Synthesis, characterization and DFT investigation of aluminum complexes of aryl-substituted-8-hydroxyquinoline

Article abstract of DOI:10.1016/j.dyepig.2011.06.033

New aryl substituted 8-hydroxyquinoline derivatives have been synthesized. The compounds were characterized by ¹H, ¹³C-NMR spectroscopy and mass spectrometry. The aluminum complexes of these derivatives show strong green emission in the range 530-535 nm with high quantum yield compared to the parent tris (8-hydroxyquinolinato)aluminum(III) complex. The structure of the aryl-substituted 8-hydroxyquinoline aluminum complexes in the ground state have been optimized at the B3LYP/6-31G (*) level of theory. The parent 8-hydroxyquinoline has also been examined using this method with the same basis set. A clear correlation between the shortening of the Al-N bond and red shift in the fluorescence emission band was observed. We have also found that substitution at the 5 and 7 position of the 8-hydroxyquinoline ligand cause simultaneous energy lowering of both highest occupied molecular orbital and the lowest unoccupied molecular orbital with the 5, 7-aryl substituted derivatives exhibiting the greatest effect.

Full text of DOI:10.1016/j.dyepig.2011.06.033

Dyes and Pigments 92 (2012) 1153e1159
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, characterization and DFT investigation of aluminum complexes
of aryl- substituted-8-hydroxyquinoline
*
FakhrEldin O. Suliman , Saleh N. Al-Busafi, Moza Al-Risi, Khalifa N. Al-Badi
Department of Chemistry, College of Science, Sultan Qaboos University, Box 36, Al-Khod 123, Sultanate of Oman, Oman
a r t i c l e i n f o
a b s t r a c t
Article history:
New aryl substituted 8-hydroxyquinoline derivatives have been synthesized. The compounds were
characterized by 1H, 13C-NMR spectroscopy and mass spectrometry. The aluminum complexes of these
derivatives show strong green emission in the range 530e535 nm with high quantum yield compared to
the parent tris (8-hydroxyquinolinato)aluminum(III) complex. The structure of the aryl-substituted 8-
hydroxyquinoline aluminum complexes in the ground state have been optimized at the B3LYP/6-31G*
level of theory. The parent 8-hydroxyquinoline has also been examined using this method with the same
basis set. A clear correlation between the shortening of the Al-N bond and red shift in the fluorescence
emission band was observed. We have also found that substitution at the 5 and 7 position of the 8-
hydroxyquinoline ligand cause simultaneous energy lowering of both highest occupied molecular
orbital and the lowest unoccupied molecular orbital with the 5, 7-aryl substituted derivatives exhibiting
the greatest effect.
Received 12 April 2011
Received in revised form
25 June 2011
Accepted 27 June 2011
Available online 3 July 2011
Keywords:
8-Hydroxyquinoline
Al(III) complex dyes
Luminescence
DFT-B3LYP
OLEDs
Suzuki coupling
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
coupled plasma (ICP) with optical and mass detection systems.
These methods, although sensitive, require expensive instrumen-
The unique fluorescence signature of 8-hydroxyquinoline has
been used as the platform for the qualitative and quantitative
analysis of several metal cations in solution. 8-hydroxyquinoline
(8-HQ) exhibits feeble fluorescence emission in aqueous media
and in many organic solvents. This weak fluorescence emission
from 8-HQ has been attributed to the photoinduced tautomeriza-
tion followed by nonradiative relaxations involving excited state
proton transfer (ESPT) [1]. The intramolecular hydrogen bonding
between the hydroxyl and the pyridyl groups is stronger in the
excited state which results in deactivation of this state via internal
conversion [2]. In chemical analysis, the bidentate 8-HQ is used to
complex metal ions such as Al(III) and Zn(II), and the resultant 8-
hydroxyquinolinato-metal complex exhibits an intense fluores-
cence signal. This allows the development of straightforward
sensitive protocols for the analysis of these metal ions where the
intensity of the emitted light is directly proportional to the amount
of the metal ion.
tation, and considerable expertise for the analysis of aluminum.
