Rod-shaped oligophenyleneethynylenes modified by donor and acceptor groups in a block manner: Synthesis and light-emitting characteristics

Article abstract of DOI:10.1246/cl.2007.794

Highly efficient blue light emmitters represented by BL-5 (Φ_f = 0.96, λ_em = 455 nm in CHC13) and BL-8 (Φ_f = 0.98, λ_em = 464nm in CHCl₃) are created by block modification of oligophenylene ethynylenes with donor and acceptor groups. A linear relationship with positive slope between Φ_f and the movability of the energetically equivalent dipolar structure unit is found for the first time. Copyright

Full text of DOI:10.1246/cl.2007.794

794
Chemistry Letters Vol.36, No.6 (2007)
Rod-shaped Oligophenyleneethynylenes Modified by Donor and Acceptor Groups
in a Block Manner: Synthesis and Light-emitting Characteristics
Takanori Ochi, Yoshihiro Yamaguchi,ꢀ Shigeya Kobayashi, Tateaki Wakamiya,
Yoshio Matsubara, and Zen-ichi Yoshidaꢀ
Department of Chemistry, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502
(Received April 9, 2007; CL-070378; E-mail: yamaguch@chem.kindai.ac.jp, yoshida@chem.kindai.ac.jp)
NC
NC
NC
1
Highly efficient blue light emmitters represented by BL-5
OMe
5
OMe
OMe
3
OMe
OMe
NC
OMe
NC
(ꢀf ¼ 0:96, ꢁem ¼ 455 nm in CHCl3) and BL-8 (ꢀf ¼ 0:98,
ꢁem ¼ 464 nm in CHCl3) are created by block modification
of oligophenylene ethynylenes with donor and acceptor groups.
A linear relationship with positive slope between ꢀf and the
movability of the energetically equivalent dipolar structure unit
is found for the first time.
2
4
NC
NC
NC
NC
OMe
OMe
OMe
OMe
BL-3
BL-1
BL-2
NC
NC
NC
NC
NC
The importance of organic light-emitting materials with
high emission efficiency in biological, chemical, and materials
science has been greatly increasing. However, a general concept
or a method for creation of highly fluorescent molecules in a de-
sired wavelength region has not been established yet, though
various attempts to achieve this goal have been made.1 Thus,
the development of method for creation of highly fluorescent
molecules should be an urgent subject. Here, we report synthesis
and light-emitting characteristics of oligophenyleneethynylenes
modified by donor and acceptor groups in a block manner
(BL-1–BL-8, see Figure 1) together with a relationship between
quantum yield (ꢀf) and the movability of the energetically
equivalent dipolar structure unit (so-called intramolecular
exciton2) in the excited singlet state.
Synthesis of block modification system (BL-1–BL-8) was
made in the following order using the Pd C–C coupling reaction3
as the key step: (1) preparation of acceptor blocks (1 and 2), (2)
preparation of donor blocks (3, 4, and 5), and (3) cross-coupling
reaction between donor blocks and acceptor blocks (Figure 2,
further detail, see the Electronic Supporting Information).7 The
structures of BLs were confirmed by spectral data (1H and
13C NMR spectroscopy and HR-FAB MS, see the Electronic
Supporting Information).7
OMe
OMe
BL-4
OMe
OMe
OMe
OMe
BL-5
NC
NC
NC
OMe
OMe
OMe
OMe
BL-7
BL-6
NC
NC
NC
OMe
OMe
OMe
BL-8
Figure 2. Structures of donor blocks, acceptor blocks, and
block modification system (BL-1–BL-8).
