Branching effect for aggregation-induced emission in fluorophores containing imine and triphenylamine structures

Article abstract of DOI:10.1039/c6nj01558a

Three new chromophores incorporating donor-π-donor-π-donor structural motifs and single, double and triple branched 4-(N,N′-dimethylamine) phenyl groups linked to triphenylamine through an imine π-bridge were synthesized. The photoluminescence properties

Full text of DOI:10.1039/c6nj01558a

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Journal Name
ARTICLE
Branching effect for aggregation-induced emission in
fluorophores containing imine and triphenylamine
structure
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a*
a
b
b
Chunchun. Li , Wen Yang , Weiqun. Zhou , Mengmeng. Zhang , Renyu. Xue , Mengying. Li ,
b
DOI: 10.1039/x0xx00000x
Zhongqin. Cheng
www.rsc.org/
Three new chromophores incorporating donor–π–donor-π-donor structural motifs and single, double and triple branched
4
-(N, N’-dimethylamine) phenyl groups linked to triphenylamine through an imine π-bridge were synthesized. The
photoluminescence properties of three chromophores were studied in solution and aggregated states. It was
demonstrated that aggregation-induced emission (AIE) effect enhanced with the increase of the branch number. The
mechanism of AIE for three compounds was explained by molecule stacking mode in crystal structures. The propeller
shaped non-planar molecular configuration of tri-branched triphenylamine inhibited face to face π-π stacking, which
provided favorable 3D structures to restrict intramolecular rotation and benefited AIE effects. TEM , DLS and fluorescent
2
microscope imaging revealed the nano-size of aggregate state in THF-H O mixture solution. Fluorescence imaging
experiments of tri-branched chromophore in living A549 cells proved its possibilities for the real-world applications.
steric interactions in the aggregate state. It has been realized
that rational RIR in the aggregate state of AIE-active
fluorophores closes the possible non-radiative decay channels
1
Introduction:
Usually, the structural design strategy of organic fluorophores
has been used to enlarge the extent of π-conjugation by
melding more and more aromatic rings together. The bigger
discotic plates can produce high fluorescence in the dilute
solutions, but fluorescence usually can be quenched in
concentrated solutions and solid state, which is named ACQ
and activated radiative decay channels and leads to the strong
fluorescence. The covalent bonds have also been used to
fasten the aryl rotors to internally set off the RIR process at the
[
13-16]
molecular level.
Some typical examples of the AIE systems
[17,18]
have been developed such as hexaphenylsilole (HPS)
which has a propeller shaped non-planar molecule.
,
(aggregation-caused fluorescence quenching) because of π-π
[
1-3]
Usually, the nonplanar triphenylamine (TPA) moiety has
been employed as an activator for aggregation-induced
emission in the construction of AIE-active luminogens
associated with its propeller-like structure and excellent
electron-donating character. The present contribution finds a
distinctive approach for multi-branched triphenylamine.
Indeed, the multi-branched strategy constitutes a relevant tool
to modulate aggregated state of AIE-active fluorophores
without drastic impacts on the energy position of the
stacking and other non-radiative pathways ,such as forming
excimers and exciplexes. The ACQ effect is generally restricted
for the real-world applications.
conventional organic materials are insoluble in water, the
materials of the ACQ effect cannot be used as sensors to
detect biological molecules in physiological buffers and as
probes to monitor ionic species in river water
effect is also a thorny obstacle to the fabrication of efficient
organic light-emitting diodes (OLEDs)
luminescent materials used as emitter layers in the light-
emitting devices must be fabricated in a solid film form.
The appearance of aggregation-induced emission (AIE)
and/or aggregation-induced enhanced emission (AIEE) effect
of fluorophore in the solution aggregated or in the solid state
have opened up a new way to fluorescence materials
[4-8]
For example, as
[
9,10]
.The ACQ
[
11]
,
because the
[19–23]
electronic transitions
. In this article, we showed that our
systems appeared as aggregation induced fluorescence
generators. The multi-branched construction was covalently
linked to triphenylamine centers to induce hindered rotation
and the desired emission enhancement. The fluorescence
enhancement increased in aggregated states with the increase
of multi-branched and the steric hindrance. Meanwhile, the
emission wavelength was red shift gradually.
