilane internal standard in CDCl3; 2.50 ppm (residual proton of the solvent) for DMSO-d6; 2.05 ppm (residual proton of the solvent) for CD3CN-d3; 8.03 ppm (residual proton of the solvent) for DMF-d7; 6.00 ppm (residual proton of the solvent) for 1,1,2,2-tetrachloroethane-d2. In 13C NMR spectrometry, the signals were referenced to the residual proton of the solvent at 77.16 ppm in CDCl3; 39.52 ppm (residual proton of the solvent) for DMSO-d6; 2.50 ppm (residual proton of the solvent) for CD3CN-d3; 163.15 ppm (residual proton of the solvent) for DMF-d7; 73.78 ppm (residual proton of the solvent) for 1,1,2,2-tetrachloroethane-d2. Positive-ion electron impact (EI) and fast-atom bombardment (FAB) mass spectra were collected from Finnigan MAT-95 mass spectrometer. UV–vis absorption spectra were collected from Varian Cary 50 UV–vis spectrophotometer. Micro-Raman spectroscopy was performed using Renishaw RM 3000 Micro-Raman system. He–Ne laser (633 nm) at 25 mW (with attenuator 10%) was used as excitation light source. The pristine SWCNTs and polymer/SWCNT hybrid samples were analyzed directly on PTFE membrane at the spectral range of 100 cm−1 to 3200 cm−1. GPC was performed using a Waters GPC system equipped with a Styragel HR 3 column, a Waters 2414 refractive index detector and a Waters 2998 photodiode array detector. The GPC system was calibrated using polystyrene standards. THF was used as the eluent with a flow rate of 0.4 ml min−1.
2.3 Monomer (3)
A mixture of 2,7-dibromo-9,9-bis(6′-bromohexyl)fluorene (1) (1.00 g, 1.54 mmol), 4′-(4-hydroxyphenyl)-2,2′:6′,2”-terpyridine 2 (1.0 g, 3.1 mmol), potassium carbonate (0.47 g, 3.4 mmol), potassium iodide (0.003 g, 0.018 mmol) and DMF (30 ml) were stirred under nitrogen at 100 °C for 16 h. After cooling to room temperature, the mixture was poured into 200 ml water and the product was precipitated. The mixture was filtered and product was recrystallized with ethanol and methanol mixture. White solid was obtained as the product. Yield 1.28 g (73%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.73–8.71 (m, 4H), 8.69 (s, 4H), 8.66 (d, 4H, J = 7.9 Hz), 7.89–7.84 (m, 8H), 7.54–7.52 (m, 2H), 7.48–7.46 (m, 4H), 7.36–7.33 (m, 4H), 6.97 (dd, 4H, J = 8.7, 7.0 Hz), 3.91 (t, 4H, J = 6.5 Hz), 2.17–2.15 (m, 4H), 1.31–1.25 (m, 4H), 1.23–1.14 (m, 4H). 13C NMR (75 MHz, CDCl3) δ (ppm): 156.5, 155.9, 149.2, 137.0, 130.4, 128.6, 126.3, 123.9, 121.5, 121.4, 118.4, 114.9, 68.1, 40.3, 29.7, 29.2, 25.8, 23.8. FAB-MS: m/z 1138.11 [M]+.
2.4 2,3-Bis(4-bromophenyl)pyrazino[2,3-f][1, 10]phenathroline (6)
Monomer 6 was synthesized according to a modified procedure [20, 21, 22]. 1,10-Phenanthroline-5,6-diamine (4) (1.00 g, 4.76 mmol) and 4,4′-dibromobenzil (5) (1.93 g, 5.23 mmol) were added into a mixture of acetic acid (60 ml) and ethanol (60 ml). The reaction mixture was stirred under nitrogen atmosphere at 60 °C for 4 h. After cooling, the crude product mixture was filtered and washed with methanol and acetone. The residue was introduced to a 250-ml round-bottom flask and was stirred in acetone (200 ml) at room temperature for 12 h to dissolve the unreacted 4,4′-dibromobenzil. The product mixture was filtered and the residue was washed with acetone. White solid was collected as the product. Yield 2.15 g (84.0%). 1H NMR (400 MHz, CDCl3) δ (ppm): 9.50 (d, 2H, J = 8.0 Hz), 9.29 (d, 2H, J = 3.0 Hz), 7.79 (dd, 2H, J = 8.1 Hz, 4.3 Hz), 7.56 (s, 8H). 13C NMR (100 MHz, CDCl3) δ (ppm): 152.381, 152.338, 151.205, 147.764, 138.199, 137.345, 133.282, 133.231, 131.842, 131.592, 126.792, 124.050, 124.036. HREI-MS: m/z 541.9591 [M]+.
2.5 Polymer (8)
Polymer 8 was synthesized by the Suzuki coupling reaction [23, 24, 25, 26, 27]. Monomer 3 (0.420 g, 0.370 mmol), monomer 6 (0.200 g, 0.370 mmol), and 7 (0.308 g, 0.738 mmol) and were added to a 50-ml Schlenk tube under a nitrogen atmosphere. Anhydrous toluene (40 ml) and tetra-n-butylammonium hydroxide (1 M solution in MeOH, 2.00 ml, 2.00 mmol) were added. The solution was further degassed by three freeze–pump–thaw cycles. Tetrakis(triphenylphosphine)palladium(0) (0.0214 g, 0.0185 mmol) was added to the reaction mixture. The reaction was carried out under nitrogen atmosphere at 60 °C for 48 h. After cooling, the mixture was filtered through a Celite bed to remove metal catalyst and insoluble polymer. The mixture was concentrated by rotary evaporation, and the polymer was obtained by precipitation in 200 ml methanol. The reprecipitation procedure was repeated twice and the residue was collected by filtration. Brown solid was obtained as product. Yield 0.182 g (29%).
