Multi-layer 3D chirality: new synthesis, AIE and computational studies

Article abstract of DOI:10.1007/s11426-019-9711-x

New synthesis of multi-layer 3D chiral molecules has been developed under novel conditions to give better outcomes. The aggregation-induced emission (AIE), UV irradiation/excitation, charge transfer (CT) and local excited (LE) ππ* transitions have been investigated on a representative individual enantiomer of pseudo C₂ asymmetry which was made possibly by differentiating moieties on phosphorous on N-phosphonyl ring of chiral sandwich framework. Meanwhile, a new tandem C-N/C-C coupling reaction was unexpectedly rendered providing a novel access to special benzo[a]carbazoles.

Full text of DOI:10.1007/s11426-019-9711-x

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-019-9711-x
SPECIAL ISSUE: Celebrating the 100th Anniversary of Chemical Sciences in Nanjing University
Multi-layer 3D chirality: new synthesis, AIE and
computational studies
Yangxue Liu1†, Guanzhao Wu1,2†, Zhen Yang2, Hossein Rouh1, Nandakumar Katakam1,
Sultan Ahmed1, Daniel Unruh1, Zhonghua Cui3,5*, Hans Lischka1,4* & Guigen Li1,2*
1Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA;
2Institute of Chemistry and Biomedical Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China;
3Institute of Atomic and Molecular Physics, Jilin University, Changchun 130021, China;
4School of Pharmaceutical Sciences and Technology, Tianjin University, Tianjin 300072, China;
5Beijing National Laboratory for Molecular Sciences, Beijing 100190, China
Received December 30, 2019; accepted February 24, 2020; published online April 2, 2020
New synthesis of multi-layer 3D chiral molecules has been developed under novel conditions to give better outcomes. The
aggregation-induced emission (AIE), UV irradiation/excitation, charge transfer (CT) and local excited (LE) ππ* transitions have
been investigated on a representative individual enantiomer of pseudo C2 asymmetry which was made possibly by differentiating
moieties on phosphorous on N-phosphonyl ring of chiral sandwich framework. Meanwhile, a new tandem C–N/C–C coupling
reaction was unexpectedly rendered providing a novel access to special benzo[a]carbazoles.
multi-layer 3D chirality, organo sandwich chirality, architecture chirality, aggregation-induced emission (AIE), charge
transfer (CT)
Citation:
Liu Y, Wu G, Yang Z, Rouh H, Katakam N, Ahmed S, Unruh D, Cui Z, Lischka H, Li G. Multi-layer 3D chirality: new synthesis, AIE and
computational studies. Sci China Chem, 2020, 63, https://doi.org/10.1007/s11426-019-9711-x
1 Introduction
very limited work on conceptually new chirality of general
interest and extensive potentials for chemical and biological
research and material applications.
Chirality has attracted widespread attention in scientific
communities for over a century because of its importance in
chemistry, biology, material, and interdisciplines [1,2]. An
increasingly large number of drugs have been showing
chirality in their structures. The asymmetric synthesis and
catalysis have thus become an active topic in synthesis and
process chemistry in the past several decades, which heavily
depend on controlling molecular chirality [3,4]. So far, the
most popular and influential chirality has been represented
by chiral BINAP/BINOL and C2 symmetric scaffolds, such
as vicinal chiral diols and diamines [5–10]. However, there is
In stereochemistry, the doubly layered chirality has been
established and proven to be effective in controlling asym-
metric reactions [11–14]. For example, chiral titanocenes and
zirconocenes have been successfully applied to asymmetric
reduction/hydrogenation reactions and asymmetric poly-
merization processes [11]. Wilkinson ferrocene-based planar
chirality has served well for asymmetric catalysis, such as
Staudinger reaction, rearrangement of O-acylated enolates,
acylation of alcohols and kinetic resolutions [13,14]. Two-
layer chiral flavinium salt has been reported on the en-
antioselective alkyne conjugate addition and oxidation of
sulfides to give modest to high enantioselectivity [15]. Very
recently, helicene dimers of two-layer chirality have been
†These authors contributed equally to this work.
