Unsymmetrical diarylethenes as molecular keypad locks with tunable photochromism and fluorescence via Cu2+ and CN- coordinations

Article abstract of DOI:10.1039/c2cc16942h

New unsymmetrical diarylethenes were synthesized and their photochromic and fluorescent properties are tailored by Cu²⁺ and CN^- coordinations. A novel molecular logic keypad lock is constructed based on the fluorescence emission changes by the inputs of UV/visible irradiation, Cu ²⁺ and CN^-.

Full text of DOI:10.1039/c2cc16942h

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COMMUNICATION
Unsymmetrical diarylethenes as molecular keypad locks with tunable
photochromism and fluorescence via Cu2+ and CNÀ coordinationsw
Qi Zou, Xin Li, Junji Zhang, Ji Zhou, Bingbing Sun and He Tian*
Received 9th November 2011, Accepted 21st December 2011
DOI: 10.1039/c2cc16942h
New unsymmetrical diarylethenes were synthesized and their
photochromic and fluorescent properties are tailored by Cu2+
and CNÀ coordinations. A novel molecular logic keypad lock is
constructed based on the fluorescence emission changes by the
inputs of UV/visible irradiation, Cu2+ and CNÀ.
with Cu2+ can recognize CNÀ by ensuring high stabilization
effects on the ligand field.8,9
Herein, we report a convenient synthesis of two novel
photochromic compounds possessing unsymmetrical molecular
architectures, P1 and P2 shown in Scheme 1. Compound P2
integrates the naphthalene moiety as a fluorophore with
interaction sites for Cu2+ on both the ends of the dithienyl-
cyclopentene to realize the photochromism and fluorescence
modulated by Cu2+ and CNÀ coordinations. Then, these
functions can be incorporated into a more complex circuitry
at the molecular level so that the logic circuit and the keypad
lock are proposed.
Electronic devices play increasing roles in our daily lives and
the development of data processing platforms using individual
molecules mimicking the operation of electronic logic gates
and circuits at the molecular level has been raised with wide
interest.1 Many researchers have therefore considered to build
important molecular scale electronic logic devices keypad
locks, which are capable of authorizing password entries, as
they open new opportunities to protect information at the
molecular scale.2 However, it is still a great challenge to design
delicate logic circuits and keypad locks using three or more
inputs for the purpose of practical applications. As a basic
binary unit, the photochromism offers a possibility for estab-
lishing such logic circuits and keypad locks.
Compound P1 is used as reference to facilitate the research
on the sensing mechanism between coordination sites and
Cu2+ in CH3CN solution. Addition of Cu2+ to the solution
of P1 produces a complex P1–Cu2+. The main absorption
band at 363 nm decreased in intensity with the addition of
Cu2+ (Fig. S1, ESIw). UV–vis titration profile of P1 and Job’s
plot indicate that P1 forms a 1 : 1 complex with Cu2+ in
CH3CN solution (Fig. S2 and S3, ESIw, respectively). Unfor-
tunately, the 1H NMR spectrum of P1–Cu2+ does not provide
Among all photochromic compounds, diarylethenes are one
of the most promising candidates for photoelectronic applica-
tions owing to their excellent thermal-stability, remarkable
fatigue-resistance, rapid response and fairly high photo-
cyclization quantum yields.3 Much progress has been made
to develop symmetrical diarylethenes with ions-modulated
photochromic properties.4 Nevertheless, for the applications
to electronic logic devices, it is essential to design unsymme-
trical diarylethenes which exhibit good optical performances
and multi-response to various ions due to their special
structures.5 Furthermore, as one of the patterns that the
fluorescence sensing of specific ions integrates into electronic
logic devices, many fluorescence enhancement chemosensors
have been designed to detect Cu2+ in the past few decades6
due to their more sensitive responses to Cu2+ than those of the
fluorescence quenching chemosensors which arise from the
paramagnetic nature of Cu2+ 7 Because the d9 electronic
.
configuration of Cu2+ displays strong binding tendencies
towards CNÀ, it is rationally concluded that the complexes
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai 200237,
China. E-mail: tianhe@ecust.edu.cn; Fax: +86 21-64252288
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc16942h
Scheme 1 Proposed sensing processes of compounds P1 and P2 with
Cu2+, complex P2–Cu2+ with CNÀ and the photochromic processes
responding to light stimuli.
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useful information due to the paramagnetic nature of Cu2+
.
To gain an insight into the stoichiometry of P1–Cu2+, the
electrospray ionization (ESI) HPLC mass spectrum was carried
out. When 1 equiv. of Cu2+ was introduced into the P1 solution,
a new peak appeared at m/z 475.9972 (Fig. S4, ESIw) that was
assigned to P1–Cu2+ with a 1 : 1 binding mode (the calculated
[C22H20NOS2ClCu]+ value was 475.9971).
