Protonation-induced cyclization of 1,8-bis(arylethynyl)anthraquinones: Monopyrylium salt formation and intensification of donor-acceptor interaction

Article abstract of DOI:10.1246/cl.2011.1456

We previously reported protonation-induced double cyclization reaction of 1,4-Ar₂Aq and 1,5-Ar₂Aq (Ar₂Aq: bis(arylethynyl) anthraquinone) with strong acid HX that generated the corresponding dipyrylium salts [1,4-Ar_2<

Full text of DOI:10.1246/cl.2011.1456

doi:10.1246/cl.2011.1456
1456
Published on the web December 5, 2011
Protonation-induced Cyclization of 1,8-Bis(arylethynyl)anthraquinones:
Monopyrylium Salt Formation and Intensification of Donor-Acceptor Interaction
Koya Prabhakara Rao, Mio Kondo, Ryota Sakamoto, Tetsuro Kusamoto, Shoko Kume, and Hiroshi Nishihara*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received October 27, 2011; CL-111052; E-mail: nisihara@chem.s.u-tokyo.ac.jp)
We previously reported protonation-induced double cycliza-
tion reaction of 1,4-Ar2Aq and 1,5-Ar2Aq (Ar2Aq: bis(aryl-
ethynyl)anthraquinone) with strong acid HX that generated the
corresponding dipyrylium salts [1,4-Ar2Pyl2]X2 and [1,5-
Ar2Pyl2]X2. In this communication we disclose the protonation
reactions of 1,8-Fc2Aq (Fc: ferrocenyl), 1,8-Am2Aq (Am:
4-N,N-bis(4-methoxyphenyl)aminophenyl), and 1,8-AmFcAq,
which is the first example of heterodonor molecules in
the Ar2Aq series, and synthesized by means of stepwise
Sonogashira-Hagihara cross-coupling reactions. In contrast to
the 1,4-Ar2Aq and 1,5-Ar2Aq series, 1,8-Ar2Aq undergoes
protonation-induced single cyclization, so that it is converted
into the corresponding monopyrylium salt [1,8-Ar2Pyl]X. [1,8-
Ar2Pyl]X features an extremely small HOMO-LUMO gap
(0.50-0.73 V), ascribable to the significant lowering of the
LUMO level upon the pyrylium formation.
a)
b)
Ar
Ar
Ar
H+
H+
Ar
Ar
Ar
O
O
O
O
O
O
O
O
O
1-ArAq
[1-ArPyl]+
1,4-Ar2Aq
[1,4-Ar2Pyl2]2+
c)
Ar
d)
Ar
O
H+
H+
Ar
O
O
O
O
?
Ar
[1,5-Ar2Pyl2]2+
Ar
Ar
1,5-Ar2Aq
1,8-Ar2Aq
Scheme 1. (a)-(c) Proton-induced single and double cyclization
in 1-ArAq, 1,4-Ar2Aq, and 1,5-Ar2Aq that afford corresponding
pyrylium and dipyrylium cations [1-ArPyl]+, [1,4-Ar2Pyl2]2+, and
[1,5-Ar2Pyl2]2+, respectively. d) The subject of the present work, the
proton response of 1,8-Ar2Aq (Ar = Fc or Am).
disclosed the protonation behavior of 1,8-Fc2Aq,7b but we could
not identify its chemical structure at that stage.
