Effect of solvent, pH and metal ions on the self-assembly process and optical properties of an A-π-D-π-A type triphenylamine carboxylic acid derivative

Article abstract of DOI:10.1039/c5tc03351a

A 'Λ' structural A-π-D-π-A type triphenylamine derivative (2E,2′E)-3,3′-(4,4′-(phenylazanediyl)bis(4,1-phenylene))diacrylic acid (abbreviated as L) with intramolecular charge transfer (ICT) character has been designed, synthesized and characterized. The m

Full text of DOI:10.1039/c5tc03351a

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Materials Chemistry C
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PAPER
Effect of solvent, pH and metal ions on the
self-assembly process and optical properties of an
A–p–D–p–A type triphenylamine carboxylic acid
derivative†
Cite this:DOI: 10.1039/c5tc03351a
ab
a
a
c
a
a
Lin Kong,* Yun Liu, Hui Wang, Xiao-he Tian, Qi-yu Chen, Yu-peng Tian,
a
a
a
Sheng-li Li, Zhao-ming Xue and Jia-xiang Yang*
0
0
0
A ‘L’ structural A–p–D–p–A type triphenylamine derivative (2E,2 E)-3,3 -(4,4 -(phenylazanediyl)bis-
4,1-phenylene))diacrylic acid (abbreviated as L) with intramolecular charge transfer (ICT) character has
(
been designed, synthesized and characterized. The morphology and linear optical properties of L
nanostructures can be tuned by changing the solvent and/or the acidity of the solution. The possible
growth mechanism is discussed through the single crystal structure and the corresponding molecular
stacking style of L molecules. Further, the strong coordination effect between the carboxylic acid group
of L and the metal ions is discussed, which also brings about changeable morphology, linear optical
properties and enhanced nonlinear optical properties, two photon absorbance (TPA) included, which
makes them suitable for applications in two-photon biological imaging, such as in HepG2 cells.
Received 15th October 2015,
Accepted 30th November 2015
DOI: 10.1039/c5tc03351a
www.rsc.org/MaterialsC
and simple approach to changes in the external environment.
To date, the self-assembly process for obtaining organic nano-
1
Introduction
In the past few decades, self-assembly of functional organic materials is still highly desirable for the advancement of
7
molecules has attracted increasing attention owing to their organic nanoscience and nanotechnology. Thus, the choice
1
unique electronic and/or photonic properties, which can be of materials based on the predication of structure–property
used as potential candidates of active components in organic relationships shows important significance in this field.
light emitting diodes (OLEDs), organic field-effect transistors
Amongst the functional organic molecules, intramolecular
OFETs), and solar cells, etc. The electric and/or photophysical charge-transfer (ICT) compounds is an important class to be
properties of these organic nanomaterials can be changed due used to regulate structure easily in the nanometer scale under
2
(
3
4
5
to the morphology including size, shape and crystallinity,
different environments through a self-assembly process. An ICT
which are considered to be associated with changes in the molecule possesses an electron-donating (D) group and an
molecular packing modes, external environments and growth electron-accepting (A) group, which are connected through a
time. The weak noncovalent intermolecular interactions such p-conjugated bridge. The charge can be partly transferred from D
as hydrogen bonding, van der Waals forces, p–p stacking and group to A group, which renders the molecule with somewhat
6
electrostatic interactions, etc. are very important for tuning the strong D–A dipole–dipole interactions. Thus, self-assembly of an
self-assembly process, which can be realized through a rational ICT compound on the nano- or micro-meter size can be con-
trolled effectively along the direction of the dipole–dipole inter-
a
8
Department of Chemistry, Key Laboratory of Inorganic Materials Chemistry of
Anhui Province, Anhui University, 111th Jiu-Long Road, Hefei, P. R. China.
E-mail: kong_lin2009@126.com, jxyang@ahu.edu.cn
actions. In addition, other weak intermolecular interactions as
described previously also play important roles in constructing
the organic nano/microstructures. So, the preparation para-
meters such as solvent, concentration, pH value of the solution,
temperature, growth time, the existence of metal ions, and so on,
can influence the self-assembly process of an ICT compound,
which can further bring about changeable and/or optimized
morphology and the corresponding optical properties.
In selecting the ICT molecule with a wealth of weak inter-
molecular interactions, the carboxylic acid derivatives are
good candidates. As it is well-known that the carboxylic acid
b
State Key Laboratory of Rare Earth Resources Utilization,
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
5
625 Renmin Street, Changchun 130022, Jilin, P. R. China
c
9
College of Life Science, Anhui University, 111th Jiu-Long Road, Hefei, P. R. China
0
†
Electronic supplementary information (ESI) available: Preparation of 4,4 -(phenyla-
zanediyl)dibenzaldehyde; selected bond lengths [Å] and angles [1] for L; the spatial
distributions of calculated results; TEM images of CPPs and L nanostructures obtained
under different pH values. Cyclic voltammograms of L; the FT-IR spectra of L and
CPPs; the powder X-ray diffraction of L and CPPs. CCDC 1440441. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5tc03351a
This journal is ©The Royal Society of Chemistry 2015
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derivatives display interesting multitudinous supramolecular
1
0
interactions, among which hydrogen bonding interaction is
the main force. Thus the self-assembly process of an ICT type
carboxylic acid derivative can be influenced by environmental
Scheme 1 Synthetic route of the target compound L.
1
1
factors such as solvents with different polarity, pH value, etc.
Moreover, the carboxylic derivative can act directly with metal
1
2
ions in constructing coordination polymer particles (CPPs)
(5 mL) and 3–5 drops of piperidine was added as a catalyzer.
based on the coordination-directed assembly of metal ions and The above mixture was refluxed for 4 h and monitored by TLC
organic carboxylic acid bridging ligands, due to the deprotona- to ensure complete reaction. Then, water was added to the
1
3
tion effect of the carboxylic acid unit. The alkaline earth metal mixture and the pH value of the solution was adjusted to 5–6,
ions, transition metal ions and rare earth metal ions are often then the precipitate was purified through recrystallization in
1
used to synthesize this type of CPP, not only because of their ethanol to yield a yellow solid. Yield: 85%. H-NMR (400 MHz,
6
fascinating architectures but also their optical and electronic DMSO-d ): d (ppm): 12.26 (s, 2H); 7.61 (d, 4H, J = 8.8 Hz); 7.53
1
4
properties. The properties of these CPPs depend not only on (d, 2H, J = 15.8 Hz); 7.39 (t, 2H); 7.19 (t, 1H, J = 7.2 Hz); 7.12 (d,
the size, shape and composition but also to a large extent on 2H, J = 7.6 Hz); 6.99 (d, 4H, J = 8.8 Hz); 6.39 (d, 2H, J = 15.8 Hz).
