A novel class of Zn(II) Schiff base complexes with aggregation-induced emission enhancement (AIEE) properties: Synthesis, characterization and photophysical/electrochemical properties

Article abstract of DOI:10.1016/j.dyepig.2012.09.020

A series of novel Zn(II) Schiff base complexes with diphenylamino and carbazole groups that exhibited aggregation-induced emission enhancement were designed and synthesized. The properties of four complexes were investigated by UV-Vis absorption and fluorescence emission spectroscopy, cyclic voltammetry and density functional theory calculations. The fluorescence intensities of the four dyes are weak in tetrahydrofuran, but become strong in a mixture of water/tetrahydrofuran (v/v = 9/1). This work constitutes the first observation of this phenomenon for Zn(II) Schiff base complexes. A simple model complex, without the possibility of intramolecular rotational motion, was prepared in order to determine the mechanism of the AIEE. The present aggregation-induced emission enhancement was attributed to restricted intramolecular rotational motions in the solid by carefully analyzing the difference in molecular structure and photophysical properties amongst the new complexes.

Full text of DOI:10.1016/j.dyepig.2012.09.020

Dyes and Pigments 96 (2013) 467e474
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Dyes and Pigments
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A novel class of Zn(II) Schiff base complexes with aggregation-induced emission
enhancement (AIEE) properties: Synthesis, characterization and photophysical/
electrochemical properties
,
,
c
,
a,
Yu-Zhong Xie a b, Guo-Gang Shan a, Peng Li a, Zi-Yan Zhou a , Zhong-Min Su
*
*
a Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China
b Department of Chemistry, Yanbian University, Yanji 133002, People’s Republic of China
c College of Chemical Engineering, Shandong University of Technology, Zibo 255049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel Zn(II) Schiff base complexes with diphenylamino and carbazole groups that exhibited
aggregation-induced emission enhancement were designed and synthesized. The properties of four
complexes were investigated by UVeVis absorption and fluorescence emission spectroscopy, cyclic
voltammetry and density functional theory calculations. The fluorescence intensities of the four dyes are
weak in tetrahydrofuran, but become strong in a mixture of water/tetrahydrofuran (v/v ¼ 9/1). This work
constitutes the first observation of this phenomenon for Zn(II) Schiff base complexes. A simple model
complex, without the possibility of intramolecular rotational motion, was prepared in order to determine
the mechanism of the AIEE. The present aggregation-induced emission enhancement was attributed to
restricted intramolecular rotational motions in the solid by carefully analyzing the difference in
molecular structure and photophysical properties amongst the new complexes.
Received 20 July 2012
Received in revised form
27 September 2012
Accepted 27 September 2012
Available online 8 October 2012
Keywords:
Aggregation-induced emission
enhancement (AIEE)
Zinc(II) complex
Ó 2012 Elsevier Ltd. All rights reserved.
Schiff base
Photophysical properties
Electrochemical properties
Density functional theory
1. Introduction
emission enhancement (AIEE) properties have been recently found
[21e27]. These compounds exhibit weak luminescence or have
Solid-state organic luminescent dyes have gained a great deal of
attention in recent years because of their potential applications in
chemosensors [1,2], field-effect transistors [3e6], live-cell imaging
[7e9], and organic light-emitting diodes (OLEDs) [10e13]. However,
a majority of organic dyes exhibit strong luminescence in their dilute
solutions. These dyes become weak luminophores in the solid state
or in high concentration in solution because of the strong interac-
tions between closely packed molecules, leading to emission
quenching [14e16]. This phenomenon may render the dyes less
suitable for some optical devices. A variety of approaches such as
attaching enhanced steric hindrance or bulky side groups into the
complexes to separate chromophoric units have been recently
investigated in an effort to overcome this problem [17e20].
However, limited success has been achieved using these approaches.
Some organic compounds exhibiting unusual aggregation-induced
almost no emission in solution, but are highly emissive in the solid
state. This characteristic of materials with AIEE properties can
provide an efficient way to resolve the problem related to emission
quenching in the condensed phase.
