Conjugated Polymers Based on Tautomeric Units: Regulation of Main

Article pubs.acs.org/Macromolecules

Conjugated Polymers Based on Tautomeric Units: Regulation of Main-Chain Conjugation and Expression of Aggregation Induced Emission Property via Boron-Complexation Ryousuke Yoshii,† Kazuo Tanaka,† and Yoshiki Chujo*,† †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: To understand the contribution of the tautomeric units to the π-conjugation through the polymer main-chain, the characteristics of the conjugated polymers containing the ketoimine moiety were investigated. Though the ketoimine skeleton usually forms the enaminoketone structure, the electronic structure in the ketoimine units can be fixed in enolimine form by boron complexation. By employing this fixing effect, we sought to regulate the electronic structure and evaluate the change in degree of main-chain conjugation. The polymerization was executed in Suzuki−Miyaura coupling reactions with the ketoimine or the boron ketoiminate monomers. The characterization and the determination of the structures of the products were performed with NMR spectrometry. The optical and electrochemical properties were examined by UV−vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The degree of main-chain conjugation was evaluated from the peak shifts in the absorption and emission bands. Initially, it was observed that the conjugated system intrinsically extended even through the enaminoketone structure in which the main-chain conjugation should be inhibited. In addition, as we expected, it was indicated that the boron complexation to the ketoimine units can contribute to the extension of π-conjugation through the main-chain. Furthermore, it was found that the boron ketoiminate polymers exhibited aggregationinduced emission properties.

■

INTRODUCTION π-Conjugated polymers have received a great deal of attention from both academic and industrial researchers. Their high charge-carrier abilities and prominent optical properties make them key materials for the developments of advanced opto and/or electronic devices such as organic light emitting diodes (OLEDs),1 photovoltaic cells (PVCs),2 field effect transistors,3 electrochromic devices,4 electrochemical supercapacitors,5 and switch elements.6 Since their electronic and optical characteristics are crucially dependent on the conjugated system through the main-chain, drastic changes can be induced to material properties if the extension of π-conjugations can be regulated. For example, Kawai and Irie et al. have established the tenability of photoluminescence and an electrical conductivity of a diarylethene-based conjugated polymer by controlling the extension of the π-conjugation in the polymer via a photoisomerization of the diarylethene moiety.7 Thus, it can be said that conjugated polymers having a tunable main-chain πconjugated system by external stimuli or chemical modifications are promising for application in advanced optoelectronic devices. However, a few examples have been reported about the conjugated polymers with a tunability of the conjugated system except for the diarylethene-based units. Therefore, the development of tunable π-conjugate systems is still of great significance not only for evolving the functionality of the © 2014 American Chemical Society

conventional devices but also for producing next generation materials. Tautomeric molecules are an important class of the building blocks to synthesize stimuli-responsive molecules for thermochromic8 and solvatochromic9 materials. Tautomerization can induce the drastic changes in electronic structures of πconjugated polymers, leading to the expression of distinct dual functions from the single material. For example, Reynolds et al. have presented that salicylideneaniline-based compounds showed the associated behavior with the light induced tautomerization,10a and Masuda et al. have reported that salicylideneaniline-based polymers exhibited multicolor emission depending on excitation wavelength due to an excited-state intramolecular proton transfer.10b,c These results propose that the tautomeric units can work as a switch for the regulation of the conjugated system through the polymer main-chains, resulting in the changes of their electronic properties. Meanwhile, there are very few reports on the main-chain type conjugated polymers involving the tautomeric units. In particular, the transforming methods from the initial tautomer to another one should be also needed for realizing the Received: January 10, 2014 Revised: March 16, 2014 Published: March 25, 2014 2268

dx.doi.org/10.1021/ma500082e | Macromolecules 2014, 47, 2268−2278

Macromolecules

Article

Scheme 1. Chemical Structures of the Tautomers of the Ketoimine Moiety and Effects of the Tautomerization Induced by the Boron Complexation

Figure 1. ORTEP drawings of (a) KI (structure I) and (b) BKI. (c) UV−vis absorption spectra of KI and BKI in THF (c = 1 × 10−5 M). (d) PL spectra of KI and BKI (c = 5.0 × 10−5 M) in THF (dashed line, water content: 0%) and THF/H2O (=1:9) mixed solvent (solid line, water content: 90%) upon the excitation at each absorption maximum; (inset) photographs of BKI in THF (water content: 0%) and in the mixed solvent of THF/ H2O (water content: 90%) under UV-irradiation.

