Circularly polarized luminescence from chiral spiro molecules: Synthesis and optical properties of 10, 10′- spirobi(indeno[1, 2-b][1]benzothiophene) derivatives

Article abstract of DOI:10.1021/acs.orglett.7b02337

Chiral spiro polycyclic aromatic compounds with thiophene and/or thiophene S, S-dioxide units were synthesized as a new family of circularly polarized luminescence (CPL) materials. Oxidation of the thiophene unit to its S, S-dioxide provided significant changes in photophysical properties such as much higher fluorescence quantum yield and positive solvatochromism in photoluminescence spectra. Density functional theory calculations qualitatively demonstrated the observed absorption properties. The prepared chiral spiro compounds exhibit intense CPL signals with a relatively large dissymmetry factor for small organic molecules.

Full text of DOI:10.1021/acs.orglett.7b02337

Letter
pubs.acs.org/OrgLett
Circularly Polarized Luminescence from Chiral Spiro Molecules:
Synthesis and Optical Properties of 10,10′-
Spirobi(indeno[1,2‑b][1]benzothiophene) Derivatives
†
‡
,†
Ko Takase, Keiichi Noguchi, and Koji Nakano*
†
‡
Department of Organic and Polymer Materials Chemistry, and Instrumentation Analysis Center, Tokyo University of Agriculture
and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
*
S Supporting Information
ABSTRACT: Chiral spiro polycyclic aromatic compounds with thiophene
and/or thiophene S,S-dioxide units were synthesized as a new family of
circularly polarized luminescence (CPL) materials. Oxidation of the
thiophene unit to its S,S-dioxide provided significant changes in photo-
physical properties such as much higher fluorescence quantum yield and
positive solvatochromism in photoluminescence spectra. Density functional
theory calculations qualitatively demonstrated the observed absorption
properties. The prepared chiral spiro compounds exhibit intense CPL signals
with a relatively large dissymmetry factor for small organic molecules.
piro polycyclic aromatic compounds consist of two planar
biaryls that are connected perpendicularly by the sp -
hybridized spiro carbon atom (Figure 1). This structural feature
(Figure 1). In fact, a number of chiral spiro polycyclic aromatic
3
S
compounds have been synthesized since the first report on7
enantiomerically pure 9,9′-spirobifluorene derivatives in 1969.
However, their chirality has attracted much less attention. Most
of them have been evaluated and used as racemic mixtures.
Nevertheless, there have been some reports that focused on
their chirality. For example, chiral spiro polycyclic aromatic
8
compounds have been applied to molecular recognition and
9
asymmetric synthesis. Chiroptical properties such as circular
dichroism (CD) and optical rotatory power (ORP) have also
7
,10
been investigated for several compounds.
However, studies
on chiroptical properties are still limited. In particular, to the
best of our knowledge, there have been only two reports on
11
circularly polarized luminescence (CPL). CPL has attracted
greater attention owing to its potential applications in display
12
13
14
devices, biological probes, sensors, and so on. Thus, a
variety of CPL materials based on chiral small organic
molecules, chiral lanthanide complexes, and helical polymers/
Figure 1. Spiro polycyclic aromatic compounds with two biaryl-
derived π-conjugated systems.
15,16
aggregates have been developed,
and new molecular
imposes high steric demand on them and hinders their
intermolecular aggregation. It also significantly limits the orbital
interaction between the two biaryl-derived π-conjugated
systems, while the spiroconjugation allows the orbital
designs are still required for further development of CPL
materials.
In this context, we designed the chiral spiro polycyclic
aromatic compound 1 as a new family of chiral building blocks
for CPL materials (Figure 1). Oxidation of a sulfide group to a
sulfonyl group in thiophene-containing π-conjugated com-
pounds has been reported to significantly change their
1
interaction to some extent. Owing to these characteristics,
spiro polycyclic aromatic compounds have been recognized as
one of the most promising building blocks for organic
2
functional materials in organic photovoltaics, organic light-
17
3
4
electronic properties. Thus, we investigated oxidation of 1
to 2 and 3 with one and two thiophene S,S-dioxide moieties,
respectively. Herein, we report the synthesis of compounds 1−
emitting diodes, and so on. Among them, 9,9′-spirobifluorene
5
is a prototypical and very widely studied skeleton. Replace-
ment of the benzene ring(s) in the 9,9′-spirobifluorene core
3
and their chiroptical properties including CPL activity.
with heterocycle(s) has also been demonstrated to develop new
6
heterocyclic building blocks.
