Study on Relationship Between Fluorescence Properties and Structure of Substituted 8-Hydroxyquinoline Zinc Complexes

Article abstract of DOI:10.1007/s10895-018-2275-7

Organic light-emitting diodes (OLEDs) produced from 8-hydroxyquinoline metal complexes play a vital role in modern electroluminescent devices. In this manuscript, a series of 8-hydroxyquinoline derivatives were synthesized by different methods and their corresponding zinc metal complexes were prepared. The UV and fluorescence properties of the complexes were measured aiming to understand the effect of substituents at the quinoline ring on the fluorescence properties of the complexes. When the C-5 of 8-hydroxyquinoline was replaced by halogen group, the fluorescence emission wavelengths had been red-shifted, at the same time, blue-shifted of fluorescence emission wavelength was observed when the C-5 position of 8-hydroxyquinoline was substituted by electron-withdrawing group. When the C-4 position of 8-hydroxyquinolie was substituted by methyl or the C-5 position was substituted by sulfonic acid group, the corresponding zinc complexes had higher fluorescence intensity than 8-hydroxyquinolie zinc.

Full text of DOI:10.1007/s10895-018-2275-7

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2275-7
ORIGINAL ARTICLE
Study on Relationship Between Fluorescence Properties and Structure
of Substituted 8-Hydroxyquinoline Zinc Complexes
He Jianbo1 & Zhou Tingting1 & Cao Yongjing1 & Zhang Yuanyuan1,2 & Yang Weiqing1 & Ma Menglin1
Received: 21 June 2018 /Accepted: 30 July 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Organic light-emitting diodes (OLEDs) produced from 8-hydroxyquinoline metal complexes play a vital role in modern elec-
troluminescent devices. In this manuscript, a series of 8-hydroxyquinoline derivatives were synthesized by different methods and
their corresponding zinc metal complexes were prepared. The UVand fluorescence properties of the complexes were measured
aiming to understand the effect of substituents at the quinoline ring on the fluorescence properties of the complexes. When the C-
5 of 8-hydroxyquinoline was replaced by halogen group, the fluorescence emission wavelengths had been red-shifted, at the
same time, blue-shifted of fluorescence emission wavelength was observed when the C-5 position of 8-hydroxyquinoline was
substituted by electron-withdrawing group. When the C-4 position of 8-hydroxyquinolie was substituted by methyl or the C-5
position was substituted by sulfonic acid group, the corresponding zinc complexes had higher fluorescence intensity than 8-
hydroxyquinolie zinc.
.
.
.
Keywords 8-hydroxyquinoline derivatives Fluorescence Substituents Zinc
Introduction
Despite several endeavors surrounding 8-hydroxyquinoline
derivatives and their metal complexes, the influence of different
substituents of 8-hydroxyquinoline metal complexes about fluo-
rescence properties were rarely reported. In the present, a com-
bined experimental and theoretical method was used to investi-
gate the influence of substituents of quinoline ring on the fluo-
rescence properties of the complexes.
Organic light-emitting diodes (OLEDs) are booming right
now: after the launch of commercial OLED displays based
on 8-hydroxyquinoline metal complexes, OLEDs are on their
way to replace LCD technology [1–3]. The studies on synthe-
sis and performances of 8-hydroxyquinoline derivative li-
gands and their different metal complexes have been widely
focused [4–7]. 8-hydroxyquinoline or its derivatives combine
with metal ions, such as Zn [8], Cd [9], Yb [10], Cu [11], Al
[12], and so on, can obtain different luminescent materials
with the properties of photoluminescence efficiency, quantum
yield, stability, light emission color, etc.
Experimental
The synthesis and characterization data of ligands were
showed in the supporting material.
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-018-2275-7) contains supplementary
material, which is available to authorized users.
General procedure of complex preparation
The 8-hydroxyquinoline derivative ligands were added to an-
hydrous methanol at room temperature then the temperature
was raised to 70 °C. After the mixture temperature reached
70 °C, an aqueous solution of zinc chloride (2: 1 ratio of the
ligand to the metal salt solution) was slowly added drop-wise,
and the mixture was refluxed at 70 °C for 20 h. The pH of the
mixture was adjusted with ammonia solution to about 7,
followed by reaction for 4 h, allowed to stand and suction
* Ma Menglin
mmlchem@163.com
1
Key Lab of Advanced Scientific Computation of Sichuan Province,
Faculty of Science, Xihua University, Chengdu 610039, China
2
Key lab of Green Chemistry and Technology of Ministry of
Education. Faculty of Chemistry, Sichuan University,
Chengdu 610064, China

