Synthesis and photophysics of novel 8-hydroxyquinoline aluminum metal complex with fluorene units

Article abstract of DOI:10.1016/j.saa.2006.12.041

A novel luminescent metal complex, (MQPF)₃Al₂, with 8-hydroxyquinoline aluminum and 9,9-diphenylfluorene was synthesized. The optical properties were investigated by UV-vis absorption and fluorescence emission spectra. The results showed that the luminescence quantum yield of (MQPF)₃Al₂ was 0.612 in THF and it emitted red light with the band gap of 3.18 eV estimated from the onset absorption. The emission spectra exhibited obvious solvent effect. With the increase of polarity of solvents the fluorescence spectra changed obviously and appeared blue shift about 60 nm at room temperature. In addition, the light-emitting can be quenched by both electron donor (N,N-dimethylaniline) and electron acceptor (Fullerene), where the processes followed the Stern-Volmer equation. However, when adding 1,4-dicyanobenzene (DCB) which was a stronger electron acceptor to the solution of (MQPF)₃Al₂, the fluorescent intensity was increased.

Full text of DOI:10.1016/j.saa.2006.12.041

Spectrochimica Acta Part A 68 (2007) 646–650
Synthesis and photophysics of novel 8-hydroxyquinoline
aluminum metal complex with fluorene units
Liheng Fenga, Xiaoju Wangb, Sanhu Zhaoc, Zhaobin Chena,∗
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b Institute of Medicine and Life Science and Technology of Shanxi, Taiyuan 030006, PR China
c Department of Chemistry, Xinzhou Teachers University, Xinzhou 034000, PR China
Received 11 October 2006; received in revised form 15 December 2006; accepted 19 December 2006
Abstract
A novel luminescent metal complex, (MQPF)3Al2, with 8-hydroxyquinoline aluminum and 9,9-diphenylfluorene was synthesized. The optical
properties were investigated by UV–vis absorption and fluorescence emission spectra. The results showed that the luminescence quantum yield of
(MQPF)3Al2 was 0.612 in THF and it emitted red light with the band gap of 3.18 eV estimated from the onset absorption. The emission spectra
exhibited obvious solvent effect. With the increase of polarity of solvents the fluorescence spectra changed obviously and appeared blue shift about
60 nm at room temperature. In addition, the light-emitting can be quenched by both electron donor (N,N-dimethylaniline) and electron acceptor
(Fullerene), where the processes followed the Stern–Volmer equation. However, when adding 1,4-dicyanobenzene (DCB) which was a stronger
electron acceptor to the solution of (MQPF)3Al2, the fluorescent intensity was increased.
© 2007 Elsevier B.V. All rights reserved.
Keywords: 8-Hydroxyquinoline aluminum; Luminescent material; Fluorene; Photophysics
1. Introduction
rene without significantly increasing of steric interactions in the
compounds backbone [6]. As a result, luminescent materials of
fluorene have emerged as the most attractive blue(red)-emitting
materials due to their high efficiency and good thermal stability
[7–10]. More recently, 8-hydroxyquinoline aluminum has been
widely used as the emissive and electron transporting material in
organic light-emitting devices (OLEDs) [11,12]. In the work, we
designed and synthesized a new metal complex, (MQPF)3Al2,
with 8-hydroxyquinoline aluminum and 9,9-diphenylfluorene in
order to improve the performance of 8-hydroxyquinoline alu-
minum. The optical properties were investigated by UV–vis
absorption and fluorescence emission spectra. Moreover, the
interactions of (MQPF)3Al2 and electron donor or electron
acceptor have been studied. The results show that the metal
complex (MQPF)3Al2 is emitting red light material compound.
Recently, thefieldofresearchintophotoluminescent(PL)and
electroluminescent (EL) materials is growing rapidly due to the
versatility and potential applications of these compounds. It is
wellknownthatagoodluminescentmaterialshouldpossesshigh
photoluminescent efficiency, excellent charge transport abil-
ity and film-forming property [1–3]. Although a great number
of luminescent compounds have been synthesized and investi-
gated, the search for new compounds that possess the combined
desirable properties, such as high brightness and luminescent
efficiency, good film-forming properties, remains a big chal-
lenge in this area [4,5]. Therefore, to meet requirement of
luminescent materials, combination of two functional groups
may be a useful approach.
