Fluorescence spectroscopic characterization of 4,7-bis(2-thienyl)-1,2,5-oxadiazolo[3,4-c]pyridine; lead structure of new red-emitting EL material

Article abstract of DOI:10.1016/S0009-2614(02)00147-1

The spectroscopic parameters of 4,7-bis(2-thienyl)-1,2,5-oxadiazolo[3,4-c]pyridine (DTOP) were determined which was designed as a red-emitting dye for the electroluminescent (EL) device. The main optical transition of DTOP is attributable to ¹L_a, and therefore, the fluorescence maximum shifted to the red side to reach to 630 nm according to the solvents. Although the non-radiative transition rate was enhanced, when the fluorescence maximum shifts to the longer than 610 nm, DTOP maintains the higher radiation transition probability.

Full text of DOI:10.1016/S0009-2614(02)00147-1

5 March 2002
Chemical Physics Letters 354 (2002) 173–178
www.elsevier.com/locate/cplett
Fluorescence spectroscopic characterization of
4,7-bis(2-thienyl)-1,2,5-oxadiazolo[3,4-c]pyridine;
lead structure of new red-emitting EL material
a,
a
a
b
Toshiaki Koga *, Akiara Takase , Seiji Yasuda , Shoji Yamashita ,
c
Hideki Gorohmaru , Thies Thiemann , Shuntaro Mataka ,
d
d
e
Kazufumi Takahashi
a
National Institute of Advanced Industrial Science and Technology, Kyushu, Shuku-machi, Tosu, Saga 841-0052, Japan
Faculty of Agriculture, Institute of Biophysics, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
b
c
Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-koh-en,
Kasuga, Fukuoka 816-8580, Japan
d
Institute of Advanced Material Study, Kyushu University, Kasuga-koh-en, Kasuga, Fukuoka 816-8580, Japan
e
Interdisciplinary Graduate School of Science and Engineering, Shimane University, Nishikawatsu-cho,
Matsue, Shimane 690-8504, Japan
Received 19 June 2001; in final form 16 January 2002
Abstract
The spectroscopic parameters of 4,7-bis(2-thienyl)-1,2,5-oxadiazolo[3,4-c]pyridine (DTOP) were determined which
was designed as a red-emitting dye for the electroluminescent (EL) device. The main optical transition of DTOP is
attributable to 1La, and therefore, the fluorescence maximum shifted to the red side to reach to 630 nm according to the
solvents. Although the non-radiative transition rate was enhanced, when the fluorescence maximum shifts to the longer
than 610 nm, DTOP maintains the higher radiation transition probability. Ó 2002 Published by Elsevier Science B.V.
Abbreviations: DTOP – 4,7-bis(2-thienyl)-1,2,5-oxadiazolo[3,4-c]pyridine; EL – electroluminescence; FWHM – full width at half
maximum
1. Introduction
in the construction of useful displays are the design
of new emitter materials and the fabrication of
material layers [1–6]. Above all, the development
of red-emitting materials for practical use in full-
color display panels [7]is an important target.
The thienyl-substituted oxadiazolopyridine,
4,7-bis(2-thienyl)-1,2,5-oxadiazolo[3,4-c]pyridine
(DTOP) (Fig. 1), is a newly designed material with
a basic skeleton suitable for a red-emitting dye.
Organic electroluminescent (EL) materials have
attracted a lot of attention due to their direct ap-
plication in display devices. The immediate goals
* Corresponding author. Fax: +81-942-81-3690.
E-mail address: t-koga@aist.go.jp (T. Koga).
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0009-2614(02)00147-1