Spiro polycyclic aromatic compounds derived from two
Received: July 28, 2017
dissymmetric biaryls are chiral molecules with C symmetry
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02337
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The synthetic route to the designed compounds 1−3 is
shown in Scheme 1. 3-Bromo-2-(2-bromophenyl)benzo[b]-
Scheme 1. Synthetic Route of Spiro Compounds 1−3 and
the Structure of Model Compounds
Figure 2. (a) UV/vis absorption and (b) normalized photo-
luminescence spectra of spiro compounds 1−3 and their model
compounds 7 and 8 in CH Cl and (c) CD and CPL spectra of spiro
2
2
compound 1 in CH Cl .
2
2
absorption bands in the wavelength region of <340 nm and a
broad absorption band in a longer wavelength region. This
clearly demonstrates that each of the perpendicularly arranged
π-conjugated systems works as an almost-independent
chromophore because of a significantly limited orbital
interaction with each other in the ground state. For all
compounds, the absorption spectra are almost independent of
1
8
thiophene (4) was dilithiated with BuLi in the presence of
N,N,N′,N′-tetramethylethylenediamine (TMEDA) and then
reacted with ester 5 to yield the tertiary alcohol intermediate
6
. The following acid-promoted intramolecular Friedel−Crafts
5
cyclization was conducted with the crude product after the
usual workup, affording the desired compound 1 as a racemic
mixture (65% yield for two steps). Next, we investigated the full
and partial oxidation of the two thiophene units of the obtained
compound 1. The full oxidation was readily achieved by using
an excess amount (10 equiv) of 3-chloroperbenzoic acid (m-
solvent polarity. In the PL spectrum, compound 1 showed an
emission maximum (λ ) at 368 nm with a Stokes shift of 2238
em
−1
cm . Similar to the absorption spectrum, the PL spectrum is
red-shifted compared to that of the model compound 7. The
PL spectrum of 3 was almost identical to that of the model
compound 8. The emission maximum was observed at 444 nm
1
7b,f,19
CPBA)
as an oxidant to give the desired compound 3 in
high yield (81%). The partial oxidation to compound 2 was
carried out with 2.0 equiv of m-CPBA. The desired compound
was obtained in 30% isolated yield along with a mixture of
unidentified byproducts which would contain the correspond-
ing thiophene S-oxide derivatives. Indeed, treatment of the
mixture with an excess amount of m-CPBA afforded compound
(Supporting Information). X-ray crystallographic analysis of 1
and 3 revealed that both enantiomers were contained in the
unit cell in each case (Figures S12 and S13 and Tables S1 and
S2). The two spiro-linked π-conjugated planes are completely
perpendicular. Enantiopure 1−3 were successfully obtained by
optical resolution of each racemic mixture with preparative
HPLC on a chiral stationary phase.
Photophysical properties of racemic 1−3 were evaluated by
UV−vis absorption and photoluminescence (PL) spectros-
copies (Figure 2 and Table 1). Compound 1 showed well-
resolved absorption bands with the longest absorption
−1
with a Stokes shift of 5255 cm , which was much larger than
that of 1. Slight red-shifts in PL spectra of 3 and the model
compound 8 were observed with an increase in solvent polarity
[λ : 427 nm (hexane) and 453 nm (MeOH) for 3; λ : 422
2
17f
em
em
2
0
nm (hexane) and 454 nm (MeOH) for 8, Figures S15], while
the absorption spectra showed little shift. Such a positive
solvatochromism in PL spectra would be attributed to the
3
21
electron-accepting property of the thiophene S,S-dioxide unit.
Compounds 3 and 8 should possess a larger dipole moment in
the excited states than compounds 1 and 7, respectively,
because of the electron-withdrawing property of the thiophene
S,S-dioxide unit. Thereby, more polar solvents induce higher
stabilization of the exited state, resulting in emission at longer
wavelength. It is noteworthy that the fluorescence quantum
1
7f,21c,d
yield of 3 (Φ = 76%) is much higher than that of 1 (6%).
The spiro compound 3 shows much higher Φ than its model
compound 8, whereas the spiro compound 1 does lower Φ
than its model compound 7. This inverse relationship may be
maximum (λ ) at 340 nm. On the other hand, the fully
abs
oxidized compound 3 gave a broader and red-shifted absorption
1
7b,c,f
spectrum.
