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Synthesis of selenopheno[3,2-c]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers
Synthetic Metals 161 (2011) 1444–1447
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis of selenopheno[3,2-c]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers Yu-Ra Jo a , Soo-Hyoung Lee b , Youn-Sik Lee b , Yun-Hwa Hwang c , Myoungho Pyo c,∗ , Kyukwan Zong a a b c
Institute of Fusion Science, Division of Science Education, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Chemical Engineering, Nanomaterials Processing Center, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Printed Electronics Engineering in World Class University Program, Sunchon National University, Chonnam 540-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 March 2011 Received in revised form 9 April 2011 Accepted 12 April 2011 Available online 14 May 2011 Keywords: Selenopheno[3,2-c]thiophene Bandgap Conjugated polymer NIR HOMO
a b s t r a c t Various selenopheno[3,2-c]thiophene (STh) derivatives functionalized by 4-n-butylphenyl, 4-npentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and visiblenear infrared (Vis-NIR) spectrophotometry. When compared with polymers of thieno[3,4-b]thiophene, poly(selenopheno[3,2-c]thiophene) (PSTh) was more easily oxidized by ca. 0.2 V (higher HOMO). On the other hand, the bandgap of electrochemically prepared PSTh varied with substituents, but showed similar values to those of poly(thieno[3,4-b]thiophene). While the resonance effect of phenyl substituents slightly lowered HOMO, the combination of resonance and inductive effects decreased LUMO more effectively, resulting in the lowest bandgap of 0.91 eV for 4-tert-butylphenyl functionalized PSTh. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The -conjugated polymers have received great attention recently as promising low bandgap polymers due to potential applications in various emerging fields such as transparent electrodes [1], OLEDs [2], DSSCs [3], and OFET [4,5]. The employment of low bandgap conjugated polymers in polymer solar cells (PSCs) was also studied and has been proven successful due to absorption of a wide range of solar spectrum, greatly improving power conversion efficiencies [6–8]. For example, bulk heterojunction PSCs based on thieno[3,4-b]thiophene units showed much improved power conversion efficiencies up to over 6% [6–8]. The stable quinoidal structure from thieno[3,4-b]thiophene resulted in a low bandgap of the polymer, which can cover the terrestrial solar spectrum from red to near infrared to harvest the maximum photon flux [9]. However, the reduction of the bandgap by raising the polymer HOMO level may not give a great impact on improving the photovoltaic efficiency, because the high HOMO level may hamper the efficient hole transport. Therefore, lowering the polymer LUMO level in obtaining the low bandgap, rather than raising the HOMO level, is more desirable. Recently, Heeney et al. reported that regioregular poly(3-hexyl)selenophene exhibited the decrease of the bandgap while maintaining the polymer HOMO level similar to that of poly(3-hexyl)thiophene [10].
∗ Corresponding author. E-mail address: [email protected] (M. Pyo). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.04.014
In this paper, we wish to report a new synthetic method for selenopheno[3,2-c]thiophene (STh) derivatives which can be easily reproduced in moderate yields. STh is a heavy homologue of thieno[3,4-b]thiophene, in which one heteroatom of a fused ring is substituted with selenium. In the previous studies of polyselenophene derivatives, it has been reported that the replacement of sulfur by selenium can profoundly affect the electronic, redox, and optical properties of polymers [11]. The involvement of filled 3d orbitals and decrease of electronegativity increased a conductivity and reduced a bandgap. In this regard, a concise synthesis of new STh monomers functionalized by 4-n-butylphenyl, 4-npentylphenyl, 4-tert-butyl, and octyl are reported. Optical and electrochemical properties of the polymers derived from them are also described. 2. Experimental STh of various substituents with different inductive and resonant effects were prepared. The starting materials (1) were prepared according to Ref. [13]. Synthetic routes for monomers were summarized in Fig. 1. 2.1. Typical procedure for preparation of 2 To a suspension of selenium powder (0.22 g, 2.75 mmol) in THF was added sodium borohydride (0.10 g, 2.75 mmol) at ice-bath temperature. After the reaction mixture was stirred for 50 min, NMP (60 mL) and 1 (1.57 mmol) was added. The reaction mixture
Y.-R. Jo et al. / Synthetic Metals 161 (2011) 1444–1447
R Br
1445
R Se
Na2Se, CuO(NP) o
S 1
NMP, 190 C S 2 n-C5H11
n-C4H9
R= (A)
(B)
tert-C4H9 (C)
n-C8H17 (D)
Fig. 1. Synthetic routes of selenopheno[3,2-c]thiophene derivatives.
