Synthesis, characterization and DFT calculation of 4-fluorophenyl substituted tris(8-hydroxyquinoline)aluminum(III) complexes

Article abstract of DOI:10.1016/j.saa.2013.08.055

New 4-fluorophenyl substituted 8-hydroxyquinoline derivatives, 5-(4-fluorophenyl)quinolin-8-ol and 5,7-bis(4-fluorophenyl)quinolin-8-ol, were synthesized and characterized by spectroscopic methods. The aluminum complexes of 5-(4-fluorophenyl)quinolin-8-ol (AlQF) and of 5,7-bis(4-fluorophenyl) quinolin-8-ol (AlQF2) exhibit strong fluorescence emission centered at 525 nm and 530 nm respectively. The quantum yield of both complexes were enhanced compared to the parent tris(8-hydroxyquinolinato)aluminum(III) complex. Electronic structures and photophysical properties of the new complexes were investigated theoretically by ab initio and density functional theory (DFT) and time dependent DFT (TD-DFT). Geometries of the ground state (S₀) and the first excited state (S₁) of the new complexes were optimized at the B3LYP/6-31G(d) functional and configuration interaction singles (CIS) method respectively. The aryl substituents were found to contribute significantly to the frontier molecular orbitals (FMOs). We have observed that in both cases the lowest occupied molecular orbital (LUMO) energy decreases while the energy of the highest occupied molecular orbital is slightly increased. The most significant increase was observed for AlQF2.

Full text of DOI:10.1016/j.saa.2013.08.055

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and DFT calculation of 4-fluorophenyl
substituted tris(8-hydroxyquinoline)aluminum(III) complexes
⇑
FakhrEldin O. Suliman , Isehaq Al-Nafai, Saleh N. Al-Busafi
Department of Chemistry, College of Science, Sultan Qaboos University, Box 36, Al-Khod 123, Sultanate of Oman, Oman
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Synthesis and characterization of new
4-fluorophenyl derivatives of 8-
hydroxyquinoline.
Emission spectra of the aluminum
complexes of the new ligands show
red shift.
Theoretical calculations were
performed using TD-DFT methods
coupled to B3LYP/6-31G(d) level of
theory.
ꢀ
ꢀ
Electronic structure and properties
were presented.
HOMO–LUMO energies and
contributions from different groups
were obtained.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 April 2013
Received in revised form 6 August 2013
Accepted 14 August 2013
Available online 26 August 2013
New 4-fluorophenyl substituted 8-hydroxyquinoline derivatives, 5-(4-fluorophenyl)quinolin-8-ol and
5,7-bis(4-fluorophenyl)quinolin-8-ol, were synthesized and characterized by spectroscopic methods.
The aluminum complexes of 5-(4-fluorophenyl)quinolin-8-ol (AlQF) and of 5,7-bis(4-fluorophenyl)quin-
olin-8-ol (AlQF2) exhibit strong fluorescence emission centered at 525 nm and 530 nm respectively. The
quantum yield of both complexes were enhanced compared to the parent tris(8-hydroxyquinolinato)alu-
minum(III) complex. Electronic structures and photophysical properties of the new complexes were
investigated theoretically by ab initio and density functional theory (DFT) and time dependent DFT
(TD-DFT). Geometries of the ground state (S0) and the first excited state (S1) of the new complexes were
optimized at the B3LYP/6-31G(d) functional and configuration interaction singles (CIS) method respec-
tively. The aryl substituents were found to contribute significantly to the frontier molecular orbitals
(FMOs). We have observed that in both cases the lowest occupied molecular orbital (LUMO) energy
decreases while the energy of the highest occupied molecular orbital is slightly increased. The most sig-
nificant increase was observed for AlQF2.
Keywords:
8-Hydroxyquinoline
Aluminum(III) complexes
Fluorescence
TD-DFT
OLEDs
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
technological interest following the introduction of small mole-
cules light emitting diodes (OLEDs) by Tang and Van Slyke [3].
8-Hydroxyquinoline (HQ), a well-known bidentate chelating
ligand, has been historically employed in gravimetric analysis to
selectively bind and precipitate metal ions by tuning of the pH of
the media [1,2]. But by the end of the eighties of last century, these
ligands has emerged as a center of extensive scientific and
The high thermal stability, excellent electron transport properties
and the unique emissive properties qualified tris(8-hydroxyquino-
line)aluminum(III) (AlQ) to play a pivotal role in the development
of OLEDs that lead to the introduction of flat panel display technol-
ogy. This step has stimulated researchers to develop many
aluminum complexes using new derivatives of 8-hydroxyquinoline
[4–12] and equally to develop complexes of HQ and its derivatives
with other metal ions such as gallium and indium has received a
⇑
Corresponding author. Tel.: +968 24141480; fax: +968 24141469.
