Fusing three perylenebisimide branches and a truxene core into a star-shaped chromophore with strong two-photon excited fluorescence and high photostability

Article abstract of DOI:10.1039/c2cc31261a

A novel star-shaped chromophore, Tr-PBI, was constructed by fusing three perylenebisimide branches and a truxene core. Tr-PBI exhibits high photostability and excellent two-photon properties: the maximum of δ_TPA is 11000 GM at 990 nm and fluorescence quantum efficiency Φ is 0.40 in THF. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31261a

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Fusing three perylenebisimide branches and a truxene core into a
star-shaped chromophore with strong two-photon excited fluorescence
and high photostabilityw
a
a
a
a
b
b
Yuanpeng Xie, Xinfu Zhang, Yi Xiao,* Youdi Zhang, Fan Zhou, Jing Qi and
Junle Qu*b
Received 20th February 2012, Accepted 7th March 2012
DOI: 10.1039/c2cc31261a
A novel star-shaped chromophore, Tr–PBI, was constructed by
fusing three perylenebisimide branches and a truxene core.
Tr–PBI exhibits high photostability and excellent two-photon
properties: the maximum of dTPA is 11 000 GM at 990 nm and
fluorescence quantum efficiency U is 0.40 in THF.
the mechanism of the intermediate-state resonant enhancement
related to the decrease in detuning energy between incident
1
0c,11
photons and the lowest one-photon allowed state.
Despite
such high d values, the short-wavelength excitation will lead to
strong scattering and absorption in biological samples, and
3,7
thus, is not applicable for TPEF microscopy.
Except for bipolar, quadrupolar, and octupolar type compounds,
star-shaped or multi-branched molecules have been studied
extensively in the field of TPA, due to their some natural
advantages: (i) the density of TPA chromophores in a star-shaped
molecule is increased without causing aggregation, and more
Currently, intensive efforts are directed toward the development
of two-photon materials, due to their important applications in
1
photonics and biophotonics, including three-dimensional optical
2
data storage, two-photon-excited fluorescence (TPEF) micro-
3
scopy, optical power limiting, and photodynamic therapy, etc.
4
5
12
effective coupling channels could be provided; (ii) electronic
Especially, the more and more widespread TPEF microscopy
6
urges chemists to search for chromophores with strong TPEF.
coupling and vibronic coupling between branches are efficient for
13
increasing d. So in this work, we construct a novel star-shaped
chromophore, Tr–PBI, as shown in Scheme 1, by fusing three PBI
units and a truxene core. Truxene not only acts as the p-bridge to
connect the three branches, but also it is a p-donor owing to its
higher electron density compared with the electron-deficient
PBI, which means that within Tr–PBI, there exists certain
intramolecular charge redistribution, which is beneficial to
As is known, the intrinsic TPEF brightness is determined by
the product of the two-photon absorption (TPA) cross section d
and the fluorescence quantum yield F. Ideal TPEF chromophores
7
should have large values of dF to improve the sensitivity.
Unfortunately, there are only a very limited number of chromo-
8
phores that possess high d and high F simultaneously. In
addition, the lower photostability of some TPEF chromophores
is rather unsuitable for the newly emerging super-resolution
biological imaging, as the single-molecule localization techniques
1
4a
enhance TPA.
Different from most known chromophores
14
of flexible structures, the unique characteristics of Tr–PBI lie in
that any two neighboring subunits are connected to each other
by two bonds, or in other words, the linker is of ladder type and
has high rigidity; the torsion angles between the nearly planar
truxene core and completely planar PBI branches are negligible,
1b
required a strong laser for photoactivation and excitation.
Perylene bisimides (PBIs) are well known for their very high F
and outstanding photostability. Several groups systematically
9
4b,10
researched the TPA properties of PBIs and their derivative.
15
and so the whole framework of Tr–PBI is nearly planar.
As a result, in the 700–1000 nm spectrum window, PBIs’ d is too
small (100–300 GM). In the short wavelength region, their d
values dramatically increase. For example, at 620 nm, the model
We synthesized Tr–PBI through two different routes where a
perylene tetraester and a perylenebisimide (PBI1) were adopted as
starting materials, respectively (see Scheme 1). For both routes
there are two similar key steps in sequence: Suzuki coupling to link
the truxene to three perylene units by single bonds, and then,
photocyclization to accomplish the fusion to form two-bond
0
10a
compound, N,N -dibutyl PBI’s d is up to 5000 GM (1 GM =
ꢀ
50
4
cm s photon molecule ), which could be ascribed to
ꢀ1 ꢀ1
10
a
16
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, 2 Linggong Rd., Dalian 116012, China.
E-mail: xiaoyi@dlut.edu.cn; Fax: +86-411-84986252;
Tel: +86-411-84986251
Key Laboratory of Optoelectronic Devices and Systems of Ministry
of Education and Guangdong Province, Shenzhen University, Nanhai
Road 3688, Shenzhen, 518060, China. E-mail: jlqu@szu.edu.cn
ladders. Route 2 is more straightforward. However, the
advantage of route 1 lies in that it produces a versatile
intermediate, Tr–PBA, which can condense with various
amines to give the corresponding fused truxene–perylenebisimides
containing different N-substituents to realize specific functions,
e.g. solubilization, bioconjugation, and molecular recognition,
etc. Tr–PBI is readily dissolved in common solvents, such as
b
w Electronic supplementary information (ESI) available: All experi-
mental procedures and compound characterizations. See DOI:
ꢀ1
10.1039/c2cc31261a
THF (1.6 mg mL ) etc., because the six side chains in central
4
338 Chem. Commun., 2012, 48, 4338–4340
This journal is c The Royal Society of Chemistry 2012