was stirred at 185–190 ◦ C for 12 h, and THF was distilled off to a trap bulb during the reaction was progressed. The mixture was cooled to room temperature and poured into saturated aqueous ammonium chloride (100 mL) solution. The resulting solution was extracted with diethyl ether (3× 30 mL) and the combined organic layers was washed with water, and dried over anhydrous MgSO4 . The solution was concentrated by rotary evaporator and the residue was purified by chromatography on silica gel using hexane/ethyl acetate as the eluent. 2.1.1. 2-(4-Butylphenyl)selenopheno[3,2-c]thiophene (2A) A white solid; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.46 (d, J = 8.4 Hz, 2H), 7.36 (d, J = 2.4 Hz, 1H), 7.32 (s, 1H), 7.22 (d, J = 2.4 Hz, 1H), 7.19 (d, J = 8.4 Hz, 2H), 2.62 (t, J = 7.2 Hz, 2H), 1.59 (m, 2H), 1.36 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H); 13 C NMR (100 MHz, CDCl3 ) ␦: 150.1, 143.5, 134.5, 133.9, 128.9, 128.7, 126.4, 126.2, 114.9, 114.2, 35.4, 33.5, 22.3, 13.9; HRMS (ESI) m/z calculated for C16 H16 SSe: 319.3, found: 320.0. 2.1.2. 2-(4-Pentylphenyl)selenopheno[3,2-c]thiophene (2B) A pale yellow solid; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.45 (d, J = 8 Hz, 2H), 7.36 (d, J = 2.4 Hz, 1H), 7.32 (s, 1H), 7.22 (d, J = 2.4 Hz, 1H), 7.19 (d, J = 8.4 Hz, 2H), 2.61 (t, J = 8 Hz, 2H), 1.62 (m, 2H), 1.34–1.25 (m, 4H), 0.90 (t, J = 6.8 Hz, 3H); 13 C NMR (100 MHz, CDCl3 ) ␦: 150.7, 150.1, 143.6, 134.5, 134.3, 133.9, 128.9, 126.4, 114.9, 114.2, 35.7, 31.5, 31.0, 22.5, 14.0; HRMS (ESI) m/z calculated for C17 H18 SSe: 333.4, found: 334.0. 2.1.3. 2-(4-tert-Butylphenyl)selenopheno[3,2-c]thiophene (2C) A white solid; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.50 (d, J = 8.4 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.37 (d, J = 2.4 Hz, 1H), 7.34 (s, 1H), 7.23 (d, J = 2.4 Hz, 1H), 1.34 (s, 9H); 13 C NMR (100 MHz, CDCl3 ) ␦: 151.7, 150.6, 150.1, 134.6, 133.7, 126.2, 125.8, 115.0, 114.3, 114.2, 34.7, 31.2; HRMS (ESI) m/z calculated for C16 H16 SSe: 319.3, found: 321.0.
Table 1 Employed substituents and isolated yields of 2. R
n-C4H9
A
41
n-C5H11
B
50
tert-C4H9
C D
Yields of 2 (%)
–n-C8 H17
25 53
2.1.4. 2-Octylselenopheno[3,2-c]thiophene (2D) A pale yellow oil; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.20 (d, J = 2.4 Hz, 1H), 7.16 (d, J = 2.4 Hz, 1H), 6.80 (s, 1H), 2.80 (t, J = 7.2 Hz, 2H), 1.67 (p, J = 7.2 Hz, 2H), 1.2–1.40 (m, 10H), 0.88 (t, J = 6.8 Hz, 3H); 13 C NMR (100 MHz, CDCl3 ) ␦: 154.5, 149.5, 135.0, 116.3, 114.0, 112.5, 34.1, 31.9, 31.0, 29.4, 29.2, 29.1, 22.7, 14.1; HRMS (ESI) m/z calculated for C14 H20 SSe: 299.3, found: 301.1. 2.2. Electrochemical and spectroscopic characterization The monomers (2A–2D) were electrochemically deposited on Pt or ITO (Delta Tech. CG-41 IN-CUV) by scanning potentials in mixed solvents (acetonitrile/CH2 Cl2 = 1/1) containing 50 mM tetrabutylammonium tetrafluoroborate (TBABF4 ). Cyclic voltammograms of polymer films were obtained in the same electrolyte solutions free from monomers. Optical bandgap of polymers prepared on ITO was recorded at various potentials using a UV-Vis-NIR spectrophotometer (JASCO V-670). The reference electrode was Ag wire and all the potentials were converted with respect to NHE using the ferrocene/ferrocenium couple. The potential was controlled via a potentiostat (BAS CV-50W).
Table 2 Fundamental properties of monomers and corresponding polymers.
