Organic super-acceptors with efficient intramolecular charge-transfer interactions by [2+2] cycloadditions of TCNE, TCNQ, and F4-TCNQ to donor-substituted cyanoalkynes

Article abstract of DOI:10.1002/chem.200802563

Thermal [2 + 2] cycloadditions of tetracyanoethene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) to N,N-dimethylanilino-substituted (DMA-substituted) alkynes bearing either nitrile, di

Full text of DOI:10.1002/chem.200802563

FULL PAPER
DOI: 10.1002/chem.200802563
Organic Super-Acceptors with Efficient Intramolecular Charge-Transfer
Interactions by [2+2] Cycloadditions of TCNE, TCNQ, and F4-TCNQ to
Donor-Substituted Cyanoalkynes**
Milan Kivala,[a] Corinne Boudon,[b] Jean-Paul Gisselbrecht,[b] Barbara Enko,[c]
Paul Seiler,[a] Imke B. Mꢀller,[d] Nicolle Langer,[d] Peter D. Jarowski,[a]
Georg Gescheidt,[c] and FranÅois Diederich*[a]
Dedicated to Professor Seiji Shinkai on the occasion of his 65th birthday
Abstract: Thermal [2+2] cycloaddi-
tions of tetracyanoethene (TCNE),
7,7,8,8-tetracyanoquinodimethane
tion spectra of selected acceptors were
nicely reproduced by applying the
spectroscopy oriented configuration in-
teraction (SORCI) procedure. The
electrochemical investigations of these
acceptors by cyclic voltammetry (CV)
and rotating disc voltammetry (RDV)
in CH2Cl2 identified their remarkable
propensity for reversible electron
uptake rivaling the benchmark com-
pounds TCNQ (Ered,1 =ꢀ0.25 V in
CH2Cl2 vs. Fc+/Fc) and F4-TCNQ
(Ered,1 =+0.16 V in CH2Cl2 vs. Fc+/Fc).
Furthermore, the electron-accepting
power of these new compounds ex-
pressed as adiabatic electron affinity
(EA) has been estimated by theoretical
calculations and compared to the refer-
ence acceptor F4-TCNQ, which is used
as a p-type dopant in the fabrication of
organic light-emitting diodes (OLEDs)
and solar cells. A good linear correla-
tion exists between the calculated EAs
(TCNQ),
7,7,8,8-tetracyanoquinodimethane
ACHTUNGTRENNUNG(F4-TCNQ) to N,N-dimethylanilino-
and
2,3,5,6-tetrafluoro-
and the first reduction potentials Ered,1.
Despite the substitution with strong
DMA donors, the predicted EAs reach
the value calculated for F4-TCNQ
(4.96 eV) in many cases, which makes
the new acceptors interesting for po-
tential applications as dopants in or-
ganic optoelectronic devices. The first
example of a charge-transfer salt be-
tween the DMA-substituted TCNQ
adduct (Ered,1 =ꢀ0.27 V vs. Fc+/Fc) and
the strong electron donor decamethyl-
ferrocene ([FeCp*2]; Cp*=pentame-
substituted (DMA-substituted) alkynes
bearing either nitrile, dicyanovinyl
(DCV; -CH=C(CN)2), or tricyanovinyl
(TCV; -C(CN)=C(CN)2) functionali-
ties, followed by retro-electrocycliza-
tion, afforded a new class of stable or-
ganic super-acceptors. Despite the non-
planarity of these acceptors, as re-
vealed by X-ray crystallographic analy-
sis
and
theoretical
calculations,
efficient intramolecular charge-transfer
(CT) interactions between the DMA
donors and the CN-containing acceptor
moieties are established. The corre-
sponding CT bands appear strongly
bathochromically shifted with maxima
up to 1120 nm (1.11 eV) accompanied
by an end-absorption in the near infra-
red around 1600 nm (0.78 eV) for
F4-TCNQ adducts. Electronic absorp-
thylcyclopentadienide;
E
ox,1 =ꢀ0.59 V
vs. Fc+/Fc) is described. Interestingly,
the X-ray crystal structure showed that
in the solid state the TCNQ moiety in
the acceptor underwent reductive s-di-
merization upon reaction with the
donor.
Keywords: charge transfer · cyclo-
addition · electrochemistry · elec-
tron acceptors · electron affinity ·
EPR spectroscopy
[a] Dr. M. Kivala, P. Seiler, Dr. P. D. Jarowski, Prof. Dr. F. Diederich
Laboratorium fꢀr Organische Chemie
ETH Zꢀrich
[c] B. Enko, Prof. Dr. G. Gescheidt
Institut fꢀr Physikalische und Theoretische Chemie, TU Graz
Technikerstrasse 4/I, 8010 Graz (Austria)
Hçnggerberg, HCI, 8093 Zꢀrich (Switzerland)
Fax : (+41)44 632 1109
E-mail: diederich@org.chem.ethz.ch
[d] Dr. I. B. Mꢀller, Dr. N. Langer
BASF SE, GVP/C—A030, 67056 Ludwigshafen (Germany)
[**] TCNE=tetracyanoethene, TCNQ=7,7,8,8-tetracyanoquinodimethane,
F4-TCNQ=2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane.
[b] Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie—UMR 7177, CNRS, Universitꢁ Louis Pasteur
4, rue Blaise Pascal, 67000, Strasbourg (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802563.
Chem. Eur. J. 2009, 15, 4111 – 4123
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4111

Introduction
DCV-substituted alkyne 9 and TCV-derivative 10 to yield
the corresponding adducts 1 (26%) and 2 (66%), respec-
tively, required heating in 1,2-dichloroethane or 1,1,4,4-tet-
rachloroethane up to 1208C (Table 1). As already observed
for 12,[17] TCNQ displays rather reduced reactivity, com-
pared to TCNE, towards formal cycloadditions with alkynes.
While an excess of TCNQ, prolonged reaction times, and
elevated temperature (608C) were applied in the synthesis
of 3, complete consumption of the starting alkyne 9 could
not be reached. Subsequent repeated chromatographic puri-
fications on SiO2 afforded 3 in a rather low yield of 14%
due to partial decomposition during column chromatogra-
phy (CC). Furthermore, reaction of TCNQ with TCV-substi-
tuted alkyne 10 under various conditions delivered only a
trace amount of the corresponding heptacyano derivative 4,
accompanied by a mixture of unidentifiable products, as re-
vealed by mass spectrometry. Gratifyingly, the stronger ac-
ceptor F4-TCNQ showed significantly higher reactivity, com-
pared to TCNQ, towards regioselective cycloadditions with
alkynes. Thus, fluorinated adducts 5–7 were obtained in
good yields ranging from 65 (5) to 88% (7), respectively,
upon reaction with DMA-substituted cyanoalkynes 8–10 in
CH2Cl2 at 258C (Table 1).
All newly prepared acceptors 1–3 and 5–7 are dark metal-
lic-like solids that are stable at ambient temperature under
air and reasonably soluble in common organic solvents such
as CH2Cl2, acetone, and acetonitrile (except for nearly in-
soluble 6). However, solutions of F4-TCNQ adducts 5–7
were found to deteriorate gradually upon contact with glass
surfaces to form green insoluble products. Thus, glassware
previously deactivated by silylation with dimethyldichlorosi-
lane (DMDCS) should be used for all manipulations involv-
ing 5–7.[21] Complete decomposition was observed during at-
tempted chromatography (SiO2 and C18-reversed phase
SiO2) of 2 and 5–7. Consequently, 2 and 5–7 were successful-
ly purified by repeated crystallization by slow diffusion of
n-hexane into CH2Cl2 solution of the compound at 258C.
The identity of 1–7 was confirmed by high-resolution
MALDI FT-ICR (for 1 and 2) or MALDI-TOF mass spec-
trometry (for 3–7), which displayed the corresponding mo-
lecular ion (see the Supporting Information), and/or NMR
spectroscopy, X-ray crystallography (for 1), and elemental
analysis. For acceptors 2, 6, and 7, the NMR spectra could
not be recorded due to the presence of paramagnetic species
in the sample, as confirmed by electron paramagnetic reso-
nance (EPR) spectroscopy (vide infra), and low solubility of
the solid (1H and 19F NMR spectra for 5 were obtained in
one case, however, they could not be reproduced). Similar
behavior has previously been described for other TCNQ-
derived strong acceptors.[6a,11]
Among the large number of strong organic acceptors that
have been described to date, cyano-based derivatives repre-
sent the most prominent class of compounds for optoelec-
tronic device applications.[1] Tetracyanoethene (TCNE)[2,3]
and 7,7,8,8-tetracyanoquinodimethane (TCNQ)[4,5] together
with their derivatives[6] were shown to form charge-transfer
(CT) complexes and salts with various organic and organo-
metallic electron donors that often exhibit technologically
interesting materials properties such as electric conductivi-
ty[7,8] and magnetic phenomena.[9] Furthermore, environmen-
tally stable acceptors are increasingly applied as p-type dop-
ants, significantly improving the performance of optoelec-
tronic devices such as organic light-emitting diodes
(OLEDs) and solar cells.[10] Although 2,3,5,6-tetrafluoro-
7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)[6a] is widely
used for this purpose, alternative dopants, such as the re-
cently reported 3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodime-
thane (F2-HCNQ) with superior thermal stability,[11] are cur-
rently being pursued by many researchers.
