Synthesis, Structure, and Chiroptical Properties of Indolo- and Pyridopyrrolo-Carbazole-Based C2-Symmetric Azahelicenes

Article abstract of DOI:10.1002/chem.202100327

Treatment of 11,12-bis(1,1’-biphenyl-3-yl or 6-phenylpyridin-2-yl)-substituted 11,12-dihydro-indolo[2,3-a]carbazole with an oxidizing system of Pd(II)/Ag(I) induced effective double dehydrogenative cyclization to afford the corresponding π-extended azahelicenes. The optical resolutions were readily achieved by a preparative chiral HPLC. It was found that the pyridopyrrolo-carbazole-based azahelicene that contains four nitrogen atoms exhibits ca. 6 times larger dissymmetry factors both in circularly dichroism (CD) and circularly polarized luminescence (CPL), |g_CD| and |g_CPL| values being 1.1×10^?2 and 4.4×10^?3, respectively, as compared with the parent indolocarbazole-based azahelicene. Theoretical calculations at the RI-CC2 level were employed to rationalize the observed enhanced chiroptical responses. The (chir)optical properties of the former helicene was further tuned by a protonation leading to remarkable red-shift with a considerable enhancement of the |g_CPL| value.

Full text of DOI:10.1002/chem.202100327

Communication
doi.org/10.1002/chem.202100327
Chemistry—A European Journal
Synthesis, Structure, and Chiroptical Properties of Indolo-
and Pyridopyrrolo-Carbazole-Based C2-Symmetric
Azahelicenes
Taisei Taniguchi,[a] Yuji Nishii,[a] Tadashi Mori,*[a] Ken-ichi Nakayama,[a] and Masahiro Miura*[a]
Dedicated to Professor Christian Bruneau
particular, circularly polarized luminescence (CPL).[5] Organic
luminescent materials with CPL characteristics are of valuable
Abstract: Treatment of 11,12-bis(1,1’-biphenyl-3-yl or 6-
phenylpyridin-2-yl)-substituted 11,12-dihydro-indolo[2,3-a]-
uses for various applications such as three-dimensional
carbazole with an oxidizing system of Pd(II)/Ag(I) induced
displays,[6] information storage systems,[7] biological probes,[8]
effective double dehydrogenative cyclization to afford the
and photocatalysis.[9] Thus, their synthesis, enantiomer separa-
corresponding π-extended azahelicenes. The optical reso-
lutions were readily achieved by a preparative chiral HPLC.
tion, and chiroptical studies have been the subjects of intense
investigations in the last two decades. Nitrogen-doped heli-
It was found that the pyridopyrrolo-carbazole-based azahe-
cenes, i.e., azahelicenes, have been often the extensive target
licene that contains four nitrogen atoms exhibits ca. 6 times
molecules among various heterohelicenes, due to the synthetic
larger dissymmetry factors both in circularly dichroism (CD)
accessibility and stability of both pyridine- and pyrrole-contain-
and circularly polarized luminescence (CPL), jgCD j and jgCPL
j
ing scaffolds.[5g,10,11]
values being 1.1×10À 2 and 4.4×10À 3, respectively, as
compared with the parent indolocarbazole-based azaheli-
cene. Theoretical calculations at the RI-CC2 level were
employed to rationalize the observed enhanced chiroptical
responses. The (chir)optical properties of the former
helicene was further tuned by a protonation leading to
remarkable red-shift with a considerable enhancement of
the jgCPL j value.
