Electrochromic switching and microkinetic behaviour of oxazine derivatives and their applications

Article abstract of DOI:10.1002/ejoc.201301182

Detailed electrochromic properties including the microkinetic switching behaviour of 2-nitro-(P1) and 2,8-dinitro-5a-[2-(4-dimethylaminophenyl)ethylene] -6,6-dimethyl-5a,6-dihydro-12H-indolo[2,1-b][1,3]benzooxazine (P2) were investigated in both solution and indium tin oxide (ITO) devices with a dual-wavelength time-dependent spectra monitoring method for the first time. These two oxazine derivatives displayed improved fatigue resistance and colour reversibility towards electrical stimulation compared with conventional spiropyran derivatives. A new electrochromic mechanism was proposed based on the experimental results and on quantum chemical calculations. Owing to an electron-withdrawing group para to the nitrogen of the indole fragment, P2 demonstrates better electro-driven reversibility of structural transformation between ring-open and ring-closed isomers than P1. The electrochromic behaviour of oxazine derivatives indicates that oxidation of the central nitrogen moiety switches the derivatives with concomitant colour change. The reversibility, chemical stability, and fatigue resistance towards electric stimulation depends on the substituents present, with an electron-withdrawing group para to the nitrogen of the indole fragment being beneficial. Copyright

Full text of DOI:10.1002/ejoc.201301182

FULL PAPER
DOI: 10.1002/ejoc.201301182
Electrochromic Switching and Microkinetic Behaviour of Oxazine Derivatives
and Their Applications
Shaoyin Zhu,[a,b] Minjie Li,*[b] Sicheng Tang,[b] Yu-Mo Zhang,[b] Bing Yang,[b] and
Sean Xiao-An Zhang*[b]
Keywords: Electrochemisty / Spectroelectrochemistry / Voltammetry / Molecular dynamics / Cyclization / Zwitterions
Detailed electrochromic properties including the micro-
kinetic switching behaviour of 2-nitro- (P1) and 2,8-dinitro-
5a-[2-(4-dimethylaminophenyl)ethylene]-6,6-dimethyl-5a,6-
dihydro-12H-indolo[2,1-b][1,3]benzooxazine (P2) were in-
vestigated in both solution and indium tin oxide (ITO) de-
vices with a dual-wavelength time-dependent spectra moni-
toring method for the first time. These two oxazine deriva-
tives displayed improved fatigue resistance and colour re-
versibility towards electrical stimulation compared with con-
ventional spiropyran derivatives. new electrochromic
A
mechanism was proposed based on the experimental results
and on quantum chemical calculations. Owing to an elec-
tron-withdrawing group para to the nitrogen of the indole
fragment, P2 demonstrates better electro-driven reversibility
of structural transformation between ring-open and ring-
closed isomers than P1.
spiro molecules from practical applications, such as their
facile photodegradation, low fatigue resistance, and slow
switching response time. These drawbacks seem to be in-
herent to conventional spiropyrans.[1a]
Introduction
The development of molecular switches for practical
applications has been accelerating. Various switchable mo-
lecules including spiro compounds, which undergo transfor-
mation from their ring-closed form (CF) to ring-open form
In contrast to conventional spiropyran derivatives, oxaz-
ine derivatives,[12] as another class of important molecular
switches, have been reported to have significantly improved
stability and response speed.[13] The zwitterionic structures
of their OF do not locate in the same conjugated structures,
and their photophysical properties are better suited for use
as chemical sensors.[14] The oxazolidine derivatives, in which
the zwitterions do not locate in the same conjugated struc-
ture either, are reported to have enhanced nonlinear optical
(NLO) properties.[15] However, until now, the stimuli for ox-
azine/oxazolidine molecules were also mainly light,[16] and
acid/base.[17] Their electrochemical behaviour, in compari-
son to conventional spiropyran derivatives, have not been
investigated with respect to the aforementioned concerns.
Herein, we represent some very interesting results on elec-
troswitching of two oxazine molecules. We have investigated
the microkinetic behaviour of the switching reactions in de-
tail with a dual-wavelength time-dependent spectra moni-
toring method, and proposed a new switching mechanism
for them.
(OF) upon external stimulus, have been investigated.[1]
A
feature of spiro compounds is their dramatic change on
light absorption during the structural transformation be-
tween their CF and OF. Amongst them, spiropyans and
their derivatives have been widely studied for use in memory
and storage,[1a,2] chemical sensors,[3] smart materials,[4] and
biological imaging.[5] Current switching stimuli for spiro-
pyrans are predominantly light,[1a,2,6] heat,[7] and acid or
base.[8] Akira Fujishima’s group investigated the photo-
electrochromic properties of a spirobenzopyran at low tem-
perature.[9] In addition, David L. Officer’s group demon-
strated the electroactivity and multiswitchable behaviour of
a spiropyran after being grafted on a polythiophene film.[10]
Recently, Braun’s group reported the ability to switch a
spiropyran upon application of stress.[11] Despite these re-
cent advance in the development of their switching meth-
ods, several critical challenges remain that prevent such
[a] State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology,
208 West Campus, 2 Ling-Gong Road, Dalian 116024, China
http://finechem.dlut.edu.cn/
In our previous work, we investigated two oxazines of 2-
nitro- (P1) and 2,8-dinitro- (P2) 5a-[2-(4-dimethylamino-
phenyl)ethylene]-6,6-dimethyl-5a,6-dihydro-12H-indolo-
[b] State Key Lab of Supramolecular Structure and Materials,
College of Chemistry, Jilin University,
[2,1-b][1,3]benzooxazine
for
chemical
sensing
(Scheme 1).[14a] P1 and P2 were found to be very sensitive
toward solvent polarity, and solvents can induce the tauto-
merization between the CF and OF, which facilitates their
application in cyanide sensing.[14a] Here, we intend to study
the electrochemical properties of P1 and P2 in comparison
Changchun 130012, China
E-mail: liminjie@jlu.edu.cn
seanzhang@jlu.edu.cn
http://supramol.jlu.edu.cn/people/zhangxiaoan/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301182.