E-mail address: fsuliman@squ.edu.om (FakhrEldin O. Suliman).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.08.055

FakhrEldin O. Suliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
67
considerable interest [4,8,13–18]. Of special interest is the tuning
Apparatus
of the emission color of OLEDs via introduction of various substit-
uents in 2, 4, or 5 and 7 positions of the quinolate ligand.
Melting points were measured by GallenKamp apparatus (UK).
1H and 13C NMR spectra were performed on a 400 MHz Bruker
spectrometer (Bruker, Germany) using tetramethylsilane (TMS)
as the internal standard and CDCl3 as the solvent at room temper-
ature. UV–Visible spectra were recorded on Varian CARY 50
spectrophotometer (VARIAN, Australia). Fluorescence spectra were
recorded on a PerkinElmer LS55 Luminescence spectrophotometer
(PerkinElmer, UK). pH of the buffer solutions were measured using
WTW pH meter pH320 (Germany).
A number of studies performed on mer-AlQ have revealed the
location of the highest molecular orbital (HOMO) to be mainly on
the phenoxide side whereas the lowest unoccupied molecular orbi-
tal (LUMO) density found in the pyridyl side [19–37]. This distribu-
tion, therefore, suggests that substitution on the phenoxide ring
will predominantly affects HOMO, while substitution on the pyridyl
ring will affect more LUMO. For instance locating electron donating
groups in 2, 3 and 4 position of HQ ligands destabilizes LUMO and as
a result its energy increases leading to larger HOMO–LUMO energy
gap. On the other side, substitution of electron withdrawing groups
in positions 5, 6 or 7 of the HQ ligand is expected to cause a blue
shift of the maximum emission wavelengths due to an increase in
the transition energy. By this notion many research groups has at-
tempted the design and synthesis of new aluminum quinolate com-
plexes with desirable characteristics such as high stability, tunable
emission color and intensity by introducing various substituents at
the phenoxide and pyridyl sides of HQ.
Synthesis
Mono substituted 4-fluorophenyl HQ (5-(4-fluorophenyl)quino-
lin-8-ol) was synthesized using literature methods [5]. While the
synthesis of a di substituted 8-hydroxyquinoline (5,7-(4-fluoro-
phenyl)-8-hydroxyquinoline) is performed similar to our previous
work [12] and is outlined below.
Investigation of efficient emitters possessing high fluorescence
intensity, higher thermal stability, proper hole transport and
charge injection properties has been the focus of an intense re-
search during the past decade [5,7,8,11,12,16,18,38,39]. Modifica-
tion of AlQ by altering the HOMO–LUMO energy gap and
consequently tuning of the emission energy has been attempted
via substitution of groups at various positions of the quninolinate
ligand. For instance, substitution of a methyl group in the pyridyl
ring resulted in enhancement of the luminescence quantum yield
of the resultant Al3+ and Ga3+ complexes [4]. On the other hand,
manipulation of the HOMO electronic density have been achieved
by substitution of electron withdrawing groups at the phenoxide
side of the quinolate group [6,7,10,11,31]. By this strategy the en-
5,7-Dibromo-8-benzyloxyquinoline (2a)
5,7-Dibromo-8-hydroxyquinoline (1a) (2.535 g, 8.37 mmol)
was mixed with benzyl chloride (1.15 g, 9.09 mmol), potassium
carbonate (1.14 g) and potassium iodide (0.135 g) in 50 mL
acetone. The mixture was refluxed under nitrogen gas for 10 h.
After reflux, the mixture was poured in water and the resulted so-
lid was filtered and washed with water and re-crystallized from
ethanol and dried in vacuum. The product was a light yellow solid
and its melting point range is 119–120 °C. The product structure
was confirmed by using NMR. dH (400 MHz, CDCl3) 5.53 (2H, s),
7.31 (2H, dd, J1 = 6.8 Hz, J2 = 14.3 Hz), 7.51 (2H, dd, J2 = 4.4 Hz,
J2 = 8.6 Hz), 7.61 (2H,d, J = 7.1 Hz) 7.96 (1H, s), 8.45 (1H, dd,
J1 = 1.3 Hz, J2 = 8.5 Hz), 8.96 (1H, dd, J1 = 1.2 Hz, J2 = 4.0 Hz).