*Corresponding authors (email: zcui@jlu.edu.cn; hans.lischka@univie.ac.at; guigen.
li@ttu.edu)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

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Liu et al. Sci China Chem
synthesized for material research; this molecular framework
contains chiral elements on each wing and can be rotated to
give both syn and anti stereoisomers [16].
solvents and/or co-solvents.
The search for more efficient GAP reagents has been ne-
cessitated for the following objectives: to better meet re-
quirements of solubility, stability and reactivity of starting
materials and products; to enhance diastereo-, enantio- and
chemoselectivities for asymmetric synthesis and catalysis; to
enable GAP protection groups to be cleaved/recycle for reuse
more effectively; and to further increase chemical yields for
reactions [19c]. GAP chemistry can not only avoid dis-
advantages for both solution-phase and solid-phase peptide
synthesis, but also increase their synthetic efficiency.
Moreover, it has benefited establishing solution-phase pep-
tide synthesizer; GAP chemistry has also enabled the Fmoc
protection group, which is the most popular in solid-phase
peptide synthesis (SPPS), to be utilized in solution-phase
peptide synthesis [19a].
So far, the design of multi-layer chirality and its chemical
synthesis have not been well documented in literature until
very recently when we became involved in this research [17].
In this work, we report the new synthesis of individual en-
antiomers of multi-layer chirality and organic sandwich
shape (Figure 1) by performing dual C–N couplings under
new catalytic systems. At the same time, we also disclose an
unexpected tandem C–N coupling/cyclization reaction of 8-
([1,1′-biphenyl]-4-yl)naphthalen-1-amine coupling with 1,2-
dibromobenzene (Scheme 1).
As displayed in Figure 1, this new chirality shows un-
precedented pseudo C2 symmetry and belongs to a type of
multi-layer organic framework (M-LOF) of single organic
molecules. Its unique characteristics are represented by three
levels of planar units arranging nearly in parallel fashion
with one on top and the other one down from the centered
planar anchor.
2 Results and discussion
This design was initiated by our ongoing project on group-
assisted purification (GAP) chemistry which is the first
concept consisting of both chemical and physical aspects:
reagent, reaction, separation, and purification [18,19]. By
taking advantage of GAP chemistry, organic syntheses in-
cluding their asymmetric versions can be conducted without
the use of traditional purification by column chromatography
and recrystallization. GAP functional groups can convert
oily and sticky products into their solid forms, and often
enable single crystals to be formed more easily when com-
pared to classic methods. Essentially, pure GAP products can
be obtained simply by washing crude mixtures with common
During our searching for more efficient GAP groups, we
found a new single molecular framework containing multi-
layered and three-dimensional chirality [17]. One of the key
steps of its synthesis involves dual Buchwald-Hartwig cou-
plings [20] of 1,2-dibromobezene with 1-amino-8-aryl-
naphthalene to get vicinal N,N-bis(8-arylnaphthalen-1-yl)
benzene-1,2-diamines, albeit in poor yields (Scheme 2).
During this synthesis, we found para-methyl anchored ar-
ylnaphthalene substrate led to the coupling product in an
improved yield; and we also observed that almost all diamino
precursors and multi-layer compounds are highly fluores-
cence-active (Figure 2; Figure S30, Supporting Information
online). These findings led us to attach a stronger electron-
donating group, MeO–, onto the arylnaphthalene substrate
trying to further enhance chemical yields. In addition, since
phenyl rings of two wings of multi-layer 3D chiral products
are conjugated by –OMe group, it would result in new
photoelectronic properties for future material research.
However, under original catalytic systems, this replacement
did not improve chemical yields that are indeed on the same
level as these of previous couplings. We thus continued
searching for new catalytic conditions in order to optimize
the key dual couplings.
Figure 1 Multi-layer 3D chiral sandwich molecules (color online).