Recently, we have shown that formal [2+2] cycloaddi-
tions, followed by retro-electrocyclization of the initially
formed cyclobutenes, of TCNE[12,13] and TCNQ[14,15] to “elec-
tronically confused” alkynes bearing one electron-
donating group and one electron-withdrawing group, such as
N,N-dimethylanilino-substituted (DMA-substituted) cya-
noalkynes, yield donor-substituted 1,1,2,4,4-pentacyanobuta-
1,3-dienes[16] (PCBDs) and the corresponding cyclohexa-2,5-
diene-1,4-diylidene-expanded derivative, respectively.[17] De-
spite the substitution with DMA donors, these compounds
are potent electron acceptors that compete with TCNE and
TCNQ in their ease of reversible electron uptake. Further-
more, intense bathochromically shifted intramolecular CT
bands are observed in their UV/Vis spectra. Based on these
results, we expected that even stronger electron acceptors
would be in reach upon i) incorporation of particularly
strong acceptor moieties, such as F4-TCNQ, and ii) increas-
ing the number of electron-withdrawing cyano functionali-
ties in the molecule.
Here, we introduce a new family of strong electron ac-
ceptors 1–7 obtained by reactions of TCNE, TCNQ, and F4-
TCNQ with DMA-substituted alkynes bearing either nitrile
(8),[17,18] dicyanovinyl (DCV; -CH=C(CN)2) (9),[13b] or tricya-
novinyl (TCV; -C(CN)=C(CN)2) (10) functionalities
(Table 1).[19] Initial attempts to prepare intermolecular CT
complexes with strong electron donors, such as decamethyl-
ferrocene
([FeCp*2];
Cp*=pentamethylcyclopentadi-
enide)[20] will be discussed as well.
Single crystals of 1 suitable for X-ray crystallographic
analysis were obtained by slow diffusion of n-hexane into
CH2Cl2 solution at 258C (Figure 1a). As already observ-
ed,[13c] in the crystal packing of highly distorted 1, two neigh-
boring molecules undergo several dipolar CN···CN interac-
tions (Figure 1b). The CN groups of one molecule interact
with the C(CN)2 moiety of its neighbor with the shortest
Results and Discussion
Synthesis and characterization: Whereas the previously re-
ported reaction of TCNE with cyanoalkyne 8 to give DMA-
substituted PCBD 11 proceeded in nearly quantitative yield
in tetrahydrofuran (THF) at 208C,[17] similar reaction with
4112
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Chem. Eur. J. 2009, 15, 4111 – 4123

Organic Super-Acceptors
FULL PAPER
Table 1. Summary of the reactions of cyano-substituted alkynes 8–10 with TCNE, TCNQ, and F4-TCNQ.
C5 of 3.17 and 3.12 ꢃ, respec-
tively, are formed. Similarly to
DMA-substituted 1,1,4,4-tetra-
cyanobuta-1,3-dienes
Cyanoalkyne
Acceptor
Product
Reaction
Time
Yield
[%]
conditions
(TCBDs)[13b,c] and TCNQ ad-
ducts,[15a] the DMA ring in 1
exhibits significant bond alter-
nation, as expressed by its qui-
noid character (dr; for its defi-
nition[22] and bond lenghts, see
caption to Figure 1) of 0.052,
indicative of efficient intramo-
lecular CT interactions in the
ground state.
[a]
TCNE
4 days
97[b]
[c]
TCNQ
12 h
27[b]
The thermal stability of se-
lected acceptors, that is essen-
tial for potential practical ap-
plications, was investigated by
[d]
[e]
[e]
F4-TCNQ
TCNE
17 h
65
26
14
thermogravimetric
analysis
(TGA). The first observable
decomposition temperatures,
as determined by derivative
thermogravimetry, ranged from
176 (for 7) to 4828C (for 2)
and presumably correspond to
the loss of co-crystallized sol-
vents. These decompositions
16 h
TCNQ
5 days
are followed by
a gradual
weight loss in most cases (see
the Supporting Information).
[d]
[c]
[f]
F4-TCNQ
TCNE
16 h
84
UV/Vis spectroscopy: Most of
the DMA-substituted accept-
ors show in CH2Cl2 intense CT
bands with end-absorptions
reaching into the near infrared
2 days
66
region
UV/Vis absorption spectrum of
hexacyano derivative fea-
(Figure 2).
The
1
tures the lowest-energy intra-
molecular CT band at 486 nm
(2.55 eV; e=43300mꢀ1 cmꢀ1).
Introduction of the cyclohexa-
2,5-diene-1,4-diylidene spacer
in 3 shifts the CT band to
[f]
TCNQ
n.d.
633 nm
(1.96 eV;
e=
[d]
F4-TCNQ
5 days
88
13900mꢀ1 cmꢀ1), with a second,
weaker CT band of lower
energy appearing at lmax
=
[a] In THF, 208C. [b] See reference [17]. [c] In 1,1,2,2-tetrachloroethane, 1208C. [d] In CH2Cl2, 258C. [e] In
1,2-dichloroethane, 608C. [f] Under various reaction conditions, only traces of 4 were formed (MALDI-TOF
MS).
930 nm
(1.33 eV;
e=
1400mꢀ1 cmꢀ1). Upon substitu-
tion of the TCNQ-derived
moiety with the strongly elec-
tron-withdrawing
fluorine
contact of 3.02 ꢃ observed between N16’ and the central vi-
nylic carbon atom C4. Consequently, two nearly orthogonal
intermolecular CN···CN contacts between N16’ and C7 or
atoms in 6, an additional red shift of both CT bands to
lmax =697 nm (1.78 eV; e=40600mꢀ1 cmꢀ1) and lmax =942 nm
(1.32 eV; e=17000mꢀ1 cmꢀ1) occurs (Figure 2). Increasing
Chem. Eur. J. 2009, 15, 4111 – 4123
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F. Diederich et al.
Figure 2. UV/Vis absorption spectra of acceptors 1, 3, 6, and 7 in CH2Cl2
at 298 K. The high-intensity band of
3
at lmax =400 nm (e=
108400mꢀ1 cmꢀ1) is cut to allow enlargement of the weaker CT transi-
tions.
tion interaction (SORCI)[24] procedure implemented in the
ORCA program suite.[25] For acceptors 1–3 and 11, the
SORCI excitation energies are in good agreement with the
experimental UV/Vis spectra recorded in CH2Cl2 (Table 2).
However, the computational analysis for 5 and 12 did not
reproduce the experimental values well (see the Supporting
Information). According to theoretical data for 1–3 and 11,
the lowest energy excitation with high extinction coefficient
(e) and calculated high oscillator strength (f) can be as-
signed as a transition from the DMA-located HOMO to the
LUMO+1 level located predominantly on the CN-contain-
ing acceptor moieties (for calculated HOMOs and LUMOs,
see the Supporting Information). The oscillator strength of
the HOMO!LUMO transition is rather small which is in
agreement with weak or no extinction observed experimen-
tally. The red shift of 1.22 eV of the longest-wavelength CT
band observed in the UV/Vis spectra upon introduction of
the cyclohexa-2,5-diene-1,4-diylidene spacer when moving
from 1 (486 nm (2.55 eV)) to 3 (930 nm (1.33 eV)) is nicely
reproduced in the SORCI spectra and amounts to 1.54 eV,
with the deviation from the experimental value being within
the usual error range of the method.
Figure 1. a) Molecular structure of 1 (ORTEP plot), arbitrary numbering,
hydrogen atoms are omitted for clarity. Atomic displacement parameters
obtained at 220 K are drawn at the 30% probability level. Selected bond
ꢀ
ꢀ
ꢀ
lengths [ꢃ] and bond angles [8]: C1 C2 1.516(5), C2 C3 1.448(5), C3 C4
ꢀ
ꢀ
ꢀ
ꢀ
1.366(5), C4 C5 1.433(5), C4 C7 1.412(6), C5 N6 1.140(5), C7 N8
ꢀ
ꢀ
ꢀ
ꢀ
1.146(5), C1 C14 1.376(5), C14 C17 1.442(5), C14 C15 1.427(6), C15
N16 1.153(5), C17 N18 1.134(5), C2 C9 1.358(5), C9 C12 1.429(6),
C12 N13 1.139(5), C1 C19 1.417(5), C9 C10 1.442(6), C10 N11
1.141(5), C19 C20 1.415(5), C20 C21 1.363(5), C21 C22 1.405(5), C22
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
C23 1.413(5), C23 C24 1.362(5), C19 C24 1.423(5), C22 N25 1.354(5),
ꢀ
ꢀ
N25 C27 1.460(5), N25 C26 1.470(5); C14-C1-C2 113.3(3), C15-C14-C17
112.9(3), C7-C4-C5 116.1(3), C12-C9-C10 115.7(3). Selected torsion
angles [8]: C14-C1-C19-C24 ꢀ2.9(6), C14-C1-C2-C9 ꢀ93.2(4), C26-N25-
C22-C21 ꢀ0.3(6). Quinoid character: dr=(((a+a’)/2-(b+b’)/2)+((c+c’)/
2-(b+b’)/2))/2.[22] dr=0.052. b) Arrangement of neighboring molecules in
the crystal packing of 1.
