- Email: [email protected]
Synthesis and optical properties of organosilicon–oligothiophene branched polymers
Journal of Organometallic Chemistry 736 (2013) 50e54
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and optical properties of organosiliconeoligothiophene branched polymers Joji Ohshita a, *, Yuta Tominaga a, Daiki Tanaka a, Tomonobu Mizumo a, Yuki Fujita b, Yoshihito Kunugi b a b
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Department of Applied Chemistry, Faculty of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2012 Received in revised form 25 February 2013 Accepted 5 March 2013
Star-shaped monomers bearing three bromobithiophenyl units linked by an organosilicon core were polymerized with 1:1 mixtures of (tributylstannyl)hexylterthiophene and bis[(tributylstannyl)thienyl] arenes under the Stille coupling conditions leading to the formation of the corresponding branched polymers bearing quinquethiophenyl side groups and an Si-linked bis(bithiophenylene)arylene backbone as yellowedark brown and red solids that are soluble in organic solvents, such as chloroform and THF. Optical properties of the resulting polymers were investigated with respect to the UVevis absorption and photoluminescence spectra, suggesting the efficient energy transfer between the p-conjugated systems at the photo-excited states. Semi-conducting properties of the polymer films were also investigated. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Organosilicon polymer Branched polymer Oligothiophene Photoluminescence Light-harvesting material
1. Introduction Polymers having alternate organosilicon and p-conjugated units are of interest because of their functionalities [1]. High carrier transporting properties are often found for those polymers, leading to the potential use of the polymer films as hole-transporting materials for organic light emitting diodes (OLEDs) and organic thin film transistors (OTFTs) [2,3]. In addition, it is known that the silicon-substitution on the p-conjugated systems often enhances the photoluminescence (PL) efficiency [4] and several Si-p alternate polymers with excellent PL and electroluminescence (EL) properties have been reported to date [3d,5,6]. For some of the polymers with different p-systems linked by organosilicon units, smooth intramolecular photo-induced energy and electron transfer between the p-systems have been also demonstrated, making it possible to utilize these polymers as light-harvesting materials [6]. In this connection, we have been interested in Si-oligothiophene alternate polymers [3] and reported the synthesis of poly(mono-, di-, and trisilanyleneoligothienylene)s (Chart 1(a), x ¼ 1e3 and m ¼ 3e5) and the utilities of their spin-coated films as the holetransport layers of multi-layered OLEDs [3aec]. More recently, we prepared similar polymers with longer oligothiophene units (x ¼ 1 and m ¼ 8e14) that were potentially useful as the active materials * Corresponding author. Tel.: þ81 82 424 7743; fax: þ81 82 424 5494. E-mail address: [email protected] (J. Ohshita). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.03.007
for OTFTs, although the mobility of the films was not very high (w105 cm2/Vs) [3d]. In an effort to obtain better understanding about the TFT activity of the Si-linked oligothiophene derivatives, we prepared a series of compounds with two or three oligothiophene units linked by an organosilicon linkage (Chart 1(b) and (c), m ¼ 3e5) [3e,f]. The TFT activity of the vapor-deposited films of the Si-linked oligothiophenes was highly dependent on the oligothiophene chain lengths and their elongation markedly improved the TFT activity. Molecular shapes also affected the activity and the starshaped compounds (Chart 1(c)) always showed higher TFT activity than the linear ones (Chart 1(b)). The highest hole mobility of 6.2 102 cm2/Vs among those Si-linked oligothiophenes was obtained for the star-shaped quinquethiophene derivative (m ¼ 5, ER ¼ SiMe) [3f]. Influence of the nature of the center element was also studied for the star-shaped compounds and it was found that the silicon-centered compound (ER ¼ SiMe) exhibited higher mobility than the carbon-centered analog (ER ¼ CH) for m ¼ 3, while elongation of the oligothiophene chain length changed the tendency and a little higher but almost comparable mobility was observed for the carbon-centered compound with m ¼ 4 as compared with the silicon-centered analog [3g]. The holetransporting properties of those Si-linked oligothiophene oligomers and polymers originate from the stabilization of the cation charge in the oxidized states. Indeed, the DFT calculations on radical cations of the model molecules indicated the delocalization of the cation charge over the Si-linked oligothiophene systems [3g].
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
51
Chart 1. Structures of Si-oligothiophene polymers (a), and linear (b) and star-shaped Si-oligothiophene compounds (c).
In this paper, we report the synthesis, and optical and semiconducting properties of branched polymers bearing Si-linked oligothiophene units. The semi-conducting properties were not as high as that we had expected. However, efficient energy transfer between the oligothiophene units at the photo-excited states was observed for these polymers, suggesting their potential as lightharvesting materials. Although monosilane-linked branched oligothiophene polymers have been reported previously [7], this is the first example of those with flexible organosilicon linkages. 2. Experimental 2.1. General All reactions were carried out in dry argon. Tetrahydrofuran (THF) and ether, and toluene used as the reaction solvents were distilled from sodium/benzophenone and calcium hydride, respectively and stored over activated molecular sieves until use. Starting monomers tris[(bromobithiophenyl)dimethylsilyl]methylsilane [3f] and -methane [3g] (E(2TBr)3, ER ¼ SiMe or CH in Scheme 1), bisstannylarenes (ArSn2 in Scheme 1), hexyl(tributylstannyl)terthiophene (3TSn1) [8], and bis(bromobutylthienyl)benzothiadiazole [9] were prepared as reported in the literature. Bis(tributylstannyl) butylthiophene (1TSn2), bis(tributylstannyl)dibutylbithiophene (2TSn2), bis(tributylstannyl)dibutylterthiophene (3TSn2), bis(tributylstannyl)dibutylquarterthiophene (4TSn2), and bis[(tributylstannyl)thienyl]benzothiadiazole) were prepared by dilithiation of the corresponding dibromooligothiophenes by the reactions with two equivalents of n-butyllithium, followed by treatment of the resulting dilithiooligothiophenes with tri(n-butyl)tin chloride, in a similar fashion to that of hexyl(tributylstannyl)terthiophene [8]. NMR spectra were recorded on Varian MR400 and System500 spectrometers. UV absorption and PL spectra were measured on Shimadzu UV3150 and HITACHI F4500 spectrophotometers, respectively. PL quantum yields were determined in an integration sphere attached to
Scheme 1. Synthesis of branched Si-linked oligothiophene polymers.
