Homo- and Copolymers Based on Carbon-Bridged Oligo(p

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

Homo- and Copolymers Based on Carbon-Bridged Oligo(p‑phenylenevinylene)s for Efficient Fluorescence over the Entire Visible Region Hiroki Nishioka,† Hayato Tsuji,*,†,‡ and Eiichi Nakamura*,† †

Department of Chemistry, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan

‡

S Supporting Information *

ABSTRACT: Highly efficient emissive conjugated polymers are desired for optoelectronic applications. While efficient blue to green emissive polymers have been successfully developed, efficient red emission is still challenging, mainly because of the increased nonradiative process rates upon decreasing the energy gap by extension of the π-system. Suppression of this nonradiative decay would be the key for efficient red-emitting polymers. We have previously developed carbon-bridged oligo(p-phenylenevinylene)s (COPVs), which could be promising backbones for efficient light-emitting polymers because of the suppression of rotational disorder by the rigidified coplanar structures. In this work, we report that a series of COPV-based homopolymers and copolymers show fluorescence over a wide range of the visible region (437−667 nm) with high to excellent fluorescence quantum yield: 0.60−0.94 for the blue to green emission and 0.50 for red. The polymers also exhibited high thermal and electrochemical stabilities, which could be promising for future application in optoelectronic devices.

I. INTRODUCTION Conjugated polymers have served as an important component for organic optoelectronic devices because of their electrical and optical properties together with processability and mechanical properties. Since a report on the organic lightemitting diode (OLED) application of poly(p-phenylenevinylene) (PPV),1,2 a variety of conjugated light-emitting polymers have been developed for optoelectronic applications, including organic light-emitting electrochemical cells (LECs)3,4 and organic solid-state lasers (OSLs).5 Although blue to green emission with fluorescence quantum yield of over 0.9 has been achieved with the compounds so far recorded in the literature,6−9 red emission has proven to be rather inefficient, with a quantum yield of typically less than 0.5 because of enhanced nonradiative decay of the excited state.4,9,10 The redemissive polymers generally consist of large π-conjugated units and/or those connected by C−C single bonds; therefore, two intrinsic causes for such a decay were considered. One originates from a problem in the red-emissive π-unit itself that suffers, more than the structurally more compact blueemissive unit, from an increase of the overlap of the vibration wave function of the ground state with that of the excited stateknown as an energy gap law.11,12 Another stems from the torsional motion along the C−C single bonds that connect the π-units in the emissive unit that accelerates the nonradiative decay.13,14 An additional problem generally seen for any molecules in their excited states is an intermolecular quenching of the excited states. In the present study, we address these © XXXX American Chemical Society

problems through the elimination of the vibrational freedoms of the emissive π-system.10,15 As a candidate of such an emissive monomer unit, we have created a family of fused ladder-type systems, carbon-bridged oligo(p-phenylenevinylene)s (COPVs; Scheme 1, where n in COPVn refers to the number of vinylene units in the molecule).16,17 They have been found to be thermally and photochemically stable because of the extreme rigidity of the ring system and the presence of organic groups protecting the top and bottom faces of the π-system. These molecules as their monomers are stable enough to serve as a stable and efficient dye in OSL applications.18−21 Here, we report the synthesis and properties of a series of homo- and copolymers using COPV as a monomer unit, for which the emission wavelength of the polymers was tuned over a wide range of the visible region through the incorporation of an acceptor unit in the polymer backbone.22 A fluorescence quantum yield of up to 0.94 in blue to green emissions was recorded for COPV homo- and copolymers with phenylenerelated linkers. Donor−acceptor (D−A) polymers incorporating COPV as a donor and benzo[c][1,2,5]thiadiazole (BT) and naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (NTz) as acceptors showed intense orange and red emissions at 602 and 634 nm with fluorescence quantum yields of 0.68 and 0.50, respectively. The polymers showed high thermal and electrochemical Received: January 15, 2018 Revised: March 13, 2018

A

DOI: 10.1021/acs.macromol.8b00102 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Synthesis of COPV-Based Polymers by Suzuki−Miyaura Polymerization of (a) COPVn-(Bpin)2 and Aryl Dibromide as a General Procedure and (b) COPVn-Br2 and NTz-(Bpin)2 to Obtain NTz-Containing Copolymers

