CL-190149
Received: February 25, 2019 | Accepted: March 25, 2019 | Web Released: May 17, 2019
Chiroptical Properties of Oligophenylenes Anchoring with Stereogenic [2.2]Paracyclophane
Sumire Ishioka,1 Masashi Hasegawa,*1 Nobuyuki Hara,2 Hiroaki Sasaki,1 Yuki Nojima,1 Yoshitane Imai,*2 and Yasuhiro Mazaki1
1Graduate School of Science, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan
2Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University,
3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
E-mail: masasi.h@kitasato-u.ac.jp (M. Hasegawa), y-imai@apch.kindai.ac.jp (Y. Imai)
To elucidate the effect of π-conjugation length on chiroptical
properties in circular dichroism (CD) and circularly polarized
luminescence (CPL) spectra, stereogenic oligophenylenes con-
taining biphenyl and terphenyl units linked by a planar chiral
[2.2]paracyclophane were synthesized. Both compounds are
highly emissive, and their chiroptical responses depend on the
conjugation length. However, extended π-conjugation exhibited
smaller dissymmetry factor (g-value) both in absorption and
emission spectra.
Highly emissive oligophenylene may contribute to fluorescent
CPL materials with high quantum yield. Previously, some
chiral or non-chiral [2.2]PCs with phenyl groups were synthe-
sized.10c,16 Nevertheless, there has been little information
concerning how π-conjugation length would affect the chirop-
tical properties. Since the present V-shaped system can be the
fundamental scaffold of an extended π-system based on
oligophenylene and chiral [2.2]PC, difference in chiroptical
properties based on the number of phenyl groups is our chief
interest. Herein we report an examination of how the length of π-
conjugation in chiral circumstances affects chiroptical properties
of CD and CPL spectra.
Keywords: Cyclophane
| Chiroptical properties |
Circularly polarized luminescence
Taking into consideration optical resolution and synthetic
facility, we chose a synthetic route shown in Scheme 1. We
started with racemic 4,12-diiodo[2.2]PC (3);16 palladium-cata-
lyzed Suzuki-coupling reaction of 3 and boronic ester 6 afforded
racemic compounds of 4 in 63% yield. This racemic compound
was successfully separated into enantiomers of (Rp) and (Sp)-4
by HPLC on a chiral stationary phase. The absolute config-
uration was determined by CD spectra, because the present V-
shaped compounds thoroughly obey a chiral exciton coupling
method, assuming the longest bisignate-shaped Cotton effect.
Recently, stereogenic π-conjugated systems have received
emerging interest due to potential applications for future optical
devices based on their chiral properties as well as their artistic
molecular architectures.1,2 The π-conjugated chromophores in
robust chiral configuration are responsible for chiroptical
properties such as optical rotation, circular dichroism (CD),
and circularly polarized luminescence (CPL).3-7 The magnitude
of the chiroptical responses is associated with the rotational
strength which is derived theoretically from a scalar product
of the electric transition dipole moment (ETDM: ®) and the
magnetic transition dipole moment (MTDM: m). On the other
hand, dissymmetry factor, g-value, is determined theoretically
as g = 4R/D = 4(«m««®«cosª)/(«m«2 + «®«2) Ä 4(«m«/«®«)cosª,
where R, D, and ª denotes rotational strength, transition
probability, and the angle between ETDM and MTDM.3 This
implies that longer m and shorter ® values can fundamentally
enhance the dissymmetry factor in chiroptical response. How-
ever, the shorter ® value usually causes the decrement of the
absorption and fluorescence properties. Hence, a subtle balance
of these two vectors is important for fine tuning of chiroptical
properties in material design being required for applications.
A planar chiral [2.2]paracyclophane (PC) framework can be
a robust stereogenic resource that can anchor π-conjugated
chromophores in a chiral position.8-10 Recently, Morisaki and
Chujo achieved a high dissymmetry factor («glum«) for CPL in
rigid cyclic compounds based on pseudo-ortho-PC and phenyl-
ene-ethynylene system.11,12 They also reported chiroptical
properties of acyclic phenylene-ethynylene and phenylene-
vinylene oligomers based on chiral PC.13 Furthermore, we
have reported redox-triggered switchable chiroptical response of
oligothiophene and tetrathiafulvalenes in a chiral PC system.14,15
However, the relationship between the molecular structure and
chiroptical properties derived from the transition dipole moments
in a particular structure has not been completely explained.
In the course of our study on the exploration of remarkable
chiroptical materials, we now design stereogenic oligopheny-
lenes (1,2) linked with a planar chiral PC framework (Figure 1).
n
n
n
n
(Sp)-1 (n = 1)
(Sp)-2 (n = 2)
(Rp)-1 (n = 1)
(Rp)-2 (n = 2)
Figure 1. Molecular structures of (Rp)/(Sp)-1 and 2.
R
R
R
I
i), ii)
+
I
R
(rac)-3
(Rp)-4 (R = TMS)
(Sp)-4 (R = TMS)
(Rp)-5 (R = I)
or
(Sp)-5 (R = I)
(Rp)-1
or
(Sp)-1
(Rp)-4
or
(Sp)-4
iii)
iv)
O
O
B
TMS
(Rp)-2
or
v)
6
(Sp)-2
Scheme 1. Synthesis and optical resolution of (Rp)/(Sp)-1 and
2; Reagent and conditions: i) 6, Cs2CO3, Pd(PPh3)4, 1,4-dioxane,
100 °C, 63%; ii) Optical resolution on chiral stationary phase;
iii) ICl, AgBF4, THF-MeOH (v/v = 1:1), 0 °C, (Rp)-5: 83%,
n
(Sp)-5: 95%; iv) BuLi, THF, ¹78 °C; then H2O, (Rp)-1: 81%,
(Sp)-1: 81%; v) Ph-Bpin, Cs2CO3, Pd(PPh3)4, 1,4-dioxane,
100 °C, (Rp)-2: 73%, (Sp)-2: 69%.
© 2019 The Chemical Society of Japan