P-phenyleneethynylene molecular wires: Influence of structure on photoinduced electron-transfer properties

Article abstract of DOI:10.1002/chem.200800159

A series of donor-acceptor arrays (exTTF-oPPE-C₆₀) containing π-conjugated oligo(phenyleneethynylene) wires (oPPE) of different length between π-extended tetrathiafulvalene (exTTF) as electron donor and fullerene (C₆₀) as electron acceptor has been prepared by following a convergent synthesis. The key reaction in these approaches is the bromo-iodo selectivity of the Hagihara-Sonogashira reaction and the deprotecting of acetylenes with different silyl groups to afford the corresponding donor-acceptor conjugates in moderate yields. The electronic interactions between the three electroactive species were determined by using UV-visible spectroscopy and cyclic voltammetry. Our studies clearly confirm that, although the C₆₀ units are connected to the exTTF donor through π-conjugated oPPE frameworks, no significant electronic interactions are observed in the ground state. Theoretical calculations predict how a simple exchange from C=C double bonds (i.e., oligo(p-phenylenevinylene) to C≡C triple bonds (i.e., oPPE) in the electron donor-acceptor conjugates considerably alters long-range electron transfer. Photoexcitation of exTTF-oPPE-C ₆₀ leads to the following features: a transient photoproduct with maxima at 660 and 1000 nm, which are unambiguously attributed to the photolytically generated radical-ion-pair state, [exTTF⁺-oPPE- C₆₀^-]. Both charge-separation and charge-recombination processes give rise to a molecular-wire behaviour of the oPPE moiety with an attenuation factor (β) of (0.2 ±0.05) A^-1.

Full text of DOI:10.1002/chem.200800159

FULL PAPER
DOI: 10.1002/chem.200800159
p-Phenyleneethynylene Molecular Wires: Influence of Structure on
Photoinduced Electron-Transfer Properties
AHCTREUNG
Mateusz Wielopolski,[a] Carmen Atienza,[a, b] Timothy Clark,[c] Dirk M. Guldi,*[a] and
Nazario Martín*[b, d]
Abstract: A series of donor–acceptor
arrays (exTTF–oPPE–C60) containing
p-conjugated oligo(phenyleneethynyl-
tive species were determined by using
UV-visible spectroscopy and cyclic vol-
tammetry. Our studies clearly confirm
that, although the C60 units are con-
nected to the exTTF donor through p-
conjugated oPPE frameworks, no sig-
nificant electronic interactions are ob-
served in the ground state. Theoretical
calculations predict how a simple ex-
change from C=C double bonds (i.e.,
triple bonds (i.e., oPPE) in the electron
donor–acceptor conjugates considera-
bly alters long-range electron transfer.
Photoexcitation of exTTF–oPPE–C60
leads to the following features: a tran-
sient photoproduct with maxima at 660
and 1000 nm, which are unambiguously
A
between p-extended tetrathiafulvalene
(exTTF) as electron donor and fuller-
ene (C60) as electron acceptor has been
prepared by following a convergent
synthesis. The key reaction in these ap-
proaches is the bromo–iodo selectivity
of the Hagihara–Sonogashira reaction
and the deprotecting of acetylenes with
different silyl groups to afford the cor-
responding donor–acceptor conjugates
in moderate yields. The electronic in-
teractions between the three electroac-
attributed to the photolytically generat-
+
C
ed radical-ion-pair state, [exTTF
–
Cꢁ
oPPE–C60 ]. Both charge-separation
and charge-recombination processes
give rise to a molecular-wire behaviour
of the oPPE moiety with an attenua-
tion factor (b) of (0.2ꢂ0.05) Šꢁ1.
ꢀ
oligo(p-phenylenevinylene) to C C
Keywords: charge transfer · cross-
coupling · donor–acceptor systems ·
fullerenes · phenyleneethynylene
Introduction
One of the major challenges in current chemistry is to find
molecules able to move charges rapidly and efficiently from,
for example, one terminus to another under the control of
an external electrical, electrochemical or photochemical
stimulus.
[a] M. Wielopolski, Dr. C. Atienza, Prof. Dr. D. M. Guldi
Department of Chemistry and Pharmacy
Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universität Erlangen-Nürnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
Fax : (+49)913-185-28307
Nature has provided impressive examples of how these
goals can be achieved. The photosynthetic reaction centre
protein moves electrons with near-unity quantum efficiency
across a lipid bilayer membrane rapidly by using several
redox cofactors, and thus serves as a model for developing
biomimetic analogues for applications in fields such as pho-
tovoltaic devices, molecular electronics and photonic materi-
als.[1]
E-mail: dirk.guldi@chemie.uni-erlangen.de
[b] Dr. C. Atienza, Prof. Dr. N. Martín
Departamento de Química Orgµnica I
Facultad de Ciencias Químicas
Universidad Complutense, 28040 Madrid (Spain)
Fax : (+34)91-394-4103
E-mail: nazmar@quim.ucm.es
[c] Prof. Dr. T. Clark
Computer-Chemie-Centrum
Interdisciplinary Center for Molecular Materials
Friedrich-Alexander-Universität Erlangen-Nürnberg
Naegelsbachstrasse 25, 91052 Erlangen (Germany)
In this context, p-conjugated oligomeric systems are of
particular interest because they provide an efficient elec-
tronic coupling between electroactive units—donor and ac-
ceptor termini—and display wirelike behaviour. Different
factors are required to make a molecule able to behave as
an ideal molecular wire: 1) matching between the donor (ac-
ceptor) and bridge energy levels, 2) good electronic coupling
[d] Prof. Dr. N. Martín
Instituto MadrileÇo de Estudios Avanzados en Nanociencia
(IMDEA-Nanociencia), 28049 Madrid (Spain)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2008, 14, 6379 – 6390
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6379

between the electron donor and acceptor units by means of
the bridge orbitals[2] and 3) a small attenuation factor (b).[3]
Among the many different p-conjugated oligomers,
oligo(p-phenylenevinylene) (oPPV)[4] has emerged as a par-
ticularly promising model system that helps to comprehend/
rationalise the basic features of poly(p-phenylenevinylene)
and also as a versatile building block for novel materials
with chemically tailored properties.[5] In this context, intra-
molecular electron transfer along conjugated chains of
oPPV has been tested in several donor–acceptor conjugates
involving anilines,[6] porphyrins[7] or ferrocenes[8] as electron
donors on one side, and [60]fullerenes, on the other side, as
the electron acceptor.[9] In fact, in a pioneering study tetra-
cene (TET; as electron donor) and pyromellitimide (PYR;
as electron acceptor) were connected by means of oPPV of
increasing length (TET–oPPV–PYR).[10] This work has dem-
onstrated the importance of energy matching between the
donor and bridge components for achieving molecular-wire
behaviour. Quantum-chemical calculations showed a compe-
tition between a direct superexchange process and a two-
step “bridge-mediated” process, whose efficiency depends
primarily on the length and nature of the conjugated
bridge.[11]
A more recent study reports on a series of p-phenylene
oligomers (Phn) that bridge phenothiazine (PTZ) and pery-
lene-3,4:9,10-bis(carboximide) (PDI) as electron donor and
acceptor, respectively.[12] In this case, the relative contribu-
tions of both the coherent superexchange and incoherent
hopping mechanisms to the overall charge-transport process
were quantified. Moreover, it has been shown that when an
oligofluorene is used as a bridge, the electronic structure of
the oligomer provides means to access the wirelike incoher-
ent hopping regime for hole transport over long distances at
nearly constant bridge energy.[2a]
Results and Discussion
Synthesis: The synthesis of exTTF–oPPE–C60 is based on
the preparation of p-conjugated oligo(p-phenyleneethynyl-
ene) building blocks using the palladium-catalysed cross-
coupling reaction of terminal alkynes with aryl halides. This
protocol provides an efficient and versatile means of extend-
ing p conjugation in organic compounds.[16]
The oligo(p-phenyleneethynylene) precursors (6, 14 and
24) were asymmetrically functionalised with terminal formyl
and alkyne groups, which are, in turn, required for connect-
ing the two electroactive units (exTTF and C60).
