Gating charge recombination rates through dynamic bridges in tetrathiafulvalene-fullerene architectures

Article abstract of DOI:10.1002/anie.201306183

An open and shut case: The competition between charge separation and recombination in artificial photosynthetic systems can be controlled by using photochromic dynamic bridge. The photoinduced opening and closing of the bridge mediates the electronic coupling between donor (D) and acceptor (A). Copyright

Full text of DOI:10.1002/anie.201306183

Angewandte
Chemie
DOI: 10.1002/anie.201306183
Artificial Photosynthesis
Gating Charge Recombination Rates through Dynamic Bridges in
Tetrathiafulvalene–Fullerene Architectures**
Sonia Castellanos, Andre A. Vieira, Beatriz M. Illescas, Valentina Sacchetti, Christina Schubert,
Javier Moreno, Dirk M. Guldi,* Stefan Hecht,* and Nazario Martꢀn*
Conversion of light into chemical energy is a process that
nature has optimized over eons in photosynthetic organisms,
delocalized p-electron system, respectively. This last feature
leads to low reorganization energies upon the reduction to the
C60 radical anion and allows for the uptake of up to six
[
1]
such as bacteria, plants, and algae. However, the search for
non-natural systems that mimic the complex overall process
of photosynthesis has remained a challenge. In particular, the
key step of the initial light-induced charge separation
between electron donors and acceptors is hampered by its
inherent microscopic reversibility, that is, competing charge
[
2b,3]
additional electrons.
The photophysical properties of
various TTF–C60 conjugates featuring different p-conjugated
molecular bridges have been investigated and charge-sepa-
rated states with lifetimes ranging from a few nanoseconds up
[
4]
to hundreds of microseconds have been realized. Of
particular interest are conjugates with p-extended TTF
derivatives, in which a conjugated p-quinoid anthracene
[2]
recombination.
Well-defined molecular model systems typically comprise
a donor (D) and an acceptor (A) covalently linked by
a bridge (B). In the resulting D–B–A structures, the role of
the bridge is ideally to facilitate the desired initial photo-
induced charge separation, yet, to slow down the undesired
charge recombination. Among the many combinations of
donors and acceptors that have been explored, those consist-
ing of proaromatic tetrathiafulvalene (TTF) and fullerene
derivatives, such as C , have shown outstanding results. The
[4c]
moiety is placed between the TTFꢀs two 1,3-dithiole rings.
Nevertheless, the design of such D–B–A architectures
features inherent drawbacks. For example, even with opti-
mized donors and acceptors, the bridge needs to play two
opposing roles. On the one hand, it should enhance the
coupling between D and A to facilitate the initial charge
separation. On the other hand, once the charge-separated
state has been formed it should prevent charge recombination
6
0
+
ꢀ
exceptional electron donating and accepting properties
originate from the aromatic stabilization of the formed TTF
radical cation and from C ꢀs unique three dimensional
by decoupling DC and AC . Clearly, conventional, static
bridges have to be a compromise of these two demands.
However, if D and A are connected by a dynamic bridge,
which can be switched between a coupled and a decoupled
form, prolonged charge-separated state lifetimes could
potentially be attained without compromising the initial
charge separation. Such improved molecular design requires
a switch entity that adopts two electronically distinct forms
and allows for precise timing of the switching, that is, when
the bridge is being coupled or decoupled. Dithienylethenes
6
0
[*] Dr. S. Castellanos, Dr. J. Moreno, Prof. Dr. S. Hecht
Department of Chemistry
Humboldt-Universitꢀt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
Dr. A. A. Vieira, Dr. B. M. Illescas, Dr. V. Sacchetti,
Prof. Dr. N. Martꢁn
Departamento de Quꢁmica Orgꢂnica, Facultad de Quꢁmica
Universidad Complutense de Madrid
Madrid 28040 (Spain)
(DTE) are ideal candidates as switchable bridges as they
reversibly interconvert between their ring-open (decoupled)
and ring-closed (p-conjugated) forms upon irradiation with
[5]
E-mail: nazmar@quim.ucm.es
light of specific wavelengths. Adopting this new strategy, we
prepared four novel D–DTE–A structures connecting either
TTF or exTTF acting as D and with C60 functioning as A by
photochromic dithienylperhydrocyclopentene or perfluoro-
cyclopentene bridges (Scheme 1).
