Switching with orthogonal stimuli: Electrochemical ring-closure and photochemical ring-opening of bis(thiazolyl)maleimides

Article abstract of DOI:10.1039/c2sc21681g

The photochemistry as well as electrochemistry of novel donor-acceptor bis(morpholinothiazolyl)-maleimides has been investigated. Proper substitution of these diarylethene-type molecular switches leads to the unique situation in which their ring-closure can only be accomplished electrochemically, while ring-opening can only be achieved photochemically. Hence, these switches operate with orthogonal stimuli, i.e. redox potential and light, respectively. The switch system could be optimized by introducing trifluoromethyl groups at the reactive carbon atoms in order to avoid by-product formation during oxidative ring closure. Both photochemical and electrochemical pathways were investigated for methylated, trifluoromethylated, and nonsymmetrical bis(morpholinothiazolyl) maleimides as well as the bis(morpholinothiazolyl)cyclopentene reference compound. With the aid of the nonsymmetrical mixed derivative, the mechanism of electrochemically driven ring closure could be elucidated and seems to proceed via a dicationic intermediate generated by two-fold oxidation. All experimental work has been complemented by density functional theory that provides detailed insights into the thermodynamics of the ring-open and closed forms, the nature of their excited states, and the reactivity of their neutral as well as ionized species in different electronic configurations. The particular diarylethene systems described herein could serve in multifunctional (logic) devices operated by different stimuli (inputs) and may pave the way to converting light into electrical energy via photoinduced pumping of redox-active meta-stable states.

Full text of DOI:10.1039/c2sc21681g

Chemical Science
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Switching with orthogonal stimuli: electrochemical
ring-closure and photochemical ring-opening of
bis(thiazolyl)maleimides†
Cite this: Chem. Sci., 2013, 4, 1028
a
b
a
a
a
Martin Herder, Manuel Utecht, Nicole Manicke, Lutz Grubert, Michael P a¨ tzel,
b
a
Peter Saalfrank* and Stefan Hecht*
The photochemistry as well as electrochemistry of novel donor–acceptor bis(morpholinothiazolyl)–
maleimides has been investigated. Proper substitution of these diarylethene-type molecular switches
leads to the unique situation in which their ring-closure can only be accomplished electrochemically,
while ring-opening can only be achieved photochemically. Hence, these switches operate with
orthogonal stimuli, i.e. redox potential and light, respectively. The switch system could be optimized by
introducing trifluoromethyl groups at the reactive carbon atoms in order to avoid by-product formation
during oxidative ring closure. Both photochemical and electrochemical pathways were investigated for
methylated, trifluoromethylated, and nonsymmetrical bis(morpholinothiazolyl)maleimides as well as the
bis(morpholinothiazolyl)cyclopentene reference compound. With the aid of the nonsymmetrical
“
mixed” derivative, the mechanism of electrochemically driven ring closure could be elucidated and
seems to proceed via a dicationic intermediate generated by two-fold oxidation. All experimental work
has been complemented by density functional theory that provides detailed insights into the
thermodynamics of the ring-open and closed forms, the nature of their excited states, and the reactivity
of their neutral as well as ionized species in different electronic configurations. The particular
diarylethene systems described herein could serve in multifunctional (logic) devices operated by
different stimuli (inputs) and may pave the way to converting light into electrical energy via
photoinduced “pumping” of redox-active meta-stable states.
Received 5th October 2012
Accepted 24th November 2012
DOI: 10.1039/c2sc21681g
www.rsc.org/chemicalscience
Among these photochromic compounds diarylethenes
belong to the most promising candidates as they typically offer
Introduction
Chemists are continuously challenged to construct and to
manipulate increasingly complex functional systems and tance. Besides the modulation of optical properties useful for
materials. For this purpose one central prerequisite is the ability information storage and processing, they are exploited as
to inuence the properties of the whole molecular ensemble by functional units for remote-controlling electronic devices,
gaining precise control over the behaviour of its single building chemical reactivity, and supramolecular binding and
blocks. One attractive way to accomplish this is the use of aggregation.6
an efficient isomerization behaviour and a high fatigue resis-
2
3
4
5
molecular units that can be "switched" between two forms
associated with different physicochemical properties by an
external stimulus. Particularly, the incorporation of photo-
Another interesting feature of diarylethenes is the possibility
of triggering their isomerization reaction not only by light but
also by electrochemical oxidation or reduction (Scheme 1).
chromic molecules into complex functional systems, in Thus, it was reported that diarylethenes bearing thiophenes as
combination with the use of light as a noninvasive stimulus aryl moieties (dithienylethenes, DTEs) can undergo oxidative
with high spatial and temporal resolution, has recently gained cyclization or cycloreversion depending on the substitution
1
7
much attention.
pattern. A similar behaviour was also reported for DTEs
8
substituted with redox-active metal centres. An isomerization
a
Department of Chemistry, Humboldt-Universit a¨ t zu Berlin, Brook-Taylor-Str. 2, 12489
Berlin, Germany. E-mail: sh@chemie.hu-berlin.de
b
Institut f u¨ r Chemie, Universit a¨ t Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-
Golm, Germany. E-mail: peter.saalfrank@uni-potsdam.de
†
Electronic supplementary information (ESI) available: Experimental details
including synthesis, photochemistry, electrochemistry, and computational
methods and data. See DOI: 10.1039/c2sc21681g
Scheme 1 Isomerization of dithienylethenes.
1
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from the ring-open to the ring-closed isomer upon electro- operates for the ring-closure and the photochemical pathway is
chemical reduction was observed for diarylethenes substituted only possible for the ring-opening reaction. A central pre-
9
with two cationic N-methylpyridinium units in the periphery.
requesite for achieving the orthogonality is the utilization of
The combination of the different strategies in one molecular highly electron donating morpholino substituents on the thia-
scaffold afforded a fully bidirectional switchable compound zole rings in combination with a highly electron decient
that undergoes isomerization in both directions either by exci- maleimide as the bridging unit. It is shown that the stability of
10
tation with light or by the electrochemical pathway. Never- 1a during the switching process can be enhanced signicantly
0
theless, to the best of our knowledge, separation of the by the introduction of triuoromethyl groups in the 5- and 5 -
electrochemical and the photochemical pathways for the position of the thiazole rings leading to structure 2a. The non-
isomerization of diarylethenes in order to precisely control symmetrically substituted derivative 3a provides distinct
the state of the switching molecule using either one of the two experimental evidence for the mechanism of the oxidative ring-
stimuli has never been achieved.
closing process. As a reference compound 4a, possessing an
While the photochemistry of diarylethenes has been exploi- electron-neutral cyclopentene bridge, was synthesized, which
ted extensively in terms of understanding their general mech- shows both electrochemical and photochemical cyclization. In
anism of operation and tuning the system in order to gain addition to experimental results, theoretical calculations based
11
12
maximum isomerization efficiencies and fatigue resistance,
on density functional theory (DFT) were performed in order to
there have been few systematic studies concerning the isomer- gain further insight into the absorption behaviour, ground and
13
ization reaction via the electrochemical pathway. Thus, it is excited state structures, reaction pathways, as well as transition
still necessary to gain deeper experimental and theoretical states for neutral and ionic species.
insight into the fundamental principles of the electrochemical
isomerization of diarylethenes as well as to optimize it in terms
of efficiency and fatigue resistance in order to combine the
Results and discussion
Synthesis
photochemical and the electrochemical switching processes in
sophisticated functional systems.