[12, 34]
application
. The AIE or AIEE effect has been attributed to
the restricted intramolecular rotation (RIR) by intermolecular
a.
School of Chemistry & Chemical Engineering and Material Science, Soochow
University, 199 Ren’ai Road, Suzhou, People’s Republic of China, 215123
School of Biology and Basic Medical Sciences, Soochow University, 199 Ren’ai
b.
Road, Suzhou, People’s Republic of China, 215123
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[
24, 25]
0
9 program
. The vibrational frequency analysis was
2
Experimental section
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DOI: 10.1039/C6NJ01558A
carried out to ensure that the obtained geometries represent
minima on the potential energy surface.
2
.1 Materials and instruments
Triphenylamine, N, N-dimethyl-p-phenylenediamine, POCl3,
DMF, NaOH, CH Cl , THF, MgSO , CH COOH, DMSO, acetone,
2
2
4
3
CDCl and MTT were used as HPLC grade purchased from J&K 3. Result and Discussion
3
(
1
CHINA).IR spectra were obtained in KBr discs using a Nicolet
3
.1 Synthesis
The synthetic route was shown in Scheme 1.
- (diphenylamino) benzaldehyde(compound 1): 30ml dry
70SX FT-IR spectrometer. Elemental analyses were performed
1
on a Yannaco CHNSO Corder MT-3 analyzer. H NMR has been
recorded on INOVA 400 at 400.13MHz, with TMS as internal
standard using CDCl3 as deuterated solvents with chemical
shifts reported as ppm. Mass spectra were recorded on a
Finnigan MAT95 mass spectrometer (ESI ). The absorption and
fluorescence spectra were recorded on a CARY50 UV-VIS
4
DMF was ice-bathed and 29.5mL POCl was added dropwise.
3
After they were stirred for 40 minutes under 0°C, the
temperature was raised to room temperature. Then 2.5g
triphenylamine was added and the reaction mixture was
heated at 45°C and stirred for 14 h. After cooling down to
room temperature, the reaction mixture was poured into ice-
bath and neutralized with NaOH, then extracted with
chloroform. The combined organic layer was dried with
magnesium sulfate anhydrous, filtered. With the organic layer
concentrated, the white purified compound was obtained by
using a column chromatograph of silica gel with ethyl acetate–
+
spectrophotometer
spectrophotometer. Fluorescence quantum yields (Φ ) in pure
and
an
FLS920
fluorescence
f
THF and THF/water mixtures determined with anthracene as
the fluorescence reference in ethanol (Φ = 0.27). The
transmission electron microscope (TEM) images of the
aggregations were obtained by Tecnai G220.The fluorescence
microscope imaging of the nano-aggregations was obtained by
Leica DM2500M. The cell images were gathered with the
inverted fluorescence microscope (Olympus, IX71) and
processed with Nikon AY software. All the experiments were
carried out at room temperature.
petroleum ether mixture (1:10 v/v) as the eluent. Yield: 2.5g
1
(90%). H-NMR (DMSO, 400MHz) δ (ppm): 9.76(s, 1H), 7.71(d,
2
H), 7.41(t, 4H), 7.21(m, 6H), 6.88(d, 2H).