A number of studies on Zn(II) Schiff base complexes as potential
OLED materials have been conducted since Hamada et al. first re-
ported on this research field in 1993 [28e31]. Recently, Chi et al.
have also developed a series of phosphorescent Pt(II) transition
metal Schiff base complexes and demonstrated that these
complexes are suitable candidates in electroluminescent applica-
tions [32,33]. In addition, the Schiff base ligands can be easily
prepared and structurally modified. Thus, various Schiff base
complexes with desired photophysical properties can be obtained
[34e41]. However, these complexes intrinsically suffer from
emission quenching in films, decreasing the device performance,
especially for non-doped devices. It is speculated that if the Schiff
base compounds with the AIEE properties can be explored, the
tendency of emission quenching for them would no longer exist.
Although the Ir(III) Schiff base complexes exhibiting AIEE proper-
ties have been reported by Park recently [42], the Zn(II) Schiff base
* Corresponding authors. Institute of Functional Material Chemistry, Faculty of
Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of
China. Tel.: þ86 431 85099108; fax: þ86 431 85684009.
E-mail address: zmsu@nenu.edu.cn (Z.-M. Su).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.020

468
Y.-Z. Xie et al. / Dyes and Pigments 96 (2013) 467e474
complexes exhibiting AIEE have never been investigated until this
present study.
tetramethylsilane as the internal standard. The complexes were also
verified using matrix-assisted laser desorptioneionization time-of-
flight mass spectrometry (MS MALDI-TOF). UVeVis absorption
spectra were obtained using a Shimadzu UVeVis spectrometer. The
PL quantum yields of the complexes in the solutions and solid-state
were measured using steady-state and time-resolved fluorescence
spectrometers (FLSP920) with an integrating-sphere photometer.
Fourier transform infrared spectrawere obtained using KBr pellets in
the 4000e400 cmꢁ1 range on an Irprestige-21 spectrometer. The
emission spectra were obtained from a Cary Eclipse spectrofluo-
rometer (Varian) equipped with a xenon lamp and quartz carrier at
room temperature. Cyclic voltammetry was performed on a BAS
In this contribution, we successfully designed and synthesized
four Zn(II) Schiff base complexes, namely, Zn(L1)2 (1), Zn(L2)2
(2), Zn (L3)2 (3) and Zn (L4)2 (4), in which the ligands L1e
L4 represent 2-(((4-(diphenylamino)phenyl)imino)methyl)phe
nol, 1-(((4-(diphenylamino)phenyl)imino)methyl)naphthalen-2-ol,
2-(((4-(9H-carbazol-9-yl)phenyl)imino)methyl)phenol, 1-(((4-(9H-
carbazol-9-yl)phenyl)imino)methyl)naphthalen-2-ol, respectively
(See Scheme 1). Herein, the diphenylamino and carbazole groups
were introduced into complexes through single bonds because the
incorporation of these aromatic moieties with rotational motions
into complexes can be expected to give rise to AIEE [25,26]. The
photophysical properties of the designed complexes have been
investigated in detail. The results demonstrate that the complexes
have AIEE properties and emitted in the yellow-green region,
100
W
electrochemical workstation. Tetrabutylammonium
perchlorate was used as supporting electrolyte (0.1 M) in dry
dichloromethane with a scan rate of 100 mV/s at room temperature
under argon. The glassy carbon rod electrode was used as the
working electrode, a Pt wire as the counter electrode, with a calomel
electrode as a reference. The oxidative potentials were calibrated by
using ferrocene as an internal standard.
indicating that they may be good candidates for OLEDs.
A
controlled experiment using compound 5 (Scheme 1) without any
aromatic rotational motions was performed to understand the
possible mechanism of the present AIEE property. Based on our
data, the most probable reasons were claimed to be restricted
intramolecular motion in the solid state, which was also considered
as the main cause of AIEE properties for reported organic small-
molecules. To the best of our knowledge, this is a first example of
Zn(II) Schiff base being shown to AIEE active. To gain insight into
the nature of their emissive excited state, quantum chemical
calculations were performed.