Synthesis of compounds 1a–4a relied on the established
strategy of connecting the aryl moieties to the bridge moiety by
cross-coupling (Scheme 3). 5-Methyl-2-morpholinothiazole 6
was easily accessed via Hantzsch's thiazole synthesis. The tri-
Here we report the oxidative isomerization of diarylethenes
that bear thiazole moieties instead of the commonly utilized
thiophenes. Thiazoles as aromatic moieties for the construction
of diarylethenes offer, besides their good synthetical avail-
ability, advantages in terms of thermal and photochemical
stability. Specically, species 1–4, as shown in Scheme 2, were
studied, with a referring to open and b to closed forms,
respectively. While until now no oxidative isomerization of such
dithiazolylethenes has been reported in the literature, our
system possesses an orthogonal behaviour towards both
switching stimuli, i.e. the electrochemical switching only

uoromethylated analogue 10 was accessed by iodination of the
5
-unsubstituted thiazole precursor and subsequent copper-
14
15
catalysed triuoromethylation using TMSCF
3
in moderate
yields. Both compounds could be converted into the corre-
sponding organostannanes 7 and 11 by direct metalation using
tert-BuLi und subsequent quenching with Bu SnCl. Finally,
3
Stille cross-coupling of 7 or 11 either with 3,4-dibromo-1-tert-
butylmaleimide 8 or with 1,2-dibromocyclopentene 12 yielded
the symmetrically substituted dithiazolylethenes 1a, 2a, and 4a,
respectively. The nonsymmetrically substituted derivative 3a
was accessed via a statistical reaction of both organostannanes
with maleimide 8 under the same conditions.
The ring-closed isomer 1b was obtained by oxidation of an
acetonitrile solution of 1a using two equivalents of ceric
ammonium nitrate and subsequent reduction with sodium
ascorbate, followed by purication using preparative HPLC.
Compounds 2b and 4b were isolated by controlled potential
electrolysis of acetonitrile solutions of the respective ring-open
isomers and subsequent purication of the crude mixtures by
precipitation or by chromatography.
Photochemical behaviour
UV/vis-spectra of ring-open compounds 1a, 2a, 3a, and 4a were
recorded in acetonitrile (Fig. 1). Besides an intense absorption
in the UV originating from the aromatic thiazole moieties, the
structurally related compounds 1a, 2a, and 3a show a broad
absorption band in the visible range between 350–500 nm. This
band can be attributed to the charge-transfer from the electron-
rich morpholinothiazoles to the strongly electron-decient
Scheme 2 Isomerization behaviour of morpholino-substituted dithiazolylethenes.
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ꢀ
ꢀ
Scheme 3 Synthesis of target photochromes 1a, 2a, 3a, and 4a: (a) 2-bromo-1,1-diethoxypropane, TsOH, (EtOH/H
2
O), 90 C, 24 h, 55%; (b)(i) tert-BuLi, (THF), ꢁ78 C,
ꢀ
ꢀ
3
9
2
1
5 min, (ii) Bu
3
SnCl, (THF), ꢁ78 C to rt, 1 h, quant.; (c) 8, Pd
2
(dba)
3
, AsPh
3
, CuI, (DMF), rt, 6 h, 33%; (d) 2-bromo-1,1-diethoxyethane, TsOH, (EtOH/H
2
O), 90 C, 24 h,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
7%; (e)(i) n-BuLi, (THF), ꢁ78 C to ꢁ10 C, 30 min, (ii) I
2
, (THF), ꢁ10 C, 30 min, 84%; (f) TMSCF
3
, CuI, KF, (DMF/NMP), 50 C, 4.5 h, 36%; (g)(i) tert-BuLi, (THF), ꢁ78 C,
ꢀ
ꢀ
ꢀ
5 min, (ii) Bu
3
SnCl, (THF), ꢁ78 C to rt, 1 h, quant.; (h) 8, Pd(PPh
3
)
4
, (toluene), 110 C, 24 h, 29%; (i) 8, Pd(PPh
3
)
4
, (toluene), 100 C, 48 h, 3%; (j) 12, PdCl
2
(PPh
3
)
2
, (DMF),
ꢀ
00 C, 20 h, 6%.
tert-butylmaleimide bridge. Compared to the parent structure respective ring-open compounds are obtained. Although the
a, for compounds 2a and 3a the intensity of this CT-band is spectra of the ring-closed and the ring-open isomers overlap
1
signicantly lower and its position is shied hypsochromically signicantly at the irradiation wavelength, complete conversion
by 40 nm and 10 nm, respectively, reecting the reduced elec- to the ring-open isomers is achieved, as these are photochem-
tron-density of the CF
3
-substituted morpholinothiazole cores.
ically inactive. The quantum yields for the ring-opening of 1b
As proposed in the literature for analogous dithienylmale- and 2b with 436 nm light were determined to be relatively high
16
imides, such a pronounced CT-behaviour between the aryl with values of 0.13 and 0.37, respectively. Noticably, the ring-
moieties and the bridge moiety of diarylethenes can lead to a opening process is signicantly enhanced for compound 2b,
diminished cyclization efficiency due to the formation of a which may be attributed to the strong electron-withdrawing
3
Twisted Intramolecular Charge Transfer (TICT) excited state, character of the CF -groups attached to the reactive carbon
18
which is characterized by a pronounced twisting of the single atoms.
