Introduction

Since the pioneering work on conducting polyacetylene films,1 p-type doping of conjugated polymers has been investigated for creating conducting and magnetic organic polymers.2, 3, 4, 5 For example, the addition of p-type doping reagents, such as tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and their derivatives, to electron-rich polymers produces open-shell polymers because of the charge-transfer (CT) complexation.6, 7, 8, 9, 10 However, a similar approach using the combination of n-type doping reagents and electron-deficient polymers is rare, probably because of the difficulty of preparing precursor polymers, namely electron-deficient conjugated polymers, and the limited chemical stability of the n-doped polymers at room temperature.

To improve the chemical stability of doped polymers, chemical reactions using cyano-containing p-type doping reagents rather than redox reactions were recently proposed.11 The chemical reactions include the thermal [2+2] cycloaddition between electron-rich alkynes and electron-deficient ethene moieties of acceptor molecules, such as tetracyanoethylene and TCNQ, followed by ring opening of the cyclobutene intermediates formed, yielding the donor–acceptor chromophores, as exemplified in Scheme 1 (see ref. 12). Application of these reactions to polymer reactions corresponds to the post-functionalization of polymers. After post-functionalization, the resulting polymers are chemically stable because of their closed-shell structures, and they possess more intense CT bands than the intermolecular CT complexes because of the strong intramolecular donor–acceptor interactions. Such polymers are promising for optical applications, such as electro-optic materials. The post-functionalization of electron-rich polymers, namely aromatic polyamines, thiophene polymers and various anilino-functionalized polymers, has previously been demonstrated.13, 14, 15, 16, 17, 18

In this paper, the post-functionalization of electron-deficient polymers using the high-yielding addition reaction between electron-rich alkynes and TCNQ moieties is reported for the first time. Because it is known that the preparation of electron-deficient conjugated polymers is difficult, TCNQ polyester19 was used as an n-type precursor polymer and a dianilino-substituted acetylene molecule as a p-type reactant. A persistent CT band appeared in the low-energy region. A detailed comparison with the model reaction of small molecules suggested that a quantitative post-functionalization was achieved under mild reaction conditions.

Experimental procedure

Materials

All reagents were purchased from Kanto (Tokyo, Japan), Wako (Osaka, Japan), Tokyo Kasei (Tokyo, Japan) and Aldrich (St Louis, MO, USA), and used without further purification. Solvents for the reactions were distilled before use.

Measurements

1H and 13C nuclear magnetic resonance (NMR) spectra were measured on a JEOL (Tokyo, Japan) model AL300 spectrometer at 20 °C. Chemical shifts are reported in p.p.m. downfield from SiMe4, using the solvent's residual signal as an internal reference. The resonance multiplicity is described as s (singlet), d (doublet), t (triplet) and m (multiplet). Attenuated total reflectance Fourier transform infrared spectra were recorded on a JASCO FT/IR-4100 spectrometer (JASCO, Tokyo, Japan). Gel permeation chromatography was measured on a JASCO system equipped with polystyrene gel columns using tetrahydrofuran as an eluent at a flow rate of 1.0 ml min−1. Relative molecular weights were determined by comparison with the calibrated standard polystyrenes. Matrix-assisted laser desorption/ionization–time of flight mass spectrometry spectra were measured on a Shimadzu/Karatos (Kyoto, Japan) AXIMA-CFR mass spectrometer using dithranol as a matrix. Melting points (m.p.) were measured in open capillaries with a Büchi Melting Point apparatus (Büchi, Flawil, Switzerland) and are uncorrected. Elemental analysis was carried out on a PerkinElmer 2400-Series II CHNS/O Analyzer (PerkinElmer, Boston, MA, USA). Thermogravimetric analysis and differential scanning calorimetry were carried out on a Rigaku ThermoPlus TG8120 and DSC8230 (Rigaku, Tokyo, Japan), respectively, under flowing nitrogen at a scanning rate of 10 °C min−1. Ultraviolet-visible-near infrared spectra were recorded on a JASCO V-670 spectrophotometer at 20 °C. Cyclic voltammetry measurements were taken at 20 °C in dehydrated CH2Cl2 containing 0.1-M (nC4H9)4NClO4 in a classic three-electrode cell. The working, reference and auxiliary electrodes were a glassy carbon disk electrode (2 mm in diameter), Ag/AgCl/CH3CN/(nC4H9)4NPF6 and a platinum wire, respectively. All potentials are referenced to the ferricinium/ferrocene (Fc+/Fc) couple used as an internal standard.

