One-electron-reduced and -oxidized stages of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes of different molecular architectures

Article abstract of DOI:10.1002/chem.200800716

A series of monomeric and oligomeric donor-substituted 1,1,4,4- tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of π-electron derealization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied π systems, limited π-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron derealization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Full text of DOI:10.1002/chem.200800716

DOI: 10.1002/chem.200800716
One-Electron-Reduced and -Oxidized Stages of Donor-Substituted 1,1,4,4-
Tetracyanobuta-1,3-dienes of Different Molecular Architectures
Milan Kivala,[a] Tsvetanka Stanoeva,[b] Tsuyoshi Michinobu,[a] Brian Frank,[a]
[b]
GeorgGescheidt,* and FranÅois Diederich*[a]
Abstract: A series of monomeric and
oligomeric donor-substituted 1,1,4,4-
(EPR/ENDOR) spectroscopy, support-
ed by density functional theory (DFT)
calculations. The extent of p-electron
delocalization in the paramagnetic spe-
cies was investigated in terms of the
EPR parameters. Despite favorable
molecular orbital (MO) coefficients,
the EPR results suggest that in radical
anions the spin and charge are con-
fined to the electron-withdrawing
TCBD moieties on the hyperfine EPR
timescale. The observed spin localiza-
tion is presumably caused by an inter-
play between the nonplanarity of the
studied p systems, limited p-electron
conjugation, and very likely counterion
effects. In radical cations, an analogous
spin and charge localization confined
to the electron-donating N,N-dialkyl-
tetracyanobuta-1,3-dienes
(TCBDs)
with various topologies have been syn-
thesized by means of thermal [2+2]
cycloaddition between tetracyanoethyl-
ACHTREUNGene (TCNE) and donor-substituted al-
kynes, followed by retro-electrocycliza-
tion. One-electron-reduced and -oxi-
dized stages of the donor-substituted
TCBDs were generated by chemical
methods. The obtained radical anions
and radical cations were studied by
using electron paramagnetic resonance/
electron nuclear double resonance
AHCTREaUGN niline moieties was found. In this
case, an efficient electron delocaliza-
tion is disabled by small MO coeffi-
cients at the joints between the donor
and acceptor portions of the studied
TCBDs.
Keywords: charge transfer · conju-
gation · cycloaddition · donor–ac-
ceptor systems · EPR spectroscopy
Introduction
lar attraction due to the ease of structural tuning to enhance
specific properties for specialized applications, such as third-
order nonlinear optical (NLO) effects.[5]
Over the decades, organic electron donor–acceptor (D–A)-
substituted systems, featuring intramolecular charge-transfer
interactions, have attracted considerable interest as promis-
ing candidates for use in next-generation electronic and op-
toelectronic devices.[1,2] Furthermore, bimolecular D–A ar-
chitectures, capable of undergoing intermolecular charge
transfer, have been utilized for the development of molecu-
lar organic conductors.[3,4] Organic materials are of particu-
Recently, we introduced a new class of potent charge-
transfer (CT) chromophores, donor-substituted 1,1,4,4-tetra-
cyanobuta-1,3-dienes (TCBDs),[6,7] accessible in very high
yields in an atom-economic synthesis by formal [2+2] cyclo-
addition between tetracyanoethylene (TCNE) and donor-
substituted alkynes, followed by retro-electrocyclization.[8]
Observed large third-order optical nonlinearities together
with high stability and easy synthetic accessibility make
these compounds attractive for fabrication of optoelectronic
devices.[6,9] Despite substitution with N,N-dialkylanilino, me-
thoxyphenyl, or thienyl donors, the TCBD moiety in these
systems remains a potent electron acceptor. Particularly re-
markable are the electrochemical properties of oligomeric
and dendritic donor-substituted TCBDs, which undergo sev-
eral reversible electron-transfer processes under electro-
chemical conditions.[7] These systems were shown to act as
“molecular batteries”, featuring exceptional electron uptake
and storage capacity.[10]
[a] Dr. M. Kivala, Dr. T. Michinobu, B. Frank, Prof. Dr. F. Diederich
Laboratorium für Organische Chemie, ETH-Hçnggerberg
Wolfgang-Pauli-Strasse 10, 8093 Zürich (Switzerland)
Fax : (+41)44-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] T. Stanoeva, Prof. Dr. G. Gescheidt
Institut für Physikalische und Theoretische Chemie, TU Graz
Technikerstrasse 4/I, 8010 Graz (Austria)
Fax : (+43)316-873-8225
E-mail: g.gescheidt-demner@tugraz.at
Herein, we report an initial investigation of one-electron-
reduced and -oxidized stages of monomeric and oligomeric
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800716.
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FULL PAPER
donor-substituted TCBDs 1–9 by means of electron para-
magnetic resonance/electron nuclear double resonance
(EPR/ENDOR) spectroscopy, supported by density func-
tional theory (DFT) calculations, to probe the extent of
electronic interaction between the D–A dyads within a mol-
ecule when an electron is either added or removed.
solution in THF to produce dark insoluble material, it must
be subjected to subsequent reactions immediately (see the
Supporting Information). Subsequent reaction of 13 with
TCNE in CH2Cl2 at 208C afforded the multivalent chromo-
phore 9 in very high yield. All new donor-substituted
TCBDs 4, 6, 8, and 9 are deep-colored solids, stable at ambi-
ent temperature and when exposed to the laboratory atmos-
phere, and melt without decomposing above 1008C. The
identity of the alkyne precursors such as 13 and the novel
Results and Discussion
1
TCBD derivatives 4, 6, 8, and 9 was confirmed by using H
Topology: Donor-substituted TCBD derivatives 1–9 can be
divided into three different classes depending on their struc-
tural features. Molecules 1–4 are monomeric, composed of a
1,1,4,4-tetracyanobuta-1,3-diene (TCBD) acceptor connect-
ed to a N,N-dialkylanilino or methoxyphenyl donor moiety.
