Dendron-containing tetraphenylethylene compounds: Dependence of fluorescence and photocyclization reactivity on the dendron generation

Article abstract of DOI:10.1002/chem.201103675

The synthesis and characterization of four dendron-containing tetraphenylethylenes (TPEs), 1₁-1₄, were synthesized, along with a TPE compound that contained four OCH₂Ph groups (referred to as 1₀) for comparison.

Full text of DOI:10.1002/chem.201103675

DOI: 10.1002/chem.201103675
Dendron-Containing Tetraphenylethylene Compounds: Dependence of
Fluorescence and Photocyclization Reactivity on the Dendron Generation
Guangxi Huang,[a, b] Baode Ma,[a, b] Jianming Chen,[a, b] Qian Peng,[a] Guanxin Zhang,*[a]
Qinghua Fan,[a] and Deqing Zhang*[a]
Abstract: The synthesis and characteri-
zation of four dendron-containing tet-
raphenylethylenes (TPEs), 11–14, were
synthesized, along with a TPE com-
pound that contained four OCH2Ph
groups (referred to as 10) for compari-
son. Photophysical studies revealed
that the TPE core became emissive
after linking dendrons onto its periph-
ery. Moreover, the fluorescence intensi-
ty was significantly enhanced when
high-generation dendrons were linked
onto the TPE core; the fluorescence in-
tensity increased in the following
order: 11 <12 <13 <14. This phenomen-
on was tentatively attributed to an en-
hancement in the energy barrier for in-
ternal rotation and torsion of the TPE
core to which four dendrons were con-
nected. In addition, the photocycliza-
tion of the TPE core into the respec-
tive 9,10-diphenylphenanthrene was fa-
cilitated when high-generation den-
drons were linked to the TPE core.
Again, the photocycliztion reactivity
increased in the following order: 11 <
12 <13 <14. We found that the fluores-
cence and photocyclization reactivity
of TPE could be modulated by cova-
lent interactions with dendrons, and
such modulation was strongly depen-
dent on the dendron-generation.
Keywords: cyclization · dendrons ·
fluorescence · photochemistry · tet-
raphenylethylene
Introduction
for OLEDs (organic light-emitting diodes).[3] Recently, a
number of chemo-/biosensors have been reported that take
advantage of such abnormal fluorescence properties of TPE
compounds.[4,5]
Tetraphenylethylene (TPE) and its derivatives have been
heavily investigated, both experimentally and theoretically.[1]
It is well-known that structural relaxations, induced mainly
by phenyl and phenylvinyl rotational/torsional motions,[2]
are effective upon photoexcitation; such structural relaxa-
tions deactivate the excited states of TPE molecules, and
therefore TPE compounds are almost non-emissive in solu-
tion. Tang and co-workers, among others, have reported that
the fluorescence of TPE compounds becomes significantly
enhanced following aggregation.[2a–c] In fact, Li and co-work-
ers have reported the fluorescence enhancement of other
types of organic molecules (e.g. styryl-type molecules).[2d,e]
Based on fluorescence spectroscopy studies under various
conditions, it is assumed that aggregation largely inhibits the
internal rotation and torsion that are responsible for non-ra-
diation pathways. In fact, TPEs are strongly emissive in the
solid state and have been used as light-emitting materials
Apart from aggregation, in principle it is also possible to
inhibit the internal rotation and torsion by modifying the
TPE framework or by linking appropriate groups to the
TPE core. For instance, Fox and co-workers[2b] reported that
the TPE framework became more rigid and the torsion of
the phenyl ring was inhibited when the phenyl rings were
connected through shorter hydrocarbon tethers within the
cyclic tetraphenylethylenes; as a result, the fluorescence of
these cyclic tetraphenylethylenes was enhanced notably.
