Introduction

Conjugated polymers have been studied extensively for use in various electronic and optoelectronic devices such as electroluminescence displays, photovoltaic cells and field-effect transistors. Many aromatic units, such as benzene, thiophene, pyridine, fluorene and carbazole, have been introduced in the conjugated polymer backbone. Among them, carbazole has many fascinating characteristics, as exemplified by the electrophilic substitution in the nitrogen atom and the aromatic ring, which produces diverse analogs and (electro)chemical and environmental stabilities. For instance, poly(N-vinylcarbazole)s have been used in xerographic applications as good photoconductors and hole-transporting materials.1 Owing to the high electron density at the 3,6-positions, the halogenation preferentially affords the 3,6-disubstituted derivatives, which enable the synthesis of poly(carbazole-3,6-diyl)s and copolymers.2 Moreover, Leclerc et al. developed the efficient preparation route of 2,7-dihalogenated carbazoles, and they revealed that poly(carbazole-2,7-diyl)s have wider pi-conjugation system than polymers with the 3,6-connection due to the nitrogen-bridged poly(p-phenylene) skeleton.3, 4 Since then, poly(carbazole-2,7-diyl)s and copolymers have been attracting much attention as reliable materials for optoelectronic applications. Other carbazole-based conjugated polymers with various connection patterns were also prepared by many research groups.5, 6, 7, 8 Recently, we reported partially conjugated fluorescent polymers that contain the carbazole-(bi)fluorene-carbazole chromophore separated by the insulating p-xylylene spacer. The report demonstrated that both the number of para-connected benzene rings and the interaction between the carbazole nitrogen lone pair and the oligophenylene moiety definitely affected the optical properties of the materials.9, 10, 11

As part of our ongoing studies on the structure–properties relationship of carbazole-based conjugated polymers, we herein synthesized three conjugated polymers that had the 1,8-difunctionalized/9-unsubstituted carbazole unit. To the best of our knowledge, although the carbazole derivatives with the free NH group and metal complexes thereof were reported,12, 13, 14 the related conjugated polymers have not been described. If the carbazole NH proton (pKa approximately 20 in dimethyl sulfoxide) forms an intramolecular hydrogen bonding with the functional group at the 1,8-positions, or if carbazole can be converted into the N-metallated species, the electronic character of the carbazole ring can be modified to influence the optoelectronic properties of conjugated polymers. Hence, the effect of intramolecular hydrogen bonding and metal ion doping on the absorption and emission spectra are discussed in this paper.

Experimental procedure

Materials

All reactions were performed in a dry nitrogen atmosphere. 1,8-Dibromo-9H-carbazole was synthesized according to the literature,15 and all synthetic procedures and characterization of carbazole monomers are described in the Supplementary Materials. 9,9-Dihexylfluorene-2,7-diboronic acid was purchased from Sigma-Aldrich (St Louis, MO, USA).

Instrumentation

The 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance 200 FT-NMR spectrometer (Bruker Biospin, Billerica, MA, USA) using tetramethylsilane (δ, 0.00) and CDCl3 (δ, 77.0) as the internal reference peaks, respectively. The infrared (IR) spectra were recorded on a Jasco FT-IR 460Plus spectrophotometer (Jasco, Tokyo, Japan) using the attenuated total reflection (ATR) method. The melting points were determined on a Yanaco MP-500D micro melting point apparatus (Yanaco, Kyoto, Japan). The elemental analyses were performed on a PerkinElmer PE2400II elemental analyzer (Perkin Elmer, Waltham, MA, USA). The gel permeation chromatographic analyses were performed on a Tosoh DP-8020 pump (Tosoh, Tokyo, Japan) using the tandem TSK Multipore HXL-M columns (with tetrahydrofuran (THF) as an eluent, flow rate=1.0 ml min−1, 40 °C), which were equipped with a refractive index detector (RI-8012). The number-averaged molecular weight (Mn) and molecular weight distribution (Mw/Mn) were estimated on the basis of the calibration curve made from standard polystyrene samples. Ultraviolet-visible (UV–Vis) and photoluminescence (PL) spectra were collected using a Shimadzu UV-1650PC spectrophotometer (Shimadzu, Kyoto, Japan) and a Shimadzu RF-5300PC spectrofluorometer (Shimadzu), respectively. The fluorescence quantum yields in the solution were determined relative to quinine sulfate in 0.5 M H2SO4 solution, which has a quantum yield of 0.55.

