New fluorophores with rod-shaped polycyano π-conjugated structures: Synthesis and photophysical properties

Article abstract of DOI:10.1021/ol0528991

Novel rod-shaped polycyano-oligo(phenyleneethynylene)s were synthesized by Pd cross-coupling reaction. Polycyano groups were found to greatly improve the emission efficiency (Φ_f) of OPEs. By the end donor modification, we achieved the creation of very intense blue light-emitting fluorophore with the SMe group (Φ_f) = 0.972, log ε 4.89, λ _em 455 nm) and very intense yellow light-emitting fluorophore with the NMe₂ group (Φ_f = 0.999, log ε 4.75, λ_em 555 nm). Contrasting Φ_f solvent dependency of 6 and 7 and a linear relationship between Φ_f and σ_p-X over the whole region of σ_p-X were also found.

Full text of DOI:10.1021/ol0528991

ORGANIC
LETTERS
2006
Vol. 8, No. 4
717-720
New Fluorophores with Rod-Shaped
Polycyano -Conjugated Structures:
Synthesis and Photophysical Properties
π
Yoshihiro Yamaguchi,* Takanori Ochi, Tateaki Wakamiya, Yoshio Matsubara, and
Zen-ichi Yoshida*
Department of Chemistry, Kinki UniVersity, 3-4-1 Kowakae, Higashi-Osaka,
Osaka 577-8502, Japan
yamaguch@chem.kindai.ac.jp; yoshida@chem.kindai.ac.jp
Received November 30, 2005
ABSTRACT
Novel rod-shaped polycyano-oligo(phenyleneethynylene)s were synthesized by Pd cross-coupling reaction. Polycyano groups were found to
greatly improve the emission efficiency ( f) of OPEs. By the end donor modification, we achieved the creation of very intense blue light-
emitting fluorophore with the SMe group (Φf ) 0.972, log 4.89, λem 455 nm) and very intense yellow light-emitting fluorophore with the NMe2
group (Φf ) 0.999, log 4.75, λem 555 nm). Contrasting Φf solvent dependency of 6 and 7 and a linear relationship between Φf and p-X over
the whole region of p-X were also found.
Φ
E
E
σ
σ
Polycyano π-conjugated systems represented by tetracyano-
ethylene (TCNE) and 7,7,8,8-tetracyano-quinodimethane
(TCNQ) continue to attract great interest as strong electron
acceptors, in particular in the fields of organic conductors1
and ferromagnets.2 However, to the best of our knowledge,
fluorescence characteristics of polycyano π-systems have not
been reported so far, even though electroluminescent polyphen-
ylenevinylenes with cyano groups have been reported.3 In
view of the increasing importance of strongly light-emitting
fluorophores (Φf ≈ 1.0, log ꢀ ≈ 5)4 in biological, chemical,
and materials science, we report here the synthesis and ex-
cellent light-emitting properties of polycyano-oligo(phen-
yleneethynylene)s (polycyano OPEs, 1-3) and polycyano-
tetrameric PEs (4-7) with a donor group substituting the
end cyano group of 3 (Figure 1) together with the parent
(1) (a) Ferraris, J.; Cowan, D. O.; Walatka, V. V., Jr.; Perlstein, J. H. J.
Am. Chem. Soc. 1973, 95, 948. (b) Coleman, L. B.; Cohen, M. J.; Sandman,
D. J.; Yamagishi, F. G.; Garito, A. F.; Heeger, A. J. Solid State Commun.
1973, 12, 1125. (c) Nakasuji, K.; Sasaki, M.; Kotani, T.; Murata, I.; Enoki,
T.; Imaeda, K.; Inokuchi, H.; Kawamoto, A.; Tanaka, J. J. Am. Chem. Soc.
1987, 109, 6970.
(2) (a) Sugimoto, T.; Murahashi, E.; Ikeda, K.; Yoshida, Z.; Nakatsuji,
H.; Yamaguchi, J.; Kai, Y.; Kasai, N. Mater. Res. Soc. Symp. Proc. 1992,
247, 417. (b) Sugimoto, T. New Funct. Mater. 1993, C, 369.
(3) For example, see: (a) Krasoviskii, B. M.; Bolotin, B. M. Organic
Luminescenct Materials; VCH: Weinheim, Germany, 1988; pp 23-29. (b)
Fahlman, M.; Salaneck, W. R.; Morakt, S. C.; Holmes, A. B.; Bredas, J. L.
Chem. Eur. J. 1997, 3, 286. (c) Kraft, A.; Grimsdale, A. C.; Holmes, A. B.
Angew. Chem., Int. Ed. 1998, 37, 402.
Figure 1. Structure of polycyano OPEs (1-3), donor-substituted
polycyano-tetrameric PEs (4-7), and parent OPEs (8-10).
OPEs (8, dimeric PE; 9, trimeric PE; 10, tetrameric PE) as
the reference systems (synthesis of 9 and 10, see Supporting
Information).
(4) (a) Yamaguchi, Y.; Tanaka, T.; Kobayashi, S.; Wakamiya, T.;
Matsubara, Y.; Yoshida, Z. J. Am. Chem. Soc. 2005, 127, 9332 and
references therein. (b) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605.
10.1021/ol0528991 CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/28/2006

