A fully conjugated TTF-π-TCAQ system: Synthesis, structure, and electronic properties

Article abstract of DOI:10.1002/chem.201002674

The synthesis of the first fully conjugated tetrathiafulvalene-tetracyano- p-quinodimethane ((TTF)-TCNQ)-type system has been carried out by means of a Julia-Kocienski olefination reaction. In particular, a tetracyanoanthraquinodimethane (TCAQ) formyl der

Full text of DOI:10.1002/chem.201002674

FULL PAPER
DOI: 10.1002/chem.201002674
A Fully Conjugated TTF–p–TCAQ System: Synthesis, Structure, and
AHCTUNGERTGENNUN lectronic Properties**
Josꢀ Santos,[a] Beatriz M. Illescas,[a] Nazario Martꢁn,*[a, b] Javier Adrio,[c]
Juan C. Carretero,*[c] Rafael Viruela,[d] Enrique Ortꢁ,*[d] Fabian Spꢂnig,[e] and
Dirk M. Guldi*[e]
Abstract: The synthesis of the first
fully conjugated tetrathiafulvalene–
tetraACHTUNGTRENNUNGcyano-p-quinodimethane ((TTF)–
at the B3LYP/6-31G** level point to
highly distorted exTTF and TCAQ that
form an almost planar stilbene unit be-
tween them. Although calculations pre-
dicted appreciable electronic communi-
cation between the donor and the ac-
ceptor, cyclic voltammetric studies did
not substantiate this effect. It was only
in photophysical assays that the elec-
tronic communication emerged in the
form of a charge-transfer (CT) absorp-
tion and emission. Once photoexcited
(i.e., the locally excited state or excited
charge-transfer state), an ultrafast, sub-
picosecond charge separation leads to
a radical ion pair state in which the
spectroscopic features of the radical
cation of exTTF as well as the radical
anion of TCAQ are discernable. The
radical ion pair is metastable and un-
dergoes a fast ((1.0ꢀ0.2) ps) charge re-
combination to reconstitute the elec-
tronic ground state. Such ultrafast
charge separation and recombination
processes come as a consequence of
the very short vinyl linkage between
the two electroactive units.
TCNQ)-type system has been carried
out by means of a Julia–Kocienski ole-
fination reaction. In particular, a tetra-
cyanoanthraquinodimethane (TCAQ)
formyl derivative and two new sulfo-
nylmethyl-exTTFs (exTTF=2-[9-(1,3-
dithiol-2-ylidene)anthracen-10(9H)-yli-
dene]-1,3-dithiole)—prepared as new
building blocks—were linked. A variety
of experimental conditions reveal that
the use of sodium hexamethyldisila-
zane (NaHMDS) as base in THF af-
forded the E olefins with excellent ste-
reoselectivity. Theoretical calculations
Keywords: charge transfer · conju-
gation · donor–acceptor systems ·
olefination · tetrathiafulvalene
Introduction
[a] Dr. J. Santos, Dr. B. M. Illescas, Prof. Dr. N. Martꢀn
Departamento de Quꢀmica Orgꢁnica
Facultad de Ciencias Quꢀmicas, Universidad Complutense
Ciudad Universitaria, 28040 Madrid (Spain)
Fax : (+34)91-394-4103
Electron donors (D) and electron acceptors (A) have been
studied in chemistry for many years. A particularly impor-
tant aspect focuses on electronic interactions, either inter- or
intramolecular. To this end, intermolecular interactions be-
tween D and A have resulted in the development of charge-
transfer (CT) complexes and/or salts that exhibit electrically
conducting, superconducting, and magnetic properties.[1] The
mode in which the two electroactive moieties interact in co-
valently connected D–bridge–A systems strongly depends
on the type of connectivity that exists between them.[2] D–
s–A systems, for example, in which the linkage between D
and A is given by a bridge of sigma bonds, have received a
great deal of attention. It was in 1974 that Aviram and
Ratner postulated that a single molecule such as 1 (TTF–s–
TCNQ (TCNQ=tetracyano-p-quinodimethane); Scheme 1)
E-mail: nazmar@quim.ucm.es
[b] Prof. Dr. N. Martꢀn
IMDEA-Nanociencia, Facultad de Ciencias
Universidad Autꢂnoma, Cantoblanco, 28049 Madrid (Spain)
[c] Dr. J. Adrio, Prof. Dr. J. C. Carretero
Departamento de Quꢀmica Orgꢁnica, Facultad de Ciencias
Universidad Autꢂnoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax : (+34)914973966
E-mail: juancarlos.carretero@uam.es
[d] Dr. R. Viruela, Prof. Dr. E. Ortꢀ
Instituto de Ciencia Molecular, Universidad de Valencia
46980 Paterna (Spain)
Fax : (+34)963543274
E-mail: enrique.orti@uv.es
[e] Dr. F. Spꢃnig, Prof. Dr. D. M. Guldi
Friedrich-Alexander-Universitꢃt Erlangen-Nꢄrnberg
Department of Chemistry and Pharmacy
& Interdisciplinary Center for Molecular Materials (ICMM)
Egerlandstrasse 3, 91058 Erlangen (Germany)
Fax : (+49)9131-852-8307
E-mail: dirk.guldi@chemie.uni-erlangen.de
[**] TTF=tetrathiafulvalene. TCAQ=tetracyanoanthraquinodimethane.