The dendrimer-like chromophores 5–7 feature three (5 and
7) or four (6) arrays of D–A dyads that are attached to a
central benzene core. Derivative 7 includes 1,2-di(1,3-di-
thiol-2-ylidene)ethane-1,2-diyl fragments as donors, which
are introduced by [2+2] cycloaddition between tetrathiaful-
valene (TTF) and the electron-deficient triple bonds in 5,
followed by retro-electrocyclization.[7,11] The third class of
compounds involves an electron-donating triphenylamine
core to which D–A dyads are connected either by ethyne-
1,2-diyl (in 8) or 1-(buta-1,3-diyn-1-yl)-3,5-diethynylbenzene
(in 9) spacers.
Extensive X-ray crystallographic investigations on mono-
meric donor-substituted TCBDs, such as 1–3,[6] showed that
the chromophores are nonplanar, thereby impairing electron
delocalization to a certain extent, and this should also hold
for the more extended, multivalent systems 5–9. Electro-
chemical investigations revealed that dendritic donor-substi-
tuted TCBDs undergo multiple redox steps at virtually
matching potentials,[7] which suggests that the individual
electron donor and acceptor moieties behave as independ-
ent electroactive centers.[12] This behavior has previously
also been observed for ferrocenyl dendrimers by Astruc and
co-workers[10] and by others.[13] Although these data suggest
little communication between individual dyads in the multi-
valent systems, such as 5–9, it was the aim of the present
study to further quantify the extent of p-electron delocaliza-
tion in radical anions and radical cations of 1–9 by means of
EPR/ENDOR spectroscopy.
and 13C NMR spectroscopy and high-resolution MALDI-FT
mass spectrometry and/or elemental analysis.
Radical anions: Generally, reduction of the parent mole-
cules 1–9 with K or Zn metal in 1,2-dimethoxyethane
(DME) and N,N-dimethylformamide (DMF) yielded EPR
spectra that could be analyzed rather well. In some cases,
EPR spectra could be obtained from the solutions of the
parent compounds even before contact with the metal took
place. These EPR spectra, however, were identical to those
observed after a short contact of the reaction solutions with
the metal mirror. Prolonged reductions led to the appear-
ance of less-resolved EPR signals. The primary spectra were
assigned to one-electron-reduced species, that is, to radical
anions. One has to bear in mind that the experimental con-
ditions and the sensitivity of EPR spectroscopy slightly
differ from those of electrochemical experiments that indi-
cate simultaneous multielectron transfers for the multivalent
derivatives (see the Supporting Information, Table 1SI).[7]
One-electron reduction of 1 with K metal in DME yielded
well-resolved and slightly temperature-dependent EPR
spectra (Figure 1). A matching simulation of the experimen-
tal EPR spectrum was achieved by taking into account four
almost equivalent 14N nuclei with 14N isotropic hyperfine
coupling constants (hfc) of approximately 1mT. The biggest
hfc of 0.253 mT is attributed to the single proton at the C(3)
position of the TCBD moiety. The phenyl protons and those
of the methoxy substituent reveal spin populations with hfc
values that are too small to be resolved in the EPR spec-
trum but distinguishable by ENDOR spectroscopy (see also
the calculated values in Figure 1).
The EPR data obtained after reduction of 2 basically
CÀ
match those attributed to 1 . A 14N hfc of 0.109 mT is as-
signed to the four virtually equivalent N atoms of the
TCBD unit. A one-proton hfc of 0.261mT corresponds to
the single alkene H atom, and an hfc of 0.029 mT can be al-
located to the two equivalent ortho protons (with respect to
the TCBD moiety) of the aryl substituent. Thus, it is not as-
tonishing that the EPR spectra obtained for 1 and 2 indicate
rather similar shapes and an identical number of lines (see
Figure 1).
