Syntheses, crystal structures, nonlinear optical properties and cis-trans isomerization of functionalized sulfur-terminal [Zn(II) and Cd?(II)] complexes based on phenothiazine-2,2′:6′,2″-terpyridine conjugated ligands

Article abstract of DOI:10.1016/j.dyepig.2014.12.011

One novel 2,2′:6′,2″-terpyridine ligand L and its complexes (Zn(S)₂L, Cd(S)₂L) were synthesized and systematically characterized. There were cis-trans isomerization of the ethylene group in compounds of L, Zn(S)₂L and Cd(S

Full text of DOI:10.1016/j.dyepig.2014.12.011

Dyes and Pigments 115 (2015) 110e119
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Syntheses, crystal structures, nonlinear optical properties and
cis-trans isomerization of functionalized sulfur-terminal [Zn(II) and
Cd (II)] complexes based on phenothiazine-2,20:60,200-terpyridine
conjugated ligands
,
,
, *
Wan Sun a, Pingping Sun a b, Yingzhong Zhu a, Hui Wang a, Shengli Li a *, Jieying Wu a
,
, c, d
Yupeng Tian a
a Department of Chemistry, Anhui Province Key Laboratory of Functional Inorganic Materials, Anhui University, Hefei 230039, China
b College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
c State Key Laboratory of Crystal Materials, Shangdong University, Jinan 250100, China
d State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
One novel 2,20:60,200-terpyridine ligand L and its complexes (Zn(S)2L, Cd(S)2L) were synthesized and
systematically characterized. There were cis-trans isomerization of the ethylene group in compounds of
L, Zn(S)2L and Cd(S)2L though some characterization. Finally it was found that the two-photon absorption
Article history:
Received 15 October 2014
Received in revised form
8 December 2014
Accepted 10 December 2014
Available online 27 December 2014
cross section values (
enhanced compared to the free De
s
) for the complexes in the near infrared region (~800 nm) were dramatically
eA type ligand.
p
© 2014 Elsevier Ltd. All rights reserved.
Keywords:
Synthesis
Crystal structure
Terpyridine
Cis-trans isomerization
Z-scan
Two-photon absorption
1. Introduction
especially in coordination chemistry, bioinorganic chemistry,
catalysis, supramolecular chemistry and molecular devices
[12e14].
Phenothiazines (PTZs) are a class of electron-rich tricylic nitro-
genesulfur hetrocycles with a low oxidation potential and a high
propensity to form stable radical cations [15]. PTZs can exhibit
relatively intense luminescence, high photoconductivities and un-
dergo reversible oxidation processes. Besides, the nonplanar
Terpyridyl compounds have ring nitrogens serving as multiple
interaction sites, and form stable coordination and hydrogen-
bonded complexes with various metal ions and molecules. They
have been widely used as ligands for transition metal cations and
building blocks in supramolecular chemistry [1]. In addition, they
are generally thermally and chemically stable. Among these ter-
pyridyl compounds, 2, 20:60, 200-terpyridine (tpy) has been most
extensively studied as a chelating compound. The coordination
chemistry of terpyridine is attracting a current attention, due to
coordinating with different metal ions and their photoluminescent
properties [2e7], in particular those of platinum or ruthenium
[8,9], zinc or cadmium [10,11]. The complexes can be of significance
phenothiazine ring structure can restrict
p-stacking aggregation,
which prevents the detrimental pure singlet excitation recombi-
nation process [16]. Consequently, PTZs have been recently used as
spectroscopic probes in supermolecular assembly for PET (photo-
induced electron transfer) studies, and as electron donor compo-
nents in the materials science [17,18].
In this article as our continuous work [19], the 2,20:60,200-ter-
pyridine ligand (L) and their complexes (Zn(S)2L, Cd(S)2L) were
synthesized and characterized by elemental analysis, 1H NMR, IR,
MALDI-TOF spectroscopy, and single crystal X-ray diffraction. A
* Corresponding authors. Tel.: þ86 551 65108151; fax: þ86 551 65107342.
