Theoretical and experimental studies on the fluorescence properties of aluminum(III), cadmium(II), zinc(II), and copper(II) complexes of substituted 8-hydroxyquinoline

Article abstract of DOI:10.1177/1747519820973601

Fifty-five 8-hydroxyquinoline (8-HQ) derivatives are synthesized and the corresponding aluminum(III), cadmium(II), copper(II), and zinc(II) metal complexes are prepared and their fluorescent activities are evaluated. The results indicate that the aluminum complexes have the best fluorescence properties, followed by zinc and cadmium complexes, while almost no fluorescence is observed with the copper complexes. The relationship between the fluorescence properties and complex structure is summarized and a predictive three-dimensional quantitative structure–property relationship model is established using the multifit molecular alignment rule of Sybyl program. With the introduction of groups at the C-2 position or electron-withdrawing groups to the 8-hydroxyquinoline framework, fluorescence wavelength blue shifts are observed with the zinc, aluminum, and cadmium complexes. At the same time, a red shift of the fluorescence emission wavelength is detected for the aluminum and zinc complexes when C-5 of 8-hydroxyquinoline was substituted with halogens or a methyl group. Moreover, the zinc, cadmium, and aluminum complexes with 2,4-dimethyl substituents on the 8-hydroxyquinoline all show good fluorescence properties. The highest occupied molecular orbital and lowest unoccupied molecular orbital energies of the complexes are also calculated and the fluorescence properties of the metal complexes are analyzed from the viewpoint of molecular orbitals.

Full text of DOI:10.1177/1747519820973601

973601CHL0010.1177/1747519820973601Journal of Chemical ResearchYuanyuan et al.
research-article2020
Research Paper
Journal of Chemical Research
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Theoretical and experimental studies on the
fluorescence properties of aluminum(III),
cadmium(II), zinc(II), and copper(II) complexes
of substituted 8-hydroxyquinoline
https://doi.org/10.1177/1747519820973601
DOI: 10.1177/1747519820973601
journals.sagepub.com/home/chl
Zhang Yuanyuan1,2, Cheng Hongrui1, Sun Qingrong1,
Chen Hongli1, Yang Weiqing1 and Ma Menglin1
Abstract
Fifty-five 8-hydroxyquinoline (8-HQ) derivatives are synthesized and the corresponding aluminum(III), cadmium(II),
copper(II), and zinc(II) metal complexes are prepared and their fluorescent activities are evaluated. The results indicate
that the aluminum complexes have the best fluorescence properties, followed by zinc and cadmium complexes, while
almost no fluorescence is observed with the copper complexes. The relationship between the fluorescence properties
and complex structure is summarized and a predictive three-dimensional quantitative structure–property relationship
model is established using the multifit molecular alignment rule of Sybyl program. With the introduction of groups at
the C-2 position or electron-withdrawing groups to the 8-hydroxyquinoline framework, fluorescence wavelength blue
shifts are observed with the zinc, aluminum, and cadmium complexes. At the same time, a red shift of the fluorescence
emission wavelength is detected for the aluminum and zinc complexes when C-5 of 8-hydroxyquinoline was substituted
with halogens or a methyl group. Moreover, the zinc, cadmium, and aluminum complexes with 2,4-dimethyl substituents
on the 8-hydroxyquinoline all show good fluorescence properties. The highest occupied molecular orbital and lowest
unoccupied molecular orbital energies of the complexes are also calculated and the fluorescence properties of the metal
complexes are analyzed from the viewpoint of molecular orbitals.
Keywords
3D-QSPR, 8-hydroxyquinoline derivatives, aluminum complexes, cadmium complexes, copper complexes,
fluorescence, zinc complexes
Date received: 11 September 2020; accepted: 26 October 2020
Introduction
Organic light-emitting diodes (OLEDs) are highly sought-
after by researchers in the monitor and lighting fields,
and developing new luminescent materials with good
properties has become an important area of research.1–5
8-Hydroxyquinoline, which possesses a heterocyclic con-
jugated planar structure, was used in multiple areas because
of its chelating performance, luminescence properties, and
1School of Science, Xihua University, Chengdu, P.R. China
2College of Chemical Engineering, Sichuan University, Chengdu, P.R.
China
Corresponding author:
Ma Menglin, School of Science, Xihua University, Chengdu 610039, P.R.
China
Email: mmlchem@163.com
Creative Commons Non Commercial CC BY-NC: This article is distributed under the terms of the Creative Commons
Attribution-NonCommercial 4.0 License (https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use,
reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and
Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).