2.2. Synthesis of ligand and complex
2.2.1. Synthesis of ligand L1
A
solution of N0,N0-diphenylbenzene-1,4-diamine (0.26 g,
1.0 mmol) and salicylaldehyde (0.12 g, 0.10 mmol) in ethanol (10 mL)
was heated under reflux for 4 h. After cooling to room temperature,
the precipitate was filtered and washed with ethanol. The precipitate
was a light yellow needles (85%). M.P.: 159e160 ꢀC, 1H NMR
2. Experimental section
(500 MHz, CDCl3)
d
13.44 (s, 1H), 8.62 (s, 1H), 7.36 (dd, J ¼ 13.1,
7.2 Hz, 2H), 27 (t, J ¼ 9.1 Hz, 4H), 7.23e7.18 (m, 2H), 7.15e7.07 (m, 6H),
7.07e6.98 (m, 3H), 6.93 (t, J ¼ 7.5 Hz, 1H). IR (KBr), v/cmꢁ1: 3444,
3032, 1614, 1587, 1489, 1454, 1328, 1278, 1190, 1149, 1112, 831, 754,
696, 533, 507. Anal. calcd. for C25H20N2O: C, 82.39; H, 5.53; N, 7.69.
Found: C, 81.78; H, 5.61; N, 7.60.
2.1. Materials and physical measurements
All reagents and solvents employed were commercially available
and used as received without further purification. The solvents for
syntheses were freshly distilled over appropriate drying reagents. All
experiments were performed under a nitrogen atmosphere using
standard Schlenk techniques. Elemental analyses (C, H, and N) were
performed on a PerkineElmer 240C elemental analyzer. Thermog-
ravimetric analyses (TGA) were performed on a PerkineElmer TG-7
analyzer heated from 30 ꢀC to 800 ꢀC under flowing nitrogen. 1H
NMR spectra were measured on a Bruker Avance 500 MHz with
2.2.2. Synthesis of ligand L2
The synthesis of ligand L2 was similar to that of ligand L1 from
N0,N0-diphenylbenzene-1,4-diamine and 2-hydroxy-1-naphthalde
hyde. The precipitate was a golden yellow powder (82%). M.P.:
158.5e160.1 ꢀC,1H NMR (500 MHz, CDCl3)
d15.67 (s,1H), 9.32 (s,1H),
8.10 (d, J ¼ 8.4 Hz, 1H), 7.79 (d, J ¼ 9.2 Hz, 1H), 7.72 (d, J ¼ 8.0 Hz,
Scheme 1. The structures of zinc(II) Schiff base complexes 1, 2, 3, 4, and 5. (For interpretation of the references to color in this legend, the reader is referred to the web version of this
article.)

Y.-Z. Xie et al. / Dyes and Pigments 96 (2013) 467e474
469
1H), 7.51 (t, J ¼ 7.7 Hz, 1H), 7.33 (t, J ¼ 7.5 Hz, 1H), 7.31e7.27 (m, 6H),
7.19e7.12 (m, 6H), 7.10 (d, J ¼ 9.1 Hz, 1H), 7.06 (t, J ¼ 7.4 Hz, 3H). IR
(KBr), v/cmꢁ1: 3415, 3053, 1622, 1587, 1489, 1421, 1327, 1278, 1244,
1174, 1074, 815, 750, 700, 545, 503. Anal. calcd. for C29H22N2O: C,
84.03; H, 5.35; N, 6.76. Found: C, 84.11; H, 5.29; N, 6.70.
After cooling to room temperature, the precipitate was filtered and
washed with ethanol. A yellow flocculent solid was obtained (75%).