Scheme 1. Synthesis of Polycyano OPEs (1-3), Polycyano-tetrameric PEs (4-7) and 35 (X ) H)
The synthesis of polycyano OPEs (1-3) was effectively
accomplished by Pd cross-coupling reaction as shown in
Scheme 1.
Emission and absorption characteristics5 of 1-3 and the
parent OPEs (8-10) together with radiative rate constant
(kr), radiationless rate constant (kd), kr/kd (our measure for
emissivity), and emission lifetime (τ) are shown in Table 1.
As seen in Table 1, it is evident that the introduction of
cyano groups at the side and end of rod-shaped OPEs
increases the Φf and λem values compared with those of the
parent OPEs (8-10). In particular, tetracyano-tetrameric OPE
3 exhibits a red shift of the fluorescence emission maximum
and a significant increase in the quantum yield, thus
providing the highly efficient violet fluorophore (Φf 0.86,
λem 392 nm).
It is worth noting that polycyano groups play an important
role for improvement of the emission characteristics (in
particular, of Φf and λem). The superior emissive properties
of 1-3 relative to 8-10 is ascribed to the increase in their
kr values and the decrease in their kd values compared with
those for 8-10, which seems to be explained by easier
formation of a dipolar unit (exciton) by CN groups (see
Supporting Information). This is obvious from the slopes for
linear plots of kr versus n (number of 2-cyanophenylene-
ethynylene units) for 1-3 (2.90 × 108) and for 8-10 (1.79
Table 1. Photophysical Properties of Polycyano OPEs (1-3)
and the Corresponding Parent OPEs (8-10) (in CHCl3)a
λem
λabs
b
comp Φf
(nm) log ꢀ (nm) τ (ns)
kr (s-1
)
kd (s-1
)
kr/kd
1
2
3
8
9
0.452 332 4.58 297 2.64 1.71 × 108 2.08 × 108 0.83
0.783 370 4.74 340 1.83 4.29 × 108 1.19 × 108 3.61
0.858 392 4.94 357 1.14 7.51 × 108 1.24 × 108 6.04
0.003 320 4.41 299 3.89 8.74 × 105 2.56 × 108 0.003
0.500 348 4.59 328 2.57 1.95 × 108 1.95 × 108 1.00
10 0.610 387 4.77 345 1.70 3.59 × 108 2.30 × 108 1.56
a All spectra were measured at 295 K. b Quantum yield is calculated
relative to quinine (Φf ) 0.55 in 0.1 M H2SO4).
(5) 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), purity
of organic fluorescent compounds was checked by constancy of the
fluorescence intensity at the maximum (λem) (10-6-10-7 impurity).
(6) (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.
Since 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.6
718
Org. Lett., Vol. 8, No. 4, 2006