Fluorene derivatives show unique chemical and physical
properties because they contain a rigid planar biphenyl unit, and
the facile substitution at C9 position of fluorene can improve
the solubility and processability of materials containing fluo-
2. Experimental
2.1. Materials and instruments
The reagents and chemicals for preparation of ligand and
complex were used as received unless noted otherwise. Ethanol,
dichloromethane, toluene, chloroform, ethyl acetate, petroleum
∗
Corresponding author. Tel.: +86 3517011492; fax: +86 3517011688.
E-mail address: zchen@sxu.edu.cn (Z. Chen).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.12.041

L. Feng et al. / Spectrochimica Acta Part A 68 (2007) 646–650
647
Scheme 1. The synthetic route of (MQPF)3Al2.
ether, etc. were purchased from Beijing Chemical Plant and
treated according to standard methods used before, which were
all applied to measurement of the light-emitting properties. The
synthesis route used was shown in Scheme 1.
edyde 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), mp 280 ◦C (lit mp 283 ◦C).
3.2. 9,9-Bis[4ꢀ-(5ꢀꢀ-methylene-8ꢀꢀ-hydroxyquinoline)
phenyl]fluorene (MQPF)
Melting points were determined on a Sanyo Gallenkamp
MPD350 melting point apparatus and uncorrected. The IR spec-
tra were determined on a PE-1700 IR spectrophotometer by
1
dispersing samples in KBr disks. H NMR spectra were mea-
A mixture of 5-chloromethyl-8-hydroxyquinoline hydro-
chloride (4.6 g, 0.02 mol) and 9,9-bis(4ꢀ-aminophenyl)fluorene
(3.5 g, 0.01 mol) was dissolved in 30 ml dry THF and stirred
at room temperture for 1 h. After adding 10 ml triethylamine,
the mixture was heated to reflex for 5 h and poured into bro-
ken ice. The mixture was extracted with ethyl acetate and
dried with anhydrous magnesium sulfate. The solvent was
removed by distillation in vacuum, giving the solid residue. The
crude product was purified by silica gel column chromatog-
raphy (eluent: ethyl acetate:n-hexane = 2:1). Yield: 33%, mp
155–156 ◦C. IR(KBr pellet) cm−1: 3120, 1610, 1580, 1456,
1420, 810, 780, 720, 690; 1H NMR (d-DMSO): δ 8.88 (s,2H),
8.53(s,2H), 7.11–8.81(m,12H), 6.53–6.95(m,10H), 5.0(s,2H),
4.0(d,2H). Element anal. calc. for MQPF (C45H34N4O2):
C, 81.55; H, 5.17; N, 8.45; found: C, 81.52; H, 5.21; N,
8.44.
sured on a Bruker ARX300 spectrometer with DMSO as solvent.
Elemental analyses were performed on a Vario EL elemental
analysis instrument (Elementar Co.) UV–vis and fluorescence
spectra were obtained on a Shimadzu UV-265 spectrophotome-
ter and Shimadzu RF-540 spectrofluorothotometer, respectively.
Luminescence spectrometer was measured with a xenon lamp as
the light source. Both excitation and emission bands were set at
10 nm. Alltheexperimentswerecarriedoutatroomtemperature.
3. Synthesis
3.1. 5-Chloromethyl-8-hydroxyquinoline hydrochloride
A mixture of 7.3 g (0.05 mol) 8-hydroxyquinoline, 10 ml of
cond. hydrochloride acid, and 8 ml (0.05 mol) of 37% formald-

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L. Feng et al. / Spectrochimica Acta Part A 68 (2007) 646–650
3.3. (MQPF)3Al2
A 10 ml solution with 0.002 mol AlCl3, 3.0 ml methanol,
7.0 ml dichloromethane was slowly added to dissolved
0.003 mol
9,9-bis[4ꢀ-(5ꢀꢀ-methylene-8ꢀꢀ-hydroxyquinoline)
phenyl]-fluorene (MQPF) dichloromethane solution at reflux-
ing. The reaction system was refluxed for 30 min and appeared
a lot deposition. After filtrating, washing and drying, the
(MQPF)3Al2 metal complex was obtained, mp >300 ◦C.
