Synthesis of Optically Active, X-Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence

Article abstract of DOI:10.1002/chem.201504270

Optically active, Fréchet-type dendrimers containing an emissive X-shaped π-electron system as the core unit were synthesized. Gram-scale optical resolution and transformations of 4,7,12,15-tetrasubstituted [2.2]paracyclophanes were also carried out. The high-generation dendrons effectively absorbed UV light and transferred energy to the core, resulting in high photoluminescence (PL) from the core. In addition, the dendrons sufficiently isolated the emissive X-shaped conjugated core and bright emission was observed from both thin films and solutions. Intense circularly polarized luminescence (CPL) was observed from the thin film. The dendrimer films exhibited excellent optical properties, such as large molar extinction coefficients, high fluorescence quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors.

Full text of DOI:10.1002/chem.201504270

DOI: 10.1002/chem.201504270
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Structure–Activity Relationships |Hot Paper|
Synthesis of Optically Active, X-Shaped, Conjugated Compounds
and Dendrimers Based on Planar Chiral [2.2]Paracyclophane,
Leading to Highly Emissive Circularly Polarized Luminescence
Masayuki Gon,[a] Yasuhiro Morisaki,*[b] Risa Sawada,[a] and Yoshiki Chujo*[a]
Abstract: Optically active, FrØchet-type dendrimers contain-
ing an emissive X-shaped p-electron system as the core unit
were synthesized. Gram-scale optical resolution and transfor-
mations of 4,7,12,15-tetrasubstituted [2.2]paracyclophanes
were also carried out. The high-generation dendrons effec-
tively absorbed UV light and transferred energy to the core,
resulting in high photoluminescence (PL) from the core. In
addition, the dendrons sufficiently isolated the emissive X-
shaped conjugated core and bright emission was observed
from both thin films and solutions. Intense circularly polar-
ized luminescence (CPL) was observed from the thin film.
The dendrimer films exhibited excellent optical properties,
such as large molar extinction coefficients, high fluorescence
quantum efficiencies, intense PL and CPL, and large CPL dis-
symmetry factors.
Introduction
second-ordered structures obtained from planar, chiral [2.2]par-
acyclophane, including V-shaped,[13b] helical,[13b] triangular,[9a,13b]
and propeller-shaped structures[13a,c] in the excited state pro-
vided large glum values in the order of 10À2 to 10À3. The CPL
profiles of the optically active [2.2]paracyclophane compounds
have been observed in dilute solutions, whereas the PL per-
formance in the solid state, in particular, the PL intensity and
Flum, decreased dramatically due to typical aggregation-caused
quenching.
Optically active, emissive, conjugated compounds exhibit a po-
tential difference between their left- and right-handed circular-
ly polarized luminescence (CPL[1]) intensities. Recently, the de-
velopment of optically active CPL-emitting organic com-
pounds[2–11] for applications in 3D organic light-emitting devi-
ces has received much attention. CPL characteristics are evalu-
ated from the photoluminescence (PL) intensity and the
luminescence dissymmetry factor (glum), defined as 2(IleftÀIright)/
(Ileft +Iright), in which Ileft and Iright represent the left- and right-
handed CPL intensities, respectively. The molar absorption co-
efficient (e), PL quantum efficiency (Flum), and large glum value
are therefore important factors in CPL materials. However, it is
generally difficult to achieve large PL intensities in combination
with large glum values due to the existing challenges in obtain-
ing large glum values in organic compounds.
We report herein the synthesis of a simple X-shaped com-
pound that contains two stacked p-electron systems. We fo-
cused on the dendritic structure[14] to investigate the optical
and chiroptical properties of its thin film. In addition to the
film-forming ability, the FrØchet-type dendrimer was selected
to overcome aggregation-caused quenching, and to take ad-
vantage of its light-harvesting effect; dendrons protect the
emissive core unit from aggregation and the benzene rings in
the dendrons effectively absorb UV light. The dendrimers ex-
hibited excellent CPL properties both in dilute solution and in
the film state, which led to highly emissive CPL materials with
large glum values. Gram-scale syntheses of enantiopure
4,7,12,15-tetrasubstituted [2.2]paracyclophanes are also dis-
closed.
