The self-aggregation of fluorophore-triphenylamine nanostructures with tunable luminescent properties: The effect of acidity and rare earth ions

Article abstract of DOI:10.1039/c3ra47893a

A D-π-A type triphenylamine derivative 3-(4-(diphenylamino)phenyl) acrylic acid (abbreviated as L) is designed, synthesized and characterized by single crystal X-ray diffraction analysis. The self-aggregation of L nanostructures in different acidity aqueous solutions and/or under the guidance of rare earth (RE) ions is studied, along with the corresponding optical properties. In strong acidic conditions, L molecules self-assemble to form two dimensional nanostructures. While in neutral aqueous solutions, L tends to form nanorods. In the strong alkali environment, nanofibers were obtained. Moreover, in acidic conditions, the existence of Tm³⁺ induces the L molecules to self-aggregate into nanoplates. Flower-like six branched structures are observed when Yb³⁺ is used. The changed morphology leads to tunable linear optical properties.

Full text of DOI:10.1039/c3ra47893a

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The self-aggregation of fluorophore-
triphenylamine nanostructures with tunable
luminescent properties: the effect of acidity and
rare earth ions†
Cite this: RSC Adv., 2014, 4, 18981
a
a
ab
a
a
Qi-yu Chen, Lin Kong,* Yu-peng Tian, Xian-yun Xu, Long-Mei Yang,
a
a
ab
Gao-bin Zhang, Wen-bin Jia and Jia-xiang Yang*
A D–p–A type triphenylamine derivative 3-(4-(diphenylamino)phenyl)acrylic acid (abbreviated as L) is
designed, synthesized and characterized by single crystal X-ray diffraction analysis. The self-aggregation
of L nanostructures in different acidity aqueous solutions and/or under the guidance of rare earth (RE)
ions is studied, along with the corresponding optical properties. In strong acidic conditions, L molecules
self-assemble to form two dimensional nanostructures. While in neutral aqueous solutions, L tends to
form nanorods. In the strong alkali environment, nanofibers were obtained. Moreover, in acidic
Received 22nd December 2013
Accepted 27th February 2014
3+
conditions, the existence of Tm induces the L molecules to self-aggregate into nanoplates. Flower-like
3+
DOI: 10.1039/c3ra47893a
six branched structures are observed when Yb is used. The changed morphology leads to tunable
www.rsc.org/advances
linear optical properties.
and/or control the morphology and/or particle size of an organic
nanostructure.
1
. Introduction
Over the past few decades, increasing interest has been received
Rare earth ions have been extensively studied in the funda-
about functional organic materials composed of optically and mental and technical eld over the past decades due to their
electronically active constituents in nanoscience and nano- particular characteristics arising from the 4f and/or 5f elec-
1
–3
14,15
technology.
nanomaterials can be controlled through their size, shape and luminescent devices, magnets, catalysts and other functional
The tunable physical properties of organic tronic shells,
which have a great potential to be used in
1
6–18
crystallinity, which are expected to provide an opportunity to materials.
Compounds consisting of RE ions also have
control the optoelectronic properties and further device signicant commercial applications in radiation intensifying
4–6
performance. A multitude of cooperative and simultaneous screens, X-ray-computed tomography, oxygen storage and
19,20
intermolecular interactions, such as hydrogen bond, van der medical imaging radiation detectors.
It is well known that
Waals, p–p stacking and electrostatic attractions, can affect the there are signicant intermolecular interactions between
2
1

nal structure and further adjust the corresponding proper- carboxylate compounds and the RE ions. According to the
7
–9
ties. In addition, the concentration, temperature, surfactants hard–so acid–base principle, RE ions should have a high
and solution acidity can also be used to alter or control the tendency to combine with the O atoms of these carboxylate
above-mentioned interactions and further cause the change of compounds. Thus, the interactions between the RE ions and the
10–13
morphology.
