A novel sensitive sensor for Cu2+ and multi-switch based on a diarylethene with a 2-(2′-hydroxyphenyl)benzothiazole unit

Article abstract of DOI:10.1016/j.tet.2016.04.007

A new asymmetrical photochromic diarylethene with a 2-(2′-hydroxyphenyl)benzothiazole unit has been synthesized. It underwent reversible cyclization and cycloreversion reactions upon alternating irradiation with lower energy UV (297 nm, 25 μW/cm²

Full text of DOI:10.1016/j.tet.2016.04.007

Accepted Manuscript
2+
A novel sensitive sensor for Cu and multi-switch based on a diarylethene with a 2-
2′-hydroxyphenyl)benzothiazole unit
(
Xiaorong Dong, Renjie Wang, Gang Liu, Peng Liu, Shouzhi Pu
PII:
S0040-4020(16)30260-5
10.1016/j.tet.2016.04.007
TET 27650
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 January 2016
Revised Date: 30 March 2016
Accepted Date: 5 April 2016
2+
Please cite this article as: Dong X, Wang R, Liu G, Liu P, Pu S, A novel sensitive sensor for Cu and
multi-switch based on a diarylethene with a 2-(2′-hydroxyphenyl)benzothiazole unit, Tetrahedron (2016),
doi: 10.1016/j.tet.2016.04.007.
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A novel sensitive sensor for Cu and multi-
switch based on a diarylethene with a 2-(2′-
hydroxyphenyl)benzothiazole unit
a
ab
a
a
ab
Xiaorong Dong, Renjie Wang,* Gang Liu, Peng Liu , Shouzhi Pu,*

ACCEPTED MANUSCRIPT
2
+
A novel sensitive sensor for Cu and multi-switch based on a
diarylethene with a 2-(2′-hydroxyphenyl)benzothiazole unit
a
ab
a
a
ab
Xiaorong Dong, Renjie Wang,* Gang Liu, Peng Liu , Shouzhi Pu,*
a
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University,
Nanchang 330013, P. R. China
b
College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, P. R. China
*
Corresponding author 1: Tel.: +86-791-3831996; fax: +86-791-3831996; E-mail address:
pushouzhi@tsinghua.org.cn (S. Pu)
*
Corresponding author 2: Tel.: +86-791-88531582; E-mail address: bio-wrj@163.com (R. Wang).
1

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Abstract: A new asymmetrical photochromic diarylethene with a 2-(2′-hydroxyphenyl)benzothiazole
unit has been synthesized. It underwent reversible cyclization and cycloreversion reactions upon
2
alternating irradiation with lower energy UV (297 nm, 25 ꢀW/cm ) and visible light, while it could
induce intramolecular proton transfer reaction upon irradiation with higher energy UV light (254 nm,
2
9
2 ꢀW/cm ) with distinct color change. The absorption maximum of the closed-ring isomer was
redshifted from 539 nm to 660 nm and the color changed from purple to cyan when the diarylethene
was stimulated with base. Furthermore, addition of acid enhanced its fluorescence intensity by 5.5 fold
and the emission peak was blueshifted from 596 nm to 570 nm with a concomitant color change from
light orange to bright green, while base could efficiently quench its fluorescence intensity. Finally, the
2
+
diarylethene was highly selective towards Cu with significant absorption and fluorescence changes,
2
+
which makes the ‘naked-eye’ detection of Cu possible.
Keywords: Diarylethene; Photochromism; Fluorescence switch; Multi-switch; Copper(Ⅱ ) recognition.
2

