The first spiroconjugated TTF- and TCNQ-type molecules: A new class of electroactive systems?

Article abstract of DOI:10.1021/ol047681d

(Chemical Equation Presented) Spiroconjugated TTF-type electron donors (13a-c) and TCNQ-type electron acceptors (10, 11) have been prepared from spiroquinone 9. Cyclic voltammetry reveals a relatively weak accepting ability for 10 and 11, and a strong electron-donor character for 13a-c. Whereas the spiroconjugation introduces a destabilization of the LUMO for compounds 9-11, the opposite is observed for compound 13, thus justifying the redox potential values.

Full text of DOI:10.1021/ol047681d

ORGANIC
LETTERS
2005
Vol. 7, No. 2
295-298
The First Spiroconjugated TTF- and
TCNQ-Type Molecules: A New Class of
Electroactive Systems?
Pilar Sand´ın,† Angeles Mart´ınez-Grau,† Luis Sa´nchez,† Carlos Seoane,†
Rosendo Pou-Ame´rigo,‡ Enrique Ort´ı,‡ and Nazario Mart´ın*,†
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad Complutense,
E-28040 Madrid, Spain, and Instituto de Ciencia Molecular, UniVersidad de Valencia,
Dr. Moliner 50, E-46100 Burjassot, Spain
nazmar@quim.ucm.es
Received November 11, 2004
ABSTRACT
Spiroconjugated TTF-type electron donors (13a
voltammetry reveals a relatively weak accepting ability for 10 and 11, and a strong electron-donor character for 13a
spiroconjugation introduces a destabilization of the LUMO for compounds 9
the redox potential values.
−
c) and TCNQ-type electron acceptors (10, 11) have been prepared from spiroquinone 9. Cyclic
c. Whereas the
11, the opposite is observed for compound 13, thus justifying
−
−
The rational design of new organic materials exhibiting
nonconventional optoelectronic properties is based on the
synthesis of molecules with appropriate geometrical and
electronic structures.1 In this context, most of the electron
donors (tetrathiafulvalene, TTF, and its derivatives such as
BEDT-TTF)2 and electron acceptors (tetracyano-p-quin-
odimethane, TCNQ, and quinones)3 reported in the literature
are mainly concerned with conjugated planar molecules.
These planar electroactive systems play an outstanding role
in the field of organic conductors forming charge transfer
(CT) complexes and salts with conducting and supercon-
ducting properties.4 The CT complex TTF-TCNQ is con-
sidered the pioneer compound that paved the way for the
field of molecular organic conducting materials.5
Typically, these electroactive derivatives give rise to quasi
one-dimensional CT complexes when they form segregated
stacks. Substitution of sulfur by selenium or tellurium atoms
on the TTF skeleton,6 the presence of chalcogen atoms on
the periphery of the TTF molecule,7 and the existence of
† Universidad Complutense de Madrid.
‡ Universidad de Valencia.
(1) (a) Handbook of Organic ConductiVe Molecules and Polymers;
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Credi, A., Eds.; Wiley-VCH: Weinheim, 2003.
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(b) TTF Chemistry. Fundamentals and Applications of TetrathiafulValene;
Yamada, Y., Sugimoto T., Eds.; Springer: New York, 2004.
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Chem. 1997, 7, 1661. (d) The Chemistry of Quinoid Compounds; Patai, S.,
Rappoport, Z., Eds.; John Wiley & Sons: New York, 1998.