Synthesis

[3,6-Bis(dicyanomethylidene)cyclohexa-1,4-diene-1,4-diyl]bis(oxyethane-2,1-diyl) dioctanoate (1)

To 2,2′-[2,5-bis(2-hydroxyethoxy)cyclohexa-2,5-diene-1,4-diylidene]dipropanedinitrile (5.5 mg, 17.0 μmol) in dehydrated dimethyl formamide (1 ml) was added octanoyl chloride (20 μl, 0.13 mmol), and the solution was stirred at 20 °C for 6.5 h. Column chromatography (SiO2, CH2Cl2/acetone 20:1) yielded the desired compound (5.1 mg, 52%).

M.p. 82–84 °C; 1H NMR (300 MHz, CDCl3, 293 K) δ 0.87 (t, J=9 Hz, 6 H), 1.29 (m, 16 H), 1.61 (m, 4 H), 2.34 (t, J=7.5 Hz, 4 H), 4.37–4.52 (m, 8 H), 6.46 p.p.m. (s, 2 H); 13C NMR (75 MHz, CDCl3, 293 K) δ 14.0, 22.5, 24.6, 28.8, 29.0, 31.6, 33.9, 60.5, 68.7, 82.8, 103.5, 112.6, 113.1, 147.5, 154.8, 173.5 p.p.m.; IR (neat): ν 2922, 2853, 2361, 2338, 2203, 1735, 1588, 1537, 1468, 1418, 1364, 1157, 942, 819 cm−1; matrix-assisted laser desorption/ionization–time of flight mass spectrometry (dithranol): m/z: calcd. for C32H40N4O6: 576.29 g mol−1; found: 575.63 g mol−1 [M−H]+; elemental analysis calcd. (%) for C32H40N4O6 (576.68): C 66.65, H 6.99, N 9.72; found: C 66.87, H 7.29, N 9.90.

[(3Z)-3{3,3-dicyano-1,2-bis[4-(dimethylamino)phenyl]prop-2-en-1-yliden}-6-(dicyanomethylidene)cyclohexa-1,4-diene-1,4-diyl]bis(oxyethane-2,1-diyl) dioctanoate (2)

Compound 1 (6.61 mg, 25.0 μmol) and 4,4′-(ethyne-1,2-diyl)bis(N,N-dimethylaniline) (14.4 mg, 25.0 μmol) in 1,2-dichloroethane (12 ml) were heated to 60 °C for 8 days. After cooling to 20 °C, the solvent was removed in vacuo, yielding the desired product (21.0 mg, quant.)

M.p. 152–153 °C; 1H NMR (300 MHz, CDCl3, 293 K) δ 0.87 (t, J=7 Hz, 6 H), 1.25–1.27 (m, 16 H), 1.60–1.62 (m, 4 H), 2.34 (t, J=7.5 Hz, 4 H), 3.08 (s, 6 H), 3.09 (s, 6 H), 3.92–4.45 (m, 8 H), 6.33 (s, 1 H), 6.55 (s, 1 H), 6.60 (d, J=9 Hz, 2 H), 6.68 (d, J=9 Hz, 2 H), 7.25 (d, J=9 Hz, 2 H), 7.72 p.p.m. (d, J=9 Hz, 2 H); 13C NMR (75 MHz, CDCl3, 293 K) δ 14.1, 22.6, 24.7, 28.91, 28.92, 29.0, 31.6, 33.9, 34.0, 39.9, 40.0, 53.4, 61.2, 61.3, 67.1, 67.6, 77.2, 83.8, 102.0, 110.5, 111.3, 111.9, 114.7, 115.5, 115.7, 116.2, 120.7, 124.7, 126.3, 132.2, 133.7, 149.4, 149.9, 152.0, 153.1, 158.3, 172.8, 173.7, 173.9 p.p.m.; IR (neat): ν 2922, 2852, 2361, 2338, 2203, 1736, 1588, 1537, 1492, 1418, 1364, 1157, 942, 820 cm−1; matrix-assisted laser desorption/ionization–time of flight mass spectrometry (dithranol): m/z: calcd. for C50H60N6O6: 840.5 g mol−1; found: 838.8 g mol−1 [M−H]+; elemental analysis calcd. (%) for C50H60N6O6 (841.05): C 71.40, H 7.19, N 9.99; found: C 69.20, H 7.11, N 9.49.

