Polymer Bulletin

, Volume 69, Issue 2, pp 137–147

Postfunctionalization of aromatic polyamine by [2+2] cycloaddition of 7,7,8,8-tetracyanoquinodimethane with side chain alkynes

Authors

  • Yusuke Washino
    • Department of Organic and Polymeric MaterialsTokyo Institute of Technology
  • Kimie Murata
    • Global Edge InstituteTokyo Institute of Technology
    • Global Edge InstituteTokyo Institute of Technology
    • PRESTO, Japan Science and Technology Agency (JST)
Original Paper

DOI: 10.1007/s00289-011-0693-7

Cite this article as:
Washino, Y., Murata, K. & Michinobu, T. Polym. Bull. (2012) 69: 137. doi:10.1007/s00289-011-0693-7
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Abstract

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.

Keywords

Aromatic polyamineCharge transferElectrochemistryPostfunctionalization

Introduction

Organic semiconducting polymers are an important class of materials for the realization of solution processible, low-cost and large area electronics [1]. Examples include aromatic polyamines for hole-transporting layers in organic light-emitting devices (OLEDs) [24], 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) [710]. 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 [11]. 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 [12]. 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 [1320]. 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” [23]. 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 [24]. 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 [25]. This study also suggested that the use of stronger acceptor molecules, such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives [2634], 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.

Experimental

Materials

Chemicals were purchased as reagent grade from Kanto, Tokyo Kasei, and Aldrich. The synthesis of P1 was previously reported [24].

General measurements

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.

Synthesis

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

Postfunctionalization

The precursor polyamine P1 was prepared by the Pd-catalyzed polycondensation between 2,7-dibromo-9,9-dioctylfluorene and a tolane-based aniline monomer [24]. The side chain alkynes were activated by both the terminal dimethylanilino groups and the main chain triarylamino moieties. Therefore, the postfunctionalization by TCNQ addition was first attempted in 1,2-dichloroethane at room temperature. However, in contrast to the TCNE addition, the reaction with TCNQ was slow. Mild heating to 50 °C gradually changed the solution color from yellow to green, suggesting the formation of donor–acceptor chromophores (Scheme 1). After the sufficient thermal stability of the products was confirmed (vide infra), the TCNQ adducts were prepared at 80 °C. It should be noted that the formed chromophores are regioisomeric mixtures, depending on the TCNQ orientation during the [2+2] cycloaddition reaction.
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Scheme 1

TCNQ addition to the side chain alkynes of P1

The postfunctionalization reaction was monitored by its 1H NMR spectra (Fig. 1). A mixture of P1 and an equivalent amount of TCNQ was dissolved into C6D6 and heated to 50 °C. As TCNQ was added to the polymer side chains, a new peak ascribed to the cyclohexa-1,4-diene rings appeared at 6.23 ppm. This peak intensity gradually increased and finally saturated after 24 h. The peak splitting of the methyl groups at 2.3–2.5 ppm supported the presence of the regioisomeric donor–acceptor moieties (vide supra). Also, peak broadening occurred as more TCNQ was added. This was never observed in the case of the TCNE addition, and accordingly, the bulky side chain chromophores probably induced the steric hindrance. In accordance with this observation, no clear peaks especially in the aromatic region were detected in the 13C NMR spectra of P2. The NMR experiments demonstrated the utility of this reaction in various research fields. Commercial reagent grade solvents can be employed without further purification. Additionally, a small amount of impurities, such as contaminated oxygen and water, do not interfere with the cycloaddition reaction, as shown by the water impurity in Fig. 1.
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Fig. 1

Reaction progress of P1 (2.85 mM repeat unit−1) and TCNQ monitored by 1H NMR spectra in C6D6 at 50 °C (a, 0 h; b, 10 h; c, 17.5 h; d, 24 h). The peak at 0.4 ppm is ascribed to water

