Alkyl-triphenylamine end-capped triazines with AIE and large two-photon absorption cross-sections for bioimaging

Article abstract of DOI:10.1039/c4tc00910j

In the present study, three new luminogens ATT-(1-3) based on 1,3,5-triazine and end-capped with multi-branched triphenylamine-containing alkyl chains have been synthesized and characterized. All the three dyes are nonemissive in solution but have a strong red fluorescent emission in the aggregate state. The two-photon absorption (2PA) cross sections measured by the open aperture Z-scan technique are determined to be as large as 2756, 4750 and 10003 GM for ATT-1, ATT-2 and ATT-3 in chloroform, respectively, showing a dramatic enhancement with an increasing number of donor branches. The relationship between their structures and properties on one- and two-photon absorption and aggregation-induced emission (AIE) is discussed, which can serve as a guideline for the development of a series of solid materials with larger two-photon cross sections and high fluorescence quantum yield. In addition, one- and two-photon fluorescence (2PF) microscopy images of HeLa cells incubated with these three dyes were obtained to demonstrate the potential applications of these fluorophores in biosensing and bioimaging.

Full text of DOI:10.1039/c4tc00910j

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Materials Chemistry C
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Alkyl-triphenylamine end-capped triazines with AIE
and large two-photon absorption cross-sections
for bioimaging†
Cite this: DOI: 10.1039/c4tc00910j
a
b
a
a
a
c
a
*
Yuting Gao, Yi Qu, Tao Jiang, Hao Zhang, Nannan He, Bo Li, Junchen Wu
a
*
and Jianli Hua
In the present study, three new luminogens ATT-(1–3) based on 1,3,5-triazine and end-capped with multi-
branched triphenylamine-containing alkyl chains have been synthesized and characterized. All the three
dyes are nonemissive in solution but have a strong red fluorescent emission in the aggregate state. The
two-photon absorption (2PA) cross sections measured by the open aperture Z-scan technique are
determined to be as large as 2756, 4750 and 10 003 GM for ATT-1, ATT-2 and ATT-3 in chloroform,
respectively, showing a dramatic enhancement with an increasing number of donor branches. The
relationship between their structures and properties on one- and two-photon absorption and
aggregation-induced emission (AIE) is discussed, which can serve as a guideline for the development of
a series of solid materials with larger two-photon cross sections and high fluorescence quantum yield. In
addition, one- and two-photon fluorescence (2PF) microscopy images of HeLa cells incubated with
these three dyes were obtained to demonstrate the potential applications of these fluorophores in
biosensing and bioimaging.
Received 3rd May 2014
Accepted 3rd June 2014
DOI: 10.1039/c4tc00910j
www.rsc.org/MaterialsC
synthesis of large s materials has become a signicant issue.3
The strategy for the design of molecules with large s has
Introduction
provided guidelines for the development of 2PA materials,
including dipolar, quadrupolar and octupolar molecules and
other archetypes.4 The accumulated knowledge and experience
has revealed that the value of s is correlated with intramolecular
charge transfer. Moreover, p systems with large conjugation
length and enforced coplanarity, the modication of donor–
acceptor groups and branching symmetry are efficient design
strategies for large s values.5 Currently, many 2PA materials
with multibranched triarylamine have been synthesized for
their good electron-donating and transporting capabilities,
which can enhance the 2PA performance owing to the increase
in p-electron density of the system.6
However, according to the strategy, many materials synthe-
sized with large 2PA cross-sections are in enforced coplanarity
structure and conformational rigidity. Most of these
compounds are hydrophobic, and their uorescence quantum
yields are considerably reduced in water on account of the
aggregation of molecules, which generally leads to uorescence
quenching.7 This shortcoming, which is known as aggregation-
caused quenching (ACQ), seriously limits their applications,
especially in uorescent chemosensors and bioimaging. Hence,
a special molecular design for 2PA materials is required not only
to ensure large two-photon activity, but more importantly, to
overcome uorescence quenching at high concentration. In
2001, Tang and his coworkers discovered a group of silole-based
anti-ACQ materials, called aggregation-induced emission (AIE)
Two-photon absorption (2PA) is a way of accessing a given
excited state by using photons of twice the wavelength of the
corresponding one-photon absorption, which increases with
the square of the light intensity. 2PA materials have various
superiorities, such as deep penetration, high three-dimensional
spatial selectivity and negligible background uorescence,
arousing considerable interest as promising materials for
applications in various areas, such as two-photon dynamic
therapy, three-dimensional optical data storage, up-converted
lasing, optical power limiting materials, two photon microscopy
and bioimaging.1 Nowadays, two-photon laser scanning uo-
rescence microscopes are commercially available and have
become common tools for biologists, which has generated an
imperative demand for new dyes with high 2PA cross-sections
(s) and a high uorescence output.2 As most conventional dyes
have only modest s and short emissive wavelength, the
aKey Laboratory for Advanced Materials, Institute of Fine Chemicals and Department
of Chemistry, East China University of Science and Technology, 130 Meilong Road,
Shanghai, 200237, China. E-mail: jlhua@ecust.edu.cn; jcwu@ecust.edu.cn; Fax:
+86-21-64250940; Tel: +86-21-64250940
bDepartment of Chemistry & Laboratory of Advanced Materials, Fudan University, 220
Handan Road, Shanghai 200433, China
cKey Laboratory of Polar Materials and Devices, Ministry of Education, East China
Normal University, Shanghai 200241, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00910j
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compounds.8 The AIE molecules are nonemissive when dis-
solved in suitable solvents but become highly luminescent
when aggregated in the solid state. They also developed a series
of AIE luminogens and identied the restriction of intra-
molecular rotation (RIR) and twisted intramolecular charge
transfer (TICT) in the aggregates as main causes for the AIE
effect through experimental and theoretical studies.9 Now AIE
luminogens have showed signicant academic value and
diverse promising applications in cell imaging,10 uorescent
sensors and bioprobes,11 and mechanouorochromic mate-
rials.12 As far as we know, most studies on the AIE uorophores
for bioapplications are focused on the TPE species with shorter
wavelength input and output signals.13 Consequently, it is still
very necessary to develop long wavelength emission AIE mate-
rials with large 2PA cross sections for bioapplications.