E-mail addresses: lsl1968@ahu.edu.cn (S. Li), jywu1957@163.com (J. Wu).
http://dx.doi.org/10.1016/j.dyepig.2014.12.011
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
111
facile synthesis route for a De
electron donor, tpy group as an electron acceptor and styrene group
p
eA system ligand with PTZ as the
were performed with an F-4500 fluorescence spectrophotometer.
The concentration of sample solution was 1.0 ꢁ 10ꢀ5 M. The fluo-
as
p
-bridge [20], and its two sulfur-terminal metal complexes are
rescence quantum yields (F) were determined relative to coumarin
presented. Sꢀ1 valence state results from charge self-regulation
behavior, which is novel and has been seldom reported before [19].
As we know, the terpyridyl ligand can easily coordinate to zinc
or cadmium ions. The metal ion binds to the acceptor to form a
307 using a standard method. Quantum yields were corrected as
follows:
ꢀ
ꢁ
Arh2s Ds
Ash2r Dr
F ¼ F
s
r
DepeA type complex, thus it should yield an increase rather than a
decrease of ICT (intramolecular charge transfer) upon excitation.
Consequently, metal ion binding is expected to realize an increased
2 PA cross section [21e24]. And then, the properties of all the
complexes were systematically investigated compared to the
original ligand. Finally, we successfully found Cd(II) complex to be
an efficient two photon absorption material with larger two-
where the s and r indices designate the sample and reference
samples, respectively, A is the absorbance at lexc is the average
,
h
refractive index of the appropriate solution, and D is the integrated
area under the corrected emission spectrum. All measurements
were carried out in air at room temperature.
photon absorption cross section values (
In addition, the nonplanar phenothiazine ring structure results
in cis- and trans-isomerization of the ethylene -bridge (see
s
¼ 1311.68 GM).
2.3. Syntheses
p
Scheme 1). The single-photon fluorescence spectra are given, and a
brief discussion on the structure-property relationship has been
described.
2.3.1. Synthesis of 10-hexyl-phenothiazin 1
10-H-phenothiazine (10.0 g, 50 mmol), 1-bromohexane (8.5 mL,
60 mmol) and sodium hydroxide powder (8.0 g, 0.2 mol) were well
mixed in 60 mL DMF solution and aliqiuat-336 was used as phase
transfer catalyst. The reaction mixture was stirred at 120 ꢂC for 24 h
and then neutralized with 2.0 M HCl, extracted three times with
100 mL of dichloromethane. The combined organic layer was
washed with aqueous brine and dried with anhydrous magnesium
sulfate. After that, the solvent was evaporated using a rotary
evaporator. The crude liquid product was then purified by flash
column chromatography, eluting with petroleum ether to give
colorless liquid (12.0 g, yield: 85%).
2. Experiments
2.1. Materials and instruments
All chemicals were purchased as reagent grade and used
without further purification. The solvents were dried and distilled
according to standard procedures. Elemental analysis was per-
formed with
a
PerkineElmer 240 analyzer. IR spectra
(4000e400 cmꢀ1), in KBr pressed pellets, were recorded on a
Nicolet FT-IR 170 SX spectrophotometer. Mass spectra were ob-
tained on a Micromass GCT-MS spectrometer. 1H NMR spectra were
performed on a Bruker 500 Hz Ultrashield spectrometer and re-
2.3.2. Synthesis of 10-hexyl-10H-phenothiazine-3-carbaldehyde 2
Put DMF (7.3 g, 0.1 mol) at 0 ꢂC, phosphorus oxychloride (30.6 g,
0.2 mol) was added and the solution was allowed to warm to room
temperature. Then, 1 (5.7 g, 0.02 mol) and distilled chloroform
(60 mL) were added. The reaction mixture was heated to 80 ꢂC and
kept for 16 h. After that, the resulting solution was poured into ice-
water, neutralized with 20% sodium hydroxide. The mixture was
stirred at room temperature for 1 h and then extracted with CH2Cl2,
dried with anhydrous magnesium sulfate. Organic phase was
filtered and concentrated. The residue was purified through column
ported as parts per million (ppm) from TMS (d). Coupling constants
J are given in Hertz. Mass spectra were acquired on a Micromass
GCT-MS (EI source).
2.2. Optical measurements
UVevis absorption spectra were measured on a UV-265 spec-
trophotometer. The fluorescence spectra in solid states and solution
Scheme 1. cis- and trans-isomerization of L.

