Rigid molecular architectures that comprise a 1,3,5-trisubstituted benzene core and three oligoaryleneethynylene arms: Light-emitting characteristics and π conjugation between the arms

Article abstract of DOI:10.1021/ja057751r

In view of increasing interest in light-emitting materials, we have investigated the light-emitting characteristics and occurrence of conjugation between arms of star-shaped rigid molecules that comprise a 1,3,5-triethynylbenzene core and methoxy group-su

Full text of DOI:10.1021/ja057751r

Published on Web 03/17/2006
Rigid Molecular Architectures That Comprise a 1,3,5-Trisubstituted Benzene
Core and Three Oligoaryleneethynylene Arms: Light-Emitting Characteristics
and π Conjugation between the Arms
Yoshihiro Yamaguchi,* Takanori Ochi, Satoshi Miyamura, 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 November 14, 2005; E-mail: yamaguch@chem.kindai.ac.jp; yoshida@chem.kindai.ac.jp
Table 1. Photophysical Data of Star-Shaped Molecules 1-9 in
Rigid molecular architectures which consist of a trivalent core
and three π extended arms (so-called star-shaped molecules) are
currently attracting considerable attention because of potential
application as devices,1 such as light-emitting diodes (LEDs), field
effect transistors (FETs), and nonlinear optics (NLO). However,
with respect to the light-emitting characteristics associated with
LEDs, only a few papers (dealing with the light-emitters with Φf
< 0.86) appear in the literature.2 Apart from this, it should be
interesting to examine if π conjugation occurs between the arms
across the 1,3,5-trisubstituted benzene core of star-shaped mol-
ecules, though such an interaction is reported for condensed
polycyclic systems.3 Thus we have investigated the light-emitting
characteristics and occurrence of π conjugation between the arms
of star-shaped rigid molecules that comprise a 1,3,5-triethynylben-
zene core and methoxy group-substituted oligo(p-phenyleneethy-
nylene) arms. Consequently, we achieved the ultimate goal (Φf ≈
1.0, log ꢀ > 5) for organic molecules with respect to light-emitting
ability by integrating the dimethoxyphenylacetylene unit with low
Φf (0.29 in cyclohexane), not the fluorene unit with high Φf (0.74
in cyclohexane). Furthermore, we have found that π conjugation
occurs between the arms.
a
CHCl3
λem
λabs
(nm)
τ
(ns)
kr
(s-
kd
1
1
compound Φf b (nm) log
ꢀ
)
(s-
)
kr/kd
1
2
3
4
5
6
7
8
9
0.15 353 4.93 305 1.17 1.28 × 108 7.23 × 108 0.18
0.14 357 5.11 320 0.78 1.80 × 108 1.11 × 109 0.16
0.23 360 4.93 316 1.17 1.96 × 108 6.55 × 108 0.30
0.24 359 4.77 314 1.70 1.41 × 108 4.48 × 108 0.32
0.46 384 4.81 334 1.55 2.97 × 108 3.49 × 108 0.85
0.83 406 5.02 377 0.95 8.69 × 108 1.78 × 108 4.88
0.85 409 5.11 380 0.78 1.10 × 109 1.93 × 108 5.67
0.97 433 5.11 405 0.78 1.25 × 109 3.86 × 107 32.33
0.98 464 5.29 426 0.51 1.93 × 109 3.93 × 107 49.00
a All spectra were measured for 10-7 M solution at 295 K. b Quantum
yield is calculated relative to quinine (Φf ) 0.55 in 0.1 M H2SO4).
kd (a measure for emissivity) and emission lifetime (τ) are
summarized in Table 1. Since the kr and kd are related to the
corresponding emission quantum yields and lifetimes by Φf ) kr
× τ and kr + kd ) τ-1, it is possible to calculate the values of kr
and kd wherever quantum yield and lifetime data are available.5
As demonstrated in Table 1, the introduction of a methoxy
(donor) group into the benzene rings of each arm of the simplest
star-shaped hydrocarbon system 1 brings about a slight red shift of
the fluorescence emission maximum (λem) and an increase in
quantum yield (Φf) (3, 4), though the introduction of a cyano
(acceptor) group does not improve the Φf value (2). Although the
emission efficiency (Φf) increases with increasing dimethoxyphe-
nyleneethynylene unit in each arm, the light-emitting characteristics
of star-shaped molecules (4-9) can be better understood by dividing
into two homologous series, MMPT (4, 6, and 8) and DMPT (5, 7,
and 9). For both series, the values of Φf, λem, log ꢀ, and λabs increase
regularly with a number of dimethoxyphenyleneethynylene units.
