Synthesis, crystal structure and characterization of photochromic 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene

Article abstract of DOI:10.1016/j.saa.2006.03.014

A novel photochromic diarylethene, 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene (BEDTP), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its photochromic and fluorescent properties were also investigated. The results showed that this compound exhibited reversible photochromism, changing from colorless to magenta after irradiation with UV light both in solution and in the crystalline phase. In hexane solution, the open-ring isomer of BEDTP exhibited relatively strong fluorescence at 400 nm when excited at 282 nm. The fluorescence intensity decreased along with the photochromism upon irradiation with 254 nm light and its closed-ring isomer showed almost no fluorescence.

Full text of DOI:10.1016/j.saa.2006.03.014

Spectrochimica Acta Part A 66 (2007) 335–340
Synthesis, crystal structure and characterization of photochromic
1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene
Shouzhi Pua,∗, Tianshe Yanga, Ruji Wangb, Fushi Zhangb, Jingkun Xua
a Jiangxi Key Lab of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, China
b Department of Chemistry, Tsinghua University, Beijing 100084, China
Received 9 August 2005; received in revised form 17 February 2006; accepted 2 March 2006
Abstract
A novel photochromic diarylethene, 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene (BEDTP), was synthesized and its
structure was determined by single-crystal X-ray diffraction analysis. Its photochromic and fluorescent properties were also investigated. The
results showed that this compound exhibited reversible photochromism, changing from colorless to magenta after irradiation with UV light both
in solution and in the crystalline phase. In hexane solution, the open-ring isomer of BEDTP exhibited relatively strong fluorescence at 400 nm
when excited at 282 nm. The fluorescence intensity decreased along with the photochromism upon irradiation with 254 nm light and its closed-ring
isomer showed almost no fluorescence.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Diarylethene; Crystal structure; Photochromism; Fluorescence
1. Introduction
anti-parallel conformers can undergo effective photocycliza-
tion reaction by a conrotatory mechanism according to the
Woodward–Hoffmann rule, while the parallel conformers are
photochemically inactive [15,16]. The photogenerated closed-
ring isomers of diarylethenes showed some colors with broad
absorption bands in the visible region, and they could regen-
erate their open-ring isomers on irradiation with appropriate
wavelength visible light. The absorption maxima of closed-ring
isomers are known to depend on the ␲-conjugate length of the
aryl groups. In fact, the longer ␲-conjugated systems were intro-
duced into the molecule of diarylethene, the longer absorption
wavelengths of the closed-ring isomers were observed.
Diarylethene crystals belong to a unique class of pho-
tochromic crystals, which exhibit thermally stable and fatigue
resistant photochromic performance [17]. In the crystalline
phase, there is no interconversion such as in solution between the
two conformers of diarylethenes and the molecules are regularly
oriented in a fixed conformation [18], except for one example
where there are two independent molecules with different con-
formations in the asymmetric unit [19]. Diarylethene crystals in
the anti-parallel conformation are very promising for practical
applications because they can not only reversibly turn various
colors (yellow, red, blue or green) from colorless, depending
on their molecular structure, upon irradiation with UV and
appropriate wavelength visible light, but also exhibit good ther-
Photochromism is referred to as a photoisomerization pro-
cess between two isomers having different absorption spec-
tra [1]. Generally, photochromic compounds are classified into
two categories, i.e., thermally reversible compounds and ther-
mally irreversible compounds [2,3]. Although various types of
photochromic compounds have been reported to date [4–6],
compounds that undergo thermally irreversible photochromic
reactions are limited to diarylethenes [4,5,7], fulgides [8,9] and
phenoxynaphthacenequinones, etc. [10,11]. Among these pho-
tochromic stable compounds, diarylethene derivatives bearing
two thiophene-derived groups are the most promising because
of their excellent thermal stability in both isomers, remarkable
fatigue resistance, and rapid response and high reactivity in the
solid state [4,12–14].