Eur. J. Org. Chem. 2014, 1227–1235
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1227

M. Li, S. X.-A. Zhang et al.
FULL PAPER
with two other references, P3 and P4, and demonstrate as that of OF induced by acid (Scheme 1). Furthermore,
their possible application in electrochromic devices. De- the purple colour in solution was observed to gradually
tailed preparation and structural characterisation of the fade away after the electric stimulation was turned off. This
four oxazine derivatives are supplied in experimental sec- might be due to the structural reversal from the OF back
tion and Supporting Information. The cyclic voltammog- to the CF after reverse electron transfer between the oppo-
rams in tetrahydrofuran (THF) were studied in a three-elec- sitely charged radical ions that are generated from both
trode system by using tetrabutylammonium hexafluoro- working/counter electrodes and caused by diffused colli-
phosphate (TBAPF6) as the electrolyte, and the related po- sion. However, under the second or third oxidation peak no
tentials were all adjusted by using ferrocene (Fc) as an in- new colour change was observed on the surface of the
ternal standard with E0 (Fc+/Fc) 0.56 V vs. SCE,[18] which working electrode by monitoring in situ with a UV/Vis
was added at the end of each CV experiment.
spectrometer.
Scheme 1. Molecular structures of P1, P2, P3, and P4.
Results and Discussion
Electroswitching Behaviour of P1 and P2
To study the feasibility of electroswitching of P1 and P2
(Scheme 1), the oxidation process was investigated first. As
shown in Figure 1, the cyclic voltammograms of P1 and
P2 showed three clear oxidation peaks (O1, O2, and O3,
respectively) between 0 and +2.0 V at a scan speed of
100 mV/s, indicating that there were three oxidation pro-
cesses involved in this potential range. Under the potential
of the first oxidation peak (O1; +1.02 V vs. SCE) for P1, a
Figure 1. Cyclic voltammograms of P1 (A) and P2 (B) (1 mm) in
THF within a scanning range from 0 V to 2.2 V {100 mV/s,
TBAPF6 (0.1 m), r.t.}. Inset: Cyclic voltammograms of P1 (A) and
P2 (B) within a smaller scanning range from 0 V to 1.2 V under
the same conditions.
Similarly, P2 was also switched with a clear colour
clear colour change from yellow to purple (with an absorp- change from light-yellow to blue under application of
tion peak at 564 nm) was observed on the surface of the +1.04 V (vs. SCE) that was related to the oxidation peak
working electrode, with some being diffused into the solu- O1 (Figure 2). The maximum absorption wavelength of this
tion. This clear purple colour was observed only on the blue solution of electrooxidised P2 also displayed similar
positive electrode side, which was further confirmed by a colour (only a 5 nm redshift) to the OF of P2 induced by
setup with two isolated electrochemical cells connected by acid. The other two oxidation peaks (O2 and O3) for P2
a salt bridge. In addition, the maximum absorption wave- did not generate any new colour either. The results men-
length of this purple solution had only 1 nm redshift com- tioned above indicate that both P1 and P2 can be switched
pared with the OF of P1 induced by acid (Figure 2).[13b,14a] by the application of an electric field, and their switchable
This result indicates that the colour of electrical stimulated potentials are closely related to the first oxidation peak
P1 is due to the generation of the same conjugated structure (O1).
1228
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 1227–1235

Electrochromic Switching of Oxazine Derivatives
Figure 2. UV/Vis spectra of P1 and P2 under the stimulation of
acid (10 equiv.) and electrical field (P1 +1.8 V; P2 +2.0 V) using
ITO electrodes.
Investigation of the Oxidative Groups of P1 and P2
Both P1 and P2 have multiple functional groups that can
be electrooxidised in the switching process. To clarify which
functional group relates to the first oxidation peak (O1) re-
sulting in the colour change, the cyclic voltammo-
grams of P3 and P4, as reference compounds, were studied
for comparison (Figure 3, and Figure S2 in the Supporting
Information). As shown in Figure 3, only two clear oxi-
dation peaks (O1 and O2) were observed for P3 and P4 in
Figure 3. Cyclic voltammograms of P3 (A) and P4 (B) (1 mm) in
the potential range from 0 to +2.0 V at 100 mV/s. The peak THF within a scanning range from 0 V to 2.2 V {100 mV/s,
potentials and shapes for these two oxidation peaks were
TBAPF6 (0.1 m), r.t.}.
very similar to those of the oxidation peaks (O2 and O3)
for P1 (Table 1). By comparison and correlation of those
oxidation peaks and the structural differences between P1,
P3, and P4, we conjecture that the first oxidation peak
(O1) of P1 is attributed to the oxidation of nitrogen in
N(CH3)2.[19] This peak assessment was further supported
by related quantum calculations. As shown in Figure 4, the
highest occupied molecular orbital (HOMO) for P1 is
mainly located on the aniline part.[20] With the highest elec-
tron density, the aniline moiety of P1 should be oxidised
preferentially over the other functional groups within the
molecule. In contrast, the HOMO of P3 and P4 are mainly
located on the indole part and the nitrophenol fragment,
due to the absence of an aniline functional group within
their structures (Figure 4); their oxidation can only take
place on those higher electron-density regions. By compar-
ing the electron density of nitrogen and oxygen atoms of
the oxazine portion of P3 and P4, we can conjecture further
that oxygen in their nitrophenol fragments will be preferen-
tially oxidised over nitrogen in the indole part due to the
higher electron density on oxygen (see the Supporting In-
formation, Table S1). Consequently, the first oxidation peak
(O1) of P3 and P4 is assigned to the oxidation of oxygen
in the nitrophenol part. Likewise, the same oxidation
process of oxygen can possibly be invoked to explain the
second oxidation peak (O2) of P1.
Table 1. Oxidation and reduction peaks (V) of P1, P2, P3, and P4.
The oxidation potential of ferrocene (Fc+/Fc) was considered to be
0.56 V vs. SCE (THF/[NBu4][PF6]) and each potential was adjusted
referring to the reference.
O1
O2
O3
R1
R2
P1
P2
P3
P4
1.02
1.04
1.38
1.40
1.38
1.38
1.65
1.66
1.69
1.82
–
0.87
0.86
1.15
–
1.11
–
–
–
–
Supplemental cyclic voltammograms of the acid-induced
OF for P1 and P4 further supported our initial oxidation P3, and P4 (in vacuo) calculated by using B3LYP/6-31+g (d, p).
