- Email: [email protected]
Synthesis and characterization of perylene-based donor–acceptor copolymers containing triple bonds
Synthetic Metals 160 (2010) 996–1001
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis and characterization of perylene-based donor–acceptor copolymers containing triple bonds E. Kozma ∗ , F. Munno, D. Kotowski, F. Bertini, S. Luzzati, M. Catellani Istituto per lo Studio delle Macromolecole – CNR, via Bassini 15, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 9 November 2009 Received in revised form 29 January 2010 Accepted 12 February 2010 Available online 15 March 2010 Keywords: Perylene diimide copolymers Conjugated polymers Donor–acceptor copolymers Acceptor materials Photovoltaic polymers
a b s t r a c t Two donor–acceptor copolymers containing perylene diimide and oligothiophenes linked through triple bonds have been designed and synthesized by palladium catalyzed Sonogashira coupling reaction. The introduction of the triple bond spacer between the electron acceptor perylene moieties and the electron donating oligothiophene moieties produces an extended conjugation along the main chain, and produces soluble and high molecular weight copolymers.The thermal, optical and electrochemical properties of these copolymers were examined. The HOMO–LUMO energy levels of these copolymers, along with the quenching of their photoluminescence in 1:1 (w/w) blends with poly(3-hexylthiophene) (P3HT), indicate that they are suitable materials for application as electron acceptor and n-type components in bulk heterojunction solar cells with P3HT. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polymers have continuously received intense attention due to their unique optical and electrical properties and have become important materials for the fabrication of low cost and large area flexible optoelectronic devices, such as solar cells [1]. For these types of applications, the electron donating or electron accepting properties of the conjugated polymer are of crucial importance. Considerable efforts have been put into the design and synthesis of various donor–acceptor architectures, because the intramolecular charge transfer and separation of -conjugated systems with donor–acceptor properties can mimic the photochemical and photophysical processes that take place in the natural photosynthetic reactions [2]. During the last few years, several groups focused their attention on perylene diimide (PDI) dyes, which act as electron acceptors and exhibit high thermal and photochemical stabilities, strong absorption of the visible light, with relatively high electron affinity and good electron transport properties. These outstanding properties have rendered them promising building blocks for donor–acceptor type small molecules [3] or polymers, by functionalizing the perylene core along the imide positions [4] or in bay positions [5]. However, previous structure calculations have revealed that nodes exist in the imide nitrogen atoms, which are unfavourable for the efficient electronic interaction between the PDI and the unit which is linked at the nitrogen atom position [6].
∗ Corresponding author. Tel.: +39 02 23 699 739; fax: +39 02 70 636 400. E-mail address: [email protected] (E. Kozma). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.02.015
Therefore, the substitution in the bay region of the perylene core through different coupling reactions became an interesting alternative to favour the electronic communication between the donor and acceptor moieties. On the other hand, it is already well known that oligothiophenes are electron donating materials and were used as hole-transport in several types of optical and microelectronic devices. The oligothiophenes can be easily prepared and their interesting optical and electronic properties, like fluorescence, semiconductance and light emission, can be tuned by changing their chemical structure [7]. There are a few studies regarding perylene–oligothiophene copolymers, in which the donor/acceptor units are directly connected and they are characterized by quite low molecular weight [8]. Among these, a narrow band gap alternating copolymer of perylene diimide–dithienothiophene, which exhibit good electron mobility and afforded very good photovoltaic performances in bulk heterojunction devices in combination with a bi(thienylenevinylene)-substituted polythiophene [8a,8b]. However, if the perylene diimide units are directly connected with the oligothiophene units, the steric hindrance between them disturbs the formation of an extended conjugation. Therefore, the incorporation of suitable spacers could contribute to a more coplanar and extended -conjugated main chain. An interesting approach is the introduction of a triple bond in the copolymer structure, which will reduce the steric hindrance between the copolymer units and will promote coplanarity. In this paper we present the synthesis of two new donor–acceptor copolymers containing N,N -bis-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide moieties
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
and bithiophene or terthiophene units linked through a triple bond. High molecular weight, soluble copolymers have been easily obtained using Sonogashira coupling reaction at room temperature. This synthetic pathway may be a useful strategy for the synthesis of other highly processable polymers based on perylene diimide systems. 2. Experimental 2.1. Materials and instruments All chemicals were purchased from Aldrich and used as received without further purification unless otherwise specified. Solvents and reagents were dried and/or distilled by the usual methods and typically used under an inert gas atmosphere. 1 H NMR spectra were recorded on a 400 MHz Bruker spectrometer operating at 11.7 T. The gel permeation chromatography (GPC) measurements were performed on a Waters 2695 chromatograph equipped with a Waters 2414 refractive index detector, using two PLGel Mixed C columns connected in series, CHCl3 as eluent and polystyrene standards as calibrants. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA 7 instrument with a platinum pan using 4–5 mg of material as probe. Before performing TGA run, the sample was held at 50 ◦ C for 1 h; the runs were carried out at heating rate of 10 ◦ C/min in a nitrogen atmosphere at a flow rate of 50 ml/min. TGA and derivate thermogravimetry (DTG) curves were recorded from 50 up to 600 ◦ C. Cyclic voltammetry measurements were performed under nitrogen atmosphere with a computer controlled Amel 2053 (with Amel 7800 interface) electrochemical workstation in a three electrode single-compartment cell using platinum electrodes and SCE as standard electrode with a tetrabutylammonium tetrafluoroborate solution (0.1 M) in acetonitrile for experiments in solid state and in CH2 Cl2 for solution. Electronic absorption spectra were performed with a Perkin Elmer Lambda 9 spectrophotometer on chloroform solutions or spin coated films on quartz. Photoluminescence (PL) spectra of polymeric solutions were recorded using 450 nm light excitation from a xenon lamp and a monochromator coupled to a N2 cooled CCD detector. 2.2. Synthesis 2.2.1. 10-Nonadecanamine 10-Nonadecanamine was obtained according to the literature procedure [9]. Briefly, a solution of 500 mg (1.77 mmol) of 10nonadecanone in 5 ml of ethanol and 3 ml of pyridine was treated with 250 mg (3.6 mmol) of hydroxylamine hydrochloride and then refluxed for 2 h. The resulting mixture was then concentrated by evaporation and the residue was partitioned between 5% HCl and hexane. The organic layer was then washed several times with water, dried over MgSO4 and concentrated to yield 480 mg of 10nonadecanone oxime as an oil (91% yield). The oxime was then dissolved in 10 ml of dry toluene and 2 ml of 70% sodium bis(2-methoxyethoxy)aluminium hydride (RedAl) in toluene was added dropwise. There is an important gas evolution as the reductive agent is added which requires a particular attention when the reaction is carried out. The reaction mixture was refluxed for 2 h, then cooled to the room temperature and carefully added to 10 ml of 5% HCl. At the end an additional quantity (3–4 ml) of concentrated HCl was added in order to dissolve the aluminium salts. The solution was extracted with hexane, washed with 5% NaOH solution, water and then dried over MgSO4 . After the evaporation of the solvent 408 mg of 10-nonadecanamine (89%) was obtained. 1 H NMR (CDCl ): ı 0.81 (t, 6H), 1.20 (m, 32H), 2.14 (br, 2H), 2.68 3 (m, 1H) IR (KBr, cm−1 ): 3372, 3256, 2494, 1463, 1377, 801, 727.