Japan Analytical Industry LC-9260 II NEXT (eluent: chloroform) with JAIGEL 1H and 2H polystyrene columns. HPLC analyses (analytical GPC for molecular weight determination) were performed on a Shimadzu LC-20A system equipped with SPD-20A UV−vis detector and a KF-805L column (Shodex). Materials. Commercial reagents were purchased from Tokyo Kasei Co., Aldrich, and other commercial suppliers and used as purchased. Anhydrous solvents were purchased from Kanto and purified by a solvent purification system (GlassContour) equipped with columns of activated alumina and copper catalyst prior to use. Other solvents were used as received. COPV1-Br2 and COPV2-Br2 were synthesized by a previously reported procedure.17 Synthesis of COPV1-(Bpin)2. A mixture of COPV1-Br2 (887 mg, 0.80 mmol), bis(pinacolato)diboron (1.03 g, 4.1 mmol), PdCl2(dppf)· CH2Cl2 (72.3 mg, 0.089 mmol), and potassium acetate (813 mg, 8.3 mmol) in 1,4-dioxane (16 mL) was stirred at 80 °C for 22 h. Water was added to the mixture, which was then extracted with dichloromethane. The organic layer was dried with MgSO4 and concentrated in vacuo. Silica gel column chromatography (eluent: hexane/ dichloromethane = 9/1, 4/1, and then 2/1), and the following reprecipitation from dichloromethane and methanol afforded the title compound as a white solid (650 mg, 67%). 1H NMR (500 MHz, CDCl3): δ 7.83 (s, 2H), 7.60 (d, J = 7.4 Hz, 2H), 7.20 (d, J = 8.6 Hz,

stability. Reduction of freedom of motion, limitation of the efficiency of intersystem crossing through elimination of heavy atoms such as sulfur,23 suppression of intrachain energy transfer to local traps,24 and reduction of interchain energy transfer by the long side chains on the top and bottom faces of the πsystem, which increases the solubility and processability of the polymer, are some of the key features of the COPV-based polymers. With such unique properties, the COPV polymers will find use in a variety of applications in the near future.

II. EXPERIMENTAL SECTION General. Proton nuclear magnetic resonance (1H NMR) and carbon nuclear magnetic resonance (13C NMR) were recorded using a JEOL ECZ-500 (500 MHz) NMR spectrometer. Chemical data for protons and carbons are reported in parts per million (ppm, δ scale) downfield from an internal standard, tetramethylsilane (δ 0.0) and CDCl3 (δ 77.0), respectively. Mass spectra were obtained using a JEOL JMS-T100LC equipped with an atmospheric pressure chemical ionization (APCI) unit and a time-of-flight mass analyzer. Melting points of solid materials were determined on a Mel-Temp II capillary melting-point apparatus and are uncorrected. Preparative gel permeation column chromatography (GPC) was performed on a B