We recently suggested that a second-ordered structure cre-
ated from the rigid and conformationally stable [2.2]paracyclo-
phane[12] structure was effective for achieving CPL.[9a,13] Despite
the stacked structure of the p-electron systems, [2.2]paracyclo-
phane-based conjugated compounds exhibit good Flum values
in addition to a large e. Furthermore, the optically active,
[a] M. Gon, R. Sawada, Prof. Dr. Y. Chujo
Department of Polymer Chemistry, Graduate School of Engineering
Kyoto University, Katsura, Nishikyo-ku
Results and Discussion
The reported synthetic procedure to obtain rac-4,7,12,15-tetra-
bromo[2.2]paracyclophane was modified as outlined in the
Supporting Information.[15] The method reported by Chow
et al. selectively produces 4,7,12,15-tetrasubstituted [2.2]para-
cyclophane compounds,[15] as shown in Scheme 1; however,
polymeric compounds are mainly obtained as byproducts. We
successfully prepared rac-4,7,12,15-tetrabromo[2.2]paracyclo-
phane in 68% yield under more dilute reaction conditions. The
Kyoto 615-8510 (Japan)
E-mail: chujo@chujo.synchem.kyoto-u.ac.jp
[b] Prof. Dr. Y. Morisaki
Department of Applied Chemistry for Environment
School of Science and Technology, Kwansei Gakuin University
2-1 Gakuen, Sanda, Hyogo 669-1337 (Japan)
E-mail: ymo@kwansei.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504270.
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Figure 1. UV/Vis absorption and PL spectra of (Sp)-3-G0 in CHCl3 (1.0”10À5
for UV and 1.0”10À7 m for PL). Excitation wavelength: 365 nm.
m
Scheme 1. Synthetic route to rac-4,7,12,15-tetrabromo[2.2]paracyclo-
phane,[15] and structures of planar chiral 4,7,12,15-tetraethynyl[2.2]paracyclo-
phanes (Sp)- and (Rp)-1.[13a]
gram-scale optical resolution of planar chiral 4,7,12,15-tetra-
substituted [2.2]paracyclophane,[16] and the successive transfor-
mations to enantiopure (Sp)- and (Rp)-1 (see Scheme 1) are also
described in the Supporting Information.
The X-shaped conjugated compound (Sp)-3-G0, in which two
p-phenylene–ethynylenes were stacked at the central phenyl-
ene units,[17,18] was prepared from the reaction of (Sp)-1 with
commercially available p-iodoanisole (2-G0; Scheme 2). In addi-
tion, the enantiomer (Rp)-3-G0 was prepared from the corre-
sponding (Rp) isomer.
The UV/Vis absorption and PL spectra of dilute solutions of
(Sp)- and (Rp)-3-G0 in CHCl3 (1.0”10À5 m) were recorded.[18]
Figure 1 shows the spectra for the (Sp) isomer. The absorption
bands derived from the p–p* transition of p-phenylene–ethy-
nylene were observed with band maxima at l=361 and
around 315 nm. Photoexcitation of (Sp)-3-G0 at l=365 nm
gave an emission band at l=416 nm with an absolute PL effi-
ciency (Flum) of 0.66. Figure 2 shows the circular dichroism
Figure 2. CD spectra of (Sp)- and (Rp)-3-G0 in CHCl3 (1.0”10À5 m) and UV/Vis
absorption spectrum of (Sp)-3-G0 in CHCl3 (1.0”10À5 m).
(CD) spectra of dilute solutions of (Sp)- and (Rp)-3-G0 in CHCl3
(1.0”10À5 m). A clearly bisignate mirror-image Cotton effects
was observed along with a relatively high [q] of 6.7”
105 degcm2 dmolÀ1, which suggested that the light-absorbing
units involved were chirally perturbed by the planar chiral
[2.2]paracyclophane. As expected, CPL was also observed. The
CPL and glum spectra of a dilute solution of (Sp)-3-G0 in CHCl3
(1.0”10À5 m) are shown in Figure 3. Mirror-image CPL signals
were observed in the PL region, and the absolute glum value
was estimated to be 1.4”10À3. The X-shaped compound of
planar, chiral [2.2]paracyclophane is an excellent CPL emitter,
that is, it has good Flum, intense CPL, and large glum properties.
However, due to crystallization, thin films of (Sp)- and (Rp)-3-G0
were not obtained, and the PL intensity and Flum observed in
the solid state decreased due to typical aggregation-caused
quenching.
Scheme 2. Synthesis of (Sp)-3-G0; dba=dibenzylideneacetone, dppf=1,1’-
bis(diphenylphosphino)ferrocene.
