An aggregation-induced emission luminophore with multi-stimuli single- and two-photon fluorescence switching and large two-photon absorption cross section

Article abstract of DOI:10.1039/c2cc36806d

A novel aggregation- and crystallization-induced emission luminophore (ENPOMe) containing tetraphenylethene and acrylonitrile moieties with high fluorescence efficiency (Φ_F of up to 0.85) has been easily synthesized. ENPOMe has an exceptionally

Full text of DOI:10.1039/c2cc36806d

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An aggregation-induced emission luminophore with
multi-stimuli single- and two-photon fluorescence
switching and large two-photon absorption cross section†
Cite this: Chem. Commun.,
2
013, 49, 273
Received 18th September 2012,
Accepted 9th November 2012
a
b
a
c
a
c
Bingjia Xu,z Mingyuan Xie,z Jiajun He, Bin Xu, Zhenguo Chi,* Wenjing Tian,
a
b
a
a
d
a
Long Jiang, Fuli Zhao,* Siwei Liu, Yi Zhang, Zhizhan Xu and Jiarui Xu
DOI: 10.1039/c2cc36806d
www.rsc.org/chemcomm
A novel aggregation- and crystallization-induced emission lumino-
phore (ENPOMe) containing tetraphenylethene and acrylonitrile
F
moieties with high fluorescence efficiency (U of up to 0.85) has
been easily synthesized. ENPOMe has an exceptionally large two-
photon absorption cross section (r) of 5548 GM, and exhibits
striking multi-stimuli-responsive single- and two-photon fluores-
cence switching with excellent reversibility in the solid state.
Scheme 1 Chemical structure of ENPOMe.
These advantages include highly spatially confined excitation,
intrinsic three-dimensional (3-D) resolution, and negligible back-
Stimuli-responsive fluorescence switching, which can be achieved ground fluorescence. These properties are extremely important in
by controlling the structure of molecular assemblies with external numerous applications such as 3-D optical data storage, sensing,
7
stimuli, has attracted considerable interest, because of its significant and imaging. However, creating such stimuli-responsive fluores-
applications in the fields of memory devices, photomodulation, cence-switching materials, which combine excellent reversibility,
1
information displays, and sensors. However, like many common high two-photon fluorescence (TPF), and large TPA cross section, is
organic luminescent materials, fluorescence-switching dyes exhibit challenging.
fluorescence quenching at high concentrations or in the solid state.
In this study, we try to develop a novel two-photon excited
This phenomenon is notoriously known as aggregation-caused fluorescence-switching compound with AIE and CIEE proper-
2
quenching (ACQ).
ties (ENPOMe, Scheme 1). Since molecules with a D–A–D
3
Several anti-ACQ materials were reported by Tang et al. and structure will have higher s, an electron acceptor (cyano group)
Park et al. in 2001 and 2002, respectively. These substances are was introduced into the molecule between tetraphenylethene
called aggregation-induced emission (AIE) or crystallization-induced and methylphenate. Furthermore, aromatic-substituted acrylo-
emission enhancement (CIEE) compounds. Recently, several AIE nitrile derivatives and tetraphenylethene derivatives have been
compounds were reported to have fluorescence switching proper- proved to be AIE active, respectively. We believe the molecule
4
8
5
4,9
10
6
ties. Fluorescence-switching materials with two-photon absorption containing tetraphenylethene and aromatic-substituted acrylonitrile
(
TPA) are more attractive than conventional single-photon moieties will possess strong TPF emission.
absorption switching dyes because of their unique advantages.
Aggregation-induced emission: the ENPOMe molecules were
genuinely dissolved in tetrahydrofuran (THF) and almost no
signal was detected from the dilute ENPOMe solution in THF,
as shown in Fig. S1 (ESI†). Fluorescence intensity remained very
low in aqueous mixtures with water fraction less than 80%, but
increased swiftly with the water fraction above 80%. Compared
with the pure THF dilute solution, the fluorescence intensity
enhancement of the water–THF mixture with 98% water fraction
reached 192-fold. UV-visible spectra of ENPOMe (Fig. S2, ESI†)
corresponded well to the fluorescence spectral changes and
suggested nanoparticle formation in the mixture solvent. The
results indicate that the emission intensity of ENPOMe is
enhanced by nanoaggregation and thus ENPOMe is AIE-active.
TPA properties: the quadratic dependence of fluorescence
intensity versus incident laser power was checked to investigate
a
PCFM Lab, DSAPM Lab, KLGHEI of Environment and Energy Chemistry, State Key
Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and
Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China.
E-mail: chizhg@mail.sysu.edu.cn; Fax: +86 20 84112222; Tel: +86 20 84112712
b
School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275,
P. R. China. E-mail: stszfl@mail.sysu.edu.cn
c
State Key Laboratory of Supramolecular Structure and Material, Jilin University,
Changchun 130012, P. R. China
d
State Key Laboratory of High Laser Physics, Shanghai Institute of Optics and Fine
Mechanics, Chinese Academy of Sciences, Shanghai 201800, P. R. China
†
Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S12, Tables S1 and S2, and structural information (NMR, MS and IR
spectra) (PDF). CCDC 898288. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc36806d
‡
These authors contributed equally to the preparation of this work.
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 273--275 273