1
3
the molecular stacking style within a material and the crystal-
linity of them.
C-NMR (100 MHz): d (ppm) 167.73, 148.22, 145.84, 143.29,
These factors can be easily regulated through 129.93, 129.64, 128.61, 125.79, 124.89, 122.82, 117.23. FT-IR
5
,15
ꢀ1
changing the external environment. The changeable properties (KBr, cm ): n = 3466 (m, nCOO), 2978, 1679 (s, nCQO), 1621
make them attractive for many scientific and industrial areas (s, nCQC), 1591 (s), 1505 (s), 1423 (m), 980 (m), 834 (m), 698 (s),
such as electronics, photonics, sensors, medicine, bio- 763 (m), 701 (m).
1
6
17
18
19
1
3,20
21
technology
and environmental technology.
Single crystals of L suitable for X-ray diffraction were obtained
Considering all the above aspects, in this study, a carboxylic by slow evaporation of dichloromethane–ethanol mixed solution
0
0
0
derivative, (2E,2 E)-3,3 -(4,4 -(phenylazanediyl)bis(4,1-phenylene))- (VCH Cl : Vethanol = 2 : 1) at room temperature for a week.
2
2
22
diacrylic acid (abbreviated as L), has been designed and synthe-
sized, among which the triphenylamine unit is employed as an
electron-donor group and the carboxylic acid unit is used as an
2.2 Preparation of L nanostructures
electron acceptor group, the two parts are connected through the L nanostructures were prepared through a simple reprecipitation
double bond to construct an A–p–D–p–A type molecule which process. For studying the effect of solvent on the aggregated style of
ꢀ
3
ꢀ1
possesses an ICT effect as the main feature. L is a typical ICT L, 200 mL of L solution in different solvents (1.0 ꢁ 10 mol L
)
compound which has attracted much interest. In 2007 and 2008, was injected into 2 mL of high-purity water under stirring. The
23
Murphy reported the synthesis of this type of compound. In samples were stirred for 10 min and then left undisturbed for 12 h
012, Singh and co-workers used it as the intermediate product at room temperature for stabilization.
2
24
for further synthesis. The main purpose of this study is to research
For studying the effect of acidity on the aggregation, 200 mL
ꢀ3
ꢀ1
the influence of the external environment on the morphology and of L–THF solution (1.0 ꢁ 10 mol L ) was injected into 2 mL
the corresponding optical properties of L. Thus, the influence of of water with different acidity, stirred for about 10 min and
solvent with different polarity, pH value of the solution and the then left undisturbed at room temperature. To avoid the
existence of metal ions on the self-assembly process of L is studied in influence of other features on the morphology of L, the acidity
detail. The possible mechanism of the changeable morphology is of the solution was adjusted just through adding different
discussed through the single crystal structure and the molecular amounts of HCl or NaOH.
stacking style. The different morphologies obtained under these
features bring about changeable linear and nonlinear optical
2
.3 Preparation of coordination polymer particles
properties. Further, the coordination effect of L and three metal
2
+
2+
3+
2+
2+
3+
ions (Ba , Cd and Yb ions are chosen in this study) in strong In this study, Ba , Cd and Yb ions were chosen to prepare
alkaline medium is used to enhance the two-photon absorption Ba–L CPPs, Cd–L CPPs and Yb–L CPPs. For the synthesis of
ꢀ
3
ꢀ1
(
TPA) cross section (d) of the material, which is useful for two- Ba–L, 50 mL of NaOH–ethanol solution (2.0 ꢁ 10 mol L ) was
ꢀ5 ꢀ1
photon microscopy imaging in living HepG2 cells.
added to 5 mL of L–ethanol solution (1.0 ꢁ 10 mol L ). The
mixture was stirred for about 30 min at room temperature.
2
+
ꢀ3
ꢀ1
Then, 50 mL of Ba aqueous solution (1.0 ꢁ 10 mol L ) was
2
Experimental section
2
5
added and a large amount of precipitate occurred immediately.
After stabilizing for 10 min, the precipitate was collected by
2.1 Synthesis of L
All chemicals used were of analytical grade. All solvents were centrifugation, washed several times with ethanol and water
purified by conventional methods before use. The synthesis alternately, and then dried under vacuum.
route of L is listed as Scheme 1.
To prepare Cd–L and Yb–L, a similar procedure was followed
2
+
2+
3+
0
For preparing L, 4,4 -(phenylazanediyl)dibenzaldehyde (the except that the metal salt of Ba was changed to Cd , Yb ,
preparation procedure is listed in ESI,† 0.5 g, 1.66 mmol) and respectively. The products were characterized through FT-IR
malonic acid (0.357 g, 3.43 mmol) were dissolved in pyridine and powder XRD.
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.4 Cell culture, incubation and 2PFM imaging
Paper
2
2.7 Nonlinear optical measurements
In this study, human liver cancer cell (HepG2) was chosen as the The nonlinear optical properties of L and three CPPs were
test candidate, which was cultured and stained with the organic measured through Z-scan and two photon excited fluorescence
ꢀ4
ꢀ1
nanomaterial and the corresponding Yb–L CPPs. HepG2 cells measurements with a concentration of 1.0 ꢁ 10 mol L
5
were seeded in 6 well plates at a density of 2 ꢁ 10 cells per well (based on mol. wt. of L) using femtosecond laser pulse and
and grown for 96 hours. For live cell imaging, cell cultures were Ti:95 sapphire system (80 MHz, 140 fs). For Z-scan measure-
incubated with the materials in PBS solution (10% PBS: 90% cell ments, the quartz-glass cell was 1.0 mm thick, and the average
media, PBS = phosphate buffered saline) at a concentration of laser power was 120 mW. The optical path length for Z-scan was
1
9
0 mM and maintained at 37 1C in an atmosphere of 5% CO and 5.0 cm from the sample. The excited wavelength was between
5% air for incubation times ranging up to 60 min. The cells 680 nm and 980 nm. For two photon excited fluorescence
2
were then washed with PBS (3 ꢁ 3 mL per well) and imaged experiments, the quartz-glass cell was 1.0 cm thick and the
using confocal laser scanning microscopy with water immersion average laser power was 500 mW.
lenses.