1H NMR (500 MHz, CDCl3):
d
8.39 (s, 2H), 7.34 (t, J ¼ 7.7 Hz, 2H), 7.21
(dd, J ¼ 14.5, 6.9 Hz, 10H), 7.05e6.97 (m, 16H), 6.93 (dd, J ¼ 17.4,
8.7 Hz, 6H), 6.65 (t, J ¼ 7.3 Hz, 2H). IR (KBr), v/cmꢁ1: 1607, 1587,
1531, 1498, 1460, 1438, 1384, 1325, 1276, 1180, 1147, 756, 696, 501.
Anal. calcd. for C50H38N4O2Zn: C, 75.80; H, 4.83; N, 7.07. Found: C,
75.92; H, 4.71; N, 7.11. MS (MALDI-TOF): m/z 791.2[Mþ].
2.2.3. Synthesis of ligand L3
The synthesis of ligand L3 was similar to that of ligand L1 from
4-(9H-carbazol-9-yl)aniline and salicylaldehyde. The precipitate
was a light yellow powder (92%). M.P.: 181.4e182.8 ꢀC, 1H NMR
2.2.7. Synthesis of complex 2
(500 MHz, CDCl3)
d
13.20 (s, 1H), 8.77 (s, 1H), 8.18 (d, J ¼ 7.8 Hz, 2H),
The synthesis of complex 2 from ligand L2 and zinc acetate was
similar to that of complex 1. The orange solid was obtained (72%).
7.65 (d, J ¼ 8.5 Hz, 2H), 7.54 (d, J ¼ 8.5 Hz, 2H), 7.51e7.42 (m, 6H),
7.38e7.29 (m, 2H), 7.10 (d, J ¼ 8.3 Hz, 1H), 7.01 (t, J ¼ 7.5 Hz, 1H). IR
(KBr), v/cmꢁ1: 3416, 3024, 1614, 1593, 1566, 1510, 1491, 1450, 1364,
1333, 1314, 1279, 1229, 1180, 1148, 1150, 841, 819, 744, 721, 655, 621,
530, 428. Anal. calcd. for C25H18N2O: C, 82.85; H, 5.01; N, 7.73.
Found: C, 81.92; H, 5.10; N, 7.68.
1H NMR (500 MHz, CDCl3):
d
9.32 (s, 2H), 8.01 (d, J ¼ 8.5 Hz, 2H),
7.78 (d, J ¼ 9.2 Hz, 2H), 7.68 (d, J ¼ 7.6 Hz, 2H), 7.47 (t, J ¼ 7.2 Hz, 2H),
7.28 (t, J ¼ 7.4 Hz, 2H), 7.20 (t, J ¼ 7.9 Hz, 8H), 7.09 (d, J ¼ 9.2 Hz, 2H),
7.02e7.00 (m,12H), 7.00e6.94 (m, 8H). IR (KBr), v/cmꢁ1: 1614,1593,
1535, 1490, 1456, 1427, 1394, 1363, 1328, 1278, 1180, 829, 748, 696,
493. Anal. calcd. for C58H42N4O2Zn: C, 78.07; H, 4.74; N, 6.28.
Found: C, 78.10; H, 4.70; N, 6.31. MS (MALDI-TOF): m/z 891.2[Mþ].
2.2.4. Synthesis of ligand L4
The synthesis of ligand L4 was similar to that of ligand L1 from
4-(9H-carbazol-9-yl)aniline and 2-hydroxy-1-naphthaldehyde. The
precipitate was an orange powder (90%). M.P.: 189.0e190.3 ꢀC, 1H
2.2.8. Synthesis of complex 3
The synthesis of complex 3 from ligand L3 and zinc acetate was
NMR (500 MHz, CDCl3)
d15.43 (s, 1H), 9.48 (d, J ¼ 3.4 Hz, 1H), 8.17
similar to that of complex 1. The yellow solid was obtained (76%). 1H
(t, J ¼ 8.2 Hz, 3H), 7.85 (d, J ¼ 9.1 Hz, 1H), 7.76 (d, J ¼ 7.8 Hz, 1H), 7.66
(d, J ¼ 8.6 Hz, 2H), 7.62e7.52 (m, 3H), 7.50e7.41 (m, 4H), 7.38
(t, J ¼ 7.7 Hz, 1H), 7.35e7.27 (m, 2H), 7.16 (d, J ¼ 9.1 Hz, 1H). IR (KBr),
v/cmꢁ1: 3418, 3042, 1602, 1534, 1512, 1477, 1450, 1358, 1335, 1315,
1229, 1165, 1134, 1115, 1001, 954, 912, 821, 750, 723, 648, 621, 501,
430. Anal. calcd. for C29H20N2O: C, 84.44; H, 4.89; N, 6.79. Found: C,
84.35; H, 4.79; N, 6.83.
