Highly emissive π-conjugated alkynylpyrene oligomers: Their synthesis and photophysical properties

Article abstract of DOI:10.1021/jo061959t

(Chemical Equation Presented) We newly prepared para- and meta-linked alkynylpyrene oligomers and examined their photophysical properties. Oligomerization of monomeric building blocks was performed by Cu ^I-promoted oxidative coupling reaction.

Full text of DOI:10.1021/jo061959t

Highly Emissive π-Conjugated Alkynylpyrene
Oligomers: Their Synthesis and Photophysical
Properties
their electrical and optical characteristics, so that the oligomers
and polymers have been used for such applications as organic
3
4
electroluminescent devices and chemical sensors. In designing
π-conjugated oligomers, an acetylenic bond is preferably used
as a linkage between aromatic nuclei because of its linearity
†
,†
‡
Hisao Shimizu, Kazuhisa Fujimoto,* Masaru Furusyo,
5
and synthetic advantages. A number of π-conjugated oligomers
§
§
§
Hajime Maeda, Yasuaki Nanai, Kazuhiko Mizuno, and
3b,6
naphthalene,7
with acetylene linkers possess benzene,
,
†
Masahiko Inouye*
8
9
thiophene, and porphyrin as a conventional core. To expand
the practicality of π-conjugated oligomers, developments of a
new class of π-conjugated oligomers bearing a variety of
aromatics are important.
Graduate School of Pharmaceutical Sciences, UniVersity of
Toyama, Sugitani 2630, Toyama 930-0194, Japan,
Analysis Technology Research Center, Sumitomo Electric
Industries Ltd., 1-1-1 Koyakita, Itami, Hyogo 664-0016, Japan,
and Department of Applied Chemistry, Graduate School of
Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan
Pyrene and its derivatives have been extensively applied to
10
11,12
photonic devices and biological probes
by virtue of their
inherent and novel photophysical characteristics. Thus, pyrene
is a fascinating core in fluorescent π-conjugated oligomers and
1
3
polymers. However, serious drawbacks exist in the photo-
physical properties of pyrene, which include the relatively short
absorption wavelength, the substantial quenching of its fluo-
fujimoto@pha.u-toyama.ac.jp; inouye@pha.u-toyama.ac.jp
ReceiVed September 22, 2006
14
rescence by the presence of oxygen, and the low fluorescence
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002, 4, 2533-2536. (c) Tam, I. W.; Yan, J.; Breslow, R. Org. Lett. 2006,
We newly prepared para- and meta-linked alkynylpyrene
oligomers and examined their photophysical properties.
Oligomerization of monomeric building blocks was per-
(9) (a) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116,
I
formed by Cu -promoted oxidative coupling reaction. The
(10) (a) Otsubo, T.; Aso, Y.; Takamiya. K. J. Mater. Chem. 2002, 12,
resulting oligomers mainly consist of 2-mer to 6-mer that
were assigned on the basis of MALDI-TOF mass spectra,
and the 2-mer, 3-mer, and 4-mer were isolated and fully
characterized. From their absorption and fluorescence spectra,
the para-linked oligomers were found to be somewhat
π-conjugated compared with meta-linked ones, and the
fluorescence quantum yields decreased with increasing
2
565-2575. (b) Ohshita, J.; Yoshimoto, K.; Tada, Y.; Harima, Y.; Kunai,
A.; Kunugi, Y.; Yamashita, K. J. Organomet. Chem. 2003, 678, 33-38.
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(
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C.; Liu, F.; Xia, Y.-J.; Lin, J.; Xie, L.-H.; Zhong, G.-Y.; Fan, Q.-L.; Huang,
W. Org. Electron. 2006, 7, 155-162.
(11) For nucleic acid probes bearing pyrenes, see: (a) Paris, P. L.;
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998, 26, 5409-5416. (c) Yamana, K.; Iwai, T.; Ohtani, Y.; Sato, S.;
oligomer length (Φ ) 0.79-0.55).
f
Nakamura, M.; Nakano, H. Bioconjugate Chem. 2002, 13, 1266-1273. (d)
Okamoto, A.; Kanatani, K.; Saito, I. J. Am. Chem. Soc. 2004, 126, 4820-
4
3
827. (e) Fujimoto, K.; Shimizu, H.; Inouye, M. J. Org. Chem. 2004, 69,
271-3275.
Size-regulated oligomers sometimes give prospective infor-
mation for chemical and physical features of the corresponding
(12) For protein probes bearing pyrenes, see: (a) Han, M. K.; Lin, P.;
1,2
Paek, D.; Harvey, J. J.; Fuior, E.; Knutson, J. R. Biochemistry 2002, 41,
longer polymers made of the same monomeric building blocks.
3
2
2
468-3476. (b) West, J. M.; Tsuruta, H.; Kantrowitz, E. R. J. Biol. Chem.
004, 279, 945-951. (c) Drury, J.; Narayanaswami, V. J. Biol. Chem. 2005,
80, 14605-14610. (d) Yang, C. J.; Jockusch, S.; Vicens, M.; Turro, N.
Among the oligomers, π-conjugated ones have been noted for
†
J.; Tan, W. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 17278-17283. (e)
Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu, H.; Inouye, M.
Chem. Eur. J. 2006, 12, 824-831.
University of Toyama.
Sumitomo Electric Industries Ltd.
Osaka Prefecture University.
‡
§
(
1) (a) Tour, J. M. Chem. ReV. 1996, 96, 537-553. (b) Electric
(13) (a) Kreyenschmidt, M.; Baumgarten, M.; Tyutyulkov, N.; M u¨ llen,
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Chem. Commun. 2004, 2800-2801.
Materials: The Oligomer Approach; M u¨ llen, K., Wegnar, G., Eds.; Willey-
VCH: Weinheim, Germany, 1998. (c) Martin, R. E.; Diederich, F. Angew.
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10.1021/jo061959t CCC: $37.00 © 2007 American Chemical Society
1
530
J. Org. Chem. 2007, 72, 1530-1533
Published on Web 01/23/2007