Dr. C. Schubert, Prof. Dr. D. M. Guldi
Department Chemie und Pharmazie
Friedrich-Alexander-Universitꢀt Erlangen/Nꢃrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
E-mail: guldi@chemie.uni-erlangen.de
Some researchers have used photochromic units as light-
responsive electronic traps that allow or prevent intramolec-
ular electron transfer from D to A depending on the adopted
[
**] S.C. and N.M. thank the Alexander von Humboldt-Foundation for
a postdoctoral fellowship and a Humboldt Research Award,
respectively. A.A.V. thanks the Conselho Nacional de Desenvolvi-
mento Cientꢁfico e Tecnolꢄgico (CNPq) of Brazil for a postdoctoral
grant. Generous support by the German Research Foundation (DFG
via SFB 658), the state of Bavaria initiative “Solar Technologies Go
Hybrid”, the Ministerio de Economꢁa y Competitividad (MINECO)
of Spain (projects CTQ2010-16959; Consolider-Ingenio CSD2007-
[6]
isomeric form. There are also some examples, in which the
electron transfer kinetics are clearly altered by the structural
modification of the bridging units by chemical inputs (chela-
[7]
tion) or, in mechanically interlocked D and A units, by
[8]
topological changes. However, herein, we show for the first
0
0010), the CAM (MADRISOLAR-2 project S2009/PPQ-1533), and
the European Research Council via ERC-2012-STG_308117
Light4Function) and ERC-2012-ADG_20120216 (Chirallcarbon) is
time that in (ex)TTF–DTE–C60 architectures, the lifetime of
the charge-separated state can be significantly shortened or
prolonged by performing light-induced structural changes in
the bridging unit.
(
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306183.
Angew. Chem. Int. Ed. 2013, 52, 13985 –13990
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13985

.
Angewandte
Communications
electron donors by an electron-deficient
dithienylperfluorocyclopentene bridge or an
electron-rich dithienylperhydrocyclopentene
bridge. Syntheses were achieved in a modular
fashion, in each case attaching C60 to an
[9]
aromatic aldehyde by a Prato reaction in
the final step (Scheme 2). Compounds 1 and
2
were assembled by sequential nucleophilic
substitution of octafluorocyclopentene with
two different lithiated thienyl residues. One
of them was connected to either TTF or
exTTF by Sonogashira cross-coupling. The
other thienyl scaffold carried an acetal-
protected aldehyde function, which, after
deprotection, allowed for the attachment of
C in the final step. Compounds 3 and 4 were
6
0
obtained by stepwise Sonogashira coupling
of the same functionalities to a pre-assem-
bled diiodo-terminated dithienylcyclopen-
Scheme 1. Open (o) and closed (c) forms of 1–4.
[10]
tene bridge. Details of the synthesis of 1–
, their purification as well as all character-
4
In the four target structures 1–4 (Scheme 1) the electron
acceptor was set to be C60 connected to either TTF or exTTF
ization data are gathered in the Supporting Information.
The photochromic properties of 1–4 were probed in
toluene (Figure 1 and Supporting Information Figure S2 and
S3). The absorption spectra of 1o–4o isomers share as
common features an intense band at 320–330 nm, a weak
sharp peak at 434 nm, and a low-intensity broad band
centered at 700 nm. The latter two are characteristics of
[
[
60]fullerene cycloadducts, as in the case of pyrrolidino-
[
3d,11]
3,4:1,2][60]fullerenes.