As for the construction of azahelicenes containing a pyrrole-
type nitrogen, a carbazole skeleton is often employed and
typically located in the center of helicene molecules (Sche-
me 1a, Cz-helicene),[5g,11a–e] while some related less symmetrical
azahelicenes have been known.[11f–i] On the other hand, we have
been interested in indolo[3.2.1,-jk]carbazole as another
carbazole motif (Scheme 1b, ICz),[12] as the derivatives of ICz are
highly fluorescent and can also be appended with donor
moieties to form D-π-A-type dyes for applications in dye-
sensitized solar cells[13] and host materials for phosphorescent
organic diodes.[14] The ICz skeleton is apparently strained and
should be a promising core scaffold of azahelicenes of the
twisted nature. Consequently, we designed a new ICz-based
azahelicene with C2-symmetry that contains a pair of ICz units
(Scheme 1c, ICz-helicene). Among two possible key routes for
the construction of the ICz-helicene by means of recently
developed step-economic palladium-catalyzed dehydrogenative
coupling, we chose a double dehydrogenation route of 11,12-
Polycyclic heteroaromatic compounds have recently attracted
significant attention as they are often relatively robust and
tractable compared with similarly π-extended aromatic hydro-
carbons and their optical, chiroptical, and electronic properties
can be tuned by the introduction of different heteroatoms as
well as the extent of π-systems. In particular, nitrogen-
containing polycyclic aromatic compounds are of special
interest as both pyridine- and pyrrole-types of nitrogen can be
involved and they contrastively act as electron-withdrawing
and -donating functions, respectively, within the structurally
similar aromatic π-systems.[1–4]
Meanwhile, π-extended helicenes with screw-shaped struc-
tures have helical handedness and exhibit unique (chir)optical
properties including high level of circular dichroism (CD) and, in
[a] T. Taniguchi, Dr. Y. Nishii, Prof. Dr. T. Mori, Prof. Dr. K.-i. Nakayama,
Prof. Dr. M. Miura
Department of Applied Chemistry, Graduate School of Engineering
Osaka University
Suita, Osaka, 565-0871 (Japan)
E-mail: tmori@chem.eng.osaka-u.ac.jp
miura@chem.eng.osaka-u.ac.jp
Scheme 1. Structures of (a) Cz-helicene, (b) ICz, and (c) ICz-helicene and (d)
possible routes to ICz-helicene.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100327
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dihydro-11,12-diphenyl-indolo[2,3-a]carbazole, as the similar
reaction of 1,2-di(9H-carbazol-9-yl)benzene underwent mono-
cyclization between the carbazole moieties and did not afford
the ICz-helicene structure (Scheme 1d).[12,15] Herein, we report
the successful preparation and optical resolution of diphenyl-
substituted ICz-helicene and its pyridopyrrolo derivatives,
together with their structural, (chir)optical, and electronic
features. Remarkably, while keeping the comparable structure,
chiroptical responses are considerably enhanced in pyridopyrro-
lo-helicenes as compared with the parent ICz-helicene.
The synthesis of ICz-helicene 7 (R=H, X=CH) was started
from the Ullmann-type reaction of 11,12-dihydro-indolo[2,3-
a]carbazole (1a) with 3-bromo-1,1’-biphenyl (2a) to give the
corresponding 11,12-bis(1,1’-biphenyl-3-yl)-substituted indolo-
carbazole 3 (Scheme 2).[16] Then, this was subjected to double
dehydrogenative cyclization with a combination of Pd(TFA)2/
AgOAc as an oxidant system to furnish the target helicene 7.[12]
The pyridopyrrolo-type helicene 8 was similarly constructed
with use of 1a and 2-bromo-6-phenylpyridine (2b) as the
starting materials. The methoxy- and chloro-substituted heli-
cenes 9 and 10 were also obtained by the similar reactions with
the corresponding indolocarbazoles 1b and 1c. The chloro-
function in 10 was reacted with di(4-tert-butylphenyl)amine
under palladium catalysis to provide the D-π-A helicene 11.[17]
The highly twisted helical structures of racemic 7 and 8
were confirmed by X-ray crystallography (Figure S14 and S15).
As anticipated, the angle between the two planes of the
°
terminal rings A and G (θAG) for 8 (42.0 ) is slightly reduced from
°
that of 7 (50.6 ), while their twist angles along the helical inner
rim (<ϕAG >) defined as average of five dihedral angles at the
°
rim are almost identical (ca. 20 ), inferring the comparable
strength of aromatic conjugations between the azahelicenes 7
and 8 (Figure 1). The distance between a centroid of A ring and
a least-squares plane for rings C, D, and E (dA-CDE) was
considerably smaller for 8 (1.00 Å) than for 7 (1.44 Å), implying
that the substantial twists being evoked at the terminal rings
(rather than overall helix structure) in the latter helicene,
primary due to the steric hindrance of CÀ H hydrogens (X=CH in
7). The optical resolutions of helicenes 7, 8, 9, and 11 to the
corresponding (P)- and (M)-enantiomers were accomplished by
preparative HPLC using conventional chiral stationary phases
(see the Supporting Information for details), while 10 was not
separable due to its low solubility into the mobile phase
solvent. The absolute configurations of the latter elutes of 7
and 8 were ambiguously determined as (M)-configuration by
the X-ray crystallographic analyses on the crystals of enantio-
merically pure samples (Figure S18 and S19). Despite of differ-
ent packing pattern, structures of the individual molecules were
essentially similar to those found for the racemic crystals. The
Flack parameters for (M)-7 and (M)-8 were found À 0.0017 and
À 0.10, respectively.
