Electron transfer through exTTF bridges in electron donor-acceptor conjugates

Article abstract of DOI:10.1039/b911028c

Rigid and soluble electron donor-acceptor conjugates combining exTTF and/or TTF as donors and C₆₀ as acceptor have been synthesized; fluorescence and transient absorption measurements confirm the generation of charge-separated radical-ion pairs

Full text of DOI:10.1039/b911028c

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Electron transfer through exTTF bridges in electron donor–acceptor
conjugatesw
a
a
b
ab
´
Beatriz M. Illescas, Jose Santos, Mateusz Wielopolski, Carmen M. Atienza,
Nazario Martı
´
n*ac and Dirk M. Guldi*b
Received (in Cambridge, UK) 4th June 2009, Accepted 28th July 2009
First published as an Advance Article on the web 13th August 2009
DOI: 10.1039/b911028c
Rigid and soluble electron donor–acceptor conjugates combining
exTTF and/or TTF as donors and C60 as acceptor have been
synthesized; fluorescence and transient absorption measurements
confirm the generation of charge-separated radical-ion pairs
with lifetimes in the ls timescale.
The incorporation of p-conjugated oligomers into electron
donor–acceptor (D–A) conjugates and/or hybrids has emerged
as a versatile strategy to design materials that find applications
in the fields of molecular electronics, photovoltaics, electro-
luminescent devices, etc.1 In this context, charge transfer—
especially over long distances—is key in developing molecules
that may function as molecular wires. Notably, a great variety
of donor and acceptor moieties have been connected through
p-conjugated oligomers.2
In pioneering work by Wasielewski et al. different oligomers
Fig. 1 C60-exTTF-exTTF (1) and C60-exTTF-TTF (2).
have been incorporated to bridge phenothiazine and perylene-
anthracenoid part of exTTF, oPPV and the pyrrolidine ring
of C60.
3,4:9,10-bis(carboximide) as donor and acceptor moieties,
respectively. Attenuation factor (b) values of 0.46 AÀ1 for
the corresponding p-phenylene oligomers and 0.093 AÀ1 for
the corresponding oligofluorenes were reported.3 b is a key
parameter often used as a benchmark to evaluate charge
transport through the molecular bridges that connect donor
This systematic study, carried out using the same D, A and
chemical connectivities,8 motivated us to test the efficiency of
charge transfer through exTTF bridges. In this context, we
have synthesized two rigid and fully conjugated electron
donor–acceptor conjugates (1 and 2)—Fig. 1—in which
p-phenyleneethynylene-exTTF serve as molecular bridges to
connect C60 with TTF or another exTTF. To guarantee
reasonable solubilities of the resulting conjugates in common
organic solvents additional octyl and alkoxy chains were
placed at the pyrrolidine nitrogen atoms and the benzene rings
respectively. Our working hypothesis was that upon light
irradiation an efficient charge transfer process should occur
from the donor to the acceptor involving a gradient of
redox centers. The efficiency of this process was expected to
determine the molecular wire character of exTTF.
and acceptor (kET = k0eÀbr , where k0 is a kinetic prefactor
DA
and rDA represents the donor–acceptor distance).4
In our groups, we have mainly focused on the synthesis of
molecular systems that integrate C60 (i.e., electron acceptor),
extended tetrathiafulvalene (exTTF) (i.e., electron donor)
and different p-conjugated oligomers (i.e., bridge). When
oligofluorenes5 were probed, b values of 0.09 AÀ1 were
derived, while oligo(p-phenyleneethynylene)s6 (oPPE) give rise
to b values in the region of 0.2 AÀ1. A particularly remarkable
finding is that oPPV7 of lengths up to 5 nm to connect the two
electroactive units result in an exceptionally low b value of
0.01 AÀ1. Such a remarkably low value has been rationalized
on the basis of an effective para-conjugation between the
The synthesis of 1 and 2 was carried out using the
palladium-catalyzed cross-coupling reaction of terminal alkynes
with aryl halides (see Scheme 1). The required building blocks,
dialkynyl-exTTF (3),9 iodo-exTTF (6)10 and iodo-TTF (10),11
had been previously reported. 1-Ethynyl-2,5-bis(hexyloxy)-
4-iodobenzene (7) was readily prepared by removal of the
TMS protecting group in ((2,5-bis(hexyloxy)-4-iodophenyl)-
ethynyl)trimethylsilane with potassium carbonate.6b
a
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias Quımicas,
Universidad Complutense, 28040 Madrid, Spain.
E-mail: nazmar@quim.ucm.es; Fax: +34 91-394-4103
b Institute of Physical and Theoretical Chemistry and Interdisciplinary
Center for Molecular Materials, University of Erlangen,
91058 Erlangen, Germany. E-mail: dirk.guldi@chemie.uni-erlangen.de;
Fax: +49 913-185-28307
All the synthetic steps involving the coupling reactions
to prepare 5, 8, 9, 11 and 12 were carried out using the
Hagihara–Sonogashira reaction, employing tetrakis(triphenyl-
phosphane)palladium as catalyst, copper(I) iodide as co-catalyst
and diisopropylamine as base (see ESIw).
c Instituto Madrilen˜o de Estudios Avanzados en Nanociencia
(IMDEA-Nanociencia), Campus UAM, Cantoblanco, E-28049,
Madrid, Spain
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b911028c
ꢀc
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5374 | Chem. Commun., 2009, 5374–5376