Both exTTF-containing conju-
gates exhibited two additional maxima at 375 and 450 nm,
[
12]
which originate from the exTTF moieties. TTFs are usually
orange colored as a result of forbidden transitions in the 400
Scheme 2. a) [Pd(PPh ) Cl ], CuI, THF/Et N; b) THF, nBuLi; c) TFA,
3
2
2
3
ꢀ
6
THF/H O; d) K CO , MeOH/H O; e) C , N-octylglycine, toluene, D;
Figure 1. Evolution of the absorption spectra of a) 1o (9.5ꢅ10 m)
2
2
3
2
60
ꢀ6
f) [Pd(PPh ) ], CuI, iPr NH, THF; g) K CO , MeOH/CH Cl ; h) [Pd-
and b) 3o (6.8ꢅ10 m) in argon-saturated toluene at 258C upon
exposure to UV light (325 nm, 70 s and 180 s, respectively, giving 1c
3
4
2
2
3
2
2
(
PPh ) Cl ], CuI, TMSA, toluene/Et N; i) nBuLi, THF, I ; j) [Pd(PPh ) ],
3 2 2 3 2 3 4
CuI, toluene/iPr NH.
and 3c). Inset: expansion of the C -cycloadduct band at 700 nm.
2
60
1
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Chemie
to 500 nm region, although in 2 and 4 this band is masked by
more intense transitions in the UV region.
originating from the pyrrolidino[60]fullerene moiety, that is,
up to three reversible one-electron reductions at almost
1
2
In the absence of oxygen, illumination of toluene sol-
utions of 1o–4o with UV light results in the growth of a new
band in the visible region accompanied by a decrease of the
intense band in the UV region (Figure 1 and Supporting
Information Figure S2). These changes are attributed to ring-
identical potentials (Ered from ꢀ0.81 V to ꢀ0.84 V, E from
red
ꢀ1.21 V to ꢀ1.27 V vs. Ag/AgNO ). On the contrary, their
3
oxidations depend on the donors present in the DTE, namely
1o and 3o undergo one two-electron quasi-reversible oxida-
1
1
tion
(Eox = 0.20 V
and
Eox = 0.28 V
vs.
Ag/
1
closure to yield 1c–4c, as could be confirmed by H NMR
AgNO ,respectively) ascribed to the formation of the
3
spectroscopy (shown for 1 in Supporting Information Fig-
ure S6). The signals for the aromatic protons of the thienyl
units in 1o shift drastically upfield by about 1 ppm as the
conjugated octatetraene system of 1c is formed. Ring-closure
is furthermore associated with a significant downfield shift of
the methyl singlets from d = 1.8 to 2.4 ppm. Note that an
analogue of 1, in which the pyrrolidinofullerene was linked to
thiophene by a single bond instead of a phenylethynyl spacer
exTTF dication, while 2o and 4o reveal two one-electron
1
2
1
oxidations (E = 0.21 V, Eox = 0.50 V and Eox = 0.21 V,
ox
2
Eox = 0.53 V vs. Ag/AgNO , respectively) attributed to the
3
successive formation of the radical cation and dication of TTF.
Upon irradiation with UV light (365 nm) until the photo-
stationary state is attained, all redox potentials in DTEs 1c
and 3c remained practically unchanged. However, in the PSS
mixtures of 2 and 4 an anodic shift on the order of 60 to
100 mV was observed in both the oxidation and reduction
potentials as compared to their ring-open analogues. Note
that after photocyclization the perfluoro DTEs 1c and 2c
display new irreversible reductions at ꢀ1.12 and ꢀ1.02 V and
oxidations at 0.87 and 1.08 V, respectively, originating from
the ring-closed form of the bridge.
(
Supporting Information Scheme S1 and Figure S5) does not
exhibit any photochromism. A similar inhibition of photo-
activity has been reported for DTEs featuring porphyrins in
[
13]
the absence of long spacers.
