Synthesis of cyclic compounds consisting of face-to-face p-oligophenyls

Article abstract of DOI:10.1016/j.tetlet.2014.01.093

Facile synthesis of xanthene-based π-stacked compounds consisting of face-to-face p-oligophenyls has been described. Just by mixing xanthene-4,5-diboronic acid and p-oligophenyls containing a benzenetetraol unit yielded cyclic compounds selectively due to the reversibility of the boronate esterification. In the ground state, weak π-π interactions between two p-oligophenyl moieties were observed, whereas their π-π interactions were clearly shown in the excited state.

Full text of DOI:10.1016/j.tetlet.2014.01.093

Tetrahedron Letters 55 (2014) 1631–1634
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of cyclic compounds consisting of face-to-face
p-oligophenyls
⇑
⇑
Yasuhiro Morisaki , Yuichi Tsuji, Yoshiki Chujo
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Facile synthesis of xanthene-based
has been described. Just by mixing xanthene-4,5-diboronic acid and p-oligophenyls containing a
benzenetetraol unit yielded cyclic compounds selectively due to the reversibility of the boronate esteri-
p-stacked compounds consisting of face-to-face p-oligophenyls
Received 30 November 2013
Revised 14 January 2014
Accepted 22 January 2014
Available online 28 January 2014
fication. In the ground state, weak
whereas their interactions were clearly shown in the excited state.
Ó 2014 Elsevier Ltd. All rights reserved.
p–p interactions between two p-oligophenyl moieties were observed,
p–p
Keywords:
Oligophenyl
p–p Interaction
Boronate esterification
Xanthene
p-Conjugated compounds exhibit unique photophysical and
To the best of our knowledge, this is the first example of cyclic
electronic properties, that are suitable for use in organic optoelec-
tronic materials such as organic light-emitting diodes,1 photovol-
taic cells,2 and organic field-effect transistors.3 Their properties
and device performances depend strongly on the transfer of charges
and excitons along not only the conjugated backbones but also
compounds consisting of face-to-face p-quinquephenyl and
p-septiphenyl.9 The simple mixing of xanthene-4,5-diboronic acid
and p-oligophenyls containing a benzenetetraol unit yielded cyclic
compounds selectively because of the reversibility of the boronate
esterification10 and hairpin shape of the xanthene scaffold. Linear-
type model compounds were also synthesized for comparison,
through the intermolecular face-to-face oriented
planes. In cases of through-bond-type conjugation,
are delocalized within a single molecule, whereas -electrons are
p
-conjugated
p-electrons
and the effect of the
investigated.
p–p stacking of the conjugated units was
p
transferred or migrated among conjugated molecules with
through-space interactions. Therefore, intermolecular orientation
p-Oligophenyl-stacked compounds were synthesized by the
boronate esterification10 as shown in Scheme 1. Xanthene was
used for the scaffold to construct the stacked structure because
of its easy functionalization at 4-,5-, and 9-positions as well as
its appropriate distance between 4- and 5-positions. p-Terphenyl
derivative 1a containing a tetramethoxy-1,4-phenylene unit was
reacted with tribromoborane to generate the corresponding tetraol
in situ, followed by the treatment with xanthene-4,5-diboronic
acid 2 to afford the corresponding cyclic compound 3a selectively
through the boronate esterification. At first, we planned to synthe-
size a p-terphenyl-stacked polymer; however, only the cyclic com-
pound was obtained, as shown in the gel permeation
chromatogram (Fig. 1).11 Due to the reversibility of the boronate
esterification, thermodynamically stable cyclic compounds were
obtained in the present system. A sharp peak was observed after
5 min; any other peaks corresponding to oligomeric and polymeric
compounds did not appear after 30 min. This sole product was iso-
lated by simple silica gel column chromatography to obtain pure
compound. The structure of the isolated compound was confirmed
by 1H, 13C, and 11B NMR spectroscopy, high-resolution mass spec-
trometric (HRMS) analysis, and elemental analysis, suggesting the
of
p-conjugated systems is important for the transfer efficiencies
of electrons and excitons. In this context, extensive studies have
recently been conducted for understanding the relationships
between the orientation of the
transannular interactions. One of the more suitable methods for
construction of the face-to-face structure is to use -stacked mole-
p-conjugated systems and their
p
cules as a scaffold, in which pairs of conjugated units are held in a
face-to-face orientation. Several rigid scaffolds, such as naphtha-
lene,4 anthracene,5 [2.2]paracyclophane,6 and bicyclo[4.4.1]unde-
cane,7 were applied to synthesize the
p-stacked oligomers and
polymers. Among them, we have focused on an easily modifiable
xanthene skeleton, which has been employed as the scaffold to
align
Herein, we report a facile and selective synthesis of xanthene-based
-stacked compounds in which two p-oligophenyls face each other.
p
-conjugated systems in a one-dimensional orientation.8
p
⇑
Corresponding authors. Tel./fax: +81 75 383 2610.
E-mail addresses: ymo@chujo.synchem.kyoto-u.ac.jp (Y. Morisaki), chujo@chujo.
synchem.kyoto-u.ac.jp (Y. Chujo).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.093