D-A, D-A-D, and A-D-A (D: donor; A: acceptor) types of
compounds are exciting materials,1 and have been intensively
studied in terms of fundamental research as well as potential
applications, such as nonlinear optics (NLOs) and organic
electronics. The electronic structure of D-A, D-A-D, and A-D-
A molecules can be systematically tuned by external stimuli,2-4
such as protons, photons, electrons, and magnetic fields. Recently,
these molecules have received unprecedented attention because of
their peculiar physical and electronic properties, which can be
employed to achieve switching behavior in molecular devices.5,6
Our recent studies have developed a new class of D-A and
D-A-D molecules, ArAq and Ar2Aq, where Ar represents an
aryl group, such as ferrocenyl (Fc), 4-N,N-bis(4-methoxyphen-
yl)aminophenyl (Am), platinadithiolene, phenyl, m-tolyl, and p-
tolyl, and Aq indicates ethynylanthraquinone or diethynyl-
anthraquinone (Schemes 1a-1c).7-12 This series of molecules
underwent single and double intramolecular cyclization reac-
tions to afford pyrylium and dipyrylium cations, respectively
(Schemes 1a-1c). For example, 1-ArAq gave rise to the
pyrylium salt [1-ArPyl]X (X: counter anion) with a quadruply
fused-ring structure, in which a cationic oxygen participated in
the aromatic six-membered ring (Scheme 1a).8 We have also
described novel D-A-D molecules, 1,4-Ar2Aq and 1,5-Ar2Aq,
which experienced unprecedented protonation-induced double
cyclization reactions to yield the dipyrylium salts, [1,4-Ar2-
Pyl2]X2 and [1,5-Ar2Pyl2]X2 (Schemes 1b and 1c).9,10 These
compounds possessed new types of quintuply fused-ring
structures, which were isoelectronic with benzo[e]pyrene and
perylene-type skeletons with 20³ electrons.10 These doubly-
condensed phases showed unique physical, chemical, and
magnetic properties. Our successful studies on D-A-D 1,4-
Ar2Aq and 1,5-Ar2Aq systems have prompted us to examine the
1,8-Ar2Aq system (Scheme 1d). We note that we previously
In the present study, we report on the new 1,8-Ar2Aq type
of molecules: a homodonor compound, 1,8-Am2Aq, and the first
heterodonor one in the Ar2Aq series, 1,8-AmFcAq. We also
address their protonation reactions, together with the exact
structure of the protonated product of 1,8-Fc2Aq. All these D-
A-D molecules undergo a protonation-induced single cycliza-
tion reaction with bis(trifluoromethanesulfon)imide (TFSIH) as
a strong acid, so as to yield the corresponding pyrylium salts,
[1,8-Ar2Pyl](TFSI). The synthesis, characterization, single-
crystal X-ray diffraction (XRD) analysis, and physical properties
of 1,8-Ar2Aq and [1,8-Ar2Pyl]X (Ar = Am and Fc, X = TFSI
and BF4) are described in this paper.
1,8-Am2Aq was synthesized by the Sonogashira-Hagihara
cross-coupling of 4-ethynyl-N,N-bis(4-methoxyphenyl)aniline
with 1,8-dibromoanthraquinone. 1,8-AmFcAq was synthesized
in two steps in order to introduce two different types of donor
moieties. In the first step, intermediate 1,8-AmBrAq was
synthesized by the Sonogashira cross-coupling of 4-ethynyl-
N,N-bis(4-methoxyphenyl)aniline with 1 equiv of 1,8-dibro-
moanthraquinone. In the second step, 1,8-AmFcAq was ob-
tained by a coupling of ethynylferrocene to the intermediate
using a similar reaction condition. We note that 1,8-AmFcAq
could not be synthesized when the order of the reactions with the
ethynyl derivatives of amine and ferrocene was reversed: In this
condition, 1,8-Fc2Aq was preferentially obtained in the first
step. This presumably stemmed from the bulkiness of Am,
which decelerate the second Sonogashira-Hagihara reaction.
1
1,8-Am2Aq and 1,8-AmFcAq were characterized by H NMR
and ESI-TOF-MS, as well as elemental analysis. Single crystals
of 1,8-AmFcAq was obtained by recrystallization from di-
chloromethane/hexane at 293 K, and its molecular structure
were determined by single-crystal XRD analysis (Figure 113 and
Table S114).
Chem. Lett. 2011, 40, 1456-1458
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1457
Figure 2. ORTEP drawing of [1,8-Fc2Pyl](BF4) with the thermal
ellipsoids set at 50% probability. Hydrogen atoms are omitted.
Figure 1. ORTEP drawing of 1,8-AmFcAq with the thermal
ellipsoids set at 50% probability. Hydrogen atoms are omitted.
(a)
H
H
Ar
Ar
H+
Ar
C C
Ar
C C
O
O
O
O
O
O
CH2Cl2
Ar
Ar
1,8-Ar2Aq
H+
CH2Cl2
Ar
second cyclization
O
O
Ar
[1,8-Ar2Pyl]+
(b)
Scheme 2. Prausible reaction mechanism of proton-induced single
cyclization in 1,8-Ar2Aq.
The protonation reaction of 1,8-Fc2Aq had been described
in the previous paper, but the chemical structure had not been
identified.7b On the other hand, in this study we succeeded in
determining the crystal structure of the protonated product: A
treatment of 1,8-Ar2Aq with TFSIH in dichloromethane
afforded the singly-cyclized form [1,8-Ar2Pyl]+ (Scheme 2).
