Solid-State Photoinduced Luminescence Switch for Advanced Anticounterfeiting and Super-Resolution Imaging Applications

Article abstract of DOI:10.1021/jacs.7b07738

Solid-state organic photoswitches with reversible luminescence modulation property are highly attractive because of their wide prospects in advanced photonic applications, such as optical data storage, anticounterfeiting and bioimaging. Yet, developing su

Full text of DOI:10.1021/jacs.7b07738

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Communication
A Solid-State Photo-Induced Luminescence Switch for Advanced
Anti-Counterfeiting and Super-Resolution Imaging Applications
Qingkai Qi, Chong Li, Xiaogang Liu, Shan Jiang, Zhaochao
Xu, Richmond Lee, Mingqiang Zhu, Bin Xu, and Wenjing Tian
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07738 • Publication Date (Web): 18 Oct 2017
Downloaded from http://pubs.acs.org on October 18, 2017
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A Solid-State Photo-Induced Luminescence Switch for Advanced
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Qingkai Qi, Chong Li, Xiaogang Liu, Shan Jiang, Zhaochao Xu, Richmond Lee, Mingqiang
§
,†
†
Zhu, Bin Xu* and Wenjing Tian
†
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, China.
‡
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
§
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong Univer-
sity of Science and Technology, Wuhan 430074, China.
⊥
Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore.
Supporting Information Placeholder
22
crystalline state. This solid-state photoswitching is attribut-
ed to the presence of a strong electron-withdrawing group
(methyl pyridinium) and the possible “dilution effect” of io-
dide anions. However, the methyl pyridinium group and the
iodide anion could also quench the emission of this com-
pound in the crystalline state. Harada et al. showed that de-
creasing temperature to 80 K enhanced the photochromism
of SP in the solid state. This enhancement is due to slowing
down the reaction rate of MC to SP photoisomerization.
Nevertheless, such a low temperature limits its practical ap-
plications.
ABSTRACT: Solid-state organic photoswitches with reversi-
ble luminescence modulation property is highly attractive
because of their wide prospects in advanced photonic appli-
cations, such as optical data storage, anti-counterfeiting and
bioimaging. Yet, developing such materials has long been a
significant challenge. In this work, we construct an efficient
solid-state photoswitch based on a spiropyran-functionalized
distyrylanthracene derivative (DSA-2SP) which exhibits ex-
ceptional reversible absorption/luminescence modulation
ability. Efficient photoswitching between DSA-2SP and its
photoisomer DSA-2MC are facilitated by large free volumes
induced by non-planar molecular structures of DSA moieties,
as well as the intramolecular hydrogen bonds between the
DSA and MC moieties. Consequently, the excellent solid-
state photochromic property of DSA-2SP is highly applicable
as both anti-counterfeiting inks and super-resolution imag-
ing agents.
23
In fact, the primary challenge in the solid-state pho-
toswitching of SP molecules concerns tight molecular pack-
ing in aggregated molecules. Such tight molecular packing
significantly impedes the cis-trans transformation, which
occurs during photochemical reactions between the SP and
MC forms. Consequently, providing a large free volume in
the solid state is crucial to facilitate efficient photoswitching.
Recently, highly twisted π-conjugated luminescent molecules
have attracted substantial research interest for their out-
standing fluorescence quantum yield in the solid state. Their
highly three-dimensional structures avoid strong intermo-
lecular π-π stacking and lead to a unique phenomenon of
Spiropyran (SP) and its derivatives represent one im-
portant class of photochromic molecules, and have attracted
considerable research interests for their versatile applications
1
-3
4-5
24
as data storage medium, chemical sensors,
imaging agents,
biological
aggregation-induced-emission (AIE). Moreover, the non-
planar conformations of AIE molecules often lead to a loose
packing in the solid state. We thus reason that linking a
highly sterically bulky AIE molecule to SP could provide con-
siderable free volume, thus facilitating efficient SP-MC pho-
to-isomerization.
6-10
11-17
and other functional materials.
These
molecules exhibit distinct absorption and luminescence
spectral changes under ultraviolet radiations, as they experi-
ence structural isomerization from the unconjugated SP form
1
8
to conjugated merocyanine (MC) form. In the past few dec-
ades, covalently attaching SP to other fluorophores has led to
Inspired by this design strategy, we covalently linked two
the remarkable development of many multi-color photo-
19-20
SP moieties to
a
popular AIE molecule—9,10-
induced luminescence switches.
However, most of these
distyrylanthrance (DSA), resulting in a novel photochromic
compound (DSA-2SP, Figure 1a). DSA-2SP exhibits highly
efficient and reversible solid-state luminescence switching
under alternating UV light and heat/visible light treatment.