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whether the ENPOMe luminophore has TPA properties. The
results are shown in Fig. S3 (ESI†). The experimental regression
coefficient of ENPOMe was estimated to be about 2.03, indicat-
ing that no saturation occurred in the experimental laser power.
1
1
Thus, fluorescence arose from a TPA process. ENPOMe has an
exceptionally large s of 5548 GM when excited by a laser pulse at
7
40 nm. This s is larger than most of the representative
1
2
materials. The large s of ENPOMe is attributed to the inter-
molecular charge transfer from the tetraphenylethene and
methylphenate to the electron-drawing cyano group. Fig. S4
(
ESI†) shows the quantum mechanical simulation carried out
1
3
by Gaussian 03W with B3LYP/6-31G(d). Most of the electrons of
tetraphenylethene and methylphenate transferred to the cyano
group at the lowest unoccupied molecular orbital (LUMO),
further confirming the aforementioned hypothesis. Fig. S5 (ESI†)
shows the TPF emission spectra of the dilute solutions of
Fig. 2 TPF spectra of ENPOMe: (B
1 1
) as-synthesized sample; (G ) ground
sample; (B2v) fumed sample (ground sample in dichloromethane vapor for five
ENPOMe in water–THF mixtures with different water fractions, minutes); (B2a) annealed sample (The ground sample was homoiothermal at
1
2
40 1C for ten minutes and cooled down at room temperature.); (G ) re-ground
which are similar to SPF. Most TPA dyes are hydrophobic and
suffer from the ACQ when dispersed in water. Thus, the unique
TPF-AIE properties, as well as the large s, endow ENPOMe with
attractive advantages in practical applications such as biological
sample; (B3v) re-fumed sample; (B3a) re-annealed sample. The insets depict the
reversibility of the TPF wavelengths of ENPOMe: by grinding–fuming treatments
(
top); and by grinding–annealing treatments (down).
7c,14
imaging, especially for two-photon microscopy.
Multi-stimuli-responsive fluorescence switching: the as-synthe- and a TPF lmax at 470 nm (Fig. 2), which are equal to those of
sized ENPOMe compound exhibits strong blue-light TPF (B -form, the pristine sample, B . Thus, the ground sample can revert
max = 469 nm) in the solid state (Fig. 1a–f and 2). However, when back to the original state, and the TPF can exhibit hypochromic
1
1
l
the sample was ground, the color changed from light green to shift back to blue. The treatments of grinding and annealing
greenish-yellow, and the maximum TPF wavelength shifted were repeated beginning with B . The results are similar to the
2
a
bathochromically to 513 nm (G -form). Aside from the red-shift first round, thus further confirming the excellent reversibility
1
of lmax, the fluorescence efficiency of ENPOMe decreased from of TPF between green and blue light, which correspond to the
0
.85 in B
metry (DSC) was performed for B
shown in Fig. S6 (ESI†). An exothermal peak at around 82 1C is peak, implying that B
observed in the DSC thermogram of G . This peak is absent in changed into an amorphous state by grinding. Aside from
the DSC curve of B . The ground sample, G -form, is metastable annealing, solvent-vapor-induced crystallized samples (B2v
and can crystallize promptly upon heating. The crystalline and B ) are obtained from the ground samples (G and G ).
1
-form to 0.66 in G
1
-form. Differential scanning calori- crystalline B-form and metastable G-form, respectively. The
and G , and the results are melting peak of G is obviously higher than the exothermal
crystals are destroyed and partially
1
1
1
1
1
1
1
3
v
1
2
sample (B ) from G exhibits a melting point at around 171 1C The properties of the crystallized samples are similar to that of
2a
1
the initial sample, suggesting that both annealing and vapor-
inducement can promptly revert TPF from green light to blue
light. Fig. 1g and h show the typical scanning electron microscopy
1 1 1
(SEM) images of B and G . The sample B of ENPOMe is formed
by crystals with a smooth surface and a rectangular shape with
different lengths that result in the blue-light emission of TPF.