A B C D a b c d e
monomer
polymer
Eox (V)a
E1/2 b (V)
HOMOc (eV)
LUMOc (eV)
Eg d (eV)
Eg e (eV)
0.82 0.81 0.89 0.82
0.52 0.56 0.45 0.29
−4.49 −4.52 −4.56 −4.28
−3.30 −3.28 −3.43 –
1.19 1.24 1.13 –
0.93 0.93 0.91 1.06
Onset of monomer oxidation. (Epa + Epc )/2 for p-doping. Onset of polymer p-doping and n-doping. LUMO–HOMO. Onset of NIR absorption.
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Y.-R. Jo et al. / Synthetic Metals 161 (2011) 1444–1447
6
3.1. Synthesis of monomers
4
In our previous work, we reported a convenient synthetic method of thieno[3,4-b]thiophene derivatives by reacting 3bromo-4-alkynylthiophene with Na2 S in the presence of catalytic amount of CuO nanoparticles. We assumed that this synthetic protocol may be extended over the synthesis of STh as a analog of thieno[3,4-b]thiophene. In our delight, the reaction of in situ generated 3-bromo-4-alkynylthiophene with Na2 Se proceeded to give the STh derivatives in a moderate yield (Table 1). In these reactions, it was turned out that the use of CuO nanoparticle was crucial for STh ring closure. The CuO nanoparticles has been known to be efficient in cross-coupling reactions of the C–N, C–O, and C–S [12]. The catalytic role of nanoparticles was considered mainly due to high surface area and dispersibility. Without CuO nanoparti-
2
Current (mA cm-2)
3. Results and discussion
A D
B C
0 -2 -4 -6 -0.6
-0.4
-0.2
0
0.2
Potenal (V vs NHE)
0.4
0.6
0.8
1
Fig. 2. Cyclic voltammograms of polymer 2 deposited on Pt in a mixed solvent (acetonitrile/CH2 Cl2 = 1/1) containing 50 mM TBABF4 . Scan rate = 50 mV/s.
Fig. 3. (top) Cyclic voltammograms and (bottom) Vis-NIR spectra of polymer 2. Voltammograms were obtained at the same condition as in Fig. 2. For Vis-NIR spectra, the potential was successively changed from −0.35 V to −0.15, +0.05, +0.25, +0.45, and +0.65 V (direction of arrow).
Y.-R. Jo et al. / Synthetic Metals 161 (2011) 1444–1447
cle or other types of Cu catalysts, the reaction was very sluggish and suffered from low yields unlike previously reported method [13]. 3.2. Electrochemical characterization The onset of monomer oxidation potentials was compared in TBABF4 by scanning potentials at 50 mV/s. The monomers began to be oxidized in a range of 0.8–0.9 V as listed in Table 2. The monomer 2C showed a little higher oxidation potential due to enhanced stabilization by t-butyl substituents. The resonance stabilization effect resulting from phenyl groups, however, was not significant because the monomer 2D demonstrated the similar oxidation potential to those of 2A and 2B. The cyclic voltammograms of polymer 2A–2D, electrochemically synthesized on Pt by potential cycling, were compared for p-doping and shown in Fig. 2. Unlike monomer oxidation, the resonance stabilization is likely to strongly affect a position of the HOMO level. Fig. 2 exhibits that polymer 2D is more easily pdoped than polymer 2A–2C by ca. 0.25 V, indicative of lowering the HOMO level by phenyl groups. E1/2 also shows a similar trend to the variation of HOMO levels, as listed in Table 2. When compared with poly(thieno[3,4-b]thiophene) derivatives [14], these changes of E1/2 values with substituents are identical, but E1/2 of poly(selenopheno[3,2-c]thiophene) (PSTh) derivatives is ca. 0.2 V lower than those of corresponding poly(thieno[3,4-b]thiophene) derivatives (i.e., higher HOMO for efficient hole transport in PSCs, as mentioned above). Apparently, this is due to the substitution of sulfur with selenium in a fused ring. The bandgap (Eg ) was also determined electrochemically. Cyclic voltammetry of polymers was obtained in a mixed solvent (acetonitrile/CH2 Cl2 = 1/1) containing 50 mM TBABF4 and tiny amount of NaH at ambient conditions. Fig. 3 demonstrates cyclic voltammograms of polymers covering p- and n-doping potential ranges, along with corresponding Vis-NIR spectra. Interestingly, while PSTh functionalized with phenyl groups (polymer 2A–2C) showed distinct n-doping redox processes, polymer 2D did not seem to experience n-doping in a potential window examined (−1.95 V to +0.75 V vs. NHE). The electrochemically determined LUMO level and bandgap are compared in Table 2. As expected, alkyl substituents on a para-position of phenyl groups only slightly affected the LUMO position. Polymer 2C revealed the lowest LUMO level as in a trend of the HOMO levels due to relatively strong inductive effect, resulting in a similar bandgap to polymer 2A and 2B. 3.3. Spectroscopic characterization The optical bandgap of polymer 2A–2D was also investigated by Vis-NIR spectroscopy. Fig. 3 demonstrates Vis-NIR spectrum changes of polymers when the potentials were successively controlled from −0.35 V to +0.65 V with a 0.20 V difference (Spectra changed to the direction of arrows.). It is obvious that onset of NIR absorption for polymer 2A–2C in a fully neutral state at −0.35 V begins to occur at ca. 1330–1360 nm (Eg = 0.91–0.93 eV), which are similar to those of corresponding poly(thieno[3,4-b]thiophene) derivatives [14]. On the other hand, the bandgap of n-octyl substituted PSTh (polymer 2D) is distinctively larger than those of other polymers (2A–2C). This behavior was also the case in poly(thieno[3,4-b]thiophene) [14], in which alkyl substitution on fused rings raised a bandgap of resulting polymers by ca. 0.1 eV.