The presence of electron-withdrawing fluorine atoms in
the bridge in 1o and 2o does not modify the absorption
features relative to the perhydro analogues 3o and 4o. In
contrast, the respective fluorinated ring-closed isomers 1c
and 2c exhibit pronounced red-shifts of the absorption
maxima when compared to their non-fluorinated counter-
parts 3c and 4c, respectively. This bathochromic shift in
perfluorocyclopentene-based DTEs leads to an overlap with
the C60 centered band at 700 nm (Figure 1).
The lack of changes in the reduction potentials after ring-
closure can be reasoned by the presence of the pyrrolidine
ring, which interrupts the conjugation between the DTE and
the C60, thus isolating the C60 electronically. However, an
increase in the HOMO energy in 1c–4c was expected as
a result of the more extended conjugated p-systems as
compared to 1o–4o. Note that these results are not in
contradiction with the observed large shifts of the absorption
of 1–4 during ring-closure, since HOMO and LUMO may be
located on the donor and acceptor units, respectively, in both
open and closed isomers, whereas the molecular orbitals
located on the bridge and responsible for the photocyclization
can correspond to levels below (HOMOꢀn) and above
(LUMO + n) the frontier orbitals. Thus the changes in the
wavelength of the optical p!p* transition in 1–4 do not
originate from changes of the HOMO and LUMO levels (see
The different nature of the bridge in the DTEs strongly
influences the photoconversion, that is, the ratio of closed/
open isomers in the photostationary state (PSS) as deter-
mined for each compound by HPLC-UV (Supporting Infor-
mation Figure S7). While close to quantitative photoinduced
ring-closure was noted for non-fluorinated DTEs 3c (95%)
and 4c (91%), their fluorinated counterparts 1c and 2c
yielded only 37 and 43%, respectively. Quantum yield values
ꢀ
3
in the order of 10 were estimated for photochemical ring-
closure, which are comparable to those reported by Osuka
and Irie for related dithienylethene-bridged diporphyrin
[14]
Supporting Information)
and so the redox potentials
remain unaltered upon ring-closure. Nevertheless and most
importantly, the photoinduced changes in the electronic
structure of the bridge proved to play a crucial role in the
electron-transfer processes of 1–4, as discussed below.
To shed light onto the electron transfer dynamics of these
novel D–DTE–A structures, in general, and onto the influ-
ence that the ring-open or the ring-closed forms of the bridges
exert on the electron transfer pathways, in particular, we
turned to transient absorption measurements upon femto-
second excitation. The distinctly different absorption changes
in the visible and in the near-infrared regions of 1–4o/c upon
light irradiation provide clear evidence for different decay
mechanisms in the two isomers of each DTE (Figures 2 and 3
and Supporting Information Figures S10–13).
[
13]
systems. A likely rationale involves the superposition of
p!p* transitions, which are responsible for the photocycli-
zation, with the absorption bands associated with donors and
acceptors in the UV region. Thus, by the absorption of
photons within this range of energy, vertical excitations from
[14]
the ground state to inactive excited states dominate.
Cycloreversion was initiated by irradiating the PSS
mixture with visible light (> 500 nm) leading to a decrease
of absorption in the visible region and an increase in the UV
as well as in the 400–500 nm range (Supporting Information
Figure S3). Full reversibility was observed for ring-opening of
1
c, while conversion of 3c into 3o was incomplete. We
hypothesize that the perhydrocyclopentene 3 undergoes
a side-reaction known for dithienylcyclopentenes that yields
a rearranged byproduct.
