A highly selective fluorescent sensor for Cd2+ based on a new diarylethene with a 1,8-naphthyridine unit

Article abstract of DOI:10.1016/j.dyepig.2016.12.023

A new asymmetrical photochromic diarylethene has been synthesized by using 1,8-naphthyridine as a functional group and perfluorodiarylethene as photoswitching unit via a peptide bond linkage, and its molecular structure was characterized by single crystal X-ray diffraction analysis. The compound exhibited favorable photochromism upon irradiation with UV/vis light, and its fluorescent behaviour could be efficiently modulated by light, base/acid, and metal ion in THF. The deprotonated derivative of the diarylethene also showed good photochromism, and displayed remarkable “turn-on” fluorescence response during photocyclization process. Furthermore, the diarylethene was highly selective toward Cd²⁺ with an obvious fluorescent color change from dark to bright green in THF. Finally, a logic circuit was fabricated with four inputs of the combinational stimuli of UV/vis and Cd²⁺/EDTA, and one output of fluorescence intensity.

Full text of DOI:10.1016/j.dyepig.2016.12.023

Accepted Manuscript
2+
A highly selective fluorescent sensor for Cd based on a new diarylethene with a
1,8-naphthyridine unit
Xiaoxia Zhang, Renjie Wang, Congbin Fan, Gang Liu, Shouzhi Pu
PII:
S0143-7208(16)31058-0
10.1016/j.dyepig.2016.12.023
DYPI 5648
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 October 2016
Revised Date: 7 December 2016
Accepted Date: 11 December 2016
Please cite this article as: Zhang X, Wang R, Fan C, Liu G, Pu S, A highly selective fluorescent sensor
2+
for Cd based on a new diarylethene with a 1,8-naphthyridine unit, Dyes and Pigments (2017), doi:
10.1016/j.dyepig.2016.12.023.
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ACCEPTED MANUSCRIPT
A highly selective fluorescent sensor for Cd2+ based on a new
diarylethene with a 1,8-naphthyridine unit
Xiaoxia Zhang,a Renjie Wang,ab* Congbin Fan,a Gang Liu,a Shouzhi Pua*
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).
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Abstract: A new asymmetrical photochromic diarylethene has been synthesized by using
1,8-naphthyridine as a functional group and perfluorodiarylethene as photoswitching unit via a
peptide bond linkage, and its molecular structure was characterized by single crystal X-ray
diffraction analysis. The compound exhibited favorable photochromism upon irradiation with
UV/vis light, and its fluorescent behaviour could be efficiently modulated by light, base/acid, and
metal ion in THF. The deprotonated derivative of the diarylethene also showed good photochromism,
and displayed remarkable “turn-on” fluorescence response during photocyclization process.
Furthermore, the diarylethene was highly selective toward Cd2+ with an obvious fluorescent color
change from dark to bright green in THF. Finally, a logic circuit was fabricated with four inputs of
the combinational stimuli of UV/vis and Cd2+/EDTA, and one output of fluorescence intensity.
Keywords: Diarylethene; Photochromism; Fluorescent sensor; Cd2+ recognition; Logic circuit.
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Introduction
Compared with traditional analytical methods, such as ion selective electrodes, voltammetric
methods and colorimetric sensors, the development of fluorescent chemosensors for sensing and
recognition metal ions of importance to the environment and health has attracted considerable
attention, due to their simplicity, high sensitivity, good selectivity and rapid response time [1-6,].
Among various metal ions, Cd2+ has been recognized as a highly toxic heavy metal ion and widely
used in many fields such as electroplating, metallurgy, weapons industry and agriculture [7-10].
Humans are chronically exposed to Cd2+ sources through the ingestion of contaminated food or
water，which can easily lead to serious injury to the kidney, lung, bone, nervous system and even
certain cancers [11-17]. Furthermore, compared with other metal cations, the detection of Cd2+ has
always been a thorny problem because it is difficult to eliminate serious interference from Zn2+, as
they have similar physical and chemical properties [18-22]. To date, only a few highly selective
fluorescent chemosensors for Cd2+ have been developed [14, 23].
