A (π-extended tetrathiafulvalene)-fluorene conjugate. Unusual electrochemistry and charge transfer properties: The first observation of a covalent D2+-σ-A.- Redox state

Article abstract of DOI:10.1021/ja012518o

The synthesis of novel electrochemically amphoteric TTFAQ-σ-A compounds (TTFAQ = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene, σ = saturated spacer, A = polynitrofluorene acceptor) is reported. Their solution redox behavior is characterized by t

Full text of DOI:10.1021/ja012518o

Published on Web 10/30/2002
A (π-Extended Tetrathiafulvalene)-Fluorene Conjugate.
Unusual Electrochemistry and Charge Transfer Properties:
The First Observation of a Covalent D2+-σ-A•- Redox State1
Dmitrii F. Perepichka,† Martin R. Bryce,*,† Igor F. Perepichka,‡
Svetlana B. Lyubchik,‡ Christian A. Christensen,† Nicolas Godbert,†
Andrei S. Batsanov,† Eric Levillain,§ Eric J. L. McInnes,| and Jing P. Zhao|
Contribution from the Department of Chemistry, UniVersity of Durham,
Durham DH1 3LE, U.K., L. M. LitVinenko Institute of Physical Organic and Coal Chemistry,
National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine, Inge´nierie Mole´culaire et
Mate´riaux Organiques, CNRS UMR 6501, UniVersite´ d’Angers,
49045 Angers Cedex 01, France, and the EPSRC c.w. EPR SerVice Centre,
Department of Chemistry, The UniVersity of Manchester, Manchester MI3 9PL, U.K.
Received November 9, 2001
Abstract: The synthesis of novel electrochemically amphoteric TTFAQ-σ-A compounds (TTFAQ ) 9,10-
bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene, σ ) saturated spacer, A ) polynitrofluorene acceptor) is
reported. Their solution redox behavior is characterized by three single-electron reduction and one two-
electron oxidation waves. Electrochemical quasireversibility of the TTFAQ2+ state and a low Eox - Ered gap
(≈0.25 V) for 3-(9-dicyanomethylene-4,5,7-trinitrofluorene-2-sulfonyl)-propionic acid 2-[10-(4,5-dimethyl-
[1,3]dithiol-2-ylidene)-9,10-dihydroanthracen-9-ylidene]-5-methyl-[1,3]dithiol-4-ylmethyl ester (10) has en-
abled the electrochemical generation of the hitherto unknown transient D2+-σ-A•- state as observed in
cyclic voltammetry and time-resolved spectroelectrochemistry. The ground state of compound 10 was shown
to be ionic in the solid but is essentially neutral in solution (according to electron paramagnetic resonance).
The X-ray structure of an intermolecular 1:2 complex between 2-[2,7-bis(2-hydroxyethoxy)-9,10-bis(4,5-
dimethyl-[1,3]dithiol-2-ylidene)-9,10-dihydroanthracene and 2,5,7-trinitro-4-bromo-9-dicyanomethylene-
fluorene, 14‚(17)2, reveals, for the first time, full electron transfer in a fluorene charge-transfer complex.
Introduction
construction of covalent donor-acceptor (D-A) compounds in
which the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) orbitals are
essentially separated and, therefore, can be independently tuned
to achieve the required values of Eox and Ered. The first strategy
is realized in the design of linear π-conjugated polymers and
oligomers, where the main synthetic principles of band gap
control have now been substantially formulated. They include
an increase in the quinoid character of the conjugated system
due to a decrease in its aromaticity, the rigidification of the
conjugated system into a nearly planar conformation, and the
creation of alternating electron-donating and electron-accepting
fragments along the conjugated chain.5 The second strategy,
which is realized for molecular D-σ-A systems, generally
explores a systematic choice of D and A units and the
introduction of electron-donating and electron-withdrawing
substituents at appropriate sites to tune independently the
HOMO and LUMO levels. Although it was widely used in the
design of intramolecular charge-transfer materials,6 high am-
photericity (Eox - Ered < 0.5 V) is quite unusual for closed-
shell organic molecules.3b,7 To achieve this, strong electron-
donor and strong electron-acceptor fragments should be linked,
Multistage organic redox systems and, in particular, electro-
chemically amphoteric compounds, for which both oxidized and
reduced states are available within a readily accessible potential
window, are of current interest in both theoretical and experi-
mental research due to their potential applications in molecular
electronics and optoelectronics.2-4
There are two basic strategies to endow high electrochemical
amphotericity: (i) by extending π-conjugation in the molecule
(in the ultimate case of graphite Eox - Ered ) 0 V) and (ii) by
* Address correspondence to this author. E-mail: m.r.bryce@durham.ac.uk
† University of Durham.
‡ National Academy of Sciences of Ukraine.
§ Universite´ d’ Angers.
| University of Manchester.
(1) (a) Electron Acceptors of the Fluorene Series, Part 14. Part 13 see:
Perepichka, I. F.; Perepichka, D. F.; Lyubchik, S. B.; Bryce, M. R.;
Batsanov, A. S.; Howard, J. A. K. J. Chem. Soc., Perkin Trans. 2 2001,
1546-1551. (b) Molecular Saddles, Part 9. Part 8 see: Godbert, N.; Bryce,
M. R. J. Mater. Chem. 2002, 12, 27-36.
(2) Nakasuji, K.; Yoshida, K.; Murata, I. J. Am. Chem. Soc. 1983, 105, 5136-
5137.
(3) (a) Tsubata, Y.; Suzuki, T.; Miyashi, T. J. Org. Chem. 1992, 57, 6749-
6755. (b) Suzuki, T.; Miyanari, S.; Tsubata, Y.; Fukushima, T.; Miyashi,
T.; Yamashita, Y.; Imaeda, K.; Ishida, T.; Nogami, T. J. Org. Chem. 2001,
66, 216-224.
(4) (a) Hu¨nig, S.; Kemmer, M.; Wenner, H.; Perepichka, I. F.; Ba¨uerle, P.;
Emge, A.; Gescheid, G. Chem. Eur. J. 1999, 5, 1969-1973. (b) Hu¨nig, S.;
Kemmer, M.; Wenner, H.; Barbosa, F.; Gesheidt, G.; Perepichka, I. F.;
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9
10.1021/ja012518o CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 14227-14238
14227

A R T I C L E S
Perepichka et al.
Chart 1
Here we report the synthesis and properties of the first
TTFAQ-σ-fluorene diads 9 and 10 and fluorene-σ-TTFAQ-
σ-fluorene triads 19 and 20. The strong electron affinity of
the fluorene fragment in compound 10 enriches the redox
properties, providing high amphotericity (Eox - Ered ≈ 0.25 V),
and a hitherto unknown redox state D2+-σ-A•- has been
observed in cyclic voltammetry (CV) and time-resolved spec-
troelectrochemistry (SEC) experiments. The X-ray crystal
structure of the TTFAQ/fluorene intermolecular complex 14‚
(17)2 reveals, for the first time, complete electron transfer in a
charge-transfer complex (CTC) of a fluorene acceptor.
but this approach faces a number of experimental hindrances.8
The conjunction of a strong donor (e.g., tetrathiafulvalene (TTF))
and a strong acceptor (e.g., 7,7′,8,8′-tetracyano-p-quinodimethane
(TCNQ)) in a D-σ-A system has been a challenge9 since an
inspiring proposal for unimolecular rectification in a TTF-σ-
TCNQ molecule.10
This class of molecules has attracted our recent interest8b,11
due to their potential to act as unimolecular rectifiers,10 single-
component electrical conductors,3a,7,12 molecular switches,13 and
artificial photosynthetic centers based upon intramolecular
photoinduced electron transfer.14 We recently reported a TTF-
σ-A diad 1 (Chart 1) where A is a polynitrofluorene moiety
Results and Discussion
Synthesis. The synthesis of diads 9 and 10 is presented in
Scheme 1. The 2-nitro group in fluorenone 2 was selectively
substituted with 3-mercaptopropionic ester in acetonitrile solu-
tion in the presence of NaHCO3 (as a basic catalyst and as a
trap for the nitronic acid formed during the reaction). These
conditions were found to be superior to those described by us
recently for the reaction with n-butanethiol where aprotic dipolar
solvents and no catalyst were used.21 The sulfide 3 was oxidized
to sulfone 4 with hydrogen peroxide in acetic acid, and the ester
group was then converted to the acid 5 by treatment with
aqueous trifluoroacetic acid and then to acid chloride 6 by
treatment with oxalyl chloride. The overall yield of 6 in the
above four steps was 70%. This acid chloride was coupled with
hydroxymethyl-TTFAQ derivative 822 with pyridine catalysis
yielding TTFAQ-fluorenone conjugate 9 in 65% yield. To
increase its acceptor ability, 9 was converted into the dicyano-
methylene derivative 10 by reaction with malononitrile in DMF
solution. For comparison, dicyanomethylene derivative 7 was
synthesized from 4. To obtain a single crystal of an inter-
molecular complex of TTFAQ with fluorene, variously substi-
tuted donor and acceptor components were explored and a
successful combination was TTFAQ derivative 14 (Scheme 2)
and a bromo-substituted fluorene acceptor 17 (Scheme 3).
Compound 14 was coupled with 18 affording, in 60% yield, an
A-σ-D-σ-A triad 19 and hence the dicyanomethylene
derivative 20 (Scheme 4). However, a stronger acceptor, 2,5,7-
trinitro-9-fluorenone-4-carbonyl chloride, gave only an oxidation
product, i.e., salt 142+‚2Cl- (90% yield) under these condi-
tions.23
with a high electron affinity (similar to TCNQ).11b Compound
0
1 possessed six reversible redox states, and the value Eox
-
0
Ered was as low as 0.3 V. In this context our attention was
drawn to an “extended” TTF donor with anthraquinoidal
separation of the two 1,3-dithiole units (TTFAQ) due to its
ability to release two electrons, simultaneously, in one quasi-
reversible process. This quasireversibility15 is due to the marked
structural rearrangement which accompanies the redox process
between a strongly bent (saddle shaped) neutral state and a fully
aromatic dication species in which the anthracene nucleus is
planar with the dithiolium rings orthogonal.16 Such a peculiarity
of TTFAQ donors could lead to unusual electrochemical
behavior in highly amphoteric D-σ-A systems. There are a
few reports on the attachment of the TTFAQ system to different
acceptor moieties, viz., fullerene,17 porphyrin (as well as
TTFAQ-porhyrin-C60 triad),14b 11,11,12,12-tetracyanoanthra-
quinodimethane,18 1,4-naphthoquinone,19 and a dicyanovinyl
group20 (which possess moderate electron affinity).
