Redox-switchable intramolecular π-π-stacking of perylene bisimide dyes in a cyclophane

Article abstract of DOI:10.1002/adma.201201266

Molecular actuation by stepwise electrochemical reduction is demonstrated for a cyclophane that exhibits a pronounced conformational transition from a closed cavity with cofacially stacked PBIs in the neutral state to an expanded open cavity in the three- and fourfold reduced state. Copyright

Full text of DOI:10.1002/adma.201201266

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Redox-switchable Intramolecular π−π-Stacking of Perylene
Bisimide Dyes in a Cyclophane
Felix Schlosser, Michael Moos, Christoph Lambert,* and Frank Würthner*
Dedicated to Professor Waldemar Adam on the occasion of his 75th birthday
Inspired by increasingly detailed insights into nature’s highly
sophisticated machinery for the transformation of chemical,
redox or light energy into mechanical motion, there is a strong
impetus on chemists to develop similarly masterful nanode-
vices.[1–9] Towards this goal there is a need for (supra)molecular
functional units whose size, shape and arrangements in space
can be controlled by external stimuli such as light, electrons or
ions. In a most general sense, such supramolecular functional
nanodevices may be called artificial molecular machines.[1]
Depending on additional design features, such units might
be further developed towards molecular motors.[2] Originating
from several decades of pioneering basic research,[3] the cur-
rently most widely investigated and already quite sophisticated
artificial molecular machines are given by catenanes and rotax-
anes,[4] for which rotational or translational motion could be
directed by photo- or electrochemical triggers and even solid
state switching devices with hysteretic (bistable) current/voltage
characteristics could be demonstrated.[5,6] Other ingenious
molecular machines include sterically congested alkenes, whose
sequential photo- and thermal isomerization processes afforded
unidirectional rotations which could be applied to spin nano-
particles and macroscopic objects.[7] Furthermore, pronounced
conformational changes have been reported for a number of
azobenzene-based photoswitches upon irradiation with light,[8]
and for calixarene-tethered oligothiophenes upon oxidation.[9]
Based on the rather limited number of molecular and
supramolecular scaffolds applied in this field of research, there
is a clear incentive to look for new building blocks whose confor-
mation and functional properties can be controlled by external
stimuli. In our opinion aggregates of functional dyes[10] are su’ch
candidates, in particular if they are composed of building blocks
such as perylene bisimides (PBI) which are known to exhibit
pronounced stability in their neutral form as well as for various
oxidized and reduced states.[11] Although a myriad of self-assem-
bled and covalently tethered PBI aggregates have been reported
during the last decade,[12] to the best of our knowledge, there
is no report that elucidates conformational changes of the PBI
aggregate structure upon electrochemical oxidation or reduc-
tion.[13] Thus, based on prior molecular modeling studies, we
have designed and synthesized PBI cyclophane 8 (Scheme 1)
and elucidated its response on stepwise electrochemical reduc-
tion, which provided unambiguous proof for a pronounced con-
formational transition from a closed cavity with co-facially stacked
PBIs in the neutral state to an expanded open cavity of the three-
and fourfold reduced state. While redox active cyclophanes have
been investigated extensively,[14,15] such a structural transforma-
tion upon charging appears to be novel in this context.
The synthesis of PBI cyclophane 8 is shown in Scheme 1. It
is based on the one-step cyclo-oligomerization that, according
to common understanding,[16] has to be carried out under high
dilution to favor the second intramolecular cyclization step
over intermolecular oligomer formation. After initial studies
with different coupling methods (i.e. oxidative Glaser and Hay
couplings) we opted for the more promising Pd/Cu-catalyzed
Sonogashira coupling conditions[17] that here, in the absence
of a haloarene, led to the oxidative diacetylene homocoupling
product 8 in a remarkably high yield of 60%. Notably, this high
yield could be obtained under rather “normal” conditions, i.e.
with 40 mg precursor PBI 7 dissolved in 60 mL THF under
argon atmosphere at 60 °C. To rationalize this excellent yield of
cyclized product, we may assume that the precursor molecules
are already templated[18] in a bimolecular PBI-PBI-complex[19]
or that at least the intermediate mono-coupled product exhibits
ideally preorganized acetylenic units for the subsequent
macrocyclization. All preceding reaction steps (Scheme 1) from
4,5-dihexyl-1,2-diiodobenzene[20] and 1,6,7,12-tetra(4-tert-butyl-
phenoxy)perylene bisanhydride 4 were rather straightforward
with mostly high yields.
