Organic Letters
Letter
fluorescence with moderate to high quantum yields (0.37−
0.68) upon excitation. The circular dichroism (CD) spectra
showed apparent Cotton signals which are characteristic to
axially chiral compounds. The pair of enantiomers apparently
draw a mirror image of each other with anisotropy factors gabs on
the order of 10−4−10−3.
The BBCz derivatives 3 did not produce clear mirror images
in their circularly polarized luminescence (CPL) spectra as
CHCl3 solutions. In sharp contrast, these compounds are
obviously CPL active in the solid state dispersed in Fomblin
PFPE (perfluoropolyether) fluid (each sample was not soluble
in the fluid and gave the expected solid-state luminescence), and
the substituents at the C3 and C3′ positions considerably affect
the luminescence property. The benchmark BBCz 3a (R = H)
showed a remarkably large glum value of 2.81 × 10−2 even though
it is rather a simple axially chiral molecule. A relatively smaller
g
lum value of 6.30 × 10−3 was given to 3b (R = t-Bu). For these
two compounds, the (S)-isomers displayed left-handed CPL
characteristics for their fluorescence emission bands, whereas
the (R)-isomers emitted right-handed CPL. Intriguingly, the
installation of OMe groups to the carbazole core (3c) resulted in
the inversion of CPL signals.20 This is possibly because of the
change in their aggregate structures (see below).21 We also
investigated the CPL activity of BBPI 5. This compound
displayed a mirror image more clearly than those of 3 as the
CHCl3 solution; however, the spectrum in the solid state was
indistinct.
Figure 2. ORTEP drawings with each binaphthyl torsion angle (left)
and short contact in molecular packing models (right) of BBCz
derivatives 3a (top), 3b (middle), and 3c (bottom).
The fact that BBCz derivatives 3 possess substantial
dissymmetry factors in the solid state indicates the carbazole
ring is an effective motif to consolidate the formation of well-
ordered aggregates.22 Accordingly, we evaluated the interaction
within their packing structures by X-ray crystallography (Figure
2). The crystal structure of 3a was underpinned by a
characteristic N−H···π hydrogen bond (ca. 2.66 Å) as well as
two C−H···π hydrogen bonds (Figure 2, top). Such a strong N−
H···π interaction has been frequently found in the crystal
structure of carbazole derivatives,23 and it is notable that the
stabilization effect of N−H···π interactions is generally higher
than that of C−H···π interactions.24 On the other hand, no N−
H···π hydrogen bonding was found between the neighboring
molecules of 3b and 3c, respectively. The tert-butyl BBCz 3b
formed 1:1 cocrystals with chloroform,25 and no obvious
intermolecular C−H···π interaction was observed between
aromatic moieties (Figure 2, middle). The methoxy BBCz 3c
exhibited a strong N−H···O interaction (2.48 Å) and two C−
H···π hydrogen bonds (Figure 2, bottom). Because the unit cell
of 3c was occupied with two independent molecules, two torsion
angles of each structure (68° and 62°) are shown. As exemplified
herein, different driving forces to consolidate each packing
system may greatly affect their chiroptical properties. Therefore,
the developed synthetic method, which can systematically and
efficiently construct various axially chiral carbazole derivatives,
would be highly valuable to explore new organic CPL materials.
We also attempted to evaluate the crystal structure of 5 but were
not successful after numerous trials.
class of axially chiral luminescence materials (Scheme 1b).
These compounds exhibited CPL characteristics with consid-
erably large dissymmetry factors in the solid state. The molecular
structure and packing system were evaluated by X-ray
crystallographic analyses.
The target axially chiral (R)- and (S)-BBCz derivatives were
synthesized from the commercially available (R)- and (S)-
BINOL in five steps, respectively (Scheme 2).
According to the literature procedure,12 BINOL was
converted to the corresponding diamine (BINAM). After
acetylation, the palladium-catalyzed direct C−H arylation
reported by Stahl13,14 was adopted to install aryl groups at the
C3 and C3′ positions to give 1a−1c. The free diamines 2a−2c
were then recovered by basic treatment. To our delight, the
cyclized BBCz derivatives 3a−3c were obtained in 20−73%
yields as optically pure compounds by utilizing the iridium-
catalyzed intramolecular amination protocol,15 which we
previously developed.16 The lower yield of 3a is probably due
to its low solubility. On the other hand, palladium-catalyzed
cyclization of 1a was not successful,17 giving 3a in only a
negligible amount. The structures of 3 were unambiguously
Additionally, a pyridine analogue of 3a was prepared (Scheme
3). BINAM was treated with 2,3-dibromopyridine in the
presence of a Pd(OAc)2/PPh3 catalyst to afford the precursor
4 in high yields. The intramolecular direct coupling was then
effected to produce the corresponding bibenzo[f ]pyrido[2,3-
b]indoles (BBPI) as a pair of pure enantiomers.18
Next, we evaluated the optical and chiroptical properties of
the synthesized poly aromatic molecules. The results are
summarized in Figure 1 and Table 1.19 Both BBCz (3a−3c)
and BBPI (5) are significantly emissive, exhibiting blue
In conclusion, we synthesized new axially chiral BBCz
derivatives adopting palladium-catalyzed direct C−H arylation
and iridium-catalyzed direct C−H amination reactions. These
compounds exhibited CPL characteristic with considerably large
dissymmetry factors up to 2.81 × 10−2 in the solid state. The
molecular structure and packing system were evaluated by X-ray
crystallographic analyses to find a strong N−H···π hydrogen
bonding interaction within 3a. Further study on the synthesis
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Org. Lett. 2021, 23, 1349−1354