4. Results and discussion
4.1. UV–vis absorption and fluorescence emission spectrum
Fig. 2. The fluorescent emission spectra of MQPF (0, 6.66 × 10−5 M) and
(MQPF)3Al2 (1, 2.22 × 10−5 M) in THF solution, 2–13 are fluorescent emiss-
sion spectra of (MQPF)3Al2 (1, 2.22 × 10−5 M) in THF solution with the
addition of Al3+, concentration of Al3+ (mol/L, M), 2, 2.34 × 10−6; 3,
7.12 × 10−6; 4, 9.67 × 10−6; 5, 1.08 × 10−5; 6, 1.26 × 10−5; 7, 1.87 × 10−5; 8,
Fig. 1 shows the UV–vis absorption spectra of MQPF and
(MQPF)3Al2 in dilute THF solution. The UV–vis absorption
spectra of MQPF was 290 and 385 nm, the UV–vis absorp-
tion spectra of (MQPF)3Al2 was 260 and 378 nm. It can be
seen from Fig. 1, the maximum wavelength of (MQPF)3Al2
is slightly blue-shift than MQPF. The photoluminescence (PL)
emission spectra of MQPF and (MQPF)3Al2 were investigated
and showed in Fig. 2. In comparing fluorescent emission spectra
of MQPF and (MQPF)3Al2, it can be found that the emis-
sion intensity and wavelength of MQPF (λmax = 465 nm) are
lower than that of (MQPF)3Al2 (λmax = 500 nm). It is because
the excited-state intramolecular proton transfer (ESIPT), from
8-hydroxyquinoline to the quinolino N atom, makes 8-
hydroxyquinoline weakly fluorescent and metal binding to
8-hydroxyquinoline blocks the ESIPT channel, thereby restor-
ing the fluorescence. So the fluorescent intensity of (MQPF)3Al2
is stronger than that of MQPF [13]. In addition, It can be known
from Fig. 2, when Al3+ is added to the MQPF (6.66 × 10−5 M)
solution in THF, the fluorescent intensity is gradually
increased until the concentration of Al3+ is 4.44 × 10−5 M.
From the fact, the format of complex is (MQPF)3Al2. The
under experiments is accorded to the proportion complex
to do.
2.31 × 10−5;9, 2,93 × 10−5;10, 3.15 × 10−5;11, 4.44 × 10−5;12, 5.23 × 10−5
;
13, 1.02 × 10−4
.
4.2. Solvent effects on photoluminescence (PL)
Complex (MQPF)3Al2 had good solubility in common
organic solvents, such as dichloromethane, THF, ethyl acetate,
ethanol, acetonitrile, methanol, etc. The emission spectra of
(MQPF)3Al2 were investigated in different solvents. It can be
seen from Fig. 3 that the emission spectra of (MQPF)3Al2 was
obvious blue shift about 60 nm with the increase of the solvents
polarity. The phenomenon may be due to its structure containing
heterocyclic system and metal center, which makes the (n–␲*)
excited state plays more important role in the (MQPF)3Al2 than
the (␲–␲*) state. It is well known that the solvolytic actions
between unshared electron-pair and polar solvent are obviously
happened, which will reduce the energy of the n orbital. Thus,
more energy will be needed for this transition of electron from
*
the n orbital to the ␲ orbital in a solvent with large polarity,
as compared with the transition in a solvent with small polar-
*
ity, which means that the energy gap between n and ␲ orbital
Fig. 1. The UV–vis absorption spectra of MQPF (6.66 × 10−5 M) and
(MQPF)3Al2 (2.22 × 10−5 M) in THF solution.
Fig. 3. The emission spectra of (MQPF)3Al2 in different solvents.

L. Feng et al. / Spectrochimica Acta Part A 68 (2007) 646–650
649
increase with the increasing of solvent polarity [14]. Further
research toward a better understanding of this effect is currently
in progress.
