Synthesis and photophysics of novel 8-hydroxyquinoline aluminum metal dye with hole transfer groups

Article abstract of DOI:10.1016/j.saa.2008.04.026

A novel luminescent dye metal complex, (CZHQ)₃Al, with 8-hydroxyquinoline aluminum and hole-transporting carbazole units was designed and synthesized. The (CZHQ)₃Al optical properties were carefully investigated by UV-vis absorption and fluorescence spectra in diluent solution. The results showed that the luminescent quantum yield of (CZHQ)₃Al was 0.62 in DMSO and it emitted red-light with the band gap of 2.89 eV estimated from the onset absorption. In addition, the light-emission of (CZHQ)₃Al can be quenched by electron acceptor (dimethylterephalate), where the process followed the Stern-Volmer equation. However, the fluorescent intensities of (CZHQ)₃Al were slowly increased with the addition of electron donor (N,N-dimethylaniline). Furthermore, the molecular interactions of (CZHQ)₃Al with fullerene (C₆₀) and carbon nanotubes (CNTs) were also respectively investigated, which indicated the metal dye can be used as new fluorescent probe.

Full text of DOI:10.1016/j.saa.2008.04.026

Spectrochimica Acta Part A 71 (2008) 1433–1437
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and photophysics of novel 8-hydroxyquinoline aluminum metal dye
with hole transfer groups
Xiaoju Wang , Liheng Feng , Zhaobin Chenb
a
b,∗
a
Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science, Shanxi University, Taiyuan 030006, PR China
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel luminescent dye metal complex, (CZHQ) Al, with 8-hydroxyquinoline aluminum and hole-
transporting carbazole units was designed and synthesized. The (CZHQ)3Al optical properties were
carefully investigated by UV–vis absorption and fluorescence spectra in diluent solution. The results
showed that the luminescent quantum yield of (CZHQ)3Al was 0.62 in DMSO and it emitted red-light
with the band gap of 2.89 eV estimated from the onset absorption. In addition, the light-emission of
3
Received 29 December 2007
Received in revised form 22 March 2008
Accepted 22 April 2008
Keywords:
(CZHQ)3Al can be quenched by electron acceptor (dimethylterephalate), where the process followed the
8
-Hydroxyquinoline
Stern–Volmer equation. However, the fluorescent intensities of (CZHQ)3Al were slowly increased with the
addition of electron donor (N,N-dimethylaniline). Furthermore, the molecular interactions of (CZHQ)3Al
with fullerene (C60) and carbon nanotubes (CNTs) were also respectively investigated, which indicated
the metal dye can be used as new fluorescent probe.
Luminescent dye
Carbazole
Photophysics
Synthesis
©
2008 Elsevier B.V. All rights reserved.
1. Introduction
nificant applications in a variety of investigations involving metal
complexes. 8-Hydroxyquinoline aluminum (Alq ) has been widely
3
Recently, the field of research about photoluminescent (PL) and
electroluminescent (EL) materials is growing rapidly due to the
versatility and practical applications of these compounds. Espe-
cially, photoluminescent and electroluminescent devices based on
organic thin layers have attracted much attention because of the
potential application to large-area flat-panel displays and light-
emitting diodes (LEDs) [1,2].
used as the emissive and electron-transporting material in organic
light-emitting devices (OLEDs) [14–16]. On the aspect, Professor
H. Tian et al. successfully obtained multifunctional material com-
pound of hole-transporting group and Alq3 by large ␲-conjugated
system [17]. However, it is well known that the excellent lumines-
cent material properties, such as efficiency, brightness, plasticity,
lie on discontinuous ␲-conjugated system on the extent.
Much progress has been made in recent years to improve the
efficiencies of the EL devices through ingenious device config-
urations, electrode modification, and new material discoveries
Herein, in order to improve the transporting performance of 8-
hydroxyquinoline aluminum, we designed and synthesized a new
metal complex, (CZHQ) Al, with 8-hydroxyquinoline aluminum
3
[
3–5]. Numerous fluorescent dyes and charge carriers have been
and hole-transporting carbazole groups. The optical properties
were investigated by UV–vis absorption and fluorescence emission
developed, yet the efforts are still going on in terms of color tun-
ing, higher efficiency, and durability. Low LE efficiency in LEDs is
attributed mainly to an imbalance in the transportation rates of
the electrons and holes in LE layers [6–9]. To overcome the prob-
lem of low LE efficiency, it is urgent and perfect for combining
the electron-injecting group, hole-transporting group and chro-
mophore in the same molecule. Carbazole and its derivatives are
good hole-transport materials and have been used in construction
of light-emitting device (LEDs) [10–13]. 8-Hydroxyquinoline is one
of the most important chelators for metal ions and has found sig-
spectra. Additionally, the interactions of (CZHQ) Al with electron
3
donor or electron acceptor have been carefully studied. Moreover,
the molecular interactions of (CZHQ) Al with excellent semicon-
3
ductor materials, such as fullerene (C60) and carbon nanotubes
(CNTs), were also carefully investigated. The results show that the
metal complex (CZHQ) Al can emit red-light.
3
2. Experimental
2.1. Materials and instruments
∗ Corresponding author. Tel.: +86 3517011492; fax: +86 3517011688.
E-mail address: zbchen@sxu.edu.cn (L. Feng).
The reagents and chemicals for preparation of ligand and
complex were used as received unless noted otherwise. Ethanol,
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.04.026