highest thermal stability of three tautomers.11 In contrast, after the boron complexation, the electronic structures of the ketoiminates have the enolimine-like one.12,13 These results propose that the alteration of electronic structure in ketoimine can be induced by the introduction of boron. Thereby, the boron complexation at the ketoimine skeleton is expected to be a trigger for the extension of the conjugated system in the polymer main-chain. In addition, boron diketonate derivatives involving boron ketoiminate are known as an important class of

regulation of the conjugated system using tautomeric systems. Even though the existence ratios of each tautomeric form are dominated by the thermal equilibrium, the transformation should be enforced, followed by the formation of the single conformation. To satisfy these demands, we employed the boron-complexation chemistry with the ketoimine skeleton. Ketoimine derivatives can form three possible tautomeric structures: ketoimine, enolimine and enaminoketone forms (Scheme 1). It is known that the enaminoketone form has the 2269

dx.doi.org/10.1021/ma500082e | Macromolecules 2014, 47, 2268−2278

Macromolecules

Article

organoboron dyes because they possess prominent photoproperties such as large molar absorption coefficients and high photoluminescence quantum yields.14 We have also reported unique optical properties of boron ketoiminate derivatives and boron-containing polymers.13,14c−f For instance, we have recently presented the boron ketoiminate derivatives exhibited aggregation−induced emission (AIE) characteristics.13 Thus, we have large possibility to obtain the AIE-active materials based on the boron ketoiminate materials. Herein, we focused on π-conjugated polymers containing tautomeric units to realize a tunable main-chain π-conjugated system. Initially, we show that the boron complexation of ketoimine is valid for changing the electronic structure from the enaminoketone form to the enolimine form. On the basis of these data, we designed and synthesized the series of the ketoimine-based conjugated polymers. Their optical and electronic properties were examined by UV−vis absorption spectrometry, photoluminescence spectrometry and cyclic voltammetry. These results showed that the conjugated system was constructed even through the ketoimine units having the enaminoketone form which should be incapable to expand main-chain conjugations. In addition, the boron ketoiminatebased polymers have more extended main-chain conjugations than those of ketoimine polymers because the electronic structure in the ketoimine units is fixed in the enolimine form which should be capable to expand main-chain conjugations by the boron complexation. Furthermore, the boron ketoiminate polymers also exhibited AIE properties resulting from the boron complexation.

Table 1. Optical Properties of Synthesized Compounds

KI BKI FP FM-a FM-b FBP FBM-a FBM-b TP TM-a TM-b TBP TBM-a TBM-b

λabs,max [nm]a

λabs,onset [nm]a

Eg [eV]b

Ered [V]c,d

HOMO [eV]e

LUMO [eV]f

368 368 416 391 402 432 425 397 430 406 412 449 425 416

412 426 461 431 444 492 464 464 484 444 458 525 488 484

3.01 2.91 2.69 2.88 2.79 2.52 2.67 2.67 2.56 2.79 2.71 2.36 2.54 2.56

−2.09 −1.48 −2.08g −1.98 −2.07 −1.29 −1.46 −1.46 −2.07g −1.9g −2.04 −1.22 −1.41 −1.46

−5.50 −6.04 −5.32 −5.48 −5.30 −5.84 −5.82 −5.82 −5.20 −5.51 −5.12 −5.59 −5.58 −5.55

−2.49 −3.13 −2.63 −2.60 −2.51 −3.32 −3.15 −3.15 −2.64 −2.72 −2.41 −3.23 −3.04 −2.99

a Measured in THF (c = 1.0 × 10−5 mol/L). bCalculated from the onset wavelength of corresponding UV−vis absorption spectra in THF. cDetermined from cyclic voltammogram in THF (c = 1 × 10−3 M) with 0.1 M Bu4NPF6 as supporting electrolyte, AgCl/Ag as reference electrode, Pt as working and counter electrodes, and scan rate at 100 mV/s. dDetermined as the onset potential of reduction. e Calculated from the empirical formula, LUMO = −E red + E1/2(ferrocene) − 4.8 (eV).18 fCalculated from LUMO and optical band gap (Eg) of corresponding compounds, HOMO = LUMO − Eg (eV). gCV of the synthesized compounds coated on an ITO glass electrode was carried out in acetonitrile with 0.1 M Et4NBF4 as supporting electrolyte, AgCl/Ag as reference electrode, Pt as counter electrode, and scan rate at 100 mV/s.