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compound 12, two quasireversible oxidation waves are
1,ox
2,ox
observed (Eap
= 0.16 V; Eap
= 0.62 V), the first of
them probably involving the oxidation of the TTF moiety
together with the two-electron oxidation of the exTTF. Again,
a quasireversible reduction wave of the formyl group is
1,red
observed at Ecp
= À1.84 V.
1 and 2 show a similar behavior to 9 and 12, respectively, in
the range of oxidation. For 2, three waves are observed that
involve the re-reduction of the oxidized species to the neutral
form (see Fig. S1w). This observation is likely associated with
the shift of the exTTF reduction to more negative potentials,
as has been mentioned above. Under reductive conditions,
1 and 2 show three reduction waves assigned to the first three
one-electron quasireversible reductions of C60 (1: À0.84,
À1.41, À2.05 V; 2: À0.86, À1.42, À2.13 V). The values of
the potentials are cathodically shifted relative to pristine C60
(À0.72, À1.12, À1.60 V), due to the saturation of a double
bond in the fullerene skeleton, which raises the LUMO energy
of the fullerene derivatives.13
Quantum chemical calculations shed light on distinguishable
differences between the two exTTF moieties in 1 and the
exTTF and TTF moieties in 2. An analysis of the molecular
orbitals of 1 revealed that both exTTFs are degenerate with
HOMO and HOMO-1 energies of À4.74 eV and À4.79 eV,
respectively. In 2 the degeneracy is cancelled with energies of
À4.75 eV for the HOMO on TTF and À4.96 eV for the
HOMO-1 on exTTF—see Fig. 2.
Given the superimposition of the ground state features of
the C60 and exTTF moieties, 360 nm was chosen as excitation
wavelength for the fluorescence experiments. In all cases
room temperature fluorescence patterns were registered that
resemble that seen for the C60 reference—a dominant *0–0
maximum develops at 715 nm. While for the C60 reference a
fluorescence quantum yield of 6.0 Â 10À4 is known, 1 and 2
reveal fluorescence quantum yields on the order of 0.55 Â 10À4
(see Fig. S5w). Implicit is a C60 excited state deactivation that
is linked to the presence of the electron donating exTTF
moieties.
Scheme 1 Reagents and conditions used in the synthesis of 1 and
2: (i) Pd(PPh3)4/CuI/iPr2NH/THF/D.
Finally, 1 and 2 were obtained by a 1,3-dipolar cyclo-
addition reaction of the corresponding azomethine ylides
(Prato’s protocol),12 generated in situ from N-octylglycine and
aldehydes 9 and 12, in 21% and 18% yields, respectively.
1
All new compounds were fully characterized by H NMR,
13C NMR, FTIR and UV–vis spectroscopy (see ESIw).
The redox potentials of 1 and 2 were studied by cyclic
voltammetry in solution at room temperature. The data are
collected in Table S1 (see Table S1 and Fig. S1 and S2 in
ESIw), together with those of C60, 9 and 12 as reference
compounds.
Femtosecond transient absorption spectroscopy—387 nm
excitation—sheds light on the electron transfer deactivation.
In line with a fairly large donor–acceptor separation, the exTTF
transient that is formed in 1 and 2 decays with nearly the same
dynamics (i.e., 1.2 ps) as seen in the exTTF reference (i.e., 1.2 ps).
Going beyond this time window, the only metastable
transient that is detected is that of the C60 singlet excited state
with a broad near-infrared absorption that extends from
around 800 nm to 1200 nm. While this transient decays rather
slowly in the C60 reference (6.0 Â 108 sÀ1) much faster decays
emerge in 1 and 2. In particular, in THF the following lifetimes
were measured: 28 ps (i.e., C60-exTTF-exTTF) and 54 ps
Compound
9 shows an electrochemically irreversible
oxidation wave which involves four electrons (Eap1,ox = 0.25 V),
and gives rise to the formation of the tetracationic species. The
reduction wave corresponding to the reversion of the tetra-
cation to the neutral molecule is observed at negative values of
1,ox
the potential (Ecp
= À0.72 V). The large separation
between the oxidation and reduction peaks suggests a rather
high activation barrier when re-reducing the dication to the
neutral form of exTTF, and has been previously observed
in related compounds.7 Apart from this, a quasireversible
reduction wave involving one electron is observed for the
1,red