However, to the best of our knowledge, little oxygen atoms of the carboxylate ligands are somewhat strong.
research has been carried about using RE metal ions to induce Up to now, the use of RE ions as efficient inductive agents to
build organic nanostructures has rarely been reported. There-
fore, in this study, the interaction between the RE ion and
carboxylate derivative is used to regulate the morphology.
a
Anhui University, Department of Chemistry, Hefei, Anhui, China. E-mail:
kong_lin2009@126.com; jxyang@ahu.edu.cn
In many functional organic materials, triphenylamine and
its derivatives are well-known for their 3-D propeller shapes
which have been widely investigated and applied in organic
b
State Key Laboratory Materials, Shandong University, Jinan 502100, P. R. China
†
Electronic supplementary information (ESI) available: selected bond lengths [ A˚ ]

eld-effect transistors and organic light-emitting diodes, as well
ꢀ
and angles [ ] for L; the packing diagram of L; SEM image of L nanostructures
prepared in pH ¼ 4; SEM image of nanostructures of L with the excess
22–24
as second order nonlinear optical devices.
In many cases,
3+
triphenylamine groups can be employed as electron-donors and
concentration of Tm ; SEM image of hexagonal prisms nanostructures when
3+
the Yb was added to acidic aqueous solution of pH ¼ 3. CCDC number used to prepare D–p–A type molecules (D ¼ donor, A ¼
9
76845. For ESI and crystallographic data in CIF or other electronic format see
acceptor) through linkage with an electron-acceptor group,
DOI: 10.1039/c3ra47893a
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which are regarded as ideal candidates for red emitting and using a eld-emission scanning electron microscope (FESEM,
25,26
nonlinear optical materials.
To increase the interactions Hitachi S-4800). The UV-vis absorption spectra were obtained
between the RE ions and the organic matrix, the carboxylic on a UV-3100 spectrophotometer in the wavelength range 200–
group is introduced into the triphenylamine group, which can 700 nm. The uorescence spectra were measured at room
also improve the solubility of the as-prepared organic temperature using a Hitachi F-7000 spectrophotometer.
compound in water as well and further present a bright future
for the application.
Considering all the aspects above, in this work, an ICT
structural triphenylamine derivative,
designed and synthesized, in which the triphenylamine group
was employed as an electron-donor unit and the carboxyl group
was employed as an electron-acceptor unit. Furthermore,
nanostructures of L were prepared under different conditions.
The inuence of the solution acidity (pH), Tm and Yb ions
on the morphology and the corresponding linear optical prop-
erties of L were discussed. It is found that pH plays an impor-
tant role in the formation of the morphology. The morphology
of L has a tendency to accumulate and form semi two dimen-
sional nanostructure in the excessive acid aqueous solution and
one dimensional nanorods in a neutral aqueous solution.
However, in the alkaline solution, the surface of the L nano-
structures was etched due to the acid–base reaction and even-
tually nanobers were observed. Furthermore, the interactions
between L and the RE ions through a simple process at room
temperature result in a completely different morphology. In
brief, by controlling the acidity of the solution and the RE metal
ions, the self-assembly of 1-D or semi 2-D organic materials can
2.2 The synthesis of L
-Diphenylaminobenzaldehyde (0.5 g, 1.8 mmol) and malonic
acid (0.0187 g, 1.8 mmol) were dissolved in pyridine (5 mL) and
–5 drops of piperidine was added as a catalyzer. The above
mixture was reuxed for 4 h and monitored by TLC to ensure a
complete reaction. Then, water was added to the mixture and
4
L (Scheme 1), was
3
the precipitate was puried through recrystallization in ethanol
3
+
3+
1
solution to get a yellow solid. Yield: 95%. H NMR (400 MHz, d
6
-
DMSO): d (ppm) 12.22 (s, 1H); 7.56 (d, 2H, J ¼ 6.8 Hz); 7.50 (d,
1
7
1
1
H, J ¼ 16.0 Hz); 7.36 (t, 4H, J ¼ 7.6 Hz); 7.13 (t, 2H, J ¼ 7.2 Hz);
.09 (d, 4H, J ¼ 8.0 Hz); 6.89 (d, 2H, J ¼ 8.0 Hz); 6.34 (d, 1H, J ¼
13
6.0 Hz); C NMR (100 MHz): d (ppm) 167.54, 146.34, 143.49,
29.75, 129.54, 127.21, 125.01, 124.15, 120.93, 116.39. FT-IR
ꢁ1
(KBr, cm ): n ¼ 3029 (m, nCOO), 1681 (s, nC]O), 1622 (w, nC]C),
1584 (s), 980 (w), 829 (w), 753 (m), 698 (s).