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T. Koga et al. / Chemical Physics Letters 354 (2002) 173–178
adiazole-N-oxide, which was deoxygenated to the
corresponding 1,2,5-oxadiazole with powdered Zn.
The ring-closure to 1,2,5-oxadiazolo[3,4-c]pyridine
was carried out with ethyl glycinate in n-butanol,
giving a mixture of the ethyl and butyl esters.
DTOP was obtained by the hydrolysis of these
esters and subsequent decarboxylation. The purity
and molecular structure of DTOP were confirmed
1
by melting point, IR spectrum, H and 13C NMR
spectra, mass spectrum, and elemental analysis.
Absorption and fluorescence excitation spectra
were measured with a HITACHI 220A spectro-
photometer and a HITACHI F3010 fluorescence
spectrophotometer, respectively. Steady-state flu-
orescence spectra were recorded on a home-made
fluorescence measurement system equipped with a
CCD camera, in order to obtain the corrected
spectra. Measurements of time-resolved fluores-
cence spectra were performed with an ultra-short
pulse laser system (Spectra-Physics) [13]. DTOP
samples were excited at 450 nm (pulse width: ꢀ100
fs, pulse energy: <1 nJ). The fluorescence signals
were detected with a streak scope (Hamamatsu
Photonics, C4334) in a single photon counting
mode. All spectroscopic measurements were per-
formed at 20 °C. Optical densities of the sample
solutions were prepared to less than 0.1 at their
absorption maximum to avoid any distortion of
the spectra.
Fig. 1. Molecular structure of 4,7-di(2-thienyl)-1,2,5-oxadiaz-
olo[3,4-c]pyridine (DTOP).
The molecule possesses a widely conjugated p-
electron system and therefore the excited singlet
state emitting the fluorescence is expected to be
shifted to a lower energy level. The red fluores-
cence of DTOP is recognized in the solid state and
is expected to be a lead structure for molecules of
great potential as red-emitting materials. Indeed,
EL device in which DTOP derivative is incorpo-
rated as a dopant has been achieved to emit
wavelength around 670 nm [8]. However, many
problems remain to be resolved, because the EL
efficiency is low and it is about 1/10 compared with
Alq3 device which is the standard EL device
emitting green light.
Here, the fundamental spectroscopic properties
of DTOP were determined in order to obtain fur-
ther information for designing the improved red-
emitter dyes on the basis of this structure. Close
measurements of the absorption and fluorescence
spectra, the fluorescence quantum yield, and the
fluorescence lifetime were carried out in this study,
because the spectroscopic characterization of
newly synthesized organic molecules is frequently
accompanied by some difficulties due to the com-
plicated behavior of their excited states [9–11].
3. Results and discussion
Fig. 2 shows the absorption spectrum of DTOP
and its second derivative in hexane. The absorp-
tion spectrum is composed of a broad band with a
full width at half maximum (FWHM) of 91 nm
and a non-symmetric tailing towards the higher
energy side. Though the absorption maximum was
observed at 458 nm, the second derivative spec-
trum suggests that a main transition component
was centered at 465 nm between sub-components
at 440 and 495 nm. Furthermore, some small
transition components can contribute to the ab-
sorption spectrum at the higher energy side. The
fluorescence excitation spectrum, monitored at
590 nm, was quite similar to the absorption
spectrum, but the tailing seen at the higher energy
2. Experimental
DTOP was synthesized according to methods
reported previously [12], although reaction condi-
tions and the experimental procedures were mod-
ified in order to protect the fragile thiophene ring.
By treatment with nitric acid, 2-acetylthiophene
was transformed to 3,4-bis(2-thienoyl)-1,2,5-ox-