1632
Y. Morisaki et al. / Tetrahedron Letters 55 (2014) 1631–1634
HO
HO
OH
MeO
OMe
OMe
BBr3
C6H13
O
C6H13O
π
π
π
π
B
B
CH2Cl2
O
O
O
O
O
O
O
O
OH
MeO
1a–c
π
a:
=
B
B
OC6H13
OC6H13
b:
c:
4a
4b
C5H11
C6H13
O
C12H25 C12H25
B
O
O
O
O
B(OH)2
O
B(OH)2
C5H11
C5H11
O
B
B
B
B
O
O
O
O
O
O
O
O
C12H25 C12H25
π
π
B
2
OC6H13
π
π
CH2Cl2 or toluene
MeOH
O
4c
Figure 2. Structures of model compounds 4a–c.
C12H25 C12H25
3a–c
Scheme 1. Synthesis of cyclic compounds 3a–c.
Figure 1. GPC charts of the synthesis of 3a: 5 min after the reaction of 1a with 2,
30 min after the reaction, and after isolation of the sole product.
Figure 3. 1H NMR spectra of 3a and 4a.
cyclic structure of 3a. The isolated yield of 3a was calculated to be
58%. p-Quinquephenyl- and p-septiphenyl-stacked cyclic com-
pounds 3b and 3c were also prepared by the same procedure in
73% and 43% isolated yields, respectively. Pentyl groups were
substituted at the para positions of the terminal phenyl unit in
3c to provide solubility in common organic solvents. In addition,
model compounds 4a–c were synthesized to compare properties
of 3a–c (Fig. 2).12
phenyl protons of 4a, indicating a ring-current shielding effect
between the face-to-face p-terphenyls in 3a. The NMR data of 3a
indicated that the four phenyl units were equivalent; thus, the xan-
thene-to-boron bonds are rotating at room temperature on the
NMR time scale. Coupling pattern indicated that the four phenyl
rings are also freely rotating.
Optical properties were investigated in detail, and the results
are shown in Figure 4 and Table 1. Figure 4 shows the UV–vis
absorption and photoluminescence (PL) spectra of compounds
3a–c and 4a–c in dilute CHCl3 (1.0 Â 10À5 M) at room temperature.
In the absorption spectra of 3a and 4a (Fig. 4A), the spectrum of 3a
became slightly broad and red-shifted (Table 1) in comparison
with that of 4a; however, differences in their absorbance wave-
The 1H NMR spectra of 3a and model compound 4a are shown
in Figure 3 as representative examples. The phenyl proton signals
of 3a were observed at d 8.15 (doublet), 7.31 (triplet), and 7.24
(triplet) ppm, whereas those of 4a appeared at d 8.31 (doublet),
7.57 (triplet), and 7.49 (triplet) ppm.13 Compound 3a exhibited a
higher magnetic-field shift of phenyl protons compared with the

