Cu-catalysed oxidative C-H/C-H coupling polymerisation of benzodiimidazoles: An efficient approach to regioregular polybenzodiimidazoles for blue-emitting materials

Article abstract of DOI:10.1039/c4cc06291d

The Cu-catalysed oxidative C-H/C-H coupling reaction of azoles has been used for the first time to develop polymerisation, which provides an efficient method for the preparation of polybenzodiimidazoles. These polymers exhibit high molecular weights, regioregularity, blue-emitting performance and thermal stability. This journal is

Full text of DOI:10.1039/c4cc06291d

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Cu-catalysed oxidative C–H/C–H coupling
polymerisation of benzodiimidazoles: an efficient
approach to regioregular polybenzodiimidazoles
for blue-emitting materials†
Cite this: Chem. Commun., 2014,
50, 13739
Received 11th August 2014,
Accepted 14th September 2014
Quan Huang, Xurong Qin, Bijin Li, Jingbo Lan,* Qiang Guo and Jingsong You*
DOI: 10.1039/c4cc06291d
www.rsc.org/chemcomm
The Cu-catalysed oxidative C–H/C–H coupling reaction of azoles
has been used for the first time to develop polymerisation, which
provides an efficient method for the preparation of polybenzodi-
imidazoles. These polymers exhibit high molecular weights, regio-
regularity, blue-emitting performance and thermal stability.
p-Conjugated polymers have been widely applied in organic opto-
electronic devices, such as organic light-emitting diodes (OLEDs),
organic photovoltaics (OPVs), organic field-effect transistors (OFETs)
and optical sensors.1 The conventional coupling approaches to
p-conjugated polymers, involving transition-metal-catalysed C–X/
C–M and C–X/C–X coupling reactions such as Yamamoto,2 Suzuki3
and Stille coupling,4 usually suffer from limitations including
tedious synthetic steps to prepare bifunctional aryl halides and/or
organometallic monomers, a stoichiometric amount of toxic bypro-
ducts, an extra end-capping procedure for removal of terminal
Scheme 1 Various coupling polymerisations. (Het)Ar = aryl or heteroaryl.
halogens or organometallic functional groups, and poor stabilities efficient synthetic strategy to build up p-conjugated polybenzodi-
of some organometallic reagents. Recently, the direct C–H arylation imidazoles (PBDI) through Cu-catalysed oxidative C–H/C–H cou-
polymerisation via the dehydrohalogenative cross-coupling reaction pling of benzobisimidazoles (Scheme 1).
of (hetero)arenes with aryl halides has become widely recognized as
Polybenzimidazoles (PBIs), polybenzodiimidazoles (PBDIs) and
an atom- and step-economic method to construct p-conjugated their derivatives have been considered as promising materials for
polymers.5 However, this method still requires the prior preparation the fabrication of optoelectronic devices and proton-exchange mem-
of aryl halides as monomers and the end-capping reaction of the branes.7,8 Normally, the synthesis of PBDI is implemented by a
halide terminus. Moreover, five-membered heterocyclic halides such polycondensation of 1,2,4,5-tetraaminobenzene tetrahydrochloride
as haloimidazoles are often seen as difficult coupling partners.6 with oxalic acid.7c However, this polycondensation involves draw-
Undoubtedly, the direct oxidative C–H/C–H coupling polymerisation backs such as the air sensitivity of substrates and high reaction
of nonpreactivated (hetero)arenes would be one of the most ideal temperature (180–200 1C). Especially, there is no effective way to
approaches to p-conjugated polymers, which would eliminate the control the regioselectivity of polymerisation and, thus, inevitably
extra steps associated with the preparation of bifunctional mono- involving formation of various regiorandom isomers (Scheme 2).
mers and end-capping reactions. But so far, the transition-metal- Therefore, the development of a highly efficient coupling poly-
catalysed oxidative C–H/C–H coupling polymerisation of azoles still merisation to access regioregular polybenzodiimidazoles, especially
remains unprecedented. Herein, we wish to explore a facile and starting from relatively stable substrates, is urgently required.
Although Pd-catalysed systems are highly efficient for the
C–H activation of azoles,9 the issues involving the probably ring-
opened isomerization of azoles in Pd-systems and removal of the
Key Laboratory of Green Chemistry and Technology of Ministry of Education,
College of Chemistry, and State Key Laboratory of Biotherapy, West China Medical
School, Sichuan University, 29 Wangjiang Road, Chengdu 610064, P. R. China.
trace Pd from products encourage us to develop a Cu-based catalyst
E-mail: jingbolan@scu.edu.cn, jsyou@scu.edu.cn; Fax: +86 28-85412203
system for the preparation of PBDI.10 Our recent efforts in direct
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc06291d
oxidative C–H/C–H cross-coupling reactions between two different
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Scheme 3 Cu-catalysed oxidative C–H/C–H coupling polymerisation of
various alkyl substituted BDIs. Reaction conditions: BDI (0.25 mmol),
