Intramolecular electron transfer mediated by a tetrathiafulvalene bridge in a purely organic mixed-valence system

Article abstract of DOI:10.1002/anie.200250587

Radical bridge design: The diquinone-tetrathiafulvalene (TTF) species shown exhibits effective electron coupling between the quinone centers which is mediated by the TTF bridge. Stepwise one-electron reduction of the molecule occurs, and the observed pote

Full text of DOI:10.1002/anie.200250587

Angewandte
Chemie
Organosulfur Conducting Bridges
studied by electrochemistry and ESR spectroscopy, and we
have also performed the synthesis of the TTF-monoquinone
2, TTF-Q, as a model compound.
Intramolecular Electron Transfer Mediated by a
Tetrathiafulvalene Bridge in a Purely Organic
Mixed-Valence System**
Syntheses of 1 and 2 were achieved using the same key
intermediate 2-oxo-1,3-dithiole derivative 7 (Scheme 1). The
iminium salt 4 was prepared according to a previously
described procedure.[12] After quantitative conversion to the
2-thioxo-1,3-dithiole 5, the hydroquinone functionalities were
protected using acetyl or silyl groups by well-established
reactions. Further transchalcogenation of 6a and 6b furnished
the 2-oxo-1,3-dithiole derivatives 7 in excellent yields. The
triethylphosphite coupling methodology was applied to 7a
and the resulting TTF derivative 8 was transformed into the
bis(1,4-hydroquinone)-TTF derivative by subsequent meth-
anolysis in a methoxide solution and subsequent treatment
with p-toluenesulfonic acid (PTSA). Without isolating the
later intermediate, the fused Q-TTF-Q system 1 was obtained
as green-blue crystals by selective oxidation using 2,3-
dichloro-5,6-dicyano-1,2-benzoquinone (DDQ).To obtain
the TTF-Q assembly 2, the Horner–Wadsworth–Emmons
olefination approach was applied to create the TTF core,[13]
which involved the 2-oxo-1,3-dithiole functionality and the
anion of phosphonate 9.[14] After deprotection of the silyl
groups of compound 10, subsequent oxidation with p-
benzoquinone afforded compound 2 as green crystals.
The cyclic voltammogram of 1 showed two one-electron
Nicolas Gautier, FrØdØric Dumur, Vega Lloveras,
JosØ Vidal-Gancedo, Jaume Veciana,
Concepció Rovira,* and PiØtrickHudhomme*
Tetrathiafulvalene (TTF) and its derivatives have been
successfully used as building blocks of low-dimensional
organic conductors and superconductors where intermolecu-
lar electron-transfer phenomena play a key role in the
electronic transport properties.[1] Important results have
been also achieved using TTFs as versatile strong p-donor
systems in materials chemistry,[2] particularly in intramolec-
ular charge-transfer materials for donor–acceptor molecules[3]
or in photoinduced electron transfer when linked to [60]full-
erene.[4] However, TTF has never been used as a bridge to
promote electron conduction between two groups. Nowadays,
unimolecular electronic devices are at the forefront of
research in nanotechnology because of the anticipated limits
to the further miniaturization of microelectronics.[5] More-
over, direct electronic conduction through single molecules
has recently been demonstrated[6] showing the potential use of
integrated molecular-sized devices. As a consequence, there is
great interest in studying the role of various parameters that
govern the intramolecular electron-transfer (IET) rates.
Mixed-valence compounds are the prototypes of molecules
able to undergo fast electron transfer through a bridge. Such
systems have been widely used to study IET between redox
centers of both inorganic[7] and organic nature,[8] with the aim
of understanding electron-transfer processes to aid the design
of molecular wires.[9]
reversible oxidation waves at E0ox1 = + 0.99 and Eo0x2
=
+ 1.36 V (versus a Ag/AgCl electrode), which are character-
istic for the TTF moiety, and two distinguishable reversible
waves at E1red = À0.20 and À0.28 V, which correspond to the
stepwise one-electron reduction of each p-quinone moiety.
The potential difference of DE = 0.08 V between the two
waves indicates that the reduction of the second p-quinone
group is somewhat affected by the presence of the first QÀC
center, and suggests that an effective electronic coupling
exists between both Q centers, which is mediated by the TTF
bridge.