Photochromism is a reversible transformation between two isomers with different absorption spectra
in response to light stimuli , and has always attracted substantial attention for a wide range of
applications as building blocks, molecular motors, photochromic lenses, nanoscale optical storage
devices, multifunctional devices and logic circuits. Among various photochromic compounds,
diarylethene derivatives have been well-recognized due to their excellent thermal stability, high
photoisomerization quantum yields and remarkable fatigue resistance [24-31]. In recent years, many
fluorescence chemosensors and multi-photoswitchable molecules based on diarylethenes have been
designed and synthesized [32-34]. For example, Zeng and coworkers have reported a
phenathrene-bridged terarylene with two crown ethers as ion recognizing moieties, which can
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efficiently detect Hg2+ via the colorimetric sensing mechanism [35]. Tian et. al have exploited an
asymmetric diarylethene with a o-amimophenol moiety to selectively recognize Cu2+ through
inducing a change in fluorescence. Moreover, its fluorescence could further increase via the specific
coordination of CN- with the Cu2+ complex, and a complicated logic circuit with three inputs (UV,
Cu2+, and CN-) and three outputs (I ≤ 100, 100 < I ≤ 150, and I ≥ 150, I = fluorescence intensity at
448 nm) was developed using these characters [36]. In our previous work, various functional units
(such as quinoline, phenanthroline, terpyridine and antipyrine) were introduced into diarylethenes,
and most could serve as chemosensors for various metal ions (Hg2+, Sn2+, Zn2+ and Al3+) [37-40].
However, only few reports on diarylethene-based fluorescent chemosensors are for Cd2+ [41-43],
because Zn2+-induced interference in Cd2+ detection is difficult to eliminate [44-46].
1,8-Naphthyridine, a simple heterocycle with two ring nitrogens is well known for typical
intramolecular change transfer fluorophore, high photostability, large Stokes’ shift and insensitivity
to pH [47]. In recent years, 1,8-naphthyridine derivatives have also attracted much attention because
they exhibit multi-functional biological activity [48-51]. The two edge-fused pyridine rings in
1,8-naphthyridine ligand have a constrained “bite” distance (2.2 Å), which is attributed to high
coordinate architecture [52]. In addition, modifications of 1,8-naphthyridine have given rise to a
great number of derivatives with tunable binding properties and a variety of fluorescent properties
via chelating with ions (such as Hg2+, Cu2+, Cr3+ and CN-) [48,53-56]. However, fluorescent sensors
based on diarylethene bearing a 1,8-naphthyridine unit for detecting Cd2+ have not hitherto been
reported. On the basis of the mentioned facts, a new photochromic diarylethene fluorescence
chemosensor bearing a 1,8-naphthyridine unit (1O) with high selectivity for Cd2+ was constructed.
The photochromic behaviour and multifunctional fluorescent switching characteristics of 1O
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induced by light, acid/base, and metal ion were systematically studied and discussed. The schematic
illustration of photochromism is shown in Scheme 1.
<Scheme 1>
2. Experimental
2.1. General methods
Unless otherwise stated, all of the materials for the synthesis of the target compound were purchased
from various commercial sources and used without further purification. The solvents used were
purified by standard methods prior to use. The solutions of metal ions (0.1 mol L-1) were prepared
by the dissolution of their respective metal nitrates in distilled water, except for Mn2+, Hg2+, K+, and
Ba2+ (their counterions were chloride ions). NMR spectra were recorded on a Bruker AV400 (400
MHz) spectrometer with CDCl3 as the solvent and tetramethylsilane as an internal standard. Infrared
spectra (IR) were recorded on a Bruker Vertex-70 spectrophotometer. 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 MUL-165 UV lamp and a
MVL-210 visible lamp. The required wavelength was isolated by the use of appropriate filters.
Fluorescence spectra were measured using a Hitachi F-4600 spectrophotometer with 5 nm slit for
both excitation and emission. Mass spectra were obtained on a Bruker AmaZon SL spectrometer.