The IR spectra also supported the postfunctionalization of P1 by the TCNQ addition. The weak vibrational peak at 2,209 cm−1 ascribed to the C≡C moieties of P1 was replaced by the C≡N vibrational one centered at 2,205 cm−1. The intensity of the new C≡N peak increased as more TCNQ was added. Moreover, the MALDI-TOF mass spectrum and elemental analysis of P2 (x + y = 1) provided strong evidence of the quantitative addition without any side reactions. Two sets of polymer peak profiles were detected for P2 (x + y = 1) (Fig. 2). The observed peak values and the intervals were in good agreement with the calculated molecular weights of the polymer structures. Molecular masses exceeding 10,000 were not clearly detected due to the limited performance of the instrument. The triangular set of peaks suggested that the termination process of the precursor polymer synthesis is debromination and concomitant hydrogenation. Also, the C, H, and N contents of P2 (x + y = 1) determined from the elemental analysis showed excellent agreement with the calculated values (see “Experimental”). All these results strongly support the quantitative TCNQ addition to P1.
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Fig. 2

MALDI-TOF mass spectrum of P2 (x + y = 1) (matrix: dithranol)

Although the MALDI-TOF mass spectrum demonstrated the clean postfunctionalization by TCNQ addition, the GPC profiles showed a different behavior. The number-average (Mn) and weight-average (Mw) molecular weights of P1, determined by GPC (THF eluent) in comparison to the calibrated standard polystyrenes, were 27,700 and 57,600, respectively (Fig. 3a). However, the Mn and Mw values estimated from the GPC charts of P2 gradually decreased with the increasing amount of the TCNQ addition, finally reaching the Mn of 18,000 and Mw of 27,000 for P2 (x + y = 1) (Fig. 3b). A similar decrease in the GPC molecular weights was previously reported for the postfunctionalization of the polystyrene derivatives bearing side chain alkynes [3537]. These results clearly suggest the strong adhesive feature of the cyano-containing chromophores to polystyrene gels. The adhesive property of the TCNQ adducts is most likely stronger than that of the TCNE adduct. Therefore, it is significant to consider the absolute molecular weights when the degree of postfunctionalization is evaluated.
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Fig. 3

a GPC charts of (a) P1, (b) P2 (x + y = 0.2), (c) P2 (x + y = 0.4), (d) P2 (x + y = 0.6), (e) P2 (x + y = 0.8), (f) P2 (x + y = 1) and b the calculated number-average and weight-average molecular weights (Mn and Mw) of the polyamines with different TCNQ addition amounts

Thermal analyses

As suggested from the GPC elution curves, the cyano-containing chromophroes induce strong dipolar-based intermolecular forces. It is previously reported that these molecular aggregation features lead to an enhancement of the polymeric thermal properties [38]. For example, the full TCNE addition to P1 improved the thermal decomposition temperature (Td) by 118 °C (291 °C → 409 °C) and glass transition temperature (Tg) by 27 °C (125 °C → 152 °C) [24]. In order to elucidate the difference between the TCNE and TCNQ, TGA and DSC measurements of P2 were performed under nitrogen at the scanning rate of 10 °C min−1. Figure 4a depicts the TGA curves of both P1 and P2 (x + y = 1). It is apparent that the TCNQ addition is effective for improving the thermal stability. The Td value of P2 (x + y = 1) was 363 °C, which is significantly higher than that of P1, but slightly lower than that of the TCNE adduct. However, the residual soot amount of P2 (x + y = 1) at 500 °C was 68%, which is approximately 6% greater than that of the TCNE adduct. Similar to the TGA results, the Tg values were also investigated by evaluating the second heating scans of the DSC curves. Since P2 (x + y = 1) did not exhibit any noticeable transition peaks, the TCNQ addition amount (x + y) was reduced. The sample P2 (x + y = 0.2) finally displayed the Tg of 150 °C (Fig. 4b), a value that is almost comparable to the full TCNE adduct. Collectively, this result suggested that the thermal improvement ability of the TCNQ adduct with the extended π-system is somewhat better than that of the compact TCNE adduct.
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Fig. 4

a Thermogravimetric analysis (TGA) curves of P1 and P2 (x + y = 1) under nitrogen flow at the heating rate of 10 °C min−1 and b differential scanning calorimetry (DSC) curves of P1 and P2 (x + y = 0.2) under nitrogen flow at the scanning rate of 10 °C min−1