Whereas the optimized geometries of acceptors of 1–3
and 11 show almost perpendicular CN-containing moieties
with respect to the DMA ring, and the cyclohexa-2,5-diene-
1,4-diylidene spacer in 3, it is not the case for 5 and 12 (see
the Supporting Information). Consequently, the LUMO and
LUMO+1 are no longer localized on either the central C
atom connecting the DMA ring with the CN-containing
moieties (LUMO+1) or the CN-acceptor itself (LUMO),
which results in a more difficult configuration interaction
problem and less accurate SORCI spectra (unless this prob-
lem is explicitly taken care of in the choice of the reference
space). Thus, the first excited state with high oscillator
strength for 5 and 12 is assigned to a mixture of a
HOMOꢀ1!LUMO+1 single excitation and an excitation
of both electrons of the HOMO to the LUMO. The
the number of the accepting CN groups, for example when
going from fluorinated 5 to 7, further lowers the energy of
the CT bands. Indeed, absorption maxima at 753 nm
(1.65 eV) and 1120 nm (1.11 eV), with an end-
absorption near 1600 nm (0.78 eV), are observed for 7. This
low optical gap is quite remarkable for a small chromophore
such as 7.[23] Protonation of the DMA moiety in 7 with tri-
fluoroacetic acid (TFA) in CH2Cl2 eliminated these long-
wavelength bands, whereas neutralization with K2CO3 re-
generated partially (due to decomposition) the original spec-
trum, thus indicating the CT character of these bands (see
the Supporting Information).
Electronic absorption spectra of 1–3, 5, 11, and 12 were
calculated by applying the spectroscopy oriented configura-
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Organic Super-Acceptors
FULL PAPER
Table 2. Experimental electronic transitions for 1–3 and 11, derived from the UV/Vis spectra in CH2Cl2, and
computed SORCI excitation spectra.
More importantly, the stud-
ied compounds undergo two (5)
or three (1–3, 6, and 7) reversi-
ble one-electron reduction steps
centered on the CN-containing
moieties, eventually followed
by the fourth irreversible elec-
tron transfer for fluorinated 6
and 7. We have previously
found that the additional CN
Experimental
Computed values[a]
l
e
l
f
Composition
of band
CI
[nm (eV)]
A
]
[nm (eV)]
coefficients[b]
1
2
486 (2.55)
317 (3.91)
304 (4.08)
43300
21400
2500
454 (2.73)
303 (4.09)
0.81
0.92
H!L+1
0.94
Hꢀ2!L
Hꢀ1!L
H!L
0.37
0.59
0.92
0.89
0.75
0.54
0.31
0.88
0.88
0.79
0.81
0.95
0.88
657 (1.89)
469 (2.65)
5600
16300
676 (1.83)
466 (2.66)
364 (3.41)
311 (3.99)
0.12
0.33
0.20
0.25
H!L+1
Hꢀ2!L
Hꢀ2!L+1
Hꢀ1!L+1
H!L
group in PCBD 11 (Ered,1
=
295 (4.21)
930 (1.33)
15400
1400
ꢀ0.30 V)[17] facilitated the first
reduction step by 390 mV when
compared to TCBD 13, which
3
1045 (1.19)
639 (1.94)
632 (1.96)
414 (3.00)
545 (2.27)
418 (2.96)
0.10
0.80
0.16
0.13
0.21
0.51
H!L+1
Hꢀ1!L
Hꢀ2!L+1
H!L
633 (1.69)
400 (3.10)
643 (1.93)
450 (2.76)
13900
108400
3200
is
reversibly
reduced
at
ꢀ0.69 V.[13c,27] As expected, the
incorporation of stronger ac-
ceptor moieties in DCV-substi-
tuted 1 and TCV-substituted 2
further shifts the observed re-
duction steps towards more
11[c]
30000
H!L+1
[a] Only excitations with oscillator strength larger than 0.10 are shown for the calculated spectra; f=oscillator
strength; H=HOMO; L=LUMO; CI=configuration interaction. [b] Correspond to the final state. [c] Taken
from ref. [17].
HOMO!LUMO+1 excitation of compounds 5 and 12 is
positive potentials. Thus, the introduction of the additional
DCV moiety upon moving from TCBD 13 to TCNE adduct
1 (ꢀ0.22 V) shifts the first reduction step anodically by
470 mV to occur at more positive potential than that of
TCNE (ꢀ0.32 V) or TCNQ (ꢀ0.25 V). Although only
poorly resolved CV traces were obtained for TCV-substitut-
ed TCNE adduct 2, the first reversible electron uptake
occurs at +0.12 V, which represents an unprecedented
anodic shift of 810 mV with respect to parent 13. On the
other hand, only less pronounced effects are observed be-
tween TCNQ-derivative 14 (ꢀ0.50 V)[15a] and expanded
PCBD 12 reduced at ꢀ0.27 V[17] or DCV-substituted 3
(ꢀ0.28 V).
calculated at higher energies.
Electrochemistry: The redox properties of acceptors 1–3, 5–
7, and the reference compounds TCNE, TCNQ,[26] and
F4-TCNQ were investigated by cyclic voltammetry (CV) and
rotating-disc voltammetry (RDV) in CH2Cl2 (+0.1m
nBu4NPF6, all potentials vs. the ferricinium/ferrocene couple
(Fc+/Fc)) and are summarized in Table 3. The DMA donor
moiety in all studied acceptors undergoes a one-electron ox-
idation step that is irreversible (CV), except for 1, 6, and
previously reported 11, and 13. An anodic shift of 140 mV is
observed for the oxidation step upon introduction of the ad-
ditional CN group into the buta-1,3-diene-1,4-diyl moiety
when going from DMA-substituted 1,1,4,4-tetracyanobuta-
1,3-diene (TCBD) derivative 13 (Eox,1 =+0.86 V),[13c] to
PCBD 11 (+1.00).[17] In contrast, the effect of substitution
Noticeable for the new chromophores 1–3 and 5–7 is the
facility of the second one-electron reduction step (Ered,2 be-
tween ꢀ0.92 (for 1) and ꢀ0.17 V (for 7)) which is irreversi-
ble for chromophore 1. In addition, unprecedented third re-
versible one-electron reduction steps are observed for 1–3,
6, and 7 ranging from ꢀ2.30 V (for 1) to ꢀ1.19 V (for 7).
As a result of F-substitution in derivatives 5–7, the rever-
sible reduction steps became notably facilitated with respect
to their TCNQ analogues.[28] Thus, the two reversible reduc-
tion steps of fluorinated cyclohexa-2,5-diene-1,4-diylidene-
expanded PCBD derivative 5 occur anodically shifted by
270 and 260 mV at ꢁ0.00 and ꢀ0.27 V, respectively, com-
pared to TCNQ derivative 12 (ꢀ0.27 and ꢀ0.53 V).[17] Fur-
thermore, spectroelectrochemical studies of 5 performed in
a optically transparent thin-layer electrode (OTTLE) sug-
gest that the electrogenerated reduced species (i.e. radical
anion and dianion) are persistent at the time scale of spec-
troelectrochemistry, namely at least for 60 s (see the Sup-
porting Information). The three one-electron transfers of
DCV-substituted 6 occurring at ꢀ0.10, ꢀ0.25, and ꢀ1.76 V
appear shifted, compared to 3, to more positive potentials
by 180, 220, and 100 mV, respectively. The introduction of
the additional CN group upon moving from 6 to 7 further
ꢀ
with stronger DCV ( CH=C(CN)2) (in 1) or TCV
ꢀ
( C(CN)=C(CN)2) (in 2) acceptor moieties is much less pro-
nounced and TCNE adducts 1 and 2 are oxidized at lower
potentials of +0.97 and +0.91 V, respectively. The extra CN
group in cyclohexa-2,5-diene-1,4-diylidene-expanded PCBD
12 (+0.52 V) shifted its oxidation step anodically by
100 mV, compared to TCNQ adduct 14, which is irreversibly
oxidized at +0.42 V. Again, DCV-substituted 3 undergoes
its oxidation step at +0.50 V, which corresponds to a shift of
only 80 mV compared to 14. An anodic shift of 90 mV is ob-
served for the oxidation step upon F-substitution of the
TCNQ moiety in 5 (+0.61 V) and 6 (+0.59 V) compared to
their TCNQ counterparts 12 and 3, respectively. Overall, the
oxidations of TCNQ adducts 3, 12, and 14 (or fluorinated
5–7) are occurring at significantly lower potentials compared
to the TCNE adducts 1, 2, and 13, which indicates less effi-
cient ground state CT interactions between the DMA donor
moiety and the CN accepting groups in these chromophores
(vide infra).