J Fluoresc
Scheme 1 Preparation of 8-hydroxyquinoline derivatives (a): HCl,
crotonaldehyde, 90 °C, 5 h; (b): SeO2, 90 °C, 16 h; (c): bromine rt.; (d):
concentrated sulfuric acid, 0-5 °C, 30 h; (e): sulfuric acid, KNO3, 10 °C,
8 h; (f): HCl, 3-penten-2-one, 90 °C, 5 h; (g): HCl, methacryladehyde,
90 °C, 5 h; (h): HCl, but-3-en-2-one, 90 °C, 3 h; (i): sulfuric acid, 2-
amino-4-methylphenol derivatives, glycerol, 90 °C, 15 min;. (j): HCl,
acrolein, 90 °C, 3 h. (k): 37%formaldehyde, hydrochloric acid; (l) aque-
ous ammonia; CH3ONa, CH3OH; KHCO3, ethanol; ammonia;
dimethylamine, triethylamine; piperidine, chloroform; morpholine, chlo-
roform; (m): p-toluidine, NaNO2; bromine; Sodium chlorite, sodium io-
dide; (n): NaNO2, sulfuric acid; (o) sulfuric acid, HNO3; (p) stannous
chloride, HCl (aq), 80 °C; (q): paraformaldehyde, piperidine, ethanol,
reflux, 5 h; (r): acetic anhydride, acetyl chloride, ice bath, 5 h; (s): acetic
anhydride, potassium carbonate, methanol, RT, 1 h; (t): POCl3, HCl; (u)
POBr3, reflux,1.5 h; (v): HBr, reflux
filtered to obtain a cake, which was recrystallized from anhy-
drous methanol to give the product.
corresponding zinc metal complexes (Scheme 2) were pre-
pared. The structures of ligands were presented in Fig. 1.
Simultaneously, the fluorescence properties of these metal
complexes were discussed in detail. The experimental results
demonstrated that different substituents on quinoline ring had
diverse influence on the fluorescence properties.
Fluorescence Measurement
For preparation of fluorescence measurement, the metal com-
plexes were dissolved in methanol at 2 × 10−5 mol/L. All fluo-
rescence spectra were recorded on a FL-4600 fluorescence
spectrometer using a 1 cm path length quartz cuvette. The
fluorescence spectra was recorded from 200 to 600 nm.
Both the widths of excitation and emission slits were set at
5.0 nm and the scanning speed was set at 1200 nm·min−1. The
maximum excitation wavelengths (λex) and the maximum
emission wavelengths (λem) for all metal complexes were
tested, respectively. All measurements were conducted at
room temperature. The fluorescence data of all metal com-
plexes were showed in the supporting material.
The fluorescence parameters of the 46 metal zinc com-
pounds were measured, in which 8 compounds exhibited im-
proved fluorescence intensity than 8-hydroxyquinoline and 28
compounds exerted decreased even no fluorescence intensity
than 800. Among them, the emission wavelengths of 2 com-
pounds were red-shifted and the wavelength of 17 compounds
were blue-shifted. The influence of different substituents of 8-
hydroxyquinoline on fluorescence emission wavelengths and
intensity were discussed below.
The Substituents of 8-hydroxyquinoline Resulted
in Red Shift of λem
Results and discussion
The emission wavelengths of 2 compounds, which had been
introduced halogen substituted groups on the benzene ring of
8-hydroxyquinoline, were all red-shifted (Fig. 2). The fluores-
cence data and corresponding fluorescence spectra of these
compounds were as following:
In the present, a series of 8-hydroxyquinoline derivatives were
synthesized through different methods (Scheme 1) and the
When the C-5 of 8-hydroxyquinoline was replaced by
chlorine or bromine, the fluorescence emission wavelengths
of the corresponding zinc complexes were redshifted from
Scheme 2 Preparation of 8-hydroxyquinoline zinc complexes