1447
Note that these optically determined values are smaller than the electrochemically determined bandgap in Table 2. This behavior (the electrochemically determined bandgap slightly larger than the optically estimated value) has been reported previously for other conjugated polymers [15,16], although the reason has not been disclosed yet. 4. Conclusion STh derivatives functionalized by 4-n-butylphenyl, 4-npentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical studies of polymers revealed that PSTh possessed the similar bandgap to corresponding poly(thieno[3,4-b]thiophene) derivatives, but significantly higher HOMO and LUMO, indicating that PSTh could be a promising hole transport material in PSCs. While the bandgap of PSTh functionalized with 4-alkylphenyl was in a range of 0.91–0.93 eV, PSTh derivatized with 4-alkyl showed the bandgap of 1.06 eV due to a lack of resonance stabilization. In all cases, however, PSTh derivatives were more easily p-doped than poly(thieno[3,4-b]thiophene) by 0.2 V, implying that both the HOMO and LUMO levels of PSTh were higher than those of poly(thieno[3,4-b]thiophene) homologues. Acknowledgments This work was supported in part by WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-10022). This work was also supported in part by the project of Regional Innovation Center (RIC) at Sunchon National University. References [1] K. Lee, G. Sotzing, Macromolecules 34 (2001) 5746. [2] Y. Yang, R.T. Farley, T.T. Steckler, S.H. Eom, J.R. Reynolds, K.S. Schanze, J. Xue, Appl. Phys. Lett. 93 (2008) 1633051. [3] Y. Cho, H. Kim, M. Oh, M.S. Lah, Y. Majima, K.-S. Sohn, M. Pyo, J. Electrochem. Soc. 158 (2011) B106. [4] T.T. Steckler, X. Zhang, J. Hwang, R. Honeyager, S. Ohira, X.H. Zhang, A. Grant, S. Ellinger, S.A. Odom, D. Sweat, D.B. Tanner, A.G. Rinzler, S. Barlow, J.L. Brédas, B. Kippelen, S.R. Marder, J.R. Reynolds, J. Am. Chem. Soc. 131 (2009) 2824. [5] Y.M. Kim, E. Lim, I.N. Kang, B.-J. Jung, J. Lee, B.W. Koo, L.-M. Do, H.-K. Shim, Macromolecules 39 (2006) 4081. [6] Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray, L. Yu, J. Am. Chem. Soc. 131 (2009) 7792. [7] J. Hou, H.-Y. Chen, S. Zhang, R.I. Chen, Y. Yang, Y. Wu, G. Li, J. Am. Chem. Soc. 131 (2009) 15586. [8] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photon. 3 (2009) 649. [9] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 109 (2009) 5868. [10] M. Heeney, W. Zhang, D.J. Crouch, M.L. Chabinyc, S. Gordeyev, R. Hamilton, S.J. Higgins, I. McCulloch, P.J. Skabara, D. Sparrowe, S. Tierney, Chem. Commun. (2007) 5061. [11] S.C. Ng, H.S.O. Chan, T.T. Ong, K. Kumara, Y. Mazaki, K. Kobayashi, Macromolecules 31 (1998) 1221. [12] S. Jammi, S. Sakthivel, L. Rout, T. Mukherjee, S. Mandal, R. Mitra, P. Saha, T. Punniyamurthy, J. Org. Chem. 74 (2009) 1971. [13] T. Kashiki, S. Shinamura, M. Kohara, E. Miyazaki, K. Takimiya, M. Ikeda, H. Kuwabara, Org. Lett. 11 (2009) 2473. [14] W.S. Lee, M.-J. Baek, M. Pyo, K. Zong, Synth. Met. 160 (2010) 1368. [15] P.M. Beaujuge, S.V. Vasilyeva, S. Ellinger, T.D. McCarley, J.R. Reynolds, Macromolecules 42 (2009) 3694. [16] P.M. Beaujuge, J. Subbiah, K.R. Choudhury, S. Ellinger, T.D. McCarley, F. So, J.R. Reynolds, Chem. Mater. 22 (2010) 2093.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis of selenopheno[3,2-c]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers Yu-Ra Jo a , Soo-Hyoung Lee b , Youn-Sik Lee b , Yun-Hwa Hwang c , Myoungho Pyo c,∗ , Kyukwan Zong a a b c
Institute of Fusion Science, Division of Science Education, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Chemical Engineering, Nanomaterials Processing Center, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Printed Electronics Engineering in World Class University Program, Sunchon National University, Chonnam 540-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 March 2011 Received in revised form 9 April 2011 Accepted 12 April 2011 Available online 14 May 2011 Keywords: Selenopheno[3,2-c]thiophene Bandgap Conjugated polymer NIR HOMO
