Synthesis and Characterization of Alternating Polymers Incorporating Boron-chelated Heterochrysene Units

The novel boron-chelated π-conjugated polymers named as BF 2-poly and BPh 2-poly were synthesized by a feasible condensation-chelation strategy. First, conjugated polymers bearing Boc group were prepared by using palladium-catalyzed Suzuki–Miyaura coupling reaction. Then, conjugated polymers (poly-1) were obtained with high efficiency by removing the Boc group from Boc-poly. Last, boron trifluoride diethyl etherate (BF3·Et2O) or triphenylboron (BPh3) chelated with poly-1 forming the target polymers. Furthermore, the boron-chelated polymers were characterized by UV−Vis absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry and thermogravimetric analysis. As expected, fluorescences peaks at 520 nm and 592 nm were observed in diluted CH2Cl2. In addition, BF 2-poly and BPh 2-poly showed strong fluorescence at 545 nm and 601 nm in homogeneous solid state. The results coming from thermogravimetric analysis and cyclic voltammetry also revealed that the conjugated polymers have favorable electrochemical and thermostable properties.


Introduction
In the past decades, π-conjugated derivatives have attracted significant interests in organic light-emitting diodes, field effect transistors, photovoltaic cell and fluorescent probes due to their unique characteristics such as high brightness, tunable emission, stability against photobleaching and high photon-emission rates [1][2][3][4][5][6][7][8].In the early 1960s, Clar developed the first synthetic acene as a kind of π-conjugated derivative [9][10][11].Since then, there has been continuing interest in the development of novel types of acenes in both academia and industry [12][13][14][15][16][17].Among these π-conjugated derivatives, chrysene is a relatively special acene with a mismatch structure compared with tetracene, which captured some scholars' attention in recent years, especially after the introduction of boron into the skeleton.
π-conjugated boron complexes are gaining increasing attention because they can be utilized as catalyst, phototherapy drugs and luminescent device [18][19][20][21].It is known that boron is strongly electrophilic by virtue of its tendency to fill the vacant orbital and complete the octet.Therefore, π-conjugated boron complexes possess more stable properties than metal-complexes [22,23].In addition, the strongly electrophilic property may result in larger Stokes shifts, stronger bathochromic shifts and higher fluorescence quantum yields.Usually, BF3•Et2O and BPh3 were used as boron chelating agents to prepare π-conjugated boron complexes.In addition, BPh2 complex possess a stronger bathochromic shift than BF2 complex because of an enhanced conjugation degree as well as the biphenyl boron chelation.Since the stronger electron-withdrawing property, BF2 complex often creates more efficient absorption and luminescence properties.In addition, boron will also bring novel function to π-conjugated polymers.Nagai et al. developed a condensation-chelation model method to prepare the first main-chain-type organoboron 1,3-diketonate polymers via Sonogashira-Hagihara coupling reaction and using BPh3 or (C6F5)BF•OEt2 as chelating agents [24].Jäkle et al. showed that novel well-defined organoboron quinolate polymers have succeeded in emission color-tuning of organoboron quinolate polymers from blue to red regions by treatment with substituted 8-hydroxyquinoline derivatives [25].
Recently, our group reported the boron-chelated N,O-bidentate chrysene analogues in Scheme 1 [26].As obtained two novel boron-based chrysene analogues (Compound 1 and 2) were synthesized by chelating with BF3•Et2O or BPh3, emitting strong green and yellow light in methylene chloride with high quantum yields.The objective of our study was to synthesize organoboron heterochrysene polymers by chelating reaction of heterochrysene based polyfluorene derivative as a polymeric ligand with BF3•Et2O or BPh3.Compound 4 was transformed into a di-brominated monomer followed by copolymerizing with di-borylated fluorene via the palladium-catalyzed Suzuki-Miyaura coupling reaction.All the boron-chelated polymers can be easily dissolved in common organic solvents such as dichloromethane and tetrahydrofuran (THF) to obtain good quality amorphous films.Focus has been drawn to the optical characteristics and electrochemical properties.Scheme 1.The synthesis route of Compounds 1 and 2.