a Hamamatsu Photonics C7473 multi-channel analyzer. Polymer molecular weights were measured by gel permeation chromatography (GPC) using Shodex KF804 and KF806 columns connected in series eluted with THF. The polymers were detected by a UV detector at 240 nm and the molecular weights were calculated relative to the polystyrene standards on a SIC-480 data station. 2.2. Polymer synthesis In an autoclave was placed a mixture of 0.285 g (0.300 mmol) of Si(2TBr)3, 0.186 g (0.300 mmol) of 3TSn1, 0.216 g (0.300 mmol) of 1TSn2, 7 mg (0.008 mmol) of Pd2(dba)3, 18 mg (0.060 mmol) of (oTol)3P, 24 mg (0.30 mmol) of CuO, 20 mL of toluene and the mixture was heated at 150 C for 48 h. The resulting precipitates were filtered and the solvent was evaporated. After hydrolysis with water, the organic layer was separated and the aqueous layer was extracted with chloroform. The organic layer and the extracts were combined and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was reprecipitated from chloroform/hexane to give 230 mg (65% yield) of pSi5T as orange solids: m.p. 95e100 C; 1H NMR (d in CDCl3) 0.41 (br s, 21H, SiCH3), 0.95 (br s, 6H, CH3), 1.29e1.41 (br m, 8H, CH2), 1.58e1.70 (br m, 4H, CH2), 2.72e2.80 (br m, 4H, CH2), 6.67e7.27 (br m, 19H, aromatic protons); 13C NMR (d in CDCl3) 11.79, 0.36, 13.64e14.14, 17.53, 22.61e22.73, 28.80, 29.30, 30.22, 31.59, 32.51, 123.59e124.87, 134.97, 135.73, 142.54. Other polymers were prepared in a fashion similar to that above. Data for pC5T: orange solid; 1H NMR (d in CDCl3) 0.43 (br s, 19H, SiCH3 and CH), 0.90 (br s, 6H, CH3), 1.31e1.39 (br m, 8H, CH2), 1.56e 1.68 (br m, 4H, CH2), 2.78 (br s, 4H, CH2), 6.65e7.26 (br m, 19H, aromatic protons); 13C NMR (d in CDCl3) 2.71, 14.05e14.14, 22.61e 22.73, 28.80, 29.27, 30.23, 31.60, 32.54, 123.40e124.85, 134.43e 135.77. Data for pSi6T: orange solid; m.p. 98e103 C; 1H NMR (d in CDCl3) 0.39 (br s, 21H, SiCH3), 0.88 (br s, 9H, CH3), 1.31e1.43 (br m, 10H, CH2), 1.57e1.67 (br m, 6H, CH2), 2.51 (br s, 4H, CH2), 2.77 (br s, 2H, CH2), 6.64e7.23 (br m, 20H, aromatic protons); 13C NMR (d in CDCl3) 11.83, 0.45, 13.98e14.13, 22.54e22.61, 28.77, 30.22, 31.58, 32.80, 123.39e123.59, 124.22e124.87, 125.14, 127.31, 135.00, 135.99, 136.76, 142.41, 143.37. Data for pC6T: yellow solid; m.p. 120e125 C; 1 H NMR (d in CDCl3) 0.39 (br s, 19H, SiCH3 and CH), 0.87 (br s, 9H, CH3), 1.30 (br s, 10H, CH2), 1.56e1.65 (br m, 6H, CH2), 2.51 (br s, 4H, CH2), 2.76 (br s, 2H, CH2), 6.65e7.11 (br m, 20H, aromatic protons); 13 C NMR (d in CDCl3) 2.68, 13.99e14.14, 22.55e22.61, 28.77, 30.22, 31.58, 32.81, 123.41e123.58, 124.19e124.87, 125.13, 135.02, 135.77e 136.77, 140.96, 143.37. Data for pSi7T: red solid; 1H NMR (d in CDCl3) 0.41 (br s, 21H, SiCH3), 0.88e0.96 (br m, 9H, CH3), 1.30e1.43 (br m, 10H, CH2), 1.57e1.68 (br m, 6H, CH2), 2.77 (br s, 6H, CH2), 6.70e7.26 (br m, 22H, aromatic protons); 13C NMR (d in CDCl3) 11.83, 0.36, 14.03e14.12, 22.61e22.77, 28.77, 29.37, 30.22, 31.58, 32.62, 123.40e 124.87, 126.50e126.67, 134.93, 135.74. Data for pC7T: red solid; m.p. 100e106 C; 1H NMR (d in CDCl3) 0.43 (br s, 19H, SiCH3 and CH), 0.88e0.96 (br m, 9H, CH3), 1.30e1.50 (br m, 10H, CH2), 1.55e1.68 (br m, 6H, CH2), 2.76 (br s, 6H, CH2), 6.65e7.25 (br m, 22H, aromatic
52
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
2.3. OTFT fabrication and evaluation
Table 1 Synthesis and properties of branched Si-oligothiophene polymers. Polymer
pSi5T pSi6T pSi7T pSi8T pSi9T pSi4TBTA pC5T pC6T pC7T pC8T pC9T pC4TBTA a b
Yield/%
65 40 58 20 51 47 63 64 28 11 32 54
Mwa
9800 32 000 6200 9500 11 000 9100 14 000 51 000 12 000 9400 9600 8900
Mw/Mna
1.97 2.47 1.51 1.37 1.57 1.34 2.33 2.87 1.90 1.30 1.39 2.29
UV lmax/nm
PL lem/nm (F)b
In THF
In THF
As solid
421 410 428 435 435 415 416 406 424 433 431 415
533 528 546 556 559 502 526 526 547 555 560 500
583 576 608 611 628 696 575 570 612 616 621 676
(0.22) (0.26) (0.30) (0.28) (0.27) (0.20) (0.16) (0.23) (0.25) (0.23) (0.25) (0.20)
(0.03) (0.02) (0.03) (0.01) (0.01) (0.01) (0.03) (0.02) (0.02) (0.01) (0.01) (0.02)
Determined by GPC, relative to polystyrene standards. Excited at the absorption maximum in THF.
protons); 13C NMR (d in CDCl3) 2.67, 14.03e14.12, 22.61e22.78, 28.80, 29.36, 30.22, 31.59, 32.67, 123.61, 124.16e124.35, 126.54, 134.99, 135.83. Data for pSi8T: red solid; 1H NMR (d in CDCl3) 0.38e 0.57 (br m, 21H, SiCH3), 0.91e1.00 (br m, 9H, CH3), 1.29e1.48 (br m, 10H, CH2), 1.58e1.72 (br m, 6H, CH2), 2.73e2.82 (br m, 6H, CH2), 6.66e7.27 (br m, 24H, aromatic protons); 13C NMR (d in CDCl3) 0.36, 1.04, 14.03e14.13, 22.60e22.75, 28.78, 29.30, 30.22, 31.58, 32.50e32.63, 123.47e123.60, 124.25e124.89, 126.43, 134.94, 135.68. Data for pC8T: red solid; 1H NMR (d in CDCl3) 0.22e0.67 (br m, 19H, SiCH3 and CH), 0.91e0.96 (br m, 9H, CH3), 1.29e1.43 (br m, 10H, CH2), 1.57e1.67 (br m, 6H, CH2), 2.60e2.82 (br m, 6H, CH2), 6.65e7.27 (br m, 24H, aromatic protons); 13C NMR (d in CDCl3) 2.62, 13.64, 14.04e14.13, 17.53, 22.61e22.76, 26.88, 27.85, 28.83, 29.31, 30.22, 31.60, 32.52, 123.37e123.84, 124.14e124.31, 124.83, 126.24e 126.44, 134.95, 135.76. Data for pSi9T: red solid; m.p. 96e100 C; 1H NMR (d in CDCl3) 0.36e0.48 (br m, 21H, SiCH3), 0.88e0.97 (br m, 9H, CH3), 1.31e1.44 (br m, 10H, CH2), 1.57e1.68 (br m, 6H, CH2), 2.78 (br s, 6H, CH2), 6.65e7.28 (br m, 26H, aromatic protons); 13C NMR (d in CDCl3) 0.34, 14.03, 31.59, 124.32. Data for pC9T: red solid; 1H NMR (d in CDCl3) 0.33e0.45 (br m, 19H, SiCH3 and CH), 0.90e0.96 (br m, 9H, CH3), 1.25e1.42 (br m, 10H, CH2), 1.61e1.66 (br m, 6H, CH2), 2.78 (br s, 6H, CH2), 6.65e7.28 (br m, 26H, aromatic protons); 13 C NMR (d in CDCl3) 13.65, 14.04e14.13, 17.53, 22.62, 26.88, 27.85, 30.21, 31.60, 124.15, 135.76. Data for pSi4TBTA: dark brown solid; 1H NMR (d in CDCl3) 0.37 (br s, 33H, SiCH3), 0.91e0.98 (br m, 12H, CH3), 1.32e1.69 (br m, 32H, CH2), 2.79 (br s, 6H, CH2), 6.67e7.27 (br m, 29H, aromatic protons), 7.74e7.97 (br m, 1H, BTA and butylthiophene aromatic protons); 13C NMR (d in CDCl3) 0.35, 14.11, 22.61e22.80, 28.82, 30.21, 31.59, 32.60, 123.60, 124.13, 124.84, 134.94. Data for pC4TBTA: brown solid; 1H NMR (d in CDCl3) 0.37 (br s, 36H, SiCH3 and CH), 0.91e0.98 (br m, 13H, CH3), 1.32e1.69 (br m, 34H, CH2), 2.79 (br m, 6H, CH2), 6.67e7.27 (br m, 32H, aromatic protons), 7.75e7.96 (br m, 1H, BTA and butylthiophene aromatic protons); 13C NMR (d in CDCl3) 2.72, 14.12, 22.60, 28.81, 30.22, 31.59, 32.68, 40.22, 123.44e124.85, 127.77, 135.03.