Figure 4. Relevant frontier molecular orbitals (HOMO) for P1, P2,
Eur. J. Org. Chem. 2014, 1227–1235
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1229

M. Li, S. X.-A. Zhang et al.
FULL PAPER
peaks assessment. Two oxidation peaks (O1 and O2) for the coloured OF, and the reversibility of the first oxidation
OF of P1 were observed in the potential range from 0 to peaks (O1) has an important effect on their switching prop-
+2.5 V after P1 was switched to its OF by treatment with erties. To reduce the complexity of multiple oxidation pro-
CH3SO3H (see the Supporting Information, Figure S3 A). cesses, a smaller scanning range (0 to +1.2 V at 100 mV/s)
Therein, the observed first oxidation peak (O1) increased was used for P1 and P2. An rather unclear reduction peak
to +1.31 V (vs. SCE) by comparison with the peak of P1 (R1) accompanying the oxidation peak O1 was observed
without acid treatment (O1; +1.02 V vs. SCE) due to the for P1 (Figure 1, A); by increasing the scanning rate, the
positive charge in the indole part of the OF. Similarly, only reversibility could be improved and a shift towards higher
one oxidation peak between 0 and +2.5 V was found after potential for O1 was observed (see the Supporting Infor-
P4 was switched to its OF by treatment with acid (see the mation, Figure S4), indicating the oxidisation process is not
Supporting Information, Figure S3 C). According to the fully reversible (Figure 1, A). However, a clear reduction
OF structure of P4, this single oxidation peak should arise peak (R1) related to peak O1 was displayed for P2 at
from oxidation of the nitrophenol part. Therefore, by anal- 100 mV/s. This result indicates that the timely reversibility
ogy with the supplemental cyclic voltammograms of the OF of P2 is much better than that of P1 owing to the influence
for P1 and P4, the first oxidation peak (O1) of P1 is con- of the nitro group para to the nitrogen of the indole part of
cluded to be the oxidation of the nitrogen in N(CH3)2. In a P2. However, the oxidation of P2 is also not fully reversible
similar way, the third oxidation peak (O3) of P1 and the (Figure 1, B).
second oxidation peak (O2) of P4 are confirmed to arise
from the oxidation of the nitrogen in the indole frag-
ment.[21]
Investigation of the Electrochromic Mechanism
Because of the structural similarities of P1 and P2, the
Based on the experimental results detailed above, a pos-
sible electroswitching mechanism for P1 and P2 is proposed
and shown in Scheme 2. Taking P1 as an example, the ni-
trogen of the aniline part first loses an electron to generate
a nitrogen radical cation as [P1-1]·+. Rapid homolytic cleav-
age of the C–O bond takes place to generate a more stable
structure with an oxygen radical in the nitrophenol part and
a positive charge on the nitrogen between the aniline and
oxidation groups related to three oxidation peaks of P2 are
presumably the same as those of P1. Due to the presence
of a nitro group with an electron-withdrawing effect in the
indole of P2, the potential of oxidation peak O3 is larger
than that of P1.
Reversibility of the Oxidation of P1 and P2
The potentials associated with oxidation peak O1 of P1 indole portions through resonance to form the coloured
and P2 could switch these derivatives from their CF to their radical ion [P1-2]·+. Accompanying this process, P1 displays
Scheme 2. Postulated mechanism for the electrochemical redox of P1 and P2.
1230
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 1227–1235

Electrochromic Switching of Oxazine Derivatives
a clear colour change in solution from yellow to purple while, another peak absorption at 426 nm arising from the
(Figure 2). Subsequently, the coloured radical ion [P1-2]·+ nitrophenol anion portion of [P1-3] also displayed a similar
(with an absorption peak at 564 nm) is converted into the trend.[23b] These results further support the aforementioned
negative oxygen ion to form the zwitterion [P1-3] (with the mechanism. Radical cation [P1-1]·+ transforms quickly into
same absorption peak at 564 nm) after accepting an elec- [P1-2]·+ through homolytic cleavage of the C–O bond with
tron. Both [P1-2]·+ and [P1-3] have the same maximum ab- the appearance of a purple colour after oxidation, which
sorption at 564 nm because their colours are mainly defined corresponds to the first oxidation process of P1 in the three-
by the positively charged portion of their conjugated nitro- electrode system. Subsequently, [P1-2]·+ transforms into
gen cations. Neither the negatively charged nitrophenol in [P1-3] after obtaining an electron when [P1-2]·+ diffuses
[P1-3] (with an absorption peak at 426 nm) nor the ni- near to the cathode.
trophenol radical in [P1-2]·+ conjugates with the positively
charged colour-dominating structure, consequently, the re-
duction process on the oxygen radical does not have a sig-
nificant effect on the colour. However, the resultant zwitter-
ion [P1-3] is less stable than [P1], and tends to change back
to [P1] thermodynamically with concomitant disappearance
of the purple colour in solution, leading to a completed
reversible switching cycle. During the whole process, the
rate-determinative step is the transformation from OF into
CF, that is, conversion from the zwitterion [P1-3] to the
initial state of [P1]. The faster this step, the better the re-
versibility of the electrochromic process.
This proposed mechanism also applies to P2. Compared
with P1, thermodynamic transformation from [P2-3] into
[P2] is enhanced. Nevertheless, this mechanism differs
greatly from that reported for spiropyran derivatives.[11] For
the spiropyran, the nitrogen of the indole part was reported
to be oxidised first to induce a colourless conjugated radical
and cation through homolytic cleavage of the C–O bond.
The colour change actually results from a further reductive
process of its oxygen radical portion to generate a conju-
gated zwitterionic isomer.[11] In contrast, for oxazine deriva-
tive P1, the nitrogen of the aniline part is initially oxidised
electrically, which immediately induces homolytic cleavage
of the C–O bond to generate an isolated, coloured nitrogen
cation of the conjugated indole and an oxygen radical of
nitrophenol. The oxygen radical of the nitrophenol portion
is converted further into the anion of nitrophenol after one-
electron reduction with the formation of a zwitterion [P1-
3]. Because the charges of the resulting zwitterionic struc-
ture do not locate in the same conjugate skeleton, both sta-
bility and colour intensity of the isomer are increased
Figure 5. (A) UV/Vis absorption spectra of P1 (1 mm) in THF
{TBAPF6 (0.1 m), ITO glass electrodes}. Applied voltage: 0, +1.0,
+1.1, +1.2, +1.3, +1.4, +1.5, +1.6, +1.7, +1.8, and +1.9 V for 30 s,
respectively. Absorbance caused by the ITO glass at each point had
been corrected. (B) Time-dependent absorption at 426 and 564 nm
of P1 (1 mm) in THF {TBAPF6 (0.1 m), ITO glass electrodes}; elec-
trifying, 600 s, +1.80 V; cutting off, 600 s; electrifying, 600 s, –1.0 V.