Previously, our group reported a 1,3,5-triazine-based 2PA
chromophore combined with AIE properties, TAPA-a, which was
fabricated into nanoparticles with PEG and used for cancer cell
imaging successfully.14 Although the energy of the IR light is
lower than that of the visible light, strong excitation can still
induce damages to the samples and bleaching of the dyes. Thus,
there is a strong need of dedicated uorophores with higher s to
keep the laser power as low as possible, in order to avoid too
high peak powers in the excitation volume.15 In the present
study, we have induced multiple alkyl chains into the large
system and synthesized polynuclear aromatic amine branched
compounds ATT-(1–3) with larger s and higher quantum yields.
Furthermore, sonication-assisted nanoparticle fabrication was
used in this study, and these materials can be fabricated into
Scheme 1 Synthetic routes to the ATT-based compounds.
(4, 5, 6) by the Horner–Wadsworth–Emmons reaction gave the
target compounds ATT-(1–3). All compounds were puried by
column chromatography or recrystallization and characterized
by 1H NMR, 13C NMR and mass spectra (shown in ESI†).
Photophysical properties
nanospheres without
a PEG coat. Different numbers of
branches attached on the 1,3,5-triazine were used to study the Fig. 1(a) shows the steady-state absorption and emission spectra
relationships between the structure and photophysical proper- of ATT-(1–3) in toluene at 298 K. All these chromophores exhibit
ties. As expected, an improvement of the 2PA cross-section was intense linear absorption in the UV/vis region with the lowest
observed for the three new structures, which involved the energy peaks located at 447, 450, 453 nm, respectively, which is
linkage of one-, two- or three-triphenylamine branch(es) slightly red-shied with the increasing number of branches.
through a common triazine core group. In particular, the star- The uorescence peaks are nearly identical at 594, 598, 602 nm
burst ATT-3 comprised a 1,3,5-triazine core and six electron- for ATT-(1–3), showing a large Stokes shi of 147, 148 and 149
donating alkyl-diphenylamines linked by p-conjugated bridges, nm, respectively. Similar behavior is observed for other
showing an extraordinarily large s of 10 031 GM, which is larger solvents, shown in Table 1. However, the extinction coefficient
than that of TAPA-a without long alkyls (8629 GM). Further is strictly proportional to the number of branches, increasing
study on the bioapplications showed that these three materials from 3.3 ꢀ 104 Mꢁ1 cmꢁ1 in ATT-1 to a doubled value of 6.1 ꢀ
were all suitable for cell imaging performed by both one- and 104 Mꢁ1 cmꢁ1 in ATT-2 and a tripled number of 1.0 ꢀ 105 Mꢁ1
two-photon laser scanning confocal microscopy.
cmꢁ1 for ATT-3 at peak wavelength in THF, as shown in
Fig. 1(b). This is independent of the number of branches in ATT-
(1–3); the lowest lying excitation is mainly localized on a specic
arm, which is demonstrated in our previous work.16 Interest-
ingly, the emission maximum (lem) of ATT-1 correspondingly
Results and discussion
Synthesis
The targeted compounds ATT-(1–3) were synthesized according shis from 594 to 675 and to 682 nm when the solvent polarity
to Scheme 1. The bromination of 2,4,6-tri(p-tolyl)-1,3,5-triazine is increased from toluene to THF and to DCM. A similar
afforded 2-(4-(bromomethyl)phenyl)-4,6-di-p-tolyl-1,3,5-triazine, phenomenon is observed in the solutions of ATT-2 and ATT-3
2,4-bis(4-(bromomethyl)phenyl)-6-(p-tolyl)-1,3,5-triazine
and (Table 1). In this case, the quantum yields (F) of emission of
2,4,6-tris(4-(bromomethyl)phenyl)-1,3,5-triazine, followed by ATT-(1–3) decrease with increasing solvent polarity. The cause
reaction with trimethyl phosphate, yielding triazine derivatives of the solvent effect is that the ATT-(1–3) derivatives comprise
4, 5 and 6. The important intermediate aldehyde 8 was triphenylamine donor (D) and triazine acceptor (A) units. Such
synthesized by an Ullmann reaction from triphenylamine D–A molecules oen show solvatochromic effects, which has
aldehyde iodide 7 and bis(4-tert-octylphenyl)amine. Finally, the been well explained by a twisted intramolecular charge transfer
condensation of aldehyde 8 with the respective triazine moiety (TICT) mechanism.17 An elevation of the HOMO level in the
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standard. It is shown in Fig. 1(c) that all the three compounds
exhibit the rst oxidation peak in the region of 0.69 V. The result
reveals that the electron density is located in a single arm,
consistent with the conclusion drawn from the absorption
measurement.