Elemental analysis was measured with a PE CHN 2400 analyzer. The fluorescence quantum yield
was measured with an Absolute PL Quantum Yield Spectrometer QY C11347-11. The pH was
measured with a PHS-3C analyzer. Crystal structure was solved through direct methods and
refined through full-matrix least-squares procedures on F2 in SHELXTL-97 program. Further details
on the crystal structure investigation have been deposited with The Cambridge Crystallographic
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Data Centre as supplementary publication CCDC 1511213 for 1O. Copy of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 0 1223
336033 or e-mail:deposit@ccdc.cam.ac.uk).
<Scheme 2>
2.2
Synthesis
of
4-(4-(2-(2,5-dimethylthiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopent-1-en-1-yl)-5-methylthiophe
n-2-yl)-N-(1,8-naphthyridin-2-yl)benzamide 1O
The synthetic route for diarylethene 1O is shown in Scheme 2. The intermediate compounds 2 and 3
were synthesized according to the similar procedures as previous descriptions [57,58]. A mixture of
compound 3 (0.56 g, 1.0 mmol), 4 (0.15 g, 1.0 mmol), EDCI (0.23 g, 1.2 mmol), HOBT (0.16 g, 1.2
mmol) and triethylamine (0.41 mL, 3.0 mmol) in anhydrous CH2Cl2 (50 mL) was stirred at room
temperature for 10 hours under a nitrogen atmosphere. The reaction mixture was washed with water,
dried over anhydrous Na2SO4, and evaporated. The crude product was purified by column
chromatography using petroleum ether/ethyl acetate (v/v = 1:1) as eluent to obtain 0.15 g of the
target compound 1O as white solid in 24% yield. M.p. 504–505 K; 1H NMR (400 MHz, THF, ppm):
δ 1.82 (s, 3H), 1.89 (s, 3H), 2.32 (s, 3H),6.71 (s, 1Hz), 7.27–7.30 (m, 1H), 7.46 (s, 1H), 7.67 (d, 2H,
J = 8.0 Hz), 8.07 (d, 2H, J = 8.0 Hz), 8.12 (d, 1H, J = 8.0 Hz), 8.20 (d, 1H, J = 8.0 Hz), 8.52 (d, 1H,
J = 8.0 Hz), 8.84 (s, 1H), 10.45 (s, 1H); 13C NMR (100 MHz, THF, ppm): δ 11.50, 11.62, 12.01,
113.63, 118.49, 118.70, 122.10, 122.44, 122.73, 123.19, 124.37, 126.98, 131.74, 134.40, 134.77,
136.38, 136.93, 138.13, 139.25, 140.62, 151.42, 152.96, 153.39, 163.66; IR (KBr, ν, cm-1):712, 735,
759, 783, 809, 825, 843, 890, 937, 988, 1050, 1102, 1138, 1187, 1264, 1275, 1336, 1424, 1493,
1608, 1674, 2860, 2928, 3217, 3517; Anal. Calcd for C31H21F6N3OS2: C, 59.13; H, 3.36; N, 6.67;
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found: C, 59.04; H, 3.41; N, 6.59; HRMS-ESI (m/z): [M + Na+]+ calcd. 652.0928, found: 652.0901.
3. Results and discussion
3.1 Photochromic and fluorescent properties
<Figure 1>
The photochromic behavior of diarylethene 1O induced by UV/vis light was investigated in THF
solution (C = 2.0 × 10-5 mol L-1) at room temperature. As shown in Figure 1, the open-ring isomer
1O exhibited a sharp absorption peak at 331 nm (ε = 3.62 × 104 L mol-1 cm-1) due to π → π*
transition [56]. Upon irradiation with 313 nm light, a new visible absorption band centered at 562
nm (ε = 9.31 × 103 L mol-1 cm-1) emerged while the original peak at 331 nm decreased due to the
formation of the closed-ring isomer 1C, accompanied with the colorless solution becoming purple.
Alternatively, the purple solution could be completely bleached upon illumination with visible light
(λ > 450 nm). After irradiating with 313 nm light for 10 min, the photostationary state (PSS) of 1
was arrived and a clear isosbestic point was observed at 353 nm, which supported the reversible
two-component photochromic reaction [59,60]. In addition, the photoconversion ratio from
1
open-ring to closed-ring isomer of 1 was analyzed by H NMR in PSS, and the conversion value
was determined as 48 %. The cyclization and cycloreversion quantum yields of 1 were determined
in THF by using 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as a reference, and the
values were measured as 0.18 and 0.10, respectively.