■

RESULTS AND DISCUSSION Crystal Structures and Optical Properties of BKI. To evaluate the feasibility of the boron complexation as a trigger for the alteration of the electronic structures of a ketoimine, we investigated the structures of the ketoimine derivatives (Figures 1, S12, and S13 and Tables S1−S4). The ketoimine KI and the boron ketoiminate BKI were prepared according to almost same procedures of our previous report.13 Initially, their crystal structures were examined by a single-crystal X-ray diffraction. The single crystals of BKI and KI were readily formed in the mixture with CH2Cl2 and hexane, and the hydrogen atoms were found from electron difference maps in the single crystal X-ray analysis. Although it was suggested that the crystal of KI involved two types of structures, both structures showed similar parameters (Table S2). Accordingly, it was found that the KI has a high planarity (dihedral angle [C15−C10−N1−C9]: structure I = −5.4°, structure II = 8.1°). In addition, KI has a C1−C9 (structure I, 1.37 Å; structure II, 1.37 Å) bond with partial double-bond character and a C1−C2 (structure I, 1.43 Å; structure II, 1.42 Å) bond with partial single-bond character. These data mean that KI possesses the enaminoketone form in the crystal. On the other hand, it was shown that BKI is a less planar molecule (dihedral angle: [C15−C10−N1−C9] = 35.8°). Since the optimized structure of BKI by a DFT calculation also indicated that BKI has less planar structure, the twisted structure should be dominantly induced by the steric repulsion between hydrogen atoms on the phenyl ring binding to the nitrogen atom and hydrogen atoms or fluorine atoms on the boron chelating ring (Figure S16). In addition, BKI has a C1−C2 (1.38 Å) bond with partial double-bond character and a C1−C9 (1.40 Å) bond with partial single-bond character, indicating BKI forms an enolimine-like electronic structure. According to the previous reports, it is known that ketoimine

Table 2. Emission Properties of Synthesized Compounds in THF (c = 1 × 10−5 M)a KI BKI FP FM-a FM-b FBP FBM-a FBM-b TP TM-a TM-b TBP TBM-a TBM-b

in the solid statea

λPL [nm]

ΦPLb

λPL(solid) [nm]c

ΦPL(solid)b

445 495 492 445 465 560 490 555 519 471, 441, 415 480 595 564 603, 440

0.01 0.01 0.01 0.01 0.04 0.10 0.05 0.03 0.09 0.05 0.19 0.04 0.35 0.03

465 485 507 471 499 562 520 560 592 539 543 683 635 646

450 nm) and shorter wavelength emission (λPL ≈ 400 nm, Figure S11). The peak 2275

dx.doi.org/10.1021/ma500082e | Macromolecules 2014, 47, 2268−2278

Macromolecules

Article

Figure 6. Optimized structures and molecular orbital diagrams for the LUMO and HOMO of (a) TM-a′ and TM-b′ (b) TBM-a′ and TBM-b′ calculated by Gaussian 03 Programs (B3LYP/6-31G (d)//B3LYP/6-31G (d)).19

polymers and b-type model compounds at the excited states, leading to the generation of the new emission bands at similar wavelength region between the polymers and the b-type models. In the solid state, the emissions of synthesized polymers were found in longer wavelength region than those in THF (Figures 7 and 8). Especially, TP and TBP showed large shifts. The redshifted emission in the film state should be derived from the enhanced intermolecular interaction among the neighboring polymer chains.20 Thus, it is likely that the thiophene-based polymers (TP and TBP) with a more planar main-chain

extension of the conjugated system through the polymer mainchains. In contrast, although most of the compounds show largely expanded LUMOs through the whole units, the LUMOs of FBM-b and TBM-b distinctly localized in the boron ketoiminate units, resulting in the strong torsion between Nside phenyl rings and boron-chelating rings in the boron ketoiminate units. It is known that a molecule having different localization manners between the frontier orbitals often exhibits the broad emission in the long wavelength region originated from the intramolecular charge transfer (CT) states.14c Thereby, it is likely that the CT states are formed in the 2276

dx.doi.org/10.1021/ma500082e | Macromolecules 2014, 47, 2268−2278

Macromolecules

Article

Figure 7. Photographs of (a) fluorene-based polymers and (b) bithiophene-based polymers in THF and solid states under UV-irradiation, and PL spectra of (c) fluorene-based polymers and (d) bithiophene-based polymers (c = 5.0 × 10−5 M) in THF (dashed line) and in film state (solid line) upon the excitation at each absorption maximum.

