Letter reSeArCH
diphenylacetylene (I′) as an alternative initiator for living APEX polym-
erization (Fig. 3e). We expected initiator I′ to react with monomer
M more rapidly than I to provide the diphenylchrysene intermediate 6.
To our delight, the living APEX polymerization of I′ and M effectively
proceeded to afford diphenyl-substituted fjord-type GNR 7 (Fig. 3e,
Extended Data Fig. 5). Amazingly, the synthesis of low-dispersity
GNR with Đ = 1.03–1.04 was accomplished with various ratios of
M/I′ (Fig. 3f). The MALDI–TOF mass measurement also supported
these rather low Đ values of GNR 7 (Extended Data Fig. 6). Such
an extremely low Đ of GNR has never been realized by other syn-
thetic methods. The use of diarylacetylenes as an initiator would have
great benefits, not only because of the small Đ, but also because of the
structural diversity and the fine-tunability of the terminal structure
of GNRs.
We also attempted to convert the length-defined fjord-type GNRs
2 to the thicker, armchair-type GNR 8 —the so-called N = 8 armchair
GNR (where N represents the number of carbon atoms within the width
of the GNR)^17 —by cyclodehydrogenation (Scholl reaction) with FeCl 3
(Fig. 4 , Extended Data Figs. 7, 8). Through the Scholl reaction, the
PS-based Mn (Đ) changed from 3.2 × 104 Da (1.21) to 2.4 × 104 Da (1.32)
(Fig. 4b, Extended Data Fig. 8a). We also prepared N = 8 armchair-
type GNR 5 with Mn = 1.4 × 103 Da (Đ = 1.31), 8. 3 × 103 Da (1.31),
6.8 × 104 Da (1.35) and 1.2 × 105 Da (1.34) from fjord-type GNR 2
with Mn = 2.9 × 103 Da (Đ = 1.25), 1.3 × 104 Da (1.23), 9.7 × 104 Da
(1.22) and 1.5 × 105 Da (1.22), respectively (Fig. 4b, Extended Data
Fig. 7a). As in the case of GNR 2 , the formation of a thicker GNR struc-
ture in 8 was supported by STM and AFM measurements with interest-
ing self-assembly patterns (see Extended Data Fig. 9, Supplementary
Figs. 12–18 for details).
In the absorption spectra of 8 with various lengths (Extended Data
Fig. 8b), the shoulder feature at longer wavelengths appeared from
400 nm to 1,200 nm, which implies the formation of a π-extended
structure in 8. The maximum emission wavelengths (λmax) changed
from 417 nm to 595 nm, and the broad emission peaks appeared in the
visible and even in the near-infrared region, which clearly indicate the
π-extension from 2 (Extended Data Fig. 8c). However, as the theoret-
ically predicted pristine N = 8 armchair-type GNR has a tiny optical
bandgap^28 of 0.42 eV, the data are suggestive of the presence of small
defects (see Extended Data Fig. 7 for the evaluation of the efficiency
of the Scholl reaction and defects). It is likely that trace defects in 8
(uncyclized fjord regions) caused the local bandgap opening, result-
ing in unexpected photoluminescence around visible-light regions.
On the other hand, IR and Raman spectroscopic analyses of GNR
8 show changes in the peripheral structure after the transformation
(see Fig. 4c, d, Extended Data Fig. 8d, e and section 12 in Supplementary
Information for details). The inherent nature of GNR 8 and the effect of
defects seems to be difficult to determine in the present work, so further
investigation into improvements in the Scholl reaction, the synthesis
of defect-free GNRs and measurements of conductivity/charge-carrier
density will be conducted in the future.
The present study not only demonstrates the first precision synthesis
of GNRs with simultaneous width, edge structure and length control,
but also introduces a completely new type of polymerization, opening
doors in polymer and materials science. The present modular GNR
synthesis will enable researchers to explore the effect of length on the
properties of GNRs and provide a range of tailor-made GNR-related
molecular nanocarbon materials.
Online content
Any methods, additional references, Nature Research reporting summaries, source
data, statements of data availability and associated accession codes are available at
https://doi.org/10.1038/s41586-019-1331-z.
Received: 31 May 2018; Accepted: 1 May 2019;
Published online 26 June 2019.
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Acknowledgements This work was supported by the ERATO programme
from JST (JPMJER1302 to K.I.), the JSPS KAKENHI (grants JP26810057,
JP16H00907, JP17K1955 and JP18H02019 to H.I.), the SUMITOMO
Foundation (141495 to H.I.) and the DAIKO Foundation (H.I.). We acknowledge
Taoka Chemical Co., Ltd for providing samples. We thank K. Kuwata (Nagoya
University) for MALDI–TOF mass measurements; M. Kamigaito and
M. Uchiyama (Nagoya University) for measurements of molecular weight
by size-exclusion chromatography multi-angle light scattering (SEC-MALS);
T. Hashizume (Hitachi Ltd and Tokyo Institute of Technology) for advice on
the STM and AFM measurements; C. M. Crudden (Queen’s University and
Nagoya University), G. J. P. Perry (Nagoya University) and A. Miyazaki (Nagoya
University) for comments and proofreading. Computations were performed
at the Research Center for Computational Science, Okazaki, Japan. ITbM is
supported by the World Premier International Research Center (WPI) Initiative,
Japan.Reviewer information Nature thanks Lawrence T. Scott and the other
anonymous reviewer(s) for their contribution to the peer review of this work.18 JULY 2019 | VOL 571 | NAtUre | 391