× 108) and the slopes for linear plots of kd versus n for 1-3
(-0.42 × 108) and for 8-10 (-0.13 × 108). This linear
relatiohship could be explained by movability of the dipolar
unit (exciton) (see Supporting Information).
OPEs4a with respect to the emission characteristics. From
the obtained results, it is suggested that strongly fluorescent
materials at a desired wavelength region could be created
by (1) selection of number and position of the appropriate
donor groups (D) at the terminal benzene ring of 36, (2)
selection of number and position of cyano groups in 36, and
(3) selection of number of phenyleneethynylene unit.
Further search was made for the more intense fluorophores
(Φf ≈ 1.0, log ꢀ ≈ 5) at the desired wavelength region, for
example, at blue (around 460 nm) and yellow (around 550
nm) regions. For this purpose we considered the end
modification of 3 with a donor group (4, SMe; 5, Me; 6,
OMe; 7, NMe2) in place of the end cyano group4a,7 and
synthesized them by the reactions shown in Scheme 1.
Emission and absorption characteristics of polycyano-
tetrameric PEs (4-7) modified by the end donor group
together with the τ, kr, kd, and kr/kd values are summarized
in Table 2. As seen in Table 2, the introduction of end-donor
Figure 3. Molecular design for the strongly fluorescent materials.
Table 2. Emission and Absorption Characteristics of
In regard to structure-emissivity relationship, we found
out that the quantum yield (Φf) linearly increases with an
increase in electron-donating ability of end-substituent X as
shown in Figure 4 where the linear relationship between Φf
Donor-Substituted Polycyano-tetrameric PEs (4-7) (in CHCl3)a
λem
λabs
b
comp Φf
(nm) log ꢀ (nm) τ (ns)
kr (s-1
)
kd (s-1
)
kr/kd
4
5
6
7
0.972 455 4.89 375 1.29 7.54 × 108 2.17 × 107 34.71
0.960 403 4.91 360 1.23 7.78 × 108 3.24 × 107 24.00
0.982 434 4.87 371 1.36 7.22 × 108 1.32 × 107 54.56
0.999 555 4.75 425 1.78 5.61 × 108 5.62 × 105 999.0
a All spectra were measured at 295 K. b Quantum yield is calculated
relative to quinine (Φf ) 0.55 in 0.1 M H2SO4).
group to tricyano-tetrameric PE significantly improves the
Φf, λem, and kr/kd values compared with those of tetracyano-
tetrameric PE 3. Thus we achieved the creation of a very
intense blue fluorophore with the SMe group (Φf 0.972, log
ꢀ 4.89, λem 455 nm) and very intense yellow fluorophore
with the NMe2 group (Φf 0.999, log ꢀ 4.75, λem 555 nm).
Fluorescence spectra and colors of 3, 4, and 7 are shown in
Figure 2.
Figure 4. The relationship between quantum yield (Φf) and
substiuent constant (σp-X) for the substiuent X of terminal benzene
ring of the donor-substituted polycyano-tetrameric PEs (4-7) and
35 (X ) H), and tetracyano-tetrameric PE 3 (X ) CN). Hammett
substituent constants (σp-X):8 0.66 (3, X ) CN), 0.00 (4, X ) SMe),
0.00 (35, X ) H), -0.17 (5, X ) Me), -0.27 (6, X ) OMe),
-0.83 (7, X ) NMe2).
and the Hammett substituent constants (σp-X)8 is kept over
the whole region of σp-X values including a cyano group.
The effect of solvent on the photophysical properties of 6
and 7 is shown in Table 3. As shown in this table, the solvent
effect on emission efficiency (Φf) of 6 (X ) OMe) and 7
(X ) NMe2) is quite contrasting. Almost no solvent effect
is observed for the absorption spectra of both 6 and 7,
Figure 2. Fluorescence spectra and colors of 3, 4, and 7 in CHCl3.
(7) Other examples of the donor-acceptor substituted OPEs: (a)
Marsden, J. A.; Miller, J. J.; Shircliff, L. D.; Haley M. M. J. Am. Chem.
Soc. 2005, 127, 2464. (b) Wilson, J. N.; Bunz, H. F. J. Am. Chem. Soc.
2005, 127, 4124.
This sort of end-donor modification of polycyano OPEs
is superior to the end-acceptor modification of polymethoxy
(8) Hansch, C.; Leo, A.; Taft, W. Chem. ReV. 1991, 91, 165.
Org. Lett., Vol. 8, No. 4, 2006
719

