Meso-disubstituted anthracenes with fluorine-containing groups: Synthesis, light-emitting characteristics, and photostability

Article abstract of DOI:10.1021/ol802337v

(Chemical Equation Presented) Synthesis, photophysical properties, and photostability of 9,10-disubstituted anthracenes with fluorine-containing groups (FCG) are described. The values of Φ_f and λ_em greatly go up by the meso-substitution with FCG, and a nice corelationship between Φ_f and A_π (magnitude of π conjugation length in the excited single state) is observed. The C₆F₅ group at the meso positions exhibits an excellent ability in the photostability as well as in the emission efficiency.

Full text of DOI:10.1021/ol802337v

ORGANIC
LETTERS
2008
Vol. 10, No. 24
5541-5544
Meso-Disubstituted Anthracenes with
Fluorine-Containing Groups: Synthesis,
Light-Emitting Characteristics, and
Photostability
Yoshio Matsubara,* Atsushi Kimura, Yoshihiro Yamaguchi, and Zen-ichi Yoshida*
Faculty of Science and Engineering Kinki UniVersity, 3-4-1 Kowakae, Higashi-Osaka,
Osaka, 577-8502, Japan
y-matsu@apch.kindai.ac.jp; yoshida@chem.kindai.ac.jp
Received October 8, 2008
ABSTRACT
Synthesis, photophysical properties, and photostability of 9,10-disubstituted anthracenes with fluorine-containing groups (FCG) are described.
The values of Φf and λem greatly go up by the meso-substitution with FCG, and a nice corelationship between Φf and Aπ (magnitude of π
conjugation length in the excited single state) is observed. The C6F5 group at the meso positions exhibits an excellent ability in the photostability
as well as in the emission efficiency.
Acenes represented by anthracenes are a family of carbon-rich
compounds of current interest1 and are attracting considerable
attention because of the light-emitting function leading to
potential application as optoelectronic materials,2 sensors,3 and
solar cells.4 However, their biggest drawback is their lack of
photostability due to ready oxygenation at the meso-positions.
Thus, creation of highly fluorescent and photostable anthracenes
should be an important and urgent subject.
anthracenes with fluorine-containing groups (abbreviated as
FCG). To the best of our knowledge, light-emitting charac-
teristics of FCG-substituted anthracenes have not been
reported so far.
In regard to 9,10-diFCG-substituted anthracenes, we
considered 2 (X ) F), 4 (X ) CF3), and 6 (X ) C6F5), and
we employed 1 (X ) H), 3 (X ) Me), 5 (X ) Ph) as the
reference compounds to 2, 4, and 6, respectively. Synthesis
of 2 (X ) F) and 4 (X ) CF3) was made by modification of
the literature methods,5,6 respectively.
We report here synthesis, light-emitting characteristics, and
photostability (in the presence air) of 9,10- disubstituted
(1) Haley, M. M.; Tykwinski, R. R.; Eds. Carbon-Rich Compounds;
Wiley-VCH: Weinheim, 2006.
For the synthesis of new compound 6, we have devised a
Cu-catalyzed coupling of pentafluorobenzene with 9,10-
dibromoanthracene using Ag2O as a base as shown in
(2) For example, see: Magnera, T. F.; Casher, D. L.; Mulcahy, M. E.;
Zheng, X.; Michl, J. ORGN-200. Abstracts of Papers, 230th ACS National
Meeting; Aug 28-Sept 1, 2005, Washington, DC; American Chemical
Society Washington, DC, 2005.
(3) For example, see: Fabbrizzi, L.; Licchelli, M.; Parodi, L.; Poggi,
A.; Taglietti, A. J. Fluoresc. 1998, 8, 263–271.
(5) Duerr, B. F.; Chung, Y. S.; Czarnik, A. W. J. Org. Chem. 1988, 53,
2120–2122
.
(4) For example: Malliaras, G.; Friend, R. Phys. Today 2005, 58, 53–
58.
(6) Loncar, L.; Burek, G.; Mintas, M.; Hergold-Brundic, A.; Nagl, A.
Acta Pharm. 1995, 45, 37–43
.
10.1021/ol802337v CCC: $40.75
Published on Web 11/13/2008
2008 American Chemical Society