Vyas and Rathore[6a] and Wang and co-workers[6b] separately
described the restriction of internal rotation and torsion for
TPEs by increasing the steric hindrance within tetrakis(pen-
taphenylphenyl)ethylene and TPE oligomers. Moreover, the
inhibition of internal rotation and torsion by encapsulation
is also characteristic of green fluorescent protein.[7]
Dendrimers, as regularly branched, 3D architectures, have
received significant attentions over the past few decades.[8,9]
Various functional groups can be positioned either at the
cores of the dendrimers, in the branching units, and/or on
the exterior surfaces. Encapsulation of various fluorophores
with dendrons has long been reported. It is anticipated that
dendron-capped fluorophores would become more-emissive
because, at least, the collisional quenching of their exited
states can be suppressed to some extent, in particular upon
incremental increase of the generation number of the den-
dron. However, fluorescence enhancement has only been
observed for a few encapsulated fluorophores with den-
[a] G. Huang, B. Ma, J. Chen, Dr. Q. Peng, Dr. G. Zhang, Prof. Q. Fan,
Prof. D. Zhang
Beijing National Laboratory for Molecular Sciences
Organic Solids Laboratory, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: dqzhang@iccas.ac.cn
[b] G. Huang, B. Ma, J. Chen
Graduate School of Chinese Academy of Sciences
Beijing 100049 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103675.
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FULL PAPER
drons.[10] Interesting dendrimer effects on fluorescence be-
havior have been demonstrated in several systems.[11]
Herein, we report TPE compounds 11–14 (see Scheme 1),
which were capped with dendrons of different generation
numbers that were constructed by using benzyl ether. For
comparison, a TPE compound with four OCH2Ph groups
(10) is also reported. We aimed to investigate whether it was
possible to inhibit the internal torsion and rotation of the
TPE core, and thus activate its excited state, by linking four
dendrons of different generations onto the periphery of
TPE (Scheme 1). Our molecular design was based on the
following considerations: 1) compared to the TPE core,
benzyl-ether-based dendrons of high generation number are
bulky, and thus the energy barriers for internal structural
torsion and rotation for dendron-containing TPEs are ex-
pected to be higher than those for TPEs without substitu-
ents; 2) high-generation dendrons may adopt hemispherical
or even spherical 3D structures, which may “clip” the
phenyl rings of the TPE core to suppress the internal struc-
tural torsion and rotation; 3) encapsulation of the TPE core
by dendrons may avoid the collision of solvent molecules.
The fluorescence of these dendron-containing TPE com-
pounds is enhanced upon incremental increase of the den-
dron generation number (see below). Moreover, the photo-
cyclization reactivity of the TPE core was dependent on the
dendron generation number. These results revealed that the
excited-state behaviors of TPE could be modulated by link-
ing dendrons onto its periphery.
Scheme 1. Chemical structures and synthesis of compounds 10, 11, 12, 13,
and 14. 18-C-6=[18]crown-6.
Results and Discussion
Synthesis: The synthesis of compounds 10, 11, 12, 13, and 14
started from tetrakis(4-hydroxyphenyl)ethylene and the cor-
responding dendritic benzyl bromides, both of which were
prepared according to literature procedures.[12] The reactions
between tetrakis(4-hydroxyphenyl)ethylene and the corre-
sponding dendritic benzyl bromides were carried out in the
presence of K2CO3, KI, and [18]crown-6 in acetone
(Scheme 1). The dendron-containing TPE compounds were
obtained after chromatographic separation. Their chemical
structures and purities were confirmed by NMR spectrosco-
py, MS, and elemental analysis.