Bis (4-benzoyl) aniline (compound 2)
、Tris (4-benzoyl-yl)
amine (compound 3): The compound 2 and 3 were prepared
according to the general procedure for compound 1 by using
1
or 5.8 mL dry DMF, and 7.6 mL POCl ,and 0.98g bis (4-benzoyl-
yl) aniline. Compound 2: Yield: 2.2g (90%). H-NMR (DMSO,
2
.2. Single crystal X-ray diffraction
The single crystals of compounds were obtained by the slow
diffusion of their respective CH Cl /cyclohexane solutions for
5.5 mL dry DMF, and 19 mL POCl , and 2.0 g triphenylamine
3
2
2
3
several days at room temperature. Since the two crystals were
stable under ambient condition, the data collection was done
without any inert gas protection at room temperature on a
Bruker SMART APEX-II CCD area detector using graphite-
monochromated Mo Ka radiation (λ= 0.71073 Å). Data
reduction and integration, together with global unit cell
refinements were done by the INTEGRATE program of the
APEX2 software. Semi-empirical absorption corrections were
applied by using the SCALE program for area detector. The
structures were solved by direct methods and refined by the
full matrix least-squares methods on F2 using SHELX.
1
4
1
00 MHz) δ (ppm): 9.87(s, 2H), 7.84(d, 4H), 7.47(t, 2 H), 7.31(s,
H), 7.21(d, 2H), 7.16(d, 4H). Compound 3: Yield: 0.86g (90%).
1
H-NMR (DMSO, 400MHz) δ (ppm): 9.94(s, 3H), 7.91(s, 6H),
[26, 27]
7
.28(s, 6H)
Compound 4 : 20mmol 4- diphenyl--amino benzaldehyde and
0 mmol N, N- dimethyl-p-phenylenediamine were added to
.
2
flask, then 10 mL ethanol and 1 to 2 drops of glacial acetic acid.
The reaction mixture was then heated at 50°C and stirred for 7
h. After cooling down to room temperature and filtered, the
green powder was obtained, and recrystallized from ethanol as
yellow solid. Yield: 0.55g (70%). Anal.calc. formula:
C H N .(%): C: 82.86; H: 6.39; N: 10.74; Found: C H N .(%):
2
.3 Cell culture and fluorescence imaging
Adenocarcinoma lung cells (A549) were purchased from the
Shanghai Institute of Cell Biology. The cells were cultured in
Roswell Park Memorial Institute culture medium (RPMI-1640),
supplemented with 6% calf serum, penicillin (100 U∙mL ),
streptomycin (100×10 g∙mL ) and 2.5×10 mol∙L glutamine
at 37°C in a 5% CO incubator. The cells were cultured in a 15
mm diameter cell culture dish for 2 days. A549 cells were
incubated with the tri-branch dye at a final concentration of 50
μg∙mL for 30minutes, and washed with PBS buffer to remove
extracellular material. Then the dish was placed under an
inverted fluorescence microscope (Olympus, Cell'R) equipped
with an objective lens (Olympus, IX71) to get long-term images
at 450 nm (λ = 380 nm).
27
25
3
27 25 3
-1
-
C: 82.30; H: 6.40; N:10.83. IR: 1609cm (vC=N), 1168cm
1
1
-1
-1
-1
-1
,
1290cm , 1325cm (vC-N), 2794cm , 2840cm (vC-H, -CH3).
-1
H-NMR (400 MHz, DMSO) δ (ppm): 8.37(s, 1H), 7.30(t, J=10,
-6
-1
-4
-1
8
2
3
H), 7.16(d, J=10.81, 6H), 7.07(d, J=11.6, 6H), 6.76(d, J=11.6,
+ +
2
H), 2.98(s, 6H). HRMS calc. for (M + H ) : 391.2048, found:
92.2144
Compound 5 and compound 6: The compound 5 and
-
1
compound
6 were prepared according to the general
procedure for compound 4 by changing the ratio of bis(4-
benzoyl)aniline and N, N- dimethyl-p-phenylenediamine.
Compound 5: Yield: 0.7g (65%). Anal. calc. formula:
C H N .(%): C:79.65; H:6.57; N:12.89; Found: C H N .(%): C:
3
6
35
5
36 35 5
2
.4 Computational methods
-1
-1
8
1
0.44; H: 6.51; N: 13.04. IR: 1620cm (vC=N), 1169cm ,
286cm , 1310cm (vC-N), 2802cm , 2853cm (vC-H, -CH3).