NMR (500 MHz, CDCl3): d 9.50 (s, 2H), 8.15e8.07 (m, 4H), 7.89
(d, J ¼ 9.3 Hz, 2H), 7.75 (d, J ¼ 7.8 Hz, 2H), 7.52 (d, J ¼ 8.3 Hz, 4H),
7.42 (d, J ¼ 8.4 Hz, 4H), 7.38e7.26 (m,14H), 7.19 (d, J ¼ 9.2 Hz, 2H). IR
(KBr), v/cmꢁ1: 1610, 1585, 1529, 1510, 1446, 1390, 1357, 1332, 1226,
1176, 1145, 842, 748, 723, 626, 567. Anal. calcd. for C50H34N4O2Zn: C,
76.19; H, 4.35; N, 7.11. Found: C, 76.02; H, 4.45; N, 7.23. MS (MALDI-
TOF): m/z 787.2[Mþ].
2.2.5. Synthesis of ligand L5
2.2.9. Synthesis of complex 4
The synthesis of ligand L5 was patterned after a published
procedure [43,44]. The precipitate was a white powder solid (80%).
The synthesis of complex 4 from ligand L4 and zinc acetate was
similar to that of complex 1. The orange solid was obtained (73%).
1H NMR (500 MHz, CDCl3):
d
8.58 (s, 2H), 8.14 (d, J ¼ 7.7 Hz, 4H),
2.2.6. Synthesis of complex 1
A solution of ligand L1 (0.36 g, 1.0 mmol) and zinc acetate (0.1 g,
0.50 mmol) in ethanol (10 mL) was heated under reflux for 4 h.
7.54 (d, J ¼ 8.6 Hz, 4H), 7.46 (t, J ¼ 7.0 Hz, 4H), 7.42 (d, J ¼ 8.6 Hz, 4H),
7.36 (d, J ¼ 3.3 Hz, 8H), 7.33e7.23 (m, 8H), 7.04 (d, J ¼ 8.6 Hz, 2H),
6.74 (t, J ¼ 7.3 Hz, 2H). IR (KBr), v/cmꢁ1: 1612,1575, 1535,1510,1452,
Scheme 2. Synthetic routes of ligands L1eL5.

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Y.-Z. Xie et al. / Dyes and Pigments 96 (2013) 467e474
Fig. 1. Photophysical properties of L1eL4 in THF solution (10ꢁ5 M). (a) Absorption spectra; (b) emission spectra.
Fig. 2. Photophysical properties of 1, 2, 3, and 4. (a) Absorption spectra. (b) Emission spectra in THF solution (10ꢁ5 M) (The excited wavelength is 329, 366, 305 and 302 nm,
respectively). (c) Emission spectra in solid state (The excited wavelength is 277, 274, 276 and 274 nm, respectively).
1427, 1392, 1363, 1336, 1228, 1182, 1165, 833, 748, 503. Anal. calcd.
for C58H38N4O2Zn: C, 78.42; H, 4.31; N, 6.31. Found: C, 78.51; H,
4.20; N, 6.40. MS (MALDI-TOF): m/z 887.2[Mþ].
