The effects of crystal structure on optical absorption/photoluminescence of bis(8-hydroxyquinoline)zinc

Article abstract of DOI:10.1016/j.ssc.2005.08.021

In this paper, the transformation processes of two types of bis(8-hydroxyquinoline)zinc: Znq₂ dihydrate and anhydrous (Znq ₂)₄ were investigated by X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG). The effects of crystal structure on optical properties of bis(8-hydroxyquinoiline)zinc were analyzed. Znq ₂ dihydrate can be transformed into anhydrous (Znq₂) ₄ during heating under vacuum. Reversal transformation occurs by the interaction between chloroform and (Znq₂)₄. But (Znq ₂)₄ was partially transformed into Znq₂ dihydrate by the interaction between ethanol and (Znq₂)₄. The different molecular structure results in different crystal stacking and electronic structure, thereby affect its optical properties.

Full text of DOI:10.1016/j.ssc.2005.08.021

Solid State Communications 136 (2005) 318–322
www.elsevier.com/locate/ssc
The effects of crystal structure on optical absorption/
photoluminescence of bis(8-hydroxyquinoline)zinc
a
Xu Bing-she , Hao Yu-ying , Wang Hua , Zhou He-feng ,
b,
a
a
*
Liu Xu-guangc, Chen Ming-weid
aCollege of Materials Science and Engineering, Taiyuan University of Technology, No. 79 Yingze Street, Taiyuan, Shanxi 030024, China
bCollege of Science, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China
cCollege of Chemical Engineering and Technology, Taiyuan University of Technology, Taiyuan 030024, China
dInstitute for Materials Research, Tohoku University, Sendai 9808577, Japan
Received 21 May 2005; received in revised form 13 August 2005; accepted 16 August 2005 by H. Eschrig
Available online 31 August 2005
Abstract
In this paper, the transformation processes of two types of bis(8-hydroxyquinoline)zinc: Znq2 dihydrate and anhydrous
(Znq2)4 were investigated by X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), differential
scanning calorimetry (DSC) and thermogravimetry (TG). The effects of crystal structure on optical properties of
bis(8-hydroxyquinoiline)zinc were analyzed. Znq2 dihydrate can be transformed into anhydrous (Znq2)4 during heating
under vacuum. Reversal transformation occurs by the interaction between chloroform and (Znq2)4. But (Znq2)4 was partially
transformed into Znq2 dihydrate by the interaction between ethanol and (Znq2)4. The different molecular structure results in
different crystal stacking and electronic structure, thereby affect its optical properties.
q 2005 Elsevier Ltd. All rights reserved.
PACS: 61.12.LD; 71.35.Cc; 78.55.Km
Keywords: A. Bis(8-Hydroxyquinoline)zinc; C. Crystal structure; D. Optical absorption; E. Photoluminescence
1. Introduction
and d-Alq3 have been identified [6–8]. Bis(8-Hydro-
xyquinoline)zinc can exist either as the Znq2 dihydrate
(Znq2$2H2O) with two water molecules occupying the
axial ligand positions on the zinc atom, or as an
anhydrous Znq2. Nicolau et al. [9] predicted by
molecular modeling studies that anhydrous Znq2 are
distorted planar or distorted tetrahedral geometries.
Merritt et al. [10] proposed that Znq2 dihydrate are
planar geometries. But Kai et al. [11] showed that
anhydrous Znq2 crystal grown from vapor phases are
tetramer structures formed via oxo-bridging of the
8-hydroxyquinoline ligands. Hopikins et al. [12] also
suggested that anhydrous Znq2 may exist as tetramers in
chloroform solvent, but partially dissociates to monomers
in nucleophilic solvent such as DMSO. Linda et al. [13,
Metal chelate of 8-hydroxyquinoline (Mqn) have drawn
considerable attention because of the excellent perform-
ance of their use in organic light-emitting devices
(OLEDs) [1–5]. Many detailed studies of the structure/
property relationships of Mqn have been reported. Two
geometric isomers of tris(8-hydroxyquinoline)aluminium
(Alq3): Meridianal and facial isomers has been proposed,
and several polymorphic phases: a-Alq3, b-Alq3, g-Alq3
*
Corresponding author. Tel.: C86 3516018942; fax: C86
3516010311.
E-mail address: haoyuying@peoplemail.com.cn (H. Yu-ying).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2005.08.021