The nonlinear absorption coefficient b was measured by the
HepG2 cells were fluorescence imaged on a Zeiss LSM 710 open aperture Z-scan technique. For the open aperture, the
META upright confocal laser scanning microscope using 40ꢁ normalized transmittance as a function of the position along
magnification water-dipping lenses for the monolayer cultures. the z axis can be calculated as eqn (1)
Image data acquisition and processing were performed using
Zeiss LSM Image Browser, Zeiss LSM Image Expert and ImageJ.
1
X
m
½
ꢀqðzÞꢂ
bI0Leff
Tz ¼
3=2; where qðzÞ ¼ h
i ;
(1)
ðm þ 1Þ
2
m¼0
1 þ ðz=z0Þ
a
2.5 TD-DFT calculation
where z is the diffraction length of the beam, I is the intensity
0 0
of the light at focus, Leff is the effective length of the sample, and
a is the linear absorption coefficient at the wavelength used.
Further, the molecular 2PA cross-section (d) could also be
determined through Z-scan measurement by using the following
relationship, eqn (2),
Molecular orbital calculations of time-dependent density functional
theory (TD-DFT) at the B3LYP/6-31g level basis set (Gaussian 03)
26
were performed to study the electronic structures and the
corresponding energy gap of L molecules in different acidity.
The molecular geometry used for the calculation was obtained
from X-ray diffraction crystallographic data, protonated and/or
deprotonated molecular structures.
d = hnb ꢁ 10ꢀ3/N d,
(2)
A
where h is the Planck’s constant, n is the frequency of input
intensity, NA is the Avogadro constant, and d is the concen-
2.6 Instrument
28
tration of the sample.
The pH value of the solution was adjusted by adding hydro-
chloric acid or sodium hydroxide in a tetrahydrofuran–water
mixture and determined with a pH meter (PHS-3CT). The X-ray
diffraction measurement of single crystals was performed on
3
Results and discussion
3.1 Effect of solvent on the self-assembly process of L
a Bruker SMART II CCD area detector using graphite- L nanostructures were simply prepared in several solvents
monochromated Mo Ka radiation (l = 0.71073 Å) at 298(2) K. (tetrahydrofuran THF, N,N-dimethyl formamide DMF, ethanol
Intensity data were collected in the variable multi-scan mode. EtOH, acetone, acetonitrile were used in this study) with no
2
9
The structures were solved by direct methods and difference addition of any surfactant, template or catalyst, the SEM
Fourier syntheses. All non-hydrogen atoms were refined aniso- images of which are shown in Fig. 1. The results revealed a
tropically and hydrogen atoms were introduced geometrically. remarkable solvent-induced morphological change of L. When
2
Calculations were performed with the SHELXTL-97 program L molecules were introduced into the THF–H O mixed solvent,
2
7
package. The powder X-ray diffraction (PXRD) patterns were rhombus-like morphology was observed as shown in Fig. 1a,
recorded on a MXP18AHF diffractometer using Cu Ka radiation the TEM image of which was shown in Fig. S1 (ESI†). The
(l = 1.54056 Å) in the 2y range from 81 to 401. The morphologies average sizes of the two diagonals were 5 mm and 1 mm,
were obtained on a field-emission scanning electron micro- respectively. Moreover, it was noteworthy that each rhombus-
scope (FESEM, Hitachi S-4800) and electron microscope (TEM, like nanostructure contained two rhombic-layers sticking
JEM-2100). UV-vis absorption spectra were obtained on a together closely with the thickness being about 500 nm. When
2
UV-3100 spectrophotometer in the wavelength range 200–800 nm. DMF–H O was used as the solvent, flexible nanowires about
Fluorescence spectra were measured at room temperature several micrometers long and B100 nm wide were observed as
using a Hitachi F-7000 spectrophotometer. The electrochemical shown in Fig. 1b. However, the main products prepared from
tests were recorded on a CHI660D workstation. Voltammo- EtOH–H O (Fig. 1c) and acetone–H O (Fig. 1d) were rectangular
2
2
grams were recorded using a Pt disc working electrode in structures with B100 nm in diameter. Furthermore, spherical
DMF containing 0.1 M Bu NClO (TBAP). Scans started at nanoparticles with a size of about 100 nm were observed from
.8 V versus a saturated calomel electrode (SCE), increasing at the acetonitrile–H O mixture (Fig. 1e). The results revealed that
4
4
1
2
ꢀ
1
a rate of 50 mV s
.
the nanostructure formation of L had a strong relationship to
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the main absorption band at 387 nm. The main absorption
band of rectangular-like nanostructures obtained from EtOH–
H O and acetone–H O solvents were centered at 389 nm and
2
2
402 nm, respectively. The spherical nanoparticles from acetonitrile–
H O showed a main absorption band at 390 nm. The nanorhombus
2
2
obtained from THF–H O showed the main absorption band at
395 nm. The strength of the interactions between L and solvent
molecules, the solvent effect, the morphology, the size and the
corresponding size distribution may be the main causes of the
changeable absorption phenomenon. The results also revealed
that the solvents influenced the ICT process of L to some extent
at the nanometer scale.
Except for UV-vis absorption, the fluorescence emission
spectra of L nanostructures obtained from the five solvents
also showed a slight shift as shown in Fig. 1g. The fluorescence
2
of L nanorhombus prepared from THF–H O showed a green
emission, the band of which centered at about 504 nm. The
emission band centered at 511 nm, 513 nm, 513 nm and
515 nm for rectangular-like nanostructures obtained from
EtOH–H O, nanowires obtained from DMF–H O, rectangular-like
Fig. 1 The morphologies of L prepared from different solvents: (a) THF–
2
2
H
H
2
O, (b) DMF–H
2 2 2
O, (c) EtOH–H O, (d) acetone–H
2
O and (e) acetonitrile– nanostructures from acetone–H O, and spheres from acetonitrile–
2
O; (f) UV-vis spectra and (g) fluorescence spectra at room temperature
H
2
O, respectively. This phenomenon can be attributed to different
of L nanostructures prepared from five different solvents in this study.