997
2.2.2. N,N -bis-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide (2) N,N -bis-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide was obtained by a modified procedure from the literature [9]. The condensation reaction of 3,4,9,10-perylenetetracarboxylic dianhydride (606 mg, 1.55 mmol) and 10-nonadecanamine (1.07 g, 3.78 mmol) was accomplished by conducting the imidazation reaction in the presence of zinc acetate (214 mg, 1.16 mmol) in quinoline (5 ml) at 160 ◦ C for 4 h under nitrogen atmosphere. The resulting N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide was isolated in 78% yield as a deep red solid after a silica gel column chromatography purification (hexane:CHCl3 = 8:2 as eluent). 1 H NMR (CDCl ): ı 0.83 (t, 12H), 1.21 (m, 56H), 1.85 (m, 4H), 2.26 3 (m, 4H), 5.25 (m, 2H), 8.51 (br, 4H, pery-H), 8.59 (br, 4H, pery-H). IR (KBr, cm−1 ): 2922, 2853, 1694, 1649, 1593, 1465, 1404, 1344, 1254, 1175. 2.2.3. 1,7-Dibromo-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide (3) 1,7-Dibromo-N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide was obtained as described in literature [10]. A mixture of N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide (100 mg, 0.108 mmol), bromine (1184 mg, 7.4 mmol) in 2 ml CH2 Cl2 was stirred at 60 ◦ C in a closed vial for 4 h. The reaction mixture was allowed to reach the room temperature and then, after removing the excess of bromine by air bubbling, the solvent was removed under vacuum. The crude product was purified by a silica gel column chromatography, using hexane:CHCl3 = 6:4 as eluent. The first band was collected to afford dibromo-N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide (as a mixture of 1,7-dibromo and 1,6-dibromo derivative 3:1) as a deep orange solid (108 mg, 92%). 1 H NMR (CDCl ): ı 0.85 (t, 12H), 1.25 (m, 56H), 1.85 (m, 4H), 2.25 3 (m, 4H), 5.20 (m, 2H), 8.70 (br, 2H, pery-H), 8.91 (br, 2H, pery-H), 9.52 (d, 2H, pery-H). IR (KBr, cm−1 ): 2919, 2849, 1700, 1660, 1589, 1465, 1382, 1327, 1237. 2.2.4. 5,5 -Dibromo-2,2 -bithiophene (4a) To a solution of 2,2 -bithiophene (1.66 g, 0.01 mol) in 50 ml CHCl3 and 50 ml acetic acid, N-bromosuccinimide (NBS, 2.05 eq) was added in one portion. The reaction started instantly at room temperature. The mixture was refluxed for 4 h, then cooled to room temperature, diluted with CHCl3 , washed several times with KOH solution until basic and then with water. The organic layer was separated, dried over MgSO4 anh. and the evaporated. After recrystallization from ethanol, colorless flakes were obtained (3 g, 92%). 2.2.5. 5,5 -Dibromo-2,2 :5 ,2 -terthiophene (4b) To a solution of 2,2 :5 ,2 -terthiophene (620 mg, 2.5 mmol) in 25 ml CHCl3 and 25 ml acetic acid, N-bromosuccinimide (NBS, 2.05 eq) was added in one portion. The mixture was refluxed for 1 h, then cooled to room temperature, diluted with CHCl3 , washed several times with KOH solution until basic and then with water. The organic layer was separated, dried over MgSO4 anh. and the evaporated. After recrystallization from ethanol, light yellow flakes were obtained (900 mg, 88%). 2.2.6. 5,5 -Bis(trimethylsilyl)ethynyl-2,2 -bithiophene (5a) 5,5 -Bis(trimethylsilyl)ethynyl-2,2 -bithiophene was obtained as described in literature [11]. 500 mg of 5,5 -dibromo-2,2 bithiophene (1.5 mmol) was dissolved in 15 ml of dry THF and 7.5 ml of fresh distilled triethylamine. Then Pd[PPh3 ]4 (86 mg, 0.08 mmol), CuI (15 mg, 0.08 mmol), trimethylsilylacethylene (TMSA, 2.05 eq) were added. After stirring the reaction mixture
998
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
at room temperature for 24 h, the reaction was quenched with water and extracted with chloroform. The solvent was removed under reduced pressure and the residue was purified by column chromatography (silica gel, hexane) to afford 490 mg 5a (88%). 1 H NMR (CDCl3 ): ı 7.11 (d, 2H), 6.99 (d, 2H), 0.27 (s, 18H). 2.2.7. 5,5 -Bis(trimethylsilyl)ethynyl-2,2 :5 ,2 -terthiophene (5b) 250 mg of 5,5 -dibromo-2,2 :5 ,2 -terthiophene (0.6 mmol) was dissolved in 10 ml of dry THF and 5 ml of fresh distilled triethylamine. Then Pd[PPh3 ]4 (40 mg, 0.035 mmol), CuI (8 mg, 0.04 mmol), and trimethylsilylacethylene (TMSA, 2.05 eq) were added. After stirring the reaction mixture at room temperature for 2 h, the reaction was quenched with water and extracted with chloroform. The solvent was removed under reduced pressure and the residue was recrystallized several times from hexane to afford 210 mg of 5b as light orange crystals (77%). 1 H NMR (TCE): ı 7.01 (d, 2H), 7.07 (s, 2H), 7.14 (d, 2H), 0.26 (s, 18H). 2.2.8. 5,5 -Diethynyl-2,2 -bithiophene (6a) To a suspension of 5,5 -bis(trimethylsilyl)ethynyl-2,2 bithiophene (190 mg, 0.53 mmol) in deoxygenated MeOH (100 ml) was added saturated K2 CO3 solution (1.5 ml). The mixture was stirred for 4 h, then diluted with CH2 Cl2 , washed several times with water, dried over MgSO4 and evaporated. 90 mg of pure product was obtained as yellow crystals (78%). 1 H NMR (TCE): ı 3.47 (s, 2H), 7.03 (d, 2H), 7.18 (d, 2H). 2.2.9. 5,5 -Diethynyl-2,2 :5 ,2 -terthiophene (6b) To a suspension of 5,5 -bis(trimethylsilyl)ethynyl-2,2 :5 ,2 terthiophene (80 mg, 0.18 mmol) in deoxygenated MeOH (50 ml) was added saturated K2 CO3 solution (1 ml). The mixture was stirred for 72 h, then diluted with CH2 Cl2 , washed several times with water, dried over MgSO4 and evaporated. The product was purified by column chromatography using hexane as eluent. 48 mg of pure product was obtained as yellow crystals (89%). 1 H NMR (TCE): ı 3.48 (s, 2H), 7.03 (d, 2H), 7.09 (s, 2H) 7.19 (d, 2H).