A polymer film was prepared by spin-coating using a 0.4 wt% solution in CHCl3 at 2000 rpm for 1 min on a doped Si wafer with a 210 nm thermally grown SiO2 as the bottom-contact type. The drain-source channel length and width were 10 mm and 2 cm, respectively. FET characteristics of the device were analyzed under vacuum at room temperature. The mobility was determined in the saturation regime. 3. Results and discussion 3.1. Polymer synthesis Branched Si-oligothiophene polymers pEmT (E ¼ Si or C, m ¼ 5e9) were prepared as yellowedark brown and red solids by the Stille coupling reactions of E(2TBr)3 (ER ¼ SiMe or CH), 3TSn1, and bis(tributylstannyl)oligothiophenes (ArSn2) in a ratio of 1:1:1, using Pb2(dba)3/(o-Tol)3P/CuO as the catalyst, followed by reprecipitation of the resulting soluble polymeric products from chloroform/hexane or ethanol (Scheme 1 and Table 1). These polymers were soluble in organic solvents such as THF, chloroform, and chlorobenzene, but the solubility gradually decreased as elongating the oligothiophene units in the backbones. They were insoluble in saturated hydrocarbons and alcohols. The polymer structures were verified mainly based on the NMR spectra. Although the NMR spectra revealed only broad signals, the proton integration ratios were almost consistent with the regularly arranged structures presented in Scheme 1. In this polymerization, random coupling reactions would occur to give termination and branched units as depicted in Chart 2. Rather small polydispersity of the polymers ranging in 1.30e2.87 seems to indicate that the polymer backbones are mainly composed of the linear components. However, the broad and multiple NMR signals for the polymers with rather long oligothiophene chains indicate that nonnegligible amounts of termination and branched units are involved, although we could not determine the ratios of these units. We tried to prepare similar branched polymers by using bis(bromobithiophenyl)(quarterthiophenyl) heptamethyltetrasilane as the monomer. However, attempted preparation of this monomer by selective Stille coupling of Si(2TB2)3 with 1 equiv of 3TSn1 was unsuccessful and the reaction proceeded with low selectivity to give mixtures of the starting compound and mono-, di-, and tri-, substituted compounds. We also examined the preparation of similar polymers with electron-deficient benzothiadiazole (BTA) units, expecting to introduce donoreacceptor (DeA) type interaction in the backbones. Thus, the Stille coupling polymerization using bis(tributylstannylthienyl)BTA as the monomer in place of bis(tributylstannyl) oligothiophenes, under the same conditions as those above (Scheme 1) afforded the corresponding DeA type polymers (pE4TBTA). In the 1H NMR spectra of these polymers, however, the integrations of the BTA ring protons were smaller than those of the
Chart 2. Possible termination (left) and branched (right) units in Si-linked oligothiophene polymers (ER ¼ SiMe or CH, Si ¼ SiMe2).
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
53
Norm. ABS or Intensity
Abs in THF 1
Abs as film
PL in THF
PL as solid
0.5
0 Chart 3. BTA-containing branched Si-linked oligothiophene polymers.
theoretical values, in contrast to pEmT whose 1H NMR spectra showed the integration ratios that were nearly consistent with the structures shown in Scheme 1. Presumably, homo-coupling of E(2TBr)3 competed with the expected cross-coupling, to give quarterthiophenylene units in the backbone. The incorporation ratios of the BTA units in the polymers were about 25% (Chart 3), regardless of the center element. Similar homo-coupling has been reported previously for the Stille coupling polymerization of bis(tributylstannylthienyl)silane and dibromobenzothiadiazole [10]. 3.2. Optical and semi-conducting properties Optical properties of the present polymers were examined with respect to the UVevis absorption and photoluminescence (PL) spectra (Table 1). The silicon-centered polymers (ER ¼ SiMe) always showed slightly red-shifted absorption bands from those of the corresponding carbon-centered polymers (ER ¼ CH). However, the differences were only 5 nm or less and the PL spectral profiles were almost the same, regardless of the center element (E ¼ Si or C), and therefore, only the spectra of silicon-centered polymers are presented in Figs. 1 and 2. For polymers pEmT, the UVevis spectra revealed broad bands probably due to the overlap of the absorptions of the different oligothiophene units in the side chains and the backbones (Fig. 1(a)). As expected, the absorption bands tended to move to lower energies as the oligothiophene chains in the backbone were elongated on going from m ¼ 5 to 9, except for pE6T whose absorption maxima were blue-shifted from those of pE5T by 10e 11 nm. It is likely that the steric repulsion between the butyl groups hinders planarity of the hexithiophene units in pE6T. From m ¼ 8 to 9, no evident shifts of the absorption maxima were observed, but pE9T exhibited more broad bands with the
250
350
450
550
650
750
Wavelength/nm Fig. 2. UVevis absorption and PL spectra in THF and as film or solid for pSi4TBTA.
absorption edges at lower energies (ledge ¼ ca. 580 nm) than those of pE8T (ca. 560 nm). The PL spectra of pExT also showed similar red-shifts as elongating the oligothiophene chain length as illustrated in Fig. 1(b) for ER ¼ SiMe. In contrast to the UVevis absorption spectra that showed broad bands composed of both the absorptions of oligothiophenes in the side and main chains, only small shoulder emission bands arising from quinquethiophene side groups at higher energy region were observed in the PL spectra of polymers pEmT with x 7. This seems indicative of the energy transfer from the photo-excited pendant quinquethiophene units to the hepta-, octa-, and nanothiophene units in the backbones (Chart 4). Similar photo-excited energy transfer between p-conjugated units linked by an organosilicon unit in polymeric and oligomeric systems has been reported [6aec,11]. Previously, highly emissive properties of monosilane-linked branched oligothiophene polymers [7], monosilane-linked monoand bithiophene dendrimers [12a], and star-shaped compounds with three or more disilanyleneebithiophenylene arms linked by a benzene [12b] or a triazine [12c] core were reported. However, the present polymers showed only moderate PL efficiency with F ¼ 0.2e0.3 in solutions, which dropped to approximately 0.02 when measured as solids. This is probably ascribed to the flexible organosilicon chains that can be easily folded to permit stacking of the neighboring oligothiophene chromophores, leading to the concentration quenching of the PL. The UVevis absorption and PL spectra of BTA-containing Silinked polymer pSi4TBTA are presented in Fig. 2. Again, the carboncentered analog exhibited quite similar optical properties. For
Fig. 1. UVevis absorption (a) and PL (b) spectra of pSimT in THF.