Yellow crystals of L suitable for X-ray diffraction were
obtained by the slow evaporation of a dichloromethane–meth-
anol mixed solution (VCH2Cl2/VMeOH ¼ 2 : 1) at room temperature
for a week.
2.3 The preparation of aggregates
3
+
be achieved. The existence of Tm induces nanoplates in acidic
ꢁ
3
A stock ethanol solution of L with a concentration of 1.0 ꢂ 10
conditions. A ower-like six branched structure is obtained
ꢁ1
3
+
mol L (abbreviated as M) was prepared. Stock aqueous solu-
tions of different acidity (pH values of which were 1, 3, 4, 5, 7, 9
and 13, respectively) and stock aqueous solutions of RE ions
when Yb is used in acidic conditions.
3
+
3+
(
Tm , Yb ) were also prepared.
2. Experimental section
The organic nanostructures were prepared through a simple
reprecipitation process. For studying the effect of the acidity on
2.1 Materials and instrument
All reagents used in synthesis route were of analytical reagent the aggregation, 200 mL of an L–ethanol solution was injected
grade. The pH of the solution was adjusted by adding hydro- into 2 mL of high-purity water with different acidity under
chloric acid or sodium hydroxide in water and demarcated with stirring. For exploring the effect of RE ions on the aggregation,
3
+
3+
the pH meter of PHS-3CT. Metal salts Tm(NO
Yb(NO $5H
O were prepared from the corresponding metal into 2 mL of high-purity water and/or aqueous solutions with
oxidate and concentrated nitric acid.
different acidity with stirring for 3 min. Then, 200 mL of L–
3 3 2
) $5H
O, the aqueous solution of Tm and/or Yb was rapidly added
3
)
3
2
The X-ray diffraction measurement of single crystal was ethanol solution was injected into the above solution under
performed on a Bruker SMART CCD area detector using stirring. The samples were stirred for 10 min and then le
˚
undisturbed for 12 h at room temperature for stabilization.
graphite-monochromated Mo Ka radiation (l ¼ 0.71073 A) at
298(2) K. The intensity data were collected in the variable u-scan
mode. The structures were solved by direct methods and 2.4 TD-DFT calculation
difference Fourier syntheses. All non-hydrogen atoms were
The molecular orbital calculations of time-dependent density
rened anisotropically and hydrogen atoms were introduced
functional theory (TD-DFT) at the pbe1pbe/6-31g level basis set
Gaussian 09) were performed to study the electronic structures
geometrically. Calculations were performed with the SHELXTL-
(
27
9
7 program package. The morphologies were obtained by
28
of L. The molecular geometry used for the calculation was
obtained from the X-ray diffraction crystallographic data.
3
. Results and discussion
3.1 Structural characterization
L crystallized in the monoclinic form with the space group
P2(1)/n as shown in Fig. 1. The crystallography data are
Scheme 1 The synthetic routes of the target compound L.
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Fig. 1 The crystal structure of L with the atom numbering scheme.
Fig. 2 The morphologies of L from different mixed solvents: (a) DMF–
2 2 2 2
H O, (b) acetonitrile–H O, (c) acetone–H O, (d) ethanol–H O.
summarized in Table 1. Selected bond lengths and bond angles
are listed in ESI Table S1.† In the molecule, the bond lengths
were all of aromatic character. The linkage bond length between
the benzene ring and the carboxyl group was quite conjugated
or catalyst. The SEM images of the L self-assembly aggregations
from different solvents are shown in Fig. 2. The solvent-induced
morphological change of L is remarkable. Monodisperse
˚
with C16–C19 being 1.399(12) A. The C19]C20 double bond
was nearly coplanar to the adjacent benzene and carboxyl group
ꢀ
spherical nanoparticles are observed in a DMF–H
solvent with an average diameter of about 80 nm and a good
dispersion (Fig. 2a). Nanobelts are observed in acetonitrile–H
with the length of about 10 mm and the width of about 500 nm
Fig. 2b), which have a tendency to aggregate. The main prod-
2
O mixed
with the torsion angle of C17–C16–C19–C20 being 176.01(16)
ꢀ
and O2–C21–C20–C19 being 179.90(15) . The structural
2
O
features suggested that all nonhydrogen atoms were highly
conjugated and nearly coplanar, which would favour the elec-
tronic delocalization in the whole molecule.