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1. Introduction
Photochromism is reversible transformations between two isomers with different characteristics
induced by alternating illumination with UV and visible light. During the past few decades, many
photochromic materials have attracted numerous attentions because of their potential applications in
1
photonic devices and optical memory media. Among various types of photochromic compounds,
diarylethenes are most promising candidates for optoelectronic applications owing to their excellent
2
thermal stability, remarkable fatigue resistance, higher conversion rate, and rapid response. Based on
these properties, many functional diarylethene molecules, which possess multi-responsive, fluorescence
probing, ion recognition, and chemosensing abilities have been designed and constructed in the past
3
few years.
It is a hot topic in current chemical research for designing and synthesizing multiple responsive
switching molecules, and integrating switchable unit into a function molecule is an effective method to
construct a multiple responsive switching molecule, whose properties can be efficiently tuned by the
switchable unit. Diaryelthene is an ideal molecule for switchable unit because it could be transformed
distinctly from one form to the other through photoirradiation. Generally, conventional switches of
4
5
6
diarylethenes include photochromic switches, fluorescence switches, and pH switches. So far, many
multi-switching molecules based on diarylethene skeletons have been developed. For example, Tian et
al. constructed a donor-photochromic bridge-acceptor (D-P-A) system based on electron-donating
triphenylamine/diethylphenylamine and electron-accepting dicyanovinyl group at each end of
dithienylethene. Its fluorescence was sharply enhanced through the nucleophilic reactions between
－
7
dicyanovinyl group and CN . In a previous work, we designed and synthesized a multi-addressable
molecular switch, in which a 2-benzothiazyl group was appended to diarylethene core via a salicylidene
3

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Schiff base. Its fluorescence emission wavelength and intensity sensitively responded to both pH and
cation stimuli. On the basis of these properties, two logic circuits with four inputs and one output were
8
constructed on a single molecule. Additionally, cation sensors based on photochromic diarylethenes
9
10
also have achieved much progress, and some functionalities such as crown ether, rhodamine,
11
12
pyridine, quinoline, and imidazole derivatives have been successfully incorporated into diarylethene
systems as ion recognition units.
Copper ion is an essential trace element, and it plays an important role in the human body and plant
1
3
tissues during life processes. However, copper ion could cause neurodegenerative diseases, such as
1
4
Alzheimer’s and Wilson’s diseases at high concentrations. In addition, it is also a widespread metal
pollutant in nature. Sometimes copper ion exhibits high toxicity even at submicromolar concentration
1
5
for certain microorganisms. Due to the dual properties of copper ion, much attention has been paid to
2
+ 16
the development of chemosensors of Cu . In past several decades, many works focused on designing
2
+
10,17
highly selective fluorescence sensors for Cu based on diarylethene . However, colorimetric sensors
2
+
18
for convenient detection of Cu are rarely reported. 2-(2′-Hydroxyphenyl)benzothiazole (HBT) group
is a fascinating functional group because the phenolate and the nitrogen atoms in the benzothiazolyl
1
9
could be used as proton donors /acceptors. Therefore, it could be used as reactive sites for acid/base
or metal ion stimuli. According to the unique physicochemical properties of HBT group, we believe
that introducing a HBT unit into a diarylethene skeleton will form a multi-responsive molecule. In this
work, we designed and synthesized a new asymmetrical diarylethene with HBT unit. Its
photochromism and fluorescence properties were studied systemically. The photochromic process of
diarylethene 1 is shown in Scheme 1.
4

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Scheme 1. Photochromism of diarylethene 1O.
2
2
. Experimental
.1. General methods
1
13
All solvents were purified by distillation before use. H NMR and C NMR spectra were recorded on a
Bruker AV400 (400 MHz) spectrometer with DMSO-d as the solvent and tetramethylsilane as an
6
internal standard. Infrared spectra (IR) were recorded on a Bruker Vertex-70 spectrometer. Melting
point was measured on a WRS-1B melting point apparatus. Absorption spectra were measured using an
Agilent 8453 UV/VIS spectrophotometer. Photoirradiation was carried out using an SHG-200 UV lamp,
a CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp. The light intensity was
2
2
2
2
5 ꢀW/cm for UV (297 nm) light, 92 ꢀW/cm for UV (254 nm) light and 13.5 mW/cm for visible
light, respectively. Fluorescence spectra were measured using a Hitachi F-4600 spectrophotometer, and
the breadths of excitation and emission slit were both selected 5 nm. Fluorescence quantumn yield was
2
+
+
measured with an Absolute PL Quantumn Yield Spectrometer QY C11347-11. Expect for Mn , K ,
2+
and Ba (their counterions were chloride ions), other metal ions were obtained by the dissolution of
their respective metal nitrates (0.10 mmol) in distilled water (10.00mL). Stock solution of NaOH (0.1
-1
mol L ) were prepared in deionized water.
.2 Synthesis of 1O
The synthetic route for diarylethene 1O is shown in Scheme 2. First, the diarylethene salicylaldehyde
2
2
0
derivative 2 was prepared by the reported method. In a 100 mL flask, diarylethene 2 (0.49 g, 1.00
5