G
The key steps in the synthesis of the oligomeric spacer are
the selective palladium-catalysed coupling of terminal al-
kynes[17] with aryl iodides in the presence of aryl bro-
mides,[18] and the selective removal of the trimethylsilyl
(TMS) protecting groups in the presence of triisopropylsilyl
(TIPS) groups. Thus, the synthesis of building block 14 was
carried out analogously to a literature procedure.[19] The ini-
tial dihalogenated derivative 4 was prepared from hydroqui-
none (1) with hexylbromide to give 1,4-dihexyloxybenzene
(2) in a yield of 80%. Bromination and subsequent iodina-
tion reactions gave 2-bromo-5-iodo-1,4-dihexyloxybenzene
(4) in a yield of 66% (Scheme 1).[20] The introduction of the
appropriate ethynyl groups at different stages of the synthet-
ic procedure was carried out by means of the Hagihara–So-
nogashira reaction.[21] The experimental conditions were op-
timised for each step to improve the yield of the target mol-
ecules and to reduce undesired homocoupling reactions.[22]
From the initial dihalogenated derivative 4 and using pal-
ladium and copper iodide as catalysts in piperidine at room
temperature, the iodine atom was substituted by p-ethynyl-
benzaldehyde by reacting with 6 to give 7 in a yield of 41%.
Compound 6 was previously synthesised from commercially
available 4-iodobenzaldehyde (5) in one step.[23] Similar con-
ditions were used to remove the bromine atom with trime-
thylsilylacetylene. However, the target product was not ob-
tained in any case. A different approach was carried out
through selective palladium-catalysed coupling of trimethyl-
silylacetylene with aryl iodide 4 leading to 8 in a yield of
87%. Further reaction of 8 with p-ethynylbenzaldehyde (6)
led to 9 together with the homocoupling product 10 in low
yields.[24] Although different experimental conditions were
tested to improve the yield, the best results were obtained
by employing tetrakis(triphenylphosphane)–palladium as
the catalyst and diisopropylamine as the base at reflux in
toluene (Tol). Under these conditions, the desired com-
pound 9 was obtained in a yield of 65% (Table 1).[25]
Our own contributions to this intriguing field include the
demonstration of molecular wirelike behaviour in a series of
oligo(p-phenylenevinylene)s (oPPVs) of variable length.
The different oPPV bridges were connected to an electron-
accepting [60]fullerene and to a p-extended tetrathiafulva-
lene (exTTF), which serves as an electron donor. From an
analysis of different electron-conduction distances up to
40 Š, we obtained exceptionally low attenuation factors (b)
of (0.01ꢂ0.005) Šꢁ1.[13] The general validity of this conclu-
sion was confirmed by testing metalloporphryins instead of
exTTF as electron donors.[14]
Encouraged by these results, we now describe a series of
novel donor–acceptor conjugates (exTTF–oPPE–C60) imple-
menting oligo(p-phenyleneethynylene) (oPPE) as
a
bridge.[15] We have started with selective palladium-catalysed
cross-coupling reactions of terminal alkynes with aryl hal-
ides to increase the length of the molecular wire systemati-
cally.[16] Electrochemical studies using cyclic voltammetry
and the determination of the photophysical properties by
means of fluorescence and transient absorption spectroscop-
ies complement the synthetic work.
An alternative route was followed for oPPE 14. Here, a
palladium-catalysed coupling reaction from 8 with triisopro-
pylsilylacetylene afforded the asymmetric alkyne 11 in a
yield of 86%. Selective removal of the TMS group, followed
by a Sonogashira reaction with 4-iodobenzaldehyde (5) led
to 13 in a yield of 67%. The target product 14 was finally
obtained by monodeprotection of either 9 or 13 with K2CO3
in THF–MeOH, or tetrabutylammonium fluoride (TBAF),
respectively, in quantitative yields.
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Molecular Wires
FULL PAPER
Scheme 1. a) C6H13Br, EtONa, EtOH, D, 24 h. b) Br2, AcONa/AcOH, 08C. c) I2, H2SO4/H2O/KIO3, AcOH/CCl4/D. d) [PdCl2
dine/RT. e) Different conditions were tested: [Pd(PPh3)4], CuI, iPr2NH, THF/D; Pd(OAc)2, PPh3, EtN3/D; [PdCl2(PPh3)2], CuI, EtN3, DMF/D. f) [Pd-
(PPh3)4], CuI, iPr2NH, Tol/D. g) [Pd(PPh3)4], CuI, iPr2NH, THF/D. h) K2CO3, THF–MeOH. i) [Pd(PPh3)4], CuI, iPr2NH, THF/D. j) Bu4NF, THF.
k) K2CO3, THF–MeOH.
A
A
N
ACHTREUNG
A
N
ACHTREUNG
The synthesis of the suitably functionalised oPPE 16 was
approached through different strategies (Scheme 2). Firstly,
the previously synthesised alkynyl 14 was reacted with 2-
bromo-5-iodo-1,4-dihexyloxybenzene (4) under Sonogashira
conditions in the presence of palladium(0)/copper(I) iodide
leading to 15 in a yield of 14%. However, the target product
16 could not be obtained by further Sonogashira reactions.
Table 1. Experimental conditions used for Sonogashira reactions.
Catalyst
Base
Solvent
T
Yield 9/10
[Pd
[Pd
[Pd
[Pd
[Pd
[Pd
U
iPr2NH
iPr2NH
–
Et3N
iPr2NH
iPr2NEt
THF
reflux 10:0
reflux 56:5
reflux 10:0
THF[a]
THF
Et3N
toluene
toluene
reflux
3:0
reflux 65:0
reflux
RT
–
55:8
45:0
ꢁ
Therefore, we carried out the C C cross-coupling reac-
A
ACHTREUNG
tions between 8 and alkynyl compound 14, from which the
homocoupled compound 17 was obtained as the main prod-
uct. This reaction sequence was studied under different ex-
H
A
acetonitrile RT
[a] Deoxygenated. [b] dba: dibenzylideneacetone.
Scheme 2. a) [Pd
A
G
N
c) [Pd(PPh3)4], CuI, iPr2NH, THF/D (Table 2). d) [Pd
A
ACHTREUNG
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6381

N. Martín, D. M. Guldi et al.
perimental conditions and, in all cases, the homocoupling
compound was obtained as the main product (Table 2).[26]
titative yields. The presence of solubilising alkoxy chains
lends the oPPEs significant solubility, which allows their full
spectroscopic characterisation (Scheme 3).
In the next phase of our synthetic strategy, a palladium-
catalysed cross-coupling reaction with copper(I) iodide and
triphenylphosphane was carried out to link 2-iodo-p-extend-
ed tetrathiafulvalenes (exTTFs) (25)[27] to the appropriate
spacer oPPEs (6, 14, 24) in good to moderate yields of 85
(26a), 50 (26b) and 38% (26c) (Scheme 4).
Table 2. Experimental conditions used for Sonogashira reactions.