J Fluoresc
Fig. 1 The structures of ligands
553 nm to 569 nm and 568 nm respectively (Table 1, Entries
1~2).
the HOMO and LUMO was reduced, so the fluorescence
emission wavelengths of 8-hydroxyquinoline derivatives zinc
complexes of 4b and 4c had been red-shifted.
It has been reported that the light emission of 8-
hydroxyquinoline metal complex originates from the ligand′s
electronic π-π* transition from a highest occupied molecular
orbital (HOMO) lying mainly on the phenoxide ring to a lowest
unoccupied molecular orbital (LUMO) located on the pyridyl
ring, which was derived from a molecular simulation of the
electronic structure of Znq2. Generally speaking, the highest
electron density of Znq2′s HOMO is located on the phenoxide
oxygen and the C-5, C-7 and C-8 positions. Therefore, it is
predicted that an electron-withdrawing or electron-donating
group at these positions will lead to a blue-shift or red shift in
the absorption and fluorescence spectra [13–16].
The red shift was mainly caused by the introduction of a π-
electron-rich substituent such as halogen at the C-5 position
[17–19]. When the C-5 of 8-hydroxyquinoline was substituted
by halogen groups, the HOMO value of the corresponding
zinc complexes would be lower than the HOMO value of 8-
hydroxyquinoline zinc complex and the energy gap between
The above data showed that some of the emission spectra
have multiple peaks. This suggests multiple transitions. This
result indicated that the ligands 4b and 4c can coordinate well
with Zn(II) ions. The emission peaks at 505 nm and 600 nm
were attributed to the5D0 → 7F1 (magneticdipole transition)
and5D0 → 7F2 (electric dipole transition) of Zn(II) ions, respec-
tively. It is known that the5D0 → 7F1 (magnetic dipole transi-
tion) in the europium complex is almost independent of the
ligands environment, whereas the5D0 → 7F2 (electric dipole
transition) is sensitive to the coordination environment. The
ligand in the Zn(II) complex absorbs energy and undergoes a
π–π* transition, the electron transitions from the ground state
of the singlet state to the lowest excited singlet state, then in-
tersystem crossing occurs to the excited state of the triplet state
in a non-radiative manner. The excited state of triplet state
transfers energy to the vibrational energy level of the europium
ion by the vibrational coupling of the chemical bond.
2000
The Substituents of 8-hydroxyquinoline Resulted
in Blue Shift of λem
8-HQ-Zn
4b-Zn
1500
4c-Zn
In the fluorescence characterization, seventeen 8-
hydroxyquinoline derivatives zinc complexes with blue-shifted
fluorescence emission wavelengths were found (Fig. 3). The
1000
500
0
Table 1 Fluorescence data of 8-hydroxyquinoline derivatives zinc
complexes with red-shifted emission wavelength
Entry
Ligand
λex nm
λem nm
Intensity
1
2
3
8-HQ
4b
264
265
265
553
569
568
1703
1492
1794
400
500
600
Wavelength(nm)
Fig. 2 Fluorescence spectra of 8-hydroxyquinoline derivatives zinc com-
4c
plexes with red-shifted emission wavelength in MeOH at 2 × 10−5 mol/L