ergy of the p–p
* transition increases leading to a shift of the emis-
sion wavelength to the blue region. However, inspection of the
literature reveals the fact that substitution of electron withdrawing
groups at C5 failed to produce such shift. Tuning of the emission
wavelength through substitution of aryl substituents at C5 have
been reported to span most of the visible region specially the green
to red part of the spectra [5,6,40]. An interesting scenario to pro-
duce blue emitting complexes based on AlQ platform has been re-
ported recently by Anzenbacher et al. [7]. The blue shifted emission
was achieved by combining substitutions of an electron donating
group at C4 together with and electron withdrawing group at C6
of the quinoline ligand. Furthermore, using a time-dependent func-
tional theory (TD-DFT) it has been demonstrated that substitution
of CH-groups in the positions 2–7 of the quinolinate ligand by
nitrogen atoms could impart significant tuning of the emission
wavelength over the whole visible range (400–700 nm) [24]. In
this case the 5-substituted derivative was predicted to produce
the most effective blue shift whereas the most important red shift
was obtained for the 4-substituted one.
5,7-Di(4-fluorophenyl)-8-benzyloxyquinoline (3a)
5,7-Dibromo-8-benzyloxyquinoline (2a) (2.50 g, 6.45 mmol)
was mixed with 4-fluorophenyl boronic acid (2.08 g, 14.86 mmol),
toluene (25.25 mL), ethanol (12.63 mL), water (18.90 mL), sodium
carbonate (2.71 g) and Pd(PPh3)3 (0.449 g). The mixture was re-
fluxed under nitrogen gas for 24 h. Then the product was extracted
with toluene and re-crystallized from ethanol and dried in vacuum.
The final product was a brownish yellow solid with mass of
1.987 g, (73.9%) yield and melting point range (106.2–106.5 °C).
NMR; dH (400 MHz, CDCl3) 5.13 (1H, s), 6.95 (4H, td, J1 = 2 Hz,
J2 = 6.8 Hz, J3 = 15.5 Hz), 7.03 (3H, dd, J1 = 2.3 Hz, J2 = 9.2 Hz), 7.11
(4H, dd, J1 = 4.2 Hz, J2 = 7.4 Hz), 7.28 (1H, dd, J1 = 4.1 Hz,
J2 = 8.5 Hz), 7.35 (1H, s), 7.48 (2H, td, J1 = 2.1 Hz, J2 = 5.5 Hz,
J3 = 12.1 Hz), 8.07 (1H, dd, J1 = 1.6 Hz, J2 = 8.5 Hz), 8.91 (1H, dd,
J1 = 1.6 Hz, J2 = 4.0 Hz); dC (100.4 MHz, CDCl3): 78 (C1), 115 (C2),
122 (C3), 126 (C4), 128 (C5), 129 (C6), 131 (C7), 132 (C8), 134
(C9), 135 (C10), 136 (C11), 138 (C12), 139 (C13), 145 (C14), 150
(C15), 153 (C16), 162 (C17), 164 (C18).
In this paper we describe the synthesis and characterization of
4-fluorophenyl mono-substituted HQ ligands at position 5 and a
disubstituted 4-fluorophenyl HQ at positions 5 and 7. Moreover
we report on the photoluminescent properties and TD-DFT theo-
retical calculations of their aluminum complexes.
5,7-Di(4-fluorophenyl)-8-hydroxyquinoline (4a)
5,7-Di(4-fluorophenyl)-8-benzyloxyquinoline
(3a)
(1.52 g,
3.6 mmol) was mixed with Pd/C (1.07 g) and cyclohexa-1,4-diene
(2.49 g, 31.08 mmol) in 34.0 mL ethanol. The mixture was refluxed
under nitrogen gas for 10 h. The product was filtered and re-crys-
tallized from ethanol/water and dried in vacuum. The final product
was white product and its mass was 0.338 g and with percent yield
of 28.4%. The melting point range of (117.8–118.3 °C). dH
(400 MHz, CDCl3): 7.13 (4H, td, J1 = 2.1 Hz, J2 = 8.6 Hz, J3 = 16 Hz),
7.38 (2H, dd, J1 = 1.5 Hz, J2 = 4.3 Hz), 7.42 (1H, dd, J1 = 2 Hz,
J2 = 5.5 Hz), 7.52 (1H, s), 7.77 (2H, td, J1 = 2.1 Hz, J2 = 5.5 Hz,
J3 = 8.8 Hz), 8.18 (1H, dd, J1 = 1.4 Hz, J2 = 8.5 Hz), 8.87 (1H, dd,
J1 = 1.4 Hz, J2 = 4.2 Hz). dC (100.4 MHz, CDCl3): 78 (C1), 117 (C2),
Experimental
Materials
All reagents were purchased from Aldrich and used without
purification. All solvents were used as received. Spectroscopic
grade solvents (Aldrich) were used for the fluorescence and
UV–Visible measurements

68
FakhrEldin O. Suliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
122 (C3), 127 (C4), 130 (C5), 131 (C6), 132 (C7), 134 (C8), 136 (C9),
138 (C10), 140 (C11), 148 (C12), 162 (C13), 165 (C14).
the exited state configurations to the electronic transitions and
to create the spectra by convolution of molecular orbitals.