On the other hand molecular luminescence techniques offer
alternative methods with many advantages for the analysis of
metal ions. These techniques are sensitive, selective, cost effective
and user friendly and require less maintenance compared to ICP
techniques.
The requirements for highly sensitive fluorescence methods
hinge in many cases on the characteristics of the complexing dye.
Therefore, development of new dyes which emit at longer wave-
lengths and with high quantum yields when complexed with metal
ions is desirable. Tuning of the tris (8-hydroxyquinolinato)alumi-
num(III) (Alq3) fluorescence is an active area of research [3e14].
A number of experimental and theoretical approaches have been
attempted to describe the geometrical characteristics of the qui-
nolinolate ligand. Density functional theory (DFT) calculations
performed on the meridional (mer) Alq3 indicated that the highest
occupied molecular orbital (HOMO) are located predominantly on
the phenoxide side of the ligand, especially at the C5 position and
C7 position of the quinolinolate ligand. The lowest unoccupied
molecular orbital (LUMO), on the other hand, is mainly localized on
the residual pyridyl side of the quinolinolate ligand [13e15].
Tris (8-hydroxyquinolinato)aluminum(III) (Alq3) is currently
entering the center of research interest especially after becoming
a key component for the development of organic light-emitting
The most widely used methods for the analysis of aluminum
are based on the sensitive atomic absorption and atomic emission
techniques, such as graphite atomic absorption and inductively
* Corresponding author. Tel.: þ968 24141480; fax: þ968 24141469.
E-mail address: fsuliman@squ.edu.om (F.O. Suliman).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.06.033

1154
FakhrEldinO. Suliman et al. / Dyes and Pigments 92 (2012) 1153e1159
diodes (OLEDs), an important material currently used as electron
injection, electron transport and emitting layer in flat panel display
systems [16]. Enhancement of the photoluminescence and elec-
troluminescence quantum yields of 8-HQ based OLEDs has been
attempted via modification of the quinolinolate ligand. Several
derivatives of 8-HQ were synthesized and used to prepare
aluminum complexes as light emitting diodes. The most attractive
strategy used depends on the introduction of electron withdrawing
and electron donating groups into the 8-HQ ligands
[4e6,8e10,17,18]. Substitution of methyl groups at C2 and C4 of the
quinolinolate ligand results in aluminum complexes that presented
high photoluminescence and electroluminescence efficiencies
[9,10]. Also tuning of the emission of the aluminum quinolinolate
system has been achieved with introduction of electron with-
drawing and electron donating phenyl moieties at the C5 position
of the 8-HQ ligand. The nature of phenyl substituents was found to
profoundly affect the emission color as well as the quantum yield of
the resultant complexes [3,5,6,17].
d
(ppm) ¼ 8.79 (dd, 1H, J ¼ 1.6 Hz and 4.2 Hz, H2), 8.46 (dd, 1H,
J ¼ 1.6 Hz and 8.6 Hz, H4), 7.97 (s, 1H, H6), 7.52 (dd, 1H, J ¼ 4.2 Hz
and 8.6 Hz, H3), 7.36e7.6 (5H, arom-H), (5.46 (s, 2H, CH2). 13C NMR
(100.4 MHz, CDCl3):
134.0, 117.2, 152.7, 137.4,116.7, 144.3, 128.5, 129.1, 128.6. EI-MS (m/z):
calculated for [C16H11Br2NO], 393.10; found, 393.12.
d
(ppm) ¼ 151.1, 122.3, 136.4, 128.8, 122.9,
2.3.2. 8-Benzyloxy-5,7-bis(p-cyanophenyl)quinoline (3b)
A mixture of 8-benzyloxy-5,7-dibromoquinoline (2b) (1.0 g,
2.5 mmol), 4-cyanophenylboronic acid (0.82 g, 5.6 mmol), sodium
carbonate (1.07 g, 10 mmol), toluene (9.96 mL), ethanol (4.98 mL),
and water (7.47 mL) was degassed under N2 for 30 min. To the
mixture was added Pd(PPh3)4 (0.18 g, 15 mmol) and the reaction
was refluxed under nitrogen for 24 h. The crude product was
extracted with toluene, recrystallized from ethanol, and dried in
vacuo (Yield ¼ 51.35%). M.P. ¼ 207.5 ꢁCe209.5 ꢁC. IR(cmꢀ1):
y
¼ 3042 (CeH)arom, 2227.55 (CN), 1601 (C]C)
.