33
strengths of weak interactions between neighbouring molecules
in different solvents as discussed previously.
the solvent used, which can be attributed to the different
strengths of noncovalent intermolecular interactions between
3
.3 Effect of acidity on the self-assembly process of L
3
0
Except for the influence of solvents on the L morphology, the
acidity of the solution changed the self-assembly process of L
molecules more obviously to some extent, which was reason-
able that L was a carboxylic acid derivative. In order to study
the influence of the solution acidity on the morphology,
experiments were conducted by adjusting the pH value of the
solution by just adding HCl and/or NaOH into the system.
Owing to the fact that the morphology of L nanostructures
L–L molecules and L–solvent. When ethanol, acetone, DMF
and acetonitrile were used, the strength of weak interactions
between L–solvent (such as hydrogen bonding) was much
higher than competing interactions between adjacent L–L
molecules, which resulted in a smaller size of the structures.
When THF was used, the interaction strength between L–solvent
was weaker (may be due to the weak dielectric constant of THF)
that brought about a somewhat larger size.
3
4
obtained from the THF–H O mixture was more regular and
2
interesting than that from the others, the following experi-
3
.2 UV-vis absorption and fluorescence properties of L
ments were carried out in THF–H O mixtures with different
2
nanostructures obtained from different solvents
acidity while keeping the other conditions unchanged.
As it is known that nanomaterials with different morphologies
When L was introduced to a weak acid solution of pH = 5.0,
are promising candidates to exhibit different optical properties the rhombic-like morphology was little changed (Fig. 2c) and
1
5,31
due to different modes of molecular stacking.
In this study, the morphology was similar to that obtained from pure THF–
O solution. When the acidity increased to pH = 3.0, the
the solvent changed not only the self-assembly process of L
H
2
molecules but also the linear optical properties. The UV-vis rhombic-like structure was also observed (Fig. 2b) with the
absorption and fluorescence properties of L nanostructures longer diagonal of the rhombus decreased to about 3 mm.
obtained from different five solvents mentioned above were However, the two rhombic-layers structure was not easily
studied and the results are shown in Fig. 1f and g.
observed and the thickness was decreased to about 100 nm.
All of the samples prepared from different solvents exhibited When the acidity reached to pH = 1.0, bead-like rhombic
two major absorption peaks at around 255 nm and 400 nm with particles were observed, the size of which was smaller than
a slight shift (Fig. 1f). The former one peak with higher energy the samples obtained in other acid solutions. The longer
corresponded to the transition of the triphenylamine group, diagonal was ca. 600 nm, the shorter diagonal was ca.
while the latter with lower energy resulted from the p–p* 400 nm and the thickness was about 200 nm (Fig. 2a). A similar
3
2
transition of the whole L molecule accompanied by the morphology was captured from the TEM microscope as shown
intramolecular charge transfer effect. When a different solvent in Fig. S2 (ESI†). The phenomenon may be due to the protona-
was used, the transition of the triphenylamine group changed tion effect caused by the interactions between hydrogen ions
little while the band at lower energy w was influenced more and carboxylic units. The more the hydrogen ions existed, the
significantly. The nanowires obtained from DMF–H
2
O showed stronger the protonation effect and the hydrogen bonds formed,
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Fig. 2 (a–e) SEM images of L nanostructures prepared in THF–H
2
O solution of (a) pH = 1.0; (b) pH = 3.0; (c) pH = 5.0; (d) pH = 9.0 and (e) pH = 13.0;
(
f) fluorescence microscopy image of L nanostructures in pH = 13.0.
which brought about the stronger interactions between L–solvent atoms were highly conjugated, which would benefit electronic
and decreased the size of the morphology to much smaller.
delocalization in the whole molecule.
On the other hand, L nanostructures obtained from alkaline
There was a O3–H3ꢃ ꢃ ꢃO4 intermolecular hydrogen bond
3
6
medium were quite different from that obtained under acid with the H3ꢃ ꢃ ꢃO4 distance of 1.864 Å (C–Hꢃ ꢃ ꢃO = 168.671),
2
medium and/or from pure THF–H O solution. When studied in which was the main driving force to form one-dimensional
the weak alkaline solution of pH = 9.0, the number of rhombus structures along the a axis as shown in Fig. 3b. The adjacent
decreased significantly and the two rhombic-layers structure molecules were also stacked through C–Hꢃ ꢃ ꢃC weak interac-
disappeared (Fig. 2d), which may be due to the erosion effect tions along the c axis at a short intermolecular distance of 2.865
caused by the interactions between alkali and carboxylic unit. Å, which was very important in supramolecular crystal packing
When L was put into strong alkaline medium of pH = 13.0, one- to form two-dimensional sheets in the bc plane as shown in
dimensional microwires with tiny branches were obtained, the Fig. 3c. There also existed weak p–p interactions as shown in
length of which was several hundred micrometers and the Fig. S4 (ESI†). All these weak interactions were supposed to be
average width was B8 mm (Fig. 2e). The microwires emitted the driving forces for the molecule self-assembly to construct
strong green light under the fluorescence microscope (Fig. 2f). bulk materials and/or micro/nanostructures.
A more interesting phenomenon was that a glaring light emis-
When L–THF solution was injected into pure water, the
sion was observed at the node points of the microwires, limited solubility of L in water induced molecular segregation
indicating that the as-prepared microwires possessed an optical from THF and precipitation in water to form tiny seed crystals.
3
5
waveguide effect when operated in the light transmission Then, in the subsequent growth process, the intermolecular
3
7
process. The corresponding TEM image shown in Fig. S3 (ESI†) interactions between adjacent L molecules acted as the driv-
clearly confirmed the node points.
ing force for the stacking of L molecules. These weak inter-
molecular interactions drove L molecules to align orderly and
assemble mainly along two different directions as shown in
Fig. 3d. There also existed O(L)–H(L)ꢃ ꢃ ꢃO (THF or water) and/or
3.4 Mechanisms of molecular stacking mode
In order to better understand the relationship between the O(water)–H(water)ꢃ ꢃ ꢃO(L) interactions between L molecules and
morphologies prepared from different solvents and the acidity THF–H O solvent, together with the C–Hꢃ ꢃ ꢃC weak interactions
2
of the solution, the weak intermolecular interactions between and weak p–p interactions, which were considered as the
adjacent L molecules were studied through single crystals of L. driving force for forming a two rhombic-layered structure.