emission lifetime (ꢂ) are summarized in Table 1. The kr and
kd were calculated from ꢂ and ꢀf.4
As shown in Table 1, it is demonstrated that the ꢀf and ꢁem
values increase effectively by the block modification, though ꢀf
values increase essentially with the ꢃ-conjugation length even
for the parent system. Thus, very high ꢀf values (>0:95) and
emission at longer wavelength (>430 nm) are observed for the
block modification system such as BL-4 (n ¼ ðl þ mÞ ꢁ 1 ¼
2), BL-5 (n ¼ ðl þ mÞ ꢁ 1 ¼ 3), BL-7 (n ¼ ðl þ mÞ ꢁ 1 ¼ 3),
The photophysical data of block modification system
(BL-1–BL-8), and parent system (PR-1–PR-3)1c together with
radiative rate constant (kr), radiationless rate constant (kd), and
Table 1. Photophysical data of block modification system and
parent system in chloroforma
Com-
pound
ꢁem Stokes
ꢁabs
ꢂ
kr
/sꢁ1
kd
/sꢁ1
b
n
ꢀf
log "
/nm shift/nm /nm /ns
Parent system
H
BL-1
BL-2
BL-3
BL-4
BL-5
BL-6
BL-7
BL-8
PR-1
PR-2
PR-3
1
2
1
2
3
2
3
4
1
2
3
0.82 403
0.93 419
0.84 392
0.95 434
0.96 455
0.95 407
0.98 443
0.98 464
0.50 348
0.61 387
0.83 388
45
49
40
54
69
42
56
72
20
42
38
4.61 358 2.45 3:34 ꢂ 108 7:33 ꢂ 107
4.87 370 1.35 6:89 ꢂ 108 5:19 ꢂ 107
4.66 352 2.19 3:84 ꢂ 108 7:31 ꢂ 107
4.81 380 1.55 6:13 ꢂ 108 3:23 ꢂ 107
4.92 386 1.20 7:98 ꢂ 108 3:33 ꢂ 107
4.90 365 1.25 7:57 ꢂ 108 3:99 ꢂ 107
4.96 387 1.10 8:94 ꢂ 108 1:82 ꢂ 107
4.99 392 1.02 9:58 ꢂ 108 1:95 ꢂ 107
4.59 328 2.57 1:95 ꢂ 108 1:95 ꢂ 108
4.77 345 1.70 3:59 ꢂ 108 2:30 ꢂ 108
4.80 350 1.58 5:24 ꢂ 108 1:07 ꢂ 108
n
PR - 1: n = 1, PR - 2: n = 2, PR - 3: n = 3
MeO
MeO
NC
Block modification system
H
m
l
BL - 1: n = 1 (l = 0, m = 2), BL - 2: n = 2 (l = 0, m = 3)
BL - 3: n = 1 (l = 1, m = 1), BL - 4: n = 2 (l = 1, m = 2), BL - 5: n = 3 (l = 1, m = 3)
BL - 6: n = 2 (l = 2, m = 1), BL - 7: n = 3 (l = 2, m = 2), BL - 8: n = 4 (l = 2, m = 3)
* n:Total number of the substituted phenylene ethynylene unit
( n = (l + m) - 1 for the block modification system)
aAll spectra were measured at 295 K. bQuantum yield is calculated relative to
Figure 1. Structures of the parent (PR) system and the block
modification (BL) system.
quinine (ꢀf ¼ 0:55 in 0.1 M H2SO4).
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.6 (2007)
795
Information).7
OMe
C
NC
OMe
CN
In conclusion, we synthesized novel rod-shaped oligophen-
ylene ethynylenes modified by donor and acceptor groups in a
block manner (BL-1–BL-8), and disclosed their light-emitting
characteristics. It is worth noting that the quantum yield (ꢀf)
increases linearly with the movability of the energetically
equivalent dipolar structure unit (intramolecular exciton).
Furthermore, it is demonstrated that the block modification is
highly effective for the improvement of light-emitting character-
istics (increase in ꢀf and the bathochromic shift of ꢁem and
C
C C
6
7
A
B
Chart 1.
r = 0.982
r = 0.865
1.0
0.8
0.6
0.4
0.2
0.0
ꢁ
abs).
We thank Mr. Akihiro Suzawa for assistance in this study.
This work was supported by Grant-in-Aid for Creative Scientific
Research (No. 16GS0209) and Scientific Research (No.
16550131) from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
Φf
: parent system
Φf = 0.165n + 0.32
: block modification system
Φf = 0.052n + 0.81
References and Notes
1
2
3
4
5
0
1
a) Y. Yamaguchi, T. Ochi, T. Wakamiya, Y. Matsubara,
Z. Yoshida, Org. Lett. 2006, 8, 717. b) Y. Yamaguchi, S.
Kobayashi, T. Wakamiya, Y. Matsubara, Z. Yoshida,
Angew. Chem., Int. Ed. 2005, 44, 7040. c) Y. Yamaguchi,
T. Tanaka, S. Kobayashi, T. Wakamiya, Y. Matsubara, Z.
Yoshida, J. Am. Chem. Soc. 2005, 127, 9332. d) J. N. Wilson,
U. H. F. Bunz, J. Am. Chem. Soc. 2005, 127, 4124. e) J. A.
Marsden, J. J. Miller, L. D. Shircliff, M. M. Haley, J. Am.
Chem. Soc. 2005, 127, 2464. f) J. N. Wilson, P. M.