Φf, λem, kr, and kd could be explained by a difference in the
dipole moment in the excited state (µe) and in the ground
state (µg) of 6 (µe ) 10.74 D < µg ) 11.52 D) and 7 (µe )
12.89 D > µg ) 12.75 D) (see Supporting Information).
In conclusion, we succeeded in the creation of a highly
efficient violet fluorophore (3, Φf 0.86, λem 392 nm) of
polycyano OPE type, in contrast to TCNE and TCNQ, which
scarcely fluoresce (Φf < 10-4). Furthermore, we created very
intense blue (4, λem 455 nm) and yellow (7, λem 555 nm)
fluorophores (Φf ≈ 1.0, log ꢀ ≈ 5) by an introduction of the
appropriate end-donor group. In addition, contrasting solvent
dependency of the emission efficiency (Φf) of 6 and 7 and
a linear relationship between Φf and σp-X over the whole
region of σp-X are other interesting findings.
Table 3. Effect of Solvent on Absorption and Fluorescence
Characteristics of 6 and 7a
λem
λabs
comp solvent
Φf (nm) log ꢀ (nm) kr (s-1
)
kd (s-1
)
kr/kd
6
benzene 0.972 420 4.84 371 6.72 × 108 1.94 × 107 34.71
CHCl3 0.982 434 4.87 371 7.22 × 108 1.32 × 107 54.56
THF
0.972 460 4.89 369 7.55 × 108 2.17 × 107 34.71
0.949 513 4.86 369 6.87 × 108 3.69 × 107 18.61
CH3CN 0.973 490 4.89 364 7.55 × 108 2.10 × 107 36.04
DMF
7
benzene 0.999 519 4.76 426 5.75 × 108 5.75 × 105 999
CHCl3 0.999 555 4.75 425 5.61 × 108 5.62 × 105 999
THF
0.084 621 4.77 422 4.96 × 107 5.39 × 108 0.092
CH3CN 0.002 682 4.78 413 9.04 × 105 6.02 × 108 0.002
DMF
0.002 699 4.73 423 1.16 × 106 5.36 × 108 0.002
a All spectra were measured at 295 K.
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.
whereas the fluorescence maximum of 6 and 7 are shifted
to longer wavelengths as solvent polarity increase. Although
the quantum yield of 6 is not altered with a change in the
solvent polarity, that of 7 remarkably decreases with an
increase in the solvent polarity. The observed effect of the
solvent on the Φf values of 6 and 7 can be interpreted by a
change in the kr and kd values of 6 and 7 with a change in
the solvent polarity. The kr and kd values of 6 are not so
altered by solvent polarity. On the contrary the kr values
decrease and the kd values increase for 7 with an increase in
solvent polarity. The contrasting solvent polarity effect on
Supporting Information Available: Synthesis, spectral
data, solvent dependency, plots of λem and Φf versus σ, plots
of kr and kd versus n, and MO calculation for polycyano
OPEs modified by donor and acceptor groups. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0528991
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Org. Lett., Vol. 8, No. 4, 2006