2.2.10. General procedure for the preparation of copolymers The two copolymers were prepared by a similar procedure by coupling 1,7-dibromo-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide (3) with 5,5 -diethynyl2,2 -bithiophene (6a) or 5,5 -diethynyl-2,2 :5 ,2 -terthiophene (6b). In a 50 ml Schlenk were added 1,7-dibromo-N,N -bis(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide 3 (0.1 mmol) and 5,5 -diethynyl-2,2 -bithiophene 6a (0.1 mmol) or 5,5”-diethynyl-2,2 :5 ,2 -terthiophene 6b (0.1 mmol), anhydrous toluene (10 ml) and freshly distilled triethylamine (5 ml). The mixture was deoxygenated with N2 for 30 min. Pd[PPh3 ]4 (0.002 mmol) and CuI (0.02 mmol) were added and the reaction mixture was stirred at room temperature for 2 h. The reaction was quenched with water and extracted with chloroform. The solution was concentrated and dropped into methanol, filtered and then subjected to Soxhlet extraction with methanol and acetone. 2.2.10.1. Poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 -bithiophene-5,5 -yl)} (P1). Black solid powder, yield: 77%. 1 H NMR (TCE): ı = 10.00 (br, 2H, pery), 8.75 (br, 4H, pery), 7.45 (br, 4H, thiophene-H), 5.10 (br, 2H, –CH–N), 2.25 (br, 4H, –CH2 –), 1.80 (br, 4H, –CH2 –), 1.25 (br, 56H, –CH2 –), 0.80 (br, 12H, –CH3 ); Mw = 74,100, Mw /Mn = 2.86.
Scheme 1. Synthetic route for the preparation of perylene- and thiophene-based monomers.
2.2.10.2. Poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 :5 ,2 -terthiophene5,5 -yl)} (P2). Black solid powder, yield: 68%. 1 H NMR (TCE): ı = 10.00 (br, 2H, pery), 8.75 (br, 4H, pery), 7.45 (br, 4H, thiophene-H), 5.10 (br, 2H, –CH–N), 2.30 (br, 4H, –CH2 –), 1.80 (br, 4H, –CH2 –), 1.25 (br, 56H, –CH2 –), 0.90 (br, 12H, –CH3 ); Mw = 46,700, Mw /Mn = 2.89. 3. Results and discussion Two novel donor–acceptor copolymers containing perylene diimide and bithiophene or terthiophene linked through triple bonds have been designed and synthesized. The intermediates and the monomers were synthesized according to the literature procedure [9–11] as outlined in Scheme 1. Poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 -bithiophene-5,5 yl)} P1 and poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 :5 ,2 -terthiophene5,5 -yl)} P2 were synthesized through a Sonogashira coupling reaction in the presence of Pd[PPh3 ]4 and CuI, as shown in Scheme 2. Both copolymers are soluble in common organic solvents. The 1 H NMR spectra of P1 and P2 copolymers exhibit the chemical shifts characteristic for the perylene protons at 10.00–8.75 ppm, with a considerable shifting due to the presence of the triple bond and confirm the chemical structures of the copolymers. The molecular weights were determined by gel permeation chromatography (GPC) using polystyrene standards as calibrants. Polymer P1 has a Mw of 74,100, Mn of 26,000 corresponding to a degree of polymerization of ca. 26 units. Polymer P2 shows a Mw value of 46,700, Mn of 16,000 corresponding to a degree of polymerization of ca. 16 units. The thermal stability of the perylene-based copolymers was determined by TGA considering the onset of thermal decomposition, i.e. the temperature corresponding to initial 5% of weight loss. TGA traces reported in Fig. 1 show that the copolymers are thermally stable in inert atmosphere up to 330–400 ◦ C. At higher
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
999
Scheme 2. Synthetic route for the preparation of copolymers P1 and P2. (a) [Pd(PPh3 )4 ], CuI, toluene–Et3 N, r.t., 2 h.
Fig. 1. TGA and DTG curves of polymer P1 (full line) and P2 (dashed line) under nitrogen atmosphere.
temperatures both polymers show a fast weight loss. Derivate thermogravimetry (DTG) curves evidence that the degradation process takes place in a single step; the Tmax values, i.e. the temperature of maximum rate of weight loss, are in the range 443–449 ◦ C. The degradation process ends at around 530 ◦ C and the residue yield calculated at 600 ◦ C is ca. 50 wt.% for both polymers. However, the residues disappear or become negligible when exposed to air atmosphere. DSC runs carried out from 50 to 250 ◦ C at 20 ◦ C/min of the polymers P1 and P2 did not show any thermal event. The electrochemical properties of the thiophene derivatives 6a and 6b and the polymers P1 and P2 were determined by cyclic voltammetry measurements at room temperature and were carried out both in solution and in solid state. The oxidation potentials for 6a and 6b were 1.18 and 1.08 V, respectively. Although no oxidation wave could be detected, two clear reversible reduction processes could be found, which can be attributed to the first and second reduction of the perylene moiety. The electrochemical properties of the copolymers are depicted in Table 1. Fig. 2 shows the absorption spectra of P1, P2 and the monomer 3 in dilute chloroform solutions and the copolymers films on quartz substrate. The absorption spectrum of the monomer 3 in solution shows the absorption peaks characteristic of the –* electronic transition of perylene diimides. The three bands above 400 nm belong to the S0 –S1 transition, with the 0–0 transition band at 525 nm with a vibronic progression at 490 nm and 459 nm [12]. As compared to monomer 3, the UV–vis spectra of the copolymers
1000
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
Table 1 Electrochemical properties of P1 and P2 in the solid state,a HOMO and LUMO orbital energy levels and optical band gap. Eox a , b
P1 P2 a b
Ered1 a , b
Ered2 a , b
Ep (V)
E0 (V)
E1/2 (V)
Ep (V)
Ep (V)
– –
−0.47 −0.49
−0.55 −0.57
−0.58 −0.60
−0.72 −0.71
opt
HOMO (eV)
LUMO (eV)
Eg
−5.93 −5.79
−4.27 −4.25
1.66 1.54
(eV)
Film on Pt in ACN/Bu4 NBF4 [0.1 M] versus SCE reference electrode. opt Ep peak potentials; E0 onset potentials; E1/2 half-wave potentials; Eg optical energy gap (calculated from the absorption edge of the copolymer films).
show a significant absorption widening through the visible range. The lowest energy absorption band of the copolymers is mainly related to the perylene core and it is ascribed to the S0 –S1 transition [3a] this transition undergo to a bathocromic shift from 525 nm (3) to 670 nm (P1) and 677 nm (P2) respectively, along with a considerable band broadening and with a less pronounced vibronic structure respect to the monomer 3. The copolymers display a high energy peak, that is mainly related to the –* transition of the donor substituent. The shift of this band, passing from 420 nm for P1 to 448 nm for P2, is due to the increased -delocalisation arising from the lengthening of the thienylene segment in the donor moiety. Moreover the presence of the triple bond is also increasing the -delocalisation respect to analogous compounds where a bithiophene or a terthiophene are directly linked to the PDI moiety (these bands are respectively at 340 nm and 380 nm for bithiophene and terthiophene substitution) [8]. The absorption spectra of the copolymers films are quite similar to the spectra observed in solution. There is just a slight red shift of the absorption edges, passing from solution to the films, likely due to interchain interactions in the solid state.