Chem. Eur. J. 2009, 15, 4111 – 4123
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4115

F. Diederich et al.
Table 3. Cyclic voltammetry (CV; scan rate v=0.1 Vsꢀ1) and rotating
disk voltammetry (RDV) data of 1–3, 5–7, and 11–14, and the reference
compounds TCNE, TCNQ, and F4-TCNQ in CH2Cl2 (+0.1m
nBu4NPF6).[a] Calculated adiabatic electron affinity (EA), based on the
shifts the first electron uptake to more positive potentials by
260 mV.[27] While the first reduction potential for 7 is the
same as that of F4-TCNQ (+0.16 V), the second is, similarly
to our previous observations[17] significantly facilitated
(ꢀ0.17 (7) vs. ꢀ0.46 V (F4-TCNQ)) and is followed by two
reversible one-electron transfers at ꢀ1.19 and ꢀ1.94 V. It
has to be mentioned that a trace of one-electron reduced
species (radical anion) has been detected by RDV in freshly
prepared CH2Cl2 solutions of 7, thus underscoring its excep-
tional electron-accepting power (see the Supporting Infor-
mation).
An electron-withdrawing group involved in a donor–ac-
ceptor p-conjugated system generally hampers the oxida-
tion, as it decreases the electron density on the oxidizable
donor moieties, and conversely, an electron-donating group
hinders the electron reduction by delivering electrons into
the acceptor. In this respect, our findings might be rather
surprising, as gradual shifts of both the oxidation and the re-
duction steps towards more positive potentials are expected
upon increasing the electron-accepting power of the CN-
containing substituents. Nevertheless, at the same time as
the acceptor strength increases, as expressed by Hammett
BP86/def-TZVP COSMO
ACHTUNGTRENNUNG
CV
EA
E8
DEp
Ep
E1/2
Slope
[V][b]
[mV][c]
[V][d]
[V][e]
[mV][f]
[eV]
4.49
1
2
3
+0.97
ꢀ0.22
90
80
+0.95 (1eꢀ)
ꢀ0.26 (1eꢀ)
ꢀ0.99 (1eꢀ)
60
90
ꢀ0.92
+0.91
ꢀ2.30
120
60
[g]
4.87
4.51
+0.12
+0.12 (1eꢀ)
65
[h]
ꢀ1.88
60
+0.50
+0.61
+0.47 (1eꢀ)
ꢀ0.32 (1eꢀ)
ꢀ0.54 (1eꢀ)
80
65
65
60
ꢀ0.28
ꢀ0.47
ꢀ1.86
60
70
60
ꢀ1.94 (1eꢀ)
[g]
5
6
4.66
4.69
ꢁ0.00
ꢀ0.27
+0.59
ꢀ0.10
ꢀ0.25
ꢀ1.76
75
70
80
60
60
70
ꢁ0.00 (1eꢀ)
ꢀ0.29 (1eꢀ)
68
61
ꢀ0.11 (1eꢀ)
55
55
ꢀ0.26 (1eꢀ)
ꢀ
ꢀ
constants sp of +0.66 ( CN), +0.84 ( CH=C(CN)2), and
ꢀ2.26
+0.64
[29]
ꢀ
+0.98 ( C(CN)=C(CN)2), the substituents become steri-
7[i]
4.98
4.36
cally more demanding. This consequently, together with
electrostatic repulsion between the negatively polarized N-
atoms in neighboring CN groups, renders the whole mole-
cule highly twisted (as revealed by X-ray crystallography
and theoretical calculations). Under these conditions, effi-
cient p-conjugation between the donor and acceptor moiet-
ies becomes impaired to a certain extent, as already previ-
ously observed,[13c] which makes any straightforward correla-
tion between the observed redox potentials and the acceptor
strength rather difficult.
+0.16
ꢀ0.17
ꢀ1.19
80
60
80
+0.16 (1eꢀ)
ꢀ0.19 (1eꢀ)
ꢀ1.24 (1eꢀ)
ꢀ1.99 (1eꢀ)
+1.00 (1eꢀ)
ꢀ0.30 (1eꢀ)
ꢀ0.85 (1eꢀ)
+0.54 (1eꢀ)
ꢀ0.28 (1eꢀ)
ꢀ0.56 (1eꢀ)
+0.87 (1eꢀ)
ꢀ0.70 (1eꢀ)
ꢀ1.38 (1eꢀ)
+0.44 (1eꢀ)
ꢀ0.55 (1eꢀ)
ꢀ0.86 (1eꢀ)
60
60
ꢀ1.94
+0.52
11[j]
12[j]
13[k]
14[l]
+1.00
ꢀ0.30
ꢀ0.85
90
90
100
65
65
100
50
60
60
70
70
140
60
70
70
4.42
ꢀ0.27
ꢀ0.53
+0.86
ꢀ0.69
ꢀ1.26
80
85
80
80
90
[m]
The optical HOMO–LUMO gaps, determined from the
end-absorption lend of the longest-wavelength UV/Vis band,
correlate reasonably well (R2 =0.895) with the electrochemi-
cal gaps D(Eox,1ꢀEred,1) suggesting that the same orbitals are
involved in both optical and electrochemical gaps for 1–3,
5–7, and 11–14 (Table 4 and Figure 3). Furthermore, the
[m]
+0.42
+1.11
ꢀ0.50
ꢀ0.76
ꢀ0.32
ꢀ1.35
ꢀ0.25
ꢀ0.81
80
80
TCNE
4.57
4.59
4.96
TCNQ
90
90
ꢀ0.26 (1eꢀ)
75
75
ꢀ0.87 (1eꢀ)
[g]
F4-TCNQ
+0.16
ꢀ0.46
90
100
+0.19 (1eꢀ)
ꢀ0.48 (1eꢀ)
75
85
Table 4. Optical and electrochemical gaps of TCNE, TCNQ, and
F4-TCNQ adducts 1–3, 5–7, and 11–14 determined from UV/Vis spectros-
copy and CV in CH2Cl2.
[a] All potentials are given versus the Fc+/Fc couple used as internal
standard. [b] E8=(Epc +Epa)/2, where Epc and Epa correspond to the
lmax [nm (eV)]
lend [nm (eV)]
D(Eox,1ꢀEred,1) [V]
cathodic and anodic peak potentials, respectively. [c] DEp =EoxꢀEred
where the subscripts ox and red refer to the conjugated oxidation and re-
duction steps, respectively. [d] Ep =Irreversible peak potential. [e] E1/2
Half-wave potential. [f] Slope=Slope of the linearized plot of E versus
log[I/ the current.
,
1
2
3
5
6
7
486 (2.55)
657 (1.89)
930 (1.33)
993 (1.25)
942 (1.32)
1120 (1.11)
643 (1.93)
859 (1.44)
570 (2.18)
759 (1.63)
920 (1.35)
1100 (1.13)
1270 (0.98)
1500 (0.83)
1400 (0.89)
1640 (0.76)
820 (1.51)
1300 (0.95)
860 (1.44)
1050 (1.18)
1.19
0.79
0.78
0.61
0.69
0.48
1.30
0.79
1.55
0.99
=
A
I
[g] Electrode inhibition during oxidation. [h] Poorly resolved second re-
duction. [i] Contains a small amount of 7 (see the Supporting Informa-
11[a]
12[a]
13[b]
14[c]
tion). [j] Taken from ref. [17]. [k] Taken from ref. [13c]. [l] Taken from
ref. [15a]. [m] Not determined.
[a] Taken from ref. [17]. [b] Taken from ref. [13c]. The originally reported
lend of 960 nm (1.29 eV) was apparently overestimated. [c] Taken from
ref. [15a].
4116
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Chem. Eur. J. 2009, 15, 4111 – 4123

Organic Super-Acceptors
FULL PAPER
Initial EPR investigations: Due to difficulties encountered
during attempted NMR characterization (vide supra), the
samples of acceptors 2 and 5–7 were investigated by means
of EPR spectroscopy. Indeed, paramagnetic character of the
samples both in the solid state and solution was found in all
cases. EPR spectra of remarkable intensity were obtained
for solid samples of 2 and 5–7 at ambient temperature. Dis-
solving 2, 6, and 7 in [D6]acetone and 5 in 1,2-dimethoxy-
ethane (DME) again led to intense EPR spectra both in
liquid and frozen solution (for EPR spectra of 2 and 7, see
Figure 5; for 6, see the Supporting Information). The ob-
tained spectra are unresolved in all cases, whereas broader
lines are observed due to anisotropic interactions in the
solid state. Nevertheless, the virtually matching g factors in-
dicate that the EPR signals observed for 2 and 5–7 in the
solid-state and solution result from compatible electronic
structures (Table 5).
Figure 3. Linear correlation between the optical gap Egap, determined
from lend, and electrochemical gap D(Eox,1ꢀEred,1) for acceptors 1–3, 5–7,
and 11–14.