Catalyst
Base
Solvent
T
Yield 16/17
[Pd
[Pd
[Pd
U
iPr2NH
iPr2NH
iPr2NH
Et3N
THF
D
17:16
8:41
–
0:58
0:60
THF[a]
D
toluene
piperidine
piperidine
D
A
G
RT
RT
C
G
piperidine
Dyads 26a–c were fully characterised on the basis of their
analytical and spectroscopic data. Importantly, they show in
addition to the oPPE features, the presence of exTTF. Thus,
[a] Deoxygenated.
1
the H NMR spectra contain singlets at d=6.30 (26a), 6.95
Moreover, the Sonogashira reaction between 8 and 12, in
spite of changing the catalyst, base and solvent, afforded ex-
clusively the homocoupling product 18 in a yield of 90%.
We focused on the synthesis of the p-conjugated system
16 by designing an asymmetric ethynyl compound, which al-
lowed selective deprotection of the silyl group and a selec-
tive Sonogashira reaction. The incentive was to avoid the
undesired homocoupling product (Scheme 3). In this way,
(26b) and 7.04 ppm (26c) corresponding to the protons of
the 1,3-dithiole rings.
Finally, the target molecules 27a–c were prepared by 1,3-
dipolar cycloadditions of the azomethine ylides, generated
in situ from sarcosine (N-methylglycine) and aldehydes
26a–c with C60, as highly soluble brown solids in yields of 33
(27a), 63 (27b) and 53% (27c) (Scheme 5).[28]
A number of spectroscopic techniques were used to deter-
mine the structure of exTTF–oPPE–C60 (27a–c). The FTIR
spectra lacked the band at around 1600 cmꢁ1—due to the
carbonyl groups—but revealed the presence of the fuller-
ꢁ
selective C C cross-coupling reactions of terminal alkynyl
12 and aryl iodide 4 and subsequent treatment with trime-
thylsilylacetylene led to asymmetric compound 20 in a yield
of 97%. Selective deprotection of the TMS protecting group
and further Sonogashira reaction with 4-iodobenzaldehyde
(5) afforded compound 16 in a yield of 64%.
1
ene-specific band at 526 cmꢁ1. The H NMR spectra of 27b,
as a representative example, showed, in addition to the aro-
ꢁ
matic signals, resonance signals of the N CH3 group at d=
This synthetic route led to high yields in all steps and
avoided the persistent and undesired homocoupling product.
Furthermore, an alternative route was developed based on
the halogen interchange to favour the Sonogashira reaction.
Thus, treatment of 8 with nBuLi and I2 at room temperature
for 12 h afforded 23 in high yields. A further Sonogashira re-
action with 14 led to 16 in a yield of 86%. The desired
oPPE 24 was obtained by deprotection of 16 and 22 in quan-
2.86 ppm and the pyrrolidine protons at d=4.98 (s, 1H),
5.03 (d, J=9.2 Hz, 1H) and 4.30 ppm (d, J=9.2 Hz, 1H) in
good agreement with other N-methylfulleropyrrolidine de-
rivatives.[13,14] We also observed the signature of the p-conju-
gated oPPEs as two singlets at d=7.03 and 7.05 ppm and
the 1,3-dithiol ring as a singlet at d=6.79 ppm (s, 4H).
All of the 13C NMR spectra showed the characteristic sig-
nals that correspond to the sp3 carbon atoms of the pyrroli-
Scheme 3. a) [Pd
A
G
N
iPr2NH, THF/D/28 h. e) nBuLi, 08C/I2/RT/12 h. f) [Pd
AHCTREUNG
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Molecular Wires
FULL PAPER
For the electrochemical and photophysical studies, refer-
ence compounds 28 and 29 were synthesised as outlined in
Scheme 6. Treatment of 9 and 15 with C60 and sarcosine (N-
methylglycine) in chlorobenzene at reflux for 3 h afforded
the reference compounds (28 and 29) in moderate yields
(Scheme 6).
Scheme 4. a) [PdCl2
N
G
CuI/iPr2NH/THF/D/3 h. c) [Pd
AHCTREUNG
Scheme 6. General conditions for the preparation of references are as fol-
lows: N-methylglycine, D, 3 h in chlorobenzene.
The electronic absorption spectra of exTTF–oPPE–C60
reveal the presence of the typical bands of C60, exTTF and
oPPE. Figure 1shows a comparison, as a representative ex-
Figure 1. UV-visible spectrum of 27b (b), 26b (c) and 28 (g).
ample, of the UV-visible spectrum of exTTF–oPPE–C60
(27b) with exTTF–oPPE (26b) and 28. A bathochromic
shift was observed for 27b relative to the corresponding ref-
erence compound (28). The increase of the p-conjugated
system between the electroactive units in 27a–c resulted in
bathochromic shifts in the wavelength range from l=431
(monomer) to 494 nm (trimer) (see the Experimental Sec-
tion in the Supporting Information).
Scheme 5. General conditions for the preparation of triads 27a–c from
(26a–c):C60 are as follows: N-methylglycine, D, 3 h in chlorobenzene.
dine ring between d=70 and 80 ppm and the tetragonal
carbon atoms of the solubilised alkoxy chains (see the Ex-
perimental Section in the Supporting Information).
It is interesting to note that 26b shows a lmax value at
460 nm that could be assigned to a charge-transfer band
from the exTTF donor to the electron-accepting p-conjugat-
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6383

N. Martín, D. M. Guldi et al.
ed oligomer with a formyl group.[29] This band is batho-
chromically shifted in 27b with the onset extending to
nearly 600 nm.
Electrochemistry: The cyclic voltammetry (CV) data of
exTTF–oPPE–C60 are collected in Table 3 along with those
of C60, exTTF and reference compounds 28 and 29. The
Table 3. Redox potentials of 28, 29, 27a–c, C60 and exTTF (V vs. SCE).[a]
Compound E1pa Ep2a E3pa E4pa
E1pc
E2pc
E3pc
E4pc
E5pc
exTTF
C60
28
0.33
–
–
–
0.29
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Figure 2. Cyclic voltammogram of 27a (c) (see Table 3 for the experi-
mental conditions).
ꢁ0.72 ꢁ1.12 ꢁ1.60
ꢁ2.05
1.16 1.47 ꢁ0.82 ꢁ1.23 ꢁ1.71 ꢁ1.76
1.14 1.44 ꢁ0.82 ꢁ1.21 ꢁ1.68 ꢁ1.74
–
–
29
27a
27b
27c
–
–
ꢁ0.82 ꢁ1.20 ꢁ1.69 ꢁ1.77 ꢁ2.03
0.310.70
0.35 0.68 0.92
–
.414
ꢁ0.82 ꢁ1.21 ꢁ1.62 ꢁ1.75
–
were also observed due to the oxidation of the oligo(phen
eneethynylene) moiety.
Overall, the redox potentials of exTTF–oPPE–C60 (27a–c)
yl-
–
–
ꢁ1.15 ꢁ1.60 ꢁ1.78 ꢁ1.94
AHCTREUNG
[a] Pa: anodic peak; pc: cathodic peak. Glassy carbon working electrode,
Ag/Ag+ as reference electrode and platinum wire as counter electrode.
Bu4NClO4 (0.1m) as supporting electrolyte in o-dichlorobenzene/acetoni-
are similar to those found for the pristine donor (exTTF)
and acceptor (C60). From this data and the absorption spec-
tra, we conclude that there are only weak electronic interac-
tions between the redox-active components in their ground
states.
ꢁ1
trile (4:1v/v) as solvent. Scan rate: 200 mVs
.
measurements were carried out in ODCB/CH3CN 4:1
(ODCB: o-dichlorobenzene) at room temperature using a
glassy carbon (GC) working electrode, standard Ag/AgCl as
the reference electrode and tetrabutylammonium hexafluor-
ophosphate (0.1m) as the supporting electrolyte (Table 3).