1H NMR
arom
(ppm) ¼ 9.08 (dd, 1H, J ¼ 1.5 Hz and 4.1 Hz,
(400 MHz, CDCl3):
d
In the present work, we report the synthesis of two new phenyl
substituted Alq3 dyes derivatized at the 5 and 7 positions of the 8-
HQ ligand. The spectroscopic properties of the complexes were
investigated and compared to those of the phenyl substituted
derivatives at C5 and the parent Alq3. Furthermore, DFT-B3LYP
methods were used to optimize and study the ground state of
these complexes.
H2), 8.17 (dd, 1H, J ¼ 1.5 Hz and 8.6 Hz, H4), 7.49 (dd, 1H, J ¼ 4.1 Hz
and 8.6 Hz, H3), 7.44 (s, 1H, H6), 7.8 (d, 2H, J ¼ 8.2 Hz, aromatic-H),
7.61 (d, 2H, J ¼ 8.2 Hz, arom-H), 7.18 (d, 2H, J ¼ 7.0 Hz, arom-H), 7.08
(d, 2H, J ¼ 7.0 Hz, arom-H), 5.32 (s, 2H, eCH2), 7.22 (m, 3H, arom-H),
7.67 (m, 2H, arom-H). 13C NMR (100.4 MHz, CDCl3):
d
(ppm) ¼ 150.7,
121.1, 134.7, 129.1, 132.7, 129.2, 127.5, 152.5, 144.0, 112.2, 137.0, 131.1,
132.8, 132.7, 134.3, 133.9, 128.5, 127.89, 132.2, 131.2, 143.8, 142.9,
119.3, 119.0. EI-MS) m/z): calculated for [C30H19N3O], 437.49; found,
437.41.
2. Experimental
2.1. Materials
2.3.3. 8-Benzyloxy-5,7-bis(p-acetylphenyl)quinoline (3d)
A
mixture of 5,7-dibromo-8-benzyloxyquinoline (2b) (2 g,
All reagents were purchased from Aldrich and used without
purifications. All solvents were used as received or were purified
using standard procedures [19]. Spectroscopic grade solvents
(Aldrich) were used for the fluorescence and UVeVisible
measurements.
5.1 mmol), 4-acetyl-phenylboronic acid (1.8 g, 11.2 mmol), sodium
carbonate (2.2 g, 20 mmol), toluene (19.96 mL), ethanol (9.9 mL),
and water (14.9 mL) was degassed under N2 for 30 min. To the
mixture was added Pd(PPh3)4 (0.35 g, 0.30 mmol) and the reaction
was refluxed under nitrogen for 24 h. The crude product was
extracted with toluene, recrystallized from ethanol, and dried in
2.2. Methods and instruments
vacuo. Yield ¼ (52.1%). M.p.: 163e164 ꢁC. IR (cmꢀ1):
y
¼ 3100
(CeH)arom, 2962 (CeH), 1682 (C]O). 1H NMR (400 MHz, CDCl3):
Infrared spectra were recorded on a PerkineElmer FT-IR BX
system in KBr pellets. Maximum absorptions were recorded in
wave numbers (cmꢀ1). Melting points were measured by Gal-
leneKamp apparatus. 1H and 13C NMR spectra were performed on
a 400 MHz Bruker spectrometer using tetramethylsilane (TMS) as
the internal standard and CDCl3 or DMSO-d6 as the solvent at room
temperature. UVeVis spectra were performed on Varian CARY 50
spectrophotometer. Fluorescence spectra were recorded on a Per-
kin Elmer LS55 Luminescence spectrophotometer. Mass spectra
were measured using Quatro ultima pt (Waters Corp. MA, USA)
instrument.