The redox potentials of 1o–4o and of their photosta-
tionary states after UV irradiation were measured by cyclic
voltammetry (Table S1 and Supporting Information Fig-
ure S8). DTEs 1o–4o display the electrochemical features
DTEs 1o and 3o were pumped with 505 nm laser pulses to
avoid local excitation of the open bridge and, thus, photo-
conversion. Compounds 1o and 3o in toluene, THF, and
benzonitrile instantaneously formed transient maxima at 515,
640, and 920 nm as well as transient minima at wavelengths
shorter than 450 nm. In line with the corresponding reference
[
5a]
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.
Angewandte
Communications
[
2b,3f,15]
experiments,
we assign these features to the
1
singlet excited states of C , ( *C at 515 and 920 nm)
6
0
60
1
and exTTF ( *exTTF at 640 and at wavelengths shorter
than 450 nm), respectively. Instead of, for example, slow
1
3
*
C60 to *C intersystem crossing, an ultrafast decay
60
occurs simultaneously with the development of new
transitions in the visible region between 550 and 700 nm
that are attributed to the one-electron oxidized bridge
+
[16]
(
oDTEC ) and the one-electron reduced radical anion
ꢀ
of C60 (C C ) in the near-infrared region at 1020 nm
60
(
Figure S10).
1
Therefore, the initially formed *C60 undergoes
charge separation in 3.5 ꢁ 0.5 and 4.7 ꢁ 0.5 ps for 1o
and 3o, respectively, in THF, and, as such, transforms
+
ꢀ
into the adjacent D–oDTEC –AC (Figure 3) charge-
separated states.
Multiwavelength analyses corroborated, however,
the metastability of the adjacent charge-separated states.
Interestingly, their lifetimes depend on the nature of the
bridge, that is, perhydro (752 ꢁ 35 ps) versus perfluoro
(
1150 ꢁ 50 ps) in THF and on the polarity of the solvent,
that is, toluene (t = 1045 ꢁ 50 ps) versus benzonitrile
3
o
(
t = 505 ꢁ 25 ps). The spectral features indicate a com-
3
o
petition between charge recombination to yield the C
6
0
3
triplet excited state ( *C at 690 nm) and charge shift to
yield the distant DC -oDTE-AC (Figure 3) charge-sepa-
6
0
+
ꢀ
rated states featuring the one-electron oxidized exTTF
+
(
exTFFC at 460/510/680 nm) and the one-electron
ꢀ
reduced radical anion of C (C C at 1020 nm). The
6
0
60
+
ꢀ
lifetimes of DC –oDTE–AC states exceed the time scale
of our femtosecond experiments and were determined in
complementary nanosecond experiments (Figure S11).
In line with the competitive formation of the C triplet
6
0
+
ꢀ
excited state and the distant DC –oDTE–AC charge-
separated state, the differential absorption spectra on
the nanosecond time scale are best described as the
3
superposition of the C60 triplet excited state ( C * at
60
+
ꢀ
6
90 nm) and the DC –oDTE–AC charge-separated state
+
[17]
ꢀ
(exTFFC at 460/510/680 nm
and C C at 1020 nm),
6
0
respectively. The kinetics described by the decays of
+
ꢀ
exTTFC at 460/510 nm and of C C at 1020 nm obey
6
0
clean unimolecular rate laws, implying no other com-
peting pathways in the charge recombination, resulting
in lifetimes of 59.8 ꢁ 5.0 and 31.5 ꢁ 3.0 ns for 1o and 3o,
[18]
respectively, in THF.
At first glance, photocyclization of DTEs 1 and 3 led
to similar changes in the transient absorption measure-
ments. Upon excitation with pulses of 387 nm, which was
meant to prevent photoreversion, the ring-closed forms
1
display, in addition to the *C60 fingerprint at 920 nm,
maxima at 485, 745, and 840 nm as well as a minimum at
6
10 nm for 1c and maxima at 475, 690, 820, and 1020 nm
Figure 2. a) Transient absorption spectra of 3o and 3o/3c mix-
ture in the photostationary state (PSS) in argon-saturated THF
ꢀ4
(
10 m) upon pump probe experiments (505 nm, 200 nJ for 3o
and 387 nm, 200 nJ for PSS). b) Time absorption profiles mon-
+
itoring the charge-transfer dynamics at the fingerprints of exTTFC
3
(
510 nm, gray), *C (680 nm, red), and C60Cꢀ (1020 nm, black).