bond between the electron donating and the electron accepting Contrasting this unidirectional photochemical switching
17
parts of the molecule. In fact, during irradiation of a yellow behaviour of diarylethenes 1a/b, 2a/b, and 3a/b, bearing a
solution of 1a in acetonitrile with UV-light (lirr ¼ 280 nm) or maleimide bridge, compound 4a/b, possessing a cyclopentene
with visible light (lirr ¼ 436 nm) no changes in the UV/vis- as bridging unit, shows photochemical bidirectionality to some
spectrum could be observed (Fig. 1a). Furthermore, analysis of extent (Fig. 1d). UV/vis spectra as well as UPLC-traces recorded
the reaction mixture by ultra-performance liquid chromato- during irradiation of a colorless solution of 4a in acetonitrile
graphy (UPLC) did not show any new photoproduct. Only when with UV-light (lirr ¼ 280 nm) indicate the formation of the ring-
exposed to UV-irradiation over an extended time period using a closed isomer 4b. However, only 36% of 4b is formed in the
high intensity lamp some unspecic degradation (bleaching) photostationary state due to a high ring-opening quantum yield
took place. According to the TICT-model, the usage of cyclo- of 0.26 while the ring-closing quantum yield was determined to
hexane as a non-polar solvent allowed for the photochemical be 0.13. Obviously the lack of a charge transfer between the
cyclization reaction to take place to some extent. Nevertheless, thiazole moieties and the cyclopentene bridge, indicated by the
its efficiency was extremely low with a quantum yield of 0.05 for absence of a visible absorption in the case of the ring-open
the ring-closure of 1a and a conversion of only 16% in the isomer, restores the reversibility of the photochemical reaction
photostationary state (see Section 2 in the ESI†). A similar lack typical for diarylethenes. Taking all of these data summarized in
of photochemical reactivity for the ring-open isomers was also Table 1 into consideration, an intramolecular charge transfer
observed for the CF
3
-substituted derivatives 2a and 3a.
interaction is clearly essential to install unidirectional photo-
Nevertheless, the ring-closed isomers 1b and 2b could be chemical behaviour.
obtained via oxidation of the respective ring-open isomers (vide
To get further insight into the photochemical processes that
infra). They show an intense absorption in the visible region lead to the observed lack of cyclization efficiency theoretical
split into two bands (Fig. 1a and b insets). Upon illumination of investigations were performed. We optimized the structures of
1
9
acetonitrile solutions of 1b and 2b with visible light (lirr
4
¼
compounds 1a–4b on the B3LYP/6-31G* and CAM-B3LYP/6-
20
36 nm) both bands diminish rapidly and the spectra of the 31G* levels of theory. Solvent effects were modeled using the
1
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ꢁ1
vertical stick spectra with Gaussians of width 1500 cm . All
results were obtained using the GAUSSIAN09 program
23
package. Further computational details can be found in the
ESI.† In Fig. 2 the broadened spectra of 1a–4b with acetonitrile
as a solvent are shown, as obtained from TD-CAM-B3LYP/PCM/
6-31G* calculations. Selected wavelengths, oscillator strengths,
and characters of the most intense transitions at wavelengths
above 240 nm are listed in Table S2 of the ESI.†
The calculated wavelengths of transitions are overall hyp-
sochromically shied compared to experimental values, but
otherwise nicely conrm the experimentally observed trends.
The overall shi is rather constant with values between about 10
and 35 nm for the ring-open isomers and between 40 and 60 nm
for the ring-closed isomers. This blue-shi is due, in part, to a
slight overestimation of excitation energies by TD-CAM-B3LYP,
and due to the idealization of the solvent as a continuum in the
PCM. By contrast, the TD-B3LYP method underestimates exci-
tation energies considerably (see Section 9 in the ESI†), due to a
wrong asymptotic one-electron potential, which is corrected for
20
in the CAM-B3LYP functional. As a consequence, the CAM-
B3LYP functional is more reliable for charge transfer excita-
tions, which are of importance here.
In particular, the TD-CAM-B3LYP spectra reproduce the
weak, broad band for the ring-open forms 1a–3a at around
400 nm (and slightly below), which is due to a charge transfer
from the thiazole units to the central maleimide bridge. This
assignment is supported by the fact that for 1a–3a the lowest-
energy transition is of the HOMO / LUMO type, where HOMO
and LUMO are localized at the thiazole and maleimide units,
respectively, as is exemplarily shown for compound 1a (Fig. 3,
le). The calculations also conrm the strong blue-shi of the
CT band of compound 2a relative to 1a, and the much weaker
blue-shi for species 3a. These hypsochromically shied tran-
sitions are due to two and one electron-withdrawing CF3-
groups, respectively, which lower the HOMO energy. The CT
transition is suppressed both in theory and experiment for
compound 4a, since cyclopentene as a bridging unit is only a
weak electron acceptor (Fig. 3, right). As a consequence, there is
Fig. 1 UV/vis-spectra during the course of irradiation of acetonitrile solutions of
(
a) 1a, (b) 2a, (c) 3a, and (d) 4a with UV-light (lirr ¼ 280 nm). Insets show UV/vis
spectra during the irradiation with visible light (lirr ¼ 436 nm) of the respective
isolated ring-closed isomers 1b, 2b, and 4b (prepared separately via oxidation of no absorption of 4a at wavelengths above 300 nm, neither
the ring-open compounds) in acetonitrile. All concentrations 5 ꢂ 10ꢁ M.
5
experimentally nor theoretically. In the UV regime (at around
260 nm according to experiment and 240 nm in theory), the
morpholinothiazole groups of the ring-open compounds show
Polarizable Continuum Model (PCM). UV/vis absorption spectra strong absorption. In further agreement between theory and
were calculated using linear-response time-dependent DFT (TD- experiment, the ring-closed compounds 1b–4b show consider-
21
22
DFT) on the TD-CAM-B3LYP level of theory, by broadening able overlap with the spectra of open forms 1a–4a. For 1b–3b,
Table 1 Photophysical properties for acetonitrile solutions of ring-open isomers 1a, 2a, 3a, and 4a as well as isolated ring-closed isomers 1b, 2b, and 4b
3
ꢁ1
ꢁ1
l
max [nm] (3 [10 L mol cm ])
a
Ring-open
isomer
Ring-closed
isomer
F
o/c
F
c/o
Conversion
(280 nm)
Comp.
(280 nm)
(436 nm)
1a/b
2a/b
3a/b
4a/b
265 (23.1), 430 (3.9)
276 (20.1), 390 (1.8)
269 (18.9), 420 (2.5)
252 (21.1), 268 (shoulder)
364 (14.0), 447 (12.9)
361 (8.9), 454 (12.9)
n.d.
<0.001
<0.001
<0.001
0.13
0.13
0.37b
n.d.
0.26
<1%
<1%
<1%
36%
b
408 (14.7)
a
b
Amount of ring-closed isomer in the photostationary state upon irradiation with UV-light determined by UPLC. Compound 3b was not isolated.
