Multicolor fluorescent π-conjugated oligomer having salicylideneaniline moieties

Article abstract of DOI:10.1246/cl.2001.916

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 condensation of 1,2-phenylenediamine with 5-dodecyloxy-2-hydroxyterephthalalde

Full text of DOI:10.1246/cl.2001.916

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

Chemistry Letters 2001
917
obtained simply by changing the wavelength of the irradiation
light. The quantum yields of photoluminescence of oligomer
1a in THF were 8.5, 7.9, and 2.1% for green, yellow, and red-
orange emissions, respectively. A similar dependence of emis-
sion color on the photoexcitation energy was observed in
toluene and CHCl3. The emission color also varied in the solid
state (Figure 1c). The excitation of the film of 1a at 380 and 420
nm resulted in green and red-orange emissions, respectively.
emission for oligomers and polymers 1b–1e, were ca. 200 nm.
Since the absorption corresponding to the visible emission can-
not be completely assigned at present, the Stokes shift cannot
be estimated for 1a. However, a very large Stokes shift can be
recognized by the fact that the red-orange emission of 1a does
not overlap with its UV absorption. These findings suggest
that the emission originates from the radiation process from the
excited-state proton-transferred tautomer to its metastable
ground state.1
In summary, we demonstrated a new photoluminescent
organic π-conjugated oligomer that emits multicolor light con-
trollable by changing the photoexcitation energy. As shown in
Figure 1, oligomer 1a exhibits very broad absorption and com-
plicated emission peaks. Thus, 1a is likely to compose plural
fluorescent segments with different structures, which is also
1
suggested by the observation of poorly resolved H NMR reso-
nances of 1a. The results presented in this study indicate that
energy transfer between the fluorescent segments is slow or
does not occur. This result differs from the general feature of
fluorescence, namely, that the emission predominantly origi-
nates from the lowest band due to the rapid energy transfer
between lumophores. Further investigation to clarify the origin
of the multicolor emission is now in progress.
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science,
Culture and Sports, Japan, and the Izumi Science and
Technology Foundation.
Analogous polymers (1b, 1c) were also prepared using m-
and p-phenylenediamines as monomers in 98 and 92% yields
respectively. These polymers were partly soluble in organic
solvents, and the soluble oligomeric part (Mn = 3170 for 1b,
3890 for 1c) showed emission in quantum yields of 0.11 and
0.058%, respectively. In contrast to oligomer 1a, there was no
dependence of the emission wavelength on the photoexcitation
energy: only orange color was observed from both oligomeric
parts irrespective of the irradiation energy. In a similar way,
soluble, high molecular weight polymers, 1d (Mn = 11800, 92%
yield) and 1e (Mn = 7900, 92% yield), were prepared by the
condensation with 3,3’,5,5’-tetramethylbenzidine and bis(4-
amino-3,5-dimethylphenyl)methane, respectively. These poly-
mers displayed orange emission with maximum around 590 nm,
which did not change upon variation of the excitation wave-
length. Thus, ortho-substitution at the diamine moieties
appears to contribute to the multicolor emission. The presence
of a hydroxy group was indispensable for generation of the
emission because the soluble part of oligomer 4 (Mn = 3120)
prepared from 3 and p-phenylenediamine in 93% yield was flu-
orescence inactive. All of the oligomers and polymers 1 hav-
ing a hydroxy group on the aromatic rings showed very large
Stokes shift. For example, the Stokes shifts of the visible
References and Notes
1
a) S. J. Formosinho and L. G. Arnaut, J. Photochem.
Photobiol. A: Chem., 75, 1 (1993). b) S. J. Formosinho
and L. G. Arnaut, J. Photochem. Photobiol. A: Chem., 75,
21 (1993).
2
3
P. W. Morgan, S. L. Kwolek, and T. C. Pletcher,
Macromolecules, 20, 729 (1987).
The structure of the oligomers in Scheme 1 was used only
for simplicity. The oligomers probably contains both
head-to-tail and head-to-head structures with respect to the
unit from the dialdehyde.
4
5
A. M. Bernard, M. R. Ghiani, P. P. Piras, and A. Rivoldini,
Synthesis, 1989, 287 (1989).
Excitation at 310 nm gave a similar emission spectrum.