Sequential double click reactions: a highly efficient post-functionalization method for optoelectronic polymers

Abstract

A new post-functionalization method based on the Huisgen's 1,3-dipolar cycloaddition between azides and alkynes, followed by the atom-economic addition reaction between electron-rich alkynes and tetracyanoethylene (TCNE) was developed to introduce the donor–acceptor chromophores into polystyrene derivatives in remarkably high yields.

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The new concept of click chemistry has attracted widespread attention in polymer science.¹ The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is one of the most frequently employed reactions, which is capable of selectively combining two components in terms of covalent bonds in excellent yields.² In order to expand the applicability of this concept, other click chemistry-type reactions, such as the Diels–Alder reaction³ and thiol-ene reaction,⁴ have also been developed and their utility in polymer chemistry was demonstrated by application to post-functionalizations. However, these click reactions do not have specific advantages regarding applications to conjugated systems due to the disruption of effective conjugation length and the resulting poor optoelectronic properties of the products.⁵

Intramolecular donor–acceptor interactions often produce optoelectronically useful chromophores. Recently, we reported the efficient introduction of nonplanar donor–acceptor chromophores, donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs), in the main chains⁶ and side chains⁷ of aromaticpolymers by high-yielding addition reactions between electron-rich alkynes and a strong acceptor, tetracyanoethylene (TCNE), under mild conditions. In particular, the TCNE addition to the polymer side chains was quantitative at room temperature, and accordingly, this reaction⁸ satisfies most of the requirements for click chemistry, as suggested in the original review.² Additional advantages of this reaction over the conventional click reactions are that no metal catalysts are necessary and the products feature strong charge-transfer (CT) bands in the visible (and near-IR) region, potent redox activities, and related optoelectronic properties. For example, the excellent second-order⁹ and third-order¹⁰ nonlinear optical effects have already been reported for the small and dendritic molecular architectures.¹¹

In this Communication, we report the first successful introduction of this donor–acceptor chromophore into polystyrene side chains using the double click reactions based on the CuAAC and TCNE addition. The double click reactions of aliphatic polymers have in the past been investigated,¹² but it is very rare to realize optoelectronic polymers.

Poly(4-azidomethylstyrene) 1 , prepared from poly(4-chloromethlystyrene) and sodium azide, was employed as the precursor polymer.¹³ The molecular weight (M_n) and the polydispersity (M_w/M_n) were 11 500 and 1.75, respectively. The azide groups were reacted with the terminal alkynes of 2p or 2m in the presence of Cu(I) catalysts, yielding the tolane-appended polymers3p and 3m (Scheme 1). Note that 2p and 2m possess both terminal and internal alkynes, and the terminal alkynes selectively react under the CuAAC conditions. Gratifyingly, we found that when the reaction is performed in DMF, the resulting high molecular weight polymers are deposited from the solutions. Thus, a simple filtration of the deposited precipitates after the reaction for 24 h provided the desired polymers3p and 3m in the fairly good yield of 84%. Evaporation of the filtrate and the subsequent precipitation into a water–methanol (4 : 1) mixture afforded the residual desired polymers. The combined yields amounted to 98%. The GPC measurements revealed a reasonable molecular weight increase (Fig. 1). The M_n and M_w/M_n values are 52 300 and 1.55 for 3p and 49 000 and 1.85 for 3m, respectively. The slightly lower molecular weight and higher polydispersity of the m-phenylene-linked 3m as compared to the p-phenylene-linked 3p are supposed to reflect the different hydrodynamic volumes in the GPC measurements. The ¹H NMR spectra confirmed the chemical structures of both polymers (Fig. 2). The benzyl proton peak of 1 at 4.60 ppm completely shifted to 5.47 ppm for 3p and 5.30 ppm for 3m. Moreover, the dialkylaniline peaks appeared after the click modification. In the IR spectra of 3p and 3m, the vibrational peak at 2095 cm⁻¹ ascribed to the azide groups of 1 disappeared, whereas the alkyne vibrational peak at 2207 cm⁻¹ remained. These results are consistent with the quantitative addition, ensuring the use of CuAAC in click chemistry reactions.

Scheme 1 Sequential click post-functionalization of polystyrene derivative 1. (a) CuSO₄·5H₂O, sodium ascorbate, DMF; (b) TCNE, 1,2-dichloroethane.

Fig. 1 GPC profiles of (a) 1, 3p, and 4p and (b) 1, 3m, and 4m.

Fig. 2 ¹H NMR spectra of (a) 1, (b) 2p, (c) 3p and (d) 4p in CDCl₃ at 20 °C.

Subsequently, the TCNE addition to the polymers3p and 3m were examined. The titration experiments of 3p and 3m with TCNE were monitored by UV/Vis spectroscopy. New CT bands appeared, and the intensities increased with the increasing amount of the added TCNE, finally leading to completely red solutions (Fig. 3). The presence of the isosbestic points at 308 and 382 nm for 3p and at 306 and 377 nm for 3m indicates no side reactions during the TCNEclick reactions. The positions of the most intense CT bands (480.5 nm for 3p and 483 nm for 3m) were consistent with the monomeric TCBD structures⁸ and did not change during the experiments, suggesting negligible interactions between the chromophores.