Preparation of aluminum complexes and measurement of
photophysical properties
Results and discussion
Synthesis
A mixture of the new ligands (0.15 mmol) and aluminum chlo-
ride hexahydrate (12 mg, 0.05 mmol) in degassed ethanol (ꢁ5 mL)
was refluxed for 3 h. The mixture was cooled to room temperature
5,7-Dibromo-8-hydroxyquinoline 1a (Scheme S1, Supplemen-
tary information) was the starting material of the synthesis of
the new ligand. Protection of hydroxyl group was carried out using
chlorobenzyl to give 2a and then the ligand underwent Suzuki
cross coupling reaction with p-fluorophenylboronic acid to give
3a. Deprotection of the new ligand gave 4a. The structure of the
new ligand was confirmed by 1H and 13C NMR. The aluminum
complexes of 8-HQ and its derivatives were prepared in methanol.
and neutralized by triethylamine (ꢁ20
lL). 10 mL of water was
added to the mixture and the precipitated complex was filtered
off. The filtered precipitate was washed thoroughly with water,
ethanol and diethyl ether and dried under vacuum to provide the
desired product (calculated (M+H)+: AlQF, 742.69; AlQF2,
1024.95; found (M+H)+: AlQF, 742.10; AlQF2, 1024.36). The
quantum yield of these complexes was determined using quinine
sulfate in dilute H2SO4 aqueous solution as standard. The lumines-
cence quantum yield of the complexes is calculated using the
following equation:
Photopysical properties
The absorption spectra of the synthesized ligands 4a and 4b and
their corresponding aluminum(III) complexes AlQF and AlQF2
were obtained in methanol (each at concentration of
2.0 ꢂ 10ꢃ5 M) at room temperature and are shown in Fig. 1. On
the other hand, the emission spectra of the ligands
([ligand] = 3.0 ꢂ 10ꢃ6 M) and their corresponding complexes
([complex] = 5 ꢂ 10ꢃ7 M) are presented in Fig. 2. The photophysi-
cal data for absorption and emission are collected in Table 1. The
main features of the absorption spectra are the strong energy
ꢀ
ꢁꢀ
ꢁ
2
As Iu gu
Au Is gu
/u ¼
/
ð1Þ
s
where /u and /s are the emission quantum yield of the complex and
standard, Au and As are the absorbance of sample and standard,
whereas Iu and Is represent the areas under the corrected spectra
of the sample and standard respectively. gu and gs are the respective
refractive index of the solvents used to prepare the samples and the
bands around 205 and 250 nm assignable to
p–
p* transitions.
standard respectively.
Moreover, the spectra exhibit a broader lower energy band cen-
tered at 333 and 326 nm for 4a and 4b respectively.
Spectrofluorometric titrations were performed by preparing
solutions of the ligands (ca 2.5 ꢂ 10ꢃ5 M) by appropriate dilution
from stock solutions. Titrations were then performed by adding
Upon chelating both ligands to Al(III) ions the bands at 250
were red shifted. Additionally a remarkable red shift was observed
for the lower energy bands for both ligands following complexa-
tion. These observations are characteristics of complexes of quino-
late ligands with metal ions such as Al(III), In(III) and Zn(II) [8,18].
The luminescence spectra of ligands and their respective alumi-
num complexes recorded in methanol at room temperature are
shown in Fig. 3. Both complexes exhibit green emission centered
at 525 and 530 nm for AlQF and AlQF2 respectively. A slight red
shift in the emission maximum was observed by the second substi-
tution of 4-fluorophenyl group at C7, which is accompanied by a
significant increase in the quantum yield (Table 1). Furthermore,
the emission maximum of AlQF and AlQF2 are red shifted by 9
and 14 nm compared to the parent compound AlQ (516 nm in
methanol). Attachment of an aryl group to the parent qunioline li-
gand results in an extension of the conjugation leading to en-
hanced quantum efficiency that is almost doubled when a second
aryl substituent was placed at C7 of the phenoxide side (Table 1).
It is well known that absorption to S1 and fluorescence involve
the same electronic states, therefore, for molecular systems with
large extinction coefficients a higher fluorescence is usually ex-
pected and as a result high quantum yields are obtained for these
systems [12].