Unlike the parent C60, the CV analysis of 28 and 29
showed four quasi-reversible reduction waves due to the
fullerene moiety. These values are shifted towards more
negative reduction potentials relative to pristine C60 due to
the saturation of a double bond in the fullerene skeleton.
Implicit is a raise of the LUMO energy of the resulting full-
erene derivatives.[30] Compounds 28 and 29 also showed two
oxidation waves (i.e., at 1.16 and 1.45 V) that correspond to
the oligomer units.
The CV analysis of 27a–c revealed the presence of four
quasi-reversible reduction waves, resembling the trend
found for the parent C60. These reduction potentials are
cathodically shifted relative to pristine C60 and appear at po-
tential values similar to those found for the reference com-
pounds. Furthermore, compounds 27a–c exhibited one addi-
tional reduction potential wave due to the presence of the
oligomer unit at ꢁ1.77 V (Figure 2).
Theoretical calculations
Molecular geometries: Results from previous quantum
chemical investigations led to the assumption that in the
trimer exTTF–oPPE–C60 significant deviation from planarity
prevails. In turn, we concluded that the lack of planarity in
the dihedral angles between the phenyl rings adjacent to the
pyrrolidine ring and to exTTF influences changes the p con-
jugation, and thus, the electronic coupling between donor
and acceptor units in 27c.[15] Further insight, by scrutinising
higher levels of theory, indicates, nevertheless, that the inac-
curate description of conjugated p systems, delocalised sys-
tems and rotation barriers as part of the previously used
semiempirical Hamiltonians PM3 and AM1might lead to a
deviation from planarity.[34] Furthermore, the previously
used geometry optimisation algorithm, as implemented in
the HyperChem software package, was found to fail in ach-
ieving complete convergence. To validate the above-men-
tioned results we employed further calculations using the
program packages Gaussian 03[35] for DFT and VAMP
10.0[36] for semiempirical calculations using the improved
AM1* Hamiltonian. Semiempirical calculations suggested
that in the minimum-energy structure the dihedral angle
formed by the phenyl rings approaches zero. These results
were confirmed by using DFT calculations at the B3LYP/6-
31G* level and crosschecked by using B3PW91/6-31G* cal-
culations. The deviation from planarity is less than ꢂ128.
Furthermore, single-point calculations at the B3LYP/6-
311G** and the B3PW91/6-311G** levels reveal a very low
rotation barrier of the phenyl rings in the p-conjugated
bridge (<2.0 kcalmolꢁ1). Rotations of the phenyl unit adja-
cent to the pyrrolidine and the benzene moiety of exTTF
The presence of exTTF leads only to one quasi-reversible
oxidation wave involving a two-electron process to form the
dication at around 0.33 V. This feature was previously con-
firmed by Coulometric analysis[31] and other related stud-
ies.[32] Attempts to generate and characterise the exTTF rad-
ical cation electrochemically caused its disproportionation
to the neutral and the dicationic form.[33] The oxidation po-
tential values in 27a–c are anodically shifted relative to the
parent exTTF, which confirms the better donor character of
the triads. However, the increase of the length on the oligo-
mer chain produces a slight cathodic shift between 0.29 and
0.35 V. Two additional oxidation waves at 0.7 and 1.4 V
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Molecular Wires
FULL PAPER
connected to the oligomer are similarly not restricted by the
the HOMO is completely conjugated throughout the whole
bridge, in the corresponding exTTF–oPPE–C60 the localiza-
tion is more pronounced. Hence, injection of an electron
into the bridge in exTTF–oPPV–C60 is favoured by the
better orbital overlap between exTTF and the oligomers.
Local-electron-affinity[3537] mappings of exTTF–oPPE–C60
and exTTF–oPPV–C60 are represented in Figure 5. Signifi-
relatively low rotation barrier of less than 1.0 kcalmolꢁ1 [34e]
.
Electronic structure and energy levels: Figure 3 shows the
calculated (AM1*) ionization potentials (IPs) and electron
affinities (EAs) of exTTF, C60–pyrrolidine, the pristine
~
Figure 3. Calculated AM1* ionization potentials IP ( ) and electron af-
Figure 5. Local-electron-affinity maps of exTTF–oPPV–C60 (left) and
&
finities EA ( ) of the pristine building blocks: monomers (m), dimers (d)
exTTF–oPPE–C60 (right) as viewed with Tramp 1.1d[38]
.
and trimers (t).
oPPE oligomers and C60–oPPE. exTTF exhibits the lowest
EA and an IP that matches the IPs of the oligomer building
blocks. With the addition of C60 to the oligomers the IP
drops to approximately the IP of C60–pyrrolidine, which sug-
gests strong electronic coupling between C60 and the bridge.
On the other hand, the electron-accepting features of the
fullerene are represented by its highest EA, which confirms
the electron-transfer pathway of the systems.
The HOMO/LUMO orbital schemes of the triads
(Figure 4) manifest this donor–acceptor character. The
HOMO is strongly localised on exTTF and reaches into the
oPPE bridge, which facilitates electron injection into the
bridge. However, in contrast to exTTF–oPPV–C60, in which
cant differences between the two systems can be seen when
comparing the conjugation in the bridge. In exTTF–oPPV–
C60, the local electron affinity is homogenously distributed
throughout the whole bridge, whereas in the oPPE systems
local maxima (red) can be found on the phenyl rings and
minima (yellow) on the triple bonds. This is due to the po-
larising character of the triple bonds and their shorter bond
lengths relative to the double bonds. Thus, the electron-
transfer pathway through the oPPE bridge is interrupted by
these ethynylene linkers. This strongly influences the
charge-separation process and explains the difference in
electron-transfer properties between exTTF–oPPE–C60 and
exTTF–oPPV–C60.
In summary, the electronic structure of all triads confirms
the donor–bridge–acceptor character of the exTTF–oPPE–
C60 systems and suggests that the HOMO!LUMO transi-
tion would represent a nearly complete charge-transfer exci-
tation with a very low extinction coefficient. To gain better
insight into these processes we carried out further calcula-
tions of the excited-state properties.
Quantum chemical modelling of excited states: To elucidate
the geometrical relaxation processes postulated on the basis
of the existence of very short-lived components in the time-
resolved spectroscopic techniques described below, we opti-
mised the minimum-energy geometries of the excited-states
in a vacuum. These singles-only configuration (CIS) calcula-
tions predict that the HOMO!LUMO transition makes a
major contribution to the charge-transfer (CT) state and
causes a large change in dipole moment (Table 4).
Figure 4. Representation of the HOMO (light grey) and LUMO (dark
grey) orbitals of exTTF–oPPE–C60 as calculated at the B3LYP/6-31G*
DFT level.
One-electron excitation from the HOMO to the LUMO
contributes to the CT state with zero oscillator strength (f)
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6385

N. Martín, D. M. Guldi et al.
Table 4. Excited-state properties predicted by quantum chemical calcula-
tions for compound 27c (top: CIS including six active orbitals; bottom:
CIS including ten active orbitals).
sect excited-state interactions with C60 in C60–oPPE and also
in exTTF–oPPE–C60. The major oPPE features include
strong visible-light fluorescence with quantum yields close
to unity (i.e., about 0.77). The fluorescence maxima, similar
to the absorption maxima, depend on the length of the oli-
gomer and reach from 400 to 490 nm in the case of the mo-
nomer and trimer, respectively.