(7) Suzuki, T.; Yamada, M.; Ohkita. M.; Tsuji, T. Heterocycles 2001, 54, 387-
394.
(8) (a) Panetta, C. A.; Heimer, N. E.; Hussey, C. L.; Metzger, R. M. Synlett
1991, 301-309. (b) de Miguel, P.; Bryce, M. R.; Goldenberg, L. M.; Beeby,
A.; Khodorkovsky, V.; Shapiro, L.; Niemz, A.; Cuello, A. O.; Rotello, V.
J. Mater. Chem. 1998, 8, 71-76.
(9) To date there is only one report on the coupling of substituted TTF and
TCNQ: Panetta, C. A.; Baghdadchi J.; Metzger, R. M. Mol. Cryst. Liq.
Cryst. 1984, 107, 103-113. However, a difficult synthetic route and
problems with purification precluded detailed characterization of the
compound.
(10) Aviram, A.; Ratner, M. Chem. Phys. Lett. 1974, 29, 277-283. For recent
discussions of this proposal see: Metzger, R. M. J. Mater. Chem. 1999, 9,
2027-2036.
(11) (a) Perepichka, D. F.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K.;
Cuello, A. O.; Gray, M.; Rotello, V. M. J. Org. Chem. 2001, 66, 4517-
4524. (b) Perepichka, D. F.; Bryce, M. R.; McInnes, E. J. L.; Zhao, J. P.
Org. Lett. 2001, 3, 1431-1434.
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Tomura, M. J. Mater. Chem. 1998, 8, 1933-1944.
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Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Raymo, F. M.; Stoddart, J. F.;
Venturi, M.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 3951-3957.
(b) Nielsen, M. B.; Nielsen, S. B.; Becher, J. Chem. Commun. 1998, 475-
476. (c) Nielsen, M. B.; Hansen, J. G.; Becher, J. Eur. J. Org. Chem. 1999,
2807-2815.
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48. (b) Kodis, G.; Liddell P. A.; de la Garza, L.; Moore, A. L.; Moore, T.
A.; Gust, D. J. Mater. Chem. 2002, 12, 2100-2108.
(15) (a) Bryce, M. R.; Coffin, M. A.; Hursthouse, M. B.; Karaulov, A. I.; Mu¨llen,
K.; Scheich, H. Tetrahedron Lett. 1991, 32, 6029-6033. (b) Pere´z, I.; Liu,
S.-G.; Mart´ın, N.; Echegoyen, L. J. Org. Chem. 2000, 65, 3796-3803. (c)
Liu, S.-G.; Pere´z, I.; Mart´ın, N.; Echegoyen, L. J. Org. Chem. 2000, 65,
9092-9102.
Electrochemistry. Compounds 9, 10, 19, and 20 in CV
experiments in MeCN and CH2Cl2 solutions show clear am-
(16) (a) Bryce, M. R.; Moore, A. J.; Hasan, M.; Ashwell, G. J.; Fraser, A. T.;
Clegg, W.; Hursthouse, M. B.; Karaulov, A. I. Angew. Chem., Int. Ed.
Engl. 1990, 29, 1450-1452. (b) Triki, S.; Ouahab, L.; Lorcy, D.; Robert,
A. Acta Crystallogr. 1993, C49, 1189-1192. (c) Bryce, M. R.; Finn, T.;
Batsanov, A. S.; Kataky, R.; Howard, J. A. K.; Lyubchik, S. B. Eur. J.
Org. Chem. 2000, 1199-1205. (d) Jones, A. E.; Christensen, C. A.;
Perepichka, D. F.; Batsanov, A. S.; Beeby, A.; Low, P. J.; Bryce, M. R.;
Parker, A. W. Chem. Eur. J. 2001, 7, 973-978. (e) Christensen, C. A.;
Batsanov, A. S.; Bryce, M. R.; Howard, J. A. K. J. Org. Chem. 2001, 66,
3313-3320.
(17) (a) Herranz, M. A.; Mart´ın, N. Org. Lett. 1999, 1, 2005-2007. (b) Mart´ın,
N.; Sa´nchez, L.; Guldi, D. M. Chem. Commun. 2000, 113-114.
(18) Herranz, M. A.; Gonzalez, S.; Perez, I.; Mart´ın, N. Tetrahedron 2001, 57,
725-731.
(19) Christensen, C. A.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K.;
Jeppesen, J. O.; Becher, J. Chem. Commun. 1999, 2433-2434.
(20) Herranz, M. A.; Mart´ın, N.; Sa´nchez, L.; Gar´ın, J.; Orduna, J.; Alcala´, R.;
Villacampa, B.; Sa´nchez, C. Tetrahedron 1998, 54, 11651-11658.
(21) Perepichka, I. F.; Popov, A. F.; Orekhova, T. V.; Bryce, M. R.; Andrievskii,
A. M.; Batsanov, A. S.; Howard, J. A. K.; Sokolov, N. I. J. Org. Chem.
2000, 65, 3053-3063.
(22) Godbert, N.; Bryce, M. R.; Dahaoui, S.; Batsanov, A. S.; Howard, J. A.
K.; Hazendonk, P. Eur. J. Org. Chem. 2001, 749-757.
9
14228 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002

2+
•-
Covalent D −σ−A Redox State
A R T I C L E S
Scheme 1 a
a
Reagents and conditions: (i) HSCH2CH2CO2Me + NaHCO3/MeCN, 20 °C, 6 h; (ii) H2O2/AcOH, 60 °C, 3.5 h; (iii) CF3CO2H/H2O, reflux, 4 h; (iv)
(COCl)2/CH2Cl2, 20 °C, 40 h; (v) CH2(CN)2/ DMF, 20 °C, 10-20 h; (vi) 6 + 8, pyridine/THF, 20 °C, 12 h; (vii) CH2(CN)2/DMF, 20 °C, 12 h.
Scheme 2 a
A•-/D0/A•-, A2-/D0/A2-, A•3-/D0/A•3- states for 20 (for 19,
possessing the lower EA, the radical trianion state is not stable).
The oxidation/reduction current ratio was 2:1 for compounds 9
and 10 and 1:1 for compounds 19 and 20, as expected for their
D/A stoichiometries.
The modification of fluorenone 9 to dicyanomethylene
derivative 10 significantly increases its acceptor properties: The
first reduction is shifted to a less negative value by ca. 0.3 V
(Table 1), making the Eoxpa - Eredpa gap as low as 0.25 V (anodic
0
peak potentials were used instead of Ered due to quasirevers-
ibility of the oxidation wave). When a solution of compound
10 was scanned negatively from +0.75 V (where the donor
fragment exists in the dication state) the first reduction wave
of the fluorene fragment merges with the reduction wave of
the TTFAQ2+ fragment, giving rise to a three-electron wave
(D2+/A0 + 3e f D0/A•-) (Figure 1). This arises from the large
separation between the anodic and cathodic oxidation peaks
a
Reagents and conditions: (i) 12 + LDA/THF, -78 °C, 1 h, then 11,
-78 °C f 20 °C, 12 h; (ii) Bu4NF /THF, 20 °C, 1.5 h, then H2O.
(.59 mV) due to the quasireversibility of the TTFAQ oxidation.
Moreover, the separation can be further increased at low
temperature and/or at high scan rate.15
Scheme 3 a
These observations led us to consider an intriguing idea:
Could this reduction wave of the TTFAQ2+ moiety be shifted
to a more negative potential than the first reduction of the
fluorene unit? If so, this could then result in the generation of
a sixth, hitherto unknown, redox state: D2+/A•-. Indeed, by
varying the experimental conditions (scan rate and temperature)
we have found that on the cathodic scan the D2+ f D0 wave
can be distinctively seen to occur after the A0 f A•- wave
even at 0 °C and scan rates higher than 500 mV s-1. A more
clear example at -15 °C and scan rate 300 mV s-1 is shown in
Figure 2. However, the low-temperature electrochemistry of 10
in MeCN (and also in CH2Cl2) is complicated by adsorption
phenomena (e.g., the sharpness and intensity of the -0.7 V peak
a
Reagents and conditions: (i) Br2/H2SO4/HNO3, 55-60 °C, 2 h; (ii)
CH2(CN)2/DMF, 20 °C, 2 h.
photeric multiredox behavior (Figure 1, Table 1), consisting of
three reversible single-electron reduction waves (corresponding
to the fluorene moiety)24 and one two-electron quasireversible
oxidation of the TTFAQ fragment. Therefore, five redox states
have been observed, and are listed as follows: neutral (D0/A0),
dication (D2+/A0), radical anion (D0/A•-), dianion (D0/A2-), and
radical trianion (D0/A•3-) for 9 and 10; A0/D0/A0, A0/D2+/A0,
D
2+/A•- f D0/A•- in Figure 2 are enhanced by adsorption).
To study in more detail the generation of the unusual species
2+/A•-, we undertook an investigation of the electrochemistry
D
(23) For synthetic details see the Supporting Information. A similar oxidation
reaction giving the TTF radical cation salt was also observed under these
conditions for 4-hydroxymethyl-TTF (ref 37).
of 10 in THF solution (where the adsorption processes are
diminished) at different temperatures, concentrations, and scan
(24) Only two reversible reductions have been observed for 19.
9
J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002 14229

A R T I C L E S
Perepichka et al.
Scheme 4 a
a Reagents and conditions: (i) pyridine/THF, 20 °C, 24 h; (ii) CH2(CN)2/DMF, 35 °C, 24 h.
Table 1. Redox Potentials in CH2Cl2 (ca. 10-4 M) vs Fc/Fc+ (20
°C, 0.2 M nBu4NPF6)
compound
E
ox
pa/V
E
pc/V
ox
E
1red0/V
E
2red0/V
E
3red0/V
4
7
8
-0.67
-0.31
-1.04
-0.91
-1.86
-1.57
-0.09
-0.07
-0.07
-0.13
-0.21
-0.32
-0.30
-0.33a
-0.39
-0.32
9
-0.65
-1.02
-0.94
-1.85
-1.53
10
13
14
16
17
19
20
-0.35a
-0.77
-0.39
-1.05
-0.61
-1.05
-0.93
-1.24
-1.15
-1.85
-1.65
-0.11
-0.11
-0.43
-0.43
-1.77
Figure 2. CV of compound 10 in MeCN at -15 °C, scan rate 300 mV s-1
(0.1 M nBu4NPF6) and a schematic representation of the electrochemical
reactions. The notation on the redox peaks refers to the redox state of the
donor/acceptor units, respectively, at that potential.
a These values could be determined only approximately due to an overlap
pc
pc
of Eox and E1red
.
confirmed by the increase in current (Figure 3, left column).
