Correlation between molecular structure and optical properties for the bis(2-(2-hydroxyphenyl)benzothiazolate) complexes

Article abstract of DOI:10.1016/j.jphotochem.2010.09.026

A series of methyl-substituted bis(2-(hydroxyphenyl)benzothiazolate)zinc derivatives [Zn(n-MeBTZ)₂, n = 3 (1a), 4 (1b), 5 (1c)] were synthesized to investigate the correlation between molecular structures and optical properties. The results indicate that the blue-emitting (λ_max = 470 nm) complex 1b is monomer with a higher PL quantum efficiency than complexes 1, 1a, 1c. Two green-emitting (λ_max = 507 nm and 499 nm) complexes 1a and 1c have special bi-molecular structures. The molecular structure for Zn(BTZ)₂ (complex 1) is dimer. Bilayer organic light-emitting devices were fabricated by using these complexes as emitting layer. The maximum emission wavelengths of the devices are in the range of 501-553 nm. The devices show turn-on voltages at 9.2, 12.7, 2.3 and 10.7 V for complex 1, 1a, 1b, and 1c, respectively. In particular, the device with complex 1b shows a higher brightness than the other complexes under the same conditions.

Full text of DOI:10.1016/j.jphotochem.2010.09.026

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Correlation between molecular structure and optical properties for the
bis(2-(2-hydroxyphenyl)benzothiazolate) complexes
a,b,∗
a,b,∗
, Fang Xiaohonga,c, Chen Liuqinga,b, Wang Hua , Hao Yuying
a
a
Xu Huixia
, Xu Bingshe
a
Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, Shanxi 030024, China
College of Materials Science and Engineering, Taiyuan University of Technology, Shanxi 030024, China
College of Mining Engineering, Taiyuan University of Technology, Shanxi 030024, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2010
Received in revised form 4 September 2010
Accepted 29 September 2010
Available online 8 October 2010
A series of methyl-substituted bis(2-(hydroxyphenyl)benzothiazolate)zinc derivatives [Zn(n-MeBTZ)2,
n = 3 (1a), 4 (1b), 5 (1c)] were synthesized to investigate the correlation between molecular structures
and optical properties. The results indicate that the blue-emitting (ꢀmax = 470 nm) complex 1b is monomer
with a higher PL quantum efficiency than complexes 1, 1a, 1c. Two green-emitting (ꢀmax = 507 nm and
4
99 nm) complexes 1a and 1c have special bi-molecular structures. The molecular structure for Zn(BTZ)2
(
complex 1) is dimer. Bilayer organic light-emitting devices were fabricated by using these complexes as
Keywords:
emitting layer. The maximum emission wavelengths of the devices are in the range of 501–553 nm. The
devices show turn-on voltages at 9.2, 12.7, 2.3 and 10.7 V for complex 1, 1a, 1b, and 1c, respectively. In
particular, the device with complex 1b shows a higher brightness than the other complexes under the
same conditions.
Bis(2-(n-methyl-2-
hydroxyphenyl)benzothiazolate)
complex
Molecular structure
Electroluminescence
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Organic light emitting diodes (OLEDs) are primarily prepared
A remarkable influences on molecular structures and proper-
ties were observed by introduction of methyl as substituent
group.
Although blue-emitting materials have been an active research
field, blue-light-emitting metal complexes with excellent perfor-
mance are still rare. It has been shown that some d10 metal
complexes with N, O-donor ligands may have high glass transi-
tion temperatures required for stable blue-light OLEDs [13]. In this
with either small molecules or polymer [1–4]. Small molecules
have plenty of advantages in that they can be highly purified, vac-
uum deposited in multilayer stacks, with long display lifetime and
high light efficiency. Luminescent metal complexes among small
molecules are currently of great interest because of their vari-
ous applications in photochemistry, OLEDs, and chemical sensors.
Zinc complexes have been regarded as excellent emitting materials
owing to their ability to achieve high luminescence, as well as good
electron transport properties [5–7].
It is well known that Zn(BTZ)2 is one of the best white elec-
troluminescent materials [8–10]. Zn(BTZ)2 has been investigated,
including substitution of metal ion with other bivalent metal
ions [11,12]. However, experimental and theoretical analysis of
the correlation between molecular structures and performance
of zinc complexes in OLED is rare to date. In this paper, with
the assistance of X-ray single-crystal diffraction, the molecular
and chemical structures of BTZ complexes were investigated.
paper, a series of Zn(n-MeBTZ)2 complexes (Zn(n-MeBTZ) = bis(2-
2
(n-methyl-2-hydroxyphenyl) benzothiazolate) zinc, n = 3 (1a), 4
(1b), 5 (1c)) were synthesized. The maximum emission wavelength
of thin film of complex 1b at room temperature is 470 nm, which
is excellent for blue-emitting material. Molecular structures of all
complexes and performances (such as thermal stability, photo-
physical and electroluminescent properties) were investigated in
detail. The chemical structures of complexes 1–1c are presented in
Fig. 1.