X. Bing-she et al. / Solid State Communications 136 (2005) 318–322
319
Calculated: C: 55.47%; N: 7.19%; H: 4.14%. Found: C:
54.65%; N: 7.04%; H: 4.30%. Weight loss percentage of
8.85% at about 110–160 8C in TG curve corresponded to
loss of two water molecules. The board endothermic peak at
130 8C in DSC curve also corresponded to the loss water
process of Znq2$2H2O.
2.2. Sample preparation
In order to obtain samples a–j, Znq2$2H2O was
divided into 10 parts and dried in vacuum oven (2!10K
2 Torr) at 100, 120, 125, 130, 135, 140, 160, 180, 200,
300 8C for 1 h, respectively, and then cooled quickly,
therefore, all samples can retain their states at high
temperatures. In fact, the sublimation temperature of
Znq2$2H2O is about 246 8C at 2!10K2 Torr, so the way
to get sample j was just conventional sublimation. The
sample k was obtained by recrystallization of sample j
from chloroform. It was dried at 100 8C in vacuum oven
for 1 h and then quickly cooled. In contrary, after sample
j was recrystallized form ethanol, two samples were
separated, one was yellowish-green, and the other was
orange. They were named sample l and m, respectively.
The sample m with orange color was obtained firstly by
filtration under vacuum pressure, while the sample l with
yellowish-green color was retained in filtrate because of
its smaller particle size, and then it was obtained by
filtration under ambient pressure. They were dried and
cooled according to the same procedure as sample k.
Scheme 1. The structure of the tetramer (Znq2)4, but full 8-
hydroxyquinoline is not shown to clarify the structure of tetramer
(Znq2)4.
14] further confirmed by X-ray diffraction, differential
scanning calorimetry (DSC) and theoretical calculations
that anhydrous Znq2 formed a sole tetrameric structure
composed of (Znq2)4 (Scheme 1), whether in films or
solid states, and also proposed that the high symmetry
and strong p–p stacking of (Znq2)4 result in enhanced
electron transport in OLEDs.
In this paper, the transformation processes of
Znq2$2H2O and (Znq2)4 were investigated by X-ray
diffraction (XRD), infrared spectra (IR), scanning electron
microscope (SEM), differential scanning calorimetry (DSC)
and thermogravimetry (TG). The effects of crystal structure
on optical properties of bis(8-hydroxyquinoiline)zinc were
analyzed. Znq2$2H2O can be transformed into (Znq2)4
during heating under vacuum. (Znq2)4 crystal began to be
built at about 135 8C, and sole (Znq2)4 crystalline phase was
obtained until 180 8C. Reversal transformation occurs by the
interaction between chloroform (purity 99.0%, water
content %wt 0.03%) and (Znq2)4, which is different
significantly from Hopikins’ report. But (Znq2)4 was
partially transformed Znq2$2H2O by the interaction
between ethanol (purity 99.7%, water content %wt 0.3%)
and (Znq2)4. The different molecular structure results in
different crystal stacking and electronic structure, thereby
affect its optical properties.
Sample
j and m were confirmed to be anhydrous
Znq2(Zn(C9H6ON)2) by elemental analysis. Sample k
and l were confirmed to be Znq2 dihydrate (Zn(C9H6-
ON)2)$2H2O) by elemental analysis. Elemental analysis
of sample j: Calculated: C: 61.125%; N: 7.92%; H:
3.42%. Found: C: 59.66%; N: 7.75%; H: 3.44%.
Elemental analysis of sample m: Calculated: C:
61.125%; N: 7.92%; H: 3.42%. Found: C: 59.90%; N:
7.68%; H: 3.42%. Elemental analysis of sample k:
Calculated: C: 55.47%; N: 7.19%; H: 4.14%. Found: C:
54.65%; N: 7.02%; H: 4.30%. Elemental analysis of
sample l: Calculated: C: 55.47%; N: 7.19%; H: 4.14%.
Found: C: 54.67%; N: 7.09%; H: 4.30%.
2. Experimental
2.1. Synthesis
2.3. Measurements
Znq2$2H2O was synthesized by following complex
reaction. The aqueous solution (40 ml) of ZnSO4
(0.01 mol) was added drop-wise to stirred acetone solution
(80 ml) of 8-hydroxyquinoline (0.02 mol) and (C2H5)3N
(0.02 mol). After refluxing the mixture at 50–60 8C for 2 h, a
crude product precipitated from the solution. The precipi-
tates were collected by filtration and washed with acetone
and deionized water for several times. The composition of
the product was pure Zn(C9H6ON)2$2H2O, which was
supported by elemental analysis, TG and DSC data.
The XRD spectra were measured by Riguku D/max
2500, Japan. The SEM images were obtained by KYKY-
1000B scanning electron microscope, Japan. The TG and
DTG curves were measured by NETZSCH STA409C
thermal analyzer, German. The DSC curve was measured
by Shimadzu DT-40 thermal analyzer, Japan. The IR spectra
were measured by FTIR-1730, America. The PL spectra
were measured by the SPR-920D spectrofluorometer,
China. The UV–vis absorption spectra were measured by
Lambda Bio 40, American PE Co.