Scheme 1. Chemical structure of the proposed Aviram–Ratner rectifier
(1) formed by TTF (tetrathiafulvalene) and TCNQ (tetracyano-p-quino-
dimethane) units covalently connected through sigma bonds.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002674.
Chem. Eur. J. 2011, 17, 2957 – 2964
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2957

could rectify the electrical current.[3] In fact, this seminal
paper has been considered to be a cornerstone of molecular
electronics.[4] D–s–A-type systems have also been studied as
artificial photosynthetic mimics. Here, photoexcitation is the
inception of an electron transfer from the electron donor to
the electron acceptor to afford a charge-separated state. The
lifetime of the latter is essential for photovoltaic applica-
tions.[5] Finally, the implications of D–s–A systems on the
field of organic molecular wires should be mentioned.[6]
Push–pull chromophores, in which p-conjugated bridges
link D and A (D–p–A), constitute an important class of
compounds. It is their properties as second- and third-order
nonlinear optical (NLO) materials that stand out among
many other physicochemical properties.[7] The possibility of
tuning their optical and electrochemical band gaps renders
D–p–A systems as appealing components for future optical-
processing device applications.[8]
ners in the subsequent Julia–Kocienski reaction with the
formyl-containing TCAQ. However, whereas the formyl-
TCAQ is readily available following the synthetic protocol
previously reported in our group,[12] the preparation of the
sulfones 5 proved to be more challenging and required us to
test different synthetic approaches.
The first attempt to obtain 5a was carried out from the
previously reported compound 2-bromomethyl-exTTF (2;
Scheme 2).[13] Reaction of 2 with 1-phenyl-1H-tetrazole-5-
Among the different electron-donor units used for the
preparation of D–p–A molecules that show NLO responses,
1,3-dithiole derivatives have been widely used since the pio-
neering work by the groups of Katz and Lehn.[9] More re-
cently, some of us introduced TTF as a donor in novel NLO
materials.[10] Similarly, p-extended TTF (2-[9-(1,3-dithiol-2-
ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole (exTTF)) has
also been used as an electroactive component in materials
that exhibit second-order NLO features.[11]
Scheme 2. Synthesis of sulfenylmethyl-exTTF 4, and failed attempt to
form sulfonylmethyl-exTTF 5a.
In the present paper, we report on a new family of D–p–
A systems in which exTTF and tetracyanoanthraquinodime-
thane (TCAQ) are used as D and A, respectively. To the
best of our knowledge, this is the first example in which
exTTF and TCAQ derivatives are covalently connected
through a fully p-conjugated bridge. The preparation of
exTTF–p–TCAQ was a synthetic challenge, because no
complementary exTTF and TCAQ derivatives endowed
with suitable groups for further olefination reactions have
so far been available. Theoretical calculations have been car-
ried out at the density functional theory (DFT) level to
gather information about the geometrical and electronic
properties of the new push–pull exTTF–p–TCAQ system.
As a complement, several physicochemical assays were con-
ducted by means of electrochemistry and photophysics to
confirm the electronic communication between D and A in
the electronic ground state as well as in the electronic excit-
ed state.
thiol (3) in THF under Mitsunobu-type reaction conditions
(PPh3/diethyl azodicarboxylate (DEAD)) led efficiently to
sulfenylmethyl-exTTF 4. However, the seemingly straight-
forward oxidation of 4 to the desired sulfone 5a suffered
from extensive decomposition regardless of the oxidant and
reaction conditions. This experimental finding could be as-
cribed to the strong reducing character of the exTTF
moiety, which, in the presence of an oxidant such as meta-
chloroperbenzoic acid (mCPBA), would undergo an initial
electron-transfer process with the formation of ion radical
intermediates that would finally provide the observed com-
plex crude reaction mixture.
The synthesis of sufonylmethyl-exTTFs 5 was successfully
achieved following an alternative synthetic strategy in which
the TTF units were introduced after the thioether/sulfone
oxidation (Scheme 3). Thus, the straightforward alkylation
of thiol 3 with 2-bromomethylanthraquinone (6) gave the
sulfenylmethylanthraquinone derivative 7, which was later
oxidized to sulfone 8 by using mCPBA (64% overall yield).
The further Wadsworth–Emmons olefination reaction of the
anthraquinone (AQ) derivative 8 with phosphonates 9a,b to
provide the desired sufonylmethyl-exTTFs 5 proved to be
highly dependent on the base and reaction conditions (see
Table S1 in the Supporting Information). The best results
were obtained by employing a great excess amount of phos-
phonate and lithium diisopropylamide (LDA; 6 equiv each)
in THF. Under these conditions, compounds 5a,b could be
isolated in moderate yields after final silica gel chromato-
Results and Discussion
Synthesis and characterization: The synthesis of the highly
functionalized, fully conjugated target D–p–A 11 was not
straightforward and required the previous preparation of
the respective D and A fragments endowed with the appro-
priate functional groups to carry out the final olefination re-
action under mild reaction conditions, compatible with the
existing sensitive functionalization. For this purpose, we de-
cided to prepare the new type of sulfonylmethyl-exTTF
compounds (5) as potential highly reactive olefination part-
2958
www.chemeurj.org
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2957 – 2964

TTF–p–TCAQ System
FULL PAPER
Scheme 3. Synthesis of sulfonylmethyl-exTTFs 5a,b.