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thiazole rings relative to the central maleimide/cyclopentene
ring are compared (Table 2). Here, C1–C6 refer to labels of
atoms involved in the respective dihedrals (see Fig. 3, upper
le). The twisting angles are indicators of whether a TICT state
is formed or not. The values in the table refer to ground and rst
excited singlet state of compounds 1a–4a, for CAM-B3LYP (gas-
phase) and CAM-B3LYP (acetonitrile).
First of all, we note that all dihedrals in the ground state S
0
ꢀ
are around 50 , reecting the geometry shown in Fig. 3 (for 1a
and 4a). A stable minimum on the rst excited state surface S1
could not be found for 4a in any case. Instead, these calcula-
tions converge towards a conical intersection, which exhibits
closer interatomic distances between the carbon atoms involved
in ring formation. Therefore, no TICT state is formed in the S
1
state of 4a, and an optical excitation should provide a reason-
able quantum yield for formation of 4b instead. In contrast, TD-
CAM-B3LYP calculations in the gas-phase show a twisting of 3a
ꢀ
towards u ¼ 72 . This is indicative of the formation of the TICT
state, which can inhibit ring closure. On the other hand, opti-
mizations of 1a and 2a in S states approach again a conical
1
intersection, i.e., one would expect reactivity towards 1b and 2b
in this case, or towards another photoproduct. TD-CAM-B3LYP/
PCM calculations in acetonitrile provide stable structures of 1a
1
and 3a in the S excited state with twisted geometries. In these
cases, however, the TICT effect appears less pronounced. Still,
the ring closure will probably be hindered since a conical
intersection cannot be reached easily. By contrast, for
compounds 2 and 4, optimizations lead into a conical inter-
section again, which may cause ring closure.
Based on these results, we conclude that 4a has a tendency to
react towards 4b independent of method and solvent. For
compounds 1a–3a the situation is less clear. In several cases we

nd stable excited state structures, some of them clearly cor-
responding to twisted (TICT) geometries. The stable structures
suggest trapped, unreactive species aer photoexcitation.
From another point of view the observed photochemical
switching behaviour can be rationalized qualitatively on the
basis of (Kohn–Sham) frontier orbitals, which are shown exem-
plarily for compounds 1a and 4a in Fig. 3. The frontier orbitals of
Fig. 2 Calculated absorption spectra of 1–4 (a in blue and b in red) in aceto-
nitrile on the TD-CAM-B3LYP/PCM/6-31G* level of theory. Line spectra (sticks) are
ꢁ1
broadened by Gaussians of 1500 cm width.
2a and 3a are similar to those of 1a (see Fig. S8 in the ESI†).
According to the Woodward–Hoffmann rules for pericyclic
24
the spectra are dominated by three well-separated bands. The reactions, the LUMO of an ideal hexatriene system dominates
lowest-energy HOMO / LUMO transition is a p / p excitation its photoreaction, which proceeds in a conrotatory fashion as the
localized within the central N–C2–C3–C4–C5–N unit (for atom sign of the corresponding orbital is opposite at C1 and C6. In 4a,
numbering see Fig. 3), with good overlap between the initial and the computed orbitals have the same overall topology as for an

nal orbitals. This explains the larger intensity of this band as ideal 6-electron p-system according to Fig. 3, right. In particular,
compared to the CT transition of the ring-open compounds.
In order to quantify the possibly destructive role of TICT
a conrotatory ring closure is favored aer photoexcitation to the
state. This leads to a product 4b with two methyl groups at the
S
1
states, which may prevent photochemical switching of formed, six-membered ring in energetically favorable trans
compounds 1a–3a to 1b–3b, we optimized geometries in the orientation. Thus, 4a is expected to photoreact by orbital

rst excited state of the open isomers (1a–4a) in the gas-phase symmetry according to the Woodward–Hoffmann rules. In
with the B3LYP and CAM-B3LYP functionals and in acetonitrile contrast, the frontier orbitals of 1a (and also those of 2a and 3a)
with CAM-B3LYP. Only S1 states are considered despite that are different from the ideal situation. In particular the LUMO is
excitation of the molecules with UV-light may lead to higher now localized strongly at the maleimide ring (which is absent in
excited states. Nevertheless, due to internal conversion these 4a), and only small coefficients are found at carbon atoms C1 and
have short lifetimes. To characterize the S
twisting angles u (C1,C2,C3,C4) and u (C3,C4,C5,C6) of the two ring closure does not occur aer photoexcitation and instead, an
0
and S
1
states the C6, which are supposed to form the C–C bond. As a consequence,
0
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Fig. 3 Isocontour plots of frontier orbitals for compounds 1a and 4a in acetonitrile, obtained on the CAM-B3LYP/PCM/6-31G* level of theory. In the upper left panel,
an atom numbering for C atoms as used in the text is shown.
0
Table 2 Dihedral angles u (C1,C2,C3,C4) and u (C3,C4,C5,C6) (the latter in
Upon oxidation of 1a a single irreversible oxidation wave at a
potential of 0.57 V is observed, which is associated with a charge
transfer of 2 electrons per molecule (Fig. 4a). As the parent
0 1
brackets if different from u) for optimized ground and first excited states S and S
of compounds 1a–4a, all calculated with (TD-)CAM-B3LYP/(PCM)/6-31G* for the
gas-phase and in acetonitrile. In cases without an entry no stable minimum was
found and a conical intersection towards ring closure was approached instead.
5-methyl-2-morpholinothiazole 6 is oxidized at a very similar
potential of 0.52 V, it can be assumed that the maleimide bridge
has only minor inuence on the oxidation potential of 1a and
thus the electron transfer originates from both electron-rich
morpholinothiazole moieties. During the return scan the step-
CAM-B3LYP (gas-phase)
CAM-B3LYP (acetonitrile)
Comp.
S
0
S
1
S
0
S
1
1a
2a
3a
4a
50.4
56.0
49.4 (53.8)
47.3
—
—
49.9
55.9
49.8 (53.3)
47.5
21.9 (22.6) wise reduction of a new species can be observed at potentials of
—
0.26 V and 0.09 V corresponding to two one-electron processes.
When a second scan-cycle is performed, reversible oxidation
72.0 (33.8)
—
44.0 (31.1)
—
Table 3 Oxidation potentials of acetonitrile solutions of 1a/1b, 2a/2b, 3a/3b,
and 4a/4b determined by cyclic voltammetry
intramolecular charge transfer from the morpholinothiazole
moiety to the maleimide ring takes place.
a
p
E
[V]
Comp.
Ring-open isomer
Ring-closed isomer
Electrochemical behaviour
1
2
3
4
a/b
a/b
a/b
a/b
0.57 (irr.)
1.14 (irr.)