The energy-minimized conformation of P1–Cu2+ is simulated
with the Gaussian 03 program package to support that P1 is
coordinated to Cu2+ by a nitrogen atom in the imide group,
an oxygen atom in the phenol group and a sulfur atom in the
thiophene group, simultaneously, as shown in Scheme 1. As
presented in Fig. S5 (ESIw), upon irradiation with 365 nm
light, a new absorption band centered at 502 nm increased
with the increase in irradiation time till the photostationary
state of P1–Cu2+ was reached corresponding to the color
change of the solution from colorless to pink (Fig. S6, ESIw),
indicating the formation of the closed isomer. Upon irradia-
tion with visible light (>500 nm), the pink solution was
bleached completely back to colorless solution, and the closed
isomer converted to the open-ring isomer (P1–Cu2+).
Fig. 1 (a) Fluorescence spectral changes of compound P2 (10 mM)
with Cu2+ (0–12 mM) in CH3CN at 25 1C, lex
= 360 nm.
(b) Fluorescence spectra of compound P2 in the presence of various
metal ions at 25 1C. [P2] = 10 mM, [Cu2+] = 10 mM, [Mn+] = 100 mM
in CH3CN, lex = 360 nm.
modulate the photochromism of diarylethene complexes by
the coordination between anions and complexes.
Interestingly, the fluorescence response of compound P2 is
relative to Cu2+ concentrations, and the fluorescence intensity
exhibits a more than 5-fold enhancement in the presence of
1 equiv. of Cu2+ (Fig. 1a). The CQN isomerization,10 which is
the predominant decay process of excited states in P2, is inhibited
through complexation of P2 with Cu2+, so compound P2 shows
a fluorescence enhancement in the presence of Cu2+. Meanwhile,
due to the reduced electron-donating ability of the phenol moiety
after the coordination of Cu2+, the photo-induced electron
transfer (PET) quenching should be effectively suppressed and
hence might yield a high fluorescence enhancement.11 Fig. 1b
demonstrates that only Cu2+ caused a prominent fluorescence
enhancement of P2, whereas very weak fluorescence variations
were observed for other metal ions. The detection limit for Cu2+
determined based on the S/B criteria is 91.1 nM (Fig. S17, ESIw).
As in a metal chelate-based sensing mechanism, copper
complexes are useful for devising fluorescent sensors for
CNÀ detection due to the high affinity of copper for CNÀ.
The sensing properties of P2–Cu2+ toward different anions
using tetrabutylammonium as counter cation were investi-
gated. As illustrated in Fig. 2, the addition of 3 equiv. of
CNÀ to P2–Cu2+ caused a distinct increase in the fluorescence
intensity, again indicating the formation of the complex
P2–Cu2+–CNÀ, while the disturbance of fluorescence intensity
for other anions could be neglected. Job’s plot experiments
reveal a 1 : 1 complex for P2–Cu2+ with CNÀ (Fig. S18, ESIw).
Under alternative illumination with ultraviolet and visible
light, P2 exhibits a typical photochromic response of diaryl-
ethenes in CH3CN solution. Fig. S7 (ESIw) shows that a new
absorption band centered at 607 nm appeared upon irradia-
tion with the light of 365 nm, indicating the formation of the
closed isomer (P3), accompanied by change of the colorless
solution to blue (Fig. S8, ESIw). Upon irradiation with visible
light ( Z 550 nm), the blue solution was bleached back to
colorless solution and the original absorption spectrum was
recovered. As depicted in Fig. S9w, addition of Cu2+ to the
solution of P2 produced a color change from colorless to
yellow with a minor absorption change (Fig. S10, ESIw).
Complex P2–Cu2+ also performs photoisomerization with
light irradiation. As seen from Fig. S11w, upon irradiation of
P2–Cu2+ with 365 nm light, the closed isomer P3–Cu2+
formed. The absorption band at 354 nm decreased and a
new band centered at 584 nm appeared, accompanied by the
color change from yellow to blue (Fig. S9, ESIw). The blue
solution returned to yellow solution with visible light ( Z550 nm)
irradiation and the original absorption spectrum was recovered.
As expected, the results from UV–Vis titration profile, Job’s
plot and HRMS (ESI+) experiment suggest a 1 : 1 binding
mode between P2 and Cu2+ (Fig. S12–S14, ESIw, respectively).
The conformation of P2–Cu2+ is also confirmed by the theore-
tical calculation shown in the ESI.w
Moreover, upon addition of excess CNÀ (100 equiv.) to P2–Cu2+
,
the increase in the fluorescence intensity is clearly observed
(Fig. S19, ESIw), which indirectly suggests that CNÀ does not
form a strong enough complex to remove Cu2+ from P2–Cu2+
.
