The Marriage of Carborane with Chalcogen Atoms: Nonconjugation, σ-πConjugation, and Intramolecular Charge Transfer

Article abstract of DOI:10.1021/acs.orglett.9b03047

A series of chalcogen-containing carborane derivatives were synthesized through a one-pot reaction. The nonconjugated six-membered ring of carborane-fused disulfide (1) can be electrochemically reduced to a dithiolate derivative. The five-membered ring of carborane-fused chalcogenophenes (2-4) showed aromaticity and considerable σ-πconjugation. The dicarborane chalcogenides (5-7) showed intramolecular charge-transfer-induced emission. These chalcogen-containing carborane derivatives provided a useful platform to study the electron interaction systematically and shed some light on the design of carborane-based optoelectronic materials.

Full text of DOI:10.1021/acs.orglett.9b03047

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
The Marriage of Carborane with Chalcogen Atoms: Nonconjugation,
σ−π Conjugation, and Intramolecular Charge Transfer
Xiaodong Yang,† Bingjie Zhang,† Sikun Zhang,† Guoping Li,† Letian Xu,† Zhijun Wang,† Pengfei Li,†
Yanfeng Zhang,‡ Zishun Liu,§ and Gang He*,†
†Frontier Institute of Science and Technology, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Key
Laboratory of Sustainable Energy Materials Chemistry, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054, China
‡Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054, China
§International Center for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, School of
Aerospace, Xi’an Jiaotong University, Xi’an 710049, China
S
* Supporting Information
ABSTRACT: A series of chalcogen-containing carborane
derivatives were synthesized through a one-pot reaction. The
nonconjugated six-membered ring of carborane-fused disulfide
(1) can be electrochemically reduced to a dithiolate derivative.
The five-membered ring of carborane-fused chalcogenophenes
(2−4) showed aromaticity and considerable σ−π conjugation.
The dicarborane chalcogenides (5−7) showed intramolecular
charge-transfer-induced emission. These chalcogen-containing
carborane derivatives provided a useful platform to study the
electron interaction systematically and shed some light on the
design of carborane-based optoelectronic materials.
icosahedral carborane and a fused 2-D π-ring system (Scheme
1).
Chalcogen atoms (E = S, Se, Te) with two inherent lone pairs
are widely used to fabricate optoelectronic materials.11 Usually,
chalcogen atoms act as two roles in organic−photoelectric
o-Carborane (1,2-closo-dicarbadodecaborane, C2B10H12) is an
electron-deficient icosahedral boron cluster containing two
adjacent carbon atoms in the cage and possesses highly
polarizable σ-aromatic characteristics.1 Carborane derivatives
with good thermal and chemical stabilities have been used as
neutron-capturing agents for boron neutron capture therapy,
due to their intrinsic properties of the boron cluster.2 Recently,
carborane has been widely used as building blocks for
constructing the π-conjugated systems3 and the carborane-
containing π-conjugated systems, which showed strong
potential application in medicine,4 catalysis,5 and nonlinear
optical materials.6 For instance, o-carborane-substituted an-
thracene or aryl derivatives exhibited aggregation-induced
emission (AIE) properties7 through an intramolecular charge
transfer (ICT) mechanism.8 Interestingly, when the carborane
was emerged into π-conjugated systems, the carborane-fused
carbo- and heterocycles showed different aromaticity. For
instance, there was no σ−π conjugation between the carborane
and the diene moiety in benzocarborane.9 In contrast, a
considerable σ−π conjugation was observed for carborane-
containing heterocyclic anion [1,2-N3R-1-CB11Cl10]−, leading
to an aromatic exo-CBN3 five-membered ring with a nucleus-
independent chemical shift (NICS(0)) value of −7.8 ppm.10
Although great achievements have been made in the study of
carborane derivatives, the electron interaction between
carborane and the π-conjugated unit is still ambiguous.