All the ATT-based derivatives are soluble in common organic
solvents such as THF, toluene and DCM but are insoluble in
water. Stable water dispersions of ATT-based nanoaggregates
were prepared by the precipitation method using THF as a
water-miscible solvent for dyes. Fig. 1(d) shows the normalized
one-photon absorption of ATT-(1–3) in THF and in dispersion of
the aggregate form (90% water) at 1 ꢀ 10ꢁ5 M. Their absorption
maxima (lmax) appeared at 441, 449 and 452 nm in THF,
respectively. The absorption of ATT-(1–3) in dispersion of the
aggregate form (90% water) was obviously broadened and their
lmax moved to 448, 458, 462 nm, which was bathochromically
shied by 7, 9, 10 nm, respectively. The absorption spectra of
ATT-(1–3) broadened, implying that the molecular stacking is
well ordered within the nanoaggregated structure. The absorp-
tion tails extending well into the long wavelength region further
indicated the aggregation of luminogen into particles in the
presence of water, as it is well known that the Mie effect of
particles causes such level-off tails in the absorption spectra.19
The corresponding emission spectrum of ATT-1 is shown in
Fig. 2(a) in aqueous THF with different water–THF ratios at 1 ꢀ
10ꢁ5 M. The emission from the THF solution of ATT-1 was so
weak that almost no photoluminescence (PL) signal was recor-
ded. However, an obvious enhancement of luminescence was
observed in the 40% (v/v) water–THF mixture, and the PL signal
gradually enhanced with increasing water–THF ratio. When the
water–THF ratio reached 90%, PL intensity increased to the
maximum. Since ATT-1 was insoluble in water, ATT-1 nano-
particles began to aggregate as the water fraction increased. The
aggregation of particles restricted the intramolecular rotation
(RIR) process, which facilitated the radiative decay of the exci-
tons, resulting in a strong increase in luminogen uorescence.
Similar behavior was observed for ATT-2 and ATT-3 in Fig. 2(b
and c), verifying that the three luminogens were AIE-active.
Moreover, the PL emissions of the luminogens were slightly
Fig. 1 (a) Normalized absorption and emission spectra of ATT-(1–3) in
toluene at room temperature at 1 ꢀ 10ꢁ5 M. (b) Absolute absorption
spectra of ATT-(1–3) in THF at 1 ꢀ 10ꢁ5 M. (c) Cyclic voltammogram of
ATT-(1–3) in DCM. (d) Normalized one-photon absorption of ATT-(1–
3) in THF (solid line) and in dispersion of the nanoaggregate form (90%
water) (dashed line) at 1 ꢀ 10ꢁ5 M.
TICT state narrows the band gap, thereby red-shiing the
emission spectrum. However, its emission intensity is weak-
ened because of the susceptibility of the TICT state to various
nonradiative quenching processes. Therefore, ATT-(1–3) shows
a relatively high F value in the nonpolar solvent toluene. In
addition, the electrochemical behavior of ATT-(1–3) was inves-
tigated by cyclic voltammetry using 0.1 M tetrabutylammonium
hexauorophosphate as the supporting electrolyte in dichloro-
methane solution with platinum button working electrodes, a
platinum wire counter electrode, and an SCE reference elec-
trode. The SCE reference electrode was calibrated using a
ferrocene/ferrocenium (Fc/Fc+) redox couple as an external
Table 1 Photophysical Properties of ATT-(1–3) at 298 K
labs (nm) (3 ꢀ 10ꢁ4 Mꢁ1 cmꢁ1
)
lem (nm)
QY, Fb
Emission decay (ns) (A1, A2)
Krad (108 sꢁ1
)
c
Solvent
ATT-1
ATT-2
ATT-3
Toluene
THF
447 (3.0)
441 (3.3)
443 (3.2)
448
450 (5.8)
449 (6.1)
447 (6.3)
458
453 (10.0)
452 (10.2)
453 (10.9)
462
594
675
682
592
598
676
693
596
602
678
695
599
0.79
3.1
0.090
<0.050
2.3, 9.7 (0.62, 0.38)
2.9
0.11
<0.050
2.6, 10.1 (0.45, 0.55)
2.8
0.11
<0.050
3.2, 11 (0.37, 0.63)
2.5
0.0081
0.0019
0.04
0.29
0.83
—
DCM
H2O–THFa
Toluene
THF
0.69
2.4
0.0091
0.0013
0.21
0.83
0.46
—
DCM
H2O–THFa
Toluene
THF
0.65
2.3
0.0073
0.0011
0.27
0.66
0.48
—
DCM
H2O–THFa
a
H2O–THF (9 : 1). b The PL quantum yield (F) was estimated with rhodamine B in ethanol as a standard. c Krad: radiative decay rate constant. The
rate constants for radiative decays can be estimated from the uorescence quantum yields and lifetimes according to ref. 18.
