Synthesis and chiroptical properties of stereogenic cyclic dimers based on 2,2′-biselenophene and [2.2]paracyclophane

Article abstract of DOI:10.1039/c9ob01907c

Stereogenic cyclic dimers based on 2,2′-biselenophene linked with [2.2]paracyclophane have been synthesized to investigate their chiroptical properties. Embedding selenophene led to the formation of intramolecular Se?π interactions between the two biselenophene strands. The resulting rigid cyclic system exhibits enhanced chiroptical properties when compared with its precursor. In addition, the electrochemical properties were also investigated.

Full text of DOI:10.1039/c9ob01907c

Organic &
Biomolecular Chemistry
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Synthesis and chiroptical properties of stereogenic
cyclic dimers based on 2,2’-biselenophene and
Cite this: Org. Biomol. Chem., 2019,
7, 8822
1
[
2.2]paracyclophane†
Received 31st August 2019,
Accepted 23rd September 2019
Masashi Hasegawa, * Kosuke Kobayakawa, Yuki Nojima and Yasuhiro Mazaki
DOI: 10.1039/c9ob01907c
rsc.li/obc
Stereogenic cyclic dimers based on 2,2’-biselenophene linked with date. Thus, the exploration and development of functional
2.2]paracyclophane have been synthesized to investigate their materials containing a selenophene scaffold remain a worth-
chiroptical properties. Embedding selenophene led to the formation while research topic.
[
of intramolecular Se⋯π interactions between the two biseleno-
In this study, we have designed stereogenic cyclic com-
phene strands. The resulting rigid cyclic system exhibits enhanced pounds based on 2,2′-biselenophene and pseudo-ortho[2.2]
chiroptical properties when compared with its precursor. In paracyclophane (PC) (1, Fig. 1). As a robust planar chiral
addition, the electrochemical properties were also investigated.
scaffold, pseudo-ortho[2.2]PC has been widely employed as a
catalyst in asymmetric synthesis and a chiroptical material.
In particular, directly connecting aryl groups to the [2.2]PC
scaffold can give rise to a helical geometry in the embedded
10–15
Stereogenic compounds containing π-conjugated units have
attracted much attention in organic materials chemistry
because the π-conjugated system in a chiral environment exhi-
bits remarkable chiroptical and optoelectronic properties. It
has been reported that stereogenic oligo- and polythiophene
π-conjugation structure, which can lead to pronounced chiropti-
1
,2
3,6,14,15
cal properties.
The combination of chirality with seleno-
phene may lead to emergent electroconductive materials based
derivatives exhibit novel properties such as circular dichroism
16–18
on a chiral organic semiconductor transistor.
To investigate
3
(
CD) in the short wave infrared (SWIR) region, circularly polar-
the intrinsic nature of selenophene in a stereogenic system, we
herein report the synthesis, structure, and chiroptical properties
of 1 using optical rotation and CD spectra. In addition, the
electrochemical properties of 1 are also described.
4
5
ized luminescence (CPL), electroactive chirality, and
magneto-optic properties. These properties are associated
6
with the electronic structure derived from oligo(or poly)-thio-
phene in the chiral scaffold.
The synthetic route used to prepare 1 is shown in
7
Selenophene is a heteroaromatic analogue of thiophene.
Scheme 1. We chose racemic 4.12-diiode[2.2]paracyclophane
Replacing sulfur with selenium alters the molecular structure
and electronic properties. The large number of electrons in the
selenium element results in high polarizability that may give
19
(
rac)-2 as a starting compound. A Suzuki coupling reaction of
2
with 3, which was derived from commercially available sele-
nophene, in the presence of Pd(PPh3)4 gave (rac)-4 in 81%
yield. The separation of (rac)-4 into (R )-4 and (S )-4 was
accomplished using HPLC on a chiral stationary phase
Chiralpak IA). The absolute configuration was determined
using X-ray analysis of (S )-4 (Fig. S6b†). Subsequently, the
8
rise to singular optoelectronic properties. Therefore, the intro-
p
p
duction of selenophene into a stereogenic system is a fascinat-
ing approach to obtain novel chiroptical materials. In addition,
the relatively large van der Waals radius of Se can lead to
strong molecular interactions in the solid state, which is
advantageous for charge-transportation in conducting
(
p
transformation of each enantiomer into 5 was carried out
9
materials. However, few studies have focused on chiroptical
properties based on selenium-containing compounds, albeit a
variety of their sulfur counterparts have been developed to
Department of Chemistry, Graduate School of Science, Kitasato University,
1
-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan.
E-mail: masasi.h@kitasato-u.ac.jp
Electronic supplementary information (ESI) available. CCDC 1939516–1939518.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/c9ob01907c
†
1
Fig. 1 Stereogenic biselenophene in pseudo-ortho[2.2]paracyclophane.