Figures S114 and 2 show the crystal structures of [1,8-Fc2Pyl]-
(TFSI) and its anion exchange product [1,8-Fc2Pyl](BF4),
respectively:13 Their crystal data are collected in Table S1.14
The C-C bond lengths in the tetracyclic moiety were in the
range of 1.358(4)-1.481(5) ¡, and the C-O bond lengths were in
the range of 1.342(3)-1.378(4) ¡, suggesting the formation of
the ³-conjugated and fused pyrylium skeleton. The C-O bond
lengths of the carbonyl group that did not participate in the
cyclization were in the range of 1.220(4)-1.233(3) ¡. The
cyclized products of the protonated salts [1,8-Am2Pyl](TFSI)
and [1,8-AmFcPyl](TFSI) were also isolated upon treatment
with TFSIH in dichloromethane. Their identification relied on
elemental analysis and ESI-TOF-MS. These materials were
NMR silent, presumably because of valence tautomerism and
resultant generation of paramagnetic species.8c,8d,9,10 Unfortu-
nately, their single crystals suitable for XRD analysis were not
obtained due to poor crystallinity. Thus, 1,8-Ar2Aq is in sharp
contrast to 1,4-Ar2Aq and 1,5-Ar2Aq that can form the
pentacyclic dipyrylium structures. This difference comes from
the fact that the the arylethynyl moieties at 1- and 8-positions are
conjugated with the same carbonyl group so that second ring
formation reaction is impossible for 1,8-Ar2Aq, whereas the
arylethynyl moieties at 4- and 5-positions are conjugated with
the other carbonyl group so that 1,4- and 1,5-derivatives can
undergo two-step ring formation reactions. We note that [1,8-
AmFcPyl](TFSI) has two possible structures where the pyry-
lium ring formed on Fc and Am sides, respectively. We already
reported that ArAq and Ar2Aq with the more strongly electron-
donating Ar needed the shorter reaction time, less harsh acid,
(c)
Figure 3. a) UV-vis-NIR spectral change of 1,8-Am2Aq in
dichloromethane upon a stepwise addition of TFSIH; b) UV-vis-
NIR spectral change of 1,8-AmFcAq in dichloromethane upon a
stepwise addition of TFSIH; (c) Overlay of the UV-vis-NIR spectra of
[1,8-Ar2Pyl](TFSI).
and smaller equivalent of acid for the pyrylium formations,
which we attributed to the stabilization of carbocationic
intermediates by the Ar group.12 1,8-Ar2Aq is also expected
to go through a similar carbocationic intermediate in the course
of the cyclization (Scheme 2). With these backgrounds, we
deduce that the cyclization preferentially occurs on the Fc side,
because the donor ability of ferrocene is stronger than that of
triarylamine by ca. 0.2 V.15
The protonation-induced cyclization reaction of 1,8-Am2Aq
and 1,8-AmFcAq with TFSIH in dichloromethane to generate
[1,8-Am2Pyl](TFSI) and [1,8-AmFcPyl](TFSI) was also moni-
tored by UV-vis-NIR absorption spectroscopy. As shown in
Figures 3a and 3b, each compound displayed a one-step spectral
change with an isosbestic point, similar to 1-ArAq,8 indicating
that only the single cyclization occurred. Figure 3c collects the
spectra of [1,8-Ar2Pyl](TFSI) in dichloromethane.
Chem. Lett. 2011, 40, 1456-1458
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1458
a)
b)
Programming)) and the Global COE Program for Chemistry
Innovation.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
Figure 4. Main transitions of [1,8-Fc2Pyl]+ estimated by TDDFT
calculation: a) in the ³-³* band; b) in the CT band. See Figure S2 and
Table S214 for the comprehensive transitions and molecular orbitals.
References and Notes
1
a) C. Joachim, J. K. Gimzewski, A. Aviram, Nature 2000, 408, 541. b)
J. M. Seminario, Nat. Mater. 2005, 4, 111. c) C. Ehli, C. Oelsner, D. M.
Guldi, A. Mateo-Alonso, M. Prato, C. Schmidt, C. Backes, F. Hauke, A.
Hirsch, Nat. Chem. 2009, 1, 243. d) B. Champin, P. Mobian, J.-P.
Sauvage, Chem. Soc. Rev. 2007, 36, 358.