6
0
1
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Angew. Chem. Int. Ed. 2013, 52, 13985 –13990

Angewandte
Chemie
+
TTF. The charge recombination in the open isomers, DC –
ꢀ
oDTE–AC , presumably occurs by a tunneling mechanism
with no participation of the non-conjugated bridging unit. On
+
ꢀ
the contrary, the fast decay in the case of the DC –cDTE–AC
states is attributed to the extended p-conjugation throughout
the ring-closed bridging unit. In other words, the lifetimes of
the charge-separated states are controlled by the photo-
chromic bridge that governs the electronic coupling between
the donors and acceptors (Figure 3). Note that the presence or
absence of fluorine atoms in the bridge has a rather minor
effect on the charge-transfer kinetics.
Figure 3. Charge separation and recombination pathways in D–B–A
compounds 1–4 featuring photochromic DTE bridges for the modu-
lation of the charge-separated states’ lifetimes.
In summary, we have prepared new molecular constructs
composed of electron-donating tetrathiafulvalene (TTF) and
p-extended-TTF derivatives covalently connected to elec-
tron-accepting pyrrolidino[3,4:1,2][60]fullerene through pho-
toswitchable dithienylethene bridges. By exposure to a light
stimulus the bridge can be interconverted between a decou-
pled ring-open and a coupled ring-closed form and, hence,
enables external control of electronic communication
between the donor and acceptor units. In the resulting
architectures, incoming photons give rise to efficient charge
separation and, furthermore, allow modulation of the life-
times of the charge-separated states over two orders of
magnitude by variation of their wavelength. The photores-
ponsive, dynamic bridge provides a means for (self-)regula-
tion through a reversible activation-deactivation mechanism
for solar energy conversion in artificial photosynthetic
as well as a minimum at 575 nm for 3c reflecting excited states
[19]
that involve the switching unit. Notably, the minima for the
excited states of 1c and 3c mirror the maxima seen in the
ground state absorption. Both of these excited states decay
rather quickly (2.5 ꢁ 0.2 ps) in THF. A closer look infers that
a likely deactivation mechanism is the evolution to the distant
+
ꢀ
DC –cDTE–AC charge-separated states by passing the adja-
+
ꢀ
cent D–cDTEC –AC charge-separated states, as supported by
the spectral features at the end of the excited state decay with
+
ꢀ
maxima of exTTFC at 510/680 nm and of C C at 1020 nm
(
6
0
+
ꢀ
Figure S10). The DC –cDTE–AC species exhibited rather
fast charge recombination and displayed shorter lifetimes in
more polar benzonitrile (t = 130 ꢁ 5 ps) than in less polar
3
c
toluene (t = 1845 ꢁ 50 ps). A qualitatively similar behavior
3
c
[
21]
was observed for DTEs 2c and 4c and their corresponding
systems. Our approach thus holds promise for the design
of improved systems for solar energy conversion.
+
PSS mixtures, which display the TTFC fingerprint at
[
4a]
[20]
4
20 nm (Figure S10 and S11).