Titration of 4a and 4b with Al(III) ions in methanol are pre-
sented in Fig. 3. It is clear from these figures that with increasing
amounts of Al(III) ions in the solution a green signal starts to grow
at 530 nm and 525 nm for 4a and 4b respectively. These titration
curves are also characterized by the presence of clear isoemissive
points at 462 for AlQF2 and less obvious isoemmisive points for
AlQF at around 400 nm and 465 nm. The presence of isoemissive
points indicates the presence of two or more species in the solu-
tion, such as the complex and the free ligand. We further investi-
gated the stoichiometry of complexation using the continuous
variation method. The molar ratio of Al(III):ligand of the predomi-
nant complex was found to be 1:3 for AlFQ and AlQF2.
increasing amounts (20 l
L) from a stock solution of Al3+ ions in
methanol. Solution lifetime measurements were obtained using a
TimeMaster fluorescence lifetime spectrometer (Photon Technol-
ogy International, NJ, USA). Excitation was at 380 nm using an
LED. The measured transient signals were fit to a multiexponential
function and the goodness of fit was judged from the value of the
reduced chi-squared (v
2). In all the experiments a quartz cell of
1 cm was used to hold the sample and all measurements are
conducted at 23 1 °C.
Computational details
The ground state geometries (S0) molecular structures of AlQF
and AlQF2 were optimized by the ab initio Hartree–Fock (HF)
method and the density functional theory (DFT) method using
the B3LYP (Becke-three parameter hybrid exchange functional
[41] combined with the Lee–Yang–Parr correlation function [42]).
In all cases the 6-31G(d) basis set usually employed for the
geometry optimization of AlQ complexes was sued. This basis set
was reported to be sufficient to estimate the various parameters
of AlQ complexes [24]. On the other hand, the first excited state
geometry (S1) molecular structure was optimized by the single
configuration interaction (CIS) [43], with 3-21G(d) basis set.
To calculate estimates of the electronic transition energies,
which include some account of the electron correlation, we used
the time-dependent density functional theory (TD-DFT) together
with the hybrid B3LYP level of theory and using 6-31G(d) basis
set. Emission energies were obtained considering the CIS opti-
mized structures of the excited state.
All of the above calculations were performed using Gamess-US
program [44,45]. To calculate the molecular orbital contributions
of various groups and atoms the program GaussSum [46] was used.
This program was further used to determine the contribution of

FakhrEldin O. Suliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
69
Fig. 1. UV–Visible absorption spectra of 2.0 ꢂ 10ꢃ5 M (1) 4a (2) 4b (3) AlQF (4) AlQF2 in methanol.
DFT calculations
were investigated by the time-dependent density functional
theory (TD-DFT). The ground state geometry of mer-AlQF and
mer-AlQF2 complexes are computed at the HF/6-31G(d) and
B3LYP/6-31G(d) levels of theory. The optimized geometries of both
complexes obtained at B3LYp/6-31G(d) level of theory (Fig. S2,
supplementary information). The labels A, B, and C are used to dis-
tinguish the three quinoline ligands attached to the central alumi-
num ion. Table 2 presents some selected parameters of the
optimized geometries of these complexes together with the parent
AlQ which is presented here for comparison purposes. The ade-
quacy of the theoretical method was checked by comparing the
parameters obtained for AlQ with those reported in the literature
and with the experimental X-ray crystallography data [32,47]. It
is evident from these results that 4-fluorophenyl substituents at
C5 and C7 exerted only slight changes on bond angles and bond
lengths. However, we observed a greater effect on bond lengths
especially for Al–N upon substitution of another aryl group at C7.
It has been reported that there is a significant degree of coplanarity
between planes of quinoline group and the aryl substituent at C5
[6]. This in turn results in the decrease of the degree of conjugation
between the quinoline group and the aryl substituent and as a con-
sequence the efficacy of the electronic interaction between the
substituent and the rest of the complex is reduced. Our DFT opti-
mized structures exhibited dihedral angles of 55° for aryl substitu-
ents at C5 which is in line with the previously reported values for
ligands of the same kind [6,12]. However, the aryl substituent at C7
exhibited a dihedral angle of 33° with the quinoline group. This re-
sult indicates that the decreased coplanarity for the substituent at
C7 lead to enhanced electronic communication with the parent
quinoline group.
