BF₃·Et₂O-mediated Friedel–Crafts C–H bond polymerization to synthesize π-conjugation-interrupted polymer semiconductors

Abstract

C–H bond functionalization offers a chance to develop a new concept of polymerization. In this article, a family of diarylfluorene-based π-conjugation-interrupted polymers (CIPs) with nonlinear or hyperbranched frameworks have been explored by BF₃·Et₂O-mediated Friedel–Crafts polymerization of AB, AB₂ and AB₄-type tertiary alcohol monomers at room temperature. Their chemical structures and optoelectronic properties have been characterized and the existence of strong intermolecular π-stacked aggregates have been observed. CIP semiconductors afford a new platform to explore novel functionality of organic devices in organic electronics or spinelectronics.

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Polymer semiconductors, as key active components, determine the functionality and performance in polymer devices, such as polymer light-emitting diodes (PLEDs),¹ bulk heterojunction polymer solar cells (PSCs),^2,3polymer memory^4,5 and polymer field effect transistors,^6,7 affording a new chance to challenge information technology and to resolve energy and control issues. Their chemical essence is π-orbital channels, which can be created via molecular engineering or total synthesis.⁸ In this framework, on one hand, it is significant to design new concept polymer semiconductors with the unique electronic structure, steric hindrance, conformation and topology as well as supramolecular interaction beyond π-conjugated polymers. For example, π-conjugation-interrupted polymers (CIPs), intermediate π-systems between π-conjugated and π-stacked configurations, exhibit intrinsic nonplanar conformations, limited conjugation length and abundant conformation isomers with the potential application of wide-bandgap host electrophosphorescent materials,^8–10 electrically memorable polymers^4,5 and micro-porous materials.^11–13

On the other hand, it is more and more emergent to develop low-cost, green and eco-friendly polymerization methodologies in terms of the acceleration of commercialization. In the past several decades, transition metal-catalyzed carbon-carbon cross-coupling reaction, such as Suzuki, Negishi, Sonogashira, Stille and other reactions, afford useful tools to synthesize the polymer semiconductors. Until now, C–H bond functionalization ^14,15 offers flexible and powerful synthetic protocols with the advantage being practical, atom-economical and environmentally-friendly to challenge the low-cost and environmental pollution. For example, Friedel–Crafts (F–C) reaction of arenes as conventional C–H bond functionalization has been a practical and atom-economical method since its discovery early in 1877.^16,17 F–C catalysts have allowed the initiation of cationic polymerization of vinyl monomers or the polycondensation of high-performance polyesters or polyketonesvia the acylation reaction of aryloyl chloride in the past century.^18–21 An alternative F–C polymerization ultilizes the tertiary alcohol to generate carbocations.²⁴ However, most of them required high-temperature and harsh acid-catalyzed conditions with complicated byproducts,²² which become a challenge when constructing polymer semiconductors for organic optoelectronics.²³

In our previous work, we have reported the developed BF₃·Et₂O-mediated F–C reaction to synthesise complicated 9,9-diarylfluorenes with special nonplanar conformations at room temperature²⁴ and F–C click post-functionalization (FCCP) of PVK to construct memorable polymers applied in nonvolatile flash memory.^4,25 We found fluorenyl tertiary alcohol with very high reactivity without steric effect, encouraging us to develop the single-monomer methodology (SMM) of F–C polymerization. In this communication, a series of diarylfluorene-based π-conjugation-interrupted polymers with unique conformations have been synthesized via the BF₃·Et₂O-mediated F–C C–H bond polymerization of AB-type, AB₂-type or AB₄-type monomers (Fig. 1). Preliminary photoluminescence characterizations exhibit anomalous photophysical properties.

Fig. 1 Friedel–Crafts C–H polymerizations of various single-monomers of tertiary alcohol.