112
W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
chromatography on silica gel using petroleum ether-ethyl acetate
(10:1) as eluent to give red oily liquid (5.0 g, yield: 80%).
2.4. X-ray crystallography
X-ray diffraction data of single crystals of compound L, Zn(S)2L
and Cd(S)2L were collected on a Siemens Smart 1000 CCD diffrac-
tometer. The determination of unit cell parameters and data col-
2.3.3. Synthesis of 10-hexyl-3-[4-(2,6-di-(pyridine-2-yl)pyridine-4-
yl)styryl]-10H- phenothiazine L
lections were performed with MoK
a
radiation (
l
¼ 0.71073 Å). Unit
4-(2,20:60,200-terpyridyl-40)-benzyl triphenyl phosphonium bro-
mide 3 was synthesized [25]. t-BuOK (1.3 g, 12.3 mmol) was placed
cell dimensions were solved by direct methods using SHELXS-97.
The other non-hydrogen atoms were located in successive differ-
ence Fourier syntheses. The final refinement was performed by
using full-matrix least-squares methods with anisotropic thermal
parameters for non-hydrogen atoms on F2. The hydrogen atoms
were added theoretically and riding on the concerned atoms.
into a dry mortar and milled to very small particles, then (1.0 g,
2
3.14 mmol) and 3 (2.0 g, 3.0 mmol) were added and mixed. The
mixture was milled vigorously for 20 min. After completion of the
reaction (monitored by TLC), the mixture was solved with 100 mL
dichloromethane, washed three times with water, and dried with
anhydrous magnesium sulfate. Then it was filtered and concen-
trated. The resulting solution was dispersed in 100 mL ethanol. The
residual solid was filtered and recrystallized from dichloro-
methane/ethanol (1:1) to give pale yellow crystals of L (1.3 g, yield:
70%). Anal. Calcd. for C41H36N4S: C, 79.87; H, 5.84; N, 9.09%. Found:
C, 79.62; H, 5.76; N, 8.95%. IR (KBr, cmꢀ1): 3051 (w), 2925(m), 2853
(w), 1583 (s), 1600 (m),1567 (m),1470 (m),1389 (m),1335 (w), 1249
(m), 1189 (m), 962 (s), 792 (m), 745 (m). 1H NMR (DMSO-d6,
400 MHz) ppm: 0.84 (m, 3H), 1.27 (m, 4H), 1.38 (m, 2H), 1.70 (m,
2H), 3.90 (m, 2H), 6.96 (m, 1H), 7.04 (d, 2H, J ¼ 9.54), 7.19 (m, 2H),
7.25 (d, 1H, J ¼ 15.56), 7.31 (d, 1H, J ¼ 16.06), 7.48 (d, 2H, J ¼ 4.52),
7.55 (m, 2H), 7.78 (d, 2H, J ¼ 8.78), 7.96 (d, 2H, J ¼ 7.53), 8.06 (m, 2H)
8.69 (m, 2H), 8.85 (m, 2H),8.90 (m, 2H). MS: m/z (%): 616 (Mþ), 545,
531.
2.5. Computational details
TD-DFT computational studies were performed to elucidate the
electronic structures of the ground state of the compound. Opti-
mizations were carried out with B3LYP [6-31G(d)] without any
symmetry restraints, and the TD-DFT {B3LYP[6-31G(d)]} calcula-
tions were performed on the optimized structure. All calculations,
including optimizations and TD-DFT, were performed with the G03
software [26]. In the calculation of the optical absorption spectrum,
the 25 lowest spin-allowed singletesinglet transitions, up to en-
ergy of about 5 eV, were taken into account.
3. Results and discussion
3.1. Preparation and spectroscopic characterization
2.3.4. Synthesis of ZnS/CdS nanocrystals
The synthetic route of L is shown in Scheme 2. Compound L was
synthesized by a solvent free Wittig reaction, which is a facile
Zn(CH3COO)2$2H2O (0.44 g, 2.0 mmol), CH3CSNH2 (0.15 mg,
2.0 mmol) and cetyltrimethylammonium bromide (CTAB, 0.01 g)
were dissolved in 80 mL of ethanol to form transparent solution.