0.70 (qr); 1.23(irr.)
0.27 (irr.)
0.15 (rev.); 0.32 (rev.)
0.47 (rev.); 0.69 (rev.)
0.30 (rev.); 0.52(rev.)
ꢁ0.29 (rev.); ꢁ0.19 (rev.)
Cyclic voltammetry was used for the electrochemical charac-
terization of all compounds. As irreversible oxidation processes
are concerned, all reported potentials (Table 3) are peak-
potentials, E , of the anodic waves and are given in reference to
p
a
Peak potentials are reported against ferrocene/ferrocenium redox
+
the ferrocene/ferrocenium (Fc/Fc ) redox couple, which was
couple as external standard; rev. ¼ reversible, qr ¼ quasireversible,
used as external standard.
irr. ¼ irreversible.
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waves arise at 0.15 V and 0.32 V (Fig. 4a, inset). In analogy to the
To test the potential of 1a as a switch with orthogonal stimuli
known behaviour of dithienylethenes this observation can be it was subjected to several switching cycles consisting of elec-
explained by a fast thermal reaction of oxidized 1a to its ring- trochemical oxidation/reduction of an acetonitrile solution of
closed analogue that is subsequently reduced in two steps to 1b, 1a in a divided H-cell and subsequent irradiation of the cell with
which possesses signicantly lower oxidation potentials due to visible light (lirr > 430 nm). During this process UPLC-traces as
its conjugated structure. A charge of 2 C per mole is required well as UV/vis-spectra of small samples of the reaction mixture
during oxidation to fully convert the ring-open isomer to its were recorded (Fig. 5). Aer the initial oxidation/reduction step
ring-closed analogue. In fact, 1b could be isolated by oxidation an absorption band in the visible region corresponding to the
of the ring-open isomer on a preparative scale using two ring-closed isomer appeared. UPLC of the reaction mixture
equivalents of ceric ammonium nitrate as oxidant and subse- indicated a conversion of 51% to the ring-closed isomer 1b, but
quent reduction with ascorbic acid. The structure was showed as well the presence of 32% of a second product 1c
conrmed via NMR-spectroscopy and MS and its isomerization (Fig. 5a). Upon irradiation with visible light ring-opening of 1b
to the ring-open isomer is observed upon irradiation with was induced, but the initial UV-spectrum of 1a could not be
436 nm light (vide supra).
restored completely due to the photoinactivity of 1c. Upon two
The oxidative ring-closure of 1a was also followed using more switching cycles the compound was almost completely
spectroelectrochemistry (Fig. S4 in the ESI†). While increasing converted to the by-product, indicated by the loss in absorbance
the potential, the formation of an absorption band centred at in the visible region and by UPLC. To isolate the by-product an
+
+
360 nm characteristic for the dicationic ring-closed isomer 1b
acetonitrile solution of 1a was subjected to electrochemical
can be observed. During the following reduction a bath- oxidation and was subsequently stirred for 2 h at room
ochromically shiet absorption band around 550–750 nm temperature. During this time the oxidized species almost
+
evolves, indicating the presence of the monoradical cation 1b_ . quantitatively converted to 1c, which was then precipitated in
Upon further reduction of the potential this band diminishes water. Analysis of a low quality X-ray crystal structure of this
again and the characteristic absorption spectrum of 1b can be material recrystallized from chloroform suggests an ionic
observed.
nature for 1c (Fig. 5c). A possible mechanism for its formation is
+
+
depicted in Scheme 4: in the ring-closed dicationic state 1b ,
which is formed upon oxidation of 1a, one of the thiazole
methyl groups loses one proton and a subsequent nucleophilic
attack of the neighboring thiazole nitrogen leads to the
formation of a 7-membered ring. This assignment is supported
by high resolution mass spectrometry proving the loss of one
proton. Additionally, NMR-spectroscopy of the rearranged
product clearly indicates the presence of two different mor-
2
pholinothiazole moieties and shows a new CH group as a
broad signal at room-temperature. At lower temperatures this
signal splits into two doublets showing the restricted
conformational exibility of the 7-membered ring (see Fig. S5 in
the ESI†).
In order to increase the fatigue resistance of the overall
switching cycle it was investigated if a substitution of the methyl
groups in 1a by triuoromethyl groups would stabilize the
dicationic species by preventing its deprotonation. In fact, the
ring-open isomer 2a, possessing triuoromethyl groups on both
thiazole rings, shows an oxidation wave centred at a peak
potential of 1.14 V (Fig. 4b). Compared to 1a this is a relatively
large shi by 570 mV to higher potentials proving the strongly
electron-withdrawing nature of the CF
the two-electron oxidation of 2a results as well in ring closing to
b as can be seen from the presence of two new reversible
3
-groups. Nevertheless,
2
oxidation waves in a second scan cycle. These oxidation waves
show a signicantly smaller shi to higher potentials of 320 mV
when compared to 1b. This is due to the formation of a
0
quaternary carbon at the 5- and 5 -position of the thiazole
3
moieties during the cyclization separating the CF -groups from
the p-system.
Fig. 4 Cyclic voltammograms of the isolated ring-open isomer (blue) and ring-
ꢁ3
When 2a is subjected to repeated electrochemical ring-
closure and photochemical ring-opening a clearly enhanced
performance of the reported system can be observed.
closed isomer (red) of (a) 1a/1b and (b) 2a/2b in acetonitrile (c ¼ 1 ꢂ 10 M).
Insets show two oxidative cycles consecutively performed on the respective ring-
open isomer.
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Fig. 5 (a) UPLC-traces and (b) absorbance in the visible region of the reaction mixture recorded during repeated electrochemical ring-closure and photochemical
ring-opening of 1a in acetonitrile / 0.1 M Bu
4
NPF
6
; (c) X-ray crystal structure of low quality crystals of by-product 1c (ORTEP-drawing, 50% probability thermal ellipsoids,
ꢁ
hydrogen atoms and ClO
4
anion were omitted for clarity); (d) UPLC-traces and (e) absorbance in the visible region of the reaction mixture recorded during repeated
NPF
electrochemical ring-closure and photochemical ring-opening of 2a in acetonitrile / 0.1 M Bu
4
6
.
indication for the formation of a by-product. Nevertheless, due
to diffusion of the compound into the cathode compartment of
the divided electrochemical cell and due to unspecic degra-
dation processes of the intermediate radical cationic species a
continuous loss in the visible absorbance is observed during

ve switching cycles. This points to a general drawback of the
electrochemical setup that has to be addressed in any future
application; however, it does not constitute an intrinsic limi-
tation of the molecular system.