State lex [nm]
f
Dm [D] Character Involved configuration
CIS=6
S1395
S2
0.3050.03 1 CT
HOMO
CT
CIS=10
!LUMO
385
0.000 136.9
HOMO!LUMO+1
Not surprisingly, when inspecting the oPPE fluorescence
in C60–oPPE under identical experimental conditions dra-
matic changes, which are as large as a 1500” reduction in
the fluorescence quantum yields, attest to an almost instan-
taneous deactivation of the quantitatively excited oPPE in
C60–oPPE. Nevertheless, it is important to note that the
fluorescence pattern of the oPPE is still preserved, despite
the presence of C60. In the near-infrared region of the fluo-
rescence spectrum the observed features resemble those
known for the C60 reference. Notably, photoexcitation at
355 nm directs the light nearly quantitatively to the oPPE
and not to C60. A reasonable explanation implies transduc-
tion of singlet excited-state energy from the oPPE (2.60 eV)
to C60 (1.76 eV). Independent confirmation for this hypothe-
sis was obtained from excitation spectra, in which the fluo-
rescence wavelength was kept constant at 715 nm and the
excitation wavelength was systematically varied. An excep-
tionally good agreement with the ground-state absorption
spectrum confirms that the C60 fluorescence evolves largely
from singlet energy transfer and, to a minor extent, from
direct excitation. Quantification of the energy-transfer reac-
tion was possible through comparing the C60 fluorescence
quantum yields in solutions of C60–oPPE in toluene with
that of the C60 reference as an internal reference under ex-
actly the same experimental conditions. Overall, the quan-
tum yield of C60 fluorescence amounts to 6.0”10ꢁ4—identi-
cal to that of a C60 reference that lacks oPPE—ruling out
any endothermic electron transfer (ꢁDGCS >ꢁ0.24 eV) be-
tween C60 (Ered: (0.61ꢂ0.02) V versus Ag/Ag+) and oPPE
(Eox: 2.09 (monomer 27a), 2.04 (dimer 27b) and 1.96 V
(trimer 27c) versus Ag/Ag+). In conclusion, on exciting the
oPPEs, a rapid intramolecular transduction of energy fun-
nels the excited-state energy to the fullerene core, generat-
S2
S6
S7
376
285
278
0.066
0.554
0.000 129.3
3.6
4.0
LE1HOMO
LE2
CT
ꢁ4!LUMO
HOMOꢁ4!LUMO+3
HOMO!LUMO,
HOMOꢁ2!LUMO
S8
277
0.00169.8
BCT
HOMO
ꢁ2!LUMO,
HOMOꢁ3!LUMO
(Table 4). In addition, in 27c two symmetric bridge charge-
transfer states (BCT) could be found close in energy to the
CT states with
a lower change in dipole moment
(69.8 debye). These states correspond to the local-electron-
affinity maxima centred on the oPPE rings, which underline
the utility of the local electron affinity at the surface as a
fast scanning method to elucidate possible CT states. These
bridge CT states can compete with the charge transfer from
exTTF to C60. The calculated excitation wavelengths (in so-
lution) for the CT states of the triads are found around l=
350 nm with a redshift that is dependent on the length of
the oligomer but independent of solvent polarity. Similar be-
haviour was found for the fluorescence maxima of 27a–c
with a solvent-independent fluorescence, which reaches a
maximum between l=450 and 500 nm and also depends on
the length of the oligomeric bridge. However, regarding the
fluorescence maximum of the BCT state of 27c at 380 nm
with a 261nm excitation in simulated diethyl ether, an over-
lap between the CT fluorescence that reaches a maximum at
450 nm is possible and depends on the quantum yield.
Scrutinising the electrostatic potential of the AM1-opti-
mised ground states and the corresponding CT states
mapped onto the molecular surface (Figure S1in the Sup-
porting Information) reveals that in all CT states of the
exTTF–oPPE–C60 triads the positive charge is localised on
exTTF (red) and the negative charge on C60 (blue). Alterna-
tively, in the BCT state the charge is localised on one of the
phenyl rings. The corresponding symmetric BCT state was
also found in the CI calculations. Further examination of
the excited states reveals a strong dependence of their
energy upon solvent polarity that follows the experimental
trends (see Supporting Information).
1
ing *C60 quantitatively. Excitation of C60, on the other hand,
1
leads directly to *C60.
In stark contrast to C60–oPPE and the C60 reference, the
fullerene fluorescence is appreciably quenched in exTTF–
oPPE–C60 (i.e., as low as 0.55”10ꢁ4 in THF). This is shown
in Figure 6. Moreover, the fluorescence quantum yields
depend strongly on the length of the oPPE bridge: 0-mer:
0.18”10ꢁ4; monomer 27a: 0.55”10ꢁ4; dimer 27b: 1.8”10ꢁ4.
No appreciable interactions were noted for the 3-mer. Set-
ting these quantum yields in relationship to the C60 refer-
ence and its lifetime, the C60 fluorescence deactivation rates
in the 0-mer, monomer 27a and dimer 27b were determined
as 2.1”10 10, 6.6”109 and 1.3”109 sꢁ1, respectively. Addition-
ally, we followed the fluorescence at 710 nm, but found only
measurable decay rates for the C60 reference (6.6”108 sꢁ1)
and the dimer 27b (2.6”109 sꢁ1). We must conclude at this
Photophysics: First, the steady-state fluorescence spectra of
the oPPE references C60–oPPE and exTTF–oPPE–C60 were
recorded upon 355 nm photoexcitation. For our photophysi-
cal 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 is directly linked to
the carbon atom of the pyrrolidine skeleton. In general, all
oPPE references fluoresce strongly throughout the visible
region, which renders this feature extremely valuable to dis-
1
point of the investigation that *C60, populated either direct-
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Chem. Eur. J. 2008, 14, 6379 – 6390

Molecular Wires
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Figure 6. Quenching of the C60 emission in exTTF–oPPE–C60 (27a, 27b)
upon excitation at 425 nm with matching optical density at 425 nm in
THF. c=C60 reference, g=exTTF–oPPE2–C60, b=exTTF–
oPPE1–C60.
ly or indirectly, powers an exothermic electron transfer
(ꢁDGCS ꢃ0.7 eV) to yield the radical-ion-pair state,
[exTTF +–oPPE–C60 ].
C
Cꢁ
The aforementioned hypothesis, namely, formation of the
radical-ion-pair state was corroborated through transient ab-
sorption spectroscopy. For oPPE, these results revealed the
nearly instantaneous generation of metastable singlet excit-
ed-state transients upon photoexcitation. The spectral char-
acteristics of these transients are ground-state bleaching in
the 400–450 nm range and new transient absorption in the
600–1200 nm range (Figure 7). The product of the aforemen-
tioned decay is the corresponding triplet excited state of
oPPE.
Figure 7. Top: differential absorption spectrum (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm) of solutions of ref-
erence trimer oPPE 16 in nitrogen-saturated THF with time delays be-
*
*
tween 0 and 3000 ps at room temperature (”=0, =1, and =2900 ps).
Bottom: time-absorption profile of the spectra shown above at 800 nm to
monitor the formation and decay of the singlet excited state of trimer
oPPE.
Upon photoexciting, C60–oPPE transient species initially
evolve that disclose features commonly seen in the oPPE
references, namely, transient bleach (i.e., below 450 nm) and
transient maxima (i.e., around 700 nm). Such an observation
is important because it attests—in close agreement with the
ground-state absorption at the 387 nm excitation wave-
length—the successful excitation of the oPPE moieties.
However, the features of the oPPE singlet excited state
decay much faster than those observed for the intersystem-
crossing (ISC) process in the oPPE references (Figure 8).