The racemization was completely prohibited for optically
resolved 7 and 8 at an ambient temperature. However,
interconversion between the enantiomers was observed for 7
°
upon heating in 1,2-dichloroethane (DCE) at 60 C and for 8 in
°
toluene at 90 C. The activation energies for racemization of
azahelicenes 7 and 8 were determined to be 26.9 and 27.8 kcal/
mol, respectively (Figure S13). As in the related systems,[18] the
terminal phenyl rings indeed provide a medium configurational
stability for the present azahelicenes.[19] It was somewhat
surprising that the activation energy for 8 was slightly, but
Scheme 2. Synthesis of racemic azahelicenes 7–11 {For clarity, (M)-config-
urations are shown as the representatives}. Reaction conditions: (x) Cu
(2.5 eq.), K2CO3 (4.0 eq.), 18-crown-6 (10 mol%) in o-dichlorobenzene at
Figure 1. Comparison of X-ray crystal structures of (a) rac-7 and (b) rac-8.
The angle between the two planes of the terminal rings A and G (θAG) is
highlighted in green. The distance between a centroid of A ring and a least-
squares plane defined by central three rings C, D, and E (dA-CDE) is highlighted
in blue. The twist angle along the helical inner rim (<ϕAG >) is defined as
average of five dihedral angles at inner rim and highlighted in red.
°
185 C for 48 h, (y) Pd(TFA)2 (30 mol%), AgOAc (6.0 eq.) in PivOH at
°
130~160 C for 24 h, (z) [(allyl)PdCl]2 (20 mol%), Mophos (80 mol%), tert-
°
BuONa (2.5 eq.) in o-xylene at 130 C for 15 h, [Mo-Phos=di-tert-butyl(2,2-
diphenyl-1-methyl-1-cyclopropyl)phosphine].
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Table 1. Photophysical properties of ICz-helicenes.
[c]
ICz
λabs (nm)[a] (log ɛ)
λ(em)fl (nm)[a,b]
Φ[a,b]
Δν (cmÀ 1
)
7
8
9
11
310 (5.00), 357 (4.53), 404 (4.15)
323 (4.79), 405 (3.43)
325 (4.81), 352 (sh, 4.45), 407 (4.11)
323 (4.96), 443 (3.82)
429
0.24
0.14
0.21
0.26
1440
880
1630
3380
420, 439 (sh)
436, 457 (sh)
521
[a] Measured in CHCl3 (1.0×10À 5 M). [b] Excitation wavelength: 7 300 nm; 8 323 nm; 9 330 nm; 11 330 nm. [c] Stokes shift.
Table 2. Experimental chiroptical properties of ICz-helicenes (M)-7, 8, 9, and 11.
ICz
CD[a]
λ (nm)
CPL[a,b]
λ (nm)
Δɛ (MÀ 1 cmÀ 1
)
g
CD (10À 3
)
Φ [a,b]
g )
CPL (10À 3
7
369
360
373
408
355
À 62
À 1.9
À 11
À 11
À 8.6
À 5.3
436
429
509
447
530
0.24
0.14
0.05
0.21
0.26
À 0.76
À 4.4
À 5.8
À 2.7
À 1.1
8
À 198
À 176
À 112
À 11
8[c]
9
11
[a] Measured in CHCl3 (1.0×10À 5 M). [b] Excitation wavelength: 7 300 nm; 8 323 nm; 9 330 nm; 11 330 nm. [c] Measured in the presence of TFA (80 mM).
meaningfully higher than that for 7, that may be ascribed to
more twisted geometry of the phenyl substituents in 7, but the
exact reason is obscure at the present stage.
