Photoswitchable organic mixed valence in dithienylcyclopentene systems with tertiary amine redox centers

Article abstract of DOI:10.1021/ja207025x

The electronic structures of the radical cations of two dithienylperfluorocyclopentene molecules with appended tertiary amine units were investigated by electrochemical and optical spectroscopic methods. The through-bond N-N distances in the photocyclized (closed) forms of the two systems are 9.3 and 17.6 A, respectively, depending on whether the nitrogen atoms are attached directly to the two thienyl units or whether xylyl spacers are in between. In the case of the radical cation with the longer N-N distance, photocyclization of the dithienylperfluorocyclopentene core induces a changeover from class I to class II mixed valence behavior. In the case of the shorter system, the experimental data is consistent with assignment of the photocyclized form to a class III mixed valence species.

Full text of DOI:10.1021/ja207025x

ARTICLE
pubs.acs.org/JACS
Photoswitchable Organic Mixed Valence in Dithienylcyclopentene
Systems with Tertiary Amine Redox Centers
Bice He and Oliver S. Wenger*
Institut f€ur Anorganische Chemie, Georg-August-Universit€at G€ottingen, Tammannstrasse 4, D-37077 G€ottingen, Germany
S Supporting Information
b
ABSTRACT: The electronic structures of the radical cations of
two dithienylperfluorocyclopentene molecules with appended
tertiary amine units were investigated by electrochemical and
optical spectroscopic methods. The through-bond NꢀN dis-
tances in the photocyclized (closed) forms of the two systems
are 9.3 and 17.6 Å, respectively, depending on whether the
nitrogen atoms are attached directly to the two thienyl units or
whether xylyl spacers are in between. In the case of the radical
cation with the longer NꢀN distance, photocyclization of the
dithienylperfluorocyclopentene core induces a changeover
from class I to class II mixed valence behavior. In the case of the shorter system, the experimental data is consistent with
assignment of the photocyclized form to a class III mixed valence species.
’ INTRODUCTION
stability (including high reversibility) and pronounced photochro-
mic behavior.9 The issue of different charge and energy transfer
properties of the open and the more π-conjugated photocyclized
form of dithienylcyclopentenes is of long-standing interest and has
stimulated much research.10 However, among the many studies
performed in this context, there are only a few that have employed
dithienylcyclopentenes as bridging units between two identical
redox centers to investigate mixed valence phenomena in the
open and closed forms of the switchable spacer.11 What is more,
until now such work has focused exclusively on inorganic redox
centers. There are two important arguments for investigation of
dithienylcyclopentene-bridged mixed valence systems with
organic redox moieties: (i) intervalence absorption bands of
organic mixed valence systems are in many cases more easily
detectable than those of coordination compounds and (ii)
organic redox centers tend to exhibit inherently greater charge
delocalization than coordination complexes.2c To underscore
the importance of this latter point, in a series of (mixed valence)
bis(triarylamine) radical cations, the effective diabatic electron
transfer distance was found to be only roughly two-thirds of the
NꢀN distance.13
Mixed valence systems have played an important role in
arriving at a detailed theoretical understanding of electron
transfer reactions.1 Although many chemists spontaneously
associate the term “mixed valence” with coordination com-
pounds, there have been investigations of purely organic mixed
valence systems since the early days of this research field.2 Aside
from being of interest from a fundamental chemistry point of
view, the extent of charge delocalization in organic mixed valence
systems is relevant in the context of molecular electronics:
Charge transfer across molecular wires longer than ∼25 Å usually
cannot occur efficiently via the tunneling mechanism and con-
sequently has to proceed via hopping from one molecular unit to
the next.3 It would be desirable to construct molecular units that
permit large charge delocalization to minimize the distance over
which charge carriers have to hop between individual units.4
Among the many organic molecular wire systems that have been
explored in the past, thiophene-based π-conjugated oligomers
and polymers represent a particularly interesting class of materi-
als since they are good hole transporters that have found
application in organic light emitting diodes and other devices.5
Recent experimental and theoretical works on bis(triarylamine)
substituted oligothiophene (mixed valence) radical cations found
that the extent of charge delocalization in these systems can
indeed be very significant.6 However, thienylene-bridged organic
mixed valence systems are still poorly explored when compared
to phenylene-bridged analogues.2c
Here, we report on the dithienylcyclopentene molecules with
appended tertiary amine units shown in Scheme 1.
In the case of molecule 1, two triarylamine redox centers are
substituted to the photoswitchable core, while in molecule 3 the
nitrogen atoms are connected directly to the thienyl rings. The
electronic structures of the one-electron oxidized (mixed
valence) forms of these molecules were investigated using optical
absorption spectroscopy and cyclic voltammetry. Our results
In the context of molecular electronics, it may be desirable to
develop molecules that exhibit reversibly switchable conductivity.7
Among the many photoswitchable molecular units investigated so
far,8 dithienylcyclopentenes have received particular attention
because of their favorable properties such as long-term chemical
Received: July 27, 2011
Published: October 03, 2011
r
2011 American Chemical Society
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Scheme 1. Chemical Structures of the Molecules Investigated in This Work
Scheme 2. Synthetic Pathways to the Molecules from
Scheme 1a
Figure 1. Cyclic voltammograms measured on acetonitrile solutions of
(a) open form of molecule 1; (b) closed form of molecule 1; (c) closed
form of reference molecule 2. In all experiments, tetrabutylammonium
hexafluorophosphate was used as an electrolyte at a concentration of
0.1 M. The waves at ꢀ0.51 V are due to decamethylferrocene (Me10Fc),
which was added to these solutions for internal voltage calibration.
’ RESULTS AND DISCUSSION
The synthetic pathway to the molecules of central interest to
this study is outlined in Scheme 2. Bromination of 2-methylthio-
phene (9)14 yields 2,4-dibromo-5-methylthiophene (10), which
can be converted to boronic acid 11.15 Reaction between the
latter and 1-bromo-4-trimethylsilyl-2,5-dimethylbenzene (13)16
yields coupling product 14,17 which can be reacted with
perfluorocyclopentene (15) to the trimethylsilyl-protected
dithienylperfluorocyclopentene 16.15 Deprotection of the trime-
thylsilyl-groups with iodine monochloride gives the diiodo-
substituted analogue 17, which can be reacted with 4,40-di-
methoxydiphenylamine (18) to the desired bis(triarylamine) 1.6a
Molecule 2 with only one amino-group is obtained as a
a (a) Br2, CH3COOH; (b) n-BuLi, THF, ꢀ78°C, triisopropyl borate,
HCl; (c) n-BuLi, THF, ꢀ78°C, TMSCl; (d) THF/H2O, Pd(PPh3)4,
Na2CO3; (e) n-BuLi, THF, ꢀ78°C; (f) ICl, CH2Cl2; (g) o-dichloro-
benzene, Cu, K2CO3.
demonstrate that in both systems the extent of charge delocaliza-
tion depends very strongly on whether the dithienylcyclopentene
unit is in its open or closed form. Molecules 2 and 4 served as
reference systems that are chemically similar but unable to exhibit
mixed valence phenomena.