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mmol) and thiol aniline (3) (0.13 g, 1.00 mmol) were dissolved in anhydrous ethanol (50.0 mL)
containing 0.02 equiv acetic acid. The mixture was stirred at 350 K under reflux for 24 h. Then, the
reaction was allowed to slowly cool to the room temperature. Then the crude product was purified by
recrystallization. The yellow soil powder was filtered off and washed with EtOH. Pure diarylethene 1O
o
-1
(
0.27 g) was obtained in 45% yield after vacuum drying. M.p. 198-199 C; IR (KBr, v, cm ): 541, 644,
1
6
87, 760, 808, 847, 896, 956, 1067, 1128, 1198, 1273, 1436, 1505, 1598, 3062; H NMR (400 MHz,
DMSO-d , ppm): δ 2.09 (s, 3H, −CH ), 2.14 (s, 3H, −CH ), 2.36 (s, 3H, −CH ), 7.15 (d, 1H, phenyl−H,
6
3
3
3
J = 8.0 Hz), 7.45 (d, 2H, phenyl−H, J = 8.0 Hz), 7.47 (s, 1H, thiophene−H ), 7.56 (t, 1H, phenyl−H, J =
.0 Hz), 7.72 (d, 1H, phenyl−H, J = 8.0 Hz), 8.11 (d, 1H, phenyl−H, J = 8.0 Hz), 8.15 (d, 1H,
8
1
3
phenyl−H, J = 8.0 Hz), 8.44 (s, 1H, phenyl−H), 11.78 (s, 1H, −OH). C NMR (100 MHz, DMSO-d ,
6
TMS): δ =10.2, 11.7, 14.0, 103.9, 117.8, 119.3, 121.7, 122.3, 124.2, 124.3, 124.9, 125.1, 126.4, 129.4,
+
1
34.9, 140.6, 141.6, 151.4, 156.2, 158.1, 163.5, 170.2; HRMS-ESI (m/z): [M+H] Calcd. For
(
C H F N O S ), 593.0792, found: 593.0709.
28 18 6 2 2 2
Scheme 2. Synthetic route to diarylethene 1O.
3
3
. Results and discussion
.1 Photochromism of 1
6

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1.0
0.8
0.6
0.4
0.2
0.0
0
0
0
0
0
0
.05
.04
.03
.02
.01
Vis
UV
.00
4
50
500
550
600
650
Wavelength (nm)
UV
Vis
500
3
00
400
600
700
Wavelength (nm)
-5
-1
Fig. 1. Absorption spectral changes of 1 by photoirradiation in acetonitrile (2.0 × 10 mol L ) at room
temperature.
-5
-1
The absorption spectra changes of 1 in acetonitrile solution (2.0 × 10 mol L ) under irradiation with
UV and visible light at appropriate wavelength are shown in Fig. 1. The open-ring isomer of 1 was
4
−1
−1
colorless in solution with an intense absorption band at 295 nm (ε, 4.60 × 10 L mol cm )
2
corresponding to the π→π* transitions. Upon irradiation with UV light (λ = 297 nm, 25 ꢀW/cm ), a
3
−1
−1
new absorption band at 539 nm (ε, 2.24 × 10 L mol cm ) was observed, indicating the formation of
the closed-ring isomer 1C with color change from colorless to light purple. The photostationary state
(PSS) was achieved after 1O was irradiated for 30 min, and the conversion was estimated to ca. 40%
1
by H NMR analysis. The original absorption spectrum could recover completely upon irradiation with
visible light (λ ≥ 500 nm). The result indicated that the closed-ring isomer (1C) could be reversed back
to the open-ring isomer (1O). The coloration–decoloration cycles of 1 could be repeated more than 100
times with negligible degradation (Fig. S1, Supporting information (SI)). The cyclization and
cycloreversion quantum yields of
1
were determined in acetonitrile by using
2
1
1
,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as a reference, and the values were
20
measured as 0.02 and 0.59, respectively. Compared to the precursor compound 2, the cyclization
7