greatly.[22]
Investigation of Spectroelectrochemistry
Derivatives P1 and P2 display similar electrochromic
properties in fabricated ITO devices and in solution (see
parts A of Figures 5 and 6). The absorption band at 426 nm
It is worth pointing out that the actual reaction kinetics
(or 429 nm) is assigned to the negatively charged nitrophen- of the electroswitching of P1 and P2 are highly dynamic
oxide portion of [P1-3] (or [P2-3]),[23] and the absorption and complex. They involve multiple dynamic processes co-
peak at 564 nm (or 590 nm) is assigned to the positively existing with various reaction rates in both solution and
charged merocyanine portion of both radical cation the interface (on the surface of both electrodes). To better
[P1-2]·+ (or [P2-2]·+) and the zwitterion [P1-3] (or [P2-3]). understand the microkinetics of the electroswitching pro-
As shown in part A of Figure 5, a clearly increased absorp- cesses and further develop their applications as potential
tion at 564 nm related to both [P1-2]·+ and [P1-3] was ob- display material, the electrochromic properties of P1 and
served with an increase in voltage from 0 to +1.9 V. Mean- P2 in the ITO device were also studied by (dual-wavelength)
Eur. J. Org. Chem. 2014, 1227–1235
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1231

M. Li, S. X.-A. Zhang et al.
FULL PAPER
indicate that the concentration of [P1-3] increases rapidly
along with [P1-2]·+ during the first 100 s, then levels out,
even though the concentration of [P1-2]·+ increases continu-
ously for the rest of the time at +1.8 V. The levelling out
of [P1-3] is an indication of an equilibrium between the
generation and consumption of [P1-3] being reached
(Scheme 2). The generation of the latter is presumably
caused by a reverse electron transfer between oppositely
charged radical ions, which were generated from both elec-
trodes (working/counter) through diffused collision. The
consumption of [P1-3] takes place through thermodynami-
cally driven ring-closing reaction to form [P1].
The absorbance at 564 nm decreased quickly from 0.45
to 0.32 at first and remained unchanged after the voltage
was off for 600 s. Meanwhile, the absorbance at 426 nm de-
creased fairly quickly from 0.023 to 0.012 initially, and then
slowly increased back to 0.021 at the end of the second
600 s period. The initial quick decreases at both 564 and
426 nm indicate that the consumption of [P1-3] is much
faster than its generation after the voltage is off. Presum-
ably, the consumption and ring-closure of [P1-3] back to
[P1] proceeds mainly in solution instead of on the electrode
surface due to less spatial restriction. The later levelling out
at 564 nm accompanied by a slow increase at 426 nm on
absorption indicates that no further ring-closing reaction of
[P1-3] is progressing even though the concentration of [P1-
3] is still increasing. Most probably, this slow increase of
[P1-3] actually occurs on the surface of the electrode
through heterogeneous one-electron transfer between [P1-
2]·+ on the electrode and the radical-counterions in solu-
tion. Those [P1-3] species on the electrode surface cannot
go further back to the CF of P1 due to spatial restriction
on the electrode. Clearly, this surface restriction makes its
colour bistable.
A voltage of –1.0 V was then applied for 600 s, and the
absorbance at 564 nm was further reduced from 0.32 to 0.20
along with a continuous increase of absorbance at 426 nm.
This results in a gradual reduction of the coloured ab-
sorbance at 564 nm, and finally leads to a partially com-
pleted electrochromic cycle for P1 at the end of 1800 s.
These results clearly show that even though the total
number of positively charged merocyanine portions from
a combination of [P1-2]·+ and [P1-3] decreases, the actual
amount of [P1-3] in the system still increases.
Figure 6. (A) UV/Vis absorption spectra of P2 (1 mm) in THF
{0.1 m TBAPF6, ITO glass electrodes}. Applied voltage: 0, +1.4,
+1.5, +1.6, +1.7, +1.8, +1.9, +2.0, and +2.1 V for 30 s, respectively.
The absorbance caused by the ITO glass at each point had been
corrected. (B) Time-dependent absorption at 429 and 590 nm of P2
(1 mm) in THF {TBAPF6 (0.1 m), ITO glass electrodes); elec-
trifying, 600 s, +2.0 V; cutting off, 600 s. (C) Real objects illustra-
tion of the electrochromism for P2 (1 mm) {TBAPF6 (0.1 m), ITO
glass electrodes}, +2.0 V, 30 s; –1.0 V, 120 s.
spectroelectrochemistry. Time-dependent absorption at 426
These results further suggest that only the net amount of
and 564 nm of P1 with the applied voltage (or spectra at [P1-2]·+ actually reduces during the process. The application
429 and 590 nm of P2) was investigated.
of the negative voltage will undoubtedly accelerate the con-
The microkinetic electrochromic behaviour and revers- version of [P1-2]·+ into [P1-3]. In addition, this reverse volt-
ibility of P1 in the ITO device were first studied in detail age will promote the ring-closing reaction of [P1-3] back to
by a dual-wavelength time-dependent spectra monitoring the CF of P1, either through pushing the surface-attached
method (Figure 5, B). A time-dependent variable absorp- zwitterion into solution or forcing it back to its CF directly
tion at 564 nm from both [P1-2]·+ and [P1-3] with a sharp on the electrode; both activities are driven by coulombic
increase before 100 s, and a relatively slow increase after- repulsion between the field and the zwitterion.