Upon photoirradiation, diarylethene derivatives can undergo
cyclization/cycloreversion photochromic reactions either in
solution or in the solid state. In solution, the open-ring iso-
mer of diarylethene has two interconverting conformations,
anti-parallel and parallel ones, in almost equal amounts. Only
∗
Corresponding author. Tel.: +86 791 3805183; fax: +86 791 3826894.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.03.014

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S. Pu et al. / Spectrochimica Acta Part A 66 (2007) 335–340
Scheme 1. Photochromism of diarylethene BEDTP.
mal stability and remarkable fatigue resistance [20]. Therefore,
investigation of diarylethene crystals have attracted much atten-
tion today, and a larger number of diarylethene crystal structures
and their properties have already been reported [17,20–22].
By far, many diarylethene compounds and their photo-
chemical/photophysical properties have been reported [4,5,23].
Diarylethenes bearing dioxoane groups on the end are of
special interest because the dioxoane group is an electron-
donating substituent, which can be effective to increase the
absorption coefficient and to decrease the ring-opening quan-
tum yield [24]. The other merit of dioxoane substituent is
that it can be easily hydrolyzed to formyl group, which
can be changed further to many other functional groups by
some simple reactions [25–27]. In this paper, on the basis of
this idea, a novel photochromic diarylethene with dioxoane
groups on the end, 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-
thienyl}perfluorocyclopentene (BEDTP, shown in Scheme 1),
was developed which exhibited good photochromism both in
solution and in the crystalline phase. In addition, its X-ray crys-
tal structure, photochromic and fluorescent properties were also
presented here.
Scheme 2. Regents and conditions: (a) Br2, CH3COOH, r.t., 20 h, 73.3%; (b)
glycol, p-toluene sulfolic acid, benzene, reflux, 16 h, 84.0%; (c) n-BuLi, THF,
−78 ◦C, C5F8, 3 h, 57.7%.
2.2.1. Synthesis of 3-bromo-2-ethyl-5-formylthiophene 2
To a stirred solution of 5-ethylthiophene-2-carbaldehyde 1
(2.0 g, 14.3 mmol) in acetic acid was added dropwise an acetic
acid solution of Br2 at room temperature. Stirring was continued
for 16 h at the temperature. Then the reaction was stopped by
the addition of water. The reaction mixture was neutralized by
Na2CO3 to neutrality. The product was extracted with ether,
dried, filtrated, evaporated and obtained in 73.3% yield as yellow
1
oil. H NMR (400 MHz, CDCl3, TMS): δ 1.316–1.354 (t, 3H,
J = 7.6 Hz, –CH3), 2.839–2.895 (q, 2H, J = 7.6 Hz, –CH2), 7.608
(s, 1H, thiophene-H), 9.789 (s, 1H, –CHO).
2.2.2. Synthesis of 3-bromo-2-ethyl-5-(1,3-dioxolane)
thiophene 3
Compound 2 (2.28 g, 10.4 mmol), glycol (2 mL, 58.5 mmol),
and p-toluenesulfonic acid (0.05 g) were dissolved in benzene
(200 mL). Under the Dean-Stark condition, the reaction mix-
ture was refluxed overnight, and then washed with NaHCO3
(5% (w/v), 2 × 50 mL) aqueous. The combined benzene lay-
ers were dried, filtered and evaporated in vacuum to yield
2. Experimental
2.1. General methods
1H NMR and 19F NMR spectra were recorded on Bruker
AV400 (400 MHz) spectrometer with CDCl3 as the solvent and
tetramethylsilane as an internal standard. Elemental analysis
was performed on a Yanaco CHN C ORDER MT-3 appa-
ratus. Fourier transform infrared (FT-IR) spectra were mea-
sured from KBr pellets using a Bruke (Vertex 70) system. The
absorption spectra were measured using a PerkinElmer Lambda-
900 UV/vis/NIR spectrometer. Photo-irradiation was carried out
using SHG-200 UV lamp, CX-21 ultraviolet fluorescence anal-
ysis cabinet, and BMH-250 Visible lamp. Light of appropriate
wavelengths was isolated by different light filters. Fluorescence
spectra were measured using a Hitachi F-4500 spectrophotome-
ter. The X-ray experiment of the single-crystal was performed on
a Bruker P4 diffractometer equipped with graphite monochro-
matized Mo K␣ radiation at room temperature (295 2 K).