We have studied the intramolecular electron-transfer
process in the Q-TTF-QÀC mixed-valence system by temper-
ature-dependent ESR spectroscopy. Electrochemical or
chemical reduction[15] of model compound 2 gives the TTF-
QÀC, which gives an ESR spectrum that is characterized by
three lines with 1:2:1 intensities that result from coupling with
two equivalent aromatic protons (aH = 2.50 G). This ESR
spectrum was singularly invariant over a wide temperature
range (270–360 K) and also with concentration, which shows
that there is no intermolecular electron transfer in this system.
In contrast, the ESR spectrum of the TTF-bridged mixed-
valence anion radical Q-TTF-QÀC changes upon cooling from
340 to 260 K (Figure 1). While the spectrum at 260 K was
essentially identical to that of model compound TTF-QÀC
(three lines in 1:2:1 intensity, aH = 2.47 G), the spectrum at
temperatures higher than 340 K is characterized by the
coupling of four equivalent aromatic protons (five lines in
1:4:6:4:1 intensity, aH = 1.23 G; Figure 1a).[16] As the temper-
ature is gradually lowered from 340 to 260 K, the alternate
lines broaden and disappear because of extensive dynamic
electron exchange between both of the Q electrophores
through the TTF bridge (Scheme 2).[17] The lowest temper-
One way to test the electron conduction through TTF is to
use it as a bridge between two redox moieties in a mixed-
valence system. Consequently, our target molecule was the
TTF-diquinone 1, Q-TTF-Q, since the p-benzoquinone
groups are particularly useful organic redox centers to
generate mixed-valence systems[10] that form persistent rad-
ical anions upon reduction.[11] The behavior of 1 has been
[*] Dr. C. Rovira, V. Lloveras, Dr. J. Vidal-Gancedo, Prof. J. Veciana
Institut de Ci›ncia de Materials de Barcelona (CSIC)
Campus Universitari de Bellaterra, 08193 Cerdanyola (Spain)
Fax: (+34)93-5805-729
E-mail: cun@icmab.es
Prof. P. Hudhomme, Dr. N. Gautier, Dr. F. Dumur
IngØnierie MolØculaire et MatØriaux Organiques
UMR 6501, 2 Bd Lavoisier, UniversitØ d'Angers
49045 Angers (France)
Fax: (+33)2-4173-5405
E-mail: pietrick.hudhomme@univ-angers.fr
[**] This work was partially supported by grants from the MENRT (for
N.G.), the “Ville d'Angers” (for F.D.), DGI-Spain BQU2000-1157,
and DURSI-Catalunya 2001SGR-00362. This research was under-
taken as part of the European collaborative COST Program
(Action D14/0004/99).
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DOI: 10.1002/anie.200250587
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Scheme 1. Syntheses of 1 and 2. Reagents: a) p-benzoquinone then glacial AcOH, 808C, 96%; b) Na2S·9H2O, MeOH, 98%; c) Ac2O, Et3N, 80%
for 6a; Ph2tBuSiCl, DMF, imidazole, 86% for 6b; d) Hg(OAc)2, CH2Cl2/glacial AcOH, 88% for 7a, 85% for 7b; e) D, P(OEt)3, 45%; f) MeONa/
MeOH, then PTSA and H2O, then DDQ, 57%; g) phosphonate 9, nBuLi, THF, À788C to 208C, 90%; h) nBu4NF/THF then p-benzoquinone, 75%.