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can act as efficient nonradiative pathways for the excited states
to decay. The uorescence lifetime data for ATT-3 in the mixture
containing 90% water are also given, which were obtained by
tting the time-resolved uorescence curves based on the
following double-exponential function:20
Y ¼ A1eꢁs/s + A2eꢁs/s
1
2
Fitting based on this function gave better tting results than
that on the single-exponential function for the sample with
water. The molecules in the mixed solvent are suspected to exist
in two different environments or decay through two relaxation
pathways. The values of A1 and A2 represent the fractional
amount of molecules in two different environments. In 90%
water–THF mixture solution, the uorescence decay curve is
well-tted biexponentially with a decay time constant of ꢂ3.2 ns
(0.37) and a rather long decay time constant of ꢂ11 ns (0.63). It
is suggested that the nanosecond decay components are
attributed to the emission from the formation of a nano-
structure, which restricts the rotation of the molecule. Similar
behavior was observed for ATT-1 and ATT-2. Furthermore, the
percentage of the molecule having a long decay is increased by
increasing the number of branches, which can be contributing
to high quantum yields.
Two-photon absorption properties
Fig. 2 (a, b and c) Corresponding emission spectra of compound ATT-
(1–3) in THF–water mixtures with different water fractions at 1 ꢀ 10ꢁ5
M. Excitation wavelength: 450 nm. (d) Plot of I/I0 vs. water content of
the solvent mixture, where I0 is the PL intensity in pure THF solution. (e)
Photographs of ATT-(1–3) in the pure THF solution and in 90% water–
THF mixture captured under UV light.
2PA cross sections of ATT-(1–3) were determined by femto-
second open aperture Z-scan technique. Fig. 3(1a–c) shows the
open-aperture Z-scan data and 2PA coefficient obtained by data
tting. The s can be calculated by using the equation of s ¼
hnbN0, where N0 ¼ NAC is the number density of the absorption
centers, NA is the Avogadro constant and C represents the solute
molar concentration. The values of s for ATT-(1–3) are 2756,
4750 and 10 003 GM at wavelength of 800 nm, respectively. A
steeper increase in the s value (1 : 1.7 : 3.6) was observed with
the increasing number of donor branches, which extended the
conjugation of the molecular structure. It is well known that
increasing the number of conjugation paths or connecting
several linear paths to form a two-dimensional (2D) or three-
dimensional (3D) conguration has been theoretically and
experimentally shown to be able to increase 2PA responses.21 In
addition, the eight terminal octyl chains have improved the
electron-donating ability and solubility of ATT-3, which results
in a considerable increase in the value of 2PA cross sections
compared to compound TAPA-a.22 The two photon uorescence
spectrum of the three compounds under different laser inten-
sities is shown in Fig. 3(2a–c). The linear dependence of uo-
rescence intensity on the square of the excitation intensity as
shown in the inset conrms that 2PA is the main excitation
mechanism of the intense uorescence emission.
blue-shied with increasing water content, which could be
attributed to its parallel packing mode. To demonstrate the PL
intensity changes of the compounds more visually, we plotted
the changes in their PL peak intensities (I/I0) versus the water
content of the mixture as shown in Fig. 2(d). The PL intensity of
ATT-(1–3) in the THF solution was very low and almost
unchanged when the water ratio was increased to 40%, 20% and
20% (v/v), respectively, but it started swily increasing upon the
addition of water to 50%, 30% and 70% (v/v), respectively. When
the water–THF ratio reached 90%, they emitted a strong orange
luminescence with 50-, 232- and 346-fold increase in the I/I0
ratio, respectively. The AIE effect is enhanced by increasing the
number of branch arms. The uorescence behavior of ATT-(1–3)
is well visualized through uorescence images of solution and
aggregate dispersions as shown in Fig. 2(e).
To get insight into the possible mechanism of AIE, time-
correlated single-photon counting method (TCSPC) was used to
elucidate the uorescence decay of ATT-based compounds in
THF and in the THF–water mixture solution (v/v ¼ 1 : 9). As can
be seen from Table 1, the uorescence of ATT-3 in THF solution
decays very fast and is well-tted by single exponential decay
with emission lifetimes as short as ꢂ110 ps. Further, it can be
inferred that intramolecular vibrational and torsional motions
Under the excitation of 80 fs, 800 nm pulse, ATT-(1–3) in a
mixture of THF and water emit intense uorescence with the
peaks located at 611 nm, 614 nm and 625 nm [Fig. (3a–c)],
respectively. The two-photon excitation uorescence is slightly
red-shied compared with one-photon excitation on account of
reabsorption of partial emissive uorescence. The overlap
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Fig. 3 (1a–c) Open-aperture Z-scan trace of ATT-(1–3) (scattered circle experimental data, straight line theoretic fitted data). (2a–c) 2PF
intensities of ATT-(1–3) under different excitation power densities in toluene. Inset: 2PF intensity versus the square of the excitation power
density. (3a–c) Two-photon fluorescence emission spectra and 2PA emission images for ATT-(1–3) in solution (THF) and in dispersion of the
nanoaggregate form (90% water) at 1 ꢀ 10ꢁ5 M, excited at 800 nm. ATT-1: 1a–3a; ATT-2: 1b–3b; ATT-3: 1c–3c.
between one- and two-photon excitation uorescence indicates shown in Fig. 4, the mean diameter for these three dyes in water
that the emissions resulted from the same excited state, was approximately 123, 139 and 142 nm, respectively. Scanning
regardless of the different modes of excitation. It shall be noted electron microscopy (SEM) was also performed to study the
that the studied three chromophores have undergone two- morphology of the nanoparticles. The nanoparticles in the SEM
photon excitation studies at one wavelength (800 nm). As the images have an average diameter of around 100, 110 and 120
shown in Fig. 4, two-photon uorescence (2PF) was remarkably nm, respectively, which is slightly smaller than those in DLS.
intensied by aggregation in 90% water–THF mixture.