Most remarkably, the recorded lifetimes (t) of the distant
Received: July 16, 2013
Revised: August 16, 2013
Published online: November 8, 2013
+
ꢀ
DC –DTE–AC charge-separated states in the ring-open forms
are up to two orders of magnitude longer than in the closed
forms (Table 1), as deduced form the distinct kinetic traces of
+
ꢀ
Keywords: charge separated state · cyclic voltammetry ·
DC and AC in each case. We attribute the different
recombination rates to the structural changes undergone by
the photochromic bridge that connects C60 and exTTF and/or
.
electron transfer · photochromism · transient absorption
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ꢀ
+
ꢀ
Table 1: Lifetimes (t) of the D–DTEC –AC and DC –DTE–AC charge-
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[
4
54400
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[
PSS
[
a] Results obtained upon photoinduced electron transfer in THF
solutions of DTEs 1–4 in their open form (o) and in their photostationary
state mixture (PSS) forms featuring perfluoro (F ) or perhydro (H )
6
6
bridges (DTE). [b] Excitation wavelengths of 505 nm (open isomer) and
+
ꢀ
3
87 nm (closed isomer). [c] t and t denote the DC –DTE–AC charge-
o
c
separated state lifetimes in the open and closed forms, respectively.
d] After irradiation with 313 nm light (ca. 45 min). [e] Measured on the
[
[4] a) N. Martꢁn, L. Sꢂnchez, M. A. Herranz, D. M. Guldi, J. Phys.
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nanosecond scale.
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www.angewandte.org 13989

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2
012, 22, 4188 – 4205.
[14] a) Gaussian03 RevisionC.02, M. J. Frisch, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, et al., see Supporting
Information, Gaussian, Inc., Wallingford CT, 2004; b) A.
Staykov, J. Areephong, W. R. Browne, B. L. Feringa, K. Yoshi-
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[
6] a) P. A. Liddell, G. Kodis, J. Andrꢃasson, L. de La Garza, S.
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7139 – 7144.
1
[18] For 2o and 4o, only the singlet excited states of C60 ( *C at 515
60
and 920 nm) were detected because of the lack of TTF ground
state absorption. These were seen to transform via the adjacent
6616 – 6619.
+
ꢀ
+
[
7] T. Oike, T. Kurata, K. Takimiya, T. Otsubo, Y. Aso, H. Zhang, Y.
Araki, O. Ito, J. Am. Chem. Soc. 2005, 127, 15372 – 15373.
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Megiatto, D. I. Schuster, S. Abwandner, G. de Miguel, D. M.
Guldi, J. Am. Chem. Soc. 2010, 132, 3847 – 3861; c) J. D.
Megiatto, D. I. Schuster, G. de Miguel, S. Wolfrum, D. M.
Guldi, Chem. Mater. 2012, 24, 2472 – 2485.
D-oDTEC -AC charge-separated state to either the distant DC -
oDTE-ACꢀ charge-separated state (TFFC at 420 nm and C C at
1020 nm) or the C60 triplet excited state ( *C60 at 690 nm) with
+
ꢀ
6
0
3
[
950 ꢁ 50 and 2780 ꢁ 140 ps for 2o, respectively. In THF, the
+
ꢀ
accordingly formed DC -oDTE-AC charge-separated states have
lifetimes of 73.1 ꢁ 5.0 ns for 2o and 54.4 ꢁ 2.0 ns for 4o.
[19] All of these transient maxima and minima are in sound
agreement with reference experiments conducted with the
closed form of the switching unit, in which they were noted to
deactivate rather quickly, within 10 ps (Supporting Information
Figure S13).
[
9] a) M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993,
1
3
15, 9798 – 9799; b) M. Prato, M. Maggini, Acc. Chem. Res. 1998,
1, 519 – 526.
[
[
[
10] G. Guirado, C. Coudret, M. Hliwa, J.-P. Launay, J. Phys. Chem. B
005, 109, 17445 – 17459.
11] L. Isaacs, A. Wehrsig, F. Diederich, Helv. Chim. Acta 1993, 76,
231 – 1250.
[20] Complementary nanosecond experiments with 1c–4c resulted in
no detectable transients.
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Moore, A. L. Moore, D. Gust, Nat. Nanotechnol. 2008, 3, 280 –
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2
1
12] N. Martꢁn, L. Sꢂnchez, C. Seoane, E. Ortꢁ, P. M. Viruela, R.
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1
3990 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 13985 –13990