2
Journal of Chemical Research 00(0)
Figure 1. The structures of the previously prepared 8-hydroxyquinoline derivatives.
coordination abilities.6 Despite many efforts having been metal complexes of 8-hydroxyquinoline with different sub-
made on the preparation of 8-hydroxyquinoline deriva- stituents. In the present work, a combined experimental and
tives and their metal complexes,7–12 only a limited number theoretical method was used to investigate the fluorescence
of comparative studies are available on the influence of spectral properties of 8-hydroxyquinoline derivatives, and
different substituents on the 8-hydroxyquinoline frame- provides a useful tool in designing metal complexes of
work and different metals on the fluorescence properties of 8-hydroxyquinoline by correlating theoretical optical prop-
the corresponding complexes. Recently, we reported the erties with their experimental values.
synthesis of 55 different substituted 8-hydroxyquinolines
(Figure 1) along with preliminary studies on the fluores-
Results and discussion
cent properties of their zinc complexes.13
For a comparative study on the fluorescence properties In general, the fluorescence intensities of the 8-hydrox-
of quinolines with different substituents and different cen- yquinoline aluminum complexes were the highest. Those
tral metals, a series of 8-hydroxyquinoline complexes with of the zinc and cadmium complexes were very similar and
aluminum(III), cadmium(II), zinc(II), and copper(II) was weaker than those of the corresponding aluminum com-
prepared using the substituted 8-hydroxyquinolines shown plexes. Fluorescence was barely observed for the copper
above as ligands (Scheme 1).
complexes of the 8-hydroxyquinoline.
The luminescence properties of the aluminum(III),
The fluorescence emissions of the 55 aluminum com-
cadmium(II), zinc(II), and copper(II) complexes were pounds were measured, 5 of which showed better fluores-
determined by fluorescence spectroscopy. The experimen- cence intensity than 8-hydroxyquinoline with fluorescence
tal results demonstrated that different substituents on the quantum yields (ɸf) above 0.171. Among the 22 complexes
quinoline ring and different central metals had diverse in which fluorescence was observed, the emission wave-
influences on the fluorescence properties.
lengths of eight compounds were red-shifted and the wave-
To our knowledge, there are no previous studies on the lengths of 14 compounds were blue-shifted. Compared
theoretical and experimental fluorescence properties of with aluminum complexes, nine zinc complexes exhibited

Yuanyuan et al.
3
.
Scheme 1. Preparation of substituted 8-hydroxyquinoline Al(III), Cu(II), Cd(II), and Zn(II) complexes.
Table 1. The fluorescence and UV data of the aluminum complexes with 1k, 1q, 1p, 1r, and 1o and 8-HQ under different
concentrations.
Entry
Ligand
UV
Fluorescence
Intensity
ɸf
λmax (nm)
λex (nm)
λem (nm)
2×10−5 molL−1
2×10−6 molL−1
2×10−7 molL−1
1
2
3
4
5
6
7
1k
1p
1q
1r
1o
272
246
258
252
248
252
258
273
256
262
260
260
370
370
417
468
464
492
502
510
496
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
1780
>10,000
>10,000
5200
5000
600
3700
400
–
–
–
0.990
0.969
0.572
0.540
0.249
0.171
0.0195
8-HQ
3a
400
–
–
–
8-HQ: 8-hydroxyquinoline.
improved fluorescence intensities in comparison to
The fluorescence intensities of the aluminum com-
8-hydroxyquinoline. Among the 16 zinc complexes in plexes were 1k>1p>1q>1r>1o>8-HQ (Table 1).
which fluorescence was observed, the emission wave- The ɸf values of the five complexes increased signifi-
lengths of three compounds were red-shifted and those of cantly compared with the parent compound 8-HQ, by
the remaining 13 compounds were blue-shifted. The fluo- 46% in the case of 1o to 475% for 1k. When a piperidyl
rescence properties of the cadmium complexes were very moiety was introduced at C-2 of 8-HQ, the fluorescence
similar to zinc, albeit the fluorescence intensity was weaker. intensity was obviously enhanced. Even when the solu-
There were eight cadmium complexes which exhibited tion of compound 1k was diluted to 2×10−6 molL−1, the
improved fluorescence intensity compared to 8-hydrox- fluorescence intensity was still more than 10,000 (Table 1,
yquinoline, and the emission wavelengths of them all were entry 1). After further diluting to 2×10−7 molL−1, com-
blue-shifted. Fluorescence was barely observed with the pound 1k still had a relatively strong fluorescence, while
copper complexes, even with 8-hydroxyquinoline (8-HQ). the other compounds had almost no fluorescence. The
The influence of different central metals and different sub- compounds with fluorescence intensities stronger than
stituents on the 8-hydroxyquinoline on the fluorescence 8-hydroxyquinoline were methylated at C-2 and C-4 posi-
emission wavelengths and intensities are discussed below.
tions (Table 1, entries 2–6). Among them, ligands 1p and
1q having bromo groups at the C-5 and C-7 positions on
quinoline benzene ring had better fluorescence intensities
(Table 1, entries 2 and 3).
There were nine 8-HQ derivatives of zinc complexes
possessing high fluorescence intensities and these were
The effects of substituents on
8-hydroxyquinoline on the fluorescence
intensity
The fluorescence data of the aluminum complexes with 1k, also characterized by UV spectroscopy analysis (Figure 3).
1q, 1p, 1r, and 1o having the greatest fluorescence intensi- The fluorescence spectra and UV spectra of their zinc com-
ties are listed in Table 1 (see also Figure 2). In order to plexes are shown below.
clearly distinguish the fluorescence intensity of these com-
pounds, the concentration of solutions was diluted.
Furthermore, the data in Table 2 reveal that the ɸf values
of complexes 3a, 1o, 1e, 1n, 1q, 1p, 4q, 5d, and 1r were

