Novel blue-light-emitting hybrid materials based on oligothiophene acids and ZnO

Article abstract of DOI:10.1016/j.cplett.2004.09.045

Novel blue-light-emitting materials based on ZnO and 2,2′-bithiophene-5,5′-dicarboxylic acid (DTDA), 4′,3″-dipentyl-5,2′: 5′,2″: 5″,2?-quaterthiophene-2,5?-dicarboxylic acid (QTDA) have been prepared. The hybrid materials show that the PL λ_max are at 450 and 425 nm for DTDA-ZnO and QTDA-ZnO, respectively.

Full text of DOI:10.1016/j.cplett.2004.09.045

Chemical Physics Letters 398 (2004) 113–117
www.elsevier.com/locate/cplett
Novel blue-light-emitting hybrid materials based on
oligothiophene acids and ZnO
a
b
a,b
Tonggang Jiu a,b, Huibiao Liu , Liming Fu , Xiaorong He a,b, Ning Wang
,
a,
a
a,
*
Yuliang Li *, Xicheng Ai , Daoben Zhu
a
CAS Key Laboratory of Organic Solids, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Zhongguancun, Beijing 100080, PR China
b
Graduate School of Chinese Academy of Sciences, Beijing, PR China
Received 13 August 2004; in final form 8 September 2004
Available online 5 October 2004
Abstract
Novel blue-light-emitting materials based on ZnO and 2,20-bithiophene-5,50-dicarboxylic acid (DTDA), 40,300-dipentyl-5,20: 50,200:
500,2000-quaterthiophene-2,5000-dicarboxylic acid (QTDA) have been prepared. The hybrid materials show that the PL kmax are at 450
and 425 nm for DTDA–ZnO and QTDA–ZnO, respectively.
ꢀ 2004 Elsevier B.V. All rights reserved.
1. Introduction
properties not to be brought from each component and
are presently largely used for advanced materials with
desirable optical, electrical, and magnetic properties
[2–4]. More recently, the possibility of combining inor-
ganic and organic components in a more intimate way
by means of covalent or non-covalent bonds has opened
wide the application fields of inorganic–organic hybrid
compounds [5]. These strong interactions not only allow
self-assembly processes but also new interesting proper-
ties can arise from the close interactions between both
components. In the present work we successfully de-
signed to synthesize organic/inorganic hybrid nanomate-
rials based on ZnO and thiophene acid oligomers. And
more interesting, the blue-light-emitting (at 450, 425
nm) was observed in the two hybrid materials.
Blue luminescent materials in particular have been of
intense interest in part because they are one of the key
color components required for full-color displays and
are still rare in applying to LEDs appropriately. Pursu-
ing efficient and stable blue-light-emitting material
remains a challenge. The materials based on inorganic
materials are favorable in the full width at half maximum
(FWHM), however, they are unstable over long periods
and the color purity of light-emitting materials is not
ideal for using in blue-light-emitting devices [1]. Poly-
meric light material is not satisfied in the color purity,
too. ZnO is an important electronic and photonic mate-
rial because of its wide band gap and high mechanical
and thermal stabilities at room temperature. Thio-
phene-based monodisperse p-conjugated oligomers have
much interest as active material in field effects transistors
or light-emitting diodes. Mixing organic and inorganic
compounds in an important hybrid material has ap-
peared for a long time as a simple way to acquire specific
2. Experimental
2.1. Materials and measurements
Most chemical reagents were purchased from Acros
or Aldrich Corporation. All solvents were purified using
standard procedures. Column chromatography was
*
Corresponding authors. Fax: +86 10 82616576.
E-mail address: ylli@iccas.ac.cn (Y. Li).
0009-2614/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.09.045