2.6 Polymer (10)
The metalation procedure is based on literature procedures. Polymer 8 (0.100 g, 0.0592 mmol) and ruthenium complex 9 (0.052 g, 0.118 mmol) were added to a 50-ml Schlenk tube. The flask was degassed and filled with nitrogen gas thrice. Dry DMF (20 ml) was added and the reaction mixture was stirred at 100 °C under nitrogen for 48 h. After cooled to room temperature, the mixture was filtered to obtain the filtrate. Excess potassium hexafluorophosphate aqueous solution was added and the mixture was stirred for 15 min. The mixture was filtered and the residue was further washed with water, methanol, ethanol and diethyl ether. Brown solid was obtained as product. Yield 0.13 g (76%).
2.7 TDDFT Calculations
All the organic molecules involved in the DFT calculations were optimized at the Becke, three-parameter, Lee–Yang–Parr (B3LYP)/6-311G(d,p) level, while B3LYP/LANL2DZ was used in geometry optimization of ruthenium-containing molecules. Vibrational analysis was done in the same basis set to verify all the optimized structures were at the minimum of the energy surface. The spin-restricted singlet–singlet transitions were calculated by TDDFT calculations using B3LYP/6-311G(d,p) or CAM-B3LYP/6-311G(d,p) for those organic molecules, and B3LYP/LANL2DZ or CAM-B3LYP/LANL2DZ were used in the calculations of ruthenium-containing molecules. CPCM was used as the solvent model and chloroform was chosen to be the solvent (ɛ = 4.7113). All calculations were performed in Gaussian 09  in the GRIDPOINT and HPC2015 system of The University of Hong Kong. The spatial plots of molecular orbitals were generated by Gaussview 5.0 and Chem3D 15.1.
3 Results and Discussion
3.1 Synthesis and Characterization of Polymers
Selected electronic excitations of model compound 8′ in chloroform with f > 0.2, calculated from TDDFT calculations at CAM-B3LYP/6-311G(d,p) level of theory
Vertical excitation energy (eV)
Oscillator strength f
Configuration interaction (CI) expansion coefficient (% contribution)
H → L
H → L + 1
H − 1 → L + 2
H − 2 → L
3.2 Polymer–CNT Hybrids
Summary of micro-Raman spectroscopic data
Intensity ratio of D- to G-bands
Major RBM peaks observed (cm−1) and the calculated nanotube diameter (nm)
152 (1.60), 193 (1.24), 216 (1.10), 256 (0.92), 288 (0.82), 386 (0.60)
152 (1.60), 199 (1.24), 219 (1.10)
152 (1.60), 197 (1.24), 218 (1.10)
Chiral SWCNTs usually show a doublet peak in G-band, and exhibit fewer Raman active modes in the region of 400–1500 cm−1, while the intensity of the D-band can be regarded as a measure of defect intensity [33, 34, 35]. The G-bands observed in Fig. 9 consists of a single sharp peak, indicating the semiconducting nature of both pristine SWCNTs and SWCNTs in the polymer/SWCNT hybrids . The intensity of D-band can be regarded as a measure of the defect intensity [32, 35]. An increase in D-band intensity indicates the presence of defect on the SWCNT surface (e.g. open tube ends or side-wall holes). Since the ratios between the D-band and G-band intensities are similar for the pristine SWCNT, polymer 8/SWCNT and polymer 10/SWCNT hybrids, indicating that there is no significant structural change or formation of defects after ultrasonication treatment. For the G- and RBM-bands, it has been reported that a frequency shift is the result of the change in SWCNT electron density [33, 34]. From Fig. 9, it can be seen that both the G- and RBM bands shift to the higher frequency after the functionalization with polymers 8 or 10 (from 1582 cm−1 for pristine SWCNT to 1587 cm−1 and 1586 cm−1 for 8/SWCNT and 10/SWCNT hybrids, respectively). This suggests a shift in electron density from the SWCNT to polymer (i.e. the SWCNTs became more electron-deficient after functionalization) [33, 34]. These results indicate there are notable interactions between the polymers and SWCNTs. Such phenomena were also observed in CNT functionalized with polythiophene based conjugated block copolymers . It is envisaged that the shift in electron density is due to the presence of electron deficient dipyrridophenanzine units on the main chain. Nevertheless, it is necessary to emphasize that at this stage, the nature of the electronic interactions is not fully understood. Further theoretical and experimental studies (e.g. ultrafast time-resolved spectroscopy) will be required in order to investigate the details of the excited states formation and the possible subsequent charge transfer processes.
A conjugated polymer incorporated with electron-deficient main chain and metal complex pendant chain were used in the dispersion of SWCNT. Due to the presence of ionic metal complex, a stable polymer–SWCNT dispersion was formed compared to the metal-free polymer. Raman spectroscopic results showed a shift in the G-band after the formation of the polymer/SWCNT hybrids. This suggests a shift in electron density from the nanotubes to the polymers and the presence of an electronic interaction between the species. In order to investigate the details of such interaction, more detailed photophysical spectroscopic studies, such as ultrafast time-resolved spectroscopic, will be required in order to understand how the polymer interact with the CNTs.
This work was supported by the Research Grants Council of Hong Kong (HKU 700311P and HKU 700613P). The computation was conducted by using the HKU Information Technology Services Research Computing Facilities supported in part by the Hong Kong UGC Special Equipment Grants (SEG HKU09). NAMD was developed by the Theoretical and Computational Biophysics Group in the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign.
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