Figure 8. PL spectra of (a) FBP and (b) TBP (c = 5.0 × 10−5 M) in THF (dashed line) and THF/H2O (=1:9) mixed solvent (solid line) upon the excitation at each absorption maximum.

investigated in the THF/H2O solvent mixture system (c = 5 × 10−5 M). The PL spectra exhibited the red-shifts of the peak position and the enhancements of the emission intensities by increasing the water content (H2O ≥ 90 vol %). Thus, we accomplished to obtain the AIE-active polymers.

structure showed larger red-shifted emission relative to the fluorene-based polymers (FP and FBP). Compared to the quantum yield of the ketoimine-containing compounds in THF (FP: ΦPL = 0.01, TP: ΦPL = 0.09), which were determined as an absolute value with the integrated sphere method, the solid sample showed the decreases of the quantum yields (FP: ΦPL < 0.01, TP: ΦPL = 0.02). The chromophores form the aggregation and the stacking in the condensed states. Thereby, the aggregation-caused quenching (ACQ) effect should be induced.15 On the other hand, the boron ketoiminate compounds showed higher quantum yields in the solid states (FBP: ΦPL = 0.13, TBP: ΦPL = 0.06) than those in THF (FBP: ΦPL = 0.10, TBP: ΦPL = 0.04). These results indicate that the boron complexation to the ketoimine units is responsible for the expression of the AIE properties. To explore the AIE behaviors of the boron ketoiminate polymers, the dependency of the emission properties on the solvent compositions was

■

CONCLUSION We have demonstrated the synthesis of the conjugated polymers based on ketoimine units as a tautomeric unit via Suzuki−Miyaura coupling reaction and investigated the effect of tautomeric units for expansion of main-chain conjugation with the measurements of optical properties. From the results, it was observed that the conjugated system was extended even though the structures of the tautomeric unit are incapable to form the main-chain conjugation. These results indicated that tautomeric units intrinsically possess the electronic properties of not only thermally stable tautomeric structures but also 2277

dx.doi.org/10.1021/ma500082e | Macromolecules 2014, 47, 2268−2278

Macromolecules

Article

(10) (a) Thompson, B. C.; Abboud, K. A.; Reynolds, J. R.; Nakatani, K.; Audebert, P. New J. Chem. 2005, 29, 1128−1134. (b) Nomura, R.; So, Y.; Izumi, A.; Nishihara, Y.; Yoshino, K.; Masuda, T. Chem. Lett. 2001, 40, 916−917. (c) Nishihare, Y.; Yoshida, Y.; Kobatashi, Y.; Fujii, A.; Ozaki, M.; Nomura, R.; Masuda, T.; Yoshino, K. Jpn. J. Appl. Phys. 2003, 42, 694−697. (11) Da Silva, M. A. V. R.; Da Silva, M. D. M. C. R.; Paiva, J. P. A.; Nogueira, I. M . C. S.; Damas, A. M.; Barkley, J. V.; Harding, M. M.; Akello, M. J.; Pilcher, C. J. Chem. Soc., Perkin Trans. 2 1993, 1765− 1769. (12) (a) Itoh, K.; Okazaki, K.; Fujimoto, M. Aust. J. Chem. 2003, 56, 1209−1214. (b) Macedo, F. P.; Gwengo, C.; Lindeman, S. V.; Smith, M. D.; Gardinier, J. R. Eur. J. Inorg. Chem. 2008, 3200−3211. (13) Yoshii, R.; Nagai, A.; Tanaka, K.; Chujo, Y. Chem.Eur. J. 2013, 19, 4506−4512. (14) (a) Xu, S.; Evans, R. E.; Liu, T.; Zhang, G.; Demas, J. N.; Trindle, C. O.; Fraser, C. L. Inorg. Chem. 2013, 52, 3597−3610. (b) Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L. J. Am. Chem. Soc. 2010, 132, 2160−2162. (c) Tanaka, K.; Chujo, Y. Macromol. Rapid Commun. 2012, 33, 1235−1255. (d) Tanaka, K.; Tamashima, K.; Nagai, A.; Okawa, T.; Chujo, Y. Macromolecules 2013, 46, 2969−2975. (e) Tokoro, Y.; Nagai, A.; Tanaka, K.; Chujo, Y. Macromol. Rapid Commun. 2012, 33, 550−555. (f) Yeo, H.; Tanaka, K.; Chujo, Y. Macromolecules 2013, 46, 2599−2605. (15) (a) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765−768. (b) Feng, X.; Tong, B.; Shen, J.; Shi, J.; Han, T.; Chen, L.; Zhi, J.; Lu, P.; Ma, Y.; Dong, Y. J. Phys. Chem. B 2010, 114, 16731−16736. (16) (a) Morisaki, Y.; Imoto, H.; Miyake, J.; Chujo, Y. Macromol. Rapid Commun. 2009, 30, 1094−1100. (b) Yoshii, R.; Nagai, A.; Tanaka, K.; Chujo, Y. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1726−1733. (17) (a) Nagai, A.; Kobayashi, S.; Nagata, Y.; Kokado, K.; Taka, H.; Kita, H.; Suzuri, Y.; Chujo, Y. J. Mater. Chem. 2010, 20, 5196−5201. (b) Morisaki, Y.; Tominaga, M.; Chujo, Y. Chem.Eur. J. 2012, 18, 11251−11257. (18) (a) Chen, C.-P.; Chan, S.-H.; Chao, T.-C.; Ting, C.; Ko, B.-T. J. Am. Chem. Soc. 2008, 130, 12828−12833. (b) Morisaki, Y.; Ueno, S.; Chujo, Y. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 334−339. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudon, K. N.; Burant, J. C.; Millam, J. M.; Iyenger, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyoto, K.; Fikuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowsky, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (20) (a) Stalder, R.; Mei, J.; Reynolds, J. R. Macromolecules 2010, 43, 8348−8352. (b) Dong, Y.; Lam, J. W. Y.; Qin, A.; Sun, J.; Liu, J.; Li, Z.; Sun, J.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. Chem. Commun. 2007, 3255−3257.

