Soft mimic gear-shift with a multi-stimulus modified diarylethene

Article abstract of DOI:10.1039/b908707a

A novel gated photochromic compound Nap-Pip-DTE was prepared to mimic a soft gear-shift based on alternative UV/Vis irradiation, protonation/ deprotonation and copper (II) ion complexation/dissociation. The fluorescence and absorption spectra were used as

Full text of DOI:10.1039/b908707a

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COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Soft mimic gear-shift with a multi-stimulus modified diarylethene†
*
Junji Zhang, Wenjuan Tan, Xianle Meng and He Tian
Received 1st May 2009, Accepted 17th June 2009
First published as an Advance Article on the web 25th June 2009
DOI: 10.1039/b908707a
modify the fluorescence and absorption outputs respectively, leading
to two other photo-active states (1Ho and 1c).
A novel gated photochromic compound Nap-Pip-DTE was prepared
to mimic a soft gear-shift based on alternative UV/Vis irradiation,
protonation/deprotonation and copper (II) ion complexation/disso-
ciation. The fluorescence and absorption spectra were used as output
signals. Copper (II) ion was introduced to mimic the parking (P)
state by complexation with two intramolecular piperazine moieties
of Nap-Pip-DTE, transforming diarylethene moieties into the
photo-inactive parallel form.
Upon irradiation with light of 254 nm, the yellowish open-ring
form Nap-Pip-DTE 1o turned red (lmax ¼ 526 nm), which was
ascribed to the transformation from open form 1o to its closed form
1c (Scheme 1(b)). Fig. 1 shows the absorption and color changes after
UV irradiation. The photostationary state, an equilibrium state of the
photocyclization reaction, was reached after irradiation at 254 nm for
25 min. Nap-Pip-DTE 1o could be photochemically regenerated
from 1c by irradiation with light of 538 nm, meanwhile the
The goal to miniaturize functional elements down to the molecular
level could result in remarkably increased performance as a conse-
quence of the higher density of functional elements compared to that
of current semiconductor devices. This has raised much attention in
the development of molecular systems simulating the operation of
electronic components such as switches, high-density data storage,
logic gates, electronic circuits, etc.1–8 Recently, mimicking functional
electronic components using photochromic compounds is of great
interest, since each isomer of the photochromic compound can
represent ‘0’ or ‘1’ of a digital code corresponding to ‘on’ and ‘off’
states stimulated by UV/Vis light. Photochromic derivatives attached
with fluorophores provide a much wider application in functional
molecular design as fluorescence can be used as the output for
detection. Besides, other external stimuli (e.g. acid/base, ions) can also
induce the photochromic derivative to undergo multi-state trans-
formations,9 thus making it a promising candidate for future complex
electronic component manufacture. Among all photochromic
compounds, diarylethene derivatives (DTE) are one of the most
attractive families due to their promising fatigue resistance, efficient
photo-isomerization, rapid response and thermally irreversible
properties.10,11 So far, a lot of work has been reported based on the
DTE systems.12–15 Although much progress has been made in
mimicking electronic components as mentioned above, reports on the
simulation of automatic control devices with molecular systems are
rather rare. In this communication, we propose a simulation of
automatic gear shifting, considered as a soft control model for
automatic driving systems. Based on this concept, a new multi-state
1,8-naphthalimide-piperazine-tethered dithienylethene (Nap-Pip-
DTE, 1o shown in Scheme 1) was designed and synthesized (ESI†
Scheme S1). Copper (II) ion was introduced as a molecular ‘‘lock’’ to
prevent photochromism after coordination with intramolecular
piperazine moieties,16 transforming the system into a photo-inactive
state (1oCu). Protonation and UV/Vis irradiation were used to
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai, 200237, P.R.
China. E-mail: tianhe@ecust.edu.cn
Scheme 1 (a) Complexation with copper (II) ion between two piperazine
moieties leads to the photo-inactive state 1oCu, which prohibits the
photocyclization. (b) Transformations from the original 1o state to the
1c and 1Ho states modulated by UV/Vis and H+, respectively.