At -20 °C, however, the two-electron reduction of TTFAQ2+
moiety (D2+ f D0) is shifted to a more negative region and
occurs between the first (A0 f A•-) and the second reduction
steps (A•- f A2-) of the fluorene moiety (Figure 3, right
column). Therefore, the current values at Epc for the first cycle
(D0/A0 f D0/A•-) and the succeeding cycles of D2+/A0
reduction almost coincide, confirming single electron processes
(A0 f A•-) in both cases. Consequently, a D2+/A•- species is
generated on the return sweep (sweeping from D2+/A0).
Oxidation of 10 is a typical example of inverted potentials
in a two-electron process;25 i.e., it is easier to remove the second
electron than the first. The reason for this is the tremendous
gain in aromaticity in formation of the dication, which does
not occur for the radical cation. The significant conformational
change accompanying this transformation explains the relative
slowness of this process, which is manifested in a large
separation between the corresponding anodic and cathodic peaks.
The oxidation process can be described by the following
equations:
Figure 1. CV of compounds 9, 10, and 20 in MeCN, 0.1 M nBu4NPF6.
rates and performed a quantitative analysis of the experimental
data. Controlled potential electrolysis (at the potential of the
anodic peak) of model TTFAQ 8 in THF consumes two
electrons per molecule and confirms a two-electron process. As
above, the CV of 10 in THF exhibits three reversible one-
electron reduction processes (only the first two were subjected
to analysis). The electrochemical potentials of these reduction
waves are independent of scan rate, concentration, and temper-
ature. In the positive direction, there is only one oxidation peak
and one reduction peak on the return sweep with a very large
separation between them, which increases with lowering the
temperature and with increasing the scan rate (Figure 3).
Thus, at +20 °C the reduction step of the TTFAQ2+ moiety
occurs at the potential of the first reduction step of the fluorene.
Therefore, the single electron reduction at the first cycle
(D0/A0 f D0/A•-) is turned into a three-electron reduction at
this potential on the succeeding cycles (D2+/A0 f D0/A•-), as
D•+-A + e- h D-A
D2+-A + e- h D•+-A
(Eox1, kox1, Rox1
(Eox2, kox2, Rox2) and
Eox2 < Eox1
)
2D•+-A h D2+-A + D-A (Kdisp, kdisp, kcop
)
and the reduction steps are composed of three reversible single-
electron transfers (D-A/D-A•-, D-A•-/D-A2-, D-A2-/D-
A•3-).
(25) Kai, H.; Evans, D. H. J. Electroanal. Chem. 1997, 423, 29-35.
9
14230 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002

2+
•-
Covalent D −σ−A Redox State
A R T I C L E S
Figure 3. Experimental (A) and simulation (B) voltammograms for 0.6 mM 10 in 0.2 M nBu4NPF6/THF at +20 °C (left column) and at -20 °C (right
column).
Table 2. Redox Potentials and Electron Transfer Parametersa for Compound 10 Estimated by Fitting with Digisim 3.05'
redox process
b
2
2-
•-
•+
2+
•+
T/°C
+20
D /cm s-1
D−A•-/D−A
D−A/D−A
D −A/D−A
D −A/D −A
3.8 × 10-6
E°/V
R
-0.955
0.5
-0.375
0.5
0.01c
-0.09c
0.55d
0.40d
k/cm s-1
E°/V
R
1.0 × 10-2
1.0 × 10-2
1.0 × 10-2
0.03c
1.0 × 10-4
0
-20
-45
3.1 × 10-6
2.4 × 10-6
1.8 × 10-6
-0.945
0.5
-0.375
0.5
-0.12c
0.60d
0.30d
k/cm s-1
E°/V
R
0.8 × 10-2
0.8 × 10-2
0.8 × 10-2
0.04c
0.8 × 10-4
-0.945
0.5
-0.375
0.5
-0.15c
0.65d
0.25d
k/cm s-1
E°/V
R
0.6 × 10-2
0.6 × 10-2
0.6 × 10-2
0.07c
0.60 × 10-4
-0.950
0.5
-0.375
0.5
-0.20c
0.70d
0.15d
k/cm s-1
0.1 × 10-2
0.1 × 10-2
0.1 × 10-2
0.1 × 10-4
a Potentials E° vs Fc/Fc+ (0.5 mM substrate in 0.1 M nBu4NPF6/THF); R is an electron-transfer coefficient; k is an electron-transfer rate constant.
b Diffusion coefficient; it was set the same for all the redox states. c These values were imposed according to the procedure of ref 25. d These values are
standard in inverted potentials in a two-electron process: ref 25.
A quantitative analysis of the data over the scan rate range
and the temperature range investigated was carried out with
Digisim 3.05. Good simulation fits were found for scan rates
from 0.1 to 5.0 V s-1 at +20, 0, -20, and -45 °C (Table 2
and Figure 3).26 Agreement was improved at faster scan rates
by inclusion of the disproportionation reaction with kdisp ) 106
M-1 s-1, although this does not provide an accurate assessment
of this parameter (Table 2).
Thus, the CV experiments demonstrate clear evidence for
existence of the D2+/A•- state for compound 10, and parameters
found from simulation confirm faster kinetics for reduction of
the fluorene moiety in the D2+/A0 state vs the TTFAQ moiety.
We note that the difference in electrochemical kinetics between
two sites in one molecule having close redox potentials was
recently used in the design of a bifunctional molecule that
receives two electrons sequentially through only one of its two
electroactive groups.27 Also, it was recently shown that in
carotenoids (which play an important role in photosynthesis28)
an inversion of the standard potentials for the first and second
electron transfer substantially depends on the structure.29
Therefore, our finding of redirection of electron transfer to the
thermodynamically less favorable site by changing electro-
(27) Zheng, Z.-R.; Evans, D. H. J. Am. Chem. Soc. 1999, 121, 2941-2942.
(28) (a) Frank, H. A.; Cogdell, R. J. In Carotenoids in Photosynthesis; Young,
A., Britton, G., Eds.; Chapman and Hall: London, 1993; Chapter 8, p 252.
(b) Faller, P.; Pascal, A.; Rutherford, A. W. Biochemistry 2001, 40, 6431-
6440.
(26) CVs for the scan rates from 0.1 to 1.0 V only are shown on Figure 3 for
clarity.
9
J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002 14231

A R T I C L E S
Perepichka et al.
zero point time), so negative absorption at 360-470 nm for
D
2+/A0 is due to consumption of D0/A0 (Figure 4). The second
cycle (300-600 s on time scale) essentially repeats the evolution
of the spectrum, confirming the reversibility of all redox
processes.
The same time-resolved SEC experiment at -30 °C indicates
that on the first cycle, the evolution of the spectrum is the same
as at +20 °C, i.e., reversible formation D0/A•- f D0/A2-
f
D0/A•- f D0/A0f D2+/A0 species (Figure 6). However, it
becomes clearly different on the second cycle, starting from
the D2+/A0 species. At +20 °C, the second cycle showed sharp
transition from D2+/A0 (disappears at E ) -0.25 V (t ) 325
s), Figure 5A,B) to D0/A•- (appears at E ) -0.35 V (t ) 335
s)) with possible existence of D0/A0 in the narrow potential
region. At -30 °C, however, a specific potential window
displays both a negative absorption in the short-wavelength
region (360-470 nm) characteristic for the D2+ moiety and a
low-intensity, but visible, positive absorption at 480-570 nm,
characteristic for the A•- moiety. Thus at 700 s, which
corresponds to Epc ) -0.3 V for the reduction of the D2+/A0
species (Figure 6A), the negative absorption at 360-470 nm
corresponding to D2+ moiety has not yet disappeared (Figure
6B) and remains observable until ca. E ) -0.75 V (t ) 790 s),
whereas a longer wavelength absorption at 500-550 nm starts
to appear at E ) -0.50 V (t ) 740 s). We attribute this to the
existence of the transient species D2+/A•-, which is expected
to combine the spectral characteristics of the D2+ and A•-
moieties. As the CV profile (Figure 6A) shows no good
resolution between D2+/A0 f D2+/A•- and D2+/A0 f D0/A0
processes, the coexistence of D2+/A•-, D0/A0, and D0/A•-
species is possible. Also, due to the method limitation for these
time-resolved SEC experiments [thin-layer (TL) conditions, low
scan rate, low substrate concentration to avoid an adsorption]
the concentration of D2+/A•- species in the solution was much
lower than that in the CV experiment preventing its precise
spectral characterization. Nevertheless, the characteristic short-
wavelength negative absorption at 730-790 s does support our
conclusions on the generation of D2+/A•- in the CV experiment.
As follows from the CV of 10 at low temperature (Figure 3),
the two-electron reduction D2+ f D0 occurs as a very broad
peak overlapping with the single-electron reduction A0 f A•-
and extending into the next reduction step (A•- f A2-). In the
time-resolved SEC experiments at -30 °C, this results in
decreased absorption (at 400-500 nm) from the D0/A2- species
at E ≈ -0.8 to -0.9 V (t ) 800-840 s) (positive absorption
from A2- is compensated by negative absorption from D2+),
Figure 4. UV-vis spectroelectrochemistry for compound 10 in thin layer
conditions (d ≈ 100 µm, +20 °C).
chemical kinetic factors may be important for studying electron
transfer in biosystems.29,30
Spectroelectrochemistry. To support our conclusions on the
unusual electrochemistry of 10 we performed UV-vis spectral
characterization of D0/A•-, D/A2-, and D2+/A0 redox states.
Figure 4 demonstrates reflection-absorption spectra for com-
pound 10 in the neutral state (D0/A0) as well as D0/A•-, D/A2-
,
and D2+/A0 redox states at room temperature. The neutral state
is featured by two peaks at 362 and 432 nm (the last shows
also a shoulder at ca. 415 nm), characteristic for the TTFAQ
nucleus.16d These remain essentially unchanged on reduction
to D0/A•- (λmax ) 368, 412, and 432 nm with near the same
intensity), and the wavelengths are essentially unchanged with
higher intensity on further reduction to D/A2- (λmax ) 369, 412
(sh), and 432 nm). Additional absorptions at 515 and 550 (sh)
nm from the radical anion of the fluorene moiety emerge for
D0/A•-, but then disappear for the D/A2- state. A similar
absorption spectrum (λmax ) 470, 515, and 550 nm) was recently
observed for the electrochemically generated radical anion of
TTF-σ-fluorene diad 1.11b On oxidation to D2+/A0, the peak at
368 nm decreases, and the peak at 432 nm almost completely
disappears, whereas low intense broad absorption is observed
as shoulders at 460 and 490 nm.