The geometries for the compounds were optimized by density
functional theory (DFT) calculations using the Becke-3-
Lee−Yang−Parr (B3LYP) at the level of 6-31G (d) with Gaussian
-1
-1
-1
-1
1
H-NMR (400 MHz, DMSO): δ (ppm): 2.92(s, 12H), 6.76(d, J=7.5,
2
| J. Name., 201x, 00, 1-3
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4
2
H), 7.24(m, 11H), 7.40(t, J=8.0, 2H), 7.80(d, J=4.4, 4H), 8.56(s,
H). HRMS calc. for (M + H ) : 537.2892, found: 538.2968.
View Article Online
DOI: 10.1039/C6NJ01558A
+
+
3
.2. Photo-physical and AIE Properties
Compound 6: Yield: 0.76g (55%). Anal. calc. formula:
C H N .(%): C: 78.94; H: 6.43; N: 13.96; Found: C H N .(%):
C: 79.06; H: 6.59; N: 14.35. IR: 1618cm (vC=N), 1167cm ,
The absorption spectra of the three compounds in THF were
shown in Fig. 1((a)). From Fig.1, the low energy side of the
spectra was dominated by a broad and structure less band at
45
45
7
45 45 7
-
1
-1
-1
-1
-1
-1
1
281cm , 1316cm (vC-N), 2797cm , 2848cm (vC-H,-CH3);
1
greater than or equal to 380 nm and underwent a
H-NMR (400 MHz, DMSO) δ (ppm): 2.92(s, 18H), 6.75(d, J=8.4,
H), 7.17(d, J=8.4,6H), 7.25(d, J=8.4, 6H), 7.85(d, J=8.4, 6H),
bathochromic shift from 4, 5 and 6. The bathochromic shift of
the transition band of all compounds could be attributed to
the enhanced electronic coupling between the 4-(N,N’-
dimethylamine)phenylimine and triphenylamine, which
6
+
+
8
6
.6(s, 3H). HRMS calc. for (M + H ) : 683.3736, found:
84.3172.
resulted in the more extended π-conjugation length on the
N
[28, 29]
long axis of the molecule.
Single branch chromophores (4)
CHO
N
exhibited a second distinctive band near 300 nm, which should
be ascribed to electronic transitions centred on the
triphenylamine moiety. The absorption peak appeared in the
manner of shoulder for both double and triple chromophores
H2N
POCl3/DMF
C2H5OH/CH3COOH
N
N
1
4
N
CHO
CHO
POCl3/DMF
(5 and 6). The fluorescent emissions were displayed under a
POCl3/DMF
3
80 nm UV lamp and the fluorescence quantum yield was
N
N
2
3
lower than 1% (Table1). Stocks shift (Δλ) was between 83 and
95 nm, which was analyzed by the intramolecular charge
transition (ICT). The band red shift clearly levelled off when
considering the multi-branched compounds for both UV and
PL spectra. Fig. 1(b) showed the normalized absorption and
fluorescence spectra of double-branched compound (5) in
various solvents. Contrary to the absorption spectra, the
fluorescence bands underwent a pronounced bathochromic
shift with the increase of solvent polarity. This solvent induced
OHC
CHO
OHC
C
C
2
NH2
2
H
NH2
H
5
O
5
O
H
H
/
C
C
H
H
3
3
C
C
O
O
O
O
H
H
N
N
N
N
N
N
N
effect provided
intramolecular charge transfer) process
fluorescence singlet states.