L crystallized in orthorhombic form with space group Pcca
as shown in Fig. 3. The crystallography data were summarized mixture with different acidity, the intermolecular interactions
2
When the L molecules were introduced into the THF–H O
38
in Table 1. Selected bond lengths and bond angles are listed in between L–L and/or L–solvent were changed. When the molecules
Table S1 (ESI†). In the molecule, the bond lengths were all of were injected into the acid solution, the carboxylic acid unit of L
aromatic character. The linkage bond lengths between the was protonated. Thus, the hydrogen bond between the carboxyl
benzene ring and the two carboxyl groups were quite conju- units of L and the solvent would be strong. The process would
gated with C10–C13 being 1.467(2) Å and C14–C15 being decrease the probability of the formation of O–HꢃꢃꢃO hydrogen
1.470(2) Å. The C13QC14 double bond was nearly coplanar to bonds between L–L molecules. In the strong acidic environment
the adjacent benzene and carboxyl group with the torsion angle (such as pH = 1.0), the hydrogen bond between the carboxyl groups
of C9–C10–C13–C14 being 169.401 and O4–C15–C14–C13 being of L and the solvent was somewhat stronger than that between L–L
176.581. The structural features suggested that all nonhydrogen molecules, thus the molecules assembled to the smaller and
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Fig. 3 (a) Crystal structure and intermolecular interactions of L with the atom numbering scheme; (b) the one dimensional framework showing the weak
O–Hꢃ ꢃ ꢃO (red) hydrogen bonds along the a axis at a distance of 1.864 Å; (c) the two dimensional architecture was connected by O–Hꢃ ꢃ ꢃO (red) hydrogen
bonds and C–Hꢃ ꢃ ꢃC weak interactions at a distance of 2.865 Å; (d) the structure clearly showed the intermolecular interactions along the ab plane;
(e) crystal photographs of L.
Table 1 Crystal data and structure refinement for L
Compound
L
b [Å]
14.740(5)
Empirical formula
Formula weight
Temperature (K)
Space group
Z
C
24
H
19NO
4
c [Å]
a [deg]
8.691(5)
385.40
298(2)
Pcca
4
1.265
808
90.000(5)
Orthorhombic
2023.2(15)
0.086
1.081
97.9%
Crystal system
3
V [Å ]
ꢀ1
m [mm
GOF
]
ꢀ3
Dcalcd [g cm
]
F(000)
Crystal size (mm )
Completeness to theta = 27.48
y range [1]
Reflections collected/unique
3
0.30 ꢁ 0.20 ꢁ 0.20
1.38–25.00
11 431/1759
Final R indices [I 4 2sigma(I)]
R
1
= 0.0432, wR = 0.1342
2
[
R
R(int) = 0.0401]
2
= 0.0556, wR = 0.1505
a [Å]
15.793(5)
R indices (all data)
1
thinner rhombic morphology, and the rapid growth of crystal seeds the existence of the two carboxyl units. The results of the linear
made it hard to form a two rhombic-layered structure. optical properties of L nanostructures prepared from different
However, when the environment changed to alkaline condi- acidity are listed in Fig. 4. As shown in Fig. 1f, in pure THF–H O
2
tions, the carboxyl unit was expected to be deprotonated, which mixed solvent, the main absorption band of L centered at
caused the intermolecular hydrogen bond between L–L molecules 257 nm and 395 nm. In acid solution, the absorption band
and L–solvents to be weakened and even disappear. Thus, the red-shifted as shown in Fig. 4a. When the pH value was 4.0, the
formation of a two-dimensional planar structure lost its driving main absorption band centered at 406 nm, which red-shifted
force. The C–HꢃꢃꢃC and p–p weak interactions became dominant in for about 11 nm from that of pure THF–H
2
O solution. When the
the molecular self-assembly process, resulting in more significant acidity of the solution increased, the extent of red-shift
3
9
one dimensional orientation growth as shown in Fig. 2e and f.
extended. When the pH value decreased to 3.0, the main band
centered at 422 nm which red-shifted for about 27 nm from
3
.5 Effect of acidity on the optical properties of L
that of pure THF–H
2
O solution, while it centered at 440 nm as
nanostructures
pH = 1.0 which showed a red-shift for about 45 nm. The lower
The acidity of the solution changed not only the self-assembly the pH value, the larger the red-shifted wavelength. However,
process of L, but the linear optical properties of it as well due to when L molecules were under alkaline conditions, the main
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Fig. 4 (a) The UV-vis absorbance, (b) normalized fluorescence spectra of L nanostructures obtained from THF–H
2
O mixed solution under different pH
values.
absorption band centred at about 379 nm until the value
increased to pH = 13.0.
As for L molecules, compared with the distribution plots of
the highest occupied molecular orbital (HOMO), the electrons
The phenomenon may be caused from the deprotonation of the lowest unoccupied molecular orbital (LUMO) were dis-
and/or protonation effect of L molecules. When the pH value tinctly different as shown in Fig. S5 (ESI†). The electron cloud
was 1.0, the whole molecule was thoroughly protonated, the density in the HOMO of L was mainly concentrated on the
existence of a protonated carboxyl unit enlarged the strength of electron-donating triphenylamine moiety with low coefficients
the acceptor unit, which caused the increased strength of D–A on the electron-withdrawing carboxylic groups. In the LUMO,
pairs, and brought about red-shifted absorption. As a result, the the electrons were mainly concentrated on the carboxylic unit
band at 440 nm red shifted for about 45 nm from 395 nm under and its adjacent benzene ring. There was low electron density
2
a pure THF–H O mixture. When the pH value increased, the distribution on the benzene ring far away from the carboxyl
protonation effect weakened, the red-shifted degree of absorp- unit. The separation between HOMO and LUMO indicated that
tion band decreased along with it until the band centered at substantial charge transferred from the donor moiety to the
about 406 nm at pH = 4.0. On the contrary, when the whole acceptor moiety when the molecules were excited. The theo-
system was under an alkaline environment, the deprotonation retical calculated band gaps between HOMOꢀ1 and LUMO was
effect controlled the molecule and the whole molecule existed 3.1423 eV with f = 0.69683, which equalled 394.56 nm and
as an alkaline structure. The strength of D–A pairs was weak- agreed with the UV-vis experimental results. In the HOMOꢀ1
ened and the absorption band centered at 379 nm which orbit, the electrons are primarily concentrated on the CQC
showed a 16 nm blue-shift from that of pure THF–H
solution. The shifted absorption was further researched Obviously, the electron transition from the HOMOꢀ1 to the
through theoretical study as shown in part 3.6. LUMO was accompanied by intramolecular charge transfer
2
O
double bond and the benzene rings near the two carboxyl units.