To have a further insight into the structure and properties of the
prepared complexes the electronic structure of these complexes
Examination of the frontier molecular orbitals (FMOs) (Fig. S3,
Supplementary information) has clearly shown that both
complexes exhibit electron distribution similar to the parent AlQ
Table 1
Summary of photophysical properties of 4a, 4b and their Al- complexes.a
Compound
kabs (nm)
kem (nm)
UF
sF (ns)
4a
4b
AlQF1
AlQF2
204, 243, 328
204, 259, 333
204, 259, 377
204, 274, 388
414
416
525
530
–
–
–
–
0.353
0.702
9.9 0.2
14.9 0.1
Fig. 2. Fluorescence spectra of: (a) (1) 5 ꢂ 10ꢃ7 M AlQF (2) 3 ꢂ 10ꢃ6 M 4a (b) (1)
5 ꢂ 10ꢃ7
M
AlQF2 (2) 3 ꢂ 10ꢃ6
M
4b in methanol (1) Emission spectra (10)
a
AlQF1, Al-4a complex; AlQF2, Al-4b complex.
Excitation spectra.

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FakhrEldin O. Suliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
HOMO orbitals are generally localized on the phenoxide side of
the ligands, whereas contribution to LUMO orbitals originate from
the pyridyl and the phenoxide groups with the former being the
major contributor. Interestingly, the aryl substituents contribute
significantly to HOMO orbitals. In the complex AlQF the aryl
substituent at C5 constitutes 11%, 12% and 13% of HOMO, H ꢃ 1
and H ꢃ 2 orbitals respectively. A higher contribution of aryl
substituent at C7 to HOMO orbitals (15–20%) was observed for
the disubstituted ligands compared to the contribution of the aryl
substituents at C5. It is evident from these results that the smaller
dihedral angle between the aryl substituent at C7 and the quino-
line moiety allows for higher electronic communications between
the substituents and the rest of the molecule. It is also observed
that the involvement of these substituents increases from HOMO
to H ꢃ 2 and even a stronger participation was observed for inner
MOs reaching 98% in H ꢃ 6 and L + 6.
We further investigated the electronic transitions by TD-DFT
calculations using B3LYP/6-31G(d) approximation which have
been reported previously to adequately simulate the absorption
and emission spectra of aluminum and gallium complexes
[12,20,25]. The computed electronic data are presented in Tables
5 and 6. For both complexes the calculations show that contribu-
tion to low energy absorptions involve the same molecular orbital
profiles. The most intense transitions are from HOMO to L + 1 and
L + 2 and from H ꢃ 2 to LUMO in both cases. On the other hand, the
major contribution to the lower energy emission at 547.8 nm for
AlQF involves a HOMO ? LUMO transition as shown in Table 6.
For AlQF2 the emission at 558.2 nm receives contributions from
HOMO ? LUMO and HOMO ? L + 1 transitions. The shift in
absorption and emission wavelengths has been well predicted by
the theoretical TD-DFT calculations. Additionally, the calculated
emission wavelengths are in good agreement with the experimen-
tally obtained values. It is well documented in the literature that
the electronic
p–
p* transitions in AlQ and analogue complexes
are localized on the quinoline ligands. These transitions are
predominantly from a phenoxide donor to a pyridyl acceptor part
of the ligand. Calculations have shown that substitution of 4-fluo-
rophenyl groups caused red shift in emission wavelength relative
to the parent AlQ complex. Furthermore, using the TD-DFT method
an additional 10 nm red shift was predicted when another 4-fluo-
rophenyl group is attached to C7. This is in good agreement with
the experimental results where the shift obtained was about
5 nm. Nonetheless, the TD-DFT method was not able to predict
the enhanced quantum yield as can be seen in Table 6. It has been
reported in the literature using theoretical and experimental re-
sults that fluorine substituents at C5 and C7 of the quinoline ligand
produce complexes that exhibited slight red shift compared to the
parent AlQ complex contradicting the expectations.
From the energy level diagram of AlQ, AlQF and AlQF2 (Fig. S4,
Supplementary information) it is clear, for both complexes, that
HOMO energy is only slightly affected by substituents at C5 and
C7, whereas the LUMO was stabilized in both cases especially for
AlQF2 where the LUMO exhibited greater decrease in energy.
Unexpectedly, these results are different than those obtained
experimentally for complexes with electron withdrawing aryl
substituents at C5 [5,6]. In these literature, it was reported that
substitution of an aryl withdrawing moiety at C5 have tuned the
emission color of the complexes by affecting the energy of HOMO
while keeping the LUMO energy almost constant as determined by
electrochemical methods.
Fig. 3. Titration of 5 ꢂ 10ꢃ5 M of (a) 4b (b) 4a with Al3+ showing growth of the
green emission in methanol; [Al3+] = 0–20 ꢂ 10ꢃ6 M.
Table 2
Selected parameters of HF/6-31G(d), B3LYP/6-31G(d) and CIS/3-21G(d) optimized
geometries of ground and excited electronic states.