4.3. Quantum yield of photoluminescence and the band gap
The fluorescence quantum yield was measured by relative
method using the quinine sulfate as the standard (0.546 in
0.5 mol L−1 H2SO4) [15]. The quantum yield was calculated
from the following equation:
ꢀ
ꢁ
2
Fs Ar nr
Φs = Φr
Fr As ns
In the above expression, Φs is the fluorescent quantum yield, F
the integration of the emission intensities, n the index of refrac-
tion of the solution, and A is the absorbance of the solution at the
exciting wavelength. The subscripts r and s denote the reference
and unknown samples, respectively. The value of quantum yield
of (MQPF)3Al2 in THF was 0.612. The band gap (Egopt) of the
Fig. 5. Fluorescence spectra of (MQPF)3Al2 at different concentration of DCB.
Concentration of (MQPF)3Al2, 2.22 × 10−5 M; concentration of DCB (mol/L,
M), 0,0.00; 1, 5.76 × 10−7; 2, 8.97 × 10−7; 3, 1.25 × 10−6; 4, 6.34 × 10−6; 5,
9.02 × 10−6; 6, 2.32 × 10−5; 7, 4.86 × 10−5; 8, 1.02 × 10−4
.
complex can be estimated from the onset absorption (UVonset
)
ined and shown in Fig. 5. It can be seen that the emission
intensity of fluorescence were increased and blue-shifted about
15 nm with gradual increasing in concentration of DCB in
THF. Meanwhile, the half peaks were broadened. From the
experimental facts and references, an explanation is proposed:
1,4-dicyanobenzene (DCB) is a ␲-bond ligand, so the interac-
tion between metal and ligands would be better stronger and
prevent the proton transfer of 8-quinolinol when DCB is grad-
ually added into the (MQPF)3Al2 solution. Further research
toward a better understanding of this effect is currently in
progress.
with Egopt (eV) = hc/λ (h = 6.626 × 10−34 Js, c = 3 × 1017 nm/s,
1 eV = 1.602 × 10−19 J). The band gap of the polymer was
3.18 eV.
4.4. The interactions of (MQPF)3Al2 with
N,N-dimethylaniline (DMA) and 1,4-dicyanobenzene (DCB)
The fluorescence quenching technique was a helpful method
for the study of the mechanism of molecular interaction, energy
transfer or charge transfer. N,N-dimethylaniline (DMA) was a
typical electron donor and 1,4-dicyanobenzene (DCB) was a
typical electron acceptor. When DMA was added to a solu-
tion of (MQPF)3Al2 in THF, the fluorescence of (MQPF)3Al2
was efficiently quenched and the quenching process followed
the Stern–Volmer equation. The apparent quenching coeffi-
cient, Ksv, was 1.88 × 104 M−1 (as shown in Fig. 4). The
quenching process of (MQPF)3Al2 with DCB was also exam-
4.5. Interaction between (MQPF)3Al2 and fullerene (C60)
Many researches show C60 bears many unusual electrochem-
ical and electronic properties. One of the most remarkable
properties of C60 related to electron transfer phenomena is that
it can efficiently induce a rapid charge separation and a further
slow charge recombination [16]. In the experiment, the inter-
actions of (MQPF)3Al2 with C60 were examined. The results
were shown in Fig. 6. As we have seen in Fig. 6, ‘0’ was
(MQPF)3Al2 in concentration (2.22 × 10−5 M) without C60,
‘1–6’ were (MQPF)3Al2 in the present in different concentration
of C60. With the gradual increasing of concentration of C60, the
fluorescence of (MQPF)3Al2 was quenched efficiently and the
process was also following the Stern–Volmer equation (Fig. 7).
The apparent quenching constant was 1.64 × 105 M−1, which
suggests that the strong interactions between (MQPF)3Al2 and
C60 happen in the excited state. This can be explained by
two reasons. Firstly, both (MQPF)3Al2 and C60 have a large
␲-conjugated system in which ␲–␲ interaction may change
the configuration of (MQPF)3Al2 [17]. Secondly, the photo-
induced charge transfer from excited (MQPF)3Al2 to C60 was
rapid. Upon this charge transfer the conjugated polymer sys-
tem may be dramatically modified and distorted because of
the strong electron–lattice interaction in the one-dimensional
system [18].
Fig. 4. Fluorescence spectra of (MQPF)3Al2 at different concentration of DMA.