Then the mixture was refluxed with vigorous stirring for 3 h and a
white turbid solution formed. Finally, the white powders of ZnS
nanocrystals were obtained by subsequent centrifugation and fol-
lowed by washing several times with water and ethanol.
In the same way, CdS nanocrystals were also prepared via
Cd(CH3COO)2$2H2O (0.50 g, 2.0 mmol) and CH3CSNH2 (0.15 g,
2.0 mmol) to afford pale yellow solid eventually.
method to prepare oligomers with extended
p-electron conjuga-
tions. ZnS nanocrystals were synthesized according to reported
methods [19]. Zn(CH3COO)2$2H2O was used to provide the zinc
source, CH3CSNH2 provide the sulfur source and cetyl trimethy-
lammonium bromide (CTAB) was added as surfactant. The M(S)2L
complex was synthesized by a solvothermal process and the MS
nanocrystals were used as optimal precursor, which can simulta-
neously provide both metal and sulfur sources. Here, NaSCN is used
as a mineralizer to improve the crystal quality.
Compound L was characterisized by EA, IR, 1H NMR and Mass
spectra. In the IR spectra, strong bands appear around 1249, 2925
and 2853 cmꢀ1, which can be assigned to vibrations of the 10-
hexyl-PTZ moiety. The bands of 1583, 1470 and 1389 cmꢀ1 for L, can
be assigned to vibrations of the terpyridyl moiety. Meanwhile, the
composition of complex M(S)2L (M ¼ Zn or Cd) was also confirmed.
In IR spectra, the bond at 513 cmꢀ1 for Zn complex and 517 cmꢀ1 for
Cd complex is attributed to the stretching vibrations of ZneS and
CdeS bond, respectively. From 1HNMR spectra of ligand L, it could
be found that the double bond protons display integration corre-
sponding to 2H, centered at 7.25 and 7.31 ppm with J ¼ 15.56 and
16.06 Hz, respectively, which indicate that trans isomer would not
isomerize to a cis isomer at room temperature even after a few days
under visible light.
2.3.5. Synthesis of M(S)2L complexes (M ¼ Zn/Cd)
Zn(S)2L complex: Ligand L (61.8 mg, 0.1 mmol), ZnS nanocrystals
(9.7 mg, 0.1 mmol) and NaSCN (32.4 mg, 0.4 mmol) was solved in
ethanol (14 mL) and DMSO (2 mL), the mixture was sealed in a
20 mL Teflon-lined reactor and heated at 130 ꢂC for 24 h. After
cooling to room temperature, red single crystals were obtained.
Yield: 65%. Anal. Calcd. for C41H36N4S3Zn: C, 66.04; H, 4.83; N,7.52%.
Found: C, 65.79; H, 4.74; N, 7.40%. IR (KBr, cmꢀ1): 3059 (w), 2922
(w), 2852 (w), 1596 (s), 1573 (m), 1469 (s), 1249 (m), 1188 (m), 1014
(m), 975 (s), 794 (m), 745 (m), 637 (w), 515 (w), 415 (w). 1H NMR
(DMSO-d6, 400 MHz) ppm: 9.14 (s, 2H), 9.01 (d, 2H, J ¼ 7.3), 8.86 (d,
2H), 8.37 (m, 4H), 7.89 (m, 4H), 7.50 (s, 2H), 7.39 (d, 1H, J ¼ 16.00),
7.32 (d, 1H, J ¼ 16.00), 7.23 (m, 2H), 7.05 (m, 2H), 6.96 (m, 1H), 3.91
(m, 2H), 1.41 (m, 2H), 1.22 (m, 6H), 0.85 (d, 3H).
The Cd(S)2L complex was also prepared similarly to Zn(S)2L via
CdS nanocrystals, L and NaSCN to afford orange-red single crystals.
Yield: 59%. Anal. Calcd. for C41H36N4S3Cd: C, 61.96; H, 4.53; N, 7.05%.