a b s t r a c t Various selenopheno[3,2-c]thiophene (STh) derivatives functionalized by 4-n-butylphenyl, 4-npentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and visiblenear infrared (Vis-NIR) spectrophotometry. When compared with polymers of thieno[3,4-b]thiophene, poly(selenopheno[3,2-c]thiophene) (PSTh) was more easily oxidized by ca. 0.2 V (higher HOMO). On the other hand, the bandgap of electrochemically prepared PSTh varied with substituents, but showed similar values to those of poly(thieno[3,4-b]thiophene). While the resonance effect of phenyl substituents slightly lowered HOMO, the combination of resonance and inductive effects decreased LUMO more effectively, resulting in the lowest bandgap of 0.91 eV for 4-tert-butylphenyl functionalized PSTh. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The -conjugated polymers have received great attention recently as promising low bandgap polymers due to potential applications in various emerging fields such as transparent electrodes [1], OLEDs [2], DSSCs [3], and OFET [4,5]. The employment of low bandgap conjugated polymers in polymer solar cells (PSCs) was also studied and has been proven successful due to absorption of a wide range of solar spectrum, greatly improving power conversion efficiencies [6–8]. For example, bulk heterojunction PSCs based on thieno[3,4-b]thiophene units showed much improved power conversion efficiencies up to over 6% [6–8]. The stable quinoidal structure from thieno[3,4-b]thiophene resulted in a low bandgap of the polymer, which can cover the terrestrial solar spectrum from red to near infrared to harvest the maximum photon flux [9]. However, the reduction of the bandgap by raising the polymer HOMO level may not give a great impact on improving the photovoltaic efficiency, because the high HOMO level may hamper the efficient hole transport. Therefore, lowering the polymer LUMO level in obtaining the low bandgap, rather than raising the HOMO level, is more desirable. Recently, Heeney et al. reported that regioregular poly(3-hexyl)selenophene exhibited the decrease of the bandgap while maintaining the polymer HOMO level similar to that of poly(3-hexyl)thiophene [10].
∗ Corresponding author. E-mail address: [email protected] (M. Pyo). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.04.014
In this paper, we wish to report a new synthetic method for selenopheno[3,2-c]thiophene (STh) derivatives which can be easily reproduced in moderate yields. STh is a heavy homologue of thieno[3,4-b]thiophene, in which one heteroatom of a fused ring is substituted with selenium. In the previous studies of polyselenophene derivatives, it has been reported that the replacement of sulfur by selenium can profoundly affect the electronic, redox, and optical properties of polymers [11]. The involvement of filled 3d orbitals and decrease of electronegativity increased a conductivity and reduced a bandgap. In this regard, a concise synthesis of new STh monomers functionalized by 4-n-butylphenyl, 4-npentylphenyl, 4-tert-butyl, and octyl are reported. Optical and electrochemical properties of the polymers derived from them are also described. 2. Experimental STh of various substituents with different inductive and resonant effects were prepared. The starting materials (1) were prepared according to Ref. [13]. Synthetic routes for monomers were summarized in Fig. 1. 2.1. Typical procedure for preparation of 2 To a suspension of selenium powder (0.22 g, 2.75 mmol) in THF was added sodium borohydride (0.10 g, 2.75 mmol) at ice-bath temperature. After the reaction mixture was stirred for 50 min, NMP (60 mL) and 1 (1.57 mmol) was added. The reaction mixture
Y.-R. Jo et al. / Synthetic Metals 161 (2011) 1444–1447
R Br
1445
R Se
Na2Se, CuO(NP) o
S 1
NMP, 190 C S 2 n-C5H11
n-C4H9
R= (A)
(B)
tert-C4H9 (C)
n-C8H17 (D)
Fig. 1. Synthetic routes of selenopheno[3,2-c]thiophene derivatives.