This is because of the shrinking of samples when transformed
into a dry state from solution.23
To demonstrate the applicability of ATT-(1–3) in cellular
imaging, the bioimaging experiments were carried out by
confocal laser scanning microscopy (CLSM) using HeLa cells as
an example. The HeLa cells were incubated with PBS solutions
(pH ¼ 7.4) of ATT-(1–3) (1 mg mLꢁ1) for 2 h at 37 ꢃC, as shown in
Fig. 5. Obviously, all three dyes can work well as uorescence
Preparation of ATT-(1–3) nanoparticles and cellular imaging
ATT-(1–3), with exceptional AIE and 2PA capacities, were
selected for one- and two-photon uorescence microscopy
imaging. The size distribution of the ATT-(1–3) nanoparticles
(NPs) was studied using dynamic light scattering (DLS). As
Fig. 4 Particle size distribution of ATT-1(a), ATT-2(b), ATT-3(c) in water studied via DLS and SEM (inserted images).
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Fig. 7 Cell viability values (%) estimated by MTT proliferation test at
different concentrations of ATT-(1–3). HeLa cells were cultured in the
presence of ATT-(1–3) at 37 ꢃC for 24 h, respectively.
emitters for bioimaging. ATT-(1–3), with exceptional AIE and
2PA capacities, were selected to undergo two-photon uores-
cence microscopy imaging, as shown in Fig. 6. The uorescent
signal from cytoplasm is clearly observed, indicating that both
ATT-(1–3) can serve as promising two-photon uorescent
Fig. 5 CLSM images of HeLa cells after incubation with ATT-1(1a–3a), biomaterials. In addition, the toxicity of ATT-(1–3) for HeLa cells
ATT-2(1b–3b) and ATT-3(1c–3c) NPs for 2 h at 37 ꢃC. Excitation at 488
nm. Fluorescence collection from 560 to 660 nm. The scale bar is 10 mm.
was measured using a standard MTT assay (Fig. 7). Aer HeLa
ꢃ
cells were cultured with ATT-(1–3) (80 mM) in PBS at 37 C for
24 h, the metabolic viability of HeLa cells were estimated to be
no less than 80%. Hence, ATT-(1–3) has good biocompatibility
with low cytotoxicity.
Conclusions
Both two-photon uorescence and aggregation-induced emis-
sion are powerful tools for biosensing and bioimaging. In this
study, we have synthesized three new triazine-based chromo-
phores with larger 2PA cross sections and bright AIE uores-
cence in water, containing different numbers of alkyl-
triphenylamine arms. As evidenced by TCSPC, intramolecular
vibrational and torsional motions can act as efficient non-
radiative pathways for the excited states to decay in the THF
solution, making emission lifetime very short. On the contrary,
uorescence decay is slowed down a lot due to the restriction of
intramolecular motions in the aggregate state. In addition, the
AIE effect is enhanced by increasing the number of branches.
More importantly, the two-photon absorption cross-sections of
ATT-(1–3) can reach as large as 2756, 4750 and 10 003 GM at
wavelength of 800 nm in chloroform, respectively, which shows
a great enhancement in the s value with the increasing number
of donor branches. Furthermore, one- and two-photon uores-
cence imaging was performed by confocal laser scanning
microscopy; ATT-3 showed the largest s value and is a potential
Fig. 6 Two-photon excited fluorescence imaging of HeLa cells after
1.5 h incubation with ATT-1(1a–c), ATT-2(2a–c) and ATT-3(3a–c) NPs
at 37 ꢃC. The images were recorded upon 800 nm excitation with a
560–660 nm band pass filter. (1a–3a) Two-photon excited fluores-
cence, (1b–3b) brightfield and (1c–3c) two-photon excited fluores-
cence/brightfield overlay. The scale bar is 20 mm.
material for bioapplications.
Experimental section
Materials and measurements
˚
Tetrahydrofuran (THF) was pre-dried over 4 A molecular
sieves and distilled under argon atmosphere from sodium
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benzophenone ketyl immediately prior to use. N,N-Dimethyl 7 h. The mixture was ltered, and the solvent was removed
formamide (DMF) and dichloromethane (DCM) were reuxed under vacuum. The residue was dissolved into trimethyl phos-
with calcium hydride and distilled before use. Starting mate- phite (10 mL) and reuxed for 9 h. The excessive trimethyl
rials 2,4,6-tri(p-tolyl)-1,3,5-triazine and 4-(diphenylamine)benz- phosphite was removed under vacuum. The residue was puri-
aldehyde were prepared according to published procedures.24 ed by column chromatography on silica (DCM) to afford the
All other chemicals were purchased from Aldrich and used as product as a white powder (2.3 g, yield: 53%). 1H NMR (CDCl3,
received without further purication.