Whereas the G -form is rough and the crystals are destroyed,
1
breaking into small pieces or even becoming amorphous. This
process leads to the green-light emission of TPF. Time-resolved
emission decay was also carried out (Fig. S7, ESI†). The B
ENPOMe has only one lifetime (1.48 ns), whereas G has two (t
exhibits two decay
is close to the B
1
of
1
1
=
1
.54 ns, t
pathways. Considering that the t
lifetime, then the t of G
2
= 2.66 ns), indicating that G
1
1
1
of G
1
2
1
may belong to the amorphous state
of ENPOMe. The results of powder X-ray diffraction (XRD) of
ENPOMe samples show that ENPOMe exhibits different mole-
cular aggregation structures before and after grinding (Fig. S8,
ESI†). The XRD results demonstrated that the B-form samples
were well-ordered microcrystalline-like structures and the
G-form samples were metastable amorphous states. Thus,
TPF reversibility for ENPOMe is ascribed to amorphization
Fig. 1 ENPOMe crystals obtained from methanol/acetonitrile under (a) ambient
light and (b) UV light with an excitation of 365 nm; (c) as-synthesized sample of
ENPOMe under ambient light (left) and UV light with an excitation of 365 nm
(
right); (d) ground sample of ENPOMe under ambient light (left) and UV light
with an excitation of 365 nm (right); (e) as-synthesized sample and (f) ground
sample of ENPOMe illuminated by a 740 nm laser pulse; (g) SEM of the
as-synthesized sample; and (h) ground sample of ENPOMe.
2
74 Chem. Commun., 2013, 49, 273--275
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and crystallization upon the grinding–annealing (or fuming) Notes and references
process. ENPOMe is proven to be CIEE-active because the
1
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yet been reported. SPF and TPF spectra of ENPOMe in B-form
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ENPOMe single crystal (four molecules per unit cell; crystal data
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diffraction. ENPOMe crystallizes layer-by-layer when assisted by
weak C–Hꢀꢀꢀp interaction. The molecules in the crystal are
arranged in slip-stack order along the long c-axis with a pitch angle
of 71.41. Molecules form antiparallel coupling in the same layer,
and establish efficient tail-to-tail interaction with adjacent mole-
cules by C–HꢀꢀꢀN and C–HꢀꢀꢀO hydrogen bonds. These bonds are
more robust than the C–Hꢀꢀꢀp interactions of the interlayer. Thus,
the molecules will first slip along the c-axis when triggered by an
external pressure, and then the crystal will crack. Furthermore,
numerous cavities are found in each layer, indicating that ENPOMe
affords loose-packing patterns in its crystals. Geometry of isolated
free molecules of ENPOMe in the ground state was obtained by
simulation based on B3LYP/6-31G(d) using Gaussian 03W (Fig. S12
and Table S2, ESI†) to validate the piezofluorochromism
mechanism. The dihedral angles of A–D and D–F pairs in the
simulated molecule are 75.21 and 5.71, respectively, which are 5.31
and 8.71 lower than those in the crystal. Thus, the twist stress of the
molecules in the crystals is greater, and readily released when
triggered by external pressure, which leads to the planarization of
the molecular conformation, an extension of molecular conjuga-
tion, and a bathochromic shift of both SPF and TPF spectra.
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fluorescence (F of up to 0.85). ENPOMe has an exceptionally
F
large TPA cross section of 5548 GM. The as-synthesized sample
exhibits striking multi-stimuli-responsive SPF and TPF switching
with excellent reversibility in the solid state. The unique reversibility
of TPF makes ENPOMe a promising material for 3-D optical data
storage or sensing.
The authors gratefully acknowledge the financial support
from the NSF of China (51173210, 51073177), the Fundamental
Research Funds for the Central Universities and NSF of
Guangdong (S2011020001190).
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This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 273--275 275