Thus we succeeded in creating very intense violet-blue (8, Φf )
0.97, log ꢀ ) 5.11) and blue (9, Φf ) 0.98, log ꢀ ) 5.29) bright
light-emitters, and thereby we achieved the ultimate goal (Φf ≈
1.0, log ꢀ > 5) for organic molecules with respect to light-emitting
ability.
In regard to the star-shaped molecules, we considered 1 (the
simplest star-shaped hydrocarbon system), 2 (cyano-substituted 1
at each terminal), 3 (methoxy-substituted 1 at each terminal), 4, 6,
and 8 (homologues containing zero, one, and two dimethoxyphe-
nyleneethynylene units and a monomethoxyphenyl terminal at each
arm) (abbreviated as MMPT homologues), and 5, 7, and 9 (homo-
logues containing zero, one, and two dimethoxyphenyleneethy-
nylene units and dimethoxyphenyl terminal at each arm) (abbrevi-
ated as DMPT homologues). Synthesis of 1-10 was achieved by
a Pd C-C cross-coupling reaction (see Supporting Information).
The photophysical data4 of star-shaped molecules 1-9 together
with radiative rate constant (kr), radiationless rate constant (kd), kr/
We examined the relations of kr and of kd with the number of
dimethoxyphenyleneethynylene units excluding the terminal arylene-
ethynylene groups (3n, a measure for π extension). Consequently,
as seen in Figure 1, we found6 linear relationships between kr and
3n (slope ) 1.85 × 108, r2 ) 0.9684) and kd and 3n (slope )
-0.68 × 108, r2 ) 0.9672) in the MMPT series, as well as between
kr and 3n (slope ) 2.79 × 108, r2 ) 0.9995) and kd and 3n (slope
) -0.51 × 108, r2 ) 1.0000) in the DMPT series. For both series,
kr values increase and kd values decrease with 3n, indicating that
emission efficiency of the homologues in both series increases with
the number of dimethoxyphenyleneethynylene units. However, from
slopes for plots of kr versus 3n and kd versus 3n, the DMPT
9
4504
J. AM. CHEM. SOC. 2006, 128, 4504-4505
10.1021/ja057751r CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
The solvent dependency on the emission characteristics of 8 and
9 is contrasting (see Supporting Information). The Φf and λem values
of 9 are remarkably altered with a change in solvent polarity, while
little solvent dependency on Φf and λem values of 8 is observed,9
which seems to be explained in terms of a marked disparity between
difference density distribution in the excited singlet and the ground
state (see Supporting Information).
In conclusion, we have synthesized the star-shaped architectures
that comprise a 1,3,5-triethynylbenzene core and methoxy group-
substituted oligo(p-phenyleneethynylene) arms. We achieved the
ultimate goal (Φf ≈ 1.0, log ꢀ > 5) for organic molecules with
respect to light-emitting ability by creating very intense violet-blue
(8, Φf ) 0.97, log ꢀ ) 5.11) and blue (9, Φf ) 0.98, log ꢀ ) 5.29)
bright light-emitters. We have found that π conjugation occurs
between the arms of 9 despite the meta-substituted system. A linear
relationship of kr (with positive slope) and of kd (with negative
slope) to the number of dimethoxyphenyleneethynylene units (3n)
for MMPT (4, 6, and 8) and DMPT (5, 7, and 9) homologues and
the contrasting solvent effect on λem of 8 and 9 are also interesting
findings. It is noted that Φf, λem, λabs, kr, kd, and solvent effect are
controlled by the number (one or two) of the methoxy groups on
the terminal benzene rings.