While for the symmetrically substituted compounds 1a and
2a an irreversible two-electron oxidation that leads to thermal
ring closure is observed in CV, the behaviour of the nonsym-
metrical derivative 3a is slightly different. Here, the cyclic vol-
tammogram (Fig. 6) shows two well separated one-electron
oxidation waves. Based on the observed oxidation potentials for
compounds 1a and 2a the wave at 0.70 V can be assigned to the
Scheme 4 Proposed mechanism for the formation of 1c.
3
UV/vis-spectra measured from small samples of the reaction oxidation of the morpholinothiazole moiety bearing the CH -
mixture show the reversible formation of the absorption bands group while the wave at 1.23 V corresponds to the morpholi-
in the visible range corresponding to the ring-closed isomer nothiazole bearing the CF -group. The origin of the small
3
(Fig. 5e). UPLC traces obtained during this experiment show shoulder at a potential of 1.05 V is currently unknown. Again,
almost quantitative conversion to 2b aer each oxidation/ the total oxidative process is irreversible yielding the ring-closed
reduction step (Fig. 5d). Subsequent irradiation with visible species, indicated by two new oxidative waves arising at 0.42 V
light leads to complete cycloreversion to 2a. There is no and 0.24 V in the second scan cycle. Most importantly, if only
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the morpholinothiazole bearing a CH
3
-group is oxidized and be seen as independent redox centres that in case of a
the return point of the potential scan is set before the second symmetrical molecule get oxidized simultaneously or at least at
28
oxidation takes place, the observed anodic wave has a quasi- very similar potentials. Thus a diradical dication of the open
ꢁ1
reversible character. At a scan rate of 1 V s the rst oxidation form is built that cyclizes to the dicationic ring-closed isomer.
step is reversible indicated by a corresponding cathodic wave on In both cases, cyclic voltammetry shows an irreversible two-
the return scan, while at lower scan rates the formed radical electron oxidation of the ring-open isomer.
cation undergoes a consecutive reaction (see Fig. S3 in the ESI†).
A rst hint that directs towards the latter EEC-mechanism is
Nevertheless, the product of this relatively slow reaction cannot the lack of any bathochromic absorbance during the spec-
be identied and is not identical with the ring-closed isomer, troelectrochemical investigation of the oxidation of 1a (Fig. S4
which only forms aer extraction of the second electron from of the ESI†), which would be characteristic for an intermediate
the ring-open species. Note that the introduction of only one monoradical cationic species either in the ring-open or in the
CF -group into the morpholinothiazole–maleimide architecture ring-closed form. A similar observation has recently been made
3
does not improve the fatigue resistance of the electrochemical for a related dithienylethene possessing amine redox centres
26a
switching process. During the oxidative cyclization of 3a a by- attached directly to the thiophene rings.
product similar to that described earlier for 1a could be
observed in UPLC/MS-measurements.
To further prove this assumption of an operating EEC-
mechanism nonsymmetrically substituted derivative is
a
necessary in order to achieve a maximum difference in the
oxidation potentials of both aryl units. The reported derivative
3a was synthesized specically for this task. The introduction of
Mechanistic considerations
the CF -unit did not alter the properties of the morpholino-
3
The mechanistic pathway for the oxidative ring-closure or ring-
opening of diarylethenes is still under discussion. Recently both
thiazole as a redox centre apart from a signicant shi of its
29
7c,13,25
oxidation potential by 570 mV. In fact, from cyclic voltam-
metry of 3a as described above (Fig. 6) it is clear that the rst
electron transfer from the molecule has a quasireversible nature
and does not lead to a chemical reaction at high scan rates.
Therefore, a stepwise double ionization to a dicationic species
has to take place in order to induce the ring-closure and thus we
conclude that the oxidative cyclization of 3a follows an EEC-
mechanism (Scheme 5).
radical cationic intermediates
as well as dicationic inter-
8d,26
mediates
were identied to be the species undergoing the
thermal isomerization reaction between the ring-open and the
ring-closed isomer. For the oxidative cyclization reaction
studied in this work both possibilities would experimentally
lead to similiar observations: for a reaction through a mono-
27
cationic state one would postulate an ECE-mechanism con-
sisting of an initial electron transfer from the ring-open isomer
giving the radical cation, which subsequently undergoes fast
cyclization. As the resulting ring-closed isomer possesses a
lower oxidation potential, a second electron is then transferred
immediately to the electrode resulting in the ring-closed dica-
tionic species. Nevertheless, an EEC-mechanism could also be
possible: supposing both morpholinothiazole moieties of the
ring-open isomer are electronically decoupled by twisting of the
ring-planes and cross-conjugation through the bridge, they can
Calculations were also performed to rationalize the electro-
chemical behaviour of compounds 1–4 and to gain more
detailed insight into the isomerization mechanism. For this
purpose, we considered different ionized species and different
electronic congurations. First, monocations with doublet spin
D
+
multiplicity (henceforth A ) were treated with the unrestricted
B3LYP (UB3LYP) method. Second, we studied various dications.
S
2+
Closed-shell dications with singlet multiplicity ( A ) were
treated with restricted B3LYP (RB3LYP) and dications with
T
2+
triplet multiplicity ( A ) were treated with UB3LYP. Finally, also
open-shell dications with singlet multiplicity were treated with
SU 2+
UB3LYP, using the broken symmetry approach ( A ). The
(broken symmetry) UB3LYP calculations for the dication serve
ꢁ
3
ꢁ1
Fig. 6 Cyclic voltammetry of 3a in acetonitrile (c ¼ 1 ꢂ 10 M, scan rate 1 V s
)
using a return potential at 0.87 V (solid line) and at 1.37 V (dotted line), respec-
tively. Inset shows two full oxidative cycles consecutively performed on the ring-
open isomer.
Scheme 5 EEC-mechanism for the oxidative ring-closure of 3a.