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Macromolecules 8H), 7.14 (d, J = 7.4 Hz, 2H), 7.01 (d, J = 8.0 Hz, 8H), 2.52 (t, J = 7.7 Hz, 8H), 1.53−1.58 (m, 8H), 1.26−1.29 (m, 64H), 0.87 (t, J = 7.2 Hz, 12H). 13C NMR (125 MHz, CDCl3): δ 157.23, 156.90, 141.50, 141.31, 139.55, 133.94, 130.56, 128.43, 128.20, 120.50, 83.52, 62.40, 35.56, 31.90, 31.35, 29.52, 29.48, 29.26, 24.78, 22.66, 14.12 (some peaks seem to be overlapped); mp 196−197 °C. HRMS (APCI+) m/z Calcd for C84H114B2O4+ ([M+]): 1206.8970. Found: 1206.8872. Synthesis of COPV2-(Bpin)2. The procedure for COPV1-(Bpin)2 was followed to afford the title compound as a yellow solid (671 mg, 41%). 1H NMR (500 MHz, CDCl3): δ 7.82 (s, 2H), 7.59 (d, J = 8.0 Hz, 2H), 7.27 (s, 2H), 7.20−7.14 (m, 22H), 7.10 (d, J = 8.6 Hz, 8H), 6.92 (d, J = 8.0 Hz, 8H), 2.48 (t, J = 7.7 Hz, 8H), 1.55−1.52 (m, 8H), 1.31−1.25 (m, 64H), 0.88 (t, J = 6.9 Hz, 12H); 13C NMR (125 MHz, CDCl3): δ 157.75, 156.53, 156.39, 154.15, 143.22, 141.80, 141.19, 139.75, 136.64, 134.05, 130.63, 128.60, 128.33, 128.15, 126.64, 119.94, 118.20, 83.49, 62.83, 62.33, 35.60, 31.93, 31.32, 29.60, 29.53, 29.28, 24.78, 22.70, 14.14 (some peaks seem to be overlapped); mp 190−192 °C (dec). HRMS (APCI+) m/z Calcd for C118H137B2O4+ ([M + H+]): 1641.0755. Found: 1641.0798. General Procedure for Polymerization. A mixture of COPVn(Bpin)2 (1 equiv), COPVn-Br2 or aryl dibromide (1.0 equiv), Pd(PPh3)4 (20 mol %), and a catalytic amount of Aliquat 336 (three drops) in toluene and 2 M K2CO3 aqueous solution (3:2, 0.025 M in toluene) was stirred at 110 °C for 24 h. An excess amount of bromobenzene was then added to the reaction mixture and stirred at 110 °C for 8 h. An excess amount of 4,4,5,5-tetramethyl-2-phenyl1,3,2-dioxaborolane was added to the reaction, and the mixture was stirred at 110 °C for another 10 h. The reaction mixture was diluted with chloroform and passed over a silica gel short plug. The solvent was removed in vacuo. The crude mixture was subjected to preparative GPC (eluent: chloroform) to remove small molecules. Reprecipitation from chloroform and acetone or methanol gave the title compound in 46−97% yield. Measurements. Thermal Properties. Thermogravimetric (TG) analysis was conducted with Rigaku ThermoPlus 2 thermal analyzer TG-8120. Sample was placed in an aluminum pan and heated to 500 °C at the rate of 10 K/min, under N2 purge at a flow rate of 100 mL/ min. Al2O3 was used as a reference material. Computational Studies. DFT calculations were performed on Gaussian 09, Revision B.01.25 Photophysical Properties. The polymers were dissolved in chloroform deoxygenated by argon gas bubbling to the concentration of 3 and 0.3 mg/L for absorption and emission measurement, respectively. Absorption spectra were measured with JASCO V-670 spectrometer. Emission spectra were measured with HITACHI F-4500 spectrometer. Photoluminescence quantum yields were measured on a Hamamatsu Photonics C9920-02 Absolute PL Quantum Yield Measurement System, and absolute quantum yields were determined by using a calibrated integrating sphere system. Fluorescence lifetimes were estimated with the time correlated single photon counting (TCSPC) operation mode which allows to measure fluorescence lifetimes from 100 ps to 10 μs. Electrochemical Properties. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were conducted with a HOKUTO DENKO HZ-7000 voltammetric analyzer. Measurements were carried out in a one-compartment cell under Ar gas, equipped with a platinum counter electrode, a glassy-carbon working electrode, and an Ag/Ag+ reference electrode. The supporting electrolyte was a 0.5 M dichloromethane solution of tetrabutylammonium hexafluorophosphate. All potentials were corrected against Fc/Fc+. CV was measured with a scan rate of 100 mV/s.