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T. Jiu et al. / Chemical Physics Letters 398 (2004) 113–117
performed on silica gel (size 160–200 meshes). FT–IR
spectra were measured on a Bruker EQUINOX55 spec-
trometer in KBr pellet at room temperature. The UV–
Vis and fluorescence spectra were obtained on a Hitachi
U-3010 and Hitachi F-4500 spectrometer, respectively.
The XRD patterns were recorded with a Japan Rigaku
D/max-2500 rotation anode X-ray diffractometer
equipped with graphite monochromatized Cu Ka radia-
47.23; H, 2.38; Found: C, 47.20; H, 2.39. MS (EI): m/z
1
254. QTDA: H NMR (CDCl3, ppm) d 7.74 (m, 2H),
7.46 (s, 2H), 7.36 (m, 2H), 2.09 (m, 4H), 1.59 (m, 4H),
1.28 (m, 8H), 0.86 (t, 6H, J = 6 Hz); FT–IR (KBr) m
(cmꢀ1) 1666 (C@O); Anal. Calc. for C28H30O4S4 C,
60.18; H, 5.41; Found: C, 60.14; H, 5.42. MS (TOF):
m/z 558.
The hybrid nanomaterials have been prepared by the
reaction of ZnO and thiophene acid oligomers. In a typ-
ical synthesis, 15 mg (0.036 mmol) QTDA/DTDA was
dissolved in 10 ml of aqueous solution of NaOH 7.2
mmol/L. The resulting brown solution were dropped
to the solution of Zn(Ac)2 Æ 2H2O under stirring and
keep for 2.5 h. After the mixture was centrifuged, the
crude product was washed with water and dried at 70
ꢁC in vacuum for 6 h. The QTDA–ZnO/DTDA–ZnO
product was obtained, which was characterized by
transmission electron microscopy (TEM), powders
X-ray diffraction measurement (XRD) and infrared
(IR).
˚
tion (k = 1.54178 A, employing a scanning rate of 0.05ꢁ
sꢀ1 in the 2h range from 10ꢁ to 70ꢁ. Transmission elec-
tron microscopy (TEM) images were collected on a
Hitachi 800 instrument with a 100 keV accelerating volt-
age. Cyclic voltammograms (CV) and photocurrent
were recorded on CHI660B voltammetric analyzer
(CH Instruments, USA). Fluorescence lifetimes were
calculated from time-resolved fluorescence intensity
decays using a photo-counting streak camera (C2909,
amamatstu). This machine uses a femto-second laser
source running at 1 kHz.The laserꢀs output wavelength
can be set to the desired excitation with OPA (OPA-
800CF, Spectra Physics).
Bithiophene acid and quaterthiophene acid were pre-
pared according to the previously reported procedure
[6,7]. The synthesis route was shown in Scheme 1. 3,30-
Dipentyl-2,20-bithiophene was synthesized using bithio-
phene as the starting material through bromination,
debromination and the Grignard coupling reaction with
corresponding reagent. Direct bromination of the
bithiophene derivative with NBS afforded the 80% yield,
after purification. Quaterthiophene was synthesized
from 2-bromothiophene through the Grignard coupling
reaction with its magnesium bromide. DTDA and
QTDA were synthesized from the corresponding bithio-
phene and quaterthiophene derivative by lithiation with
butyllithium and followed by carboxylation with dry ice.
The chemical structures of the organic compounds were
verified by 1H NMR, FT–IR, Mass Spectra and Elemen-
3. Results and discussion
The TEM image (Fig. 1a) shows that the morphology
of DTDA–ZnO is nanocuboids, which the average size
is 100 nm · 250 nm. The size distribution histograms
of nanocuboids are shown in Supplementary files. Fig.
1b displays the morphology of the QTDA–ZnO is nano-
rods. The diameter of these 1D rods is 80–200 nm, and
the length is 250–600 nm. The powders XRD patterns of
DTDA–ZnO and QTDA–ZnO (Fig. 2) show that the
two prepared hybrid ZnO are wurtzite structure (JCPDS
36–1451). In Fig. 2, all diffraction peaks can be indexed
perfectly to this crystal system. The sharp peaks of those
patterns indicate the crystallites in the hybrid materials
are high.
1
tal Analyses. DTDA: H NMR (CDCl3, ppm) d 10.34
Shown in Fig. 3 are the UV–vis spectra recorded in
room temperature colloid of the hybrid materials pre-
pared from the oligothiophene acids and zinc oxide.
(m, 2H), 7.76 (m, 2H), 7.00 (m, 2H); FT–IR (KBr) m
(cmꢀ1) 1664 (C@O); Anal. Calc. for C10H6O4S2 C,
Scheme 1. The synthesis of DTDA and QTDA.

T. Jiu et al. / Chemical Physics Letters 398 (2004) 113–117
115
Fig. 1. TEM images of as prepared ZnO: (a) DTDA–ZnO nanocu-
boids; (b) QTDA–ZnO nanorods.
Fig. 3. Absorption spectra of DTDA–ZnO, QTDA–ZnO in suspen-
sion and DTDA, QTDA in ethanol solution.
Fig. 2. XRD patterns of DTDA–ZnO, QTDA–ZnO.
The absorption spectra of DTDA–ZnO and QTDA–
ZnO exhibit a promontory, centering at 369 and 353
nm. While the absorption peaks of the DTDA and
QTDA are at 331 and 364 nm, respectively. There is
obvious difference from the absorption of ZnO nano-
crystals. It suggests that the hybrid materials containing
ZnO and thiophene acid oligomer are formed.
Fig. 4 illustrates the room temperature photolumines-
cence (PL) spectra recorded DTDA, QTDA and the hy-
brid ZnO, respectively. It is clear that the PL peak of
DTDA–ZnO at 450 nm and the peak of QTDA–ZnO
at 425 nm are in the blue emission region. It is well
known that the PL peaks of ZnO are about 385 and
500 nm, which the peak at 385 nm is resulted from the
direct integration of induced exciton [8], and the green
emission was caused by impurity and different intrinsic
defects in ZnO, such as oxygen and zinc vacancies in
the crystal [9,10], respectively. We observed at the first
the blue emission at 450 and 425 nm of ZnO 1D nano-
materials, respectively, which were only observed in
ZnO film [11]. In ZnO film, the blue emission included
from the donor to the valence band and from the con-
duction band to the acceptor. In Fig. 4 the peaks of
oligothiophene acids disappeared, which may attribute
Fig. 4. The room temperature PL spectra: DTDA (kex = 330 nm),
QTDA (kex = 350 nm) in ethanol, DTDA–ZnO (kex = 325 nm) and
QTDA–ZnO (kex = 345 nm) colloid in ethanol.
to the charge transfer from ZnO to DTDA and QTDA,
as a result that the PL of DTDA and QTDA are
quenched. This is consistent with the next result of
electrochemical calculation. In our design, the oligothio-
phene acids play an ꢁimpurityꢀ role. DTDA and QTDA
are connected with the surface of ZnO and Zn2+ have a
tendency to the surface of ZnO crystal. The excess Zn2+
form the surface fluorescence centers and thereby the
new blue emission band is excitated. The coordination
of DTDA and QTDA make positive Zn2+ ions adsorbed
at the carrier particle promote visible fluorescence by
deforming interatomic bonds, thus it alters the surface
energy level structure of ZnO by charge transfer. There-
fore, the concept of surface fluorescence centers seems to
be more realistic for blue emission band. As for the posi-
tion difference of the two hybrid material PL peaks, this
is maybe result from intrinsic defects depending on
hybrid ZnO forms and preparation conditions.