was stirred at 185–190 ◦ C for 12 h, and THF was distilled off to a trap bulb during the reaction was progressed. The mixture was cooled to room temperature and poured into saturated aqueous ammonium chloride (100 mL) solution. The resulting solution was extracted with diethyl ether (3× 30 mL) and the combined organic layers was washed with water, and dried over anhydrous MgSO4 . The solution was concentrated by rotary evaporator and the residue was purified by chromatography on silica gel using hexane/ethyl acetate as the eluent. 2.1.1. 2-(4-Butylphenyl)selenopheno[3,2-c]thiophene (2A) A white solid; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.46 (d, J = 8.4 Hz, 2H), 7.36 (d, J = 2.4 Hz, 1H), 7.32 (s, 1H), 7.22 (d, J = 2.4 Hz, 1H), 7.19 (d, J = 8.4 Hz, 2H), 2.62 (t, J = 7.2 Hz, 2H), 1.59 (m, 2H), 1.36 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H); 13 C NMR (100 MHz, CDCl3 ) ␦: 150.1, 143.5, 134.5, 133.9, 128.9, 128.7, 126.4, 126.2, 114.9, 114.2, 35.4, 33.5, 22.3, 13.9; HRMS (ESI) m/z calculated for C16 H16 SSe: 319.3, found: 320.0. 2.1.2. 2-(4-Pentylphenyl)selenopheno[3,2-c]thiophene (2B) A pale yellow solid; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.45 (d, J = 8 Hz, 2H), 7.36 (d, J = 2.4 Hz, 1H), 7.32 (s, 1H), 7.22 (d, J = 2.4 Hz, 1H), 7.19 (d, J = 8.4 Hz, 2H), 2.61 (t, J = 8 Hz, 2H), 1.62 (m, 2H), 1.34–1.25 (m, 4H), 0.90 (t, J = 6.8 Hz, 3H); 13 C NMR (100 MHz, CDCl3 ) ␦: 150.7, 150.1, 143.6, 134.5, 134.3, 133.9, 128.9, 126.4, 114.9, 114.2, 35.7, 31.5, 31.0, 22.5, 14.0; HRMS (ESI) m/z calculated for C17 H18 SSe: 333.4, found: 334.0. 2.1.3. 2-(4-tert-Butylphenyl)selenopheno[3,2-c]thiophene (2C) A white solid; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.50 (d, J = 8.4 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.37 (d, J = 2.4 Hz, 1H), 7.34 (s, 1H), 7.23 (d, J = 2.4 Hz, 1H), 1.34 (s, 9H); 13 C NMR (100 MHz, CDCl3 ) ␦: 151.7, 150.6, 150.1, 134.6, 133.7, 126.2, 125.8, 115.0, 114.3, 114.2, 34.7, 31.2; HRMS (ESI) m/z calculated for C16 H16 SSe: 319.3, found: 321.0.
Table 1 Employed substituents and isolated yields of 2. R
n-C4H9
A
41
n-C5H11
B
50
tert-C4H9
C D
Yields of 2 (%)
–n-C8 H17
25 53
2.1.4. 2-Octylselenopheno[3,2-c]thiophene (2D) A pale yellow oil; 1 H NMR (400 MHz, CDCl3 ) ␦: 7.20 (d, J = 2.4 Hz, 1H), 7.16 (d, J = 2.4 Hz, 1H), 6.80 (s, 1H), 2.80 (t, J = 7.2 Hz, 2H), 1.67 (p, J = 7.2 Hz, 2H), 1.2–1.40 (m, 10H), 0.88 (t, J = 6.8 Hz, 3H); 13 C NMR (100 MHz, CDCl3 ) ␦: 154.5, 149.5, 135.0, 116.3, 114.0, 112.5, 34.1, 31.9, 31.0, 29.4, 29.2, 29.1, 22.7, 14.1; HRMS (ESI) m/z calculated for C14 H20 SSe: 299.3, found: 301.1. 2.2. Electrochemical and spectroscopic characterization The monomers (2A–2D) were electrochemically deposited on Pt or ITO (Delta Tech. CG-41 IN-CUV) by scanning potentials in mixed solvents (acetonitrile/CH2 Cl2 = 1/1) containing 50 mM tetrabutylammonium tetrafluoroborate (TBABF4 ). Cyclic voltammograms of polymer films were obtained in the same electrolyte solutions free from monomers. Optical bandgap of polymers prepared on ITO was recorded at various potentials using a UV-Vis-NIR spectrophotometer (JASCO V-670). The reference electrode was Ag wire and all the potentials were converted with respect to NHE using the ferrocene/ferrocenium couple. The potential was controlled via a potentiostat (BAS CV-50W).
Table 2 Fundamental properties of monomers and corresponding polymers.