Table 1. Photophysical Data of 9,10-Di-X-substituted Anthracenes in Cyclohexanea
c
c
compd
Φfb
λem 0-0 (nm)
λabs 0-0 (nm)
λem 1-0 (nm)
λabs 0-1 (nm)
τd(ns)
kr × 107 (s-1
)
knr × 107 (s-1
)
Aπ(Å)
1
2
3
4
5
6
0.28
0.54
0.58
0.68
0.72
0.66
377
394
402
410
406
400
376
393
398
400
394
390
398
418
426
431
428
422
357
372
378
376
371
370
4.08
8.07
8.11
6.89
5.67
6.15
6.87
6.70
7.15
9.87
12.7
10.7
17.7
5.70
5.18
4.64
4.94
5.53
-0.94
0.16
0.32
0.75
0.94
0.66
a All spectra were measured for 10-7 M solution at 295 K in contact with air. For details of the photophysical measurements, see the Supporting
Information. b Quantum yield was calculated relative to quinine (Φf ) 0.55 in 0.1 M H2SO4). c The log ε values for λabs (0-0 band) and λabs (0-1 band)
of 1-6 were approximately 3.6-4.0 and among 3.9-4.2, respectively. d τ values were obtained by time-resolved fluorescence spectroscopy using LaserStrobo
fluorescence lifetime system (Photon Technology International). For details, see the Supporting Information.
Scheme 1. Synthesis of New Compound 6
Figure 1. 9,10-Di-X-substituted anthracenes.
increases with the conjugative interaction between these
substituents and the anthracene ring in the ground and excited
states, even if the substituents (C6F5 and Ph) are not
completely coplanar at both states.
The fluorescence quantum yields (Φf) also greatly increase
by meso-substitution with FCG compared that of the parent
anthracenes. Noteworthy is a considerably high emission
efficiency (Φf: 0.54-0.68) of 9,10-di-FCG-substituted an-
thracenes in contrast to those (Φf: 0.1 for X ) Br, Φf: 0.48
for X ) Cl) of 9,10-dihaloanthracenes.17
It is noted that the Me, CF3, Ph, and C6F5 groups increase
the kr values and decrease the knr values compared with those
of the parent anthracene. To get an insight into the emission
efficiency, we have examined the relationship between Φf
and magnitude (Aπ) of π conjugation length in the S1 state
of 9,10-disubstituted anthracenes in relation to the theoreti-
cally derived equation18 Φf ) 1/(exp(-Aπ) + 1), where Aπ
) ln(kr /knr).
Scheme 1 (see the Supporting Information). Other bases
(including K3PO4)7 are not effective. Suzuki coupling and
its modified method,8,9 Goossen coupling,10 and copper- and
iron-catalyzed cross coupling using 1,3-bis(2,6-diisopropy-
lphenyl)imidazolinium chloride11-13 were not appropriate for
the synthesis of 6.
The photophysical data of 9,10-disubstituted anthracenes
(1-6)14 together with radiative rate constant (kr), radiation-
less rate constant (knr), and emission lifetime (τ) are sum-
marized in Table 1. Since kr and knr are related to the
corresponding emission quantum yields and lifetimes by Φf
) kr × τ and kr + knr ) τ-1, it is possible to calculate the
values of kr and knr wherever quantum yield and lifetime data
are available.15
As seen in Table 1, both λabs (0-0 and 0-1 bands) and
λem (0-0 and 1-0 bands) values arising from the S0-S1
transition oriented along the short axis16 in 1-6 increased
by meso-substituents regardless of their electron-donating
and -attracting nature, suggesting that the π extension
(7) Do, H.-Q.; Daugulis, O. J. Am. Chem. Soc. 2008, 130, 1128–1129.
(8) Korenaga, T.; Kosaki, T.; Fukumura, R.; Ema, T.; Sakai, T. Org.
Lett. 2005, 7, 4915–4917
.
(9) Ie, Y.; Nitani, M.; Aso, Y. Chem. Lett. 2007, 36, 1326–1327
.
(10) Goossen, L. J.; Deng, G.; Levy, L. M. Science 2006, 313, 662–664.
(11) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5,
2417–2420
.
(12) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics
2004, 23, 1157–1160
.
(13) Hatakeyama, T.; Nakamura, M. J. Am. Chem. Soc. 2007, 129, 9844–
9845
.
(14) The compounds (1-6) were purified by repeated column chroma-
tography followed by recrystallization. Purity was checked by constancy
of the fluorescence intensity at the maximum λem
.
Figure 2. Absorption (left) and fluorescence (right) spectra of 4
(15) (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–
1506.
(red) and 6 (blue) in cyclohexane. All spectra were measured for
10-7 M solution at 295 K in contact with air.
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Org. Lett., Vol. 10, No. 24, 2008