The structural properties of PBI 8 were investigated by NMR,
UV-vis and fluorescence spectroscopy, which all supported
the assumed dye arrangement based on molecular modeling
(Figure 1). Thus, in the UV-vis absorption spectra (Figure 1a), a
pronounced excitonic coupling of the two PBI dyes is evident by
the reversal of the band intensities at around 540 and 580 nm.
This reversal in intensity is highly indicative for PBI dyes that
are helically stacked at close distance (Figure 1b) and originates
from a complex interplay of excitonic and vibronic couplings
(so-called intermediate coupling regime).[21,22] Notably, this kind
of cofacial PBI-PBI stacking arrangement has not been observed
before for self-assembled PBIs equipped with four phenoxy
substituents in bay positions,[19] but is the energetically favored
PBI-PBI arrangement for the parent PBIs that bear hydrogen
atoms at bay positions.[13] No spectral broadening is observed
for the PBI-related absorption bands from 400−650 nm for PBI
F. Schlosser, M. Moos, Prof. C. Lambert,
Prof. F. Würthner
Universität Würzburg
Institut für Organische Chemie & Center for
Nanosystems Chemistry
Am Hubland, 97074 Würzburg, Germany
E-mail: christoph.lambert@uni-wuerzburg.de;
wuerthner@chemie.uni-wuerzburg.de
DOI: 10.1002/adma.201201266
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DOI: 10.1002/adma.201201266
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a 100
90
100
90
80
70
60
50
40
30
20
10
0
80
70
60
50
40
ε
30
20
10
0
400
500
600
700
800
λ [nm]
b
Figure 1. a) UV-vis absorption (solid lines) and fluorescence spectra
(dashed lines, λex = 545 nm) of PBI 8 (red) and PBI 5 (black), and fluo-
rescence excitation spectrum of PBI 8 (blue, dashed-dotted line, λdet
=
700 nm) in dichloromethane at room temperature. b) Structural model
according to MM+ calculations with assignment of magnetically non-
equivalent PBI protons (marked in red and green); for better clarity, the
lower lying carbons are of light blue color and the bulky tert-butylphenoxy
groups are replaced by hydrogen. Notably, only the arrangement with
P-helicity is depicted although the molecules consist of a 1:1 mixture of
P- and M-helical PBI stacks.
Even more direct evidence for the structural model comes
from NMR spectroscopy. Accordingly, the characteristic 1H sin-
glet signals of PBI core protons 2, 5, 8 and 11 (marked red and
green in Figure 1b) are split into two singlet signals, and the
AA’BB’ spin systems of aromatic protons of the tert-butylphe-
noxy subunits are split into two sets of signals (see Supporting
Information, Figure S1), which is in agreement with a D2-sym-
metric chiral scaffold as depicted in Figure 1b. To examine the
Scheme 1. Synthesis of perylene bisimide cyclophane 8. Reagents and
conditions: i) trimethylsilylacetylene, Pd(PPh3)2Cl2, CuI, HNiPr2, rt, 15 h;
ii) triisopropylsilylacetylene, Pd(PPh3)2Cl2, CuI, HNiPr2, rt, 20 h; iii) THF,
MeOH, KOH, H2O, rt, 1 h; iv) 4-iodoaniline, imidazole, 120 °C, 5 h;
v) Pd(PPh3)2Cl2, CuI, HNiPr2, THF, 60 °C, 20 h; vi) nBu4NF, THF, rt, 4 h;
vii) Pd(PPh3)2Cl2, CuI, HNiPr2, THF, 60 °C, 22 h.
1
flexibility of the PBI macrocycle 8 temperature-dependent H
NMR measurements were performed (see Supporting Infor-
mation, Figure S2). At higher temperatures coalescence into a
single signal is observed for the signals of perylene protons 2, 5,
8 and 11, as well as for the aromatic proton signals of tert-butyl-
phenoxy subunits. These changes suggest a higher symmetry
species and may be attributed to a flipping process between the
P- and M-helical PBI-PBI arrangements that is now faster than
the NMR timescale (for structural model, see Figure S3 in Sup-
porting Information). In the transition state the PBI chromo-
phores are expected to arrange in a parallel orientation with
respect to their N-N-axis concomitant with a loss of chirality
and an increase of distance between the PBI π-faces.