J Fluoresc
14000
12000
10000
8000
6000
4000
2000
0
8-HQ-Zn
1c-Zn
1d-Zn
1f-Zn
1g-Zn
1i-Zn
10000
8000
6000
4000
2000
0
Fig. 3 Fluorescence spectra of 8-
hydroxyquinoline derivatives
zinc complexes with blue-shifted
emission wavelength in MeOH at
2 × 10 −5 mol /L −5 mol/L
8-HQ-Zn
3a-Zn
3b-Zn
3c-Zn
5m-Zn
5o-Zn
8-HQ-Zn
1k-Zn
10000
8000
6000
4000
2000
0
1m-Zn
1n-Zn
1o-Zn
1p-Zn
1q-Zn
1j-Zn
400
500
Wavelength(nm)
600
400
500
Wavelength(nm)
600
400
500
Wavelength(nm)
600
fluorescence data and corresponding fluorescence spectra of
their zinc complexes are as following:
2 position of the pyridine ring, such as ligands 1c, 1d, 1f, 1 g,
1i, 1j, and 1 k, produced a certain spatial steric resistance to the
molecular structure of the complex, which were not conducive
to the binding of the ligand to the metal to form a stable five
member ring structure. At the same time, the N atom of pyri-
dine ring had a solitary pair of electrons, and this atom was
directly involved in the complexation of the metal. If electron-
donating groups were introduced to its ortho position, the dis-
tribution of electron cloud would transform, significantly alter
the LUMO value, expanded the gap between HOMO and
LUMO, which resulted in a significant blue shift of fluores-
cence emission wavelength. Therefore, when electron-donating
groups were introduced into C-2 position on pyridine ring, the
wavelength of complexes had a significant blue shift (Table 2,
Entries 2~8).
A blue shift of fluorescence emission wavelengths of com-
pounds 1 m, 1n, 1o, 1p, 1q, and 3a, for which the methyl at C-
4 position [22]. Among them, 1 m had more blue shift λem
than 3a, for them joint action of substituents at C-2 and C-4
position, not as 3a only methyl at C-4 position (Table 3,
Entries 9, 14). Compared with compound 3a, the blue shift
of 1n was weaker, which was due to the effect of
methylsubstituent at C-5 position (Table 2, Entries 10, 14).
For ligand 5 m was effected only by sulfonic acid group at
C-5 position and 1 g was effected by sulfonic acid group at C-
5 position and hydroxyl group at C-2 position (Table 2,
Entries 5, 17), so 1 g had more blue shift λem than 5 m
(Table 2, Entries 3,4).
The maximum emission peaks shows that a good correlation
exists between the electron withdrawing and donating nature of
the substituent group on the 8-hydroxyquinoline and emission
wavelength [17]. The electron-donating groups were intro-
duced to pyridine ring or electron-withdrawing groups were
introduced to benzene ring all would led to a fluorescence
emission wavelength blue shift of zinc complexes [20, 21].
Due to existence of hydroxyl group at quinoline benzene ring,
resulted in the molecule electron richer. The ligands with dif-
ferent electron-withdrawing groups, such as acetyl group and
sulfonic acid group, were introduced to benzene ring, the elec-
tron cloud of benzene ring was deflected, the electron cloud of
quinoline ring moved toward the substituents, the distribution
of electron cloud in the whole molecular structure was uneven,
which led to the increase of HOMO value of quinoline benzene
ring. The energy gap between HOMO and LUMO increased,
and the fluorescence emission wavelength of compounds had a
blue shift, such as compounds 1i, 1 k, 5 m and 5o (Table 2,
Entries 2~4). The addition of methyl and hydroxyl groups at C-
Table 2 Fluorescence data of 8-hydroxyquinoline derivative zinc com-
plexes with blue-shifted emission wavelength
Entry
Ligand
λex nm
λem nm
Intensity
1
2
8-HQ
1c
264
237
377
231
216
227
260
270
210
235
374
210
216
259
386
280
215
230
553
550
540
536
515
491
416
452
512
545
531
513
510
537
532
500
527
519
1703
1148
1277
1444
7953
4507
4442
3118
6704
1792
9894
5170
6609
14,289
875
The blue-shifted of the fluorescence wavelengths was due to
the destabilizing action of the methyl group onto the pyridine
ring on the LUMO value and the conjugation effect of the
substituents onto the phenoxy ring on the HOMO [20–23],
the energy gap between the HOMO and LUMO of the zinc
complexes had an increase compared to that of 8-
hydroxyquinoline zinc complex, so the fluorescence emission
wavelengths of the above complexes had been blue-shifted.
3
1d
4
1f
5
1 g
1i
6
7
1j
8
1 k
1 m
1n
9
10
11
12
13
14
15
16
17
18
1o
1p
The Fluorescence Intenstiy of 8-Hydroxyquinoline
Derivatives Zinc Complexes
1q
3a
3b
There were eight 8-hydroxyquinoline derivatives zinc complexes
which owed high fluorescence intensity and they were also char-
acterized by UV spectroscopy analysis using 8-
hydroxyquinolone zinc complexes as contrast (Fig. 4). The
3c
1248
5000
4632
5 m
5o