Scheme 4. Synthesis of reference compound 10 and exTTF–p–TCAQ 11
by Julia–Kocienski olefination reaction.
graphic purification (42 and 32% yield, respectively). At
least in part, the difficulty of this reaction probably stems
from the high acidity of the benzylic hydrogen atoms in a
position to the sulfone functionality.
moieties. In addition, the vinyl protons appear at d=
7.20 ppm (J=16.2 Hz), thus confirming the E stereochemis-
try (see the Supporting Information).
All new compounds were fully characterized on the basis
1
of analytic and spectroscopic methods. The H NMR spectra
of sulfonylmethyl-exTTFs 5a,b show, in addition to the aro-
matic protons, the presence of the exTTF moiety as two sin-
glets around d=6.3 ppm for 5a and the four SMe groups at
around d=2.40 ppm for 5b. The methylene group in a to
the sulfone functionality appears as an AB system for 5a (d
ꢁ4.9 ppm, J=10.5 Hz) and as a singlet at dꢁ5.0 ppm for
5b. The chemical structure of compounds 5a,b was ascer-
tained by means of mass spectrometry, which showed the
molecular ions at 602.0 and 786.0, respectively.
1-Phenyl-1H-tetrazol-5-yl sulfones 5a,b are suitably func-
tionalized for further Julia–Kocienski olefinations by reac-
tion with formyl-containing compounds. This reaction leads
to E olefins with excellent stereoselectivity when benzylic
sulfones and aromatic aldehydes are employed.[14] To check
the performance of compounds 5a,b in the Julia-Kocienski
reaction, styryl-exTTF 10 was first synthesized by reaction
of 5a with benzaldehyde by employing sodium hexamethyl-
disilazane (NaHMDS) as base (THF, ꢂ788C to RT; 71%
yield).
Molecular and electronic structures: The molecular geome-
try of compound 11 was theoretically optimized starting
from different geometrical conformations. The tetra-
AHCTUNGTREGmNNUN ethylthio-exTTF (TTM-exTTF) and TCAQ molecules of
which 11 is comprised were also calculated as reference
compounds. B3LYP/6-31G** calculations predicted that the
most stable conformation of compound 11 corresponds to
that depicted in Figure 1, in which the exTTF and TCAQ
moieties are folded up and down, respectively. The confor-
mation in which both moieties are folded in the same direc-
tion was found to be basically isoenergetic and differed less
than 0.2 kcalmolꢂ1. Other possible conformations that re-
ꢂ
sulted from the internal rotation around the C C single
bonds that link the ethylene bridge to the exTTF and
TCAQ units were calculated within 2.0 kcalmolꢂ1.
Next, we tested the Julia–Kocienski reaction in the syn-
thesis of the desired exTTF–p–TCAQ derivative 11
(Scheme 4). Previous attempts in our group to prepare this
type of compound, either by means of Wittig olefination or
Heck coupling reaction between appropriately functional-
ized exTTF and TCAQ derivatives, were unsuccessful, likely
due to the rapid formation of the charge-transfer complex
formed between the two strong electroactive moieties. Grat-
ifyingly, when we carried out the Julia–Kocienski reaction of
5b with TCAQ-CHO under the mild reaction conditions de-
veloped for the model alkene 10 (NaHMDS, THF, ꢂ788C),
exTTF–p–TCAQ 11 was obtained as a black solid in almost
quantitative yield (94%).
1H and 13C NMR spectra of compound 11 show the ex-
pected signals that correspond to both exTTF and TCAQ
Figure 1. Minimum-energy conformation calculated for 11 at the B3LYP/
6-31G** level.
Chem. Eur. J. 2011, 17, 2957 – 2964
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2959

D. M. Guldi et al.
The exTTF unit in compound 11 adopts a butterfly- or
saddlelike structure, similar to that calculated for unsubsti-
tuted exTTF,[15,16] and consistent with the crystal structures
observed for different exTTF derivatives.[17–19] To relieve the
short contacts between the sulfur atoms and the hydrogen
atoms in the peri positions, the central ring of the anthra-
ꢂ
cene unit folds in a boat conformation along the C9 C10
vector by an average angle of 38.48. This value compares
well to the X-ray value experimentally found for the TTM-
exTTF molecule (388),[17] for which an almost identical value
(38.38) is theoretically calculated. The dithiole rings are
tilted by 33.98 with respect to the plane defined by the an-
thracene atoms C11-C12-C13-C14. Furthermore, the dithiole
ꢂ
rings are folded inward by 18.78 along the S S axes.