The synthesis of tertiary alcohol monomers and polymers are outlined in Scheme 1. We integrated thiophene and/or carbazole reactive groups into fluorene at the 2 or 7 position to construct tertiary alcohol single-monomers and to investigate the BF₃·Et₂O-mediated F–C C–H polymerization. Firstly, AB-type, AB₂-type or AB₄-type tertiary alcohol monomers, including 2-(thiophen-2-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-ol (TPFOH), 2,7-di(thiophen-2-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-ol (DTPFOH), 2-(carbazol-9-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-ol (CzPFOH) and 2,7-dicarbazol-9-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-ol (DCzPFOH), were designed and prepared by the Grignard reaction of ketone substituted by the electron-rich thiophene or carbazole groups with the reactivity of the F–C reaction. Pd(PPh₃)₄-catalyzed Suzuki cross-coupling reaction of thiophen-2-yl boronic acid starting from 2-bromofluorenone and 2,7-dibromofluorenone and consequently the typical Grignard nucleophilic addition were carried out to give corresponding AB-type monomer TPFOH and AB₂-type monomer DTPFOH with the overall yields of 75% and 79%, respectively. AB₂-type carbazole monomer CzPFOH and AB₄-type monomer DCzPFOH were synthesized via the key step of an Ullmann C–N coupling reaction in the conditions of CuI/DCB/BPy, followed by the same Grignard reaction in the overall yields of 80% and 68%, respectively.

Scheme 1 Total synthesis routes for tertiary alcohol monomers and π-conjugation-interrupted polymers.

After that, the F–C C–H polymerization of AB-type, AB₂-type and AB₄-type tertiary alcohol monomers were carried out according to our previous conditions using BF₃·Et₂O complex as Lewis acid catalysts.²⁴Dichloromethane (DCM) was used as the polymerization solvent owing to its good solubility of the monomers and target polymers. A new series of π-conjugation-interrupted polymers, including poly[2-(thiophen-2-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-yl] (PTPF), poly[2,7-di(thiophen-2-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-yl] (PDTPF), poly[2-(carbazol-9-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-yl] (PCzPF) and poly[2,7-di(carbazol-9-yl)-9-(4-(octyloxy)phenyl)-fluoren-9-yl] (PDCzPF), have been synthesized at ambient temperature with high yields of 65%, 80%, 73% and 68%, respectively. The gel permeation chromatography (GPC) results of the four polymers were measured using THF as eluent and polystyrene as standard. It is found that number average molecular weight (M_n) of polymers PCzPF (9472 g mol⁻¹) and PDCzPF (8894 g mol⁻¹) are obviously higher than that of PTPF (1989 g mol⁻¹) and PDTPF (2236 g mol⁻¹) (Table 1). Polymer degree of repeat unit in PCzPF is up to 18 and PTPF has only about 4 repeat units. These results indicated that carbazole-based tertiary alcohol monomers exhibit good reactivity with respect to thiophene-based analogues under the same reaction conditions.

Table 1 The molecular weights, optical properties and electrochemical data for the pure polymers

Polymer	M n	M w	PDI	λ abs, max/nm		λ PL , max/nm		Cyclic voltammetry
				Sol.	Film	Sol.	Film	E ox/(V)	E red/(V)	HOMO (eV)	LUMO (eV)	E g (eV)
PTPF	1989	2762	1.39	332	346	401	470	1.08	−2.59	−5.80	−2.13	3.67
PDTPF	2236	2876	1.29	357	362	433	470, 524	0.91	−2.44	−5.63	−2.28	3.35
PCzPF	9472	13698	1.45	330	333	376	395, 550	0.94	−2.59	−5.66	−2.13	3.53
PDCzPF	8894	12346	1.39	348	350	377	400, 480, 550	0.92	−2.60	−5.65	−2.14	3.50

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The molecular structures of these polymers were further confirmed by ¹H and ¹³C NMR and FT-IR spectra (see ESI† for detailed analysis data). Using ¹H NMR and IR spectra of PCzPF as an example, together with its monomer CzPFOH for the purpose of comparison. In NMR, the resonance peak at δ 2.5 assigned to –OH proton resonance of the monomer completely disappeared in the spectrum of its corresponding polymer. Furthermore, the resonance peaks at δ 3.80 assigned to –OCH₂– proton resonance and other alkane and aromatic regions become broader, which indicated that polymerizations occur under BF₃·Et₂O catalyst.

In IR analysis, the strong absorption band observed at 3536 cm⁻¹ in CzPFOH is associated with its –OH stretching vibration, the broad absorption peak at 3400 cm⁻¹ come from the intermolecular hydrogen bonds of tertiary alcohols. For corresponding PCzPF, these bands weaken significantly, indicating that most of the hydroxyl groups have been consumed by the polymerization. All these characterizations have confirmed that the π-conjugation-interrupted polymers, including PTPF, PDTPF, PCzPF and PDCzPF, have been successfully prepared via the BF₃·Et₂O-catalyzed F–C C–H bond polymerization. All the CIPs exhibited high thermal stability with decomposition temperatures of up to 400 °C (see ESI†).

