Construction of the Luminescent DonorâAcceptor Conjugated

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Construction of the Luminescent Donor−Acceptor Conjugated Systems Based on Boron-Fused Azomethine Acceptor Shunsuke Ohtani, Masayuki Gon, Kazuo Tanaka,* and Yoshiki Chujo Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

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S Supporting Information *

ABSTRACT: We report the boron-fused acceptor compound involving the azomethine structure prepared through the facile synthetic method. Additionally, we constructed the donor− acceptor (D−A) conjugated polymers which have the boronfused azomethine acceptors in the main chains. Cyclic voltammetry revealed that the electron-accepting ability was enhanced by boron coordination. Because of the strong D−A interaction, the conjugated polymers showed significant redshifts of optical bands both in the UV−vis absorption and PL spectra. Moreover, these polymers exhibited intense emission in both the solution and film states; meanwhile, the monomeric unit or non-boron-containing model polymers showed weak emission. It is suggested that these highly efficient emissive properties should be caused not only by the rigid skeleton but also by the strong D−A conjugated system originating from the boron-fused structure. This paper demonstrates that the combination of the azomethine structure and boron atom should be a promising strategy for obtaining an accepting monomeric unit, leading to luminescent conjugated polymer materials.

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INTRODUCTION Conjugated polymers have received a great deal of attention from both academic and industrial researchers because of their unique characters, such as luminescence, electrical conductivities, and nonlinear optical properties.1−6 In particular, donor− acceptor (D−A) polymers with an alternating array of electronrich (donor) and electron-poor (acceptor) units have been intensively studied because the optical band gaps of these polymers can be finely controlled to adjust the wavelengths of absorption and emission bands through tuning of their in-chain D−A interactions.7−10 These conjugated copolymers have been applied in organic light-emitting diodes,11 organic field effect transistors,12,13 and organic photovoltaics.14,15 Therefore, for designing D−A copolymers which have desired functions, many researchers have devoted much of their efforts to developing new donor and acceptor units. Recently, we proposed the concept of “element blocks”, which are structural functional units consisting of various groups of elements, to create functional materials according to designs.16−19 Based on this strategy, introduction of heteroatoms into organic scaffolds is one of the facile and effective methods to vary the properties of functional materials. In particular, a series of luminescent polymers with useful properties, such as dual emission,20−22 solid-state emission,23−27 high electron-carrier ability,28−30 narrow band gaps,31−37 narrow absorption and emission bands,38 and sensing properties,39−41 have been obtained by employing boron “element blocks”. The one possible approach to construct conjugated molecules is connecting the aryl compounds with a vinylene (−CC−) linkage.42,43 A conjugated monomeric unit having a vinylene © XXXX American Chemical Society

linkage such as stilbene or carbon-bridged oligo(p-phenylenevinylene) usually works as a donor unit in the D−A polymers because of the relatively high energy level of the HOMO (highest occupied molecular orbital) at the vinylene.44 On the other hand, once the carbon atoms are replaced by more electronegative elements, nitrogen atoms, the role of the unit in the D−A conjugated system should change. Azomethine (−C N−) and azo (−NN−) linkages are known to be interesting alternatives to vinylenes. The theoretical study indicated that their energy levels of HOMO and LUMO (lowest unoccupied molecular orbital) were stabilized in comparison to those of the vinylene derivatives.45 Consequently, azomethine and azo compounds intrinsically possess electron-deficient character because of the nitrogen atom; that is, they can work as acceptor units in the conjugated system. In general, the azomethine compounds were readily prepared by simple condensation reactions between primary amines and aldehydes.46−48 In addition to the synthetic easiness, the azomethine compounds have unique properties, such as absorption properties49 and coordinating ability with metal ions through the lone pairs of the nitrogen atoms.50 The azomethine structures in conjugated systems are therefore a versatile platform for creating functional acceptor materials given their comparable optoelectronic properties to their homologues. However, most of the azomethine compounds have been regarded as one of the components in the conjugated systems rather than functional Received: February 6, 2019 Revised: April 16, 2019

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DOI: 10.1021/acs.macromol.9b00259 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes of the BAmM and AmM

Scheme 2. Synthetic Routes of the Polymers

group reported that fused boron−azobenzene (BAz) worked as a strong electron acceptor because of the intrinsic electron deficiency of the NN double bond and the boron−nitrogen (B−N) coordination which dramatically lowered the energy of the LUMO of the azobenzene ligand.52 As a result, the

acceptor moieties. In this research, we designed the azomethine compounds whose nitrogen atoms are directly modified by the complexation of the electron-deficient boron atom. The effect of boron complexation in the conjugated molecules is known to give a substantial decrease in LUMO energy.51 Actually, our B