Found: C, 62.07; H, 4.45; N, 6.93%. IR (KBr, cmꢀ1): 3059 (w), 2922
(w), 2851 (w), 1596 (s), 1574 (m), 1467 (s), 1402 (m), 1247 (m), 1189
(m),1014 (m), 979 (s), 793 (m), 743 (m), 517 (w). 1H NMR (DMSO-d6,
400 MHz) ppm: 9.08 (s, 3H), 8.78 (d, J ¼ 4.2, 3H), 8.34 (s, 3H), 7.83
(d, 4H), 7.49 (s, 2H),7.33 (d, 1H, J ¼ 18),7.31 (d, 2H, J ¼ 14), 7.20 (m,
3H), 7.05 (m, 2H), 6.96 (m, 1H), 3.90 (m, 2H), 1.25 (m, 6H), 0.85 (m,
2H).
3.2. Crystal structure
The structure of L is shown in Fig. 1. Pale yellow single crystals
suitable for X-ray diffraction analysis were obtained by slow
evaporation of L solution (CH2Cl2 covered with ethanol) at room
temperature after several days. Data collections and refinements
were listed in Table S1, selected bond distances and angles were
given in Table S2. The center pyridinyl plane (labeled P2) is ar-
ranged with two pyridine planes (P1 and P3) being 8.65ꢂ (P2 and
P1) and 9.32ꢂ (P2 and P3). The result indicates that the terpyridyl

W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
113
Scheme 2. Synthetic route for L and M(S)2L.
group is barely coplanar. The dihedral angle between P2 and phenyl
ring (labeled P4) is 41.57ꢂ. At the PTZ donor end, the dihedral angle
of 33.37ꢂ between one phenyl ring (labeled P5) and P7, the dihedral
angle of 31.34ꢂ between other phenyl ring (labeled P6) and P7,
which indicate that there are some distortion between terpyridyl,
PTZ and styrene. The C]C linkage between P4 and P5 exists in
trans-isomer and was quite conjugated with bond lengths of
C(24)eC(22): 1.484(1) Å, C(22)]C(23): 1.305(1) Å and C(23)e
C(21): 1.550(1) Å. These structural features suggest that all non-
hydrogen atoms between donor and acceptor are conjugated,
complex Cd(S)2L, the dihedral angles between P2 and P1, P2 and P3,
P2 and P4, P4 and P5, P5 and P6 are 11.62ꢂ, 3.35ꢂ, 18.92ꢂ, 32.94ꢂ and
25.00ꢂ, respectively. These dihedral angles of two complexes are
lower than that of the ligand L molecule. In two complexes, it is
similar that the C]C linkage between P4 and P5 exists in trans-
isomer and was quite conjugated with bonds of C(15)eC(13):
1.493(1) Å, C(13)]C(14): 1.229(1) Å and C(14)eC(17): 1.488(1) Å for
Zn complex, and C(34)eC(36): 1.466(1) Å, C(34)]C(28): 1.338(1) Å
and C(28)eC(2): 1.459(1) Å for Cd complex. The results indicate
that the molecules of complexes have better planarity than the
ligand L in spite of a degree of distortion to some extent. In addition,
the planarity of Zn complex is better than that of Cd complex. These
structure features provide a theoretical foundation for their optical
properties.
leading to a p-bridge for the charge transfer from PTZ to terpyridyl.
However, the whole molecule has a certain degree of distortion
because of the intermolecular interaction in solid states.
The structures of two complexes are shown in Fig. 1. Data col-
lections and refinements were also listed in Table S1, selected bond
distances and angles were given in Table S2. The structures of
M(S)2L (M ¼ Zn, Cd) are similar. In each molecule, the center metal
is in a distorted five-coordinated trigonal-bipyramidal geometry
with the two apical sites occupied by two sulfur atoms, respec-
tively. The SeM bond lengths are almost equal (2.290 and 2.310 Ǻ
for Zn; 2.446 and 2.437 Ǻ for Cd), and the sulfur atoms are most
likely to be identical Sꢀ1 valence states as a result of charge self-
regulation behavior. In the two complexes, it is observed that the
N atom torsion angle of terpyridyl group (electron-acceptor) was
changed owing to the highly versatile and adaptive coordination
modes of the terpyridyl moiety. In complex Zn(S)2L, the dihedral
angles between P2 and P1, P2 and P3, P2 and P4, P4 and P5, P5 and
P6 are 2.49ꢂ, 3.09ꢂ, 9.56ꢂ, 16.89ꢂ and 16.13ꢂ, respectively. In
3.3. The linear absorption spectra and TD-DFT studies
The photophysicle properties of Ligand L and its complexes in
different solvents are collected in Table 1. The linear absorption
spectra of the ligand L and its complexes were measured in THF, as
shown in Fig. 2. The calculated frontier orbitals of L, Zn(S)2L and
Cd(S)2L are shown in Fig. 3. In the spectra of ligand L, there are two
absorption shoulders. One shoulder at ca. 383 nm can be assigned
to ICT from PTZ to terpyridyl group, the shoulder originating from
HOMO / LUMO þ 1 (calculated at 375.3 nm) (Table S3).