Because the D2+/A•- species was observed in CV only as a
transient at low temperatures, we performed comparative
simultaneous CV-UV-vis spectral (time-resolved SEC) ex-
periments for 10 at +20 °C and at -30 °C, in thin-layer
conditions at scan rate 5-10 mV/s31 (in reflection spectral
mode). Figure 5 shows evolution of the spectrum during the
cyclic voltammetry at +20 °C: starting from the neutral state
D0/A0 and sweeping the potential to the negative direction, the
first cycle results in reversible formation of D0/A•- f D0/A2-
f (reverse sweep direction) f D0/A•- f D0/A0 f D2+/A0
species with their characteristic absorptions. A spectrum of the
neutral D0/A0 state was used as a background (yellow color at
whereas an oxidation on the back scan (D0/A2- f D0/A•-
f
D0/A0f D2+/A0) nicely reproduces the spectral features of all
consequent redox states (Figure 6B).
IR Spectra. The solid-state IR spectral studies (in KBr
pellets) show that the CtN stretching frequency of compound
10, νCtN ) 2185 cm-1 (and a second very weak signal at 2154
cm-1), is at significantly lower wavenumber as compared to
that in a neutral fluorene moiety (represented by compounds 7
and 21, νCtN ) 2233 cm-1) and is consistent with that of the
ion radical salt (21•-)Li+ (2185 cm-1).11b A similar change in
the CtN stretching frequency from 2225 to 2180 cm-1 was
also found for the TCNQ0/TCNQ•- system.32,33 The CN
(29) An inversion of the standard potentials for the first and second electron
transfer substantially depends on the structure of carotenoids: an inversion
occurs in the oxidation of â-carotene and in the reduction of canthaxanthin,
but not in the reduction of â-carotene or in the oxidation of canthaxanthin:
Hapiot, P.; Kispert, L. D.; Konovalov, V. V.; Save´ant, J.-M. J. Am. Chem.
Soc. 2001, 123, 6669-6677.
(30) Dwyer, T. M.; Mortl, S.; Kemter, K.; Bacher, A. Fauq, A.; Frerman, F. E.
Biochemistry 1999, 38, 9735-9745.
(31) A higher scan rate which would provide a higher concentration of D2+
/
A•- species could not be achieved in the thin layer SEC cell due to an
increase in the uncompensated resistance, especially in a low-polar solvent
(e.g., THF, which was used to prevent the adsorption of the D2+/A0 species)
and at low electrolyte concentration (to prevent its precipitation at low
temperatures).
(32) Chappell, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M.; Poehler, T.
O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442-2447.
9
14232 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002

2+
•-
Covalent D −σ−A Redox State
A R T I C L E S
Figure 5. Time-resolved UV-vis spectroelectrochemistry for compound 10 in thin layer conditions (d ≈ 50 µm, +20 °C), in 2D (B) and 3D (C) presentation.
CV conditions (A): scan rate 10 mV/s, potentials range -0.9 to +0.6 V, vs Ag wire scanning in the negative direction starting at 0 V.
Figure 6. Time-resolved UV-vis spectroelectrochemistry for compound 10 in thin layer conditions (d ≈ 50 µm, -30 °C), in 2D (B) and 3D (C) presentation.
CV conditions (A): scan rate 5 mV/s, potentials range -0.9 to +0.7 V, vs Ag wire scanning in the negative direction starting at 0 V.
ing frequency for the weaker band at 2154 cm-1 in 10 and
Chart 2
14‚(17)2 may indicate more negative charge accumulated on
the cyano groups, as would be the case for a dianion species.
Due to the lower electron affinity of the acceptor moiety in
compound 20 there is a small shift of the CtN peak (to νCtN
) 2227 cm-1) due to much weaker charge transfer in this
compound.
A key difference between 10 and 14‚(17)2, apart from the
covalent linkage of the donor and acceptor components in the
former, is the different D/A stoichiometry. The 1:2 D/A ratio
in complex 14‚(17)2 satisfies the two-electron donor and one-
electron acceptor characteristics of the components, whereas this
is not the case for 10, where the D/A ratio was designed to be
1:1. This dilemma is solved in the solid state by intermolecular
interaction with another fluorene moiety from an adjacent
molecule accepting the second electron from the TTFAQ moiety.
However, in solution the donor and acceptor fragments of one
molecule are expected to interact intramolecularly only, which
should result in very different CT behavior. Unfortunately, the
extremely low intensity of the CN signal in solution (an IR
spectrum of 10 in CH2Cl2 shows no significant absorption in
the region expected for the CN group, Figure S2 in Supporting
Information)35 prevented a detailed investigation of this interest-
ing point.
stretching frequencies for 10 are in excellent agreement with
νCtN ) 2185 and 2154 cm-1 found for the ionic conductive
complex of a similar fluorene with BEDO-TTF (22)2‚21.34
Similar values of the CtN stretching were obtained for another
ionic complex, (21)2‚23 (2182 cm-1),33 whereas an increase of
νCtN to 2203 cm-1 was observed in TTF-fluorene CTC with
partial charge transfer, 21‚TTF11b (for neutral acceptor 21 νCtN
) 2233 cm-1). These data suggest a complete electron transfer
in the diad 10 with formation of the fluorene radical anion in
the solid state. This conclusion is in full agreement with X-ray
analysis (see below) of the CTC 14‚(17)2 (the CtN frequencies
of which are almost identical to those of 10) where the TTFAQ
and fluorene components exist as the dication and radical anion
species, respectively. The further lowering of the CtN stretch-
(35) We also observed a dramatic decrease in the intensity of the CN stretching
in the IR spectra in solution (compared to that in KBr pellets) for other
fluorene-9-dicyanomethylene acceptors, e.g., for compounds 7 νCN ) 2233
cm-1 as an extremely weak band. Variation of the solvent did not restore
the peak intensity. A marked variation in the intensity of CN absorption
from strong to undetectable in different solvents and with the incorporation
of certain substituents is well documented: Bellamy, L. J. In The infrared
spectra of complex molecules, 3rd ed.; Chapman and Hall: London and
New York, 1975; Vol. 1, p 297.
(33) Saito, G.; Hirate, S.; Nishimura, K.; Yamochi, H. J. Mater. Chem. 2001,
11, 723-735.
(34) Horiuchi, S.; Yamochi, H.; Saito, G.; Sakaguchi, K.; Kusunoki, M. J. Am.
Chem. Soc. 1996, 118, 8604-8622.
9
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A R T I C L E S
Perepichka et al.
Figure 8. Temperature dependence of the intensity of the EPR signal of
10 in the solid state.
Figure 7. EPR spectrum of 10 in CH2Cl2 at 105 K (A) and in 0.1 M Bu4-
NPF6/THF at 298 K (B).
CTCs), which is stable in the solid state but easily dissociates
in solution.
EPR experiments for 10 in 0.2 M nBu4NPF6/THF solution
showed hyperfine structure of the spectrum even at room
temperature (g ) 2.0024), although the signal was very weak
(Figure 7B). The spectrum observed is very similar to that of
the ion radical salt (21•-)Li+,37 which displays hyperfine
coupling to two sets of two equivalent 14N nuclei (a2N ) 3.25
and 0.51 MHz, respectively) and two sets of two equivalent 1H
nuclei (a2H ) 3.38 and 0.14 MHz, respectively, from EPR and
electron nuclear double resonance measurements). We assigned
Electron Paramagnetic Resonance (EPR) Studies. The high
donor and acceptor ability of the TTFAQ and fluorene moieties,
respectively, in 10 results in the appearance of new properties
in this donor-acceptor diad. Whereas isolated TTFAQ 8 and
fluorene 7 are both EPR silent (as expected), facile electron
transfer in 10 results in a strong EPR signal in the solid state
and a less intense signal in solution. It also manifests in
paramagnetic broadening of the signals in the 1H NMR spectra
for protons adjacent to the fluorene nucleus of 10 (but not for
9, 19, and 20 which possess weaker acceptor moieties). Since
the TTFAQ radical cation is not stable16d (it rapidly dispropor-
tionates to form the diamagnetic dication TTFAQ2+), only one
EPR signal is expected, corresponding to the radical anion of
the fluorene moiety. Indeed, both acetone and CH2Cl2 solutions
as well as a powdered sample (even at Q-band frequency) give
a broad single line36 centered at g ) 2.0028-2.0034 which is
close to the measured g-value of the ion radical salt (21•-)Li+ 37
(TTF-like radical cations have g ≈ 2.007 in fluid solution38).
When a CH2Cl2 solution is cooled to 250 K, a poorly resolved
hyperfine structure is observed, but the signal broadens on
further cooling, and on freezing to 105 K, a single line is
observed at g ) 2.0034 (Figure 7A). Spin-counting experiments
reveal that only ca. 1% of the molecules exist in a radical form
in CH2Cl2 solution at room temperature, thus indicating an
essentially neutral ground state for 10 in solution.39 Despite the
low radical concentration, we believe that the EPR signal in
solution is a result of electron transfer in compound 10 itself
and not due to impurity. Most probably, electron transfer occurs
only in the donor-acceptor complexed state (head-to-tail
intramolecular or intermolecular, or a combination of those
1
the larger of the two H couplings to the 3 and 6 positions of
the fluorene ring. The EPR spectrum of 10 can be simulated
with two equivalent 14N (3.4 MHz) and two equivalent 1H nuclei
1
(3.4 MHz).40 By analogy with (21•-)Li+, we assign the H
coupling to the 3,6 positions of the fluorene. Even though these
are no longer chemically equivalent in 10, an SO2R group has
a very similar electronic influence to NO2,41 and we do not
resolve the difference between the two sites.
Simultaneous electrochemistry and EPR (SEEPR) experi-
ments for the above solution were performed by continuous
scanning of the potential from +0.8 to -0.2 V vs Ag wire at
1000 mV/s to generate a quasistationery concentration of D0/
A•- and D2+/A•- radicals in the EPR cell (see Supporting
Information).42 The population of the D2+/A•- species was
expected to increase with lowering the temperature. However,
there was no significant difference in the spectral shape or
position for +20, -10, and -40 °C and only a small increase
in intensity was observed (as expected for ground-state radicals).