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Figure 2. Optical absorption spectra of acetonitrile solutions of (a) open form of molecule 1; (b) closed form of molecule 1; (c) closed form of reference
molecule 2 following addition of copper(II) perchlorate. (Left) Absorption spectra obtained after addition of 0.0ꢀ1.0 equivalents of copper(II) oxidant.
(Right) Absorption spectra measured after addition of 1.0ꢀ2.0 equivalents of oxidant.
byproduct from the same reaction and can be separated from
molecule 1 by column chromatography.
The potential splitting between the two waves is 150 mV,
suggesting that unlike in the open form, in the closed isomer of
molecule 1 there is significant mutual interaction between the
two triarylamine units. To eliminate any doubts that this splitting
into two waves could have a different origin, reference molecule 2
with only one triarylamine unit was investigated. The cyclic
voltammogram of the closed isomer (2c) is shown in Figure 1c
and exhibits a single oxidation wave at 0.27 V vs Fc+/0, in
agreement with our expectation. The observation of a potential
splitting of 150 mV between the two triarylamine oxidations in
molecule 1c is in line with a recent study of a ferrocene-
disubstituted dithienylperfluorocyclopentene.11c,d At a compar-
able (geometrical) separation distance between the two redox-
active units, a potential splitting of 160 mV has been reported for
the closed form of the photoisomerizable bridging unit. Voltam-
metry sweeps to potentials more positive than 0.7 V vs Fc+/0 that
show the effect of photochemical ring-closure on dithienylper-
fluorocyclopentene-localized oxidations in a suitable reference
molecule are given in Figure S7 of the Supporting Information.
As commonly observed, the open dithienylcyclopentene moiety
is oxidized irreversibly and at high potential, while the closed
form exhibits a reversible wave at significantly less positive
potential.
Synthesis of target molecules 3 and 4 departs from 3-bromo-2-
methyl-5-trimethylsilylthiophene (19), which is put into reaction
with perfluorocyclopentene (15) to yield dithienylperfluorocy-
clopentene 20.15 Trimethylsilyl-iodo exchange affords diiodo-
substituted molecule 21, which is reacted with 4,40-dimethoxy-
diphenylamine (18) to obtain a mixture of target molecules 3 and
4.6a This mixture is separable by column chromatography.
Figure 1a shows the cyclic voltammogram of the open form of
molecule 1 (hereafter referred to as 1o) in acetonitrile with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as the
electrolyte. The wave at ꢀ0.51 V is due to decamethylferrocene
(Me10Fc), which was used as a reference in all electrochemical
experiments reported herein;18 however, all experimentally de-
termined potentials are reported relative to the ferrocenium/
ferrocene (Fc+/0) couple as is commonly done for electrochem-
istry in nonaqueous solvents. The wave centered at 0.27 V vs Fc+/0
is readily assigned to oxidation of the two triarylamine units; this
is evident both from comparison to literature data and from
comparison to a tertiary amine reference molecule comprised of
two anisyl- and one xylyl-substituent (Figure S1 in Supporting
Information).12a,19
The optical absorption spectra shown in Figure 2 provide
further evidence for significantly different extents of electronic
communication between the triarylamine redox centers in the
open and closed isomers of molecule 1, but the evidence in this
case is somewhat more subtle and requires more careful inspec-
tion of the experimental data. Figure 2a shows the result of a
titration of molecule 1o with copper(II) perchlorate in acetoni-
trile solution.
Irradiation of molecule 1 with UV light induces a photocycli-
zation reaction as commonly observed for dithienylperfluorocy-
clopentenes. In the specific case of molecule 1, this leads to a
photostationary state containing a ratio between the closed
isomer (hereafter denoted 1c) and its open form (1o) greater
than 20:1, as indicated by 1H NMR spectroscopy (Figure S4 in
Supporting Information). The cyclic voltammogram of isomer
1c is shown in Figure 1b. Instead of a single oxidation wave, two
quasi-reversible oxidations are now observed in the potential
range where triarylamine oxidations typically occur.19,31
Upon addition of the copper oxidant, an absorption band at
12 900 cmꢀ1 becomes observable. This band is also observed for
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at 10 000 cmꢀ1 reaches its maximum exactly after addition of
1 equivalent of oxidant is in line with assignment of the broad near-
infrared absorption of 1c•+ to an intervalence transition. Indeed,
Figure 2c (left) shows that for the closed form of molecule 2,
which is unable to exhibit mixed valence phenomena due to the
presence of only a single triarylamine unit, an analogous near-
infared absorption band cannot be detected. This eliminates the
(theoretical) possibility that the additional near-infrared absorp-
tion intensity in Figure 2b is due to an electronic transition
primarily associated with the bridging dithienylperfluorocyclo-
pentene unit. There are no electronic absorptions at wavenum-
bers lower than 8000 cmꢀ1 in any of the species reported here;
we measured absorption spectra down to 3700 cmꢀ1 on a
suitable instrument.
In the middle and lower thirds of Figure 3, the experimental
absorption data from Figure 3a are disentangled by multipeak
fitting with Gaussian functions. The experimental spectrum of
1o•+ between 7800 and 18 000 cmꢀ1 (solid trace in Figure 3b)
can be fitted adequately with three Gaussian functions (dotted
traces in Figure 3b). Albeit due to a single electronic transition,
satisfactory fitting of the triarylamine radical cation absorption
band at 12 900 cmꢀ1 requires the use of two Gaussians. These
two Gaussians were kept fixed for the fit of the experimental
spectrum of 1c•+ (Figure 3c), because they describe absorptions
that are localized on a given triarylamine unit and should there-
fore depend only little on whether the photoswitchable bridging
unit is in its open or closed form. A total of four Gaussian
functions is necessary to obtain a reasonable fit to the experi-
mental spectrum in Figure 3c. The lowest energy Gaussian is of
central interest here, as it describes the additional near-infrared
intensity present in 1c•+, which was attributed above to an
intervalence transition.33 The characteristics of this Gaussian
are summarized in Table 1.