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the benzene rings on the dendrons to enhance the PL per-
formance of the system. FrØchet-type bromo-substituted den-
drons 4-Gn (n=1–4) were prepared and treated with p-iodo-
phenol (5) to obtain dendrons 2-Gn (Scheme 3). Sonogashira–
Hagihara coupling[23] of (Sp)-1 with 2-Gn afforded the target
dendrimers (Sp)-3-Gn (n=1–4), which contained the X-shaped
conjugated moiety as the core unit (Scheme 3). The yields of
different enantiomers varied according to experimental error
because of difficulties in handling as a result of increased hy-
groscopicity with increasing generations. The dendrimers were
highly soluble in common organic solvents, including THF, tol-
uene, CH2Cl2, and CHCl3. In addition, they possessed film-form-
ing abilities through spin coating of solutions in CHCl3 (2.0”
10À3 m).
The optical and chiroptical properties of (Sp)- and (Rp)-3-Gn
(n=1–4) were evaluated by means of UV/Vis absorption, PL,
CD, and CPL spectroscopy. The optical and chiroptical data are
summarized in Tables 1 and 2, respectively. Figures 4A and 4B
show the UV/Vis absorption spectra of dilute solutions of all
compounds in CHCl3 (1.0”10À5 m) and spin-coated films, re-
spectively. Absorption bands at lꢀ275 and 300–400 nm were
assigned to the p–p* bands of the benzene rings in the den-
drons and p-phenylene–ethynylene moieties of the core unit,
respectively. The absorption bands at lꢀ275 nm increased
with increasing dendrimer generation (i.e., the number of ben-
zene rings). As shown in Figure 4B, the absorption edges of
the (Sp)-3-G1 and G2 films were redshifted relative to those of
the G3 and G4 dendrimers because of core unit aggregation in
the solid state.
Figure 3. CPL and glum spectra of (Sp)- and (Rp)-3-G0 in CHCl3 (1.0”10À5 m)
and the PL spectrum of (Sp)-3-G0 in CHCl3 (1.0”10À5 m). Excitation wave-
length: 279 nm.
From the viewpoint of film formation for the X-shaped com-
pound, we focused on the dendritic structure.[14,19,20] FrØchet-
type[20] dendrons were introduced into the X-shaped com-
pound to take advantage of the light-harvesting effect[21,22] of
Scheme 3. Synthesis of dendrimers (Sp)-3-Gn (n=1–4).
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Table 1. Spectroscopic data for the optical properties of the (Sp) isomers.
[a]
[b]
[c]
[d]
[e]
[f]
labs [nm] (e”10À5 [mÀ1 cmÀ1])
labs [nm]
llum [nm]
llum [nm]
Flum
Flum
(Sp)-G0
(Sp)-G1
(Sp)-G2
(Sp)-G3
(Sp)-G4
361 (0.68)
–
368
278, 367
279, 369
283, 368
416
415
416
417
416
–
0.66
0.63
0.66
0.66
0.67
–
279 (0.40), 363 (0.67)
279 (0.61), 364 (0.69)
279 (1.00), 365 (0.68)
279 (1.76), 363 (0.68)
425, 445
423, 435
421, 438
420, 435
0.20
0.54
0.65
0.58
[a] In CHCl3 (1.0”10À5 m). [b] Thin film prepared by the spin-coating method from solutions in CHCl3 (1.0”10À3 m). [c] In CHCl3 (1.0”10À7 m), excited at l=
279 nm. [d] Thin film, excited at l=279 nm. [e] Absolute PL quantum efficiency in CHCl3 (1.0”10À7 m) excited at the band maximum of the core unit (l=
365 nm). [f] Absolute PL quantum efficiency of the film prepared by the spin-coating method from solutions in CHCl3 (1.0”10À3 m), excited at the band
maximum of the core unit (l=370 nm).
Table 2. Spectroscopic data for the chiroptical properties of the (Sp) iso-
mers.
[a]
[d]
gabs ”103
glum ”103
Solution[b]
Film[c]
Solution[b]
Film[c]
(Sp)-G0
(Sp)-G1
(Sp)-G2
(Sp)-G3
(Sp)-G4
+1.3
+1.3
+1.3
+1.6
+1.3
–
+1.4
+1.4
+1.4
+1.4
+1.4
–
+1.6
+1.6
+1.6
+1.5
+2.1
+2.0
+1.8
+2.0
[a] gabs =2De/e, in which De indicates differences in absorbance between
left- and right-handed circularly polarized light, respectively. The gabs
value of the band maximum at lꢀ360 nm was estimated. [b] In CHCl3
(1.0”10À5 m). [c] Film prepared by the spin-coating method from solu-
tions in CHCl3 (5.0”10À3 m). [d] glum =2(IleftÀIright)/(Ileft +Iright), in which Ileft
and Iright indicate the luminescence intensities of left- and right-handed
CPL, respectively. The excitation wavelength was 279 nm in the CPL spec-
tra. The glum value of the PL band maximum at lꢀ415 nm was estimat-
ed.