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Macromolecules

coupling is an effective approach to introduce boron-fused azomethine units into the main chain of conjugated polymers. Optical properties of the synthesized polymers and monomer were examined (Table 2). In UV−vis absorption spectra (chloroform, 1.0 × 10−5 M) of the synthesized boron-fused azomethine polymers (BAmP-F and BAmP-T), the longest absorption bands assigned as S0 → S1 (π−π*) transitions showed red-shifts and significant increase in the absorbance compared to in that of the monomer BAmM (Figure 1a). This result suggests that the conjugated systems should be effectively elongated through the polymer main chains. In addition, timedependent density functional theory (TD-DFT) calculation (see the Supporting Information for details) of model compounds BAmM′, BAmP-F′, and BAmP-T′ indicated that the HOMOs of synthesized polymers are delocalized on the entire molecules, while the LUMOs are localized on a boronfused azomethine unit (Figure 2). It is suggested that the HOMO−LUMO transitions of the synthesized polymers changed into the charge-transfer transitions by copolymerization with the bithiophene and fluorene comonomers. Furthermore, the energy levels of HOMOs are strongly affected by the comonomer units, while those of LUMOs are constant regardless of the comonomer units. This result indicated that boron-fused azomethine units could play a role as an electron acceptor in the conjugated system. The bithiophene-based polymers BAmP-T showed much red-shifted absorptions as compared with the fluorene-based polymers BAmP-F, in which fluorene unit worked as a weaker electron donor than the bithiophene unit.67 That is, the red-shifts followed by polymerization are originated from the D−A interaction between the comonomer units. Moreover, the boron-fused azomethine polymer showed further red-shifts compared to the corresponding non-boron-containing azomethine polymers (AmP-F and AmP-T). This should be because introduction of boron center to the complex with azomethine increased the electron affinity of this electron-accepting unit, leading to a stronger D−A interplay, compared to the control polymers without boron. To evaluate the electrochemical properties of the synthesized monomers and polymers, we performed cyclic voltammetry (CV) in dichloromethane (1.0 × 10−3 M of samples and 0.1 M of tetrabutylammonium hexafluorophosphate (NBu4PF6)) and estimated LUMO orbital levels from the onset potentials in cyclic voltammograms (Figure S26). It was found that the LUMO levels of the boron-fused azomethine derivatives (BAmM: −3.27 eV; BAmP-F: −3.18 eV; BAmP-T: −3.31 eV) significantly decreased by the complexation of boron atoms compared to the non-boron-containing derivatives (AmM: −2.73 eV; AmP-F: −2.67 eV; AmP-T: −2.73 eV). These values are lower than that of the conventional electron acceptor, benzothiadiazole (−2.86 eV). Those results indicated that the electron acceptor ability of the azomethine derivatives was significantly enhanced by boron complexation, and the red-shifts of the boron-fused azomethine polymers in the UV−vis absorption spectra were ascribed to the D−A interactions in the polymer main chains. The photoluminescence (PL) spectra (Figure 1b) of the synthesized polymers were measured in chloroform (1.0 × 10−4 M). In PL spectra, the boron-fused azomethine polymers also showed emission with a single broad peak and significant redshift compared to the monomer and corresponding non-boroncontaining azomethine polymers (BAmP-F: λem= 542 nm; BAmP-T: λem = 627 nm; BAmM: λem = 476 nm; AmP-F: λem = 461 nm; AmP-T: λ em = 501 nm). According to the

conjugated D−A copolymer with the NN double bond in the main chain exhibited highly efficient near-infrared emission. More recently, we synthesized the boron-fused azomethine complex modified with a diethylamino group and found that the crystal showed more efficient emission than the amorphous or diluted solution state.53 The phenomenon is known as crystallization-induced emission enhancement (CIEE) property.54−57 This finding suggests the possibility that the boronfused azomethine complex is utilized for creating a luminescent material. Hence, to demonstrate the feasibility of the azomethine structure as an acceptor in polymeric systems, we constructed a series of copolymers with fluorene and bithiophene donors and evaluated their electronic properties. Several researches based on complexation of azomethine and the boron atom were reported; however, to the best of our knowledge, there were no studies about an electron-accepting conjugated polymer scaffold.56−64