Furthermore, reduction of 5 with K metal in DME at
270 K yielded an intense EPR spectrum that is compatible
in terms of its shape and g factor to that of 5 recorded both
in the solid state and DME solution (see the Supporting In-
formation). The data is in good agreement with the pub-
lished values for radical anions of TCNE, TCNQ, and
F4-TCNQ.[32] Owing to the very low first reduction poten-
tials of 2, and 5–7, it can be anticipated that the correspond-
ing radical anions are at least partly present under the ap-
plied experimental conditions, as observed by others.[6a,11]
electrochemical gap decreases in the sequence 13>11>1>
2 reflecting the increased acceptor strength of the CN-con-
taining substituents as mentioned above.
The electron-accepting power, expressed as adiabatic elec-
tron affinity (EA), has been calculated (BP86/def-TZVP
COSMOACHTUNGTRENNUNG
(e=4.5)//BP86/def-SV(P))[30] for acceptors 1–7, 11
and 12 as well as for the reference compound F4-TCNQ
(Table 3).[31] A good linear correlation (R2 =0.857) exists be-
tween the calculated EAs and the first reduction potentials
Ered,1 (Figure 4). Despite the substitution with strong DMA
2ꢀ
Charge-transfer salt {[FeCp*2]+}2[12]2 (15): While explor-
ing the ability of the TCNQ-derived acceptor 12 to form
charge-transfer complexes with various electron donors, we
found that the TCNQ moiety in 12 undergoes reductive
s-dimerization upon reaction with the strong electron donor
decamethylferrocene ([FeCp*2]; Eox,1 =ꢀ0.59 V),[20] as previ-
ously observed for TCNQ.[33]
Thus, an intense green solution was obtained upon addi-
tion of yellow [FeCp*2] in CH2Cl2 to the originally deep-
purple solution of 12 in dry acetonitrile at 258C. The solid
obtained after evaporation of the solvents was recrystallized
by slow diffusion of n-hexane into CH2Cl2 solution to afford
2ꢀ
dark-green crystals of {[FeCp*2]+}2[12]2 (15) in 68% yield.
The obtained crystals of 15 incorporated, even after pro-
longed drying in vacuo, remaining co-crystallized CH2Cl2
molecules, as indicated by elemental analysis and TGA that
are in good agreement with the formula {[FeCp*2]+}2-
[12]22ꢀ·1.4 CH2Cl2 (see the Supporting Information).
The X-ray crystal structure of 15 consists of two inde-
2ꢀ
Figure 4. Linear correlation between the calculated adiabatic electron
affinity EA (BP86/def-TZVP COSMOACTHNUTRGENUGN(e=4.5)//BP86/def-SV(P)) for 1–3,
5–7, 11, and 12, F4-TCNQ, TCNQ, and TCNE against Ered,1 determined
by CV.
pendent [12]2 ions (designated as molecule A and B), four
[FeCp*2]+ molecules, one of which is disordered, and three
disordered CH2Cl2 molecules (Figure 6, for details, see the
Experimental Section; for molecule B, see the Supporting
2ꢀ
Information).[34] The two dimeric ions [12]2 feature long
ꢀ
ꢀ
ꢀ
central C C bonds (C105 C106 and C156 C157) of 1.63 ꢃ
which are comparable to those in reported (TCNQ)2
2ꢀ
donors, the predicted EAs for 2 (4.87 eV) and 7 (4.98 eV)
rival the value calculated for the state-of-the-art p-type
dopant F4-TCNQ (4.96 eV),[10] which makes them interesting
for potential applications in optoelectronic devices.
s-dimers.[33] Consequently, the involved C-atoms are practi-
cally tetrahedral with the bond angles ranging from 105.38
(C135-C105-C106) to 113.08 (C102-C105-C106). The uni-
Chem. Eur. J. 2009, 15, 4111 – 4123
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4117

F. Diederich et al.
Figure 5. EPR spectra of 2 and 7 in the solid state (a and c) and in [D6]acetone solution (b and d) recorded at ambient temperature (ca. 298 K).
Table 5. The g factors extracted from the EPR spectra of 2, 5, and 7, re-
corded under various conditions, and the g factors for the reference com-
pounds TCNE, TCNQ, and F4-TCNQ.
lar effects have previously been described for other anionic
TCV-substituted systems.[35] This is further supported by cal-
culations at the B3LYP/6-31G(d) level[36] performed on the
Compound
Conditions
g factor
2ꢀ
[12]2 s-dimer. The bond lengths and torsional angles of the
2
solid state
[D6]acetone solution (1 mm)
solid state
DME solution (270 K)
after K metal reduction in DME (270 K) 2.0035
2.0036
2.0034
2.0032
2.0034
calculated dianionic structure compare well to those ob-
tained by X-ray analysis (see caption to Figure 6a). The cal-
culated HOMO of the dianionic species has most of the
electron density on the TCV moieties with only small coeffi-
5
7
solid state
2.0033
ꢀ
cients on the central C C bond (see the Supporting Infor-
mation).
[D6]acetone solution (1 mm)
2.0032
[a]
Cꢀ
TCNE
2.0026ꢁ0.0002
2.0027ꢁ0.0002
2.0029ꢁ0.0001
In the crystal packing of 15, multiple short contacts with
N···C distances ranging from 3.35 to 3.58 ꢃ between the
[b]
[b]
Cꢀ
TCNQ
F4-TCNQ
Cꢀ
[12]2 ions (A and B) and the neighboring [FeCp*2]+ mole-
2ꢀ
[a] Taken from ref. [32a]. [b] Taken from ref. [32b].
cules are observed. For example, molecule A is surrounded
by eight [FeCp*2]+ units with ten intermolecular N···C con-
tacts between 3.36 and 3.61 ꢃ (Figure 6b; for molecule B,
formity of the distances in the originally quinoid TCNQ
rings in [12]22ꢀ, averaging to 1.39 ꢃ for both independent
molecules, suggests that their aromatization occurred upon
s-dimer formation. Furthermore, the significant lengthening
ꢀ
see the Supporting Information). The average Fe C bond
lengths of 2.097 ꢃ, based on the three ordered [FeCp*2]+
units, are consistent with the presence of FeIII atoms.[37]
Naturally, the s-bond formation between two radical cen-
ters results in disappearance of paramagnetic properties. In
ꢀ
of the C=C bonds in the TCV ( C(CN)=C(CN)2) moieties
to an average value of 1.44 ꢃ with the concurrent shorten-
ꢀ
ꢀ
ing of the adjacent C C bonds to 1.37 ꢃ indicate delocaliza-
tion of the negative charge over the entire TCV unit, hence
forming tricyano-substituted allylic anions (Figure 6a). Simi-
analogy to the reported TCNQ s-dimers, the central C C
bond in diamagnetic [12]2 is expected to be rather weak
2ꢀ
due to the long bond length, considerable delocalization of
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Organic Super-Acceptors
FULL PAPER
and 2129 cmꢀ1, accompanied by a band at 806 cmꢀ1 resulting
ꢀ
from the d
N
ꢂ
band at n
(C N)=2178 cm and a weak band at 839 cmꢀ1,
which most likely results from the d
Supporting Information), thus indicating dissociation of the
central s-bond to form two radical anions 12 in solution.
Moreover, the solid obtained upon evaporation of the
CHCl3 solution gave identical IR spectrum to that of pris-
tine 15. This is further supported by preliminary EPR inves-
tigations both in the solid state and solution. Whereas the
2ꢀ
solid sample of 15 containing diamagnetic s-dimers [12]2 is
EPR silent both at 290 and 180 K, dissolving 15 in DME re-
sults in appearance of intense EPR spectra apparently re-
Cꢀ
sulting from radical anion 12 (see the Supporting Informa-
tion). This is in agreement with the fact that no NMR spec-
tra of 15 could be obtained in CD2Cl2 or [D6]acetone solu-
tion.
The influence of the counter cation, solvent polarity, con-
centration, and temperature on the s-dimerization process is
currently under investigation.[38]
Conclusions
A series of stable organic super-acceptors has been synthe-
sized by thermal [2+2] cycloadditions of TCNE, TCNQ, and
F4-TCNQ to DMA-substituted alkynes bearing either nitrile,
dicyanovinyl, or tricyanovinyl functions, followed by retro-
electrocyclization of the initially formed cyclobutenes. De-
spite the nonplanarity of these acceptors, as revealed by
X-ray crystallographic analysis (for 1) and theoretical calcu-
lations, efficient intramolecular charge-transfer (CT) inter-
actions are established. The corresponding CT bands appear
strongly bathochromically shifted with an end-absorption
reaching far into the near infrared region, down to 1600 nm
(0.78 eV) for F4-TCNQ adduct 7. Electronic absorption
spectra of 1–3, 5, 11, and 12 were calculated by applying the
spectroscopy oriented configuration interaction (SORCI)
procedure. In most cases the SORCI excitation energies are
in good agreement with the experimental UV/Vis spectra re-
corded in CH2Cl2. The electrochemical investigations of
these acceptors by CV and RDV in CH2Cl2 identified their
remarkable propensity for reversible electron uptake rival-
ing in some cases even the benchmark compound F4-TCNQ
(Ered,1 =+0.16 V vs. Fc+/Fc). Furthermore, the electron-
2ꢀ
Figure 6. a) ORTEP plot of dianion [12]2 (molecule A) in the X-ray
crystal structure of 15. Atomic displacement parameters obtained at
220 K are drawn at the 30% probability level. Hydrogen atoms and
CH2Cl2 molecules are omitted for clarity. Selected experimental[34] and
calculated (in parentheses; B3LYP/6-31G(d))[36] bond lengths [ꢃ]: a 1.41
(1.41), b 1.41 (1.42), c 1.44 (1.45), d 1.45 (1.45), e 1.37 (1.39), f 1.48 (1.48),
g 1.63 (1.67). Selected experimental[34] and calculated (in parentheses;
B3LYP/6-31G(d))[36] torsion angles [8]: C81-C82-C83-C84 162.7 (163.5),
C99-C81-C82-C83 167.7 (161.2), C82-C81-C90-C95 140.6 (137.8), C82-
C81-C99-C104 135.2 (141.0). b) Arrangement of neighboring molecules
in the crystal packing of 15 showing ten intermolecular N···C contacts be-
2ꢀ
tween [12]2 (molecule A) and the eight surrounding [FeCp*2]+ mole-
cules. The shortest contact between two neighboring molecules of type A
ꢀ
(N117 C129 3.16 ꢃ) is not shown.