wards were observed during the first 600 s electrooxidation
It is worth pointing out that even though the partial re-
at +1.8 V. Meanwhile, the absorbance at 426 nm from the versibility of P1 will limit its application in reversible elec-
nitrophenoxide portion of [P1-3] displayed a different trend trochromic devices, it might find use in one-time switches
with an initial increase and subsequent levelling out. The or bistable devices. In addition, the dual-wavelength time-
time-dependent spectra changes at both wavelengths clearly dependent spectra monitoring method employed herein is a
1232
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 1227–1235

Electrochromic Switching of Oxazine Derivatives
very powerful tool with which to investigate the microki-
The antifatigue characteristics of P2 in the ITO device
netic behaviour of a wide range of highly complex dynamic were also investigated by spectroelectrochemistry. As re-
chemical reactions. vealed in Figure 7, the amplitude of the reversible colour
In contrast, P2 displays an improved response to electric absorption of P2 at 590 nm did not change significantly
stimulus and much better electrochromic reversibility than during 8000 s treatment with cycle voltage, then gradually
P1. As shown in Figure 6 (A), its absorbance both at 590 declined afterwards. This result reveals that P2 has some
and 429 nm displays a similar trend, which increases upon antifatigue properties when incorporated into an ITO de-
rising voltage after +1.4 V. This indicates that the genera- vice, but these characteristics need to be enhanced further
tion rate of both [P2-2]·+ and [P2-3] is increased with the for practical application. This inspires us to further improve
applied voltage. These results correspond to the oxidation the electrochromic behaviour of such compounds by struc-
process of P2 in the three-electrode system, and are consis- ture modification.
tent with the aforementioned hypothesis of a new electro-
chromic mechanism.
The microkinetic electrochromic behaviour of P2 was
also studied in an ITO device with the same dual-wave-
length time-dependent spectra monitoring method. Inter-
estingly, the corresponding spectra of P2 displayed a signifi-
cantly different phenomenon (Figure 6, B) to that of P1
(Figure 5, B). Both absorbances for the coloured species in-
creased synchronously to 1.6 at 590 nm and 0.15 at 429 nm
within 200 s at +2.0 V, and became relatively invariable af-
terwards during the 600 s energization. The maximum ab-
sorption intensity of P2 at 429 nm (0.15 in Figure 6, B,
which arises from the nitrophenol anion portion of [P2-3])
is nearly 6.5 times that of P1 (0.023 in Figure 5, B, which
arises from the nitrophenol anion portion of [P1-3]). This
suggests that over 6 times the amount of [P2-3] has been
generated during the electrooxidation of P2 in comparison
Figure 7. Time-dependent absorbance of P2 (1 mm) at 590 nm in
THF {TBAPF6 (0.1 m)} measured under a cyclic voltage (+1.8 and
to the amount of [P1-3] from P1. It further implies that
–1.0 V). A voltage of +1.8 V was applied for 30 s and then –1.0 V
was applied for 120 s for each cycle.
[P2-2]·+ is much easier and nearly 5.5 times faster to convert
into [P2-3] than [P1-2]·+. This might be due to the initially
generated [P2-2]·+ detaching more easily than [P1-2]·+ from
the surface of the electrode into solution, which will cer-
tainly speed up the conversion from [P2-2]·+ into [P2-3]
Conclusions
driven by diffused collision and intercounterion electron
transfer.
The electrochromic behaviour of oxazine derivatives P1
By further comparing the absorption intensity at 590 nm and P2 were investigated by spectroelectrochemistry in situ
(1.6 in Figure 6, B, which is presumably from the merocyan- in both solution and in ITO devices. Distinct improvements
ine portion of [P2-3] and [P2-2]·+) with that at 564 nm (0.45 in colour reversibility for P1 and P2 have been observed
in Figure 5, B, which is presumably from the merocyanine in comparison with spiropyrans. A dual-wavelength time-
portion of [P1-3] and [P1-2]·+), we can conclude that the dependent spectra monitoring method is introduced to
blue colour observed upon electrooxidation of P2 is mainly probe the microkinetics of these highly dynamic and com-
caused by [P2-3], and a very small amount of [P2-2]·+ actu- plex colour-switching reactions. A new electrochromic
ally exists in the process. This conclusion is consistent with mechanism is proposed that is firmly supported by both
the fact that both curves (at 590 and 429 nm) synchronously experimental results and theoretical simulation, which lead
level out (with slight downwards slope) when the equilib- us to conclude that the oxidation of nitrogen in the aniline
rium between the generation and the consumption of [P2- part (related to the oxidation peak O1) is responsible for
3] is reached from the 200 to 600 s electrooxidation of P2. the colour and structure switching between their CF and
This conclusion is supported further by the fact that the OF forms. P1 shows an electrical colour bistability, whereas
colour faded quickly along with its absorbance decrease P2 demonstrates better colour reversibility, chemical sta-
from 1.48 back to 0.06 at 590 nm and from 0.15 to 0.02 at bility, and fatigue resistance than P1 due to the presence of
429 nm within another 600 s after the voltage being cut off. an electron-withdrawing nitro group in the indole part. The
The colour fading is due to the net consumption of col- demonstrated results not only open a new avenue for elec-
oured [P2-3] through thermodynamically driven deionis- trochromic materials, but will also inspire and accelerate
ation. These results, in combination with structure analysis, further development of oxazine derivatives for applications
reveal that the nitro group para to the nitrogen of the indole such as display, memory, and storage. In addition, the dual-
part of P2 has an important effect on enhancing the electro- wavelength time-dependent spectra monitoring method in-
chromic reversibility.
troduced herein will be a very powerful tool that could be
Eur. J. Org. Chem. 2014, 1227–1235
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1233

M. Li, S. X.-A. Zhang et al.
FULL PAPER
Synthesis of 2-Nitro-5a-6,6-trimethyl-5a,6-dihydro-12H-indolo-
[2,1-b][1,3]benzooxazine (P3): The synthesis of P3 was conducted
according to the reported procedure.[20,23a] M.p. 162.7–163.8 °C; Rf
used to truly understand what is going on in a broad range
of chemical processes.
1
= 0.46 (EtOAc/hexane, 1:15). H NMR (300 MHz, CDCl3, 25 °C,
TMS): δ = 1.19 (s, 3 H), 1.54 (s, 3 H), 1.59 (s, 3 H), 4.62 (s, 2 H),
6.57 (d, J = 7.8 Hz, 1 H), 6.71 (d, J = 9 Hz, 1 H), 6.83 (m, 1 H),
7.08 (m, 1 H), 7.12 (m, 1 H), 7.95 (dd, J = 9, 3 Hz, 1 H), 8.06 (d,
J = 3 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 16.44, 25.96,
39.88, 47.89, 102.67, 108.27, 118.01, 118.70, 120.40, 122.15, 123.16,
123.86, 127.44, 137.88, 140.27, 146.51, 158.99 ppm. LC-HRMS:
calcd. for C18H19N2O3 [M + H]+ 311.1390; found 311.1380.