Windscherf, U. Evans, M. L. Myrick, U. H. F. Bunz, Macro-
molecules 2002, 35, 8681. g) K. Koishi, T. Ikeda, K. Kondo,
T. Sakaguchi, K. Kamada, K. Tawa, K. Ohta, Macromol.
Chem. Phys. 2000, 201, 525. h) J. M. Tour, Acc. Chem.
Res. 2000, 33, 791. i) J.-S. Yang, T. M. Swager, J. Am.
Chem. Soc. 1998, 120, 5321. j) M. Biswas, P. Nguyen,
T. B. Marder, L. R. Khundkar, J. Phys. Chem. A 1997,
101, 1689.
n
Figure 3. The relationship between quantum yield (ꢀf) and the
movability (number, n) of the dipolar structure unit.
and BL-8 (n ¼ ðl þ mÞ ꢁ 1 ¼ 4). This is ascribable to the
increase in kr value and decrease in kd value as shown in
Table 1. The Stokes shift also increases by the block modifica-
tion.
Although the parent system (PR-1–PR-3) only emits in
the UV region, this disadvantage was surmounted by the block
modification. Thus, the block modification is highly effective
not only for the bathochromic shift of ꢁem, but also for the
increase in the emission efficiency, leading to creation of the
highly efficient light-emitters in the longer wavelength region
(e.g., ꢀf ¼ 0:98, ꢁem ¼ 464 nm for BL-8, ꢀf ¼ 0:96, ꢁem
¼
455 nm for BL-5) (see the Electronic Supporting Information).7
In regard to the relation between ꢀf and intramolecular
exciton, we previously proposed a concept1b that the emission
efficiency (ꢀf) depends on the movability of the energetically
equivalent dipolar structure unit (so-called intramolecular
exciton) in the excited singlet state shown by A and B in the
present case (Chart 1).
2
a) E. E. Nesterov, Z. Zhu, T. M. Swager, J. Am. Chem. Soc.
2005, 127, 10083. b) J. G. Muller, J. M. Lupton, J. Feldmann,
¨
U. Lemmer, U. Scherf, Appl. Phys. Lett. 2004, 84, 1183. c)
J. G. Muller, U. Lemmer, G. Raschke, M. Anni, U. Scherf,
¨
J. M. Lupton, J. Feldmann, Phys. Rev. Lett. 2003, 91,
267403. d) Q. Zhou, T. M. Swager, J. Am. Chem. Soc.
1995, 117, 12593.
To establish this concept, we examined the relationship
between the light-emitting efficiency (ꢀf) and the movability
of the dipolar structures (A and B). Consequently, we found5 that
ꢀf values linearly increase with an increase in n as shown
in Figure 3. It is worth noting that a linear correlation is seen
between ꢀf and n, because this is a first successful example of
the quantification of contribution of intramolecular exciton
to ꢀf.
3
4
K. Sonogashira, Comprehensive Organic Synthesis, ed. by
B. M. Trost, I. Fleming, Pergamon, Oxford, 1991, Vol. 3,
p. 521.
a) N. Leventis, A.-M. M. Rawashdeh, I. A. Elder, J. Yang,
A. Dass, C. Sotiriou-Leventis, Chem. Mater. 2004, 16,
1493. b) Principles of Photochemistry, ed. by J. A. Barltrop,
J. D. Coyle, Wiley, New York, 1978, p. 68.
The energy migration6 between different excitons (A and B)
in the block modification system is ascribed to their energetic
equivalence as indicated by the overlap of fluorescence and
absorption spectra of 6 and 7 (see the Electronic Supporting
Information).7 The high intercept values (0.81) for the block
modification system imply that the block modification is effec-
tive for creation of light-emitters with high emission efficiency
even if n is small. A linear relationship was also found between
ꢁem (or ꢁabs) and n as expected (see the Electronic Supporting
5
6
The lower (ꢀf ¼ 0) and upper (ꢀf ¼ 1) limit of quantum
yield should be taken into consideration for the application
of linear relationship between ꢀf and n.
a) M. I. Sluch, A. Godt, U. H. F. Bunz, M. A. Berg, J. Am.
Chem. Soc. 2001, 123, 6447. b) N. P. Balsara, L. J. Fetters,
N. Hadjichristidis, D. J. Lohse, C. C. Han, W. W. Graessley,
R. Krishnamoorti, Bull. Chem. Soc. Jpn. 1999, 32, 6137.
Supporting Information is available electronically on the
CSJ-Journal web site, http://www.csj.jp/journals/chem-lett/.
7
Published on the web (Advance View) May 19, 2007; doi:10.1246/cl.2007.794