Tuning electron transfer through p-phenyleneethynylene molecular wires

Article abstract of DOI:10.1039/b603149h

Weak wire-like behavior - with a damping factor (β) of 0.2 ± 0.05 A^-1 - has been found in a series of C₆₀-wire- exTTF systems (i.e., p-phenyleneethynylene): these results contrast with previous observations involving p-phenylenevinyl

Full text of DOI:10.1039/b603149h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Tuning electron transfer through p-phenyleneethynylene molecular
wires{
Carmen Atienza,ab Nazario Mart´ın,*a Mateusz Wielopolski,b Naomi Haworth,c Timothy Clarkc and
Dirk M. Guldib
Received (in Cambridge, UK) 2nd March 2006, Accepted 30th May 2006
First published as an Advance Article on the web 23rd June 2006
DOI: 10.1039/b603149h
soluble (i.e., introducing long alkoxy chains) and to bear two
functional termini (i.e., linking exTTF and C60). En route towards
Weak wire-like behavior—with a damping factor (b) of 0.2 ¡
0.05 A21—has been found in a series of C60–wire–exTTF
˚
target molecules 9a–c
a stepwise approach involving tail
systems (i.e., p-phenyleneethynylene): these results contrast with
previous observations involving p-phenylenevinylene systems.
functionalization was followed. The syntheses of 3, 7 and 8 were
carried out starting from 2-iodo-exTTF (1)6 by Hagihara–
Sonogashira palladium catalyzed cross-couplings with oPPE
(2, 5, 6). The oPPE were prepared from p-ethynylbenzaldehyde
(2) by reacting them with 4—palladium(II), copper(I) iodide and
PPh3 in piperidine or other secondary amine—to afford 5.{ 5 was
further reacted with 4 to obtain 6. The final step of synthesizing
9a–c involved treating exTTF-containing aldehydes (3, 7, 8) with
C60 and sarcosine (N-methylglycine) in refluxing chlorobenzene for
3–6 h. The reaction takes place by 1,3-dipolar cycloaddition of the
‘‘in situ’’ generated azomethine ylides to C60,7 affording 9a–c as
stable and highly soluble brown solids in moderate yields
(33–63%)—see ESI for details on the characterization.
Testing electronic conduction through molecular wires constitutes
a major challenge in physics, chemistry and engineering. Despite
this general interest, only a handful of molecules have been
thoroughly studied and their electron transfer dynamics and
transport properties evaluated.1
Conceptually, an ideal molecular nanowire should resemble the
features of a monodisperse p-conjugated oligomer.2 Hereby,
p-conjugation emerges as a particularly crucial factor to realize
(i) small attenuation factors (i.e., electron conduction over large
distances), (ii) good contacts with the termini (i.e., electron donors
and electron acceptors) and (iii) good orbital mixing (i.e., with
donor and acceptor states). Synthetic methodologies—mostly
involving cross-coupling reactions—are powerful tools for fine-
tuning most of the aforementioned criteria. Notably, we have
recently succeeded in demonstrating wire-like behavior in a series
of oligo-p-phenylenevinylenes (oPPV). The different oPPV bridges
connect an electron accepting [60]fullerene with an extended
tetrathiafulvalene (exTTF), that serves as an electron donor. When
Importantly, in 9a–c the LUMOs (i.e., 22.7 ¡ 0.05 eV) are
nearly identical to that of the C60 reference, while the HOMOs
basically resemble that of exTTF (i.e., 27.4 ¡ 0.05 eV). A
summary is given in Fig. S1 and Table S1.{
Compounds 9a–c were probed in fluorescence and transient
absorption studies. For our photophysical assays we added the
properties previously determined for the 0-mer, that is, the electron
donor acceptor system, in which the anthracenoid core of exTTF
analyzing variable electron conduction distances, that is, of up to
21
˚
˚
40 A, exceptionally low attenuation factors (b = 0.01 ¡ 0.005 A
)
were established.3 In follow-up work, where the exTTF moieties
were replaced by metalloporphryins, these general findings were
further corroborated.4
In this communication we wish to report on the rational design
of a series of novel donor–bridge–acceptor systems using oligo-
p-phenyleneethynylenes (oPPE) bridges,5 which have been system-
atically increased from the monomer to the trimer. Our current
study sheds light for the first time onto the difference between the
behavior of oPPV and that of oPPE in comparable ensembles (i.e.,
C60–oPPV–exTTF versus C60–oPPE–exTTF (9a–c)).
