Introduction
π-conjugated polymers are important materials for organic electronic applications, and thienyl-derived moieties possess excellent electronic properties. A number of synthetic routes and physical properties of soluble poly(dialkylcyclopentadithiophene) polymers, pCPDT, a Csp3-bridged bithiophene framework, have been extensively studied (Coppo & Turner, Reference Coppo and Turner2005). Current use of pCPDT homopolymer is far less extended than its donor-acceptor copolymer analogue, e.g., cyclopentadithiophene-benzothiadiazole, due to its high HOMO energy level which makes it sensitive to oxidation. Nonetheless, since the first chemical polymerisation report of dialkyl-substituted cyclopentadithiophenes to form soluble pCPDT (Asawapirom & Scherf, Reference Asawapirom and Scherf2001) its use as active layer in organic field-effect transistors (Horie et al., Reference Horie, Majewski, Fearn, Yu, Luo, Song, Saunders and Turner2010), organic photovoltaics (Bijleveld et al., Reference Bijleveld, Fonrodona, Wienk and Janssen2010), and tunning of its bandgap via protonation of terminal α-positions with trifluoroacetic acid (Tang et al., Reference Tang, Lin, Wang and He2015) suggest there is room for novel applications.
Despite its non-toxic reagents and by-products, among other advantages, the Suzuki-Miyaura cross-coupling (Suzuki, Reference Suzuki2011) remains under explored for the synthesis of thienyl-derived polymers, compared to other transition-metal catalysed cross-coupling reactions such as Kumada and Stille methodologies, partly due to the difficulty in synthesising the required organoboryl-containing monomers and their undesired protodeborylation during the cross-coupling polymerisation reaction. In this regard, N-methyliminodiacetic acid (MIDA) boronate esters have proven to be excellent nucleophilic partners for the Suzuki-Miyaura reaction because they can slowly deliver the reactive boronic acid under mild conditions (Burke & Gillis, Reference Burke and Gillis2009).
Objective
To synthesise cyclopentadithiophene homopolymers, pCPDT, via Suzuki-Miyaura polymerisation using MIDA boronate esters as the key boron masking group in AB-type monomers, and thus demonstrate their potential as efficient substrates for the formation of thienyl-derived conjugated polymers in high yields and high polymer molecular weights.
Methods
The Direct C-H Electrophilic Borylation synthetic methodology (Bagutski et al., Reference Bagutski, Del Grosso, Ayuso Carrillo, Cade, Helm, Lawson, Singleton, Solomon, Marcelli and Ingleson2013) was employed for the preparation of the MIDA boronate ester monomer, 1, according to the procedure previously reported (Ayuso Carrillo et al., Reference Ayuso Carrillo, Ingleson and Turner2015). pCPDT homopolymer was prepared via Suzuki-Miyaura polymerisation methodology (Suzuki, Reference Suzuki2011). Further details are provided in the Supplementary Materials.
Results
Monomer 1 quantitatively produces pCPDT (99% isolated yield, crude polymer) under identical polymerisation reaction conditions that were optimised for poly(3-hexylthiophene), P3HT (Ayuso Carrillo et al., Reference Ayuso Carrillo, Ingleson and Turner2015). Table 1 shows polymer molecular weights recorded at different reaction times.
Selected results from the Suzuki-Miyaura polymerisation of 1 to form pCPDT.a
a Reaction conditions: T: 55 °C, [1] = 6.1 × 10−2 M, K3PO4: 3 equiv, H2O: 40 equiv, Pd2(dba)3: 2.5 mol%, SPhos (CAS 657408–07-6): 5 mol%, Solvent: THF. bDetermined by Gel Permeation Chromatography (THF at 35 °C, polystyrene calibration). Quoted values corresponding to Soxhlet-fractionated chloroform fraction after sequential extractions with methanol, and n-hexane, for 14 h each. cNumbers in parenthesis correspond to crude samples.
