New functionalized and soluble bis-tetrathiafulvalene derivatives as building blocks in the construction of fullerene-derived electroactive triads

Article abstract of DOI:10.1016/S0040-4039(00)01309-5

Novel functionalized bis-tetrathiafulvalene (TTF) derivatives are built on the 1,3,5-benzene core. Long alkyl chains have been incorporated on peripheral positions of the TTF moieties in order to increase the solubility of the final products. After approp

Full text of DOI:10.1016/S0040-4039(00)01309-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 7737–7741
New functionalized and soluble bis-tetrathiafulvalene
derivatives as building blocks in the construction of
fullerene-derived electroactive triads
Jose´ L. Segura, Eva Ma6 Priego and Nazario Mart´ın*
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad Complutense, E-28040, Madrid, Spain
Received 27 July 2000; accepted 1 August 2000
Abstract
Novel functionalized bis-tetrathiafulvalene (TTF) derivatives are built on the 1,3,5-benzene core. Long
alkyl chains have been incorporated on peripheral positions of the TTF moieties in order to increase the
solubility of the final products. After appropriate functionalization of the bis-TTF it has been covalently
attached to the C60 core yielding the first C60–TTF2 triad. Electrochemical and spectroscopic studies of the
target compounds suggest that there are no significant interactions between the TTF units and the C60
moiety in the ground state. © 2000 Elsevier Science Ltd. All rights reserved.
Many efforts have been devoted in recent years to the development of new systems
containing two or more tetrathiafulvalene (TTF) units.1–3 Because of the intramolecular
through-bond or through-space interactions between the TTF units, this kind of system may
display multi-stage redox behavior which might provide the possibility of controlling the
stoichiometry, band filling and molecular assembly in desired conductive complexes. On the
other hand, extensive efforts have been dedicated to the development of new C60-based
donor–acceptor systems endowed with various electron donors which present light-induced
intramolecular charge-separation (CS).4 Among them, we have paid special attention to the
development of a strategy probing organic donor molecules (e.g. TTF derivative 1 and
p-extended TTF derivatives 2 and 3, Fig. 1) linked to the fullerene core that gain rather than
lose aromaticity on charge separation.5
* Corresponding author. Tel: +34-91-3944227; fax: +34-91-3944103; e-mail: nazmar@eucmax.sim.ucm.es
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01309-5

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Figure 1.
Different strategies have been proposed as efficient ways of decreasing charge recombination
rates, thus increasing lifetimes of charge-separated states. One way of achieving this is to
increase the interchromophore separation. However, this cannot easily be done using simple
bichromophoric systems without compromising the efficiency of formation of the CS state. This
problem can be circumvented, (i) by stabilizing the radical cation species of the donor fragment
by gaining of aromaticity upon oxidation as in the TTF derivatives and (ii) by using polychro-
mophoric systems, i.e. triads, tetrads, pentads, etc.6 The synthesis of appropriately functional-
ized bis-tetrathiafulvalene (TTF) derivatives built on the 1,3,5-benzene core allows two
conjugated electroactive TTF units to be covalently attached to the C60 core yielding the first
C60–TTF2 triad.
Upon treatment of a refluxing solution of 3,5-dimethylbenzoate (4)7 in carbon tetrachloride
with NBS and AIBN under argon atmosphere, the corresponding dibromomethyl derivative 5
was obtained in a 31% yield. Subsequent treatment of 5 with triphenylphosphine in refluxing
N,N-dimethylformamide (DMF) gives the bis(triphenylphosphonium) salt derivative 6 in a 60%
yield. Lithium ethoxide catalyzed Wittig reaction of the bis(triphenylphosphonium
methyl)benzene bromide 6 with bis(dodecylthio)substituted formyltetrathiafulvalene 78 was
carried out in chloroform:ethanol (3:5) at 50°C, leading to a mixture of cis/trans isomers.
Attempts at isomerization to the all-trans stereoisomer by using the usual iodine-catalyzed
conditions were not successful due to the rapid formation of a charge-transfer salt between the
TTF donor fragments and iodine. However, the major isomer (tlc) formed was finally isolated
by careful chromatography in a 37% yield. Spectroscopic analyses confirmed that the isolated
isomer corresponds to the more stable all-trans configuration, the coupling constants measured
in the 1H NMR spectrum for the vinyl protons being 15.7 Hz. Reduction of the ester group with
DIBAL-H in dry methylene dichloride yielded the hydroxymethyl derivative 9, which is an
interesting building block given that it will allow the incorporation of this bis-TTF into more
complex molecular or macromolecular systems by using simple reactions for the synthesis of
esters or ethers. Thus, reaction of 9 with a fullerene derivative endowed with a solubilizing
polyether chain and a carboxylic group (10) in chloroform in the presence of dicyclohexylcar-
bodiimide (DCC) and dimethylaminopyridine (DMAP) afforded the triad 11 as the main
reaction product in a 30% yield (Scheme 1). Interestingly, despite the general lack of solubility
of rigid TTF dimers and fullerene derivatives, the presence of long alkyl chains on the TTF