Y. Morisaki et al. / Tetrahedron Letters 55 (2014) 1631–1634
1633
lengths were small. It is implied that, in the ground state,
p–p
interactions between p-terphenyl units in 3a are weak because of
the relatively long distance (ca. 4.5 Å) between the 4- and 5-posi-
tions of the xanthene skeleton. Similar tendencies were observed
in the cases of 3b and 3c; the spectra of cyclic compounds were
slightly red-shifted (Table 1) in comparison with model com-
pounds 4b and 4c, and the spectra of cyclic and model compounds
were almost identical regardless of the size of their
planes.
p-conjugated
On the other hand, the PL spectra of 3a–c differed from those of
4a–c, as shown in Figure 4. The peaks were clearly red-shifted
(Table 1) and broadened without vibrational structures in compar-
ison with the spectra of model compound spectra. PL decay studies
for 3a–c and 4a–c were carried out; their time-resolved PL spectra
are shown in Figures S22–24 in Supporting information. Long-lived
PL decays were observed in the spectra of 3a–c; for example, PL
lifetimes (s) of 3a and 4a were calculated to be 3.77 and 0.88 ns
(Table 1), respectively. In particular, the time-resolved PL spectra
of 3b exhibited two kinds of decay pathways; thus, the decay curve
of 3b at 388 nm was fitted with the double exponential equation,
and the
s was calculated to be 0.74 ns (43.3%) and 6.75 ns
(56.7%) (Table 1), which included the long-lived component. In
addition, the PL at 432 nm was found to be the single exponential
decay, and the
s was calculated to be 6.94 ns (Table 1). These
results suggest the existence of two radiative decay processes.
The shapes of the time-resolved PL spectra of 3a–c were not iden-
tical due to the different
depending on the -conjugated plane. The PL quantum efficiencies
UPL) of cyclic compounds 3a–c (UPL = 0.71–0.78) were larger than
p–p stacking modes of each compound,
p
(
those of model compounds 4a–c (UPL = 0.32–0.35), as shown in
Table 1. The radiative rate constants (kr) of 4a–c were smaller than
the non-radiative rate constants (knr), whereas the kr of 3a–c were
larger than the knr. It is implied that formation of the stacked struc-
ture in 3a–c is relatively faster than the non-radiative decay by
photo-excitation, because of the face-to-face structure of p-olig-
ophenyls, and they emit from the stacked structure like excimer
emission. Excimer emission depends on the
p-conjugation length
of a parent chromophore and the corresponding dimeric orienta-
tion in the cyclic structure; namely, through-bond (TB) conjugation
of the chromophore and through-space (TS) conjugation between
chromophores.14 When energy band gap of TB is larger than that
of TS, emission takes place from TS via the internal conversion from
TB to TS. On the other hand, in the case of the
p-conjugation-
extended chromophore, energy band gap of TB is narrow and those
of TB and TS are close, resulting in an extensive delocalization
between the corresponding chromophores because of the
Figure 4. UV–vis absorption and PL spectra of (A) 3a and 4a, (B) 3b and 4b, and (C)
3c and 4c in CHCl3 (1.0 Â 10À5 M) at room temperature.
Table 1
Results of optical measurements
sd (ns)
v2
kr  108
knr  108
kabs (nm)
a
b
c
e
f
a
Compound
kabs (nm)
kPL (nm)
UPL
3a
4a
294
292
386
370
0.78
0.32
3.77
0.88
1.06
1.19
2.07
3.64
0.58
7.73
294
292
3b
329
444
0.67
0.74 (43.3%)
6.75 (56.7%)
at 388 nm
6.94
at 432 nm
1.42
1.16
0.97
0.48
329
1.11
1.13
4b
322
382
0.35
2.46
4.58
322
3c
4c
339
335
423
390
0.71
0.33
3.72
0.81
1.02
1.02
1.91
4.07
0.78
8.27
339
335
a
b
c
d
e
f
Absorption maximum in CHCl3 (1.0 Â 105 M).
PL peak top in CHCl3 (1.0 Â 105 M).
Absolute PL quantum efficiency.
PL life time at PL peak top, unless otherwise noted.
Radiative rate constant.
Non-radiative rate constant.