Moreover, it also emits bright fluorescence. Accordingly, we
have successfully demonstrated its application in both opti-
cal anti-counterfeiting and super-resolution imaging.
switches function only in the solution phase. Developing
efficient solid-state luminescence switches remains signifi-
cantly challenging and at the same time increasingly impera-
21
tive for numerous applications.
To this end, Yu et al. reported that two cationic microcrys-
talline pyrido-spiropyrans exhibited photochromism in the
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peaked at 670 nm appeared, corroborating the photoswitch-
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ing from SP to MC moiety (Figure 1c and 1d; detailed dynam-
ic studies of these absorption and emission changes are
available in Figures S3 and S4). Moreover, the UV exposed
DSA-2SP could be restored after brief heating at 90 °C for 5
min. The fluorescence quantum efficiencies of the DSA-2SP
powders through 10 cycles of UV-Heat switching were shown
in Figure S5.
Similar to the spectral changes in its powders, both the ab-
sorption and emission bands of DSA-2SP thin film demon-
strate reversible changes under alternated UV irradiations
and heat treatment (Figure 1e and 1f). Multiple cycles of such
treatment indicate efficient reversibility and anti-fatigue
capability of DSA-2SP. We also found that upon pho-
toswitching, DSA-2MC in the thin film exhibit much im-
proved stability than that in the solution phase. For example,
its color could be retained for >24 h in a dark room at ambi-
ent temperature without much fading (Figure S2b). Similar
characteristics are also found in DSA-2SP doped PMMA films
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(
Figure S6).
Powder X-ray diffraction (PXRD) experiments on pristine
DSA-2SP powder showed no obvious diffraction peaks, sug-
gesting that the solid DSA-2SP stay in an amorphous state
and lack close molecular packing (Figure 2a). Moreover, X-
ray photoelectron spectroscopy (XPS) experiments on DSA-
Figure 1. a) Reversible structural isomerization between
DSA-2SP and DSA-2MC under different stimuli. b) Visible
and fluorescent pictures of DSA-2SP powders and films. c)
UV-vis reflection and d) PL spectra of DSA-2SP powders be-
fore and after UV irradiation, ex: 410 and 560 nm. e) UV-vis
absorption and f) PL spectra of a spin-coated DSA-2SP film
before and after UV irradiation, ex: 410 and 560 nm. Insets of
e) and f): reversible switching of the absorption and emission
by the UV-Heat cycle.
2SP thin film reveal a noticeable increase in binding energy
of nitrogen 1s electron after UV irradiation (Figure 2b). This
increase in binding energy could be understood by consider-
ing that as UV irradiation convert SP to MC moiety, the posi-
tively charged indoline cation in the zwitterionic MC moiety
render it more difficult to extract the nitrogen 1s electrons.
We also noticed that the absorption band of MC substantial-
ly overlapped with the emission band of DSA (Figure 2c),
while the distance between these two moieties was only
We first synthesized DSA-2SP using SP-COOH and DSA-
2OH via esterification reaction (Scheme S1-S3), and studied
its photoswitching characteristics in the solution phase. Be-
fore UV irradiation, DSA-2SP dissolved in tetrahydrofuran
~
1.463 nm (Figure S7). Consequently, there exists considera-
ble intramolecular energy transfer from DSA to MC moieties
Figure 2e). Indeed, after UV irradiations, the average fluo-
(
(
THF) showed a strong absorption peaked at 410 nm and an
rescence lifetime of DSA-2SP thin film at 530 nm (emissions
from the DSA moieties) exhibits a significant decrease from
emission centered at 550 nm (Figure S1b and S1c), which are
attributed to the DSA moiety. Upon continuous 365 nm UV
light irradiation, DSA-2SP exhibited a noticeable change in
both its visual color (from colorless to cyan) and fluorescence
1
.35 ns to 0.47 ns (Figure 2d), suggesting significant energy
25
transfer to the MC moieties. The energy transfer process
from the DSA donor to the MC acceptor was further demon-
strated by the PL excitation spectra (Figure S8) and ultrafast
transient absorption spectra (Figure S9).
(
(
(
from yellow to orange). Accordingly, a new absorption band
maximized at 590 nm) and a corresponding emission band
centered at 630 nm) started to emerge and gradually inten-
sified (Figure S1b and S1d). These new absorption and emis-
sion bands are consistent with those of the MC moieties
which possess a more extended π-conjugation than their SP
isomers, thereby indicating the photoswitching from SP to
MC moieties. It is noteworthy that MC isomers are unstable
in THF and their colors fade off within 20 seconds after UV
light was turned off (Figure S2a). Such rapid switching from
MC to SP isomers is due to strong vibrational and rotational
freedom of DSA-2SP in the solution phase.