1
acetal 3 as a yellow oil (84.0%). H NMR (400 MHz, CDCl3,
TMS): δ 1.246–1.284 (t, 3H, J = 7.6 Hz, –CH3), 2.7496–2.806
(q, 2H, J = 7.6 Hz, –CH2), 3.968–4.022(t, 2H, J = 7.2 Hz, –CH2),
4.093–4.147 (t, 2H, J = 7.2 Hz, –CH2), 6.006 (s, 1H, –CH), 6.976
(s, 1H, thiophene-H).
2.2.3. Synthesis of 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-
3-thienyl}perfluorocyclopentene BEDTP
To a stirred solution of 3 (2.296 g, 8.73 mmol) in THF
was added dropwise a 2.5 mol/L n-BuLi (3.5 mL; 8.75 mmol)
at −78 ◦C under nitrogen atmosphere. Stirring was continued
for 30 min, perfluorocyclopentene (0.59 mL; 4.36 mmol) was
slowly added to the reaction mixture, and the mixture was
stirred for 2.5 h at −78 ◦C. The reaction was stopped by the
addition of water. The product was extracted with ether. The
organic layer was washed sequentially by 1.0 mol/L HCl aque-
ous solution and water. The organic layer was dried over MgSO4,
filtrated and evaporated. The crude product was purified by
column chromatography on SiO2 using CHCl3 as the eluent
and 1.2 g of BEDTP obtained in 57.7% yield. The compound
2.2. Synthesis of BEDTP
The synthesis method of diarylethene BEDTP was described
in Scheme 2 and experimental details were carried out as
following.

S. Pu et al. / Spectrochimica Acta Part A 66 (2007) 335–340
337
crystallized from chloroform/hexane (v/v = 1/2) at room temper-
Table 1
Crystal data and structure refinement
ature and produced the crystals suitable for X-ray analysis. Mp:
127.6–127.4 ◦C; Anal. Calcd for C23H22F6O4S2 (%); Found
(calcd): C, 51.11; H, 4.10. Found: C, 51.23; H, 4.20. 19F NMR
(400 MHz, CDCl3, TMS): δ 110.48 (4F), 131.94 (2F). 1H NMR
(400 MHz, CDCl3): δ 0.924–0.961 (t, 6H, J = 7.8 Hz, –CH3),
2.192–2.248 (q, 4H, J = 7.5 Hz, –CH2), 4.002–4.036 (t, 4H,
J = 6.8 Hz, –CH2), 4.101–4.136 (t, 4H, J = 7.0 Hz, –CH2), 6.021
(s, 2H, –CH), 7.094 (s, 2H, thiophene-H). IR (KBr): ν 1081,
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C23H22F6O4S2
540.53
295(2) K
0.71073 A
˚
Monoclinic
P21/n (no. 14)
◦
˚
Unit cell dimensions
a = 13.353(2) A, α = 90.00
◦
˚
˚
b = 13.370(2) A, β = 96.79◦6(11)
1131, 1274, 1566 and 2976 cm−1
.
c = 13.600(3) A, γ = 90.00
3
˚
Volume
2411.0(8) A
Z
4
2.3. Determination of the crystal structure of diarylethene
BEDTP
Density (calculated)
Absorption coefficient
F (0 0 0)
1.489 g/cm3
0.296 mm−1
1112
Crystal data were collected by a Bruker P4 diffractometer
equipped with graphite monochromatized Mo K␣ radiation at
room temperature. Unit cell was obtained and refined by 24
well centered reflections with 4.8◦ < θ < 12.5◦. Data collection
was monitored by three standards every 100 reflection collected.