SCHEME 1. Synthetic Schemes for 1a-d and 2a-ca
a
Reagents and conditions: (a) n-C12H25OH, DEAD, PPh3, THF, rt, 1 h; (b) (i) 2-methy-3-butyn-2-ol, Pd(PPh3)4, CuI, Et3N, 80 °C, 15 h for 4, 4 h for
8
, (ii) (tert-butyldimethylsilyl)acetylene, Pd(PPh3)4 for 4, Pd(tert-Bu3P)2 for 8, CuI, Et3N, 80 °C, 2.5 h, (iii) NaH, toluene, reflux, 30 min for 4, 1.5 h for 8;
(c) ICl, ether, rt, 1 h; (d) (i) Pd(PPh3)4 for 5, Pd(tert-Bu3P)2 for 9, CuI, morpholine, 110 °C, 4 h, (ii) n-Bu4NF, THF, H2O, rt, 5 min; (e) CuCl, O2, TMEDA,
THF, acetone, rt, 1 h. DEAD ) diethyl azodicarboxylate, THF ) tetrahydrofuran, TMEDA ) N,N,N′,N′-tetramethylethylenediamine.
quantum yields.15 Recently, we synthesized various alky-
nylpyrenes and found that the introduction of alkynyl groups
into pyrene nuclei induced effective extension of π-conjugation
and a large increase of fluorescence intensities compared with
the parent pyrene.1 Furthermore, the alkynylpyrenes almost
maintain their fluorescence intensities even under aerated
conditions, giving rise to an additional useful feature for practical
uses. Taking these features into account, we herein report the
synthesis and photophysical properties of new acetylene-linked
π-conjugated oligomers based on alkynylpyrene skeletons.
2e
Figure 1 shows the chemical structures of alkynylpyrene
oligomers 1 and 2. The monomeric building blocks of the
oligomers, 1a and 2a, are made up of a pyrene core and two
diethynylbenzene derivatives. The structural difference between
1
a and 2a is the linkage position of terminal acetylene groups
FIGURE 1. Alkynylpyrene oligomers 1a-d and 2a-c.
on the benzene rings, i.e., para for 1a and meta for 2a. Long
alkyl side chains were introduced on the benzene rings to make
the alkynylpyrene-based oligomers soluble in common organic
solvents. Monomeric building blocks 1a and 2a were synthe-
by using a CuCl-TMEDA-promoted oxidative coupling reaction
(Scheme 1). In this reaction, the 2-mer (1b and 2b), 3-mer (1c
and 2c), and 4-mer (1d) were mainly formed, which were
separated by recycling preparative gel permeation chromatog-
raphy (GPC) with CHCl3 as an eluent.
The optical properties of the oligomers were investigated by
using CHCl3 as a solvent. When the concentrations of 1a, 1c,
16
sized by repeating the Sonogashira and deprotection reactions
1
7
18
starting from 3 and 6, respectively. Each oligomer was
obtained from the corresponding monomeric building blocks
2
a, and 2c were varied, the absorbances at their absorption
(
15) (a) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977,
-5
maxima (λmax) obeyed Beer’s law at e2.0 × 10 M (Figures
S1A and S1B in Supporting Information). Thus, the chro-
mophore aggregation was negligible below that concentration.
The electronic absorption spectra of 1a-d and 2a-c in CHCl3
are shown in Figure 2. The absorption maximum and its
coefficient (log ꢀ) of the monomeric building block 1a are 436
nm and 4.84, respectively (Figure 2a). The absorption maxima
of 1b-d consecutively shifted to longer wavelengths, and their
absorption coefficients increased with increasing of oligomer
length. The 4-mer 1d has an absorption maximum at 454 nm,
9
1
9, 2039-2044. (b) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem.
977, 81, 2176-2180. (c) Kusumoto, Y.; Takeshita, Y.; Kurawaki, J.;
Satake, I. Chem. Lett. 1997, 349-350. (d) Oton, J. M.; Acuna, A. U. J.
Photochem. 1980, 14, 341-343. (e) Silva, M. A. D. R.; da Silva, D. C.;
Machado, V. G.; Longhinotti, E.; Frescura, V. L. A. J. Phys. Chem. A 2002,
1
06, 8820-8826.
16) Sonogashira, K. In Metal-Catalyzed Cross-Coupling Reactions;
Diederich, F., Stang, P. J., Eds.; Wiley-VCH: New York, 1998; pp 203-
(
2
29.
(
17) Zhu, Z.; Swager, T. M. Org. Lett. 2001, 3, 3471-3474.
(
18) Sandin, R. A.; McKee, R. A. Organic Syntheses; Wiley: New York,
1
943; Collect. Vol. II, pp 100-101.
J. Org. Chem, Vol. 72, No. 4, 2007 1531