The UV-vis absorption and fluorescence spectra of four polymers in solutions and in thin films are displayed in Fig. 2, and their optical data are summarized in Table 1. Transparent thin films of these polymers on glass substrates (quartz plates) were prepared by spin coating from the THF solution. They have the electronic absorption peak in the range from 330–360 nm and exhibit violet and blue emission with the emission peaks of 380–430 nm in solution. These results are consistent with their limited effective conjugation length with wide bandgaps owing to the conjugation-interrupted conformations of all the polymers. This phenomenon is due to the fact that PDTPF and PDCzPF have longer conjugation lengths than PTPF and PCzPF, which have been also confirmed by their bathochromic shift in UV-vis spectra. All the polymers in thin films have a bathochromic-shift of the UV spectra and a red-shifted PL spectra with respect to their corresponding solution. Furthermore, the PL spectra exhibit new shoulders and/or tailing peaks with respect to that in solution, except for the polymer PTPF. For example, polymer PDTPF exhibits the maximum peak of 524 nm with a shoulder peak of 470 nm. In addition, for PTPF and PDTPF, they exhibit large Stokes shifts of 123 nm for PTPF and 162 nm for PDTPF in thin films, respectively, which indicate that the strong interchain interactions occur. Similarly, with respect to solution, both PCzPF and PDCzPF in films exhibit a new tailed peak of 550 nm and a shoulder peak of 480 nm added for PDCzPF. This anomalous PL can probably be attributed to the intermolecular π-stacked interaction resulting in the aggregate or excimer emission. It can be deduced that this kind of polymer semiconductor might serve as single-molecular white electroluminescent materials if further fine design is conducted.

Fig. 2 Absorption and PL spectra of four polymers in DCM and in thin films. (a), (b) in DCM (dilute solutions); (c), (d) in thin films.

The electrochemical behavior of four polymers PTPF, PDTPF, PCzPF and PDCzPF was investigated by cyclic voltammetry (CV). CV curves are displayed in Fig. 3 and the data are summarized in Table 1. The highest occupied molecular orbital (HOMO) energy levels are estimated to be −5.80, −5.63, −5.66, and −5.65 eV according to their oxygen onset position of polymers are 1.08, 0.91, 0.94, and 0.92 V, respectively. The lowest unoccupied molecular orbital (LUMO) energy levels are estimated to be −2.13, −2.28, −2.13, and −2.14 eV according to their oxygen onset position of polymers are −2.59, −2.44, −2.59, and −2.60 V, respectively. Their bandgaps are in the range of 3.35–3.67 eV, which major contributed to the conjugation-interrupted backbones.

Fig. 3 Cyclic voltammograms of thin films of four polymers coated onto platinum plate electrodes in acetonitrile containing n-Bu₄N⁺PF₆⁻ (0.1 M); counter electrode: platinum wire; reference electrode: Ag/AgNO₃ (0.1 M in acetonitrile), scan rate: 100 mV s⁻¹.

In conclusion, BF₃·Et₂O-mediated Friedel–Crafts (F–C) polymerization as a new concept C–H polymerization has been successfully developed. A series of π-conjugation-interrupted polymers (CIPs) have been synthesized starting from tertiary alcohol monomers. Carbazole-based tertiary alcohol monomers exhibit more reactivity than that of thiophene-based analogues under the same reaction conditions to give CIPs with higher molecular weight. These CIPs exhibit wide bandgap more than 3.0 eV with the aggregate or excimer emission in the thin film owing to their tangle interchain interaction of congested conformations, which offer new models to investigate the magnetic effects of polymer semiconductors. We will work on optimizing synthetic conditions for further increasing the molecular weight and exploring the potential value in the field of PLEDs or nonvolatile resistance-type memory cells. C–H bond polycondensations offer promising methods to develop the polymer semiconductors.

Acknowledgements

The authors wish to thank the National Key Basic Research Program of China (973) (2009CB930600), National Natural Science Foundation of China (60876010, 20774043, 20704023, 20974046), Key Project of the Ministry of Education of China (208050), and Natural Science Foundation of Jiangsu Province (BK2008053, 10KJB510013, SJ209003).

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Footnote

† Electronic supplementary information (ESI) available: Materials, experimental details and the NMR, GPC FT-IR and TGA characterization data. See DOI: 10.1039/c1py00203a

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