Figure 1. Emission spectra (lex =340 nm) of compounds 10, 11, 12, 13, and
14 at RT. The concentration of each compound was 1.0ꢁ10À5 molLÀ1 in
THF. Inset: photographs of solutions of compounds 11–14 in THF taken
under illumination from a UV lamp.
sion expected for TPE compounds. Fluorescence began to
emerge for compounds 11 and 12, but the intensities were
rather weak. Interestingly, compounds 13 and 14 exhibited
relatively strong fluorescence, with lmax =487 nm. Their fluo-
rescence intensities increased in the order: 10 <11 <12 <13 <
14. As shown in Figure 1 inset, the fluorescence of com-
pounds 13 and 14 were easily detectable by the naked-eye,
whilst compounds 11 and 12 were almost non-emissive under
UV-light irradiation. The corresponding fluorescence quan-
tum yields (Table 1) increased notably upon incremental in-
crease of the generation number for these dendron-contain-
ing TPE compounds. For instance, the fluorescence quantum
yield of compound 14 was 22 times higher than that of com-
pound 11 in THF. The fluorescence quantum yields of com-
pounds 11–14 were slightly dependent on solvent properties,
as shown in Table 1; for instance, that of compound 14
Absorption and fluorescence spectroscopy: The Supporting
Information, Figure S1 shows the absorption spectra of solu-
tions of compounds 10, 11, 12, 13, and 14 in THF. All of these
TPE compounds exhibited absorption peaks at 238, 276, and
330 nm. Compared to compound 10, the different absorption
intensities at 238 and 276 nm were due to the presence of
different benzene rings in the corresponding dendrons of
compounds 11–14. Thus, the electronic coupling between the
TPE core and the respective dendron within compounds 11–
14 can be neglected.
Figure 1 shows the fluorescence spectra of solutions of
compounds 10, 11, 12, 13, and 14 in THF. Compound 10 was
almost non-emissive, owing to the internal rotation and tor-
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G. Zhang, D. Zhang et al.
Table 1. Photophysical parameters of compounds 11–14.
11 12 13
330 330 332
14
328
labs [nm][b]
lem [nm][b]
t [ns][b]
487
487
487
487
0.087
0.085
0.222 (86.14%)
2.090 (13.86%)
0.033
0.069
0.071
0.654 (62.79%)
2.929 (37.21%)
0.093
0.195
0.193
Ff[a,b]
Ff[a,c]
Ff[a,d]
0.004
0.007
0.011
0.011
0.018
0.029
[a] The fluorescence quantum yield (Ff) was measured with 9,10-diphe-
nylanthracene (DPA) in EtOH (Ff =90%) as the standard. The absorb-
ance of each solution at 330 nm (the excitation wavelength) was adjusted
to be less than 0.05. [b] In THF. [c] In toluene. [d] In DMSO.
Figure 2. Stable conformation of compound 13 obtained by molecular dy-
namic (MD) simulations.
reached 0.195 in toluene. On the basis of the fluorescence-
decay data, the fluorescence lifetimes of compounds 11–14
were evaluated and shown in Table 1. Compounds 11 and 12
exhibited single lifetimes of 0.087 and 0.085 ns, respectively,
but two fluorescence lifetimes were obtained for compounds
13 and 14.[13] Importantly, the fluorescence lifetimes of com-
pounds 13 and 14 were much longer than those of com-
pounds 11 and 12. These photophysical data for these den-
dron-containing TPE compounds suggested that the fluores-
cence of the TPE core could be enhanced by linking den-
drons of high generation onto its periphery.
that the fluorescence of these TPE compounds increased
upon increasing the dendron generation number in the
order: 11 <12 <13 <14. Variation of the fluorescence quan-
tum yields of these TPE compounds in different solvents
may be due to the fact that the dendrons adopted different
conformations and, as a result, the TPE core was differently
encapsulated.