T. Koga et al. / Chemical Physics Letters 354 (2002) 173–178
175
Fig. 3. Fluorescence spectra of DTOP. Excitation wavelength,
460 nm. Hex – hexane; diEth – diethyl ether; PhH – benzene;
Ace – Acetone; pyrid – pyridine; MeOH – methanol; PhNH2 –
aminobenzene.
1
1
there exist two excited singlet states, La and Lb,
and that the fluorescence emission is due to the
transition 1La ! A. It is generally known that
the larger dipole moment is induced at 1La and the
energy level of the emission state is lowered by the
resulting enhanced dipole–dipole interaction with
polar solvents.
Fig. 2. Absorption spectrum and its second derivative of
DTOP in hexane: curve a – absorption spectrum; curve b – the
second derivative.
region (<400 nm) in the absorption spectrum was
not observed in the excitation spectrum. This
finding indicates that this transition has no con-
tribution to the fluorescence emission of DTOP.
Probably, the transition band tailing towards the
higher energy side would be attributable to the
n–pꢁ transition, the resulting n–pꢁ state being
strongly polarized.
The fluorescence spectra of DTOP were inves-
tigated in various organic solvents and the spec-
trum in hexane is shown in Fig. 3. The fluorescence
maximum can be recognized at 564 nm with an
FWHM of 110 nm. The Stokes shift estimated by
the energy difference between the absorption and
fluorescence maximum was ca. 4000 cmꢂ1. The
fluorescence maximum shifted to longer wave-
lengths with an increase in the solvent polarity.
The fluorescence spectrum in methanol showed its
maximum at 616 nm and the Stokes shift is larger
(5600 cmꢂ1) than in the spectra in hexane. The
observed solvent polarity dependence of the ab-
sorption and fluorescence spectra suggests that
When the wavelength of the fluorescence max-
imum of DTOP is plotted against the dielectric
constant of various solvents, a dependence of the
wavelength of the maximum on the polarity of
the solvent becomes evident (see Fig. 4). Thus, the
fluorescence maximum was observed at 564 nm in
hexane (D ¼ 1:9) and at 611 nm in chloroform
(D ¼ 4:8). In the more polar solvents with a di-
electric constant higher than D ¼ 4:8, no further
shift could be noted. Exceptions were quinoline
and benzyl alcohol, which showed the maximum
at 625 and 628 nm, respectively. In solvents par-
ticipating in hydrogen bonding interaction such as
methanol, ethanol, and propanol, the fluorescence
due to an n–pꢁ transition normally shifts to shorter
wavelengths, on the other hand the maximum due
to the p–pꢁ transition shifts to longer wavelengths.
The spectral shift in the fluorescence of DTOP in
these solvents followed this trend.
The fluorescence quantum yields (/) of DTOP
were determined by using rhodamine-B in ethanol

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T. Koga et al. / Chemical Physics Letters 354 (2002) 173–178
DTOP was dependent on the solvent polarity.
The plots of fluorescence quantum yields against
the dielectric constants of the solvents are given in
Fig. 4. It is interesting to note that the fluores-
cence quantum yields decreased almost linearly
with the increase in the solvent polarity. The
fluorescence of DTOP was suppressed in alcoholic
solvents. While the fluorescence maximum was
seen at almost the same wavelength for DTOP in
acetone and propanol, of which dielectric con-
stants are nearly equal, the fluorescence quantum
yields in acetone and propanol were very differ-
ent, namely 0.39 and 0.22, respectively. The for-
mation of the hydrogen bond between the
hydroxy group of the alcohol and the O-atom in
the oxadiazolopyridine ring should quench the
fluorescence of DTOP. Further solvent effects due
to specific molecular interaction between the ex-
cited DTOP and the solvents were noted. No
fluorescence due to DTOP could be detected in
aniline. The amino derivatives of benzene, espe-
cially methyl-substituted aminobenzenes, are
known to be the strong electron-donors. Most
likely, the formation of a charge transfer complex
in the excited state between aniline and DTOP
Fig. 4. Solvent polarity dependence of the fluorescence maxi-
mum wavelength of DTOP. The maximum position was de-
termined from the fluorescence spectrum of DTOP in the
various solvents listed in Table 1.
as a fluorescence standard (/ ¼ 0:97). These were
summarized together with other optical parame-
ters in Table 1. The fluorescence quantum yield of
Table 1
Spectroscopic parameters of DTOP in various solutions
Abs.
(nm)
Em.
(nm)
/
s (ns)
550–600 nm
sr
(ns)
si
(ns)
Solvent
D
(20 °C)
Max
FWHM
Max
FWHM
Hexane
CycloHex
CCl4
1.9
458
461
465
465
465
462
464
474
477
472
473
466
461
460
460
476
471
90
88
91
91
92
92
93
103
97
96
98
92
93
94
94
91
105
564
567
580
601
600
587
611
–
110
109
115
120
118
117
132
–
0.54
0.54
0.43
0.40
0.35
0.52
0.35
–
5.72
5.70
6.60
6.50
6.50
6.98
6.21
–
10.59
10.56
15.35
16.25
18.57
13.42
17.74
–
12.43
12.39
11.58
10.80
10.00
14.36
9.56
–
2.02
2.24
2.28
2.38
4.34
4.81
6.89
9.0
Benzene
PhCH3
DiEth
CHCl3
PhNH2
Quinoline
Pyridine
PhCH2OH
PrOH
625
617
628
611
612
614
616
614
633
136
131
108
135
128
135
135
125
148
0.25
0.23
0.12
0.22
0.39
0.19
0.15
0.20
0.03
5.28
5.70
2.96
4.11
6.57
4.22
3.01
6.16
1.88
21.12
24.78
24.67
18.68
16.85
22.18
20.06
30.80
62.27
7.04
7.43
3.36
5.27
10.76
5.21
3.54
7.71
1.94
12.3
13.1
20.1
20.7
24.3
32.6
34.84
42.5
Acetone
EtOH
MeOH
PhNO2
GlyOH
D – dielectric constant, Abs. – absorption; Em. – fluorescence excited at 460 nm; / – fluorescence quantum yield; s – natural
fluorescence lifetime; sr – calculated radiative lifetime; si – non-radiative lifetime.