The PL spectra of dilute solutions of (Sp)-3-Gn in CHCl3 (1.0”
10À7 m) and the spin-coated films are shown in Figure 5A and
B, respectively. Excitation was carried out at l=279 nm to
ensure excitation of the benzene rings in the dendrons. In Fig-
ure 5A, emission bands of the dendrimers (Sp)-3-Gn appeared
at lꢀ415 nm, which was comparable to the spectrum of the
dendron-free (Sp)-3-G0; thus indicating that emission occurred
from the p-phenylene–ethynylene moieties in the core unit.
Because the emitting species of (Sp)-3-Gn (n=0–4) were identi-
cal (i.e., the same core unit), their Flum values were also com-
parable and calculated to be approximately 0.65 (Table 1). The
band intensity of the dendrimers increased with an increasing
number of benzene rings in the dendrons because of the
light-harvesting effect and energy transfer from the dendrimer
surface to the core. The PL intensity of the films also increased
with increasing dendrimer generation (Figure 5B). In contrast
with the observations in solution, the Flum of the dendrimer
film increased with increasing dendrimer generation. The Flum
values of 3-G3 and -G4 were different, despite both cores
being well protected by dendrons (see Figure 8, below). It is
considered that, in the films, the conformations of the core
units are slightly different in the excited states due to the re-
stricted mobility of the cores by the congested dendrons. Ag-
gregation of the core phenylene–ethynylene moieties is inhib-
ited by the dendrons, and emission occurs from the isolated
Figure 4. A) UV/Vis absorption spectra of solutions (1.0”10À5 m) of (Sp)-den-
drimers in CHCl3. B) UV/Vis absorption spectra of (Sp)-dendrimer thin films.
The spectra were normalized at lꢀ365 nm.
phenylene–ethynylene moieties, despite their film state. As the
result, the intense blue emission derived from large e and
good Flum values was obtained from the higher generation
dendrimers.
The CD spectra of dendrimers (Sp)- and (Rp)-3-Gn in CHCl3
(1.0”10À5 m) and as films are shown in Figure 6A and B, re-
spectively. In all cases, mirror-image Cotton effects with large
De values were observed in the CD spectra. The dissymmetry
factors of absorbance (gabs =De/e) are also plotted in Figure 6.
The gabs values of the p–p* transition bands of the benzene
rings at l=250–300 nm decreased with increasing dendrimer
generation (insets in Figure 6A and B). Because the Cotton
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the gabs values in the p–p* transition bands of the phenylene–
ethynylene moieties were identical.
The X-shaped, second-order structure of the p-electron sys-
tems of planar chiral [2.2]paracyclophane was CPL active, as
previously shown in Figure 3. Intense CPL signals were there-
fore observed for the dendrimers in CHCl3 (1.0”10À5 m) and
the spin-coated films (Figure 7A and B, respectively). All den-
drimers exhibited high glum values in the order of 10À3. Thus,
the FrØchet-type dendron provided planar, chiral, [2.2]paracy-
clophane-based p-electron systems with unprecedented PL
and CPL properties such as large e, good Flum, and excellent
glum values both in solution and as films. Figure 8 shows the si-
mulated structures of the dendrimers (Sp)-3-Gn (n=1–4) ob-
tained by using PM3[24] with MOPAC2012.[25] The dendrons of
(Sp)-3-G1 and -G2 do not sufficiently protect the core unit,
whereas those of (Sp)-3-G3 and -G4 isolate the core unit; thus
inhibiting aggregation-caused quenching.
