Naphthalene
Diimide Incorporated Thiophene-Free Copolymers
with Acene and Heteroacene Units: Comparison of Geometric Features
and Electron-Donating Strength of Co-units
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Abstract
A family of naphthalene diimide (NDI)-based
donor (D)-acceptor
(A) copolymers with various acene- (benzene (Bz), naphthalene (Np),
and pyrene (Py)) and heteroacene-type (selenophene (Se) and thiophene
(Th)) donor rings has been designed and synthesized as a means to
systematically understand structure–property relationships
on the subject of the structural factor and electron-donating capability
of the donor portions for applications in organic field-effect transistors
(OFETs) based on NDIs. Alongside of two categories dealing with the
lack or existence of the heteroatoms in the donor framework, the resulting
copolymers can also be classified into ‘thiophene-free’
D–A copolymers (<b>PNDI-Bz</b>, <b>PNDI-Np</b>, <b>PNDI-Py</b>, and <b>PNDI-Se</b>) and thiophene-containing
copolymer (<b>PNDI-Th</b>). The results from optical and electronic
properties lead to the determination that the empirical electron-donating
strength of donor co-units is in the order of Bz < Np < Py <
Th < Se. In contrast with the similarity of the LUMO levels (−3.73∼−3.82
eV) due to the dominant NDI contribution to the polymer backbone,
the HOMO levels are sensitive to the relative electron-donating ability
and shown to primarily influence whether unipolar <i>n</i>-channel (<b>PNDI-Bz</b> and <b>PNDI-Np</b>) or ambipolar
charge transport (<b>PNDI-Py</b>, <b>PNDI-Se</b>, and <b>PNDI-Th</b>) is observed in OFETs of the NDI-based copolymers.
Intriguingly, regardless of the strong electron donors toward efficient
intramolecular charge transfer (ICT), the best OFET performance is
observed in the acene-based centrosymmetric copolymer <b>PNDI-Np</b> (5.63 × 10<sup>–2</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>) when compared to those of the other copolymers
with axisymmetric units. Thus, the present work highlights that the
geometric features of the donors in NDI D–A copolymers strongly
reflect the carrier mobility dynamics rather than inserting electron-rich
donor moieties into the backbone to lower the band gap and further
strengthen ICT