Cu(OAc)2 (20 mol%), Ag2CO3 (0.5 equiv.), and xylene (1.0 mL) under O2
(1 atm) at 140 1C for 24 h; the products were obtained by reprecipitation
from CHCl3–MeOH after Soxhlet extraction; Mn and PDI were estimated
by GPC on polystyrene standards.
Scheme 2 The polycondensation route and the oxidative C–H/C–H
coupling approach to PBDIs.
azoles have demonstrated that the Cu-based catalyst system is a
great substitute for Pd-catalysed systems.9b,11 Therefore, the initial
investigation started with the coupling polymerisation of 1,7-dioctyl-
1H,7H-benzo[1,2-d:4,5-d]diimidazole (BDI-8) in the presence of
Cu(OAc)2. Delightedly, the Cu-catalysed coupling polymerisation is
apt to occur under the conditions using Ag2CO3 and O2 as oxidants,
affording the polymer PBDI-8 with a moderate number-average
molecular weight (Mn = 8100) in 63% yield (soluble fraction in
chloroform) (Table S1, entry 1, ESI†). Moreover, reducing the
amount of Ag2CO3 to 0.5 equivalent led to a higher Mn of 32 000
(PDI of 1.52) with a yield of 66% (Table S1, entry 3, ESI†). But further
reducing the loading of Ag2CO3 afforded a decreased yield and Mn
(Table S1, entry 4, ESI†). Upon increasing the amount of Cu(OAc)2 to
one equivalent and without using Ag2CO3, only oligomeric products
were obtained (Table S1, entries 5 and 6, ESI†). Trace water might
retard the catalytic activity of copper and as a result, employing
Cu(OAc)2ꢀH2O as the catalyst resulted in a significant reduction of
Mn (Table S1, entry 7, ESI†). Among the solvents screened, xylene
proved to be a more efficient solvent (Table S1, entries 3 and 8–11,
ESI†). Further prolonging reaction time had no obvious effect on Mn
and yield of PBDI-8 (Table S1, entry 12, ESI†).
signal of the terminal units was observed, which suggested a
high molecular weight and regioregularity of the polymer.
Utilizing the optimized reaction conditions of polymerisation,
dibutyl, didodecyl and dioctadecyl substituted PBDIs were pre-
pared (Scheme 3). All polymers were characterized and identified
by GPC, 1H and 13C-NMR spectroscopies. It was found that the Mn
of polymers increased with the increase of alkyl chain lengths.
The polymer PBDI-18 exhibits the most narrow molecular weight
distribution (PDI = 1.32) as compared with the other polymers.
The regioisomeric PBDI-80 was prepared through the poly-
merisation of 1,5-dioctyl-1H,7H-benzo[1,2-d:4,5-d]diimidazole
(BDI-80) under the optimized reaction conditions [eqn (1)].
The regiorandom copolymer COPBDI-8 was also obtained
through the copolymerisation of BDI-8 and BDI-80 using the
same reaction conditions [eqn (2)]. Both polymers exhibit small
number-average molecular weights as compared with the
others. The 1H-NMR spectrum confirms that COPBDI-8 con-
sists of two different repeating units.
1
The structure of PBDI-8 was identified by H and 13C-NMR
spectroscopy. The 1H–1H NOESY and 13C–1H COSY were also
investigated to determine the assignment of the NMR chemical
shifts (Fig. S4, ESI†). As shown in Fig. 1, all the signals could be
assigned to protons and carbons in the repeating unit, and no
(1)
(2)
Subsequently, the photophysical properties of these poly-
mers were investigated and the results are summarized in
Table 1. The emission spectra and fluorescence images are
shown in Fig. 2. The UV-vis spectra of all polymers in CHCl3
give the absorption maxima from 405 to 428 nm (Fig. S1, ESI†).
The absorption spectra are gradually red-shifted with the increase
of alkyl chain lengths of polymers except for COPBDI-8. It is
noteworthy that the six polymers exhibit similar emission spectra
in the blue-light region with high fluorescence quantum yields in
CHCl3 solution. The films of polymers PBDI-8, PBDI-80, PBDI-12
and PBDI-18 show significantly red-shifted emissions compared
Fig. 1 1H- and 13C-NMR spectra of PBDI-8 in CDCl3.
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Table 1 Photophysical and thermal properties of polymers
C–H/C–H coupling polymerisation of other heteroarenes to
construct various p-conjugated polymer materials.
In CHCl3 solution
In PS film
This work was supported by grants from the National Basic
Research Program of China (973 Program, 2011CB808601), the
National High Technology Research and Development Program
of China (863 Program, 2013AA031901), the National NSF of
China (No. 21432005, 21372164, 21172155, 21025205, J1310008
and J1103315/J0104), and Sichuan Provincial Foundation
(2012JQ0002).
a
b
c
Polymer
labs (nm)
lem (nm)
FF
lem (nm)
Td (1C)
PBDI-4
PBDI-8
PBDI-12
PBDI-18
PBDI-80
COPBDI-8
405
411
415
428
417
406
449
447
448
450
458
457
0.20
0.28
0.28
0.13
0.16
0.21
470
480, 507, 622
521
524, 622
522, 622
481, 507
352
362
373
404
359
279
a
Absolute fluorescence quantum yields were measured in CHCl3 at
B10ꢁ6 M with an integrating sphere. Emission maximum in PS film
b
Notes and references
(1 wt%). Detected by TGA analysis and heating rate = 10 1C minꢁ1
.
c
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Chem. Commun., 2014, 50, 13739--13741 | 13741