8 in comparison with PBI 5, which is consistent with a rather
rigid supramolecular architecture. Likewise the sharp features
of the diphenyldiacetylene absorption band at <400 nm com-
plies with rigidity. The fluorescence spectrum of PBI 8 shows
a large Stokes shift, a lack of vibronic structure and a decrease
in fluorescence quantum yield from 100% (PBI 5) to 6% (PBI
8). In compliance with our earlier work on self-assembled
PBI π-stacks these features may be attributed to a change in
the mutual arrangement of the two PBI chromophores in the
excited state.[13]
The dynamic of the flipping process of PBI 8 was investi-
gated further by calculating the free activation enthalpy ΔG≠
using Equation (1) according to the coalescence method[23]
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ꢀ
ꢁ
√
a) 1.5x10-6
R · T ·
2
c
ꢀG ≠ R · T · ln
=
c
(1)
π· NA · h · ꢀv
digital fit
exp.
1.0x10-6
5.0x10-7
0.0
where R, NA and h are the gas constant, the Avogadro constant
and the Planck constant, respectively, T is the coalescence tem-
c
perature and Δν is the frequency difference between the inter-
converting protons observed in the low temperature regime.
The perylene core protons, as well as the tert-butylphenoxy
protons, are well suited for this purpose because of the simple
determination of their coalescence temperatures from line
broadening. Coalescence temperatures for perylene and tert-
butylphenoxy protons of 344 K and 333 K, respectively, have
been determined. The frequency differences of the respective
proton signals at low temperature are 131.3 Hz and 62.3 Hz.
With these values, a free activation enthalpy ΔG≠ of 68 kJ mol−1
was calculated in each case (see Table S1 in Supporting Infor-
mation) which corresponds to a rate constant of 138 s−1 for
the oscillating movement of the PBIs between the two M- and
P-helical conformations at a temperature of 333 K.[23]
-5.0x10-7
-1.0x10-6
-1.5x10-6
-1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
E / V vs. Fc/Fc+
wavelength / nm
2000
1000
500
b)
1.2
While proton NMR measurements thus give evidence for a
thermally activated structural rearrangement (interconversion
of P- and M-helical cyclophane 8), reduction of the PBI moieties
in 8 induces another molecular motion in the PBI cyclophane,
a widening of the macrocyclic ring with a concomitant separa-
tion of the PBI π-faces. Cyclic voltammetry (CV) of 8 in dichlo-
romethane shows dynamic behavior with increasing scan rate
(0.250–10 V s−1 , see Figure 2a and also Figure S4 in Supporting
Information) which can be digitally simulated using DigiSim[24]
by assuming an EEECE mechanism. This mechanism involves
stepwise reduction of the cyclophane (PBI…PBI → PBI−…PBI→
PBI−…PBI−) to the dianion in which both PBIs are singly
charged (see Figure 3). These two processes are well resolved
in the CV at −0.964 V and −1.148 V vs Fc/Fc+ (values given are
those from the digital fit with an estimated accuracy of 5 mV.
The analogous first redox potential of the reference compound
7 is at −1.112 V, see Figure S5. The difference to −0.964 V of 8
could be interpreted by a stabilizing effect of the negative charge
by the second neutral PBI in the mixed valence 8−). The third
reduction occurs with E1/2 = −1.426 V at much more negative
potential (PBI−…PBI−→ PBI2-…PBI−), but is immediately fol-
lowed by a chemical reaction (K = kf/kb = 19.9 with kf > 106 s−1 )
that we relate to the separation of the two PBI π-faces and
concomitant opening of the cyclophane cavity (PBI2-…PBI−⇔
PBI2-……PBI−, see below) which is caused by the increased
electrostatic repulsion in the trianion. The subsequent fourth
reduction which leads to the tetraanion, occurs at a formally
more positive potential (E1/2 = −1.124 V) than the third reduc-
tion because of less Coulomb repulsion in the widened cyclo-
phane. These last two reduction processes can be seen in the
CV by a scan rate dependent (see Figure S4) third wave around
−1.5 V. Consequently, back oxidation is at even more positive
potentials than the second reduction and covers the transfer of
three electrons. This is seen as a strong anodic signal at −1.1 V.
The following anodic peak at −0.95 V refers to the fourth oxi-
dation step to yield the neutral cyclophane 8. The whole redox
cycle is chemically fully reversible on a long term scale (30 min)
as has been proved by thin-layer measurements (see inset in
Figure 3 and Figure S6 in Supporting Information).