4
Journal of Chemical Research 00(0)
2.0
1.5
1.0
0.5
0.0
1k-Al
1o-Al
30000
20000
10000
0
1k-Al
1o-Al
1p-Al
1q-Al
1r-Al
1p-Al
1q-Al
1r-Al
8-HQ-Al
8-HQ-Al
250
300
350
400
400
500
Wavelength (nm)
Wavelength (nm)
Figure 2. Fluorescence (left) and UV (right) spectra of aluminum complexes with 1k, 1q, 1p, 1r, and 1o and 8-HQ in
2×10−6 molL−1 MeOH.
8-HQ-Zn
2.5
1e-Zn
16000
8-HQ-Zn
1n-Zn
1e-Zn
14000
12000
10000
8000
6000
4000
2000
0
2.0
1.5
1.0
0.5
0.0
1o-Zn
1p-Zn
1q-Zn
3a-Zn
4q-Zn
5d-Zn
1n-Zn
1o-Zn
1p-Zn
1q-Zn
3a-Zn
4q-Zn
5d-Zn
300
400
500
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 3. Fluorescence (left) and UV (right) spectra of 8-hydroxyquinoline derivative zinc complexes with high fluorescence
intensities in MeOH at 2×10−5 molL−1.
Table 2. The fluorescence and UV data of zinc complexes
with 3a, 1o, 1e, 1n, 1q, 1p, 4q, 5d, 1r, and 8-HQ at a
concentration of 2×10−5molL−1.
dependent on the electronic properties and steric factors of
substituents, for example, methyl, halogen, and sulfonic
acid groups at the C-4, C-5, and C-7 positions of 8-HQ. For
instance, the ɸf values of the eight complexes increased sig-
nificantly relative to the parent 8-HQ from 20% in the case
of 1r to 842% for 3a. The ɸf values of 3a was 0.377, which
is greater than that of 8-hydroxyquinoline (0.040) with zinc
Entry Ligand UV
Fluorescence
Intensity
ɸf
λmax λex
λem
(nm)
2×10−5 molL−1
(nm)
256
(nm)
1
3a
1o
1e
1n
1q
1p
4q
5d
1r
259
374
216
210
216
210
215
230
235
263
237
537
531
515
512
510
513
527
519
545
556
–
14,289
9894
7953
6704
6609
5170
5000
4632
1792
1703
<400
0.377 (Table 2, entries 1 and 10).
2
3
4
5
6
7
8
9
262
260
262
260
266
258
258
260
0.248
0.226
0.174
0.153
0.138
0.130
0.116
0.048
0.040
–
There were eight fluorescent 8-hydroxyquinoline cad-
mium complexes that were characterized by UV spectro-
scopic analysis using the 8-hydroxyquinolone cadmium
complex for comparison (Figure 4). The fluorescence and
UV spectra of the cadmium complexes are as follows.
The fluorescence intensities of the cadmium complexes
of 8-hydroxyquinoline derivatives and 8-HQ were much
lower than those of the corresponding aluminum and zinc
complexes (Table 3). The complexes of 3a, 1o, 1e, 1n, 1q,
1p, 4q, and 5d all showed higher fluorescence intensities
than 8-HQ. This is in good agreement with the zinc
10
11
8-HQ 260
1k 257
8-HQ: 8-hydroxyquinoline.

Yuanyuan et al.
5
2000
1500
1000
500
0
2.5
2.0
1.5
1.0
0.5
0.0
8-HQ-Zn
1e-Zn
1n-Zn
1o-Zn
1p-Zn
1q-Zn
3a-Zn
4q-Zn
5d-Zn
8-HQ-Cd
1e-Zn
1n-Zn
1o-Zn
1p-Zn
1q-Zn
3a-Zn
5m-Zn
5o-Zn
300
400
500
400
450
500
550
600
Wavelength (nm)
Wavelength(nm)
Figure 4. Fluorescence (left) and UV (right) spectra of 8-hydroxyquinoline derivative cadmium complexes with high fluorescence
intensity in MeOH at 2×10−5 molL−1.
Table 3. The fluorescence and UV data of cadmium
complexes of 3a, 1o, 1e, 1q, 1n, 4q, 5d, 1p, and 8-HQ at the
concentration of 2×10−5 molL−1.
completely different fluorescence intensities compared
with those of aluminum complexes. The aluminum com-
plex of compound 1k exhibited good fluorescence inten-
sity, while those zinc and cadmium showed lower
fluorescence (Table 1 entry 1; Table 2 entry 1; Table 3 entry
10). The reason may be that the aluminum complexes were
tri-coordinated and the piperidine could participate in metal
complexation, while the 8-hydroxyquinoline zinc and cad-
mium complexes were bi-coordinated and piperidine ring
was not involved due to the spatial relationships. In the alu-
Entry Ligand UV
Fluorescence
Intensity
ɸf
λmax λex
(nm) (nm)
λem
(nm)
2×10−5 molL−1
1
2
3
4
5
6
7
8
9
3a
1o
1e
1q
1n
4q
5d
1p
258
262
262
264
258
256
258
260
262
374
218
218
218
218
232
212
263
245
556
544
526
520
522
530
520
530
–
1785
1319
1223
1099
1031
1000
926
861
<400
<400
0.048
0.035
0.033
0.029 minum complex of 1k, the piperidine ring had a strong
0.028
0.027
0.025
0.023
–
electron-donating effect and formed a large p–π conjuga-
tion with quinoline ring, which made the conjugation effect
over the whole molecule stronger and the fluorescence
intensity increased. In addition, introduction of a piperidine
ring increased the rigidity of whole molecular structure and
reduced the molecular vibrations, which decreased the
probability of collisions with other molecules and increased
the fluorescence intensity.
8-HQ 260
1k 262
10
–
–
8-HQ: 8-hydroxyquinoline.
However, with compound 3a as the ligand, the zinc
complex exhibited almost 10 times the fluorescence inten-
sity of the 8-HQ zinc complex (Table 2, entry 1), and the
fluorescence intensity of which was also the best for the
cadmium complex (Table 3, entry 1). However, the fluores-
cence properties of the 3a aluminum complex were not
good, being almost 10 times worse than 8-HQ aluminum
(Table 1, entry 7). When C-5 was substituted by a sulfonic
acid group, such as in 1e, 4q, and 5d, only the zinc and
cadmium complexes exhibited high fluorescence intensi-
ties and fluorescence quantum yields (Table 2, entries 3, 7,
and 8). The possible reason was that the rigidity of the zinc
and cadmium complexes was greater than that of the alu-
minum complexes, and the introduction of a methyl group
could stabilize the complexes.
complexes, but the fluorescence intensities of the cadmium
complexes were lower than those of zinc.
As almost all copper complexes of the 8-hydroxyquino-
line derivatives were not fluorescence active, no further
studies are discussed herein.
There are three compounds—1o, 1p, and 1q—for which
the zinc, cadmium, and aluminum complexes all showed
good fluorescence properties (Table 1, entries 2, 3, and 5;
Table 2, entries 2, 5, and 6; Table 3, entries 2, 4, and 8).
Their common structural features were 2,4-dimethyl substi-
tution of the parent 8-hydroxyquinoline. The introduction of
substituents at C-2 and C-4 of the pyridine ring, such as in
ligands 1o, 1p, and 1q, produced a certain spatial steric
resistance to the molecular structure of the complex, which
was not conducive for metal binding and forming a stable
five-membered ring structure. At the same time, the N atom
of the pyridine ring, having lone pair electrons, was directly
involved in complexation of the metal. Thus, steric hin-
drance has a great influence on the fluorescence properties.
Some zinc and cadmium complexes—such as those of
1k, 1e, 3a, 4q, and 5d—exhibited the corresponding
Three-dimensional quantitative structure–
property relationship of the metal
complexes
Three-dimensional quantitative structure–property rela-
tionship (3D-QSPR) analysis is widely applied due to its