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T. Jiu et al. / Chemical Physics Letters 398 (2004) 113–117
Table 1
Photophysical Parameters of the hybrid materials (298 K) investigated
in ethanol
Hybrid material
s1 (ns)
s2 (ns)
sav (ns)
kET
DTDA–ZnO
QTDA–ZnO
0.64 (34%)
0.05 (48%)
3.37 (66%)
0.27 (52%)
2.44
0.16
4.1 · 108
6.0 · 109
When the ZnO nanoparticles are irradiated, electrons
of ZnO can be excited from valence band (Ev) to con-
duct band (Ec). The charges at the Ec of ZnO can eas-
ily transfer to the ground state of oligothiophene acids.
The energy levels of oligothiophene acid (HOMO and
LUMO) and ZnO (Ev and Ec) are compatible for
charge transfer. ZnO nanoparticles are as donors,
and DTDA and QTDA are as acceptors in this system.
Charge transfer processes from ZnO to oligothiophene
acids result in the fluorescence of oligothiophene acid
molecules are quenched.
The dynamics of energy-transfer behavior between
the two chromophore units was examined by employing
the time-resolved nanosecond fluorescence spectroscopy
as shown in Fig. 5 and Table 1. The experiments on life-
time measurements are carried out by exciting hybrid
materials at 355 nm and monitoring the visible band.
ZnO emission should be a multiexponential process with
a series of short and long (nanosecond–microsecond)
component. But in the present investigation we did not
observe any emission decay in microsecond time
domain. The both hybrid materials show dual life from
Table 1. For DTDA–ZnO the lifetime spectrum includes
two components with 0.64 and 3.37 ns, respectively.
Similarly the two component of QTDA–ZnO are 0.05
and 0.27 ns. We have determined the average time con-
stant (sav) and energy-transfer rate constants (kET) out
of these two decay components for the hybrids of differ-
ent thiophene acids.
Scheme 2. Electron transfer mechanism of oligothiophene acids and
ZnO.
The direction of charge transfer may be concluded
from energy levels calculated according to electrochem-
ical data. The CV was performed in a solution of
Bu4NPF6 (0.1 M) in DMF at a scan rate of 20 mV/s
at room temperature under the protection of nitrogen.
One platinum electrode was used as the working elec-
trode and the other Pt wire was used as the counter
electrode. An argentum electrode was used as the refer-
ence electrode. The HOMO energy level of DTDA is at
5.96 eV, which is obtained by cyclic voltammetry. Con-
sidering its onset of absorption peak at 386 nm, the
LUMO energy level is at 2.74 eV, and the energy gap
is 3.22 eV. In the same manner, the LUMO energy le-
vel of QTDA is at 2.79 eV, and the energy gap is 2.89
eV from the onset of UV absorption at 431 nm. The
band gap of nanometer scale ZnO is 3.55 eV [12].
The conduction band (Ec) and valence band (Ev) of
ZnO is at ꢀ0.30 and 3.25 eV, respectively, with respect
to the vacuum energy level. The mechanism of the
energy-transfer process between ZnO and oligothio-
phene acids can be proposed (shown in Scheme 2).
Fig. 5. Time-resolved photoluminescence decay profile of DTDA–ZnO and QTDA–ZnO in ethanol (circle).

T. Jiu et al. / Chemical Physics Letters 398 (2004) 113–117
117
4. Conclusion
article can be found, in the online version at
doi:10.1016/j.cplett.2004.09.045.
In conclusion, both DTDA–ZnO and QTDA–ZnO
show strong blue fluorescent emission with high bright-
ness and good efficiencies, and they may be used as an
advanced material for blue-light-emitting diode devices.
The photophysical properties of hybrid materials have
been monitored using steady-state and time-resolved
emission and absorption spectroscopy. The hybrid
materials may make it a candidate for thermally stable
and solvent resistant blue fluorescent material for its
low solubility in most common solvents.
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