<Figure 2>
In order to observe molecular conformation and evaluate the photochromic reactivity in the crystal,
X-ray crystallographic analysis was carried out. The single crystal of 1 was obtained by slow
evaporation of methanol solution. Table S1 shows crystallographic data for 1O, which belonged to
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space groups of Triclinic P-1 and contain a molecule of water. The crystal density of 1O was
determined to be 1.442 g cm–3. Table S2 lists the corresponding dihedral angles and distances
between the reactive carbon atoms. For 1O, the dihedral angles between the hexafluorocyclopentene
ring and the heteroaryl ring [thiophene ring (S2) and thiopehene ring (S1)] are 46.7° for
S2/C26-C27-C29-C30 and 41.5° S1/C16- C17-C18-C19. The dihedral angle between the thiophene
ring (S1) and its adjacent benzene ring is 20.5°. Notably, the dihedral angle between the benzene
ring and the naphthyridine is 1.6°, indicating the two rings are near coplanarity. Figure 2A shows the
molecular structure of 1 in the crystal. It can be clearly seen that 1O molecule adopts a
photoreactive anti-parallel conformation. The double bond (C21–C25) of the
hexafluorocyclopentene ring is 1.355 Å, which is significantly shorter than other carbon-carbon
single bonds (1.503 Å to 1.517 Å) of the central ring. The intramolecular distance between the two
reactive carbon atoms (C19–C27) is 3.573 Å. Therefore, the conformation and the distance fulfill
the requirement for diarylethene molecule to undergo photocyclization reaction in the single
crystalline phase [61]. Upon irradiation with UV light (λ = 365 nm), the light yellow single crystal
of 1O quickly turned to blue due to the formation of the closed-ring isomer 1C. Conversely,
irradiation of visible light (λ > 450 nm) completely erased the blue color of the crystal, indicating
that the process was reversible (Figure. 2B).
<Figure 3>
Like most other reported diarylethenes [62,63], 1O exhibited fluorescence switching characteristics
upon alternating irradiation of UV and visible light. Figure 3 shows the emission spectral changes of
1O upon photoirradiation when excited at 360 nm light. The fluorescence emission peak of 1O was
observed at 439 nm in THF, and the absolute fluorescence quantum yield was determined to be
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0.003. Upon irradiation with 313 nm UV light, the emission intensity of 1O gradually decreased due
to the formation of non-fluorescence closed-ring isomer 1C. The back irradiation with appropriate
visible light (λ > 450 nm) regenerated the open-ring isomer 1O, and recovered the original emission
intensity. In the PSS, the fluorescence intensity of 1O was quenched to ca. 35%. The residual
fluorescence in the photostationary state may be attributed to the incomplete cyclization and the
existence of isomers with parallel conformation.
3.2 Changes in absorption and fluorescence by base/acid stimuli
<Figure 4>
Just as the reported diarylethenes [38,64], diarylethene 1 also exhibited multi-responsive
characteristic upon stimulation with base/acid and UV/vis light. The absorption spectral changes of
1O induced by tetrabutyl ammonium hydroxide (TBAH) and hydrochloric acid (HCl) in THF were
measured at room temperature and shown in Figure 4A. When TBAH solution was added to the
colorless solution of 1O, the absorption maximum at 331 nm gradually decreased and a new
absorption band was observed at 387 nm due to the formation of deprotonated diarylethene 1O′.
This change could be easily seen visually, as the colourless solution of 1O became pale yellow. In
an absorption titration experiment, the absorption intensity of 1O′ at 387 nm was enhanced
accompanied with the TBAH concentration increased from 0 to 20.0 equiv (8.0 µL, 0.1 mol L-1),
followed by a plateau with further titration (Figure S1, SI). Reversely, 1O′ could be neutralized with
20.0 equiv HCl (8.0 µL, 0.1 mol L-1) and recover the absorption spectrum of 1O. Notably, 1O′ also
could undergo reversible photoisomerization (Figure 4B). Upon irradiation with 313 nm light, the
pale yellow solution of 1O′ turned to magenta and a new obvious visible absorption band was
observed at 550 nm (ε = 2.81 × 104 L mol-1 cm-1) due to the formation of the closed-ring isomer 1C′.