† Electronic supplementary information (ESI) available: Synthetic details
and spectroscopic data. See DOI: 10.1039/b908707a
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The anti-parallel form is photoactive while the other one is photo-
inactive. Gated photochromic reaction of diarylethene can be realized
by turning the photoactive anti-parallel form into the photo-inactive
parallel form with hydrogen bonds17 or metal coordination bonds
with crown ether moieties18 etc. By intramolecular complexation of
copper (II) ion with two piperazine moieties,16 the photo-inactive
parallel form was in the majority resulting in the prevention of
photochromism (Scheme 1(a)). Furthermore, this Nap-Pip-DTE
system had selectivity for the copper (II) ion. Other ions such as Zn2+,
Hg2+, Fe3+, Co3+ etc. had no effect on the photochromism of this
Nap-Pip-DTE system (ESI† Fig. S7). Several factors might cooperate
to achieve the selectivity for Cu2+, such as square-planar coordination
structure and the appropriate dimension of Cu2+. This is the first
example of modifying photochromic properties of diarylethene by
direct copper (II) ion complexation. The photochromic performance
can be recovered by extracting the copper (II) ion from Nap-Pip-
DTE ligand using EDTA. The Nap-Pip-DTE-Cu complex showed
excellent thermal stability after being kept at a temperature of 333K
for about 10 days. The structure of Nap-Pip-DTE-Cu was
determined by TOF-MS ES+: calculated for (C69H71N6O4S2–Cu)
1173.42, found: 1173.4225 (ESI† Fig. S3).
Fig. 1 UV/Vis absorption spectral changes of Nap-Pip-DTE (1 ꢀ 10ꢁ5 M)
in CH3CN at 298 K upon irradiation under 254 nm. The inset shows
the reversible color change between the open form (yellow) and closed
form (red) after irradiation under 254 nm and 538 nm, respectively.
characteristic red of the closed form returned to its original yellowish
of the open form. The quantum yield corrected for the active
conformer for the photocyclization was determined (F254 ¼ 0.176),
which was found to be much higher than that for photo-
cycloreversion (F538 ¼ 0.025). The closed form 1c showed excellent
thermal stability at 333K for 10 days in the dark.
Besides complexation with copper (II), the piperazine moieties of
Nap-Pip-DTE 1o can also response to protons. As shown in Fig. 3B,
Nap-Pip-DTE 1o showed weak fluorescence due to the strong
photoinduced electron transfer (PET) effect with piperazine moieties
as electron donors.19 The PET effect can be efficiently blocked by
protonation of piperazine moieties, consequently the strong fluores-
cence of 1,8-naphthalimide moieties resumed (Scheme 1(b)). After
adding 6 equivalents of trifluoroacetic acid (TFA) to 1o solution
(CH3CN), the fluorescence emission at 510 nm (s ¼ 8.05 ns, ESI†
Fig. S8) was significantly enhanced and reached its peak of about
80ꢀ that of the unprotonated form, due to the formation of
protonated Nap-Pip-DTE (1Ho). The fluorescence can be reduced to
its initial unprotonated state 1o after neutralization of 1Ho by
triethylamine (TEA). Also after protonation, the absorption wave-
length of 1,8-naphthalimide in 1o (400 nm) was blue shifted to 378 nm
in 1Ho, while the characteristic absorption of 1c at 526 nm was red
shifted to 539 nm in 1Hc (Fig. 3A).
The photochromism of Nap-Pip-DTE 1o was prevented when
20 equivalents of copper (II) ion were added. A minor change had
taken place in the absorption spectrum of 1oCu (Fig. 2) under UV
irradiation compared with that of 1o at the wavelengths of 526 nm
and 368 nm, which were characteristic for the closed form of
Nap-Pip-DTE (1c). This color change can also be seen directly from
the inset pictures presented in Fig. 2, the color of the yellowish
solution of Nap-Pip-DTE-Cu (1oCu) is almost the same after UV
irradiation. In other words, the photochromism of Nap-Pip-DTE-Cu
(1oCu) was prevented to a large extent by copper (II) ion complex.