6
Journal of Chemical Research 00(0)
Al complexes
Zn complexes
Cd complexes
Figure 5. 3D views of metal complex congeners by RMSD-based fitting.
Table 4. The statistical parameters for the 3D-QSPR CoMFA modelsa.
Entry
Complex
Statistical results
Field contribution
R2
F
SE
q2
Steric
(%)
Electrostatic
(%)
1
2
3
Al
Zn
Cd
0.888
0.935
0.912
28.471
45.632
44.781
1.236
0.892
0.955
0.444
0.539
0.505
90.1
69.2
84.4
9.90
30.8
15.6
R2: the predictive correlation coefficient; F: test value; SE: standard error of estimate; q2: the leave-one-out (LOO) cross-validation coefficient.
aMethod is CoMFA.
well-established predictive power.14–16 In particular, the
Al
Zn
Cd
comparative molecular field analysis (CoMFA) is an effec-
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Al: n=25, R=0.928, SD=0.09956, P<0.0001
Zn: n=22, R=0.974, SD=0.02013, P<0.0001
Cd: n=12, R=0.952, SD=0.01183, P<0.0001
tive method based on statistical techniques.17
In order to further modify the molecule, 3D-QSPR stud-
ies were applied on the ɸf using the CoMFA protocol for the
first time. The data-based fitting procedure was finally
adopted through careful comparison. Each metal complex,
in which fluorescence could be observed, was superim-
posed to the template based on the common substructure of
aluminum(III), cadmium(II), zinc(II), and copper(II) com-
plexes of 8-hydroxyquinoline. The aligned molecules are
illustrated in Figure 5, and the statistical parameters are
listed in Table 4.
For metal complexes, as shown in Table 4, better predic-
tions are obtained with 3D-QSPR/CoMFA for the absorp-
tion properties. The model has a high R2 value (0.935) for
zinc complexes, with a low standard deviation (SE, 0.892)
and a high Fischer’s ratio (F, 45.632), while a QSPR model
is generally acceptable if R2 is approximately 0.85 or
higher. Specially, the cross-validation-related coefficient q2
0.0
0.2
0.4
0.6
0.8
1.0
Observed fluorescence quantum yield
Figure 6. Plots of experimental data versus the corresponding
predicted data from the QSPR/CoMFA models.
is 0.4501 (>0.45), suggesting a good prediction ability of showed that the steric hindrance has a significant influence
this model. on the fluorescence properties, especially the fluorescence
The contribution of the steric field of aluminum(III), properties of aluminum complexes which are almost deter-
zinc(II), and cadmium(II) is 90.1% (Table 4, entry 1), mined by the steric factors. The plots of the predicted ver-
69.2% (Table 4, entry 2), and 84.4% (Table 4, entry 3) and sus actual ɸf value for the QSPR/CoMFA model are
the electrostatic field of aluminum(III), zinc(II), and displayed in Figure 6, which shows a good correlation. The
cadmium(II) is 9.90% (Table 4, entry 1), 30.8% (Table 4, electrostatic field is the main factor influencing the maxi-
entry 2), and 15.6% (Table 4, entry 3), respectively. This mum absorption.