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The magenta solution could be thoroughly reverted to pale yellow upon irradiation with visible light
(λ > 450 nm), indicating the original open-ring isomer 1O′ was restored. The cyclization and
cycloreversion quantum yields of 1′ were measured to be 0.27 and 0.005 (Table 1). Compared with
1, the molar absorption coefficient (1C′, ε = 2.81 × 104) and the cyclization quantum yields (Φ 1′, o-c
= 0.27) exhibited notable enhancement, while the cycloreversion quantum yields (Φ1′, c-o = 0.005)
distinctly decreased. The result indicated that the deprotonation effect could efficiently promote the
photocyclization reaction and heavily suppress the photocycloreversion reaction of 1. The structures
and color changes of 1′ induced by base/acid and UV/vis light are illustrated in Scheme 3. Moreover,
the reversible transformation between 1C and 1C′ could be realized by the stimulation of TBAH
and HCl (Figure 4C). After adding 21.0 equiv (8.4 µL, 0.1 mol L-1) TBAH to the purple solution of
1C, the color changed to magenta and the absorption intensity notably enhanced with a obvious
blue-shifted from 562 nm to 550 nm due to the formation of the deprotonated 1C′, which could also
return to 1O by stimulation with 21.0 equiv of HCl (8.4 µL, 0.1 mol L-1).
<Table 1>
<Scheme 3>
Notably, the deprotonation effect not only influenced the absorption spectra of 1 but also heavily
affected the fluorescence properties. Like the open-ring isomer of 1O, 1O′ also showed very weak
fluorescence in THF solution when excited at 360 nm light. However, the closed-ring isomer 1C′
exhibited brilliant red fluorescence at around 678 nm when excited at 560 nm light. After reaching
the PSS, the intensity of 1O was enhanced 135 fold (Fiugre 5), and the fluorescence quantum yield
of 1C′ was determined to be 0.061 in THF through using an Absolute PL Quantumn Yield
Spectrometer QY C11347-11. The photoinduced electron transfer and the high coplanarity of the
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closed-ring isomer (1C′) may be responsible for this phenomenon. On the one side, the
deprotonation effect maybe interfere the electron transfer from 1,8-naphthyridine group to the
diarylethene, and block the photoinduced electron transfer (PET) process [18,65], which maybe
induce the fluorescence of 1C′ notable enhancement. On the other side, the structure of hexadiene in
the closed-ring isomer 1C′ had much higher coplanarity than the open-ring isomer 1O′ [24,66,67],
and the benzene ring and the 1,8-naphthyridine ring were near coplanarity. Therefore, the molecular
structure of 1C′ displayed a high coplanarity, which further induced the fluorescence enhancement
effect. The result was completely contrary to that reported for a diarylethene with a salicylidene
Schiff base substituent, in which the deprotonated closed-ring isomer exhibited non-fluorescence,
while the open-ring isomer showed strong fluorescence emission and bright orange emission [68].
Back irradiation by appropriate visible light (λ > 450 nm) regenerated the open-ring isomers 1O′,
and recovered the original emission intensity. Therefore, the switching behavior of the
deprotonation diarylethene (1O′) is a promising candidate for potential application in “turn-on”
photoswitchable devices.
<Figure 5>
3.3 Fluorescence response to metal ions
<Figure 6>
The fluorescence changes of diarylethene 1 induced by addition of various metal ions in THF was
investigated at room temperature. Figure 6 shows the emission spectra and fluorescence color
changes of 1O induced by the addition of various metal ions (3.0 equiv), including Fe3+, Sr2+, Co2+,
Ni2+, Mn2+, Ba2+, Pb2+, Hg2+, Ca2+, Mg2+, Sn2+, Cd2+, Zn2+, Al3+, Cr3+, Cu2+, K+ and Ag+ in THF (2.0
× 10-5 mol L-1), respectively. It could be seen that none of these ions induced any significant
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changes in the fluorescence spectrum of 1O except for Cd2+, Ag+, and Zn2+. Compared to Cd2+, the
fluorescence emission intensity of 1O exhibited minor change with the addition of the same amount
of Ag+ and Zn2+. The results indicated that the binding ability of Cd2+ with diarylethene 1 was
higher than that of other metal ions. Additionally, in order to elucidate the response time of Cd2+ to
1O, the relationship curve between the duration time of Cd2+ and emission intensity change of 1O
were carried out. Figure S2 (SI) shows the emission intensity of 1O as a function of time in the
presence of 3.0 equiv of Cd2+ in THF. It could be easily seen that the fluorescence intensity of 1O
showed no obvious change during prolonging the time from 0 to 30 min. The results indicated that
the fluorescence of 1O in response to Cd2+ was independent to time.