Synthesis: The syntheses of compounds 1–3, 5, and 7 have
already been described.[6,7] Compounds 4 and 6 were ob-
tained in near-quantitative yield by efficient cycloaddition of
TCNE with the corresponding alkyne precursors, followed
by in situ retro-electrocyclization, as previously reported. In
the synthesis of the dendritic systems 8 and 9, which feature
a central triphenylamine core, sequences of iterative Sono-
gashira cross-couplings and silyl deprotections were ap-
plied.[14] Thus, triiodotriarylamine 10[15] was cross-coupled
with (trimethylsilyl)buta-1,3-diyne[16] to yield tris-alkynylat-
ed product 11, which, after desilylation and cross-coupling
with iodoarene 12,[7] afforded dendritic 13 in 46% yield
(Scheme 1). As desilylated 11 deteriorates readily even as a
CÀ
The EPR spectrum attributed to the radical anion 3 , fea-
turing two types of N,N-dimethylanilino (DMA) donors, one
directly attached to the TCBD framework and the second
through an acetylenic linker, can be simulated in a straight-
forward way by using one 14N hfc of 0.114 mT for four virtu-
ally equivalent N atoms. Reduction of 4 with K in DME or
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Donor-Substituted 1,1,4,4-Tetracyanobuta-1,3-dienes
FULL PAPER
values are rather similar and in
good agreement with their cal-
culated counterparts. The 1H
CÀ
CÀ
À
hfc of H C(3) in 1 and 2
amounts to approximately 0.25–
0.26 mT, which demonstrates
that spin transfer by means of
this position is possible.[17] The
1
remaining H hfc values charac-
terizing the amount of delocali-
zation of the spin into the adja-
cent substituents are small, thus
indicating that spin delocaliza-
tion into the donor moieties is
not significant. The shapes and
the widths of EPR signals ob-
CÀ
tained for radical anions 3 to
9
CÀ
are almost identical, point-
ing to very similar spin distribu-
tions in these charged species
(Figure 2).
In conclusion, the EPR spec-
CÀ
CÀ
tra attributed to 1 to 9 indi-
cate that in the one-electron-re-
duced species, the spin and the
charge are essentially confined
to one 1,1,4,4-tetracyanobuta-
1,3-dienyl moiety. There is no
significant temperature depend-
ence of the EPR spectral shape,
thus dynamic phenomena such
as spin transfer between identi-
CÀ
CÀ
cal acceptor units in 5 to 9 on
the hyperfine timescale can be
ruled out.
According to the electro-
Scheme 1. Synthesis of dendritic charge-transfer chromophore 9. a) 1,4-Bis(trimethylsilyl)buta-1,3-diyne,
MeLi·LiBr, THF, 3 h, 208C, then H+/H2O; b) 10, [PdCl
CHTREUNG
2A
chemical measurements (see
the Supporting Information),[7]
over two steps). c) nBu4NF, THF, 20 min, 08C. d) 12, [PdCl CTHREUNG
(yield over two steps). e) TCNE, CH2Cl2, 11 h, 208C, 100% (9).
simultaneous electron-transfer
processes should take place
Zn in DMF led to unresolved EPR signals (Figure 2). Their
upon reduction of dendritic 5–9, impairing the observation
CÀ CÀ
CÀ
CÀ
CÀ
width is compatible with those of 1 , 2 , and 3 , thus the
of the primary radical anions 5 to 9 . However, it has to
be kept in mind that the experimental conditions applied in
the electrochemical measurements and the environment in
the diffusion layer at the working electrode are not identical
to those of the EPR experiment. In the latter case, non-ex-
haustive reduction of the parent compound allows the for-
mation of equilibria with EPR-detectable amounts of one-
electron-reduced paramagnetic species, that is, the radical
CÀ
spin distribution in 4 is very likely to be similar to the
former radical anions.
CÀ CÀ CÀ
CÀ
The EPR spectra assigned to 1 , 2 , 3 , and 4 show
that the character of the donor group at the phenyl substitu-
ent (methoxy in 1, N,N-dialkylamino in 2–4) does not signif-
icantly alter the spin distribution. The same electron distri-
bution can also be established in derivatives 3 and 4, in
which the residual hydrogen atom on the TCBD moiety is
replaced by either a 4-(N,N-dimethylamino)phenylethynyl
or a (triisopropylsilyl)ethynyl group, respectively. With their
CÀ
CÀ
anions 1 to 9 .
Radical cations: The starting point for describing the prop-
erties of the donor–acceptor molecules in this study are the
radical cations of 2 and 4 because they resemble the electro-
active unit in most of the derivatives introduced here (unex-
pectedly, oxidation of 1 did not yield any clearly distinguish-
able EPR spectra).
CÀ
CÀ
well-defined electronic structure, radical anions 1 to 4
serve as ideal paradigms for the investigation of the multiva-
lent derivatives 5–9 carrying multiple units of 4.
CÀ
CÀ
The EPR data of 1 to 9 are summarized together with
the calculated hfc values in Table 1. In all cases, the 14N hfc
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7641

G. Gescheidt, F. Diederich et al.
that the tendency of spin and
charge delocalization from the
DMA donor group attached to
the TCBD acceptor in the 2-po-
sition is not pronounced. If a
second donor moiety is at-
tached as in 3 or 4, hardly any
spin is transferred to the second
donor. This can be rationalized
1
by the rather low H hfc estab-
À
lished for H C(3) (0.089 mT) in
2, indicating a low molecular
orbital (MO) coefficient of the
HOMO at this position (for cal-
+
C
culated HOMO (2
)
and
CÀ
LUMO (2 ), see the Support-
ing Information, Figure 1SI).
Generally, such low coefficients
do not allow an efficient spin
and charge transfer.[19]
Figure 1. Experimental (exp) EPR spectra obtained after one-electron reduction of 1 (T=280 K) and 2 (T=
280 K) with K metal in DME together with the corresponding simulations (sim). The experimental (bold) and
calculated hfc values in mT and their assignments are displayed below the spectra. The inset shows the
This behavior is well reflect-
ed in the EPR spectra of den-
CÀ
ENDOR spectrum of 1 .
+
C
dritic radical cations 5
and
+
C
6
.