The TCAQ moiety in compound 11 also adopts a saddle-
like structure to alleviate the steric contacts between the
cyano groups and the peri hydrogen atoms. The anthracene
ꢂ
unit is folded up along the C9 C10 vector by 35.68, and the
C(CN)2 units are tilted down by an average angle of 30.98
with respect to the C11-C12-C13-C14 plane. These folding
angles are in very good correlation with the X-ray values re-
ported for the TCAQ molecule (33.9 and 30.48, respective-
ly).[20]
The lateral benzene rings of the anthracene unit in both
the exTTF and the TCAQ moieties present an aromatic
ꢂ
structure because all the C C bonds have a length of
(1.40ꢀ0.01) ꢆ. It is important to note that the benzene
rings in the central part of the molecule are conjugated
through the ethylene bridge and form an almost planar
trans-stilbene unit (see Figure 1).
Figure 2. Electron-density contours (0.03 ebohrꢂ3) and orbital energies
calculated for the HOMOs and LUMOs of 11 at the B3LYP/6-31G**
level. H and L denote HOMO and LUMO, respectively.
Figure 2 shows the atomic orbital (AO) composition of
the highest-occupied (HOMOꢂ2 to HOMO) and lowest-un-
occupied (LUMO to LUMO+2) molecular orbitals of 11.
The HOMO (ꢂ5.10 eV) and HOMOꢂ1 (ꢂ5.63 eV) are lo-
calized on the electron-donor exTTF unit and are calculated
at slightly lower energies than the HOMO (ꢂ4.90 eV) and
HOMOꢂ1 (ꢂ5.43 eV) of the TTM-exTTF molecule. In con-
trast, the LUMO (ꢂ3.64 eV) and LUMO+1 (ꢂ2.71 eV)
spread over the electron-acceptor TCAQ unit and are ob-
tained at slightly higher energies than the LUMO
(ꢂ3.75 eV) and LUMO+1 (ꢂ2.80 eV) of the TCAQ mole-
cule. Therefore, compound 11 presents a small HOMO–
LUMO energy gap of 1.46 eV, and low-energy charge-trans-
fer absorption bands are to be expected in the electronic
spectrum. The electronic communication between the donor
and acceptor units in 11 is evidenced by the shift to lower
energies of the HOMOs and to higher energies of the
LUMOs and by the charge transfer of 0.10 e that takes
place from the exTTF-ethylene environment to the TCAQ
moiety in the electronic ground state. It is also to be noted
that the HOMOꢂ2 and the LUMO+2 are mainly localized
over the central stilbene unit, and their AO compositions
are indeed similar to those calculated for the HOMO and
LUMO of stilbene.
pared with those of TTM-exTTF and TCAQ (Figure 3 and
Table S2 in the Supporting Information). The measurements
were carried out in dichloromethane as solvent, using a
glassy carbon working electrode, a standard Ag/AgNO3 ref-
erence electrode, and tetrabutylammonium perchlorate
(0.1m) as supporting electrolyte.
Both exTTF and TCAQ undergo, in contrast to related
TTF and TCNQ, single two-electron redox processes that
lead to the corresponding dication (exTTF2+) and dianion
Figure 3. Cyclic voltammograms of 11 (c), TTM-exTTF (a), and
TCAQ (d) recorded in CH2Cl2 at 100 mVsꢂ1 containing 0.1m
nBu4NClO4 as supporting electrolyte.
Electrochemistry: The redox properties of exTTF–TCAQ
(11) were examined by cyclic voltammetry (CV) and com-
2960
www.chemeurj.org
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2957 – 2964

TTF–p–TCAQ System
FULL PAPER
(TCAQ2ꢂ) species. To this end, TTM-exTTF shows a quasi-
1
G
2
whereas the TCAQ reference undergoes a reversible (DE
1
ꢁ41 mV) two-electron reduction around ꢂ0.56 V (E = ) that
2
corresponds to the formation of the dianion.
1
For 11, oxidation is discernable at +0.22 V (E = ) together
2
1
with a reduction at ꢂ0.54 V (E = ). Both redox processes are
2
reversible and involve two electrons. Electrochemically, we
could not detect any appreciable impact on the oxidation
and reduction of exTTF and TCAQ, respectively, despite
the presence of both electroactive moieties that are connect-
ed through a conjugated ethylene bridge. The electrochemi-
cal band gap for 11 is approximately 0.80 V, which resembles
those of related TTF–TCAQ systems.[21]
The nature of the HOMO and LUMO, which are respec-
tively localized on the electron-donor and -acceptor units
(Figure 2), indicates that oxidation of 11 should mainly
affect the exTTF moiety, whereas reduction concerns the
TCAQ moiety. In a first approach, Koopmanꢇs theorem ena-
bled us to relate the HOMO and LUMO energies with the
electrochemical potentials of the first oxidation and reduc-
tion processes, respectively. Using this theorem and in con-
trast to what is experimentally observed, more positive/neg-
ative oxidation/reduction potentials had to be expected for
11 due to the lower energy of the HOMO (ꢂ5.10 eV) and
the higher energy of the LUMO (ꢂ3.64 eV) than TTM-
exTTF (ꢂ4.90 eV) and TCAQ (ꢂ3.75 eV), respectively. The
reason for this apparent discrepancy is that Koopmanꢇs the-
orem is a one-electron approach and does not apply to the
oxidation/reduction processes of 11, which involve two elec-
trons.