Figure 7A shows the fluorometric titration profile of 1 with Cd2+. Upon gradual addition of Cd2+ to
the solution of 1, the original weak fluorescence signal in the range of 400–500 nm was remarkably
red-shifted to 541 nm with a marked enhancement in fluorescecnce intensity. Upon excitation at 360
nm, a concomitant color change from dark to bright green was observed due to the formation of the
complex 1O-Cd2+. When the concentration of Cd2+ increased to 3.0 equiv, the fluorescence intensity
of 1O-Cd2+ at 541 nm reached a plateau. There was a good linearity between the concentration of
Cd2+ (0 → 3.0 equiv) and the ratio of the fluorescence intensities (I541 nm/I439 nm) (Figure S3, SI),
demonstrating that diarylethene 1 could quantitatively detect Cd2+ at the relevant concentration. In
addition, the binding constant (Ka) of 1O with Cd2+ was determined to be 8.60 × 103 L mol-1 with
good linear relationship (R =0.998, Figure S4, SI), as obtained by fitting the data to the
Benesi-Hildebrand expression [69]. Meanwhile, the absolute fluorescence quantum yield of
1O-Cd2+ was determined to be 0.039. After EDTA was added to 1O-Cd2+, the fluorescence of 1O
was restored. The result was attributed to the complexation-dissociation reaction between Cd2+ and
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EDTA. Moreover, the complex 1O-Cd2+ exhibited a notable fluorescence switch by photoirradiation
(
Figure 7B). Upon irradiation with 313 nm light, the emission intensity of 1O-Cd2+ was quenched to
ca. 87% due to the formation of the non-fluorescence 1C-Cd2+ when arrived at the PSS.
Alternatively, the back irradiation with appropriate visible light regenerated the open-ring isomer
1O-Cd2+ and recovered its original emission intensity. Therefore, dual-controlled photoswitching
circulation behavior of 1O induced by Cd2+/EDTA and UV/vis light stimuli could be constructed
(Scheme 4). Furthermore, in order to check the suitable pH range for Cd2+ detection, the pH effect
on the complexation of 1O with Cd2+ were carried out at room temperature. When pH was increased
from 2.0 to 7.5, the fluorescence emission intensity of 1O gradually enhanced with the addition of
3.0 equiv Cd2+. The emission intensity showed slowly decreased and arrived at a plateau with the
increase of pH value from 7.5 to 13.5 (Figure S5, SI). The results indicated that 1O could be used as
a good Cd2+ probe even in acid and base environment.
<Figure 7>
<Scheme 4>
Subsequently, a competitive experiment was carried out by adding Cd2+ to the solution of 1O in the
presence of other metal ions, and the results was shown in Figure 8A. Before the introduction of
Cd2+, there were almost no fluorescence changes at 541 nm in the presence of other metal ions.