The adjacent molecules are stacked through C–H/C weak
interactions along the same direction (b axis) at a short inter-
(
ucts in acetone–H O are nanorods with lengths of about 1.3 mm
2
and an average width of about 140 nm (Fig. 2c), which are
directionally arranged one by one. Moreover, nanorods with
lengths of about 10 mm and an average width of about 200 nm
are obtained in ethanol–H O (Fig. 2d). This solvent-based
2
morphological variation can be attributed to the different
˚
molecular distance between 2.804 and 2.854 A to form a 1-D
structure as shown in Fig. S1a.† There is also an O2–H2/O1
˚
intermolecular hydrogen bond with the H/O distance of 1.842 A
ꢀ
(
C–H/O ¼ 178.59 ), which is very important in supramolecular
intensity of the noncovalent intermolecular interactions
crystal packing to form 2-D sheets in the ab plane (Fig. S1b†).
There also exists a C10–H10/O1 intermolecular hydrogen bond
29
between L–L and L–solvent. Owing to the fact that the
morphology of L obtained from ethanol was more irregular than
that obtained from the other solvents, the following experi-
ments were carried out in an ethanol solution in this work.
˚
ꢀ
with the H/O distance of 2.349 A (C–H/O ¼ 163.56 ), which
connects with the C–H/C weak interactions mentioned above to
form 2-D sheets in the bc plane. Such weak interactions are
somewhat strong and supposed to be the main driving forces for
the molecules self-assembly along a certain direction to form
bulk materials and/or nanomaterials.
3.3 The effect of the acidity on the aggregation of L
As L is a carboxylic acid derivative, the acidity of the solution
may change the weak intermolecular interactions and further
inuence the morphology. In order to study the inuence of the
3.2 The effect of the solvent on the aggregation of L
In the present work, the L nanostructures are simply prepared acidity of the solution on the morphology, experiments were
in several solvents with no addition of any surfactant, template conducted by adjusting the acidity of the EtOH–water mixed
Table 1 The crystal data and structure refinement for L
˚
Empirical formula
Formula weight
Crystal system
Space group
Temperature (K)
Z
C
21
H
17NO
2
a [A]
13.716(5)
8.084(5)
15.868(5)
100.602(5)
1729.4(14)
1.211
˚
315.36
Monoclinic
P2(1)/n
298(2)
4
664
b [A]
˚
c [A]
b [deg]
3
˚
V [A ]
ꢁ3
D
calcd[g cm
Goodness of t on F
]
2
F (000)
1.032
Crystal size (mm)
Final R indices [I > 2sigma(I)]
0.30 ꢂ 0.20 ꢂ 0.20
Reections collected/unique
R indices (all data)
2366
R
1
¼ 0.0409, wR
2
¼ 0.1329
R
1
¼ 0.0549, wR ¼ 0.1497
2
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formation of one-armed nanotubes was considered as coming
from the erosion effect of the nanorods. The surface of the
nanotubes was rough, which may be due to the erosion effect
caused by the interactions between the alkali and carboxylic
group. These one-armed nanotubes arranged orderly and
stacked into a quasi 2-D structure. Furthermore, when the
acidity reached to pH ¼ 13, nanobers arose with the length of
ꢃ30 mm and width of about 200 nm (Fig. 3f), which was a
marked distinction compared with the nanorods obtained in
pH ¼ 7. Moreover, the growth direction of these nanobers was
basically the same in the visual eld, which revealed that the
intermolecular interactions along this direction were the main
driving forces to form the nanobers.
Fig. 3 The SEM images of the L nanostructures prepared in an
aqueous solution with different acidities (a) pH ¼ 1, (b) pH ¼ 3, (c) pH ¼
5
, (d) pH ¼ 7, (e) pH ¼ 9, (f) pH ¼ 13.