Table 1. Redox potentials of 1,8-Ar2Aq and [1,8-Ar2Pyl]+ in 0.1 M
n-Bu4NClO4-CH2Cl2
Oxidation
E0¤ (ox1)a E0¤ (ox2)a E0¤ (red)a
Reduction
HOMO-LUMO
2
a) S. Kume, M. Murata, T. Ozeki, H. Nishihara, J. Am. Chem. Soc. 2005,
127, 490. b) R. Sakamoto, M. Murata, S. Kume, H. Sampei, M. Sugimoto,
H. Nishihara, Chem. Commun. 2005, 1215. c) A. Sakamoto, A. Hirooka,
K. Namiki, M. Kurihara, M. Murata, M. Sugimoto, H. Nishihara, Inorg.
Chem. 2005, 44, 7547. d) R. Sakamoto, S. Kume, M. Sugimoto, H.
Nishihara, Chem.®Eur. J. 2009, 15, 1429.
Compound
gapc
1,8-Am2Aq
0.31b
0.40b
0.12
®
®
¹1.38
¹0.33
¹1.41
¹0.30
¹1.33
¹0.31
1.69
0.73
1.53
0.56
1.40
0.50
[1,8-Am2Pyl]+
1,8-AmFcAq
0.28
0.48
0.14
0.30
[1,8-AmFcPyl]+ 0.26
1,8-Fc2Aq
[1,8-Fc2Pyl]+
3
4
a) I. Ratera, D. Ruiz-Molina, F. Renz, J. Ensling, K. Wurst, C. Rovira, P.
Gütlich, J. Veciana, J. Am. Chem. Soc. 2003, 125, 1462. b) J. Jiao, G. J.
Long, F. Grandjean, A. M. Beatty, T. P. Fehlner, J. Am. Chem. Soc. 2003,
125, 7522. c) H. Nishihara, Adv. Inorg. Chem. 2002, 53, 41. d) E. Hillard,
A. Vessières, L. Thouin, G. Jaouen, C. Amatore, Angew. Chem., Int. Ed.
2006, 45, 285.
0.07
0.19
aIn V vs. ferrocenium/ferrocene. bA two-electron process. cDe-
fined as E0¤(ox1) ¹ E0¤(red), in V.
a) J. E. Green, J. W. Choi, A. Boukai, Y. Bunimovich, E. Johnston-
Halperin, E. DeIonno, Y. Luo, B. A. Sheriff, K. Xu, Y. S. Shin, H.-R.
Tseng, J. F. Stoddart, J. R. Heath, Nature 2007, 445, 414. b) M. Irie,
Chem. Rev. 2000, 100, 1685. c) M. B. Duriska, S. M. Neville, B.
Moubaraki, J. D. Cashion, G. J. Halder, K. W. Chapman, C. Balde, J.-F.
Létard, K. S. Murray, C. J. Kepert, S. R. Batten, Angew. Chem., Int. Ed.
2009, 48, 2549.
The common feature lies on the intense bands around 500-
550 nm: According to a DFT calculation for [1,8-Fc2Pyl]+, these
bands are ascribed to the ³-³* transtions of the pyrylium ring
(Figure 4a). In addition, broad absorption bands spanning 600-
1400 nm are also commonly observed, which stem from charge-
transfer (CT) transitions from Fc or Am to Pyl(³*) (Figure 4b).
These kinds of absorption bands were also observed in
other pyrylium and dipyrylium salts.8-10 Upon addition of
base (MeOH), [1,8-Ar2Pyl](TFSI) underwent spectral changes
(Figure S3)14 similar to those of [1-ArPyl](TFSI), which
generated methoxide adducts 1-ArPyl-OMe.8b Therefore, we
deduce that 1,8-Ar2Pyl-OMe was also generated in this case.
The existence of the NIR CT bands infers that [1,8-
Ar2Pyl]X possesses quite a narrow HOMO-LUMO gap. Then
1,8-Ar2Aq and [1,8-Ar2Pyl](TFSI) were subjected to electro-
chemical measurements in 0.1 M n-Bu4NClO4-CH2Cl2, and
formal potentials (E0¤) were collected in Table 1. The cyclization
significantly impacted the reduction: It positively shifted E0¤ for
the first reduction by ca. 1.0 V, whereas the positive shifts of E0¤
for the first and second oxidations were no more than ca. 0.2 V.
This series of results indicates that a significant lowering of the
LUMO level is induced by the expansion of the ³-conjugation
upon the cyclization. As a result, [1,8-Fc2Pyl](TFSI), [1,8-
AmFcPyl](TFSI), and [1,8-Am2Pyl](TFSI) featured extremely
small HOMO-LUMO gaps (Table 1).