When 3.0 equiv Cd2+ was introduced to the above mentioned solution, a notable fluorescence
enhancement effect at 541 nm was observed. The results clearly demonstrated any metal ions
considered in the study did not interfere with the detection of Cd2+. In order to understand the
binding stoichiometry of 1O-Cd2+, Job's plot experiment was performed. As shown in Figure 8B, the
emission intensity at 541 nm was plotted as a function of the mole fraction of 1 under a constant
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total concentration. The concentration of the complex 1O-Cd2+ approached the maximum value
when the molar fraction of [1O]/([1O]+[Cd2+]) was 0.5, suggesting that a 1:1 bind stoichiometry
1
ratio for 1O-Cd2+ complex. To further confirm 1O binding form with Cd2+, the H NMR titration
experiment in THF-d8 was performed. The chemical shift of the amide NH can be used to
distinguish whether Cd2+ is bound to carbonyl oxygen or imidic acid nitrogen [70]. As shown in
Figure 8C, the chemical shift of the amine NH exhibited resonance upfield with the addition of Cd2+
increased from 0 to 1.0 equiv. The results indicated that the carbonyl oxygen was participated in the
complexation with Cd2+ because the Cd2+ blocked the amide resonance and induced the NH
resonance upfield. The Hb-e protons showed downfield shifts in THF-d8 (Hb 8.08→8.36; Hc
8.14→8.50; Hd 8.84→8.87, and He 7.30→7.57), indicating that Cd2+ coordinated with two
naphthyridine nitrogen-atoms. Furthermore, the formation of 1O-Cd2+ complex was confirmed by
- +
using ESI-MS in which the peak at m/z = 804.8 for [1 + Cd2+ + NO3 ] (Figure S6, SI). The result
indicated that 1O bound to Cd2+ with a bind stoichiometry of 1:1. The binding mechanism between
1O and Cd2+ is shown in Scheme 4. Moreover, to determine the limit of detection of 1O toward Cd2+,
we recorded the fluorescence data starting from the Cd2+ concentration as low as 10-9 M using 0.04
µM solution of 1O (Figure S7, SI). From the concentration-dependent fluorescence titration
experiment, the detection limit of 1O was found to be 1.97 × 10-7 mol L-1 for Cd2+, by the reported
method [71].
<Figure 8>
3.4 Application as a logic circuit
As mentioned above, the fluorescence of 1O could be independently modulated by the stimulation
of UV/vis light and Cd2+ / EDTA. On the basis of this fact, a combinational logic circuit by the
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stimulation of light irradiation (In1: 313 nm light and In2: λ > 450 nm visible light) and chemical
species (In3: Cd2+ and In4: EDTA) as input signals and the change of fluorescence intensity at
541nm as output signal (Figure 9) were constructed. The emission intensity of 1O at 541 nm was
regarded as an initial. When the emission intensity at 541 nm was 3-fold larger than the initial value,
the output signal could be regarded as ‘on’ state with a Boolean value of ‘1’; Otherwise, it was
regarded as ‘off’ state with a Boolean value of ‘0’. Under the stimuli of different inputs, the
diarylethene showed an ‘on’ - ‘off’ - ‘on’ fluorescence switching behavior. Consequently, 1O could
read a string of four inputs and write one output. For example, when the string is ‘0, 0, 1, and 0’ the
corresponding input signals of In1, In2, In3, and In4 are ‘off, off, on, and off’. Under these
conditions, 1O was converted to 1O-Cd2+ by the stimulation of Cd2+ and its emission intensity
enhanced dramatically. As a result, the output signal is ‘on’ and the output digit is ‘1’. Similarly,
other stimulations also resulted in the same ‘on’ - ‘off’ - ‘on’ fluorescence switch and all possible
logic strings were concluded in the combinational logic circuit as shown in Table 2.
<Figure 9>
<Table 2>
4. Conclusions
In summary, a novel fluorescent sensor based on a photochromic diarylethene with a
1,8-naphthyridine unit was developed, and it exhibited multi-responsive behaviour when triggered
by light, acid/base, and Cd2+. The deprotonated derivative not only showed excellent photochromic
performance but also displayed notable near infrared fluorescence emission during the
photocyclization. In addition, the diarylethene could be utilized as a fluorescent sensor for detection of
Cd2+ with high selectivity, and a logic circuit was constructed on the basis of the unimolecular
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platform. This work provides a useful design strategy for the construction of diarylthene-based
fluorescent chemosensor for the recognition of specific metal ions with a certain functionalized
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), and the Masters' Innovative Foundation of Jiangxi Science and Technology
Normal University (YC2014-X03).
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Table Captions:
Table 1. Absorption spectral properties of diarylethenes 1 and 1′ in THF (2.0 × 10-5 mol L-1) at room
temperature.