2. Experimental
2.1. Materials and instruments
All materials were purified by high-vacuum gradient-
temperature sublimation before analysis. C, H, and N microanalysis
were carried out with an Elemental Vario EL Elemental analyzer.
∗ Corresponding authors at: Key Laboratory of Interface Science and Engineering
in Advanced Materials, Taiyuan University of Technology, Ministry of Education,
Shanxi 030024, China. Tel.: +86 0351 6014852; fax: +86 0351 6010311.
E-mail addresses: xuhuixia0824@163.com (X. Huixia), xubs@tyut.edu.cn
1
H NMR data were recorded with Switzerland Bruker DR × 300
(
X. Bingshe).
NMR spectrometers. FT-IR spectra were determined with a Nicolet
1
010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.09.026

X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
109
S
S
N
N
O
CH3
O
Zn
Zn
H3C
O
O
N
S
N
S
Zn(BTZ) (1)
Zn(3-MeBTZ)2 (1a)
2
CH3
S
S
CH3
N
N
O
O
Zn
O
Zn
O
N
S
N
S
H C
3
H3C
Zn(5-MeBTZ) (1c)
Zn(4-MeBTZ) (1b)
2
2
Fig. 1. The chemical structures of all Zn complexes in this study.
7
4
199B spectrometer. Thermal analysis was conducted using STA
09PC thermogravimeter under dry argon atmosphere. Melting
and electroluminescent (EL) spectra were measured by SPR-920D
spectrofluorometer. The fluorescence spectra were examined by
Cary Eclipse fluorescence spectrophotometer. The lifetimes (ꢁF) in
tetrahydrofuran (THF) solution were determined with Edinburgh
Analytical Instruments FL900.
points (Tm) of the complexes were determined by differential scan-
ning calorimeter (DSC). UV–vis absorption spectra were recorded
by Lambda Bio 40, American PE Co. The photoluminescent (PL)
Fig. 2. (a) Molecular structure; (b) crystal diagram between two adjacent molecules of complex 1.

1
10
X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
Fig. 3. (a) Molecular structure of complex 1b; (b) the XRD patterns of complex 1b; (c) crystal diagram between adjacent molecules of complex 1b along c axis.
Quantum efficiency measurements were carried out at room
Cyclic voltammetry was performed with Autolab/PG STAT302
in a one-compartment electrolysis cell consisting of a platinum
wire as working electrode, a platinum electrode as counter, and
a calomel electrode as reference. The solution of 0.1 M TBAPF6 was
used as electrolyte. Cyclic voltammetric behaviors were monitored
at scan rate of 50 mV/s.
All the calculations were performed using the density functional
theory (DFT) with the Dmol3 program [14]. The geometry opti-
mization was carried out on the DND basis set for all atoms and
GGA level. The powder XRD was simulated by the Reflex module of
material studio.
temperature in THF solution. Zn(BTZ)2 was used as reference.
The concentrations of sample were carefully adjusted so that the
optical densities at the intersection of spectra between sample
and reference (excitation wavelength) were <0.1 absorption units.
PL quantum efficiency were calculated relative to the value of
Zn(BTZ)2 (complex 1) in THF (set to 1). The equation
ꢀ
ꢁ
2
ꢂ ArIs
s
˚
s = ˚r
2
ꢂ AsIr
r
was used to calculate quantum efficiency, where ˚ is the quantum
efficiency, ꢂ is the refractive index of solution, A is the absorbance
at the wavelength of excitation, I is the integrated areas of emission
bands, and the subscripts s and r stand for sample and reference,
respectively.
The OLED structures employed in this study were double layer.
−
4
Organic layer was fabricated by high-vacuum (10 Pa) thermal
evaporation onto a glass substrate precoated with an indium–tin-
oxide (ITO) layer. Prior to use, the ITO surface was ultrasonicated in
a detergent followed by deionzed water rinse and dip into acetone.

X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
111
Fig. 4. (a) Molecular structure of complex 1a; (b) the XRD patterns of complex 1a; (c) crystal diagram between adjacent molecules of complex 1a along b axis.
Luminance-current–voltage characteristics of OLED were recorded
on Keithley 2400 Source Meter and L-2188 spot Brightness Meter.
