916
Chemistry Letters 2001
Multicolor Fluorescent π-Conjugated Oligomer Having Salicylideneaniline Moieties
Ryoji Nomura,* Yoshiki So, Atsushi Izumi, Yusuke Nishihara,† Katsumi Yoshino,† and Toshio Masuda*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501
†Department of Electronic Engineering, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871
(Received June 19, 2001; CL-010582)
A multicolor luminescence active π-conjugated oligomer,
in which the emission color can be controlled by changing the
wavelength of the excitation light, was synthesized by the con-
densation of 1,2-phenylenediamine with 5-dodecyloxy-2-
hydroxyterephthalaldehyde.
The fluorescence of molecules originates from the decay
process from the singlet excited state to the ground state, and if
the emission is sufficiently strong and appears in the visible
region, such fluorescent molecules are applicable as light-emit-
ting materials. Since the luminescence is readily detected even
by the naked eye, fluorescent molecules are utilized as sensors,
displays, probes and so on. In the present study, we synthe-
sized a new π-conjugated oligomer having salicylideneaniline
moieties in the main chain and found that its emission color
changes from green to yellow, and in addition, to red-orange,
by changing the wavelength of the irradiation light, that is, by
simply varying the photoexcitation energy.
The target molecule (1a) is an oligomer prepared by the
condensation of 5-dodecyloxy-2-hydroxyterephthaldehyde (2)
with o-phenylenediamine (Scheme 1). The incorporation of a
dodecyloxy group is to enhance the solubility of 1a because the
aromatic polyazomethines generally show very poor solubility.
The introduction of a hydroxy group at the ortho position to the
formyl group of 2 is to allow the oligomer 1a to undergo excit-
ed-state intramolecular proton transfer (ESIPT).1 ESIPT is a
photochemically induced tautomerization process of molecules
having a cyclic hydrogen bond. Photochemical excitation of
the ground state of such molecules is followed by an extremely
rapid tautomerization process (keto–enol tautomerization) to
give energetically more stable excited-state tautomers. The tau-
tomers radiatively or nonradiatively relax to the metastable
ground state which is then thermally converted to the normal
form. If the relaxation of the excited-state tautomers is accom-
panied with a radiation process, fluorescence with a large
Stokes shift is attainable. Therefore, the loss of the emission
energy by the reabsorption of the emitted fluorescence photons
can be reduced.
Scheme 1 shows the synthetic route to the monomers (2 or
3) and the oligomer (1).3 The synthesis of monomer 2 involves
the alkylation of 4-hydroxyphenol, iodination of the 2- and 5-
positions of the aromatic ring, formylation of the C–I bonds,
and the removal of the methyl group in the presence of LiCl in
N,N-dimethylformamide.4 The oligomerization was conducted
in a mixed solvent (hexamethylphosphoramide and N-methyl-2-
pyrrolidinone) in the presence of LiCl as a condensation agent.2
Orange-colored oligomer 1a was obtained in 49% yield as a
methanol/water insoluble part, and its molecular weight and
polydispersity estimated by GPC (polystyrene standards) were
1690 and 1.60, respectively.
Oligomer 1a displayed very unique fluorescence behavior.
As shown in Figure 1a, the UV–visible spectrum exhibited a
broad absorption with two major peaks around 310 and 360 nm.
When 1a was excited by UV light (360 nm) in THF, four fluo-
rescent peaks were observed at 390, 418, 440, and 527 nm.5
Because the former three peaks do not significantly contribute
to its luminescence color, the fluorescence with a maximum
centered at 527 nm corresponded to the green emission that was
clearly recognized by the naked eye. Irradiation at 390 nm
gave three emission peaks at 418, 440, and 517 nm, similarly to
the photoirradiation at 360 nm. However, a shoulder was
detected at 558 nm in the emission peak in the visible region
(470–700 nm). This slight spectral difference caused a drastic
change in the emission color. Namely, irradiation at 390 nm
results in yellow emission. Emphasis should be placed on the
fact that the further decrease in the photoexcitation energy to
420 nm provided red-orange emission with a fluorescent peak
centered at 576 nm. Figure 2 clearly demonstrates the fluores-
cence color variation upon manipulation of the photoexcitation
energy: green, yellow, and red-orange colors are readily
Copyright © 2001 The Chemical Society of Japan