The dication and dianion species of 11 were computed at
the B3LYP/6-31G** level to provide a deeper understanding
of the oxidation/reduction processes. As depicted in Fig-
ure 4a, the minimum-energy structure calculated for 112+
corresponds to a conformation in which the TCAQ unit re-
mains folded and the exTTF unit adopts an orthogonal con-
formation similar to those previously found for exTTF dicat-
ions both theoretically[15,16,22] and experimentally.[18,19] Upon
oxidation, the exocyclic C=C bonds that connect the dithiole
rings to the anthracene unit elongate from 1.365 ꢆ in neu-
tral 11 to 1.482 ꢆ in charged 112+. This lengthening allows
for the rotation of the dithiole rings to minimize the steric
interactions, and, as a consequence, the anthracene unit be-
comes planar. In the resulting conformation, the dithiole
rings are nearly perpendicular to the anthracene plane and
the SMe groups lie in the plane of the dithiole rings. This
disposition of the SMe groups was already reported for the
TTM-TTF dication.[23,24] The net atomic charges calculated
for 112+ by using the natural population analysis (NPA) al-
gorithm indicate that the electrons have been mainly re-
moved from the SMe-dithiole rings, which accumulate a
charge of +0.91 e each, whereas the anthracene unit re-
mains mainly neutral (+0.05 e). The exTTF moiety in 112+
is therefore constituted by an aromatic anthracene unit (14
p electrons) substituted by singly charged dithiole rings.
This minimizes the coulombic repulsion between the posi-
Figure 4. Minimum-energy conformations calculated for a) 112+ and
b) 112ꢂ at the B3LYP/6-31G** level.
tive charges in the dication and explains the fact that almost
identical oxidation potentials are observed for 11 and for
the reference compound TTM-exTTF.
Reduction of 11 to the dianion produces structural
changes on the molecular geometry similar to those predict-
ed upon oxidation, but they are now localized on the elec-
tron-acceptor TCAQ unit (see Figure 4b). For 112ꢂ, the
exTTF unit preserves the saddlelike structure of the neutral
molecule and the TCAQ unit becomes planar with the
C(CN)2 groups twisted out of the anthracene plane by an
average angle of 458. As for the exTTF unit, this geometri-
cal reorganization is made possible by the elongation of the
C=C exocyclic bonds that link the C(CN)2 groups to the an-
thracene unit that lengthen from approximately 1.380 ꢆ in
11 to approximately 1.475 ꢆ in 112ꢂ and allow rotation of
the C(CN)2 groups. The NPA charges calculated for 112ꢂ in-
dicate that the extra electrons are mainly supported by the
C(CN)2 groups (ꢂ1.80 e).
Photophysics: The electronic absorption spectra of exTTF–
TCAQ (11) reflect the well-known absorptions of exTTF,
with absorption maxima at 376 and 445 nm, redshifted
about 10 nm in comparison to the reference,[19] and TCAQ,
with maxima in the UV region of the spectrum at 345, 302,
and 280 nm,[17] marginally redshifted.[25] Additionally, new
absorption features evolve in the red part of the spectrum,
that is, between 500 and 800 nm, in which neither exTTF
nor TCAQ absorbs. When varying the solvent polarity from
nonpolar toluene to polar acetonitrile, the maximum of this
new band shifts from about 590 nm to 620 nm. In light of
the aforementioned results and in analogy with previous re-
sults,[26] we assigned this new band to a charge-transfer fea-
ture, namely, a redistribution of charge density from the
Chem. Eur. J. 2011, 17, 2957 – 2964
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2961

D. M. Guldi et al.
Figure 6. Three-dimensional fluorescence plot of 11 recorded in THF
under argon (this figure is reproduced in color in the Supporting Infor-
mation).
Figure 5. Absorption and emission (lexc =600 nm) spectra of 11 recorded
in toluene.
shortlived excited state (1.2 ps) are transient maxima around
465, 605, and 990 nm, as well as transient bleaching at
around 450 nm (not shown). The short lifetimes (i.e., after
5 ps, no transient absorption remains) are rationalized by
the presence of the sulfur atoms, with a strong second-order
vibronic spin–orbit coupling. Going beyond our femtosec-
ond experiments (i.e., 3 ns) we tested exTTF in nanosecond
experiments following 355 nm excitation. However, outside
of the 10 ns time window of the instrumental time resolu-
tion, no notable transients were detected.
When investigating 11 (Figure 7), instantaneously upon
photoexcitation at 387 nm the singlet excited state features
of exTTF are discernable (vide supra). These features of the
localized exTTF excited state transform within the next
0.2 ps into a new transient with a minimum at 450 nm,
maxima at 480/620 nm, and a shoulder at 680 nm. The tran-
sient—including the 450 nm minima and 680 nm shoulder—
electron-donating exTTF to the electron-accepting TCAQ
(Figure 5).
Vertical transitions to the lowest-energy electronic states
were calculated for 11 in the presence of the solvent using
the time-dependent DFT (TD-DFT) approach and the
B3LYP/6-31G**-optimized ground-state geometry. TD-DFT
calculations predict four CT states above 500 nm that result
from electron excitations from the HOMO and HOMOꢂ1,
localized on the exTTF moiety, to the LUMO and
LUMO+1, localized on the TCAQ unit. The first excited
state corresponds to the HOMO!LUMO excitation and is
calculated too low in energy (around 950–1000 nm) with an
oscillator strength (f) of 0.08. The second excited state re-
sults from the HOMOꢂ1!LUMO transition and occurs
around 700 nm (f=0.07). The third and fourth excited states
are very close in energy and result from a mixing of the
HOMOꢂ2!LUMO and HOMO!LUMO+1 excitations.