The single crystals of complexes 1 and 1a–1c were grown by
tively and collected at room temperature. They were measured on
SMART APEX CCD diffratomater, Mo K␣ (ꢀ = 0.71073 A˚ ). Cell con-
stants and an orientation matrix for data collections were obtained
◦
2
vacuum sublimation at 390, 390, 415 and 395 C for 5 h, respec-
by Full-matrix least-squares on F . The structures were solved
Fig. 5. (a) Molecular structure of complex 1c; (b) the intermolecular interactions in the solid state of complex 1c along c axis.

1
12
X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
by direct methods with SHELXL-97. The powder X-ray diffraction
XRD) spectra were measured by Riguku D/max 2500.
s), 6.899–6.927(2H, d), 7.172–7.198(8H, m), 7.404–7.430(2H, d),
7.767–7.790(2H, d); Anal. calcd for C28H20N O S Zn: C, 61.59; H,
(
2
2 2
−
1
3
.69; N, 5.13; found: C, 61.40; H, 3.602; N, 4.562; FT-IR (KBr) cm
:
2.2. Preparation of the ligands
3431, 2857, 1620, 1535, 1495, 1444, 1393, 1246, 1187, 982, 823,
7
24, 707, 591 (Fig. 2).
BTZ was synthesized by the reaction of 2-aminobenzenethiol
Bis(2-(hydroxyphenyl)benzothiazolate)zinc (1) Zn(BTZ) was syn-
2
and salicylic acid. The methyl-substituted ligands were obtained
from the reaction between 2-aminobenzenethiol and correspond-
ing n-methyl salicylic acid in toluene solution. At last, the final
products were purified by vacuum sublimation, and product yields
were 75–85%.
thesized by the reaction between BTZ and Zn(CH COO) ·2H O [10].
3
2
2
3. Results and discussion
2
-(2-Hydroxyphenyl)benzothiazolate (BTZ): 1H NMR (300 MHz,
CDCl ): ı 6.99(1H, d), 7.14(1H, m), 7.45(1H, d), 7.5(1H, d), 7.7(2H,
3.1. Molecular structures
3
−
1
m), 7.9(1H, d), 8.02(1H, d), 12.5(1H, S). FT-IR (KBr) cm : 3056,
The crystal data, collection information and refinement data for
complexes 1a–1c are summarized in Table 1. The crystal structures
are described in Figs. 3a, 4a and 5a, respectively. Selected bond
lengths and angles are listed in Table 2. Although the four molecules
are in the same crystal system and space groups, their molecular
structures have distinct differences. The molecular conformation,
molecular packing, bond lengths and bond angles of complex 1
are basically identical to previous reference [10]. Its crystal exists
as anhydrous dimer, which consists of two zinc ions with five-
2
9
833, 2595, 1622, 1588, 1482, 1437, 1272, 1250, 1218, 1151, 1129,
70, 862, 757.
2
-(3-Methyl-2-hydroxyphenyl)benzothiazolate (3-MeBTZ): 1H
NMR (300 MHz, CDCl ):
ı
2.357(3H, s), 7.037–6.933(1H, s),
3
7
8
6
.271–7.201(3H, m), 7.536–7.439(1H, m), 7.933–7.793(1H, m),
.010–7.982(1H, d), 12.334(1H, s); Anal. calcd for C14H11NOS: C,
9.68; H, 4.59; N, 5.80; found: C, 70.76; H, 5.365; N, 5.307; FT-IR
−
1
(
KBr) cm : 3054, 2915, 2855, 2350, 1618, 1506, 1438, 1242, 1085,
61, 628.
-(4-Methyl-2-hydroxyphenyl)benzothiazolate (4-MeBTZ): 1H
7
coordinate geometry. The Zn(1) and Zn(1A) ions are bridged by two
Zn–O bonds (2.017(3) A˚ ), as can be seen in Fig. 2a. The monomer 1b
2
NMR (300 MHz, CDCl ): ı 2.148–2.361(3H, s), 6.745–6.772(1H,
is four-coordinated by two deprotonated 4-MeBTZ ligand (Fig. 3a).
3
d), 6.907(1H, s), 7.348–7.399(1H, m), 7.452–7.503(1H, m),
Complexes 1a and 1c have two crystallographically independent
2+
7
1
.540–7.567(1H, m), 7.857–7.883(1H, d), 7.933–7.960 (1H, d),
2.471(1H, s). Anal. calcd for C14H11NOS: C, 69.68; H, 4.59; N, 5.80;
Zn ions, each in distorted four-coordinate geometry, as shown in
Figs. 4a and 5a. These peculiar molecular structures are named as
bimolecule. Moreover, there is distinct intermolecular ␲–␲ inter-
action between molecules. The steric hindrance provided by the 3-,
4- and 5-methyl groups of phenoxide ring prohibits effectively the
formation of pentacoordinate complex.
