Synthesis of Tellurium-Containing σ-Extended Aromatics with Room-Temperature Phosphorescence

Article abstract of DOI:10.1021/acs.orglett.9b03106

A synthesis of tellurium-embedded -extended aromatics from tellurium powder and readily available cyclic diaryliodonium salts has been developed. The versatility of this method has been demonstrated by the synthesis of various functionalized dibenzotellurophenes (DBTe's), a ladder-type -system, and a heterosumanene. These compounds demonstrated good air/moisture stability and high thermal stability. Remarkably, many DBTe's exhibited interesting tunable roomerature phosphorescence (RTP) in the solid state.

Full text of DOI:10.1021/acs.orglett.9b03106

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis of Tellurium-Containing π‑Extended Aromatics with
Room-Temperature Phosphorescence
Mengjing Jiang, Jimin Guo, Bingxin Liu, Qitao Tan,* and Bin Xu*,†,‡
†
,§
†,§
†
,†
†
Department of Chemistry, Innovative Drug Research Center, Shanghai University, 99 Shangda Road, Shanghai 200444, China
‡
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
*
S Supporting Information
ABSTRACT: A synthesis of tellurium-embedded π-extended
aromatics from tellurium powder and readily available cyclic
diaryliodonium salts has been developed. The versatility of
this method has been demonstrated by the synthesis of
various functionalized dibenzotellurophenes (DBTe’s), a
ladder-type π-system, and a heterosumanene. These com-
pounds demonstrated good air/moisture stability and high
thermal stability. Remarkably, many DBTe’s exhibited
interesting tunable room-temperature phosphorescence
(
RTP) in the solid state.
ellurium has been widely used in fields ranging from
T
metallurgy to electronics, including applications as
1
thermoelectric materials and in solar panels. Recently,
2
organotellurium has also attracted pharmaceutical interests.
Tellurium is a unique element among chalcogens, and the
properties of tellurium-containing compounds cannot simply
be extrapolated from the behaviors of their lighter chalcogen
3
sulfur and selenium analogues. Tellurium-containing aromatic
compounds often exhibit unique properties, such as notable
4
5
heavy-atom effects, strong intermolecular interactions, and
6
narrow band gaps. Among tellurium-containing aromatics,
tellurophene-based small molecules and polymers have been
7
extensively investigated in relation to their redox and
8
9
photochemical reactivity, optoelectronic properties, and
10
anion recognition ability (Figure 1). This may partially be
attributed to the facile preparation of tellurophenes by various
Figure 1. Selected examples of tellurophenes and a few π-extended
tellurium-containing aromatics as materials.
1
1−13
effective methods.
However, little was known about the
properties of larger π-extended tellurium-containing aromatic
1
6
systems, although their sulfur and selenium analogues are well
1). The reaction of dihalobiphenyl with organolithium
reagents, followed by the treatment of tellurium represents a
general synthetic method to DBTe’s, which suffers from poor
functional group tolerance, tedious procedures, and low yields
14
known to be eminent materials. Recently, dibenzotellur-
ophene (DBTe) and larger π-extended systems have emerged
as promising materials in hydrogen evolution, Li-ion batteries,
15
17
organic thin-film transistors, and solar cells (Figure 1).
(route b). Reactions of 2,2′-diiodobiaryls with an in-situ-
However, the challenge in the preparation and the resultant
lack of structural diversity is the main reason, if not the entire
reason, that limits their wide applications.