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quantum yield of 1 (Φ1, O-C = 0.02) was much lower than that of 2 (Φ2, O-C = 0.10), while the
cycloreversion quantum yield of 1 (Φ1, C-O = 0.59) was much higher than that of 2 (Φ2, C-O = 0.08). The
results shown that the benzothiazolyl group at the terminal phenol could remarkably inhibit the
photocyclization of 1O, but notably increase the ring-opening reaction of 1C. Additionally, it is worth
noting that the light purple solution of 1C could be changed to red upon further irradiation with high
2
energy UV light (λ = 254 nm, 92 ꢀW/cm ). Upon illumination with 254 nm light, the absorption band
of 1C at 539 nm hypochromatically shifted to 483 nm accompanied with the molar absorption
3
−1
−1
4
−1
−1
coefficient sharply increased from ε (1C, 2.24 ×10 L mol cm ) to ε′ (1C′, 1.24 × 10 L mol cm )
Fig. 2A). It was possible that the closed-ring isomer of 1C underwent intramolecular proton transfer
from the phenol to the benzothiazolyl group, inducing the structure of 1C to change from the enol form
(
22
to ketone form (Scheme 3). After photoirradiation with 254 nm light for 20 min, the PSS of 1C′ was
arrived. Subsequently, the absorption band of 1C′ at 483 nm gradually decreased when it was irradiated
with appropriate visible light (λ ≥ 500 nm). Unfortunately, the red color solution of 1C′ could not be
bleached entirely after irradiation with visible light for 2 hrs (Fig. 2B). The results indicated that the
2
3
ketone form closed-ring isomer of 1C′ was stable and less photoactive to visible light. In order to
determine the structure of ketone form (1C′), the 2-(2′-hydroxyphenyl)benzothiazole (HBT) group was
synthesized. Fig. S2 shows the absorption spectral change of HBT group induced by 254 nm UV light.
The results demonstrated that the original peak at 272 nm and 349 nm gradually decreased when
1
irradiation with 254 nm light. It is worth noting that the process is irreversible. In addition, the H
NMR change of HBT group illuminated by 254 nm UV light was also obtained. As shown in Fig. S3, a
new signal peak appeared at 5.22 ppm (N−H), while the integral area of the signal peak at 12.80 ppm
(O−H) was become lower than before photoirradiation. The results demonstrated that the hydroxy
8

ACCEPTED MANUSCRIPT
proton (O−H) of HBT group was transfer to the benzothiazolyl group (N−H) during this process.
Moreover, the MS-ESI date shown that 1O and 1C′ with the same molecular weight (Fig. S4, SI),
which further verified only photo-induced intramolecular proton transfer reaction was take place during
the process of 1O changed to 1C′.
1
.0
1.0
0.8
0.6
0.4
0.2
0.0
1O
1O
0.8
0.6
0.4
UV
UV
Vis
1
C'
1C'
0.2
1C
1O'
0.0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
(A)
(B)
-5
-1
Fig. 2. Absorption spectral changes of 1C by photoirradiation in acetonitrile (2.0 × 10 mol L ): (A)
absorption spectral changes of 1C upon irradiation with 254 nm light; (B) absorption spectral changes
of 1C′ upon alternating irradiation with visible light and 254 nm light.
F
F
F F
F
F
F
F
F
F
F
297 nm
Vis
F
S
S
N
N
N
N
O
S
F
O
S
OH
1
O
1C
OH
2
54 nm
F
F
F F
F
F
F
F
F
F
254 nm
Vis
F
S
S
N
N
H
N
H
N
O
S
O
S
O
1
O'
1C'
O
Scheme 3. Structural and color changes among different forms of 1 with different UV light in
-5
-1
acetonitrile (2.0 × 10 mol L ).
3.2 Multi-photochromism of 1 with stimulation of base/acid
Diarylethene 1 also exhibited multi-responsive characteristics when it was stimulated with base/acid
9