Both absorption spectra reflect specific features
due to the much larger radiative rate constant (k ) of 3 and the
r
of the non-spiro-linked model compounds 7 and 8 (Scheme 1),
much smaller k of 1 than those of 8 and 7, respectively,
r
respectively, while they are slightly red-shifted compared to
although the reason behind such rate constants is unclear yet
(Table S3). The horizontally dissymmetric compound 2
exhibited the emission maximum at 459 nm. The positive
solvatochromism of the emission band was observed as in the
case of 3, which would reflect the feature of the thiophene S,S-
dioxide containing fragment of the molecule (Figure S15). In
4b
those of the corresponding model compounds. Such a red-
shift would indicate the spiroconjugation between the two1
spiro-linked π-conjugated systems in each spiro compound.
The horizontally dissymmetric compound 2 showed composite
features of the model compounds 7 and 8, that is, well-resolved
B
DOI: 10.1021/acs.orglett.7b02337
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Photophysical Properties of Spiro Compounds 1−3 and Their Model Compounds 7 and 8
a
b
Stokes shift (cm−1
c
d
e
|[α]D25|f
λabs (nm)
λem (nm)
)
Φ (%)
|gabs
3
|
|glum
−3
|
1
2
3
7
8
340
g
368
459
444
354
444
2238
6 (330 nm)
1 (380 nm)
76 (360 nm)
11 (280 nm)
46 (350 nm)
2.0 × 10− (348 nm)
1.5 × 10 (252 nm)
1.2 × 10− (293 nm)
1.5 × 10 (355 nm)
3.0 × 10 (534 nm)
1.5 × 10 (497 nm)
175 (c = 0.22)
153 (c = 0.38)
121 (c = 0.20)
−3
−3
3
−4
360
330
355
5255
2054
5646
a
b
c
The longest absorption maximum in CH Cl . Emission maximum in CH Cl . Absolute quantum yield determined by a calibrated integrating
2
2
2
2
d
−5
−5
−5
sphere system. Excitation wavelength in parentheses. In CH Cl (1.2 × 10 M for 1, 6.0 × 10 M for 2, and 5.1 × 10 M for 3). Wavelength for
the calculated |g | in parentheses. In CH Cl . Excitation at 300 nm for 1, 330 nm for 2, and 350 nm for 3. Wavelength for the calculated |glum| in
parentheses. In CH Cl . Absorption maximum was not determined since the spectrum showed no obvious maximum in longer wavelength region.
2
2
e
abs
2
2
f
g
2
2
addition, a new emission band in a longer wavelength region
∼520 nm), that is, dual emission, was observed in higher polar
compound gave mirror-image CPL signals. The dissymmetry
23
(
factors in luminescence, glum [2ΔI/I = 2(I − I )/(I + I )],
L
R
L
R
−3 −3
solvents. This emission band may be attributed to the
photoinduced intramolecular charge transfer (ICT) where the
thiophene and thiophene S,S-dioxide containing fragments of
the molecule serve as electron-donor and electron-acceptor
of 1−3 were estimated to be ±1.5 × 10 , ±3.0 × 10 , and
−4
±1.5 × 10 , respectively, which are comparable to those of the
previously reported compounds with a large glum value for small
15a
organic molecules in a monomeric state. It is notable that
compound 3 demonstrates CPL activity with a high
fluorescence quantum yield. The wavelength of the CPL
21a,b
units, respectively.
The k of 2 is much smaller than those of
r
1
and 3, resulting in lower Φ.
In order to obtain a better understanding of the photo-
24
maximum of 2 is longer than that of the PL maximum and
corresponds to the shoulder band at 520 nm which would be
derived from the photoinduced ICT (vide supra). Such a red-
shift in the CPL maximum indicates more significant CPL from
the ICT state.
physical properties of 1−3, we conducted density functional
theory (DFT) and time-dependent (TD) DFT calculations at
the B3LYP/6-31G(d) level of theory. The HOMOs and the
LUMOs of 1, 3, and their model compounds 7 and 8 are
delocalized over the entire skeleton (Figures S16). A slightly
larger distribution on the thiophene S,S-dioxide moieties was
observed in the LUMOs of 3 and 8, which would be attributed
to the electron-withdrawing ability of the thiophene S,S-dioxide
unit. The TD−DFT calculation demonstrated the red-shifted
longest absorption wavelength of 3 compared to that of 1
In conclusion, we have synthesized chiral spiro polycyclic
aromatic compounds with thiophene and/or thiophene S,S-
dioxide units. Oxidation of the thiophene units to the S,S-
dioxide units resulted in a significant change in photophysical
properties. In particular, full oxidation of the two thiophene
units of 1 was found to increase the fluorescence quantum
yield. The synthesized chiral spiro compounds exhibited CPL
(Table S10). In addition, the longest absorption wavelengths of
−3
with a glum value of <3.0 × 10 . The previously reported CPL-
1
and 3 were estimated to be red-shifted compared to those of
active spiro polycyclic aromatic compounds possess a silicon
model compounds 7 and 8, respectively. These calculation
results are qualitatively consistent with the observed absorption
spectra of them. The orbital symmetry of the HOMOs of 7 and
11
spiro center. Therefore, the present work is the first example
of CPL property of spiro polycyclic aromatic compounds with a
spiro carbon atom. The observed glum values are comparable to
those of the reported chiral small organic molecules which have
been known to exhibit significant CPL properties. Considering
the rich chemistry of spiro polycyclic aromatic compounds, the
molecular design with a chiral spirocyclic π-conjugated skeleton
would be promising to develop novel CPL materials.