thermally unstable ones. In addition, we demonstrate the feasibility of the boron complexation to elongate the mainchain conjugation. The degree of the conjugation length of the boron ketoiminate polymers developed by the boron complexation larger than those of ketoimine polymers because the electronic structures in the ketoimine units were fixed in the enolimine-like form which is favorable to elongate the mainchain conjugation by the boron complexation. Moreover, the boron ketoiminate derivatives showed AIE property derived from boron ketoiminate units. This is the first example to offer the intrinsic properties of the tautomer-containing conjugated polymers as well as to apply the boron complexation as a trigger for alteration in the electronic structure of the tautomers, leading to the expansion of the conjugated systems through the polymer main-chains. Our findings would expand the applicability as a new building block for conjugated polymer with a tunable main-chain conjugated system and a controllable optical property. Furthermore, we can say the boron ketoiminate polymers with prominent AIE properties have high potentials to be advanced all-organic opto and/or electronic materials.

■

ASSOCIATED CONTENT

S Supporting Information *

Text giving the specific experimental procedure used, PL spectra of synthesized compounds with a bithiophene unit on the excitation at 300 nm, CV measurements, IR spectra, crystallographic data, the results of DFT calculations, and 1H, 13 C, and 11B NMR spectra of some synthesized compounds and two .cif files. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(Y.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by the JFE 21st Century Foundation (for K.T.), the Adaptable and Seamless Technology Transfer Program through target-driven R&D, Japan Science and Technology Agency (JST), and a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No.2401)” (24102013) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan.

■

REFERENCES

(1) Mitschke, U.; Bäuerle, P. J. Mater. Chem. 2000, 10, 1471−1507. (2) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15−26. (3) Stutzmann, N.; Friend, R. H.; Sirringhaus, H. Science 2003, 299, 1881−1884. (4) Groenendaal, L.; Zotti, G.; Aubert, P. −H.; Waybright, S. M.; Reynolds, J. R. Adv. Mater. 2003, 15, 855−879. (5) Estrada, L. A.; Liu, D. Y.; Salazar, D. H.; Dyer, A. L.; Reynolds, J. R. Macromolecules 2012, 45, 8211−8220. (6) Kar, A. K. Polym. Adv. Technol. 2000, 11, 553−559. (7) (a) Kawai, T.; Kunitake, T.; Irie, M. Chem. Lett. 1999, 905−906. (b) Kawai, T.; Nakashima, Y.; Irie, M. Adv. Mater. 2005, 17, 309−314. (8) Datta, S.; Bhattacharya, S. Chem. Commun. 2012, 48, 877−879. (9) Kwon, J. E.; Park, S. Y. Adv. Mater. 2011, 23, 3615−3642. 2278

dx.doi.org/10.1021/ma500082e | Macromolecules 2014, 47, 2268−2278