From the electronic absorption onset at lower energy is possible to derive an energy gap of 1.66 eV for P1 and 1.54 eV for P2. The HOMO and LUMO energy levels of the two copolymers can be calculated from the electrochemical properties and the electronic absorption; LUMO level from the onset of the first reduction potential during cyclic voltammetry measurements and HOMO level from LUMO energies and the optical band gaps. As reported in Table 1, the copolymers show the following HOMO and LUMO energy levels: −5.93 eV and −4.27 eV for P1 and −5.79 eV and −4.25 eV for P2. Moreover, the LUMO orbital energy level of the copolymers is lower than the P3HT one, thus from the electronic point of view P1 and P2 are possible candidates as electron acceptor components in bulk heterojunction solar cells in blend with P3HT. The potentialities as photoactive acceptor materials of the two perylene-based copolymers were investigated by spectroscopical methods. It has to be noticed that charge photogeneration in the photoactive blends is based on photoinduced charge transfer between the donor and electron accepting components of the device active layer [1]. Therefore, photoluminescence measure-
Fig. 2. Electronic absorption spectra of copolymers P1, P2 and monomer 3 in chloroform solution (left figure) and in films on quartz substrate (right figure).
Fig. 3. Photoluminescence spectra (films) of P3HT (dashed line); (A) copolymer P1 (solid line) and of P1:P3HT/1:1 blend (circles); (B) copolymer P2 (solid line) and P2:P3HT/1:1 blend (squares) (the PL intensity is normalized to the number of absorbed photons).
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
ments were used to probe the occurrence of photoinduced charge transfer in P3HT/P1 or P2 blends. The photoluminescence spectra of the pristine copolymers and P3HT films and of the 1:1 blends are depicted in Fig. 3. It can be seen that upon blending the photoluminescence intensity is drastically reduced. This PL quenching suggests that photoinduced charge transfer is occurring between the two material components and that the charge transfer is fast enough to compete with the radiative recombination of the excitons, thus bringing support that the copolymers act as acceptors with respect to P3HT. 4. Conclusions We have presented the preparation of two novel high molecular weight copolymers with perylene diimide and oligothiophene units, containing a triple bond linker, which have been synthesized by Sonogashira coupling reaction. This synthetic route may be a useful strategy for the preparation of a wide family of processable perylene-based copolymers containing different electron donor groups in which will be possible to tune the electronic affinity of the materials. These copolymers show higher molecular weight compared with similar perylene-based copolymers reported in literature and are easily processable from solution. They exhibit interesting electrochemical properties and a wide electronic absorption in the visible spectral range. The HOMO and LUMO electronic levels of these copolymers, derived from the electrochemical and optical characterizations, show that they posses suitable electron affinities for photovoltaic applications, to be used as electron acceptors in bulk heterojunction solar cell with P3HT as donor material. Acknowledgments The authors want to thank Mr. Alberto Giacometti Schieroni for GPC characterization. The research was supported by the E.U. Marie Curie RTN project “SolarNtype” MRTN-CT-2006-035533,
1001
by the projects of Fondazione Cariplo “Danae”, “Nuovi Sistemi fotoluminescenti nanostrutturati” and “Fabrication of innovative metal-oxide-nanowire-based dye-sensitized solar cells”. References [1] (a) S. Günes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324–1338; (b) C.J. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftci, Organic Photovoltaics. Concepts and Realization, Springer-Verlag, Berlin, 2003; (c) G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789–1791; (d) C. Lungeschmied, G. Dennier, H. Neugebauer, N.S. Sariciftci, M. Glaathaar, T. Meyer, A. Meyer, Energy Mater. Sol. Cells 91 (2007) 379–384. [2] H. Meier, Angew. Chem. Int. Ed. 44 (2005) 2482–2506. [3] (a) S. Chen, Y. Liu, W. Qiu, X. Sun, Y. Ma, D. Zhu, Chem. Mater. 17 (2005) 2208–2215; (b) J. Huang, H. Fu, Y. Wu, S. Chen, F. Shen, X. Zhao, Y. Liu, J. Yao, J. Phys. Chem. C 112 (2008) 2689–2696. [4] (a) E.H.A. Beckers, S.C.J. Meskers, A.P.H.J. Schenning, Z.J. Chen, F. Würtner, R.A.J. Janssen, J. Phys. Chem. A 108 (2004) 6933–6937; (b) N. Wang, F. Lu, C. Huang, Y. Li, M. Yuan, X. Liu, H. Liu, L. Gan, L. Jiang, D. Zhu, J. Polym, Science Part A: Polym. Chem. 44 (2006) 5863–5874; (c) H. Langhals, S. Saulich, Chem. Eur. J. 8 (2002) 5630–5643. [5] (a) E.E. Neuteboom, S.C.J. Meskers, P.A. van Hal, J.K.J. van Duren, E.W. Meijer, R.A.J. Janssen, H. Dupin, G. Pourtois, J. Cornil, R. Lazzaroni, J. Bredas, D. Beljonne, J. Am. Chem. Soc. 125 (2003) 8625–8638; (b) H.Y. Wang, K.Y. Pu, S. Huang, F. Liu, B. Peng, W. Wei, React. Funct. Polym. 69 (2009) 117–123. [6] F. Würtner, Chem. Commun. 14 (2004) 1564–1579. [7] D. Fichou (Ed.), Handbook of Oligo- and Polythiophenes, Wiley-VCH, Weinheim, 1999. [8] (a) X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, J. Am. Chem. Soc. 129 (2007) 7246–7247; (b) J. Huang, Y. Wu, F. Hongbing, Z. Xiaowei, Y. Jiannian, S. Barlow, S.R. Marder, J. Phys. Chem. A 113 (2009) 5039–5046; (c) Z. Tan, E. Zhou, X. Zhan, X. Wang, Y. Li, S. Barlow, S.R. Marder, Appl. Phys. Lett. 93 (2008) 073309/1–073309/3; (d) L. Huo, Y. Zhou, Y. Li, Macromol. Rapid Commun. 29 (2008) 1444–1448; (e) E. Kozma, D. Kotowski, G. Grieco, R. Consonni, S. Luzzati, M. Catellani, Mat. Res. Soc. Symp. Proc. 1114 (2009), 1114-G08-15. [9] L.D. Wescott, D.L. Mattern, J. Org. Chem. 68 (2003) 10058–10066. [10] P. Rajasingh, R. Cohen, E. Shirman, L.J.W. Shimon, B. Rybtchinski, J. Org. Chem. 72 (2007) 5973–5979. [11] U. Dahlmann, R. Neidlein, Helv. Chim. Acta 79 (1996) 755–766. [12] R. Gvishi, R. Reinsfeld, Z. Burshtein, Chem. Phys. Lett. 213 (1993) 338.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis and characterization of perylene-based donor–acceptor copolymers containing triple bonds E. Kozma ∗ , F. Munno, D. Kotowski, F. Bertini, S. Luzzati, M. Catellani Istituto per lo Studio delle Macromolecole – CNR, via Bassini 15, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 9 November 2009 Received in revised form 29 January 2010 Accepted 12 February 2010 Available online 15 March 2010 Keywords: Perylene diimide copolymers Conjugated polymers Donor–acceptor copolymers Acceptor materials Photovoltaic polymers