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Sato, A.; Turner, S. S.; Le Pevelen, D.; Day, P.; Laukhin, V.; Klehe, A.-
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10.1021/ol047681d CCC: $30.25
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hydrogen bonds8 have been used to increase the dimension-
ality of the donor partner. On the acceptor side, substitution
on the central ring as well as coordination of the nitrogen
atoms of the cyano groups to metal atoms have afforded the
formation of multidimensional networks.3,9
Another interesting and much less studied approach to
increase dimensionality is based on the spiroconjugative
effect.10 Although the interest of spirocompounds has focused
on aspects such as conformational transmission, helical
structure, or photochromism,11 there are only a few examples
in the literature in which spiroconjugation has been used to
prepare electroactive molecules.12 In this paper we describe
the first examples of spiroconjugated analogues of TCNQ
(10) and DCNQI (11) acceptors, as well as analogues of TTF
(13a-c) (Scheme 3). The multistep synthetic procedure and
the spectroscopic and electrochemical characterization are
now reported.
reaction of 8 under the same reaction conditions or modifying
the solvent, temperature, and reaction time was unsuccessful.
Attempts to obtain 9 using different oxidants such as Br2/
py, NBS/AIBN, and SeO2 were fruitless. Finally, compound
9 was prepared in 21% yield by oxidation with an excess of
benzeneseleninic anhydride (BSA) by refluxing for 4 days
(Scheme 2).
Scheme 2
The preparation of spiro[5,5]undecatetraenedione (9) was
carried out by following an improved version of the
procedure reported by Farges et al.13 A full characterization
of the intermediate compounds, which were not totally
characterized in the prior work, is also provided (see
Supporting Information). Creation of the spirane system takes
place in the first synthetic step by Michael addition of
1,2,5,6-tetrahydrobenzaldehyde (1) to methylvinyl ketone (2)
and subsequent intramolecular aldol reaction to yield 3. Birch
reduction of ketone 3 yielded alcohol 4, whose acetylation
followed by treatment with peracetic acid gave rise to
epoxide 6. Reduction of 6 with LiAlH4 led to a mixture of
alcohols (7a and 7b), which were separated by several careful
chromatographies. Diol 7a was submitted to oxidation by
treatment with Jones’ reagent to afford spirodione 8 (Schemes
1 and 2).
The synthesis of the target spiro-donor and acceptor
molecules is depicted in Scheme 3. The spiroanalogue of
TCNQ (10) was prepared from 9 by reaction with Lehnert’s
reagent (malononitrile, TiCl4, pyridine).14 It is interesting to
note that this reaction proceeds cleanly without competitive
nucleophilic attack of the anion of malononitrile to the
conjugated olefinic carbons.
Scheme 3
Scheme 1
The spiro DCNQI analogue 11 was synthesized from 9
by following Hu¨nig’s procedure,15 by reaction with bis-
(trimetrylsilyl)carbodiimide (BTC) and TiCl4 (Scheme 3).
1
In contrast to compound 10, the H NMR spectrum of 11
(6) (a) Fabre´, J. M. Chem. ReV. 2004, 104, 5133. (b) Alberola, A.;
Coronado, E.; Gala´n-Mascaro´s, J. R.; Gime´nez-Saiz, C.; Go´mez-Garc´ıa,
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A.; Jubault, M.; Gorgues, A.; Duguay, G. Angew. Chem., Int. Ed. Engl.
1997, 36, 878.
Oxidation of spiro[5,5]undecan-3,9-dione (8) to 9 was a
hard task. We first used the conditions followed by Farges
et al.,13 who claimed to obtain 9 in 34% yield by treatment
with dichlorodicyano-p-benzoquinone (DDQ). In our hands,
(8) (a) Morita, Y.; Maki, S.; Ohmoto, M.; Kitagawa, H.; Okubo, T.;
Mitani, T.; Nakasuji, K. Org. Lett. 2002, 4, 2185. (b) Bryce, M. R. J. Mater.
Chem. 1995, 5, 1481.
296
Org. Lett., Vol. 7, No. 2, 2005

presents a broad doublet at δ 6.46-6.43 corresponding to
the four hydrogens adjacent to the spiro carbon. Now, the
four hydrogens close to the cyanoimino groups are not
equivalent and appear as two broad doublets at 7.24-7.20
and 6.79-6.55 ppm due to the presence in solution of both
syn and anti isomers. This syn/anti isomerism is well-
documented in DCNQ and is due to the flipping of the cyano
groups attached to nitrogen atoms.16
acceptor character as a result of the presence of the
dicyanomethylene and cyanoimino groups, respectively,
which significantly shift the reduction potentials toward less
negative values (see Table 1 and Figure 1).