As shown in Table 1, several factors for this catalytic
coupling need to be adjusted carefully. It should be men-
tioned that in our initial synthesis, the use of 1-bromo-8-
phenylnaphthalene to react with vicinal benzene diamine did
not result in any diamino product. This lesson suggested us to
directly use 1,2-dibromobenzene and 8-(4-methoxyphenyl)
naphthalen-1-amine as reactants for present optimizations.
The coupling in toluene with 3.0 equiv. of t-BuONa at
110 °C only afforded 11% yield. The yield was slightly im-
proved to 15% when the reaction was performed in p-xylene.
Scheme 1 Tandem catalytic C–N coupling/cyclization reaction (color
online).

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Liu et al. Sci China Chem
Table 1 Palladium-catalyzed coupling conditions and resultsa)
Scheme 2 Dual Buchwald-Hartwig Diamino couplings [16b] (color on-
line).
Temperature Time Yield
Entry
Base
Solvent
(°C)
110
133
133
133
133
133
(h)
24
24
24
24
19
14
(%)
1
2
3
4
5
6
t-BuONa (3.0 equiv.) toluene
t-BuONa (3.0 equiv.) p-xylene
t-BuOK (3.0 equiv.) p-xylene
t-BuOK (2.1 equiv.) p-xylene
t-BuOK (2.1 equiv.) p-xylene
t-BuOK (2.1 equiv.) p-xylene
11
15
19
24
33
41
a) All reactions were performed in tubes which are tightly capped by
septum and protected with balloons filled with argon gas; during the re-
action, a small amount of hexane employed for preparation of (t-Bu)3P
solution flew into balloon through needle on septum.
Figure 2 Fluorescence showings of multi-layer 3D chiral amide (16a)
(color online).
Replacing t-BuONa with t-BuOK resulted in a higher yield
to 19%. Decreasing the loading of t-BuOK to 2.1 equiv.
further increased chemical yield to 24%. Shortening reaction
time from 24 to 19 and 14 h, chemical yields were increased
to 33% and 41%, respectively (Table 1).
Scheme 3 Diamino coupling using 1-amino-8-phenylnaphthalene (color
online).
After achieving the above optimization, we next re-in-
vestigated the original coupling of 1,2-dibromobenzene with
1-amino-8-phenylnaphthalene under this new catalytic sys-
tem. We found this system resulted in vicinal N,N-bis(8-
phenylnaphthalen-1-yl)benzene-1,2-diamine in a chemical
yield of 27% which almost doubled the original yield of 14%
(Scheme 3). It is interesting that as shown in the X-ray
structure of this diamino product, the phenyl ring on position
N1 of (8-phenylnaphthalen-1-yl)benzene 1,2-diamine is ar-
ranged in parallel to the corresponding naphthyl ring on N2 of
this benzene 1,2-diamine, and vice versa. However, as shown
in Scheme 4, this conformation has to be adjusted to become
those of 9 and 10 so as to enable the next cyclization to occur.
The serious steric effects by two phenyl groups of vicinal N,
N-bis(8-aryl- naphthalen-1-yl)benzene-1,2-diamines would
be responsible not only for low chemical yields of dual
couplings (27% and 41% for 9 and 10, respectively), but also
for relatively low yields of the next cyclization step (39%
and 69% for 11 and 12, respectively).
resulting 1H-naphtho[1,8-de][1,2,3]triazine (2) was treated
with metal copper in hydrogen bromide to give 8-bromo-
naphthalen-1-amine (3) which was converted into N-(8-
bromonaphthalen-1-yl)acetamide (4) by protection with
acetyl chloride. Suzuki coupling of 4 with (4-methox-
yphenyl)boronic acid gave N-(8-(4-methoxyphenyl)naph-
thalen-1-yl)acetamide (6) followed by acidic hydrolysis with
concentrated aqueous HCl to afford 8-(4-methoxyphenyl)
naphthalen-1-amine (8). The optimized conditions of dual
Buchwald-Hartwig C–N couplings resulted in vicinal N,N-
bis(8-(4-methoxyphenyl)naphthalen-1-yl)benzene-1,2-dia-
mine (10) which exists in an equilibrium of enantiomeric
conformers. Two of these conformers are represented, which
is based on the conformations of next cyclized N-phosphonyl
chlorides (Figure S29) and X-ray structural analysis of its
derived phosphoramide. As usual, we made several efforts
on obtaining these enantiomeric conformers by chiral high
performance liquid chromatography (HPLC), but we fizzled
due to their unfixed flexibility. However, after they are cy-
clized to 2-chloro-1,3-bis(8-(4-methoxyphenyl)naphthalen-
1-yl)-1,3-dihydrobenzo[d][1,3,2]diazaphosphole 2-oxides
(12), we were able to separate these two enantiomers via
chiral prep-HPLC (Scheme 4).