To gain further insights into the photochromic behaviors
of DSA-2SP, we performed quantum chemical calculations
based on the density functional theory (DFT). Our DFT cal-
culations show that the DSA group possesses a highly non-
planar molecular structure (Figure 2f). Such a steric structure
could afford significant free volume in the DSA-2SP powder,
thus enabling efficient photo-isomerization from the SP to
the MC moieties in the solid-state. We also noticed that up-
on photo-isomerization, hydrogen bonding interactions be-
tween the MC and DSA moieties in the resulted DSA-2MC
were present (CH…O distance: ~3.34 Å; Figure 2f). These
hydrogen bond interactions further stabilize the MC form by
The successful photoswitching of DSA-2SP in THF solution
motivated us to explore its photochromic behaviors in the
solid-state. It was exciting to discover that the solid powder
of DSA-2SP exhibited efficient photoswitching at room tem-
perature, under the irradiation of a hand-held ultraviolet
lamp (365 nm). We noticed that after UV exposure, the color
of the DSA-2SP powder immediately changed from yellow to
dark purple (Figure 1b). At the same time, a new UV-Vis ab-
sorption band centered at 590 nm and an emission band
~0.57 kcal/mol, leading to more efficient photoswitching
from the SP to MC (Figure 2g). Moreover, this enhanced sta-
bility endows the MC form a longer lifetime after UV is
turned off. Indeed, our calculations reveal that the energy
differences between the SP and MC forms is reduced to ~6.91
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Figure 2. a) PXRD pattern of the pristine DSA-2SP powder. b) XPS spectra of DSA-2SP film before and after 365 nm UV irradia-
tions. c) Overlap of MC Absorption and DSA emission spectra, ex: 410 nm. d) Fluorescence decays of DSA-2SP film before and
after 365 nm UV irradiations. e) Mechanism of photochromic luminescence of DSA-2SP under UV irradiations. f) Optimized
molecular structures of various conformations and g) their relative enthalpy levels.
kcal/mol, in comparison to ~14.9 kcal/mol in a reference
printed images taken a) under visible light and b) after 1 min
UV irradiation, and their corresponding fluorescent images
in c) and d). e) The reflective UV-vis spectra and f) corre-
sponding PL spectra of the filter paper printed with DSA-2SP
before and after 365 nm UV irradiation, respectively. Insets
in e) a plot of the reflectance at 580 nm and f) switches of the
peak emission wavelength through 10 cycles of UV-writing
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compound SP /MC . All these factors, including the relative
stabilization of DSA-2MC with respect to DSC-2SP and the
availability of large free volumes in the solid-state enables
efficient photoswitching in DSC-2SP.
The excellent photochromic property of DSC-2SP was
demonstrated in its applications as an anti-counterfeit ink.
Consequently, we printed an umbrella onto a filter paper
with DSA-2SP solution (Figure 3). After a complete drying
under ambient conditions, we observed a pale yellow pattern
on the paper (Figure 3a). The light color and the associated
low background interference suggest that DSA-2SP is a suit-
able anti-counterfeit ink. Next, exposing the printed paper to
365 nm UV light immediately changed the visible color of the
umbrella dye to blue violet (Figure 3b and Video). This color
change is easily distinguishable by a naked eye. Furthermore,
the visible color of the printed pattern was restored to its
°
and heat-erasing at 90 C.
We would like to point out that DSA-2SP has a large mo-
lecular weight (>1000) and is highly difficult to crystallize.
These properties render it extremely challenging to charac-
terize the molecular structure of DSA-2SP. The potential
security features and excellent solid-state photochromic
properties of DSA-2SP make it a promising anti-counterfeit
ink with either visual color or emission changes.
The photoswitching characteristics of DSA-2SP also endow
it with a substantial potential in super-resolution imaging. In
many super-resolution imaging techniques such as stochastic
°
initial state after heating at 90 C for 1 min. This color revers-
ibility was reproduced for many cycles without fail (Figure
27
optical reconstruction microscopy (STORM) and photoac-
3e). Finally, the corresponding fluorescence changes between
28
tivated localization microscopy (PALM) , photoswitchable
bright yellow to red emissions provided even higher contrast
and clearer signals than the visual colors of DSA-2SP during
this process (Figure 3c-3d).