We attribute this signal to the D0/A•- radical anion. Thus, we
were unable to register the transient D2+/A•- species by SEEPR
spectroscopy, which could be due to the fact that its signal is
very similar to that of D0/A•-.
Variable-temperature studies on powdered samples of 10
show that the intensity of the signal increases monotonically
with decreasing temperature down to 10 K (Figure 8), indicating
a ground (not thermoexcited) state character of the radical
species. The ionic CTC 14‚(17)2 gives rise to almost the same
EPR spectra: A single line corresponding to the fluorene radical
anion was found in the solid state (g ) 2.0031) and a similar,
(36) A much less (1/10) intense signal at g ) 2.007 (the value expected for a
TTF-like radical cation) was also observed at 300 K in CH2Cl2 only. Given
its very low concentration and the fact that it disappears on changing the
solvent, concentration, or temperature, we cannot unambiguously assign
this signal.
(37) Perepichka, D. F.; Bryce, M. R.; Batsanov, A. S.; McInnes, E. J. L.; Zhao,
J. P.; Farley, R. D. Chem. Eur. J. 2002, 8, 4656-4669.
(38) (a) Ribera, E.; Rovira, C.; Veciana, J.; Tarre´s, J.; Canadell, E.; Rousseau,
R.; Molins, E.; Mas, M.; Schoeffel, J.-P.; Pouget, J.-P.; Morgado, J.;
Henriques, R. T.; Almeida, M. Chem. Eur. J. 1999, 5, 2025-2039. (b)
Cavara, L.; Gerson, F.; Cowan, D. O.; Lerstrup, K. HelV. Chim. Acta 1986,
69, 141-151.
(39) The free radical concentration in the solid state was estimated at 298 K by
spin counting experiment to be ca. 15%, as a result of antiferromagnetic
interaction between neighboring fluorene moieties. Lowering of the
observed spin number (in respect to the theoretical value) is typical for
many radical ion salts, e.g., ref 3b.
(40) The significantly larger line width of 10, cf. (21•-)Li+, precludes observation
of the smaller couplings.
(41) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(42) Fast scanning of the potential with continuous recording of the spectra was
employed as a practically achievable alternative to time-resolved pulse
SEEPR.
9
14234 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002

2+
•-
Covalent D −σ−A Redox State
A R T I C L E S
though less intense, signal in CH2Cl236 (g ) 2.0029) and acetone
(g ) 2.0027) solutions. Taking into account the results of X-ray
studies (see below) and IR spectroscopy, it is evident that a
full electron transfer giving rise to the fluorene radical anion
and dication of TTFAQ in the solid state of the diad 10 and the
CTC 14‚(17)2. Due to the 1:1 donor/acceptor ratio in 10, the
TTFAQ moiety in this diad presumably exists as a mixture of
the neutral and dication states, which is also consistent with
the UV-vis spectrum of 10 in solid films (see below). The
stability of this charge-separated species can be explained by
aromatic stabilization of the TTFAQ2+ 43 unit and also by
increased crystalline forces for the ionic molecules.
Another manifestation of the CT character of these com-
pounds is their pronounced electrical conductivity. Both com-
pound 10 and complex 14‚(17)2 show very similar conductivity
values: σrt ) 2 × 10-6 and 8 × 10-6 S cm-1, respectively
(two-probe method on compressed pellets). This conductivity
of 10 is quite high for a single-component purely organic
monomeric material,3,12,44 although substantially higher values
(10-2 S cm-1) have been reported recently.45
Figure 9. Dication, radical anion, and solvent in the crystal structure of
14‚(17)2‚2MeCN.
This is in agreement with suggested disproportionation of 10
in the solid state to give a mixture of D2+-σ-A•- and D0-
σ-A•-.
A solution of the intermolecular CT complex 14‚(17)2 in THF
(1.5 × 10-3 M) does not give any observable long-wavelength
absorption band due to dissociation of the complex. However,
a strong donor-acceptor interaction in the solid state of this
complex results in full electron transfer with formation of
dication 142+ and two radical anions 17•- as confirmed by X-ray
analysis. It presents the first definite example of a fully ionic
fluorene CTC.48 This is in contrast to analogous CTCs TTF‚17
and Me3BrTTF‚17 which show only partial charge transfer (see
Supporting Information). In the structure of 14‚(17)2‚2MeCN,
the TTFAQ2+ unit has crystallographic Ci symmetry, while the
fluorene and the acetonitrile molecule of crystallization occupy
general positions (Figure 9). As with other TTFAQ systems,16
the conformation of 14 drastically changes on oxidation: The
folded anthracenediylidene moiety is converted into a planar
(within (0.03 Å), fully aromatic anthracene system (details of
the X-ray structural analysis of isolated 14 and 17 are given in
Supporting Information). Each dithiole ring acquires a +1
charge, which is manifested in the shortening of C-S bonds
(ca. 1.68-1.72 Å) and lengthening of the CdC bonds (1.360(4)
Å) in the dithiole ring (for more details see Table S1 in
Supporting Information), as well as planarization of the ring
(Figure 9). The correlation between positive charge and C-S
distances is well studied for TTF,49 and the same approach can
be applied to TTFAQ (e.g., in 2,6-dibutoxy-9,10-bis(1,3-dithiol-
2-ylidene)-9,10-dihydroanthracene, C(2)-S(1), C(3)-S(1), and
C(3)dC(4) distances are of 1.770(3), 1.745(3), and 1.319(4)
Å, respectively, for the neutral state and of 1.680(2), 1.710(2),
and 1.346(3) Å, respectively, in its charge-transfer salt with
TCNQ2‚2MeCN).16c Thus, the dithiole ring in 142+ has es-
sentially the same geometry as that in (TTFMe4)(ClO4)2.50 The
CT Complexation in Solution and in the Solid State.
Examination of the electronic absorption spectra of compounds
9 and 10 in different solvents (CH2Cl2, CHCl3, MeCN) revealed
only very weak CT interaction for both compounds. There were
very broad weak bands (Figure S3 in Supporting Information)
in the region ca. 500-1300 nm (λmax ≈ 650 nm, ꢀ ≈ 60 M-1
cm-1) for 9 and ca. 500-2000 nm (λmax 780 and 1150 nm, ꢀ ≈
50 M-1 cm-1, in CH2Cl2) for 10. A linear dependence was found
in the concentration range 5 × 10-4 to 5 × 10-3 M consistent
with the intramolecular character of these bands.46 Taking into
account the EPR data, it is very likely that absorption of the
fluorene radical anion contributes to the long-wavelength bands
of 10.47 This behavior is in contrast to the analogous TTF-
σ-fluorene diad 1,11b where the ICT band (λmax ) 1230 nm) is
considerably more intense (ꢀ ) 4700 M-1 cm-1). The reason
for this difference is probably a highly bent geometry of the
TTFAQ moiety, which prevents efficient π-complexation
between the donor and acceptor fragments in 10. In the solid
state, however, the UV-vis spectrum of 10 (see Supporting
Information) shows very strong long-wavelength absorbance
consistent with superposition of TTFAQ0, TTFAQ2+ 16d
and
,
fluorene•- 11b characteristic bands (see also SEC part) but with
no peak at 1150 nm for a CT band (as observed in solution).
(43) Although formation of cation radical also yields one aromatic dithiolium
ring, the largest gain of aromaticity occurs on transition from cation radical
(which essentially preserves the geometry of the neutral species) to fully
aromatic dication: (a) Reference 16d. (b) Mart´ın, N.; Sa´nchez, L.; Seoane,
C.; Ort´ı, E.; Viruela, P. M.; Viruela, R. J. Org. Chem. 1998, 63, 1268-
1279.
(44) (a) Inokuchi, H.; Imaeda, K.; Enoki, T.; Mori, T.; Marayama, Y.; Saito,
G.; Okada, N.; Seki, K.; Higuchi, Y.; Yasuoka, N. Nature 1987, 329, 39-
40. (b) Cordes, A. W.; Haddon, R. C.; Oakley, R. T.; Schneemeyer, L. F.;
Waszczak, J. V.; Young, K. M.; Zimmerman, N. M. J. Am. Chem. Soc.
1991, 113, 582-588. (c) Yamashita, Y.; Tanaka, S.; Imaeda, K.; Inokuchi,
H. Chem. Lett. 1991, 1213-1216. (d) Cordes, A. W.; Haddon, R. C.;
Oakley, R. T. AdV. Mater. 1994, 6, 798-802.
(45) (a) Chi, X.; Itkis, M. E.; Patrick, B. O.; Barclay, T. M.; Reed, R. W.; Oakley,
R. T.; Cordes, A. W.; Haddon, R. C. J. Am. Chem. Soc. 1999, 121, 10395-
10402. (b) Chi, X.; Itkis, M. E.; Kirschbaum, K.; Pinkerton, A. A.; Oakley,
R. T.; Cordes, A. W.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 4041-
4048.
(46) The low accuracy accompanying measurements of such weak bands does
not exclude the possibility that intermolecular complexation may contribute
to these absorption bands.
(47) The radical anion of the fluorene moiety has moderately intense bands in
the region 500-2000 nm (ꢀ800 nm ≈ 4 × 10-3 M-1 cm-1, ref 11b and 37),
and TTFAQ2+ does not absorb beyond 600 nm (ref 16d).
(48) All the known X-ray crystal structures of nitrofluorene-TTFs CTCs are
neutral or have only partial charge transfer: (a) Reference 21. (b)
Perepichka, I. F.; Perepichka, D. F.; Lyubchik, S. B.; Bryce, M. R.;
Batsanov, A. S.; Howard, J. A. K. J. Chem. Soc., Perkin Trans. 2 2001,
1546-1551. (c) Bryce, M. R.; Moore, A. J.; Batsanov, A. S.; Howard, J.
A. K.; Robertson, N. R.; Perepichka, I. F. In Supramolecular Engineering
of Synthetic Metallic Materials: Conductors and Magnets; Veciana, J.,
Rovira, C., Amabilino, D. B., Eds.; NATO ASI Series; Kluwer Publish-
ers: Dordrecht, 1999; Vol. 518, pp 437-449. (d) Perepichka, I. F.;
Kuz’mina, L. G.; Perepichka, D. F.; Bryce, M. R.; Goldenberg, L. M.;
Popov, A. F.; Howard, J. A. K. J. Org. Chem. 1998, 63, 6484-6493. (e)
Moore, A. J.; Bryce, M. R.; Batsanov, A. S.; Heaton, J. N.; Lehmann, C.