As shown in Schemes 4 and 5, nine steps were needed
to achieve the GAP amide product of 2-amino-1,3-bis(8-(4-
methoxyphenyl)naphthalen-1-yl)-1,3-dihydrobenzo[d]
[1,3,2]diazaphosphole 2-oxide (16). The synthesis was star-
ted from an inexpensive commercial chemical, naphthalene-
1,8-diamine, via oxidative cyclization by treating with so-
dium nitrite in aqueous media containing acetic acid. The
At the cyclization step, the reaction of diamines with
phosphoryl trichloride in triethylamine or pyridine did not

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Liu et al. Sci China Chem
Scheme 4 Synthesis of Pseudo C2 & 3D N,N-phosphonyl chlorides (color online).
give any cyclization product, although they worked well for
cases of (1R,2R)-cyclohexane-1,2-diamine and (1R,2R)-1,2-
diphenylethane [18,19]. This reaction has to be performed by
pre-deprotonating N,N-bis(8-(4-methoxyphenyl)naphthalen-
1-yl)benzene-1,2-diamine (10) with n-butyl lithium followed
by the treatment with phosphorus oxychloride at −78 °C in
dried tetrahydrofuran (THF). In our original cyclization of
N,N-bis(8-phenylnaphthalen-1-yl)benzene-1,2-diamine,
a
yield of 39% was obtained. However, in the present case, the
corresponding yield was substantially enhanced to 69%,
which is likely due to the strong-electron donating effect of
the methoxy group making diamino moieties more nucleo-
philically reactive toward the N-phosphonyl protection in
both intramolecular and intermolecular manners at this step.
This observation is consistent with the case of the methyl
derivative under the same conditions which gave a higher
chemical yield of 45% than that of the unsubstituted case
(39%) [19b]. The resulting azide of this para-methoxy case
was found to be not so stable and showed complex unknown
species after being isolated and stored for a short period of
time at room temperature. It was thus directly subjected to
Pd/C catalyzed hydrogenation with the crude azide (14a) to
give the final product of 2-amino-1,3-bis(8-(4-methox-
yphenyl)naphthalen-1-yl)-1,3-dihydrobenzo[d][1,3,2]diaza-
phosphole 2-oxide (Scheme 5). Similarly, the multi-layer 3D
compounds also showed strong fluorescence activity inside
column and in solution (Figure 2). 16a was selected at ran-
dom for further aggregation-induced emission (AIE) study
[21]. The photoluminescence (PL) spectra of 16a in THF/
water mixture with different water fractions (fw) and the
corresponding plot of emission enhancement versus fw de-
monstrate the AIE activity (Figure 3). It is non-fluorescent
when dissolved in THF solution but becomes emissive at
651 nm with a 37,500 a.u. intensity when fw=30 vol% in
THF/water mixture. While more water is gradiently added
(fw=50 vol%, 70 vol% and 90 vol%), the emission intensities
are significant increase to 49,500, 56,100 and 61,800, re-
spectively.