The oligo-p-phenyleneethynylene building blocks 2, 5 and 6
were designed—see Schemes 1 and S1 (ESI{)—to be sufficiently
aDepartamento de Qu´ımica Orga´nica, Facultad de Ciencias Qu´ımicas,
Universidad Complutense, E-28040, Madrid, Spain.
E-mail: nazmar@quim.ucm.es; Fax: (+34)913-944-103
bInstitute for Physical and Theoretical Chemistry, University of
Erlangen, 91058, Erlangen, Germany. E-mail: dirk.guldi@chemie.
uni-erlangen.de; Fax: (+49)9131-852-8307
cFriedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Computer-
Chemie-Centrum, Naegelsbachstrasse 25, 91052, Erlangen, Germany
{ Electronic supplementary information (ESI) available: Spectroscopic
and MO data. See DOI: 10.1039/b603149h
Scheme 1 Building blocks used in the preparation of triads 9a–c.
This journal is ß The Royal Society of Chemistry 2006
3202 | Chem. Commun., 2006, 3202–3204

View Article Online
is linked directly to the carbon of the pyrrolidine skeleton.3a 355 nm
excitation was chosen, although it populates the singlet excited
states of C60 (1.76 eV) and that of the oPPE (2.6 eV), due to over-
lapping absorptions. We know, however, from past experiments
with C60–oPPE that when the oPPEs are excited, a rapid
intramolecular transduction of energy funnels the excited state
energy to the fullerene core, generating 1*C60 quantitatively.8
Excitation of C60, on the other hand, leads directly to 1*C60.
Overall the quantum yield of C60 fluorescence amounts to 6.0 6
1024—identical to that of a C60-reference that lacks the oPPE
however, clear that no radical ion pair is formed for the 3-mer—
2
?
vide infra. Both radical ion pair features, that is, C60 and
+
6
exTTF , decay in the 0-mer (4.9 6 10 s21), 1-mer (1.1 6
?
106 s21), and 2-mer (3.8 6 105 s21) with similar rates (2DGCR
1.0 eV) to reinstate the singlet ground states.
#
Relating the charge separation and charge recombination
dynamics in THF to the electron donor acceptor separation (i.e.,
center-to-center—RCC) enabled us to evaluate the damping factor
of the oPPE bridges—see Fig. 2.10 Both relationships reveal linear
dependences and provided damping factors (b) that are in perfect
agreement with each other: 0.21 ¡ 0.05 A21 for charge separation
bridge—ruling out any endothermic electron transfer (2DGCS
.
˚
20.24 eV) between C60 (Ered: 0.61 ¡ 0.02 V versus Ag/Ag+)8
and oPPE (Eox: 1-mer: 2.09 V, 2-mer: 2.04 V; 3-mer: 1.96 V versus
Ag/Ag+).8
versus 0.2 ¡ 0.05 A21 for charge recombination. Please note that
˚
these values are more than an order of magnitude higher than
what we have established earlier for oPPV bridges (i.e., 0.01 ¡
0.005 A21).3
In stark contrast to C60–oPPE and the C60 reference, the
fullerene fluorescence is appreciably quenched in 9a–c (i.e., as low
as 0.55 6 1024 in THF)—Fig. S2.{ Moreover, the fluorescence
quantum yields depend strongly on the length of the oPPE bridge:
0-mer: 0.18 6 1024; 1-mer: 0.55 6 1024; 2-mer: 1.8 6 1024.§
Setting these quantum yields in context to the C60-reference and its
lifetime, the C60 fluorescence deactivation rates in the 0-mer,
1-mer, and 2-mer were determined as 2.1 6 1010 s21, 6.6 6 109 s21
and 1.3 6 109 s21, respectively. Additionally, we followed the
fluorescence at 710 nm, but found only measurable decay rates for
the C60 reference (6.6 6 108 s21) and the 2-mer (2.6 6 109 s21).