Typical rate constants for this decay are of the order of
ꢃ1012 sꢁ1. Such values confirm the quantitative quenching of
the oPPE fluorescence in C60–oPPE. At the conclusion of
the oPPE decay only the C60 singlet excited-state features
are discernable, that is, a transient maximum at 880 nm. On
a timescale of up to 3.0 ns the C60 singlet excited-state inter-
system crosses to the corresponding triplet manifold. Impor-
tantly, the kinetics of the singlet decay and the triplet
growth match each other reasonably well to yield ISC rates
in C60–oPPE of 6.5”108 sꢁ1. The most prominent feature of
the C60 triplet excited state is a 700 nm maximum as it
evolves towards the end of the timescale of our femtosecond
experiments (i.e., 3.0 ns). In complementary nanosecond ex-
periments with C60–oPPE the same triplet transient is seen,
which in the absence of molecular oxygen, decays with mul-
tiexponential kinetics.
The additional detection from results of exTTF–oPPE–C60
of the instantaneous grow-in of the 880 nm absorption af-
firms the successful C60 excitation. Instead of seeing, howev-
er, the slow ISC dynamics that C60–oPPE and C60 exhibit,
the singlet–singlet absorption decays in the presence of
exTTF donors with accelerated dynamics. The singlet excit-
ed-state lifetimes, as they were determined from an average
of first-order fits of the time-absorption profiles at various
wavelengths (850–950 nm), are listed in Table 5.
Spectroscopically, the transient absorption changes, taken
after the completion of the decay, bear no resemblance to
the C60 triplet excited state. Importantly, Figure 9 corrobo-
rates the spectral signatures of the one-electron oxidised
+
C
Cꢁ
exTTF and the one-electron reduced C60 , which were de-
tected as new transient maxima at 660 and 1010 nm, respec-
tively. The spectral identification holds for the 0-mer, 1-mer
and 2-mer, although for the 3-mer only a very broad transi-
ent, whose identity remains unknown to us at this stage,
dominates the region of interest. It is, however, clear that no
Chem. Eur. J. 2008, 14, 6379 – 6390
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6387

N. Martín, D. M. Guldi et al.
Figure 8. Top: differential absorption spectrum (visible and near-infrared)
obtained upon femtosecond flash photolysis (420 nm) of solutions of ref-
erence C60–oPPE 28 in nitrogen-saturated THF with time delays between
*
*
0
and 3000 ps at room temperature (”=0, =1, and =2900 ps).
*
)
Bottom: time-absorption profile of the spectra shown above at 690 (
*
and 900 nm ( ) to monitor the formation and decay of the singlet excited
state of C60.
Table 5. Photophysical properties of the exTTF–oPPE–C60 triads and the
fulleropyrrolidine.
[a]
Ffl
t1 [ns]
kCS [sꢁ1
]
kCR [sꢁ1
]
C60 reference
0-mer
monomer 27a
dimer 27b
trimer 27c
6.0”10ꢁ4
0.18”10ꢁ4
0.55”10ꢁ4
1.8”10ꢁ4
–
1.233
–
–
0.380
(0.793)
–
–
2.1”10 10
6.6”109
1.3”109
3.9”108
4.9”106
1.1”106
3.8”105
Figure 9. Top: differential absorption spectrum (visible and near-infrared)
obtained upon femtosecond flash photolysis (477 nm) of solutions of mo-
nomer 27a in nitrogen-saturated THF with time delays between 0 and
A
not detectable
[a] Singlet lifetime.
*
*
3000 ps at room temperature (”=0, =1, and =2900 ps). Middle: dif-
ferential absorption spectrum (visible and near-infrared) obtained upon
femtosecond flash photolysis (477 nm) of solutions of dimer 27b in nitro-
gen-saturated THF with several time delays between 0 and 3000 ps at
radical ion pair is formed for the 3-mer (see below). Both
Cꢁ
C
radical-ion-pair species, that is, C60 and exTTF +, decay in
the 0-mer (4.9”106 sꢁ1), monomer 27a (1.1”106 sꢁ1) and
dimer 27b (3.8”105 sꢁ1) with similar rates (ꢁDGCR ꢃ1.0 eV)
to reinstate the singlet ground states.
*
*
room temperature (”=0, =1, and =2900 ps). Bottom: time-absorp-
tion profile of the spectra shown above at 1010 nm to monitor the forma-
tion of the radical-ion-pair state (* =monomer 27a, * =dimer 27b).
Figure 10 documents how relating the charge-separation
and charge-recombination dynamics in THF to the electron
donor–acceptor separation (i.e., centre-to-centre: RCC
)
in perfect agreement with each other. In particular, (0.21ꢂ
allows the evaluation of the attenuation factor (b) of the
oPPE bridges. From both relationships, which reveal linear
dependences, attenuation factors (b) were derived that are
0.05) Šꢁ1 was determined for the charge separation, whereas
ꢁ1
(0.2ꢂ0.05) Š
evolved for the charge recombination.
Please note that these values are factors of 20 and 2 higher
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Molecular Wires
FULL PAPER
dynamics reveal a wirelike behaviour with an attenuation
factor (b) of (0.2ꢂ0.05) Šꢁ1.
Acknowledgements
This work was supported by the MEC of Spain (project CT2005-02609/
BQU and Consolider-Ingenio 2010C-07-25200), the CAM (project P-
PPQ-000225-0505), the Deutsche Forschungsgemeinschaft (SFB 583),
FCI and the Office of Basic Energy Sciences of the U.S. Department of
Energy.
[1] a) F. L. Carter, Molecular Electronic Devices, Dekker, New York,
1987; b) J. Jortner, M. A. Ratner, Molecular Electronics, Blackwell,
London, 1997; c) Photoinduced Electron Transfer (Eds.: M. A. Fox,
M. Chanon), Elsevier, Amsterdam, 1998; d) R. L. Carrol, C. B.
Gorman, Angew. Chem. 2002, 114, 4556; Angew. Chem. Int. Ed.
2002, 41, 4378; e) Electron Transfer in Chemistry (Ed.: V. Balzani),
Wiley-VCH, Weinheim, 2001; f) W. W. Parson, Photosynth. Res.
2003, 76, 81–92; g) The Special Issue on Organic Electronics:
Chem. Mater. 2004, 16, 4381–4846; h) The Special Issue on Molecu-
lar Wires and Electronics: Top. Curr. Chem. 2005, 257, 1, 33–62.
[2] a) R. H. Goldsmith, L. E. Sinks, R. F. Kelley, L. J. Betzen, W. Liu,
E. A. Weiss, M. A. Ratner, M. R. Wasielewski, Proc. Natl. Acad. Sci.
U.S.A. 2005, 102, 3540–3545; b) E. A. Weiss, M. J. Tauber, R. F.
Kelley, M. J. Ahrens, M. A. Ratner, M. R. Wasielewski, J. Am.
Chem. Soc. 2005, 127, 11842–11850; c) M. J. Tauber, R. F. Kelley,
J. M. Giaimo, B. Rybtchinski, M. R. Wasielewski, J. Am. Chem. Soc.
2006, 128, 1782–1783; d) R. H. Goldsmith, M. R. Wasielewski,
M. A. Ratner, J. Phys. Chem. A 2006, 110, 20258–20262.
[3] a) K. Müllen, G. Wegner, Electronic Materials: The Oligomer Ap-
proach, Wiley-VCH, Weinheim, Germany, 1998.
Figure 10. Centre-to-centre distances (RCC) dependence of charge-separa-
tion (lnkCS) and charge-recombination (lnkCR) rate constants in exTTF–
oPPE–C60 in nitrogen-saturated THF at room temperature. The slope
represents b. The dashed line represents the singlet lifetime of C60.
than what we have established earlier for oPPV bridges
(i.e., (0.01ꢂ0.005) Šꢁ1)[13,14] and oligofluorene [oFl] bridges
(i.e., (0.09ꢂ0.005) Šꢁ1),[39] respectively.