Scheme 5 Synthesis of pseudo C2 & 3D N,N-phosphonyl amides (color
online).
yphenyl)naphthalen-1-yl)-1,3-dihydrobenzo[d][1,3,2]diaza-
phosphole 2-oxide (12) and the corresponding final amide
product (16), but failed in both cases. Fortunately, the ab-
solute stereochemistry can be assigned based on the analo-
gous structure of 2-amino-1,3-bis(8-phenylnaphthalen-1-yl)-
1,3-dihydrobenzo[d][1,3,2]diazaphosphole 2-oxide, which
has been confirmed by X-ray diffraction analysis [19b]. The
enantiomerically pure isomer of 2-chloro-1,3-bis(8-(4-
methoxyphenyl)naphthalen-1-yl)-1,3-dihydrobenzo[d]
[1,3,2]diazaphosphole 2-oxide (12b) was further reacted
with lithium benzylamine which was pre-generated by
treating benzylamine with butyl lithium in THF at −78 °C
(Scheme 6). The reaction was performed at −78 °C for
30 min followed by gradually increasing temperature from
−78 °C to room temperature. The reaction proceeded to
completion by stirring at room temperature for 6 h to give 2-
(benzylamino)-1,3-bis(8-(4-methoxyphenyl)naphthalen-1-
yl)-1,3-dihydrobenzo[d][1,3,2]diazaphosphole 2-oxide (17)
with an 81% yield. It was also subjected to the reaction with
methyl lithium under the same conditions to afford 1,3-bis(8-
(4-methoxyphenyl naphthalen-1-yl)-2-me thyl-1,3-dihy-
drobenzo[d][1,3,2]diazaphosphole 2-oxide (18) in a yield of
We also made several efforts on obtaining single crystals
of individual enantiomers of 2-chloro-1,3-bis(8-(4-methox-

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Liu et al. Sci China Chem
combination with the PBE0-D3 functional [23,24] and the
cc-pVDZ basis set [25] was used to optimize the geometry of
the complex (Figure 4). To preserve the main features of the
crystal structure, selected atoms indicated in this figure were
fixed at their positions in the crystal structure during the
optimization process. Second-order algebraic diagrammatic
construction (ADC(2)) calculations [26–28] were performed
using the TVZP basis set [29] to compute the excited-state
spectrum. Natural transition orbitals (NTOs) [30] were used
to characterize the electronic transition at orbital level. En-
vironmental effects were considered by the conductor-like
screening model (COSMO) [31], based on the state specific
approach [32]. For the dielectric constant ε and the refractive
index n, respectively, the values ε=8.93 and n=1.42 were
used, which correspond to the weakly polar di-
chloromethane. For the calculation of the vertical emission
transition from S1, water was chosen as solvent (ε=78.4 and
n=1.33). The calculations were carried out using the program
package TURBOMOLE [33].
Figure 3 Photoluminescence (PL) spectra of 16a in THF/water mixtures
with different water fractions (fw). c=1 μM; excitation wavelength (λex):
532 nm (color online).
The selected bond and interring distances are displayed in
Figure 4. Compared to the crystal structure, the calculated
bond distances of the aromatic systems do not show sig-
nificant differences, whereas the four bonds associated with
the phosphorus atom are slightly longer by 0.03 Å. The two
interring distances given in Figure 4 are 0.2–0.3 Å shorter
than in the crystal, demonstrating the crystal packing effect
on these weak nonbonded interactions.
Scheme 6 Reactions of N,N-phosphonyl chloride with lithium reagents
(color online).
The computed UV/Vis spectrum is displayed in Figure 5.
The spectrum is characterized by two low-lying charge
transfer (CT) bands at 332 and 319 nm. The CT involved in
these transitions is quite significant and amounts to 0.8e. It is
directed from the central ring to the naphthalene units (see
insets in Figure 5). The location of these two weak transitions
clearly indicates that the broadened absorption observed at
around 320–330 nm can be ascribed to CT transitions. The
next group of transitions around 280 nm are responsible for
the next band and represent local excited (LE) ππ* transitions
within the naphthalene ring. The strongest band is formed by
a series of closely-spaced transitions between 216 to 228 nm,
which are characterized as LE in naphthalene coupled with
CT character. The UV/Vis spectrum predicted by ADC(2) is
consistent with the experimental data (Figure S31), which
shows the broadened absorption at 330 nm (CT transitions)
and very pronounced absorption at around 290 nm owing to
the LE transitions. The sharp absorption at around 240 nm in
computed spectrum can be ascribed to the closely-spaced
transitions with LE coupled with CT character.