Synthesis of Monomer 5
Thiamine (175 mg, 1.08 mmol) and tetrabutylammonium hexafluorophosphate (4 mg, 0.01 mmol) were dissolved in methanol-water (15 mL:3 mL) mixed solvent.Then sodium hydroxide aqueous solution (10%) was dropped into the above mixture to achieve a weakly alkaline solution (pH 9).After the addition of 6-bromopyridine-2-carboxaldehyde (Compound 7, 200 mg, 1.08 mmol), the mixture was stirred and heated at 65 °C for 3 h.Then the reaction was quenched by addition of excess water to favor the precipitation of a yellow solid.The crude product was collected through centrifugation and washed thoroughly by methanol.The residue was chromatographically purified on silica gel eluting with petroleum ether/CH2Cl2 (1:1) to afford a yellow powder (50 mg).The obtained powder was directly dissolved in anhydrous THF (4 mL) containing di-tert-butyl dicarbonate (292 mg, 1.34 mmol) and N,N-Dimethylaminopyridine (5 mg, 0.04 mmol).The reaction mixture was heated at 45 °C and stirred for 10 h.Excess methanol was added to quench the reaction to give a white precipitation.After centrifugation, the crude product was recrystallized from CH2Cl2/methanol to give a white needle of monomer 5 (55 mg, 0.096 mmol, 17.8%). 1

Characterizations
1 H (600 MHz) and 13 C (150 MHz) nuclear magnetic resonance (NMR) were performed on a Bruker AV600 spectrometer using tetramethylsilane as internal standard.Fourier transform infrared spectroscopy (FT-IR) of the samples was recorded using a Varian 640-IR instrument (Varian, Palo Alto, CA, USA).The electrochemical behaviors were investigated in the solid state by cyclic voltammetry (CV) in anhydrous acetonitrile solution at room temperature.A three-electrode electrochemical cell was used, which was comprised of a working electrode (glassy carbon electrode), a platinum mesh auxiliary electrode, and an Ag/AgNO3 reference electrode.Furthermore, 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) was used as a supporting electrolyte.The oxidation potentials were measured versus a standard ferrocene/ferrocenium redox system as the internal standard for estimating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the boron-chelated polymers films.The scan rate was 50 mV•s −1 .Thermogravimetric analysis (TGA) was measured by TGA (Pyris-6, Perkin-Elmer, Waltham, MA, USA) operated under 20 mL/min nitrogen flow rate at the heating rate of 10 °C/min.UV-vis and fluorescence spectra were obtained through a UV2550 UV-Vis spectrophotometer (Shimazu, Japan) and an RF5301PC fluorescence spectrophotometer (Shimazu, Japan), respectively.Number-average molecular weight (Mn) and the molecular weight distribution (Mw/Mn) were measured via gel permeation chromatography (GPC) (Waters, Milford, MA, USA) with a system-equipped Waters HT4 and HT3 column-assembly and a Waters 2414 refractive index detector.The column was placed under 30 °C; the system was operated at a flow rate of 1.0 mL/min with THF as an eluent.Polystyrene was used as standards for the calibration.Film X-ray diffraction (XRD) patterns were performed at room temperature with a D8 ADVANCE X-ray power diffractometer system (Bruker, Faellanden, Switzerland).The patterns were recorded on a quartz plate at a tube voltage of 40 kV and a current of 40 mA over a 2θ range of 10°-90° using a step size of 0.01° at a scan speed of 1 s/step.