W.; Howard, J. A. K.; Robertson, N.; Underhill, A. E.; Perepichka, I. F. J.
Mater. Chem. 1998, 8, 1541-1550. (f) Soriano-Garc´ıa, M.; Toscano, R.
A.; Robles Mart´ınez, J. G.; Salmero´n, U. A.; Lezama R. R. Acta Crystallogr.
1989, C45, 1442-1444.
(49) Clemente, D. A.; Marzotto, A. J. Mater. Chem. 1996, 6, 941-946.
9
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A R T I C L E S
Perepichka et al.
three-electrode cell (Ag/Ag+ reference). The working electrodes were
1, 1.6, or 2 mm diameter platinum disks polished to a mirror finish.
Platinum wire was used as an auxiliary electrode. An ohmic drop
compensation was applied when necessary. Numerical simulations of
the voltammograms were performed with Digisim 3.05 using the default
numerical options with the assumption of planar diffusion. The Butler-
Volmer law was applied for the electron-transfer kinetics.
Spectroelectrochemistry. Routine UV-vis SEC experiments have
been performed with a Perkin-Elmer Lambda 19 NIR spectrometer.
Time-resolved spectroelectrochemistry was performed using the re-
ported experimental setup.54 In both cases, CV was performed in thin-
layer conditions (50-100 µm) using 0.1 M nBu4NPF6/THF as an
electrolyte, and spectra were recorded in reflection mode using a Pt
disk 5 mm in diameter as working electrode and a reflector and Ag
wire as a quasireference electrode. For each experiment, the samples
(cell and solution) were prepared in a glovebox containing dry, oxygen-
free (<1 ppm) argon.
EPR Experiments. X- and Q-band EPR spectra were recorded on
a Bruker EMX and a Bruker ESP 300E spectrometer, respectively. The
solid-state variable-temperature (300-10 K) studies on 10 were
performed at Q-band under nonsaturating conditions determined at 10
K (0.1 mW microwave power; 100 kHz and 5 G modulation frequency
and amplitude, respectively). Spin-counting experiments on 10 in
degassed CH2Cl2 were performed at X-band and room temperature by
calibration against a range of concentrations of standard 2,2-di(4-tert-
octylphenyl)-1-picrylhydrazyl solutions in benzene measured under
identical, nonsaturating conditions. The experimental protocol was tested
for accuracy against solutions of known concentration of other radical
species.
SEEPR experiments were carried out on a Bruker ESP 300
spectrometer driven by Bruker’s ESP 1600 software. The design of
the SEEPR-electrochemical cell was inspired by the work of Fiedler
et al.55 Experiments were performed in the X-band frequency range
(∼9.4 GHz) in a TE102 cavity. Low-temperature measurements
involved a standard variable-temperature cryostat using liquid nitrogen
evaporation. The samples (cell and solution) were prepared in a
glovebox containing dry, oxygen-free (<1 ppm) argon.
Methyl 3-(4,5,7-Trinitro-9-oxo-2-fluorenylsulfanyl)propanoate
(3). To a solution of fluorenone 2 (2.00 g, 5.62 mmol) in acetonitrile
(200 mL), finely powdered NaHCO3 (2.0 g, 24 mmol) was added,
followed by methyl 3-mercaptopropionate (1.5 mL, 14 mmol). The
reaction mixture was stirred at 20 °C for 6 h, then left at 0 °C for 15
h. The complete consumption of the starting fluorenone was confirmed
by thin-layer chromatography. The reaction mixture was filtered from
the inorganic salts, concentrated in vacuo to ca. 15 mL, and diluted
with hot propan-2-ol (50 mL). The yellow precipitate which formed
on cooling was filtered off, washed with propan-2-ol and with water,
and dried in vacuo, giving pure sulfide 3 (1.86 g, 76%), mp 163-165
°C.56 m/z (EI): 433 (M+, 11%), 149 (100%). 1H NMR (200 MHz,
CDCl3): δ 8.93 (1H, d, J ) 2 Hz), 8.75 (1H, d, J ) 2 Hz), 7.95 (1H,
d, J ) 2 Hz), 7.92 (1H, d, J ) 2 Hz), 3.75 (3H, s), 3.42 (2H, t, J ) 7
Hz), 2.79 (2H, t, J ) 7 Hz). 13C NMR (75 MHz, CDCl3): δ 185.8,
171.1, 151.1, 148.6, 147.1, 146.9, 145.5, 139.5, 137.61, 137.56, 128.5,
127.0, 125.51, 125.50, 122.4, 52.3, 33.0, 27.4. Anal. Calcd for
C17H11N3O9S: C, 47.12; H, 2.56; N, 9.70. Found: C, 46.76; H, 2.54;
N, 9.73.
dithiolium and anthracene systems form a dihedral angle of 62°
and are connected by the essentially single C(23)-C(24) bond
of 1.488(4) Å (1.34(2) Å for the neutral state, see Table S1 in
Supporting Information).51 Comparison of the 17•- anion with
the neutral molecule (Table S1 in Supporting Information) shows
a shortening of the previously single bonds a, c, and e (Scheme
3) and lengthening of the b and c bonds, resulting in a nearly
uniform delocalization of π-electron density in the five-
membered fluorene ring and the dicyanomethylene group. These
changes resemble those which occur on reduction of TCNQ to
the anion radical.52 The Br and NO2 substituents are disordered
between positions 4 and 5, in an 85(A):15(B) ratio (similar “4/5
disorder” occurs in every structure studied, reducing the
precision). Repulsion between these substituents causes the
fluorene system to warp, but slightly less than that in neutral
17. The average deviation of the 13 carbon atoms from their
mean plane is 0.08 Å, with a maximum deviation of 0.19 Å.
There is a 7° twist around d.
Conclusions
We have shown that extended-TTF donors and fluorene
acceptors can be used as convenient synthons in the design of
D-σ-A systems with high amphotericity. We have synthesized
TTFAQ-σ-fluorene diad 10 and have demonstrated that it
displays clear multistage redox behavior (three single-electron
reduction and one two-electron oxidation processes): Eox - Ered
is as low as 0.25 V, which is to the best of our knowledge the
lowest known value for closed shell D-σ-A systems. It has
been demonstrated by CV experiments on diad 10, and
supported by time-resolved UV-vis spectroelectrochemistry,
that under certain conditions a previously unknown D2+-σ-
A•- redox state can been generated. This finding represents an
important example of preferable kinetically controlled electron
transfer onto a thermodynamically less favorable center, which
suggests a new approach for controlling electron-transfer
processes. In contrast to TTF-fluorene intermolecular CTCs
and D-σ-A diads, the donor-acceptor interaction between the
TTFAQ and the fluorene moieties in the solid state results in
complete one-electron transfer, as demonstrated by X-ray
analysis (for the intermolecular complex 14‚(17)2), Fourier
transform infrared spectroscopy (FTIR), and EPR data [for 10
and 14‚(17)2]. In solution, however, only a low-spin concentra-
tion (1%) and no CT absorption were observed for 10, indicating
the facile dissociation of the complex.
Experimental Section
General details are the same as described previously.53 UV-vis-
near-IR spectra were recorded on Varian Cary 5 spectrophotometer.
Electrochemistry. Cyclic voltammetric experiments have been
carried out at 298 and 253 K ((0.1 K) with EG&G PAR 273A
potentiostat/galvanostat or BAS CV50 electrochemical analyzers in a
Methyl 3-(4,5,7-Trinitro-9-oxo-2-fluorenylsulfonyl)propanoate
(4). To a hot solution of sulfide 3 (0.43 g, 0.92 mmol) in acetic acid
(15 mL), H2O2 (33% aqueous solution; 0.7 mL) was added and the
reaction mixture was stirred at 60 °C for 3.5 h. The solution was then
concentrated in vacuo to 5-7 mL, and the hot solution was diluted
(50) Shibaeva, R. P. Kristallografiya 1984, 29, 480-484 (in Russian).
(51) Similar changes in C(2-dithiole)-C(9-anthracene) distances for the neutral
and the charged states were observed for other TTFAQ derivatives: donor
21 (1.366(3) Å) and its charge-transfer salt 21‚TCNQ2‚2MeCN (1.486(2)
Å), (ref 16c); 2,6-di(2-hydroxyethoxy)-9,10-bis[4,5-di(methylthio)-1,3-
dithiol-2-ylidene]-9,10-dihydroanthracene (22) (1.361(4) Å) and 222+‚(ClO4-
)
2
(1.487(6) Å) (ref 16e).
(52) (a) Flandrois, S.; Chasseau, D. Acta Crystallogr. 1977, B33, 2744-2750.
(b) Kistenmacher, T. J.; Emge, T. J.; Bloch, A. N.; Cowan, D. O. Acta
Crystallogr. 1982, B38, 1193-1199.
(53) Moore, A. J.; Chesney, A.; Bryce, M. R.; Batsanov, A. S.; Kelly, J. F.;
Howard, J. A. K.; Perepichka, I. F.; Perepichka, D. F.; Meshulam, G.;
Berkovich, G.; Kotler, Z.; Mazor, R.; Khodorkovsky, V. Eur. J. Org. Chem.
2001, 1927-1935.
(54) Gaillard, F.; Levillain, E. J. Electroanal. Chem. 1995, 398, 77-87.
(55) Fiedler, D. A.; Koppenol, M.; Bond, A. M. J. Electrochem. Soc. 1995,
142, 862-867.
(56) An additional portion of 3 (100 mg, 4%) can be isolated from the mother
liquor by flash chromatography on silica, eluting with CH2Cl2, mp 158-
160 °C.
9
14236 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002

2+
•-
Covalent D −σ−A Redox State
A R T I C L E S
with water (3 mL) and cooled in an ice bath. The precipitate was filtered
off, washed sequentially with water and NaHCO3 solution, then with
hot water, and, finally, with methanol (1 mL), giving sulfone 4 (0.42
g, 90%), mp 187-188 °C. m/z (EI): 465 (M+, 5%), 379 (100%). m/z
(CI): 483 ([MNH4]+, 85%), 453 (100%). 1H NMR (200 MHz, acetone-
d6): δ 9.05 (1H, d, J ) 2 Hz), 8.86 (1H, d, J ) 2 Hz), 8.76 (1H, d, J
) 2 Hz), 8.64 (1H, d, J ) 2 Hz), 3.87 (2H, t, J ) 7.5 Hz), 3.62 (3H,
s), 2.86 (2H, t, J ) 7.5 Hz). 13C NMR (75 MHz, acetone-d6): δ 185.4,
170.9, 151, 147.4, 147.2, 144.8, 139.8, 139.7, 138.3, 137.8, 131.5, 128.4,
126.7, 123.4, 52.4, 51.6, 28.1. IR (KBr): ν 1741 (CdO), 1620 (CdO),
1544, 1367, 1348, 1317 cm-1. Anal. Calcd for C17H11N3O11S: C, 43.88;
H, 2.38; N, 9.03. Found: C, 43.62; H, 2.32; N, 8.98.