The fluorescence of compound 14 was enhanced further
through the addition of water to its solution in THF
(Figure 3). The fluorescence intensity of compound 14 in-
The influence of dendrons on the fluorescence properties
of the TPE core may be attributed to the following points:
1) encapsulation of TPE core by the dendrons may shield
the core from the solvent, and thus collisional quenching
may be eliminated; 2) the presence of dendrons may make
the TPE core more rigid and enhance the barrier for inter-
nal rotation and torsion, thereby inhibiting these non-radia-
tive pathways. The fluorescence spectra of compounds 11–14
were also recorded in other solvents, and the fluorescence
quantum yields varied accordingly (Table 1). For instance,
the Ff value of compound 14 increased from 0.093 in THF
to 0.195 and 0.193 in toluene and DMSO, respectively. Such
solvent-dependence implied that the fluorescence enhance-
ment of compounds 11–14 was not solely caused by inhibi-
tion of the collisional quenching by shielding the TPE core
with four dendrons. Therefore, the inhibition of internal ro-
tation and torsion owing to the presence of the dendrons
presumably also contributed to the fluorescence enhance-
ment for compounds 11–14. In fact, the structures of com-
pounds 11–14 were investigated with molecular-dynamics
(MD) simulations (see the Supporting Information). As an
example, Figure 2 shows the simulated structure of com-
pound 13. It was clear that the TPE core was tightly sur-
rounded by the third-generation dendrons. Thus, we expect-
ed that the internal rotation and torsion of the TPE core
would be significantly restricted. According to previous re-
ports, the TPE core would be expected to be even-more-
tightly encapsulated by dendrons of higher generation (e.g.
14), and thus the energy barriers for internal rotation and
torsion would be higher, thereby leading to the inhibition of
these internal motions and thus to fluorescence enhance-
ment of the TPE core. Accordingly, it was understandable
Figure 3. Fluorescence spectra of compound 14 (1.0ꢁ10À5 molLÀ1; lex
340 nm) in a THF/water mixture with various amounts of water.
=
creased gradually with increasing water content. This result
was different from the parent TPE compound and its deriva-
tives, for which the fluorescence intensities increased only
slightly when the water content was below a critical amount,
but then increased abruptly when the water content was
above the critical amount.[5a–b] For instance, the fluorescence
intensity of compound 14 at 444 nm was enhanced by 0.6,
3.6, and 7.3 times when the volume ratio of water in the so-
lution reached 20%, 60%, and 90%, respectively. However,
when the water volume ratio increased to 95%, the fluores-
cence intensity was slightly reduced. This result was proba-
bly due to the formation of aggregates with different mor-
phologies within which the intermolecular packing may be
changed.[6b,14] Moreover, the fluorescence maximum was
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blue-shifted along with the enhancement in fluorescence in-
tensity. Similar fluorescence enhancement and blue-shifts
were detected for compounds 11–13 (see the Supporting In-
formation, Figure S7). For compound 10, the fluorescence in-
tensity was only enhanced when the water content was
higher than 40%.
According to previous reports, compounds 10, 11, 12, 13,
and 14 were not water-soluble and thus the addition of water
to their solutions in THF would lead to aggregation. In fact,
solutions of compounds 11–14 became opaque when the
water content was higher than 30%. After the addition of
water, the solutions of compounds 11–14 were subjected to
DLS (dynamic light scattering) studies. For instance, parti-
cles with sizes of around 212.5 nm were detected for the so-
lution of compound 14 in THF with 95% water content (see
the Supporting Information, Figure S8). Previous studies[4,5]
indicated that the aggregation of TPE compounds could
induce fluorescence enhancement, which was typically at-
tributed to the restriction of internal rotation after aggrega-
tion. However, for compounds 13 and 14, we anticipated that
their aggregation would lead to interdendron interactions,
which would impose additional barriers to the internal rota-
tion and torsion of the TPE cores. The shifts in the emission
maxima may be due to the fact that the TPE core became
more twisted because of the interdendron interactions, in
particular for high-generation dendrons, and, as a result, the
p-conjugation within the TPE core was reduced. However,
for compound 10, the TPE core was not well-shielded by the
substituents, and thus the intermolecular interactions of the
benzene rings may affect the p-conjugation within the TPE
core, thus leading to variation of the emission maximum.
More studies on this property are underway.
Figure 4. a) Emission spectra and b) absorption spectra of compound 14
(1.0ꢁ10À5 molLÀ1 in THF) after UV-light irradiation (350 nm) for differ-
ent periods of time. Inset: photographs of solutions of compounds 14 and
24 in THF taken under illumination from a UV lamp.