Even though the through-bond nitrogenꢀnitrogen distance
(dNN) is usually a bad measure for the effective electron transfer
distance (dAB) in bis(triarylamine) radical cations (and also in
other organic mixed valence species),2c a brief comparison of
dNN in our system to nitrogenꢀnitrogen distances in previously
explored organic mixed valence systems appears useful for our
analysis here. The radical cation of N,N,N,0N0-tetramethyl-p-
phenylenediamine (TMPD) and the radical anion of 1,4-dini-
Figure 3. (a) Experimental optical absorption spectra of 1o•+ (solid
trace) and 1c•+ (dashed trace), extracted from the titration end points in
the left halves of Figure 2a and b (that is after addition of exactly
1 equivalent of oxidant). The same experimental data are also shown in
(b) (1o•+) and (c) (1c•+), along with the results of multipeak fits with a
minimal number of Gaussian functions (dotted traces).
the above-mentioned reference amine comprised of two anisyl-
and one xylyl-substituents (Figure S2 in Supporting In-
formation) and is typical for triarylamine radical cations.19
Comparison of the titration end points in the left and right parts
of Figure 2a shows that addition of 1 equivalent of oxidant to a
solution of 1o leads to an absorbance roughly half as large as
addition of 2 equivalents of oxidant. This observation is in line
with the expectation that in the dicationic form of 1o there are
two singly oxidized triarylamine radical cations with equal
extinction coefficients at 12 900 cmꢀ1. The occurrence of an
isosbestic point at 28 500 cmꢀ1 suggests that at any given
titration point there are only two species which contribute
significantly to the optical absorption spectrum.
Figure 2b shows analogous titration data for the closed isomer
of molecule 1. The data for 1c is qualitatively similar to that
shown for the open isomer, but with a subtle difference ob-
servable in the left part of Figure 2b: Underlying the triarylamine
radical cation absorption at 12 900 cmꢀ1, there is a broad and
somewhat weaker band (marked by an upward arrow) extending
from 8000 cmꢀ1 to at least 15 000 cmꢀ1. The absorbance at
10 000 cmꢀ1 is caused to a significant extent by this broad band
and is found to maximize after addition of 1 equivalent of oxidant.
This becomes evident by the appearance of an isosbestic point at
10 800 cmꢀ1 in the right part of Figure 2b. It is to be noted that
the triarylamine radical cation band centered at 12 900 cmꢀ1
trobenzene are both fully delocalized class III systems with dNN
=
5.7 Å.2c,20 The N,N,N,0N0-tetraanisyl-p-phenylenediamine radi-
cal cation has similar dNN and was also interpreted as a class III
system.21 Most systems with larger dNN, for example a biphenyl-
bridged bis(triarylamine) analogue with dNN = 10.0 Å or a tolane-
bridged congener with dNN = 10.9 Å, are clear cases of class II
systems.12a A bis(triarylamine) radical cation with a ladder-type
pentaphenylene bridge and a nitrogenꢀnitrogen distance of 22 Å
was described as a class II system with HAB = 548 cm .
ꢀ1 12d Two
bis(triarylamine) radical cations with [2.2]paracyclophane
bridges and dNN = 9.9 and 15.1 Å, respectively, were also
interpreted as class II species.34 Among dinitroaromatic radical
does also contribute to the overall absorption at 10 000 cmꢀ1
,
hence the absorbance at this wavenumber decreases less sig-
nificantly between 1 and 2 equivalents of oxidant than might be
anticipated.
A direct superposition of the near-infrared absorption spectra
of 1o•+ and 1c•+ is shown in Figure 3a, from which it can more
clearly be seen that after addition of one equivalent of oxidant, 1c
exhibits significantly stronger absorption between 7800 cmꢀ1
and 12 600 cmꢀ1 than 1o.32 This falls into the spectral range in
which intervalence absorption bands of bis(triarylamine) radical
cations are commonly observed.12a,b The fact that the absorbance
anions, there are a few examples of class III systems with dNN
=
10.0 Å,22 but these are clearly exceptions to the commonly
observed behavior.35 In molecule 1o, dNN = 17.6 Å and conse-
quently the expectation of a fully delocalized electronic structure
for the radical cation of 1o•+ appears unrealistic.3c,12c
Usually there are several experimental observables which can
be used to distinguish between class II and class III mixed valence
forms, but most of them are difficult to elucidate in the specific
case of 1o•+.1b,d,f,hꢀj For instance, the intervalence absorption
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Table 1. Intervalence Band Data for the Radical Monocations of 1c and 3c along with Effective Electron Transfer Distances (dAB
ARTICLE
)
a
and Electronic Coupling Matrix Elements (HAB
)
compound
νmax [cmꢀ1
]
εmax [Mꢀ1cmꢀ1
]
Δν1/2 [cmꢀ1
]
dAB [Å]
17.6
0.66 ꢁ 17.6d
9.3
HAB [cmꢀ1
]
1c•+
11 250
4250
3500
476 b
722 b
5550 c
1511 b
2289 b
3c•+
11 100
18 400
2300
9.3
0.66 ꢁ 9.3d
a Parameters νmax, εmax, and Δν1/2 are defined in the text. b Calculated based on eq 1. c Calculated based on eq 3. d Multiplication factor 0.66 is used
because prior studies on related bis(triarylamine) radical cations have led to the conclusion that dAB ≈ 0.66 ꢁ dNN
13
.
bands of class II systems commonly exhibit stronger solvent
dependence than those of class III radicals, but in our case this
effect is superimposed by the solvent dependence of the triar-
ylamine radical cation band at 12 900 cmꢀ1. Likewise, an
asymmetry of the intervalence absorption band, as reported for
several systems near the borderline of class II and class III, is
masked.12a Vibrational fine structure, frequently appearing in
class III systems, is not observed. If occurring, this should be
detectable even in our case since it would be expected close to the
electronic origin of the near-infrared absorption band around
8000 cmꢀ1. Taken together, these facts support assignment of
1c•+ to Robin-Day class II.23 The potential splitting of 150 mV
reported above and a comproportionation constant of 344
provides further support for this assignment. For reference, the
potential splitting observed between the two oxidation waves of
the class III system TMPD•+ amounts to 600 mV, resulting in a
comproportionation constant of 1.4 ꢁ 1010.2c
Figure 4. Cyclic voltammograms measured on acetonitrile solutions of
(a) open form of molecule 3; (b) closed form of molecule 3; (c) closed
form of reference molecule 4. In all experiments, tetrabutylammonium
hexafluorophosphate was used as an electrolyte at a concentration of
0.1 M. The waves at ꢀ0.51 V are due to decamethylferrocene (Me10Fc),
which was added to these solutions for internal voltage calibration.