3.2. IR and 1H NMR spectra
IR spectra of ligands L1eL4 show a very strong absorption band
due to the imine C]N stretching at 1614, 1622,1614 and 1602 cmꢁ1
,
2.2.10. Synthesis of complex 5
respectively. It is shifted by about 10 cmꢁ1 in the complex 1e4,
shows at 1607, 1614, 1610 and 1612 cmꢁ1, respectively, suggesting
that coordination of the Schiff base groups through nitrogen atoms
with the zinc ion occurred. 1H NMR spectra of L1eL4 show the
important proton signals at 8.62, 8.10, 8.77 and 9.48 ppm, respec-
tively, which are assigned to the imine CH]N proton. The CH]N
protons of complexes 1e4 appear as singlet at 8.39, 9.32, 9.50 and
8.58 ppm, respectively.
The synthesis of complex 5 from ligand L5 and zinc acetate was
similar to that of complex 1. The white powder was obtained (78%).
1H NMR (300 MHz, CDCl3):
d
8.23 (s, 2H), 7.32 (t, J ¼ 6.9 Hz, 2H), 7.13
(d, J ¼ 9.5 Hz, 2H), 6.87 (d, J ¼ 8.5 Hz, 2H), 6.61 (t, J ¼ 7.0 Hz, 2H),
3.63 (q, J ¼ 7.2 Hz, 4H), 1.27 (t, J ¼ 7.3 Hz, 6H). Anal. calcd. for
C18H20N2O2Zn: C, 59.76; H, 5.57; N, 7.74. Found: C, 59.51; H, 5.20;
N, 7.40.
3.3. Photophysical properties
2.3. Theoretical calculations
3.3.1. Photophysical properties of ligands
The absorption and photoluminescent (PL) spectra of free
ligands L1eL4 recorded in THF solution are presented in Fig. 1. The
bands around 300 nm are assigned to the
aromatic rings. The low-energy absorption peaks are situated at
The electronic structures of the singlet ground state (S0) for
complexes 1e4 were investigated by performing density functional
theory (DFT) calculations at the B3LYP level [45,46]. The 6-31G*
basis sets were employed to optimize the C, H, N, and Zn atoms. The
excitation energies were then studied by time-dependent DFT (TD-
DFT). All calculations were performed with the C.02 version of the
Gaussian 03 program package [47].
pep* transition of the
389 nm for L1, 416 nm for L2, 342 nm for L3 and 387 nm for L4,
Table 1
Characters of the absorption and fluorescence spectra of ligands and complexes.
3. Results and discussion
aLigands
labs lem (nm)
Complexes
3.1. Synthesis
/
alabs(nm)
lem (nm)
cFsolution
Fpowder
1
2
3
4
389/537
416/544
342/418(526)
387/503
422
441
409
430
a518/b527
a544/b554
a496/b504
a503/b521
3.0%
16.0%
3.0%
16.7%
20.9%
21.4%
4.5%
Ligands L1eL5 were synthesized according to the procedures
described in the literature [43] (Scheme 2). Compounds 1e5 can be
readily achieved from the reaction of ligands L1eL5, respectively,
with zinc acetate in ethanol and then heated under reflux for 4 h.
The molecular structures of these complexes were characterized by
1H NMR, MS, and elemental analysis.
2.0%
a
b
c
THF solution (10ꢁ5 M).
Powder.
CH2Cl2 solution.

Y.-Z. Xie et al. / Dyes and Pigments 96 (2013) 467e474
471
Fig. 3. Emission spectra of 3 (a) and 5 (b) in water/THF mixtures with different water fractions.
respectively, which can be attributed to the
p
ep* transition
3.4. Aggregation-induced emission enhancement (AIEE) properties
resulting from the conjugation between the aromatic rings and
nitrogen atoms. The ligands display relatively weak emission at
room temperature, with peaks at 537, 544, 418/526 and 503 nm for
L1, L2, L3 and L4 in the solution, respectively.
The obtained compounds (1e4) readily dissolve in common
organic solvents, such as chloroform, THF, acetone, acetonitrile, and
dichloromethane, but are insoluble in water. Thus, the aggregated
particles should be precipitated as the fraction of water is increased.