the bonding electrons, and electrostatic repulsions.[33c,38]
Breaking this bond by external stimuli should yield two rad-
ical anions 12 (Scheme 1). Indeed, the solid-state IR spec-
Cꢀ
ꢂ
trum of 15 displays two bands in the nACTHNUTRGNE(NUG C N) region at 2169
Scheme 1. Formation and dissociation of dimeric {[FeCp*2]+}2[12]22ꢀ (15).
Chem. Eur. J. 2009, 15, 4111 – 4123
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4119

F. Diederich et al.
dro-2,6-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1H,4H,5H,8H-2,3a,4a,6,7a,
accepting power of the new compounds expressed as adia-
batic electron affinity has been estimated by theoretical cal-
culations. Despite the substitution with strong DMA donors,
the predicted EAs for acceptors 2 and 7 reach the value cal-
culated for F4-TCNQ (4.96 eV), which makes these com-
pounds interesting for potential applications as p-type dop-
ants in the fabrication of OLEDs or solar cells.
The first example of a charge-transfer salt between the
DMA-substituted TCNQ adduct 12 and strong electron
donor decamethylferrocene has been prepared. The X-ray
crystallographic analysis revealed that the TCNQ moiety in
the acceptor underwent reductive s-dimerization upon reac-
tion with the donor. Initial investigations by EPR and IR
spectroscopy both in the solid state and solution suggest,
that the dimeric species dissociates in solution. Detailed in-
vestigations of this phenomenon as well as attempts to pre-
pare conductive or magnetic charge-transfer salts between
these potent acceptors and various organic and organome-
tallic donors are currently being pursued. We believe that
even stronger acceptors are in reach upon careful choice of
reacting partners in the [2+2] cycloadditions.
+
8a-hexaazacyclopenta
N
,
m/z calcd 503.3816; Uvinul 4049 H Ciba Speciality Chemicals). The most
important peaks are reported in m/z units with M as the molecular ion.
Thermogravimetric analyses (TGA) were carried out on a TA Instru-
ments TGA Q500 V5.3 instrument in air, at a heating rate of 208Cminꢀ1
between 508C and 9008C. Elemental analyses were performed by the
Mikrolabor at the Laboratorium fꢀr Organische Chemie, ETH Zꢀrich,
with a LECO CHN/900 instrument, fluorine was estimated by ionchro-
matography on a Metrohm 761 Compact IC instrument, chlorine was es-
timated by argentometric titration.
Electrochemistry: The electrochemical measurements were carried out at
208C in CH2Cl2, containing 0.1m nBu4NPF6 in a classical three-electrode
cell. CH2Cl2 was purchased in spectroscopic grade from Merck, dried
over molecular sieves (4 ꢃ), and stored under Ar prior to use. nBu4NPF6
was purchased in electrochemical grade from Fluka and used as received.
The working electrode was a glassy carbon disk electrode (3 mm in diam-
eter) used either motionless for cyclic voltammetry (0.1 to 10 Vsꢀ1) or as
rotating-disk electrode for rotating disk voltammetry (RDV). The auxili-
ary electrode was a Pt wire, and the reference electrode was either an
aqueous Ag/AgCl electrode or a platinum wire used as a pseudo-refer-
ence electrode. All potentials are referenced to the ferricinium/ferrocene
(Fc+/Fc) couple, used as an internal standard, and are uncorrected from
ohmic drop. The cell was connected to Autolab PGSTAT30 potentiostat
(Eco Chemie BV, Utrecht, The Netherlands) controlled by the GPSE
software running on a personal computer.
EPR measurements: EPR spectra were recorded on a Bruker ESP 300
X-band spectrometer. All measurements were performed at ambient
temperature (ca. 298 K), unless otherwise stated. 1,2-Dimethoxyethane
(DME) was heated to reflux over Na/K alloy and stored over Na/K alloy
under high vacuum. [D6]acetone was used as received. Compounds 2, 6,
and 7 were dissolved in [D6]acetone and deoxygenated thoroughly by
argon bubbling through for 10 min. Reduction of 5 was performed by
contact of the DME solution of the parent compound with a K-metal
mirror under high vacuum.
Experimental Section
Materials and general methods: Reagents and solvents were purchased at
reagent grade from Acros, Aldrich, and Fluka, and used as received.
CH2Cl2 was freshly distilled from CaH2 under N2. All reactions were per-
formed under an inert atmosphere by applying a positive pressure of N2.
Column chromatography (CC) and plug filtrations were carried out with
SiO2 60 (particle size 0.040–0.063 mm, 230–400 mesh ASTM; Fluka) or
SiO2 60 (particle size 0.063–0.200 mm, 70–230 mesh ASTM; Merck) and
distilled technical solvents. 3-[4-(Dimethylamino)phenyl]-2-propynenitrile
(8),[17,18] {3-[4-(dimethylamino)phenyl]-2-propyn-1-ylidene}propanedini-
trile (9),[13b] 4-[4-(dimethylamino)phenyl]-1-buten-3-yne-1,1,2-tricarboni-
trile (10),[19] and 3-(dicyanomethylidene)-2-[4-(dimethylamino)phenyl]-
1,4-pentadiene-1,1,5,5-tetracarbonitrile (12),[17] were prepared according
to literature procedures. Thin-layer chromatography (TLC) was conduct-
ed on aluminum sheets coated with SiO2 60 F254 obtained from Macher-
ey–Nagel; visualization with a UV lamp (254 or 366 nm). Melting points
(m.p.) were measured on a Bꢀchi B-540 melting-point apparatus in open
capillaries and are uncorrected. 1H NMR, 13C NMR, and 19F NMR spec-
tra were measured on a Varian Gemini 300 or on a Bruker DRX400 or
on a Bruker DRX500 spectrometer at 298 K. Chemical shifts (d) are re-
ported in ppm relative to the signal of tetramethylsilane (TMS). Residual
solvent signals in the 1H and 13C NMR spectra were used as an internal
reference. Coupling constants (J) are given in Hz. The apparent reso-
nance multiplicity is described as s (singlet), br s (broad singlet), d (dou-
blet), t (triplet), q (quartet), and m (multiplet). Infrared spectra (IR)
were recorded on a Perkin–Elmer BX FT-IR spectrometer; signal desig-
nations: s (strong), m (medium), w (weak). UV/Vis spectra were record-
ed on a Varian Cary-5 spectrophotometer. The spectra were measured in
CH2Cl2 in a quartz cuvette (1 cm) at 298 K. The absorption maxima
(lmax) are reported in nm with the extinction coefficient (e) mꢀ1 cmꢀ1 in
brackets; shoulders are indicated as sh. High-resolution (HR) EI-MS
spectra were measured on a Micromass AutoSpec-Ultima spectrometer.
HR FT-ICR-MALDI spectra were measured on a IonSpec Ultima Fouri-
er transform (FT) instrument with [(2E)-3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malononitrile (DCTB) or 3-hydroxypicolinic acid (3-
HPA) as matrix. HR MALDI-TOF spectra were recorded on a Bruker
Ultraflex II mass spectrometer using 7,7,8,8-tetracyanoquinodimethane
(TCNQ) as matrix and internal calibration (Bruker High Precision Cali-
bration Mode) with TCNQ ([M]+, C12H4N4+, m/z calc. 204.0430; Fluka),
terthiophene ([M]+, C12H8S3+, m/z calcd 247.9783; Fluka), and hexahy-
Calculations: Electronic absorption spectra of 1–3, 5, 11, and 12 were cal-
culated by applying the spectroscopy oriented configuration interaction
(SORCI)[24] procedure implemented in the ORCA program suite.[25] The
geometries of all molecules were optimized by using the TURBOMOLE
program suite, version 5.10[40] at the BP86/def-SV(P) level. A single-point
Hartree–Fock calculation using the TZVP basis set[41] was performed to
generate the input for the SORCI calculation. The active space was con-
structed from all Slater determinants distributing altogether six electrons
in the HOMOꢀ2, HOMOꢀ1, HOMO, LUMO, and LUMO+1 of the
Hartree–Fock calculation, preserving the HF-occupation of all other orbi-
tals. The three configuration selection parameters were set to 10ꢀ6 (tsel),
10ꢀ4 (tpre), and 10ꢀ5 (tnat), respectively.
