reSeArCH Letter
Information). Dimer 3a was also successfully synthesized in 67% yield
in the same fashion (Extended Data Fig. 2c). Although the reason for
this excellent regioselectivity is not fully understood at this stage, each
regioselective APEX propagation step was observed to enable highly
controlled polymerization.
The absorption and fluorescence spectra of all sizes of GNR
2 show that they were both red-shifted as GNRs became longer
(Fig. 1g, h). In particular, wide-range emission covering the visible
region (wavelength λ = 400–700 nm) was observed in longer
GNRs with Mn = 1.5 × 105 Da. Thus, we have revealed that a length
effect exists in the photophysical properties of GNRs, especially in
fluorescence.
The highly soluble nature of GNR 2 enables microscopic visuali-
zation, such as scanning tunnelling microscopy (STM) and atomic
force microscopy (AFM) of individual molecules and self-assembled
structures, respectively, by simply evaporating GNR solutions of
1,2,4-trichlorobenzene on highly oriented pyrolytic graphite (HOPG)
(Fig. 2 , Supplementary Figs. 1–11). After deposition of GNR 2 on
HOPG, measurements by STM showed the existence of a pair of
two waving wires (Fig. 2a, Supplementary Fig. 1). Judging from the
height (~0.3 nm) and widths of the single- and double-waving wires
(1.7–2.2 nm and 3.9 nm, respectively), these are considered to be two
isolated fjord-type GNR 2 wires that appear to be flexible, arising from
the helical and waggling conformations (Fig. 2b)^23. In other STM meas-
urements, we obtained various self-assembly images (Supplementary
Figs. 1–11), such as rope-like helical wires of GNR 2 (Supplementary
Figs. 6, 7) and a longitudinally assembling long straight wire of GNRs
2 (Supplementary Figs. 3, 4, 9–11). We also observed by AFM uni-
form stripe images of close-packed GNR 2 with intervals of about
5 nm (Fig. 2c–f). The longer wires might be bundled GNR 2. Although
the detailed assembly model remains unclear, such diversity in the
assembly pattern indicates attractive applications in materials science
and supramolecular chemistry.
The living nature of APEX polymerization allows us to synthesize
a range of previously inaccessible molecular nanocarbon materials.
For example, living APEX block copolymerization was also possible
by reacting methoxytriethylene glycol (TEG)-substituted benzonaph-
thosilole monomer M′′ to fjord-type GNR 2 with Mn = 3.1 × 104 Da
(Đ = 1.21) (Fig. 3a, Extended Data Fig. 4). The polymerization afforded
An–Bm type block copolymer 5 with Mn = 6.8 × 104 Da (Đ = 1.23).
The introduction of a TEG group on the monomer did not affect the
high efficiency and narrow dispersity range achieved in the synthesis of
2 with a 3-ethyl-3-undecyl group (Fig. 3b). The one-pot synthesis of the
GNR block copolymer was also possible, starting from a phenanthrene
initiator I with a monomer M, followed by sequential addition of a
second monomer M′′ (Fig. 3c, d). The Mn and Đ values of GNRs before
and after the addition of monomer M′′ were almost identical to those
obtained in the experiments shown in Fig. 3a. These results are promis-
ing for the prospects of living APEX polymerization, such as application
to highly precise synthesis of graft and star copolymers, fine-tuning
of polymer units and properties, and supramolecular assembly using
amphiphilic GNR polymers.
Although the proposed APEX polymerization system certainly
yields precise GNRs, there still exists room for improvement in terms of
dispersity (length control). Unlike the well developed living polymeri-
zation of olefins^26 , APEX polymerization provides GNR polymers with
up to Đ = 1.21. Inspired by our previously developed Pd-catalysed
APEX reaction of diarylacetylenes with dibenzosilole^27 , we testedaRRR
R = n-octylEt EtR RRRRCH 2 Cl 2 /MeCN98%
n–3FeCl 3n–3
Fjord-type GNR 2
Mn = 32 kDa (Ð = 1.21)Armchair-type GNR 8
Mn = 24 kDa (Ð = 1.32)Mna
(kDa)2.9
13
32
97
1501.25
1.23
1.21
1.22
1.221.4
8.3
24
68
1201.31
1.31
1.32
1.35
1.34Yield
(%)b95
96
98
95
96GNR 2 GNR 8b200250 300350 400Intensity300900 1,5002,100 2,7003,300RamanD
1,360 cm–1G
1,600 cm–1G′ (2D)
2,670 cm–1D + D′
2,988 cm–12D′
3,264 cm–1RBLM
339 cm–1G + D
2,880 cm–1532 nmWavenumber (cm–1) Wavenumber (cm–1)Wavenumber (cm–1)3,400 3,200 3,000 1,400 1,000 600780757895
805RR
9 -octylEt772748R = nEt
9
(simulated)8
(observed) 884796IntensitycdIRÐaMna
(kDa) ÐaFig. 4 | Transformation of fjord-type GNR 2 to armchair-type GNR 8.
a, Scholl reaction of 2 in the presence of FeCl 3 (7.0 equiv. with respect
to the number of hydrogen atoms in the fjord regions). b, Results of all
Scholl reactions of GNR 2 with various polymer lengths. aDetermined
using PS standards. bIsolated yield after purification. c, Observed IR
spectra of 8 (red line) and DFT-calculated (B3LYP/6-31G(d)) IR spectra
of dimeric armchair GNR 9 (black line) (see section 12 in Supplementary
Information for details). d, Raman spectrum of 8 , obtained with by a
532-nm excitation laser. In the Raman spectrum of 8 , a sharp and intense
G-band peak and a larger D-band peak were observed at 1,600 and
1,360 cm–1, respectively, which are typical for armchair-type GNRs^17.
Well resolved double-resonance signals were also observed at 2,670, 2,880,
2,988 and 3,264 cm–1, which were assigned to G′ (2D), G + D, D + D′
and 2D′ peaks, respectively. Furthermore, a radial breathing-like mode
(RBLM) stretch, in which the wavenumber is well known to correspond to
the width of the armchair-type GNR^29 ,^30 , was observed at 339 cm–1. This
value was in between those observed in armchair-type GNRs with N = 7
(396 cm–1)^11 and 9 (312 cm–1)^14 and in good agreement with the calculated
value of armchair-type GNRs with N = 8 (328 cm–1)^29.390 | NAtUre | VOL 571 | 18 JULY 2019