54
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
design of new functional polymers for organic electronics, based on Si-oligothiophene systems. Acknowledgment This work was supported in part by a GranteineAid for Scientific Research (No.23350097) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References Chart 4. Intramolecular energy-transfer in branched Si-linked oligothiophene polymers.
pE4TBTA, broad shoulder peaks were observed at 540 nm and 670 nm probably due to bis(bithiophenyl)benzothiadiazole units in the UVevis absorption and PL spectra, respectively, together with the major peaks ascribed to the emission of the shorter oligothiophene units in the side groups and the main chains (Chart 3). The PL bands based on the oligothiophene units of pE4TBTA observed as the major peaks in solutions were relatively suppressed in the solid state spectra as illustrated in Fig. 2 for pSi4TBTA, indicating smooth energy transfer from quarter- and quinquethiophene units to the bis(bithiophenyl)BTA units. This can be explained by stacking of the p-conjugated units in the solid states. Red-shifts of the PL bands were also observed in the solid states from those in solutions, again indicative of the p-stacking. Although the UVevis absorption spectrum of a pSi4TBTA film revealed a slightly blue-shifted maximum from that in THF, enhanced shoulder approximately at 550 nm seemed to again indicate the p-stacking in the film state. Bottom-contact type TFT devices with the polymer spin-coated film as the active layer were fabricated and their p-type field effect mobility (mFET) and on/off ratio (Ion/Ioff) were noted. Unfortunately, these devices showed rather low activity with mFET ¼ 2.7e 7.2 106 cm2/Vs and Ion/Ioff ¼ 10e102 for the silicon-centered polymers, and mFET ¼ 1.2e4.4 106 cm2/Vs and Ion/Ioff ¼ 10e102 for the carbon-centered ones. The mobility tended to increase as the expansion of oligothiophene chains and the best performance of the devices were obtained with pSi8T. It was also noted that the silicon-centered polymers always showed higher mobility than those of the carbon-centered analogs. This is most likely due to the enhanced conjugation in silicon-centered polymers, as evidenced by the red-shifted absorption maxima, although there may be many factors other than polymer electronic states including film morphology that affected the FET activity. Annealing of the films did not affect the results.
4. Conclusions In summary, we prepared branched polymers composed of silicon-linked oligothiophene units and investigated their optical and carrier-transporting properties. Although their PL properties were not surprisingly high and the carrier mobility of the polymer films in TFTs was rather low, they showed efficient photo-induced energy transfer between the different chromophores in the polymer main and side chains, being potentially useful as the lightharvesting materials. It was also noteworthy that the siliconcentered polymers exhibited slightly red-shifted absorption bands and higher TFT activity than those of the carbon-centered polymers. These studies seem to provide an idea for molecular
[1] (a) Y. Shang, L.-Q. Yang, G.-L. Zhang, H. Zhang, B. Liu, J. Polym. Res. 18 (2011) 1889; (b) J.C. Sanchez, W.C. Trogler, Macromol. Chem. Phys. 209 (2008) 1528; (c) W. Uhlig, Prog. Polym. Sci. 27 (2002) 255; (d) J. Ohshita, A. Kunai, Acta Polym. 49 (1998) 379. [2] (a) S.A. Manhart, A. Adachi, K. Sakamaki, K. Okita, J. Ohshita, T. Ohno, T. Hamaguchi, A. Kunai, J. Kido, J. Organomet. Chem. 592 (1999) 52; (b) J. Ohshita, K. Yoshimoto, Y. Tada, Y. Harima, A. Kunai, Y. Kunugi, K. Yamashita, J. Organomet. Chem. 678 (2003) 33; (c) X. Ren, J. Li, R.J. Holmes, P.I. Djurovich, S.R. Forrest, M.E. Thompson, Chem. Mater. 16 (2004) 4743. [3] (a) A. Kunai, T. Ueda, K. Horata, E. Toyoda, J. Ohshita, M. Ishikawa, K. Tanaka, Organometallics 15 (1996) 2000; (b) J. Ohshita, A. Takata, H. Kai, A. Kunai, K. Komaguchi, M. Shiotani, A. Adachi, K. Sakamaki, K. Okita, Y. Harima, Y. Kunugi, K. Yamashita, M. Ishikawa, Organometallics 19 (2000) 4492; (c) J. Ohshita, K. Yoshimoto, M. Hashimoto, D. Hamamoto, A. Kunai, Y. Harima, Y. Kunugi, K. Yamashita, M. Kakimoto, M. Ishikawa, J. Organomet. Chem. 665 (2003) 29; (d) J. Ohshita, D.-H. Kim, Y. Kunugi, A. Kunai, Organometallics 24 (2005) 4494; (e) D.-H. Kim, J. Ohshita, Y. Kunugi, A. Kunai, Chem. Lett. 35 (2006) 266; (f) J. Ohshita, Y. Izumi, D.-H. Kim, A. Kunai, T. Kosuge, Y. Kunugi, A. Naka, M. Ishikawa, Organometallics 26 (2007) 6150; (g) J. Ohshita, Y. Hatanaka, S. Matsui, T. Mizumo, Y. Kunugi, Y. Honsho, A. Saeki, S. Seki, J. Tibbelin, H. Ottosson, Dalton Trans. 39 (2010) 9314. [4] (a) S. Kyushin, M. Ikarugi, M. Goto, H. Hiratsuka, H. Matsumoto, Organometallics 15 (1996) 1067; (b) H. Maeda, Y. Inoue, H. Ishida, K. Mizuno, Chem. Lett. (2001) 1224. [5] (a) J. Ohshita, T. Uemura, D.-H. Kim, A. Kunai, Y. Kunugi, M. Kakimoto, Macromolecules 38 (2005) 730; (b) T. Sasan, H. Sogawa, K. Tamura, M. Shiotsuki, T. Maeda, F. Sanda, J. Polym. Sci. A Polym. Chem. 48 (2010) 1815; (c) A. Mori, E. Takahisa, Y. Yamamura, T. Kato, A.P. Mudalige, H. Kajiro, K. Hirabayashi, Y. Nishihara, T. Hiyama, Organometallics 23 (2004) 1755; (d) X.-H. Zhou, Y.-H. Niu, F. Huang, M.S. Liu, A.K.-Y. Jen, Macromolecules 40 (2007) 3015; (e) K.L. Paik, N.S. Baek, H.-K. Kim, J.-H. Lee, Y. Lee, Macromolecules 35 (2002) 6782. [6] (a) T.-Y. Luh, Y.-J. Chen, Chem. Commun. (2006) 4669; (b) T.-Y. Luh, Pure Appl. Chem. 82 (2010) 613; (c) J. Ohshita, R. Taketsugu, K. Hino, A. Kunai, J. Organomet. Chem. 691 (2006) 3065; (d) K. Takagi, H. Kakiuchi, Y. Yuki, M. Suzuki, J. Polym. Sci. A Polym. Chem. 45 (2007) 4786. [7] (a) X.-M. Liu, C. He, X.-T. Hao, L.-W. Tan, Y. Li, K.S. Ong, Macromolecules 34 (2004) 5965; (b) S.A. Ponomarenko, N.N. Rasulova, Y.N. Luponosov, N.M. Surin, M.I. Buzin, I. Leshchiner, S.M. Peregudova, A.M. Murafarov, Macromolecules 45 (2012) 2014. [8] M.-S. Choi, Tetrahedron Lett. 49 (2008) 7050. [9] S. Steinberger, A. Mishra, E. Reinold, C.M. Müller, C. Uhrich, M. Pfeiffer, P. Bäuerle, Org. Lett. 13 (2011) 90. [10] J. Ohshita, S. Kangai, H. Yoshida, A. Kunai, S. Kajiwara, Y. Ooyama, Y. Harima, J. Organomet. Chem. 692 (2007) 801. [11] (a) M. Fujitsuka, D.W. Cho, J. Ohshita, A. Kunai, T. Majima, J. Phys. Chem. C 110 (2006) 12446; (b) M. Fujitsuka, D.W. Cho, J. Ohshita, A. Kunai, T. Majima, J. Phys. Chem. C 111 (2007) 1993; (c) J. Ohshita, K. Kawashima, A. Iwata, H.-Q. Tang, M. Higashi, A. Kunai, Z. Naturforsch. B 59 (2004) 1332. [12] (a) O.V. Borshchev, S.A. Ponomarenko, N.M. Surin, M.M. Kaptyug, M.I. Buzin, A.P. Pleshkova, N.V. Demchenko, V.D. Myakushev, A.M. Muzafarov, Organometallics 26 (2007) 5165; (b) A. Naka, Y. Matsumoto, T. Itano, K. Hasegawa, T. Shimamura, J. Ohshita, A. Kunai, T. Takeuchi, M. Ishikawa, J. Organomet. Chem. 694 (2009) 346; (c) A. Naka, R. Fukuda, R. Kishimoto, Y. Yamashita, Y. Ooyama, J. Ohshita, M. Ishikawa, J. Organomet. Chem. 702 (2012) 67.