When an L–EtOH solution was injected into pure water, the
sudden change in the environment of the L molecules induced
solution as shown in Fig. 3. When the experiment was carried molecular segregation from ethanol and precipitation in water
out in the neutral conditions of pH ¼ 7, nanorods with smooth to form seed crystals. In the subsequent growth process, the
surfaces were observed with lengths and diameters of ꢃ10 mm strong intermolecular D–A dipole–dipole interactions between
and ꢃ200 nm, respectively (Fig. 3d), which t well with the adjacent L molecules acted as the driving force for the aggre-
result shown in Fig. 2d.
gation of the L molecules. The limited solubility of L in the poor
As the acidity of the solution varied, a remarkable change in solvent (water), together with the strong intermolecular dipole–
the L self-aggregation was observed. When the acidity of the dipole interactions, drove L molecules to align orderly and
solution increased, the morphology gradually changed from assemble mainly along the direction of the dipole moment, and
nanorods to ordered superimposed ake-like semi 2-D struc- thus 1-D nanorods were formed.
tures. In detail, when the acidity was pH ¼ 5 (Fig. 3c), there is a
While in the acid solution, the carboxylic acid unit of L was
slight change in the morphology compared to that obtained in fully protonated. Thus, the hydrogen bond between the carboxyl
the neutral conditions. The nanorods in the decentralized state groups of L and water would be strong. The process would
appeared and the length decreased to ꢃ1.5 mm. When the decrease the probability of the formation of O–H/O hydrogen
acidity increased to pH ¼ 4, two different morphologies of a bonds. Thus, the proportion and the corresponding strength of
stacked structure and dispersed nanorods coexisted (Fig. S2†). the C–H/C weak interactions between the triphenylamine
The stacked structure was made of multiple nanorods arranged group and the p–p weak interactions between the conjugated
orderly with a length of about 3 mm and a thickness of about groups of adjacent molecules would increase and be compa-
1.5–2 mm. The dispersed nanorods had an average length of rable to that of O–H/O hydrogen bonds. The three types of
about 4 mm. The results indicated that the driving force of the weak interactions would be the main driving force for the
formation of different nanostructure was probably a complex molecular packing which leads to the creation of accumulation
30
balance of intermolecular interactions.
from 1-D morphology to 2-D structures. However, when the
Furthermore, as the acidity increased to pH ¼ 3, the aggre- environment changed to alkaline condition, the carboxyl group
gated nanorods were observed to form a semi 2-D structure was expected to be deprotonated, which caused the intermo-
(Fig. 3b). The length and diameter of a single nanorod were lecular hydrogen bond to weakened and even disappeared.
ꢃ1 mm and 200 nm, respectively. These nanorods gathered to Thus, the C–H/C weak interactions were dominant in the
form a semi 2-D structure with ꢃ20 mm length and ꢃ15 mm molecular self-assembly, resulting in a more obviously 1-D
31
width. The border between the adjacent nanorods was clearly orientational growth.
observed, which showed the aggregation and weak intermo-
lecular interactions along two directions. When the acidity of
3
ions
.4 The morphology of the L nanostructure induced by RE
the preparation environment increased to pH ¼ 1, super-
imposed ake-like structures were obtained (Fig. 3a). A single
nanoake possessed a width of about 300 nm, a thickness of Through the above study, it is concluded that the solvent and
0–80 nm and an average length of about 1.2–1.6 mm. Several acidity of the solution have a signicant effect on the self-
7
nanoakes stacked closely and the interface was clearly aggregation of the L molecules. Furthermore, the effect of the
observed. The nanoake had a high morphological purity as RE metal ions on the morphology of L was studied. A more
other morphologies were seldom captured under such interesting change in the morphology was expected based on
conditions.
the strong interactions between the RE metal ions and the O
3
+
Another interesting trend of morphological change was atoms of L. To explore the aim, the aqueous solution of Tm
ꢁ
4
observed when the L molecules were put in an alkaline medium. (4 ꢂ 10 M) was added to the reaction system. A cross-like
When the acidity of the aqueous solution reached to pH ¼ 9, structure was observed, which had a length of about 6–10 mm
ꢀ
one-armed nanotubes were obtained (Fig. 3e) with a length of and the larger angle made of two nanorods was about 100
3
+
about 3–5 mm and an average bore diameter of 100 nm. The (Fig. 4a). As the concentration of Tm increased, the molecular
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structure was obtained, among which six nanorods extended in
six directions revolved round the centre point (Fig. 5a) to form
3
+
the six-branched ower. When the concentration of Yb
ꢁ
4
increased to 2 ꢂ 10 M, the length of the nanorods increased
to ꢃ40 mm and the nanostructure gathered gradually (Fig. 5b).