It is noteworthy that the photochromic properties of
P2–Cu2+ can be modulated by CNÀ. Fig. S15w shows that
the bands at 354 nm decreased along with the addition of
CNÀ, indicating the formation of complex P2–Cu2+–CNÀ. It
exhibits a typical photochromic response: upon irradiation of
P2–Cu2+–CNÀ with 365 nm light, the absorption band at
350 nm decreased, and a new band at 581 nm, which corres-
ponded to the ring-closed isomer P3–Cu2+–CNÀ, appeared at
the same time (Fig. S16, ESIw). This process was accompanied
by a color change from yellow to blue. Upon irradiation with
visible light ( Z 550 nm), the blue solution returned to yellow
solution and the original absorption spectrum was recovered.
To the best of our knowledge, it is the first report that anions
Fig. 2 (a) Fluorescence spectral changes of complex P2–Cu2+
(10 mM) with CNÀ (0–30 mM) in CH3CN solution at 25 1C, lex
=
360 nm. (b) Fluorescent response of P2–Cu2+ (10 mM) to various anions
at 30 mM concentration in CH3CN solution: 1, FÀ; 2, ClÀ; 3, BrÀ; 4, IÀÀ;
5, HSO4À; 6, NO3À; 7, ClO4À; 8, CNÀ; 9, HPO42À; 10, H2PO4
11, P2O74À; 12, CH3COOÀ; 13, OHÀ. lex = 360 nm, lem = 448 nm.
;
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Therefore, the significant fluorescence enhancement might
result from the electrostatic interaction between CNÀ and
Besides, compound P2 has been used to construct a logic circuit
and a molecular keypad lock with UV irradiation, Cu2+ and CNÀ
as inputs, at the first time. This present work provides deeper
insights for the future design of unsymmetrical photochromic
compounds as multifunctional nanoscale logic devices.
This work was financially supported by NSFC (21190033),
National 973 Program (2011CB808400).
P2–Cu2+ 8a,12
As shown in Fig. S20, the detection limit for
.
CNÀ is 22.5 nM and there is a fairly linear relationship
between the fluorescence intensity and concentrations of
CNÀ from 4.0 to 20.0 mM in CH3CN solution (R = 0.993),
which can be applied as a calibration curve for CNÀ detection.
Therefore, compound P2 and its complex P2–Cu2+ can serve
as fluorescence enhancement chemosensors for Cu2+ and
CNÀ with high selectivity and sensitivity, respectively.
Notes and references
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Additionally, there is no effect on the fluorescence intensity
of P2 and P3 in the presence of CNÀ only; nevertheless,
sequentially adding Cu2+ to the solution can also induce a
‘‘turn-on’’ signal (Fig. S21 and S22, ESIw, respectively).
Besides, the fluorescence of P2–Cu2+ and P2–Cu2+–CNÀ is
quenched when they are exposed to UV light (Fig. S23 and
S24, ESIw, respectively), which is attributed to the efficient
energy transfer from the excited fluorophore core to the
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These studies inspire us to utilize compound P2 to develop a
complicated logic circuit for a molecular traffic signal with three
inputs: UV irradiation, Cu2+ and CNÀ as input-1, input-2 and
input-3, respectively. The outputs are designated as output-1
(I r 100), output-2 (100 o I r 150) and output-3 (I > 150),
respectively, where I is the value of fluorescence intensity at
448 nm. If the value of I locates in a specific range, the
corresponding output is considered as the ‘‘1’’ state, otherwise, it
is considered as the ‘‘0’’ state. Thus, according to the truth table
(Table S1, ESIw), a logic circuit with AND, OR and NOT gates
integrated within a single molecule is approached (Fig. 3a).
Depending on the different combinations of three inputs (UV
irradiation, Cu2+ and CNÀ designated as ‘‘U’’, ‘‘C’’ and ‘‘T’’,
respectively), compound P2 can switch between different
fluorescence emission states ‘‘On and Off’’. A molecular keypad
lock is constructed to visualize these sequence-dependent pheno-
mena directly. Out of six possible input combinations i.e. UCT,
UTC, CUT, CTU, TUC, TCU, only the UCT input combi-
nation (i.e. it is the password) gives birth to an instinct
fluorescent output signal (Fig. S25, ESIw). As illustrated in
Fig. 3b, when the sequence of three input keys UCT is inserted,
the keypad lock ‘‘opens’’, corresponding to a strong fluores-
cence signal. All other sequences (as wrong passwords), which
give weak fluorescence signal outputs, fail to open the lock.
In summary, two novel diarylethenes P1 and P2 with unsym-
metrical structures have been successfully synthesized. The photo-
chromic properties of compound P2 and its complex P2–Cu2+
can be easily modulated by employing Cu2+ and CNÀ, respec-
tively. P2 and P2–Cu2+ can be considered as fluorescence
enhancement chemosensors for Cu2+ and CNÀ, respectively.
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Fig. 3 (a) A logic circuit and (b) a fluorescence keypad lock with
inputs of UV irradiation, Cu2+ and CNÀ. Details are shown in the text.
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Chem. Commun., 2012, 48, 2095–2097 2097