1434
X. Wang et al. / Spectrochimica Acta Part A 71 (2008) 1433–1437
Scheme 1. The synthetic route of (CZHQ)3Al.
dichloromethane, toluene, chloroform, ethyl acetate, DMSO,
petroleum ether, etc. were purchased from Beijing Chemical Plant
and treated according to standard methods before use, which were
all applied to measurement of the light-emitting properties. The
synthesis route used was shown in Scheme 1.
Melting points were determined on a Sanyo Gallenkamp
MPD350 melting point apparatus and uncorrected. The IR spectra
were determined on a PE-1700 IR spectrophotometer by dispers-
ing samples in KBr disks. 1H NMR spectra were measured on a
Bruker ARX300 spectrometer with DMSO as solvents. Elemen-
tal analyses were performed on a Vario EL elemental analysis
instrument (Elementary Co.) UV–vis absorption and fluorescence
spectra were obtained on a Shimadzu UV-265 spectrophotome-
ter and Shimadzu RF-540 spectrofluorophotometer, respectively.
Luminescence spectrometer was measured with a xenon lamp as
the light source. Both excitation and emission bands were set at
10 nm. All the experiments were carried out at room tempera-
ture.
2.2. Synthesis
2.2.1. 5-Chloromethyl-8-hydroxyquinoline hydrochloride
A mixture of 7.3 g (0.05 mol) 8-hydroxyquinoline, 10 ml of 36.5%
hydrochloride acid and 8 ml (0.05 mol) of 37% formaldehyde was
treated with hydrogen chloride gas for 90 min. The yellow solid
was collected on a filter and dried to give 8.9 g (77.5% yield), m.p.
◦ ◦
280 C (lit. m.p. 283 C).
Fig. 1. The UV–vis absorption spectra of (CZHQ)3Al (5.24 × 10−5 M) in DMSO.