−
1
found: C, 68.92; H, 4.613; N, 5.690; FT-IR (KBr) cm : 3063, 2916,
2
7
855, 1628, 1576, 1482, 1439, 1275, 1242, 1221, 1134, 978, 808,
55.
2
-(5-Methyl-2-hydroxyphenyl)benzothiazolate (5-MeBTZ): 1H
NMR (300 MHz, CDCl ): ı 2.359(3H, s), 6.996–6.7.024(1H, d),
Though the zinc ions in complex 1 are five-coordinated with
distorted trigonal bipyramidal geometry, the Zn(II) ions bearing
the methyl-substituted ligands are coordinated by two nitrogen
atoms from thiazolate rings and two oxygen atoms from phenoxide
rings, forming distorted tetrahedral geometry. The dihedral angles
between the phenoxide and benzothiazolate rings in complex 1b
3
7
7
.182–7.212(1H, d), 7.405–7.431(2H, m), 7.479–7.505(1H, d),
.891–7.919(1H, d), 7.971–7.994(1H, d), 12.331(1H, s). Anal. calcd
for C14H11NOS: C, 69.68; H, 4.59; N, 5.80; found: C, 70.56; H, 4.49;
−1
N, 5.276; FT-IR (KBr) cm : 3442, 2921, 2360, 1624, 1594, 1497,
1
438, 1274, 1222, 984, 828, 724.
◦
◦
are 2.166 and 3.754 . The dihedral angles including in Zn(1) ion for
◦
◦
◦
2.3. Preparation of zinc complexes
complex 1a are 1.194 and 4.280 , while in Zn(2) ion are 4.736 and
1
◦
.949 . The dihedral angles between the phenoxide and benzothia-
zolate rings are 11.702 and 16.626 for the nonbridging ligands for
complex 1, the others are 12.693 and 11.016 for bridging ligands.
Therefore, X-ray crystallographic structures have proved that the
introduction of methyl led to higher degree of conjugation.
◦ ◦
Bis(2-(3-methyl-2-hydroxyphenyl)benzothiazolate)zinc
(1a)
◦
◦
(
Zn(3-MeBTZ) ): Zn(3-MeBTZ) was synthesized from the reaction
2
2
between 3-MeBTZ and Zn(CH COO) ·2H O in methanol solution.
3
2
2
The mixed solution was heated in reflux and stirred for 2 h. The
yellow powder was filtered and purified by vacuum sublimation.
The Zn–N bond lengths are remarkably different, ranging from
1.996(3) A˚ (Zn(1)–N(1) for complex 1b) to 2.176 A˚ (Zn(1)–N(1) for
complex 1). The Zn(1)–O(1) bond length for complex 1 (2.056(3) A˚ )
is significantly longer than that for complex 1b (1.909(3) Å), com-
plex 1a (1.919(4) Å) and complex 1c (1.8951(2) A˚ ). Therefore,
introducing methyl group as a substituent on the phenoxide ring
effectively enhances the coordination between Zn and N atoms
and electrovalent action between Zn and O atoms. The values of
bond lengths and angles are changed in bi-molecular structures,
suggesting the difference in the coordination effect of zinc in the
same complex. For all complexes discussed in this study, Zn–O
1
H NMR (300 MHz, DMSO): ı 1.902(6H, s), 6.488(4H, d), 7.137(2H,
s), 7.408–7.527(4H, m), 8.062(2H, d), 8.273(2H, d); MALDI-TOF
+
m/z: [MH ] calcd for C28H20N O S Zn, 544.03; found: 545.3; Anal.
2
2 2
calcd for C28H20ZnS N O : C, 61.59; H, 3.69; N, 5.13; found: C,
2
2
2
−1
6
1
1.40; H, 3.70; N, 5.12; FT-IR (KBr) cm : 3043, 2926, 1653, 1458,
412, 1225, 1088, 897, 750.
Bis(2-(4-methyl-2-hydroxyphenyl)benzothiazolate)zinc
(1b)
(
Zn(4-MeBTZ) ): A procedure similar to that for complex 1a was
2
1
used to prepare and purify complex 1b. H NMR (300 MHz, CDCl ):
3
ı 2.325–2.386(6H, d), 6.556–6.583(2H, d), 6.774–6.834(2H, m),
7
7
3
3
.194–7.298(4H, m), 7.421–7.504(2H, m), 7.572–7.661(2H, d),
bonds are shorter than Zn–N bonds. For example, Zn–N lengths
of 1.996(3) and 2.023(3) A˚ are longer than Zn-O lengths of 1.909(3)
.771–7.797(2H, d). Anal. calcd for C28H20N O S Zn: C, 61.59; H,
2
2 2
−
1
.69; N, 5.13; found: C, 61.60; H, 3.748; N, 5.076; FT-IR (KBr) cm
009, 1611, 1536, 1477, 1245, 1205, 1148, 1016, 943, 860, 752.