Photocyclization reactions: The emission color of a solution
of compound 14 in THF gradually transformed from green
to blue on standing under ambient conditions for several
hours (Figure 4a, inset). In fact, the fluorescence spectrum
of compound 14 varied significantly upon UV-light (350 nm)
irradiation: the intensity of the fluorescence at around
487 nm became gradually weaker and a new emission at
around 387 nm emerged simultaneously (Figure 4a). The ab-
sorption spectrum of compound 14 also changed (Figure 4b):
the absorption at around 330 nm gradually decreased and
two weak absorption peaks at around 354 and 372 nm ap-
peared. Appealingly, these new fluorescence and absorption
spectra were rather similar to those of 9,10-diphenylphenan-
threne and its derivatives,[15,16] which indeed can be generat-
ed by the oxidative photocyclization of tetraphenylethy-
lenes.[15] Therefore, these results suggested that the TPE
core of compound 14 could be converted into its correspond-
ing 9,10-diphenylphenanthrene derivative (24) upon UV-
light irradiation, in the presence of oxygen (Scheme 2).[17]
Such a photochemical reaction has long been known, but it
typically requires long irradiation times for the transforma-
tion to be completed.[15]
Scheme 2. The photocyclization reactions and chemical structures of the
photocyclization products (2, 20, 21, 22, 23, and 24).
14. A signal at m/z 13518 was detected for the solution of
compound 14 after UV-light irradiation with exposure to air,
which corresponded to the molecular weight of the respec-
tive photocyclization product (24) plus K+. Moreover, the
1H NMR signal owing to the protons of the TPE core (d=
6.87 ppm) disappeared, and signals at d=6.77, 6.94, and
8.17 ppm emerged after UV irradiation for just 11 min (see
the Supporting Information, Figure S18). To confirm the as-
signment for compound 14 after UV irradiation, the
1H NMR spectrum of compound 1 (tetrakis(p-methoxylphe-
nyl)ethylene) was recorded after UV-light irradiation for
different periods of time (see the Supporting Information,
Figure S13). The signals attributed to the aromatic protons
of compound 1 (d=6.67 and 6.83 ppm) disappeared, and
1
Both MS and H NMR spectroscopic data were collected
to support the above photocyclization reaction of compound
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G. Zhang, D. Zhang et al.
new signals at d=6.82, 7.03, 7.18, 7.28, and 8.23 ppm were
observed after irradiation for 3 h. The signal assigned to
four methoxy groups (d=3.66 ppm) split into two signals at
d=3.72 and 3.99 ppm (see the Supporting Information, Fig-
ure S13). In fact, the 1H NMR spectrum of compound 1
after UV-light irradiation was almost identical to that of the
which contained high-generation dendrons, increased rapidly
and reached their maxima after UV-light irradiation for just
short periods of time. Based on these results, we concluded
that the photocyclization reactivity of the TPE core within
compounds 10, 11, 12, 13, and 14 could be modulated by the
dendrons on their peripheries; the TPE core became more-
photoreactive with increasing dendron-generation number,
and the photoreactivity increased in the order: 10 <11 <12 <
13 <14. This observation agreed with the 1H NMR data of
compounds 10, 11, 12, 13, and 14 (see the Supporting Informa-
tion, Figures S14–S18): the signal at d=6.90 ppm disap-
peared and new signals at around d=6.80, 7.00, and
8.20 ppm emerged more-quickly for compounds 13 and 14
after UV-light irradiation compared to compounds 10, 11,
and 12. This result was understood by considering that con-
necting high-generation dendrons to the TPE core signifi-
cantly restricts the internal rotation and torsion, thereby
rendering the excited state of TPE more-emissive and -reac-
tive. This photocyclization reaction is currently under fur-
ther investigation.