5
6
a) V. A. Dediu, L. E. Hueso, I. Bergenti, C. Taliani, Nat. Mater. 2009, 8,
707. b) D. J. Gundlach, J. E. Royer, S. K. Park, S. Subramanian, O. D.
Jurchescu, B. H. Hamadani, A. J. Moad, R. J. Kline, L. C. Teague, O.
Kirillov, C. A. Richter, J. G. Kushmerick, L. J. Richter, S. R. Parkin, T. N.
Jackson, J. E. Anthony, Nat. Mater. 2008, 7, 216. c) Y.-s. Huang, S.
Westenhoff, I. Avilov, P. Sreearunothai, J. M. Hodgkiss, C. Deleener,
R. H. Friend, D. Beljonne, Nat. Mater. 2008, 7, 483.
a) R. B. Ross, C. M. Cardona, D. M. Guldi, S. G. Sankaranarayanan,
M. O. Reese, N. Kopidakis, J. Peet, B. Walker, G. C. Bazan, E.
Van Keuren, B. C. Holloway, M. Drees, Nat. Mater. 2009, 8, 208. b) G.
Qian, Z. Zhong, M. Luo, D. Yu, Z. Zhang, Z. Y. Wang, D. Ma, Adv. Mater.
2009, 21, 111.
7
8
a) M. Murata, T. Fujita, M. Yamada, M. Kurihara, H. Nishihara, Chem.
Lett. 2000, 1328. b) M. Murata, M. Yamada, T. Fujita, K. Kojima, M.
Kurihara, K. Kubo, Y. Kobayashi, H. Nishihara, J. Am. Chem. Soc. 2001,
123, 12903.
a) M. Kondo, M. Uchikawa, W.-W. Zhang, K. Namiki, S. Kume, M.
Murata, Y. Kobayashi, H. Nishihara, Angew. Chem., Int. Ed. 2007, 46,
6271. b) M. Kondo, M. Uchikawa, S. Kume, H. Nishihara, Chem.
Commun. 2009, 1993. c) M. Kondo, M. Uchikawa, K. Namiki, W.-W.
Zhang, S. Kume, E. Nishibori, H. Suwa, S. Aoyagi, M. Sakata, M.
Murata, Y. Kobayashi, H. Nishihara, J. Am. Chem. Soc. 2009, 131, 12112.
d) K. P. Rao, T. Kusamoto, R. Sakamoto, Y. Yamamoto, S. Kume, M.
Nihei, H. Oshio, H. Nishihara, Chem. Commun. 2011, 47, 2330.
K. P. Rao, T. Kusamoto, F. Toshimitsu, K. Inayoshi, S. Kume, R.
Sakamoto, H. Nishihara, J. Am. Chem. Soc. 2010, 132, 12472.
9
10 K. P. Rao, M. Kondo, R. Sakamoto, T. Kusamoto, M. Nishikawa, S.
Kume, M. Nihei, H. Oshio, H. Nishihara, Chem.®Eur. J. 2011, in press.
doi:10.1002/chem.201101708.
In conclusion, we described the protonation-induced cy-
clization reaction of new D-A-D conjugated molecules, 1,8-
Ar2Aq, to yield the corresponding pyrylium salts [1,8-Ar2Pyl]X
with a tetracyclic structure. This single cyclization was in sharp
contrast to 1,4-Ar2Aq and 1,5-Ar2Aq having two Ar groups and
undergoing double cyclization. The pyrylium formation resulted
in more intense D-A interaction, such as smaller HOMO-
LUMO gaps and NIR CT transitions.
11 M. Kondo, M. Murata, H. Nishihara, E. Nishibori, S. Aoyagi, M. Yoshida,
Y. Kinoshita, M. Sakata, Angew. Chem., Int. Ed. 2006, 45, 5461.
12 a) R. Sakamoto, K. P. Rao, H. Nishihara, Chem. Lett. 2011, 40, 1316.
b) M. Kondo, Ph.D. Thesis, The University of Tokyo, Tokyo, 2007.
13 Crystallographic data have been deposited with The Cambridge Crystallo-
graphic Data Centre: Deposition numbers CCDC-852735-852737 for
[1,8-Fc2Pyl](TFSI), [1,8-Fc2Pyl](BF4), and 1,8-AmFcA. Copies of the
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
data_request/cif.
14 Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
15 S. Amthor, B. Noller, C. Lambert, Chem. Phys. 2005, 316, 141.
This work was supported by Grants-in-Aid from MEXT of
Japan (Nos. 20245013 and 21108002, area 2107 (Coordination
Chem. Lett. 2011, 40, 1456-1458
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/