all provided a quite comparable pattern with some differences
in intensities and excitation energies. Apparently, the intensities
of CD and CPL were found generally stronger in the
pyridopyrrolo-helicene 8 (and 9 in moderate degrees). It is also
to note that the signs of the CPL and the lowest-energy CD
bands are always the same, as is typically observed for the π-π*
transition of the rigid aromatic systems.[22] As for the typical
pristine helicenes, the absolute configurations were assigned as
The lowest-energy absorption bands measured in chloro-
form (CHCl3) solutions were red-shifted in an order of 8<7<
9!11 and the corresponding fluorescence emissions were also
in the same order (Figure 2 and Table 1). An introduction of
electron-donor substituent on the indolocarbazole skeleton (9
and 11) lead to a considerable bathochromic shift due to their
DÀ A structure. As expected from the CT nature, helicene 11
showed an apparent solvatochromism in the exited state,
exhibiting blue, green, and yellow emissions in cyclohexane,
CHCl3, and DMSO, respectively (Figure S20 and Table S5). A
large dipole moment of 11 in the exited state (μe) was
estimated to be 23.7 D, using the experimental differential
dipole moments between the excited and ground states (Δμ=
μe-μg =22.9D) from the Lippert-Mataga plot,[20] and the ground
state dipole moment (μg) of 0.8 D calculated at the B3LYP/6-
31G(d) level of theory. The fluorescence quantum yields for
azahelicenes 7, 8, 9, and 11 for CHCl3 solutions were in a range
of 0.14–0.26 that are at least one order of magnitude larger
than those of pristine carbohelicenes of the similar size.[21]
Notably, azahelicene 11 exhibited the emission with a relatively
high quantum yield of 0.50 in DMSO (Table S5). A moderate
Stokes shift of 1440 cmÀ 1 was observed for azahelicene 7, which
was further enhanced by the introduction of the DÀ A character
(in 9 and 11). The smaller value observed for 8 (880 cmÀ 1)
indicates the relatively minor structural relaxation of this
helicene in its exited state. A relatively rigid nature of
pyridopyrrolo-helicenes 8 and 9 is in accord with the structured
fluorescence spectra (Figure 2b).
$
$
(+) P and (À ) M,[5c] that are in accord with those determined
by the anomalous X-ray scattering described above.
Dissymmetry factors for CD and CPL spectra are frequently
used for more accurately evaluating the degree of molecular
Figure 2. (a) UV-vis and (b) fluorescence spectra of ICz-helicenes in CHCl3
(1.0×10À 5 M).
The results of the CD and CPL spectral measurements for
the chiral ICz-helicenes are compared in Figure 3 and Table 2.
All the azahelicenes provided mirror-imaged spectra between
the (M)- and (P)-enantiomers. While a very complex signal of
multiple negative and positive Cotton effects was apparent in
the CD spectra for all the helicene derivatives investigated, they
Figure 3. (a) CD and (b) CPL spectra of ICz-helicenes in CHCl3 (1.0×10À 5 M).
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chirality, that are defined as gCD =Δɛ (λ) /ɛ (λ) {ɛ (λ)=1/2 [ɛL
(λ)+ɛR (λ)]} and gCPL =2×ΔI (λ)/I (λ) {ΔI (λ)=IL (λ)-IR (λ)},
respectively,[22] where ɛL and ɛR denote the molecular extinction
coefficients for left and right circularly polarized light, and ΔI is
the intensity difference between the left (IL) and right (IR)
circularly polarized spontaneous emissions from a fluorophore
at a given wavelength. The jgCD j and jgCPL j values of helicene
derivatives are usually in a range of 10À 4 to 10À 3, and only
occasionally becomes 10À 2 in the order. As such, the jgCD j value
of 7 at 369 nm was found in an ordinal value of 1.9×10À 3, and
interestingly, a 5.8 times larger value of 1.1×10À 2 was observed
for the pyridopyrrolo helicene 8, despite of the negligible
structural differences (vide supra). The jgCPL j value of helicene 8
was also 5.8 times larger than that of 7, being 4.4×10À 3 and
7.6×10À 4, respectively. In a view of CPL-responsive materials
science, the CPL brightness[22e] or luminocity[22f] shall better
gauge the quantitative performance of the relevant chiral
molecules. While the pyridine moiety generally reduces molec-
ular absorptivity, the value was estimated to be around 3 times
improved in the pyridopyrrolo-helicene 8 than that of parent
helicene 7.
It is worth citing that a pyridine-containing S-shaped
azahelicene was reported to show a relatively large (absolute)
dissymmetry factor value compared with the corresponding
carbohelicene,[23] for which none of a possible rationale was
discussed. A simultaneous introduction of electron-withdrawing
and -donating functions to[7] helicene was found to significantly
enhance its CPL features, which was ascribed to a proper
control of the relevant molecular orbitals.[24a] In chiral carbazolo-
phanes, the relative geometries of two carbazole moieties were
recently shown to impact on the chiroptical responses and
excimer emission in particular.[24b]
Table 3. Transition energy, dipole transition moments, and dissymmetry
factor for the S1!S0 transition of (M)-7 and (M)-8 calculated at the RI-CC2/
def2-TZVP level.
Parameter/ICz
7
8
Transition energy (eV)
2.92 (425 nm)
266.4
17.4
2.94 (422 nm)
179.0
51.0
[a]
jμj
[b]
jmj
cosθ[c]
À 1.0
À 1.0
[d]
gCPL
À 2.6×10À 4
À 1.1×10À 3
[a] Magnitude of electric dipole transition moment in 10À 20 esu cm units.