No decay was observed except the statistic fluctuation in the
range of 4.2%. Raw intensities were corrected for Lorentz
and polarization effects. Direct phase determination yielded the
positions of all non-hydrogen atoms. Hydrogen atoms were
generated theoretically and ridden on their parent atoms in
the final refinement. All non-hydrogen atoms were subjected
to anisotropic refinement. The final full-matrix least-square
refinement on F2 converged with R1 = 0.0653 and wR2 = 0.1201
for 3377 observed reflections [I ≥ 2σ(I)]. The final difference
electron density map shows no features. The crystallographic
data obtained and experimental details employed were sum-
marized in Table 1. Selected bond lengths (A) and angles ( )
were listed in Table 2. Further details on the crystal structure
investigation have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication number
CCDC 274837 for this paper. These data can be obtained free of
charge via http://www.ccdc.can.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Centre, 12 Union Road,
Cambridge CB21EZ, UK [fax: (+44) 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk]).
Crystal size
θ range for data collection
Limiting indices
0.4 mm × 0.5 mm × 0.5 mm
4.8–12.5◦
−1 < h < 8, −9 < k < 9, −11 < l < 11
5323
4231 [R(int) = 0.0255]
Full-matrix least-squares on F2
4231/14/323
Reflections collected
Unique reflections
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2δ(I)]
R indices (all data)
Largest diff. peak and hole
1.027
R1 = 0.0653, wR2 = 0.1201
R1 = 0.0778, wR2 = 0.1265
−3
˚
1.033 and −0.742 e A
Table 2
◦
˚
Selected bond length (A) and angles ( )
S(1)–C(18)
S(1)–C(15)
F(3)–C(3)
F(3ꢀ)–C(3)
F(4)–C(3)
F(4ꢀ)–C(3)
O(1)–C(10)
O(2)–C(12)
C(1)–C(5)
C(1)–C(7)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(6)–C(13)
C(13)–C(14)
C(18)–S(1)–C(15)
C(11)–O(1)–C(10)
C(5)–C(1)–C(2)
F(4ꢀ)–C(3)–C(2)
F(4)–C(3)–C(2)
F(4ꢀ)–C(3)–C(4)
F(4)–C(3)–C(4)
F(4)–C(3)–F(3)
C(2)–C(3)–F(3)
C(4)–C(3)–F(3)
F(4ꢀ)–C(3)–F(3ꢀ)
C(2)–C(3)–F(3ꢀ)
C(4)–C(3)–F(3ꢀ)
C(1)–C(5)–C(4)
O(1)–C(11)–C(12)
C(14)–C(13)–C(6)
1.719(4)
1.727(4)
1.513(7)
1.533(8)
1.252(6)
1.207(9)
1.408(5)
1.398(6)
1.339(5)
1.468(5)
1.506(5)
1.489(6)
1.503(6)
1.497(5)
1.504(6)
1.480(7)
92.58(15)
107.7(4)
110.6(3)
128.8(8)
120.4(6)
120.6(7)
125.3(6)
96.7(6)
◦
˚
3. Results and discussion
3.1. Photochromic properties of BEDTP in solution
The photochromic behavior of BEDTP was investigated and
its absorption spectra and color changes in hexane solution
(C = 5.0 × 10−5 mol/L) were shown in Fig. 1. The absorption
maximum of its colorless open-ring isomer (BEDTP-a) was
observed at 238 nm. Upon irradiation with 254 nm UV light,
the colorless solution turned to magenta with a new broad
absorption band centered at 542 nm. This can be assigned to
the formation of the closed-ring isomer (BEDTP-b), as shown
in Fig. 1(a). The magenta colored solution of BEDTP-b returned
to colorless upon irradiation with visible light (λ > 510 nm).
This indicates that BEDTP-b returned to the initial open-ring
form which the absorption maximum was observed at 238 nm.