a
clear evidence for [ TICT (twisted
N
N
30-32]
of the
N
N
5
N
6
Scheme 1 the synthetic route of branched compounds
(
a)
(b)
-
5
Fig. 1 (a) UV- spectra and fluorescent emissions of single- (4) double- (5) and triple-branched (6) triphenylamine derivatives in THF solution (1×10 M); (b) absorption
and normalised emission spectra of double branch compound (5) in various solvents
A
solvent-poor-solvent photoluminescence (PL) test, THF/water mixtures with high water contents because of its
which was commonly used for studying the AIE phenomenon, insolubility in water, and the luminescence of such systems
[33-35]
was performed to investigate the luminescent behavior was primarily attributed to molecule aggregation. The
of the purpose compounds. The molecules would aggregate in dispersion system of nano-aggregated was obtained by the
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[
36]
gradual addition of water into THF with different ratios. AIE properties was investigated by introduction of multi-branched
View Article Online
DOI: 10.1039/C6NJ01558A
[37]
effect (I/I0) of three derivatives was drawn in Fig.2 (a-c). The donor substituted into triphenylamine central. The
formation of aggregates can also be confirmed by the changes outstanding fluorescence enhancement in aggregation state
of UV−vis, PL spectra and fluorescence microscope imaging in was exhibited with increasing the branch numbers of the
THF/water mixtures with various water volume fractions (fw). donor. As shown in the Fig.2 (b), when the water fraction
Single-branched chromophore (compound 4) (Fig.2(a)) increased to 91%, double-branched triphenylamine
exhibited the strong absorption peaks at 395 nm and the weak (compound 5) exhibited emission enhancements of 25.1- fold
absorption peaks at 301 nm in pure THF solution. With an (Fig.2 (b) and Table 1) in THF/water mixture solutions,
increase of fw from 0% to 90%, the strong peaks decreased compared to that in pure THF (Fig.2(c)), and the fluorescence
and the weak peak increased. At the same time, in the solvent quantum yield (Φ ) was determined to be 5.1% using
f
mixtures, the levelled-off tails appeared in the visible region anthracene as a fluorescence reference (Φ = 0.27 in ethanol)
f
[38]
when fw was equal to 90%, indicating that the aggregates (Table 1) . When fw was equal to 91%, the fluorescence
were probably at nanoscale sizes. However, there were no microscope imaging of 5 in aggregated state was shown in the
levelled-off spectral tails in the long-wavelength region in inserted picture of Fig.2 (b). AIE effect enhancement of 5
UV−vis spectra with fw equal to 80% or less than. Single- should be due to their more twisted conformations caused by
branch chromophore (compound 4) in dilute THF solution (5× the increase of the substituent group on the phenyl ring.
-5
1
0 mol/L) emitted blue fluorescence at 483 nm (QY=0.79%) Fig.2(c) showed the AIE effect of tri-branched triphenylamine
(Table 1). The fluorescence intension decreased when the fw (compound 6). When fw was equal to 93%, 6 exhibited the
increased to 40%, and then increased as the content of water fluorescence enhancements of 157- fold (Fig.2(d) and Table 1)
increased. When fw was equal to 80%, the fluorescence of the compared to that in THF (Fig.2(c)), and the Φf were
[39, 40]
solution increased to a maximum value, the fluorescence determined to be 16.2% (Table 1)
strength was equivalent to that in pure THF (Fig.2 (d) and Table microscope imaging showing bright green fluorescence of 6
). In addition, the levelled-off tails appeared in the visible was inserted in Fig.2 (c) (fw = 93%). The strongest AIE effect of
. Fluorescence
1
region of UV spectra when fw was equal to 80%, indicating 6 in all three compounds confirmed further that steric
that the aggregates were probably at nanoscale sizes. Inserted hindrance was advantageous to the enhancement of the AIE
[
41]
picture in Fig.2 (a) was fluorescence microscope imaging of 4 effect.
when fw was equal to 80%, which indicated the appearance of
the aggregated fluorescence.
The influence of steric hindrance effects on the AIE
-
5
Fig. 2(a-c) The UV- spectra (left) and Emission spectra (right) of triphenylamine derivatives in THF–water mixtures (5×10 M) with different water fraction (fw).
inset: fluorescence microscopic picture of triphenylamine derivatives water under 365 nm irradiation). (d) AIE effect (I/I ) of triphenylamine derivatives.