The synthesis of DBTe, a fundamental unit for large
tellurium-containing aromatics, was first investigated. The
preparation of DBTe’s usually suffers from a multistep
synthesis, substrates that are not readily available, or low
yields. The previous synthesis of DBTe gave a poor yield by
the reaction of dibenzo[b,d]thiophene 5,5-dioxide with
tellurium at an elevated temperature (route a in Scheme
prepared tellurium−copper slurry provide a more straightfor-
18
ward method to DBTe’s (route c), whereas the preparation
of functionalized 2,2′-diiodobiaryls is nontrivial in some
19
cases. The reduction of dibenzotellurophene dichlorides,
prepared through the reaction of 2-halobiphenyl with organo-
metallic reagents, followed by quenching with TeCl and
cyclization, can also give DBTe’s (route d). The direct
4
20
Received: August 31, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03106
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Synthesis of Dibenzotellurophenes (DBTe’s)
Table 1. Optimization of Reaction Conditions
b
entry
solvent
additive
yield (%)
t
1
2
3
4
5
6
7
8
9
DMSO (3.0 mL)
DMSO (3.0 mL)
DMSO (3.0 mL)
DMSO (2.0 mL)
DMSO (2.0 mL)
DMSO (2.0 mL)
DMSO (2.0 mL)
DMSO (3.0 mL)
DMSO (3.0 mL)
DMSO (3.0 mL)
DMSO (3.0 mL)
NMP (2.0 mL)
DMF (2.0 mL)
DMSO (2.0 mL)
KO Bu (4 equiv)
Cs CO (4 equiv)
trace
10
30
56
74
70
43
29
54
trace
7
trace
39
2
3
pyridine (3 equiv)
pyridine(1.0 mL)
2-picoline (1.0 mL)
2,6-lutidine (1.0 mL)
2-MeOPy (1.0 mL)
bipyridine (3 equiv)
1,10-phen (3 equiv)
10
11
12
13
14
Et N
3
2-picoline (1.0 mL)
2-picoline (1.0 mL)
2-picoline (1.0 mL)
c
50
a
Reaction conditions: 1a (0.3 mmol), tellurium powder (0.45 mmol),
additive, solvent, temperature, under a nitrogen atmosphere, 12 h.
Isolated yield. Air atmosphere. 1,10-phen = 1,10-phenanthroline. 2-
treatment of 1,1′-biphenyl with TeCl4 at an elevated
temperature gave the dichloride; however, this method gave
b
c
21
only pristine DBTe. Therefore, it is imperative that a highly
efficient synthesis affording DBTe’s of structural diversity is
developed.
MeOPy = 2-methoxypyridine.
a
Scheme 2. Synthesis of Functionalized DBTe’s
The ready availability and the air/moisture stability of cyclic
diaryliodonium salts have made them ideal starting materials
22,23
for the construction of various heterofluorenes.
We herein
report a convenient synthesis of tellurium-bridged aromatic
compounds from cyclic diaryliodonium salts and tellurium
powder under transition-metal-free conditions (Scheme 1b).
Diversified DBTe’s, tellurium-containing ladder-type π-con-
jugated systems, and heterosumanene could also be prepared
via multifold cyclization. These compounds demonstrated
good air/moisture and high thermal stabilities. Additionally,
many DBTe’s exhibit interesting tunable room-temperature
phosphorescence (RTP) in the solid state, which has triggered
tremendous research efforts due to their potential applications
2
4,25
in many fields.
We commenced our investigation by the reaction of 1a and
elemental tellurium in the presence of bases. Considering the
limited solubility of tellurium and iodonium salts, DMSO was
selected as the solvent. Inorganic bases such as potassium tert-
butoxide and cesium carbonate gave only a trace or negligible
amount of the desired product (Table 1, entries 1 and 2). We
next turned our attention to organic bases. Fortunately, the
addition of 3.0 equiv of pyridine gave 2a in 30% yield, and
increasing the amount of pyridine afforded 56% yield (entries 3
and 4). Among the commercial and inexpensive pyridine
derivatives, 2-picoline gave the best yield (entries 5−7).
Bidentate ligands such as 2,2′-bipyridine and 1,10-phenanthro-
line gave 29 and 54% yield, respectively (entries 8 and 9).
Surprisingly, trialkyl amines such as Et N were not effective
a
3
Reaction conditions: diaryliodonium salts 1b−1q (0.3 mmol),
tellurium powder (0.45 mmol), 2-picoline (1.0 mL), DMSO (2.0
mL), 120 °C, under a nitrogen atmosphere, 12 h. Yields shown are of
additives (entry 10). Only a trace amount of 2a was obtained
in the absence of a base, indicating its vital role in this
transformation (entry 11). Considering the efficiency and cost,
b
the isolated products. 2q (0.2 mmol) and tellurium (0.4 mmol) were
used.
2
-picoline was selected as the base for the following synthesis.
Other solvents led to trace or much lower yields (entries 12
and 13). The yield of the reaction decreased when conducted
under open air (entry 14).
