Light-emitting efficiency tuning of rod-shaped π conjugated systems by donor and acceptor groups

Article abstract of DOI:10.1021/ja051588i

In view of the increasing importance of highly efficient light-emitting materials in chemistry, biological science, and materials science, we investigated the light-emitting efficiency tuning of rod-shaped oligo(p-phenylene ethynylene)s (OPEs, trimeric to pentameric systems) by donor and acceptor groups, so that they emit the very intense fluorescence (Φ_f ≈ 1.0, log ε ≈ 5) at 460 nm as the desired wavelength region. This goal was achieved by side modification by MeO (donor) groups and end modification by a CN-substituted benzene ring or CF₃-substituted pyridine ring (acceptor) of tetrameric p-phenylene ethynylene rod-shaped molecules (Φ_f = 0.96, λ_em = 458 nm, log ε = 4.96 for the former and Φ_f = 0.99, λ_em = 459 nm, log ε = 4.92 for the latter). The high Φ_f values for 11 and 12 are interpreted in terms of k_r (radiative rate constant) and k_d (radiationless rate constant). The linear relationship with a positive slope between Φ_f and the Hammett σ constant was found for the first time. It is found that k_d rather than k_r varies with σ_p-X. The photophysical properties (Φ_f, λ_em, λ_abs, log ε) were not so altered with the solvent polarity, which could be explained by the dipole moments in the excited and ground states. The results would be valuable for the molecular design of highly efficient light-emitting materials. Copyright