62%. As revealed by chiral HPLC, there is no racemization
observed in these two reactions.
Interestingly, when we utilized a phenyl group to replace
para MeO– for the coupling, the reaction underwent toward
another direction, i.e., intermolecular C–N bond formation
was followed by intramolecular C–C cyclizing (Scheme 1).
The desired product was barely formed in the resulting
mixture. Instead, 1-([1,1′-biphenyl]-4-yl)-11H-benzo[a]car-
bazole (19) was generated as the major product in a 35%
yield, and its structure was determined by X-ray diffraction
analysis (Scheme 1). Product 19 appeared as thin snowflake-
shaped solids, which benefited to generating single crystals
for X-ray analysis. This product is anticipated to serve as a
structural lead for material and medicinal chemistry because
similar structural units showed applications in these fields
[22]. As shown in its structure, the N–H bond on the indole
ring is nearly perpendicular to the phenyl ring on position-8
of phenylnaphthalene; this unique arrangement will also
have applications for catalyst design by introducing func-
tional groups onto nitrogen atoms.
The luminescence properties of the model 15a structure
(Figure 4) have been investigated at ADC(2) level. For that
purpose, the structure of 15a has been optimized for the S1
state in gas phase without geometry restrictions. Structural
changes in comparison to the ground state and the vertical
emission energy for aqueous solution have been calculated.
To further understand the electronic behavior of these new
chiral molecules, we conducted computational studies by
selecting 15a as a representative model which has been un-
ambiguously confirmed by X-ray structural analysis. In our
computational methods, density functional theory (DFT) in

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Liu et al. Sci China Chem
been investigated on a representative individual enantiomer.
We have found that the UV/vis spectrum predicted by ADC
(2) is very well consistent with the experimental data, which
show the broadened absorption at 330 nm (CT transitions)
and very pronounced absorption at around 290 nm owing to
the LE transitions. The sharp absorption at around 240 nm in
experimental spectrum can be ascribed to the closely-spaced
transitions with LE coupled with CT character. The com-
parison of the calculated emission energy for the monomer
unit with the experimental band maximum for the aggregated
state shows a substantial influence of the aggregation of
about 60 nm.
Figure 4 Optimized structures with selected bond distances in Å com-
puted at the PBE0-D3/cc-pVDZ level. Hydrogen atoms connected to car-
bon are not shown and the carbon atoms fixed in the optimized process are
marked by circles (color online).
Acknowledgements This work was supported by the National Natural
Science Foundation of China (91956110, 21672100) and Robert A. Welch
Foundation (D-1361, USA). Z. C. acknowledges financial support from
Beijing National Laboratory for Molecular Sciences (BNLMS201910). H.
L. are grateful for support from the School of Pharmaceutical Science and
Technology (SPST), Tianjin University, Tianjin, China, including computer
time on the SPST computer cluster Arran. We thank Dr. Kaz Surowiec for
the analytic assistance discussion.
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
Figure 5 Simulated UV/Vis spectrum at the ADC(2)/cc-pVDZ level in
dichloromethane, and pictures of the NTOs are given for S1, S2 (CT tran-
sitions) and S5 (LE) states (color online).
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3 Conclusions
In summary, we developed more efficient synthesis of or-
ganic sandwich molecules with unique pseudo C2-symmetry
by searching for new dual catalytic couplings. The pseudo C2
design was made possibly by differentiating two moieties on
phosphorous on the centred five-membered N-phosphonyl
ring. Meanwhile, a new catalytic reaction was established for
the formation of 1-([1,1′-biphenyl]-4-yl)-11H-benzo[a]car-
bazole as confirmed by X-ray diffraction analysis. The AIE,
UV irradiation/excitation, CT and LE ππ* transitions have
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