Yuanyuan et al.
7
active as their cadmium complexes, which were blue-
shifted as well.
Al Green75.80%, Yellow 24.20%
Blue 13.23%, Red 86.77%
The blue shift of complex 1i is 97nm, which is the largest
of the aluminum complexes, while that of the zinc complex
is 137nm which is also the largest among the zinc com-
plexes (Table 5, entry 13). The largest blue shift in the cad-
mium complexes is compound 1q with a blue shift of 70nm
(Table 5, entry 12). The blue shifts for different substituted
compounds with different metal complexes were almost the
same. The effect of substituents on the fluorescence wave-
lengths is discussed later. In terms of the blue shift of the
maximum fluorescence emission wavelength, the results for
the zinc, cadmium, and aluminum complexes were almost
identical and the common points are discussed below.
The maximum emission indicated that the electron-donat-
ing and electron-withdrawing nature of the substituents on
the 8-HQ and the emission wavelengths had a good correla-
tion.18 There were two factors causing the blue shifts of the
aluminum, cadmium, and zinc complexes, which were the
introduction of electron-donating groups on the pyridine ring
and electron-withdrawing groups on the benzene ring.19,20
The addition of substituents at C-2 of the pyridine ring, such
as in ligands 1b, 1c, 1g, 1i, 1l, 1n, 1o, 1p, 1q, and 1r, pro-
duced a certain spatial steric resistance to the complex struc-
ture, which was not conducive to the formation of a stable
five-membered ring structure. At the same time, the N atom
of the pyridine ring, having a lone pair of electrons, was
directly involved in complexation of the metal. A blue shift
of the fluorescence emission wavelengths of compounds 1n,
1o, 1p, 1q, 1r, and 3a with a methyl group at the C-4 position
was observed.21 Among them, 1n, 1p, 1q, and 1r had larger
blue shifts λem value than 3a, which might be because they
all had two substituents at C-2 and C-4 (Table 5, entries
8–12), while 3a only has a methyl group at C-4 (Table 5,
entry 7). Compared with compound 3a, the blue shift of 1o
was weaker, which might be due to the effect of the chlorine
Zn
Green 87.74%, Yellow 12.56%
Blue 49.00%, Red 51.00%
Cd Green 67.23%, Yellow 32.77%
Blue 35.34%, Red, 64.66%
Steric field
Electrostatic field
Figure 7. Stereo views of CoMFA steric field and electrostatic
field of metal complexes.
To view the field effect on the target property, CoMFA substituent at C-5 (Table 5, entries 3 and 7).
contour maps were generated and shown in Figure 7. For
In the same way, the λem values of the zinc and cad-
steric fields, green means that a bulky group is favorable and mium complexes of 1e were blue-shifted, but the wave-
yellow indicates that a bulky group is unfavorable. Similarly, length of aluminum complexes could not be observed due
blue means that an electropositive charge is favorable and to weak fluorescence (Table 5, entry 14). The structures of
red indicates that an electronegative charge is favorable.
the above compounds were consistent with the previous
For example, for the aluminum complexes, the steric field blue shifts ligand, of which the electron-donating groups
of the complexes has green contours (75.80%) indicating steri- were introduced on the pyridine ring while the electron-
cally favored regions for fluorescence properties, and the yel- withdrawing groups were introduced on the benzene ring.
low contours (24.20%) highlight the sterically unfavorable
regions. The electrostatic field has blue contours (13.23%)
The substituents on 8-hydroxyquinoline
representing electropositively preferred regions to better fluo-
resulted in a red shift of λ emission
rescence performance, and red contours (86.77%) indicate
examples (λem)
regions where more electronegative substituents are favored to
increase fluorescence. The data for the CoMFAsteric field and Since no red shift was observed in the cadmium complexes,
electrostatic field of the cadmium and zinc complexes of only zinc and aluminum complexes were compared and
8-hydroxyquinoline are also shown in Figure 7.
discussed. The emission wavelength of nine aluminum
complexes was red-shifted, while three of the nine zinc
complexes were red-shifted. The wavelengths of the alu-
minum and zinc complexes of 2d, 4m, and 4n were red-
shifted. The fluorescence data for these compounds are
given in Table 6.
The substituents on 8-hydroxyquinoline
resulted in a blue shift of λ emission
examples (λem)
The λem of the 13 aluminum and zinc complexes were both
When halogens or methyl groups were introduced at
blue-shifted (Table 5). Seven of them were fluorescence the C-5 position of 8-hydroxyquinoline, the fluorescence