:
and 1.898(3) A˚ in complex 1a.
Packing in the solid state depends on the molecular structures of
the complexes. Close ␲–␲ stacking interactions between adjacent
ligands facilitate carrier transport. The ␲–␲ stacking of complexes
1 and 1b occurs in the phenoxide and benzothiazolyl moieties.
The aromatic rings between adjacent ligands in complex 1b are
separated with a distance of 3.643 A˚ . The complex 1a displays an
Bis(2-(5-methyl-2-hydroxyphenyl)benzothiazolate)zinc
(1c)
(
Zn(5-MeBTZ) ): Complex 1c was synthesized from the reaction
2
between 5-MeBTZ and Zn(CH COO) ·2H O in ethanol solution.
3
2
2
The mixed solution was stirred for 2 h at room temperature.
The precipitate of complex 1c was obtained, and then purified
by vacuum sublimation. 1H NMR (300 MHz, CDCl ): ı 2.325(6H,
effective face-to-face overlap between adjacent molecules besides
3

X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
113
Table 1
Crystal data and structure refinements for complexes 1a, 1b and 1c.
Identifical code
1a
1b
1c
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
Unit cell dimensions
a, Å
C56H40N4O4S4Zn2
1091.90
298(2)
0.71073
Triclinic
P-1
C28H20N2O2S2Zn
961.95
298(2)
0.71073
Triclinic
P-1
C56H40N4O4S4Zn2
1091.90
293(2)
0.71073
Triclinic
P-1
7.7789(9)
13.8027(16)
23.181(3)
104.811(2)
96.345(2)
92.730(2)
2384.1(5)
2
8.9899(11)
12.1617(15)
12.8719(16)
63.492(2)
84.825(2)
71.187(2)
1189.7(3)
2
2.685
1.380
9.719(2)
11.248(2)
11.902(2)
73.099(3)
81.498(3)
76.476(3)
1205.8(4)
11
b, Å
c, Å
˛
, deg
ˇ, deg
ꢃ, deg
Volume, Å3
Z
3
Density, calcd mg/cm
Absorption coefficient, mm
1.521
1.235
2.112
5.889
−1
F(0 0 0)
1120
976
737
Reflections collected/unique
Data/restraints/parameters
Goodness-of-fit on F
13287/9048 [R(int) = 0.0326]
9048/0/635
0.987
7503/4529 [R(int) = 0.0295]
4529/0/318
1.066
4385/3838 [R(int) = 0.0314]
3838/0/631
1.014
2
Final R indices [I > 2ꢄ(I)]
R indices (all data)
R1 = 0.0663, wR2 = 0.1782
R1 = 0.1127, wR2 = 0.2304
R1 = 0.0534, wR2 = 0.1077
R1 = 0.0802, wR2 = .1340
R1 = 0.0670, wR2 = 0.1634
R1 = 0.0970, wR2 = 0.2127
initial-to-tail stacking (in Fig. 4c), resulting in a shortest distance
of about 3.564 A˚ . There is also weak S. . .O interaction besides ␲–␲
stacking interactions for complex 1c, as can be seen in Fig. 5b. On
the other hand, complex 1 exhibits a longer intermolecular ␲–␲
interaction of 3.7653 A˚ [10]. The intermolecular ␲–␲ interaction
30°C/min
has a direct influence on luminescent properties [15,16], discussed
below.
To confirm that the single crystal structures are represen-
tative of the entire sample, a global optimization method by
Rietveld refinement for powder diffraction spectra were carried
out. Figs. 3b and 4b reveal that the simulated powder XRD pat-
tern of complex 1b and 1a which were calculated from the single
crystal data, and the measured powder XRD pattern were prac-
tically identical. To further evidence the unique presence of the
bi-molecular structure for complex 1a, the thermal analysis was
carried out. From the DSC curves for powder obtained at different
heating rates, a single endothermic transition is obtained, which
narrows with a decreasing heating rate, as shown in Fig. 6. Simi-
1
5°C/min
5°C/min
1
50
200
250
300
350
Temperature (ºC )
Fig. 6. DSC curves of complex 1a powder at different heating rates.
Table 2
Experimental structure parameters for complexes 1, 1a, 1b and 1c.