Fig. 3 UV/Vis spectral changes of (a) 3p and (b) 3m upon titration with TCNE (0–1.0 equiv.) in chloroform at 20 °C.

The UV/Vis spectral results enabled us to prepare the full TCNEadduct polymers. The addition of the stoichiometric amount of TCNE, followed by evaporation of the solvents, quantitatively furnished the desired TCBD polymers, 4p and 4m, without any purification processes (Scheme 1). The molecular weights of the TCBD polymers were slightly increased from the precursor polymers, as shown in the GPC profiles (Fig. 1). Thus, the M_n values of 4p and 4m were 58 500 and 51 000, respectively. The polydispersities were almost similar to those of the precursors. In the ¹H NMR spectra, some peaks, such as the benzyl protons, became broader as compared to the precursors, implying the increased rotational barriers of part of the single bonds due to the bulky and twisted TCBD moieties (Fig. 2). The IR spectra also substantiated the chemical structures. A weak alkyne vibrational peak of the precursor tolane polymers was replaced by a strong cyano one at 2214 cm⁻¹. Moreover, the elemental analysis results of 4p and 4m showed a good agreement with the calculated values. All these results support the perfect post-functionalization by the alkyne-TCNEclick reactions.

In order to investigate the importance of the reaction order, we also tried the other double click reactions based on the first TCNE addition to 2p and 2m, followed by the CuAAC addition to 1 (Scheme 2). The TCNE addition to 2p and 2m quantitatively afforded the corresponding TCBD molecules, 5p and 5m, at room temperature. The terminal alkynes of 5p and 5m were chemically stable and therefore subjected to the reaction with 1 in the presence of Cu(I) catalysts. The resulting polymers, 6p and 6m, showed absorption spectral shapes similar to 4p and 4m, respectively, but the peak instensities were apparently lower (Fig. S1‡ ). Furthermore, the M_n values of 6p (42 800) and 6m (44 800) were lower than those of 4p and 4m, and the polydispersities were much higher (Table S1‡ ). All these results indicate that it is difficult to densely attach the bulky TCBD chromophores as the polystyrene side chain. Accordingly, the original route based on the first incorporation of the linear tolane side chains by CuAAC, followed by the TCNE addition is a more efficient method to construct the high density donor–acceptor side chains. Based on the measured molecular weights, the reaction order is more significant for the p-linked derivative than the m-linked analogue.

Scheme 2 Post-functionalization by TCNEclick reactions followed by CuAAC. (a) TCNE, 1,2-dichloroethane; (b) 1, CuSO₄·5H₂O, sodium ascorbate, DMF.

Another important feature of the donor–acceptor systems is the redox activities. The tolane polymers, 3p and 3m, displayed the only aniline-centered, quasi-reversible oxidation waves at 0.28 and 0.17 V (vs. Fc/Fc⁺), respectively, in the cyclic voltammograms (CVs) recorded in CH₂Cl₂ (+0.1 M (nC₄H₉)₄NClO₄) at 20 °C (Table S2 and Fig. S2‡ ). On the other hand, the TCBD polymers, 4p and 4m, showed anodically shifted oxdation potentials at 0.84 and 0.73 V, respectively, as well as the TCBD-centered reduction potentials at −0.91 and −0.98 V, respectively. In contrast to the small TCBD molecules showing two well-resolved reduction steps,⁸ the polymers displayed only the single reduction waves. The electrochemical band gaps of both 4p and 4m, calculated from the oxidation and reduction potentials, are ∼1.7 V. This value is comparable to the optical band gaps determined by the end absorptions (1.72 eV).

In conclusion, we have developed a highly efficient post-functionalization method using two different click reactions, which can be applicable to various polymers. The importance of the reaction order was rationally explained by the steric reasons. It was found that the alkyne-TCNEclick reactions proceeds even in sterically congested environments and provides the potent donor–acceptor chromophores in excellent yields. All the obtained polymers were thermally stable with decomposition temperatures exceeding 340 °C (Table S1‡ ). Combined with the high thermal stability, they are promising for a variety of optoelectronic device applications, which are our future projects.

This work was supported by a Grant-in-Aid for Scientific Research and the Special Coordination Funds for Promoting Science and Technology from MEXT, Japan.

Notes and references

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13. The azide content in the polymer was determined to be 96% by the elemental analysis.

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Footnotes

† All new molecules and polymers were fully characterized by IR, UV/Vis, ¹H and ¹³C NMR, mass spectrometry or GPC and elemental analysis.

‡ Electronic supplementary information (ESI) available: Synthetic procedures, polymer analyses, UV-Vis spectra, and electrochemistry results. See DOI: 10.1039/b9py00238c

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