Experimental Section
General Procedures: Melting points (uncorrected) were determined
with a SGW X-4B microscopy melting-point apparatus (Shanghai).
NMR spectra (1H and 13C) were obtained with a Varian 300M.
Spectra were referenced to the residual proton solvent peaks using
shifts reported by Fulmer et al.[24] UV/Vis absorption spectra were
recorded with a Shimadzu UV-2550 PC double-beam spectropho-
tometer and the path length was 1 cm. LC-HRMS was obtained
with an Agilent 1290-microTOF QII. Cyclic voltammograms were
obtained with a Bio-logic electrochemical work station. Experi-
ments were performed under protection of Argon in THF contain-
ing TBAPF6 (0.1 m) as supporting electrolyte. The three-electrode
cell consisted of a glass-carbon working electrode (Shenhua, China)
with a surface area of 7.07 mm2, a Pt-wire counter electrode (Shen-
hua, China), and an Ag/AgNO3 reference electrode (Shenhua,
China). The potential was calculated relative to ferrocene (Fc+/Fc)
as an internal standard with E0 (Fc+/Fc) 0.56 V vs. SCE.[18]
Synthesis of P1: Synthesized according to the literature[23b] with
improved procedures and higher yield. 1-(2-Hydroxy-5-ni-
trobenzyl)-2,3,3-trimethylindoleninium
176.9 mg) and 4-dimethylaminobenzaldehyde (0.5 mmol, 74.5 mg)
were heated to reflux in ethanol (10 mL) for 3.5 h. The solvent was
removed with a rotary vacuum evaporator and treated with aq.
Na2CO3. Ethyl acetate was added to extract the product, then the
organic phase was separated and the solvent was distilled off under
reduced pressure. The residue was recrystallized (ethyl acetate/hex-
ane) to give the product (196 mg, 89%) as an orange solid. Rf =
Synthesis of 2-Nitro-5a-(2-phenylethylene)-6,6-dimethyl-5a,6-di-
hydro-12H-indolo[2,1-b][1,3]benzooxazine (P4): 1-(2-Hydroxy-5-
nitrobenzyl)-2,3,3-trimethylindoleninium
chloride
(1 mmol,
347 mg) and benzaldehyde (1.9 mmol, 200 mg) were heated to re-
flux in ethanol (10 mL) for 16 h, then the solvent was removed
with a rotary evaporator. The excess benzaldehyde was treated with
diethyl ether, then the residue was washed with aq. NaHCO3 and
ethyl acetate was added to extract the product. The organic phase
was separated and the solvent was distilled off under reduced pres-
sure after being dried with anhydrous Na2SO4. The crude product
was purified by column chromatography to give P4 (280 mg, 70%)
as a yellow solid; m.p. 79–80 °C; Rf = 0.65 (ethyl acetate/hexane,
1:15). 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ = 1.25 (s, 6 H),
1.53 (s, 6 H), 4.58 (s, 2 H), 6.35 (d, J = 15 Hz, 1 H), 6.61 (d, J =
9 Hz, 1 H), 6.80 (d, J = 18 Hz, 1 H), 6.86 (t, J = 6 Hz, 2 H), 7.09
(d, J = 9 Hz, 1 H), 7.12 (d, J = 9 Hz, 1 H), 7.27–7.41 (m, 5 H),
7.97 (dd, J = 9, 6 Hz, 1 H), 8.00 (d, J = 3 Hz, 1 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 40.58, 49.99, 103.60, 108.66, 117.55, 119.88,
120.61, 122.21, 123.12, 123.84, 123.88, 126.76, 127.56, 128.51,
128.64, 135.43, 136.06, 138.05, 140.53, 146.33, 159.09 ppm. LC-
chloride
(0.51 mmol,
HRMS: calcd. for C25H23N2O3 [M
399.1691.
+
H]+ 399.1703; found
1
0.33 (ethyl acetate/hexane, 1:2). H NMR (300 MHz, 25 °C, [D6]-
DMSO, TMS): δ = 1.35 (s, 6 H), 2.92 (s, 6 H), 4.8 (s, 2 H), 6.39
(d, J = 18 Hz, 1 H), 6.67 (d, J = 9 Hz, 2 H), 6.84 (m, 1 H), 6.86
(d, J = 18 Hz, 1 H), 6.87 (d, J = 9 Hz, 1 H), 6.94 (d, J = 6 Hz, 1
H), 7.11 (m, 1 H), 7.24 (d, J = 9 Hz, 1 H), 7.42 (d, J = 9 Hz, 2 H),
7.93 (dd, J = 9, 3 Hz, 1 H), 8.09 (d, J = 3 Hz, 1 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 40.3, 40.6, 49.9, 108.7, 112.1, 117.6, 118.4,
120.1, 120.6, 122.3, 123.2, 123.7, 123.9, 127.5, 127.9, 136.2, 138.4,
140.2, 146.5, 150.7, 159.7 ppm. LC-HRMS: calcd for [M + H]+
442.2125; found 442.2121.
Testing Sample Preparation: THF was first stirred with CaH2 at
room temperature overnight, and was further purified by refluxing
with sodium under the protection of N2 overnight. Then it was
distilled to be storeed under N2 prior to use. Tetrabutylammonium
hexafluorophosphate (TBAPF6), which was used as electrolyte
(0.1 m), was crystallized three times from ethanol. The concentra-
tion of each substance in THF was 1ϫ 10–3 m.
Preparation of the Device: The ITO glass electrodes were washed
according to a standard method prior to use. Two ITO glass elec-
trodes were stuck together by AB adhesive as space (ca. 1 mm).
The centre of the two ITO glass electrodes was hollow. THF solu-
tion with TBAPF6 (0.1 m) containing the target molecules (1ϫ
10–3 m) was injected into the hollow centre after the solution was
deaerated with argon for ca. 20 min.[25] The spectroelectrochemis-
try and reaction dynamic processes were measured with a double-
beam spectrophotometer with an electrochemical workstation.