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Scheme 1 Synthesis of (R
ditions: (a) Cs CO (aq), Pd(PPh
using HPLC on a chiral stationary phase (Chiralpak IA); (c) ICl, THF, 0 °C,
99% (for (R )-5) and >99% (for (S )-5); (d) Ni(cod) , 2,2’-bipyridyl, THF,
0 °C, 20% (for (R ,R )-1) and 18% (for (S ,S )-1).
p
,R
p
)-1 and (S
p
,S
p
)-1. Reagents and con-
Fig. 2 (a) The crystal structure of (R
p
,R
p
)-1 (thermal ellipsoids at
50% probability). Selected dihedral angles (°): C(18)–C(17)–C(1)–Se(1) =
38.3(7), C(30)–C(29)–C(8)–Se(2) = 38.4(7), C(23)–C(24)–C(9)–Se(3) =
−37.7(7), C(35)–C(36)–C(16)–Se(4) = −37.6(7), Se(1)–C(4)–C(5)–Se(2) =
19.6(6), and C(11)–C(12)–C(13)–Se(4) = −12.1(9). (b) Intermolecular con-
tacts between the stacks. Selected short contacts (Å) i: Se(2)⋯least
2
3
3
)
4
, toluene, 120 °C, 81%; (b) separation
>
8
p
p
2
p
p
p
p
squares plane (LSP) of Se(4)–C(13)–C(14)–C(15)–C(16)
= 3.373(2),
ii: Se(1)⋯LSP of Se(3)–C(9)–C(10)–C(11)–C(12) = 3.382(3), iii: C(12)⋯H(1)
upon treatment with iodomonochloride (ICl) followed by a 2.878, iv: C(13)⋯H(6) 2.892, and v: C(13)⋯H(6) 2.642.
Ni-catalysed coupling reaction to give the stereogenic homo-
coupling products (R ,R )-1 and (S ,S )-1 together with un-
p
p
p
p
identified products. (R
p
,R
p
)-1 and (S
p
,S
p
)-1 were sufficiently
sym- and 4 in CH
metric structures using H, C, and Se NMR spectroscopy. revealed an absorption maximum at 380 nm together with a
The solid state structure of (R ,R )-1 was determined using shoulder at ∼436 nm. In contrast, 2,2′-biselenophene exhibits
X-ray crystallography (Fig. 2). The orientation of the biseleno- an absorption maximum at 320 nm (CHCl
phene was restricted by the selenium atoms; compound 1 the UV-vis spectrum of (R )-4 exhibits maxima at 262 and
Subsequently, we measured the UV-vis and CD spectra of 1
stable under ambient conditions and characterized as D
2
2
Cl (Fig. 3). The UV-vis spectrum of (R ,R )-1
2
p
p
1
13
77
p
p
20a
3
).
Furthermore,
p
1
adopts a C symmetric structure composed of a pair of bisele- 315 nm. The observed bathochromic shift in 1 was attributed to
nophene units in a cisoid and transoid arrangement. These the extension of the π-conjugation in the selenophene moieties.
two biselenophene moieties are arranged in an edge-to-face As expected, embedding biselenophene into a stereogenic
fashion with Se⋯π interactions; the distances between Se(2) [2.2]PC scaffold remarkably enhanced the intensity of the CD
and the least-squares plane of selenophene A, and Se(1) and spectra. We found mirror image CD spectra for both 1 and 4.
selenophene B were found to be 3.373(2) and 3.382(3) Å, The CD spectrum of (R ,R )-1 exhibits a roughly bisignate shape
p
p
respectively (i and ii in Fig. 2b). Both biselenophene units are with small splitting peaks in the range corresponding to the
distorted from the benzene ring of the cyclophane in the range absorption band found in its UV-vis spectrum. Peaks or valleys
of 37.6–38.4°, leading to helical arrays between the two [2.2]PC were found at 432, 409, 375, and 360 nm. The maximum inten-
units, whereas the dihedral angles within two selenophenes sity of the Cotton effect was recorded as Δε = 57 (at 432 nm) for
p p
are relatively small. In the packing diagram there are inter- (R ,R )-1. On the other hand, a very small first Cotton effect was
molecular C–H⋯π interactions (Fig. 2b). Overall, the biseleno- found in the CD spectrum of (R )-4 at ∼360 nm (Fig. 3b, inset).
p
phene units arrange in an edge-to-face fashion along the c-axis Fig. 3c shows gabs plots, which are defined as Δε/ε obtained
in the crystal structure.
from CD and UV-vis spectra. The largest |gabs| values were
Compound 1 exhibits a relatively large optical specific found at 434 nm in 1. Thus, this indicates that the magnetic
rotation when compared with its acyclic precursor (4). The transition dipole moment in the corresponding transition band
2
5
rotation value for (R
p
,R
p
)-1 was [α] = +563 (CH
2
Cl
2
, c = may be enhanced in the present rigid cyclic system.
D
.8 × 10− ), while that of (R )-4 was [α] = +26 (CH Cl , c = Unfortunately, compounds 1 and 4 were not fluorescent, which
3
25
0
5
p
D
2
2
.0 × 10− ). Embedding the selenophene moieties into the was attributed to the internal heavy atom effect.
3
rigid stereogenic system may lead to an increase in the optical
rotation properties.