3
+
ꢁ5
Also, to eliminate the hydrolysis effect, Yb (6 ꢂ 10 M) was
added in a solution of pH ¼ 3, and regular hexagonal (Fig. 5c)
and hexagonal prism (Fig. S4†) images can be observed with the
average side length being 2 mm. These hexagonal akes were
believed to be the gem of the ower-like six-branched
structures.
3.5 Linear optical properties and theoretical calculations
3
.5.1 The linear optical properties in different acidity
3 3 2
Fig. 4 SEM images of the nanostructures of L with Tm(NO ) $5H O: in
solutions. The UV-vis absorption spectrum of L in an EtOH
solution exhibited two major absorption bands centred at
293 nm and 364 nm (Fig. 6a), respectively. The latter corre-
sponded to the p–p* transition of the whole L molecule, while
3+
ꢁ4
an aqueous solution with a concentration of Tm at 4 ꢂ 10 M (a), in
3+
an acidic aqueous solution of pH ¼ 3 with a concentration of Tm at
ꢁ
6
ꢁ6
ꢁ5
2
ꢂ 10 M (b); 6 ꢂ 10 M (c); and 2 ꢂ 10 M (d), respectively. The
virtual arrow inset of (c) is the virtual direction.
32
the former resulted from the triphenylamine fragment. To
further investigate the molecular level, and to better understand
the relationship between the optical properties and the elec-
tronic structure of L, theoretical calculations on the energy
levels were performed by the TD-DFT method at the pbe1pbe/6-
interactions of a certain direction became enhanced and led to
the increased length of the nanorods and a small quantity of
impurities. The phenomenon was possibly caused by the
3
+
3
1g level based on the molecule conformation in the crystal
hydrolysis of Tm (Fig. S3†). In order to eliminate this hydro-
lysis effect, the solution was adjusted to acidic conditions.
33
structure. In the highest occupied molecular orbital (HOMO),
the electrons were mainly concentrated on the electron-
donating triphenylamine moiety with low coefficients on the
electron-withdrawing cyanovinyl group. While in the lowest
unoccupied molecular orbital (LUMO) plots, the electrons were
mainly located on the electron-withdrawing groups. The sepa-
ration between the HOMO and LUMO indicated that a
substantial charge transfer from the donor moiety to the
acceptor moiety occurred when the molecules were excited.
Furthermore, the theoretical spectral characteristic showed
two main transitions. One was from the conguration HOMO to
3
+
When the Tm ions were added into the acidic aqueous solu-
tion, the morphology of L further changed. For example, when
the sample was prepared in pH ¼ 3 and the concentration of
3
+
ꢁ6
Tm was 2 ꢂ 10 M, unordered rectangular nanoplates were
3
+
obtained (Fig. 4b). However, as the concentration of Tm
ꢁ
6
gradually increased to 6 ꢂ 10 M, the nanoplate structures
grew along a particular direction shown in Fig. 4c (the white
solid lines are the virtual growth direction of the nano-
3
+
structures). When the concentration of Tm increased to 2 ꢂ
ꢁ5
10
M, the nanoplate structure nally grew into a large size
LUMO with labs ¼ 367 nm (oscillator strength f
being
sheet-like structure with a length of about 30–50 mm and a width
of about 4–20 mm (Fig. 4d). To further explore the inuence of
the different RE ions on the aggregation of the L molecules,
HOMO–LUMO
0
.6745). The other optical transition corresponded to the
3
+
another kind of RE ion, Yb , was investigated. A completely
3
+
different morphology compared with that of Tm was observed
3
+
as shown in Fig. 5. Moreover, the concentration of the Yb ions
also inuenced the L aggregation mode. When an aqueous
3
+
ꢁ5
solution of Yb (8 ꢂ 10 M) was used, a ower-like nanorod
3 3 2
Fig. 5 SEM images of the nanostructures of L with Yb(NO ) $5H O: (a)
3+
ꢁ5
in an aqueous solution with a concentration of Yb of 8 ꢂ 10 M, (b) Fig. 6 The UV-vis spectra of L measured in ethanol and the spatial
3+
in an aqueous solution with the concentration of Yb increased to 2 ꢂ distributions of the calculated HOMO and LUMO (a), the UV-vis (b) and
ꢁ
4
1
0
M, (c) in an acidic aqueous solution of pH ¼ 3 with the fluorescence spectra (c) of an EtOH–water mixed solution of L at
3+ ꢁ5
concentration of Yb at 6 ꢂ 10 M.
different pH values.