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Y. Morisaki et al. / Tetrahedron Letters 55 (2014) 1631–1634
Effect Transistors; Groza, J. R., Locklin, J. J., Eds.; CRC Press Taylor & Francis
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compound 3b (p-quinquephenyl dimer) both emissions were
4. (a) Arnold, R.; Matchett, S. A.; Rosenblum, M. Organometallics 1988, 7, 2261; (b)
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observed. We confirmed that intermolecular
p–p interactions
could be ignored under this concentration. Hence, in the excited
state, the p-oligophenyls in the cyclic structure were stacked on
account of the xanthene unit and interacted intramolecularly.
In conclusion, we synthesized new cyclic compounds based on
xanthene-based compounds as a scaffold, in which p-oligophenyls
faced each other. The reaction of xanthene diboronic acid with
tetraol-containing p-oligophenyl afforded the cyclic compound
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selectively. In the ground state, weak
two p-oligophenyl moieties were observed, while clear intramolec-
ular interactions were observed in the excited state. The
synthetic procedure is simple, and the face-to-face structure of
-electron systems can be readily constructed. This method will
be applied for the construction of face-to-face structures not only
of -conjugated oligomers but also of -conjugated polymers,
leading to an improved understanding of the orientations
of -conjugated systems and their interactions.
p–p interactions between
p–p
6. For pseudo-para-linked [2.2]paracyclophane: (a) Guyard, L.; Audebert, P.
Electrochem. Commun. 2001, 3, 164–167; (b) Morisaki, Y.; Chujo, Y.
Macromolecules 2002, 35, 587; (c) Guyard, L.; Audebert, P.; Dolbier, W. R., Jr.;
Duan, J.-X. J. Electroanal. Chem. 2002, 537, 189; (d) Salhi, F.; Lee, B.; Metz, C.;
Bottomley, L. A.; Collard, D. M. Org. Lett. 2002, 4, 3195; (e) Salhi, F.; Collard, D.
M. Adv. Mater. 2003, 15, 81; (f) Morisaki, Y.; Chujo, Y. Angew. Chem., Int. Ed.
2006, 45, 6430; (g) Morisaki, Y.; Chujo, Y. Polym. Chem. 2011, 2, 1249; (h)
Morisaki, Y.; Ueno, S.; Saeki, A.; Asano, A.; Seki, S.; Chujo, Y. Chem. Eur. J. 2012,
18, 4216; (i) Morisaki, Y.; Chujo, Y. Chem. Lett. 2012, 41, 840; For pseudo-ortho-
linked [2.2]paracyclophane: (j) Morisaki, Y.; Hifumi, R.; Lin, L.; Inoshita, K.;
Chujo, Y. Polym. Chem. 2012, 3, 2727; For pseudo-geminal-linked
[2.2]paracyclophane: (k) Jagtap, S. P.; Collard, D. M. J. Am. Chem. Soc. 2010,
132, 12208; (l) Jagtap, S. P.; Collard, D. M. Polym. Chem. 2012, 3, 463.
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13680; (b) Jagtap, S. P.; Mukhopadhyay, S.; Coropceanu, V.; Brizius, G. L.;
Brédas, J.; Collard, D. M. J. Am. Chem. Soc. 2012, 134, 7176.
p
p
p
p
p–p
Acknowledgments
Financial support from The Asahi Glass Foundation is gratefully
acknowledged. Y.T. appreciates Research Fellowships from the
Japan Society for the Promotion of Science for Young Scientists.
8. (a) Morisaki, Y.; Chujo, Y. Tetrahedron Lett. 2005, 46, 2533; (b) Giaimo, J. M.;
Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.; Wasielewski, M. R. J. Phys.
Supplementary data
Chem.
A 2008, 112, 2322; (c) Morisaki, Y.; Murakami, T.; Chujo, Y.
Macromolecules 2008, 41, 5960; (d) Morisaki, Y.; Murakami, T.; Sawamura, T.;
Chujo, Y. Macromolecules 2009, 42, 3656; (e) Yoo, H.; Yang, J.; Yousef, A.;
Wasielewski, M. R.; Kim, D. J. Am. Chem. Soc. 2010, 132, 3939; (f) Morisaki, Y.;
Sawamura, S.; Murakami, T.; Chujo, Y. Org. Lett. 2010, 12, 3188.
9. Cyclic compound comprising face-to-face biphenyls based on 1,8-linked
naphthalene was reported: (a) Iyoda, M.; Kondo, T.; Nakao, K.; Hara, K.;
Kuwatani, Y.; Yoshida, M.; Matsuyama, H. Org. Lett. 2000, 2, 2081; Partly p-
terphenyl-stacked structure based on [2.2]paracyclophane was constructed:
(b) König, B.; Knieriem, B.; de Meijere, A. Chem. Ber. 1993, 126, 1643.
10. Boronic acids are useful building blocks to construct a variety of self-assembled
systems. For example, see: (a) Fujita, N.; Shinkai, S.; James, T. D. Chem. Asian J.
2008, 3, 1076; (b) Nishiyabu, R.; Kubo, Y.; James, T. D.; Fossey, J. S. Chem.
Commun. 2011, 1124.
Supplementary data (synthetic details, NMR spectra, and the
data of PL decay analysis) associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
093.
References and notes
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