known compound
2
(3,6-dimethoxy-9,10-bis(4-methoxy-
phenyl)-phenanthrene).[15c] Thus, we concluded that com-
pound 1 was converted into compound 2 after UV-light irra-
1
diation for a certain period of time. The H NMR signal at
around d=8.20 ppm was used as a characteristic peak to
verify the formation of the photocyclization reaction prod-
uct. As shown in the Supporting Information, Figure S18,
1
new H NMR signals at d=6.77, 6.94, and 8.17 ppm (in par-
ticular) gradually appeared for the solution of compound 14
after UV-light irradiation. This 1H NMR data[18] provided
direct evidence for the transformation of the TPE core of
compound 14 into 9,10-diphenylphenanthrene.
Similar changes in the fluorescence and absorption spec-
tra were observed for compounds 10, 11, 12, and 13 after UV-
light irradiation (see the Supporting Information, Figur-
es S9–S12): 1) the fluorescence intensity at around 487 nm
decreased and a new emission at around 387 nm appeared
simultaneously, and 2) the absorption at around 330 nm
gradually weakened. These changes implied that the TPE
core of compound 14 had undergone photocyclization into
Conclusion
Four dendron-containing tetraphenylethylene (TPE) com-
pounds (11–14) were synthesized and investigated as well as
a TPE compound that contained four OCH2Ph groups (10)
for comparison. Photophysical studies were performed for
compounds 11–14, which revealed that the fluorescence of
the TPE core (which was non-emissive in solution) was
“switched on” after linking dendrons onto its periphery.
Moreover, the fluorescence intensity was significantly en-
hanced when dendrons of high generation number were
linked onto the TPE core; the fluorescence intensity in-
creased in the following order: 11 <12 <13 <14. Based on
previous studies, we presumed that this phenomenon was
caused by the enhancement of the energy barrier for inter-
nal rotation and torsion of the TPE core to which four den-
drons were connected. In addition, compared to compound
10, photocyclization of the TPE core into the respective
9,10-diphenylphenanthrene was facilitated for TPE com-
pounds with high-generation dendrons. Again, the photocy-
cliztion reactivity increased in the order: 11 <12 <13 <14.
Therefore, the fluorescence and photocyclization reactivity
of TPE could be modulated by covalent connection with
dendrons, and such modulation was strongly dependent on
the generation number of the dendrons.
1
9,10-diphenylphenanthrene. In addition, the H NMR spec-
tra of compounds 10, 11, 12, and 13 were also recorded after
UV-light irradiation with exposure to air (see the Support-
ing Information, Figures S14–S17). Their 1H NMR spectra
showed similar changes following UV-light irradiation: the
signals at d=6.90 ppm completely disappeared and new sig-
nals at around d=6.80, 7.00, and 8.20 ppm emerged. Hence,
the TPE cores of compounds 10, 11, 12 and 13 underwent
transformation into 9,10-diphenylphenanthrene after UV-
light irradiation for different periods.
Interestingly, the fluorescence intensity at 387 nm of com-
pounds 10, 11, 12, 13, and 14 (1.0ꢁ10À5 molLÀ1 in THF)
reached their maxima after UV-light irradiation for 62, 44,
50, 17, and 11 min, respectively (Figure 5). Clearly, the fluo-
rescence intensities at 387 nm of compounds 13 and 14,
Experimental Section
Physical measurements and instrumentation: Melting points were mea-
sured on a Shimadzu DTG-60. 1H NMR and 13C NMR spectra were re-
corded on a Bruker Avance 400 MHz spectrometer. MS spectra were de-
termined with BEFLEX III for TOF-MS. Elemental analysis was per-
formed on a Flash EA 1112 Elemental Analyzer. Absorption spectra
were recorded on a Hitachi (model U-3010) spectrophotometer. Fluores-
cence measurements were carried out on a Hitachi (model F-4500) spec-
Figure 5. Plot of the fluorescence intensity at 387 nm versus the UV-light
irradiation time (350 nm) for compounds 10, 11, 12, 13, and 14 (1.0ꢁ10À5
molLÀ1 in THF).