The dynamics of the intramolecular exchange process was
theoretically simulated[18] (Figure 1b) and first-order rate
constants for the thermally activated IET process were
extracted by fitting the experimental ESR spectra. The
simulation leads to a linear Arrhenius plot over the 330–
270 K temperature range, which gives the activation param-
eters:
E
act = 7.96 kcalmolÀ1
;
logA = 13.9
(DG°298K
=
6.4 kcalmolÀ1; DH° = 7.4 kcalmolÀ1; DS° = 3.1 e.u.). The acti-
vation parameters for compound 1 are close to those pre-
viously reported for the anthracene-diquinone anion radi-
cal.[10e] This is one of the very few cases of an intervalence
system for which the IET process can be followed by ESR
spectroscopy,[8a,b,d,l,10e] which allows an accurately determina-
tion of the rate constant for thermally activated electron
transfer. Figure 1 shows the results obtained in a 10:1 mixture
of ethyl acetate/tert-butanol. However, the rate constants of
the electron-transfer (ET) process have been determined in
different solvents, which gives rise to different rate constants
because of energy variations resulting from solvent reorgan-
ization. For example, the rate constants at 300 K are very
similar in dichloromethane and ethyl acetate (kET = 2.58 ”
108 sÀ1 and 2.10 ” 108 sÀ1, respectively) and of the same order
in the 10:1 mixture of ethyl acetate/tert-butanol (kET = 1.30 ”
108 sÀ1), whereas in tert-butanol the ET process is slower
(kET = 2.89 ” 107 sÀ1). It is important to indicate that the
thermally activated IET process can only be fully studied
using the 10:1 ethyl acetate/tert-butanol mixture, since other
solvent systems exhibit interfering precipitation processes. As
a result of the high insolubility of the monoanion radical
species in any solvent, we have not been able to generate the
corresponding dianion by chemical reduction or electro-
chemical reduction at À0.5 V.
Figure 1. Experimental (a) and simulated (b) ESR spectra of 1ÀC at dif-
ferent temperatures in ethyl acetate/tert-butanol (10:1) with PPh4Br.
Scheme 2. Electron exchange through the TTF bridge.
ature spectrum unequivocally demonstrates that the odd
electron of Q-TTF-QÀC is localized on one particular Q unit
on the ESR time scale, and as the temperature is raised the
activation energy barrier is overcome, which promotes a
faster IET process between the two Q moieties.
The high insolubility of the Q-TTF-QÀC mixed-valence
species also prevents the quantitative study of the IET process
by Vis/NIR spectroelectrochemical methods, since the com-
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Angewandte
Chemie
1996 271, 1705 – 1707; b) M. A. Reed, C. Zhou, C. J. Muller, T. P.
pound starts to precipitate after passing a very small quantity
of charge. Nevertheless, by deconvoluting the Vis/NIR
spectra at different reduction steps, a broad charge-transfer
band centered around 1300 nm is clearly observed for the
generated Q-TTF-QÀC mixed-valence species. Moreover, this
band is completely absent for both the neutral species and in
the TTF-QÀC model compound.
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In summary, we have obtained a new, purely organic
mixed-valence compound in which the existence of a temper-
ature-dependent intramolecular electron transfer has not only
been undoubtedly established but for which the rate constant
for the thermally activated IET process has been accurately
determined. In addition, this is the first time that a bridge
exhibiting a strong electron-donating character, such as TTF,
has been used in a mixed-valence system. The results here
demonstrate that TTF is a suitable molecular bridge for the
promotion of intramolecular electron transfer between two
redox centers and constitutes a natural route towards the
measurement of an electrical current through metal-TTF-
metal nanojunctions. In that sense, TTF can be considered as
the prototype of a huge family of electron donors in which the
ionization potential (ID) is small and can be tuned by changing
the substituents and the conjugated extension. These charac-
teristics are very important since one of the criteria for a good
donor molecular wire is that the ID value must be small, and
must closely match the work function of the metal layer that
acts as an electrode.[6c] We are currently working on the
introduction of conjugated spacers between the 1,3-dithiole
moieties of compound 1 to elongate the TTF bridge, and also
on the utilization of other redox centers to replace
p-benzoquinone groups.
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[12] Yields were improved by the use of piperidine instead of
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Received: November 20, 2002
Revised: March 3, 2003 [Z50587]
Keywords: electron transfer · mixed-valent compounds ·
.
molecular electronics · quinones · tetrathiafulvalene
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(0.2m in CH2Cl2).
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[18] J. Heinzer, Mol. Phys. 1971, 22, 167; Quantum Chemistry
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[19] M. B. Robin, P. Day, Adv. Inorg. Chem. Radiochem. 1967, 10,
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delocalized and the redox centers show intermediate valence
states.
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