A diarylethene with five-membered heterocyclic rings has two
conformations with the two rings in mirror symmetry parallel
conformation and in C2 symmetry anti-parallel conformation.10
When the Nap-Pip-DTE-H 1Ho was irradiated with UV light of
254 nm, the emission at 510 nm was remarkably quenched to 10%
of the fully protonated state (Fig. 3B). Intramolecular fluorescent
resonance energy transfer (FRET) was responsible for this fluores-
cence quenching.20 The maximum absorption wavelength of the
light-induced product 1Hc (539 nm), the closed form of the proton-
ated Nap-Pip-DTE, overlaps well with the emission wavelength of
the 1,8-naphthalimide fluorophore (510 nm), which results in a strong
FRET effect leading to an efficient fluorescence quenching
(Scheme 1(b)). The fluorescence can be recovered by irradiation with
visible light at 538 nm. One thing should be pointed out: although
FRET has a photocycloreversion effect on 1c, it is not so obvious as
that of direct irradiation with 538 nm light. The photocycloreversion
quantum yield (F538 ¼ 0.025) is much lower than that of photo-
cyclization (F254 ¼ 0.176) and the energy transferred to 1c by FRET
is much weaker than that of direct 538 nm irradiation. Under such
circumstances, the FRET induced photocycloreversion has little
influence on 1c and can be neglected.
Fig. 2 UV/Vis absorption spectra of 1o (1 ꢀ 10ꢁ5 M, solid line), 1oCu (1
ꢀ 10ꢁ5 M, dash-dotted line), 1c (1 ꢀ 10ꢁ5 M, dotted line) and 1oCu after
UV irradiation (1 ꢀ 10ꢁ5 M, dashed line) in CH3CN at 298 K. The inset
shows the color changes of 1o, 1oCu and 1oCu after UV irradiation.
It is worth noting that, besides preventing the photochromism,
copper (II) ion complexation can also impact the protonation of
piperazine moieties. After the addition of 20 equivalents of copper (II)
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Fig. 4 Emission spectral changes of (a) 1Ho (1 ꢀ 10ꢁ5 M, solid line),
(b) Nap-Pip-DTE-Cu 1oCu (1 ꢀ 10ꢁ5 M, dash-dotted line) in CH3CN:
H2O ¼ 9:1 (v/v), (c) 1oCu after protonation to 1oCu + H (dashed line),
and (d) after Cu2+ was extracted from 1oCu + H by EDTA (dotted line).
All emissions are upon excitation at 400 nm at 298 K. The inset shows the
Job plot: the total concentration of Nap-Pip-DTE-H and Cu2+ ion is
0.2 mM. The inset picture shows the fluorescence changes of 1o, 1oCu,
1oCu + H and 1Ho upon excitation at 400 nm at 298K.
referring to the interconversion of the protonation/deprotonation,
cyclization/cycloreversion and complexation/dissociation, can be
illustrated as follows. First, the original non-fluorescent Nap-Pip-
DTE (1o) form was set as the neutral (N) state. This N gear state
could be shifted into other gear states (D, R or P) by any of the three
stimuli (H+, UV or Cu2+) respectively. After protonation, the fluo-
rescent Nap-Pip-DTE-H (1Ho) was formed which simulated the
drive (D) state. On this occasion, the significant fluorescence
enhancement at 510 nm due to protonation (Fig. 3) can be used as the
output which directs the shifting of the gear-box from the N state (1o)
to the D state (1Ho). Since the emission peak of 1Ho was at 510 nm
(excited at 400 nm) where 1Ho has no absorption, it met the addi-
tional requirement that the emission wavelength being measured
must reside far enough outside the spectral regions where the
photochromic reactions are induced.12a,21 On another occasion, the
closed 1c form generated upon photocyclization reaction could
modify the reverse (R) state with UV irradiation at 254 nm as the
input and absorption at 526 nm as the output. The appearance of an
absorption peak at 526 nm after photocyclization (Fig. 1) could act as
a director for the shifting procedure from the N state (1o) to the
R state (1c). Besides the protonation and UV irradiation, the
phenomenon of adding copper (II) ion was even more interesting
since it could perform a ‘‘lock’’ behavior. As mentioned above, after
adding 20 equivalents of copper (II) ion into the system, the formed
Fig. 3 (A) Absorption spectra of 1o (1 ꢀ 10ꢁ5 M, solid line), 1c
(1 ꢀ 10ꢁ5 M, dashed line), 1Ho (1 ꢀ 10ꢁ5 M, dash-dotted line) and 1Hc
(1 ꢀ 10ꢁ5 M, dotted line) in CH3CN. (B) Emission spectral changes of (a)
1o (1 ꢀ 10ꢁ5 M, dotted line), (b) 1Ho (1 ꢀ 10ꢁ5 M, solid line), (c) 1Hc
(1 ꢀ 10ꢁ5 M, dot-dashed line) and (d) 1Ho after neutralization (dashed
line) in CH3CN. All emissions are upon excitation at 400 nm at 298 K.