(
0
4
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Table 1 The maximum absorption (λa, nm), DFT calculation results (nm), the maximum emission wavelength, (λem, nm), fluorescence
View Article Online
quantum yield (Φ , %) and AIE effect (I/I ) of three compounds in THF and THF/H O
DOI: 10.1039/C6NJ01558A
f
0
2
THF
2
THF-H O
a
λ
a,max (nm)
DFT
409
λ
em (nm)
Φ
f (%)
λ
em(nm)
H
2
O%
Φ
f (%)
I/I
0
1
2
3
395
483
0.79
0.21
0.11
480
80
91
95
0.81
5.1
1.2
25.1
157
412
413
446
454
495
508
503
514
16.2
a I/I : the proportion of Φ in THF to Φ in THF–water.
energies of HOMO and LUMO. Table1 showed the energy gaps
of the HOMO and LUMO orbital of DFT predicted and UV
absorption. In Table 1, the calculated energy gaps between the
HOMO and LUMO were close to the corresponding UV
absorption energies. The energy gap (HOMO to LUMO) were
0
f
f
3
.3 Theoretical studies
To better understand the spectral behavior, the fully optimized
structures of three compounds were carried out at the level of
B3LYP/6-31G (d). Molecular orbital amplitude plots of the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of three derivatives
were depicted in Fig.3. All three dyes showed quite similar
3
.03 eV (409nm), 2.78 (446 nm) and 2.73(454 nm) for single-
branched (4), double-branched (5) and triple-branched
compound (6) respectively. The energy gap decreased with
branch number enhancement, which corresponding to the
bathochromic shift in the UV spectra with branch number
enhancement. As shown in Fig. 3, the longest wavelength
electronic transition indicated an electronic delocalization all
along the compound structure (π–π* type) with a very slight
charge transfer from the amino group to the phenylimine
moieties.
structures since the triphenyl moiety globally present a D -like
3
symmetry with twisted angles around the bond connecting the
[42, 43]
central nitrogen atom to the phenylene rings.
Time-
dependent (TD) combining with a SCRF method (CPCM, by THF
as solvent) at the same level were applied to indicate the
Fig. 3 HOMO(bottom) and LUMO(top) orbital from left to right, single, double and triple branch compounds computed by the B3LYP/6-31G(d) level of the theory.
3
.4 Crystal structures and mechanisms of AIE
395, 412 and 413 nm and the maximum emission wavelength
was 483, 495 and 508 nm for single-, double- and tri-branched
triphenylamine in THF respectively. X-ray molecular structure
and crystallographic packing of single branch compound (4,
CCDC number: 1495312) were depicted in Fig.4 (a) and Fig.4
In order to better understand the relationship between the
photophysical properties and the molecular packing, the single
crystal of three triphenylamine derivatives were prepared
through slow evaporation of acetonitrile solution at room
[
44,
45]
(b). The triphenyl moiety globally present a D -like symmetry
3
temperature.