The participation of the HOMOꢂ2, which spreads over the
central stilbene unit (Figure 2), enhances the intensity of the
electronic transitions. The fourth state, which bears most of
the intensity (f=0.27), shifts from 568 nm in toluene to
559 nm in acetonitrile.
The confirmation for the CT hypothesis came from fluo-
rescence studies. Although the emissions of exTTF and
TCAQ are almost completely quenched, a new emission in
the 600 to 700 nm range—depending on the solvent—is dis-
cernable. In Figure 6, a 3D fluorescence plot of 11 is shown.
Here, the strongly quenched exTTF emission around 500 nm
is followed by the much stronger CT emission, with its maxi-
mum at 650 nm, as it evokes from the correlating CT ab-
sorption, respectively in THF.
is in perfect agreement with the pulse-radiolysis-generated
+
C
exTTF radical cation (Figure S1 in the Supporting Infor-
mation),[27] whereas the transition at 620 nm is assigned to
Cꢂ
the TCAQ radical anion (Figure S2).[25] Notably, the addi-
Cꢂ
tional spectral features of TCAQ in the visible region with
a minimum around 480 nm are buried beneath the much
+
C
stronger exTTF fingerprint. The correspondingly formed
+
C
Cꢂ
exTTF –TCAQ radical ion-pair state is shortlived and
decays through recovery of the ground state. Times of 0.2 ps
and (1.0ꢀ0.2) ps have been determined for the charge-sepa-
ration and charge-recombination processes, respectively, in
THF. Likewise, excitation at 660 nm, which generates the
excited state of the CT state, causes these to convert into
+
C
Cꢂ
those of the exTTF -TCAQ radical ion-pair state (Fig-
ure S3).
Next, ultrafast laser flash photolysis experiments were car-
ried out by employing either the 387 nm excitation wave-
length (to photoactivate the singlet excited states of 11) or
the 660 nm excitation wavelength (to excite exclusively the
CT transition).
The ultrafast charge-separation and charge-recombination
dynamics are a result of the very short vinyl linkage. This
provides not only strong electronic coupling, as it is discern-
able in form of CT absorption and emission, but also planar-
ity of the molecule. As discussed above, an almost planar
trans-stilbene unit is formed between the exTTF and TCAQ
units through the ethylene bridge (Figure 1). Electronic de-
Photoexcitation of exTTF at 387 nm generates an exTTF-
centered excited state. Spectral characteristics of this very
2962
www.chemeurj.org
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2957 – 2964

TTF–p–TCAQ System
FULL PAPER
the TCAQ unit. As a complement, the electronic excited
state was probed by means of femtosecond transient-absorp-
tion spectroscopy. To this end, 11 reveals in the excited state
ultrafast charge separation (0.2 ps)—that is, the formation of
the radical cation of exTTF and the radical anion of
TCAQ—and fast charge-recombination ((1.0ꢀ0.2) ps) pro-
+
C
Cꢂ
radical ion-pair state is
cesses. The exTTF –TCAQ
formed either through a localized exTTF excited state or ex-
cited CT state.
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft (SFB 583),
Cluster of Excellence (EAM), the Office of Basic Energy Sciences of the
U.S., the MICINN of Spain (project nos. CTQ2008-00795/BQU,
CTQ2009-08790, CTQ2009-07791, and Consolider-Ingenio CSD2007-
00010 on Molecular Nanoscience), the CM (project nos. P-PPQ-000225-
0505 and S2009/PPQ-1634), and the EU (FUNMOL FP7-212942-1) is
greatly appreciated. J.S. is indebted to the MEC for a research grant.
[1] a) Special issue on “Molecular Conductors” (Ed.: P. Batail), Chem.
Rev. 2004, 104, 4887–5782; b) For a review on magnetic materials,
see: J. S. Miller, Inorg. Chem. 2000, 39, 4392–4408; c) J. M. Williams,
J. R. Ferrar, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang,
A. M. Kini, M.-H. Whangbo, Organic Superconductors (including
Fullerenes), Prentice Hall, Englewood Cliffs, 1992; d) M. R. Bryce,
Chem. Soc. Rev. 1991, 20, 355–390; e) P. Day, M. Kurmoo, J. Mater.
Chem. 1997, 7, 1291–1295.
[2] For general reviews, see: a) J. Garꢀn, N. Martꢀn in TTF Chemistry.
Fundamentals and Applications of Tetrathiafulvalene, (Eds.: J.
Yamada, T. Sugimoto), Springer, 2004; b) J. L. Segura, N. Martꢀn,
Angew. Chem. 2001, 113, 1416–1455; Angew. Chem. Int. Ed. 2001,
40, 1372–1409.
[3] A. Aviram, M. A. Ratner, Chem. Phys. Lett. 1974, 29, 277–283.