J Fluoresc
2.5
2.0
1.5
1.0
0.5
0.0
Fig. 4 Fluorescence spectra and
UVof 8-hydroxyquinoline deriv-
atives zinc complexes with high
fluorescence intensity in MeOH
at 2 × 10−5 mol/L
8-HQ-Zn
1g-Zn
1m-Zn
1o-Zn
1p-Zn
1q-Zn
3a-Zn
5m-Zn
5o-Zn
16000
14000
12000
10000
8000
6000
4000
2000
0
8-HQ-Zn
1g-Zn
1m-Zn
1o-Zn
1p-Zn
1q-Zn
3a-Zn
5m-Zn
5o-Zn
400
450
500
Wavelength(nm)
550
600
300
400
500
Wavelength(nm)
fluorescence data and corresponding fluorescence spectra and
UV spectra of their zinc complexes are as following:
the introduction of a sulfonic acid group at the C-5 position
would change the LUMO value and the HOMO value of the
complexes [14, 23, 24]. Larger LUMO and HOMO value
resulted in the confinement of holes and electrons in the or-
ganic light-emitting layer, which increased the electron-hole
recombination efficiency. At the same time, the mobility of
electrons in the framework of the complex was enhanced and
the internal electronic transitions got reduced, so the corre-
sponding zinc complexes owed high fluorescence properties.
The ligands 1 g, 1 m, 1o, 1p, 1q and 1a, which were all
substituted by methyl at C-2 of 8-hydroxyquinoline, had dif-
ferent fluorescence properties. The ligands 1 g, 1 m, 1o, 1p
and 1q owed high fluorescence properties (Table 3, Entries
2~6), however the fluorescence property of 1a was not good
(Table 3, Entry 11), which confirmed that excellent fluores-
cent properties of 1 g and 1 m were caused by the introduction
of sulfonic acid group at the C-5 of 8-hydroxyquinoline and
the introduction of the methyl group at C-4 position. The
excellent fluorescent properties of 1o, 1p and 1q were caused
by the introduction halogen at C-5 and C-7. When halogens
are added via a heavy atom effect, which would be helpful to
have available molecular structures of the HOMO and LUMO
electron densities, which resulted good fluorescent properties.
When 8-hydroxyquinoline was substituted by introducing
sulfonic acid group at C-5 position, such as 1 g, 5 m and 5o
(Table 3, Entries 3, 7, 8), their zinc complexes exhibited good
fluorescence properties. But 1 h and 5n due to the introduction
of a nitro group at C-7 position, the electron mobility of its
zinc complex skeleton was weakened and the electron-hole
recombination efficiency was reduced, so 1 h and 5n owed
poorer fluorescence properties.
Furthermore, close inspection of the data in Table 3 reveals
that the fluorescence quantum yield (ɸ) of complexes 3a, 1o,
1 g, 1 m, 1q, 1p, 5 m, 5o and 1n depend on the electronic
properties of the substituted methyl, halogen and sulfonic acid
group moieties connected to C-4, C-5 and C-7 of 8-HQ. For
instance, the fluorescence quantum yield of the eight com-
plexes has increased significantly relative to the parent 8-HQ
from 20% in the case of 1n to 842% for 3a.
When methyl was introduced into C-4 position on 8-
hydroxyquinoline pyridine ring, the fluorescence intensity of
compound was obviously enhanced. The fluorescence inten-
sity of 3a was almost 10 times better than 8-hydroxyquinoline
(Table 3, Entry 1). The fluorescence quantum yields of 3a was
0.377, which far greater than 0.040 of 8- hydroxyquinoline
zinc (Table 3, Entry 1).
When the C-2 or C-4 of 8-hydroxyquinoline was substitut-
ed by methyl group or the C-5 was substituted by halogen and
sulfonic acid group, the corresponding zinc complexes exhib-
ited high fluorescence intensity and fluorescence quantum
yields compared with 8-hydroxyquionline zinc complex
(Table 3, Entries 2~8). We speculated that the introduction
of a methyl group at C-4 position of 8-hydroxyquinoline or
Table 3 Fluorescence data of 8-hydroxyquinoline zinc complexes
Entry
Ligand
λex nm
λem nm
Intensity
ɸ
1
2
3a
259
374
216
210
216
210
215
230
235
263
271
247
265
537
531
515
512
510
513
527
519
545
556
–
14,289
9894
7953
6704
6609
5170
5000
4632
1792
1574
<800
<800
<800
0.377
0.248
0.226
0.174
0.153
0.138
0.130
0.116
0.048
0.040
1o
3
1 g
1 m
1q
4
5
Conclusion
6
1p
7
5 m
5o
The 8-hydroxyquinoline ligands of zinc compounds with dif-
ferent substituents or different positions had different fluores-
cence properties. Firstly, when C-5 of 8-hydroxyquinoline was
replaced by halogen, the fluorescence emission wavelengths of
the corresponding zinc complexes had been red-shifted.
Secondly, when the C-5 position of 8-hydroxyquinoline was
substituted by electron-withdrawing groups, the fluorescence
wavelengths of their zinc complexes had been blue-shifted.
8
9
1n
10
11
12
13
8-HQ
1a
<0.010
<0.010
<0.010
1 h
5n
–
–
B-^ means the fluorescence intensity was too low to get the accurate value

J Fluoresc
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Acknowledgements The financial support of this work was supplied by
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