R(Å)//(°)
HF
B3P
CIS
AlQF1
AlQF2
AlQF1
AlQF2
AlQF
AlQF2
Al–Na
Al–Nb
Al–Nc
Al–Oa
Al–Ob
Al–Oc
Nb–Al–Nc
Na–Al–Oc
Oa–Al–Ob
U1
2.097
2.146
2.063
1.826
1.854
1.860
171.0
171.7
164.3
53
2.090
2.147
2.072
1.859
1.868
1.871
170.3
171.4
165.4
52
2.084
2.128
2.063
1.852
1.880
1.889
171.1
172.6
166.6
52
2.070
2.113
2.059
1.859
1.885
1.883
170.1
172.3
166.5
53
2.002
2.094
2.098
1.892
1.850
1.847
171.2
172.3
166.5
52
2.069
2.101
2.084
1.873
1.872
1.852
171.3
169.0
165.6
51
U2
33
–
33
24
complex [20,21,29,30]. As expected all MOs are ligand centered
and without significant contribution from the central metal ion
as has been observed previously [21,22,29,30]. Inspection of the
highest occupied molecular orbital (HOMO) reveals that the aryl
substituents at C5 and C7 have considerable contribution to these
orbitals, however, this contribution is absent in case of LUMO orbi-
tals. To evaluate the role of the aryl substituents in the electronic
structure of these complexes we further calculated the contribu-
tions from various regions of the complexes at B3LYP/6-31G(d) le-
vel using methods reported previously [22]. The results are shown
in Tables 3 and 4 for AlQF and AlQF2 respectively. As expected
OLEDs materials are expected to generate a strong electrolumi-
nescence due to the efficient electron–hole combination. This phe-
nomenon accompanies injection of electrons from the cathode into
the LUMO orbitals while the holes are injected into the HOMO
orbitals. Enhancing the electron injection efficacy can be achieved
by lowering of the LUMO energy. Generally vertical ionization

FakhrEldin O. Suliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
71
Table 3
Molecular orbital components of AlFQ (%) obtained at B3LYP/6-31G(d,p).
Orbital
Energy (eV)
Phenol A
Phenol B
Phenol C
Pyridine A
Pyridine B
Pyridine C
Phenf A
Phenf B
Phenf C
Al
L + 2
L + 1
ꢃ1.52
ꢃ1.61
ꢃ1.83
ꢃ4.99
ꢃ5.18
ꢃ5.27
ꢃ1.61
21
7
0
69
9
1
6
18
4
9
66
0
2
3
22
0
0
74
3
52
18
0
9
1
14
43
12
1
9
1
4
7
57
0
1
10
7
1
0
0
11
2
0
0
1
0
1
12
0
0
0
1
0
0
0
0
0
0
1
0
0
LUMO
HOMO
H ꢃ 1
H ꢃ 2
L + 1
0
18
13
0
7
18
43
0
1
Table 4
Molecular orbital components of AlF2Q (%) obtained at B3LYP/6-31G(d,p).
Energy (eV) Phenol A Phenol B Phenol C Pyridine A Pyridine B Pyridine C Phenf 5A Phenf 5B Phenf 5C phenf 7A Phenf 7B Phenf 7C Al Al
L + 2
L + 1
ꢃ1.74
ꢃ1.84
22
5
0
58
6
0
5
19
4
7
56
0
2
6
22
0
1
59
53
22
0
8
1
10
34
11
1
8
0
3
7
55
0
1
9
1
0
0
7
1
0
0
1
0
1
8
0
0
0
1
0
0
8
2
1
0
15
2
0
1
3
1
2
17
0
0
1
3
0
0
0
0
0
1
0
0
1
3
0
0
LUMO ꢃ2.06
HOMO ꢃ5.01
H ꢃ 1
H ꢃ 2
ꢃ5.21
ꢃ5.3
1
20
20
other descriptors such as the hardness (
g), the chemical potential
Table 5
and the global electrophlicity index ( ) are presented in Table 7.
x
Calculated absorption wavelengths (k), oscillator strengths (f) and major contribution
at TD-B3LYP/6-31G(d) level of theory using HF/6-31G(d) geometry for AlQF and
AlQF2.