To pursue the photostability of anthracenes (1-6), we
examined the relationship between fluorescence intensity and
irradiation time using the reported apparatus.20 The results
are illustrated in Figure 5. The initial rate of decrease in
fluorescence intensity is also shown in Table 2.
Figure 3. Correlation between fluorescence quantum yield (Φf) and
magnitude (Aπ) of π conjugation length in the S1 state of 9,10-
disubstituted anthracenes. Solid line: theoretical line based on Φf
) 1/(exp (-Aπ) + 1).18
Consequently, we have found that all the plots of Φf
against Aπ fall on the theoretical line (solid line) as shown
in Figure 3. The Aπ values19 obtained from kr and knr values
are shown in Table 1.
The result clearly indicates the increase in the π conjuga-
tion between 9,10-substituents and anthracene ring in the S1
state. This is also supported by the observed linear relation-
ship with a positive slope between λem and Aπ (Figure 4).
Figure 5. Relationship between fluorescence intensity and irradia-
tion time: 1 (black), 2 (green), 3 (pink), 4 (light blue), 5 (dark blue),
6 (purple) Photoirradiation conditions: High-pressure Hg lamp (100
W) was placed 50 cm from the sample tube. The sample was
dissolved in cyclohexane in the presence of air. Concentration of
all anthracenes was 1 × 10-6 M.
Both Figure 5 and Table 2 coincidentally indicate that the
photostability is in the following order: 6 . 5 > 4 > 1 > 2
> 3. Thus, the C6F5 group exerts an excellent effect on
photostability, although CF3 and F groups are not so effective
compared with the reference anthracenes.
Table 2. Initial Rate of Decrease in Fluorescence Intensity (h-1
)
(Order of Photostability)
compd (X)
6 (C6F5) 5 (Ph) 4 (CF3) 1 (H) 2 (F) 3 (Me)
) 0.005 0.056 0.062 0.085 0.163 0.596
initial rate (h-1
It is worth noting that 9,10-di(pentafluorophenyl)an-
thracene is much superior to the others with respect to both
emission efficiency and photostability because high photo-
stability is known to be incompatible with high emissivity.20
From HPLC, NMR, and peroxide tests of the sample
solution under irradiation (see the Supporting Information),
it is demonstrated that the decrease in fluorescence intensity
is ascribed to the formation of the endoperoxide (abbreviated
as EP). It is observed that EP is then converted to the
corresponding 10-hydroxyanthrone, and eventually to an-
thraquinone for 1 (X ) H) and even for 4 (X ) CF3). It is
interesting that our photooxygenation products are very
similar to those of the photosensitized oxygenation of
anthracenes.21
Figure 4. Correlation between fluorescence emission maximum
(λem) and Aπ.
In regard to the solvent effect on photophysical properties
of 9,10-di-FCG-substituted anthracenes (2, 4,and 6), we have
observed that the Φf, λem, and λabs values are slightly
increased with solvent polarity (see the Supporting Informa-
tion).
(16) Valeur, B. Molecular Fluorescence; Wiley-VCH: Weinheim, 2002;
p 27.
(17) Krasovitskii, B. M.; Bolotin, B. M. Organic Luminescent Materials;
VCH: Weinheim, Germany, 1988; p 26.
(18) Yamaguchi, Y.; Matsubara, Y.; Ochi, T.; Wakamiya, T.; Yoshida,
Z. J. Am. Chem. Soc. 2008, 130, 13867–13869.
(19) Aπ values range from positive values to negative values because
of the logarithmic values. However, the negative sign only means the value
is smaller than the positive one. The absolute Aπ value could be given by
ln (kr/knr) - {ln (kr/knr) at Φf ) 0.01}. The situation is similar to the case
of temperature. The positive values (practical Aπ, Aπ*) can be obtained by
Aπ* ) Aπ + 4.6, which is given by Aπ ) ln (kr/knr) - {ln (kr/kd) at Φf )
0.01}.18
The steric energy (kcal/mol) of EP of 9,10-di-FCG-
substituted anthracenes (a measure for the difficulty in
EP formation) is as follows: 51.9 for EP of 6 > 39.5 for
EP of 4 > 32.3 for EP of 5 > 32.2 for EP of 2 > 21.5
for EP of 1.
Org. Lett., Vol. 10, No. 24, 2008
5543

Interestingly a linear relationship seems to be observed
between the initial rate of fluorescence intensity decrease
and the MM2 steric energy of EP of 9,10-di-FCG-substituted
anthracenes as shown in Figure 6, although the electronic
effect of substituent should also be taken into consideration
for EP formation.
In conclusion, (1) we prepared 6 by a Cu-catalyzed C-C
cross-coupling between pentafluorobenzene and 9,10-dibromo-
anthracene. (2) The values of Φf and λem are greatly increased
by the meso-substitution with FCG. (3) We found a nice
correlation between Φf and Aπ. (4) The C6F5 groups at the meso
positions exhibit an excellent ability in the photostability as well
as in the emission efficiency. (5) The decrease in the fluores-
cence intensity is found to be due to the formation of 9,10-
endoperoxide. The initial rate of decrease in fluorescence
intensity correlates to the steric energy of the endoperoxides of
9,10-di-FCG-substituted anthracenes.
Supporting Information Available: Synthesis, NMR
data, HRMS data, absorption and fluorescence spectra,
solvent effect, photooxygenation products, and MM2 steric
energy. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL802337V
Figure 6. Relationship between initial rate of fluorescence intensity
decrease and MM2 steric energy of EP of 9,10-di-FCG-substituted
anthracenes.
(20) Matsubara, Y.; Matsuda, T.; Hatta, S.; Yamaguchi, Y.; Yoshida,
Z. AdV. Mater. 2002, 14, 1211–1213.
(21) Kotani, H.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004,
126, 15999–16006.
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Org. Lett., Vol. 10, No. 24, 2008