Synthesis and Light-Emitting Characteristics of Doughnut-Shaped π-Electron Systems

Article abstract of DOI:10.1002/anie.200352749

Doughnut-shaped fluorescent molecules: Highly symmetrical, functionally and structurally interesting octakis-m-cyclynes 1 and octakis-p-cyclynes 2 are shown to be a new class of light-emitting materials (see scheme). A pentacoordinate Cu¹¹ comp

Full text of DOI:10.1002/anie.200352749

Communications
Light-Emitting Macrocycles
Synthesis and Light-Emitting Characteristics of
Doughnut-Shaped p-Electron Systems**
Yoshihiro Yamaguchi,* Shigeya Kobayashi,
Satoshi Miyamura, Yoshifumi Okamoto,
Tateaki Wakamiya, Yoshio Matsubara, and
Zen-ichi Yoshida*
Molecular electronics owes its remarkable development in
utilization to organic optoelectronic materials, and in partic-
ular, to light-emitting devices.[1] To date, many types of
organic light-emitting material have been designed and
fabricated for this purpose, most of which have a linear
molecular shape, such as poly(p-phenylene vinylene). In
addition to their optoelectronic applications, circular-shaped
light-emitting molecules[2] may also be applicable in detecting
circular-shaped nanodefects on the surface of materials. These
possibilities have inspired us to create the doughnut-shaped
light-emitting molecule 1,[3,4] which is constructed by the
formal insertion of pyridine and benzene groups into the
single bonds of cyclohexadecayne, and to examine their light-
emitting characteristics.
Compounds 2 (abbreviated as octakis-m-cyclynes) and 3
(abbreviated as octakis-p-cyclynes) were synthesized by using
[5]
À
the Sonogashira C C coupling reaction as the key step
(Scheme 1). Thus, octakis-m-cyclyne 2 was obtained by the
intramolecular cyclization of 6ac (or 6bc) under high dilution
conditions, while octakis-p-cyclynes 3 were afforded by the
intermolecular cyclization of 7ac (or 7bc), which have p-
phenylethynyl units.
[*] Prof. Dr. Y. Yamaguchi, Dr. S. Kobayashi, S. Miyamura, Y. Okamoto,
Prof. Dr. T. Wakamiya, Prof. Dr. Y. Matsubara, Prof. Dr. Z.-i. Yoshida
Faculty of Science andEngineering
Kinki University
Higashi-Osaka, Osaka 577-8502 (Japan)
Fax: (+81)6-6723-2721
E-mail: yamaguch@chem.kindai.ac.jp
yoshidaz@sc.sumitomo-chem.co.jp
[**] We thank Professor Munakata (Kinki University) for the single-
crystal X-ray analysis of CuII complex 8. This work was supportedby
a Grant-in-Aidfor Scientific Research (Nos. 13305062 and
14540507) from the Ministry of Education, Science, Sport, and
Culture of Japan.
366
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200352749
Angew. Chem. Int. Ed. 2004, 43, 366 –369

Angewandte
Chemie
with AM1 calculations. The
AM1 calculation also suggests
that the octakis-p-cyclynes 3 are
also strain-free. The typical
absorption and fluorescence
spectra of 2b and 3b are shown
in Figure 1, which demonstrates
that the absorption maximum
(labs
) and fluorescence maxi-
mum (lem) appears at longer
wavelength for the p-cyclyne 3b
than for the m-cyclyne 2b,
although the difference (Dlabs
=
51 nm, Dlem = 49 nm) is rela-
tively small in spite of the meta-
fused structure of 2b.
The photophysical data of 2[6]
and 3 are summarized in Table 1,
together with other related sys-
tems. The table shows that not
only long conjugated octakis-p-
cyclynes (3a, 3b), but also short
conjugated octakis-m-cyclynes
(2b and particularly 2c) emit an
unusually strong fluorescence, in
contrast to the acyclic homo-
logue 1,3-PBP, whose labs and
lem values are nonetheless sim-
1
ilar. H NMR spectroscopy con-
firms that compounds 2 and 3 do
not aggregate in chloroform sol-
ution (10À2 m–10À4 m). A small
Stokes shift is also a character-
istic photophysical property of
these cyclynes.
Scheme 1. Reagents and conditions: a) [PdCl2(Ph3P)2], CuI, Et3N/THF (1:1), 53% (for 6aa), 62% (for
6ba); b) i) I2, CH2ClCH2Cl, ii) KOH, MeOH, CHCl3, iii) [PdCl2(Ph3P)2], CuI, Et3N/THF (1:1), <5%
(steps i–iii for 2a), 35% (steps i–iii for 2b), 11% (steps i–iii for 3a), 5% (steps i–iii for 3b); c) Na,
BuOH, CHCl3, 95% (for 2c). TMS=trimethylsilyl.
It is noteworthy that com-
pound 2b can include two mole-
cules of [CuII(hfac)2] (Hhfac =
1,1,1,5,5,5-hexafluoro-2,4-penta-
1
Compounds 2 and 3 were fully characterized by H and
13C NMR spectroscopy and mass spectrometry (see Support-
ing Information). Although the 1H and 13C chemical shifts of
the tetrakis-m-cyclynes 9 (R = CO2Me)[4d] (strained system)
nedione) provide the greenish-blue pentacoordinate CuII
complex 8 upon standing in CHCl3 for a week at room
appear at lower field (1H shift of inner-ring protons: d = 8.67–
8.85 ppm; 13C shift of sp carbon atoms: d = 89–93 ppm) than is
usually the case (strain-free system), the chemical shifts of 2
were observed at the same field as those of the acyclic
homologue, 1,3-di(pyridylethynyl)benzene (1,3-PBP). This
suggests 2 to be an almost strain-free system, in accordance
Figure 1. Absorption (Abs) andfluorescence (Flu) spectra of octakis-
m-cyclyne 2b andoctakis- p-cyclyne 3b in CHCl3 (orange: 2b; green:
3b).
Angew. Chem. Int. Ed. 2004, 43, 366 –369
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
367