(CdO), 1617 (CdO), 1541, 1443, 1343 cm-1. Anal. Calcd for
C40H27N3O11S5: C, 54.23; H, 3.07; N, 4.74. Found: C, 54.11; H, 3.23;
N, 4.61.
Dicyanomethylenefluorene 10. To a solution of fluorenone 9 (52
mg, 0.059 mmol) in dry DMF (0.5 mL), malononitrile (30 mg, 0.45
mmol) was added and the mixture was stirred at 20 °C for 12 h, after
which period it was diluted with methanol and left for 2-3 h at 0 °C.
The precipitate was then filtered, washed with methanol, and dried in
vacuo. The resulting brown powder was extracted with dry CHCl3 (4
× 5 mL), the extract was evaporated, and the residue was taken up in
CH2Cl2. This solution was stirred for 1-2 min with dry silica (10 mg)57
and filtered. The silica was then washed with CH2Cl2,58 and the filtrate
was evaporated and dried in vacuo, giving pure product 10 (47 mg,
86%), mp > 300 °C. m/z (ES): 933 (M+, 100%). 1H NMR (300 MHz,
CDCl3): δ 9.74 (1H, s), 9.39 (1H, s), 9.04 (1H, br), 8.70 (1H, br),
7.59-7.67 (2H, m), 7.48-7.57 (2H, m), 7.20-7.32 (4H, m), 4.82 (1H,
d, J ) 13 Hz), 4.70 (1H, d, J ) 13 Hz), 3.62 (2H, t, J ) 7 Hz), 2.94
(2H, t, J ) 7 Hz), 2.00 (3H, s), 1.92 (6H, s). IR (KBr): ν 2185 (CtN),
2154 (CtN), 1741 (CdO), 1541, 1445, 1342 cm-1. Anal. Calcd for
C43H27N5O10S5: C, 55.30; H, 2.91; N, 7.50. Found: 55.10; H, 2.98;
N, 7.54.
3-(4,5,7-Trinitro-9-oxo-2-fluorenylsulfonyl)propanoic Acid (5).
Ester 4 (0.40 g, 0.86 mmol) was dissolved in a mixture of trifluoroacetic
acid (4 mL) and water (4 mL) and refluxed for 4 h (a white precipitate
formed after 0.5-1 h). After the mixture cooled, the precipitate was
filtered off and washed with hot water and methanol affording acid 5
as a white powder (0.39 g, 100%), mp 247 °C (dec). m/z (EI): 451
(M+, 24%), 379 ([M-CH2CH2CO2]+, 100%). 1H NMR (200 MHz,
acetone-d6): δ 9.05 (1H, d, J ) 2 Hz), 8.86 (1H, d, J ) 2 Hz), 8.76
(1H, d, J ) 2 Hz), 8.65 (1H, d, J ) 2 Hz), 3.85 (2H, t, J ) 7.5 Hz),
2.85 (2H, t, J ) 7.5 Hz). Anal. Calcd for C16H9N3O11S: C, 42.58; H,
2.01; N, 9.31. Found: C, 42.40; H, 1.96; N, 9.32.
2,6-Bis{2-[(tert-butyldiphenylsilyl)oxy]ethoxy}-9,10-bis(4,5-di-
methyl-1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (13). To a
solution of the phosphonate ester 1259 (4.15 g, 17.3 mmol) in dry THF
(200 mL) at -78 °C under argon was added lithium diisopropylamide
(1.5 M solution in cyclohexane; 12.7 mL, 19.0 mmol), and the resultant
cloudy yellow-brown mixture was stirred for 1 h at -78 °C. Anthra-
quinone 1116e (5.56 g, 6.90 mmol) was dissolved in dry THF (50 mL)
and added to the reaction mixture via a syringe over 30 min. The
reaction mixture was stirred at -78 °C for another 1 h, whereupon it
was allowed to attain room temperature over 12 h. Evaporation of the
solvent gave a red residue which was dissolved in CH2Cl2 (300 mL),
washed with water and brine, dried (MgSO4), and concentrated in vacuo.
Column chromatography (silica gel, CH2Cl2-hexane, 1:1 v/v) afforded
13 as a yellow foam (4.13 g, 58%), mp 128-131 °C (from CHCl3-
3-(4,5,7-Trinitro-9-oxo-2-fluorenylsulfonyl)propanoyl Chloride
(6). To a suspension of acid 5 (100 mg, 0.22 mol) in dry CH2Cl2 (2
mL), oxalyl chloride (1 mL, excess) and N,N-dimethylformamide
(DMF) (2 µL) were added and the reaction mixture was stirred for 40
h. The solvent was removed in vacuo, and the residue was suspended
in dry CH2Cl2 (1 mL), filtered off, washed with CH2Cl2 (2 mL), and
dried in a flow of Ar, giving acid chloride 5b (105 mg, 100%), which
1
was used in the next step without purification. H NMR (300 MHz,
acetone-d6): δ 9.05 (1H, d, J ) 2 Hz), 8.86 (1H, d, J ) 2 Hz), 8.80
(1H, d, J ) 2 Hz), 8.68 (1H, d, J ) 2 Hz), 4.03 (2H, t, J ) 7.5 Hz),
3.63 (2H, t, J ) 7.5 Hz).
Methyl 3-(9-Dicyanomethylene-4,5,7-trinitro-2-fluorenylsulfonyl)-
propanoate (7). A solution of fluorenone 4 (61 mg, 0.13 mmol) and
malononitrile (31 mg, 0.47 mmol) in DMF (0.5 mL) was stirred at 20
°C for 6 h and then left at 0 °C overnight. The dark green reaction
mixture was diluted with MeOH (3 mL) and stirred at 0 °C for 1 h,
and the precipitate was filtered off and washed with MeOH. The yellow-
greenish powder was dissolved in hot acetone (1.5 mL), diluted with
MeOH (3 mL), and left to crystallize at 0 °C, giving pure product 7
(50 mg, 75%), mp 240 °C (dec). m/z (EI): 513 (M+, 4%), 389 (100%).
1
methanol). m/z (EI): 1032 (M+, 100%). H NMR (CDCl3): δ 7.76-
7.74 (8H, m), 7.52 (2H, d, J ) 8.5 Hz), 7.43-7.38 (12H, m), 7.18
(2H, d, J ) 2.5 Hz), 6.77 (2H, dd, J1 ) 2.5 Hz, J2 ) 8.5 Hz), 4.17
(4H, t, J ) 5 Hz), 4.04 (4H, t, J ) 5 Hz), 1.92 (6H, s), 1.91 (6H, s),
1.09 (18H, s). 13C NMR (CDCl3): δ 156.7, 136.8, 135.6, 133.6, 131.2,
129.6, 128.3, 127.7, 126.3, 121.2, 120.8, 120.6, 111.7, 111.2, 69.2,
62.6, 26.8, 19.2, 13.10, 13.07. Anal. Calcd for C60H64O4S4Si2: C, 69.72;
H, 6.24. Found: C, 69.46; H, 6.24.
1
MS (CI): m/z 531 (MNH4+, 16%), 274 (100%). H NMR (300 MHz,
2,6-Bis(2-hydroxyethoxy)-9,10-bis(4,5-dimethyl-1,3-dithiol-2-
ylidene)-9,10-dihydroanthracene (14). Compound 13 (3.70 g, 3.57
mmol) was dissolved in dry THF (50 mL), and a solution of
tetrabutylammonium fluoride (10.7 mL, 10.7 mmol of a 1.0 M solution
in THF) was added dropwise over 15 min, causing the reaction mixture
to change color from yellow to brown. Further stirring for 1.5 h,
followed by addition of water (0.1 mL) and evaporation of the solvent,
afforded a brown residue. It was dissolved in CH2Cl2 (200 mL), washed
with water and brine, dried (MgSO4), and concentrated in vacuo.
Chromatography using a short column (silica gel, initially CH2Cl2 to
remove the byproducts, then CH2Cl2-acetone, 9:1 v/v) afforded 14 as
a yellow powder (1.65 g, 83%), mp 218-220 °C (dec). m/z (EI): 556
(M+, 100%). 1H NMR (CDCl3): δ 7.53 (2H, d, J ) 8.5 Hz), 7.17 (2H,
d, J ) 2.5 Hz), 6.81 (2H, dd, J 8.5 and 2.5 Hz), 4.15 (4H, t, J ) 4.5
Hz), 4.01-3.98 (4H, m), 2.06 (2H, t, J ) 6.0 Hz), 1.923 (6H, s), 1.916
(6H, s). 13C NMR (CDCl3): δ 156.4, 136.9, 131.7, 128.7, 126.5 (2C),
acetone-d6): δ 9.74 (1H, d, J ) 2 Hz), 9.44 (1H, d, J ) 2 Hz), 9.10
(1H, d, J ) 2 Hz), 8.82 (1H, d, J ) 2 Hz), 3.86 (2H, t, J ) 7.5 Hz),
3.62 (3H, s), 2.89 (2H, t, J ) 7.5 Hz). 13C NMR (75 MHz; acetone-
d6): δ170.8, 154.1, 150.3, 147.7, 147.4, 144.2, 140.0, 139.7, 135.3,
134.6, 130.8, 130.0, 126.0, 125.0, 113.2, 113.1, 85.9, 52.24, 51.9, 28.1.
IR (KBr): ν 2233 (CtN), 1736 (CdO), 1545, 1344 cm-1. Anal. Calcd
for C20H11N5O10S: C, 46.79; H, 2.16; N, 13.64. Found: C, 46.76; H,
2.13; N, 13.67.
Fluorenone 9. To a solution of acid chloride 6 (118 mg, 0.250 mmol)
in dry THF (6 mL) at -20 °C, pyridine (23 µL, 0.285 mmol) and then
a solution of alcohol 822 (106 mg, 0.225 mmol) in THF (10 mL) were
added and the reaction mixture was stirred at -20 °C for 2 h and then
stirred overnight at 20 °C. The solvent was then removed in vacuo,
and the residue was chromatographed on silica, eluting with CH2Cl2.