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X. Bing-she et al. / Solid State Communications 136 (2005) 318–322
amorphous diffraction spectrum (two board peaks) for
sample c, d and e, indicating that sample c, d, and e are
composed of crystal constituent and amorphous constitu-
ent. The later is apparently observed at around 125–
135 8C, which means that with decomposition of
Znq2$2H2O, the transition from crystalline to amorphous
phase occurred. The crystal diffraction peaks of sample c
are similar to those for sample a, and b, which should be
attributed to residual Znq2$2H2O. When the temperature
reached 135 8C, two new diffraction peaks at 2qZ8.560
and 9.8608 became increasingly apparent, this indicates
that crystal structure of (Znq2)4 began to be built. When
the temperature reached 140 8C, the crystal structure of
(Znq2)4 formed basically, its diffraction peaks located at
about 2qZ8.800, 9.675, 10.403, 14.504 and 15.1138,
respectively. The diffractions at 2qZ6.912, 21.0658 were
still attributed to residual Znq2$2H2O. When the
temperature reached 160 8C, the peaks at 2qZ6.912,
21.0658 disappeared, whereas the diffraction peaks of
(Znq2)4 became strong. When the temperature reached
180 8C, these peaks became much stronger, indicating an
increase in overall degree of crystallinity. Fig. 1(4)
displays that little changes in XRD spectra were found
above 180 8C. The diffraction spectra of sample heated at
300 8C is basically in accordance with Linda’s measure-
ments. It can be seen that heating temperature of samples
must be above 180 8C to obtain sole (Znq2)4 crystal
phase.
Fig. 1. XRD spectra of all sample. Samples a–j were dried at
different temperature, a: 100 8C, b: 120 8C, c: 125 8C, d: 130 8C, e:
135 8C, f: 140 8C, g: 160 8C, h: 180 8C, i: 200 8C, j: 300 8C,
respectively. k: Sample attained by recrystallization of sample j
from chloroform. l and m: Samples attained by recrystallization of
sample j from ethanol.
Fig. 1(5) and (6) show that the XRD spectra of sample k
and l are completely different from that of sample j, but the
same as that of samples a, b. XRD spectrum of sample m is
completely identical with that of sample j. In order to
investigate the component of sample j through m, their
infrared spectra were measured. A broad band was observed
at 3369 cmK1 in infrared spectra of sample k or l, which is
due to O–H group of water molecular. But no apparent
stretching vibration of O–H group was detected in infrared
spectra of sample j and m. This mean that (Znq2)4 was fully
dissociated to monomer by recrystallization of sample j
from chloroform. While (Znq2)4 was partially dissociated to
monomer after recrystallization of sample j from ethanol.
The monomer of Znq2 is not stable, further formed Znq2
dihydrate. The image of SEM also supported above results.
3. Results and discussion
3.1. XRD analysis
XRD spectra and XRD data of samples are shown in
Fig. 1 and Table 1. It can be seen from Fig. 1(1) that
XRD spectra of sample a, b are basically identical, the
first sharp diffraction peak occurs at 2qZ6.9608, the
second one at 2qZ21.058, other little diffraction peaks
are very weak. XRD spectrum of sample a or b was
attributed to that of Znq2$2H2O. Fig. 1(2) shows a
superposition of the crystal diffraction spectrum and the
Table 1
The XRD data of sample a, f, j
Sample a
Sample f
Sample j
2q
I/I0
2q
I/I0
2q
8.77
I/I0
2q
I/I0
6.960
100
25
10
9
6.980
21.010
16.520
18.260
35.400
28.960
100
20
13
13
9
15
12
12
11
12
14.40
15.20
8.80
100
72
66
59
57
55
21.000
16.500
18.260
35.400
29.020
9.69
10.20
14.28
15.18
9.68
9
15.46
10.22
8
8