˚
˚
Extrapolating the relationships to RCC of 28.46 A, which
corresponds to the center-to-center distance in the 3-mer, helps to
explain the lack of electron transfer activation in the latter: The
rate of electron transfer would be with 3.9 6 108 s21, notably
slower than the intrinsic deactivation of the C60 singlet excited state
(6.6 6 108 s21)—see marked data point (i.e, large square) in Fig. 2.
To shed light on this observation we conducted calculations on
the topological and electronic structure of C60–oPPE–exTTF.
Theoretical calculations at DFT and semiempirical AM1 levels
revealed that the dihedral angle formed by the phenyl ring adjacent
to the pyrolidine and the benzene moiety of the exTTF connected
to the oligomer is not planar, and this deviation from planarity
increased significantly when semiempirical PM3 was used
(Fig. S3{). This lack of planarity could break the electronic
coupling between the donor and acceptor units in 9c, thus
accounting for the photophysical outcome.
1
We must conclude at this point of the investigation that *C60,
populated either directly or indirectly, powers an exothermic
electron transfer (2DGCS # 0.7 eV) to yield the radical ion pair
state, C60 –oPPE–exTTF .
2
?
+
?
The aforementioned hypothesis, namely, formation of the
radical ion pair state, was corroborated through transient
absorption spectroscopy. Importantly, Fig. 1 corroborates the
The investigation of the electronic structure revealed significant
differences between the oPPE and oPPV systems. The HOMO in
both systems is strongly localized on the exTTF and the LUMO
on the C60. According to the common, but oversimplified, one-
electron concept, the HOMO–LUMO transition would represent a
complete charge-transfer excitation with a very low extinction
coefficient. In addition, the HOMO in the C60–oPPV–exTTF triad
reaches into the oPPE bridge, whereas the HOMO in the C60–
oPPE–exTTF triad is completely localized on the exTTF. This
+
?
spectral signatures of the one-electron oxidized exTTF and the
one-electron reduced C60 2, which were detected as new transient
?
maxima at 660 and 1000 nm, respectively.3,9 The spectral
identification holds for the 0-mer, 1-mer, and 2-mer, while for
the 3-mer only a very broad transient, whose identity remains
unknown to us at this stage, dominates the region of interest. It is,
Fig. 1 Differential absorption spectrum (visible and near-infrared)
obtained upon nanosecond flash photolysis (355 nm) of y1.0 6 1025
M
Fig. 2 Center-to-center distances (RCC) dependence of electron transfer
rate constants (ln kCR) in C60–oPPV–exTTF in nitrogen saturated THF at
room temperature. The dotted line represents the singlet lifetime of C60.
solutions of C60–oPPV–exTTF (2-mer) in nitrogen saturated THF with a
time delay of 200 ns at room temperature. Insert shows charge
recombination dynamics at 660 nm.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 3202–3204 | 3203

View Article Online
P-PPQ-000225-0505), the Deutsche Forschungsgemeinschaft
(SFB 583).
Notes and references
{ Compound 5 was obtained together with the dialdehyde homocoupling
compound of 2. However, when the reaction is carried out in refluxing
toluene for 17 h, only 5 is obtained in 65% yield.
§ For the 3-mer no appreciable interactions were noted.
1 (a) Special issue on Organic Electronics, Chem. Mater., 2004, 16, 4381;
(b) R. L. Carrol and C. B. Gorman, Angew. Chem., Int. Ed., 2002, 41,
4378; (c) Special issue on Molecular Wires and Electronics, Top. Curr.
Chem., 2005, 257, 1.