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and optical properties of organosiliconeoligothiophene branched polymers Joji Ohshita a, *, Yuta Tominaga a, Daiki Tanaka a, Tomonobu Mizumo a, Yuki Fujita b, Yoshihito Kunugi b a b
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Department of Applied Chemistry, Faculty of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2012 Received in revised form 25 February 2013 Accepted 5 March 2013
Star-shaped monomers bearing three bromobithiophenyl units linked by an organosilicon core were polymerized with 1:1 mixtures of (tributylstannyl)hexylterthiophene and bis[(tributylstannyl)thienyl] arenes under the Stille coupling conditions leading to the formation of the corresponding branched polymers bearing quinquethiophenyl side groups and an Si-linked bis(bithiophenylene)arylene backbone as yellowedark brown and red solids that are soluble in organic solvents, such as chloroform and THF. Optical properties of the resulting polymers were investigated with respect to the UVevis absorption and photoluminescence spectra, suggesting the efficient energy transfer between the p-conjugated systems at the photo-excited states. Semi-conducting properties of the polymer films were also investigated. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Organosilicon polymer Branched polymer Oligothiophene Photoluminescence Light-harvesting material
1. Introduction Polymers having alternate organosilicon and p-conjugated units are of interest because of their functionalities [1]. High carrier transporting properties are often found for those polymers, leading to the potential use of the polymer films as hole-transporting materials for organic light emitting diodes (OLEDs) and organic thin film transistors (OTFTs) [2,3]. In addition, it is known that the silicon-substitution on the p-conjugated systems often enhances the photoluminescence (PL) efficiency [4] and several Si-p alternate polymers with excellent PL and electroluminescence (EL) properties have been reported to date [3d,5,6]. For some of the polymers with different p-systems linked by organosilicon units, smooth intramolecular photo-induced energy and electron transfer between the p-systems have been also demonstrated, making it possible to utilize these polymers as light-harvesting materials [6]. In this connection, we have been interested in Si-oligothiophene alternate polymers [3] and reported the synthesis of poly(mono-, di-, and trisilanyleneoligothienylene)s (Chart 1(a), x ¼ 1e3 and m ¼ 3e5) and the utilities of their spin-coated films as the holetransport layers of multi-layered OLEDs [3aec]. More recently, we prepared similar polymers with longer oligothiophene units (x ¼ 1 and m ¼ 8e14) that were potentially useful as the active materials * Corresponding author. Tel.: þ81 82 424 7743; fax: þ81 82 424 5494. E-mail address: [email protected] (J. Ohshita). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.03.007
for OTFTs, although the mobility of the films was not very high (w105 cm2/Vs) [3d]. In an effort to obtain better understanding about the TFT activity of the Si-linked oligothiophene derivatives, we prepared a series of compounds with two or three oligothiophene units linked by an organosilicon linkage (Chart 1(b) and (c), m ¼ 3e5) [3e,f]. The TFT activity of the vapor-deposited films of the Si-linked oligothiophenes was highly dependent on the oligothiophene chain lengths and their elongation markedly improved the TFT activity. Molecular shapes also affected the activity and the starshaped compounds (Chart 1(c)) always showed higher TFT activity than the linear ones (Chart 1(b)). The highest hole mobility of 6.2 102 cm2/Vs among those Si-linked oligothiophenes was obtained for the star-shaped quinquethiophene derivative (m ¼ 5, ER ¼ SiMe) [3f]. Influence of the nature of the center element was also studied for the star-shaped compounds and it was found that the silicon-centered compound (ER ¼ SiMe) exhibited higher mobility than the carbon-centered analog (ER ¼ CH) for m ¼ 3, while elongation of the oligothiophene chain length changed the tendency and a little higher but almost comparable mobility was observed for the carbon-centered compound with m ¼ 4 as compared with the silicon-centered analog [3g]. The holetransporting properties of those Si-linked oligothiophene oligomers and polymers originate from the stabilization of the cation charge in the oxidized states. Indeed, the DFT calculations on radical cations of the model molecules indicated the delocalization of the cation charge over the Si-linked oligothiophene systems [3g].
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
51
Chart 1. Structures of Si-oligothiophene polymers (a), and linear (b) and star-shaped Si-oligothiophene compounds (c).
In this paper, we report the synthesis, and optical and semiconducting properties of branched polymers bearing Si-linked oligothiophene units. The semi-conducting properties were not as high as that we had expected. However, efficient energy transfer between the oligothiophene units at the photo-excited states was observed for these polymers, suggesting their potential as lightharvesting materials. Although monosilane-linked branched oligothiophene polymers have been reported previously [7], this is the first example of those with flexible organosilicon linkages. 2. Experimental 2.1. General All reactions were carried out in dry argon. Tetrahydrofuran (THF) and ether, and toluene used as the reaction solvents were distilled from sodium/benzophenone and calcium hydride, respectively and stored over activated molecular sieves until use. Starting monomers tris[(bromobithiophenyl)dimethylsilyl]methylsilane [3f] and -methane [3g] (E(2TBr)3, ER ¼ SiMe or CH in Scheme 1), bisstannylarenes (ArSn2 in Scheme 1), hexyl(tributylstannyl)terthiophene (3TSn1) [8], and bis(bromobutylthienyl)benzothiadiazole [9] were prepared as reported in the literature. Bis(tributylstannyl) butylthiophene (1TSn2), bis(tributylstannyl)dibutylbithiophene (2TSn2), bis(tributylstannyl)dibutylterthiophene (3TSn2), bis(tributylstannyl)dibutylquarterthiophene (4TSn2), and bis[(tributylstannyl)thienyl]benzothiadiazole) were prepared by dilithiation of the corresponding dibromooligothiophenes by the reactions with two equivalents of n-butyllithium, followed by treatment of the resulting dilithiooligothiophenes with tri(n-butyl)tin chloride, in a similar fashion to that of hexyl(tributylstannyl)terthiophene [8]. NMR spectra were recorded on Varian MR400 and System500 spectrometers. UV absorption and PL spectra were measured on Shimadzu UV3150 and HITACHI F4500 spectrophotometers, respectively. PL quantum yields were determined in an integration sphere attached to
Scheme 1. Synthesis of branched Si-linked oligothiophene polymers.