a b s t r a c t Two donor–acceptor copolymers containing perylene diimide and oligothiophenes linked through triple bonds have been designed and synthesized by palladium catalyzed Sonogashira coupling reaction. The introduction of the triple bond spacer between the electron acceptor perylene moieties and the electron donating oligothiophene moieties produces an extended conjugation along the main chain, and produces soluble and high molecular weight copolymers.The thermal, optical and electrochemical properties of these copolymers were examined. The HOMO–LUMO energy levels of these copolymers, along with the quenching of their photoluminescence in 1:1 (w/w) blends with poly(3-hexylthiophene) (P3HT), indicate that they are suitable materials for application as electron acceptor and n-type components in bulk heterojunction solar cells with P3HT. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polymers have continuously received intense attention due to their unique optical and electrical properties and have become important materials for the fabrication of low cost and large area flexible optoelectronic devices, such as solar cells [1]. For these types of applications, the electron donating or electron accepting properties of the conjugated polymer are of crucial importance. Considerable efforts have been put into the design and synthesis of various donor–acceptor architectures, because the intramolecular charge transfer and separation of -conjugated systems with donor–acceptor properties can mimic the photochemical and photophysical processes that take place in the natural photosynthetic reactions [2]. During the last few years, several groups focused their attention on perylene diimide (PDI) dyes, which act as electron acceptors and exhibit high thermal and photochemical stabilities, strong absorption of the visible light, with relatively high electron affinity and good electron transport properties. These outstanding properties have rendered them promising building blocks for donor–acceptor type small molecules [3] or polymers, by functionalizing the perylene core along the imide positions [4] or in bay positions [5]. However, previous structure calculations have revealed that nodes exist in the imide nitrogen atoms, which are unfavourable for the efficient electronic interaction between the PDI and the unit which is linked at the nitrogen atom position [6].
∗ Corresponding author. Tel.: +39 02 23 699 739; fax: +39 02 70 636 400. E-mail address: [email protected] (E. Kozma). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.02.015
Therefore, the substitution in the bay region of the perylene core through different coupling reactions became an interesting alternative to favour the electronic communication between the donor and acceptor moieties. On the other hand, it is already well known that oligothiophenes are electron donating materials and were used as hole-transport in several types of optical and microelectronic devices. The oligothiophenes can be easily prepared and their interesting optical and electronic properties, like fluorescence, semiconductance and light emission, can be tuned by changing their chemical structure [7]. There are a few studies regarding perylene–oligothiophene copolymers, in which the donor/acceptor units are directly connected and they are characterized by quite low molecular weight [8]. Among these, a narrow band gap alternating copolymer of perylene diimide–dithienothiophene, which exhibit good electron mobility and afforded very good photovoltaic performances in bulk heterojunction devices in combination with a bi(thienylenevinylene)-substituted polythiophene [8a,8b]. However, if the perylene diimide units are directly connected with the oligothiophene units, the steric hindrance between them disturbs the formation of an extended conjugation. Therefore, the incorporation of suitable spacers could contribute to a more coplanar and extended -conjugated main chain. An interesting approach is the introduction of a triple bond in the copolymer structure, which will reduce the steric hindrance between the copolymer units and will promote coplanarity. In this paper we present the synthesis of two new donor–acceptor copolymers containing N,N -bis-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide moieties
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
and bithiophene or terthiophene units linked through a triple bond. High molecular weight, soluble copolymers have been easily obtained using Sonogashira coupling reaction at room temperature. This synthetic pathway may be a useful strategy for the synthesis of other highly processable polymers based on perylene diimide systems. 2. Experimental 2.1. Materials and instruments All chemicals were purchased from Aldrich and used as received without further purification unless otherwise specified. Solvents and reagents were dried and/or distilled by the usual methods and typically used under an inert gas atmosphere. 1 H NMR spectra were recorded on a 400 MHz Bruker spectrometer operating at 11.7 T. The gel permeation chromatography (GPC) measurements were performed on a Waters 2695 chromatograph equipped with a Waters 2414 refractive index detector, using two PLGel Mixed C columns connected in series, CHCl3 as eluent and polystyrene standards as calibrants. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA 7 instrument with a platinum pan using 4–5 mg of material as probe. Before performing TGA run, the sample was held at 50 ◦ C for 1 h; the runs were carried out at heating rate of 10 ◦ C/min in a nitrogen atmosphere at a flow rate of 50 ml/min. TGA and derivate thermogravimetry (DTG) curves were recorded from 50 up to 600 ◦ C. Cyclic voltammetry measurements were performed under nitrogen atmosphere with a computer controlled Amel 2053 (with Amel 7800 interface) electrochemical workstation in a three electrode single-compartment cell using platinum electrodes and SCE as standard electrode with a tetrabutylammonium tetrafluoroborate solution (0.1 M) in acetonitrile for experiments in solid state and in CH2 Cl2 for solution. Electronic absorption spectra were performed with a Perkin Elmer Lambda 9 spectrophotometer on chloroform solutions or spin coated films on quartz. Photoluminescence (PL) spectra of polymeric solutions were recorded using 450 nm light excitation from a xenon lamp and a monochromator coupled to a N2 cooled CCD detector. 2.2. Synthesis 2.2.1. 10-Nonadecanamine 10-Nonadecanamine was obtained according to the literature procedure [9]. Briefly, a solution of 500 mg (1.77 mmol) of 10nonadecanone in 5 ml of ethanol and 3 ml of pyridine was treated with 250 mg (3.6 mmol) of hydroxylamine hydrochloride and then refluxed for 2 h. The resulting mixture was then concentrated by evaporation and the residue was partitioned between 5% HCl and hexane. The organic layer was then washed several times with water, dried over MgSO4 and concentrated to yield 480 mg of 10nonadecanone oxime as an oil (91% yield). The oxime was then dissolved in 10 ml of dry toluene and 2 ml of 70% sodium bis(2-methoxyethoxy)aluminium hydride (RedAl) in toluene was added dropwise. There is an important gas evolution as the reductive agent is added which requires a particular attention when the reaction is carried out. The reaction mixture was refluxed for 2 h, then cooled to the room temperature and carefully added to 10 ml of 5% HCl. At the end an additional quantity (3–4 ml) of concentrated HCl was added in order to dissolve the aluminium salts. The solution was extracted with hexane, washed with 5% NaOH solution, water and then dried over MgSO4 . After the evaporation of the solvent 408 mg of 10-nonadecanamine (89%) was obtained. 1 H NMR (CDCl ): ı 0.81 (t, 6H), 1.20 (m, 32H), 2.14 (br, 2H), 2.68 3 (m, 1H) IR (KBr, cm−1 ): 3372, 3256, 2494, 1463, 1377, 801, 727.