The main gray-yellow fraction was evaporated, and the residue was
stirred with petroleum ether and filtered giving a brown solid of 9 (160
mg, 80%), mp 210 °C (dec). m/z (FAB): 885 (M+, 55%), 123 (100%).
1H NMR (300 MHz, CDCl3): δ 9.00 (1H, d, J ) 2 Hz), 8.83 (1H, d,
J ) 2 Hz), 8.67 (1H, d, J ) 2 Hz), 8.58 (1H, d, J ) 2 Hz), 7.59-7.67
(2H, m), 7.49-7.57 (2H, m), 7.20-7.32 (4H, m), 4.82 (1H, d, J ) 13
Hz), 4.70 (1H, d, J ) 13 Hz), 3.58 (2H, t, J ) 7 Hz), 2.90 (2H, t, J )
7 Hz), 2.00 (3H, s), 1.927 (3H, s), 1.921 (3H, s). IR (KBr): ν 1739
(57) Hydrolysis of compound 10 to fluorenone 9 on silica (which we have not
observed for other fluorene-9-dicyanomethylene derivatives) does not allow
chromatographic purification.
(58) Sometimes compound 10 (and also 9) is retained on silica; in this case, a
few drops of methanol were added to CH2Cl2 to elute all the compound.
(59) Moore, A. J.; Bryce, M. R. J. Chem. Soc., Perkin Trans. 1 1991, 157-
168.
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A R T I C L E S
Perepichka et al.
120.9, 120.8, 111.5, 111.4, 69.3, 61.5, 13.2, 13.1. Anal. Calcd for
C28H28O4S4: C, 60.40; H, 5.07. Found: C, 60.15; H, 5.06.
H2O and dried, and the solvent was evaporated. Column chromatog-
raphy (silica gel, CH2Cl2) of the residue (140 mg) afforded 19 (104
mg, 60%), mp >200 °C (dec). m/z (FAB): 1148 (M+, 10%), 152
4-Bromo-2,5,7-trinitrofluorene-9-one (16).60 Fluorenone 15 (5.00
g, 15.9 mmol) was dissolved in a mixture of fuming nitric acid (5.3
mL, 127 mmol; d ) 1.51 g mL-1) and concentrated sulfuric acid (150
mL; d ) 1.84 g mL-1) with heating at 50 °C. Bromine (13.0 g, 81
mmol) was added dropwise over 10 min, and the mixture was stirred
at 55-60 °C for 2 h. The mixture was cooled to ambient temperature
and poured onto 1 kg of crushed ice. The solid was filtered off and
washed with water yielding compound 16 (5.70 g, 91%). Recrystalli-
zation of this crude product by dissolution in acetic acid (65 mL) and
further addition of hot water (15 mL) afforded pure 16 (3.65 g, 58%),
mp 194-195 °C (191-192 °C).61 1H NMR (300 MHz, acetone-d6): δ
8.97 (1H, d, J ) 2 Hz), 8.80 (1H, d, J ) 2 Hz), 8.76 (1H, d, J ) 2
Hz), 8.55 (1H, d, J ) 2 Hz). 13C (50 MHz, acetone-d6): δ 185.8 (CdO),
150.9, 150.4, 145.6, 140.5, 140.2, 139.9, 136.3, 126.4, 122.4, 121.7,
118.9.
1
(100%). H NMR (300 MHz, CDCl3): δ 8.91 (2H, d, J ) 2.5 Hz),
8.66 (2H, d, J ) 8.5 Hz), 8.59 (2H, d, J ) 2.5 Hz), 8.49 (2H, d, J )
2 Hz), 8.39 (2H, dd, J ) 8.5 Hz, J ) 2.1 Hz), 7.33 (2H, d, J ) 8.5
Hz), 7.06 (2H, d, J ) 2.5 Hz), 6.78 (2H, dd, J ) 8.5 Hz, J ) 2.4 Hz),
4.90 (4H, m), 4.49 (4H, m), 1.835 (6H, s), 1.827 (6H, s). 13C NMR
(75 MHz, CDCl3): δ 187.6 (CdO), 164.2 (CdO), 156.0, 149.4, 148.5,
146.6, 146.0, 137.5, 136.8, 136.1, 132.5, 131.7, 130.3, 128.8, 128.7
(2C), 126.3, 122.0, 121.0, 120.7, 120.0, 119.2, 111.7, and 111.6 (dithiole
C-4 and C-5), 65.9 and 65.3 (CH2CH2), 13.04 (CH3), 13.00 (CH3). IR
(KBr) ν: 1732 (CdO), 1609, 1590, 1528 (NO2), 1343 (NO2), 1275,
1219, 1158, 1086, 737 cm-1. Anal. Calcd for C56H36N4O16S4: C, 58.53;
H, 3.16; N, 4.88. Found: C, 58.46; H, 3.22; N, 4.97.
Dicyanomethylenefluorene 20. Fluorenone 19 (50 mg, 0.044 mmol)
and malononitrile (10.5 mg, 0.159 mmol) in DMF (0.40 mL) were
stirred at 35 °C for 24 h, the solvent was removed in high vacuum,
and the residue was chromatographed (silica gel, CH2Cl2) affording
dicyanomethylene derivative 20 (37 mg, 68%), mp 190-195 °C. m/z
4-Bromo-2,5,7-trinitro-9-dicyanomethylenefluorene (17). Fluo-
renone 16 (500 mg, 1.27 mmol) and malononitrile (180 mg, 2.73 mmol)
in DMF (2.0 mL) were stirred at ambient temperature for 2 h (immediate
dissolution followed by precipitation in 30 min). The mixture was
diluted with ethanol (8 mL) and allowed to stay at +5 °C for 3 h. The
solid was filtered off and washed with ethanol affording compound 17
(530 mg, 94%), mp 340 °C (dec) (MeCN). m/z (CI-): 442, 440 ([M -
1
(FAB): 1244 (M+, 50%), 613 (100%). H NMR (300 MHz, CDCl3):
δ 9.47 (2H, d, J ) 2 Hz), 9.29 (2H, d, J ) 2 Hz), 8.88 (2H, d, J ) 2
Hz), 8.52 (2H, d, J ) 8.5 Hz), 8.35 (2H, dd, J ) 8.5 Hz, J ) 2 Hz),
7.27 (2H, d, J ) 8.5 Hz), 7.05 (2H, d, J ) 2.5 Hz), 6.81 (2H, dd, J )
8.5 Hz, J ) 2.5 Hz), 4.95 (4H, m), 4.53 (4H, m), 1.880 (6H, s), 1.872
(6H, s). IR (KBr) ν (cm-1): 2227 (CtN), 1731 (CdO), 1602, 1529
(NO2), 1342 (NO2), 1272, 1164, 1102, 732.
1
H]-, 100%, 100%). H NMR (200 MHz, acetone-d6 + 0.5 drop of
CF3CO2D): δ 9.66 (1H, d, J ) 2 Hz), 9.47 (1H, d, J ) 2 Hz), 9.03
(1H, d, J ) 2 Hz), 8.86 (1H, d, J ) 2 Hz). 13C (50 MHz, acetone-d6):
δ 158.8, 158.0, 154.9, 150.4, 142.7, 140.5, 140.0, 135.5, 125.9, 124.2,
122.1, 120.7, 118.7, 113.4, 113.3, 113.0. Anal. Calcd for C16H4-
BrN5O6: C, 43.46; H, 0.91; Br, 18.07; N, 15.84. Found: C, 43.34; H,
0.84; Br, 17.80; N, 15.77.
Acknowledgment. We thank Dr. Francois X. Sauvage (CNRS
UMR 8516, Lille) for his help with SEEPR experiments and
the following for funding: EPSRC (D.F.P. and C.A.C.); The
Royal Society for funding visits to Durham (I.F.P. and S.B.L.);
The Royal Society of Chemistry for a Grant for International
Authors (I.F.P.); The University of Durham (N.G.).
CTC 14‚(17)2. To the solution of acceptor 17 (18.0 mg, 41 mmol)
in hot acetonitrile (1.2 mL), a solution of donor 14 (11.4 mg, 20 mmol)
in acetonitrile (0.8 mL) was added, and the solution was left to stand.
The black crystalline product was collected, washed with acetonitrile,
and dried, affording CTC 14‚(17)2. It was not possible to remove all
the MeCN even by drying in vacuo at 100 °C. Anal. Calcd for C60H36-
Br2N10O16S4‚1/3MeCN: C, 50.09; H, 2.56; N, 9.95. Found: C, 49.60;
H, 2.53; N, 10.39. IR (KBr) ν: 2230 (MeCN), 2179 and 2153 (CN),
Supporting Information Available: Experimental details of
the reaction of 2,5,7-trinitrofluorene-9-one-4-carbonyl chloride
with TTFAQ 14,1H NMR spectrum for compound 20, FTIR
transmission spectra of compound 10 in KBr pellets and in
CH2Cl2 solution, UV-vis-near-IR spectra of compounds 9 and
10, EPR and SEEPR spectra of 10 in 0.1 M nBu4NPF6/THF,
the description of the X-ray structures of 14‚1/2PhMe, 17‚PhMe,
17‚TTF, and 17‚TTFMe3Br (PDF) with a table of selected bond
distances, experimental details of the X-ray analysis, X-ray
crystallographic files in CIF format for compounds 14‚1/2PhMe,
17‚PhMe, 17‚TTF, 17‚TTFMe3Br, and 14 (17)2‚2MeCN are
available free of charge via the Internet at http://pubs.acs.org.
1625, 1580, 1511, 1445 1333, 1310, 1077 cm-1
.
Fluorenone 19. Carbonyl chloride 19 (157 mg, 0.45 mmol) and diol
14 (84 mg, 0.15 mmol) in dry THF (20 mL) were stirred under nitrogen
until full dissolution. Dry pyridine (0.080 mL, 1.0 mmol) was added,
and the solution was stirred at 20 °C for 24 h. It was diluted with H2O
and then extracted with CH2Cl2, the organic layer was washed with
(60) Procedure adapted from: Andrievskii, A. M.; Gorelik, M. V.; Avidon, S.
V.; Al’tman, E. Sh. Russ. J. Org. Chem. 1993, 29, 1519-1524; Zh. Org.
Khim. 1993, 29, 1828-1834 (in Russian).
(61) Newman, M. S.; Blum, J. J. Am. Chem. Soc. 1964, 86, 5600-5602.
JA012518O
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14238 J. AM. CHEM. SOC. VOL. 124, NO. 47, 2002