d
(ppm) ¼ 9.05 (dd, 1H, J ¼ 1.6 Hz and 3.9 Hz, H2), 8.22 (dd, 1H,
J ¼ 1.6 Hz and 8.6 Hz, H4), 8.10 (d, 2H, J ¼ 5.1 Hz, arom-H), 7.99 (d,
2H, J ¼ 6.7 Hz, arom-H), 7.73(d, 2H, J ¼ 5.1 Hz, arom-H), 7.61 (d, 2H,
J ¼ 6.7 Hz, arom-H), 7.52 (s, 1H, H6), 7.44 (dd, 1H, J ¼ 3.9 Hz and
8.6 Hz, H3), 7.17 (m, 5H, arom-H), 5.27 (s, 2H, CH2), 2.68 (s, 3H, CH3),
2.65 (s, 3H, CH3). 13C NMR (100.4 MHz, CDCl3):
d
(ppm) ¼ 150.44,
128.56,135.35,128.93,130.75,128.73,128.47,152.26,144.26,128.23,
137.42, 130.58, 131.52, 130.68, 129.48, 129.02, 198.28, 198.05, 134.56,
133.28, 143.90, 143.25, 136.75, 136.31, 127.86, 127.49. EI-MS) m/z):
calculated for [C33H29NO3], 471.55; found, 471.70.
2.3.4. 5, 7-Bis(p-cyanophenyl)-8-hydroxyquinoline (4b)
2.3. Synthesis
A mixture of 8-benzyloxy-5,7-bis(p-cyanophenyl)quinoline (3b)
(0.57 g, 1.3 mmol), Pd/C (10 wt%, 0.40 g), and cyclohexa-1,4-diene
(2.0 g) in absolute ethanol (15.0 mL) was refluxed under N2 for
5 h. The mixture was filtered, and ethanol was evaporated. The
product was recrystallized from ethanol/water and dried in
5-(p-Cyanophenyl)-8-hydroxyquinoline (4a) and 5-(p-acetyl-
phenyl)-8-hydroxyquinoline (4c) were synthesized by the recently
reported method [3,6].
vacuum. (Yield ¼ 61%). M.p. ¼ 264e265 ꢁC, IR (cmꢀ1):
y
¼ 3042
2.3.1. 8-Benzyloxy-5,7-dibromoquinoline(2b)
(CeH)arom, 1359 (CN), 1602 (C]C)arom. 1H NMR (400 MHz, CDCl3):
A
mixture of 5,7-dibromo-8-hydroxyquinoline (1b) (3.0 g,
d
(ppm) ¼ 8.85 (dd, 1H, J ¼ 4.2 Hz and 1.5 Hz, H2), 8.21 (dd, 1H,
9.9 mmol), benzyl chloride (1.38 g, 10.9 mmol), potassium
carbonate (1.37 g) and potassium iodide (0.162 g, 0.98 mmol) in
acetone (60 mL) was refluxed under N2 for 10 h. The mixture was
poured into water, filtered, and washed with water. The crude
product was recrystallized from ethanol and dried in vacuo. Yield:
J ¼ 8.5 Hz and 1.5 Hz, H4), 7.96 (d, 2H, J ¼ 8.2 Hz, arom-H), 7.81 (d,
2H, J ¼ 8.1 Hz, arom-H), 7.78 (d, 2H, J ¼ 8.2 Hz, arom-H), 7.61 (d, 2H,
J ¼ 8.1 Hz, arom-H), 7.52(dd, 1H, J ¼ 4.2 Hz and 8.5 Hz, H3), 7.26 (s,
1H, H6). 13C NMR (100.4 MHz, CDCl3):
d
(ppm) ¼ 148.6, 122.8, 134.0,
129.1, 130.7, 126.0, 120.2, 149.79, 138.69, 132.5, 132.2, 111.6, 111.0,
129.9, 129.3, 143.7, 141.8, 118.9, 118.6. EI-MS) m/z): calculated for
[C23H13N3O], 347.11; found, 347.18.