Miyaura coupling polymerization of COPVn-Br2 and COPVn(Bpin)2 (Scheme 1a) in 80% and 77% yield, respectively. For the broad tuning of the photophysical properties, copolymers of COPVs with various linkers were synthesized. Durene (dur) and bibenzyl (BB) units were used to blue-shift the emission wavelength by the separation of π-conjugation of COPV fluorophores, which comes from the orthogonality of the molecular orbitals and the nonconjugative ethylene connector unit, respectively. The phenylene (ph) moiety gives conjugation over the polymer backbone, albeit with reduced effective conjugation between the COPV units, which also leads to blue-shifted emission. On the other hand, to obtain redshifted emission, electron-accepting linkers such as quinoxaline (Qx), benzo[c][1,2,5]thiadiazole (BT), naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (NTz), and 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole (DTBT) units were utilized. The synthesis was accomplished by the use of COPVn-(Bpin) 2 and appropriate aryl dibromides, which gave the polymers (Scheme 1a). For the synthesis of poly(COPV1-NTz) and poly(COPV2NTz), a readily available NTz-(Bpin)2 was copolymerized with COPVn-Br2 (Scheme 1b).26 The polymers were obtained in moderate to good yield of 46−97%. Most of the polymers were well soluble in common organic solvents, such as toluene, THF, or chloroform. The molecular weight (Mn, Mw) of the polymers was estimated by GPC using polystyrene as a standard (Table 1). The polymers possess more than six repeating units as Table 1. Molecular Weights of the Polymers poly(COPV1-X), X = dur BB ph (none)b Qx BT NTz DTBT poly(COPV2-X), X = dur BB ph (none)b Qx BT NTz DTBT

Mn

Mw

PDIa

6000 11700 6800 5800 14400 11100 16800 10800

10700 16800 9700 8100 28200 22600 29400 17200

1.61 1.43 1.42 1.40 1.96 2.03 1.76 1.59

17300 19600 23100 26000 31900 22700 6900 14200

27500 26200 47100 35900 56200 48600 12100 21800

1.59 1.34 2.04 1.38 1.77 2.14 1.76 1.53

a PDI refers to polydispersity index. bX = (none) stands for the homopolymers.

judged from the Mn (five for poly(COPV2-NTz)). The polymer chain is long enough to show the photophysical behavior expected for polymeric materials (vide infra). The Mw of the polymer can be tuned as examined for COPV2 homopolymer poly-COPV2. While the palladium catalyst loading did not significantly affect the Mn, an increase of the reaction concentration resulted in larger Mn (Tables S1 and S2); however, this caused no significant difference in the photophysical properties (absorption and emission) beyond a Mn of 26 000 (Figure S1). The thermal stabilities of the polymers were tested by thermogravimetric analysis (Figure S2). It was found that poly-

III. RESULTS AND DISCUSSION Synthesis and Characterization. We relied on the Suzuki−Miyaura cross-coupling reaction to synthesize the polymers as shown in Scheme 1. COPVn-Br2 (n = 1 or 2) were synthesized by a previously reported procedure17 and were subjected to borylation to obtain COPVn-(Bpin)2. The COPV1 and COPV2 homopolymers were obtained by Suzuki− C


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Figure 1. Calculated HOMOs, LUMOs, and their energy levels for the model compounds tri-COPV1, tri(COPV1-dur), and tri(COPV1-BT), obtained at the B3LYP/6-31G(d,p) level of theory.

COPV1 showed 5 wt % loss at 375 °C, similar to monomeric COPV1. Other polymers showed an even higher temperature for 5 wt % loss. These results indicate that the high thermal stability of COPVs was well maintained in the polymers. Molecular Orbital Studies. DFT calculations were carried out to obtain insights into the electronic properties of the homo- and copolymers (Figure 1 and Table 2). We examined