2 (a) K. Mu¨llen and G. Wegner, Electronic Materials: The Oligomer
Approach, Wiley-VCH, Weinheim, Germany, 1998; (b) J. L. Segura,
N. Mart´ın and D. M. Guldi, Chem. Soc. Rev., 2005, 34, 31.
3 (a) F. Giacalone, J. L. Segura, N. Mart´ın and D. M. Guldi, J. Am. Chem.
Soc., 2004, 126, 5340; (b) F. Giacalone, J. L. Segura, N. Mart´ın,
J. Ramey and D. M. Guldi, Chem.–Eur. J., 2005, 11, 4819.
4 G. De La Torre, F. Giacalone, J. L. Segura, N. Mart´ın and D. M. Guldi,
Chem.–Eur. J., 2005, 11, 1267.
5 For representative recent examples of aryleneethynylene molecular
wires, see: (a) J.-F. Nierengarten, T. Gu, G. Hadziioannou and
V. Krasnikov, Helv. Chim. Acta, 2004, 87, 2948; (b) Y. Shirai,
Y. Zhao, L. Cheng and J. M. Tour, Org. Lett., 2004, 6, 2129; (c)
Ch. Wang, A. S. Batsanov, M. R. Bryce and I. Sage, Org. Lett.,
2004, 6, 2181; (d) W. Hu, H. Nakashima, K. Furukawa, Y. Kashimura,
K. Ajito, Y. Liu, D. Zhu and K. Torimitsu, J. Am. Chem. Soc., 2005,
127, 2804.
6 M. C. D´ıaz, B. Illescas, C. Seoane and N. Mart´ın, J. Org. Chem., 2004,
69, 4492.
7 M. Prato and M. Maggini, Acc. Chem. Res., 1998, 31, 519.
8 C. Atienza, B. Insuasty, C. Seoane, N. Martin, J. Ramey and
D. M. Guldi, J. Mater. Chem., 2005, 15, 124.
9 D. M. Guldi and M. Prato, Acc. Chem. Res., 2000, 33, 695.
10 R. Marcus, Angew. Chem., Int. Ed. Engl., 1993, 32, 1111.
11 B. Ehresmann, B. Martin, A. M. H. C. Horn and T. Clark, J. Mol.
Model., 2003, 9, 342.
12 (a) K. Pettersson, J. Wiberg, T. Ljungdahl, J. Martensson and
B. Albinsson, J. Phys. Chem. A, 2006, 110, 319; (b) K. Pettersson,
A. Kyrychenko, E. Ronnow, T. Ljungdahl, J. Martensson and
B. Albinsson, J. Phys. Chem. A, 2006, 110, 310.
Fig. 3 Electron affinity maps of C60–oPPV–exTTF and C60–oPPE–
exTTF was calculated with Parasuf and the surfaces were viewed with
Tramp1.1d.
might have a strong influence on the charge-separation process
because in the C60–oPPV–exTTF triad the injection of an electron
into the bridge is facilitated through better orbital overlap between
the exTTF and the oligomer in comparison to the oPPE system
(Fig. S4{). This fact was also observed from electron affinity maps
(Fig. 3).11
In conclusion, we have demonstrated that through a simple
exchange of C–C double bonds (oPPV) for C–C triplet bonds
(oPPE) the long-range electron transfer (i.e., charge separation and
charge recombination) in electron donor–acceptor conjugates can
be considerably altered. Notably, the HOMOs of oPPE (i.e.,
between 28.9 and 28.4 eV) are slightly below those in oPPV (i.e.,
between 28.4 and 27.9 eV), which might diminish possible
interactions with C60. It is important, however, to consider the
temporal limits of the precursor state (i.e., singlet excited state of
C60). Very similar damping factors (i.e., 0.29 A21) have recently
˚
been reported by Albinsson et al.12
This work has been supported by the MEC of Spain and
Comunidad de Madrid (Projects CTQ2005-02609/BQU and
3204 | Chem. Commun., 2006, 3202–3204
This journal is ß The Royal Society of Chemistry 2006