400 MHz), d: 8.72 (d, J ¼ 7.9 Hz, 4H), 8.65 (d, J ¼ 7.1 Hz, 2H),
1H and 13C NMR spectra were recorded on a Bruker AM-400 7.51 (d, J ¼ 7.6 Hz, 4H), 7.37 (d, J ¼ 7.7 Hz, 2H), 3.71 (d, J ¼ 10.9
spectrometer using chloroform-d (CDCl3), tetrahydrofuran-d8 Hz, 12H), 3.30 (d, J ¼ 22.1 Hz, 4H), 2.48 (s, 3H). 13C NMR (CDCl3,
(THF-d8) or dimethylsulfoxide-d6 (DMSO-d6) as solvent and tet- 100 MHz), d: 171.48, 171.05, 143.16, 136.03, 135.93, 135.13,
ramethylsilane (d ¼ 0) as internal reference. The UV/vis spectra 133.37, 130.04, 129.20, 128.91, 53.09, 33.79, 32.42. MS (EI) (m/z):
were recorded on a Varian-Cary 500 spectrophotometer with 2 [(M + H)+] calcd for C28H31N3O6P2, 568.5; found: 568.1.
nm resolution at room temperature. The uorescence spectra
(4,40,400-(1,3,5-Triazine-2,4,6-triyl)tris(benzene-4,1-diyl))tris-
were taken on a Varian-Cary uorescence spectrophotometer. (methylene)triphosphonate (6). In a 100 mL round-bottom
Time-resolved uorescence measurements in this study were ask, 2,4,6-tri(p-tolyl)-1,3,5-triazine (3.51 g, 10 mol), NBS (5.34 g
performed using the Edinburgh OB 900-L time-correlated single 30 mmol) and BPO (0.3 g, 1.2 mmol) were dissolved into 50 mL
ꢃ
photon counting system (TCSPC). Emission was collected at chlorobenzene and heated at 110 C for 7 h. The mixture was
right angle with respect to the pump. Aer deconvolution from ltered and the solvent was removed under vacuum. The
the system response function, a temporal resolution of ꢂ30 ps residue was dissolved into trimethyl phosphite (10 mL) and
can be reliably obtained. The 2PA cross sections of ATT-(1–3) reuxed for 9 h. The excessive trimethyl phosphite was removed
were measured by femtosecond open-aperture Z-scan technique under vacuum. The residue was puried by column chroma-
according to a previously described method.24 Two-photon tography on silica (ethanol–DCM ¼ 1 : 10, v/v) to afford the
excited uorescence (2PF) was excited by fs pulses with different product as a white powder (5.3 g, yield: 78%). 1H NMR (CDCl3,
intensities at a wavelength of 800 nm. The repetition rate of the 400 MHz), d: 8.71 (d, J ¼ 8.0 Hz, 6H), 7.51 (m, 6H), 3.61 (d, J ¼
laser pulses was 250 kHz, and the pulse duration was 80 fs. The 10.8 Hz, 18H), 3.30 (d, J ¼ 22.0 Hz, 6H). 13C NMR (CDCl3, 100
measurement was performed with a xed scattering angle of MHz), d: 171.3, 136.3, 136.2, 135.0, 135.0, 130.1, 130.1, 129.3,
90ꢃ. The size of the nanoparticles was determined by an ALV- 129.2, 53.1, 53.0, 33.9, 32.5. HRMS (EI) (m/z): [M+] calcd for
5000 laser light scattering spectrometer (DLS). The SEM
micrographs were obtained on a JEOL JSM-6360 scanning
C
30H36N3O9P3, 675.1664; found: 675.1663.
4-[N,N-Bis(4-iodophenyl)amino]benzaldehyde (7). A modi-
electron microscope (SEM). The cell imaging experiments were ed version of a previously reported method was used.25 In a
carried out with an Olympus FV1000 laser scanning confocal 500 mL three-necked round-bottom ask, 4-(N,N-diphenyla-
microscope and a 60ꢀ oil immersion objective lens.
mino)benzaldehyde (14.00 g, 51.28 mmol), potassium iodide
(11.43 g, 68.85 mmol), acetic acid (210 mL) and water (20 mL)
were heated to 80 ꢃC. Aer stirring for 1 h, potassium iodate
Synthesis
(10.97 g, 51.26 mmol) was added, and the reaction was stirred at
ꢃ
(4-(4,6-Di-p-tolyl-1,3,5-triazin-2-yl)benzyl)dimethylphosphine 80 C for 4 h. The solution was allowed to cool; the solid was
oxide (4). In a 100 mL round-bottom ask, 2,4,6-tri(p-tolyl)-1,3,5- collected, washed with water and recrystallized from DCM–
triazine (3 g, 8.5 mmol), N-bromobutanimide (NBS) (1.5 g, 8 ethanol (1 : 5, v/v), giving the product as a yellow powder (20.05
mmol), and benzoyl peroxide (BPO) (0.3 g, 1.2 mmol) were g, yield: 75%). 1H NMR (CDCl3, 400 MHz), d: 9.89 (s, 1H), 7.75 (d,
dissolved into 50 mL of chlorobenzene and heated at 110 ꢃC for J ¼ 9.0 Hz, 2H), 7.67 (d, J ¼ 9.0 Hz, 4H), 7.09 (d, J ¼ 9.0 Hz, 2H),
7 h. The mixture was ltered, and the solvent was removed 6.93 (d, J ¼ 9.0 Hz, 4H).