For class II systems with Gaussian-shaped intervalence ab-
sorption bands, an estimate of the matrix element HAB quantify-
ing the electronic coupling between the two redox-active centers
can be obtained using eq 1.24
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ1
HAB ¼ 2:05 ꢁ 10ꢀ2
ꢁ
εmax ꢁ ν~max ꢁ Δν~1=2 ꢁ dAB
ð1Þ
matrix elements reported for these molecular bridges compared
to OPVs or OPE wires.3c,12b,26 Tunneling-energy gaps and
bridge redox potentials play a key role in this context.20,27
Significantly stronger electronic couplings have been reported
for mixed valence systems in which the nitrogen centers of
tertiary amines are attached directly to bridging thiophene units.6
This prompted our interest in molecule 3, which differs from
molecule 1 by the absence of two bridging xylyl units, resulting in
a through-bond NꢀN distance of roughly 9.3 Å.
νmax is the energy of the near-infared absorption band max-
imum in wavenumbers, Δν1/2 is the full width at half-maximum
(in cmꢀ1), εmax is the molar extinction coefficient at the band
maximum, and dAB is the electron transfer distance. Using the
Gaussian fit parameters given in Table 1 and dAB = dNN = 17.6 Å,
one obtains HAB = 476 cmꢀ1. However, as mentioned above,
investigations of various kinds of organic mixed valence systems
have demonstrated that dAB is usually significantly shorter than
the distance between atoms which may formally be regarded as
Figure 4a shows the result of a cyclic voltammetry experiment
with the open form of molecule 3 (hereafter referred to as 3o) in
acetonitrile solution in the presence of tetrabutylammonium
hexafluorophosphate electrolyte and Me10Fc as a reference. The
solid trace represents the initially recorded voltammogram. The
first oxidation wave occurs at 0.29 V vs Fc+/0, at essentially the
same potential for which triarylamine oxidation was observed for
molecule 1o (Figure 1a). However, contrary to all other triar-
ylamine oxidations reported in this work so far, the oxidation
process considered here is irreversible. Instead, the subsequent
voltage sweep from 0.7 to ꢀ0.7 V vs Fc+/0 reveals two weaker
reduction waves at ꢀ0.05 and ꢀ0.28 V vs Fc+/0. In the
subsequent second oxidative sweep from ꢀ0.7 to +0.7 V
(dotted trace in Figure 4a), weak oxidation waves now occur
at ꢀ0.22 and 0.01 V vs Fc+/0, and at the same time the initially
observed oxidation wave at 0.29 V has become less prominent.
redox-active centers.2c,13 For several dinitroaromatic radical
35
anions, dAB was found to be only roughly 40% of dNN
,
while
for a series of bis(triarylamine) radical cations dAB was estimated
to be roughly two-thirds of dNN
.
13 When using dAB = 0.66 ꢁ dNN
= 11.6 Å, one obtains HAB = 722 cmꢀ1 for 1o•+. It appears
appropriate to conclude that 476 and 722 cmꢀ1 represent
reasonable lower and upper limits for the electronic coupling
matrix element (HAB) of the closed form of molecule 1 (Table 1,
last column). HAB values on this order were previously reported
for bis(triarylamine) radical cations with oligo-p-phenylene
vinylene (OPV) and oligo-p-phenylene ethynylene (OPE)
bridges and similar geometrical distance between redox-active
centers.2c,12a,12b Oligo-p-phenylene bridged organic mixed va-
lence systems exhibit significantly weaker electronic coupling at
comparable geometrical distance between redox units,12a,25 in
line with the stronger distance dependence of electronic coupling
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Figure 5. Optical absorption spectra of acetonitrile solutions of (a) open form of molecule 3; (b) closed form of molecule 3; (c) closed form of reference
molecule 4 following addition of copper(II) perchlorate. (Left) Absorption spectra obtained after addition of 0.0ꢀ1.0 equivalents of copper(II) oxidant.
(Right) Absorption spectra measured after addition of 1.0ꢀ2.0 equivalents of oxidant.
This behavior appears peculiar at first glance but can be under-
stood readily upon inspection of the data in Figure 4b, measured
on the closed form of molecule 3 (hereafter named 3c). The
voltammogram observed for this species exhibits two reversible
oxidation waves centered at ꢀ0.25 and ꢀ0.02 V vs Fc+/0. This is
precisely at the potentials at which reduction and oxidation waves
are observed after initial oxidation of molecule 3o (Figure 4a).
The conclusion which imposes itself is that application of
electrochemical potentials more positive than 0.29 V vs Fc+/0
to solutions of the open form of molecule 3 induces ring closure
of the dithienylperfluorocyclopentene unit. This ring closure
appears to be electrochemically irreversible, atleast in the potential
range considered here. From the decrease of the oxidation wave
at 0.29 V in Figure 4a with increasing number of voltammetry
cycles, one may estimate that ultimately more than 80% of all
molecules undergo electrochemically induced ring closure.36
Both tertiary amine redox units are oxidized simultaneously at
0.29 V vs Fc+/0; hence, ring closure is likely to be the result of a
two-electron oxidation process.
The observation of ring closure of dithienylcyclopentenes
after two-electron oxidation is not without precedent, in fact
there have been several very detailed investigations on this
particular subject.28 The efficiency of electrochemical ring clo-
sure has been found to increase with increasing electron-donat-
ing character of substituents attached to the C5-position of the
thienyl units,28a,b and therefore efficient electrochemical ring
closure in the amino-substituted molecule 3o is not a major
surprise. The redox waves of the closed form fall into a potential
range which is typical for thienyl-localized redox processes in
dithienylcyclopentenes.28a,b Thus, after initial oxidation of the
tertiary amine units of 3o, ring closure occurs, and a species is
formed for which the observed oxidation waves look like thienyl-
localized redox processes. However, as will be discussed below, in
the resulting species there is in fact significant charge delocaliza-
tion between the amino-groups and the thienyl-rings. The
potential splitting of 230 mV observed between the two oxida-
tion waves of 3c indicates that there is significant interaction
between its two redox units. Thus, the one-electron oxidized
form (3c•+) may potentially be considered a mixed valence
system comprised of two thienyl/amine redox centers bridged
by a perfluorocyclopentene unit.
The cyclic voltammogram of the closed form of reference
molecule 4 in Figure 4c is in line with all observations and
interpretations made above, although in this case the most
important redox process appears to remain localized on the
tertiary amine. Moreover, electrochemical cyclization of the open
form of molecule 4 does not occur, indicating that electron-
donating substituents on both thienyl units are necessary for
efficient electrochemical ring closure.