COPV1. Meanwhile, the LUMO of tri(COPV1-BT) was localized on the BT linker unit, which has inherently low LUMO energy level by its electron accepting character, and its energy level is lowered by as much as 0.89 eV than that of triCOPV1. The HOMO−LUMO gap of tri(COPV1-BT) is therefore lower than that of tri-COPV1, resulting in longer wavelength absorption and emission. The lowering of the LUMO level was more significant in stronger electronaccepting NTz unit (Table 2). On the other hand, in tri(COPV1-DTBT), separation of the donor (COPV) and acceptor (BT) unit by thiophene rings led to narrower bandgap compared to tri(COPV1-BT) (Table 2). The effect of the number of repeating units on energy levels and excitation energies was also examined. COPV1 with methyl side groups was also employed as a model. The change in both HOMO and LUMO energy levels was significant when the number of repeating units was less than three (Figure S4a). However, when the number of repeating units was larger than six, the change becomes rather small. A similar tendency was also observed in the excitation energies (Figure S4b). Because the polymers obtained experimentally have more than six repeating units, we expect that the degree of polymerization did not affect the observed photophysical properties much. Photophysical Properties. The absorption and emission spectra of COPV-based polymers in deoxygenated chloroform are shown in Figure 2, and their properties are summarized in Table 3. The homopolymer poly-COPV1 exhibited broad absorption at the peak wavelength of 428 nm and efficient lightblue emission at the peak wavelength of 470 nm with fluorescence quantum yield of 0.79. The phenylene-type linkers, such as the dur, BB, or ph groups, made the absorption and emission blue-shifted, down to deep-blue region (410 and 437 nm for poly(COPV1-dur), 415 and 438 nm for poly(COPV1-BB)). The high fluorescence quantum yield of poly(COPV1-BB) suggests the localization of emissive system on COPV units, with minimum effect of flexible BB units on emission. The spectra of homopolymer (poly-COPV1) and copolymers with phenylene-related linkers (dur, BB and ph) showed vibronic fine structure, ascribed to rigid COPV backbone in these polymers. On the other hand, electronwithdrawing linkers (Qx, BT, NTz, and DTBT) resulted in redshift and broadened emission bandstypical characteristics for D−A-type materials. Yellow and orange emissions of 545, 575, and 594 nm with fluorescence quantum yields of over 0.60 were achieved. However, when thiophene rings were incorporated

Table 2. HOMO and LUMO Energy Levels and Energy Gap (ΔE) of the Model Compounds Obtained at the B3LYP/631G(d,p) Level of Theory tri(COPV1-X), X = dur BB ph (none)a Qx BT NTz DTBT a

HOMO [eV]

LUMO [eV]

ΔE [eV]

−5.10 −4.97 −4.86 −4.73 −4.87 −4.87 −4.98 −4.71

−1.05 −1.36 −1.53 −1.50 −2.02 −2.39 −2.77 −2.64

4.04 3.61 3.32 3.23 2.85 2.48 2.21 2.08

X = (none) stands for the homopolymers.

model compounds bearing methyl groups in place of 4alkylphenyl side chains and molecules bearing three COPV1. In Figure 1, comparison of three oligomers (COPV1 homotrimer (tri-COPV1), co-oligomer with bulky aromatic linkers (tri(COPV1-dur)), and co-oligomer with electron-withdrawing linkers (tri(COPV1-BT)) are shown, while results for other systems are shown in the Supporting Information (Figure S3). Tri-COPV1 showed extended HOMO and LUMO over the whole molecule, which is consistent with the longer wavelength absorption and emission than monomeric COPV1, as experimentally observed (discussed later). The dihedral angle between the two COPV1 units was 37°. On the other hand, the bulky linkers in tri(COPV1-dur) stand nearly orthogonal to the COPV1 units (dihedral angle ≈90°), and the HOMO and LUMO were more localized on a single COPV1 unit. As a result, tri(COPV1-dur) shows a larger energy gap, leading to blue-shifted emission compared with tri-COPV1. Similar localization of the molecular orbitals was found in tri(COPV1-BB) (Figure S3b). In the case of tri(COPV1-BT), the HOMO was delocalized over the whole molecule, and the HOMO energy level was lowered by 0.14 eV than that of triD


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Figure 2. Normalized absorption, emission spectra in degassed chloroform, photographs under 365 nm light irradiation, and fluorescence decay profiles of poly(COPV1-X) (a−d) and poly(COPV2-X) (e−h).