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+
and light. In acetonitrile, the deprotonated 1O-H was obtained after the addition of NaOH solution
-1
(
8.0 µL, C = 0.10 mol L ). The absorption maximum of 1O at 295 nm gradually decreased, when
NaOH solution was added (0 to 20.0 equiv). Meanwhile, two new absorption bands appeared at 345
and 427 nm with color change from colorless to yellow. The results demonstrated that the deprotonated
+
20,24
-1
1
O-H was formed (Fig. 3A).
Neutralization with hydrochloric acid (HCl, 5.0 µL, C = 0.10 mol L )
+
recovered the absorption spectrum of 1O. In acetonitrile, diarylethene 1O-H also underwent
photoisomerization upon alternating UV and visible light. Upon irradiation with 297 nm UV light, the
+
original absorption bands of 1O-H at 295 nm, 345 nm, and 427 nm decreased. Simultaneously, a new
3
−1
−1
visible absorption band centered at 660 nm (ε, 6.90 × 10 L mol cm ) increased due to the formation
+
of a closed-ring isomer 1C-H accompanied with color change from yellow to cyan (Fig. 3B). The
colored solution could be bleached completely upon irradiation with visible light (λ ≥ 500 nm),
+
+
suggesting the closed-ring isomer 1C-H returned to the open-ring isomer 1O-H . The quantum yields
+
of cyclization and cycloreversion of 1O-H were determined to 0.03 and 0.26, respectively. Compared
+
to 1O, the cyclization quantum yield of 1O-H showed negligible change, but the cycloreversion
+
quantum yield notably decreased. After 50 repeated cycles, no remarkable decomposition of 1O-H
+
was detected by UV/Vis spectral analysis, indicating 1O-H has good fatigue resistance.
1
.0
1.0
0.8
0.6
0.4
0.2
0.0
1O
1O
-
+
OH
H
0.8
0.6
UV Vis
+
1
O-H
Vis
UV
0.4
+
-
1O-H
+
OH
H
2
0 equiv/NaOH
Vis UV
0.2
+
1C-H
0
0.0
300
400
500
600
700
800
3
20
400
480
560
Wavelength (nm)
Wavelength (nm)
(A)
(B)
1
0

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Fig. 3. Absorption spectral and color changes of 1 by NaOH/HCl and light stimuli in acetonitrile (2.0 ×
-5
-1
1
0 mol L ) at room temperature: (A) absorption spectral changes of 1O upon addition of NaOH/HCl,
+
(
B) absorption spectral changes 1O-H by photoirradiation.
1.0
0.8
0.6
0.4
0.2
0.0
1
O
-
+
H
OH
2
0 eq/NaOH
+
H
-
+
OH
1C-H
0
1C
300
400
500
600
700
800
Wavelength (nm)
-5
-1
Fig. 4. Absorption spectral and color changes of 1C in acetonitrite (2.0 × 10 mol L ) with the
stimulation of NaOH and HCl.
+
Moreover, the colored isomer of 1C also could be transformed to deprotonated 1C-H via the
stimulation of base. During the addition of NaOH (0 to 20.0 equiv) to the solution containing 1C, the
+
light purple color of solution changed to cyan due to the formation of deprotonated 1C-H , whose
absorption maximum was remarkably redshifted from 539 nm to 660 nm (Fig. 4). The results were well
+
consistent with the absorption spectrum of 1O-H in PSS upon irradiation with 297 nm light. Compared
+
to 1C, the absorption maximum of 1C-H was bathochromically shifted by 121 nm. The color and
+
absorption spectrum of 1C-H could be converted back to 1C by neutralization with HCl (12.5
equiv.).The structure and color changes between neutral and deprotonated states induced by base/acid
and light are described in Scheme 4.
11