Complex
1b
1
1a
1c
Bond distances (Å)
Zn(1)–O(2)
Zn(1)–N(1)
Zn(1)–O(1)
Zn(1)–N(2)
Zn(2)–O(4)
Zn(2)–O(3)
Zn(2)–N(3)
Zn(2)-N(4)
1.898(3)
1.996(3)
1.909(3)
2.023(3)
1.957(3)
2.176(4)
2.056(3)
2.098(4)
1.897(4)
2.002(5)
1.919(4)
2.002(5)
1.905(4)
2.000(7)
2.007(5)
2.008(5)
1.8979(2)
2.0149(2)
1.8951(2)
2.0144(2)
1.957(2)
1.863(1)
1.975(1)
1.985(2)
Bond angles (deg)
O(2)–Zn(1)–O(1)
O(2)–Zn(1)–N(2)
O(1)–Zn(1)–N(2)
O(2)–Zn(1)–N(1)
O(1)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
O(4)–Zn(2)–O(3)
O(4)–Zn(2)–N(4)
O(3)–Zn(2)–N(4)
O(4)–Zn(2)–N(3)
O(3)–Zn(2)–N(3)
N(4)–Zn(2)–N(3)
127.64(14)
93.92(13)
107.70(14)
115.89(13)
94.69(13)
118.82(13)
171.89(14)
87.36(16)
100.16(15)
96.77(16)
81.98(15)
121.33(16)
128.3(2)
111.0(2)
94.9(2)
94.73(19)
112.47(18)
117.04(19)
127.6(2)
95.2(2)
117.5(7)
89.7(8)
132.1(9)
123.8(8)
92.1(7)
104.7(8)
116.5(8)
95.2(7)
122.6(8)
120.5(7)
97.1(6)
112.7(3)
113.2(2)
93.9(2)
109.7(2)
106.5(6)

1
14
X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
1.2
1.0
0.8
0.6
0.4
0.2
0.0
complex 1b absorption
complex 1a absorption
complex 1b emission
complex 1a emission
1.0
0.8
0.6
0.4
0.2
0.0
complex 1
complex 1a
complex 1b
complex 1c
400
500
600
700
250
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 7. The PL spectra for thin films of complexes 1, 1a, 1b and 1c.
Fig. 8. The absorption and emission spectra of complex 1a and 1b in THF solution.
lar behavior was observed for complex 1c. In addition, the 1H NMR
tion spectra. The meta-position substitution to the hydrogen
(complex 1b) results in blue-shift emission and absorption spectra
and ortho- (complex 1a) and para-position (complex 1c) substitu-
tion led to distinct red shift. The optical transition responsible for
metal (2-hydroxyphenyl)benzothiazolate complex is centered on
the organic ligand [10,12]. This transition is due to a ␲–␲* charge
transfer from the electron rich phenoxide ring (HOMO) to the elec-
tron deficient benzothiazolate ring (LUMO). Therefore, observed
blue- or red-shifted in absorption and emission spectra can be
explained by correlating the electronic effects due to the methyl
substituent.
spectroscopy of the single crystal and powder samples in CDCl had
3
identical results. No detectable changes were observed in the FT-IR
spectra comparing the single crystal with powder sample dispersed
in KBr. These results support the uniqueness in the single crystal
and in the powder purified by the vacuum sublimation.
3.2. Thermal stability
Complex 1 exhibits reasonable stability upon exposure to air
and high thermal stability. The melting temperature (Tm) and
◦
decomposition temperature (T ) were located at 307 and 500 C,
d
respectively. Upon methyl substitution, Tm decreases in the order
3.4. Electronic structure
1
c > 1 > 1b > 1a, and Td decreases in the order 1 > 1b > 1c > 1a. The
values Td of methyl-substituted complexes are lower than that of
complexes 1, indicating that five-coordinate molecular structure
results in a higher thermal stability than four-coordinate struc-
tures. Thermalanalysisresults show that 1a exhibitedthe lowest Tm
and Td.
To investigate the electronic effects caused by the addition of
methyl group, cyclic voltammetry experiments were carried out
for all complexes in THF solution. For the electrochemical behav-
ior of all complexes, it has been proposed that the oxidative and
reductive processes involve the Zn-phenoxide and -benzothiazolyl
moiety. Based on the DFT calculations on monomeric complexes
1b, the HOMO levels are mostly due to ␲-orbital contributions
from the phenoxide rings, while the LUMO levels are mainly due
to the benzothiazolyl rings, as shown in Fig. 9. The experimen-
tal data of energy gap, oxidation potentials for all complexes
are listed in Table 3. It is evident that all of methyl-substituted
complexes show lower oxidative potentials than complex 1, indi-
cating the relative ease of oxidation. This can be ascribed to the
great electro-donating ability of the methyl group. In particu-
lar, complexes 1a and 1c show lower oxidative potentials than
1b.