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Dendron-Containing Tetraphenylethylene Compounds
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trophotometer. Fluorescence decays were recorded by using an Edin-
burgh Analytical Instruments (FLS920) apparatus; the fluorescence life-
times were estimated by fitting the fluorescence decays. Dynamic light
scatting (DLS) was carried out on an ALV5000 Laser Light-Scattering
Instrument. Preparative HPLC was performed with a recycling prepara-
tive HPLC system (JAIGEL, LC-9101).
trated under reduced pressure and further purified by preparative HPLC
(CHCl3). Compound 14 was obtained as a white powder (0.11 g, 54.5%).
M.p. >3428C (decomp); 1H NMR (400 MHz, CDCl3, 258C, TMS): d=
7.51–7.07 (m, 320H), 6.87 (d, J=6.4 Hz, 8H), 6.69–6.50 (m, 120H), 6.50–
6.25 (m, 68H), 4.85 (s, 128H), 4.80–4.44 ppm (m, 120H); 13C NMR
(100 MHz, CDCl3, 258C, TMS): d: 160.2, 160.1, 157.1, 139.5, 139.3, 138.6,
137.2, 136.9, 132.7, 128.6, 128.0, 127.6, 114.1, 106.5, 101.7, 70.1, 70.0 ppm;
MS (MALDI-TOF): m/z calcd. for C894H764O124K: 13520.3 [M+K]+;
found: 13520; elemental analysis calcd (%) for C894H764O124: C 79.59,
H 5.71; found: C 79.16, H 5.77.
Photocyclization: A solution of the dendron-containing TPE compound
under investigation was placed into a quartz cell (path length: 1.0 cm) at
RT for UV-light irradiation. Irradiation was carried out by using a PLS-
SXE300UV xenon arc lamp (Beijing Trusttech Co. Ltd). Irradiation at
350 nm was achieved by using the appropriate light filters. The distance
between the center of the quartz cell and the lamp was 20.0 cm; at this
position, the light intensity without passing through the filters was
92 mWcmÀ2
.
Acknowledgements
Molecular-dynamic (MD) simulation: To find stable configurations of
these dendron-containing TPE compounds, MD simulations under or-
thogonal projections to latent structures force field (OPLSFF) were per-
formed. For compound 13, the total energy converged after 5 ns NPT
steps. All calculations were done with GROMACS.[19]
This research was financially supported by the NSFC, the Chinese Acad-
emy of Science, and the State Key Basic Research Program. This work
was also supported by the Sino–German joint project (TRR61).
Synthesis of compound 11: A mixture of tetrakis(4-hydroxyphenyl)ethy-
lene (32 mg, 80 mmol), G1-Br (153 mg, 0.4 mmol), KI (212 mg,
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1.28 mmol), K2CO3 (177 mg, 1.28 mmol), and
a catalytic amount of
[18]crown-6 was heated to reflux in acetone (6 mL) overnight. The prod-
uct was concentrated under reduced pressure and purified by column
chromatography on silica gel (100–200, petroleum ether/CH2Cl2, 1:2).
Compound 11 was obtained as a white powder (0.10 g, 77.9%). M.p.
>3338C (decomp); 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.49–
7.28 (m, 40H), 6.94 (d, J=6 Hz, 8H), 6.70 (d, J=7.6 Hz, 8H), 6.65 (s,
8H), 6.56 (s, 4H), 5.01 (s, 16H), 4.90 ppm (s, 8H); 13C NMR (100 MHz,
CDCl3, 258C, TMS): d=160.3, 157.2, 139.6, 138.6, 137.3, 136.9, 132.7,
128.7, 128.2, 127.7, 114.1, 106.6, 101.7, 70.3, 69.9 ppm; MS (MALDI-
TOF): m/z calcd. for C110H93O12: 1605.7 [M+H]+; found: 1605.7; elemen-
tal analysis calcd (%) for C110H92O12: C 82.27, H 5.77; found: C 81.85,
H 6.21.