It is interesting to note that by extrapolating the linear re-
lationship in Figure 10 to the trimer 27c a charge-separation
rate would be obtained that would, in fact, be slower than
the singlet lifetime of C60 (i.e., dashed line). This, in turn,
helps to rationalise the lack of C60 fluorescence quenching
in the trimer 27c.
[4] a) J. F. Nierengarten, J. F. Eckert, J. F. Nicoud, L. Ovali, V. Krasni-
kov, G. Hadziioannou, Chem. Commun. 1999, 617–618; b) D. M.
Guldi, C. Luo, A. Swart, R. Gómez, J. L. Segura, N. Martín, C.
Brabec, N. Saricifti, J. Org. Chem. 2002, 67, 1141–1152; c) J. L.
Segura, N. Martín, D. M. Guldi, Chem. Soc. Rev. 2005, 34, 31 –47;
d) T. M. Figueira-Duarte, A. GØgout, J. F. Nierengarten, Chem.
Commun. 2007, 109–119.
[5] a) E. Peeters, P. A. van Hal, J. Knol, C. J. Brabec, N. S. Sariciftci,
J. C. Hummelen, R. A. J. Janssen, J. Phys. Chem. B 2000, 104,
10174–10190; b) T. Gu, D. Tsamouras, C. Melzer, V. Krasnikov, J. P.
Gisselbrecht, M. Gross, G. Hadziioannou, J. F. Nierengarten, Chem-
PhysChem 2002, 124–127.
[6] a) K. G. Thomas, V. Biju, D. M. Guldi, P. V. Kamat, M. V. George, J.
Phys. Chem. B 1999, 103, 8864–8869; b) D. M. Guldi, A. Swartz,
Ch. Luo, R. Gomez, J. L. Segura, N. Martín, J. Am. Chem. Soc.
2002, 124, 10875–10886.
[7] a) N. P. Redmore, I. V. Rubstov, M. J. Therien, J. Am. Chem. Soc.
2003, 125, 8769; b) T. O. Screen, J. R. G. Thorne, R. G. Denning,
D. G. Bucknall, H. L. Anderson, J. Mater. Chem. 2003, 13, 2796–
2808.
Conclusion
We have successfully developed the synthesis of novel
donor–bridge–acceptor (exTTF–oPPE–C60) systems based
on oligo(p-phenyleneethynylene) (oPPE) as a bridge. The
combination of the bromo–iodo selectivity in the Hagihara–
Sonogashira reaction and the selective deprotection of
TMS-protected acetylene in the presence of a TIPS-protect-
ed acetylene is a powerful tool for the preparation of asym-
metric oPPEs. A further cross-coupling reaction in a conver-
gent manner leads to the respective dyads, endowed with an
aldehyde group and an exTTF unit, which undergo a 1,3-di-
polar cycloaddition reaction with C60 to afford the final mo-
lecular arrays 27a–c.
The electrochemical study reveals amphoteric redox be-
haviour and a lack of significant electronic communication
between the donor (exTTF) and the acceptor (C60) moieties
through the p-conjugated oligomer in the ground state.
However, quantum chemical calculations disclose the
donor–bridge–acceptor character of the exTTF–oPPE–C60
systems and suggest that the HOMO!LUMO transition
would represent a nearly complete charge-transfer excita-
tion with a very low extinction coefficient. Photoexcitation
is followed by a rapid intramolecular charge separation to
generate a radical-ion-pair state (exTTF and C60 at 660 and
1000 nm, respectively). The radical ion pairs decay in 4.9”
106 (27a) and 1.1”106 sꢁ1 (27b). The charge-recombination
[8] T.-Y. Dong, S.-W. Chang, S.-F. Kin, M.-C. Lin, Y.-S. Wen, L. Lee, Or-
ganometallics 2006, 25, 2018–2024.
[9] a) N. Martín, L. Sµnchez, B. Illescas, I. PØrez, Chem. Rev. 1998, 98,
2527–2548; b) D. M. Guldi, N. Martín, J. Mater. Chem. 2002, 12,
1978–1992; c) Fullerenes: From Synthesis to Optoelectronic Proper-
ties (Eds.: D. M. Guldi, N. Martín), Kluwer Academic Publishers,
Dordrecht, 2002; d) F. Giacalone, N. Martín, Chem. Rev. 2006, 106,
5136–5190.
[10] W. B. Davis, W. A. Svec, M. A. Ratner, M. R. Wasielewski, Nature
1998, 396, 60–63.
[11] G. Pourtis, D. Beljonne, J. Cornil, M. A. Ratner, J. L. BrØdas, J. Am.
Chem. Soc. 2002, 124, 4436–4447.
[12] E. A. Weiss, M. J. Ahrens, L. W. Sinks, A. V. Gusev, M. A. Ratner,
M. R. Wasielewski, J. Am. Chem. Soc. 2004, 126, 5577–5584.
Chem. Eur. J. 2008, 14, 6379 – 6390
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6389

N. Martín, D. M. Guldi et al.
[13] F. Giacalone, J. L. Segura, N. Martín, D. M. Guldi, J. Am. Chem.
Soc. 2004, 126, 5340–5341.
[28] a) M. Prato, M. Maggini, Acc. Chem. Res. 1998, 31, 519–526; b) N.
Tagmatarchis, M. Prato, Synlett 2003, 768–779.
[14] a) G. de la Torre, F. Giacalone, J. L. Segura, N. Martín, D. M. Guldi,
Chem. Eur. J. 2005, 11, 1267–1280; b) F. Giacalone, J. L. Segura, N.
Martín, D. M. Guldi, Chem. Eur. J. 2005, 11, 4819–4834.
[15] For a preliminary communication, see: C. Atienza, N. Martín, M.
Wielopolski, N. Haworth, T. Clark, D. M. Guldi, Chem. Commun.
2006, 30, 3202–3204. See also: a) C. Wang, A. S. Batsanov, M. R.
Bryce, I. Sage, Org. Lett. 2004, 6, 2181–2184; b) Y. Shirai, Y. Zhao,
L. Cheng, J. M. Tour, Org. Lett. 2004, 6, 2129–2132; c) J.-F. Nieren-
garten, T. Gu, G. Hadziioannou, V. Lrasnikov, Helv. Chim. Acta
2004, 87, 2804–2805; d) W. Hu, H. Nakashima, K. Furukawa, Y. Ka-
shimura, K. Ajito, Y. Liu, D. Zhu, K. Torimitsu, J. Am. Chem. Soc.
2005, 127, 2804–2805; e) C. Wang, L.-O. Palsson, A. S. Batsanov,
M. R. Bryce, J. Am. Chem. Soc. 2006, 128, 3789–3799.
[16] a) K. Sonogashira in Comprehensive Organic Synthesis (Eds.: B. M.
Trost, I. Fleming), Pergamon Press, Oxford, 1991; b) K. Sonogashira
in Metal-Catalyzed Cross-Coupling Reactions (Eds.: F. Diederich,
P. J. Stang), Wiley-VCH, Weinheim, 1998, 203; c) K. Sonogashira, J.
Organomet. Chem. 2002, 653, 46–49; d) S. Eisler, N. Chahal, R.
McDonald, R. R. Tykwinski, Chem. Eur. J. 2003, 9, 2542–2550;
e) W. Mohr, J. Stahl, F. Hampel, J. A. Galysz, Chem. Eur. J. 2003, 9,
3324–3340; f) S. Eisler, A. D. Slepkov, E. Elliott, T. Luu, R. McDo-
nald, F. A. Hegmann, R. R. Tykwinski, J. Am. Chem. Soc. 2005, 127,
2666–2676.