The fluorescence emission also revealed shifted phenomenon from the central triphenylamine moiety to the peripheric
as shown in Fig. 4b. When the system was in an acidic environ- carboxyl unit.
ment, the main fluorescence emission band existed at 487 nm,
When L molecules were thoroughly protonated in a strong
while it centred at B515 nm when the material was under acidic environment, the theoretical calculation results revealed
alkaline conditions. The possible reason was also attributed to that the main absorption band gap was 2.7457 eV (that was
the deprotonation and/or protonation effect of the L molecule. 451.55 nm) with f = 1.0481, which was excited from the
The different stacking modes of the molecules under acidic and/ HOMOꢀ2 to the LUMO orbital as shown in Fig. S6 (ESI†). In
or alkaline conditions (as discussed previously) resulted in the the HOMOꢀ2 orbit of the protonated L molecule, the electrons
different shifted trend.
were primarily concentrated all around the molecule. In the
LUMO, the electrons are primarily concentrated on the car-
boxylic unit and its adjacent benzene ring. When L molecules
3.6 Theoretical calculations
To better understand electron structure of L and the electron were partly protonated in a weakly acidic environment, the
absorption under different acidity conditions, the ICT process calculated main absorption band gap was 2.9430 eV (that was
was interpreted directly by theoretical calculations on the 421.28 nm), which was excited from the HOMO to LUMO
energy levels, which were performed by a time-dependent orbital (shown in Fig. S7, ESI†).
density functional theory (TD-DFT) calculation method at the
When L was deprotonated to be the carboxylic acid anion,
B3LYP/6-31g level based on the L molecule conformation in the the charge transfer process was significantly changed as shown
4
0
crystal structure, deprotonated and protonated structures. in Fig. S8 (ESI†). The theoretical calculation results revealed
The calculated results and the spatial distributions of the corres- that the main absorption band gap was 3.2377 eV with
ponding molecular orbital diagrams are listed in Fig. S5–S8
ESI†).
f
=
0.61247, the corresponding wavelength equalled
(
382.93 nm, which was from the HOMOꢀ4 to the LUMO orbit.
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In the HOMOꢀ4 orbit, the electrons are primarily concentrated 3–7 mm (Fig. 5a and Fig. S12, ESI†). Spherical nanoparticles of
on the carboxylic units. In the LUMO, the electrons are mainly Cd–L with diameters varying from 100 nm to 300 nm were
concentrated on the oxygen atoms of the carboxylic units. The observed as shown in Fig. 5b and Fig. S13 (ESI†). While for
changing structure of the electron cloud density in different Yb–L, silk-like structures with the sizes of around dozens of
conditions may bring about the changeable optical properties. microns were observed as shown in Fig. 5c. The results indicated
that the morphology was strongly associated with the metal ion
2
+
3.7 The preparation and morphology of coordination polymer
used. As Ba interacted with L through the two carboxylic units,
the O–Hꢃ ꢃ ꢃO hydrogen bonds between L molecules disappeared,
which resulted in the disappearance of the main driving forces
of the rhombic morphology formation. The newly created inter-
molecular interactions amongst Ba–L molecules drove them
particles
The results discussed above revealed that the solvent and
solution acidity influenced the weak intermolecular interactions,
the self-assembly process, the molecular stacking mode and the
corresponding linear optical properties of the L nanostructures.
Further, the next main purpose of this study was to research the
coordination effect between the L molecule and metal ions on
the stacking mode and the corresponding optical properties.
Through the intermolecular interactions between metal ions and
carboxylic acid units of the L molecule, size- and morphology-
dependent nanoparticles can be obtained. Among the numerous
alternative metal ions, three metal salts, Ba , Cd and Yb ,
were chosen considering the outer electron structure, the possi-
ble coordination number and the fluorescence of L under the
existence of the metal ions (shown in Fig. S9, ESI†). There were
some factors that played vital roles in affecting the morphology
of the resulting CPPs, such as the amount of solvent, the molar
ratio of reactants used in the reaction, the additives for adjusting
4
2
stacked into a rectangular structure, which can also be con-
sidered as the distorted rhombic morphology. That is to say, the
2
+
introduction of Ba ion changed the morphology slightly. When
2
+
Cd was introduced into the synthetic system with the coordi-
2
+
nation number of Cd being about 4 or 6, the increased number
of L molecules led them to be uniformly distributed around the
2
+
central core Cd ions. Thus, the differences of the intermole-
cular interactions between adjacent Cd–L molecules along three
dimensions were small, which resulted in the spherical nano-
2
+
2+
3+
3
+
particles. Further, as Yb ions were introduced, whose coordi-
nation number was high, about 8 to 12, several L molecules
3
+
attached to the central core Yb ions. Thus, steric hindrance led
to the weak crystallinity of Yb–L molecules and the growth along
three dimensions was difficult which brought about the for-
mation of a two dimensional silk-like morphology with low
crystallinity.
1
3,41
the deprotonation rate, etc.
factors, the solvent was the same as that used when described
in part 3.3, which was a THF–H O mixed solvent with the water
fraction f = 90%. The amount of metal ions to L was fixed at 1 : 1
To reduce these influencing
2
3.8 Optical properties of the as-prepared three CPPs
w
and NaOH was used to deprotonate L molecules, which can The construction of CPPs brought about not only the change-
maximize the protonation speed. Thus, three CPPs, Ba–L, Cd–L able morphology, but also the optical phenomena, linear and
and Yb–L, were prepared through the coordination process nonlinear optical properties included.
2
+
2+
3+
between L and Ba , Cd and Yb ions, respectively.