under vacuum. The residue was dissolved into trimethyl phos-
4-(Bis(4-(bis(4-(2,2,3,3-tetramethylbutyl)phenyl)amino)phenyl)
phite (10 mL) and reuxed for 9 h. The excessive trimethyl amino)benzaldehyde (8). A modied version of a previously
phosphite was removed under vacuum. The residue was puri- reported method was used.26 In a 250 mL round-bottom ask, 4-
ed by column chromatography on silica (DCM) to afford the [N,N-di(4-iodophenyl) amino]benzaldehyde (3 g, 5.7 mmol) bis-
product as a white powder (2 g, yield: 55%). 1H NMR (CDCl3, 400 (4-(2,2,3,3-tetramethylbutyl)phenyl)amine (6 g, 17.1 mmol),
MHz), d: 8.72 (d, J ¼ 7.7 Hz, 2H), 8.65 (d, J ¼ 8.2 Hz, 4H), 7.50 potassium carbonate (6.76 g, 49.02 mmol), activated copper
(dd, J ¼ 8.4, J ¼ 2.4 Hz, 2H), 7.37 (d, J ¼ 8.0 Hz, 4H), 3.73 (d, J ¼ bronze (1.6 g, 25.7 mmol) and 18-crown-6 (0.11 g, 0.042 mmol)
3.3 Hz, 6H), 3.30 (d, J ¼ 22.1 Hz, 2H), 2.48 (s, 6H). 13C NMR were reuxed in 1,2-dichlorobenzene (100 mL) for 48 h under
(CDCl3, 100 MHz), d: 171.43, 170.98, 143.01, 133.52, 132.80, argon atmosphere. The mixture was ltered, and the solvent
129.87, 129.34, 129.18, 128.89, 128.24, 52.34, 33.73, 29.60. MS was removed under vacuum. The residue was puried by
(EI) (m/z): [(M + H)+] calcd for C26H26N3O3P, 460.2; found: 460.1. column chromatography on silica [petroleum ether (PE)–DCM
(((6-(p-Tolyl)-1,3,5-triazine-2,4-diyl)bis(4,1-phenylene))bis- ¼ 1 : 1, v/v] to afford the product as a yellow powder (2 g, yield:
(methylene))bis(dimethylphosphine oxide) (5). In a 100 mL 34%). 1H NMR (DMSO-d6, 400 MHz), d: 9.73 (s, 1H), 7.69 (d, J ¼
round-bottom ask, 2,4,6-tri(p-tolyl)-1,3,5-triazine (3 g, 8.5 8.9 Hz, 2H), 7.33 (d, J ¼ 8.6 Hz, 8H), 7.12 (d, J ¼ 8.9 Hz, 4H), 6.97
mmol), NBS (3 g, 16 mmol) and BPO (0.3 g, 1.2 mmol) were (d, J ¼ 8.6 Hz, 8H), 6.91 (d, J ¼ 8.8 Hz, 4H), 6.83 (d, J ¼ 8.8 Hz,
dissolved into 50 mL of chlorobenzene and heated at 110 ꢃC for 2H), 1.71 (s, 8H), 1.55 (s, 24H), 0.75 (s, 36H). 13C NMR (THF-d8,
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Journal of Materials Chemistry C
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126 MHz), d: 189.21, 154.12, 146.13, 145.62, 145.18, 140.37, was puried by column chromatography on silica (PE–DCM ¼
139.25, 131.29, 127.78, 127.62, 124.40, 124.09, 118.29, 57.52, 4 : 1, v/v) the product as a yellow powder. 1H NMR (THF-d8, 400
38.60, 32.74, 31.86, 31.62. HRMS (ESI) (m/z): [(M + H)+] calcd for MHz), d: 8.65 (d, J ¼ 8.4 Hz, 6H), 7.62 (d, J ¼ 8.4 Hz, 6H), 7.44–
C
75H97N3O, 1056.7632; found: 1056.6593.
7.34 (m, 6H), 7.26–7.01 (m, 30H), 6.97–6.75 (m, 54H), 1.63 (s,
(E)-N1-(4-(Bis(4-(2,2,3,3-tetramethylbutyl)phenyl)amino)- 24H), 1.30 (s, 72H), 0.66 (s, 108H). 13C NMR (THF-d8, 100 MHz),
phenyl)-N1-(4-(4-(4,6-di-p-tolyl-1,3,5-triazin-2-yl)styryl) phenyl)- d: 171.04, 145.20, 143.94, 143.79, 142.39, 141.62, 134.93, 130.58,
N4,N4-bis(4-(2,2,3,3-tetramethylbutyl)phenyl) benzene-1,4- 129.15, 127.61, 126.85, 126.20, 125.55, 124.12, 123.28, 121.56,
diamine (ATT-1). In a 100 mL round-bottom ask, 4 (150 mg, 56.86, 37.90, 32.10, 31.24, 31.02. MALDI-TOF: [M+] calcd for
0.32 mmol), 8 (400 mg, 0.38 mmol), potassium tert-butoxide
(230 mg, 1.8 mmol), 18-crown-6 (20 mg, 0.08 mmol) and 50 mL
dichloromethane were added under argon atmosphere. Aer
C249H306N12, 3466.4381; found: 3466.4153.