X-ray analysis: The structures were solved by direct methods (SIR-97)[42]
and refined by full-matrix least-squares analysis (SHELXL-97),[43] using
an isotropic extinction correction. All non hydrogen atoms were refined
anisotropically; hydrogen atoms were refined isotropically, whereby hy-
drogen positions are based on stereochemical considerations.
X-ray crystal structure of 1: Crystal data at 220(2) K for C20H11N7, Mr =
349.36, triclinic, space group P1, 1calcd =1.278 gcmꢀ3, Z=2, a=7.0331(18),
¯
b=7.7652(19), c=16.756(2) ꢃ, a=86.996(14), b=83.408(15), g=
88.385(16)8, V=907.6(3) ꢃ3. Bruker–Nonius Kappa-CCD diffractometer,
MoKa radiation, l=0.7107 ꢃ, m=0.082 mmꢀ1. A black crystal of 1 (linear
dimensions ca. 0.16ꢄ0.07ꢄ0.06 mm) was obtained by slow diffusion of
n-hexane into a solution of 1 in CH2Cl2. Numbers of measured and
unique reflections are 3942 and 2466, respectively (Rint =0.062). Final
R(F)=0.072, wR(F2)=0.168 for 247 parameters and 1636 reflections
with I>2s(I) and 2.92<q<23.238 (corresponding R values based on all
2466 reflections are 0.112 and 0.193, respectively).
X-ray crystal structure of 15: Crystal data at 220(2) K for 2(C46H28N12)·4-
A
ACHTUGNERTN(NGNU CH2Cl2), Mr =3010.04, monoclinic, space group P21/c
A
, Z=4, a=21.8649(13), b=31.0432(15),
c=26.3554(14) ꢃ, b=114.293(11)8, V=16305(2) ꢃ3. Bruker-Nonius
Kappa-CCD diffractometer, MoKa radiation, l=0.7107 ꢃ, m=
4120
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 4111 – 4123

Organic Super-Acceptors
FULL PAPER
0.489 mmꢀ1. A green crystal of 15 (linear dimensions ca. 0.15ꢄ0.09ꢄ
0.05 mm) was obtained by slow diffusion of n-hexane into a solution of
15 in CH2Cl2. Numbers of measured and unique reflections are 33275
and 19452, respectively (Rint =0.044). Final R(F)=0.072, wR(F2)=0.165
for 1949 parameters and 14056 reflections with I> 2s(I) and 1.66<q<
21.988 (corresponding R values based on all 19452 reflections are 0.106
and 0.186, respectively). CCDC-711841 (1) and CCDC-711842 (15) con-
tain the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Silylation of glass surfaces with dimethyldichlorosilane (DMDCS):[21] The
glassware was soaked in a toluene solution of DMDCS (5% v/v) for
15 min at 258C. Subsequently, the glassware was rinsed twice with tolu-
ene, soaked for 15 min in MeOH, rinsed with MeOH, and finally dried in
a nitrogen stream.
UV/Vis (CH2Cl2): lmax (e)=379 (sh, 68000), 400 (108400), 633 (13900),
930 nm (1400mꢀ1 cmꢀ1); HR-MALDI-TOF-MS (TCNQ): m/z calcd for
+
C26H15N7 [M]+: 425.1383; found: 425.1374.
3-[4-(Dicyanomethylidene)-2,3,5,6-tetrafluoro-2,5-cyclohexadien-1-yl-
AHCTUNGTERGiNNUN dene]-3-[4-(dimethylamino)phenyl]-1-propene-1,1,2-tricarbonitrile (5):
F4-TCNQ (57 mg, 0.206 mmol) was added to a solution of 8 (35 mg,
0.206 mmol) in CH2Cl2 (50 mL) in a flask deactivated with DMDCS. The
mixture was stirred for 17 h at 258C. After that time, n-hexane (50 mL)
was added slowly forming a second layer on the top of the reaction solu-
tion and the mixture was allowed to stand for two days at 258C. The
mother liquor was carefully removed using a Pasteur pipette, and the
solid was washed with n-hexane. Repeated (3ꢄ) crystallization by slow
diffusion of n-hexane into CH2Cl2 solution at 258C afforded 5 (60 mg,
65%) as a black metallic-like solid. Rf =0.13 (SiO2, CH2Cl2/EtOAc 95:5;
decomp); m.p.>4108C (decomp); 1H NMR (300 MHz, CD2Cl2): d=3.39
(s, 6H), 6.95 (d, J=9.4 Hz, 2H), 7.33 ppm (d, J=9.4 Hz, 2H); 13C NMR
(125 MHz, CD2Cl2): not available due to low solubility of the solid;
19F NMR (282 MHz, CD2Cl2): d=ꢀ140.53 (m), ꢀ133.24 ppm (brs); IR
(neat): n˜ =2652 (w), 2197 (s), 2181 (m), 1635 (m), 1602 (s), 1532 (m),
1387 (s), 1271 (s), 1200 (s), 1161 (s), 1072 (m), 978 (m), 960 (m), 869 (w),
834 (s), 822 cmꢀ1 (m); UV/Vis (CH2Cl2): lmax (e)=330 (12400), 370 (sh,
9300), 391 (12200), 539 (25600), 993 nm (22300mꢀ1 cmꢀ1); HR-MALDI-
3-(Dicyanomethylidene)-2-[4-(dimethylamino)phenyl]-1,4-pentadiene-
1,1,5,5-tetracarbonitrile (1): TCNE (48 mg, 0.380 mmol) was added to a
solution of 9 (42 mg, 0.190 mmol) in 1,2-dichloroethane (35 mL). The
mixture was stirred for 16 h at 608C. The solvent was evaporated in
vacuo to afford a black solid that was purified by repeated (3ꢄ) slow dif-
fusion of n-hexane into CH2Cl2 solution at 258C. Subsequent CC (SiO2,
CH2Cl2!CH2Cl2/EtOAc 95:5; decomp) afforded 1 (17 mg, 26%) as a
black metallic-like solid. Rf =0.48 (SiO2, CH2Cl2/EtOAc 95:5; decomp);
m.p.>2508C (decomp); 1H NMR (500 MHz, CD2Cl2): d=3.21 (s, 6H),
6.80 (d, J=9.4 Hz, 2H), 7.67 (d, J=9.4 Hz, 2H), 8.02 ppm (s, 1H);
13C NMR (125 MHz, CD2Cl2): d=40.83, 74.33, 97.59, 100.24, 109.54,
109.82, 110.22, 112.27, 113.36, 114.22, 114.32, 117.29, 132.97, 147.72,
155.16, 155.85, 158.85 ppm; IR (neat): n˜ =3031 (w), 2923 (w), 2852 (w),
2214 (s), 1603 (s), 1476 (s), 1457 (s), 1382 (s), 1359 (s), 1275 (m), 1218 (s),
1167 (s), 1059 (m), 943 (m), 898 (w), 825 cmꢀ1 (s); UV/Vis (CH2Cl2):
lmax (e)=317 (21400), 304 (25000), 486 nm (43300mꢀ1 cmꢀ1); HR-
+
TOF-MS (TCNQ): m/z calcd for C23H10N6F4 [M]+: 446.0898; found:
446.0886; elemental analysis calcd (%) for C23H10N6F4 (446.37): C 61.89,
H 2.26, N 18.83, F 17.02; found: C 61.92, H 2.43, N 18.65, F 17.12.
2-{[4-(Dicyanomethylidene)-2,3,5,6-tetrafluoro-2,5-cyclohexadien-1-yl-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
of 9 (50 mg, 0.226 mmol) in CH2Cl2 (40 mL) in a flask deactivated with
DMDCS. The mixture was stirred for 16 h at 258C. After that time,
n-hexane (50 mL) was added and the mixture was allowed to stand for
two days at 258C. The mother liquor was carefully removed, the solid
was washed with n-hexane (3 ꢄ) and dried in vacuo to give 6 (94 mg,
ꢀ
MALDI-MS (DCTB): m/z calcd for C20H11N7 [M]ꢀ: 349.1081; found:
349.1084.