a Hamamatsu Photonics C7473 multi-channel analyzer. Polymer molecular weights were measured by gel permeation chromatography (GPC) using Shodex KF804 and KF806 columns connected in series eluted with THF. The polymers were detected by a UV detector at 240 nm and the molecular weights were calculated relative to the polystyrene standards on a SIC-480 data station. 2.2. Polymer synthesis In an autoclave was placed a mixture of 0.285 g (0.300 mmol) of Si(2TBr)3, 0.186 g (0.300 mmol) of 3TSn1, 0.216 g (0.300 mmol) of 1TSn2, 7 mg (0.008 mmol) of Pd2(dba)3, 18 mg (0.060 mmol) of (oTol)3P, 24 mg (0.30 mmol) of CuO, 20 mL of toluene and the mixture was heated at 150 C for 48 h. The resulting precipitates were filtered and the solvent was evaporated. After hydrolysis with water, the organic layer was separated and the aqueous layer was extracted with chloroform. The organic layer and the extracts were combined and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was reprecipitated from chloroform/hexane to give 230 mg (65% yield) of pSi5T as orange solids: m.p. 95e100 C; 1H NMR (d in CDCl3) 0.41 (br s, 21H, SiCH3), 0.95 (br s, 6H, CH3), 1.29e1.41 (br m, 8H, CH2), 1.58e1.70 (br m, 4H, CH2), 2.72e2.80 (br m, 4H, CH2), 6.67e7.27 (br m, 19H, aromatic protons); 13C NMR (d in CDCl3) 11.79, 0.36, 13.64e14.14, 17.53, 22.61e22.73, 28.80, 29.30, 30.22, 31.59, 32.51, 123.59e124.87, 134.97, 135.73, 142.54. Other polymers were prepared in a fashion similar to that above. Data for pC5T: orange solid; 1H NMR (d in CDCl3) 0.43 (br s, 19H, SiCH3 and CH), 0.90 (br s, 6H, CH3), 1.31e1.39 (br m, 8H, CH2), 1.56e 1.68 (br m, 4H, CH2), 2.78 (br s, 4H, CH2), 6.65e7.26 (br m, 19H, aromatic protons); 13C NMR (d in CDCl3) 2.71, 14.05e14.14, 22.61e 22.73, 28.80, 29.27, 30.23, 31.60, 32.54, 123.40e124.85, 134.43e 135.77. Data for pSi6T: orange solid; m.p. 98e103 C; 1H NMR (d in CDCl3) 0.39 (br s, 21H, SiCH3), 0.88 (br s, 9H, CH3), 1.31e1.43 (br m, 10H, CH2), 1.57e1.67 (br m, 6H, CH2), 2.51 (br s, 4H, CH2), 2.77 (br s, 2H, CH2), 6.64e7.23 (br m, 20H, aromatic protons); 13C NMR (d in CDCl3) 11.83, 0.45, 13.98e14.13, 22.54e22.61, 28.77, 30.22, 31.58, 32.80, 123.39e123.59, 124.22e124.87, 125.14, 127.31, 135.00, 135.99, 136.76, 142.41, 143.37. Data for pC6T: yellow solid; m.p. 120e125 C; 1 H NMR (d in CDCl3) 0.39 (br s, 19H, SiCH3 and CH), 0.87 (br s, 9H, CH3), 1.30 (br s, 10H, CH2), 1.56e1.65 (br m, 6H, CH2), 2.51 (br s, 4H, CH2), 2.76 (br s, 2H, CH2), 6.65e7.11 (br m, 20H, aromatic protons); 13 C NMR (d in CDCl3) 2.68, 13.99e14.14, 22.55e22.61, 28.77, 30.22, 31.58, 32.81, 123.41e123.58, 124.19e124.87, 125.13, 135.02, 135.77e 136.77, 140.96, 143.37. Data for pSi7T: red solid; 1H NMR (d in CDCl3) 0.41 (br s, 21H, SiCH3), 0.88e0.96 (br m, 9H, CH3), 1.30e1.43 (br m, 10H, CH2), 1.57e1.68 (br m, 6H, CH2), 2.77 (br s, 6H, CH2), 6.70e7.26 (br m, 22H, aromatic protons); 13C NMR (d in CDCl3) 11.83, 0.36, 14.03e14.12, 22.61e22.77, 28.77, 29.37, 30.22, 31.58, 32.62, 123.40e 124.87, 126.50e126.67, 134.93, 135.74. Data for pC7T: red solid; m.p. 100e106 C; 1H NMR (d in CDCl3) 0.43 (br s, 19H, SiCH3 and CH), 0.88e0.96 (br m, 9H, CH3), 1.30e1.50 (br m, 10H, CH2), 1.55e1.68 (br m, 6H, CH2), 2.76 (br s, 6H, CH2), 6.65e7.25 (br m, 22H, aromatic
52
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
2.3. OTFT fabrication and evaluation
Table 1 Synthesis and properties of branched Si-oligothiophene polymers. Polymer
pSi5T pSi6T pSi7T pSi8T pSi9T pSi4TBTA pC5T pC6T pC7T pC8T pC9T pC4TBTA a b
Yield/%
65 40 58 20 51 47 63 64 28 11 32 54
Mwa
9800 32 000 6200 9500 11 000 9100 14 000 51 000 12 000 9400 9600 8900
Mw/Mna
1.97 2.47 1.51 1.37 1.57 1.34 2.33 2.87 1.90 1.30 1.39 2.29
UV lmax/nm
PL lem/nm (F)b
In THF
In THF
As solid
421 410 428 435 435 415 416 406 424 433 431 415
533 528 546 556 559 502 526 526 547 555 560 500
583 576 608 611 628 696 575 570 612 616 621 676
(0.22) (0.26) (0.30) (0.28) (0.27) (0.20) (0.16) (0.23) (0.25) (0.23) (0.25) (0.20)
(0.03) (0.02) (0.03) (0.01) (0.01) (0.01) (0.03) (0.02) (0.02) (0.01) (0.01) (0.02)
Determined by GPC, relative to polystyrene standards. Excited at the absorption maximum in THF.
protons); 13C NMR (d in CDCl3) 2.67, 14.03e14.12, 22.61e22.78, 28.80, 29.36, 30.22, 31.59, 32.67, 123.61, 124.16e124.35, 126.54, 134.99, 135.83. Data for pSi8T: red solid; 1H NMR (d in CDCl3) 0.38e 0.57 (br m, 21H, SiCH3), 0.91e1.00 (br m, 9H, CH3), 1.29e1.48 (br m, 10H, CH2), 1.58e1.72 (br m, 6H, CH2), 2.73e2.82 (br m, 6H, CH2), 6.66e7.27 (br m, 24H, aromatic protons); 13C NMR (d in CDCl3) 0.36, 1.04, 14.03e14.13, 22.60e22.75, 28.78, 29.30, 30.22, 31.58, 32.50e32.63, 123.47e123.60, 124.25e124.89, 126.43, 134.94, 135.68. Data for pC8T: red solid; 1H NMR (d in CDCl3) 0.22e0.67 (br m, 19H, SiCH3 and CH), 0.91e0.96 (br m, 9H, CH3), 1.29e1.43 (br m, 10H, CH2), 1.57e1.67 (br m, 6H, CH2), 2.60e2.82 (br m, 6H, CH2), 6.65e7.27 (br m, 24H, aromatic protons); 13C NMR (d in CDCl3) 2.62, 13.64, 14.04e14.13, 17.53, 22.61e22.76, 26.88, 27.85, 28.83, 29.31, 30.22, 31.60, 32.52, 123.37e123.84, 124.14e124.31, 124.83, 126.24e 126.44, 134.95, 135.76. Data for pSi9T: red solid; m.p. 96e100 C; 1H NMR (d in CDCl3) 0.36e0.48 (br m, 21H, SiCH3), 0.88e0.97 (br m, 9H, CH3), 1.31e1.44 (br m, 10H, CH2), 1.57e1.68 (br m, 6H, CH2), 2.78 (br s, 6H, CH2), 6.65e7.28 (br m, 26H, aromatic protons); 13C NMR (d in CDCl3) 0.34, 14.03, 31.59, 124.32. Data for pC9T: red solid; 1H NMR (d in CDCl3) 0.33e0.45 (br m, 19H, SiCH3 and CH), 0.90e0.96 (br m, 9H, CH3), 1.25e1.42 (br m, 10H, CH2), 1.61e1.66 (br m, 6H, CH2), 2.78 (br s, 6H, CH2), 6.65e7.28 (br m, 26H, aromatic protons); 13 C NMR (d in CDCl3) 13.65, 14.04e14.13, 17.53, 22.62, 26.88, 27.85, 30.21, 31.60, 124.15, 135.76. Data for pSi4TBTA: dark brown solid; 1H NMR (d in CDCl3) 0.37 (br s, 33H, SiCH3), 0.91e0.98 (br m, 12H, CH3), 1.32e1.69 (br m, 32H, CH2), 2.79 (br s, 6H, CH2), 6.67e7.27 (br m, 29H, aromatic protons), 7.74e7.97 (br m, 1H, BTA and butylthiophene aromatic protons); 13C NMR (d in CDCl3) 0.35, 14.11, 22.61e22.80, 28.82, 30.21, 31.59, 32.60, 123.60, 124.13, 124.84, 134.94. Data for pC4TBTA: brown solid; 1H NMR (d in CDCl3) 0.37 (br s, 36H, SiCH3 and CH), 0.91e0.98 (br m, 13H, CH3), 1.32e1.69 (br m, 34H, CH2), 2.79 (br m, 6H, CH2), 6.67e7.27 (br m, 32H, aromatic protons), 7.75e7.96 (br m, 1H, BTA and butylthiophene aromatic protons); 13C NMR (d in CDCl3) 2.72, 14.12, 22.60, 28.81, 30.22, 31.59, 32.68, 40.22, 123.44e124.85, 127.77, 135.03.