8
Journal of Chemical Research 00(0)
Table 5. Data for the aluminum, cadmium, and zinc complexes both with blue shifted of λem.
Entry Ligand Aluminum complexes
Zinc complexes
Cadmium complexes
λex (nm)
λem (nm) Intensity
λex (nm)
λem (nm) Intensity
λex (nm)
λem (nm) Intensity
1
2
3
4
5
6
7
8
8-HQ
1g
1o
1b
1c
4q
3a
1r
1n
1l
1p
1q
1i
370
268
260
263
263
210
370
260
260
260
256
262
259
270
510
507
502
501
499
497
496
492
490
471
468
464
413
–
>10,000
1400
>10,000
2730
264
231
374
237
377
215
259
235
210
270
210
216
260
216
553
536
531
550
540
527
537
545
512
452
513
510
416
515
1703
1444
9894
1148
1277
5000
14,289
1702
6704
1688
5170
6609
1442
7953
263
–
374
–
590
–
544
–
<400
<400
1319
<400
<400
1000
1785
<400
1031
<400
861
1600
1490
1780
–
–
218
262
–
218
–
212
218
–
530
556
–
522
–
530
520
–
>10,000
600
9
10
11
12
13
14
1640
>10,000
>10,000
1690
1099
<400
1223
1e
<400
218
526
8-HQ: 8-hydroxyquinoline.
Table 6. Data for the aluminum and zinc complexes both red-shifted λem.
Entry
Ligand
Aluminum complexes
Zinc complexes
λex (nm)
λem (nm)
Intensity
λex (nm)
λem (nm)
Intensity
1
2
3
4
5
6
7
8
9
2d
4m
4n
4l
4a
5g
5f
2b
8-HQ
233
384
234
232
370
242
225
383
370
539
528
522
538
532
522
517
531
510
2078
1990
1870
1300
1490
2150
1279
1300
210
265
265
256
261
261
235
260
264
570
569
568
–
–
–
–
–
553
2100
1492
1694
<400
<400
<400
<400
<400
1703
>10,000
8-HQ: 8-hydroxyquinoline.
Table 7. Energies of the molecular orbitals and the ɸf values of
1o, 1q, and 1p.
emission wavelength of the corresponding metal alu-
minum and zinc complexes was red-shifted. Compared
with the λem of the 8-HQ aluminum complex (Table 6,
entry 9), the fluorescence emission wavelengths of com-
pounds 2d, 4m, and 4n with methyl, chlorine, and bro-
mine substituents, respectively, at the C-5 position of
quinoline, exhibited changes in the range of 12–29nm for
the red shifts of the aluminum complexes, and 15–17nm
red shifts for the zinc complexes (Table 6, entries 1–3).
The fluorescence wavelengths of the aluminum com-
plexes of 2b, 4a, 4l, 5g, and 5f were red-shifted too (Table 6,
entries 4–8). The common feature was a methyl or halogen
group at C-5 of the 8-hydroxyquinoline. For the zinc com-
plexes of 2b, 4l, 4a, 5g, and 5f, the fluorescence intensities
were less than 400 and thus almost fluorescent inactive.
Entry Metal Ligand ɸf
EHOMO
*
ELUMO
Egap
1
Al
8-HQ 0.171 −0.18683 0.00791 0.19474
2
3
4
1o
1q
1p
0.249 −0.20389 0.00588 0.20977
0.572 −0.20338 0.01523 0.21861
0.969 −0.21896 0.01046 0.22942
5
6
7
8
Zn
Cd
8-HQ 0.040 −0.24034 0.01259 0.25293
1p
1q
1o
0.138 −0.24719 0.03914 0.28633
0.153 −0.24363 0.04285 0.28648
0.248 −0.28329 0.06099 0.34428
9
8-HQ
1p
1q
–
−0.23957 0.03951 0.27908
10
11
12
0.023 −0.23972 0.04333 0.28305
0.029 −0.24506 0.04115 0.28621
0.035 −0.24636 0.04055 0.28691
1o
*The Hartree was used as the energy unit; energies in subsequent tables
are also in Hartrees.
Analysis of the effect of the structure on
the fluorescence properties in terms of
molecular orbitals
There are three compounds—1o, 1p, and 1q—for which parent compound, the calculated values of the EHOMO
,
the zinc, cadmium, and aluminum complexes all showed ELUMO, and Egap of the complexes of 1o, 1p, and 1q are
good fluorescence properties. In Table 7, with 8-HQ as the given (Figure 8).

Yuanyuan et al.
9
8-HQ
1o
1p
1q
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
Al Complexes
Zn Complexes
Cd Complexes
Figure 8. The HOMOs and LUMOs of metal complexes 8-HQ, 1o, 1p, and 1q.
The effects of the different energies of the molecular orbit-
Another phenomenon was that the λem values of the
als on the fluorescence properties of the corresponding zinc, cadmium, and aluminum complexes, in which the
aluminum(III), cadmium(II), copper(II), and zinc(II) metal fluorescence intensity was increased, almost showed blue
complexes are summarized as follows: the electron density of shifts. This might due to the larger energy gap between the
the pyridine ring was improved by the introduction of methyl LUMO and HOMO of the above metal complexes.
groups at C-2 and C-4 of 8-HQ and would thus change the
The addition of substituents at the C-2 position of the
LUMO and HOMO values of the complexes.19,21 At the same pyridine ring, such as in ligands 1b, 1c, 1g, 1i, 1l, 1n, 1o,
time, the N atom of the pyridine ring, having a lone pair of 1p, 1q, and 1r (Table 5, entries 3–11), results in a certain
electrons, was directly involved in complexation of the metal. spatial steric resistance to the complex structure, which was
If electron-donating groups were introduced at ortho posi- not conducive to the formation of a stable five-membered
tions, the distribution of the electron cloud would be changed, ring structure. If electron-donating groups were introduced
which might significantly alter the LUMO value and expand at ortho positions, the distribution of the electron cloud
the gap between the HOMO and LUMO. For example, the ɸf would change, which might significantly alter the LUMO
values of aluminum complexes from 8-HQ, 1o, and 1p to 1q value and expand the HOMO and LUMO gaps, such as in
increased from 0.171, 0.249, and 0.572 to 0.969 and the Egap the zinc, cadmium, and aluminum complexes 1o, 1p, and
increased from 0.19474, 0.20977, and 0.21861 to 0.22942 1q (Table 7), which resulted in a significant blue shift of the
(Table 7, entries 1–4), that is, the higher the Egap value, the fluorescence emission wavelength. Therefore, when elec-
higher the ɸf value. The same results were obtained for cad- tron-donating groups were introduced at C-2 position on
mium (Table 7, entries 9–12) and zinc metal complexes the pyridine ring, the wavelength of the complexes had a
(Table 7, entries 5–8). A larger energy gap between the significant blue shift.
LUMO and HOMO resulted in the confinement of holes and
However, the regularity of the wavelength changing with
electrons in the organic light-emitting layer, which could the Egap is not as good as that of ɸf. In order to analyze the
increase the electron–hole recombination efficiency. Finally, cause of the blue shift of the metal complexes from the
the molecular total energy was decreased, which facilitated viewpoint of molecular orbitals, the HOMOs and LUMOs
the π–π* transition, increased the number of effective migra- of complexes with similar structures were calculated and
tions, and enhanced the fluorescence intensity of the com- compared, and the results are shown in Figure 9 and Table 8.
plexes. At the same time, the mobility of electrons in the
The ligands, which possess different electron-withdraw-
framework of the complex was enhanced and the internal ing groups, such as sulfonic acid, could induce deflection
electronic transitions were reduced, so the corresponding of the electron cloud, which leads to an increase of the
metal complexes had high fluorescence properties.22
HOMO value of the quinoline benzene ring. As a result, the