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+
+
Scheme 4. Structural changes between 1O, 1O-H , 1C, and 1C-H .
3.2 Fluorescence properties of 1
2
5
Like most diarylethenes have been reported, diarylethene 1 exhibited notable fluorescence in
-5
-1
acetonitrile (2.0 × 10 mol L ). Its fluorescence spectrum was measured at room temperature, and the
result is shown in Fig. 5A. The emission peak of 1O was observed at 596 nm when excited at 380 nm.
The fluorescence quantum yield of 1O was determined to be 0.003. Upon irradiation with 297 nm UV
light, the emission intensity at 596 nm decreased gradually along with the photoisomerization due to
the production of the weakly fluorescent closed-ring isomer 1C. After exposure to UV light (λ = 297
nm) for 30 min, the emission intensity of 1O was quenched to ca. 88%. The residual fluorescence in the
PSS may be attributed to the incomplete cyclization reaction and the existence of parallel
2
6
conformation. Back irradiation with visible light of appropriate wavelength (λ > 500 nm) regenerated
the open-ring isomer 1O and recovered the original emission spectrum. It should be noted that the
fluorescence of 1O could be modulated by acid/base stimulation. Fig. 5B shows the emission spectral
-5
-1
changes of 1O by alternating addition of acid and base in acetonitrile (C = 2.0 × 10 mol L ). Addition
-1
+ 27
of acid (HCl, 9.6 µL, C = 0.10 mol L ) to 1O produced the protonated diarylethene 1O+H . The
+
emission peak hypochromatically shifted from 596 (1O) nm to 570 (1O+H ) nm, and its fluorescence
1
2

ACCEPTED MANUSCRIPT
emission intensity was enhanced significantly by 5.5 fold with a concomitant change of color from
+
light orange to bright green. The absolute fluorescent quantum yield of the 1O+H was determined to
+
be 0.01. The fluorescence spectrum of 1O+H could return to the initial state 1O by neutralizing with
-1
+
base (NaOH, 9.6 µL, C = 0.10 mol L ). Similar to 1O, the emission intensity of 1O+H also could be
modulated by illumination UV/Vis light (Fig. 5C). Upon irradiation with 297 nm light, the emission
+
intensity of 1O+H was quenched to ca. 53% when arrived at the PSS. Compared to 1O, the
+
fluorescent modulation efficiency of 1O+H was significantly improved, which may be attributed to
2
8
the intermolecular charge transfer. Furthermore, base could also tune the fluorescence properties of 1.
Upon addition of base (NaOH, 0 to 20.0 equiv.), the emission intensity of 1O was distinctly quenched
+
to 74% due to the formation of deprotonated compound 1O-H (Fig. 5D). The emission intensity of 1O
-1
could be restored to the initial state via neutralizing with acid (HCl, 5.0 ꢀL, C = 0.10 mol L ). The
results indicated that an efficient multi-responsive fluorescence switch could be constructed based on
diarylethene 1 using acid/base and light as stimuli.
1
20
600
450
300
150
0
UV Vis
+
6
5
4
3
2
1
1O+H
9
0
-
+
H
OH
0
5
10 15 20 25
6
0
Equiv. H+
3
0
1
O
0
550
600
650
700
750
4
80
540
600
660
720
Wavelength (nm)
Wavelength (nm)
(A)
(B)
1
3

ACCEPTED MANUSCRIPT
1
20
+
1O
6
5
4
3
2
1
00
00
00
00
00
00
1O+H
-
+
H
100
OH
UV Vis
8
0
6
0
+
1
C+H
40
20
0
+
1
O-H
1O
0
4
50
500
550
600
650
700
750
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
(C)
(D)
Fig. 5. Emission spectral changes and photos of 1 by stimulation of acid/base and light in acetonitrile
-5
-1
(
2.0 × 10 mol L ) excited at 380 nm: (A) emission intensity changes of 1O; (B) emission intensity
+
changes of 1O with the stimulation of HCl; (C) emission intensity changes of 1O+H by
photoirradiation; (D) emission intensity changes of 1O with the stimulation of NaOH.
2
+
3.3 Photochromism and fluorescence response to Cu
2
+
Furthermore, diarylethene 1 shows high selectivity to Cu with obvious absorption spectral and color
2
+
changes. Upon addition of Cu (0 to 10.0 equiv.), the absorption maximum of 1O at 295 nm gradually
2
+
increased and a new visible absorption appeared at 420 nm due to the formation of 1O-Cu complex
accompanied with distinct color change from colorless to light yellow (Fig. 6A). The subsequent
addition of excess of EDTA did not recover its original color and absorption spectrum, indicating that
2
+
the sensing process of 1O in response to Cu was irreversible. Interestingly, the closed-ring isomer 1C
2
+
2+
also showed obvious selectivity towards to Cu . When Cu was added to the solution of 1C, the
2
+
absorption intensity at 539 nm decreased and reached a plateau after 10.0 equiv of Cu was added.
Meanwhile, a new absorption band at 420 nm was detected by UV/Vis absorption and the solution
color changed from purple to light yellow. The results indicated that the closed-ring isomer 1C could be
2
+
2+
transformed into 1O-Cu complex, and the absorption spectrum of 1C-Cu was well consistent with
1
4