The TTF-type spiro compounds 13a-c were synthesized
by Wittig-Horner reaction of the carbanion generated in situ
from phosphonate esters (12a-c),17 in the presence of n-BuLi
at -78 °C, with dione 9 (Scheme 3). The analytical and
spectroscopic data clearly support the proposed structures
(see Supporting Information).
The redox properties of the novel spiro derivatives were
determined by cyclic voltammetry (CV) measurements
recorded at room temperature. The data obtained are collected
in Table 1 together with the redox potentials measured for 9
Table 1. Redox Potentials of the Novel Spiro Donor and
Acceptor Moleculesa
Figure 1. CVs for compounds 9 and 10 measured in CH2Cl2 at
room temperature (V vs Ag/Ag+; 100 mV/s).
compoundb
E1red
E2red
E1ox
E2ox
(pa)
(pc)
(pc)
(pa)
-2.07 (-1.94)c
-0.89 (-1.09)c -1.28 (-1.47)c
It is well-established that TCNQ and DCNQI present a
similar electrochemical behavior showing two one-electron,
reversible reduction waves to the corresponding radical anion
and dianion species.3 The spiro analogues 10 and 11 exhibit
a first irreversible reduction wave followed by a second
reversible wave (Table 1 and Figure 1). The reduction
potentials are cathodically shifted by about 1 V with respect
to the values measured for the parent TCNQ and DCNQI
and are in the range of those shown by some quinone
9
10
11
-1.03
0.22
-1.50
-0.35
-0.41
TCNQ
DCNQI
13a
13b
13c
0.21
0.55
1.03
0.66
0.60
0.44
1.09
0.98
0.77
TTFd
a Experimental conditions: GCE as working electrode, NBu4+ClO4- (0.1
M) as supporting electrolyte. b V vs SCE, CH2Cl2, 200 mV/s. c V vs Ag/
Ag+, CH2Cl2, 100 mV/s. d Anodic peaks
acceptors such as 9,10-anthraquinone derivatives (Er1ed
-0.9 V vs SCE).18
≈
The novel spiro donor molecules 13a-c show oxidation
potentials slightly more positive than those of the parent TTF
and quite similar to the well-known BEDT-TTF (E1ox ≈ 0.6
V vs SCE),2 thus exhibiting a strong electron-donor character.
However, the CV measurements are accompanied by an
immediate darkness of the solution, which apparently
indicates an extensive decomposition process due to the
electrochemical instability of the oxidized species. This
finding is in agreement with the irreversible character
observed for the first oxidation wave in 13a-c.
Compounds 13a-c show a similar behavior. In the first
scan, they exhibit an irreversible oxidation wave (∼0.6 V),
which should be reasonably attributed to the generation of
the radical cation. This wave could also correspond to the
formation of a diradical dication, in which the two orthogonal
dithiole moieties were simultaneously oxidized to a radical
cation in a manner similar to that found for some π-extended
TTF derivatives.19 In the following scans, a progressive
lowering of the first oxidation wave is observed and a second
oxidation wave appeared at more positive potentials.
The above electrochemical behavior suggests an ECE
(electrochemical-chemical-electrochemical) process as has
and for the parent compounds TTF, TCNQ, and DCNQI.