1a
8
is suitable for spiroconjugation in 1 and 3, respectively.
Indeed, the bonding and antibonding combinations were
shown in the HOMO−1 and HOMO, respectively, for each
spiro compound (Figure S16). The energy differences between
HOMO−1 and HOMO (0.27 eV for 1; 0.22 eV for 3) are close
22
1b
to the experimental (0.30 eV) and calculated (0.27 eV)
spiro splittings of 9,9′-spirobifluorene and much smaller than
the HOMO−1/HOMO energy differences of 7 and 8. These
facts clearly indicate the spiroconjugation in 1 and 3 which
would induce the red-shifted absorption spectra of 1 and 3
compared to those of 7 and 8, respectively. For the horizontally
dissymmetric compound 2, the HOMO and LUMO are
localized in thiophene and thiophene S,S-dioxide containing
fragments, respectively, by reflecting its donor−acceptor type
structure (Figure S16).
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.7b02337.
1
Experimental procedures, characterization data, H and
13
C NMR spectra for all new products (1−3, 7, and 8),
theoretical calculation results, and PL, CD, and CPL
spectra (PDF)
X-ray data for compound 1 (CIF)
The chiroptical properties of 1−3 were investigated by CD
and CPL spectroscopies and optical rotation measurement
(Table 1, Figure 2c, and Figure S17). The CD spectra of (+)-1
X-ray data for compound 3 (CIF)
exhibited two large positive Cotton effects at 343 and 329 nm
followed by a series of smaller negative and positive ones in the
range of <318 nm. As expected, (−)-1 gave a mirror-image CD
spectrum. In the cases of compounds 2 and 3, the CD spectra
of the enantiomers also exhibited complete mirror image. The
dissymmetry factors in absorption, gabs (Δε/ε = (ε − ε )/[(ε
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: k_nakano@cc.tuat.ac.jp.
L
R
L
ORCID
+
ε )/2]), were estimated to be approximately ±(1.2−2.0) ×
R
−3
10 . In the CPL spectra, two enantiomers of each spiro
Koji Nakano: 0000-0003-4166-7074
C
DOI: 10.1021/acs.orglett.7b02337
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
(12) (a) Peeters, E.; Christiaans, M. P. T.; Janssen, R. A. J.; Schoo, H.
F. M.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1997, 119,
The authors declare no competing financial interest.
9
909−9910. (b) Grell, M.; Oda, M.; Whitehead, K. S.; Asimakis, A.;
Neher, D.; Bradley, D. D. C. Adv. Mater. 2001, 13, 577−580.
13) Heffern, M. C.; Matosziuk, L. M.; Meade, T. J. Chem. Rev. 2014,
14, 4496−4539.
14) Accetta, A.; Corradini, R.; Marchelli, R. Top. Curr. Chem. 2011,
00, 175−216.
15) For recent reviews, see: (a) San
ACKNOWLEDGMENTS
(
1
(
3
■
This work was partially supported by MEXT KAKENHI Grant
No. 16H00824. The computations were performed at the
Research Center for Computational Science, Okazaki, Japan.
We are grateful to Prof. Kyoko Nozaki (The University of
Tokyo) for chiral HPLC separation and Prof. Satoshi Minakata
and Prof. Youhei Takeda (Osaka University) for fluorescence
lifetime measurements.
(
́
chez-Carnerero, E. M.;
Agarrabeitia, A. R.; Moreno, F.; Maroto, B. L.; Muller, G.; Ortiz, M.
J.; de la Moya, S. Chem. - Eur. J. 2015, 21, 13488−13500. (b) Kumar,
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DOI: 10.1021/acs.orglett.7b02337
Org. Lett. XXXX, XXX, XXX−XXX