Their signals are rather
similar (Figure 4), and the cor-
+
C
Oxidation of 2 with [bis(trifluoroacetoxy)iodo
ACHTREUNGzene (PIFA) in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) or
AlCl3 in CH2Cl2 afforded a well-resolved EPR spectrum
(Figure 3). The observed spectral pattern is produced by
four different hfc values. The largest hfc of 1.36 mT can be
assigned to the six equivalent b protons of the two methyl
(III)]
U
responding data closely resemble those of 4 (Table 2).
Therefore, it can be concluded that one-electron oxidation
of 5 and 6 by several methods (see the Experimental Sec-
tion) leads to the formation of radical cations with the spin
and the charge being confined to one N,N-dialkylanilino
donor moiety.
groups attached to the amino nitrogen atom. The smaller H
Dendritic system 7 possesses 1,2-di(1,3-dithiol-2-ylidene)-
ethane-1,2-diyl fragments (vinylogously extended tetrathia-
fulvalenes) as donors inserted between the central phenyl
core and the peripheral DMA-substituted TCBD dyads. Not
unexpectedly, only unresolved EPR signals could be detect-
ed after the oxidation of 7. Their shape and the correspond-
ing g factor (2.0060) are compatible with radical cations car-
rying the dominating amount of the spin and the charge at
sulfur atoms. Unfortunately, we were not able to obtain
ENDOR spectra of these species and can therefore not de-
termine the amount of electron delocalization. Yet, it can be
anticipated that the dominating amount of electron popula-
tion resides at the 1,3-dithiol-2-ylidene moieties.[20] This is in
very good agreement with the markedly lower first-oxida-
tion potential of 7 (+0.41V in CH 2Cl2 versus Fc+/Fc (ferri-
cinium/ferrocene couple)) compared with 5 and 6 (+0.88
and +0.89 V in CH2Cl2 versus Fc+/Fc, respectively; see the
Supporting Information).[7] The first-oxidation potential of
7, however, is considerably higher than that of 14, which is
approximately À0.30 V in MeCN (versus Fc+/Fc)
(Figure 5).[21] Thus, a considerable interaction of the vinylo-
hfc values of 0.56 and 0.18 mT are attributed to the pairwise
equivalent aromatic protons in the ortho and meta positions
with respect to the N,N-dimethylamino substituent, respec-
tively. These hfc values were verified by ENDOR spectros-
copy. The EPR simulation reveals a 14N hfc of 1.18 mT for
the amino nitrogen atom. This spin distribution is in very
good agreement with related alkyl-substituted aniline deriv-
atives and forms the basis for the interpretation of the EPR
spectra of the remaining molecules described herein.[18]
Silyl derivative 4 reveals an almost identical 14N hfc
(1.15 mT) as found in 2 +. The rather small 1H hfc of
C
+
À
C
0.089 mT assigned to H C(3) in 2 is missing, and addition-
al hfc values attributable to the triisopropylsilyl group are
not distinguishable. Importantly, the dominating six-proton
hfc of approximately 1.3 mT in 2 is replaced by a lower H
hfc of 0.702 mT for the four equivalent b protons of the ad-
jacent methylene groups in the n-hexyl substituents attached
to the amino nitrogen atom (Table 2).
+
1
C
Remarkably, oxidation of 3 bearing two different DMA
donor units leads to an EPR spectrum resembling that of
+
1
2
(Figure 3). The corresponding 14N and the six methyl H
gous
tetrathiafulvalene-type
C
+
C
hfc values are not significantly lower than those for 2
donor with the 1,1,4,4-tetracya-
nobuta-1,3-dienyl moiety exists.
This is reasonable since the
HOMO of 14 possesses rather
(0.979 and 1.05 mT, respectively; Table 2). The remaining
+
C
data resemble those of 2 as well. Thus, spin and charge are
confined to only one DMA moiety. The above data show
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Donor-Substituted 1,1,4,4-Tetracyanobuta-1,3-dienes
FULL PAPER
CÀ
CÀ
lated counterparts (UB3LYP/6-31G*) for 1 , 2 , and 3 (in italics).
Table 1. EPR data of radical anions 1 to 9 together with their calcu-
CÀ CÀ CÀ
14N hfc [mT] (4N)
1H hfc [mT] (1H)
g factor
CÀ
CÀ
CÀ
CÀ
CÀ
CÀ
CÀ
CÀ
CÀ
1
2
3
4
5
6
7
8
9
0.104/0.144
0.109/0.144
0.117/0.096
0.13
0.092
0.109
0.253/0.258
0.261/0.252
2.0027
2.0027
2.0026
2.0027
2.0026
2.0028
2.0026
2.0026
2.0028
–
–
–
–
–
–
–
0.107
0.136
[a]
[a] Unresolved signal.
central core and the peripheral N,N-dihexylanilino (DHA)
substituents exists. The potential of the first three-electron
oxidation of 8 centered at the DHA moieties (+0.89 V in
CH2Cl2 versus Fc+/Fc) is essentially identical to those of de-
rivatives 2, 5, and 6 (see the Supporting Information;
Figure 5).[6,7] Furthermore, a second one-electron oxidation
step located on the central triphenylamine core in 8 was ob-
served at +1.02 V. Indeed, immediately after oxidation of 8
with PIFA in HFP, an EPR spectrum similar to that assigned
+
+
+
C
C
C
to 2 , 5 , and 6 is observed. However, this signal rapidly
converts into a three-line spectrum reflecting one 14N nu-
cleus with an hfc of 0.89 mT (Figure 6). Presumably, the pri-
mary radical cation is rapidly converted to a triphenyl-
AHCTREaUNG mine-type radical cation under our experimental condi-
tions. On the other hand, the first-oxidation potential of
dendrimer 9 (+0.75 V versus Fc+/Fc) is lower than those of
derivatives 2, 5, and 6 and corresponds to the one-electron
oxidation of the central triphenylamine core (see the Sup-
porting Information). Thus, chemical oxidation of 9 with
PIFA in HFP leads to the immediate appearance of a three-
line EPR signal reflecting electron removal from the
triphenylACHTERaUNG mine core.