Synthetic Approach and Characterizations
As shown in Scheme 2, a symmetrical di-brominated monomer (5) was synthesized by a benzoin condensation of 6-bromopyridine-2-carboxaldehyde (7), where thiamine and tetrabutylammonium hexafluorophosphate were used as catalysts.The product was directly treated with excess di-tert-butyl dicarbonate ester (Boc2O), aiming to eliminate the intermolecular cross-linking effect by hydrogen bonding between hydroxyl proton and oxygen atom in enol forms.The structure of 5 was confirmed by FT-IR (shown in Figure S1), elemental analysis and NMR (shown in Figures S2 and S3).Monomer 6 was synthesized following a procedure described elsewhere [27].Monomers 5 and 6 were equimolarly mixed, where Pd(PPh3)4 was used as catalyst.Obtained Boc-poly was treated with piperidine under mild conditions.The reacting solution was centrifuged to remove some insolubles formed in deprotection reaction and the supernatant was concentrated and then precipitated into methanol to give poly-1.Poly-1 was further reacted with excess BF3•Et2O or BPh3 in anhydrous CH2Cl2 or THF to give BF 2 -poly and BPh 2 -poly.All the polymers exhibited desirable solubility in THF, CH2Cl2 and chloroform.Scheme 2. The synthesis route of conjugated polymers.
The elution curves and GPC Data for obtained polymers were shown in Figure 1 and Table 1, respectively.The Mn and Mw/Mn of Boc-poly were determined as 1.41 × 10 4 Da and 3.17, respectively.Poly-1 exhibited a lower molecular weight (Mn = 1.03 × 10 4 Da) and a narrower distribution (Mw/Mn = 1.93).After chelating reactions, the Mn of BF 2 -poly and BPh 2 -poly were 1.13 × 10 4 Da and 1.24 × 10 4 Da with polydispersity indexes being 1.86 and 2.12, respectively.The signals in the 1 H NMR spectra of the polymers can be classified into two groups (shown in Figures S4-S7): the signals that emerged at 7.3-8.3ppm can be attributed to the aromatic protons; and the peaks ranging from 0.5 to 2.5 ppm can be assigned to the hydrogen atoms in alkyl groups.It is worth noting that the distinct signal at 1.29 ppm in the 1 H NMR spectrum of Boc-poly represents the hydrogen in Boc groups (shown in Figure S4).The disappearance of the peak at 1.29 ppm in the 1 H NMR spectrum of poly-1 suggested that the deprotection reaction was completed (shown in Figure S5).The 1 H NMR spectrum of BF 2 -poly showed a similar tendency to that of poly1 (shown in Figure S6).However, compared to those of poly-1 and BF 2 -poly, the integral of the aromatic protons in BPh 2 -poly increased more than double, which coincided with expected structures (shown in Figure S7).The 13 C NMR spectra of BF 2 -poly and BPh 2 -poly were illustrated in Figures S8 and S9, respectively.
The FT-IR spectra of the polymers were shown in Figure 2. Boc-poly displayed a characteristic absorption band around 1768 cm −1 in FT-IR spectrogram, which can be attributed to the C=O stretching in Boc groups.After deprotection, the disappearance of the C=O stretching in FT-IR spectrogram of poly-1 also suggested that the deprotection reaction was completed.Meanwhile, FT-IR analysis showed that poly-1 contained -OH group (3307 cm −1 ), which can be assigned to the hydroxyl group coming from 2,2'-pyridoin-based ligand.The FT-IR spectra of BF 2 -poly and BPh 2 -poly showed the disappearance of -OH group, suggesting the chelating reactions were completed.The XRD patterns of BF 2 -poly and BPh 2 -poly are shown in Figure 3.The samples displayed no distinct peaks from 10° to 90°, suggesting the polymers existed in an amorphous form rather than crystals.
The thermal properties of monomer 5 and polymers were studied by thermogravimetric analysis (shown in Figure 4).The monomer showed a desirable thermostability with an onset temperature of weight loss (5%) at 375 °C.The onset temperatures of weight loss (5%) of BF 2 -poly and BPh 2 -poly were 288 °C and 248 °C, respectively, suggesting a higher thermal stability than poly-1 (220 °C).