Their solution color changes upon UV light irradiation were
99.0(5)
101.9(4)
97.8(7)
96.4(5)
94.9(5)
110.9(3)
107.4(4)
115.4(4)

338
S. Pu et al. / Spectrochimica Acta Part A 66 (2007) 335–340
Fig. 2. ORTEP drawings of crystal BEDTP. The ellipsoids were drawn at 35%
probability level.
conformations so that two fluorine atoms of C(3) are split to two
sets named as F(3), F(4) and F(3ꢀ), F(4ꢀ). The molecule has five
planar ring, including one perfluorocyclopentene ring, two thio-
phene rings and two 1,3-dioxoane rings. So, they can form four
dihedral angles between every two adjacent rings. The dihedral
angle between perfluorocyclopentene ring and the left thiophene
ring is 56.3(1)◦, and that of between the perfluorocyclopen-
tene ring and the right thiophene ring is 49.0(2)◦. The dihedral
angle between C10–O1–C11–C12–O2 and C6–C7–C8–C9–S2
plane is 79.9(2)◦, and that between C19–O3–C20–C21–O4 and
C15–C16–C17–C18–S1 plane is 84.0(2)◦.
Fig. 1. Absorption spectra and color changes of diarylethene BEDTP in hexane
solution (5.0 × 10−5 mol/L). (a) Absorption spectra: open-ring form (BEDTP-
a, solid line), closed-ring form (BEDTP-b, dot line), and in the photostationary
state (dash line) upon irradiation with 254 nm UV light; (b) color changes before
and after UV irradiation.
In the perfluorocyclopentene ring of BEDTP-a, distances
of C1–C2, C2–C3, C3–C4, C4–C5 and C1–C5 are 1.506(5),
˚
1.489(6), 1.503(6), 1.497(5) and 1.339(5) A, respectively. These
data clearly indicate that the C1–C5 bond is a double bond, being
shown in Fig. 1(b). The coloration–decoloration cycle could
be repeated more than 20 times and a clear isosbestic point
was observed at 314 nm even after 20 times. All these results
described above indicated that it owned good photochromic
properties. The quantum yields of cyclization and cyclorever-
sion reactions of BEDTP were determined by comparing the
reaction yields of the diarylethene in hexane against 1,2-bis(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene in hexane at
room temperature [28], and their values were 0.362 (254 nm)
and 0.024 (510 nm), respectively.
3.2. Relationship between crystal structure and
photochromic properties
Final structural confirmation of diarylethene BEDTP was
provided by X-ray crystallographic analysis. Its structure fea-
tures were shown in Fig. 2 and a packing diagram was shown
in Fig. 3 [18]. As shown in Fig. 2, the molecule occupied
approximately in C2 symmetry, and it was packed in a photoac-
tive anti-parallel conformation in crystal, which can undergo
photocyclization reaction [29]. It was found that the perfluo-
rocyclopentene is planar which is the average of two envelope
Fig. 3. A packing views along the b direction.

S. Pu et al. / Spectrochimica Acta Part A 66 (2007) 335–340
339
Fig. 4. Photographs of photochromic process of BEDTP in crystalline phase.
significantly shorter than the other carbon–carbon single bonds.
Two justly symmetrical thiophene moieties were linked by a
double bond C1–C5. The two-ethyl groups located on different
sides of the double bond and trans-direction of the thiophene
planes. This kind conformation was crucial to its photochromic
and photoinduced properties [15]. The distance of C6–C15 is
been reported [32–36]. In this work, the fluorescence prop-
erties in the solution of the title compound were measured
using a Hitachi F-4500 spectrophotometer, and the breadths of
excitation and emission slit were selected 5.0 nm. The fluores-
cence excitation and emission spectra of BEDTP-a in hexane
(C = 1.0 × 10−4 mol/L) at room temperature was illustrated in
Fig. 5. From this figure, one can clearly see that the hexane
of BEDTP exhibited relatively strong fluorescence at 400 nm
when excited at 282 nm. The Stokes shift of the fluorescence
was relatively large and the fluorescence spectral edge showed
a red-shift in comparison with the absorption edge. This kind of
large Stokes shift has already been discussed in detail by Sekiya
and co-workers [6].