1
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30
S 2+
as models for triplet and singlet biradicals, respectively,
and always more stable than the closed-shell singlet ( A ). For
within a single-determinant formalism. When applied to the the ring-closed forms 1b–4b no broken symmetry solution can
open forms, the singlet biradical is characterized by two be found for the singlet state, i.e. the UB3LYP and RB3LYP
unpaired a and b electrons which are well separated in space, calculations give the same result with only paired electrons. For
localized on either the le or the right morpholino thiazole the closed form the triplet dication is always signicantly less
moiety, as shown in Fig. S9 of the ESI.† Similarly, the two excess stable than the singlet.
a spins of the triplet dication are also localized at these units as
shown there. Since only ground states (and no long-range experimentally observed potential splitting DE
Interestingly, for the ring-closed compounds 1b–4b the
, dened as the
p
charge transfer excitations) were considered here, the B3LYP difference between the rst and the second oxidation potential
method was used as it is reliable in this case. Again, PCM Ep,2 ꢁ Ep,1, is signicantly overestimated by the quantum
embedding was used to model solvents.
mechanical calculations. While experimental DE vary between
p
We rst performed DSCF calculations with reoptimized 0.22 V (for 2b) and 0.10 V (for 4b) the according differences
geometries for ions giving adiabatic ionization potentials betweenthe theoretical rst and second ionization potentials DIP
1
+
2+
0
1+
¼
IP(A / A ) ꢁ IP(A / A ) are in the range of 0.79 eV (for 2b)
n+
m+
m+
n+
IP(A / A ) ¼ E(A ) ꢁ E(A ),
(1)
and 0.72 eV (for 4b), referring to the formation of the most stable
SU 2+
A
dications. The DIP-values for gas-phase calculations (see
where n and m denote the positive charge (n,m ¼ 0, 1, or 2) of the
Table S5 in the ESI†) are 4–5 times higher (3.43–3.56 eV) showing
0
2+
0
1+
1+
molecule. Note that IP(A / A ) ¼ IP(A / A ) + IP(A
/
the extraordinary role of the solvent for the stabilization of the
2
+
A ). The adiabatic IPs for the different switches exposed to a
PCM (acetonitrile) eld were calculated (Table 4). Furthermore,
the corresponding gas-phase values, as well as DSCF energy
31
charge of a dicationic species. Note that it has been described
earlier that DFT-calculations on this level of theory using PCM to
consider solvent effects underestimate this stabilization energy
and thus, the resulting compression of potential splitting
v
n+
m+
differences for vertical ionizations, IP (A / A ), i.e. without
reoptimization of ion geometries, were determined (see Section
32
observed in the experiment is not reproduced accurately.
10 of the ESI†).
As described above, the ring-open compounds, except for the
nonsymmetrical derivative 3a, do not show any potential split-
ting in their two electron oxidation waves. This behaviour can
be rationalized in terms of electronic decoupling of the two
identical electrophores in the ring-open form. Thus, only elec-
trostatic effects play a role for the additional energy required for
extracting the second electron from the molecule. For localized
charges this may be compensated by the solvent and to a lesser
extent by the supporting electrolyte leading to a situation where
First of all we note that the calculated IPs (Table 4) reveal very
nicely the trends observed experimentally for the impact of
substitution on the electrochemical behaviour of compounds
1
–4. For example, when going from bis-CH
bis-CF -substituted 2a the experimental oxidation potential
increases by 0.57 V, while theory predicts a shi of 0.54 eV for
3
-substituted 1a to
3
0
1+
the rst ionization potential IP(A / A ). Furthermore, the
signicantly lower ionization potentials for the ring-closed
species 1b–4b are reproduced very well by the quantum
E
p,2 # Ep,1, which allows for the simultaneous oxidation of both
0
mechanical calculations: the rst ionization potentials IP(A /
electrophores via a concerted electron transfer or a dispropor-
1
+
A ) are lower by between 0.39 eV (for 1) and 0.68 eV (for 4) than
the corresponding values for the ring-open species, while
experimentally potential shis between 0.42 V (for 1) and 0.56 V
28
tionation mechanism. This is revealed as well in the theoret-
ical DIP-values for the ring-open forms (0.35–0.68 eV) that are
considerably smaller than the corresponding values for the
ring-closed forms. Nevertheless, they fail to reproduce the
experimentally observed complete compression of potential
splitting for compounds 1a, 2a, and 4a. However, note that the
above arguments are valid only for strictly reversible oxidation
processes. As in our experiment the chemical reaction subse-
quent to the second oxidation step is very fast, the equilibrium
may be shied to the dicationic product despite the absence of
an exergonic second electron transfer.
(
for 4) are found. Concerning doubly ionized species, from the
0
2+
IP(A / A )-values it can be seen that for the ring-open isomers
1
least equally stable as compared to the triplet biradical ( A ),
SU 2+
a–4a the singlet biradical ( A ) is slightly more stable or at
T
2+
n+
m+
Table 4 Adiabatic ionization potentials IP(A / A ) in eV calculated as
0
+
2+
energy differences between neutral (A ) and reoptimized ions (A and A ),
obtained on the B3LYP/PCM(acetonitrile)/6-31G* level of theory. For the dication
three different configurations were used (see text).
The calculations also nicely explain the reactivity of dications
towards ring closure. For this purpose, we determined transition
states for the ring closure reaction of neutral as well as charged
0
2+
+
2+
IP(A / A
)
IP(A / A )
0
+
IP(A / A )
A
33
D
+
S
2+
SU 2+
T
2+
S
2+
SU 2+
T
2+
species. For this purpose the so-called QST3 method, again in
conjunction with B3LYP/PCM/6-31G*, was used. There are in
A
A
A
A
A
A
1
1
2
2
3
3
4
4
a
b
a
b
a
b
a
b
5.02
4.63
5.56
4.90
5.16
4.76
4.71
4.03
10.82
10.00
11.98
10.59
11.34
10.30
Instable
8.78
10.52
10.00
11.47
10.59
10.99
10.30
10.10
8.78
10.54
11.53
11.47
11.98
11.00
11.74
10.16
10.53
5.80
5.37
6.42
5.69
6.18
5.54
—
5.50
5.37
5.91
5.69
5.83
5.54
5.39
4.75
5.52 principle at least two possible reaction products, arising from
6.90
either the conrotatory or the disrotatory pathway, where the two
ligands at C1 and C6 (–CH or –CF ) end up in a trans or in a cis
3 3
orientation, respectively. Therefore, one also expects at least two
transition states. The energies of the conrotatory products (b)
5.91
7.08
5.84
6.98
5.45 and corresponding transition states (‡) were calculated relative
8.78
6.50
to the reactants (open, a) (Table 5). Table 5 further contains an
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entry DE
Edge Article
SU 2+
g
for each species, which is the maximal possible energy, temperatures. For singlet biradical dications ( A ) the
which can be gained in the ionized state by geometry relaxation. activation energies towards ring closure are between 0.05 eV
SU 2+
SU 2+
The latter is given as the reorganization energy
(for 4a ) and 0.52 eV (for 2a ). Nevertheless, also in this
case reorganization energies DEg are larger than activation
energies, suggesting high reaction rates again. Since the ring-
v
n+
m+
n+
m+
DE
g
¼ IP (A / A ) ꢁ IP(A / A ).