10
Journal of Chemical Research 00(0)
Table 9. Energies of the molecular orbitals and the λem values
of 2d, 4m, and 4n.
Entry Metal Ligand λem EHOMO
ELUMO
Egap
4q
1l
1
2
3
4
5
6
7
8
Al
8-HQ 510 −0.18683
0.00791 0.19474
4n
4m
2d
522 −0.22910 −0.00459 0.22451
528 −0.23883 −0.00864 0.23019
539 −0.24375 −0.00996 0.23375
Zn
8-HQ 553 −0.24034
0.01259 0.25293
4n
4m
2d
568 −0.25414 −0.00592 0.24822
569 −0.26035 −0.00741 0.25294
570 −0.26603 −0.01094 0.25509
HOMO
LUMO
HOMO
LUMO
Al Complexes
Zn Complexes
8-HQ: 8-hydroxyquinoline.
Figure 9. The HOMOs and LUMOs of metal complexes 4q
and 1l.
Table 8. Energies of molecular orbitals and the λem values of
4q and 1l.
4n
Entry Metal Ligand λem EHOMO
ELUMO
Egap
1
2
3
4
5
6
Al
8-HQ 510 −0.18683
0.00791 0.19474
4q
1l
497 −0.23567 −0.03417 0.20253
471 −0.25381 −0.02341 0.23040
4m
Zn
8-HQ 553 −0.24034
0.01259 0.25293
4q
1l
527 −0.29282 −0.02636 0.26646
452 −0.29806 −0.02180 0.27626
2d
8-HQ: 8-hydroxyquinoline.
energy gap between the HOMO and LUMO is increased,
and the fluorescence emission wavelength of compounds
had a blue shift, such as in compounds 4q and 1l (Table 8).
The Egap of the aluminum and zinc complexes of 1l were
0.23040 and 0.27626, respectively, which are larger than
those of the corresponding complex 4q, so the fluorescence
emission wavelength of 1l has a bigger blue shift than that
of 4q (Table 8, entries 2, 3, 5, and 6). Ligand 4q was
HOMO
LUMO
HOMO
LUMO
Al Complexes
Zn Complexes
Figure 10. The HOMO and LUMOs of metal complexes 2d,
4m, and 4n.
affected only by a sulfonic acid group at C-5 and 1l was lower the HOMO value of the complexes, the greater the
affected by a sulfonic acid group at C-5 and by a hydroxy red-shifted fluorescence emission wavelength. For exam-
group at C-2 at the same time, so 1l had a greater blue shift ple, the HOMO of the aluminum complexes decreases from
of λem than 4q. The results are consistent with the Egap 4n, 4m to 2d at −0.22910, −0.23883, and −0.24375, respec-
analysis.
tively, and the wavelength changes from 522, 528 to 539
The luminescence of the 8-hydroxyquinoline metal (Table 9, entries 2–4). The HOMO of zinc complexes
complex was caused by the electron π–π* transition from decreases from 4n, 4m to 2d at −0.25414, −0.26035, and
the HOMO of the phenol ring to the LUMO located on the −0.26603, respectively, and the wavelength changes from
pyridyl ring.23 The highest electron density of the HOMO 568, 569 to 570 (Table 9, entries 5–8).
of the 8-hydroxyquinoline metal complex was at the phenol
When the C-5 position of the 8-hydroxyquinoline ben-
oxygen and the C-5, C-7, and C-8 positions. Thus, it can be zene ring was substituted with a halogen, such as in 4m and
predicted that electron-withdrawing groups or electron- 4n, which has three lone pairs of electrons, it was easy to
donating groups at these positions would lead to blue or red form a large p–π conjugation with the quinoline benzene
shifts of the absorption and fluorescence spectra.24 There ring, which led to a more uniform distribution of the elec-
are three compounds, 2d, 4m, and 4n, for which the fluo- tron cloud and decreased the HOMO value of substituted
rescence wavelength of the zinc and aluminum complexes 8-hydroxyquinoline metal complex. The lower HOMO
was red-shifted. In Table 9, with 8-HQ as the parent com- value led to the fluorescence emission wavelengths of the
pound, the EHOMO, ELUMO, and Egap of the complexes of 4m, metal complexes being red-shifted. When a methyl group
4n, and 2d were calculated (Figure 10).
was introduced at C-5 of the 8-hydroxyquinoline, due to the
The Egap of most of the red-shifted complexes was also electron donor effect, the HOMO value of the phenol ring
larger than that of 8-hydroxyquinoline, and was irregular decreased. The fluorescence emission wavelength of the
with the wavelength changes. However, we found that the metal complexes had a red shift.