The difference in the oxidation potentials of the central
triphenylamine cores in 8 and 9 (+1.02 and +0.75 V versus
Fc+/Fc, respectively) might be explained in the following
way: Upon introduction of the TCBD acceptor moieties, the
oxidation potential of the triphenylamine core shifts to
more positive potentials (the oxidation potential of triphe-
nylamine is +0.39 V in MeCN versus Fc+/Fc).[22] This shift
is 630 mV in 8 and only 360 mV in the more extended 9.
The significantly smaller shift observed for 9 versus 8 is pre-
sumably due to the less-pronounced electron-withdrawing
effect of the more distant TCBD units on the central triphe-
nylamine core in 9 when compared with 8 (see the Support-
ing Information).
CÀ
CÀ
Figure 2. EPR spectra of 3 to 9 obtained by reduction with K metal in
DME (T=260–270 K).
Conclusion
large MO coefficients at the central carbon atoms (see the
One-electron-reduced and -oxidized stages of D–A chromo-
phores 1–9 (radical anions and radical cations, respectively),
obtained by chemical reduction and oxidation of the corre-
sponding neutral species, were studied by using EPR
ure 2SI).[21]
Supporting Information, FigCAHTREUNG
In dendritic 8 and 9, which posses the oxidizable triphe-
nylamine core, a competition between the oxidation of the
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7643

G. Gescheidt, F. Diederich et al.
impaired by rather small MO
coefficients in the HOMO at
the positions where the donor
and acceptor moieties are
linked.
These results suggest that
highly charged stages of D–A
chromophores 1–9 may display
remarkable magnetic properties
depending on the character of
the environment (e.g., solvent,
counterions, etc.). The explora-
tion of these phenomena is now
intensively being pursued.
Experimental Section
+
+
Materials and general methods: Re-
agents and solvents were purchased at
C
C
Figure 3. EPR spectra of 2 and 3 obtained by oxidation with PIFA in HFP (T=280 K) together with their
simulations.
reagent grade from Acros, Aldrich,
and Fluka, and used as received. Tet-
rahydrofuran (THF) was freshly dis-
tilled from Na/benzophenone and
+
+
C
C
+
Table 2. EPR data of radical cations 1 –9 together with their calculat-
CH2Cl2 from CaH2 under N2. All reactions were performed under an
inert atmosphere by applying a positive pressure of N2 or Ar. Column
chromatography (CC) and plug filtrations were carried out with silica gel
60 (particle size 0.040–0.063 mm, 230–400 mesh; Fluka) and distilled
technical solvents. Tris(4-iodophenyl)amine (10),[15] iodoarene 12,[7] and
N,N-dihexyl-4-[(triisopropylsilyl)buta-1,3-diyn-1-yl]aniline (15)[7] were
prepared according to literature procedures. Thin-layer chromatography
(TLC) was conducted on aluminum sheets coated with silica gel 60 F254
obtained from Macherey-Nagel; visualization with a UV lamp (254 or
366 nm). Melting points (m.p.) were measured on a Büchi B-540 melting-
point apparatus in open capillaries and are uncorrected. “Decomp”
refers to decomposition. 1H and 13C NMR spectra were measured on a
Varian Gemini 300 or on a Bruker DRX500 spectrometer at 298 K unless
otherwise stated. Chemical shifts (d) are reported in ppm relative to the
+
C
C
ed counterparts (UB3LYP/6-31G*) for 2 and 3 (in italics; all hfc
values in mT).
14N (amino)
1H (Hb)[a]
–
1H (H aromatic)[b]
–
g factor
+
+
C
C
1
2
–
–
1.18/0.982 (1N) 1.36/ 1.18 (6H) 0.56/À0.53 (2H, o)
2.0028
0.18/À0.21 (2H, m)
+
C
3
0.98/1.30 (1N)
1.0/ 0.775 (6H) 0.49/À0.26 (2H, o)
0.17/À0.09 (2H, m)
2.0030
+
C
C
C
C
C
4
5
6
7
8
1.15 (1N)
1.17 (1N)
0.98 (1N)
–
1.1 (1N)
0.89 (1N)
0.89 (1N)
0.70 (4H)
0.70 (4H)
0.52 (4H)
–
0.54 (2H, o)
0.52 (2H, o)
0.43 (2H, o)
–
2.0040
2.0023
2.0027
2.0060
2.0030
+
+
+
+[a]
0.7 (4H)
0.4 (2H)
1
signal of tetramethylsilane (TMS). Residual solvent signals in the H and
13C NMR spectra were used as an internal reference. Coupling constants
(J) are given in Hz. The apparent resonance multiplicity is described as s
(singlet), brs (broad singlet), d (doublet), t (triplet), q (quartet), sept
(septuplet), and m (multiplet). Infrared spectra (IR) were recorded on a
Perkin–Elmer FT1600; signal designations: s (strong), m (medium), w
(weak). UV/Vis spectra were recorded on a Varian Cary-5 spectropho-
tometer. The spectra were measured for samples in CH2Cl2 in a quartz
cuvette (1cm) at 298 K. The absorption maxima ( lmax) are reported in
nm with the extinction coefficient (e) in mÀ1 cmÀ1 in brackets; shoulders
are indicated as sh. High-resolution (HR) FT-ICR-MALDI spectra were
measured on an IonSpec Ultima Fourier transform (FT) instrument with
[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile
(DCTB), or 3-hydroxypicolinic acid (3-HPA) as the matrix. The most im-
portant peaks are reported in m/z units with M as the molecular ion.