A B C D a b c d e
monomer
polymer
Eox (V)a
E1/2 b (V)
HOMOc (eV)
LUMOc (eV)
Eg d (eV)
Eg e (eV)
0.82 0.81 0.89 0.82
0.52 0.56 0.45 0.29
−4.49 −4.52 −4.56 −4.28
−3.30 −3.28 −3.43 –
1.19 1.24 1.13 –
0.93 0.93 0.91 1.06
Onset of monomer oxidation. (Epa + Epc )/2 for p-doping. Onset of polymer p-doping and n-doping. LUMO–HOMO. Onset of NIR absorption.
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6
3.1. Synthesis of monomers
4
In our previous work, we reported a convenient synthetic method of thieno[3,4-b]thiophene derivatives by reacting 3bromo-4-alkynylthiophene with Na2 S in the presence of catalytic amount of CuO nanoparticles. We assumed that this synthetic protocol may be extended over the synthesis of STh as a analog of thieno[3,4-b]thiophene. In our delight, the reaction of in situ generated 3-bromo-4-alkynylthiophene with Na2 Se proceeded to give the STh derivatives in a moderate yield (Table 1). In these reactions, it was turned out that the use of CuO nanoparticle was crucial for STh ring closure. The CuO nanoparticles has been known to be efficient in cross-coupling reactions of the C–N, C–O, and C–S [12]. The catalytic role of nanoparticles was considered mainly due to high surface area and dispersibility. Without CuO nanoparti-
2
Current (mA cm-2)
3. Results and discussion
A D
B C
0 -2 -4 -6 -0.6
-0.4
-0.2
0
0.2
Potenal (V vs NHE)
0.4
0.6
0.8
1
Fig. 2. Cyclic voltammograms of polymer 2 deposited on Pt in a mixed solvent (acetonitrile/CH2 Cl2 = 1/1) containing 50 mM TBABF4 . Scan rate = 50 mV/s.
Fig. 3. (top) Cyclic voltammograms and (bottom) Vis-NIR spectra of polymer 2. Voltammograms were obtained at the same condition as in Fig. 2. For Vis-NIR spectra, the potential was successively changed from −0.35 V to −0.15, +0.05, +0.25, +0.45, and +0.65 V (direction of arrow).
Y.-R. Jo et al. / Synthetic Metals 161 (2011) 1444–1447
cle or other types of Cu catalysts, the reaction was very sluggish and suffered from low yields unlike previously reported method [13]. 3.2. Electrochemical characterization The onset of monomer oxidation potentials was compared in TBABF4 by scanning potentials at 50 mV/s. The monomers began to be oxidized in a range of 0.8–0.9 V as listed in Table 2. The monomer 2C showed a little higher oxidation potential due to enhanced stabilization by t-butyl substituents. The resonance stabilization effect resulting from phenyl groups, however, was not significant because the monomer 2D demonstrated the similar oxidation potential to those of 2A and 2B. The cyclic voltammograms of polymer 2A–2D, electrochemically synthesized on Pt by potential cycling, were compared for p-doping and shown in Fig. 2. Unlike monomer oxidation, the resonance stabilization is likely to strongly affect a position of the HOMO level. Fig. 2 exhibits that polymer 2D is more easily pdoped than polymer 2A–2C by ca. 0.25 V, indicative of lowering the HOMO level by phenyl groups. E1/2 also shows a similar trend to the variation of HOMO levels, as listed in Table 2. When compared with poly(thieno[3,4-b]thiophene) derivatives [14], these changes of E1/2 values with substituents are identical, but E1/2 of poly(selenopheno[3,2-c]thiophene) (PSTh) derivatives is ca. 0.2 V lower than those of corresponding poly(thieno[3,4-b]thiophene) derivatives (i.e., higher HOMO for efficient hole transport in PSCs, as mentioned above). Apparently, this is due to the substitution of sulfur with selenium in a fused ring. The bandgap (Eg ) was also determined electrochemically. Cyclic voltammetry of polymers was obtained in a mixed solvent (acetonitrile/CH2 Cl2 = 1/1) containing 50 mM TBABF4 and tiny amount of NaH at ambient conditions. Fig. 3 demonstrates cyclic voltammograms of polymers covering p- and n-doping potential ranges, along with corresponding Vis-NIR spectra. Interestingly, while PSTh functionalized with phenyl groups (polymer 2A–2C) showed distinct n-doping redox processes, polymer 2D did not seem to experience n-doping in a potential window examined (−1.95 V to +0.75 V vs. NHE). The electrochemically determined LUMO level and bandgap are compared in Table 2. As expected, alkyl substituents on a para-position of phenyl groups only slightly affected the LUMO position. Polymer 2C revealed the lowest LUMO level as in a trend of the HOMO levels due to relatively strong inductive effect, resulting in a similar bandgap to polymer 2A and 2B. 3.3. Spectroscopic characterization The optical bandgap of polymer 2A–2D was also investigated by Vis-NIR spectroscopy. Fig. 3 demonstrates Vis-NIR spectrum changes of polymers when the potentials were successively controlled from −0.35 V to +0.65 V with a 0.20 V difference (Spectra changed to the direction of arrows.). It is obvious that onset of NIR absorption for polymer 2A–2C in a fully neutral state at −0.35 V begins to occur at ca. 1330–1360 nm (Eg = 0.91–0.93 eV), which are similar to those of corresponding poly(thieno[3,4-b]thiophene) derivatives [14]. On the other hand, the bandgap of n-octyl substituted PSTh (polymer 2D) is distinctively larger than those of other polymers (2A–2C). This behavior was also the case in poly(thieno[3,4-b]thiophene) [14], in which alkyl substitution on fused rings raised a bandgap of resulting polymers by ca. 0.1 eV.