The concentration dependence of fluorescence emission
spectra was measured in hexane solution at room tempera-
ture, and it was exhibited that the fluorescence intensity held
the maximum value when the concentration of BEDTP was
1.0 × 10−4 mol/L. So, we used this optimal concentration solu-
tion to investigate its fluorescence spectral changes upon irra-
diation with 254 nm UV light, and the results were shown
in Fig. 6, and inset figure showed the relationships between
−log(int) and the exposal time upon irradiation with 254 nm
˚
3.828(6) A, which is short enough theoretically for the reaction
to take place in the crystalline phase [30]. It is well known that
the absorption spectrum and color are dependent on the sub-
stituent effects and the ␲-conjugation length in molecule. The
arrangement described above was very beneficial to form the
extended conjugation. After irradiation with UV light, the ␲-
conjugation extended throughout the whole molecule, and the
UV–vis absorption spectra displayed drastic changes resulting
in displaying remarkable different color.
As a mater of fact, crystal of BEDTP showed photochromic
reaction coincident with the theoretical analysis. Their color
changes upon photo-irradiation are shown in Fig. 4. Upon irradi-
ation with 254 nm light, the colorless crystal of BEDTP-a turned
to magenta quickly. When the magenta crystal was dissolved
in hexane, the solution turned to magenta, and the absorption
maximum was observed at 542 nm, which was the same as that
of the closed-ring isomer BEDTP-b shown in Fig. 1. Unfor-
tunately, the crystal structure of BEDTP-b could not be given
because it returned easily to BEDTP-a under the condition of
experiment. The magenta color disappeared upon irradiation
with 510 nm light and the absorption spectrum of the solution
containing the colorless crystal was the same as that of the open-
ringisomerBEDTP-a. Comparingthetwophotographsshownin
Fig. 4, it clearly showed that the photochromic process had taken
place in BEDTP crystalline phase upon irradiation with UV and
visible light. Furthermore, the diarylethene crystals exhibited
many times of coloration/decoloration cycles by alternate irra-
diation with UV and visible light. So, the crystals of diarylethene
BEDTP will be good candidates for optoelectronic applications
[17].
3.3. Fluorescence properties of BEDTP
Fluorescent properties can be useful not only for track-
ing individual molecules within a microenvironment, but
also in molecular-scale optoelectronics [31]. To date, many
diarylethene derivatives and their fluorescent properties have
Fig. 5. Excitation spectra(dash line) and fluorescence spectra (solid line) of
BEDTP-a in hexane (1.0 × 10−4 mol/L) at room temperature.

340
S. Pu et al. / Spectrochimica Acta Part A 66 (2007) 335–340
dation of Jiangxi, China (No. 050017) and by the Science Funds
of the Education Office of Jiangxi, China (No. [2005] 140).
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Fig. 6. Change in the intensity of the fluorescence spectra of BEDTP in hexane
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UV light. From this figure, it can be easily found that BEDTP-
a exhibited relatively strong fluorescence at 400 nm. How-
ever, along with the photochromism from open-ring isomers
BEDTP-a to closed-ring isomers BEDTP-b upon irradiation
with 254 nm light, the fluorescence intensity decreased dra-
matically and −log(int) and the exposal time showed good
linearity at the first 6 min, and the photostationary state (PS)
showed weak fluorescence. The closed-ring isomers BEDTP-b
showed almost no fluorescence. This fluorescent characteristics
which use external photostimulation to switch the fluorescence
on and off from open-ring isomers to closed-ring isomers is sim-
ilar to those of some other diarylethene compounds reported
[37,38]. It had been reported that this kind of diarylethenes
which showed fluorescence in the open-ring isomer and showed
no fluorescence or weak fluorescence in the closed-ring isomer
could be potentially applied to optical memory with fluores-
cence readout method and fluorescence modulation switches
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In summary, the diarylethene described here has been first
synthesized and its structure was confirmed by X-ray crystal-
lographic analysis. The compound took on good photochromic
behavior both in solution and in crystalline phrase. The open-
ring isomers of the diraylethene exhibited relatively strong
fluorescence and the fluorescence intensity decreased upon irra-
diation with 254 nm light, and its closed-ring isomers showed on
fluorescence. So, it can be potential to use as an optical recording
material possessing fluorescence readout.
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This work was financially supported by the Natural Science
Foundations of China (No. 20564001), Natural Science Foun-