HOMO for complex 1b has node at 4-positions of the phenoxide
rings, and has bonding at 3- or 5-position, but the LUMO has node
at 3- or 5-position and has bonding at 4-position. Therefore, com-
plex 1a and 1c show obvious decrease in oxidation potential. The
reductive potential of complex 1b was expected to be lower than
that of complexes 1a, 1c. Theoretical results indicate that complex
1b has a lower LUMO energy of −3.27 eV compared with complex
3.3. Photophysical properties
The solid-state PL spectra of all complexes can be seen from
Fig. 7. Interestingly, complex 1b has blue-shifted emission rela-
tive to other complexes (ꢀmax = 470 nm). While the complexes 1a
and 1c have ꢀmax values at 507 and 499 nm, respectively. Table 3
lists the photophysical data of all complexes. The UV–vis spectra
of all complexes exhibit intense absorption bands in the range of
4
10–430 nm in THF solution, corresponding to the electronic tran-
sitions from phenoxide to benzothiazolyl. The absorption bands in
50–380 nm correspond to intraligand transitions [11]. The absorp-
2
tion patterns of all materials are similar in 250–435 nm regions. The
maximum absorption bands of 1a and 1c appear at lower energies,
while complex 1b occur at much higher energy, as can be seen in
Fig. 8.
The results suggest the different substitutional positions of
methyl on phenoxide ring can produce different effect on absorp-
Table 3
Photophysical and electrochemical data for complexes (Em: emission; ˚PL: PL quantum efficiency relative to complex 1 (set to 1); Eox: oxidation potential; Eg: energy gap;
F: lifetime in THF and voltage: the turn on voltage in the bilayer devices).
ꢁ
Complexes
Absorption (nm)
Solution
Em (ꢀmax)
Relative ˚PL
Eox (eV)
Eg (eV)
ꢁF (ns)
Voltage (V)
Powder
Solution
Powder
1
250, 311, 381, 417
260, 321, 372, 431
260, 311, 376, 423
252, 318, 384, 417
303, 380
322, 414
310, 386
309, 395
466
471
460
477
480
507
470
496
1
1.36
1.28
1.34
1.30
2.32
2.25
2.37
2.28
4.008
3.977
4.724
5.354
9.2
12.7
2.3
1
1
1
a
b
c
0.70
1.56
1.36
10.7

X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
115
4
3
2
1
0
0
0
0
0
complex 1a
complex 1b
complex
1
complex 1c
0
5
10
15
20
25
30
Voltage (V)
Fig. 11. Current vs voltage plots for all complexes.
Fig. 9. Molecuar orbital population of HOMO and LUMO for complex 1b.
2
1
1
cd/m ) of complexes 1, 1a, 1b and 1c are 9.2 V, 12.7 V, 2.3 V and
0.7 V, respectively.
1
−2.73 eV. The calculated values of HOMO energy is −4.96 eV for
The ability of the emissive layer to trap charges is considered
complex 1b.
to be the most important factors which determine the operating
voltage. The device operating voltage (at a fixed current) increases
in the order 1b < 1a < 1 < 1c, as can be seen in Fig. 11. The reason for
the higher operating voltages is the poorer carrier-transfer ability
at the metal/organic interface. The device based on 1b has a lower
operating voltage.
3
.5. Electroluminescent performance
The electroluminescent properties of complexes 1 and 1a–1c
were investigated. The performances of devices were measured
using zinc complexes as emitting material and NPB as hole-
transport material. The structures of OLEDs used in this work are of
typical bilayer, employing ITO as anode and Al as cathode. The ꢀmax
of the complexes are in the range of 501–553 nm of EL spectra. No
characteristic emission peak of NPB (ꢀmax = 446 nm) was observed,
indicating that the light emissions originate from emitting materi-
als (in Fig. 10). The EL spectrum of complex 1b splits into two peaks
located at 501 and 544 nm. All the EL spectra have large full width
at half maximum (FWHM), e.g. 168 nm for complex 1, 135 nm for
complex 1a, 110 nm for complex 1b, and 100 nm for complex 1c,
almost covering the main part of the visible-light range from 420
to 780 nm. The broaden EL spectra relative to PL spectra may be
caused by the formation of exciplex at the interface between NPB
and Zn complexes [17,18]. The exciplex is a result of charge trans-
fer interaction between the excited state of donor and ground state
of acceptor. In this paper, the donor is hole-transfer material (NPB)
and the acceptor is emitting-material (Zinc complexes). There is lit-
tle evidence for exciplex formation in the NPB/Alq bilayer because
NPB is a weaker donor [19]. The turn-on voltages (measured at
4
. Conclusions
A group of methyl-substituted [Zn(BTZ)2]2 derivatives were
synthesized, forming dimer monomer, and bi-molecular structures.
