A tetraphenylethene-based caged compound: Synthesis, properties and applications

Article abstract of DOI:10.1039/c4cc03337j

A tetraphenylethene-based caged compound (TPE-C) is designed and synthesized. TPE-C is non-fluorescent either in solution or in aggregated state, but its emission can be induced to emit strong cyan emission in the aggregated state by UV irradiation. This property enables TPE-C to be applied in photo-patterning and anti-counterfeiting related areas. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03337j

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A tetraphenylethene-based caged compound:
synthesis, properties and applications†
Cite this: Chem. Commun., 2014,
50, 8134
Chris Y. Y. Yu,a Ryan T. K. Kwok,a Ju Mei,a Yuning Hong,a Sijie Chen,a
Jacky W. Y. Lama and Ben Zhong Tang*abc
Received 5th May 2014,
Accepted 2nd June 2014
DOI: 10.1039/c4cc03337j
www.rsc.org/chemcomm
A tetraphenylethene-based caged compound (TPE-C) is designed and group is the most representative quencher for caged fluorophores.5–9
synthesized. TPE-C is non-fluorescent either in solution or in aggregated Because of the strong electron-withdrawing ability of the 2-nitro-
state, but its emission can be induced to emit strong cyan emission in the benzyl group, the emission of fluorophores in the caged compound
aggregated state by UV irradiation. This property enables TPE-C to be is quenched through the photo-induced electron transfer (PeT)
applied in photo-patterning and anti-counterfeiting related areas.
process. Once the 2-nitrobenzyl group is cleaved by UV irradiation,
the emission of the fluorophore will be recovered. Conventional
The development of fluorescent materials with high solid-state fluorophores such as BODIPY, fluorescein, and rhodamine have
quantum yield is of practical importance because most of their real- been widely used in the caged systems.6–9 Their emissions can be
world applications require the use of materials in aggregated state recovered almost completely after UV treatment in solution. How-
or even in solid state.1,2 Fluorophores with aggregation-induced ever, the recovery efficiency is not satisfactory in the aggregated state.
emission (AIE) characteristics have emerged as a novel class of Only weak or no emission can be observed even after a long time of
materials which could meet such requirements due to their good UV irradiation because the emission of uncaged fluorophores is
photostability, high photo-bleaching resistance and high quantum quenched by aggregation-caused quenching (ACQ). Hence such an
yields in aggregated state.3 So far, a variety of functional materials ACQ effect greatly limits the applications of the caged compounds in
based on AIE fluorophores have been extensively reported and a the aggregated state.
number of stimuli responsive systems have been constructed
with AIE materials.3,4 A photo-activatable AIE system, however, is was constructed by using an AIE-active tetraphenylethene (TPE)
still rare. derivative as a fluorophore with a 2-nitrobenzene group as a
Herein, we reported a new caged compound (TPE-C), which
Caged fluorophores, whose emission is partially or completely quencher. TPE-C is non-fluorescent either in solution or in
quenched by a quencher but can be recovered upon cleavage of the aggregated state but its emission in the aggregated state can be
quencher under certain stimuli, such as UV or thermal treatment, photoactivated upon UV irradiation and consequently a strong cyan
are a type of typical photoactivatable materials and have been well emission is observed (Scheme 1). The fluorescence response to the
studied and applied in many technological fields especially those UV-irradiation endows the TPE-C with the potential to be applied in
related to biological applications such as macromolecular move- photo-patterning and anti-counterfeiting related areas.
ment tracking and super-resolution imaging.5–9 A 2-nitrobenzyl
a Department of Chemistry, HKUST Jockey Club Institute for Advanced Study,
Division of Biomedical Engineering, Division of Life Science, State Key Laboratory
of Molecular Neuroscience and Institute of Molecular Functional Materials,
The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong, China. E-mail: tangbenz@ust.hk
b Guangdong Innovative Research Team, SCUT-HKUST joint Research Laboratory,
State Key Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou 510640, China
c HKUST Shenzhen Research Institute, No. 9 Yuexing First RD, South Area,
Hi-tech Park, Nanshan, Shenzhen 518057, China
† Electronic supplementary information (ESI) available: Detailed synthesis and
characterization of TPE-P and TPE-C; UV and PL spectra of TPE-C; photographs of
TPE-C under UV irradiation; HPLC of TPE-C with different irradiation times and
Scheme 1 Uncaging process of TPE-C.