UV-vis Absorption and Photoluminescence Studies
The UV-vis absorption spectra were measured in diluted CH2Cl2 solution, as shown in Figure 5a.The detail data were shown in Table 2. Boron-based heterochrysene derivatives (1 and 2) displayed maximum absorption peak at 410 and 468 nm.The wavelengths of maximum absorption of BF 2 -poly (433 nm) and BPh 2 -poly (481 nm) were high compared to those of 1 and 2, because of the extended conjugation after polymerization.Otherwise, the relative electron-donating ability of the phenyl units can form an intramolecular charge-transfer system in D-p-A system, resulting in a significant difference between BF 2 -poly and BPh 2 -poly [26].The biphenyl boron chelation in BPh 2 -poly resulted in a stronger bathochromic-shift.Photoluminescence behaviors were also investigated in diluted CH2Cl2, as shown in Figure 6.BF 2 -poly and BPh 2 -poly displayed emission peaks at 520 nm and 592 nm, respectively.BF 2 -poly and BPh 2 -poly showed obvious bathochromic-shift compared to the corresponding compounds.The quantum yields (Φf) of BF 2 -poly and BPh 2 -poly were calculated as 0.23 and 0.11, respectively, where N,N'-di(2,6-diisopropylphenyl)-perylene-3,4,9,10-tetracarboxylic acid bisimide (Φf = 1.00 in chloroform) was used as the reference standard [29].It should be noted that boron-chelated polymers exhibited decreased fluorescence quantum compared to those of 1 and 2, respectively.The decreased fluorescence quantum yields of boron-chelated polymers may be partly because of the unrestricted rotation around the C-C bond connecting chrysene unit to fluorene unit in noncoplanar conjugated polymers.Otherwise, 100% chelating ratios of BF 2 -poly and BPh 2 -poly were difficult to achieve.A small quantity of 2,2'-pyridoin groups may still exist in the boron-chelated polymers, which may also contribute to decreased fluorescence quantum yields.Films of boron-chelated polymers on quartz plates used for absorption and fluorescence investigations were prepared by spin-coating from a CH2Cl2 solution.The absorption maxima of the boron-chelated polymers in thin films shown in Figure 5b were almost identical to those in solution, which indicated that there was nearly no change in conformation of the boron-chelated polymers from the solution to the solid state.In addition, the absorption results of boron-chelated polymers in thin films were used to calculate the band gap values in electrochemical analysis.The fluorescence spectra of the boron-chelated polymers showed significant differences between in CH2Cl2 solution and in the solid state (Figure 6).BF 2 -poly and BPh 2 -poly showed emission peaks at 545 nm and 601 nm, respectively.The maximum emission peaks of BF 2 -poly and BPh 2 -poly exhibited red shift of 25 and 9 nm in the solid state.This can be ascribed to the stronger self-absorption effect and a higher degree of ordering in solid state than those in solution.However, the reason for the difference in red shift values from solution to the solid state between two different boron-chelated polymers is still not clear.

Electrochemical Analyses
The electrochemical behaviors of BF 2 -poly and BPh 2 -poly were investigated by cyclic voltammetry (CV) with a standard three-electrode electrochemical cell in anhydrous acetonitrile solution at room temperature with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte to test their electron affinity.The oxidation potentials were measured versus Ag/AgNO3 as the reference electrode and a standard ferrocene/ferrocenium redox system as the internal standard for estimating the HOMO of the boron-chelated polymers films.A glassy carbon electrode was used as a working electrode.A platinum sheet was used as an auxiliary electrode.

Figure 6 .
Figure 6.Fluorescence spectra of BF 2 -poly and BPh 2 -poly in CH2Cl2 and the solid state.

Table 1 .
Summary of gel permeation chromatography (GPC) data for obtained polymers.

Table 2 .
Absorption and fluorescence data for relating compounds in CH2Cl2 solution.