Postfunctionalization of aromatic polyamine by [2+2] cycloaddition of 7,7,8,8-tetracyanoquinodimethane with side chain alkynes
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- Washino, Y., Murata, K. & Michinobu, T. Polym. Bull. (2012) 69: 137. doi:10.1007/s00289-011-0693-7
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Electron-rich, side chain alkynes of an aromatic polyamine were functionalized by a [2+2] cycloaddition, followed by retro-cyclization with the electron-accepting 7,7,8,8-tetracyanoquinodimethane (TCNQ). 1H NMR studies were used to optimize the reaction conditions. Mild heating to >50 °C afforded the postfunctionalized aromatic polyamines with the desired acceptor amounts. The quantitative TCNQ addition was demonstrated by the MALDI-TOF mass spectrum and elemental analysis. Introduction of the cyano-based acceptor moieties into the polymer side chains resulted in unusually strong intermolecular interactions. In addition to the π–π interactions of the extended acceptor moieties, these intermolecular forces were supposed to improve the thermal stability of the aromatic polymers. Furthermore, the donor–acceptor chromophores formed by this postfunctionalization displayed low energy charge-transfer bands and redox activities in both the anodic and cathodic directions. The straightforward postfunctionalization technique using the alkyne–TCNQ addition is useful for the preparation of narrow band gap polymers in one step.
KeywordsAromatic polyamineCharge transferElectrochemistryPostfunctionalization
Organic semiconducting polymers are an important class of materials for the realization of solution processible, low-cost and large area electronics . Examples include aromatic polyamines for hole-transporting layers in organic light-emitting devices (OLEDs) [2–4], regio-regular poly(3-alkylthiophene)s for p-type semiconductors in photovoltaic cells (PCs) [5, 6], and donor–acceptor type conjugated polymers for ambipolar type thin film transistors (TFTs) [7–10]. In contrast to a large number of p-type semiconducting polymers, there are few guidelines for constructing n-type polymers. Poly(benzobisimidazobenzophenanthroline) (BBL) is a commercially available n-type polymer, which shows an electron mobility (μe) of 0.1 cm2 V−1 s−1 . However, this polymer has a solubility problem due to its highly rigid ladder structure. It was recently demonstrated that thiophene-based conjugated polymers possessing imide moieties in the conjugated main chain display n-channel TFT performances . This successful result was achieved by the careful selection of monomer structures on the basis of computational modeling. These recent studies suggest that there is still room for pursuing new semiconducting polymers with controlled energy levels.
We have been trying to develop an experimental technique to control the polymer energy levels by adapting semiconducting polymer doping techniques [13–20]. The key reagent of this project is cyano-containing strong acceptor molecules, which are originally p-type doping reagents, but undergo [2+2] cycloaddition with electron-donor-activated alkynes [21, 22]. The cyclobutene intermediates formed by the [2+2] cycloaddition are smoothly transformed into donor–acceptor type products via ring opening. The important features of this reaction are high efficiency, no byproducts, and easy purification without chromatographic techniques, which are also the main criteria of “click chemistry” . For example, when one of the most compact acceptor molecules, tetracyanoethylene (TCNE), was added to a solution of the side chain alkyne-appended aromatic polyamine, the reaction quantitatively proceeded at room temperature and an intense red color ascribed to the intramolecular charge transfer of the product occurred . In other words, the LUMO level of the aromatic polyamine was lowered by this postfunctionalization reaction. It is important to note that there was a clear correlation between the polymer LUMO level and the added TCNE amount. A similar result was observed for the postfunctionalization of polythiophene derivatives . This study also suggested that the use of stronger acceptor molecules, such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives [26–34], is effective for the dramatic lowering of the LUMO levels or the enhancement of the n-type characteristics.
Based on these previous studies, we decided to apply the alkyne–TCNQ addition reaction to the postfunctionalization of the aromatic polyamine to create potent n-type polymers. The energy level decrease of the polyamine, as elucidated by its electrochemical redox potentials, was significant when compared to the TCNE-adducted case. The enhanced thermal stability by this postfunctionalization is also described.
Chemicals were purchased as reagent grade from Kanto, Tokyo Kasei, and Aldrich. The synthesis of P1 was previously reported .
1H and 13C NMR spectra were measured on a JEOL model AL300 spectrometer at 20 °C. Chemical shifts are reported in ppm downfield from SiMe4, using the solvent’s residual signal as an internal reference. Infrared (IR) spectra were recorded on a JASCO FT/IR-4100 spectrometer. MALDI-TOF mass spectra were measured on a Shimadzu AXIMA-CFR mass spectrometer. The operation was performed at an accelerating potential of 20 kV by a linear positive ion mode with dithranol as a matrix. Gel permeation chromatography (GPC) was measured on a JASCO system (PU-980, CO-965, RI-930, UV-970, and AS-950) equipped with polystyrene gel columns using THF as an eluent at the flow rate of 1.0 mL min−1 after calibration with standard polystyrenes. Elemental analysis was performed on a PerkinElmer 2400-Series H CHNS/O analyzer. Thermogravimetric analysis (TGA) was carried out on a Seiko SII TG/DTA 6200, at the heating rate of 10 °C min−1 between 20 °C and 500 °C. Differential scanning calorimetry (DSC) measurements were carried out on a Seiko SII DSC 6220 with a Seiko SII EXSTAR 6000 PC Station. UV–Vis–near infrared (NIR) spectra were recorded on a JASCO V-670 spectrophotometer. Fluorescence spectra were recorded on a JASCO FP6500 spectrophotometer. Cyclic voltammograms (CVs) were measured at 20 °C in a classical three-electrode cell. The working, reference, and auxiliary electrodes were a glassy carbon disk electrode (0.07 cm2), Ag/Ag+/CH3CN/(nC4H9)4NClO4, and Pt wire, respectively. All potentials were referenced to the ferrocene/ferricinium (Fc/Fc+) couple used as an internal standard.
To a solution of P1 (15 mg) in 1,2-dichloroethane (30 mL), a desired amount of TCNQ solution in 1,2-dichloroethane (1.5–1.7 mM) was added. After the mixture was stirred for 24 h at 80 °C, the solvent was removed in vacuo.
P2 (x + y = 1): 1H NMR (300 MHz, C6D6): δ = 0.80–1.24 (m, 30n H), 1.77 (br s, 4n H), 2.34–2.54 (m, 6n H), 6.23 (br s, 4n H), 6.75–7.75 ppm (m, 14n H); IR (KBr): ν = 2925, 2852, 2205, 1579, 1560, 1543, 1521, 1508, 1499, 1491, 1464, 1439, 1373, 1313, 1273, 1175, 942, 862, 819, 669 cm−1; Elemental analysis: calcd for (C57H58N6)n: C 82.77, H 7.07, N 10.16; found: C 82.30, H 7.16, N 9.70%.
Results and discussion
The clean postfunctionalization of the aromatic polyamine was achieved by the alkyne–TCNQ addition reaction. The quantitative reaction without any side products was demonstrated by a series of analytical methods including MALDI-TOF mass and elemental analyses. The postfunctionalized polyamines displayed significantly lowered energy levels elucidated by the electrochemical redox potentials. In particular, the dramatic decrease in the LUMO level led to the narrow band gap polymers with the electrochemical band gap of 1.14 eV. Further lowering of the LUMO levels or enhancement of the n-type characteristics will be possible through this postfunctional methodology using fluorinated TCNQ derivatives. Application of these n-type polymers to organic electronic devices is also worthwhile for future studies.
This work was supported, in part, by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.