photo-patterning and pattern erasing. See DOI: 10.1039/c4cc03337j
8134 | Chem. Commun., 2014, 50, 8134--8136
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Scheme 2 Synthetic route to TPE-P and TPE-C. (I) Pd(PPh3)4, Na2CO3,
THF–H2O, reflux overnight; (II) Cs2CO3, MeCN, 80 1C, 8 h.
Fig. 2 (A) PL spectra of TPE-C in 95% fw with different irradiation times. (B)
Change in PL intensities of TPE-C at 488 nm in different fw versus
irradiation times. Concentration: 10 mM; excitation wavelength: 320 nm.
Inset: photographs of TPE-C in 95% fw excited using a hand-held UV lamp
at 365 nm under UV irradiation for 0 s and 240 s.
with fw = 85% and 95%. Their PL peak intensities at 488 nm
gradually enhanced along with the UV irradiation time, indicating
that the TPE-P is formed during UV treatment (Fig. 2A and Fig. S3,
Fig. 1 (A) PL spectra of TPE-P in the THF–water mixture with different
water fractions (fw). (B) The plot of relative PL intensities (I/I0) versus fw. I0 are
the PL intensities of the dyes in THF solutions at 488 nm; dye concentration:
10 mM; excitation wavelength: 320 nm. Inset: photographs of (a) TPE-P and
(b) TPE-C in fw = 95% excited using a hand-held UV lamp at 365 nm.
ESI†). The PL enhancement in fw = 95% is faster than that in fw
=
85%, this could be because the TPE-P aggregates in 95% water
content are more compressed and the intramolecular motions are
more restricted (Fig. 2B).3,4 The photos of TPE-C fluorescence in the
THF–water mixture with fw = 95% taken under UV illumination also
The synthetic route of TPE-C is depicted in Scheme 2. TPE-P is demonstrate such an uncaging process (Fig. S4, ESI†).
synthesized via Suzuki coupling between 4-bromotetraphenylethene To further verify whether the cyan fluorescence is attributed to
(1) and (4-hydroxyphenyl) boronic acid (2). The resultant TPE-P is the formation of TPE-P, we conducted high-performance liquid
then reacted with 2-nitrobenzyl bromide (3) in the presence of chromatography (HPLC) to monitor the uncaging process. We first
Cs2CO3 to furnish TPE-C. The structure and purity of the TPE-P ran the pure TPE-P and TPE-C using acetonitrile as the references.
and TPE-C are fully characterized by NMR and mass spectrometry The peaks for TPE-P and TPE-C were observed at 1.5 and 2.0 min,
(Fig. S10–S17, ESI†).
respectively (Fig. S5, ESI†). The TPE-C aggregates in the of THF–
To investigate the photophysical properties of TPE-C and water mixture with fw = 95% were then irradiated by UV light and
TPE-P, we firstly conducted UV absorption measurements. The the samples were taken out for HPLC analysis every minute. The
UV spectra of both TPE-C and TPE-P in THF solution exhibit HPLC spectra show that the peak area for TPE-C decreases while
an absorption maximum at 320 nm (Fig. S1, ESI†). We then the peak area for TPE-P increases along with the UV irradiation
investigated their photoluminescence (PL) properties. As shown time (Fig. S6, ESI†), which is consistent with the PL results. As
in Fig. 1, both TPE-C and TPE-P are non-fluorescent in pure THF indicated by the mass analysis, the isolated product from HPLC at
solution. The emission of TPE-P increases swiftly when the water 1.5 min has an exact mass of 424.1822 (Fig. S7, ESI†), which
fraction ( fw) in the THF–water mixture exceeds 85%. When fw
=
corresponds to the mass of TPE-P (Fig. S14, ESI†). Based on the
95%, the PL intensity at 488 nm is 80-fold higher than that in above results, it can be concluded that TPE-C is uncaged upon UV
pure THF solution. The fluorescence enhancement of TPE-P is irradiation and the released TPE-P accounts for the fluorescence
attributed to the formation of nanoaggregates (Fig. S2, ESI†), enhancement.