Yuanyuan et al.
11
ligand (1.0mmol), water, and MeOH in a capped vial was
heated at 80°C for 1day. Colored block–like crystals were
collected by filtration, washed with ether, and dried to give
the desired product.
Conclusion
Fifty-five 8-hydroxyquinoline (8-HQ) derivatives were
synthesized and the corresponding aluminum(III),
cadmium(II), copper(II), and zinc(II) metal complexes
were prepared and their fluorescent activities were evalu-
ated. The aluminum complexes had the best fluorescence
properties, followed by zinc. The fluorescence can also be
observed in cadmium complexes, while almost no fluores-
cence was observed in copper complexes. The fluorescence
properties of the few aluminum, zinc, and cadmium com-
plexes were different. The 3D-QSPR model was proposed
using CoMFA based on the molecular simulation, which
also exhibited good stability and good prediction ability.
The optimum models were statistically significant with
cross-validated coefficients (q2 =0.444, 0.539, and 0.505)
and conventional coefficients (R2 =0.888, 0.935, and
0.912), indicating that they were reliable enough for the
prediction of the fluorescence quantum yields.
The relationship between the fluorescence properties
and the complex structure has been summarized. The
effects of the structure and energies of the molecular orbit-
als on the fluorescence properties were analyzed. The effect
of the ligand substituents on the fluorescence properties of
8-hydroxyquinoline zinc, cadmium, and aluminum com-
plexes shows some common points: first, the introduction
of a halogen or a methyl group at C-5 of the 8-hydroxyqui-
noline ring results in a red shift of the fluorescence emis-
sion wavelength of aluminum complexes due to the
increased conjugation effect and the decreased EHOMO
value. Second, when electron-withdrawing groups were
introduced on the 8-hydroxyquinoline ring, the conjugation
effect of the quinoline ring was decreased and the Egap value
increased, which led to a blue shift of the fluorescence
emission wavelength and a weak fluorescence intensity.
Third, a stronger intensity of the fluorescence would be
achieved by introducing a methyl groups at position of 2
and 4 of the 8-hydroxyquinoline ring, which led to steric
issues and a bigger gap between the HOMO and LUMO.
General procedure for the preparation of the copper com-
plexes. A pale green solution of CuCl2·2H2O (0.5mmol)
dissolved in methanol was added slowly to a solution of the
8-hydroxyquinoline ligand (1.0mmol) in methanol. After
stirring for 30min, the solid was filtered, washed with
ether, and dried to give the desired product.
Photoluminescence quantum yields. Quantum yields of fluo-
rescence (ɸf) can be measured in two different ways: rela-
tive to a fluorescent standard material with a known ɸf or as
an absolute quantity. The measurement of relative fluores-
cence quantum yields has exclusively been done by optical
methods, most prominently using a spectrofluorometer and
the comparative method as proposed by Parker and Rees.25
In this paper, the ɸf was calculated by a comparative method
using the following equation6,26,27
nx2 Fx
A
std
Φf =
.
.
.Φfstd
ns2td
F
AX
std
In this experiment, nx was approximately equal to the
refractive index of the solvent, the solvent used in this
experiment was MeOH, so the nx value was approximately
1.360, the nstd for the refractive of water index of water had
a value of about 1.337. F denotes the integrated area in the
fluorescence spectrum, and A denotes the absorbance in the
ultraviolet spectrum. ɸfstd is the fluorescence quantum yield
of the standard solution, quinine sulfate (1.0μgmL−1) in
sulfuric acid solution (0.1molL−1) was used as a standard
reference, and the ɸfstd was 0.55. Fluorescence spectra were
measured in MeOH solution, and the slit width was 5.0nm.
Computational details. All computations of optimized
geometry were carried out using the Gaussian 0928 com-
puter software package. The electronic descriptors were
obtained from a single-point calculation at the B3LYP/6-
311+g (d) level. The structural parameters—including the
energy of highest occupied molecular orbital (EHOMO), the
energy of the lowest unoccupied molecular orbital (ELUMO),
and the energy difference between the LUMO and HOMO
(Egap)—were also calculated. 3D-QSPR studies on the
metal complexes were performed using the literature proce-
dure reported by us using CoMFA performed on the Sybyl
8.0 package.29
Experimental
Details of the synthesis and characterization of ligands are
provided in the supporting material. The procedures for the
preparation of the zinc complexes and recording the fluo-
rescence measurements are consistent with those of previ-
ous our report.13
General procedure for the preparation of the aluminum com-
plexes. The 8-hydroxyquinoline ligand (1.0mmol) was
added to anhydrous methanol at room temperature, and
then the temperature was raised to 70°C. An aqueous solu-
tion of aluminum sulfate (1.0mmol) was slowly added
dropwise, and the mixture was maintained at 70°C for 20h.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
The pH of the mixture was adjusted with ammonia solution article.
to about 7–8. After stirring for 4h, the obtained precipitate
was filtered and recrystallized from anhydrous methanol to Funding
give the pure product.
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
work was financially supported by the Ministry of Education,
General procedure for the preparation of the cadmium com-
plexes. A mixture of CdNO3 (0.5mol), 8-hydroxyquinoline Chunhui Project of China (no. 192635).

12
Journal of Chemical Research 00(0)
13. He JB, Zhou TT, Cao YJ, et al. J Fluoresc 2018; 5: 1121
14. Yang J, Ma M, Wang XD, et al. Eur J Med Chem 2015;
ORCID iD
Ma Menglin
https://orcid.org/0000-0002-1482-2078
99: 82.
15. Ma M, Jiang XJ, Wang XD, et al. Chem Biol Drug Des 2016;
Supplemental material
88: 451.
Supplemental material for this article is available online.
16. Mahmood U, Rashid S, Ali S, et al. Int J Mol Sci 2011;12:
8862.
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