A polymer film was prepared by spin-coating using a 0.4 wt% solution in CHCl3 at 2000 rpm for 1 min on a doped Si wafer with a 210 nm thermally grown SiO2 as the bottom-contact type. The drain-source channel length and width were 10 mm and 2 cm, respectively. FET characteristics of the device were analyzed under vacuum at room temperature. The mobility was determined in the saturation regime. 3. Results and discussion 3.1. Polymer synthesis Branched Si-oligothiophene polymers pEmT (E ¼ Si or C, m ¼ 5e9) were prepared as yellowedark brown and red solids by the Stille coupling reactions of E(2TBr)3 (ER ¼ SiMe or CH), 3TSn1, and bis(tributylstannyl)oligothiophenes (ArSn2) in a ratio of 1:1:1, using Pb2(dba)3/(o-Tol)3P/CuO as the catalyst, followed by reprecipitation of the resulting soluble polymeric products from chloroform/hexane or ethanol (Scheme 1 and Table 1). These polymers were soluble in organic solvents such as THF, chloroform, and chlorobenzene, but the solubility gradually decreased as elongating the oligothiophene units in the backbones. They were insoluble in saturated hydrocarbons and alcohols. The polymer structures were verified mainly based on the NMR spectra. Although the NMR spectra revealed only broad signals, the proton integration ratios were almost consistent with the regularly arranged structures presented in Scheme 1. In this polymerization, random coupling reactions would occur to give termination and branched units as depicted in Chart 2. Rather small polydispersity of the polymers ranging in 1.30e2.87 seems to indicate that the polymer backbones are mainly composed of the linear components. However, the broad and multiple NMR signals for the polymers with rather long oligothiophene chains indicate that nonnegligible amounts of termination and branched units are involved, although we could not determine the ratios of these units. We tried to prepare similar branched polymers by using bis(bromobithiophenyl)(quarterthiophenyl) heptamethyltetrasilane as the monomer. However, attempted preparation of this monomer by selective Stille coupling of Si(2TB2)3 with 1 equiv of 3TSn1 was unsuccessful and the reaction proceeded with low selectivity to give mixtures of the starting compound and mono-, di-, and tri-, substituted compounds. We also examined the preparation of similar polymers with electron-deficient benzothiadiazole (BTA) units, expecting to introduce donoreacceptor (DeA) type interaction in the backbones. Thus, the Stille coupling polymerization using bis(tributylstannylthienyl)BTA as the monomer in place of bis(tributylstannyl) oligothiophenes, under the same conditions as those above (Scheme 1) afforded the corresponding DeA type polymers (pE4TBTA). In the 1H NMR spectra of these polymers, however, the integrations of the BTA ring protons were smaller than those of the
Chart 2. Possible termination (left) and branched (right) units in Si-linked oligothiophene polymers (ER ¼ SiMe or CH, Si ¼ SiMe2).
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
53
Norm. ABS or Intensity
Abs in THF 1
Abs as film
PL in THF
PL as solid
0.5
0 Chart 3. BTA-containing branched Si-linked oligothiophene polymers.
theoretical values, in contrast to pEmT whose 1H NMR spectra showed the integration ratios that were nearly consistent with the structures shown in Scheme 1. Presumably, homo-coupling of E(2TBr)3 competed with the expected cross-coupling, to give quarterthiophenylene units in the backbone. The incorporation ratios of the BTA units in the polymers were about 25% (Chart 3), regardless of the center element. Similar homo-coupling has been reported previously for the Stille coupling polymerization of bis(tributylstannylthienyl)silane and dibromobenzothiadiazole [10]. 3.2. Optical and semi-conducting properties Optical properties of the present polymers were examined with respect to the UVevis absorption and photoluminescence (PL) spectra (Table 1). The silicon-centered polymers (ER ¼ SiMe) always showed slightly red-shifted absorption bands from those of the corresponding carbon-centered polymers (ER ¼ CH). However, the differences were only 5 nm or less and the PL spectral profiles were almost the same, regardless of the center element (E ¼ Si or C), and therefore, only the spectra of silicon-centered polymers are presented in Figs. 1 and 2. For polymers pEmT, the UVevis spectra revealed broad bands probably due to the overlap of the absorptions of the different oligothiophene units in the side chains and the backbones (Fig. 1(a)). As expected, the absorption bands tended to move to lower energies as the oligothiophene chains in the backbone were elongated on going from m ¼ 5 to 9, except for pE6T whose absorption maxima were blue-shifted from those of pE5T by 10e 11 nm. It is likely that the steric repulsion between the butyl groups hinders planarity of the hexithiophene units in pE6T. From m ¼ 8 to 9, no evident shifts of the absorption maxima were observed, but pE9T exhibited more broad bands with the
250
350
450
550
650
750
Wavelength/nm Fig. 2. UVevis absorption and PL spectra in THF and as film or solid for pSi4TBTA.
absorption edges at lower energies (ledge ¼ ca. 580 nm) than those of pE8T (ca. 560 nm). The PL spectra of pExT also showed similar red-shifts as elongating the oligothiophene chain length as illustrated in Fig. 1(b) for ER ¼ SiMe. In contrast to the UVevis absorption spectra that showed broad bands composed of both the absorptions of oligothiophenes in the side and main chains, only small shoulder emission bands arising from quinquethiophene side groups at higher energy region were observed in the PL spectra of polymers pEmT with x 7. This seems indicative of the energy transfer from the photo-excited pendant quinquethiophene units to the hepta-, octa-, and nanothiophene units in the backbones (Chart 4). Similar photo-excited energy transfer between p-conjugated units linked by an organosilicon unit in polymeric and oligomeric systems has been reported [6aec,11]. Previously, highly emissive properties of monosilane-linked branched oligothiophene polymers [7], monosilane-linked monoand bithiophene dendrimers [12a], and star-shaped compounds with three or more disilanyleneebithiophenylene arms linked by a benzene [12b] or a triazine [12c] core were reported. However, the present polymers showed only moderate PL efficiency with F ¼ 0.2e0.3 in solutions, which dropped to approximately 0.02 when measured as solids. This is probably ascribed to the flexible organosilicon chains that can be easily folded to permit stacking of the neighboring oligothiophene chromophores, leading to the concentration quenching of the PL. The UVevis absorption and PL spectra of BTA-containing Silinked polymer pSi4TBTA are presented in Fig. 2. Again, the carboncentered analog exhibited quite similar optical properties. For
Fig. 1. UVevis absorption (a) and PL (b) spectra of pSimT in THF.
54
J. Ohshita et al. / Journal of Organometallic Chemistry 736 (2013) 50e54
design of new functional polymers for organic electronics, based on Si-oligothiophene systems. Acknowledgment This work was supported in part by a GranteineAid for Scientific Research (No.23350097) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References Chart 4. Intramolecular energy-transfer in branched Si-linked oligothiophene polymers.
pE4TBTA, broad shoulder peaks were observed at 540 nm and 670 nm probably due to bis(bithiophenyl)benzothiadiazole units in the UVevis absorption and PL spectra, respectively, together with the major peaks ascribed to the emission of the shorter oligothiophene units in the side groups and the main chains (Chart 3). The PL bands based on the oligothiophene units of pE4TBTA observed as the major peaks in solutions were relatively suppressed in the solid state spectra as illustrated in Fig. 2 for pSi4TBTA, indicating smooth energy transfer from quarter- and quinquethiophene units to the bis(bithiophenyl)BTA units. This can be explained by stacking of the p-conjugated units in the solid states. Red-shifts of the PL bands were also observed in the solid states from those in solutions, again indicative of the p-stacking. Although the UVevis absorption spectrum of a pSi4TBTA film revealed a slightly blue-shifted maximum from that in THF, enhanced shoulder approximately at 550 nm seemed to again indicate the p-stacking in the film state. Bottom-contact type TFT devices with the polymer spin-coated film as the active layer were fabricated and their p-type field effect mobility (mFET) and on/off ratio (Ion/Ioff) were noted. Unfortunately, these devices showed rather low activity with mFET ¼ 2.7e 7.2 106 cm2/Vs and Ion/Ioff ¼ 10e102 for the silicon-centered polymers, and mFET ¼ 1.2e4.4 106 cm2/Vs and Ion/Ioff ¼ 10e102 for the carbon-centered ones. The mobility tended to increase as the expansion of oligothiophene chains and the best performance of the devices were obtained with pSi8T. It was also noted that the silicon-centered polymers always showed higher mobility than those of the carbon-centered analogs. This is most likely due to the enhanced conjugation in silicon-centered polymers, as evidenced by the red-shifted absorption maxima, although there may be many factors other than polymer electronic states including film morphology that affected the FET activity. Annealing of the films did not affect the results.