TCNQ polyester (P1)

2,2′-[2,5-Bis(2-hydroxyethoxy)cyclohexa-2,5-diene-1,4-diylidene]dipropanedinitrile (302 mg, 0.932 mmol) and sebacoyl chloride (0.200 ml, 0.937 mmol) were stirred in dehydrated N,N-dimethylacetamide (10 ml) at 20 °C for 48 h. The solution was poured into MeOH, and the precipitate was collected. Reprecipitation into MeOH yielded the desired TCNQ polyester (302 mg, 92%).

Gel permeation chromatography (tetrahydrofuran) Mw 5000, Mn 4400; 1H NMR (300 MHz, CDCl3, 293 K) δ 1.27 (s, 8n H), 1.59 (m, 4n H), 2.36 (t, J=7.5 Hz, 4n H), 4.00–4.80 (m, 8n H), 6.45 p.p.m. (s, 2n H); 13C NMR (75 MHz, CDCl3, 293 K) δ 24.6, 28.9, 29.0, 33.9, 60.6, 68.8, 82.8, 103.6, 112.7, 113.2, 147.6, 154.9, 173.5 p.p.m.; IR (neat): ν 2915, 2849, 2359, 2217, 1727, 1562, 1527, 1457, 1369, 1332, 1239, 1173, 1153, 1042, 993, 843, 801, 719 cm−1; elemental analysis calcd. (%) for (C26H26N4O6)n: C 63.66, H 5.34, N 11.42; found: C 62.82, H 5.22, N 11.42.

Donor–acceptor polyester (P2)

P1 (51.6 mg, 0.105 mmol) and 4,4′-(ethyne-1,2-diyl)bis(N,N-dimethylaniline) (27.1 mg, 0.103 mmol) in 1,2-dichloroethane (7 ml) were heated to 80 °C for 3 days. After cooling to 20 °C, the solvent was removed in vacuo, yielding the desired polymer (78.7 mg, quant.).

Gel permeation chromatography (tetrahydrofuran) Mw 4500, Mn 3700; 1H NMR (300 MHz, CDCl3, 293 K) δ 1.26 (m, 8n H), 2.31 (m, 4n H), 3.06 (s, 6 H), 3.07 (s, 6 H), 3.61 (m, 4n H), 3.93–4.23 (m, 8n H), 6.30 (s, n H), 6.54–6.89 (m, 5n H), 7.25 (s, 2n H), 7.69 p.p.m. (s, J=4.4 Hz, 2n H); 13C NMR (75 MHz, CDCl3, 293 K) δ 24.7, 29.0, 29.1, 29.6, 33.8, 34.0, 39.9, 40.0, 40.3, 61.2, 61.3, 67.1, 67.6, 77.2, 102.0, 110.7, 111.3, 111.9, 114.8, 115.6, 115.7, 116.3, 120.6, 124.6, 126.3, 133.8, 134.0, 149.4, 149.7, 150.0, 152.1, 153.1, 158.4, 172.8, 173.6, 173.8 p.p.m.; IR (neat): ν 2925, 2359, 2341, 2202, 1732, 1586, 1530, 1487, 1414, 1362, 1333, 1207, 1155, 1060, 943, 819, 731, 650 cm−1; elemental analysis calcd. (%) for (C44H46N6O6)n: C 70.01, H 6.14, N 11.13; found: C 68.21, H 6.27, N 10.58.

Results and Discussion

Synthesis and characterization

A precursor polymer, the TCNQ polyester, was prepared by polycondensation between sebacoyl chloride and the commercially available TCNQ derivative with two OH groups, according to a previous report on a similar TCNQ polyester synthesis19 (Scheme 2). Although the previous report did not describe the molecular weights of the TCNQ polymers obtained, the molecular weights of P1 were measured by gel permeation chromatography. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were 5000 and 4400, respectively, corresponding to a monomer repeat unit (n) of ∼8. The relatively low polydispersity (Mw/Mn) suggests the partial adsorption of P1 on the polystyrene gel.