56.8%. M.p.: 117.8 ꢁCe119.2 ꢁC. IR(cmꢀ1):
y
¼ 3090 (CeH)arom, 2962
(CeH), 1574 (C]C)arom, 1490 (CeN). 1H NMR (400 MHz, CDCl3):

FakhrEldinO. Suliman et al. / Dyes and Pigments 92 (2012) 1153e1159
1155
2.4. Computational details
The general structure of the investigated complexes is shown in
Fig. 1. The labels aec are used to distinguish the three quinoline
ligands complexed to the central aluminum atom. The results of the
theoretical calculations are reported according to the labeling
shown here in Fig. 1. The ground state geometries of Alq3 and its
phenyl substituted derivatives were fully optimized using Becke’s
three parameter hybrid functional combined with Lee, Yang and
Parr (LYP) correlation [20,21] denoted as B3LYP with the 6-31G*
basis set. All computations were performed using Gamess-US
package [22].
3. Results and discussion
3.1. Synthesis
The synthesis of the aryl substituted ligands 4ae4b starts from
5-bromo-8-hydroxyquinoline and 5, 7-dibromo-8-hydroxyquino-
line (1a, 1b), respectively. The Benzyl derivative of 1a (2a) and 1b
(2b; Scheme 1) were first synthesized from the bromo-substituted
8-hydroxyquinolines 1a and 1b and then underwent a Suzuki
coupling reaction with 4-cyanophenylboronic acid or 4-acetyl-
phenylboronic acid to give 3ae3d. Debenzylation gave 4ae4d. The
structures of the ligands were confirmed by 1H, 13C-NMR, and IR
spectroscopy and mass spectrometry. The aluminum complexes of
8-HQ, and derivatives 4ae4d were prepared in methanol.
Fig. 1. The geometry of mer-Alq3, labels a-c a for the quinolinate ligands.
2.3.5. 5, 7-Bis(p-acetylphenyl)-8-hydroxyquinoline (4d)
3.2. Formation of (qunolinolato) aluminum (III) and their spectral
properties
A mixture of 8-benzyloxy-5, 7-bis(p-acetylphenyl)quinoline
(3d) (0.62 g, 1.3 mmol), Pd/C (10 wt%, 0.40 g), and cyclohexa-1,4-
diene (0.93 g) in absolute ethanol (12.0 mL) was refluxed under
N2 for 5 h. The mixture was filtered, and ethanol was evaborated.
The product was recrystallized from ethanol/water and dried in
We used UVeVisible and fluorescence spectroscopy to investi-
gate the formation of the aluminum (III) quinolinates of the aryl
substituted derivatives of 8-hydroxyquinoline. The absorption
vacuum. (Yield ¼ 65%). M.p. ¼172.9e174.8 ꢁC. IR(cmꢀ1):
y
¼ 3064
spectra of all derivatives are characterized by a very intense pep*
(CeH)arom
,
2929 (CeH), 1680 (C]O), 1603 (C]C). 1H NMR
absorption bands at around 250 nm accompanied by a less intense
and broader band above 300 nm. In all cases, the absorption bands
shift to longer wavelengths by 5e13 nm when cyanophenyl and
acetylphenyl groups are attached to C5 and C7 of the 8-
hydroxyquinoline ligand. Upon coordination of these ligands with
(400 MHz, CDCl3):
d
(ppm) ¼ 8.86 (dd,1H, J ¼ 1.1 Hz and 4.1 Hz, H2),
8.26 (dd, 1H, J ¼ 1.1 Hz and 8.5 Hz, H4), 8.10 (d, 2H, J ¼ 8.4 Hz, arom-
H), 8.08 (d, 2H, J ¼ 8.4 Hz, arom-H), 7.94 (d, 2H, J ¼ 8.4 Hz, arom-H),
7.64 (s, 1H, H6), 7.61 (d, 2H, J ¼ 8.4 Hz, arom-H), 7.48 (dd, 1H,
J ¼ 4.1 Hz and 8.5 Hz, H3), 2.87 (s, 3H, CH3), 2.68 (s, 3H, CH3). 13C
aluminum in methanol the
pep* absorption maxima exhibited
NMR (100.4 MHz, CDCl3):
d
(ppm) ¼ 149.7, 122.9, 136.2, 129.1, 130.7,
a red shift compared to the ligands as well as a noticeable increase
in the quantum yield due to the extension of conjugation of the
quinoline chromophores. Also the aryl-substituted complexes
exhibited a red shift compared to the parent Alq3 complex.