The left part of Figure 5a shows the optical absorption spectra
of an acetonitrile solution of molecule 3o obtained after addition
of up to 1 equivalent of copper(II) perchlorate. The initial
solution is nearly colorless, and with increasing addition of
oxidant there is only the build-up of a very broad absorption
feature at wavenumbers higher than 15 500 cmꢀ1, and the
solution turns red. Addition of a further equivalent of oxidant
(Figure 5a, right) simply further increases the intensity of this
absorption band. In other words, already the very first oxidation
step in Figure 5a gives a spectrum that is essentially identical to
that obtained at the titration end point, only with weaker overall
intensity.
Figure 5b shows two analogous sets of data for the closed form
of molecule 3. Based on cyclic voltammetry data, it can be
estimated that the photostationary state in this case contains
more than 90% of 3c, and this is corroborated by 1H NMR data
(Figure S5 in Supporting Information). The initially present
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mixed-valence species with low-energy intervalence transitions.
In order to test this hypothesis, an analogous titration was
performed with the closed form of reference molecule 4, which
has only one amino-group attached to the dithienylperfluoro-
cyclopentene core. The result of this titration is shown in
Figure 5c. Instead of the two near-infrared absorptions seen in
Figure 5b, there is only an absorption band at 13 300 cmꢀ1 for
4c•+. Based on its great similarity to the absorption bands
observed in Figure 2a/c, this band is attributed to absorption of
a charge-localized tertiary amine radical cation. Consistent with
this interpretation, this band remains essentially unaltered
upon addition of a second equivalent of oxidant (Figure 5c,
right). It is therefore plausible that the near-infrared bands of
3c•+ in Figure 5b are indeed associated with the mixed valence
character of this particular radical cation.
None of the forms of molecule 3 absorbs at wavenumbers
lower than 9000 cmꢀ1 (absorption spectra were measured down
to 3700 cmꢀ1), hence the unique absorptions between
9000 cmꢀ1 and 21 000 cmꢀ1 from the left part of Figure 5b,
shown in more detail in Figure 6a, are indeed the lowest-
energetic absorptions. The overall appearance of these two bands
is reminiscent of the lowest energetic absorptions reported
recently for the radical cations of (E)-1,2-bis[5-[bis(4-butoxyphenyl)-
amino]-2-thienyl]ethylene (5) and 2,6-bis[bis(4-methoxyphenyl)
amino]-2,20-bithiophene (6) (Scheme 3).6a Time-dependent
DFT calculations for 5•+, 6•+ and two related species (7•+, 8•+)
suggested that their near-infrared absorption bands have
significant intervalence charge transfer character, while the
absorptions occurring at shorter wavelengths are of different
origin.6a
Figure 6. (a) Solid trace is the experimental absorption spectrum
measured on an acetonitrile solution of 3c•+ as extracted from the
titration end point in the left part of Figure 5b. The dotted traces
represent the result of a multipeak fit to the experimental data with a
minimal number of Gaussian functions. (b) Solvent dependence of the
experimental absorption spectrum of 3c•+ (solid trace, acetonitrile;
dotted trace, ethanol; dashed trace, dichloromethane).
absorption band in the visible spectral range (λmax
=
17 300 cmꢀ1) typical for the closed form of dithienylperfluoro-
cyclopentenes decreases in intensity upon oxidation. Simulta-
neously, two new absorption bands at lower energies gain in
intensity up to the titration point at which exactly one equivalent
of oxidant has been added (left part of Figure 5b). Further
addition of oxidant leads to a decrease of the intensity of the two
low-energy absorptions, and at 2 equivalents of copper(II)
added, they have disappeared completely (Figure 5b, right).
The end points of the titrations of 3c and 3o yield nearly identical
absorption spectra. The last absorption spectrum in the right part
of Figure 5a essentially corresponds to the last absorption
spectrum in Figure 5b, suggesting that the same species is probed
in absorption in both cases. This finding is in line with the
electrochemical results from above, which indicated that two-
electron oxidation of 3o leads to cyclization. It may be concluded
that 3c2+ is the species that is detected at the end points of the
two titrations in Figure 5a and b.
The solvent dependences of the two lowest-energetic absorp-
tions of 3c•+ are significantly different (Figure 6b): While the
maximum of the higher energy absorption shifts from
15 700 cmꢀ1 in acetonitrile to 15 400 cmꢀ1 in ethanol and
14 700 cmꢀ1 in dichloromethane, the maximum of the weaker
near-infrared absorption stays essentially constant at 11100 cmꢀ1
.
Multipeak fitting with Gaussian functions reveals that the minor
shift to lower wavenumbers of the near-infrared band is merely
the result of the significant red-shift of the higher-energy
absorption band. In acetonitrile, the characteristics of the Gauss-
ian profile fitted to the near-infrared absorption band (dotted
trace in Figure 6a) are as listed in Table 1. An important finding is
that the fitted band profile is significantly narrower than what
would be expected for a class II intervalence band: One would
normally expect a class II intervalence band to satisfy eq 2,1a but
in the present case the bandwidth Δν1/2 is calculated to
5064 cmꢀ1, while the value obtained from the Gaussian fit
Above we noted already that the very first oxidation step
performed on molecule 3o (Figure 5a, left) results in an
absorption spectrum which is nearly identical to that obtained
at the titration end points of 3o and 3c (Figure 5a,b, right), only
with weaker overall intensity. The logical conclusion is therefore
that the 3c2+ species is formed already at the very first oxidation
step of 3o.37 This interpretation is in agreement with the cyclic
voltammetry data in Figure 4a which exhibits a single oxidation
wave at 0.29 V vs Fc+/0, indicating that 3o•+ and 3o2+ are formed
at very close potential. The latter reacts irreversibly to 3c2+, and
thus neither 3o•+ nor 3o2+ are observed in the optical absorption
spectra of Figure 5a. The clean observation of monocationic 3c•+
is certainly facilitated by the relatively large potential separation
between the first and second oxidation process (230 mV, see
above).