Synthesis of P2: 2,8-Dinitro-5a,6,6-trimethyl-5a,6-dihydro-12H-
indolo[2,1-b][1,3]benzooxazine (0.7 mmol, 247.8 mg) and 4-(N,N-
dimethyl)benzaldehyde (1.4 mmol, 208.6 mg) were heated to reflux
in ethanol (10 mL) with CH3SO3H as catalyst (200 mg) for 12 h
under an N2 atmosphere. The solvent was distilled off under re-
duced pressure, and the residue was washed with satd. aq. Na2CO3
until the pH of the aqueous solution was neutral. The aqueous
layer was treated with aqueous KOH solution and extracted with
ethyl acetate, then the organic phase was separated and dried with
anhydrous Na2SO4. Ethyl acetate was evaporated to give the crude
product, which was purified by column chromatography to give P2
(190 mg, 56%) as a yellow solid; m.p. 128–131 °C; Rf = 0.56 (ethyl
acetate/hexane, 1:2). 1H NMR (400 MHz, [D6]DMSO, 25 °C,
TMS): δ = 1.58 (s, 6 H), 3.25 (s, 6 H), 5.76 (s, 2 H), 6.95 (d, J =
8 Hz, 2 H), 7.04 (d, J = 8 Hz, 1 H), 7.37 (d, J = 16 Hz, 1 H), 7.72
(d, J = 8 Hz, 1 H), 8.11 (d, J = 8 Hz, 3 H), 8.24 (s, 1 H), 8.36 (d,
J = 8 Hz, 1 H), 8.49 (d, J = 16 Hz, 1 H), 8.72 (s, 1 H) ppm. 13C
DFT Calculations: The quantum chemical calculations were per-
formed with the Gaussian 09 program.[20] The geometries of P1,
P2, P3, and P4 were optimised (in vacuo) by the density functional
theory (DFT) method by using B3LYP/ 6-31+g (d, p). Frequency
calculations were carried out to confirm the local minima possess
all real frequencies.
Supporting Information (see footnote on the first page of this arti-
cle): Cyclic voltammograms of P1, P2, P3, and P4 (Figure S1, Fig-
NMR (75 MHz, D6MSO): δ = 27.8, 43.9, 54.3, 95.1, 108.8, 113.6, ure S2, and Figure S4), cyclic voltammograms of acid-reduced
117.1, 122.1, 123.1, 123.7, 125.7, 128.1, 128.7, 129.1, 130.8, 133.3,
141.2, 144.1, 145.6, 145.9, 155.8, 158.0, 163.5 ppm. LC-HRMS:
calcd. [M + H]+ 487.1976; found 487.1962.
ring-open form for P1, P2, and P4 (Figure S3), quantum chemistry
calculation on the electron distribution of P1, P2, P3, and P4
(Table S1), copies of the 1H and 13C NMR spectra (Figure S4–S6).
1234
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 1227–1235

Electrochromic Switching of Oxazine Derivatives
[15] a) F. Mançois, V. Rodriguez, J.-L. Pozzo, B. Champagne, F.
Castet, Chem. Phys. Lett. 2006, 427, 153–158; b) F. Mançois,
J.-L. Pozzo, J. Pan, F. Adamietz, V. Rodriguez, L. Ducasse, F.
Castet, A. Plaquet, B. Champagne, Chem. Eur. J. 2009, 15,
2560–2571; c) F. Mançois, L. Sanguinet, J.-L. Pozzo, M. Guil-
laume, B. Champagne, V. Rodriguez, F. Adamietz, L. Ducasse,
F. Castet, J. Phys. Chem. B 2007, 111, 9795–9802; d) L. San-
guinet, J.-L. Pozzo, M. Guillaume, B. Champagne, F. Castet,
L. Ducasse, E. Maury, J. Soulié, F. Mançois, F. Adamietz, V.
Rodriguez, J. Phys. Chem. B 2006, 110, 10672–10682; e) L.
Sanguinet, J.-L. Pozzo, V. Rodriguez, F. Adamietz, F. Castet,
L. Ducasse, B. Champagne, J. Phys. Chem. B 2005, 109, 11139–
11150.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant number 21072025) and Jilin Univer-
sity.
[1] a) G. Berkovic, V. Krongauz, V. Weiss, Chem. Rev. 2000, 100,
1741–1754; b) K. Szaciłowski, Chem. Rev. 2008, 108, 3481–
3548; c) F. M. Raymo, M. Tomasulo, Chem. Soc. Rev. 2005,
34, 327–336.
[2] a) I. Yildiz, E. Deniz, F. M. Raymo, Chem. Soc. Rev. 2009, 38,
1859–1867; b) F. M. Raymo, R. J. Alvarado, S. Giordani,
M. A. Cejas, J. Am. Chem. Soc. 2003, 125, 2361–2364; c) L.
Zhu, M.-Q. Zhu, J. K. Hurst, A. D. Q. Li, J. Am. Chem. Soc.
2005, 127, 8968–8970.
[3] a) N. Shao, Y. Zhang, S. Cheung, R. Yang, W. Chan, T. Mo,
K. Li, F. Liu, Anal. Chem. 2005, 77, 7294–7303; b) Y. Li, Y.
Duan, J. Li, J. Zheng, H. Yu, R. Yang, Anal. Chem. 2012, 84,
4732–4738; c) B. Champagne, A. Plaquet, J.-L. Pozzo, V. Rodri-
guez, F. Castet, J. Am. Chem. Soc. 2012, 134, 8101–8103.
[4] a) A. Setaro, P. Bluemmel, C. Maity, S. Hecht, S. Reich, Adv.
Funct. Mater. 2012, 22, 2425–2431; b) M. Zhang, X. Hou, J.
Wang, Y. Tian, X. Fan, J. Zhai, L. Jiang, Adv. Mater. 2012, 24,
2424–2428; c) G. Liu, J. Wang, Angew. Chem. 2010, 122, 4527;
Angew. Chem. Int. Ed. 2010, 49, 4425–4429; d) S. Silvi, A. Ard-
uini, A. Pochini, A. Secchi, M. Tomasulo, F. M. Raymo, M.
Baroncini, A. Credi, J. Am. Chem. Soc. 2007, 129, 13378–
13379; e) I. Yildiz, S. Impellizzeri, E. Deniz, B. McCaughan,
J. F. Callan, F. M. Raymo, J. Am. Chem. Soc. 2010, 133, 871–
879; f) R. Byrne, D. Diamond, Nat. Mater. 2006, 5, 421–424.
[5] a) L. Zhu, W. Wu, M.-Q. Zhu, J. J. Han, J. K. Hurst, A. D. Q.
