effective in generating molecular glass materials. However,
besides preventing molecules from crystallization, none of
these structural moieties or shapes per se have a direct control
on the photophysical properties of the fluorophores. In this
Letter, we take one step further to show a molecular strategy,
namely the ortho-substitutent effect of arylamine, which can
effectively enhance fluorescence in the solid state while
preserving the intrinsic Φf in solution with a concomitant
hypsochromic emission shift. The structural strategy is
successfully applied to two kinds of fluorophores, coumarin
and stilbene. In addition, bright and efficient blue electro-
luminescence (EL) observed for nondoping organic light-
emitting diodes (OLEDs) fabricated from structure-tailored
coumarin and stilbene derivatives further strengthens the
validity of the approach.
treatment of BBr3 to give the corresponding phenols, which
were further condensed with ethyl acetoacetate under a
Pechmann condition to yield the aryl-substituted coumarins.
Low reaction yields were observed if Lewis acids such as
zinc chloride were used as catalysts for the condensation.
PhC1 displays relatively low fluorescence yield (Φf )
48%) and a large bathochromic shift relative to C1 in
emission wavelength, indicating there is partial conjugation
between the chromen-2-one and the diphenylamino group.
The solid-state fluorescence intensity of PhC1 was improved
relatively (Figure 1), although fluorescence quantum yield
Coumarin 1 (7-diethylamino-4-methylcoumarin, C1) is an
appropriate fluorophore to serve as the molecular platform
to illustrate the strategy owing to its near unit blue
fluorescence efficiency in solution, but weak fluorescence
in the solid state.7 Accordingly, diaryl appended C1 deriva-
tives (denoted by PhC1, MeC1, and ClC1) are synthesized
(Scheme 1). Although Pd-catalyzed aromatic amination
Scheme 1a
Figure 1. Left: Fluorescence emission spectra of Coumarin 1 (C1),
ClC1, MeC1, and PhC1 as solid films on quartz. Right: Color
photographs of the same solid films under UV irradiation (365 nm)
qualitatively revealing relative fluorescence intensity and varied blue
hue.
a Reagents and conditions: (i) 1,2-dichlorobenzene, copper
bronze, 18-crown-6, K2CO3, reflux, 4 days, 85% (X ) H), 77%
(X ) CH3), 30% (X ) Cl); (ii) BBr3, CH2Cl2, -78 °C f rt, 8 h;
(iii) acetoacetate, 70% H2SO4(aq).
in the solid state awaited for measurement. Nevertheless, the
fluorescence emission maximum (λmaxem) of the solid film
appeared at 474 nm corresponding to the greenish blue hue.
The color purity is thus not appropriate for the application
in OLED, strictly requiring good blue-color purity. In the
cases of MeC1 and ClC1, the ortho substituents (i.e., methyl
and chloro groups) of phenyl groups augment the non-
planarity of the triarylamino center to prevent crystallization,
as well as reduce the extent of conjugation between the
chromen-2-one and the diarylamino group.
The structural features are reflected in the photospectro-
scopic and electrochemical data. The fluorescence quantum
yields of MeC1 and ClC1 are 77% and 86%, respectively,
and emission wavelengths are much shorter than that of
PhC1 (Table 1, Figure 1), either in solutions or in solid films.
The gradually increasing oxidation potentials, implying wider
energy gaps between HOMO and LUMO, are observed at
0.56, 0.64, and 0.88 V in the order of PhC1, MeC1, and
ClC1 (Table 1) consistent with blue-shifted wavelengths in
absorption and fluorescence spectra. The LUMO energy level
of the compounds remained mostly the same due to the very
similar reduction potential. This is understandable because
of the reduction site, assuming the carbonyl of chromenone
has the same chemical structure in three molecules. Besides
the energy gap law, the rather pronounced fluorescence
reactions8 are commonly employed in the synthesis of
triarylamines,9 they failed to give 3-bisarylamino anisole, but
only a monosubstituted compound isolated even after a
prolonged reaction time or by the addition of an excess of
chelating or nonchelating phosphine ligands. The aryl groups
were thus introduced to the amino group via the Ullmann
reaction10 starting with 3-methoxyaniline followed by the
(6) For reviews, see: (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1. (b)
Thelakkat, M. Macromol. Mater. Chem. Eng. 2002, 287, 442
(7) Jones, G., II; Jackson, W. R.; Choi, C.-Y.; Bergmark, W. R. J. Phys.
Chem. 1985, 89, 294.
(8) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998,
37, 2046.
(9) For recent examples, see: (a) Yamamoto, T.; Nishiyama, M.; Koie,
Y. Tetrahedron Lett. 1998, 39, 2367. (b) Goodson, F. E.; Hauck, S. I.;
Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 7527. (c) Watanabe, M.;
Nishiyama, M.; Yamamoto, T.; Koie, Y. Tetrahedron Lett. 2000, 41, 481.
(d) Harris, M. C.; Buchwald, S. L. J. Org. Chem. 2000, 65, 5327. (e)
Watanabe, M.; Yamamoto, T.; Nishiyama, M. Chem. Commun. 2000, 133.
(10) (a) Goodbrand, H. B.; Hu, N.-X. J. Org. Chem. 1999, 64, 670. (b)
Bacon, R. G. R.; Hill, H. A. O. Chem. ReV. 1965, 19, 95. (c) Bushby, R.
J.; McGill, D. R.; Ng, K. M.; Taylor, N. J. Mater. Chem. 1997, 7, 2343.
(d) Gauthier, S.; Frechet, J. M. J.; Synthesis, 1987, 383.
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