ACCEPTED MANUSCRIPT
2
+
2+
the absorption of 1O-Cu . The sensing process of 1C in response to Cu could not recover the
original color after addition of excess of EDTA (Fig. 6B). In addition, the light yellow solution of
2+
1
O-Cu could not turn purple when irradiated with 297 nm UV light for more than 30 min, suggesting
2
+
that the 1O-Cu complex exhibited no photochromism. The reason might be attributed to the
2
+
high-affinity binding of diarylethene 1 with Cu , which restricted the interconversion efficiency from
2
+ 29
the parallel to the anti-parallel conformationof 1O-Cu . These results indicated that the ‘naked-eye’
2
+
sensor for Cu based on diarylethene 1 could be constructed in acetonitrile.
1
.2
1.2
0.9
0.6
0.3
0.0
2
+
Cu
0.9
2
+
1O-Cu
2+
C-Cu
1
0.6
2+
10 eq/Cu
0.3
2+
Cu
1
o
0
2+
Cu
1C
1o
0.0
3
00
350
400
450
500
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
(A)
(B)
2
+
Fig. 6. Absorption spectral and color changes of 1O (A) and 1C (B) upon addition of Cu (0−10.0
-5
-1
equiv.) in acetonitrile (2.0 × 10 mol L ).
It is noteworthy that the fluorescence properties of 1 could be modulated by metal ions. The binding
2
+
3+
3+
3+
2+
+
2+
2+
2+
ability of 1O with different ions, such as metal ions Cu , Al , Fe , Cr , Mn , K , Ca , Mg , Co ,
2
+
2+
2+
2+
2+
2+
2+
2
Ni , Ba , Cd , Hg , Sr , Zn , Pb (Fig. 7), and anions Fꢁ, Clꢁ, Brꢁ, Iꢁ, NO ꢁ, NO ꢁ, SO ꢁ, HSO ꢁ,
3
2
4
4
2
2
SO ꢁ, CO ꢁ, HCO ꢁ, CH COOꢁ, and SCNꢁ (Fig. S5, SI) were tested by fluorescence spectroscopy in
3
3
3
3
2
+
acetonitrile. It is clearly seen that 1O exhibits highly selective for Cu . As shown in Fig. 7, the
-5
-1
emission intensity change of 1O (2.0 × 10 mol L ) induced by the addition of different metal ions
(10.0 equiv.). It can be seen that the fluorescence emission intensity of 1O was notably decreased only
1
5

ACCEPTED MANUSCRIPT
2+
3+
2+
2+
when Cu was added. Except that Al , Cd , and Zn can slightly enhance the emission intensity,
3
+
3+
2+
+
2+
2+
2+
2+
2+
2+
2+
2+
other cations, such as Fe , Cr , Mn , K , Ca , Mg , Co , Ni , Ba ,Hg , Sr , and Pb lead to
negligible decrease in the emission intensity of 1O. The fluorescence intensity of 1 declined
2
+
remarkably after the Cu concentration increased from 0 to 10.0 equiv. followed by a plateau with
further titration, and the emission intensity of 1O at 596 nm was remarkably quenched to ca. 11% (Fig.
8). The results may be attributed to the ligand-to-metal charge transfer, in which the charge was
3
0
transferred from the ligand towards the coordinating metal ion. The inset of Fig.8 depicts the effect of
2
+
Cu concentration on the emission intensity at 596 nm. A linear relationship was observed between the
2
+
fluorescence intensity of 1O and Cu concentration in the range of 0 to 4.0 equiv., indicating that 1O
2
+
could be potentially used as a colorimetric fluorescent probe for Cu .
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-5
-1
Fig. 7. The (F−F )/F ratio of diarylethene 1O at 596 nm in acetonitrile (2.0 × 10 mol L ) in the
0
0
presence of respective metal ions (10.0 equiv.).
1
6