84%) as
a copper-like solid. Rf =0.12 (SiO2, CH2Cl2/EtOAc 95:5;
3-(Dicyanomethylidene)-4-[4-(dimethylamino)phenyl]-1,4-pentadiene-
1,1,2,5,5-pentacarbonitrile (2): TCNE (13 mg, 0.101 mmol) was added to
decomp); m.p.>2708C (decomp); 1H NMR, 13C NMR, and 19F NMR not
available due to contamination with paramagnetic species and low solu-
bility of the solid; IR (neat): n˜ =3039 (w), 2925 (w), 2662 (w), 2189 (s),
2162 (s), 1635 (m), 1603 (s), 1505 (s), 1480 (m), 1401 (s), 1357 (s), 1222
(s), 1179 (s), 1079 (m), 1002 (s), 964 (s), 920 (m), 859 (m), 833 cmꢀ1 (s);
UV/Vis (CH2Cl2): lmax (e)=289 (sh, 20600), 321 (27000), 697 (40600),
942 nm (17000mꢀ1 cmꢀ1); HR-MALDI-TOF-MS (TCNQ): m/z calcd for
a
solution of 10 (25 mg, 0.101 mmol) in 1,1,2,2-tetrachloroethane
(15 mL), and the mixture was stirred for two days at 1208C. After that
time, n-hexane (20 mL) was added slowly forming a second layer on the
top of the reaction solution and the mixture was allowed to stand for two
days at 258C. The mother liquor was carefully removed using a Pasteur
pipette, and the solid was washed with n-hexane. Repeated (3ꢄ) crystalli-
zation by slow diffusion of n-hexane into CH2Cl2 solution at 258C afford-
ed 2 (25 mg, 66%) as a black solid. Rf not determined due to rapid de-
composition on SiO2; m.p.>2328C (decomp); 1H NMR and 13C NMR
not available due to contamination with paramagnetic species and low
solubility of the solid; IR (neat): n˜ =2923 (w), 2214 (m), 1601 (s), 1485
(s), 1437 (m), 1375 (s), 1191 (s), 1171 (s), 1121 (m), 1064 (w), 941 (w),
822 cmꢀ1 (w); UV/Vis (CH2Cl2): lmax (e)=295 (15400), 469 (16300),
657 nm (5600, mꢀ1 cmꢀ1); HR-MALDI-MS (DCTB): m/z calcd for
C21H10N8ꢀ [M]ꢀ: 374.1023; found: 374.1028.
C26H11N7F4 [M]+: 497.1007; found: 497.1019; elemental analysis calcd
(%) for C26H11N7F4 (497.41): C 62.78, H 2.23, N 19.71, F 15.28; found: C
62.28, H 2.37, N 19.27, F 15.34.
+
3-{[4-(Dicyanomethylidene)-2,3,5,6-tetrafluoro-2,5-cyclohexadien-1-yl-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
of 10 (15 mg, 0.061 mmol) in CH2Cl2 (10 mL) in a flask deactivated with
DMDCS. The mixture was stirred for five days at 258C. After that time,
n-hexane (10 mL) was added slowly forming a second layer on the top of
the reaction solution and the mixture was allowed to stand for three days
at 258C. The mother liquor was carefully removed using a Pasteur pip-
ette, and the solid was washed with n-hexane. Repeated (3ꢄ) crystalliza-
tion by slow diffusion of n-hexane into CH2Cl2 solution at 258C afforded
7 (28 mg, 88%) as a black metallic-like solid. Rf not determined due to
rapid decomposition on SiO2; m.p.>3608C (decomp); 1H NMR,
13C NMR, and 19F NMR not available due to contamination with para-
magnetic species and low solubility of the solid; IR (neat): n˜ =2196 (s),
2171 (m), 1633 (m), 1595 (s), 1502 (m), 1476 (m), 1352 (s), 1304 (s), 1200
(s), 1171 (s), 1111 (m), 1030 (m), 972 (m), 869 (w), 836 cmꢀ1 (w); UV/Vis
(CH2Cl2): lmax (e)=306 (sh, 15900), 342 (19400), 420 (11800), 753
(36700), 1120 nm (9400mꢀ1 cmꢀ1); HR-MALDI-TOF-MS (TCNQ): m/z
2-{[4-(Dicyanomethylidene)-2,5-cyclohexadien-1-ylidene][4-(dimethyl-
ACHTUNGTRENNUNGamino)phenyl]methyl}-1,3-butadiene-1,1,4,4-tetracarbonitrile (3): TCNQ
(92 mg, 0.450 mmol) was added to a solution of 9 (50 mg, 0.226 mmol) in
1,2-dichloroethane (35 mL). The mixture was stirred for five days at
608C. The solvent was evaporated in vacuo and the residue purified by
repeated (3ꢄ) slow diffusion of n-hexane into CH2Cl2 solution at 208C.
The obtained solid was divided into four portions, which were individual-
ly purified by multiple CC (SiO2, 3ꢄCH2Cl2!CH2Cl2/EtOAc 1:1;
decomp) to afford 3 (13 mg, 14%) as a deep-blue metallic-like solid. Rf =
0.43 (SiO2, CH2Cl2/EtOAc 1:1; decomp); m.p. 140–1428C; 1H NMR
(400 MHz, 1,1,2,2-[D2]tetrachloroethane): d=3.11 (s, 6H), 6.70–6.74 (m,
3H), 7.13 (d, J=9.2 Hz, 2H), 7.21 (dd, J=9.5, 1.9 Hz, 1H), 7.27 (dd, J=
9.5, 1.9 Hz, 1H), 7.40 (dd, J=9.5, 1.9 Hz, 1H), 7.96 ppm (s, 1H);
13C NMR (125 MHz, 1,1,2,2-[D2]tetrachloroethane): d=40.59, 97.26,
98.05, 109.48, 110.49, 110.62, 111.99, 113.40, 114.89, 122.14, 126.21, 126.71,
133.18, 133.29, 135.09, 135.65, 143.32, 151.10, 153.65, 153.77, 161.78 ppm;
IR (neat): n˜ =2857 (w), 2198 (s), 1607 (w), 1575 (s), 1519 (m), 1395 (m),
1367 (s), 1347 (s), 1162 (s), 1002 (w), 940 (m), 909 (m), 820 cmꢀ1 (m);
calcd for C27H10N8F4 [M]+: 522.0959; found: 522.0960.
+
Charge-transfer salt {[FeCp*2]+}2[12]22ꢀ·1.4 CH2Cl2 (15): To a solution of
12 (50.0 mg, 0.133 mmol) in dry MeCN (20 mL), [FeCp*2] (43.4 mg,
0.133 mmol) in dry CH2Cl2 (10 mL) was added dropwise. The originaly
purple solution became intense green while stirred for 30 min at 258C.
The solvent was evaporated in vacuo and the dark residue dissolved in
Chem. Eur. J. 2009, 15, 4111 – 4123
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4121

F. Diederich et al.
CH2Cl2 (15 mL). n-Hexane (ca. 15 mL) was added slowly on the top of
the CH2Cl2 solution forming a second layer, and the mixture was allowed
to stand for five days at 08C. The formed solid was collected by filtration,
washed with n-hexane, and dried in vacuo (2ꢄ10ꢀ6 mbar) to give an ana-
lytical sample of 15 (69.2 mg, 68%) as dark green crystals readily soluble
in CH2Cl2. Crystals of 15 in equilibrium with the supernatant were used
for X-ray crystallographic analysis to prevent loss of co-crystallized sol-
utes. Interestingly, red-brown crystals that were obtained in comparable
yields in different runs, featured identical elemental composition and
crystallographic parameters as the original sample of 15. M.p.>1708C
(decomp); IR (neat): n˜ =2961 (w), 2920 (w), 2853 (w), 2796 (w), 2169 (s),
2129 (s), 1601 (s), 1581 (s), 1519 (s), 1476 (m), 1444 (m), 1424 (m), 1338
(s), 1283 (s), 1172 (s), 1124 (m), 1061 (m), 1022 (m), 945 (m), 905 (w),
834 (m), 822 (m), 806 cmꢀ1 (m); IR (CHCl3): n˜ =3010 (m), 2920 (w),
2868 (w), 2178 (s), 1601 (m), 1582 (s), 1525 (w), 1505 (w), 1477 (w), 1424
(m), 1356 (s), 1334 (s), 1182 (m), 1174 (m), 1022 (w), 989 (w), 946 (w),
904 (w), 839 cmꢀ1 (w); UV/Vis (CH2Cl2): lmax (e)=277 (52500), 320 (sh,
37700), 437 (33700), 470 (sh, 29000), 620 (39600), 690 (sh, 26000),
1307 nm (28300mꢀ1 cmꢀ1); elemental analysis calcd (%) for
C86H88N12Fe2·1.4 CH2Cl2 (1520.29): C 69.05, H 6.02, N 11.06, Cl 6.53;
found: C 69.11, H 6.31, N 11.11, Cl 6.54.
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Acknowledgement
This research was supported by the ETH Research Council, the NCCR
“Nanoscale Science”, Basel, and FWF (Austria, project no. P20019).
B. E. thanks the “Doktorandinnenkolleg FreChe Materie” for her schol-
arship. Prof. Dr. P. Walde and Dr. T. Schweizer (ETH Zurich) are grate-
fully acknowledged for the TGA measurements, and L. Bertschi (ETHZ)
for the measurements of the challenging MALDI-TOF spectra. We thank
Prof. Dr. J. S. Miller (University of Utah) for many fruitful discussions.
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2ꢀ
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[FeCp*2]+ molecules, the ESDs for Fe C and C C bond lengths
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ꢀ
ꢀ
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Received: December 6, 2008
Published online: March 5, 2009
Chem. Eur. J. 2009, 15, 4111 – 4123
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4123