amounts to only 2300 cmꢀ1
.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Δν~
¼
2310 ꢁ ν~
ð2Þ
max
1=2
This, along with the observation of the solvent-independence
of the near-infrared absorption band suggests that analysis in
the framework of a fully delocalized class III system may be adequate
for 3c•+, similar to what has been done for the mixed valence
radical cations of 5 and 6.6a As noted above, in class III systems
intervalence bands are frequently asymmetric or even exhibit
vibrational fine structure.1f,12a Whether or not such asymmetry is
present in the near-infrared absorption band of Figure 6a cannot
be established based on the available data, and therefore the data
is fitted with symmetric Gaussian functions which require a
minimum of independent fit parameters. For fully delocalized
The occurrence of two near-infrared absorptions after addition
of exactly 1 equivalent of oxidant to the closed form of molecule
3 (Figure 5b) suggests that 3c•+ may indeed be considered a
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Journal of the American Chemical Society
ARTICLE
Scheme 3. Chemical Structures of Four Molecules with Thienyl-linkers Previously Investigated in the Context of Organic Mixed
Valence6a
Scheme 4. Graphical Summary of the Oxidation and Cycli-
zation Processes Observed for Molecule 3
the radical cations of 5ꢀ8 as well as in comparison to the shorter
system 1c•+ discussed in the first half of this paper. Although it
has been noted that in some cases the two-state model may be
inadequate for analysis of intervalence bands in strongly coupled
(class III) mixed valence system,29 we restrict the current analysis
to this model for the sake of simplicity. On a final note regarding
the mixed valence character of 3c•+, we point out that a
comproportionation constant of 7.8 ꢁ 103 (based on ΔE =
230 mV, see above) is not in contradiction with interpretation of
3c•+ as a class III system.
Scheme 4 summarizes our findings for molecule 3. The open
form (3o) photoisomerizes reversibly to the closed form (3c).
The one-electron oxidized form of the open species (3o•+) and
the corresponding two-electron oxidized species (3o2+) are
formed at very close potential, likely because the two amine
units in molecule 3o are only comparatively weakly interacting.
Thus, in Scheme 4, radical monocation 3o•+ is drawn as a charge-
localized (class I) species. The dication 3o2+ undergoes sponta-
neous cyclization to 3c2+. One-electron oxidation of 3c gives 3c•+
which may be described adequately by two resonance structures:
one implying full, the other implying partial delocalization of the
odd electron. The discussion above leads to the conclusion that
the fully delocalized structure is reasonable, although we cannot
exclude definitively a significant contribution from the partially
delocalized resonance structure. Further oxidation of 3c•+ leads
to 3c2+, which is the common two-electron oxidation product of
both 3o and 3c. Interestingly, a solution containing a 1:1 ratio of
3o and copper oxidant is photoswitchable to the 3c•+ state
(Figure S8 in Supporting Information).
systems, HAB may be calculated according to eq 3.1a
ν~
max
HAB
¼
ð3Þ
2
For 3c•+, this yields a value of 5550 cmꢀ1 (Table 1), which is
similar to what has been reported for 6•+ (5050 cmꢀ1) based on
’ SUMMARY AND CONCLUSIONS
the same analysis.6a For 5•+ a value of 4375 cmꢀ1 was reported,
Two dithienylperfluorocyclopentenes substituted symmetri-
cally with two tertiary amino-groups have been explored from the
perspective of mixed valence chemistry. While in molecule 1 it is
possible to make a formal distinction between redox-active
triarylamine groups and a dithienylperfluorocyclopentene brid-
ging unit, this distinction is not meaningful for molecule 3.
Instead, when the nitrogen centers are attached directly to the
•+
while for 7
H
= 5250 cmꢀ1 and for 8•+ HAB = 6270 cmꢀ1 6a
.
AB
Alternatively, if one were to interpret the intervalence data of
3c•+ in the framework of a class II mixed valence species, one
would obtain HAB = 1511 cmꢀ1 based on eq 1 and dAB = dNN
=
9.3 Å, or HAB = 2289 cmꢀ1 for dAB = 0.66 ꢁ dNN = 6.1 Å. These
values appear low both in comparison to those reported above for
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Journal of the American Chemical Society
ARTICLE
thienyl rings, there is strong charge delocalization into the
dithienylperfluorocyclopentene core, and one might consider
the resulting radical monocations as partially “bridge-localized”
species. Nevertheless, our study shows that thanks to the
symmetrical nature of molecule 3 with its two easily oxidizable
termini, the monocationic closed form of this compound exhibits
spectroscopic and electrochemical properties that are typical for
mixed valence species. For 3c•+, the experimental data is con-
sistent with delocalization of the odd electron over the entire
molecule (Robin-Day class III), while in the case of 1c•+, there is
only partial charge delocalization (Robin-Day class II). In the
open forms of both cations, the odd electron is localized on one
branch of the otherwise symmetrical systems (Robin-Day class I).
Because the open and closed forms can be interconverted by
irradiation with UV and visible light, the extent of electronic
communication is photoswitchable. Notably, intervalence ab-
sorptions in the molecules studied herein can be detected at long
wavelengths (>900 nm) at which no photoisomerization reac-
tions are induced, thereby allowing nondestructive interrogation
of the cyclization state. This has been an important goal of much
research on dithienylcyclopentenes.30
Attachment of the redox-active tertiary amine units directly to
the thienyl-rings permits construction of molecular mixed va-
lence systems with significantly shorter (geometrical and
effective) electron transfer distances than what has previously
been achieved for dithienylcyclopentene-based mixed valence
molecules with inorganic redox centers.11 What is more, it also
introduces the interesting possibility of electrochemically indu-
cing the cyclization reaction by a two-electron process.
In molecular wires, tertiary amine-substituted dithienylcyclo-
pentenes may represent interesting photoswitchable building
blocks. In their closed forms, they may serve as good hopping
stations for long-range charge transfer between distant donors
and acceptors because within these units, charge is strongly
delocalized. In their open forms, by contrast, charge is localized
and hopping events involving these molecular units as stations
(also called stepping stones)3b have to occur over greater
molecular distances.
Varian or on a Cary 50 instrument from the same company. Copper(II)
perchlorate was used as the preferred oxidant as described earlier.19
Identical results were obtained with NOBF4, but this compound is more
difficult to handle and titration errors were generally larger.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic protocols and char-
b
acterization data for compounds 1, 2, 3, 4, 14, 16, 17 and for two
reference compounds; optical absorption and cyclic voltammetry
data for the two reference compounds mentioned in the text.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
oliver.wenger@chemie.uni-goettingen.de.
’ REFERENCES
(1) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Creutz, C.
Prog. Inorg. Chem. 1983, 30, 1. (c) Creutz, C.; Taube, H. J. Am. Chem. Soc.
1969, 91, 3988. (d) Brunschwig, B. S.; Sutin, N. Coord. Chem. Rev. 1999,
187, 233. (e) Nelsen, S. F. Chem.—Eur. J. 2000, 6, 581. (f) Demadis,
K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101, 2655.
(g) Launay, J.-P. Chem. Soc. Rev. 2001, 30, 386. (h) Brunschwig, B.