ACCEPTED MANUSCRIPT
1
0
0
0
.0
.8
.6
.4
120
90
60
30
0.2
0
2 4 6 8 10 12 14
2+
Equiv. Cu
0
2+
10 eq/Cu
0
5
00
550
600
650
700
750
Wavelength (nm)
2
+
−5
−1
Fig. 8. Emission intensity changes of 1O induced by Cu in acetonitrile (2.0 × 10 mol L ) (Inset
2
+
shows its emission intensity changes along with the addition of Cu from 0 to 14.0 equiv.).
2+
In order to confirm the stoichiometry between 1O and Cu , a Job’s plot was performed in acetonitrile
3
1
according to the reported method, and the results are depicted in Fig. 9A. It can be easily calculated
2
+
2+
that the complex rations of compounds between 1O and Cu is 1:1. The binding sites for Cu to
coordinate with diarylethene 1 are probably the -OH group of phenol and the N atom of benzothiazole.
3
2
(
Scheme 5). In addition, the 1:1 binding stoichiometry was further confirmed by ESI-Mass analysis.
+
As shown in Fig. 9B, the free 1O had a main peak at 593.0 m/z for [1O+H] . When excess amounts of
2
+
2+
+
Cu were added into 1O, a new peak at 654.9 m/z was observed due to the formation of [1O–Cu -H] .
2
+
Additionally, the association constant (K ) of 1O with Cu was obtained using the liner
a
6
−1
Bensi-Hildebrand expression, and the value of K was found to be 3.6 × 10 L mol (Fig. S6, SI). The
a
2
+
result further confirmed that 1O bound to Cu with a higher affinity.
1
7

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40
30
20
10
0
0.0
0.2
0.4
0.6
2+
0.8
1.0
2+
χ = [Cu ]/[Cu ]+[1O]
(A)
(B)
2
+
Fig. 9. Job’s plot showing the 1:1 complex between 1O and Cu (A), and ESI-MS spectrum changes of
2
+
1
O upon addition of Cu (B).
Scheme 5. Proposed mechanism of the absorption and emission intensity changes for 1O in the
2
+
presence of Cu by photo-irradiation.
4
. Conclusions
In summary,
-(2′-hydroxyphenyl)benzothiazole unit was synthesized, and its multi-responsive behaviors to light,
a
novel
asymmetrical
photochromic
diarylethene
containing
a
2
2
+
acid/base, and Cu were systematically investigated. The results demonstrated that different energy
UV light could regulate its conformational transition between enol form and ketone form. Induced by
the stimulation of base, the maximum absorption of the closed-ring isomer exhibited remarkable
1
8

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bathochromic shift of 121 nm. However, the fluorescence intensity of the diarylethene remarkably
increased by 5.5 fold when it was protonated with acid. Moreover, the diarylethene exhibited dramatic
2
+
changes in its absorption spectra and color upon addition of Cu , indicating it could be potentially
2
+
applied as highly selective ‘naked-eye’ colorimetric sensors for Cu . Our experimental results
demonstrated that multi-responsive signal transmission at the unimolecular platform could be realized
by using a photochromic diarylethene with a HBT functional group.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51373072, 21362013
and 21262015), the Project of Jiangxi Advantage Sci-Tech Innovative Team (20142BCB24012), the
Science Funds of Natural Science Foundation of Jiangxi Province (20132BAB203005), the Science
Funds of Jiangxi Science & Technology Normal University (2014XJZD001), and the Masters'
Innovative Foundation of Jiangxi Science and Technology Normal University (YC2014-X03).
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