The scope of this reaction was investigated next. As shown
in Scheme 2, various cyclic diaryltellurophenes were prepared
in good to excellent yield. Cyclic diaryliodonium salts with
monosubstitution at the three-position, regardless of their
electronic characters, afforded the desired DBTe’s (2b−i) in
B
DOI: 10.1021/acs.orglett.9b03106
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
moderate to good yield. 1-Monosubstituted, 3,6- and 3,7-
disubstituted diaryliodoniums gave the desired DBTe’s (2j−n)
in good yield. Diaryltellurophenes 2o with an isoquinoline
motif could also be prepared. The introduction of a tellurium
atom to flavonoids with biological activities would be
interesting. To our delight, tellurium-containing flavones 2p
and 2q were also achieved. The structures of 2a, 2e, 2g, 2i, and
afford pristine tritellurasumanene. We applied this newly
developed method for the synthesis of tritellurasumanene 2s.
3
0b
Fortunately, 2s was achieved from 1s
under similar
1
conditions (Figure 2e). The H NMR spectrum of 2s showed
26
13
only one singlet peak at 8.31 ppm, and the C NMR spectrum
exhibited three peaks, indicating its high symmetry.
Chalcogen bonding has emerged as a potential alternative to
5
2
q were confirmed by X-ray crystallographic analysis (Figure 3
hydrogen bonding and halogen bonding. Tellurium atoms
and Figures S1−S6). It should be noted that most DBTe’s
were stable enough toward air and moisture to be purified by
chromatography without special precautions. The control
reactions and a plausible mechanism for this reaction can be
found in the Supporting Information (Scheme S1).
usually form the strongest chalcogen bonds. We obtained
several single crystals of DBTe’s and ladder-type 2r, allowing
the direct observation of the chalcogen bonding (Figure 3 and
We next successfully extended the method to the synthesis
of larger π-conjugated systems. The transformation of ladder-
19
type bis-iodonium 1r to 2r was achieved in 31% yield under
the standard condition with TBAB as an additive (Figure 2a).
Figure 3. Chalcogen bonding in the crystals of 2a, 2g, 2i, and 2r. Gray
dashed line for Te···H or heteroatom···H interactions.
Figures S1−S6). In DBTe 2a, strong Te···Te interactions were
observed with a distance of 4.06 Å, which is 0.24 Å shorter
than the sum of the van der Waals distance of Te (4.30 Å). An
obvious tendency of 2a to form tellurium clusters reflects the
strong metallic character of tellurium. Strong intermolecular
Te···Te interactions were observed for 2r with extremely short
distances (3.61 and 3.65 Å). To the best of our knowledge, this
is the shortest Te···Te distance reported in tellurium-
containing compounds; distances of around 3.80 to 4.04 Å
31
were previously found in 3,4-dimethoxytellurophene. In 2g
and 2i, interactions between tellurium and heteroatoms were
also observed (Figure 3 and Figures S3 and S4).
Figure 2. Synthesis of ladder-type semiconductor 2r and tritellur-
asumanene 2s. (a) Synthesis of 2r. (b) TGA spectroscopy of 2r. (c)
X-ray structure of 2r. (d) Sumanene and heterosumanene. (e)
Synthesis of 2s. TBAB = tetrabutylammonium bromide.
All DBTe’s showed weak and broad absorption peaks at the
long wavelengths in their UV−vis spectra (Figure 4a and
Figures S15−S25). The absorption of 2a was red-shifted as
32
Compound 2r exhibited impressive thermal stability (up to
compared with its analogue dibenzothiophene. The most
striking property of DBTe’s we observed was the RTP,
although tellurophenes have recently been revealed as
3
50 °C) by thermogravimetric analysis (TGA) (Figure 2b),
which is a prerequisite for materials. The structure of 2r was
unambiguously confirmed by X-ray crystallographic analysis
9
interesting RTP materials by He and Rivard. Whereas
(
Figure 2c). Sumanene, a C -symmetric buckybowl resem-
DBTe’s were nonemissive in solution, many DBTe’s exhibited
color-tunable solid-state RTP in the presence or absence of
oxygen (Figure 4b, Table S1, and Figures S15−S23). DBTe’s
2a and 2f emitted bright green and yellow colors with one
major emission band under UV excitation. However, 2i
exhibited interesting dual-emission bands with maxima at
468 and 600 nm, respectively, resulting in a whitish orange
3v
bling a basic fragment of C60 (Figure 2d), has received
significant interest not only for its unique geodesic structure
but also for its properties.