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conguration HOMO to LUMO+1 with labs ¼ 296 nm. The
calculated results t with the experimental data and the corre-
sponding absorption bands were observable on the absorption
spectra of the L–EtOH solution.
Furthermore, the UV-vis absorption spectra of the L nano-
structures obtained in different acidity aqueous solutions were
thoroughly studied. The results revealed that the p–p* transi-
tion of the whole conjugated structure changed a little with the
change of the acid and alkali environment. The absorption
band centred at 362 nm in acidic solution of pH ¼ 1 and/or 3,
which was 356 nm in the neutral environment of pH ¼ 7, while
it centred at 359 nm in an alkaline solution of pH ¼ 13. At the
Fig. 7 The fluorescence spectra of L–Tm (a), L–Yb (b) with different
concentrations of RE metal ions.
3
+
same time, the absorption band from the triphenylamine nanohybrid (the ratio of L to Tm was 167 : 1 at this moment),
fragment varied much more, which centred at 284 nm, 293 nm the emission showed a signicant red shi to 470 nm with a
3
+
and 300 nm in acidic, neutral and alkaline solutions, respec- quenched intensity. In the presence of the Tm ions, the heavy
3
+
tively. Moreover, the ratio of the two peak values showed uc- atom effect and the energy transfer from L to the Tm ions in
tuation. In an acidic solution of pH ¼ 1 and 3, the absorbance the process of an f–f transition may cause the uorescence
34
ratio of A /A
was 0.3768, where the suffix represents the quenching. The red shi might arise from the coordination
2
84 362
3
+
monitor wavelength. In a neutral solution of pH ¼ 7, the effect between L and the Tm ions. The action ‘hot spot’ was
absorbance ratio of A /A increased to 0.5687. In an alkaline the O atoms of the carboxyl group, which was the electron-
2
93 365
solution of pH ¼ 13, the absorbance ratio showed a further acceptor unit of L. The interactions may inuence the electron
increase to A300/A359 ¼ 1.0869. The results meant that the elec- distribution of L, which increased the electron cloud density of
tronic structure of the whole conjugated structure changed little the carboxyl group and further increased the strength of the D–
with the different acidity of the solution. The phenomenon was A pair, the direct result of which was the red-shi of the
reasonable in that the acidity of the solution changed the H uorescence.
3
+
atom of the carboxylic group, which was not included in the
As the usage of Tm increased, the uorescence quenched
whole conjugated structure. On the other hand, the different regularly. Along with this quenched emission, a more inter-
acidity of the solution violently inuenced the electronic esting phenomenon appeared. When the concentration
ꢁ
5
3+
structure of the triphenylamine fragment. The higher the increased to 2 ꢂ 10 M (the ratio of L to Tm was 50 : 1), the
acidity used, the more the inuence occurred. uorescence emission showed a wide peak, which centred at
As for the uorescence emission, there was a stable blue- 470 nm with a shoulder peak at 513 nm. To further increase the
shied wavelength band which varied from acidic conditions to concentration, the shoulder peak became more dominant.
ꢁ
3
a neutral medium, and then to an alkaline medium. When the When the concentration increased to 2 ꢂ 10 M (the ratio of L
3
+
pH value of the solution was 1, the emission band centred at to Tm was 1 : 2), the uorescence band mainly centred at
84 nm. When the pH value increased to 7, the emission band 513 nm. Aer this point, the emission band and the uores-
blue shied for about 9 nm to 475 nm, which further blue cence intensity did not change any more.
4
shied for about 23 nm to 461 nm as pH ¼ 13. The results
The emission band at 513 nm can be considered as the
meant that there was a great environmental inuence on the inherent uorescent emission of the L–Tm complex. When a
3
+

uorescence with the increased pH value.