[4] Special issue on “Organic Electronics and Optoelectronics” (Eds.:
S. R. Forrest, M. E. Thompson), Chem. Rev. 2007, 107, 923–1386.
[5] a) Special issue on “Organic Photovoltaics” (Eds.: J. L. Brꢈdas, J. R.
Durrant), Acc. Chem. Res. 2009, 42, 1689–1857; b) N. Martꢀn, L.
Sꢁnchez, M. A. Herranz, B. Illescas, D. M. Guldi, Acc. Chem. Res.
2007, 40, 1015–1024; c) J. L. Segura, N. Martꢀn, D. M. Guldi, Chem.
Soc. Rev. 2005, 34, 31–47; d) J. L. Delgado, P.-A. Bouit, S. Filippone,
M. A. Herranz, N. Martꢀn, Chem. Commun. 2010, 46, 4853; e) M. R.
Wasielewski, Chem. Rev. 1992, 92, 435–461; f) M. R. Wasielewski in
Photoinduced Electron Transfer, (Eds.: M. A. Fox, M. Chanon),
Elsevier, Amsterdam, 1988.
[6] a) D. M. Guldi, B. M. Illescas, C. M. Atienza, M. Wielopolski, N.
Martꢀn, Chem. Soc. Rev. 2009, 38, 1587–1597; b) E. A. Weiss, M. R.
Wasielewski, M. A. Ratner, Top. Curr. Chem. 2005, 257, 103–134;
c) D. K. James, J. M. Tour, Top. Curr. Chem. 2005, 257, 33–62.
[7] S. Barlow, S. R. Marder in Functional Organic Materials, (Eds.:
T. J. J. Mꢄller, U. H. F. Bunz), Wiley-VCH, Weinheim, 2007,
pp. 393–437.
Figure 7. Top: differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of 11 in THF
under argon with a time delay of 1.5 ps. Bottom: time absorption profiles
of spectra shown above at 630 nm (filled circles), and 1060 nm (empty
circles) monitoring the charge separation and charge recombination.
localization across this p-conjugated unit therefore enables
strong interactions between the donor and acceptor parts of
the molecule.
Conclusion
In summary, we have carried out the synthesis of a new
building block for exTTFs, namely, sulfonylmethyl-exTTFs
(5), suitably functionalized for Julia–Kocienski olefination
reactions with different aldehydes. The reaction proceeds
with an excellent stereoselectivity of the resulting trans
olefin. As a proof-of-principle, the reaction of 5b with the
formyl-containing TCAQ afforded for the first time a fully
conjugated TTF–p–TCAQ-type system (11). Theoretical cal-
culations at the B3LYP/6-31G** level show a highly distort-
ed molecule 11 at the exTTF and TCAQ moieties with an
almost planar central stilbene unit. Importantly, the central
vinyl spacer guarantees efficient connection of the electroac-
tive donor and acceptor in the electronic ground state, and
in turn gives rise to the formation of charge-transfer (CT)
bands—both in absorption and emission—in the visible
region of the spectrum. These experimental findings were
nicely supported by TD-DFT calculations, which predict
four CT states above 500 nm that result from electronic ex-
citations from the HOMO and HOMOꢂ1, localized on the
exTTF moiety, to the LUMO and LUMO+1, localized on
[8] M. Kivala, F. Diederich, Acc. Chem. Res. 2009, 42, 235–248.
[9] a) H. E. Katz, K. D. Singer, J. E. Sohn, C. W. Dirk, L. A. King,
H. M. Gordon, J. Am. Chem. Soc. 1987, 109, 6561–6563; b) M.
Blanchard-Desce, I. Ledoux, J.-M. Lehn, J. MalthÞte, J. Zyss, J.
Chem. Soc. Chem. Commun. 1988, 737–739.
[10] a) R. Andreu, A. I. de Lucas, J. Garꢀn, N. Martꢀn, J. Orduna, L.
Sꢁnchez, C. Seoane, Synth. Met. 1997, 86, 1817–1818; b) A. I. de Lu-
cas, N. Martꢀn, L. Sꢁnchez, C. Seoane, R. Andreu, J. Garꢀn, J.
Orduna, R. Alcalꢁ , B. Villacampa, Tetrahedron 1998, 54, 4655–
4662; c) J. Garꢀn, J. Orduna, J. I. Rupꢈrez, R. Alcalꢁ, B. Villacampa,
C. Sꢁnchez, N. Martꢀn, J. L. Segura, M. Gonzꢁlez, Tetrahedron Lett.
1998, 39, 3577–3580; d) M. Gonzꢁlez, N. Martꢀn, J. L. Segura, J.
Chem. Eur. J. 2011, 17, 2957 – 2964
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2963

D. M. Guldi et al.
Garꢀn, J. Orduna, Tetrahedron Lett. 1998, 39, 3269–3272; e) M.
Gonzꢁlez, N. Martꢀn, J. L. Segura, C. Seoane, J. Garꢀn, J. Orduna, R.
Alcalꢁ, C. Sꢁnchez, B. Villacampa, Tetrahedron Lett. 1999, 40, 8599–
8602; f) M. Gonzꢁlez, J. L. Segura, C. Seoane, N. Martꢀn, J. Garꢀn, J.