MALDI-TOF spectra were recorded on a Bruker Daltonics Ultraflex
mass spectrometer using DCTB as matrix. Elemental analyses were per-
formed by the Mikrolabor at the Laboratorium für Organische Chemie,
ETH Zürich, with a LECO CHN/900 instrument.
+
C
9
–
–
2.0030
[a] b Protons are separated from the spin-bearing 2pz-atom orbital by an
sp3-hybridized C atom. [b] The upper values represent the primary spec-
trum, those in the second line are consistent with the secondary spec-
trum, which is observed upon warming the sample (see the main text).
(ENDOR) spectroscopy. Although the MO coefficients in
the LUMO would allow electron delocalization in radical
anions 1 to 9 , EPR investigations together with DFT cal-
culations indicate that spin and charge are confined to the
electron-withdrawing 1,1,4,4-tetracyanobuta-1,3-dienyl moi
CÀ
CÀ
ACHTREeUNG -
ACHTREUNGty on the hyperfine EPR timescale. The observed spin locali-
CÀ
CÀ
zation in 1 to 9 is presumably due to 1) substantial devia-
tion of the p-system constituents from planarity as observed
previously,[6] 2) limited p-electron delocalization, and very
likely 3) counterion effects. An analogous spin and charge
EPR and ENDOR measurements: EPR and ENDOR spectra were re-
corded on a Bruker ESP 300 spectrometer. The isotropic doublet EPR
spectra were simulated with Winsim,[23] a public-domain program.
localization confined to the electron-donating N,N-dialkyl-
Sample preparation
aniline moieties is present in radical cations 2 –9 +; unex-
+
C
C
Reductions: 1,2-Dimethoxyethane (DME) was heated to reflux over an
Na/K alloy and stored over an Na/K alloy under high vacuum. DMF was
stored over molecular sieves. All reductions were performed by contact
pectedly, oxidation of 1 did not yield any clearly distinguish-
able EPR spectra. In this case, delocalization is additionally
7644
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Chem. Eur. J. 2008, 14, 7638 – 7647

Donor-Substituted 1,1,4,4-Tetracyanobuta-1,3-dienes
FULL PAPER
+
+
C
C
Figure 4. EPR spectra assigned to cations 5 and 6 obtained by oxidation with PIFA in HFP (T=280 K) together with their simulations.
Figure 5. Comparison of the first-oxidation potentials of 2,3, and 5–9.
of the solutions of the parent compounds with a K- or Zn-metal mirror
under high vacuum.
Oxidations: CH2Cl2 was heated to reflux over molecular sieves and
stored under high vacuum. 1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP), tri-
fluoroacetic acid (TFA), and [bis(trifluoroacetoxy)iodoACTHRE(UNG III)]benzene
(PIFA) were purchased from Aldrich and used without further purifica-
tion. Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (“magic
blue”) was synthesized according to a literature procedure.[24] All oxida-
tions were performed under high vacuum by reacting solutions of the oxi-
dants in CH2Cl2 or HFP with those of the substrates. For oxidations in
CH2Cl2, the solvent was distilled under high vacuum into the Pyrex-glass
reaction cuvette charged with the oxidant and the substrate. The solu-
tions in CH2Cl2 were stirred at 190 K (dry ice/2-propanol bath) and the
sample was immediately transferred into the temperature-controlled mi-
crowave cavity of the EPR spectrometer. When HFP was used as the sol-
vent, it was placed into the sample tube under Ar, sealed, and degassed
by 3 or 4 freeze–pump–thaw cycles. The follow-up procedure was as de-
scribed for CH2Cl2.
+
C
Figure 6. EPR spectrum of 8 obtained by oxidation with PIFA in HFP
(T=300 K) together with its simulation.
UB3LYP/6-31g(d) protocol and are shown in the Supporting Informa-
tion.