Full text of DOI:10.1021/ja051588i

Published on Web 06/09/2005
Light-Emitting Efficiency Tuning of Rod-Shaped π Conjugated Systems by
Donor and Acceptor Groups
Yoshihiro Yamaguchi,* Takahiro Tanaka, Shigeya Kobayashi, Tateaki Wakamiya,
Yoshio Matsubara, and Zen-ichi Yoshida*
Department of Chemistry, Kinki UniVersity, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
Received March 11, 2005; E-mail: yamaguch@chem.kindai.ac.jp; yoshida@chem.kindai.ac.jp
Scheme 1 a
To date, research on light-emitting materials has been animated
not only in chemistry1 (e.g., molecular materials, probes, sensors,
and tracers) but also in biological science (e.g., biosensing2) and
materials science (e.g., optoelectronics3). For further development
of these fields, creation of highly efficient light-emitting materials
at the desired wavelength region should be indispensable, though
other factors should also be taken into consideration. Since no
general concept or method for this purpose has been established
yet, first we have chosen oligo(p-phenylene ethynylene)s (OPEs,
trimeric to pentameric systems) as the rod-shaped π systems and
tried to tune their electronic structures by donor and acceptor groups,
so that they emit very intense fluorescence (Φf ≈ 1.0, log ꢀ > 4.5)
at 460 nm as an example of the desired wavelength region. In
connection with this trial, we examined whether any relationship
would be found between the structure and fluorescence quantum
yield (Φf) of OPEs modified by donor and acceptor groups, for
the first time. Marder,4 Bunz,5 and others6 have reported the relation
of the spectra (λabs and λem) to the structure of donor-acceptor
oligo- and poly(p-arylene ethynylene)s (OAEs and PAEs).
After various trials, we achieved our goal by a combination of
side modification by MeO (donor) groups and end modification
by a CN-substituted benzene ring or a CF3-substituted pyridine ring
(acceptor) of OPE rod-shaped molecules.7 Herein we wish to report
the results.
General synthesis of trimeric, tetrameric, and pentameric PE
modified by EWG (X) at the end and by EDGs (2,5-dimethoxy
groups at each benzene ring) at the side (abbreviated as E (EWG)/
S (EDG) PEs) was effectively accomplished by a repeating
Sonogashira Pd-coupling reaction,8 as shown in Scheme 1 (for
details, see Supporting Information).
Emission and absorption characteristics of the parent and the
modified PEs together with emission lifetime (τ), radiative rate
constant (kr), and radiationless rate constant (kd) are demonstrated
in Table 1.
As seen in Table 1, the E (EWG)/S (EDG) modification evidently
increases Φf and λem compared with those of the parent PEs (TriPE
and TetraPE).9 This tendency is bigger for the tetrameric system
(n ) 2) than for the trimeric system (n ) 1). In particular, E
(CN/Ph)/S (MeO) (9) and E (CF3/Py)/S (MeO) (10) modifications
are remarkable and effective to provide the highly efficient blue
light emitters. We tried to examine the relation of Φf to the Hammett
substituent constant (σp-X).10
The result is worth noting because the quantum yield (Φf) linearly
increases with an increase in the electron-withdrawing ability of
X, as shown in Figure 1 (correlation factor: r ) 0.986 for n ) 1,
r ) 0.943 for n ) 2; slope: F ) 0.0431 for n ) 1, F ) 0.121 for
n) 2). To our knowledge, the finding of the linear relationship
between Φf and σ values is unprecedented. An approximately linear
relation for the plot of λem versus σ was also found as expected
(see Supporting Information).
a Reagents and conditions: (a) 2,5-dimethoxy phenylacetylene,
Pd(Ph3P)2Cl2, CuI, Et3N, THF, rt, 7 h; (b) trimethylsilyl acetylene,
Pd(Ph3P)2Cl2, CuI, Et3N, THF, rt, 7 h; (c) 1 M aq. KOH, MeOH, CHCl3,
rt, 0.5 h; (d) 13, Pd(Ph3P)2Cl2, CuI, Et3N, THF, rt, 7 h; (e) Pd(Ph3P)2Cl2,
CuI, Et3N, THF, reflux.
Table 1. Emission and Absorption Characteristics of Parent and
Modified PEs (trimeric, tetrameric, and pentameric systems) by
EWG at the End and by EDGs (MeOs) at the Side (in CHCl3)
λem
(nm)
λabs
(nm)
τ
(ns)
kr
(s-
kd
1
1
compound
Φfa
log
ꢀ
)
(s-
)
TriPE
0.50 348 4.59 328 2.57 1.95 × 108 1.95 × 108
0.76 401 4.58 370 2.63 2.89 × 108 9.12 × 107
0.78 428 4.70 376 2.00 3.91 × 108 1.10 × 108
0.78 411 4.56 376 2.75 2.83 × 108 7.99 × 107
0.79 434 4.80 381 1.58 4.98 × 108 1.33 × 108
0.76 436 4.66 383 2.19 3.47 × 108 1.10 × 108
0.81 430 4.74 390 1.82 4.45 × 108 1.04 × 108
0.85 441 4.84 395 1.45 5.88 × 108 1.04 × 108
0.86 435 4.71 395 1.95 4.41 × 108 7.18 × 107
0.91 447 4.71 398 1.95 4.67 × 108 4.62 × 107
0.82 455 4.71 401 1.95 4.21 × 108 9.23 × 107
1
2
3
4
5
TetraPE 0.61 387 4.77 345 1.70 3.59 × 108 2.30 × 108
6
7
8
9
10
PentaPE 0.83 388 4.80 350 1.58 5.24 × 108 1.07 × 108
11
12
0.96 458 4.96 414 1.10 8.76 × 108 3.65 × 107
0.99 459 4.92 415 1.20 8.23 × 108 8.32 × 106
a Quantum yield is calculated relative to quinine (Φf ) 0.55 in 0.1 M
H2SO4).
It was found that kd rather than kr11 (see Supporting Information)
varies with σp-X, as seen in Table 1. Thus, E (EWG)/S (EDG)
modification of tetrameric PE was shown to bring about the
significant improvement of light-emitting efficiency in the trimeric
and tetrameric systems.
Figure 1 and Table 1 suggest a possibility that E (CN/Ph)/
S (MeO) and E (CF3/Py)/S (MeO) modification of tetrameric PE
may provide the very intense blue light emitter (Φf ≈ 1.0, λem
)
460 nm, log ꢀ ≈ 5), which is our final goal in the present inves-
tigation. This possibility was realized by preparing the pentameric
systems 11 and 12 (see Scheme 1 and Table 1). The absorption
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C O M M U N I C A T I O N S
state (µg ) 0.09 D), as expected. On the other hand, calculated
dipole moments of E (EWG)/S (EDG) PEs in the excited state do
not exceed those in the ground state (for example, µe ) 0.66 D
and µg ) 1.99 D for 11; see Supporting Information for others).
Such a dipole moment change in the excited and ground states
of E (EWG)/S (EDG) PEs appears to be responsible for little solvent
dependency to the spectra of 11 and 12. From these arguments, it
is inferred that the acetylenic structure of E (EWG)/S (EDG) PEs
in the ground state (see Supporting Information) may also be held
in the excited state as reported for TriPE.13
In conclusion, we succeeded in the creation of highly efficient
light emitters by E (EWG)/S (EDG) modification of rod-shaped
OPEs. We also made very interesting findings on the relationships
between (1) Φf and Hammett σ constant, and (2) light-emitting
characteristics (λem, Φf) and solvent polarity. These should be
valuable for the molecular design of highly efficient light emitters.
Figure 1. The relationship between quantum yield (Φf) and substituent
constant for the electron-withdrawing substituent X (σp-X) for E (EWG)/S
(EDG) PEs.
Acknowledgment. This work was supported by Grant-in-Aid
for Creative Scientific Research (No. 16GS0209) and Scientific
Research (No. 16550131) from the Ministry of Education, Science,
Sports, and Culture of Japan.
Supporting Information Available: Synthesis, NMR data, HR MS
data, absorption and fluorescence spectra, plot of λem versus σ and Φf
versus σ, and MO calculation for oligo-PEs modified by donor and
acceptor groups. This material is available free of charge via the Internet
at http://pubs.acs.org.
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