3.8.1 Linear optical properties. The UV-vis absorption and
These three CPPs were analysed through infrared spectro- single-photon-excited fluorescence emission of L and three
scopy (Fig. S10, ESI†) and powder X-ray diffraction (Fig. S11, CPPs were studied and compared as shown in Fig. 6. The UV-vis
ESI†), the results of which revealed the intermolecular coordi- absorption spectrum of L in ethanol solution exhibited two major
nation effect between the L molecule and these different metal absorption bands centred at 258 nm and 390 nm, respectively. The
ions, which also confirmed the formation of CPPs. In infrared absorption band of the three corresponding CPPs showed a slight
ꢀ
1
spectroscopy, the C–O stretching frequency at 1680 cm for blue-shift to about 383 nm. There were also some differences in the
ꢀ1
uncoordinated L shifted to B1634 cm for the three CPPs, emission wavelength of L and the CPPs. L–EtOH solution exhibited a
which illustrated the complete deprotonation of the carboxylate green–blue emission centred at 491 nm. The Ba–L and Cd–L CPPs
unit after the formation of these CPPs.
showed the main emission band at 523 nm and 527 nm,
The morphology of these three CPPs was studied through respectively, the fluorescence of which was higher than that
SEM and TEM analysis as shown in Fig. 5, Fig. S12 and S13 of pure L–ethanol solution. On the other hand, Yb–L showed a
(ESI†), respectively. The rectangular microcubes with a clear
surface of Ba–L were observed with the average length being
Fig. 6 (a) UV-vis spectra and (b) single photon fluorescence spectra of L
and three CPPs dispersed in ethanol solution at the concentration of 1.0 ꢁ
Fig. 5 SEM images of coordination polymer particles (a) Ba–L, (b) Cd–L
and (c) Yb–L obtained by dispersing the samples in ethanol.
ꢀ
3
ꢀ1
10 mol L .
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green emission centred at 540 nm, which was weaker than that
Two photon absorbance. The second-order nonlinear optical
of pure L–ethanol solution. One possible reason for the red- susceptibilities of L and the three CPPs were measured by the
shifted fluorescence emission for the three CPPs may be open Z-scan method as shown in the middle row of Fig. 7.
8
ꢀ2
attributed to the organic linker chelating to the metal centre, Based on eqn (1) and (2), b of L at I = 1.115 ꢁ 10 GW cm was
0
ꢀ
1
which effectively increased the rigidity of the ligand and further 0.003224 cm GW and d was 133.01 GM. Whereas, b of the
8
ꢀ2
ꢀ1
enhanced the ICT process of the ligand molecules.
Ba–L CPPs at I
0
= 1.308 ꢁ 10 GW cm was 0.005076 cm GW
= 1.308 ꢁ
optical properties, L and the corresponding three CPPs exhibited 10 GW cm was 0.006306 cm GW and d was 289.06 GM,
3
.8.2 Nonlinear optical properties. Except for the linear and d was 232.65 GM, b of the Cd–L CPPs at I
0
8
ꢀ2
ꢀ1
8
ꢀ2
excellent nonlinear optical properties. Through the open- and and b of the Yb–L CPPs at I
0
= 1.238 ꢁ 10 GW cm was
ꢀ1
closed-Z-scan method, the sign and magnitude of the second- 0.01299 cm GW and d was 579.50 GM, respectively. Clearly,
and third-order nonlinear optical susceptibilities of L and the the TPA coefficient and TPA cross section of the three kinds of
three CPPs were measured, including the nonlinear refractive CPPs were larger than that of pure L. For the TPA cross section,
index (g), nonlinear absorption coefficient (b), TPA cross section it increased approximately 1.75, 2.17, 4.35 times respectively.
(3)
(
d), and third-order nonlinear susceptibility (w ). The concen- For Ba–L, the coordination effect can enlarge the p-conjugated
ꢀ
4
ꢀ1
44
tration of each sample was 1.0 ꢁ 10 mol L . The two photon degree of L molecules, which caused the increased d data.
excited fluorescence was captured under the best TPA wave- While for Cd–L and Yb–L, except for the enlarged p-conjugated
length confirmed from the open-Z-scan experiment, which was degree, the larger coordination number may bring about more
800 nm for L, 720 nm for Ba–L, 720 nm for Cd–L and 740 nm for ligand molecules around the centered metal ions, which
Yb–L CPPs, respectively. The results are shown in Fig. 7. The resulted in increased d data. The results agree with the discus-
nonlinearity parameters for L and the CPPs were extracted and sions mentioned in part 3.7.
summarized in Table 2.
Third-order nonlinear susceptibility. For the closed aperture,
Two photon excited fluorescence. The two photon excited
the calculation of the nonlinear refractive index g fitting can be
obtained as in eqn (3).
fluorescence (2PEF) spectra of the four samples are shown in
the top row of Fig. 7, respectively. The maximum emission
wavelength located at 503 nm for L, 530 nm for Ba–L, 531 nm
for Cd–L and 510 nm for Yb–L, respectively. The 2PEF peak
0.25
TP–V = 0.406(1 ꢀ s) |DF|, where |DF| = KLeffgI ,
(3)
0
position of the samples showed a shift of differing extent where DTP–V is the peak–valley transmittance difference from
compared to that of 1PEF (see Fig. 6b), which was 12 nm red- the closed-aperture scan. The difference between normalized
shift for L, 7 nm red-shift for Ba–L and 4 nm red-shift for Cd–L, transmittance values at valley and peak positions can be clearly
respectively. On the contrary, for that of Yb–L, the 2PEF peak seen. The calculated nonlinear refractive indices g were 3.033 ꢁ
ꢀ
ꢀ
15
15
2
ꢀ1
ꢀ15
ꢀ15
position showed a 30 nm blue-shift from that of 1PEF. The 10
results indicated the difference of the excited states for 1PEF 10
cm W for L and 5.107 ꢁ 10 , 5.794 ꢁ 10 , 4.597 ꢁ
2
ꢀ1
cm W for Ba–L, Cd–L and Yb–L, respectively. It was
and 2PEF, mainly during the excitation process: single-photon clear that g increased when the coordination polymer particles
4
3
absorption vs. two-photon absorption.
formed.
Fig. 7 (up) Two photon excited fluorescence; (middle) open-aperture Z-scan response curves (solid lines show fitting to experimental data); (down)
close-aperture Z-scan response curves (solid lines show fitting to experimental data), for L, Ba–L, Cd–L and Yb–L, respectively.