2-[4-(Dihexylamino)phenyl]-3-[(triisopropylsilyl)ethynyl]buta-1,3-diene-
1,1,4,4-tetracarbonitrile (4): A mixture of N,N-dihexyl-4-[(triisopropyl-
AHCTREsUNG ilyl)buta-1,3-diyn-1-yl]aniline (15) (100 mg, 0.22 mmol) and TCNE
(27 mg, 0.22 mmol) in CH2Cl2 (30 mL) was stirred for 10 h at 208C. The
solvent was evaporated under vacuum and the residue subjected to CC
(silica gel, CH2Cl2) to give 4 (112 mg, 88%) as a deep-purple solid. Rf =
0.67 (silica gel, CH2Cl2); m.p. 124–1268C; 1H NMR (300 MHz, CDCl3):
d=0.91(t, J=6.6 Hz, 6H), 1.07–1.10 (m, 21H), 1.33 (m, 12H), 1.63 (m,
Calculations: Calculations were performed with the Gaussian 03 pack-
age.[25] For geometry optimization and single-point determinations of the
Fermi contacts (hfc), the UB3LYP/6-31G* protocol was used. HOMOs
and LUMOs for selected compounds were calculated by using the
Chem. Eur. J. 2008, 14, 7638 – 7647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7645

G. Gescheidt, F. Diederich et al.
4H), 3.39 (t, J=7.8 Hz, 4H), 6.67 (d, J=9.4 Hz, 2H), 7.72 ppm (d, J=
9.4 Hz, 2H); 13C NMR (75 MHz, CDCl3): d=11.21, 14.18, 18.65, 22.78,
26.81, 27.44, 31.72, 51.63, 72.79, 96.70, 100.11, 110.17, 111.20, 112.18,
113.45, 114.64, 116.85, 126.39, 132.76, 151.09, 153.25, 159.56 ppm; IR
(neat): n˜ =2928 (m), 2864 (m), 2213 (s), 2140 (w), 1604 (s), 1543 (m),
1487 (s), 1449 (s), 1419 (s), 1349 (s), 1310 (m), 1262 (m), 1220 (s), 1187
(s), 1162 (s), 1116 (m), 1074 (w), 1022 (m), 999 (w), 883 (m), 828 (m),
803 cmÀ1 (m); UV/Vis (CH2Cl2): lmax (e)=262 (13800), 311 (14500), 444
(25000), 468 (sh, 23000), 549 nm (sh, 7200mÀ1 cmÀ1); HR-MALDI-MS
(3-HPA): m/z calcd for C37H52N5Si+ [M+H]+: 594.3987; found: 594.3994;
elemental analysis calcd (%) for C37H51N5Si (593.92): C 74.82, H 8.65, N
11.79; found: C 74.76, H 8.68, N 11.87.
147.37, 150.31, 153.33, 158.12 ppm; IR (neat): n˜ =2925 (m), 2855 (m),
2212 (m), 2186 (m), 1600 (s), 1533 (w), 1486 (s), 1446 (s), 1413 (s), 1339
(s), 1290 (m), 1212 (m), 1182 (s), 1117 (s), 1016 (w), 980 (w), 884 (m),
818 cmÀ1 (m); UV/Vis (CH2Cl2): lmax (e)=288 (172000), 374 (236000),
414 (sh, 230000), 456 (272300), 568 nm (20800mÀ1 cmÀ1); HR-MALDI-
+
MS (3-HPA): m/z calcd for C216H202N31 [M+H]+: 3231.6816; found:
3231.6722; elemental analysis calcd (%) for C216H201N31 (3231.17): C
80.29, H 6.27, N 13.44; found: C 80.64, H 6.28, N 13.12.
Acknowledgements
2,2’,2’’-[(5-{5,5-Dicyano-3-(dicyanomethylene)-4-[4-(dihexylamino)phe-
nyl]pent-4-en-1-yn-1-yl}benzene-1,2,4-triyl)triethyne-2,1-diyl]tris{3-[4-(di-
This research was supported by the ETH Research Council, FWF (Aus-
tria, project no. P 20019-N17) and the NCCR “Nanoscale Science”,
Basel. Dr. Jean-Paul Gisselbrecht, Prof. Corinne Boudon, and Prof. Dr.
Maurice Gross (UniversitØ Louis Pasteur, Strasbourg, France) are grate-
fully acknowledged for providing the electrochemical data of the new
compounds and Dr. Carlo Thilgen (ETHZ) for his help with the nomen-
clature of the compounds described in this paper.
hexylamino)phenyl]buta-1,3-diene-1,1,4,4-tetracarbonitrile} (6):
A mix-
ture of oligoalkyne 16 (50 mg, 0.038 mmol) and TCNE (20 mg,
0.153 mmol) in CH2Cl2 (25 mL) was stirred for 18 h at 208C. The solvent
was evaporated under vacuum and the residue subjected to CC (silica
gel, CH2Cl2/EtOAc 98:2) to give 6 (68 mg, 98%) as a black metallic
solid. Rf =0.59 (silica gel, CH2Cl2/EtOAc 98:2); m.p. 129–1318C;
1H NMR (300 MHz, CDCl3): d=0.91(t, J=6.4 Hz, 24H), 1.33 (s, 48H),
1.63 (m, 16H), 3.40 (t, J=7.6 Hz, 16H), 6.67 (d, J=9.3 Hz, 8H), 7.82 (d,
J=9.3 Hz, 8H), 8.03 ppm (s, 2H); 13C NMR (75 MHz, CDCl3): d=14.18,
22.78, 26.81, 27.51, 31.68, 51.70, 71.60, 92.00, 98.81, 107.03, 109.93, 111.87,
112.53, 114.31, 114.78, 116.87, 125.61, 133.48, 139.26, 150.03, 153.79,
157.72 ppm; IR (neat): n˜ =2927 (m), 2856 (m), 2213 (s), 2186 (m), 1600
(s), 1540 (m), 1484 (s), 1445 (s), 1414 (s), 1341 (s), 1262 (s), 1213 (s), 1182
(s), 1118 (s), 977 (m), 900 (m), 821 cmÀ1 (m); UV/Vis (CH2Cl2): lmax (e)=
287 (72100), 372 (138600), 471 (189900), 634 nm (19400mÀ1 cmÀ1); HR-
[1] For reviews on organic electronics and optoelectronics, see: a) Spe-
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+
MALDI-MS (3-HPA): m/z calcd for C118H123N20 [M+H]+: 1820.0234;
found: 1820.0185; elemental analysis calcd (%) for C118H122N20 (1820.40):
C 77.86, H 6.75, N 15.39; found: C 77.97, H 7.01, N 15.10.