The changes of the absorption and emission spectra in Tm complex was very small in the solution or the L–Tm complex
different acidity solutions indicated that, to a certain extent, the had not yet been generated. Thus, the uorescence showed only
small amount of Tm was used, either the proportion of the L–
3
+
intermolecular interactions between the L–L molecules and/or a red shi emission. On increasing the usage of the Tm ions, a
L–solvent were different in the acid–base aqueous solution strong coordination ability emerged and the inherent uores-
environment, as mentioned above, which then brought about cent emission at 513 nm appeared. When the concentration of
3
+
ꢁ3
3+
signicant changes in the morphology, size and optical the added Tm ions increased to 2 ꢂ 10 M, the ratio of Tm
properties. to L was 2 : 1 in this condition. At this point, all of the L
.5.2 The linear optical properties of L–RE nanohybrids. In molecules may be coordinated with the Tm ions according to
3
+
3
addition, from the results and discussions mentioned above, the results mentioned in the SEM analysis. Aer this point, little
the interactions between the RE metal ions and the L molecules change could be observed in the emission band and the uo-
were somewhat strong and could bring about signicant rescence intensity. The results further conrmed that the
3
+
changes in the morphology and/or size. Besides the optimum ratio of Tm to L was 2 : 1 in this study. To achieve
morphology/size research, a more important goal in this work the perfect coordination of the Tm atom, some of the water
was to investigate the effect of the interactions on the optical molecules may be involved in the coordination effect.
properties (Fig. 7). The maximum uorescence emission band
The uorescence properties of the L–Yb nanohybrid were
of the L–EtOH solution was observed at 450 nm when it was similar to those that occurred for L–Tm. With the increasing
3
+
excited at 360 nm. As was expected, when a small amount of concentration of Yb , the emission was gradual quenched.
3
+
ꢁ6
ꢁ1
ꢁ6
3+
Tm (6 ꢂ 10 mol L ) was introduced to synthesize the L–Tm When the concentration was 2 ꢂ 10 M (the ratio of L to Yb
18986 | RSC Adv., 2014, 4, 18981–18988
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was 500 : 1), there appeared an emission band centred at 467
nm, which red shied for about 17 nm compared to that of the
4. Conclusion
L–EtOH solution. When the concentration increased to 8 ꢂ In this work, the self-aggregation of the uorophore-triphenyl-
ꢁ5
3+
10
M (the ratio of L to Yb was 12.5 : 1), the emission band at amine (L) nanostructures was studied in different conditions,
4
67 nm disappeared, a new band centred at 515 nm appeared including different solvents, different acidities of the solution,
3
+
3+
and the uorescence intensity reached the minimum. Further- and under guide with the RE ions Tm and/or Yb . The results
more, the emission band and the uorescence intensity did not revealed that the controlled synthesis of self-assembled organic
change any more when the concentration continually increased. materials can be realized through changing the growth envi-
In terms of the mechanism, the driving forces for the ronment, which can also optimize the optical properties. This
formation of L–RE probably related to several kinds of forces, type of controlled synthesis method also opens up the possi-
such as the RE ions induced changing of the intermolecular bility of utilizing organic nanomaterials for a multitude of
interactions and the corresponding crystal growth process, applications in bio- and chemical sensing and nanoelectronics.
because X-ray powder diffraction (XRD) patterns of the three
samples (Fig. S5†) showed slight variations, such as the marked
parts. Based on the results, the possible conversion process was
Acknowledgements
illustrated in Scheme 2. In this process, when the RE ions were This work was supported by the NSFC (no: 21101001, 21271004,
introduced, the metal ions inuenced the intermolecular 51372003), the 211 project of AnHui University and the Natural
interactions of the L molecules and further changed the crystal Science Foundation of Anhui Province (1208085MB22).
growth mode of them. When the usage of the RE ions increased,
35
the trend of change increased to result in L–RE nanohybrids.
Notes and references
Moreover, different RE ions had a different inuencing model.
Thus, the crystal growth can be controlled by altering the
species of the RE ion, which resulted in crystals with a different
size and morphology. On the basis of the above analysis and
SEM results, the possible conversion process may be as follows.
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+
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