Communications
emitting characteristics. In addition, the pentacoordinate CuII
complex 8 of octakis-m-cyclyne 2b emits remarkably strong
fluorescence, contrary to scientific knowledge, which suggests
that various transition-metal complexes of octakis-m-cyclynes
2 could be utilized to fabricate useful luminescent materials.
The optoelectronic properties and other applications of the
doughnut-shaped nano-fluorophores will be reported in due
course.
Table 1: Photophysical data of cyclynes and related compounds in
CHCl3.
Compound
labs [nm]
loge
lem [nm]
Ff[a]
Stokes shift [nm]
2b[b]
2c[b]
3a[b]
3b[b]
8[b]
315
313
334
366
313
322
307
5.19
4.92
5.13
4.27
5.14
4.64
4.70
344
344
371
393
344
356
329
0.18
0.37
0.59
0.33
0.30
0.11
0.03
29
31
37
27
31
34
22
9[b]
1,3-PBP[b]
[a] Quantum yieldis calculatedrelative to quinine ( Ff =0.55 in 0.1m
H2SO4). [b] lex =312 (2b), 311 (2c), 333 (3a), 364 (3b), 313 (8), 311 (9),
and301 nm ( 1,3-PBP).
Experimental Section
Phenyltriazane deprotection: A sealable flask was charged with the
starting material (6aa, 6ba or 7aa, 7ba), I2 (1.2 equiv), and
ClCH2CH2Cl.[7] The solution was degassed, back-filled with Ar,
sealed, and stirred at 808C for 12 h. The reaction mixture was washed
three times with 10% aqueous Na2S2O3, dried over MgSO4, and
concentrated in vacuo. The residue was purified by column chroma-
tography on SiO2 (eluent: CHCl3/EtOAc) to give the product (6ab,
6bb or 7ab, 7bb).
(Trimethylsilyl)acetylene deprotection: The starting material
(6ab, 6bb or 7ab, 7bb) was dissolved in an approximate 10:1 mixture
of CHCl3 and MeOH with 2m aqueous KOH (ꢀ 10 equiv), and the
reaction mixture was stirred at room temperature. The reaction was
monitored by TLC, and when the reaction was complete the reaction
mixture was carefully washed with water. The organic fractions were
then dried over MgSO4 and evaporated. Chromatography on SiO2
(eluent: CHCl3/EtOAc) yielded the product (6ac, 6bc or 7ac, 7bc).
Cyclization:
A Schlenk flask charged with [PdCl2(Ph3P)2]
(0.1 equiv) and CuI (0.05 equiv) was evacuated and back-filled with
Ar three times. Then dry Et3N and THF (2:1, v/v) was added to the
flask. While the mixture was stirred under Ar at 758C, a solution of
the sequence precursor (6ac, 6bc or 7ac, 7bc) in dry THF was added
to the flask by a syringe pump at a rate of ꢀ 8 mLhÀ1. After addition,
the reaction mixture was stirred at the same temperature for 2 h and
then the solvent was removed with a rotary evaporator. The product
was purified by column chromatography on SiO2 (eluent: CHCl3/
EtOAc and/or benzene/EtOAc) followed by recrystallization from
CHCl3. Spectral data of 2a–2c, 3a, and 3b can be found in the
Supporting Information.
Figure 2. ORTEP representation of the pentacoordinate CuII complex
8: a) front view, b) side view.
temperature. Its X-ray crystal structure (Figure 2; see also the
Supporting Information) indicates that two opposing pyridine
nitrogen atoms of 2b coordinate to the CuII ions in
[Cu(hfac)2], and that the cyclyne moiety has a coplanar
structure with expected bond lengths and angles. The N···N
separation between each Cu-coordinated pyridine ring is
15.56 Š and that between each free pyridine ring is 14.16 Š,