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RESULTS AND DISCUSSION The boron-fused azomethine monomer, polymers, and the model compounds were synthesized according to Schemes 1 and 2 as well as Figures S1−S25. To improve solubility and film formability of the polymers, the monomer BAmM was modified with two alkyl chains. Polymerizations were accomplished by the palladium-catalyzed Migita−Kosugi−Stille65,66 coupling with (9,9-didodecyl-9H-fluorene-2,7-diyl)bis(trimethylstannane) (F) or (3,3′-didodecyl[2,2′-bithiophene]-5,5′-diyl)bis(trimethylstannane) (T) in toluene in the presence of 2dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (X-Phos) and Pd2(dba)3 (dba = dibenzylideneacetone) (Scheme 2). The synthesized polymers (BAmP-F, BAmP-T) showed good solubility in common organic solvents such as tetrahydrofuran (THF), toluene, chloroform, and dichloromethane. To investigate the effect of the boron complexation, we synthesized the model azomethine polymers with the non-boron-containing azomethine monomer (AmM) as a starting compound. The size-exclusion chromatography (SEC) using chloroform as an eluent with the polystyrene standards revealed the numberaverage weight (Mn) and the molecular weight distribution (Mw/Mn) of 9500 and 1.9 for BAmP-F, 12900 and 2.2 for BAmP-T, 16100 and 1.5 for AmP-F, and 19300 and 1.8 for AmP-T, respectively (Table 1). The yield of AmP-F was low Table 1. Polymerization Resultsa BAmP-F BAmP-T AmP-F AmP-T

yieldb (%)

Mn

Mw

Mw/Mn

nc

58 70 8 51

9500 12900 16100 19300

18400 28600 24500 34500

1.9 2.2 1.5 1.8

10 13 21 26

a

Estimated by SEC with the polystyrene standards in CHCl3. Isolated yields. cAverage number of repeating units calculated from Mn and molecular weights of repeating units.

b

because high molecular weight fraction was selectively collected by HPLC (high-performance liquid chromatography) using chloroform as an eluent. The chemical structures of the polymers were confirmed by 1H, 13C, and 11B NMR as well as MALDI-TOF mass spectroscopies. All the purified polymers gave satisfactory spectroscopic data corresponding to their expected molecular structures. These observations strongly suggest that the palladium-catalyzed Migita−Kosugi−Stille C


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Macromolecules Table 2. Optical Properties of Synthesized Monomer and Polymers in CHCl3 Solution λmax,absa (nm) BAmM BAmP-F BAmP-T AmM AmP-F AmP-T

385 439 483 339 387 409

εb (105 M−1 cm−1) 0.058 0.41 0.44 0.15 0.82 0.40

λemc (nm) 473 542 627 461 501

ΦFd

χ2

krf (s−1)

knrf (s−1)

0.6 (33%), 1.7 (67%) 0.5 0.6 (14%), 1.5 (86%)

1.04 1.05 1.01

7

1.0 × 10 4.7 × 108 3.3 × 108

6.5 × 108 1.5 × 109 3.8 × 108

0.8 (25%), 1.4 (75%) 0.5 (14%), 1.8 (86%)

1.00 1.08

4.3 × 106 1.1 × 106

7.6 × 108 5.8 × 108

τe (ns)

0.016 0.24 0.46 600 nm) fluorescent materials are advantageous for bioimaging as they cause minimum photodamage to biological samples, low deep tissue penetration, and negligible interference from the background autofluorescence of biomolecules in the living organisms.69

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Tokyo Kasei Chemical Promotion Foundation (for K.T.) and a Grant-in-Aid for EarlyCareer Scientists (for M.G.) (JSPS KAKENHI Grant 18K14275), for Scientific Research (B) (JP17H03067), for Scientific Research on Innovative Areas “New Polymeric Materials On the basis of Element-Blocks (No. 2401)” (JP24102013), and for Challenging Research (Pioneering) (JP18H05356).

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CONCLUSION We synthesized a new boron-fused azomethine acceptor through the facile synthetic method. The polymers containing the acceptor units were also synthesized using Migita−Kosugi− Stille coupling reaction. Boron coordination provided structural rigidity and enhanced electron-accepting property. As a result, the optical measurement revealed that the polymers showed significant red-shift from the non-boron-containing azomethine polymers in both the UV−vis absorption and PL spectra. It can be explained by a D−A interaction originating from the electronaccepting property of boron-fused azomethine units. From electrochemical measurements, it was found that these polymers had low LUMO levels and the boron−azomethine moieties functioned as strong electron acceptors in the conjugated systems. Additionally, the boron-fused azomethine polymers showed strong emission in both the solution and film states because of the rigid main-chain conjugated structure. These findings could be widely applicable in the design of luminescent polymer materials with wide versatility.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00259. Experimental details; Figures S1−S31 and Table S1 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shunsuke Ohtani: 0000-0002-8669-1089 Masayuki Gon: 0000-0002-5540-5908 Kazuo Tanaka: 0000-0001-6571-7086 E


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