7739
Scheme 1.

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moieties as well as the polyether chain on the fullerene system provide enhanced solubility to
11, thus allowing full spectroscopic and electrochemical characterization of the system.†
Although systems containing two TTF units should be oxidizable up to the tetracation
because each TTF unit can release two electrons, the number of electrochemical oxidation
processes observed by using cyclic voltammetry measurements in different TTF dimers
reported in the literature is strongly variable depending upon the electronic interaction
between the two electroactive units. Only two electrochemical processes corresponding to
oxidation to the bis(radical cation) and tetracation are observed for those systems in which
the TTF units are not interacting.9 Thus, two quasireversible redox waves are observed in the
cyclic voltammetry measurements performed for the functionalized bis-TTFs 8 and 9 (mea-
surements performed in dichloromethane, reference electrode: SCE; working electrode: glassy
carbon; supporting electrolite: Bu4N+ClO− , at 200 mV/s). Anodic peaks of E1ox=0.50 V and
4
E2ox=0.91 V were measured for dimer 9. Cyclic voltammetry of triad 11 shows an ampho-
teric redox-active behaviour. In particular, it shows, together with two quasireversible oxida-
tion waves (E1ox=0.51 V and E2ox=0.92 V) corresponding to the bis-TTF system, three
quasireversible waves (E1red=−0.75 V, E2red=−1.15 V and E3red=−1.67 V), which can be
assigned to the fulleropyrrolidine core.
The absorption spectrum of triad 11 in dichloromethane solution discloses superimposed
features of the C60 and the bis-TTF unit, respectively. Taking the electrochemical data and
the absorption features into consideration, we reached the conclusion that no significant
interactions arise between the two redox-active moieties in the ground state of the combined
units, which is in agreement with that observed for previously reported TTF–C60 dyads.5
In conclusion, we have reported the synthesis of soluble functionalized bis-TTF derivatives
which are appealing building blocks for the preparation of more complex molecular and
macromolecular systems containing this multiple electroactive unit. As a practical application,
we have prepared the C60–TTF2 triad by formation of an ester linkage between the hydroxy-
methyl substituted TTF-dimer and a C60 derivative endowed with a carboxylic acid function-
ality. Given the good lifetimes found for the charge separated (CS) states observed for
previous examples of TTF–C60 dyads,5 work is in progress in order to explore the photoin-
duced electron transfer properties of 11 to compare the lifetime of the CS state with that of
analogous systems containing only one TTF unit.
Acknowledgements
We are indebted to DGESIC (Project PB98-0818) for financial support.
† Compounds 8, 9 and 11 gave satisfactory analytical and spectroscopic data. Selected spectroscopic data for 11: 1H
NMR (C6D6, 200 MHz): l (ppm): 7.85 (d, 2H, J=8.5 Hz), 7.57 (d, 2H, J=8.5 Hz), 7.40 (s, 2H), 7.37 (s, 1H), 6.91
(d, 2H, J=15.7 Hz), 6.36 (d, 2H, J=15.7 Hz), 6.30 (s, 2H), 5.30 (s, 2H), 5.20 (s, 2H), 4.7 (d, 1H, J=9.5 Hz), 4.3 (d,
(d, 1H, J=9.5 Hz), 3.8–3.5 (m, 11H), 2.77 (t, 8H), 1.19 (m, 80H), 0.81 (t, 12H). IR (KBr): 2926, 2853, 1700, 1626,
1110, 527 cm−1. UV–Vis (CH2Cl2): 260, 318, 374, 420, 436 nm.

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