997
2.2.2. N,N -bis-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide (2) N,N -bis-(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide was obtained by a modified procedure from the literature [9]. The condensation reaction of 3,4,9,10-perylenetetracarboxylic dianhydride (606 mg, 1.55 mmol) and 10-nonadecanamine (1.07 g, 3.78 mmol) was accomplished by conducting the imidazation reaction in the presence of zinc acetate (214 mg, 1.16 mmol) in quinoline (5 ml) at 160 ◦ C for 4 h under nitrogen atmosphere. The resulting N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide was isolated in 78% yield as a deep red solid after a silica gel column chromatography purification (hexane:CHCl3 = 8:2 as eluent). 1 H NMR (CDCl ): ı 0.83 (t, 12H), 1.21 (m, 56H), 1.85 (m, 4H), 2.26 3 (m, 4H), 5.25 (m, 2H), 8.51 (br, 4H, pery-H), 8.59 (br, 4H, pery-H). IR (KBr, cm−1 ): 2922, 2853, 1694, 1649, 1593, 1465, 1404, 1344, 1254, 1175. 2.2.3. 1,7-Dibromo-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide (3) 1,7-Dibromo-N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide was obtained as described in literature [10]. A mixture of N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide (100 mg, 0.108 mmol), bromine (1184 mg, 7.4 mmol) in 2 ml CH2 Cl2 was stirred at 60 ◦ C in a closed vial for 4 h. The reaction mixture was allowed to reach the room temperature and then, after removing the excess of bromine by air bubbling, the solvent was removed under vacuum. The crude product was purified by a silica gel column chromatography, using hexane:CHCl3 = 6:4 as eluent. The first band was collected to afford dibromo-N,N -bis-(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide (as a mixture of 1,7-dibromo and 1,6-dibromo derivative 3:1) as a deep orange solid (108 mg, 92%). 1 H NMR (CDCl ): ı 0.85 (t, 12H), 1.25 (m, 56H), 1.85 (m, 4H), 2.25 3 (m, 4H), 5.20 (m, 2H), 8.70 (br, 2H, pery-H), 8.91 (br, 2H, pery-H), 9.52 (d, 2H, pery-H). IR (KBr, cm−1 ): 2919, 2849, 1700, 1660, 1589, 1465, 1382, 1327, 1237. 2.2.4. 5,5 -Dibromo-2,2 -bithiophene (4a) To a solution of 2,2 -bithiophene (1.66 g, 0.01 mol) in 50 ml CHCl3 and 50 ml acetic acid, N-bromosuccinimide (NBS, 2.05 eq) was added in one portion. The reaction started instantly at room temperature. The mixture was refluxed for 4 h, then cooled to room temperature, diluted with CHCl3 , washed several times with KOH solution until basic and then with water. The organic layer was separated, dried over MgSO4 anh. and the evaporated. After recrystallization from ethanol, colorless flakes were obtained (3 g, 92%). 2.2.5. 5,5 -Dibromo-2,2 :5 ,2 -terthiophene (4b) To a solution of 2,2 :5 ,2 -terthiophene (620 mg, 2.5 mmol) in 25 ml CHCl3 and 25 ml acetic acid, N-bromosuccinimide (NBS, 2.05 eq) was added in one portion. The mixture was refluxed for 1 h, then cooled to room temperature, diluted with CHCl3 , washed several times with KOH solution until basic and then with water. The organic layer was separated, dried over MgSO4 anh. and the evaporated. After recrystallization from ethanol, light yellow flakes were obtained (900 mg, 88%). 2.2.6. 5,5 -Bis(trimethylsilyl)ethynyl-2,2 -bithiophene (5a) 5,5 -Bis(trimethylsilyl)ethynyl-2,2 -bithiophene was obtained as described in literature [11]. 500 mg of 5,5 -dibromo-2,2 bithiophene (1.5 mmol) was dissolved in 15 ml of dry THF and 7.5 ml of fresh distilled triethylamine. Then Pd[PPh3 ]4 (86 mg, 0.08 mmol), CuI (15 mg, 0.08 mmol), trimethylsilylacethylene (TMSA, 2.05 eq) were added. After stirring the reaction mixture
998
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
at room temperature for 24 h, the reaction was quenched with water and extracted with chloroform. The solvent was removed under reduced pressure and the residue was purified by column chromatography (silica gel, hexane) to afford 490 mg 5a (88%). 1 H NMR (CDCl3 ): ı 7.11 (d, 2H), 6.99 (d, 2H), 0.27 (s, 18H). 2.2.7. 5,5 -Bis(trimethylsilyl)ethynyl-2,2 :5 ,2 -terthiophene (5b) 250 mg of 5,5 -dibromo-2,2 :5 ,2 -terthiophene (0.6 mmol) was dissolved in 10 ml of dry THF and 5 ml of fresh distilled triethylamine. Then Pd[PPh3 ]4 (40 mg, 0.035 mmol), CuI (8 mg, 0.04 mmol), and trimethylsilylacethylene (TMSA, 2.05 eq) were added. After stirring the reaction mixture at room temperature for 2 h, the reaction was quenched with water and extracted with chloroform. The solvent was removed under reduced pressure and the residue was recrystallized several times from hexane to afford 210 mg of 5b as light orange crystals (77%). 1 H NMR (TCE): ı 7.01 (d, 2H), 7.07 (s, 2H), 7.14 (d, 2H), 0.26 (s, 18H). 2.2.8. 5,5 -Diethynyl-2,2 -bithiophene (6a) To a suspension of 5,5 -bis(trimethylsilyl)ethynyl-2,2 bithiophene (190 mg, 0.53 mmol) in deoxygenated MeOH (100 ml) was added saturated K2 CO3 solution (1.5 ml). The mixture was stirred for 4 h, then diluted with CH2 Cl2 , washed several times with water, dried over MgSO4 and evaporated. 90 mg of pure product was obtained as yellow crystals (78%). 1 H NMR (TCE): ı 3.47 (s, 2H), 7.03 (d, 2H), 7.18 (d, 2H). 2.2.9. 5,5 -Diethynyl-2,2 :5 ,2 -terthiophene (6b) To a suspension of 5,5 -bis(trimethylsilyl)ethynyl-2,2 :5 ,2 terthiophene (80 mg, 0.18 mmol) in deoxygenated MeOH (50 ml) was added saturated K2 CO3 solution (1 ml). The mixture was stirred for 72 h, then diluted with CH2 Cl2 , washed several times with water, dried over MgSO4 and evaporated. The product was purified by column chromatography using hexane as eluent. 48 mg of pure product was obtained as yellow crystals (89%). 1 H NMR (TCE): ı 3.48 (s, 2H), 7.03 (d, 2H), 7.09 (s, 2H) 7.19 (d, 2H).