It is apparent from these results that VEA is greatly affected by sub-
stitution of aryl groups at C5 and C7 position of the quinoline ligand
compared to VIP which is less affected. Substitution of a 4-fluoro-
phenyl group at C5 resulted in a 0.3 eV decrease in VEA compared
to the parent molecule, AlQ. Interestingly, a further 0.33 eV de-
crease in VEA was observed when a second substituent was placed
at C7 of the quinoline ligand. It can be inferred from these results
that the di-substitution of electron withdrawing aryl groups at C5
and C7 positions could be an effective way to improve the electron
injection abilities of AlQ complexes.
k (nm)
fcalc
Major contribution
ALOF
433.88
422.20
413.11
411.25
387.43
375.17
314.51
0.0193
0.1151
0.0487
0.0211
0.0146
0.0119
0.0207
HOMO ? LUMO (91%)
HOMO ? L + 1 (60%), HOMO ? L + 2 (24%)
H ꢃ 2 ? LUMO (81%)
H ꢃ 1 ? LUMO (73%)
H ꢃ 1 ? L + 1 (71%)
H ꢃ 2 ? L+ (97%)
HOMO ? L + 3 (91%)
Chemical hardness (g) and global electrophilicity index (x) are
ALQF2
465.80
447.42
439.57
438.40
customarily used to describe the chemical stability and reactivity
of chemical species. These descriptors are calculated by the follow-
ing equations [33]:
0.0211
0.0707
0.0606
0.0201
HOMO ? LUMO (92%)
HOMO ? L + 1 (56%), HOMO ? L + 2 (21%)
H ꢃ 2 ? LUMO (80%)
HOMO ? L + 2 (70%), HOMO ? L + 1 (21%)
VIP ꢃ VEA
g
¼
ð4Þ
ð5Þ
2
l2
Table 6
x
¼
2g
Calculated emission wavelengths (k), oscillator strengths (f) and major contribution at
TD-B3LYP/6-31G(d) level of theory using CIS/3-21G(d) geometry for AlQF and AlQF2.
The results in Table 7 reveals the fact that hardness of the alumi-
num complexes included in this study decrease upon substitution
with the highest hardness value obtained for the parent molecule.
k (nm)
fcalc
Major contribution
ALOF
547.8
424.2
0.0730
0.0275
HOMO ? LUMO (96%)
H ꢃ 1 ? L + 1 (90%)
The global reactivity index (
of the molecule. The results shown in Table 7 clearly indicate that
increases with substitution at C5 and C7 positions. Therefore, this
x) measures the electrophilic power
AQF2
558.2
542.5
x
0.0650
0.0790
HOMO ? LUMO (61%), HOMO ? L + 1 (37%)
H ꢃ 2 ? LUMO (76%), H ꢃ 2 ? L + 1 (18%)
global electrophilicity index can be utilized to assess the electron
injection properties of OLEDs.
potential (VIP) and electron affinities (VEA) are used to evaluate
the energy barriers associated with the injection of electrons and
holes into OLEDs. These parameters can be calculated using the
DFT methods using the following equations:
Table 7
Energy of the Ionization potential, electron affinity, global hardness, global electro-
philicity index and other descriptors.
Parameter (eV)
AlQ
6.32
0.47
2.93
AlQF
6.18
0.77
2.71
AlQF2
VIP ¼ EþN ꢃ E0N
VEA ¼ E0N ꢃ EꢃN
ð2Þ
ð3Þ
Ionization potential (VIP)
Elecrn Affiniy(VEA)
6.10
1.10
2.50
Hardness (
Chemical potential (
Electronegativity (
Global electrophilicty index (
g
)
where EþN , EꢃN , and E0N are the total energy of the cationic, anionic and
neutral states calculated at the optimized neutral geometry,
respectively. The calculated values of VIP and VEP as well as the
l
)
)
ꢃ3.40
ꢃ3.46
ꢃ3.60
v
3.40
3.48
3.60
x
)
1.97
2.23
2.60

72
FakhrEldin O. Suliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 66–72
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Conclusion
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We have developed new 4-fluorophenyl substituted derivatives
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of 8-hydroxyquinoline and their aluminum complexes. The
absorption and emission spectroscopy properties of the aluminum
complexes of the new ligands were further studied. A higher quan-
tum yield was obtained for both complexes compared to the parent
tris(8-hydroxyquinolinolate)aluminum(III) with AlQF2 exhibiting
the highest quantum yield. On the basis of their photophysical
properties these complexes can be considered as efficient green
light emitters. Moreover, using DFT methods the ground state
structure was calculated using the B3LYP/6-31G(d) level of theory.
On the other hand the first excited state was optimized by CIS
methods at 3-21G(d) level of theory. The aryl substituents were
predicted to contribute significantly to the HOMO orbitals.
Additionally, using TD-DFT methods the predicted emission and
absorption wavelengths were in good agreement with the
experimental results.
Acknowledgement
We would like to thank SQU for the research grant #IG/SCI/09/
05. Thanks are also to CAARU for the ESI-MS.
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