Therefore, it is very important to develop novel carborane
derivatives to study the electron interaction between 3-D
Scheme 1. Strategies for the Synthesis of Carborane-Fused
Disulfide, Carborane-Fused Chalcogenophenes, and
Dicarborane Chalcogenides
Received: August 27, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03047
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molecules: one is as a π-donor to modulate the molecular band
gap by regulating the LUMO level,12 and the other is as an n-unit
to induce charge transfer.11d Benefiting from the special
properties of chalcogen atoms, the combination of chalcogen
atoms and carborane was studied recently. Xie and co-workers
synthesized several carborane-fused carbo- and heterocycles via
the reaction of carborane-fused zirconacyclopentane with
chalcogen atoms (E = S, Se, Te); however, these carborane
derivatives did not show any conjugation or other electron
interaction.13 More recently, the same group developed a series
of π-block atom (X = NH, O, S)-containing carborane
derivatives, which showed considerable σ−π conjugation in
the aromatic systems.14 Evidently, it is still necessary to explore
the electron interaction, such as σ−π conjugation or electron
transfer between carborane and chalcogen atoms. Thus, a
combination of chalcogen atoms and carborane might construct
a series of carborane derivatives with different electron
properties, which will provide a useful platform to study the
electron interaction systematically and shed some light on the
design of carborane-based optoelectronic materials.
C2′ position, we guess that the C−H bond can be selectively
activated. The carbon−lithium bond of carborane moiety
formed11a,16 when S1 was lithiated with n-BuLi (1.05 equiv).
Followed by the addition of S2Cl2 (1.1 equiv), dicarborane
trisulfide (5) was afforded in 21% yield.17 With minor change,
dicarborane selenide (6) and dicarborane telluride (7) were
obtained with 73 and 45% yield, respectively. These compounds
were all confirmed by single-crystal X-ray analysis (Figures
S1−S7), NMR (1H, 13C, 11B), HRMS, and thermogravimetric
analysis (Figure S8).
As shown in Figure 1a, the single-crystal structure of 1 exhibits
a nonplanar backbone with moderate S−S bond lengths (i.e.,
Based on the considerations, we explored a one-pot reaction
for the synthesis of carborane derivatives from readily available
3-(carborane-1-yl)-1-methyl-1H-indole S115 and a chalcogen
source to discuss the electron interaction between carborane
and chalcogen atoms. According to Scheme 2, the carborane-
Scheme 2. Synthetic Approach toward Carborane-Fused
Disulfide, Carborane-Fused Chalcogenophenes, and
Dicarborane Chalcogenides
Figure 1. (a) Crystal structures of 1. (b) Reductive cleavage of disulfide
bonds about 1. (c) Reduction-active orbital of 1 occupied with 0, 1, and
2 electrons; top is the LUMO of 1, middle is the SOMO of 1−, and
bottom is the HOMO of 12−. (d) Energy rotation profile, with respect
to the S2−C7−C10−S3 torsion angle for all of the oxidation states of 1.
2.07 Å), and the dihedral angle is 27.4°, which is highly
consistent with the previous results.18 To study the electron
interaction in 1, the NICS(0) value was calculated first. The
NICS(0) value of −3.8 ppm suggested that there was almost
nonconjugation between the carborane and the nonconjugated
disulfide dithiin ring.9 Considering the special structure and
nonconjugated backbone and the importance of the reductive
cleavage of disulfide bonds in many biological and chemical
processes,19 a plausible mechanism for molecule 1 is depicted in
Figure 1b. In detail, a reversible oxidation peak at E1/2 −2.79 V vs
Fc/Fc+ and an irreversible oxidation peak at Epc −2.55 V
indicated that two-electron reduction of the disulfide bond exists
(Figure S10b). The calculated LUMO for 1 is shown in Figure
1c, consisting mostly of S−S antibonding character with
minimal heterocyclic ring π-character. The first electron in the
reduction went into an orbital dominated by S−S antibonding
character, which would promote S−S bond cleavage to produce
a sulfur radical and an anion (Figure 1c). The potential energy as
a function of the breaking of the S−S bond and concomitant
rotation about the S2−C7−C10−S3 torsion angle is shown in
Figure 1d. Upon addition of the first electron to 1, the
S2−C7−C10−S3 torsion angle increased from 27.4 to 38.5° for
optimized geometries, whereas a second shallow well for 1− was
present 0.13 eV higher in energy at a torsion angle of 58.5°.
a
b
c
d
e
f
S2Cl2. SeCl2. TeCl4 (1.1 equiv). S2Cl2. SeCl2. TeCl4 (0.6 equiv).