FIGURE 2. Electronic absorption spectra of (a) 1a-d and (b) 2a-c
in CHCl
FIGURE 4. Normalized fluorescence spectra of (a) 1a-d and (b) 2a-c
3
.
-6
in CHCl
3
(1.0 × 10 M). Excitation wavelength was 436, 451, 452,
and 454 nm for 1a-d and 440, 443, and 444 nm for 2a-c, respectively.
with the electronic absorption spectra. The fluorescence quantum
yields (Φf) and fluorescence lifetimes (τs) of the oligomers were
measured in degassed CHCl3 and THF (Table 1). The Φf values
of the monomers 1a and 2a are very high in both the solvents,
although the values of the oligomers somewhat decreased,
especially for the meta-linked ones. Fluorescence quantum yields
can be described by the equations Φf ) kfτs and Φf ) kf/(kf +
kisc + knr) under the degassed conditions, in which kf, kisc, and
knr represent the rate constants for fluorescence radiation,
intersystem crossing, and nonradiative decay, respectively. From
the experimental values of Φf and τs, we calculated the values
of kf and kisc + knr. As can be seen in Table 1, the kf values of
the para-linked oligomers decrease with increasing oligomer
length. The longer the length of the oligomer, the more flexible
the structure becomes. Thus, in the longer oligomers, intersystem
crossing and nonradiative decay processes might to some extent
compete with the fluorescence radiation process. This situation
will be responsible for the relationship between the observed
FIGURE 3. Absorption maxima of oligomers 1a-d (circles and solid
line) and 2a-c (squares and dashed line) plotted against reciprocal
chain length and fitted linearly.
revealing that the magnitude of the bathochromic shift from 1a
to 1d is ca. 18 nm. In the case of the meta-linked oligomers,
only a slight bathochromic shift was observed probably because
of partial insulation of the π-conjugation on these oligomers
(Figure 2b). To anticipate the absorption maxima of the
corresponding polymers, each λmax of the oligomers was plotted
against the reciprocal chain length 1/n (Figure 3).1b Although
both oligomers have a linear relationship between λmax and 1/n,
the slope for the para-linked oligomers is steeper than that for
meta-linked ones. In the para-linked oligomers, the theoretical
absorption maximum for infinite chain length is ca. 460 nm,
where the para-linked polymers of enough length may absorb.
However, the bathochromic shift of the para-linked oligomers
Φ values and the oligomer lengths. Nevertheless, the oligomers
f
indeed possess high fluorescence quantum yields, and only slight
quenching of the fluorescence occurred even under aerated
22
conditions, as in the cases for simple alkynylpyrenes previously
(19) Computational modeling was performed by DFT calculation in order
to examine the π-conjugation of the para-linked oligomers. The two OR
groups on the benzene ring can exist in two conformers, syn and anti, against
the pyrene plane of the oligomers. We assumed that the free rotation of the
pyrene and benzene planes on the oligomers may influence the π-conjuga-
tion. The structures of the para-linked oligomers were simplified as 15-
18 shown in Supporting Information to make geometry optimization
uncomplicated. The ZPE difference between the syn and anti conformers
was calculated to be e2.4 kJ/mol on the basis of B3LYP/6-31G(d) level
optimized structure. Furthermore, rotational barriers were estimated based
on RHF/3-21G(d) level optimized structure (data not shown). The rotational
barriers were also very small (syn to anti, 12 kJ/mol; anti to syn, 3.1 kJ/
mol), implying that the pyrene and benzene planes can easily rotate on the
oligomers at room temperature.
is not so large possibly because of easy rotation of the benzene
ring with respect to the pyrene core.19 Nevertheless, this finding
indicates that the para-linkage is preferable for the π-conjugation
on alkynylpyrene-based oligomers.
The fluorescence spectra of the oligomers were measured in
degassed CHCl3 (Figure 4). In all of the spectra, two strong
emission bands are seen in the visible region. The emission
maxima of the para-linked oligomers 1a-d consecutively
shifted to longer wavelengths, in a manner similar to their
absorption maxima. On the other hand, the fluorescence spectra
for the meta-linked oligomers 2a-c scarcely varied in agreement
(20) Morris, J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem. 1976,
80, 969-974.
1
532 J. Org. Chem., Vol. 72, No. 4, 2007