Concentration of (MQPF)3Al2, 2.22 × 10−5 M; concentration of DMA (mol/L,
M), 0,0.00; 1, 6.72 × 10−6; 2, 1.34 × 10−5; 3, 2.65 × 10−5; 4, 6.78 × 10−5; 5,
9.86 × 10−5; 6, 2.31 × 10−4
.

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L. Feng et al. / Spectrochimica Acta Part A 68 (2007) 646–650
research showed the absorption has multiple absorptions ranging
from 260 to 385 nm and the emission is located 500 nm in THF.
The luminescence quantum yield was 0.612 in THF. The emis-
sion spectra exhibit obvious solvent effect. With the increase
of polarity of solvents, the fluorescence spectra change obvi-
ously and appear blue-shifted about 60 nm at room temperature
In addition, the light-emitting can be quenched by both electron
donor, electron acceptor fullerene (C60), and the quenched pro-
cesses followed the Stern–Volmer equation. 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
Foundation of China (30000112); Laboratory of Organic Solid,
Chinese Academy of Sciences and The Returned Student Sci-
ence Foundation of Shanxi Province of China.
Fig. 6. Fluorescent spectra of (MQPF)3Al2 at different concentration of C60
.
Concentration of (MQPF)3Al2, 2.22 × 10−5 M; concentration of C60 (mol/L,
M), 0, 0.00; 1, 9.81 × 10−7; 2, 2.54 × 10−6; 3, 3.62 × 10−6; 4, 6.38 × 10−6; 5,
8.74 × 10−6; 6, 1.42 × 10−5
.
References
[1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay,
R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539.
[2] S. Son, A. Dodabalapur, A.J. Lovinger, M.E. Galvin, Science 269 (1995)
376.
[3] X. Zhan, Y. Liu, G. Yu, X. Wu, D. Zhu, R. Sun, D. Wang, A.J. Epstein, J.
Mater. Chem. 11 (2001) 1606.
[4] L. Zheng, R.C. Urian, Y. Liu, A.K.Y. Jen, L. Pu, Chem. Mater. 12 (2000)
13.
[5] C. Weder, M.S. Wrighton, Macromolecules 29 (1996) 5157.
[6] G. Klaerner, R.D. Miller, Macromolecules 31 (1998) 2007.
[7] Y. Yang, Q. Pei, J. Am. Chem. Soc. 118 (1996) 7416.
[8] L.H. Feng, Z.B. Chen, Spectrochim. Acta Part A 63 (2006) 15.
[9] K. Weinfurtner, H. Fujikawa, S. Tokito, Y. Taga, Appl. Phys. Lett. 76 (2000)
2502.
[10] L.H. Feng, Z.B. Chen, Polymer 46 (2005) 3952.
[11] H.D. Burrows, M. Fernandes, J. Seixas de Melo, A.P. Monkman, S.
Navaratnam, J. Am. Chem. Soc. 125 (2003) 15310.
[12] G.M. Credo, D.L. Winn, S.K. Buratto, Chem. Mater. 13 (2001) 1258.
[13] E. Nardez, L. Devol, B. Larrey, B. Valeur, J. Phys. Chem. B 101 (1997)
7786.
Fig. 7. Dependence of I0/I on the concentration of C60. Concentration of
(MQPF)3Al2 2.22 × 10−5 M.
[14] H.C. Song, P. Huan, B.N. Ying, Spectrochim. Acta Part A 59 (2003) 1509.
[15] J.N. Dmas, G.A. Crobys, J. Phys. Chem. 71 (1971) 991.
[16] B.W. Jing, D.Q. Zhang, D.B. Zhu, Tetrahedron Lett. 41 (2000) 8559.
[17] S.H. Gallagher, R.S. Armstrong, P.A. Lay, C.A. Reed, J. Phys. Chem. 99
(1995) 5817.
5. Conclusions
A novel metal complex (MQPF)3Al2 containing hole trans-
port groups, 8-hydroxyquinoline aluminum and fluorophore
fluorene units was designed and synthesized. The optical
[18] S. Morita, A.A. Zakhidov, K. Yoshino, Solid State Commun. 82 (1992)
249.