fused disulfide (1) was prepared via lithium−halide exchange
reaction using S2Cl2 as the chalcogen source in 53% yield, and a
trace amount carborane-fused thiophene (2) was generated
concomitantly. After the reaction time was prolonged, 2 was
quantitatively obtained in 38% yield. Similarly, the carborane-
fused selenophenophene (3) and tellurophenophene (4) were
prepared successfully in moderate yields of 61 and 46%,
respectively. Considering the different C−H bond dissociation
energies of carborane at the C2 position and indole ring at the
B
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Thus, the 1− rapidly reduced to the dianion 12− with almost no
calculated barrier. Based on the current and previous
mechanistic studies,20 it can be seen that the disulfide bond of
compound 1 is reduced directly by intermolecular electron
transfer from an external reducing agent via three steps.
noting that the indole plane of 6 is twisted through the C1−C2
bond of an o-carborane with an angle of ψ = 33.8°, which means
ICT probably occurs.22
According to the previous work,14 the carborane-fused
chalcogenophenes (2−4) may have strong electron communi-
cations. To study their electron interactions, the crystal
structures were measured first. The fused five-membered rings
in 2−4 were coplanar (Figure 2a−c). In addition, the UV−vis
Figure 2. (a−c) Crystal structures of 2−4. (d) HOMO and LUMO
energy levels of 2−4. (e) Calculated NICS(0) values (ppm) of the
aromatic rings of 2−4 at the B3LYP/6-311++G(d,p) level of theory.
absorption spectra of 2−4 were measured in THF at room
temperature (Figure S9), and the theoretical maxima are highly
consistent with those in the experiment (Figures S15−S17).
Moreover, the electrochemical characteristics of 2−4 in THF
were also probed through CV (Figure S10), and the results are
summarized in Table S8. Based on these data, the LUMO and
HOMO of 2−4 could be estimated (Table S8). As can be seen,
they exhibit similar HOMO and LUMO level distributions,
which are in good agreement with DFT calculations (Figure 2d).
The LUMO of 2−4 decreased from −2.59 to −3.04e due to the
stronger electron-donating character of the chalcogen atoms and
the enhancement of metallic properties from S to Te,11a,21 which
showed that 2−4 may have a strong σ−π conjugation between
carborane and chalcogenophenes.
For further proof of the aromatic properties of molecules 2−4,
the NICS(0) values of the carboranyl cages and rings A, B, and C
in carborane-fused systems were calculated (Figure 2e). As the
elements of E change from sulfur to tellurium, NICS(0) values
of cages decreased from −27.6 to −26.9 ppm; ring A decreased
from −8.4 to −7.5 ppm. For rings B and C, NICS(0) values were
around −12.2 and −10.6 ppm, respectively. The calculated
NICS(0) values of 2−4 suggested that they showed aromaticity
and considerable σ−π conjugation.
Figure 3. (a−c) Crystal structures of 5−7. (d) Calculated structure and
molecular orbitals of 6 in the excited state. Differences of total energy
(ΔE) and energy levels of each orbital were calculated at the B3LYP/6-
31G(d) level by DFT and the B3LYP/6-31+G(d) level by TD-DFT.
(e) PL spectra of 6 in different solvents. (f) PL spectra of 6 in different
THF/water ratios. (g) Emission intensity of 6 as the THF/water ratio is
altered. Inset: aggregates under UV light (λexcit = 365 nm); [6] = 10−4
M. (h) Photographs of graphic security made from 5 (left side) and 6
(right side).
To estimate the relationship between the conformation and
optical properties, DFT and TD-DFT were tested in the gas
phase. By altering the angle, changes in electronic structure were
investigated. On this basis, two optimized geometries of
molecule 6 with approximately parallel (ψ1 = 4°, ψ2 = 6°) and
perpendicular (ψ1 = 82°, ψ2 = 120°) distributions were obtained
(Figure 3d). For the spatial influence of selenium atoms, it is
difficult to have completely planar and perpendicular con-
formation. It was shown that HOMOs of two geometries were
localized at the indole moiety or selenium atoms. Meanwhile,
the LUMO in the perpendicular distribution was delocalized
over the partial o-carbarane and indole moiety. In contrast, the
LUMO in the parallel distribution was localized at selenium-
bridged o-carborane through σ*−π* conjugation. It is evidence
of the notable characteristic ICT behavior. The planar
conformer was 0.81 kcal mol−1 higher than the perpendicular
Although σ−π conjugation between carborane and chalcogen
atoms has been discussed, the charge transfer properties have
not been determined. To clarify the charge transfer process, the
crystal structures of 5−7 were studied (Figure 3a−c). It is worth
C
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Letter
conformer. From these data, the twisted ICT (TICT) process
should be responsible for the emission of the dicarborane
chalcogenides dyads.