128.9,126.4,197.0,142.5,130.1,129.8,198.1,198.0,134.7,132.9,148.7,
144.3, 139.0, 136.6, 122.8, 121.4. EI-MS) m/z): calculated for
[C26H23NO3], 381.42; found, 381.48.
Scheme 1. Syntheses of aryl-substituted 8-hydroxyquinoline.

1156
FakhrEldinO. Suliman et al. / Dyes and Pigments 92 (2012) 1153e1159
Fig. 2 shows the fluorescence emission of Alq3 and for the
complexes of 4ae4d with aluminum in methanol. Typically the
emission bands for the aryl substituted aluminumquinolinates shift
to longer wavelength by 8e13 nm. The 5, 7-aryl substituted alu-
minumquinolinates exhibited shifts to longer wavelengths
compared to the 5-aryl-substituted derivatives. The fluorescence
emission is greatly enhanced when an electron poor group is
attached to C5 and C7 of the quinoline ligand. It is clear from Fig. 2
that the cyanophenyl substituted derivatives exhibited a slightly
more intense fluorescence compared to the acetylphenyl
substituted derivative. Moreover, the 5, 7-aryl substituted deriva-
tives show a much higher enhancement in the fluorescence emis-
sion. The addition of aryl rings to 8-hydroxyquinoline results in the
extension of the conjugation which results in an increase in the
fluorescence intensity of the chelates of these ligands with
aluminum (III).
The rate of fluorescence photon emission, FF, is related to the
fluorescence quantum yield, 4F, by the following equation [23]
FF ¼ nS kA4FV
(1)
0
where nS is the number density of the ground state (in molecules
0
per cm3), kA is the rate of absorption, V is the sample volume and 4F
is the quantum efficiency.
The rate constant of fluorescence, kF, and 4F are expected to be
large if the transition probability between S0 and S1 is large. It is
known that absorption to S1 and fluorescence involve the same
electronic states, therefore both kF and kA (S0 / S1) are larger for
molecular systems that possess large molar absorptivity (
other hand, the most efficient fluorescence usually associates with
pp* transitions and these are characterized by large , kA and kF.
3). On the
3
Therefore, there is always an increase in 4F with the extension of
the conjugation system.
Fig. 3 shows the titration curves for the 5-acetylphenyl and 5, 7-
acetylphenyl substituted derivatives with Al3þ in methanol using
an excitation wavelength of 290 nm. This figure shows a clear
growing green band at 530 nm accompanied by a band at shorter
wavelength (390 nm) with the presence of clear isoemissive points.
The presence of an isoemissive point indicates that at least two
excited species are involved in the emission spectra. In fact, the
Fig. 3. Titration curve of 2 ꢂ 10ꢀ6 M of (a) 4c and (b) 4d with Al3þ showing a growing
green emission in methanol. [Al3þ] ¼ 0, 0.1
mM, 0.2 mM, 0.4 mM, 0.6 mM, 0.8 mM, 1.0 mM,
2.0 mM and 3.0 mM, from bottom to top.
observation of an isoemissive point is a clear indication that two
emitting species are present in equilibrium in the reaction mixture,
possibly the free ligand and its aluminum complex. The stoichi-
ometry of the metal: ligand studied by the molar ratio and the
continuous variation methods was found to be 1: 3 for all deriva-
tives. Similar results were obtained for the p-cyanophenyl
substituted ligands. These ligands can, therefore, be used as
excellent candidates for sensing aluminum ions in various media.