Synthesis of compound 12: A mixture of tetrakis(4-hydroxyphenyl)ethy-
lene (16 mg, 40 mmol), G2-Br (161 mg, 0.2 mmol), KI (106 mg,
0.64 mmol), K2CO3 (88.3 mg, 0.64 mmol), and a catalytic amount of
[18]crown-6 was heated to reflux in acetone (6 mL) overnight. The prod-
uct was concentrated under reduced pressure and further purified by
column chromatography on silica gel (100–200, CH2Cl2/EtOAc, 40:1).
Compound 12 was obtained as a white powder (105 mg, 79.5%). M.p.
>3318C (decomp); 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.58–
7.26 (m, 80H), 6.92 (d, J=5.6 Hz, 8H), 6.77–6.58 (m, 32H), 6.57–6.41 (m,
12H), 4.98 (s, 32H), 4.92 (s, 16H), 4.86 ppm (s, 8H); 13C NMR
(100 MHz, CDCl3, 258C, TMS): d=160.3, 160.1, 157.1, 139.6, 139.4,
138.6, 137.2, 136.9, 132.8, 128.7, 128.1, 127.7, 114.1, 106.6, 106.5, 101.7,
70.2, 70.1, 69.8 ppm; MS (MALDI-TOF): m/z calcd. for C222H188O28Na:
3324.3 [M+Na]+; found: 3324.0; elemental analysis calcd (%) for
C222H188O28: C 80.71, H 5.74; found: C 80.37, H 5.89.
Synthesis of compound 13: A mixture of tetrakis(4-hydroxyphenyl)ethy-
lene (8 mg, 20 mmol), G3-Br (165 mg, 0.1 mmol), KI (53.1 mg,
0.32 mmol), K2CO3 (44.2 mg, 0.32 mmol), and a catalytic amount of
[18]crown-6 was heated to reflux in acetone (6 mL) overnight. The prod-
uct was concentrated under reduced pressure and further purified by
column chromatography on silica gel (100–200 mesh, CH2Cl2/EtOAc,
40:1). Compound 13 was obtained as a white powder (113 mg, 84.3%).
M.p. >3458C (decomp); 1H NMR (400 MHz, CDCl3, 258C, TMS): d=
7.42–7.20 (m, 160H), 6.89 (d, J=5.6 Hz, 8H), 6.68–6.54 (m, 64H), 6.54–
6.42 (m, 28H), 4.92 (s, 64H), 4.89–4.81 (m, 40H), 4.78 ppm (s, 16H);
13C NMR (100 MHz, CDCl3, 258C, TMS): d=160.2, 160.1, 157.1, 139.6,
139.3, 138.6, 137.2, 136.9, 132.7, 128.7, 128.1, 127.7, 114.1, 106.6, 106.5,
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6695.7 [M+H]+; found: 6695.0; elemental analysis calcd (%) for
C446H380O60: C 79.95, H 5.72; found: C 79.59, H 5.87.
Synthesis of compound 14: A mixture of tetrakis(4-hydroxyphenyl)ethy-
lene (6 mg, 15 mmol), G4-Br (251 mg, 75 mmol), KI (39.8 mg, 0.24 mmol),
K2CO3 (33.1 mg, 0.24 mmol), and a catalytic amount of [18]crown-6 was
heated to reflux in acetone (6 mL) overnight. The product was concen-
Chem. Eur. J. 2012, 18, 3886 – 3892
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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been directly from the air. In addition, compound 24 exhibited no
AIE properties as the addition of water to the solution in THF led
to a fluorescence decrease (see the Supporting Information, Fig-
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[18] 1H NMR signals that were attributed to protons at positions a and b
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of the phenyl protons of the dendrons.
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Received: November 22, 2011
Published online: February 23, 2012
3892
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Chem. Eur. J. 2012, 18, 3886 – 3892