X. Wang et al. / Spectrochimica Acta Part A 71 (2008) 1433–1437
1435
ꢀ
2
.2.2. 5-[(Carbazol-9 -yl)methyl]-8-hydroxyquinoline (CZHQ)
A mixture of 3.34 g (0.02 mol) carbazole, 0.6 g (0.025 mol) NaH
and 30 ml DMSO was stirred until no bubble generated at room
temperature. After filtering the mixture, 1.12 g (0.02 mol) KOH
was added to the solution. 4.6 g (0.02 mol) 5-Chloromethyl-8-
hydroxyquinoline hydrochloride, which was dissolved by 10 ml
DMSO, was slowly dropped into the above solution. The mixture
◦
was stirred at room temperature for 3 h and heated to 70 C for 2 h.
The reaction solution was filtrated after adding to 200 ml water, the
precipitate was dissolved by ethanol, then adding the ethanol solu-
tion to 100 ml ice water gave the crude product. The crude product
was purified by silica gel column chromatography (eluent: ethyl
−
1
acetate:n-hexane = 1:4). Yield: 51.8%. IR (KBr pellet) cm : 3050,
660, 1590, 1575, 1530, 1170, 795; 1H NMR (d-DMSO): ı 8.88 (s,
H), 8.22 (s, 1H), 7.52–6.87 (m, 11H), 5.95 (m, 2H). Element Anal.
Calcd. for CZHQ (C22H16 N O): C, 81.46; H, 4.97; N, 8.64. Found: C,
1
1
2
Fig. 2. Fluorescence spectra of (CZHQ)3Al at different concentration of DMTP.
8
1.52; H, 5.13; N, 8.55.
−5
Concentration of (CZHQ)3Al, 5.24 × 10 M; concentration of DMTP (mol/l, M), 0,
−7
−6
−6
−6
−5
0
8
.00; 1, 7.33 × 10 ; 2, 2.16 × 10 ; 3, 5.12 × 10 ; 4, 9.44 × 10 ; 5, 2.87 × 10 ; 6,
.14 × 10−
5
.
2
.2.3. (CZHQ) Al
3
A
mixture of 0.002 mol AlCl3, 3.0 ml methanol, 7.0 ml
dichloromethane was slowly added to 30 ml dichloromethane
under refluxing which was dissolved in 0.002 mol 5-((9H-carbazol-
3
.3. The interactions of (CZHQ) Al with dimethylterephalate and
3
N,N-dimethylaniline
9
-yl)methyl)-8-hydroxyquinoline (CZHQ). The reaction mixture
was refluxed for 30 min and a lot of deposition appeared. After
The fluorescence quenching technique was a helpful method
for the study of the mechanism of molecular interaction, energy
transfer or charge transfer. Dimethylterephthalate (DMTP) was a
typical electron acceptor and N,N-dimethylaniline (DMA) was a
typical electron donor. When DMTP was added to a solution of
filtrating, washing and drying, the (CZHQ) Al metal complex was
3
◦
obtained (m.p. >300 C).
3. Results and discussion
(
CZHQ) Al in DMSO, the fluorescence of (CZHQ) Al was efficiently
3
3
3
.1. UV–vis absorption and fluorescence emission spectrum
quenched and the quenching process followed the Stern–Volmer
equation (as shown in Fig. 2). The apparent quenching coefficient,
4
−1
Fig. 1 shows the UV–vis absorption spectra of (CZHQ) Al in
Ksv, was 1.54 × 10 M . The quenching process of (CZHQ) Al with
3
3
dilute DMSO solution. The maximum UV–vis absorption peak of
CZHQ was 343 nm with shoulder peak 330 nm, which was simi-
lar with the absorption spectra of carbazole and it should mainly
come from carbazole unit absorption. However, the fluorescence
DMA was also examined and shown in Fig. 3. It can be seen that
the fluorescence emission intensity of (CZHQ) Al were increased
3
with gradual addition of DMA in DMSO, which was obviously differ-
ent phenomenon with DMTP adding to (CZHQ) Al solution. From
3
emission peak of (CZHQ) Al was 522 nm that rooted in the emis-
the experimental facts and references, an explanation is possible
reasonable: DMA is a rich electron group with stronger ligand capa-
bility. There are competitive complex processes with metal Al3+ and
ligand atoms or molecules from DMA and CZHQ when DMA is grad-
3
sion peak of 8-hydroxyquinoline aluminum. In addition, due to
the charge transfer process of hole-transfer group (carbazole) and
electron-transfer group (8-hydroxyquinoline), the maximum emis-
3+
sion peak of (CZHQ) Al was obviously red-shift comparing to that
ually added into the (CZHQ) Al solution. The action of DMA and Al
3
3
of 8-hydroxyquinoline aluminum.
can reduce the quenching fluorescent intensities of (CZHQ) Al from
3
metal Al3+ ions.
3.2. Quantum yield of photoluminescence and the band gap
The photoluminescent quantum yield of (CZHQ) Al was mea-
3
sured by relative method using the quinine sulfate as the standard
0.546 in 0.5 mol/l H SO ) [18]. The quantum yield was calculated
(
2
4
from the following equation:
ꢀ
ꢁ
2
Fs Ar nr
˚s = ˚r
Fr As ns
In the above expression, ˚s is the fluorescent quantum yield,
F is the integration of the emission intensities, n is the index of
refraction of the solution, and A is the absorbance of the solu-
tion at the exciting wavelength. The subscripts r and s denote
the reference and unknown samples, respectively. The value
of quantum yield of (CZHQ) Al in DMSO was 0.62. The band
3
opt
gap (Eg ) of the complex can be estimated from the onset
opt
g
−34
absorption (UVonset) with E (eV) = hc/ꢀ (h = 6.626 × 10
J s,
Fig. 3. Fluorescence spectra of (CZHQ)3Al at different concentration of DMA.
−5
c = 3 × 10 nm/s, 1 eV = 1.602 × 10−19 J). The band gap of the metal
1
7
Concentration of (CZHQ)3Al, 5.24 × 10 M; concentration of DMA (mol/l, M), 0,
−
6
−6
−6
−5
−5
0
4
.00; 1, 5.78 × 10 ; 2, 1.97 × 10 ; 3, 9.88 × 10 ; 4, 1.16 × 10 ; 5, 2.28 × 10 ; 6,
complex was 2.89 eV.
−5
−4
.
.96 × 10 ; 7, 7.83 × 10