The inset diagram shows the emission intensity of 1o and 1Ho after
neutralization. The inset picture shows the fluorescence changes of 1o,
1Ho and 1Hc.
ion (1oCu) and then 6 equivalents of TFA subsequently (1oCu + H),
the fluorescence intensity just rose to less than 35% of 1Ho (s ¼ 8.01
ns, ESI† Fig. S8). Competitive complexation between copper (II) ion
and proton with piperazine moieties contributed to this phenomenon.
After partial complexation with copper (II) ion, fewer lone-pair
electrons were left for protonation. Thus the formation of Nap-Pip-
DTE-Cu (1oCu) prevented further protonation and remarkably
decreased the concentration of 1Ho, resulting in a significant reduc-
tion of the fluorescence intensity. The different sequence of adding
copper (II) ion and proton did not affect the phenomenon; in other
words, no sequence-ordered behavior was observed (ESI† Fig. S6).
The fluorescence intensity can be restored to over 80% of the initial
intensity by adding EDTA. The dynamic quenching constant of
Cu2+, Ksv ¼ 1.65 ꢀ 104 L$molꢁ1, which can be calculated using the
Stern–Volmer formula: I0/I ¼ 1 + Kqs0CQ ¼ 1 + KsvCQ. The Job plot
depicted in Fig. 4 demonstrates that the complexation ratio of copper
(II) ion with Nap-Pip-DTE ligand is 1:1.
It should be pointed out that this Nap-Pip-DTE system could
mimic the soft control of an automatic gear-shift. This concept,
Fig. 5 The scheme of mimicking the soft gear-shift by Nap-Pip-DTE.
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chromic behavior (Fig. 2) but also reduced the effect of protonation
of Nap-Pip-DTE (1o) (Fig. 4). In other words, 1oCu could be
considered as the parking (P) state, since neither proton nor UV
irradiation had effects on shifting the gear state. The whole system
was locked by copper (II) ion complexation. In this situation,
coordination of 1o with copper (II) ion modified the shifting from the
N state (1o) to the P state (1oCu). After extracting the copper (II) ion
from 1oCu by EDTA, the P state returned to its initial N state, which
was the original 1o form. As illustrated in Fig. 5, all four molecular
states could be interconverted reversibly, thus simulating the soft
control of gear-shift.
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In summary, we have synthesized Nap-Pip-DTE composed of
N-butyl-1,8-naphthalimide, piperazine and dithienylethene units and
demonstrated that this novel compound can response to UV irradi-
ation, proton and copper (II) ions. It is noteworthy that this Nap-Pip-
DTE system can mimic the soft control of a gear-shift with reversible
UV/Vis irradiation, protonation/deprotonation and complexation/
dissociation processes, providing the prospect to manufacture
automotive driving systems with molecular devices. Currently, efforts
directed toward designing more sophisticated molecular driving
systems of this kind are under further investigation in our laboratory.
Acknowledgements
This work is financially supported by National Basic Research
973 Program (2006CB806200), NSFC/China (50673025), Scientific
Ministry of China (2008DFA41390) and partially by Scientific
Committee of Shanghai. J. J. Z. thanks Z. Q. Guo, Dr Z. J. Ning, L.
L. Zhu, D. Zhang, X. Li, and C. Gao in our laboratory for their
assistance in the experiments.
€
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