Polymerization

Imine-functionalized polymer (P1) (Typical procedure): to a THF (10 ml) solution of monomer 4 (0.12 g, 0.20 mmol) and 9,9-dihexylfluorene-2,7-diboronic acids (0.08 g, 0.20 mmol), Pd(PPh3)4 (2.3 mg, 2.0 μmol) and 2 M Na2CO3 (2 ml) were added, and the mixture was heated to reflux for 48 h. After the solution was neutralized with 1 M HCl, an aqueous phase was extracted with CHCl3. The combined organic phase was washed with saturated NH4Cl solution and brine, and then it was dried over MgSO4. The polymer was purified by precipitating in hot CH3OH and subjected to Soxhlet extraction with acetone for 6 h to obtain an orange powder. The yield was 0.10 g (65%). Mn=11 100 (Mw/Mn=1.6). 1H-NMR (δ, CDCl3) 0.63–0.95 (10H, Alkyl), 0.97–1.28 (12H, Alkyl), 1.95–2.45 (22H, Alkyl), 6.87 (4H, bs, Ar), 7.65–8.05 (8H, Ar), 8.64 (2H, bs, Ar), 8.68 (2H, bs, N=CH), 12.64 (1H, bs, NH). IR (ATR) 665, 817, 852, 875, 1012, 1144, 1198, 1264, 1377, 1462, 1587, 2854, 2927, 3019, 3373. Similarly, other conjugated polymers were prepared by the Suzuki coupling polymerization and characterized by gel permeation chromatographic, 1H-NMR spectra and IR spectra as follows:

1,3-Oxazoline-functionalized polymer (P2): Mn=6800 (Mw/Mn=1.4). 1H-NMR (δ, CDCl3) 0.60–0.95 (10H, Alkyl), 0.97–1.32 (12H, Alkyl), 1.56 (12H, bs, Me), 1.95–2.35 (4H, Alkyl), 4.27 (4H, bs, OCH2), 7.62–7.94 (6H, Ar), 8.25 (2H, bs, Ar), 8.59 (2H, bs, Ar), 12.27 (1H, bs, NH). IR (ATR) 681, 722, 820, 878, 999, 1055, 1146, 1194, 1262, 1364, 1464, 1602, 1646, 2856, 2929, 2957, 3022, 3326.

Pyridine-functionalized 9-H polymer (P3): Mn=6700 (Mw/Mn=1.3). 1H-NMR (δ, CDCl3) 0.53–1.01 (10H, Alkyl), 1.01–1.35 (12H, Alkyl), 1.95–2.37 (4H, Alkyl), 7.22–7.40 (2H, Ar), 7.61–8.48 (12H, Ar), 8.61 (2H, bs, Ar), 8.97 (2H, bs, Ar), 13.33 (1H, bs, NH). IR (ATR) 738, 788, 820, 872, 1140, 1203, 1260, 1416, 1456, 1476, 1588, 2853, 2926, 3359.

Pyridine-functionalized 9-Me polymer (P4): Mn=5100 (Mw/Mn=1.5). 1H-NMR (δ, CDCl3) 0.57–0.92 (10H, Alkyl), 0.94–1.25 (12H, Alkyl), 1.94–2.30 (4H, Alkyl), 2.99 (3H, bs, Me), 7.22–7.40 (2H, Ar), 7.63–8.04 (12H, Ar), 8.58 (2H, bs, Ar), 8.80 (2H, bs, Ar). IR (ATR) 683, 747, 789, 820, 874, 1061, 1128, 1186, 1260, 1402, 1429, 1454, 1562, 1584, 2852, 2926, 3050.