There are visible red-shifts from the peak positions of two
complexes to those of L ligand in the same solvent. As shown in the
molecular structures of two complexes, the metal (Zn/Cd) ions

114
W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
Fig. 1. The molecular structures of L, Zn(S)2L and Cd(S)2L.
Table 1
coordinate with terpyridyl group of ligand L, which makes the
molecule better planarity relative to the ligand, contributing to
increase the intramolecular charge transfer degree and enhance
molecular polarity, especially in the excited state [27]. The two
peaks located at 405 nm for Zn(S)2L and 399 nm for Zn(S)2L can be
assigned to MLCT, and the calculated frontier orbitals can also
proved the band for Zn(S)2L originating from HOMO / LUMO þ 3
(calculated at 403.8 nm) (Table S3), the band for Cd(S)2L originating
from HOMO-1 / LUMO þ 3 (calculated at 402.3 nm) (Table S3).
Therefore, it can be concluded that the results of experiments are
well consistent with the theoretical calculations.
Single-photon-related photophysical properties of L, Zn(S)2L and Cd(S)2L.
Compound
L
Solvent
Lmax
ε
Lmax
Fc
a
b
Benzene
CH2Cl2
THF
CH3CN
DMF
Benzene
CH2Cl2
THF
CH3CN
DMF
Benzene
CH2Cl2
THF
CH3CN
DMF
275, 324, 379
280, 325, 375
282, 325, 379
279, 326, 380
281, 324, 380
327, 416
329, 424
323, 404
322, 410
320, 396
2.83, 1.18, 0.43
2.92, 1.31, 0.54
3.01, 1.34, 0.481
3.05, 1.38, 0.56
2.93, 1.16, 0.54
3.19, 1.43
3.56, 1.93
3.28, 1.94
3.39, 2.07
2.96, 2.00
527
556
545
563
564
513
543
0.75
0.54
0.63
0.28
0.31
0.09
0.01
0.31
0.06
0.04
0.09
0.06
0.32
0.01
0.15
Zn(S)2L
Cd(S)2L
459, 538
473, 556
474, 560
432, 517
539
324, 406
325, 417
322, 402
321, 408
1.16, 0.48
3.96, 2.14
3.67, 2.15
3.91, 2.35
The absorption spectra of L, Zn(S)2L and Cd(S)2L in four solvents
after 3 h and 5 days are presented in Figs. S1 and S2. 5days later, the
462, 536
474
band of
p / p* red shifted along with the increase in trans
structure, because trans structure being planar conjugate, cis
structure being the not planar conjugated system [28].
315, 394
3.23, 2.32
487
a
Peak position of the absorption band.
Peak position of Single-photon fluorescence, excited at the absorption maximum.
Quantun yields determined by using fluorescein as standard.
b
c

W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
115
Fig. 2. Linear absorption spectra of L, Zn(S)2L and Cd(S)2L in THF
solution(c ¼ 1.0 ꢁ 10ꢀ5 mol$Lꢀ1).