Orduna, R. Alcalꢁ, B. Villacampa, V. Hernꢁndez, J. T. Navarrete, J.
Org. Chem. 2001, 66, 8872–8882.
1199–1205; d) C. A. Christensen, A. S. Batsanov, M. R. Bryce,
J. A. K. Howard, J. Org. Chem. 2001, 66, 3313–3320.
[19] A. E. Jones, C. A. Christensen, D. F. Perepichka, A. S. Batsanov, A.
Beeby, P. J. Low, M. R. Bryce, A. W. Parker, Chem. Eur. J. 2001, 7,
973–978.
[20] U. Schubert, S. Hꢄnig, A. Aꢄmuller, Liebigs Ann. Chem. 1985,
1216–1222.
[21] J. Wu, S. Liu, A. Neels, F. Le Derf, M. Sallꢈ, S. Decurtins, Tetrahe-
dron 2007, 63, 11282–11286.
[22] M. C. Dꢀaz, B. M. Illescas, N. Martꢀn, I. F. Perepichka, M. R. Bryce,
E. Levillain, R. Viruela, E. Ortꢀ, Chem. Eur. J. 2006, 12, 2709–2721.
[23] C. Wartelle, R. Viruela, P. M. Viruela, F. X. Sauvage, M. Sallꢈ, E.
Ortꢀ, E. Levillain, F. Le Derf, Phys. Chem. Chem. Phys. 2003, 5,
4672–4679.
[11] a) M. A. Herranz, N. Martꢀn, L. Sꢁnchez, J. Garꢀn, J. Orduna, R.
Alcalꢁ, B. Villacampa, C. Sꢁnchez, Tetrahedron 1998, 54, 11651–
11658; b) M. Otero, M. A. Herranz, C. Seoane, N. Martꢀn, J. Garꢀn,
J. Orduna, R. Alcalꢁ, B. Villacampa, Tetrahedron 2002, 58, 7463–
7475.
[12] B. M. Illescas, N. Martꢀn, J. Org. Chem. 2000, 65, 5986–5995.
[13] T. Furuta, H. Torigai, M. Sugimoto, M. Iwamura, J. Org. Chem.
1995, 60, 3953–3956.
[14] C. Aꢉssa, Eur. J. Org. Chem. 2009, 1831–1844.
[15] N. Martꢀn, L. Sꢁnchez, C. Seoane, E. Ortꢀ, P. M. Viruela, R. Viruela,
J. Org. Chem. 1998, 63, 1268–1279.
[24] a) H. Endres, Z. Naturforsch. B 1986, 41, 1437–1442; b) K. Brunn,
H. Endres, J. Weiss, Z. Naturforsch. B 1988, 43, 224–230; c) P. G.
Jones, Z. Naturforsch. B 1989, 44, 243–244.
[16] M. C. Dꢀaz, B. M. Illescas, N. Martꢀn, R. Viruela, P. M. Viruela, E.
Ortꢀ, O. Brede, I. Zilbermann, D. M. Guldi, Chem. Eur. J. 2004, 10,
2067–2077.
[17] A. S. Batsanov, M. R. Bryce, M. A. Coffin, A. Green, R. E. Hester,
J. A. K. Howard, I. K. Lednev, N. Martꢀn, A. J. Moore, J. N. Moore,
E. Ortꢀ, L. Sꢁnchez, M. Savirꢂn, P. M. Viruela, R. Viruela, T.-Q. Ye,
Chem. Eur. J. 1998, 4, 2580–2592.
[18] a) M. R. Bryce, A. J. Moore, M. Hasan, G. J. Ashwell, A. T. Fraser,
W. Clegg, M. B. Hursthouse, A. I. Karaulov, Angew. Chem. 1990,
102, 1493–1495; Angew. Chem. Int. Ed. Engl. 1990, 29, 1450–1452;
b) S. Triki, L. Ouahab, D. Lorcy, A. Robert, Acta Crystallogr. Sect. C
1993, 49, 1189–1192; c) M. R. Bryce, T. Finn, A. S. Batsanov, R.
Kataky, J. A. K. Howard, S. B. Lyubchik, Eur. J. Org. Chem. 2000,
[25] A. M. Kini, D. O. Cowan, F. Gerson, R. Moeckel, J. Am. Chem. Soc.
1985, 107, 556–562.
[26] a) P. de Miguel, M. R. Bryce, L. M. Goldenberg, A. Beeby, V. Kho-
dorkovsky, L. Shapiro, A. Niemz, A. O. Cuello, V. Rotello, J. Mater.
Chem. 1998, 8, 71–76; b) M. A. Herranz, S. Gonzꢁlez, I. Pꢈrez, N.
Martꢀn, Tetrahedron 2001, 57, 725–731; c) G. Zerza, A. Cravino, H.
Neugebauer, N. S. Sariciftci, R. Gꢂmez, J. L. Segura, N. Martꢀn, M.
Svensson, M. R. Andersson, J. Phys. Chem. A 2001, 105, 4172–4176.
[27] D. M. Guldi, L. Sꢁnchez, N. Martꢀn, J. Phys. Chem. B 2001, 105,
7139–7144.
Received: September 17, 2010
Published online: February 3, 2011
2964
www.chemeurj.org
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2957 – 2964