Phenyl-functionalized polymer (P0): Mn=8400 (Mw/Mn=1.6). 1H-NMR (δ, CDCl3) 0.60–0.95 (10H, Alkyl), 0.95–1.30 (12H, Alkyl), 1.95–2.33 (4H, Alkyl), 7.36–8.00 (18H, Ar), 8.51 (2H, bs, Ar), 8.58 (1H, bs, NH). IR (ATR) 701, 770, 818, 871, 1028, 1156, 1261, 1415, 1461, 1496, 1599, 2851, 2952, 3028, 3463.

Results and Discussion

Polymer synthesis

Three carbazole monomers that had the imine (4), 1,3-oxazoline (6), and pyridine (8) functional group at the 1,8-positions were synthesized from 1,8-dibromocarbazole. As the N-methyl analog of monomer 8, monomer 9 was prepared to investigate the relationship between the intramolecular hydrogen bonding and the optical properties of the conjugated polymer. Monomer 11, which bore the phenyl group, was also synthesized. The monomer synthetic procedure is described in the Supplementary Materials. These carbazole monomers were subjected to the Suzuki coupling polymerization with 9,9-dihexylfluorene-2,7-diboronic acid (Scheme 1). In preparing the imine-functionalized conjugated polymer, we initially prepared 3,6-dibromo-1,8-bis((phenylimino)methyl)carbazole as the monomer and performed the Suzuki coupling polymerization. However, the imine C=N bond was partially hydrolyzed to produce the formyl group, as it was confirmed by the 1H-NMR spectra, where the remaining imine functional group was calculated to be 80% from the integral ratio. Meanwhile, the polymerization that used monomer 4 with the sterically demanding imine functional group did not undergo hydrolysis to give the corresponding polymer P1, and the signal assignable to the imine proton was observed at 8.68 p.p.m. in the theoretical integral ratio (Figure 1). Similarly, other conjugated polymers were prepared and purified by precipitation in hot CH3OH, followed by Soxhlet extraction with acetone. All conjugated polymers had good solubility in THF, CHCl3 and toluene. The number-averaged molecular weight of the polymers ranged from 5100 to 11 100. The conjugated polymers P1–P3 showed the NH proton signal at 12–13 p.p.m., which was shifted considerably to the downfield region than that of the conjugated polymer P0 (8.58 p.p.m.). The stretching N–H vibration signal of the conjugated polymers P1–P3 were observed below 3400 cm−1, while that of conjugated polymer P0 was observed below 3463 cm−1. These results indicate that intramolecular hydrogen bonding occurred between the carbazole NH proton and the C=N moiety that were included in the functional group at the 1,8-positions.

Figure 1
figure 1

1H-NMR spectrum of conjugated polymer P1 in CDCl3 (200 MHz).