A monomer unit 1 was also prepared for comparison with P1. The addition reaction of 1 with Me2NPhC≡CPhNMe2 was first conducted in CH2Cl2 at 20 °C. A slight coloration occurred after stirring overnight, but no isolatable amount of 2 was obtained. On the basis of the quantitative addition yield of Me2NPhC≡CPhNMe2 to TCNQ at 20 °C (see ref. 20), it was probably caused by the decreased reactivity of the TCNQ moiety because of the substitution of the two electron-donating alkyloxy groups. Replacement of the solvent by 1,2-dichloroethane and slight heating started to change the solution color from orange to green. After a sufficient reaction time, the desired product 2 was quantitatively obtained. On the basis of this knowledge, the post-functionalization of P2 was attempted. The heating of P2 and Me2NPhC≡CPhNMe2 in 1,2-dichloroethane to 80 °C (near boiling point) gradually changed the solution color, implying that the reaction was occurring. The time-dependent spectral change in the ultraviolet-visible-near infrared absorption was monitored (Figure 1a). P1 possessed absorption maxima at 434 and 412 nm, which are ascribed to the dialkyloxy-substituted TCNQ moieties. When the post-functionalization occurred, the intensity of these peaks started to increase and they fused into a single broad peak. In contrast, the weak peaks at <360 nm decreased. The presence of the isosbestic point at 360 nm indicated that there were no undesired side reactions during post-functionalization. In addition, a well-defined low-energy band appeared at 630 nm, which is ascribed to the CT transition from the dimethylanilino donors to the cyano-based acceptor moieties. The intensity of this CT band was plotted against the reaction time (Figure 1b). The plots suggest that the reaction was almost completed after 30 h, and prolongation did not result in any degradation of the donor–acceptor chromophores formed.

Figure 1
figure 1

(a) Ultraviolet-visible-near infrared spectral change during the preparation of P2 from P1 and an equivalent amount of 4,4′-(ethyne-1,2-diyl)bis(N,N-dimethylaniline) in 1,2-dichloroethane at 80 °C. (b) Plots of time versus intensities of the charge-transfer band at 630 nm.

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The obtained donor–acceptor chromophores were fully characterized. In the 1H and 13C NMR spectra of 2, all peaks were assigned to the single chemical structure, despite the possibilities of E- and Z-isomers (Figures 2a and c). To estimate the relative stability of these isomers, computational calculations were carried out for the simplified model structures at the ωB97XD/6-31G* level (see Supplementary Information). These calculations suggested that the Z-form is energetically preferred by 11.4 kJ mol−1 compared with the E-form. Therefore, the obtained product is assumed to be the Z-isomer. The matrix-assisted laser desorption/ionization–time of flight mass spectrometry and elemental analysis were also consistent with the chemical structure. The 1H NMR spectrum of P2 was similar to that of 2, although some peaks were broader because of the polymer effect (Figure 2b). The number of 13C NMR peaks in the aromatic region agreed with that of 2 or with the chemical structure of the monomer repeat unit (Figure 2d). The IR spectra of P1 and P2 revealed the difference in the cyano vibrational peak positions. The cyano peak of P1 detected at 2217 cm−1 completely disappeared after the post-functionalization, and a new cyano peak appeared at 2202 cm−1 (Figure 3). This shift is consistent with the previous report.15 All these results support the clean post-functionalization of the TCNQ polyester by an addition reaction with electron-rich alkyne molecules.

Figure 2
figure 2

1H nuclear magnetic resonance (NMR) spectra of (a) 2 and (b) P2 in CDCl3 at 20 °C and the aromatic region of 13C NMR spectra of (c) 2 and (d) P2 in CDCl3 at 20 °C.

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Figure 3
figure 3

Attenuated total reflectance Fourier transform infrared spectra (neat) of (a) P1 and (b) P2.

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Thermal properties

The obtained polymers were chemically and thermally stable. The thermogravimetric analysis of P1 and P2 was carried out at a heating rate of 10 °C min−1 under flowing nitrogen. Both polymers showed no decomposition at least up to 200 °C, suggesting that the high thermal stability was retained after the post-functionalization (Figure 4a). This result is consistent with the previous results for the post-functionalization of p-type-conjugated polymers.13, 17 The residual soot amount of P2 (63%) was greater than that of P1 (53%), reflecting the aromatic contents in the polymers. The enhanced intermolecular interactions of P2 with additional aromatic rings were implied by the melting points of the small molecules (1: 82–84 °C, 2: 152–153 °C). The weak intermolecular interactions based on the π–π interactions and dipolar interactions might also be associated with the increased amount of soot at 500 °C. Both polymers P1 and P2 displayed a glass transition temperature (Tg) at 87 and 123 °C, respectively, in the differential scanning calorimetry thermograms (Figure 4b). It is thought that the higher Tg of P2 is also derived from the enhanced intermolecular interactions.

Figure 4
figure 4

(a) Thermogravimetric analysis and (b) differential scanning calorimetry thermograms (second heating) of P1 and P2 at the scanning rate of 10 °C min−1 under nitrogen flow.

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Optical and electrochemical measurements

The formation of stable CT bands was evaluated by ultraviolet-visible-near infrared absorption spectra in various solvents, and the energy levels were estimated from the end absorptions and electrochemical redox potentials.

