Unusual fluorescent properties of 3,4,6-triphenyl-α-pyrones

Article abstract of DOI:10.1246/cl.2001.8

Novel fluorophores, 3,4,6-triphenyl-α-pyrone derivatives, were synthesized and their spectroscopic properties have been investigated. In the solid state they show intense greenish-yellow fluorescence, but not in solution. It is concluded that the 6-phenyl

Full text of DOI:10.1246/cl.2001.8

8
Chemistry Letters 2001
Unusual Fluorescent Properties of 3,4,6-Triphenyl-α-pyrones
Keisuke Hirano, Satoshi Minakata, and Mitsuo Komatsu*
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
(Reveived September 22, 2000; CL-000880)
Novel fluorophores, 3,4,6-triphenyl-α-pyrone derivatives,
were synthesized and their spectroscopic properties have been
investigated. In the solid state they show intense greenish-yel-
low fluorescence, but not in solution. It is concluded that the 6-
phenyl group is fixed at an angle of about 26° to the pyrone
plane in the solid state by molecular packing and this fixation is
required for this unique fluorescent property.
The α-pyrone structure is one of basic skeletons found in
natural products and is also well-known as a dye in yellow
flowers.1,2 This chromophore has been known to be biological-
ly active since Corey carried out Diels–Alder reaction between
the diene portions of α-pyrone and dienophiles.3 It was recent-
ly found that a low molecular weight α-pyrone functions as an
inhibitor for HIV-protease, which makes this compound of even
more interest from the biological point of view.4 However only
a few reports have appeared in which the α-pyrone derivatives
are used as functional materials.
Recently we found that some α-pyrone derivatives show
intense fluorescence, but only in the solid state. Fluorescence
has been a major topic of recent research for photofunctional
materials, and has found wide application in a variety of fields,
such as photochemistry, laser chemistry, fluorescent probes,
electroluminescence devices and related areas.5 In this paper,
we report the synthesis of the novel functional fluorophores,
derivatives of 3,4,6-triphenyl-α-pyrone, and their unusual spec-
troscopic properties.
nm, compared with those in dichloromethane. Pyrones 3d and
3f whose Y groups were electron-donating, were red-shifted,
compared with 3a and 3b. Based on the above, we conclude
that the intramolecular charge-transfer occurs from an electron-
releasing group to the lactone moiety which is enrolled as an
electron-withdrawing group. However, pyrone 3e whose Y
group was a nitro group, showed a red shift in its absorption
band, compared with that of 3d. It is assumed that elongated
conjugation by the nitro group affects this spectrum change.
Interestingly, the α-pyrone derivatives did not emit fluores-
cence in solutions at all. Only a few reports have appeared in
which such a type of fluorescent property is expressed, so far as
we are aware. Tris(8-hydroxyquinolino)aluminum (Alq3),
which is used in electroluminescence devices showed a largely
similar phenomenon in which an intense fluorescence was
observed in the solid state and only weak one in solution.7
Evaporated films of the α-pyrone derivatives on glass plates
were prepared and their spectroscopic properties were investi-
gated. The results are listed in Table 2. Compared with the
solution properties, these evaporated films showed a slight red-
shift in their absorption maxima, but tendency of the shift by Y
was similar to that in solutions. These films all showed fluores-
cence properties and their large Stokes shifts of about 150 nm
were assumed to be caused by contributions of intermolecular
interactions. The relative fluorescence intensity was evaluated
in the solid state with excitation at 380 nm using Alq3 as the
The title compounds were synthesized using sulfonium
ylides.6 The sulfonium ylides 1 were prepared in situ from the
corresponding sulfonium salts followed by reactions with
diphenylcyclopropenone 2 to give the desired compounds 3.
The results are summarized in Table 1. In the case where
Y was NO2, the yield of 3e was slightly lower than those of the
other derivatives. It is assumed that nucleophilicity of the sul-
fonium ylide 1c toward 2 is weak because of the presence of the
electron-withdrawing substituent. The α-pyrone derivatives
thus synthesized were yellow solids and emitted a greenish yel-
low fluorescence. The only exception was 3f which appeared
orange and emitted a reddish fluorescence.
The absorption spectra of the α-pyrone derivatives were
measured in dichloromethane and acetonitrile. The absorption
maxima in acetonitrile showed a slight blue shift by about a few
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
9
This suggests that the phenyl group is fixed at an angle of about
26º to the pyrone plane in the solid state by molecular packing
and the path of fluorescence was thus increased. On the other
hand, in solution, the free rotation of the 6-phenyl group pro-
vides a path for non-radiative decay, and, hence, the α-pyrone
derivatives do not fluoresce, when in solution.
In summary, novel fluorophores, α-pyrone derivatives,
were synthesized and their spectroscopic properties were inves-
tigated. An electron-donating substituent on the 6-phenyl
group, Y, causes a red-shift in the absorption and a fluorescent
maximum. The unusual property of fluorescence only in the
solid state can be explained by the fact that the 6-phenyl group
is fixed at an angle of about 26° to pyrone plane by the molecu-
lar packing, which caused intense fluorescence in the solid
state. Fluorescent material in the solid state has potential for
use as a fluorescent pigment and in EL devices. These issues
are currently under investigation.
reference. Some α-pyrone derivatives showed more intense
fluorescence than Alq3 as shown in Figure 1. Therefore these
compounds are of considerable promise for use in EL applica-
tions instead of Alq3.
X-ray structure analysis was performed on 3a and Figure 2
shows the molecular packing diagram.8 Two phenyl groups at
the 3- and 4-positions were twisted from the plane of the pyrone
ring about 50°. On the other hand, the other one at 6-position is
also twisted out of the pyrone plane by only 26°. This figure
clearly showed that the free rotation of the 6-phenyl group was
blocked because of the rather short intermolecular distance
between molecules. Based on these results, it is assumed that
the behavior of the phenyl group is relevant to the unusual fluo-
rescent properties observed, i.e. emission only in the solid state.
References and Notes
1
R. A. Hill, “Progress in the Chemistry of Organic Natural
Products”; Speringer-Verlag, Weinheim-New York (1986);
Vol. 49, pp. 1–78.
2
3
4
5
L. A. Collect, M. T. Davies-Coleman, and D. E. A. Rivett,
Prog. Chem. Org. Nat. Prod., 75, 181 (1998).
E. J. Corey and A. P. Kozikowski, Tetrahedron Lett., 48,
5373 (1983).
D. L. Romero, P. R. Menninen, F. Han, and A. G. Romero,
J. Org. Chem., 64, 4980 (1999).
K. Araki, T. Mutai, Y. Shigematsu, M. Yamada, T.
Nakajima, S. Kuroda, and I. Shimano, J. Chem. Soc.,
Perkin Trans. 2, 1996, 613.
6
7
Y. Hayashi and H. Nozaki, Tetrahedron, 27, 3085 (1971).
C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett., 51, 913
(1987).
8
X-ray crystallographic data : C23H16O2, MW = 324.38, yel-
low, prismatic, monoclinic, space group P21/c (#14), a =
18.267(4) Å, b = 20.298(5) Å, c = 8.975(3) Å, β = 90.01
(3)°, V = 3327(1) Å3, Z = 8, Dc 1.295 g/cm3, F(000) =
1360.00, µ(Mo Kα) = 0.81 cm–1, graphite monochromated
Mo Kα (λ = 0.71069 Å), T = 23 °C, Final discrepancy fac-
tor: R = 0.062 and Rw = 0.105 This crystal consists of
crystallographically independent two molecules in the
asymmetrical unit. The structure was solved by direct
method (MITHRIL90).