We performed a TD-DFT calculation study to validate the
experimental UV-vis and CD spectra. We initially optimised
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p p
Fig. 4 The molecular orbitals of (R ,R )-1.
state, the S and S excitation contributes to the positive sign,
1
2
while S
3
and S
4
p
result in a negative Cotton effect in the R con-
figuration. S and S are composed of transannular transitions,
1
3
which are associated with the electronic transition from one
biselenophene to the other. Although these are basically for-
bidden in the absorption spectra, these transitions have a
strong influence on the Cotton effect in the CD spectra. Thus,
this indicates that the magnetic transition dipole moment may
be predominant in this transition. This is consistent with the
larger gabs values found in 1, in particular that observed at
434 nm (Fig. 3c).
The electrochemical properties of 1 were investigated using
cyclic voltammetry (CV) and differential pulse voltammetry
n
(
DPV) in CH
2 2 4 6
Cl containing Bu PF . The CV curve obtained
5
Fig. 3 (a) The UV-vis spectra of (R
p
,R
p
)-1 (2.6 × 10− M) and (R
p
)-4 for (R ,R )-1 exhibited two sets of reversible and semi-revers-
p
p
2.8 × 10− M) in CH
5
Cl
together with the simulated spectrum of
,R )-1
)-4 (4.1 × 10 M) together
)-1 using TD-B3LYP/6-311+G(2d,p);
the inset shows the enlarged spectra of (R )-4 and (S
)-1, (R )-4, and (S
(
2
2
ible peaks (Fig. 5), although compound 4 did not exhibit any
oxidation peak within the potential window. Similar oxidation
(
R
p
,R
p
)-1 using TD-B3LYP/6-311+G(2d,p). (b) The CD spectra of (R
p
p
−5
−5
and (S
p
,S
p
)-1 (1.4 × 10 M), (R
p
)-4, and (S
,R
p
waves were observed in the DPV curve (E = 0.52 and E2
=
1
with the simulated spectrum of (R
p
p
+
)-4. (c) The gabs 0.71 V, vs. Fc/Fc ) together with a further oxidation peak at a
higher potential (E = 0.99 V). The multi-redox process corres-
p
p
p p p
(Δε/ε) plots obtained for (R ,R )-1, (S ,S
p
p
p
)-4.
3
ponds to the redox reactions derived from the two biseleno-
phene moieties. Since the redox potential of biselenophene
the geometry of (R
theory. The most stable geometry was consistent with the
molecular structure obtained during X-ray crystallography. The
p
,R
p
)-1 at the B3LYP/6-31G(d,p) level of
was recorded as a much higher value (>1.20 V) in the litera-
20
ture,
the reduction in the first and second potentials
suggests favourable interactions between the two biseleno-
phenes, which are present in a face-to-face arrangement.
1
obtained structure adopts a C symmetric geometry composed
of cisoid and transoid 2,2′-biselenophene moieties. Both the
highest occupied molecular orbital (HOMO) and lowest occu-
pied molecular orbital (LUMO) are spread over one side of the
2
,2′-biselenophene and phenyl rings in the cyclophanes. The
next HOMO and LUMO were also delocalized in a similar
manner, but located on the other biselenophene (Fig. 4).
Time dependent (TD)-DFT calculations were performed at
the B3LYP/6-311+G(2d,p) level of theory based on the opti-
mized C geometry. TD-DFT calculations for the R configur-
1 p
ation revealed that the longest absorption band of 1 involved
four transition bands associated with HOMO(H) → LUMO(L)
(
S
1
), H−1 → L (S
2
3 4
), H−1 → L+1 (S ), and H → L+1 (S ). These
transitions gave a qualitatively reproducible absorption spec-
trum, as shown in Fig. 3a. The calculated CD spectrum also
reproduced a qualitatively similar Cotton effect to that
Fig. 5 CV (100 mV s−1
)
p p
and DPV curves recorded for (R ,R )-1
observed experimentally. From the analysis of the transition (5.0 × 10− M) in CH
4
containing Bu
n
(0.1 M) at 25 °C.
2
Cl
2
4
NPF
6
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A similar tendency has been observed in an oligothiophene-
containing cyclophane scaffold.
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We have prepared rigid stereogenic cyclic dimers based on
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tallography revealed that (R ,R )-1 adopted a C symmetric geo-
p
p
1
metry bearing cisoid and transoid biselenophene moieties.
Embedding 2,2′-biselenophene into a robust chiral scaffold
remarkably enhanced the chiroptical properties when com-
pared to its acyclic chiral precursor. In addition, CV revealed
stepwise redox properties, which suggest that interactions were
formed between the two intramolecular biselenophene arrays.
To the best of our knowledge, this is the first example of
chiroptical properties based on a biselenophene scaffold. Our
results provide a foundation for the synthesis of new seleno-
phene-containing materials.
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Conflicts of interest
There are no conflicts of interest to declare.
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Acknowledgements
9
This work was partially supported by the JSPS KAKENHI Grant
Number 18K05092 and Kitasato Research Centre for
Environment Science. All calculations were performed at the
Research Centre for Computational Science, Okazaki (Japan).
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