overlap between π* (LUMO) and π (HOMO). A similar tendency was observed in COPV2-based polymers. Importantly, unlike PV derivatives, COPV polymers showed low nonradiative decay rates. In the case of PPV, a large increase in knr up to 18.6 × 108 s−1 was found upon the elongation of conjugation, ascribed to the intramolecular rotation.13,14 The COPV skeleton eliminates C−C rotation in its π-system; therefore, it reduces the nonradiative decay rate (0.9 × 108 s−1 in poly-COPV1). COPV2-based polymers showed slightly larger knr values than COPV1-based polymers. One possible reason for this faster nonradiative decay is the reduced quinoidal contribution in the excited state originating from the longer π-conjugated backbone of COPV2. Longer COPV homologues favor benzenoid form rather than quinoidal forms in their photoexcited and oxidized states because of aromatic stabilization.27−29 We assume that this reduced quinoidal contribution in the COPV2 polymer as compared to in the COPV1 polymer invokes a less double-bond character of the linkage between the COPV units. This raises the possibility of torsional motion along the C−C bonds at the linkage, which enhances the nonradiative decay. The thiophene-containing poly(COPV1-DTBT) and poly(COPV2-DTBT) showed largely decreased fluorescence quantum yield of 0.44 and 0.23, respectively (vide supra), and a value of knr that is larger than kr, probably because of the enhanced intersystem crossing by the heavy-atom effect of sulfur in thiophene rings.10,23 Electrochemical Studies. The electrochemical properties of the polymers were examined by cyclic voltammetry (CV)

into the polymer chain (poly(COPV1-DTBT)), the fluorescence quantum yield dropped to 0.44, although the wavelength was further red-shifted to 657 nm in the deep-red region. The trend seen for the COPV1-based polymers was also observed in the COPV2-based polymers, whose luminescence was expectedly red-shifted (Figure 2e−h). Importantly, intense orange and red emissions at 602 and 634 nm with good fluorescence quantum yields of 0.68 and 0.50 were achieved with the copolymer of COPV2 with BT or NTz units, respectively. To obtain a deeper insight into the nature of the intense fluorescence, the lifetime of the fluorescence was measured. The lifetimes (τ) are shown in column 5 in Table 3, and the decay profiles are shown in Figure 2d,h. All fluorescence decay profiles were well fitted by a single decay component. The rate constants for radiative and nonradiative decay were calculated (kr and knr, respectively, Table 3, columns 6 and 7). As compared with the monomeric COPV1 (τ = 3.15 ns), polyCOPV1 exhibited a significantly shorter lifetime of τ = 0.75 ns. The shortening of the lifetime by the extension of the π-system was previously seen in phenylenevinylene (PV) oligomers and polymers and also the reported family of COPVs.13,14,17 In this context, well-conjugated poly(COPV1-ph) showed similar lifetime of τ = 0.64 ns, while less conjugated poly(COPV1dur) and poly(COPV1-BB) exhibited longer lifetimes of τ ≈1.2 ns. On the other hand, D−A-type polymers show longer lifetimes of over 2 ns mainly because of the reduction of kr, possibly derived from the charge transfer nature by the lower E


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Macromolecules Table 3. Summary of the Photophysical Properties of COPV-Based Polymers λabsa [nm] COPV1g

λemb [nm]

ΦFc

τd [ns]

Table 4. Energy Levels of COPV-Based Polymers

kre knrf [108 s−1] [108 s−1]

323, 336, 352

385

0.98

3.15

3.1

0.1

poly(COPV1X), X = dur

351, 367

0.93

1.21

7.7

0.6

BB

373

0.89

1.19

7.5

0.9

ph (none)b Qx BT NTz DTBT COPV2g

409 428 442 486 517 548 376, 396, 419

410, 437 415, 438 449 470 545 575 594 657 433, 458

0.94 0.79 0.60 0.72 0.71 0.44 1.00

0.64 0.75 2.75 2.91 2.83 2.34 2.10

14.8 10.5 2.2 2.5 2.5 1.9 4.8

0.9 2.8 1.5 1.0 1.0 2.4 0.0

0.73

1.15

7.5

1.2

0.86 0.72 0.65 0.64 0.68 0.50 0.23

1.21 0.69 0.67 2.35 2.75 3.48 1.96

6.0 9.5 10.9 2.7 2.5 1.4 1.2

2.3 5.1 4.0 1.5 1.2 1.4 3.9

poly(COPV2X), X = dur

433

BB ph (none)b Qx BT NTz DTBT

446 464 485 474 506 538 548

445, 481 462 481 501 561 602 634 667

poly(COPV1-X), X = dur BB ph (none)c Qx BT NTz DTBT poly(COPV2-X), X = dur BB ph (none)c Qx BT NTz DTBT

HOMOa [eV]