X-ray molecular structures and
with twisted angles around the bond connecting the central
nitrogen atom to the phenylene rings, which agreed with the
result of DFT calculations. The angle between two phenyl rings
was 82.6°, and the angles between two phenyl ring and 4-N,
N’-dimethylphenylimido were 62.6° and 64.1°respectively. In
crystallographic packing (Fig. 4(b)), on the one side, adjacent
molecules were stacked by C22–H22/C11 weak interactions (d
crystallographic packing of three chromophores were depicted
in Fig.4-6. As shown in Fig.4-6, all three compounds showed
quite
similar
structures
since
the
4-(N,N’-
imethylamine)phenylimine branches adopted a quasi-planar
conformation, the angles between the benzylidene and 4-N,N’-
dimethylphenylimido were 56.29° (single branched), 9.2°
(
double branched) and 21.3°, 33.0° and 35.0°(triple branched),
which indicated the conjugation between the benzylidene and
-N,N’-dimethylphenylimido with the increased of branches. It
=2.875Å), which led to the weak π–π stacking interaction and
the distance between two planes (C6C7C8C9C10C11) of the
adjacent molecules was 3.333 Å. The face-to-face stacking
caused the quenched emission in the crystalline aggregated
state. On the other side, another weak interactions C4–H4/N2
4
led to the gradually bathochromis-shifts from single-, double-
and tri-branched triphenylamine of both absorption and
emission bands. The maximum absorption wavelength was
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(
2.643 Å) between adjacent molecules compelled a J stacked aggregated state. So, the fluorescence strength inVieawgAgrtricelegOantleinde
model and the J-type aggregate equally resulted in the state was equivalent to that in pure THF. DOI: 10.1039/C6NJ01558A
increase of the fluorescence intensity in the crystalline
(a)
(b)
Fig. 4 X-ray molecular structure (a) and crystallographic packing (b) of single branch compound (4)
The molecule of 5(CCDC number: 1495311) had a C2V H22/C11 weak interactions (d =2.867Å). The aggregation of 5
symmetry (Fig .5(a)). The angles between phenyl ring and two did not involve π–π interactions. Every two molecules in 5
4
-N, N’-dimethylphenylimido were 63.2° and the angles form a dimer and pack in a parallel but staggered style, which
[
46, 47]
between two 4-N, N’-dimethylphenylimido were 80.7°. The inhibited the π–π interactions in crystal structure.
Thus,
symmetry structures were beneficial to co-plane between two the stacking manner not only avoided the maximum face-to-
imine branches of adjacent molecules, which were the main face π–π stacking that caused the quenched emission, but also
driving force for the formation of symmetric dimers, which decreased the vibration of molecular framework, which make
[
48]
also locked the intramolecular rotations of the phenyls (Fig. for the increase of AIE effect.
(b)). The adjacent molecules were stacked through C22–
5
(a)
(b)
Fig. 5 X-ray molecular structure (a), crystallographic packing (b) of double branch compound (5)
Fig. 6 depicted crystal structure and crystallographic dimethylphenylimido exhibiting more twisting structure of 6
packing of the 6. In Fig.6 (a), 6 present a propeller shaped non- than that in 5. And, one branch, 4-N, N’-dimethylphenylimido
planar asymmetry molecular structure due to the propeller- chain of one molecular inserted between two 4-N, N’-
shaped triphenylamine unit. The angles between three 4-N, N’- dimethylphenylimido chain in another molecule reduce the
dimethylphenylimido were 62.1°, 78.8° and 76.2 °. In the conjugation of molecules and prevent face-to-face π–π
[
32, 49]
crystal of 6 (Fig.6 (b)), the dimers between two imine branches stacking in aggregations.
Therefore, the vibration of
of adjacent molecules in double branch molecule could not be molecular framework was also decreased and AIE increased.
found. On the other hand, the torsion angle between 4-N, N’-
(a)
(b)
Fig. 6 X-ray molecular structure (a), crystallographic packing (b) of triple branch compound (6)
6
| J. Name., 201x, 00, 1-3
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ARTICLE
3
.5 Nanoparticles fabrication and cell imaging
enhancement. The molecules of compounds can slowly
assemble in an ordered fashion to form microcrystalline but
less-emissive particles in the THF–water mixtures with low
water contents, while in mixtures containing very large
amounts of water, the molecules of compounds abruptly
agglomerate to form less-crystalline or even amorphous but
more-emissive clusters, which effectively avoid π-π stacking.
In order to further gain insight into the AIE mechanism of
the three compounds, the size and growth process of particles
with different water fractions were studied using TEM and.