while the Cu···Cu distance is 11.58 Š.
Received: August 29, 2003 [Z52749]
The CuII complex 8 emits remarkably strong fluorescence
(Ff: 0.30 for 8, 0.18 for 2b), which is surprising as CuII ions are
known to quench fluorescence. The reason for this observa-
tion is not clear at present. Complex 8 does not disassociate in
solution, since the Rf value of complex 8 is quite different
from that of parent 2b in thin-layer chromatography (TLC)
analysis with several solvents. Furthermore, the increase of
quantum yield in complex 8 is definitive evidence that no
dissociation takes place, because the quantum yield decreases
in the solution of 2b and Cu(OTf)2. Therefore, the surpris-
ingly high quantum yield of complex 8 seems to be a
consequence of the inhibition of internal conversion, brought
about by an increase in the rigidity of the cyclyne ring that
stems from the steric-bulk effects of the CuII-coordinated hfac
ligands.
In conclusion, the functionally and structurally interesting
octakis-m-cyclynes 2 and octakis-p-cyclynes 3 are doughnut-
shaped p-electron systems that are constructed by the formal
insertion of pyridine and benzene groups into the single bonds
of cyclohexadecayne. These doughnut-shaped azamacro-
cycles with cavity dimensions of ꢀ 1 nm have strong light-
Keywords: alkynes · copper · fluorescence · insertion ·
macrocycles
.
[1] For relevant reviews, see: a) S. A. Jenekhe, Adv. Mater. 1995, 7,
309; b) F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, A. J. Heeger,
Acc. Chem. Res. 1997, 30, 430; c) A. Kraft, A. C. Grimsdale, A. B.
Holmes, Angew. Chem. 1998, 110, 416; Angew. Chem. Int. Ed.
1998, 37, 402; d) Y. Wada, H. Yamada, K. Matsushige, Oyo
Butsuri 2001, 70, 1395; e) R. L. Carroll, C. B. Gorman, Angew.
Chem. 2002, 114, 4556; Angew. Chem. Int. Ed. 2002, 41, 4378.
[2] a) Cyclophane Chemistry for 21st Century (Ed.: H. Takemura),
Research Signpost, Trivandrum, 2002; b) Interactions in Mole-
cules, Electronic, and Steric Effects (Ed.: S. D. Peyerimhoff),
Wiley-VCH, Weinheim, 2003.
[3] For reviews on the synthesis of related carbomacrocycles, see:
a) J. S. Moore, Acc. Chem. Res. 1997, 30, 402; b) D. Zhao, J. S.
Moore, Chem. Commun. 2003, 807.
[4] For recent papers on the synthesis of related arene–azaarenecy-
clynes in which pyridine N atoms are directed inside macrocycles,
see: a) Y. Tobe, A. Nagano, K. Kawabata, M. Sonoda, K.
Naemura, Org. Lett. 2000, 2, 3265; b) O. Henz, D. Lentz, A. D.
Schlꢀter, Chem. Eur. J. 2000, 6, 2362; c) O. Henz, D. Lentz, A.
368
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 366 –369

Angewandte
Chemie
Schater, P. Franke, A. D. Schlꢀter, Chem. Eur. J. 2002, 8, 357; d) S.
Kobayashi, Y. Yamaguchi, T. Wakamiya, Y. Matsubara, K.
Sugimoto, Z. Yoshida, Tetrahedron Lett. 2003, 44, 1469.
[5] For a review, see K. Sonogashira in Comprehensive Organic
Synthesis, Vol. 3 (Eds.: B. M. Trost, I. Fleming), Pergamon,
Oxford, 1991, pp. 521 – 549.
[6] Reliable photophysical data for 2a are not obtained because of
the difficulty of its purification.
[7] Z. Wu, J. S. Moore, Tetrahedron Lett. 1994, 35, 5539.
Angew. Chem. Int. Ed. 2004, 43, 366 –369
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
369