Imide-functionalized
arenes, exemplified by naphthalene diimides
(NDIs), perylene diimides (PDIs), and bithiophene imides (BTIs), are
the most promising building blocks for constructing high-performance
n-type polymers. In order to reduce the steric hindrance associated
with NDI- and PDI-based polymers and to address the high-lying LUMO
issue of BTI-based polymers, herein a highly electron-deficient imide-functionalized
bithiazole, N-alkyl-5,5′-bithiazole-4,4′-dicarboximide
(BTzI), was successfully synthesized via an efficient
C–H activation. Single crystal of BTzI model compound
showed a planar backbone with close π-stacking distances (3.2–3.3
Å). The N,N′-bis(2-alkyl)-2,2′-bithiazolethienyl-4,4′,10,10′-tetracarboxdiimide
(DTzTI) was also used for constructing polymer semiconductors.
Compared to DTzTI, BTzI is more electron-deficient,
rendering it highly appealing for enabling n-type polymers. On the
basis of BTzI and DTzTI, a series of polymers,
including acceptor–acceptor homopolymers, and donor–acceptor
and donor–acceptor–acceptor copolymers, were synthesized,
which feature different contents of acceptor units in polymeric backbones.
As imide content increases, the polymer FMO levels were gradually
lowered, yielding a transition of charge carrier from ambipolarity
to unipolar n-type in organic thin-film transistors (OTFTs). The acceptor–acceptor
homopolymer PBTzI possesses the deepest LUMO/HOMO level
of −3.94/-6.17 eV, enabling minimal off-current (I
off) of 10–10–10–11 A in OTFTs. The highest electron mobility of 1.61 cm2 V–1 s–1 accompanied by small I
off of 10–10–10–11 A and high on-current/off-current ratio (I
on/I
off) of 107–108 was achieved from OTFTs using PDTzTI homopolymer, showing the pronounced advantages of acceptor-acceptor
homopolymer approach for developing unipolar n-type polymer semiconductors.
The correlations between the FMO levels and the transistor performances
underscore the significance of FMO tuning for enabling unipolar electron
transport. The results demonstrate that imide-functionalized thiazoles
are excellent units for constructing high-performance n-type polymers.
Moreover, the synthetic routes to these highly electron-deficient
imide-functionalized thiazoles and the polymer structure–property
correlations developed here are informative for materials invention
in organic electronics.