suggesting that TPE-P is AIE-active.3 On the other hand, TPE-C
Inspired by rapid and highly efficient release of TPE-P from
remains non-emissive although the nanoaggregates formed the caged compound TPE-C in aggregated state, we explored the
when 95% of water was added to the THF solution. The PL possibility to utilize TPE-C as a kind of UV activatable fluorescent
results indicated that the emission of TPE-C in the aggregated material for photo-patterning and anti-counterfeiting related
state is quenched by 2-nitrobenzene through the PeT process.
applications. First of all, we tried to use filter paper as a substrate
Inspired by the uncaging process in conventional systems, for writing. As shown in Fig. S8 (ESI†), the letter ‘‘I’’ is written
TPE-C is expected to respond to UV irradiation. As the proposed with TPE-C while the letters ‘‘A’’ and ‘‘E’’ are written with TPE-P
uncaging mechanism shown in Scheme S2 (ESI†), the 2-nitrobenzyl for comparison. Before UV treatment, the letters ‘‘A’’ and ‘‘E’’ are
group in TPE-C is cleaved by UV irradiation and TPE-P and 2-nitroso- highly emissive but the letter ‘‘I’’ is still non-fluorescent. With
benzaldehyde are readily formed.9 Since TPE-P is highly emissive in the increase of UV irradiation time, the emission of letter ‘‘I’’
the aggregated state, we utilized PL measurements to monitor the becomes stronger. Although the emission of letter ‘‘I’’ is still
uncaging process of TPE-C in the aggregated state. As shown in weaker than the letters ‘‘A’’ and ‘‘E’’, it is understandable that
Fig. 2, TPE-C shows a weak emission in both the THF–water mixtures the photoactivation process may occur only on the surface of the
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caged fluorophore and the synthesis of other caged compounds
with AIE moieties with long-wavelength emission are in
progress.
This work was partially supported by the National Basic
Research Program of China (973 Program; 2013CB834701), the
Research Grants Council of Hong Kong (604711, 604913, HKUST2/
CRF/10 and N_HKUST620/11), Innovation and Technology
Commission (ITCPD/17-9), and the University Grants Committee
of Hong Kong (AoE/P-03/08). B. Z. T. is grateful for the support
from the Guangdong Innovative Research Team Program of
China (201101C0105067115).
Fig. 3 Photographs of the process of (A and C) photo-patterns by a mask
with the HKUST logo under UV irradiation and (B and D) patterns erasing
the process after removing the mask under further UV irradiation.
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addition to the fluorescent writing, TPE-C possesses the
potential to be used in anti-counterfeiting applications. Since
we have demonstrated that the photo-activation can be carried
out on the filter paper, we can conveniently fabricate patterns or
erase patterns by adding or removing a mask. Filter papers are
firstly soaked with the THF solution of TPE-C and dried by using
compressed air. Two projector films with the HKUST logo, one a
transparent image (Fig. 3A) while the other one a dark image
(Fig. 3C), were covered onto the filter papers. The HKUST logo
gradually emerged on the filter papers upon UV irradiation. For
the film with the transparent logo, the frame structure displays
brighter emission than the surroundings. In contrast, the frame
structure shows dimmer emission than the surroundings when a
mask with a dark logo is used. Moreover, the patterns can be
erased by further UV irradiation after removing the masks
(Fig. 3B and D). Since the caged fluorophores in both the logo
and surrounding areas are activated, the whole filter paper is
emissive and the patterns cannot be seen as a result. To
demonstrate the flexibility of this method, we used other films
with different logos to perform the same experiment (Fig. S9,
ESI†). All the frame structures of patterns can be presented and
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8136 | Chem. Commun., 2014, 50, 8134--8136
This journal is ©The Royal Society of Chemistry 2014