fluorophores play a key role for achieving good imaging qual-
ity. To demonstrate the application of DSA-2SP in super-
resolution imaging, we uniformly doped this compound in
the diblock copolymer of polystryrene-block-poly(ethylene
oxide) (PSt-b-PEO). The amphiphilic nature of PSt-b-PEO
leads to self-assembly of water-dispersed cylindrical micelles
in aqueous solution (Figure 4a; see Supporting Information
for detailed preparation process). In these cylindrical micelle
nanostructures, continuous 302 nm ultraviolet light (~0.40
2
2
mW/cm ) and intermittent 561 nm laser (~525.7 mW/cm )
irradiations enabled the on/off switching of red fluorescence
(
from the fluorescent MC moiety and non-fluorescent SP
moiety of DSA-2SP, respectively), and permitted the recon-
struction of super-resolution images (Figure S10). Our results
showed that the super-resolution image of PSt-b-PEO cylin-
drical micelles exhibited greatly enhanced resolution com-
pared to conventional fluorescence image (Figure 4 and Fig-
ure S11). Per Gaussian deconvolution, the full-width at half-
Figure 3. Illustration of DSA-2SP as an anti-counterfeiting
ink on a filter paper printed with an umbrella-shape. The
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maximum (fwhm) of a fluorescent micelle is ~49.5 nm in the
This work was supported by 973 Program (grant no.
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super-resolution imaging mode, cf. ~427.4 nm in the conven-
tional fluorescence imaging mode (Figure 4e). During our
experiment, the overall STORM resolution of the super-
resolution image amounts to ~53.3±0.42 nm, as calculated
from the Fourier Ring Correlation (FRC) curve (Figure 4f).
2013CB834701), the Natural Science Foundation of China
(grant no. 21674041, 51373063, 51573068, 21221063, 21708039),
Program for Changbaishan Scholars of Jilin Province, and
Outstanding Postdoctoral Award DMTO Project from DICP.
X.L. and R.L. acknowledged Singapore University of Tech-
nology and Design for a Startup Research Grant.
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In summary, we report a highly efficient photochromic
spiropyran-functionalized distyrylanthracene derivative,
DSA-2SP which photoswitching in both absorption and
fluorescence of DSA-2SP is highly reversible under
alternating UV and heat/visible light treatment. Notably,
such switching is very efficient not only in the solution-state
but also in the solid-state (such as in solid powders and
films). We showed that the efficient photoswitching between
DSA-2SP and DSA-2MC are facilitated by large free volumes
induced by non-planar molecular structures of DSA moieties,
as well as the intramolecular hydrogen bond interactions
between the DSA and MC moieties. Finally, we successfully
demonstrated the application of DSA-2SP as an anti-
counterfeit ink, and as super-resolution imaging agent. We
believe that these results have significant implications for
developing advanced solid-state photochromic materials.
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Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Synthesis and characterizations, UV-vis and PL spectra, dy-
namic studies, transient absorption, super-resolution imag-
ing, fluorescence quantum efficiency (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Email: xubin@jlu.edu.cn.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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Figure 1. a) Reversible structural isomerization between DSAꢀ2SP and DSAꢀ2MC under different stimuli. b)
Visible and fluorescent pictures of DSAꢀ2SP powders and films. c) UVꢀvis reflection and d) PL spectra of
DSAꢀ2SP powders before and after UV irradiation, ex: 410 and 560 nm. e) UVꢀvis absorption and f) PL
spectra of a spinꢀcoated DSAꢀ2SP film before and after UV irradiation, ex: 410 and 560 nm. Insets of e) and
f): reversible switching of the absorption and emission by the UVꢀHeat cycle.
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Figure 2. a) PXRD pattern of the pristine DSA-2SP powder. b) XPS spectra of DSA-2SP film before and after
365 nm UV irradiations. c) Overlap of MC Absorption and DSA emission spectra, ex: 410 nm. d)
Fluorescence decays of DSA-2SP film before and after 365 nm UV irradiations. e) Mechanism of
photochromic luminescence of DSA-2SP under UV irradiations. f) Optimized molecular structures of various
conformations and g) their relative enthalpy levels.
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Figure 3. Illustration of DSA-2SP as an anti-counterfeiting ink on a filter paper printed with an umbrella-
shape. The printed images taken a) under visible light and b) after 1 min UV irradiation, and their
corresponding fluorescent images in c) and d). e) The reflective UV-vis spectra and f) corre-sponding PL
spectra of the filter paper printed with DSA-2SP before and after 365 nm UV irradiation, respectively. Insets
in e) a plot of the reflectance at 580 nm and f) switches of the peak emission wavelength through 10 cycles
of UV-writing and heat-erasing at 90 °C.
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Figure 4. Super-resolution imaging of cylindrical micelles formed from PSt-b-PEO copolymer self-assembly
staining by DSA-2SP. a) Bright field image. b) Conventional fluorescence image. c) Merged bright field and
fluorescence image. d) Super-resolution imaging corresponding to the same field of fluorescence image. e)
Fluorescence cross-sectional profiles of single cylindrical micelle. f) Fourier Ring Correlation (FRC) curve
calculated from d).
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