Fig. 4. Single-photon fluorescence spectra of L in THF solution at different times
(c ¼ 1.0 ꢁ 10ꢀ5 mol$Lꢀ1).
and 5 days are presented in Fig. 5. As shown, there are two emission
peaks in weaker-polar solvent and one single-emission peak in
stronger-polar solvent like DMF and acetonitrile at short wave after
3 h. However, there is only one emission peak at short wave in
almost all solvents after 5 days. As shown in Fig. 6, the solid fluo-
rescence spectra of L is also one emission peak and is red-shifted
40 nm compared with DMF solution. In the crystal structure of L,
the styrene double bond of solid L exists with Trans structure. We
can speculate that the short wave emission would be trans
configuration and the long wave is induced by Cis structure. This
suggests that styrene double bond can flip cis configuration in
weaker-polar solvent, due to the smaller molecular forces between
the solvent and solute. The weaker of polarity, the higher propor-
tion of cis-lycopene. So in stronger-polar solvent and solid, L only
exists as trans-lycopene. In addition, the emission peak position of
3.4. The fluorescence property
The fluorescence spectra of L and its complexes were investi-
gated and found to change along with time in weaker-polar solvent.
For instance, there are two emission peaks in new THF solution of L
(see Fig. 4): one at 454 nm with low-intensity and the other at
541 nm with high-intensity. The peak-intensity at 454 nm
increased and that at 541 nm reduced along with the time. Finally,
the fluorescence peak at 541 nm disappeared and the single-
emission peak of L is at 454 nm. From 1HNMR of L, Zn(S)2L and
Cd(S)2L, the ligand and its two complexes all exist trans isomeri-
zation stably. The two emission peaks of L in weaker-polar solvent
reflect body's tautomerization from cis isomer to trans isomer.
The luminescence behaviors of L are different in different sol-
vent polarity. The fluorescence spectra of L in five solvents after 3 h
Fig. 3. Molecular orbital energy diagram for L, Zn(S)2L, Cd(S)2L.

116
W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
Fig. 5. Single-photon fluorescence spectra of L in five solvents after 3 h (left) and five days (right) (c ¼ 1.0 ꢁ 10ꢀ5 mol$Lꢀ1).
cis-trans isomerization increasing with the solvent polarity was
positively solvatochromism. This explains that the two emission
peaks would be assigned to ICT [29].
excitation also trans structure is increased, which provides an
explanation for this interesting phenomenon.
In Fig. 7, the fluorescence spectra of Zn complex is similar to L in
THF, acetonitrile and DMF due to intramolecular charge transfer of
the ligand. The metal coordinates with acceptor group of
3.6. Two-photon absorption (TPA) properties
The nonlinear properties of the compounds were studied by
using an open aperture Z-scan technique under a femtosecond
laser pulse and Ti: 95 sapphire system (680e1080 nm, 80 MHz,
140 fs). We chose the testing wavelengths from 700 nm to 900 nm,
but only in some wavelengths were the TPA found which was
shown in Fig. S4. The following experiment datas were measured
under the optimum wavelengths. The experimental spectra,
measured at 740 nm for L, 780 nm for Zn(S)2L and 790 nm for
Cd(S)2L in DMF at a concentration of 1.00 mM, are shown in Fig. 8.
The theoretical data were fitted using the following equations:
donorepeacceptor (DepeA) models which increased the charge
transfer degree and the emission peaks showed slight red-shifting.
Just start testing in the benzene and methylene chloride, fluores-
cence emission is given priority to short wave. This indicates that
the introduction of the metal ions, planarity enhancement, more
stable and trans configuration are afforded by complex formation.
So, the transformation time of complex from cis to trans configu-
ration is less than that of the ligand.
∞
m
X
3.5. Infrared optical properties
½ꢀq0ðzÞꢃ
Tðz; s ¼ 1Þ ¼
for jq0j < 1
(1)
m¼0 ðm þ 1Þ3=2
According to the results of infrared test, L, Zn(S)2L and Cd(S)2L
appear peaks at about 962 (s), 792 (m), 975 (s), 794 (m), 979 (s) and
793 (m) cmꢀ1, respectively. It is characterized as the trans and cis
where q0(z) ¼ bI0Leff/(1 þ x2), x ¼ z/z0, z0
¼
pu20
/l,
b
is the two-
photon absorption (TPA) coefficient of the solution, I0 is the in-
tensity of laser beam at focus (z ¼ 0), Leff ¼ [1ꢀexp(ꢀa0L)]/a0 is the
effective length with a0 the linear absorption coefficient, L is the
asymmetric deformation vibration
d
¼
CeH (out-plane) of
R1CH ¼ CHR2, indicating that the samples contains both cis- and
trans-structures, which has been examined by XRD. As shown in
Fig. S3, there are some overlap between the sample and single
crystal. It also proved the fluorescence test of the sample in solvent,
excitation wave also cis structure is reduced and short wave
sample length, z0 ¼ pu02
/l
is the diffraction length of the beam with
l the wavelength of the beam and z is the
u0 the spot size at focus,
sample position. Based on (1)Eq., the molecular TPA coefficient
can be deduced. Furthermore, the TPA cross-section for a given
sample is determinated by using the following relationship:
b
hbv ¼ sNAd0 ꢁ 10ꢀ3
(2)
here h is the Planck constant,
n
is the frequency of incident laser, NA
is the Avogadro number, and d0 is the sample concentration. Ac-
cording to (2)Eq., the TPA cross-section can be got [30].