Table 2. Truth table of all possible strings of four binary-input data and the corresponding output
digit.
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Table 1.
Conversion
at PSS in
THF
THF
Φc
Compd.
λo,max/nma (ε/L mol-1 cm-1)
λc,max/nmb (ε/L mol-1 cm-1)
562 (9.31 × 103)
Φo-c
Φc-o
331 (3.62 × 104)
387 (9.27 × 103)
1
0.18
0.10
48%
72%
550 (2.81 × 104)
1′
0.27 0.005
a Absorption maxima of open-ring isomers.
b Absorption maxima of closed-ring isomers.
c Quantum yields of cyclization reaction (Φo-c) and cycloreversion reaction (Φc-o), respectively.
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Table 2.
Input
Output a
In3 (Cd2+)
In4 (EDTA)
λem = 541 nm
In1 (UV)
In2 (Vis)
0
1
0
0
0
1
1
1
0
0
0
1
1
1
0
1
0
0
1
0
0
1
0
0
1
1
0
1
1
0
1
1
0
0
0
1
0
0
1
0
1
0
1
1
0
1
1
1
0
0
0
0
1
0
0
1
0
1
1
0
1
1
1
1
0
0
0
1
0
0
0
0
1
0
0
1
0
0
0
0
a
At 541 nm, the emission intensity above the original value of 1C–Cd2+ (250) is defined as 1,
otherwise defined as 0.
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Figure Captions:
Scheme 1. Photochromism of diarylethene 1O.
Scheme 2. Synthetic route to diarylethene 1O.
Scheme 3. Schematics of molecular structures and fluorescence changes of 1O induced by TBAH /
HCl and light stimuli.
Scheme 4. Schematics of molecular structures and fluorescence changes of 1O induced by Cd2+ /
EDTA and light stimuli.
Figure 1. Absorption spectral and color changes of 1 by photoirradiation in THF (C = 2.0 × 10-5
mol L-1) at room temperature.
Figure 2. ORTEP drawing of crystals of 1O (A) and photographs of photochromic for diarylethene
1 (B) in the crystalline phase.
Figure 3. Changes in the fluorescence of 1O upon alternating irradiation with UV and visible light
in THF (C = 2.0 × 10-5 mol L-1), excited at 360 nm.
Figure 4. Absorption spectral changes of 1 induced by TBAH / HCl and light stimuli in THF (C =
2.0 × 10-5 mol L-1) at room temperature: (A) absorption spectral changes of 1O upon addition of
TBAH / HCl, (B) absorption spectral changes of 1O′ by photoirradiation, (C) absorption spectral
changes of 1C upon addition of TBAH / HCl.
Figure 5. Changes in the fluorescence changes of 1 induced by TBAH / HCl and light stimuli in
THF (C = 2.0 × 10-5 mol L-1) at room temperature, excited at 650 nm: (A) 1O′ induced by
photoirradiation.
Figure 6. Changes in the fluorescence of 1O induced by the addition of various metal ions (3.0
equiv) in THF (C = 2.0 × 10-5 mol L-1): (A) emission spectral changes, (B) emission intensity
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changes, (C) photos demonstrating changes in its fluorescence.
Figure 7. Emission spectral changes of 1O induced by Cd2+ / EDTA and light stimuli in THF (C =
2.0 × 10-5 mol L-1) at room temperature, excited at 360 nm: (A) 1O induced by Cd2+ / EDTA, (B)
emission intensity changes of 1O-Cd2+ by alternating irradiation with UV / vis light.
Figure 8. (A) Competitive tests for the fluorescent responses of 1O to various metal ions in THF:
Red bars represent the addition of 3.0 equiv of various metal ions to the solution of 1O, Black bars
represent the addition of Cd2+ (3.0 equiv) to the above solution, (B) Job’s plot showing the 1:1
1
complex between 1O and Cd2+, and (C) The H NMR spectrum changes of 1 by addition of Cd2+
solution in THF-d8.
Figure 9. The combinational logic circuit equivalent to the truth table given in Table 1: In1 (297 nm
light), In2 (500 nm visible light), In3 (Cd2+), In4 (EDTA) and O (Output signal).
29