4. Conclusions In summary, we prepared branched polymers composed of silicon-linked oligothiophene units and investigated their optical and carrier-transporting properties. Although their PL properties were not surprisingly high and the carrier mobility of the polymer films in TFTs was rather low, they showed efficient photo-induced energy transfer between the different chromophores in the polymer main and side chains, being potentially useful as the lightharvesting materials. It was also noteworthy that the siliconcentered polymers exhibited slightly red-shifted absorption bands and higher TFT activity than those of the carbon-centered polymers. These studies seem to provide an idea for molecular
[1] (a) Y. Shang, L.-Q. Yang, G.-L. Zhang, H. Zhang, B. Liu, J. Polym. Res. 18 (2011) 1889; (b) J.C. Sanchez, W.C. Trogler, Macromol. Chem. Phys. 209 (2008) 1528; (c) W. Uhlig, Prog. Polym. Sci. 27 (2002) 255; (d) J. Ohshita, A. Kunai, Acta Polym. 49 (1998) 379. [2] (a) S.A. Manhart, A. Adachi, K. Sakamaki, K. Okita, J. Ohshita, T. Ohno, T. Hamaguchi, A. Kunai, J. Kido, J. Organomet. Chem. 592 (1999) 52; (b) J. Ohshita, K. Yoshimoto, Y. Tada, Y. Harima, A. Kunai, Y. Kunugi, K. Yamashita, J. Organomet. Chem. 678 (2003) 33; (c) X. Ren, J. Li, R.J. Holmes, P.I. Djurovich, S.R. Forrest, M.E. Thompson, Chem. Mater. 16 (2004) 4743. [3] (a) A. Kunai, T. Ueda, K. Horata, E. Toyoda, J. Ohshita, M. Ishikawa, K. Tanaka, Organometallics 15 (1996) 2000; (b) J. Ohshita, A. Takata, H. Kai, A. Kunai, K. Komaguchi, M. Shiotani, A. Adachi, K. Sakamaki, K. Okita, Y. Harima, Y. Kunugi, K. Yamashita, M. Ishikawa, Organometallics 19 (2000) 4492; (c) J. Ohshita, K. Yoshimoto, M. Hashimoto, D. Hamamoto, A. Kunai, Y. Harima, Y. Kunugi, K. Yamashita, M. Kakimoto, M. Ishikawa, J. Organomet. Chem. 665 (2003) 29; (d) J. Ohshita, D.-H. Kim, Y. Kunugi, A. Kunai, Organometallics 24 (2005) 4494; (e) D.-H. Kim, J. Ohshita, Y. Kunugi, A. Kunai, Chem. Lett. 35 (2006) 266; (f) J. Ohshita, Y. Izumi, D.-H. Kim, A. Kunai, T. Kosuge, Y. Kunugi, A. Naka, M. Ishikawa, Organometallics 26 (2007) 6150; (g) J. Ohshita, Y. Hatanaka, S. Matsui, T. Mizumo, Y. Kunugi, Y. Honsho, A. Saeki, S. Seki, J. Tibbelin, H. Ottosson, Dalton Trans. 39 (2010) 9314. [4] (a) S. Kyushin, M. Ikarugi, M. Goto, H. Hiratsuka, H. Matsumoto, Organometallics 15 (1996) 1067; (b) H. Maeda, Y. Inoue, H. Ishida, K. Mizuno, Chem. Lett. (2001) 1224. [5] (a) J. Ohshita, T. Uemura, D.-H. Kim, A. Kunai, Y. Kunugi, M. Kakimoto, Macromolecules 38 (2005) 730; (b) T. Sasan, H. Sogawa, K. Tamura, M. Shiotsuki, T. Maeda, F. Sanda, J. Polym. Sci. A Polym. Chem. 48 (2010) 1815; (c) A. Mori, E. Takahisa, Y. Yamamura, T. Kato, A.P. Mudalige, H. Kajiro, K. Hirabayashi, Y. Nishihara, T. Hiyama, Organometallics 23 (2004) 1755; (d) X.-H. Zhou, Y.-H. Niu, F. Huang, M.S. Liu, A.K.-Y. Jen, Macromolecules 40 (2007) 3015; (e) K.L. Paik, N.S. Baek, H.-K. Kim, J.-H. Lee, Y. Lee, Macromolecules 35 (2002) 6782. [6] (a) T.-Y. Luh, Y.-J. Chen, Chem. Commun. (2006) 4669; (b) T.-Y. Luh, Pure Appl. Chem. 82 (2010) 613; (c) J. Ohshita, R. Taketsugu, K. Hino, A. Kunai, J. Organomet. Chem. 691 (2006) 3065; (d) K. Takagi, H. Kakiuchi, Y. Yuki, M. Suzuki, J. Polym. Sci. A Polym. Chem. 45 (2007) 4786. [7] (a) X.-M. Liu, C. He, X.-T. Hao, L.-W. Tan, Y. Li, K.S. Ong, Macromolecules 34 (2004) 5965; (b) S.A. Ponomarenko, N.N. Rasulova, Y.N. Luponosov, N.M. Surin, M.I. Buzin, I. Leshchiner, S.M. Peregudova, A.M. Murafarov, Macromolecules 45 (2012) 2014. [8] M.-S. Choi, Tetrahedron Lett. 49 (2008) 7050. [9] S. Steinberger, A. Mishra, E. Reinold, C.M. Müller, C. Uhrich, M. Pfeiffer, P. Bäuerle, Org. Lett. 13 (2011) 90. [10] J. Ohshita, S. Kangai, H. Yoshida, A. Kunai, S. Kajiwara, Y. Ooyama, Y. Harima, J. Organomet. Chem. 692 (2007) 801. [11] (a) M. Fujitsuka, D.W. Cho, J. Ohshita, A. Kunai, T. Majima, J. Phys. Chem. C 110 (2006) 12446; (b) M. Fujitsuka, D.W. Cho, J. Ohshita, A. Kunai, T. Majima, J. Phys. Chem. C 111 (2007) 1993; (c) J. Ohshita, K. Kawashima, A. Iwata, H.-Q. Tang, M. Higashi, A. Kunai, Z. Naturforsch. B 59 (2004) 1332. [12] (a) O.V. Borshchev, S.A. Ponomarenko, N.M. Surin, M.M. Kaptyug, M.I. Buzin, A.P. Pleshkova, N.V. Demchenko, V.D. Myakushev, A.M. Muzafarov, Organometallics 26 (2007) 5165; (b) A. Naka, Y. Matsumoto, T. Itano, K. Hasegawa, T. Shimamura, J. Ohshita, A. Kunai, T. Takeuchi, M. Ishikawa, J. Organomet. Chem. 694 (2009) 346; (c) A. Naka, R. Fukuda, R. Kishimoto, Y. Yamashita, Y. Ooyama, J. Ohshita, M. Ishikawa, J. Organomet. Chem. 702 (2012) 67.