[17] a) K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett. 1975,
16, 4467–4470; b) Y. Tohda, K. Sonogashira, N. Hagihara, Synthesis
1977, 777–778.
[18] a) R. Diercks, K. P. C. Vollhardt, Angew. Chem. 1986, 98, 268–270;
Angew. Chem. Int. Ed. Engl. 1986, 25, 266–268; b) R. D. Bradshaw,
L. Guo, C. A. Tessier, W. G. Youngs, Organometallics 1996, 15,
2582–2584.
[19] a) Q. Zhou, P. J. Carroll, T. M. Swager, J. Org. Chem. 1994, 59,
1294–1301; b) S. Hçger, A.-D. Meckenstock, S. Müller, Chem. Eur.
J. 1998, 4, 2423–2434; c) M. Mayor, C. Didschies Angew. Chem.
2003, 115, 3284–3287; Angew. Chem. Int. Ed. 2003, 42, 3176–3179;
Angew. Chem. Int. Ed. 2003, 42, 3176–3179; d) F. Maya, J. M. Tour,
Tetrahedron 2004, 60, 81–92.
[20] a) H. Meier, D. Ickenroth, U. Stalmach, K. Koynov, A. Bathiar, C.
Bubeck, Eur. J. Org. Chem. 2001, 23, 4431–4443; b) D. Ickenroth, S.
Weissmann, N. Rumpf, H. Meier, Eur. J. Org. Chem. 2002, 16, 2808–
2814.
[29] a) M. C. Diaz, M. A. Herranz, B. M. Illescas, N. Martín, N. Godbert,
M. R. Bryce, C. Luo, A. Swartz, G. Anderson, D. M. Guldi, J. Org.
Chem. 2003, 68, 7711–7721; b) B. Insuasty, C. Atienza, C. Seoane,
N. Martín, J. Garin, J. Orduna, R. Alcala, B. Villacampa, J. Org.
Chem. 2004, 69, 6986–6995.
[30] L. Echegoyen, L. E. Echegoyen, Acc. Chem. Res. 1998, 31, 593–601.
[31] a) M. R. Bryce, A. J. Moore, J. Chem. Soc. Perkin Trans. 1 1991,
157–168; b) M. R. Bryce, A. J. Moore, J. Chem. Soc. Chem.
Commun. 1991, 22, 1638–1639.
[32] a) A. S. Batsanov, M. R. Bryce, M. A. Coffin, A. Green, R. E.
Hester, J. A. K. Howard, I. K. Lednev, N. Martín, A. J. Moore, J. N.
Moore, E. Ortí, L. Sµnchez, M. Savirón, P. M. Viruela, R. Viruela, T.
Ye, Chem. Eur. J. 1998, 4, 2580; b) I. PØrez, S.-C. Liu, N. Martín, L.
Echegoyen, J. Org. Chem. 2000, 65, 3796–3803; c) N. Martín, E.
Ortí in Handbook of Advanced Electronic and Photonic Materials
and Devices, Vol. 3 (Ed.: H. S. Nalwa), Academic Press, New York,
2001, Chapter 6.
[33] a) D. M. Guldi, L. SanchØz, N. Martín, J. Phys. Chem. B 2001, 105,
7139–7144; b) A. E. Jones, C. A. Cristensen, D. F. Perepichka, A. S.
Batsanov, A. Beeby, P. J. Low, M. R. Bryce, A. W. Parkle, Chem.
Eur. J. 2001, 7, 973–978; c) C. Wang, L.-O. Palsson, A. S. Batsanov,
M. R. Bryce, J. Am. Chem. Soc. 2006, 128, 3789–3799.
[34] a) J. P. Stewart in Reviews in Computational Chemistry, Vol. 3 (Eds.:
K. B. Lipkowitz, D. B. Boyd), VCH, New York, 1991, p. 45–48;
b) N. T. Anh, G. Frison, A. Solladie-Cavallo, M. Patrick, Tetrahedron
1998, 54, 12841–12849; c) P. Burk, K. Herodes, I. Koppel, Int. J.
Quantum Chem. Symp. 1993, 27, 633; d) M. J. S. Dewar, K. M.
Dieter, J. Am. Chem. Soc. 1986, 108, 8075; e) Rotations in the mo-
nomer, dimer and trimer do not seem influence the electronic struc-
ture and the coupling between donor and acceptor. The rotational
barriers comply with these found in the exTTF–oPPV–C60 triads
(0.34 kcalmolꢁ1).
[35] Gaussian 03 (Revision D.02), M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomer-
y, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyen-
gar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[21] a) K. Sonogashira, Comprehensive Organic Synthesis, Vol. 3, Perga-
mon, Oxford, 1990; b) Q. Zhou, P. J. Carroll, T. M. Swager, J. Org.
Chem. 1994, 59, 1294–1301; c) Modern Acetylene Chemistry (Eds.:
P. J. Stang, F. Diederich), VCH, Weinheim, 1995; d) K. Sonogashira,
J. Organomet. Chem. 2002, 653, 46–49.
[22] D. B. Wertz, R. Gleiter, F. Rominger, J. Org. Chem. 2004, 69, 2945–
2952.
[23] M. Ravikanth, J.-P. Strachan, F. Li, J. S. Lindsey, Tetrahedron 1998,
54, 7721–7734.
[24] a) S. Kawabata, N. Tanabe, A. Osuka, Chem. Lett. 1994, 101, 1797–
1800; b) A. Osuka, N. Tanabe, S. Kawabata, I. Yamazaki, Y. Nishi-
mura, J. Org. Chem. 1995, 60, 7177–7185; c) D. A. M. Egbe, C. Ul-
bricht, T. Orgis, B. Carbonnier, T. Kietzke, M. Peip, M. Metzner, M.
Gericke, E. Birckner, T. Pakula, D. Neher, U.-W. Grummt, Chem.
Mater. 2005, 17, 6022–6032.
[36] T. Clark, A. Alex, B. Beck, F. Burkhardt, J. Chandrasekhar, P.
Gedeck, A. Horn, M. Hutter, B. Martin, G. Rauhut, W. Sauer, T.
Schindler, T. Steinke, VAMP 10.0, Erlangen (Germany), 2003.
[37] B. Ehresmann, B. Martin, A. H. C. Horn, T. Clark, J. Mol. Model.
2003, 9, 342–347.
[25] F. W. Hartner, Y. Hsiao, K. K. Eng, N. R. Rivera, M. Palucki, L.
Tan, N. Yasuda, D. L. Hughes, S. Weissman, D. Zewge, T. King, D.
Tschaen, R. P. Volante, J. Org. Chem. 2004, 69, 8723–8730.
[26] D. V. Kosynkin, J. M. Tour, Org. Lett. 2001, 3, 993–995.
[27] a) R. W. Higgins, C. L. Hilton, S. D. Deochar, J. Org. Chem. 1951,
16, 1275–1277; b) H. O. House, D. G. Koepsell, W. J. Campbell, J.
Org. Chem. 1972, 37, 1003–1011; c) M. C. Díaz, B. M. Illescas, C.
Seoane, N. Martín, J. Org. Chem. 2004, 69, 4492–4499.
[38] H. Lanig, R. Koenig, T. Clark, Tramp 1.1d, 2005.
[39] C. Atienza-Castellanos, M. Wielopolski, D. M. Guldi, C. van der
Por, M. R. Bryce, S. Filippone, N. Martín, Chem. Commun. 2007, 48,
5164–5166.
Received: January 25, 2008
Published online: June 13, 2008
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