To investigate the dendritic effect on PL, X-shaped (Sp)-3-G0
was dispersed in a polystyrene (PS; Mn =8.1”104, Mw =2.1”
105) matrix. The PL spectra of 5 and 10 wt% (Sp)-3-G0 in PS
were compared with those of (Sp)-3-G3 and (Sp)-3-G4 spin-
coated films (Figure 9A). The PL intensities of 5 and 10 wt%
(Sp)-3-G0 in PS were clearly lower than those of the dendrimer
films, despite the benzene-rich environment. As shown in the
excitation spectra (Figure 9B), only weak bands derived from
the PS phenylene moieties were observed at lꢀ270 nm,
which implied that the benzene rings of PS did not participate
significantly in emission from the core unit. On the other hand,
as shown in Figure 9B, bands appeared at lꢀ275 nm in the
excitation spectra of the (Sp)-3-G3 and (Sp)-3-G4 dendrimer
films. Figures S13A and S13B in the Supporting Information
show the excitation spectra of (Sp)-3-Gn (n=1–4) in CHCl3 and
as spin-coated films, respectively. The band intensity derived
Figure 5. A) PL spectra of solutions (1.0”10À7 m) of (Sp)-dendrimers in CHCl3.
B) PL spectra of (Sp)-dendrimer thin films. Excitation wavelength: 279 nm.
effect of the benzene rings is induced by the optically active
[2.2]paracyclophane core, the planar chirality of the core is not
effective for benzene rings on the dendron surface. In contrast,
Figure 6. A) CD, gabs, and UV/Vis absorption spectra of solutions (1.0”10À5 m) of dendrimers in CHCl3. B) CD, gabs, and UV/Vis absorption spectra of the thin
films. Expanded gabs spectra (from l=250 to 300 nm) are shown in the insets.
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Figure 7. A) CPL, glum, and PL spectra of solutions (1.0”10À5 m) of dendrimers in CHCl3. B) CPL, glum, and PL spectra of dendrimer thin films. Excitation wave-
length: 279 nm.
Figure 8. Space-filling model of the dendrimers simulated by means of
PM3.[24] The conjugated core units are shown in dark gray.
from the benzene units in the dendrons increased as the den-
drimer generation increased. These results clearly indicate the
contribution of the benzene rings to emission from the core
unit in the dendrimers through the light-harvesting effect.
Conclusion
We achieved the gram-scale optical resolution and transforma-
tions of planar, chiral, 4,7,12,15-tetrasubstituted [2.2]paracyclo-
phanes. Optically active FrØchet-type dendrimers containing an
X-shaped conjugated core with the planar chiral [2.2]paracyclo-
Figure 9. A) PL spectra of (Sp)-3-G3 and -G4 films. PL spectra of PS films con-
phane moiety were prepared. The dendrons effectively ab-
taining 5 and 10 wt% (Sp)-3-G0. B) Excitation spectra of (Sp)-3-G3 and -G4
films. Excitation spectra of PS films containing 5 and 10 wt% (Sp)-3-G0. Exci-
tation spectra were obtained at the PL band maximum wavelength.
sorbed UV light and transferred energy to the core. In addition,
the dendrons sufficiently isolated the emissive X-shaped conju-
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gated core, which resulted in high emission from the thin
films. Intense CPL by the planar chiral [2.2]paracyclophane was
observed with a high dissymmetry factor in the order of 10À3
.
The combination of FrØchet-type dendrons with planar chiral
[2.2]paracyclophane provided emissive conjugated compounds
with large e and De values, good Flum values, intense PL and
CPL, and large glum factors. This strategy is promising for the
design of organic CPL materials.
[6] For reports on induced CPL from inherently achiral compounds by an
axially chiral binaphthyl moiety, see: a) E. M. Sµnchez-Carnerero, F.
Moreno, B. L. Maroto, A. R. Agarrabeitia, M. J. Ortiz, B. G. Vo, G. Muller, S.
de La Moya, J. Am. Chem. Soc. 2014, 136, 3346–3349; b) By cyclodex-
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M. Iwamura, K. Nozaki, Angew. Chem. Int. Ed. 2014, 53, 14392–14396;
Angew. Chem. 2014, 126, 14620–14624; c) for CPL from supramolecular
gels of an achiral gelator, see: Z. C. Shen, T. Y. Wang, L. Shi, Z. Y. Tang,
M. H. Liu, Chem. Sci. 2015, 6, 4267–4272.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas (no. 15H00992) “p-Figuration” from
the Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. M.G. acknowledges Research Fellowships (no.
14J02514) from the Japan Society for the Promotion of Science
for Young Scientists. We are grateful to Professor Kazuo Akagi
and Dr. Kazuyoshi Watanabe (Department of Polymer Chemis-
try, Graduate School of Engineering, Kyoto University) for CPL
data analysis.
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Keywords: chirality
·
conjugation
·
dendrimers
·
luminescence · paracyclophanes
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Received: October 23, 2015
Published online on January 11, 2016
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