2.2.10. General procedure for the preparation of copolymers The two copolymers were prepared by a similar procedure by coupling 1,7-dibromo-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide (3) with 5,5 -diethynyl2,2 -bithiophene (6a) or 5,5 -diethynyl-2,2 :5 ,2 -terthiophene (6b). In a 50 ml Schlenk were added 1,7-dibromo-N,N -bis(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic acid diimide 3 (0.1 mmol) and 5,5 -diethynyl-2,2 -bithiophene 6a (0.1 mmol) or 5,5”-diethynyl-2,2 :5 ,2 -terthiophene 6b (0.1 mmol), anhydrous toluene (10 ml) and freshly distilled triethylamine (5 ml). The mixture was deoxygenated with N2 for 30 min. Pd[PPh3 ]4 (0.002 mmol) and CuI (0.02 mmol) were added and the reaction mixture was stirred at room temperature for 2 h. The reaction was quenched with water and extracted with chloroform. The solution was concentrated and dropped into methanol, filtered and then subjected to Soxhlet extraction with methanol and acetone. 2.2.10.1. Poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 -bithiophene-5,5 -yl)} (P1). Black solid powder, yield: 77%. 1 H NMR (TCE): ı = 10.00 (br, 2H, pery), 8.75 (br, 4H, pery), 7.45 (br, 4H, thiophene-H), 5.10 (br, 2H, –CH–N), 2.25 (br, 4H, –CH2 –), 1.80 (br, 4H, –CH2 –), 1.25 (br, 56H, –CH2 –), 0.80 (br, 12H, –CH3 ); Mw = 74,100, Mw /Mn = 2.86.
Scheme 1. Synthetic route for the preparation of perylene- and thiophene-based monomers.
2.2.10.2. Poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 :5 ,2 -terthiophene5,5 -yl)} (P2). Black solid powder, yield: 68%. 1 H NMR (TCE): ı = 10.00 (br, 2H, pery), 8.75 (br, 4H, pery), 7.45 (br, 4H, thiophene-H), 5.10 (br, 2H, –CH–N), 2.30 (br, 4H, –CH2 –), 1.80 (br, 4H, –CH2 –), 1.25 (br, 56H, –CH2 –), 0.90 (br, 12H, –CH3 ); Mw = 46,700, Mw /Mn = 2.89. 3. Results and discussion Two novel donor–acceptor copolymers containing perylene diimide and bithiophene or terthiophene linked through triple bonds have been designed and synthesized. The intermediates and the monomers were synthesized according to the literature procedure [9–11] as outlined in Scheme 1. Poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 -bithiophene-5,5 yl)} P1 and poly{[1,7-diethynyl-N,N -bis-(10-nonadecyl)perylene3,4,9,10-tetracarboxylic acid diimide]-alt-(2,2 :5 ,2 -terthiophene5,5 -yl)} P2 were synthesized through a Sonogashira coupling reaction in the presence of Pd[PPh3 ]4 and CuI, as shown in Scheme 2. Both copolymers are soluble in common organic solvents. The 1 H NMR spectra of P1 and P2 copolymers exhibit the chemical shifts characteristic for the perylene protons at 10.00–8.75 ppm, with a considerable shifting due to the presence of the triple bond and confirm the chemical structures of the copolymers. The molecular weights were determined by gel permeation chromatography (GPC) using polystyrene standards as calibrants. Polymer P1 has a Mw of 74,100, Mn of 26,000 corresponding to a degree of polymerization of ca. 26 units. Polymer P2 shows a Mw value of 46,700, Mn of 16,000 corresponding to a degree of polymerization of ca. 16 units. The thermal stability of the perylene-based copolymers was determined by TGA considering the onset of thermal decomposition, i.e. the temperature corresponding to initial 5% of weight loss. TGA traces reported in Fig. 1 show that the copolymers are thermally stable in inert atmosphere up to 330–400 ◦ C. At higher
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
999
Scheme 2. Synthetic route for the preparation of copolymers P1 and P2. (a) [Pd(PPh3 )4 ], CuI, toluene–Et3 N, r.t., 2 h.
Fig. 1. TGA and DTG curves of polymer P1 (full line) and P2 (dashed line) under nitrogen atmosphere.
temperatures both polymers show a fast weight loss. Derivate thermogravimetry (DTG) curves evidence that the degradation process takes place in a single step; the Tmax values, i.e. the temperature of maximum rate of weight loss, are in the range 443–449 ◦ C. The degradation process ends at around 530 ◦ C and the residue yield calculated at 600 ◦ C is ca. 50 wt.% for both polymers. However, the residues disappear or become negligible when exposed to air atmosphere. DSC runs carried out from 50 to 250 ◦ C at 20 ◦ C/min of the polymers P1 and P2 did not show any thermal event. The electrochemical properties of the thiophene derivatives 6a and 6b and the polymers P1 and P2 were determined by cyclic voltammetry measurements at room temperature and were carried out both in solution and in solid state. The oxidation potentials for 6a and 6b were 1.18 and 1.08 V, respectively. Although no oxidation wave could be detected, two clear reversible reduction processes could be found, which can be attributed to the first and second reduction of the perylene moiety. The electrochemical properties of the copolymers are depicted in Table 1. Fig. 2 shows the absorption spectra of P1, P2 and the monomer 3 in dilute chloroform solutions and the copolymers films on quartz substrate. The absorption spectrum of the monomer 3 in solution shows the absorption peaks characteristic of the –* electronic transition of perylene diimides. The three bands above 400 nm belong to the S0 –S1 transition, with the 0–0 transition band at 525 nm with a vibronic progression at 490 nm and 459 nm [12]. As compared to monomer 3, the UV–vis spectra of the copolymers
1000
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
Table 1 Electrochemical properties of P1 and P2 in the solid state,a HOMO and LUMO orbital energy levels and optical band gap. Eox a , b
P1 P2 a b
Ered1 a , b
Ered2 a , b
Ep (V)
E0 (V)
E1/2 (V)
Ep (V)
Ep (V)
– –
−0.47 −0.49
−0.55 −0.57
−0.58 −0.60
−0.72 −0.71
opt
HOMO (eV)
LUMO (eV)
Eg
−5.93 −5.79
−4.27 −4.25
1.66 1.54
(eV)
Film on Pt in ACN/Bu4 NBF4 [0.1 M] versus SCE reference electrode. opt Ep peak potentials; E0 onset potentials; E1/2 half-wave potentials; Eg optical energy gap (calculated from the absorption edge of the copolymer films).
show a significant absorption widening through the visible range. The lowest energy absorption band of the copolymers is mainly related to the perylene core and it is ascribed to the S0 –S1 transition [3a] this transition undergo to a bathocromic shift from 525 nm (3) to 670 nm (P1) and 677 nm (P2) respectively, along with a considerable band broadening and with a less pronounced vibronic structure respect to the monomer 3. The copolymers display a high energy peak, that is mainly related to the –* transition of the donor substituent. The shift of this band, passing from 420 nm for P1 to 448 nm for P2, is due to the increased -delocalisation arising from the lengthening of the thienylene segment in the donor moiety. Moreover the presence of the triple bond is also increasing the -delocalisation respect to analogous compounds where a bithiophene or a terthiophene are directly linked to the PDI moiety (these bands are respectively at 340 nm and 380 nm for bithiophene and terthiophene substitution) [8]. The absorption spectra of the copolymers films are quite similar to the spectra observed in solution. There is just a slight red shift of the absorption edges, passing from solution to the films, likely due to interchain interactions in the solid state.