The emission spectra of 1e4 in water/THF mixtures with different
water content (0e90%) were obtained to verify if the compounds
displayed AIEE properties [7,52]. For instance, 3 exhibits a blue
emission at 496 nm in THF solvent. The emission intensity is very
weak and almost does not change when the water fraction (fw) is
below 50% (Fig. 3(a)). However, a significant enhancement in
emission is observed in the water/THF mixture with a water fraction
above 50%. The emission intensity of the mixture with a water
fraction of 90% was about 25 times higher than that in pure THF
solvent, indicating that 3 has a strong AIEE property. Complexes 1, 2,
and 4 also exhibit AIEE properties (Figs. S2eS4).
Previous studies have shown that AIE and/or AIEE might be
related to the restriction of intermolecular rotation, formation of
specific aggregates, and effects of intramolecular planarization.
However, one of the most probable reasons accounting for AIE and/or
AIEE is restriction of intramolecular motion, which is usually
considered as the main cause of AIE in small organic molecules
[21,53]. Herein, we also suppose that the present AIEE phenomenon
was caused by the restricted intermolecular rotation. Complex 5 was
synthesized with no intermolecular rotation to investigate the
possible reason for the present AIEE(Scheme 1). The emissionspectra
of 5 in water/THF mixtures with different water content (0e90%)
were also performed. Complex 5 is highly emissive when dissolved
in the solvent, but the emission intensity sharply decreases upon
3.3.2. Photophysical properties Zn(II) Schiff base complexes
The UVeVis and photoluminescent spectra of complexes 1e4 in
tetrahydrofuran (THF) solutions (10ꢁ5 M) and in the solid state at
room temperature were measured to investigate their optical
properties. As shown in Fig. 2, all of the absorption spectra exhibit
two major absorption bands. The absorption bands in the UV region
ranging from 250 nm to 350 nm correspond to C]N pep* transi-
tions. The low-energy absorption band approximately 400 nm
extending to the visible region is tentatively attributed to the
intramolecular charge transfer of ligands. After coordination with
Zn(II), the absorption maxima are significantly red-shifted in
comparison with their corresponding free ligands. The red-shift
may be due to more efficient conjugation in the Zn(II) complex
compared with that in the free ligands. The complexes display
relatively weak emission in the solution at room temperature, with
peaks at 518, 544, 496, and 503 nm for 1, 2, 3 and 4, respectively
(The excited wavelength is 329, 366, 305 and 302 nm, respectively.
Excitation spectra see Fig. S1). It is noted that the maximum
emission of complexes 2 and 4 in solution are same as their cor-
responding ligands L2 and L4, respectively. However, complexes 1
and 3 exhibits blue-shifted emission compared with ligands L1 and
L3, respectively. It is noted that the maximum emission of
complexes in the solid state are red-shift as compared to those in
solution (See Table 1.), which may be attributed to the different
molecular conformations in solid state and solution [48,49]. The
emission of 1 is obviously red-shifted compared with that of 3
because the diphenylamino group is a much stronger electron-
donor than the carbazole group, resulting in an increase in the
HOMO energy [50,51]. A similar result was also observed when the
emission spectrum of 2 was compared with that of 4. Replacing
phenol (1 and 3) with naphthalene (2 and 4) also resulted in
remarkable bathochromic shifts in the emission because a more
extended conjugation of the naphthalene destabilizes the LUMO
energy level, leading to a decrease in the energy gap. The experi-
mental results have been confirmed by quantum chemical
calculations.
In the emission spectra, the four dyes show weak fluorescence
when dissolved in organic solvents, the fluorescence quantum
yields (
F) of the four dyes, are only 3.0%, 16.0%, 3.0% and 2.0% in the
dilute CH2Cl2 solutions, respectively (Table 1). However, the fluo-
rescence quantum efficiencies of four powdered dyes are 16.7%,
20.9%, 21.4% and 4.5%, respectively, which clearly indicates the AIEE
characteristics of 1e4.
Fig. 4. TGA curves of complexes 1e4. Inset: DSC curves of 1e4.