1436
X. Wang et al. / Spectrochimica Acta Part A 71 (2008) 1433–1437
Fig. 4. Fluorescent spectra of (CZHQ)3Al at different concentration of C60. Con-
Fig. 6. Fluorescent spectra of (CZHQ)3Al at different concentration of CNTs. Con-
−5
−5
−1
centration of (CZHQ)3Al, 5.24 × 10 M; concentration of C60 (mol/l, M), 0, 0.00;
centration of (CZHQ)3Al, 5.24 × 10 M; concentration of CNTs (mg ml ), 0, 0.00; 1,
−
6
−5
3, 2.87 × 10−5
−5
−5
−6 −6 −6 −5 −5
1
1
,
4.36 × 10
;
2, 1.27 × 10
;
;
4, 3.41 × 10
;
5, 6.11 × 10
;
6,
5.36 × 10 ; 2, 8.21 × 10 ; 3, 9.67 × 10 ; 4, 3.08 × 10 ; 5, 7.23 × 10
.
−
4
−4
.31 × 10 ; 7, 1.76 × 10
.
3
.5. Interaction between (CZHQ) Al and carbon nanotubes
3
The investigation on the interaction between (CZHQ) Al and
3
3
.4. Interaction between (CZHQ) Al and fullerene (C60)
3
CNTs was helpful to understand the optical property of (CZHQ) Al
3
and apply it to LEDs. The interaction between them in diluted solu-
tions was examined by fluorescence spectrophotometer and the
Many researches show C60 bears many unusual electrochemical
and electronic properties. One of the most remarkable prop-
erties of C60 related to electron transfer phenomena is that it
can efficiently induce a rapid charge separation and a further
slow charge recombination [19]. In the experiment, the inter-
result indicated that the fluorescence of (CZHQ) Al can also be
3
quenched by carbon nanotubes (Fig. 6). It can be seen that the emis-
sion peak intensities of (CZHQ) Al were decreased with gradual
3
increase in concentration of CNTs in DMSO. Meantime, the emis-
sion peaks had no obvious change. The phenomenon indicated that
actions of (CZHQ) Al with C60 were examined. It can be seen
3
−5
in Fig. 4, ‘0’ was (CZHQ) Al (5.24 × 10 M) without C60, ‘1–7’
3
the intense interaction of (CZHQ) Al and CNTs in excited state hap-
3
were (CZHQ) Al in the present in different concentration of C60.
3
pened [22]. Further research toward a better understanding of this
action is currently in progress.
With gradual increase in the concentration of C60, the fluores-
cence of (CZHQ) Al was quenched efficiently and the process was
3
also following the Stern–Volmer equation (Fig. 5). The apparent
4
−1
4. Conclusions
quenching constant was 1.19 × 10 M , which indicated the strong
interactions between (CZHQ) Al and C in the excited state had
3
60
A
novel metal complex (CZHQ) Al containing 8-
3
happened. This can be explained by two reasons. Firstly, both
CZHQ) Al and C had a large ␲-conjugated system in which ␲–␲
hydroxyquinoline aluminum electron transport groups, and
carbazole hole-transport groups was designed and synthesized.
The optical research showed the absorption had two absorptions
peaks (343 nm, 330 nm) and the emission was located at 522 nm
in DMSO. The luminescence quantum yield was 0.62 in DMSO.
The light emitting can be quenched by electron acceptor and
the quenched processes followed the Stern–Volmer equation.
(
3
60
interaction may change the configuration of (CZHQ) Al [20]. Sec-
3
ondly, the photo-induced charge transfer from excited (CZHQ) Al
3
to C60 was rapid. Upon this charge transfer the conjugated sys-
tem may be dramatically modified and distorted because of the
strong electron–lattice interaction in the one-dimensional system
[
21].
The molecular interactions of (CZHQ) Al with fullerene (C60)
3
and carbon nanotubes both happened in the excited state. It
can be anticipated that the metal complex will have a potential
application as an emitting red-light material.
Acknowledgements
The present work was supported by the Natural Science Foun-
dation of China (30000112); Laboratory of Organic Solid, Chinese
Academy of Sciences and the Returned Student Science Foundation
of Shanxi Province of China.
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