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Note that these optically determined values are smaller than the electrochemically determined bandgap in Table 2. This behavior (the electrochemically determined bandgap slightly larger than the optically estimated value) has been reported previously for other conjugated polymers [15,16], although the reason has not been disclosed yet. 4. Conclusion STh derivatives functionalized by 4-n-butylphenyl, 4-npentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical studies of polymers revealed that PSTh possessed the similar bandgap to corresponding poly(thieno[3,4-b]thiophene) derivatives, but significantly higher HOMO and LUMO, indicating that PSTh could be a promising hole transport material in PSCs. While the bandgap of PSTh functionalized with 4-alkylphenyl was in a range of 0.91–0.93 eV, PSTh derivatized with 4-alkyl showed the bandgap of 1.06 eV due to a lack of resonance stabilization. In all cases, however, PSTh derivatives were more easily p-doped than poly(thieno[3,4-b]thiophene) by 0.2 V, implying that both the HOMO and LUMO levels of PSTh were higher than those of poly(thieno[3,4-b]thiophene) homologues. Acknowledgments This work was supported in part by WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-10022). This work was also supported in part by the project of Regional Innovation Center (RIC) at Sunchon National University. References [1] K. Lee, G. Sotzing, Macromolecules 34 (2001) 5746. [2] Y. Yang, R.T. Farley, T.T. Steckler, S.H. Eom, J.R. Reynolds, K.S. Schanze, J. Xue, Appl. Phys. Lett. 93 (2008) 1633051. [3] Y. Cho, H. Kim, M. Oh, M.S. Lah, Y. Majima, K.-S. Sohn, M. Pyo, J. Electrochem. Soc. 158 (2011) B106. [4] T.T. Steckler, X. Zhang, J. Hwang, R. Honeyager, S. Ohira, X.H. Zhang, A. Grant, S. Ellinger, S.A. Odom, D. Sweat, D.B. Tanner, A.G. Rinzler, S. Barlow, J.L. Brédas, B. Kippelen, S.R. Marder, J.R. Reynolds, J. Am. Chem. Soc. 131 (2009) 2824. [5] Y.M. Kim, E. Lim, I.N. Kang, B.-J. Jung, J. Lee, B.W. Koo, L.-M. Do, H.-K. Shim, Macromolecules 39 (2006) 4081. [6] Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray, L. Yu, J. Am. Chem. Soc. 131 (2009) 7792. [7] J. Hou, H.-Y. Chen, S. Zhang, R.I. Chen, Y. Yang, Y. Wu, G. Li, J. Am. Chem. Soc. 131 (2009) 15586. [8] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photon. 3 (2009) 649. [9] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 109 (2009) 5868. [10] M. Heeney, W. Zhang, D.J. Crouch, M.L. Chabinyc, S. Gordeyev, R. Hamilton, S.J. Higgins, I. McCulloch, P.J. Skabara, D. Sparrowe, S. Tierney, Chem. Commun. (2007) 5061. [11] S.C. Ng, H.S.O. Chan, T.T. Ong, K. Kumara, Y. Mazaki, K. Kobayashi, Macromolecules 31 (1998) 1221. [12] S. Jammi, S. Sakthivel, L. Rout, T. Mukherjee, S. Mandal, R. Mitra, P. Saha, T. Punniyamurthy, J. Org. Chem. 74 (2009) 1971. [13] T. Kashiki, S. Shinamura, M. Kohara, E. Miyazaki, K. Takimiya, M. Ikeda, H. Kuwabara, Org. Lett. 11 (2009) 2473. [14] W.S. Lee, M.-J. Baek, M. Pyo, K. Zong, Synth. Met. 160 (2010) 1368. [15] P.M. Beaujuge, S.V. Vasilyeva, S. Ellinger, T.D. McCarley, J.R. Reynolds, Macromolecules 42 (2009) 3694. [16] P.M. Beaujuge, J. Subbiah, K.R. Choudhury, S. Ellinger, T.D. McCarley, F. So, J.R. Reynolds, Chem. Mater. 22 (2010) 2093.