used to replace the carbons of sumanene to tune the structures
and intrinsic properties.
poses a great challenge to chemists.
the synthesis of sulfur- and selenium-doped sumanenes;
however, tellurium-containing sumanene could be obtained
under similar conditions. In 2016, Shao et al. reported the
first ultrasound-assisted synthesis of tritellurasumanene with
hexa-alkoxyl groups on its periphery through the lithiation of
hexa-alkoxy-substituted triphenylene, followed by treatment
27,28
Various heteroatoms have been
29,30
The synthesis of heterosumanene
29,30
Recently, we reported
́
color with Commission Internationale de l’Eclairage 1931
coordinates of (0.36, 0.33) (Figure S13). Compounds 2a and
2i had been purified to >99% purity by preparative high-
performance liquid chromatography (HPLC) (Figure S14).
The lifetimes on a microsecond scale, as determined by time-
resolved technology (Figure S27), indicated that the emissions
might originate from phosphorescence or thermally activated
30b
29h
with tellurium powder;
however, this method could not
C
DOI: 10.1021/acs.orglett.9b03106
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
conjugated systems such as nanographenes by this method is
currently underway in our laboratory.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03106.
Experimental procedures, spectral data, photophysical
properties, and calculation data (PDF)
Accession Codes
CCDC 1892105−1892106, 1892119, and 1942804−1942806
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.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.
Figure 4. Photophysical properties of representative DBTe’s. (a)
UV−vis spectra. (b) Photoluminescence spectra of DBTe’s in the
solid state at room temperature (red curves) and in THF at 77 K for
AUTHOR INFORMATION
■
2
a (dashed line) and photographs (insets) of powder samples taken
Corresponding Authors
under 365 nm. The measurements were conducted under air.
*
*
E-mail: qttan@shu.edu.cn (Q.T.).
E-mail: xubin@shu.edu.cn (B.X.).
33
delay fluorescence (TADF). The long emission wavelength
ORCID
3
4
of DBTe 2a and the retention of the emission band at 77 K
Figure 4b) ruled out the possibility of TADF. Crystallization
has been proven to be an effective approach to trigger pure
Qitao Tan: 0000-0002-6220-651X
Bin Xu: 0000-0002-9251-6930
Author Contributions
§M.J. and J.G. contributed equally.
(
3
5
organic RTP. The crystal of 2a exhibited green phosphor-
escence similar to that of the amorphous powder or ground
sample (Figure S28), which indicates that it was not
crystallization-induced RTP. Powder and ground samples of
Notes
2
i similarly emitted (Figure S29). The offset face-to-face
The authors declare no competing financial interest.
stacking of 2i (interplanar distance of 3.57 Å) might be one of
the reasons for its dual-emission behavior through excimer
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
nos. 21971159, 21672140, and 21672136) for financial
support. We thank Prof. Hongmei Deng (Laboratory for
Microstructures, SHU) for NMR measurements and Prof.
Baoli An (SHU) for transient luminescence measurements. We
thank Prof. Chang-Hua Ding (SHU), Prof. Zhi-Ming Duan
SHU), and Prof. Yuan-Zhi Tan (Xiamen University) for
helpful discussions. The calculations were supported by the
High Performance Computing Center, Shanghai University
■
36
formation (Figure S4). To confirm this, we measured the
emission spectra of 2a, 2f, and 2i in PMMA matrix (1 wt %)
(
(
Figures S30−S32). The emission spectra of 2a and 2f in
PMMA matrix were similar to powder samples; however, the
emission of 2i in PMMA film was different, and the emission
band at ∼600 nm found in the powder disappeared. As a result,
the color of the PMMA film of 2i was different from that of the
powder sample (Figure S33). These results supported the
excimer formation for DBTe 2i in powder. The intensity of the
solid-state emission of DBTe’s was low, thus obviating the
measurement of a reliable quantum yield. Density functional
theory (DFT) calculations revealed that multiple energetically
low-lying triplet states (T ) to the excited singlet state (S ) and
(
(
zq4000).
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