Their interesting structural features were characterized by single-
crystal and powder X-ray diffraction. Lower oxidation potentials
of methyl complexes than that of complex 1 were observed, indi-
cating that the methyl group induces a relatively easy oxidation
and effective hole trapping. Complex 1b, which has methyl group
at the 4-position of phenoxide, exhibits brighter blue emission,
stronger EL intensity, lower turn-on and operating voltages than
other complexes. The luminescent mechanisms were discussed
with molecular orbital calculations and redox potentials, showing
that the luminescent properties are ligand-based and can be tuned
by introducing functional groups.
Acknowledgments
This work was financially supported by National Natural Sci-
entific Foundation of China (20671068, 21071108, 60976018);
Natural Science Foundation of Shanxi Province (2008011008,
2010021023-2); Program for Changjiang Scholar and Innovation
Research Team in University (IRT0972); Innovation Foundation for
Graduate Student of Shanxi Province(20081007).
complex
1
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
complex 1a
complex 1b
complex 1c
References
[
[
1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913–915.
2] J. An, J. Chang, J. Han, C. Im, Y.-J. Yu, D.H. Choi, J. Jin, T. Majima, J. Photochem.
Photobiol. A: Chem. 200 (2008) 371–376.
[
[
3] S.R. Forrest, Nature 428 (2004) 911–918.
4] B.S. Xu, H. Wang, Y.Y. Hao, Z.X. Gao, H.F. Zhou, J. Lumin. 122/123 (2007)
663–666.
[
[
5] T.T. Wang, G.C. Zeng, H.P. Zeng, P.Y. Liu, R.X. Wang, Z.J. Zhang, Y.L. Xiong, Tetra-
hedron 65 (2009) 6325–6329.
6] H. Xu, Z.F. Xu, Z.Y. Yue, P.F. Yan, B. Wang, L.W. Jia, G.M. Li, W.B. Sun, J.W. Zhang,
J. Phys. Chem. C 112 (2008) 15517–15525.
400
500
600
700
Wavelength (nm)
[
[
7] B.S. Xu, Y.Y. Hao, X.H. Fang, H. Wang, X.G. Liu, Appl. Phys. Lett. 90 (2007) 053903.
8] Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Takahashi, K. Shibata, Jpn. J. Appl. Phys.
35 (1996) L1339–L1341.
Fig. 10. The EL spectra of all complexes.

1
16
X. Huixia et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 108–116
[
9] T. Sano, Y. Nishio, Y. Hamada, H. Takahashi, T. Usuki, K. Shibata, J. Mater. Chem.
0 (2000) 157–161.
10] G. Yu, S.W. Yin, Y.Q. Liu, Z.G. Shuai, D.B. Zhu, J. Am. Chem. Soc. 125 (2003)
4816–14824.
[15] S. Jung, Y. Kang, H.S. Kim, Y.H. Kim, C.L. Lee, J.J. Kim, S.K. Lee, S.K. Kwon, Eur. J.
Inorg. Chem. (2004) 3415–3423.
[16] V.V. Grushin, N. Herron, D.D. LeCloux, W.J. Marshal, V.A. Petrov, Y. Wang, Chem.
Commun. (2001) 1494–1495.
1
[
1
[
[
11] Y.P. Tong, S.L. Zheng, X.M. Chen, Inorg. Chem. 44 (2005) 4270–4275.
12] B.S. Xu, H.X. Xu, X.H. Fang, L.Q. Chen, H. Wang, X.G. Liu, Org. Electron. 9 (2008)
[17] X. Xu, Y. Liao, G. Yu, H. You, C.A. Di, Z.S. Su, D.G. Ma, Q. Wang, S.Y. Li, S.Q. Wang,
J.P. Ye, Y.Q. Liu, Chem. Mater. 19 (2007) 1740–1748.
9
06–910.
[18] S.P. Singh, Y.N. Mohapatra, M. Qureshi, S.S. Manoharan, Appl. Phys. Lett. 86
(2005) 113505.
[19] H. Wang, K.P. Klubek, C.W. Tang, Appl. Phys. Lett. 93 (2008) 093306.
[
[
13] H.J. Son, W.S. Han, J.Y. Chun, B.K. Kang, S.N. Kwon, J. Ko, S.J. Han, C. Lee, S.J. Kim,
S.O. Kang, Inorg. Chem. 47 (2008) 5666–5676.
14] Y. Kan, G.S. Wu, L.D. Wang, Y. Qiu, Int. J. Quantum Chem. 98 (2004) 325.