The quinone-type spiro compound 9 shows only one ir-
reversible reduction wave at -1.94 V. This value is higher
than that measured for p-benzoquinone (-0.60 V), suggesting
a weak electronic interaction between the two cyclohexenone
rings. Spirocompounds 10 and 11 show a stronger electron-
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Bauer, T.; Tillmanns, E.; Lux, F. R.; Hollstein, M.; Gross, H. G.; Langohr,
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Org. Lett., Vol. 7, No. 2, 2005
297

been previously reported for other dithiafulvene derivatives.20
The mechanism of dimerization of 1,4-dithiafulvenes into
TTF vinylogues has been well established by Hapiot et al.21
and later skillfully used by other authors to obtain TTF-based
dimeric or polymeric species.22 These electrochemical cou-
pling processes are associated to a high electronic density
of unpair electrons at the vinyl carbon in the radical cation,
as it could occur in compounds 13a-c.
The electronic structures of compounds 9, 10, and 13a
were theoretically investigated at the B3LYP/aug-cc-pVDZ
level.23 The LUMO orbital is doubly degenerate for the
electron acceptors 9 and 10 (see Figure 2). The energies
Upon reduction of 9 and 10, the first electron enters in
one of the degenerate LUMO orbitals, giving rise to a
dissymmetric C2V radical anion, in which the charge and the
unpaired electron are located over one of the π-subsystems
while the other remains mostly neutral. For 10, the charges
accumulated by each subsystem are -0.85 and -0.15 e,
respectively. This result agrees with the localization of the
spin density on one dienone moiety evidenced for 9- from
ESR measurements.24 The entrance of the second electron
occurs on the second LUMO orbital, and calculations indicate
that the 102- dianion consists of two orthogonal radical anion
subsystems bearing identical charges and spin densities.
Oxidation of 13a takes place in a similar way. The first
electron is removed from one of the degenerate HOMOs,
forming a localized radical cation, and the extraction of the
second electron leads to two identical radical cation sub-
systems.
In summary, the first examples of spiroconjugated TTF-
type electron donors (13a-c) and TCNQ- and DCNQI-type
electron acceptors (10, 11) have been synthesized from
spirodione 9. The electrochemical study reveals a relatively
weak electron acceptor character in 10 and 11 and a strong
donor ability of 13a-c, similar to BEDT-TTF. However, in
contrast to the parent TTF, the novel donors 13a-c are
electrochemically unstable, leading upon oxidation to un-
known electrochemically active species. Work is currently
in progress to determine the degree of electronic interaction
through the spiro connectivity in the novel electroactive
systems, as well as their potential application as building
blocks in molecular materials science.
Figure 2. Electron density contours showing the degenerate nature
of the LUMO of compound 9 (D2d symmetry).
calculated for this orbital (9, -2.53 eV; 10, -3.78 eV) are
higher than those obtained for p-benzoquinone (-3.92 eV)
and TCNQ (-5.12 eV). Spiroconjugation therefore intro-
duces a destabilization of the LUMO of 1.3-1.4 eV that
justifies the large cathodic shift recorded for the reduction
potentials of 9 and 10 (see Table 1). For the electron donors
13, the effect of spiroconjugation on the HOMO orbital is
completely different. The HOMO of 13a is calculated at
-4.97 eV, slightly more stable than the HOMO of TTF
(-4.83 eV) and close to the HOMO of BEDT-TTF (-4.98
eV). These values explain the fact that 13a-c exhibit
electron-donor strengths slightly weaker than that of TTF
and similar to BEDT-TTF.
Acknowledgment. This work has been supported by the
MCyT of Spain (Projects BQU2002-00855, BQU2003-
05111, and CTQ2004-03760/BQU).
Supporting Information Available: Experimental details
and spectroscopic characterization of all compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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Mart´ın, N. J. Phys. Chem. B 2001, 105, 7139.
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OL047681D
(23) Density functional theory (DFT) B3LYP calculations were per-
formed using the Gaussian 03 program (Frisch, M. J. et al., GAUSSIAN-
03; Gaussian Inc.: Pittsburgh, PA, 2003). Molecular geometries were
optimized using the cc-pVDZ basis set (see ref S-1).
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Soc. 1972, 94, 2919.
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