LUMOb [eV]

−5.54 −5.47 −5.44 −5.31 −5.64 −5.58 −5.54 −5.26

(−2.26) (−2.43) (−2.65) (−2.90) −2.68 (−3.11) −2.84 (−3.27) −3.16 (−3.35) −3.14 (−3.28)

−5.26 −5.23 −5.28 −5.15 −5.30 −5.27 −5.30 −5.16

(−2.47) (−2.61) (−2.72) (−2.79) −2.70 (−2.88) −2.77 (−3.05) −3.18 (−3.22) −3.14 (−3.19)

a

HOMO levels obtained from oxidation potentials determined by DPV. bLUMO levels obtained from oxidation potentials determined by DPV. Numbers in the parentheses shows the LUMO levels estimated from HOMO levels and optical bandgap obtained from absorption onset. cX = (none) stands for the homopolymers.

could be also determined for D−A polymers, while those for non-D−A polymers which did not show reduction waves were estimated using the optical bandgaps from absorption onsets. The trend for oxidation and reduction potentials correlates well with the values obtained by DFT calculation. The electrochemical stabilities were also investigated. Monomeric COPV1 and COPV2 showed excellent electrochemical stability after 50 cycles (Figure S6) as reported previously,17 and poly-COPV1 was stable after 50 cycles (Figure S6). In contrast, a structurally related PPV derivative MDMO-PPV rapidly decomposed after three cycles. Similarly, poly(COPV1-dur), poly-COPV2, and poly(COPV2-dur) also showed stable CV traces. Poly(COPV1-BT) and poly(COPV2BT) were stable against oxidation, but unstable against reduction, probably because of the BT unit where the LUMO is localized. Thus, COPV serves as an electrochemically stable donor in the D−A polymer design.

a

Absorption maximum wavelengths measured in chloroform solution. Fluorescence maximum wavelengths measured in chloroform solution. cFluorescence quantum yield determined using an absolute method. dFluorescence lifetime measured in chloroform solution. e Radiative decay constant kr calculated from ΦF and τ. fNonradiative decay constant knr calculated from ΦF and τ. gValues were taken from ref 17. b

and differential pulse voltammetry (DPV). The CV traces are shown in Figure 3. The polymers showed quasi-reversible

IV. CONCLUSION In summary, homo- and copolymers based on COPV molecules have been synthesized effectively with the aid of the Suzuki−Miyaura coupling, and their emission wavelengths have been tuned in the visible region with good to excellent fluorescence quantum yields. The homopolymers of COPV1 and COPV2 exhibited light-blue and green emissions, respectively. Copolymers with phenylene-related aromatic linkers showed blue-shifted emissions with high fluorescence quantum yields of over 0.7. The D−A copolymer showed a reduced optical bandgap. COPV2-based D−A polymers poly(COPV2-BT) and poly(COPV2-NTz) exhibited intense red emissions at peak wavelengths of 602 and 634 nm with fluorescence quantum yields of 0.68 and 0.50, respectively. The light-emitting COPV core is thermally and electrochemically stable, and the COPV-containing polymers are also stable. Replacement of a variety of known chromophores with such a

Figure 3. CV traces of COPV-based polymers.

oxidation peaks in CV. Reduction peaks were also observed at −2.12, −1.96, −1.64, and −1.66 eV for COPV1-based D−A copolymers with Qx, BT, NTz, and DTBT linkers, respectively. Oxidation potentials determined by DPV (Figure S5 and Table S3) and by using these values, HOMO levels of the polymers were calculated which are summarized in Table 4. LUMO levels F


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stable COPV unit is an interesting future possibility in materials research.

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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.8b00102. Details for instruments, Figures S1−S6, Tables S1−S3, and NMR spectra for COPVn-(Bpin)2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.T.). *E-mail [email protected] (E.N.). ORCID

Hayato Tsuji: 0000-0001-7663-5879 Eiichi Nakamura: 0000-0002-4192-1741 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Grants-in-Aid for scientific research from MEXT (15H05754 to E.N. and 16H04106 to H.T.).

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REFERENCES

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Homo- and Copolymers Based on Carbon-Bridged Oligo(p

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