As shown in Figure 1a, the addition reaction produced a low-energy band in the ultraviolet-visible-near infrared region. These bands were chemically persistent in various solvents, and the λmax values are summarized as a function of solvent polarity ET(30) (Table 1). Similar to many donor–acceptor chromophores prepared by this class of addition reactions,21, 22, 23 both 2 and P2 displayed a positive solvatochromism with a correlation coefficient of ∼0.90 (Figure 5). In contrast, the λmax values of the precursors 1 and P1 did not show such behaviors.

Table 1 Summary of λmax values in different solvents at 20 °C
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Figure 5
figure 5

Relationship between the solvent polarity parameter ET(30) and the charge-transfer band maxima of 2 and P2.

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To compare the chemical stability of the CT bands with those prepared by the conventional n-type doping, complexation of the TCNQ polyester P1 with tetrathiafulvalene was attempted. However, P1 did not react with tetrathiafulvalene even when the solution was heated to the boiling point of CHCl3, probably because of the cathodic shift of Ered,1 of the TCNQ moieties relative to the unsubstituted TCNQ (Ered,1=−0.25 V) (see ref. 24; vide infra).

Redox potentials are often used to estimate the energy levels of aromatic compounds. Cyclic voltammograms of the polymers and model monomers were measured in CH2Cl2 containing 0.1-M (nC4H9)4NClO4 at 20 °C. Figure 6 shows the typical cyclic voltammogram curves, and the redox potentials are summarized in Table 2. The redox behaviors of the polymers were basically the same as those of the corresponding monomers. The precursors displayed the only two reversible reduction steps at the first reduction potential (Ered,1) of −0.46 and the second reduction potential (Ered,2) of −0.93 V (vs Fc+/Fc) for 1 and at Ered,1 of −0.47 and Ered,2 of −0.96 V for P1, ascribed to the TCNQ moieties (Figures 6a and c). After the addition reaction with the electron-rich alkyne molecule, the reduction potentials cathodically shifted, and the current intensity of Ered,2 significantly decreased. In addition, a new irreversible oxidation peak (Eox,1) ascribed to the dimethylanilino groups appeared at 0.34 V for 2 and at 0.19 V for P2. Different peak top values of Eox,1 might reflect the stronger adsorption features of P2 to the working electrode. The calculated electrochemical band gaps (Δ∣Eox,1−Ered,1∣) of 2 and P2 were 1.24 and 1.11 V, respectively, which are consistent with the optical band gaps determined by the end absorption in CH2Cl2. The energy levels estimated from the redox potentials and optical band gaps indicated that both HOMO and LUMO levels were elevated after the addition reaction. In contrast to the acceptor addition to the p-type precursor polymers resulting in a marked decrease in the LUMO levels,11, 25 the addition of electron-rich alkyne molecules to the n-type precursor polymers leads to a more significant elevation of HOMO levels than LUMO levels.

Figure 6
figure 6

Cyclic voltammograms of (a) 1, (b) 2, (c) P1 and (d) P2 in CH2Cl2 with 0.1-M (nC4H9)4NClO4 at the scan rate of 0.1 V s−1 under nitrogen.

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Table 2 Summary of electrochemical and optical measurements
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Conclusion

The post-functional preparation of persistent intramolecular CT interactions in the n-type precursor polymer was achieved using the addition reaction between the electron-rich alkynes and TCNQ moiety. The quantitative conversion ensures a facilitated preparation protocol without special purification, such as column chromatography. The conventional analytical methods and a comparison with the model reaction of the monomer moiety demonstrated the high purity of the post-functionalized polymer. A sufficiently high thermal stability with the onset decomposition temperature of ∼200 °C was retained even after post-functionalization. The electrochemical and optical measurements clearly revealed the formation of intramolecular CT bands, accompanying the marked change in energy levels. With designable electron-rich alkyne molecules, the TCNQ polymers can serve as a reactive functional polymer for preparing various CT energies.

scheme 1

Reaction mechanism of a high-yield addition reaction between alkynes activated by electron-donating groups (EDGs) and 7,7,8,8-tetracyanoquinodimethane.

scheme 2

Synthesis and reaction of 7,7,8,8-tetracyanoquinodimethane diester and polyester. (a) Octanoyl chloride, dimethyl formamide, 20 °C, 6.5 h, 52%; (b) Me2NPhC≡CPhNMe2, 1,2-dichloroethane, 60 °C, 8 days, 100%; (c) sebacoyl chloride, N,N-dimethylacetamide, 20 °C, 48 h; (d) Me2NPhC≡CPhNMe2, 1,2-dichloroethane, 80 °C, 3 days.