3.3. Molecular geometries
To further scrutinize the structure of the synthesized complexes
the ground state geometry of Alq3 and its aryl substituted deriva-
tives were optimized using B3LYP methods. In Table 1, we present
some selected parameters of the optimized geometries of Alq3 and
Fig. 2. Fluorescence spectra (lex 390) of (1) Alq3 (2) Al-4c (3) Al-4a (4) Al-4d (5) Al-4b
in methanol.

FakhrEldinO. Suliman et al. / Dyes and Pigments 92 (2012) 1153e1159
1157
Table 1
and emission wavelength depends on a fine balance of electronic
transition between the quinolinolate dyes and the aryl substituents
at C5 and C7. It has been reported that some C5 substituted qui-
nolinolate ligands exhibit a significant degree of non-coplanarity
with dihedral angles ꢃ 50ꢁ between the planes of the quinoline
and the aryl substituents as shown by X-ray crystallography [3].
This results in the reduction of the degree of conjugation between
the two moieties and consequently the efficacy of electronic
interactions. Inspection of the DFT optimized structures reveals
that the dihedral angles between the planes of the aryl substituent
and the quinoline groups at C5 are between 49ꢁe53ꢁ (Table 1).
These results correlate well with those obtained by X-ray crystal-
lography [3]. On the other hand the dihedral angles between the C7
substituent and the quinoline group is 33ꢁ and 38ꢁ for the p-cya-
nophenyl and p-acetylphenyl derivatives respectively. Clearly, from
these results one can conclude that the presence of another aryl
substituent at C7 increases the efficiency of electronic communi-
cation with the quinoline group.
We also calculated the frontier molecular orbitals (FMOs) for
Alq3 and its aryl substituted derivatives using the optimized ground
state geometries. The highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) for Alq3 and
its aryl substituted derivatives are shown in Fig. 4. The HOMO in all
complexes shows almost the same trend and is localized at ligand a.
On the other hand, the LUMO is predominantly localized at ligand b.
These results are in agreement with the previously reported
Selected B3LYP/6-31G* optimized geometries of the ground electronic state.
R (Å),
4
(deg)
Al-8HQ
Al-4a
Al-4b
Al-4c
Al-4d
Experimentala
AleN1
AleN2
AleN3
AleO1
AleO2
AleO3
2.084
2.126
2.064
1.855
1.881
1.884
171.6
172.5
166.6
e
2.080
2.122
2.066
1.858
1.881
1.879
170.9
172.4
166.8
49
2.071
2.089
2.054
1.894
1.913
1.916
169.0
171.7
166.8
52
2.086
2.121
2.063
1.865
1.893
1.893
170.4
172.1
165.9
50
2.076
2.066
2.144
1.890
1.883
1.891
171.4
172.4
166.7
53
2.050
2.087
2.017
1.850
1.860
1.857
N1eAleN3
N2eAleO1
O2eAleO3
F1o
F2o
e
e
33
e
38
a
Alq3 X-ray data from reference [22]. Fo1 dihedral angle between 5- substituted
phenyl group and the quinoline nucleus. Fo2 dihedral angle between 7-substituted
phenyl group and the quinoline nucleus.
its aryl substituted derivatives. For comparison we also included
the X-ray experimental results of the parent Alq3 complex in Table 1
[24]. We clearly observe from this table the good agreement of
experimental and calculated data for Alq3. This indicates the reli-
ability of the computational method adopted in this study for the
delineation of the structure of the aryl substituted derivatives of
aluminum quinolinate.