Figure 1. The relationship of kr and kd to 3n in MMPT and DMPT
homologues (blue line for MMPT homologues, red line for DMPT
homologues).
Table 2. Photophysical Data of Star-Shaped System (9),
Banana-Shaped System (10), and Rod-Shaped System (11) in
a
CHCl3
λem
(nm) log
λabs ∆λabs
kr
(s-
kd
1
1
compound
Φf b
ꢀ
(nm)
(nm)
ꢀ
/
ꢀ11
)
(s-
)
9
10
11
0.98 464 5.29 426 36 3.50 1.93 × 109 3.93 × 107
0.90 432 4.96 402 12 1.59 8.02 × 108 8.92 × 107
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,
Sport, and Culture of Japan.
0.81 430 4.75 390
0
1.00 4.55 × 108 1.07 × 108
a All spectra were measured for 10-7 M solution at 295 K. b Quantum
yield is calculated relative to quinine (Φf ) 0.55 in 0.1 M H2SO4).
Supporting Information Available: Synthesis, NMR data, HR MS
data, fluorescence spectra for 6, 7, 8, and 9, solvent effect on spectra
of 8 and 9, and MO and MM2 calculation for 8 and 9. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
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homologues are favored over the MMPT homologues with respect
to the emission characteristics. The emissive superiority of the
DMPT homologues to the MMPT homologues might be explained
by movability (number) of the dipolar dimethoxyphenylacetylene
unit (donor, OMe; acceptor, CtC) in the excited singlet state as
suggested in the previous paper.7
To know if any π conjugation occurs between the arms of star-
shaped molecules, we have examined the ∆λabs and ꢀ/ꢀ11 values
for 1,3,5-trisubstituted benzene (9, star shape), 1,3-disubstituted
benzene (10, banana shape), and monosubstituted benzene (11, rod
shape)8 with the trimeric(dimethoxyphenyleneethynylene) group.
The results are summarized in Table 2.
As seen in Table 2, the values of ∆λabs and ꢀ/ꢀ11 evidently
increase in the order of 9 (star shape) > 10 (banana shape) > 11
(rod shape). Difference in λabs between 9 and 11 becomes 36 nm,
and ꢀ9/ꢀ11 becomes 3.50. These data clearly indicate an extension
of the π conjugation between the arms. Although an increase in
∆λabs is reported for condensed polycyclic systems,3 the star-shape
molecule (9) seems to be a more straightforward example for meta
conjugation.
(3) For example, see: (a) Scherf, U.; Mu¨llen, K. Polymer 1992, 33, 2443.
(b) Cao, X.-Y.; Hong, Z.; Zhang, W.; Lu, H.; Pei, J. J. Org. Chem. 2005,
70, 3645.
(4) The compounds were purified by repeated column chromatography
followed by recrystallization. Quite different from the usual case (purity
checked by NMR, elemental analysis, etc., 10-2-10-3 impurity), purifica-
tion of light-emitting materials was checked by constancy of the
fluorescence intensity at the maximum (λem) (10-6-10-7 impurity).
(5) (a) Barltrop, J. A.; Coyle, J. D. Principles of Photochemistry; Wiley: New
York, 1978; p 68. (b) Leventis, N.; Rawashdeh, A.-M. M.; Elder, I. A.;
Yang, J.; Dass, A.; Sotiriou-Leventis, C. Chem. Mater. 2004, 16, 1493.
(6) A similar relation is found for the donor/acceptor rod-shaped light-emitting
materials.7
(7) Yamaguchi, Y.; Kobayashi, S.; Wakamiya, T.; Matsubara, Y.; Yoshida,
Z. Angew. Chem., Int. Ed. 2005, 44, 7040.
(8) Yamaguchi, Y.; Tanaka, T.; Kobayashi, S.; Wakamiya, T.; Matsubara,
Y.; Yoshida, Z. J. Am. Chem. Soc. 2005, 127, 9332.
(9) We have confirmed that the Φf and λem values of 8 and 9 in polar solvents
are not so altered with change in concentration from 10-6 to 10-8 M.
JA057751R
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