(2)
SU 2+
open isomers treated as singlet biradical dications ( A )
show the lowest total energy as demonstrated above, we
expect, on this level of theory, that the electrochemical ring
closure leading to a conrotatory (trans) product is dominated
by a reaction on the biradical singlet potential energy surface
of the dication. Reactions of the dication on the triplet surface
are not favored. The monocationic surface will also play only a
minor role. For the thermal cyclization reaction of compound
This quantity is calculated for the open (a) species only. If
m+ ‡
m+
DE > E ¼ E((A ) ) ꢁ E(A ) at least a purely energetic crite-
g
act
rion to overcome the activation energy Eact for ring closure is
fullled. Of course, this does not necessarily imply that the
reaction actually takes place.
From the results it is apparent that most of the ring closure
reactions are exergonic. Exceptions are compound 4 in its
neutral form, as well as all triplet dications. Singly charged
cations, and even more so singlet dications give large, negative
3a these ndings are summarized in Fig. 7.
For compound 3 we also considered a disrotatory pathway
leading to the cis-product (see Table S8 of the ESI†). We note
that the energies for ring-closed species 3b(dis) are always less
favorable in comparison to the respective conrotatory prod-
ucts. In most cases the reaction is endergonic. The corre-
sponding activation energies Eact(dis) are larger than the
S
2+
reaction energies up to ꢁ1.47 eV (for the singlet dication 2a ).
Similarly, there are also large variations in activation ener-
gies for ring closure. For neutral molecules we nd high barriers
of around 2 eV for 1a–3a, and an even higher barrier of 2.61 eV
for compound 4a, reecting the fact that the a / b reactions of
neutral species are all thermally forbidden by the Woodward–
Hoffmann rules (vide infra). For the monocations, we nd
E
act(con) values for all cations, but smaller for the neutral
species. Thus, for neutral species a thermal reaction would
proceed faster (albeit still slow) along the disrotatory pathway,
in agreement with Woodward–Hoffmann rules. In general,
however, the disrotatory pathway is neither kinetically nor
thermodynamically attractive in comparison to the conrotatory
one.
Thermally induced neutral and monocationic ground state
reactions are dominated by the HOMO. From Fig. 3 one can
see that for the compounds under discussion one nds that
the HOMO, at least as far as the carbon atoms C1 and C6 are
concerned, have a phase similar to what one expects from
ideal hexatriene, i.e., the phase on C1 and C6 are of the same
sign. As a consequence, the ring closure would have to proceed
in a disrotatory manner. Still, this process is associated with
D
+
activation energies between 0.77 eV (for 3a ) to 0.92 eV (for
D
+
4
a ). For a purely thermal reaction these are high barriers as
well, suggesting small reaction rates. The maximal energy gain
DE is signicantly smaller than the activation energies, making
g
it very unlikely that the molecules can overcome this ground-
state barrier.
S
2+
The smallest barriers are found for singlet dications ( A ).
The classical barriers, i.e. without vibrational corrections, are
S
2+
close to zero for the singlet closed-shell dications 1a to
S
2+
S
2+
3
a . For the 4a singlet closed-shell dication, no stable
isomer and transition state could be found. Due to the
considerable energy gained during relaxation, these rather
small barriers should readily be overcome, even at lower
large barriers due to the presence of the two additional CH - or
3
3
CF -groups at C1 and C6, while the barrier for the conrotatory
pathway is even larger, in agreement with the Woodward–
Hoffmann rules. In the end, a thermal ring-closure reaction is
unfavorable for neutral molecules and monocations. For the
dications reacting thermally, one expects a prominent role for
HOMO-1. From Fig. 3 we note that both for 1a and 4a the
coefficients at C1 and C6 in this orbital are large and have
opposite sign, in agreement with what one expects for an ideal
Table 5 Energies (in eV) relative to open forms (a) for compounds 1–4 according
g
to B3LYP/PCM(acetonitrile)/6-31G* calculations. The DE values are maximum
energy gains of ionic ring-open species as defined in eqn (2). Product and tran-
sition state energies refer to the conrotatory pathway for the cyclization reaction.
D
+
S
2+
SU 2+
T 2+
Neutral
A
A
A
A
1
1
1
a
0.00
ꢁ0.29
2.09
0.00
0.00
0.00
ꢁ0.81
0.30
0.66
0.00
0.00
0.70
6-electron conjugated p-system. Therefore, a conrotatory
b
ꢁ0.68
ꢁ1.11
0.01
0.84
0.00
ꢁ1.47
0.01
0.67
0.00
ꢁ1.25
0.01
0.77
Instable
—
‡
0.86
1.80 pathway leading to the trans product should be easily acces-
DE
g
0.25
0.00
0.66 sible, which explains the shallow barriers observed for the
S
2+
2
2
2
a
0.00
ꢁ0.08
2.10
0.00
0.43
1.62
0.56
0.00
singlet dications
A
according to Table 5. Of course, this
b
ꢁ0.74
ꢁ0.96
analysis holds strictly for restricted orbitals only, which are
shown in Fig. 3. For the unrestricted (UB3LYP) orbitals used
for the open-shell singlet and triplet dications the situation is
‡
0.77
0.52
DE
g
g
g
0.21
0.00
0.56
0.00
3
3
3
a
0.00
ꢁ0.22
2.02
b
ꢁ0.61
ꢁ0.91
0.52 more complicated. This leads eventually to unfavorable path-
‡
0.92
0.35
1.68
0.62
0.00
0.41
ways for the triplet, and to nite, but not too large, barriers for
DE
0.29
0.00
0.62
0.00
the biradical singlet state. Due to the favorable reaction ener-
4
4
4
a
0.00
0.07
2.61
SU 2+
gies, the biradical singlet
A
appears to be the most prob-
b
ꢁ0.63
ꢁ1.28
‡
0.82
—
—
0.05
1.80 able reaction channel for electrochemical ring closure as
0.42 outlined above.
DE
0.18
0.44
1
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reactions (“drivers”). Work to exploit this and similar systems
for solar energy conversion and storage is currently ongoing in
our laboratories.
Acknowledgements
The authors are grateful to Stefan Mebs (Department of
Chemistry, Humboldt-Universit ¨a t zu Berlin) for obtaining the
Xray crystal structures of 1a and 1c. Generous support by the
German Research Foundation (DFG via SFB 658, projects B8
and C2) is gratefully acknowledged. BASF AG, Bayer Industry
Services, and Sasol Germany are thanked for generous dona-
tions of chemicals.
Notes and references
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Conclusions
We have described the rst dithiazolylethene derivative, which
undergoes an electrochemically driven conversion from the
ring-open to its ring-closed form. While oxidative ring-closure is
facilitated by the presence of the terminal electron-donating
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designing multifunctional devices with logic functions
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spatiotemporal control over a given function (“trigger”), but
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