Optical properties

Postfunctional TCNQ addition led to a dramatic change in their optical spectra due to the formation of the donor–acceptor chromophores. The precursor polyamine P1 possessed the lowest energy absorption maximum (λmax) at 403 nm in 1,2-dichloroethane (Fig. 5). When TCNQ was added to the polymer side chains, new low energy peaks with the λmax of 669 nm appeared and this peak intensity increased in proportion to the added TCNQ amount. The presence of an isosbestic point supported the absence of any side reactions, which was already demonstrated by the MALDI-TOF mass spectrum and elemental analysis (vide supra). The position of the λmax or charge-transfer (CT) band is caused by the combination of the donor and acceptor structures. Due to the more extended π-system of TCNQ, the λmax bathochromically shifted relative to the TCNE-adduct and the end absorption reached into NIR. However, due to the twisted donor–acceptor systems, the TCNQ-adducted polymers were only slightly fluorescent.
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Fig. 5

UV–Vis–NIR spectra of P1 and P2 with different TCNQ-adducted amounts in 1,2-dichloroethane

In order to highlight the difference in the optical properties after postfunctionalization, the absorption spectra of P1 and P2 (x + y = 1) were measured in various organic solvents, and the λmax values were plotted as a function of solvent polarity ET(30) (Fig. 6). The precursor polyamine P1 revealed no relationship between the absorption spectra and solvent polarity. However, similar to some small molecules and polymers containing this class of donor–acceptor chromophores [39, 40], P2 (x + y = 1) displayed a clear positive solvatochromism. This result indicates the greater dipole moment for P2, which is the origin of high performance nonlinear optics [41].
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Fig. 6

Relationship between the solvent polarity parameter ET(30) (tetrahydrofuran, 37.4; ethyl acetate, 38.1; chloroform, 39.1; dichloromethane, 41.1; acetone, 42.2; N,N-dimethylformamide, 43.8) and the λmax values of aP1 and bP2 (x + y = 1)

Electrochemistry

A change in both the HOMO and LUMO levels by this postfunctionalization was evaluated by cyclic voltammetry (CV) in CH2Cl2 with 0.1 M (nC4H9)4NClO4 at 20 °C. It is reported that the CV of P1 displays only reversible oxidation steps with the first oxidation potential (Eox,1) of 0.10 V (vs. Fc/Fc+) [24]. This Eox,1 value corresponds to the HOMO level of −4.90 eV based on the assumption of the Fc/Fc+ couple = −4.80 eV. For the TCNE addition, a gradual decrease in the polymer energy levels had been realized by the addition amount [24]. On the other hand, a small amount of TCNQ addition significantly lowered the energy levels, especially the LUMO level. Consequently, it was difficult to tune the energy levels of P1 using TCNQ. Figure 7 shows the typical CV curve of P2 (x + y = 1), accounting for the reversible oxidation and reduction peaks ascribed to the aromatic amino donor and TCNQ-based acceptor moieties, respectively. Due to the strong adhesive feature of P2 onto the working electrode, the anodic and cathodic measurements were conducted separately. The Eox,1 of P2 (x + y = 1) was anodically shifted to 0.37 V compared to P1 because of the efficient intramolecular donor–acceptor interactions. The first reduction potential (Ered,1) of P2 (x + y = 1) was clearly detected at −0.77 V. This potential is comparable to those of the small donor–acceptor molecules [4244], indicating that the side chain chromophores of P2 are almost independent of each other. This is probably caused not only by the sp3-hybridized nitrogen atoms in the polymer main chain, but also by the significant twist between the repeating units. The HOMO and LUMO levels of P2 (x + y = 1) calculated from the redox potentials were −5.17 and −4.03 eV, respectively. The electrochemical band gap (1.14 V in CH2Cl2) was consistent with the optical band gap determined by the end absorption (1,034 nm; 1.20 eV in 1,2-dichloroethane).
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Fig. 7

Cyclic voltammogram of P2 (x + y = 1) in CH2Cl2 with 0.1 M (nC4H9)4NClO4 at the scanning rate of 0.1 V s−1 under nitrogen

Conclusion

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.

Acknowledgments

This work was supported, in part, by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.

Copyright information

© Springer-Verlag 2011