reduction of the formyl group (Ecp
= À1.90 V). In
contrast to exTTF, which undergoes a two-electron oxidation
process leading to the dication, TTF reveals two one-electron
oxidation processes, giving rise to the radical cation
and dication species in two successive steps. In the case of
Fig. 2 HOMO-1 (left) and HOMO (right) of 1 (upper part) and 2
(lower part) resulting from DFT calculations (B3PW91/6-31G*).
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the one-electron oxidized TTF evolves in the 400 to
450 nm range. This spectral observation corroborates the
energetic assumption that, indeed, a C60 -exTTFꢁ+-TTF
À
ꢁ
À
+
ꢁ
to C60 -exTTF-TTFꢁ
charge shift occurs. The
C60 -exTTF-TTFꢁ+ decays monoexponentially on the lower
ms timescale to give rise to a lifetime of 5.8 ms (1.7 Â 105 sÀ1).
New donor–acceptor conjugates (1, 2) connected through a
rigid bridge formed by exTTF and p-phenyleneethynylene units
have been prepared in a multistep synthesis. Photophysical
studies reveal thatÀin 1 charge-transfer processes involve two
À
ꢁ
isoenergetic C60 -exTTFꢁ+-exTTF/C60 -exTTF-exTTFꢁ
À
ꢁ
+
ꢁ
states that are equilibrated. Consequently, two distinct charge
recombination processes evolve. In 2, on the other hand, the
situation changes due to lowering the oxidation potential of the
terminal TTF. In fact, in 2 a stepwise electron transfer
À
charge shift from C60 -exTTFꢁ+-TTF to
ꢁ
powers
À
a
+
C60 -exTTF-TTFꢁ
resulting in only a single charge
ꢁ
recombination process. Thus, exTTF acts as an efficient moiety
in electron transfer processes allowing the generation of
separated ion radical pairs with remarkable lifetimes.
Financial support from the Deutsche Forschungsge-
meinschaft (SFB 583), the Office of Basic Energy Sciences of
the US, the MICINN of Spain (projects CTQ2008-00795/
BQU and Consolider-Ingenio 2010C-07-25200, Nanociencia
Molecular), the CM (project P-PPQ-000225-0505) and the EU
(FUNMOL FP7-212942-1) is greatly appreciated.
Fig. 3 Upper part: differential absorption spectra (visible and near-
infrared region) obtained upon femtosecond flash photolysis (387 nm,
150 nJ) of C60-exTTF-TTF (2) (B10À6 M) in deoxygenated THF with
several time delays between 0 and 500 ps at room temperature. Lower
part: time–absorption profiles of the spectra shown above at 1000 nm,
monitoring the charge separation.
Notes and references
(i.e., C60-exTTF-TTF). At the conclusion of the C60 singlet
excited state stands a transient that reveals a set of two
maxima—Fig. 3. One maximum is seen at 670 nm, which agrees
well with the one-electron oxidized form of exTTF, while the
second maximum, which evolves around 1010 nm, corresponds
to the one-electron reduced form of C60. The transient is in all
cases stable on the timescale of 3.0 ns. Thus, we hypothesize
that any underlying charge shift reaction, as may happen, must
develop on a timescale outside of the 3.0 ns time window.
During the early stages of the C60-exTTF-exTTF nano-
second experiments the same spectroscopic characteristics
are discernable that were already seen at the end of the
femtosecond experiments—the radical ion pair states with
maxima at 670 and 1010 nm. Their decays differ, however,
significantly: C60-exTTF-exTTF features, for example, two
components (see Fig. S4w). For one of the two decays
a multi-wavelength analysis yields a lifetime of 485 ns
(2.1 Â 106 sÀ1). The second component, on the other hand,
is much longer lived with a lifetime of 58 ms (1.7 Â 104 sÀ1).
Since the two species are spectroscopically indistinguishable
we assign the shorter livÀed component to the adjacent radical
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ꢁ
´
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À
radical ion pair state (i.e., C60 -exTTF-exTTFꢁ+). Considering
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quantum yield for forming the remote radical ion pair state
is only 25 Æ 2%. The radical ion pair state decays in
C60-exTTF-TTF in only one step (see Fig. S5w). But instead
of registering the fingerprint absorption of the one-electron
oxidized exTTF characteristics with a maximum at 670 nm,
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This journal is The Royal Society of Chemistry 2009
5376 | Chem. Commun., 2009, 5374–5376