Preparation of AIE nanoparticles
ꢃ
stirring at 45 C for 6 h, the mixture was poured into distilled
A solution of ATT-(1–3) in THF (30 mL, 1 mg mLꢁ1) was injected
into 30 mL PBS buffer (pH ¼ 7.4) under ultrasonication. THF
water and extracted with dichloromethane and water. The
combined organic phases were dried over anhydrous MgSO4
and concentrated using a rotary evaporator. The residue was
puried by column chromatography on silica (PE–DCM ¼ 4 : 1,
v/v) the product as a yellow powder (160 mg, yield: 33%). 1H
NMR (THF-d8, 400 MHz), d: 8.63 (d, J ¼ 8.2 Hz, 2H), 8.56 (d, J ¼
8.0 Hz, 4H), 7.60 (d, J ¼ 7.9 Hz, 2H), 7.36 (d, J ¼ 7.8 Hz, 2H), 7.27
(d, J ¼ 7.8 Hz, 4H), 7.16 (d, J ¼ 8.2 Hz, 8H), 6.87 (dd, J ¼ 14.4 Hz,
7.9 Hz, 20H), 3.47 (s, 6H), 2.41 (s, 8H), 1.25 (s, 24H), 0.66 (s,
36H). 13C NMR (THF-d8, 100 MHz), d: 171.32, 171.04, 145.21,
143.98, 143.81, 142.89, 142.32, 141.72, 134.78, 133.75, 130.47,
129.10, 128.76, 127.67, 126.84, 126.15, 125.53, 124.12, 123.28,
121.63, 32.08, 31.22, 31.01, 24.55, 24.35, 24.15. HRMS (ESI)
(m/z): [M+] calcd for C99H116N6, 1389.9295; found: 1389.9337.
[2,4-Tris((4-(N,N-bis(4-(N,N-bis(4-tert-octylphenyl)amino))
phenylamine)(benzene-4,1-diyl)(ethene-2,1-diyl)(benzene-4,1-
diyl)))-6-p-toluene]-1,3,5-triazine (ATT-2). In a 100 mL round-
bottom ask, 5 (150 mg, 0.26 mmol), 8 (700 mg, 0.66 mmol),
potassium tert-butoxide (230 mg, 1.8 mmol), 18-crown-6 (20 mg,
0.08 mmol) and 50 mL dichloromethane were added under
ꢃ
was evaporated by N2 ow at 70 C, and the solution was fol-
lowed by ltration through a 0.2 mm lter.
Live cell imaging
HeLa cells were plated on 14 mm glass coverslips and allowed to
adhere for 12 h. The cells were washed with PBS and then
incubated ꢃwith ATT-(1–3), respectively, in PBS (pH 7.4) for 90
min at 37 C. Cell imaging was then carried out aer washing
the cells with PBS.
Acknowledgements
This work was supported by NSFC/China (21372082,
2116110444, 21172073 and 91233207) and the National Basic
Research 973 Program (2013CB733700 and 2013CB834701). We
are grateful to Wen Lv from Nanjing University of Posts and
Telecommunications for signicant help.
ꢃ
argon atmosphere. Further, aer stirring at 45 C for 6 h, the
Notes and references
mixture was poured into distilled water and extracted with DCM
and water. The combined organic phases were dried over anhy-
drous MgSO4 and concentrated using a rotary evaporator. The
residue was puried by column chromatography on silica (PE–
DCM ¼ 4 : 1, v/v) the product as a yellow powder (200 mg, yield:
23%). 1H NMR (THF-d8, 400 MHz), d: 8.66 (d, J ¼ 8.3 Hz, 4H), 8.59
(d, J ¼ 8.1 Hz, 2H), 7.63 (d, J ¼ 8.1 Hz, 4H), 7.38 (s, 4H), 7.29 (d, J ¼
8.1 Hz, 2H), 7.14 (t, J ¼ 20.9 Hz, 19H), 7.04–6.56 (m, 40H), 2.36 (s,
3H), 1.65 (s, 16H), 1.26 (s, 48H), 0.66 (s, 72H). 13C NMR (THF-d8,
100 MHz), d: 170.89, 170.62, 147.74, 144.77, 143.53, 143.37,
142.53, 141.90, 141.20, 134.34, 133.28, 130.03, 129.98, 129.93,
128.70, 128.41, 127.18, 126.44, 125.77, 125.12, 125.06, 123.69,
122.86, 121.15, 30.81, 30.60, 24.14, 23.94, 23.74, 23.54. HRMS (ESI)
(m/z): [M+] calcd for C174H211N9, 2427.6821; found: 2427.6984.
2,4,6-Tris[(4-(N,N-bis(4-(N,N-bis(4-tert-octylphenyl)amino))
phenylamine)(benzene-4,1-diyl)(ethene-2,1-diyl)(benzene-4,1-
diyl))]-1,3,5-triazine (ATT-3). In a 100 mL round-bottom ask, 6
(190 mg, 0.25 mmol), 8 (1.12 g, 1 mmol), potassium tert-bu-
toxide (336 mg, 3.0 mmol), 18-crown-6 (20 mg, 0.08 mmol) and
50 mL dichloromethane were added under argon atmosphere.
Aer stirring at 45 ꢃC for 6 h, the mixture was poured into
distilled water and extracted with dichloromethane and water.
The combined organic phases were dried over anhydrous
MgSO4 and concentrated using a rotary evaporator. The residue
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