S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002, 31, 168. (i) D’Alessandro,
D. M.; Keene, F. R. Chem. Rev. 2006, 106, 2270. (j) D’Alessandro, D. M.;
Keene, F. R. Chem. Soc. Rev. 2006, 35, 424.
(2) (a) Cowan, D. O.; Levanda, C.; Park, J.; Kaufman, F. Acc. Chem.
Res. 1973, 6, 1. (b) Mazur, S.; Sreekumar, C.; Schroeder, A. H. J. Am.
Chem. Soc. 1976, 98, 6713. (c) Hankache, J.; Wenger, O. S. Chem. Rev.
2011, 111, 5138–5178.
(3) (a) Gray, H. B.; Winkler, J. R. Proc. Natl. Acad. Sci. U.S.A. 2005,
102, 3534. (b) Cordes, M.; Giese, B. Chem. Soc. Rev. 2009, 38, 892.
(c) Lloveras, V.; Vidal-Gancedo, J.; Figueira-Duarte, T. M.; Nierengarten,
J. F.; Novoa, J. J.; Mota, F.;Ventosa, N.; Rovira, C.; Veciana, J. J. Am. Chem.
Soc. 2011, 133, 5818.
(4) Funston, A.; Kirby, J. P.; Miller, J. R.; Pospíꢀsil, L.; Fiedler, J.;
Hromadovꢁa, M.; Gꢁal, M.; Pecka, J.; Valꢁaꢀsek, M.; Zawada, Z.; Rempala,
P.; Michl, J. J. Phys. Chem. A 2005, 109, 10862.
(5) (a) Roncali, J. Chem. Rev. 1992, 92, 711. (b) Tour, J. M. Chem.
Rev. 1996, 96, 537. (c) Roncali, J. J. Mater. Chem. 1999, 9, 1875. (d) Lu,
J. P.; Xia, P. F.; Lo, P. K.; Tao, Y.; Wong, M. S. Chem. Mater. 2006,
18, 6194. (e) Chen, J. W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709.
(f) Gao, P.; Beckmann, D.; Tsao, H. N.; Feng, X. L.; Enkelmann, V.;
Baumgarten, M.; Pisula, W.; M€ullen, K. Adv. Mater. 2009, 21, 213.
(6) (a) Odom, S. A.; Lancaster, K.; Beverina, L.; Lefler, K. M.;
Thompson, N. J.; Coropceanu, V.; Brꢁedas, J. L.; Marder, S. R.; Barlow, S.
Chem.—Eur. J. 2007, 13, 9637. (b) N€oll, G.; Avola, M.; Lynch, M.; Daub,
J. J. Phys. Chem. C 2007, 111, 3197. (c) Lacroix, J. C.; Chane-Ching, K. I.;
Maquꢂere, F.; Maurel, F. J. Am. Chem. Soc. 2006, 128, 7264.
(7) (a) Raymo, F. M.; Tomasulo, M. Chem. Soc. Rev. 2005, 34, 327.
(b) Milder, M. T. W.; Areephong, J.; Feringa, B. L.; Browne, W. R.;
Herek, J. L. Chem. Phys. Lett. 2009, 479, 137.
(8) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, 2001.
(9) (a) Irie, M. Chem. Rev. 2000, 100, 1685. (b) Tian, H.; Yang, S. J.
Chem. Soc. Rev. 2004, 33, 85.
(10) (a) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. J. Chem. Soc., Chem.
Commun. 1993, 1439. (b) Tsivgoulis, G. M.; Lehn, J. M. Chem.—Eur. J.
1996, 2, 1399. (c) Endtner, J. M.; Effenberger, F.; Hartschuh, A.; Port, H.
J. Am. Chem. Soc. 2000, 122, 3037. (d) Peters, A.; McDonald, R.; Branda,
N. R. Chem. Commun. 2002, 2274. (e) Liddell, P. A.; Kodis, G.; Moore,
A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 2002, 124, 7668.
(f) Wenger, O. S.; Henling, L. M.; Day, M. W.; Winkler, J. R.; Gray, H. B.
Polyhedron 2004, 23, 2955. (g) Jukes, R. T. F.; Adamo, V.; Hartl, F.;
’ EXPERIMENTAL SECTION
Commercially available chemicals were used as received without
further purification. All reactions were carried out under nitrogen using
solvents which were dried by routine methods. Thin-layer chromatog-
raphy was performed using Polygram SIL G/UV254 plates from
Machery-Nagel. For column chromatography, Silica Gel 60 from
Macherey-Nagel was employed. Reaction products were characterized
1
by H and 13C NMR spectroscopy (Avance DRX 300 spectrometer,
using the deuterated solvent as the lock and residual solvent as an
internal reference), by electron ionization mass spectrometry (EI-MS)
using a Finnigan MAT8200 instrument, and by elemental analysis.
Detailed synthetic protocols and product characterization data are given
in the Supporting Information. Cyclic voltammetry was performed using
a Versastat3ꢀ100 potentiostat from Princeton Applied Research
equipped with a Pt disk working electrode, and a silver counter
electrode. A silver wire did also serve as a quasi-reference electrode.
Decamethylferrocene (Me10Fc) was used as an internal reference,18 but
all experimentally determined potentials are reported relative to the
ferrocenium/ferrocene (Fc+/0) couple as is commonly done in electro-
chemistry. Prior to voltage scans at rates of 100 mV/s, nitrogen gas was
bubbled through the dried solvent. The supporting electrolyte was a
0.1 M solution of tetrabutylammonium hexafluorophosphate. Optical
absorption spectra were recorded on a Cary 5000 spectrometer from
17035
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Journal of the American Chemical Society
ARTICLE
Belser, P.; De Cola, L. Coord. Chem. Rev. 2005, 249, 1327. (h) Belser, P.;
De Cola, L.; Hartl, F.; Adamo, V.; Bozic, B.; Chriqui, Y.; Iyer, V. M.;
Jukes, R. T. F.; Kuhni, J.; Querol, M.; Roma, S.; Salluce, N. Adv. Funct.
Mater. 2006, 16, 195. (i) Roberts, M. N.; Carling, C. J.; Nagle, J. K.;
Branda, N. R.; Wolf, M. O. J. Am. Chem. Soc. 2009, 131, 16644.
(11) (a) Fraysse, S.; Coudret, C.; Launay, J. P. Eur. J. Inorg. Chem.
2000, 1581. (b) Launay, J. P.; Fraysse, S.; Coudret, C. Mol. Cryst. Liq.
Cryst. 2000, 344, 125. (c) Tanaka, Y.; Inagaki, A.; Akita, M. Chem.
