Organic Letters
Subsequently, 1,4-bis[3-(3-bromonaphthalen-2-yl)-5-dode-
cylthiophen-2-yl]buta-1,3-diyne (s6) was furnished through
the Suzuki coupling of s5 and 2-bromo-3-naphthaleneboronic
acid in 68% yield. Then the Pt-induced cyclization of s6
The key intermediate compound 4 was subsequently
subjected to the synthesis of precursor compounds for the
final sulfur-doped NGs (Scheme 2). Treatment of 4 with n-
butyllithium gave 2,2′-didodecyl-10,10′-biphenanthro[2,1-b]-
thiophene (5) in 92% yield. The 2-fold Suzuki coupling of 4
afforded 4,4′-di([1,1′-biphenyl]-2-yl)-2,2′-didodecyl-10,10′-
biphenanthro[2,1-b]thiophene (6) and 2,2′-didodecyl-4,4′-
bis[2-(naphthalen-2-yl)phenyl]-10,10′-biphenanthro[2,1-b]-
thiophene (7) in 88% and 79% yields, respectively (Scheme 2).
Finally, the well-controlled Scholl reactions allowed the
formation of 1−3 containing multiple subhelicenes with good
yields, including double, hextuple, and quadruple helicenes,
respectively (Scheme 2). Compound 1 was achieved from 5 in
1
Figure 2. H NMR spectra (aromatic region) of (a) 1, (b) 2, and (c) 3
in dichloromethane-d at 30 °C.
2
8
9% yield by fusing two C−C bonds, leading to the formation
which is attributed to the inversion of thieno[4]helicenes. The
additional ring fusion of the thienyl moiety results in flattened
thieno[4]helicenes in 3 compared to 1 [15.0° vs 16.7° (Scheme
of a pair of thieno[4]helicenes. Compound 2 was synthesized
from 6 in 78% yield via the cyclization of four C−C bonds,
resulting in two additional pairs of carbo[4]helicenes and
thieno[5]helicenes. Compound 3 was obtained from 7 in 52%
yield, in which six C−C bonds were fused and an extra pair of
carbo[5]helicenes was generated accordingly. Interestingly,
compound 3 fused two more C−C bonds than 2 during the
cyclodehydrogenation. This is likely due to the different
distance between the thiophene ring and opposite benzene
ring in their precursors 6 and 7, which is driven by their
respective steric hindrance. All targeted compounds (1−3)
were purified by preparative thin-layer chromatography and
then recrystallized from dichloromethane (DCM) and
methanol. The chemical identities of targeted compounds 1−
2
)]. Therefore, a faster interconversion is expected, which
range from 30 to 120 °C (Figure S20). Compound 2 combines
six subhelicenes with three different types of helicene
1
substructures. The H NMR spectrum of 2 is clearly influenced
by the dynamic processes (Figure 2b), which were observed
(
Figure S11)]. It can be assumed that the additional
carbo[4]helicenes (with a dihedral angle of 24.1°) are the
main cause of line broadening in the NMR spectra of 2, which is
also supported by the broadened signals of H and H (Figure
2
4
25
2b). The effect is most pronounced for H −H , which are
8
10
1
13
3
were confirmed by H and C NMR spectroscopy (one- and
involved in the carbo[4]helicene and thieno[4]helicene
motions. The fixed helicity of the thieno[5]helicenes in 2 is
indicated by signals of two isomers for H16 with an almost
constant ratio and rather narrow signals for H −H , which
On the basis of the DFT [B3LYP/6-31G(d)] calculations,
1
6
the dihedral angles of different helicene substructures were
examined. Among them, the dihedral angle of thieno[4]-
helicene in 1 (Scheme 2, yellow) was predicted to be ∼16.7°,
which is smaller than that of the pristine carbon analogue
narrow further with an increase in temperature (Figure S11).
The complex overall dynamic process of 2 stands for its
multilevel helicene substructures in which all individual
30
motions seem to interlock.
16
[4]helicene (∼37°) due to the weakened steric interactions
The UV−vis absorption spectra of 1−3 in anhydrous DCM
solutions are presented in Figure 3a. The spectrum of 1
exhibited a maximum absorption peak (λmax) at 449 nm and an
absorption onset at 465 nm, corresponding to an optical energy
gap of 2.67 eV. In comparison with the reported pristine carbon
by the introduction of thiophene rings. Likewise, 2 and 3 were
estimated to have similar angles of 18.6° and 15.0°, respectively,
at the same cove position. The dihedral angle of thieno[5]-
helicenes in 2 (Scheme 2, green) was calculated to be 33.4°,
which is similar to that of carbo[5]helicenes in 3 (Scheme 2,
blue). The carbo[4]helicenes in 2 (Scheme 2, orange) showed
a dihedral angle of 24.1°, smaller than that of the carbon
analogue [4]helicene (∼37°), which is probably affected by the
adjacent thieno[4]helicenes. Because of the multiple helicene
substructures and the corresponding curvatures, compounds
16
analogue of 1 [A (Figure 1a)], which showed a maximum
absorption of 502 nm and an optical energy gap of 2.36 eV,
sulfur-doped 1 exhibited an enlarged energy gap. The similar
tendency was also found for both 2 and 3 based on the
calculation results (Figure S41), compared with their respective
carbon analogues. Due to the extended π-conjugation, both 2
and 3 displayed obvious red-shifted absorption peaks compared
to 1, with λmax values of 485 and 500 nm. The optical energy
gaps of 2 and 3 are estimated to be 2.45 and 2.30 eV,
respectively. Compounds 1−3 exhibited blue, green, and
orange fluorescence, respectively, and their maximum emission
peaks were observed at 460, 505, and 569 nm, respectively
(Figure 3a, dashed lines). Interestingly, compound 2 containing
three types of subhelicenes exhibited an absolute quantum yield
of 13.8%, higher than those of 1 (4.5%) and 3 (3.8%). The
electrochemical behaviors of 1−3 were investigated by cyclic
1
−3 have good solubility in common organic solvents, such as
DCM, THF, toluene, etc.
The existence of different helicene substructures results in
multiple chirality centers and thus a variety of possible isomers
1
disappears at a slightly increased temperature (Figure S5),
2
071
Org. Lett. 2021, 23, 2069−2073
Niu, Wenhui