As shown in Table 1, the bond lengths of AleN and AleO are only
slightly affected upon substitution of an aryl moiety at the C5 and
C7 positions of the quinoline ligand. The spectral shift of absorption
Fig. 4. The molecular orbital surfaces of the (a) HOMO (b) LUMO of mer-Alq3 and its aryl-substituted derivatives.

1158
FakhrEldinO. Suliman et al. / Dyes and Pigments 92 (2012) 1153e1159
Fig. 4. (continued).
complexes [7,12,14,25]. In Table 2 we list the calculated energies of
the HOMO and LUMO of the studied complexes. It is observed from
these results that the presence of an aryl moiety at C5 stabilizes
simultaneously the HOMO and the LUMO eigenvalues. Further
stabilization of the HOMO and the LUMO is observed when
substitution of another aryl moiety occurs at C7. Interestingly, the
HOMO and LUMO energy gap (Eg) of the investigated complexes
show a decrease with substitution that correlates well with the red
shift observed for emission bands. The calculated energy gaps for
the 5-aryl substituted derivatives also agree well with the experi-
mental values [3] as shown in Table 2. Also it is clear from Table 2
that the absorbance wavelengths of the complexes correlate well
with the energy gap and a clear red shift is observed for all
complexes. The maximum shift was obtained for Al-4b which is in
line with results obtained by the theoretical model. It is worth
mentioning here that more accurate simulation of the electronic
spectra requires further computations using time-dependent
density functional theory (TD-DFT) [26].
It has been reported that the energy of the LUMO greatly influ-
ence the turn-on voltage in organic light emitting diodes (OLED)
[12]. Theinjection energyof electronsfromthecathodeAl tothe Alq3
surface is calculated to be 2.4 eV [12]. This is indicative of the pres-
ence of a large energy barrier for the charge injection for Alq3 which
entails the use of a high turn-onvoltage. Therefore, the occurrence of
a lower LUMO will enhance the ability of electron injection. It is clear
from this study that the simultaneous substitution at C5 and C7 of
the quinolinate ligand could be an effective way to lower the LUMO
levels in the alumiumquinolinate compounds. The p-cyanophenyl
substituents lowered the LUMO by 0.79 eV and 1.16 eV for the 5-aryl
substituted and 5, 7-aryl substituted ligands respectively. The ace-
tylphenyl substituted ligands, however, lowered the LUMO level by
0.48 eV and 0.66 eV for the 5-aryl substituted and the 5, 7-aryl
substituted derivatives, respectively.
4. Conclusion
We have prepared two new aryl substituted 8-hydroxyquinoline
derivatives and their metal complex dyes with Al(III). Furthermore,
their spectroscopic properties were compared with those of the 5-
aryl substituted derivatives and the parent Alq3 complex. It has
been shown that these new dyes exhibit a higher quantum yield as
compared to Alq3. These compounds emit in the range of
530e535 nm. The ground state of these metal complex dyes have
been optimized using the B3LYP/6-31G* level of theory. The
calculations indicated some structural changes due to attachment
Table 2
The HOMO, LUMO and gap energies calculated by the DFT-B3LYP/6-31G* method.
Compound
labs (nm)
EHOMO (au)
ELUMO (au)
Eg (au)
Alq3
380
393
405
389
397
ꢀ0.1806
ꢀ0.1992
ꢀ0.2148
ꢀ0.1905
ꢀ0.1964
ꢀ0.0602
ꢀ0.0831
ꢀ0.1052
ꢀ0.0777
ꢀ0.0831
0.1204
Al-4a
Al-4b
Al-4c
Al-4d
0.1161 (0.1128)a
0.1096
0.1128 (0.1091)a
0.1133
a
From reference [3].

FakhrEldinO. Suliman et al. / Dyes and Pigments 92 (2012) 1153e1159
1159
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The AleN bond lengths were correlated to the experimental spec-
tral shifts. Stabilization of the HOMO and the LUMO eigenvalues is
also observed and the energy gap Eg correlates well with the
observed spectral changes.
Acknowledgements
The authors would like to thank SQU for the financial support
(SQU grant # IG/SCI/09/05).
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