Commun. 2007, 1169. (d) Tanaka, Y.; Ishisaka, T.; Inagaki, A.; Koike, T.;
Lapinte, C.; Akita, M. Chem.—Eur. J. 2010, 16, 4762. (e) Motoyama, K.;
Li, H.; Koike, T.; Hatakeyama, M.; Yokojima, S.; Nakamura, S.; Akita, M.
Dalton Trans. 2011, DOI: 10.1039/c1dt10727e.
(12) (a) Lambert, C.; N€oll, G. J. Am. Chem. Soc. 1999, 121, 8434.
(b) Barlow, S.; Risko, C.; Chung, S. J.; Tucker, N. M.; Coropceanu, V.; Jones,
S. C.; Levi, Z.; Brꢁedas, J. L.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 16900.
(c)Nelsen, S. F.; Weaver, M. N.; Telo, J. P. J. Phys. Chem. A 2007, 111, 10993.
(d) Zhou, G.; Baumgarten, M.; M€ullen,K.J. Am. Chem. Soc. 2007,129, 12211.
(13) (a) Nelsen, S. F.; Konradsson, A. E.; Weaver, M. N.; Telo, J. P.
J. Am. Chem. Soc. 2003, 125, 12493. (b) Lancaster, K.; Odom, S. A.;
Jones, S. C.; Thayumanavan, S.; Marder, S. R.; Brꢁedas, J. L.; Coropceanu,
V.; Barlow, S. J. Am. Chem. Soc. 2009, 131, 1717.
irradiation of concentrated solutions of 1c in acetonitrile (which are
needed for cyclic voltammetry) leads to the appearance of an increas-
ingly intense wave around 0.27 V vs Fc+/0. This wave is likely due to
increasing contamination of the solution with a photodegradation
product. For the UVꢀvis experiments reported below, this problem
does not occur because the respective solutions are less concentrated,
and much shorter irradiation times can be employed.
(32) A subtraction of the near-infrared portions of the absorption
spectra of 1c•+ and 1o•+ is shown in Figure S6 in the Supporting
Information.
(33) Without in-depth computational work, the highest energy
Gaussian cannot be assigned to a specific electronic transition with
certainty. However, we note that the triarylamine radical cation band
around 12 900 cmꢀ1 has been previously observed to exhibit a shoulder
on its high energy side, see for example references 12a, 12b, and 34. The
necessity to use a total of four Gaussians for the overall fit is therefore by
no means unusual.
(34) Amthor, S.; Lambert, C. J. Phys. Chem. A 2006, 110, 1177.
(35) As pointed out by a reviewer, comparison to dinitroaromatic
radical anions should be made with caution, not only because of their
anionic nature but also because of the different amounts of ion-pairing in
the bis(triarylamines) and the dinitroaromatic mixed valence species, as
well as the different reorganization energies associated with intervalence
charge transfer in the two types of systems.
(14) Lantz, R.; H€ornfeldt, A. B. Chem. Scripta 1972, 2, 9.
(15) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. Chem.—Eur. J. 1995,
1, 275.
(16) (a) Hensel, V.; Schl€uter, A. D. Liebigs Ann. 1997, 303.
(b) Hanss, D.; Wenger, O. S. Inorg. Chem. 2009, 48, 671. (c) Walther,
M. E.; Wenger, O. S. ChemPhysChem 2009, 10, 1203. (d) Hanss, D.;
Wenger, O. S. Eur. J. Inorg. Chem. 2009, 3778. (e) Hanss, D.; Wenger, O.
S. Inorg. Chem. 2008, 47, 9081.
(36) The remaining signal at 0.3 V vs Fc+/0 is thus attributed to a
remaining fraction of the open form of molecule 3. The origin of the
apparent wave at 0.4 V vs Fc+/0 is unknown.
(37) An anonymous reviewer is acknowledged for correcting our
initial interpretation of the experimental data in Figure 5a.
(17) Ye, B. H.; Naruta, Y. Tetrahedron 2003, 59, 3593.
(18) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay,
P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713.
(19) Sreenath, K.; Thomas, T. G.; Gopidas, K. R. Org. Lett. 2011,
13, 1134.
(20) Nelsen, S. F.; Tran, H. Q.; Nagy, M. A. J. Am. Chem. Soc. 1998,
120, 298.
(21) Szeghalmi, A. V.; Erdmann, M.; Engel, V.; Schmitt, M.; Amthor,
S.; Kriegisch, V.; N€oll, G.; Stahl, R.; Lambert, C.; Leusser, D.; Stalke, D.;
Zabel, M.; Popp, J. J. Am. Chem. Soc. 2004, 126, 7834.
(22) Nelsen, S. F.; Weaver, M. N.; Telo, J. P. J. Am. Chem. Soc. 2007,
129, 7036.
(23) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967,
10, 247.
(24) Hush, N. S. Coord. Chem. Rev. 1985, 64, 135.
(25) (a) Rosokha, S. V.; Sun, D.-L.; Kochi, J. K. J. Phys. Chem. A
2002, 106, 2283. (b) Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi,
J. K. J. Am. Chem. Soc. 2002, 124, 843.
(26) Wenger, O. S. Chem. Soc. Rev. 2011, 40, 3580.
(27) Wenger, O. S. Acc. Chem. Res. 2011, 44, 25.
(28) (a) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.;
Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.—Eur. J.
2005, 11, 6414. (b) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko,
M.; Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.—
Eur. J. 2005, 11, 6430. (c) Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003,
125, 3404. (d) Peters, A.; Branda, N. R. Chem. Commun. 2003, 954.
(e) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J. M. Chem.—Eur. J. 1995,
1, 285.
(29) Nelsen, S. F.; Weaver, M. N.; Zink, J. I.; Telo, J. P. J. Am. Chem.
Soc. 2005, 127, 10611.
(30) (a) Murguly, E.; Norsten, T. B.; Branda, N. R. Angew. Chem., Int.
Ed. 2001, 40, 1752. (b) Tsivgoulis, G. M.; Lehn, J. M. Angew. Chem., Int.
Ed. 1995, 34, 1119. (c) K€arnbratt, J.; Hammarson, M.; Li, S. M.;
Anderson, H. L.; Albinsson, B.; Andrꢁeasson, J. Angew. Chem., Int. Ed.
2010, 49, 1854.
(31) We note that the two oxidation waves have different intensities,
suggesting that the photostationary state in the electrochemistry experi-
ment differs from the 95% purity of 1c derived from 1H NMR
spectroscopy. We have made the observation that prolonged UV-
17036
dx.doi.org/10.1021/ja207025x |J. Am. Chem. Soc. 2011, 133, 17027–17036