T. Koga et al. / Chemical Physics Letters 354 (2002) 173–178
177
will perfectly quench the fluorescence emission of
DTOP.
The measurement of time-resolved fluorescence
of DTOP was performed by using a combination
of a sub-pico second laser and a streak scope. The
fluorescence decay curve of DTOP could best be
fitted to a single exponential function for all sol-
vents examined here. According to the Strickler–
Berg equation (Eq. (1)), the radiative lifetime (sr) is
given by the ratio of the natural lifetime (s) versus
the fluorescence quantum yield (/) [14]
sr ¼ s=/:
ð1Þ
The radiative lifetime is an important spectro-
scopic parameter to give the transition probability
of S1 ! S0. DTOP gave sr ¼ 10:59 ns in hexane.
Since this value corresponds to an oscillator
strength of (f) ꢀ10ꢂ1, it is clear that DTOP has a
very high transition probability around 664 nm.
The radiative lifetimes were plotted against the
fluorescence maximum wavelength. They in-
creased linearly with a shift of the fluorescence
maximum to longer wavelengths. However, all
values are in the range of 10–30 ns. These results
show that the radiative transition probability is
lowered with a decrease of energy difference be-
tween the lowest excited singlet state and the
ground state and, at the same time, that the lowest
singlet state emitting the fluorescence is attribut-
1
able to the La state in the p–pꢁ transition.
By using the natural lifetime (s) and the radia-
tive lifetime (sr), the non-radiative lifetime (si) can
be calculated (Eq. (2)),
1=s ¼ ð1=srÞ þ ð1=siÞ:
ð2Þ
The obtained sr and si are summarized in Table 1.
When si was plotted against the fluorescence
maximum wavelength of DTOP in various sol-
vents, it can be seen that the non-radiative lifetime
of DTOP decreases rapidly at 610 nm (Fig. 5).
This result demonstrates that some relaxation
channel is opened at this energy level. The triplet
states n–pꢁ and/or p–pꢁ are situated between the
lowest excited singlet state and the ground state.
Fig. 5. Plots of non-radiative transition time (a) and radiative
transition time (b) of DTOP against the fluorescence maximum
wavelength. sr and si were calculated based on Eqs. (1) and (2)
using the experimentally decided fluorescence quantum yield
and natural lifetime (see text).
4. Conclusion
1
When the energy level of La state is lowered to
A new organic molecule, DTOP, has been syn-
thesized. Its dominant feature is a heterocyclic ring,
which includes O- and N-atoms. The spectral line
around 610 nm by dipole–dipole interaction, the
intersystem crossing rate to the triplet state of
DTOP is remarkably enhanced.

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T. Koga et al. / Chemical Physics Letters 354 (2002) 173–178
[2]L. Smilowitz, A. Hays, A.J. Heeger, G. Wang, J.E. Bowers,
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shapes of absorption and fluorescence are broad
and structureless even in non-polar solvents, similar
to spectra of strongly polarized species. Regardless,
the ordinal transition of DTOP can be attributed to
p–pꢁ and the fluorescence emitting state is 1La which
has a high radiative transition probability covering
a wide wavelength region. The spectroscopic prop-
erties measured above indicate that DTOP has po-
tential as a red-emitter dye useful in EL devices.
[3]K. Tada, H. Sawada, J. Kyokane, K. Yoshino, Jpn. J.
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Acknowledgements
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This research has been supported by the grant of
R&D consortium project on organic EL project.
We thank Dr. S. Saito and Research and Devel-
opment Laboratory of Kyushu Matsushita Elecrtic
Co., Ltd for helpful discussion on EL devices.
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