Li, J. Am. Chem. Soc. 2007, 129, 3524–3526; b) N. Shao, J. Jin,
H. Wang, J. Zheng, R. Yang, W. Chan, Z. Abliz, J. Am. Chem.
Soc. 2010, 132, 725–736.
[16]
a) M. Tomasulo, E. Deniz, T. Benelli, S. Sortino, F. M. Raymo,
Adv. Funct. Mater. 2009, 19, 3956–3961; b) M. Tomasulo, S.
Sortino, F. M. Raymo, Adv. Mater. 2008, 20, 832–835; c) E.
Deniz, S. Ray, M. Tomasulo, S. Impellizzeri, S. Sortino, F. M.
Raymo, J. Phys. Chem. A 2010, 114, 11567–11575; d) E. Deniz,
M. Tomasulo, J. Cusido, I. Yildiz, M. Petriella, M. L. Bossi, S.
Sortino, F. M. Raymo, J. Phys. Chem. C 2012, 116, 6058–6068;
e) E. Deniz, S. Sortino, F. M. Raymo, J. Phys. Chem. Lett.
2010, 1, 3506–3509.
[17]
a) M. Tomasulo, S. Sortino, F. M. Raymo, J. Photochem. Pho-
tobiol., A 2008, 200, 44–49; b) J. Pennakalathil, T.-H. Kim, K.
Kim, K. Woo, J.-K. Park, J.-D. Hong, Langmuir 2010, 26,
11349–11354; c) M. Tomasulo, I. Yildiz, S. L. Kaanumalle,
F. M. Raymo, Langmuir 2006, 22, 10284–10290.
[18]
[19]
[20]
N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877–910.
A. Rasche, J. Heinze, Electrochim. Acta 2008, 53, 3812–3819.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Starov-
erov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Ad-
amo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Mar-
tin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09,
revision A.02, Gaussian, Inc., Wallingford CT, 2009.
[6] a) V. I. Minkin, Chem. Rev. 2004, 104, 2751–2776; b) E. Deniz,
M. Tomasulo, R. A. DeFazio, B. D. Watson, F. M. Raymo,
Phys. Chem. Chem. Phys. 2010, 12, 11630–11634.
[7] Y. Shiraishi, M. Itoh, T. Hirai, Phys. Chem. Chem. Phys. 2010,
12, 13737–13745.
[8] A. Doron, E. Katz, G. Tao, I. Willner, Langmuir 1997, 13,
1783–1790.
[9] J. F. Zhi, R. Baba, K. Hashimoto, A. Fujishima, J. Photochem.
Photobiol., A 1995, 92, 91–97.
[10] K. Wagner, R. Byrne, M. Zanoni, S. Gambhir, L. Dennany, R.
Breukers, M. Higgins, P. Wagner, D. Diamond, G. G. Wallace,
D. L. Officer, J. Am. Chem. Soc. 2011, 133, 5453–5462.
[11] C. K. Lee, D. A. Davis, S. R. White, J. S. Moore, N. R. Sottos,
P. V. Braun, J. Am. Chem. Soc. 2010, 132, 16107–16111.
[12] a) Y. Prostota, J. Berthet, S. Delbaere, P. J. Coelho, Dyes Pigm.
2013, 96, 569–573; b) M. Å. Petersen, E. Deniz, M. B. Nielsen,
S. Sortino, F. M. Raymo, Eur. J. Org. Chem. 2009, 4333–4339;
c) E. Deniz, J. Cusido, S. Swaminathan, M. Battal, S. Impelliz-
[21] a) M. A. González-Fuentes, B. R. Díaz-Sánchez, A. Vela, F. J.
González, J. Electroanal. Chem. 2012, 670, 30–35; b) R. Sala-
zar, P. A. Navarrete-Encina, C. Camargo, J. A. Squella, L. J.
Núnez-Vergara, J. Electrochem. Soc. 2008, 155, 103–108.
[22] L. Sheng, M. Li, S. Zhu, H. Li, G. Xi, Y.-G. Li, Y. Wang,
Q. Li, S. Liang, K. Zhong, S. X.-A. Zhang, Nature Commun.,
accepted.
zeri, S. Sortino, F. M. Raymo, J. Photochem. Photobiol., A [23] a) M.-B. Suhr Kirketerp, M. Åxman Petersen, M. Wanko, L.
2012, 229, 20–28; d) F. M. Raymo, J. Phys. Chem. A 2012, 116,
Andres Espinosa Leal, H. Zettergren, F. M. Raymo, A. Rubio,
M. Brøndsted Nielsen, S. Brøndsted Nielsen, ChemPhysChem
2009, 10, 1207–1209; b) E. Deniz, M. Tomasulo, S. Sortino,
F. M. Raymo, J. Phys. Chem. C 2009, 113, 8491–8497.
[24] G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb,
A. Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg, Orga-
nometallics 2010, 29, 2176–2179.
[25] a) Y.-M. Zhang, W. Li, X. Wang, B. Yang, M. Li, S. X.-A.
Zhang, Chem. Commun. 2013. DOI: 10.1039/C3CC44924F; b)
Y.-M. Zhang, M. Li, W. Li, Z. Huang, S. Zhu, B. Yang, X.-C.
Wang, S. X.-A. Zhang, J. Mater. Chem. 2013, 1, 5309–5314.
Received: August 6, 2013
11888–11895.
[13] a) M. Tomasulo, S. Sortino, A. J. P. White, F. M. Raymo, J.
Org. Chem. 2005, 70, 8180–8189; b) M. Tomasulo, S. Sortino,
F. M. Raymo, J. Org. Chem. 2007, 73, 118–126; c) E. Deniz, S.
Sortino, F. M. Raymo, J. Phys. Chem. Lett. 2010, 1, 1690–1693;
d) M. Tomasulo, S. Sortino, F. M. Raymo, Org. Lett. 2005, 7,
1109–1112.
[14] a) S. Zhu, M. Li, L. Sheng, P. Chen, Y. Zhang, S. X.-A. Zhang,
Analyst 2012, 137, 5581–5585; b) M. Tomasulo, S. Sortino,
A. J. P. White, F. M. Raymo, J. Org. Chem. 2005, 71, 744–753;
c) M. Tomasulo, F. M. Raymo, Org. Lett. 2005, 7, 4633–4636;
d) J. Ren, W. Zhu, H. Tian, Talanta 2008, 75, 760–764.
Published Online: December 20, 2013
Eur. J. Org. Chem. 2014, 1227–1235
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1235