TABLE 1. Photophysical Data of 1a-d and 2a-ca
absorptionb
fluorescence
log ꢀ (M- cm-1
1
)
λem (nm)
c
Φf (CHCl3)
d
Φf (THF)
d
τs (ns)
e
kf (× 10 ) (s-1)
f
8
kisc + knr (× 10 ) (s-1)
f
8
compound
λabs (nm)
1
1
1
1
2
2
2
a
b
c
d
a
b
c
436
451
452
454
440
443
444
4.84
5.28
5.47
5.58
4.82
5.02
5.19
448
465
470
473
455
459
461
0.74
0.67
0.62
0.55
0.70
0.56
0.35
0.79
0.68
0.64
0.55
0.75
0.61
0.44
0.98
1.33
1.48
1.35
1.52
1.11
1.51
8.06
5.11
4.32
4.07
4.93
5.50
2.91
2.14
2.41
2.44
3.34
1.65
3.51
3.71
a
All measurements were performed under degassed conditions. b 1.0 × 10 M in CHCl3, λabs is the absorption band appearing at the longest wavelength.
-6
c
-6
d
1
.0 × 10 M in CHCl3, λem is the fluorescence band appearing at the shortest wavelength. Fluorescence quantum yield, determined by using 9,10-
20 21 -6
diphenylanthracene (Φf ) 0.95 in EtOH) and coumarin102 (Φf ) 0.74 in EtOH) as reference compound. e Fluorescence lifetime, measured at 1.0 × 10
f
M in THF. Calculated by the following equations: Φf ) kfτs and τs ) 1/(kf + kisc + knr).
reported by us.12e These findings encouraged us to apply the
corresponding polymers as novel electric and photonic mater-
ials.
devices. Introduction of extra recognition sites for biological
species into our oligomers is also under consideration in order
to develop novel chemical sensors.
In conclusion, we have developed π-conjugated oligomers
based on bis(phenylethynyl)pyrene skeletons. The para-linked
oligomers showed longer absorption wavelengths and intense
fluorescence emissions. These fascinating properties of the
alkynylpyrene-based oligomers will facilitate the synthesis of
alkynylpyrene polymers and the application to new optical
Acknowledgment. The present work was partly supported
by the Grant-in-Aid for Scientific Research (KAKENHI) in
Priority Area “Molecular Nano Dynamics” from Ministry of
Education, Culture, Sports, Science and Technology.
Supporting Information Available: Experimental details for
1
the syntheses and analyses, Figures S1A and S1B, H NMR spectra,
(21) Jones, G., II; Jackson, W. R.; Halpern, A. M. Chem. Phys. Lett.
1
980, 72, 391-395.
and details of the calculations for 15-18. This material is available
(
22) Integrations of the fluorescence spectra of 1a-d in aerated THF
free of charge via the Internet at http://pubs.acs.org.
showed that 94%, 93%, 88%, and 85% of the levels observed from the
corresponding carefully degassed solutions were maintained, respectively.
JO061959T
J. Org. Chem, Vol. 72, No. 4, 2007 1533