Suzuki-Miyaura polymerisation of 1 displayed reaction features analogously to other MIDA boronate esters, e.g., slow release of the corresponding thienyl boronic acid, step-growth of polymer chains over time, high yield and high molecular weight of isolated polymer. For example, Figure 1 shows the increase of polymer molecular weights of pCPDT over time (Ayuso Carrillo et al., Reference Ayuso Carrillo, Ingleson and Turner2015; Reference Ayuso Carrillo, Turner and Ingleson2016; Foster et al., Reference Foster, Bagutski, Ayuso-Carrillo, Humphries, Ingleson and Turner2017). Likewise, the 1H NMR spectra of the filtered solutions after polymer precipitation (Figure S1) showed evidence of 1 only but no deboronated monomer, after 4 h and 8 h of reaction.
GPC traces at different times of 1-derived pCPDT. Crude samples. Reaction conditions as in Table 1.
Discussions
Combination of highly active phosphines and palladium precatalysts with cyclopentadithiophene-derived MIDA boronate ester AB-type monomers afforded pCPDT under mild reaction conditions. The molecular weights of isolated polymers after Soxhlet fractionation (typically producing ≥90% yield of the chloroform fraction) are superior to those reported in the literature synthesised by Yamamoto reductive polymerisation (e.g., M n = 5–9 kDa, Ð M = 1.8–2.6) (Asawapirom & Scherf, Reference Asawapirom and Scherf2001), oxidative polymerisation with FeCl3 (e.g., M n = 10.5 kDa, Ð M = 3.9) (Horie et al., Reference Horie, Majewski, Fearn, Yu, Luo, Song, Saunders and Turner2010), and comparable to those obtained by Kumada catalyst-transfer polymerisation (e.g., M n = 31.2 kDa, Ð M = 2.0) (Willot et al., Reference Willot, Govaerts and Koeckelberghs2013).
It is noteworthy that 1 was prepared via a two-step synthesis (Scheme 1, left) where 2 was obtained during the purification of the diborylated cyclopentadithiophene unit via chromatographic column. In contrast, as shown in Scheme 1, top-right, multiple attempts to borylate 3 in a controlled fashion were unsuccessful, leading to in situ deborylation and uncontrolled electrophilic polymerisation of the cyclopentadithiophene fragment (see Supplementary Materials).
Possible synthetic routes for cyclopentadithiophene-derived AB-type monomers for Suzuki-Miyaura polymerisation.
Conclusions
The synthesis of soluble poly(cyclopentadithiophene) polymers employing N-methyliminodiacetic acid boronate ester/bromo heterobifunctionalised AB-type monomers was demonstrated. Thus, the use of MIDA boronate esters in Suzuki-Miyaura polymerisations holds a promising future for the efficient and non-toxic preparation of π-conjugated polymers relevant for organic electronics applications.
Acknowledgements
JA-C acknowledges: Prof M Ingleson and Prof M Turner for guidance and support during his PhD studies, Dr J Esquivel-Guzman for synthesis of precursors, and the School of Chemistry, University of Manchester for access to instrumentation facilities.
Author Contributions
JA-C designed, performed the experiments, collected and analysed the data, and wrote the manuscript.
Funding Information
This work was partially supported by Conacyt-Mexico (scholarship 311311).
Conflict of Interest
JA-C declares none.
Data Availability Statement
The data that support the findings of this study are openly available in Zenodo at http://doi.org/10.5281/zenodo.3735473
Supplementary Materials
To view supplementary materials for this article, please visit http://dx.doi.org/10.1017/exp.2020.30.
Open access
Comments
Comments to the Author: The authors report an original and new PCPDT synthesis via Suzuki-Miyaura polymerisation of MIDA boronate esters. The method is well outlined and explained. The results will be of interest and significant benefit for the community working in the synthesis of conjugated polymers especially polythiophenes. The report is, therefore, recommended for publication after some minor revision:(1) PCPDT synthesis via Yamamoto-type homocoupling that starts from dibromoCPDT monomers should be included into the discussion of advantages and shortcomings of the several methods that have been applied for PCPDT generation until now (see e.g. Udom Asawaprom et al. 2001, DOI: 10.1002/1521-3927(20010701)22:10<746::AID-MARC746>3.0.CO;2-H).(2) Please add the solution UV-vis spectra of the PCPDTs of entries 1-3.