[b] Magnitude of magnetic dipole transition moment in 10À 23 erg GÀ 1 units.
[c] θ: angle between the electric and magnetic dipole transition moments.
2
2
[d] Dissymmetry factor, gCPL =4jμj jmjcosθ/(jμj + jmj ).
remain antiparallel. A configurational analysis on 8 revealed
that a considerable amount (4%) of HOMO-1 to LUMO
contribution is apparent in addition to the HOMO-LUMO (87%),
while the former contribution was completely missing in 7
(nearly pure HOMO-LUMO transition). The A-symmetry nature
of HOMO-1 orbital (while HOMO and LUMO orbitals are both B-
symmetry) seems responsible for the enhanced magnetic dipole
transition moment in pyridopyrrolo-helicene 8 (Figure S22).
In order to externally control the chiroptical responses by
modifying electron-withdrawing effect of pyridine-nitrogen,[26]
we performed the CD and CPL spectral examination for 8 with a
gradual addition of trifluoroacetic acid (TFA) (Figure 4). As
expected, a slight red-shift was observed in the absorption
spectra upon an increased addition of the acid. In addition, the
fluorescence spectra are more significantly red-shifted with a
decrease of emission intensity. The original blue emission was
completely disappeared and the green emission became
apparent in the presence of an 80 mM of TFA. Nevertheless, the
observed CPL was remained in the comparable intensity,
implying that the (absolute) dissymmetry factor value consid-
erably increased in the protonated form. Such a phenomenon
Consequently, we performed the theoretical calculations by
the approximate coupled cluster singles and doubles model at
the RICC2/def2-TZVP level to explain the enhanced CPL
responses for 8 against 7 (see the Supporting Information for
details).[25] The electronic transition relevant for the CPL
response was determined as the A-symmetry for both of the
helicenes, which means the transition is along the Z-axis. The
calculated transition energies for 8 (422 nm vs 429 nm) and a
relative shift between helicenes 7 and 8 (Δλ= +3 nm vs
+7 nm) all well reproduced the experimental observations. It is
to note that
a considerable amount (~15%) of double
excitation character was identified for both of the helicenes,
which somewhat justifies our failure of the (conventional) TD-
DFT method to reproduce the experimental results. The
dissymmetry factor can be theoretically approximated as gCPL
=
2
2
4jμj jmjcosθ/(jμj + jmj ), where μ and m are the electric
and magnetic dipole transition moments of the relevant
electronic transitions.[22] The calculation predicted very well the
experimental CPL spectra both for helicenes 7 and 8, the
dissymmetry factor for the latter being calculated to be 4.2
times higher than that of the former (Table 3), again satisfac-
torily reproduced the observation. This is attributed to the
introduction of pyridine-nitrogens, for which a considerable
enhancement of jmj and slight reduction of jμj are respon-
sible, while the angles between both dipole transition moments
Figure 4. (a) UV-vis and (b) fluorescence spectral changes of 8 in CHCl3
(1.0×10À 5 M) by an incremental addition of TFA. (c) CD and (d) CPL spectra
of 8 in CHCl3 (1.0×10À 5 M) in the presence (80 mM) and absence of TFA.
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may be quite valuable for possible applications in the
chiroptical switching devices.[27] It is to note that an azahelicene
Keywords: azahelicenes
polarized luminescence · dehydrogenative coupling · X-ray
·
circular dichroism
·
circularly
involving an imidazole moiety was reported to show
a
structures
decreased dissymmetry factor upon an addition of TFA.[26d]
Thus, the effect of protonation to basic nitrogen on chiroptical
responses, in particular the jgCPL j values, is not straightforward
and details of observed enhancement in the current system are
remain to be elucidated.
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In summary, we successfully obtained the enantiomerically
pure forms of a series of C2-symmetic π-extended azahelicenes
bearing the double indolo[3.2.1,-jk]carbazole scaffold by means
of palladium-mediated double dehydrogenative cyclization,
followed by chiral HPLC. The chiral helicene 8 possessing
pyridopyrrolo-carbazole moieties exhibited much improved
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chiroptical responses with ca. 6 times higher jgCD j and jgCPL
j
values compared with those from the non-pyridine azahelicene
7. Theoretical calculations confirmed that the pyridine nitrogen
effectively increases the magnetic versus electric contribution in
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for designing novel azahelicenes and derivatives, with superior
optical, chiroptical, as well as electronic properties.
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Conflict of Interest
The authors declare no conflict of interest.
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Manuscript received: January 26, 2021
Accepted manuscript online: March 28, 2021
Version of record online: April 14, 2021
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