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Y.-Z. Xie et al. / Dyes and Pigments 96 (2013) 467e474
diluted solutions, the aromatic rotors in 1e4 can rotate freely,
resulting in weak emission. The intramolecular rotational motions
should be suppressed effectively during aggregation. Thus, radiation
decay is enhanced by promoting a relatively small structural relax-
ation, leading to enhanced emission.
3.5. Thermal analysis
The thermal properties of the four complexes under a nitrogen
gas flow were also evaluated by TGA and differential scanning
calorimetry (DSC). The decomposition temperatures (Td) were 364,
389, 370, and 395 ꢀC with a weight loss of 5% for 1, 2, 3, and 4,
respectively, indicating that they also exhibit good thermal stability
(Fig. 4). Noticeably, replacement of phenyl group by a naphthalene
moiety results in complexes 2 and 4 with much higher Td values
than 1 and 3. The four complexes have better thermal properties
than 5 and other imines compounds [44,54,55]. All complexes,
except 1 with high glass-transition temperature (Tg) of 156 ꢀC, show
no phase transition during the heating scan period (Fig. 4, inset:
DSC curves). The excellent thermal stability and amorphism of the
complexes improve the operating lifetime as well as efficiency of an
electroluminescent device.
Fig. 5. Cyclic voltammetry curves of complexes 1e4 in 0.1 M Bu4NClO4/CH2Cl2 solution
at scan rate of 100 mV/s.
Table 2
HOMO and LUMO energy level of dyes 1e4.
aHOMO
(eV)
bEg
(eV)
cLUMO
(eV)
dHOMO
(eV)
dLUMO
(eV)
dEg
(eV)
ox
E
(V)
onset
3.6. Electrochemical properties
1
2
3
4
0.33
0.31
0.63
0.34
ꢁ5.13
ꢁ5.11
ꢁ5.43
ꢁ5.14
2.67
2.53
3.00
2.73
ꢁ2.46
ꢁ2.58
ꢁ2.43
ꢁ2.41
ꢁ5.02
ꢁ4.98
ꢁ5.41
ꢁ5.36
ꢁ1.80
ꢁ1.88
ꢁ1.93
ꢁ1.98
3.22
3.10
3.48
3.38
The anodic scanning cyclic voltammetry was employed to
investigate the electrochemical behavior, and to estimate the
HOMO energy levels of 1e4. Fig. 5 shows the cyclic voltammetry
curves of 1e4, which exhibit reversible processes in the oxidation
scan that correspond to the oxidations of the triphenylamine or
carbazole units in the molecule [50,56]. The corresponding LUMO
levels were obtained by the HOMO values and the energy gaps (Eg),
which were deduced from the edges of the absorption spectra. The
values obtained are listed in Table 2. Through the cyclic voltam-
mograms, the oxidation potentials of 1e4 were measured to be
0.33, 0.31, 0.63 and 0.34 V, and the energy band gaps of about 2.67,
2.53, 3.00 and 2.73 eV can be obtained from extrapolated UVeVis
absorption edge, respectively. The HOMO energy levels of all
a
b
c
Determined from the onset of the oxidation voltages.
Calculated from the edges of the absorption spectra.
obtained by the HOMO values and the energy gaps (Eg).
Calculated from the theoretical simulations.
d
increasing the water fractions due to the aggregation quenching
emission (Fig. 3(b)). Careful analysis of the structures of 1e5
reveals the presence of aromatic rotors (marked in red color,
Scheme 1) in 1e4, but not in 5. The excited intramolecular motions in
1e4 may be considered as one of the possible reasons for AIEE. In
Table 3
Selected frontier orbitals of complexes 1e4.

Y.-Z. Xie et al. / Dyes and Pigments 96 (2013) 467e474
473
complexes were calculated by EHOMO ¼ ꢁ(Eox þ 4.8) eV. As result,
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4. Conclusions
In summary, four Zn(II) Schiff base complexes (1e4) with
diphenylamino groups for 1 and 2, and carbazole groups for 3 and
4, respectively that can undergo rotation and the model complex 5
without rotational motion were prepared. Their photophysical and
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