s
From the linear absorption spectra (see Fig. 2), it is discovered
that there is no linear absorption in the wavelength range
680e900 nm for all the chromophores. Therefore, any absorption at
this wavelength range should be derived from multiphoton ab-
sorption process. In Fig. 8, the open-aperture transmittance was
symmetric with respect to the focus (z ¼ 0). The obvious minimum
transmittance unambiguously indicated a biggish nonlinear ab-
sorption which was attributed to TPA effect.
It has been found that the two-photon absorption cross sections
of coordination compounds (Zn(S)2L and Cd(S)2L) are remarkably
enhanced compared with that of L. The behavior of TPA spectra can
Fig. 6. Solid fluorescence spectra of L, Zn(S)2L and Cd(S)2L.

W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
117
Fig. 7. Single-photon fluorescence spectra of Zn(S)2L (upleft) and Cd(S)2L(upper right) in five different solvents after 3 h, Zn(S)2L and Cd(S)2L in THF after five days (down)
(c ¼ 1.0 ꢁ 10ꢀ5 mol$Lꢀ1).
Fig. 8. Open aperture Z-scan curves of L, Zn(S)2L and Cd(S)2L in DMF (c ¼ 1.0 ꢁ 10ꢀ3 mol$Lꢀ1).

118
W. Sun et al. / Dyes and Pigments 115 (2015) 110e119
properties, supramolecular constructs, and biomedical activities. Chem Rev
Table 2
2008;108(6):1834e95.
Two-photo absorption coefficient
b
and cross section
s
of L, Zn(S)2L and Cd(S)2L.
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Compound
Best nonlinear absorption
wavelength/nm
b
/(cm$Wꢀ1 /GMa
)
s
€
[4] Bhowmik Sandip, Ghosh Biswa Nath, Marjomaki Varpu, Rissanen Kari. Nano-
L
740
780
790
0.012
0.023
0.031
535.19
973.20
1311.68
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Cd(S)2L
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1 GM ¼ 1 ꢁ 10ꢀ50 cm4 s per photon.
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Cd(II) compared with Zn(II) ion which possibly gives a more
extended delocalization. The experimental results are also in
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s
s
response comes from the marked core to periphery charge redis-
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complexation with Zn(II) and Cd(II) enhances the electron-acceptor
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In this article, the compounds L, Zn(S)2L and Cd(S)2L have
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proved by single-photon fluorescence spectra, IR spectra and XRD.
The transformation time of complex from cis into trans type is
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to dramatically enhanced two-photon absorption, especially for the
complex Cd(S)2L. In addition, the functionalized complexes can be
used for excellent fluorogenic and potential applications due to a
simple preparation. It provides a new idea and effective pathway to
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Acknowledgments
[19] Gao Yuanhao, Wu Jieying, Li Yiming, Sun Pingping, Zhou Hongping,
Yang Jiaxiang, et al. A sulfur-terminal Zn (II) complex and its two-photon
This work was supported by a grant for the Education Revitali-
zation Plan Talent Project (2013) and the National Natural Science
Foundation of China (21271004, 51372003, 21271003, 51432001),
and the Anhui Provincial Natural Science Foundation
(1308085MB24), and Scientific Innovation Team Foundation of
Educational Commission of Anhui Province of China (KJ2012A025,
2006KJ007TD).
microscopy biological imaging application.
J Am Chem Soc 2009;131:
5208e13.
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.12.011.
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