DLS. Fig.7 showed the TEM images of triphenylamine
chromophores. As shown in Fig. 7, all nanoparticles were in
spherical shape with an average size of around 150 nm, 350
nm and 200 nm respectively for single-, double- and triple-
branched chromophores. As can be seen from the data in table
[50-52 ]
2
, it was at close proximity with TEM results while the water
content increased to 80%-90%. And this result gives direct
evidence of molecular aggregation during emission
4
5
6
-
5
Fig. 7 TEM images of compound 4, 5 and 6 in THF–water mixtures at concentrations of 5×10 M with different water fractions (4 in THF–water (2/8, v/v); 5, 6 in THF–
water (1/9, v/v)).
1
1
1
4
2
0
8
6
4
2
0
0
200
400
600
800
1000
Particle size(nm)
(
a )
(b)
(c)
-
1
Fig.8 Fluorescence images of A549 treated with 50 μg∙mL triple branch compound (6) under visible light (a) and UV irradiation (b) and DLS data for tri-branched
chromophore in the cell culture fluid(c).
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Table 2 The average diameter of aggregates with different water fractions (fw ) of compound 4-6
View Article Online
DOI: 10.1039/C6NJ01558A
4
5
6
dfw=40%(nm)
dfw=60%(nm)
dfw=80%(nm)
dfw=90%(nm)
105
140
152
110
160
210
320
357
143
146
174
199
We furthered to study the potential applications of three compounds in THF-H O mixture solutions was shown by TEM
2
triphenylamine chromophores in bio-imaging. Compound 6 and DLS which indicated the aggregation of three
was selected to investigate the applicability in fixed cell chromophores. We employed the X-ray single crystal
imaging for 6 having the highest fluorescence quantum among structures of three compounds and theoretical calculations to
the three dyes. Living cell imaging of A549 was obtained at 450 reveal the AIE mechanism. In the crystal packing, we found
nm (λ=380 nm) by inverted fluorescence microscope. The cells that the inhibited face to face π-π stacking and favorable 3D
displayed a bright green fluorescence image. The fluorescence structures would restrict intramolecular rotation and make AIE
images were depicted in Fig.8(a) and Fig8.(b). The fluorescence effect enhance with the increase of branch number.
and bright-field images revealed a high cell membrane Fluorescent microscope imaging also indicated AIE effect.
permeability of 6.
Furthermore, fluorescence imaging of the 6 in living A549 cells
To confirm the formation of aggregation inside the cells, provided evidence of its potential for practical applications in
small amount of 6 was added to the cell culture fluid to biological systems.
simulate the cell environment, and dynamic light scattering
(
DLS) measurement was conducted. As it was shown in Fig.8(c),
Acknowledgments
the average diameter of the particle was 192.3nm, which
indicated the formation of aggregation inside the cells. 6 could
probably permeate the cell membrane in the solution state,
This work was supported by the National Natural Science
Foundation of China (No. 51079094) and the Natural Science
Foundation of Jiangsu Province (No. BK2010215). The Project
was funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
[53-55]
and aggregate inside the cell.
As for intracellular imaging applications, the cytotoxicity
of the probe should be taken into consideration. The MTT
assay was performed to evaluate the toxicity of 6 by A549 cells.
8
-16-24 h incubation was taken at the same condition. The
experimental data were expressed in Table 3. Compound 6 Notes and references
exhibited a little cytotoxicity for cells, and at the concentration
of 20 µM, the cell viability was still more than 83%, which
indicated a good biocompatibility of 6.
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New Journal of Chemistry
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DOI: 10.1039/C6NJ01558A
Sentence:
Three compounds linked to triphenylamine and single, double triple branched
ꢀ(N,N’ꢀdimethylamine) phenyl groups through an imine πꢀbridge were synthesized,
4
showing effective AIE effect and good fluorescence imaging.
Image
properties
N
CHO
N
2
H N
3
POCl /DMF
2 5 3
C H OH/CH COOH
N
N
1
4
N
CHO
CHO
POCl
3
/DMF
3
POCl /DMF
N
3
N
2
OHC
CHO
OHC
NH
2
NH
2
N
N
N
N
N
N
N
N
N
N
N
5
N
6