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Table 2 Open- and closed-aperture Z-scan measurement data for the nonlinearity parameters of L and the three CPPs
L
Ba–L
Cd–L
Yb–L
Wavelength [nm]
800
720
720
740
ꢀ1
TPA coefficient b extrapolated to solute [cm GW
]
0.003224
1.115 ꢁ 10
133.01
0.005076
1.308 ꢁ 10
232.65
0.006306
1.308 ꢁ 10
289.06
0.01299
1.238 ꢁ 10
579.50
ꢀ
1
8
8
8
8
0
I [GW cm ]
d [GM]
2
ꢀ1
ꢀ15
ꢀ15
ꢀ15
ꢀ15
g [cm W
]
3.033 ꢁ 10
5.107 ꢁ 10
5.794 ꢁ 10
4.597 ꢁ 10
(3)
ꢀ13
ꢀ8
ꢀ13
ꢀ8
ꢀ13
ꢀ8
ꢀ13
ꢀ8
Re(w ) [esu]
ꢀ1.574 ꢁ 10
ꢀ1.039 ꢁ 10
ꢀ2.651 ꢁ 10
ꢀ1.510 ꢁ 10
ꢀ3.007 ꢁ 10
ꢀ1.897 ꢁ 10
ꢀ2.385 ꢁ 10
ꢀ3.973 ꢁ 10
(3)
Im(w ) [esu]
(
3)
The third-order nonlinear susceptibility (Re(w )) was also for the two-photon absorption. In this study, the samples’ absolute
(3)
determined through the closed-aperture Z-scan method. The value of the imaginary part of w was larger than that of the real
5
value of the real part of the third-order nonlinear susceptibility, part. The ratio of the imaginary part to real part was as large as 10 .
(
3)
(
Re(w )), can be calculated by the experimental measurements This meant that L and these CPPs possessed a high TPA activity
of g as in eqn (4)
and could be a good TPA material for optical-limiting applications
and/or protective devices.
2
2
e0c n0 g
ð3Þ
Rew ðesuÞ ¼
;
(4)
p
3.9 Two-photon microscopy bio-imaging applications
where e
vacuum, n
was calculated to be ꢀ1.574 ꢁ 10
of Ba–L, Cd–L, Yb–L, the values were ꢀ2.651 ꢁ 10 , ꢀ3.007 ꢁ (HepG2) were the test candidates, cultured and stained with the
0
is the vacuum permittivity, c is the velocity of light in a
(
3)
is the linear refractive index. The value of (Re(w )) To evaluate the performance of the as-prepared material in live
0
ꢀ
13
esu for L. As for the CPPs cells, a 2PFM imaging was conducted. Human liver cancer cells
ꢀ
13
ꢀ
13
ꢀ13
10
, ꢀ2.385 ꢁ 10
esu, respectively, the absolute values of samples. The large 2PA action cross-sections of these samples
which were all higher than that of pure L. The results indicated made all of them suitable attractive candidates for two photon
that the CPPs could reasonably be a better candidate for third- imaging. Here, the ligand L and Yb–L CPPs were chosen as the
order NLO materials.
two photon imaging agents as the TPA cross section d of Yb–L
The value of the imaginary part of the third-order nonlinear was the largest among these three CPPs.
(
3)
susceptibility, Im(w ), can be calculated from the value of b as
given in eqn (5)
3.9.1 Cytotoxicity tests (MTT assay). Considering the appli-
cation in two-photon fluorescence imaging, the MTT assay was
performed at the first stage to ascertain the cytotoxic effect of L
and the CPPs. Cytotoxicity is a potential side effect of dyes that
must be controlled when dealing with living cells or tissues. In
2
2
e0c n0 lb
ð3Þ
Imw ðesuÞ ¼
;
(5)
4
p2
Owing to the existence of coordination interactions between this study, all of the experiments were repeated three times.
(
3)
L and metal ions, an increase in Im(w ) value was expected. The measured data for each set of experiments were expressed
(
3)
Indeed, the absolute value of Im(w ) of Ba–L CPPs, Cd–L CPPs, with the average value. Fig. 8a showed the cell viability for
ꢀ
8
ꢀ8
ꢀ8
Yb–L CPPs were 1.510 ꢁ 10 , 1.897 ꢁ 10 , and 3.973 ꢁ 10
HepG2 cells when different concentrations of the samples were
esu, respectively, all of which were higher than the absolute used. The results clearly indicated that the cells incubated with
ꢀ8
value of pure L (1.039 ꢁ 10 esu).
L and the CPPs under the concentration of 5 mM remained
(
3)
It is known that the real part of w leads to an induced about 85–90% viable after 24 h of feeding time, demonstrating
refractive-index change whereas the imaginary part is responsible the preferable biocompatibility. When the concentration of the
Fig. 8 (a) MTT assay of HepG2 cells treated with L and/or the CPPs at different concentrations for 24 h; (b) one-photon image, two-photon
fluorescence microscopy (2PFM) image, bright-field image of HepG2 cells stained with the samples and merged image and of HepG2 cells incubated
with: (upper) 10 mM of L, (down) 10 mM of Yb–L CPPs, after 60 min of incubation, washed by PBS buffer, with excitation at 800 nm and 740 nm,
respectively.
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samples was up to 60 mM, the viable cells gradually decreased to suitable for the uptake of the lysosome part of HepG2 cells more
about 60%. The cytotoxicity tests definitely indicated that the widely and evenly.
low-concentration of the samples would not give rise to obvious
toxic effects on living cells over a period of 24 h, which has great
4
5
Acknowledgements
potential for further biological studies.
.9.2 Two-photon microscopy bio-imaging. Two-photon
3
This work was supported by the Natural Science Foundation of
Anhui Province (1508085SMB208), the NSFC (No. 51432001),
and the Educational Commission of Anhui Province of China
excitation cell imaging was achieved by using HepG2 cancer
cells through two-photon laser scanning confocal microscopy
(Fig. 8b). Treated cells were excited at an appropriate 2PA
(KJ2014ZD02).
wavelength, which was 800 nm for L, 740 nm for Yb–L,
respectively. The green channels in Fig. 8b showed 1PFM
imaging. The red channels in Fig. 8b showed 2PFM imaging.
These micrographs clearly revealed that the samples success-
fully internalised with HepG2 cells. Two-photon induced
fluorescence was observed predominantly from the cellular
cytoplasmic region, which was consistent with the images
Notes and references
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evenly uptake the lysosome part all around the HepG2 cells.
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in less material entering and an uneven distribution. On the
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from a THF–H O mixed solvent and the morphology could be
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2
further tuned by changing the acidity of the growing solution. 11 (a) W. Du, A. J. Cruz-Cabeza, S. Woutersen, R. J. Davey and
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