8
8-
1.0
0.8
0.6
82-
84-
Δ
0.4
0.2
0.0
-0.2
5000
10000
wavenumbers / cm-1
15000
20000
25000
Figure 2. a) Cyclic voltammogram of 8 in DCM/ 0.2 M TBAH at v =
250 mV s−1 vs Fc/Fc+ and digital fit by DigiSim. b) Spectroelectrochem-
istry of 8; the spectra shown were obtained by deconvolution with SpecFit
and refer to the hypothetically isolated species.
While the interpretation of the CVs seems plausible, proof
for this postulated mechanism is gained by UV/Vis/NIR spec-
troelectrochemistry of the reduced species in a thin-layer (ca.
100 μm) at a reflective platinum working electrode.[25] In this
experiment, a series of UV/Vis/NIR spectra were recorded in
reflection at a polished platinum working electrode while the
potential was varied stepwise in 20 mV steps. The spectra of the
hypothetically isolated reduced species were then evaluated by
deconvolution with SpecFit (see Figure 2b and also Figure S7
in Supporting Information). As expected from the model in
Figure 3, the spectrum of the monoanion 8− (Figure 2b, red)
shows spectral features of both the neutral PBI (two bands
at ca. 17000−19000 cm−1 ) and of the singly reduced PBI (two
sharp peaks at ca. 10000 cm−1 and a very prominent signal at
14000 cm−1 ). In addition, there is the onset of a weak band vis-
ible below 4000 cm−1 which has intervalence charge transfer
(IV-CT) character,[26] and is due to an optically induced electron
transfer from PBI− to the neutral PBI in the mixed valence
cyclophane 8. This feature is in agreement with the one recently
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Figure 3. Reduction processes and reversible contraction/expansion of cyclophane 8. The redox potentials and equilibrium constant were obtained
by digital fit to the cyclic voltammograms at different scan rate. The inset shows 10 thin-layer voltammograms vs time at v = 10 mV s−1 . Switching
potentials are −0.6/−1.5 V.
observed in a PBI dimer linked by a single σ-bond at the bay
positions.[27] In the dianion 82− the spectral feature of the PBI−
at 14000 cm−1 increases in intensity, whilst that of neutral PBI
at 17000 cm−1 decreases (Figure 2b, blue). Furthermore, a mod-
erately strong absorption at 5000 cm−1 appears, which is also
seen in the σ-bonded PBI dimer, which is caused by a “quasi-
CT” process in which an electron of this biradical dianion is
excited from the one PBI to the other and vice versa.[27] The tri-
anion is not visible in the spectroelectrochemistry because its
intermediate concentration vanishes due to the very fast follow-
up process (ring expansion) and reduction to the tetraanion at
even higher potential. The latter shows spectral features (broad
and intense band at ca. 15000 cm−1 , Figure 2b, green) very sim-
ilar to the dianion of the reference compound 7 (Figure S7) and
other tetraphenoxy PBIs.[28]
The changes in Coulomb repulsion between the closed and the
expanded cyclophane macrocycle involves the conversion of
about 30 kJ/mol of electrical into mechanical energy and sug-
gests a bistability[29,30] for species 83−. Our current research is
devoted towards the elucidation of this bistability, the evaluation
of the distance changes upon expansion/contraction (‘molecular
actuation’)[9,31] and the capturing of small molecules/ions in the
expanded macrocycle.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
The UV/Vis/NIR spectra of reduced species 8 impressively
support the above outlined redox-dependent ring expansion/
contraction mechanism. Most importantly, they prove that
pronounced electronic interactions between the two PBIs are
manifested by IV-CT and quasi-CT processes in the mono- and
dianion which requires spatial proximity, whereas no inter-
actions are evident in the fully reduced tetraanion due to the
larger PBI separation (almost identical spectra to that of 72−).
Acknowledgements
We gratefully acknowledge financial support for this project by the DFG
Research Unit “Light-Induced Dynamics in Molecular Aggregates” (FOR
1809).
Received: March 27, 2012
Published online:
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[30] This value is an estimation based on ΔG0 = − n × F × ΔE that only
considers the difference in the redox potential for the third electron
(n = 1) in the oxidative and reductive step ΔE ≈ 1.4–1.1 V = 0.3 V
with the Faraday constant F = 96485 C/mol.
[31] K. Miwa, Y. Furusho, E. Yashima, Nature Chem. 2010, 2,
444–449.
©
wileyonlinelibrary.com
Adv. Mater. 2012,
DOI: 10.1002/adma.201201266
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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