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Fig. 2 Two-photon absorption spectra of PBI1 and Tr–PBI in THF.
Moreover, the emission spectrum of Tr–PBI loses the vibration
structures, which is quite different from PBI1’s mirror symmetry
of emission and absorption; this reveals certain charge-transfer
characteristics of Tr–PBI. The fluorescence quantum yield F of
Tr–PBI in THF is 0.40. Although this F value is remarkably
lower than that (100%) of PBI1, it is sufficiently high for practical
applications.
The two photon absorption (TPA) spectra (700–1000 nm) of
PBI1, and Tr–PBI in THF are plotted in Fig. 2. The TPA d
values are obtained by the method of two-photon-excited
fluorescence (TPEF) measurement technique (see ESIw). Limited
by the femtosecond laser excitation source, we cannot measure d
in the short wavelength spectral range below 700 nm. The PBI1’s
TPA spectrum shows that d values are close to zero in the range
of 800–1000 nm, but increase in shorter wavelengths; in the range
of 700–800 nm, there is a TPA peak with a moderate d of
Scheme 1 Synthesis of Tr–PBI: (a) Br
b) Pd(PPh , KOH, THF/H O, reflux, 16 h; (c) I
reflux, 6 h; (d) 4-methylbenzenesulfonic acid, 110 1C, 6 h; (e) 3-pentanamine,
butanol, reflux, 10 h; (f) Br , CHCl , K CO , reflux, 6 h.
2
, CH
2
Cl
2
, K
2 3
CO , rt, 2 h;
(
3
)
4
2
2
, toluene, sunlight,
530 GM at around 770 nm. Our TPA data of PBI1 collected by
this method are very similar to those of other PBI derivatives
2
3
2
3
10
previously measured by a femtosecond Z-scan technique. In
sharp contrast to PBI1’s inaction, Tr–PBI shows very large TPA
d values (2000–11 000 GM) across the whole 700–1000 spectral
window. The maximum of d is 11 000 GM at 990 nm. Since the
fluorescence quantum yield F of Tr–PBI is 0.40, the values of dF
will also be large, e.g. up to 4400 GM at 990 nm. To our
knowledge, the value of dF of Tr–PBI is fairly large compared
truxene efficiently increase solubility and prevent the planar
1
frameworks from aggregation.
7
The absorption and emission spectra of PBI1 and Tr–PBI
are shown in Fig. 1. Firstly, the absorption magnitudes of
Tr–PBI at different wavelengths in the visible range are far
from the three folds of PBI1’s, which indicates that three PBI
branches in Tr–PBI are no longer the single and independent
PBI chromophores, but have been integrated into a larger p
system. Secondly, Tr–PBI exhibits considerably broadened
absorption and emission bands compared with those of PBI;
though Tr–PBI has such a large conjugated system, the aborption
of Tr–PBI is shifted little bathochromically compared to that
of PBI1; several groups have observed similar phenomenons
8
,13c,18
to those of other chromophores that have been reported.
Also importantly, the Tr–PBI’s broad TPA spectrum indicates
that there exist multiple favored TPA transitions, and thus,
Tr–PBI has the capability to be efficiently excited at many
different wavelengths in the NIR region.
In contrast to PBI1, the significant enhancement of the TPA
for Tr–PBI could be attributed to its structural features: that is
a large conjugated system, overall molecular rigidity and
planarity. The structural features of Tr–PBI can be beneficial to
p-electronic delocalization and afford more effective coupling
9
based on the enlarged perylene bisimides at the bay-position.
19
channels to increase d. On the other hand, Cho et al. considered
that vibronic coupling, not electronic coupling, between branches
played an important role in the enhancement of TPA absorption
in the triphenylamine-based multibranched structure, because
the central amino group breaks the conjugation of the whole
1
3c
molecules.
However, Tr–PBI is entirely conjugated and
highly rigid, so vibronic coupling and electronic coupling
1
3
could exert a collaborative contribution to increase d.
The photostability of Tr–PBI was preliminarily evaluated
on a confocal fluorescence microscope by a comparison study
with three reference compounds including PBI1, BODIPY
(a boron dipyrromethene dye, shown in ESIw), and fluorescein.
Fig. 1 UV-vis absorption spectra (a) and fluorescence emission
spectra (b) of PBI1 and Tr–PBI in THF.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4338–4340 4339

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2
5% of its initial emission after irrdiation for 10 minutes;
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1
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1
1
00% intensity after 16 minutes. Thus the photostability of
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In summary, we developed a novel star-shaped chromophore,
Tr–PBI, by fusing three perylenebisimide branches and a truxene
core. Tr–PBI’s d is 11 000 GM at 990 nm and fluorescence
quantum efficiency F is 0.40 in THF; and so, the maximum value
of dF is high up to 4400 GM. In addition, photostability of
Tr–PBI is outstanding, even better than the perylenebisimide
precursor, because of its large, rigid, nearly planar conjugation
system and ladder-type framework. These excellent properties
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21
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We thank National Natural Science Foundation of China
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340 Chem. Commun., 2012, 48, 4338–4340
This journal is c The Royal Society of Chemistry 2012