From the electronic absorption onset at lower energy is possible to derive an energy gap of 1.66 eV for P1 and 1.54 eV for P2. The HOMO and LUMO energy levels of the two copolymers can be calculated from the electrochemical properties and the electronic absorption; LUMO level from the onset of the first reduction potential during cyclic voltammetry measurements and HOMO level from LUMO energies and the optical band gaps. As reported in Table 1, the copolymers show the following HOMO and LUMO energy levels: −5.93 eV and −4.27 eV for P1 and −5.79 eV and −4.25 eV for P2. Moreover, the LUMO orbital energy level of the copolymers is lower than the P3HT one, thus from the electronic point of view P1 and P2 are possible candidates as electron acceptor components in bulk heterojunction solar cells in blend with P3HT. The potentialities as photoactive acceptor materials of the two perylene-based copolymers were investigated by spectroscopical methods. It has to be noticed that charge photogeneration in the photoactive blends is based on photoinduced charge transfer between the donor and electron accepting components of the device active layer [1]. Therefore, photoluminescence measure-
Fig. 2. Electronic absorption spectra of copolymers P1, P2 and monomer 3 in chloroform solution (left figure) and in films on quartz substrate (right figure).
Fig. 3. Photoluminescence spectra (films) of P3HT (dashed line); (A) copolymer P1 (solid line) and of P1:P3HT/1:1 blend (circles); (B) copolymer P2 (solid line) and P2:P3HT/1:1 blend (squares) (the PL intensity is normalized to the number of absorbed photons).
E. Kozma et al. / Synthetic Metals 160 (2010) 996–1001
ments were used to probe the occurrence of photoinduced charge transfer in P3HT/P1 or P2 blends. The photoluminescence spectra of the pristine copolymers and P3HT films and of the 1:1 blends are depicted in Fig. 3. It can be seen that upon blending the photoluminescence intensity is drastically reduced. This PL quenching suggests that photoinduced charge transfer is occurring between the two material components and that the charge transfer is fast enough to compete with the radiative recombination of the excitons, thus bringing support that the copolymers act as acceptors with respect to P3HT. 4. Conclusions We have presented the preparation of two novel high molecular weight copolymers with perylene diimide and oligothiophene units, containing a triple bond linker, which have been synthesized by Sonogashira coupling reaction. This synthetic route may be a useful strategy for the preparation of a wide family of processable perylene-based copolymers containing different electron donor groups in which will be possible to tune the electronic affinity of the materials. These copolymers show higher molecular weight compared with similar perylene-based copolymers reported in literature and are easily processable from solution. They exhibit interesting electrochemical properties and a wide electronic absorption in the visible spectral range. The HOMO and LUMO electronic levels of these copolymers, derived from the electrochemical and optical characterizations, show that they posses suitable electron affinities for photovoltaic applications, to be used as electron acceptors in bulk heterojunction solar cell with P3HT as donor material. Acknowledgments The authors want to thank Mr. Alberto Giacometti Schieroni for GPC characterization. The research was supported by the E.U. Marie Curie RTN project “SolarNtype” MRTN-CT-2006-035533,
1001
by the projects of Fondazione Cariplo “Danae”, “Nuovi Sistemi fotoluminescenti nanostrutturati” and “Fabrication of innovative metal-oxide-nanowire-based dye-sensitized solar cells”. References [1] (a) S. Günes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324–1338; (b) C.J. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftci, Organic Photovoltaics. Concepts and Realization, Springer-Verlag, Berlin, 2003; (c) G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789–1791; (d) C. Lungeschmied, G. Dennier, H. Neugebauer, N.S. Sariciftci, M. Glaathaar, T. Meyer, A. Meyer, Energy Mater. Sol. Cells 91 (2007) 379–384. [2] H. Meier, Angew. Chem. Int. Ed. 44 (2005) 2482–2506. [3] (a) S. Chen, Y. Liu, W. Qiu, X. Sun, Y. Ma, D. Zhu, Chem. Mater. 17 (2005) 2208–2215; (b) J. Huang, H. Fu, Y. Wu, S. Chen, F. Shen, X. Zhao, Y. Liu, J. Yao, J. Phys. Chem. C 112 (2008) 2689–2696. [4] (a) E.H.A. Beckers, S.C.J. Meskers, A.P.H.J. Schenning, Z.J. Chen, F. Würtner, R.A.J. Janssen, J. Phys. Chem. A 108 (2004) 6933–6937; (b) N. Wang, F. Lu, C. Huang, Y. Li, M. Yuan, X. Liu, H. Liu, L. Gan, L. Jiang, D. Zhu, J. Polym, Science Part A: Polym. Chem. 44 (2006) 5863–5874; (c) H. Langhals, S. Saulich, Chem. Eur. J. 8 (2002) 5630–5643. [5] (a) E.E. Neuteboom, S.C.J. Meskers, P.A. van Hal, J.K.J. van Duren, E.W. Meijer, R.A.J. Janssen, H. Dupin, G. Pourtois, J. Cornil, R. Lazzaroni, J. Bredas, D. Beljonne, J. Am. Chem. Soc. 125 (2003) 8625–8638; (b) H.Y. Wang, K.Y. Pu, S. Huang, F. Liu, B. Peng, W. Wei, React. Funct. Polym. 69 (2009) 117–123. [6] F. Würtner, Chem. Commun. 14 (2004) 1564–1579. [7] D. Fichou (Ed.), Handbook of Oligo- and Polythiophenes, Wiley-VCH, Weinheim, 1999. [8] (a) X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, J. Am. Chem. Soc. 129 (2007) 7246–7247; (b) J. Huang, Y. Wu, F. Hongbing, Z. Xiaowei, Y. Jiannian, S. Barlow, S.R. Marder, J. Phys. Chem. A 113 (2009) 5039–5046; (c) Z. Tan, E. Zhou, X. Zhan, X. Wang, Y. Li, S. Barlow, S.R. Marder, Appl. Phys. Lett. 93 (2008) 073309/1–073309/3; (d) L. Huo, Y. Zhou, Y. Li, Macromol. Rapid Commun. 29 (2008) 1444–1448; (e) E. Kozma, D. Kotowski, G. Grieco, R. Consonni, S. Luzzati, M. Catellani, Mat. Res. Soc. Symp. Proc. 1114 (2009), 1114-G08-15. [9] L.D. Wescott, D.L. Mattern, J. Org. Chem. 68 (2003) 10058–10066. [10] P. Rajasingh, R. Cohen, E. Shirman, L.J.W. Shimon, B. Rybtchinski, J. Org. Chem. 72 (2007) 5973–5979. [11] U. Dahlmann, R. Neidlein, Helv. Chim. Acta 79 (1996) 755–766. [12] R. Gvishi, R. Reinsfeld, Z. Burshtein, Chem. Phys. Lett. 213 (1993) 338.