Detailed experimental procedures, NMR spectra, and X-
ray crystallographic data for new compounds (PDF)
Accession Codes
For further insight into the charge transfer process, their
emission properties were studied. Take 6 as an example:
photoluminescence (PL) spectra were measured in various
solvents, such as THF, CHCl3, acetone, CH3CN, and DMF
(Figure 3e). The emission bands around 580 nm were emerging
and slightly bathochromically shifted by increasing the solvent
polarity. In contrast, the emission band around 440 nm was
hardly influenced by changing solvent types. However, yellow−-
green emission bands centered at 538 nm with a PL quantum
yield (ΦPL) of 11.5% and a delayed decay with a lifetime (τ) of
3.79 ns were observed in the solid state. These data indicate that
the luminescent bands around 440 and 580 nm should be
assigned to the LE state of indole moieties and the ICT state23 in
the whole molecule. In addition, the emission of 6 was strongly
dependent on the polarity of solvents, which might be attributed
to the TICT process.8f,22b,24 The typical AIE properties inspired
us to study the aggregation behaviors in mixed solvents, in which
THF and water were used as good and poor solvents (Figure
3f,g). At low water contents (fh ≤ 50%), the intensity of the LE
emission bands was observable. Although the water content
exceeded 60%, originating from the lowest energy ICT excited
state, the emission started to increase swiftly. It became very
strong at fh of 95%, with a bright yellow emission band emerging
at longer wavelengths (≈546 nm). Moreover, the PL emission
intensity of 6 is about 8 times higher than that of S1 in the
crystalline state (ΦPL 15.3 vs 1.8%, Figure S11a). This fact
strongly suggests that the Se atom should induce charge transfer.
For 5, similar PL changes were detected in aqueous mixtures
(Figure S12). In the PL spectra of 7, almost no emission could
be observed, either in solution or in the solid state at room
temperature. In the frozen (at 77 K) matrix of 2-MeTHF with
10−4 M concentration, compound 7 displayed vibrationally
resolved luminescence (Figure S13), which might be interpreted
in terms of the heavy atom effect of tellurium.
To show the promising applications of 6 as temperature-
sensitive anticounterfeiting materials in information protection
and graphic security, a proof of concept graphic security system
(the Chinese “Taiji” map) has been made (Figure 3h). A
detailed production process is detailed in the Supporting
Information. This process was reversible, and the PL spectra of 6
was measured after each step (Figure S11b).
In conclusion, we successfully synthesized a series of
chalcogen-containing carborane derivatives through a one-pot
reaction. The experiment and calculation indicated that the
nonconjugated six-membered ring in 1 could be electrochemi-
cally reduced to a dithiolate derivative in three steps. NICS(0)
values and DFT calculations have proven that 2−4 had
considerable 3-D aromatic character for σ−π conjugation. The
dicarborane chalcogenides 5−7 showed AIE properties for the
TICT mechanism. The design strategy for 6 might show
promising applications of temperature-sensitive anticounterfeit-
ing materials in information protection and graphic security.
Research on other applications of chalcogen-containing
carborane derivatives is currently underway.
CCDC 1839586 and 1899799−1899804 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ganghe@mail.xjtu.edu.cn.
ORCID
Pengfei Li: 0000-0002-4736-2477
Yanfeng Zhang: 0000-0003-4711-8690
Zishun Liu: 0000-0003-4669-8347
Gang He: 0000-0002-5319-7084
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation of
China (21875180, 21704081), the Fundamental Research
Funds for the Central Universities (1191329805), Natural
Science Foundation of Shaanxi Province (2018JM2026), and
the Cyrus Chung Ying Tang Foundation. We thank Gang Chang
and Axin Lu at the Instrument Analysis Center of Xi’an Jiaotong
University for their assistance with HRMS and PL spectra.
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* Supporting Information
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ACS Publications website at DOI: 10.1021/acs.orglett.9b03047.
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DOI: 10.1021/acs.orglett.9b03047
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