2,2’-{[(4-{5,5-Dicyano-3-(dicyanomethylene)-4-[4-(dihexylamino)phenyl]-
[2] For some recent work on conjugated electron donor–acceptor sys-
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pent-4-en-1-yn-1-yl}phenyl)imino]bis(4,1-phenyleneethyne-2,1-diyl)}bis{3-
[4-(dihexylamino)phenyl]buta-1,3-diene-1,1,4,4-tetracarbonitrile} (8):
A
mixture of oligoalkyne 17 (50 mg, 0.043 mmol) and TCNE (33 mg,
0.257 mmol) in CH2Cl2 (30 mL) was stirred for 16 h at 208C. The solvent
was evaporated under vacuum and the residue subjected to CC (silica
gel, CH2Cl2/EtOAc 99:1) to give 8 (68 mg, 100%) as a black solid. Rf =
0.55 (silica gel, CH2Cl2/EtOAc 99:1); m.p. 122–1268C; 1H NMR
(500 MHz, CDCl3): d=0.91(t, J=6.9 Hz, 18H), 1.32 (s, 36H), 1.61 (m,
12H), 3.37 (t, J=7.9 Hz, 12H), 6.66 (d, J=9.4 Hz, 6H), 7.10 (d, J=
8.8 Hz, 6H), 7.57 (d, J=8.8 Hz, 6H), 7.75 ppm (d, J=9.4 Hz, 6H);
13C NMR (125 MHz, CDCl3): d=13.96, 22.57, 26.63, 27.27, 31.50, 51.46,
72.67, 87.10, 93.69, 110.34, 111.58, 112.08, 113.48, 114.53, 115.49, 116.77,
116.96, 124.58, 132.67, 135.41, 148.86, 150.65, 153.15, 159.25 ppm; IR
(neat): n˜ =2925 (m), 2855 (m), 2214 (m), 2160 (s), 1600 (s), 1483 (s), 1446
(s), 1412 (s), 1351 (s), 1317 (s), 1286 (s), 1211 (s), 1179 (s), 1116 (s), 1016
À1
(m), 991(m), 900 (w), 820 (m), 804 cm
(m); UV/Vis (CH2Cl2): lmax
(e)=279 (57500), 321(sh, 52700), 347 (54500), 461(517800), 522 nm
(131100mÀ1 cmÀ1); HR-MALDI-MS (3-HPA): m/z calcd for C102H103N16
+
[M+H]+: 1551.8552; found: 1551.8582; elemental analysis calcd (%) for
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C
102H102N16 (1552.04): C 78.94, H 6.62, N 14.44; found: C 79.11, H 6.75, N
14.26.
2,2’,2’’,2’’’,2’’’’,2’’’’’-[Nitrilotris(4,1-phenylenebuta-1,3-diyne-4,1-diylben-
zene-5,1,3-triyldiethyne-2,1-diyl)]hexakis{3-[4-(dihexylamino)phenyl]-
buta-1,3-diene-1,1,4,4-tetracarbonitrile} (9): A mixture of 13 (16.0 mg,
0.006 mmol) and TCNE (12.5 mg, 0.097 mmol) in CH2Cl2 (20 mL) was
stirred for 11 h at 208C. The solvent was evaporated under vacuum and
the residue subjected to CC (silica gel, CH2Cl2/EtOAc 99:1) to give 9
(21mg, 100%) as a black solid. Rf =0.53 (silica gel, CH2Cl2/EtOAc 99:1);
m.p. >1308C (decomp); 1H NMR (500 MHz, CDCl3): d=0.91(t, J=
6.5 Hz, 36H), 1.34 (s, 72H), 1.64 (m, 24H), 3.40 (t, J=7.8 Hz, 24H), 6.70
(d, J=9.3 Hz, 12H), 7.06 (d, J=8.7 Hz, 6H), 7.46 (d, J=8.7 Hz, 6H),
7.75–7.78 (m, 15H), 7.85 ppm (d, J=1.5 Hz, 6H); 13C NMR (125 MHz,
CDCl3): d=13.96, 22.57, 26.63, 27.28, 31.49, 51.52, 72.35, 73.25, 77.46,
78.10, 83.75, 86.08, 96.58, 109.79, 111.02, 111.35, 112.27, 113.52, 114.33,
116.06, 116.41, 121.21, 124.17, 124.75, 132.65, 134.16, 136.33, 139.15,
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7646
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Donor-Substituted 1,1,4,4-Tetracyanobuta-1,3-dienes
FULL PAPER
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Received: April 15, 2008
Published online: July 18, 2008
Chem. Eur. J. 2008, 14, 7638 – 7647
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7647