X. Bing-she et al. / Solid State Communications 136 (2005) 318–322
321
tetrameric unit, but also involve strong intermolecular
p–p stacking. The inter- and intra-molecular p–p inter-
actions result in extended network of overlapping phenolato
and pyridyl moieties, thereby lower LUMO level energy and
narrower band gap of (Znq2)4 than that of Znq2$2H2O.
Fig. 3(1) also shows that sample d (130 8C) has the strongest
band-tail absorption, in relation to other samples, which
originates from band tail localized states and intrinsic band-
gap states resulting from amorphous component.
Fig. 2. The images of sample j in chloroform (1) and ethanol
solution (2), respectively.
Fig. 3(2) displays that the optical absorption peak of
sample k or l is at about 447 nm, apparently different from
that of sample j. However, the absorption peak of sample m
is at about 475 nm, identical with that of sample j. These
results also indicate that the (Znq2)4 was dissociated
monomer in chloroform, but partially dissociated monomer
in ethanol. The blueshift of absorption peaks of sample k or l
in relation to that of sample a can be attributed to distortion
of planar geometries of Znq2$2H2O molecules.
3.2. The image of SEM of Znq2$2H2O and (Znq2)4
The images of SEM of sample j in chloroform and
ethanol solution, respectively, are shown in Fig. 2. It can be
seen that the sample j in chloroform solution exists as
needle, but sample j in ethanol solution simultaneously exist
as sheet and needle. The needles correspond to Znq2$2H2O,
and the sheets correspond to (Znq2)4.
3.4. Photoluminescence properties
3.3. Optical absorption properties
The photoluminescences (PL) of samples a–j were
measured. Fig. 4(1) shows the PL spectra of sample a, d, f
and j. It can be seen that emitting peaks redshifted from
sample a to d, blueshifted from sample d to j. The brightness
of them reduced from sample a to d, enhanced from sample
d to j. The FWHM (full width at half-maximum) became
wider from sample a to d, and then became narrower from
sample d to j. The sample d has the longest emitting peak
wavelength, the widest FWHM and the lowest brightness,
which also result from band tail localized states and intrinsic
band-gap states of sample d. Since the optical absorption
spectrum of sample d extends into the PL region, intense
self-absorption results in a decrease in brightness. The
emitting peak wavelength of sample f is about 549.5 nm.
The reason why the green emitting from Znq2$2H2O was
not observed in PL spectrum of sample f could be thought to
be fluorescence quenching due to the energy transfer from
Znq2$2H2O to tetramer (Znq2)4, consequently only yellow
emitting from tetramer was detected. The lower emission
brightness of sample f also results from self-absorption due
to band tail localized states and intrinsic band-gap states,
which is attributed to high defect densities in sample f.
Fig. 3 shows the UV–vis absorption spectra of samples a,
d, f, j, k, l, m in the solid state. The optical absorption peak of
sample a is at around 459 nm. This absorption peak
corresponds to the charge transfer transition from oxide-
containing phenolato moiety of the quinolate ligand to its
nitrogen-containing pyridyl moiety [15]. With lower water
content, this optical absorption peak redshifted to 465 nm
(130 8C) and 476 nm (300 8C). The optical absorption peak
of 476 nm should correspond to (Znq2)4. The optical
absorption band of sample f (140 8C) is wider than that of
sample a and j. The UV–vis absorption spectrum of sample f
should be the superposition of the absorption spectra of
Znq2$2H2O and (Znq2)4, consequently leading to wider
absorption band. According to the optical absorption peaks
of samples, the optical band-gap of sample a, j can be
estimated to be about 2.708 and 2.611 eV, respectively. As
discussed by Linda et al., the HOMO of (Znq2)4 is localized
on the two non-bridging, terminal ligands, and the LUMO of
(Znq2)4 is located on the bridging, terminal ligands. The
bridging, terminal ligands not only exhibit intramolecular
p–p stacking interactions with center ligands of the
Fig. 3. The UV absorption spectra of samples a, d, f, j, k, l, m. a:
Samples dried at 100 8C, d: Sample at 130 8C, f: Sample at 140 8C, j:
Sample dried at 300 8C, k: Sample attained by recrystallization of
sample j from chloroform. l and m: Samples attained by
recrystallization of sample j from ethanol.
Fig. 4. The PL spectra of sample a: 100 8C, d: 130 8C, f: 140 8C, j:
300 8C, k, l, m. k: Samples attained by recrystallization of sample j
from chloroform. l and m: Samples attained by recrystallization of
sample j from ethanol.

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X. Bing-she et al. / Solid State Communications 136 (2005) 318–322
Fig. 4(2) display that the PL peak of sample k is similar
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sample k in relation to sample l is attributed to self-
absorption, and the reduction brightness of sample j in
relation to sample m also is the results of self-absorption.
The sample k or j has more impurities and defects than
sample l or m.
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Znq2$2H2O can be transformed into (Znq2)4 by heating
at above 180 8C under vacuum. Reversal transformation can
occur by the interaction between chloroform and (Znq2)4.
But (Znq2)4 was partially transformed into Znq2$2H2O by
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This work was supported by the National Excellent
Youth Foundation of China (50025103), the National
Natural Science Foundation of China (20271037 and
90306014), Shanxi Province Natural Science Foundation
(20041066).