Full size image

Optical properties

The UV–Vis and PL spectra of conjugated polymers P1–P4 were measured in THF solution (Figure 2), and the maximum wavelengths are listed in Table 1. In comparison with the conjugated polymer P0 (λmax=340 nm, the UV–Vis and PL spectra are only given in the Supplementary Materials), which bore the phenyl group at the 1,8-positions, the conjugated polymers P1–P3 showed the absorption maxima at the longer wavelength region. More specifically, the conjugated polymers P1 and P3 had absorption maxima >360 nm. Meanwhile, the conjugated polymer P4, which cannot form intramolecular hydrogen bonding, also showed a slightly red-shifted UV–Vis spectra (and the onset wavelength), and the absorption maximum was observed at 349 nm. This result can be explained by the electronic interaction between the donor (carbazole) and the acceptor (pyridine).16 Therefore, the absorption maximum at 363 nm observed in the conjugated polymer P3 most likely originated from the intramolecular hydrogen bonding. All conjugated polymers emitted weak fluorescence, and the emission maximum wavelengths depended on the functional group at the 1,8-positions of the carbazole unit. The conjugated polymers P1 and P3, which can form strong intramolecular hydrogen bonding (vide supra), showed similar PL spectra, and the fluorescence quantum yields were quite low (QY<0.01). As previously reported for 7-(2′-pyridyl)indole,17 the excited state intramolecular proton transfer might quench the fluorescence emission from these conjugated polymers. In contrast, the conjugated polymer P4, which cannot form intramolecular hydrogen bonding, emitted visible blue light with peak maxima at 410 and 418 nm. Because the conjugated polymer P0 emitted purple light (λmax=399 nm), the conjugated polymer P4 was proven to have the wider pi-conjugated system due to the donor–acceptor interaction as previously described. The conjugated polymer P2 showed the broad PL spectrum with three peak maxima at 410, 431 and 458 nm. Again, the fluorescence quantum yield was low (QY=0.02) because of the intramolecular hydrogen bonding.

Figure 2
figure 2

(a) UV–Vis and (b) PL spectra of the polymers in the THF solution (10−5 M) (circle: P1, triangle: P2, square: P3 and diamond: P4).

Full size image
Table 1 Optical properties of conjugated polymers in THF
Full size table

Subsequently, the influence of the metal ion doping was investigated using the conjugated polymer P3, which bore the pyridine functional group. After the carbazole NH proton at the 9-position was abstracted with NaH, an equimolar amount of ZnCl2 or CuCl2 was added to measure the UV–Vis and PL spectra in the THF solution (Figure 3). The conjugated polymer complex P3–Zn with the coordinated zinc(II) ion had a similar UV–Vis spectrum to the undoped polymer P3. The absorption maximum wavelength was 360 nm, and a small tailing to the longer wavelength region was observed. The conjugated polymer complex P3–Cu with a coordinated copper(II) ion showed new absorption peaks at 406 and 450 nm, which may be assigned to the metal-to-ligand charge transfer transition. Although the conjugated polymer complex P3–Cu was non-fluorescent, the conjugated polymer complex P3–Zn emitted a green light with a maximum wavelength at 500 nm. The fluorescence quantum yield was approximately 0.02, which was larger than that of the undoped polymer P3 (<0.01). Thus, the inhibition of the intramolecular hydrogen bonding and the rigidification by the complex formation with the zinc(II) ion resulted in the redshift and the fluorescence enhancement of the conjugated polymer complex P3–Zn.

Figure 3
figure 3

(a) UV–Vis and (b) PL spectra of the polymers in the THF solution (10−5 M) (circle: P3, triangle: P3–Zn and square: P3–Cu).

Full size image

Conclusions

Three conjugated polymers with alternating carbazole and fluorene units were synthesized using the Suzuki coupling polymerization, where the 1,8-positions of the carbazole were functionalized with imine, 1,3-oxazoline and pyridine. The intramolecular hydrogen bonding between the carbazole NH proton and the C=N moiety included in the functional group was confirmed by the 1H-NMR and IR spectra. Compared with the polymer bearing the phenyl group at the 1,8-positions, all polymers showed the redshift of the UV–Vis spectra, which could be explained by the intramolecular hydrogen bonding and/or the donor–acceptor interaction. The formation of an intramolecular hydrogen bonding decreased the fluorescence quantum yields of the conjugated polymer. Metal ion doping had a large impact on the PL spectra, where the coordination of copper(II) ion completely quenched the fluorescence emission, but the coordination of zinc(II) ion resulted in an increase and a redshift of the PL spectra.

scheme 1

Synthesis of conjugated polymers (P1–P4 and P0) using Suzuki coupling polymerization.