New strong organic acceptors by cycloaddition of TCNE and TCNQ to donor-substituted cyanoalkynes

Article abstract of DOI:10.1039/b714731g

Donor-substituted 1,1,2,4,4-pentacyanobuta-1,3-dienes and a cyclohexa-2,5-diene-1,4-diylidene-expanded derivative were prepared by a [2 + 2] cycloaddition of tetracyanoethene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) to anilino-substituted cyanoal

Full text of DOI:10.1039/b714731g

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New strong organic acceptors by cycloaddition of TCNE and TCNQ to
donor-substituted cyanoalkynes{
Philippe Reutenauer,a Milan Kivala,a Peter D. Jarowski,a Corinne Boudon,b Jean-Paul Gisselbrecht,b
Maurice Grossb and Franc¸ois Diederich*a
Received (in Cambridge, UK) 25th September 2007, Accepted 22nd October 2007
First published as an Advance Article on the web 31st October 2007
DOI: 10.1039/b714731g
substituted by one EDG and one electron-withdrawing group
(EWG) have not been studied. In the present work, such a reaction
is shown to be possible.
Donor-substituted 1,1,2,4,4-pentacyanobuta-1,3-dienes and a
cyclohexa-2,5-diene-1,4-diylidene-expanded derivative were pre-
pared by a [2 + 2] cycloaddition of tetracyanoethene (TCNE) or
7,7,8,8-tetracyanoquinodimethane (TCNQ) to anilino-substi-
tuted cyanoalkynes, followed by retro-electrocyclisation; they
feature intense bathochromically-shifted intramolecular charge-
transfer bands and undergo their first one-electron reductions
at potentials similar to those reported for TCNE and TCNQ.
We found that the anilino donor-substituted cyanoalkynes 5–7
(see the ESI for their preparation{) reacted with TCNE at room
temperature in THF to give the donor-substituted PCBDs 1–3 in
good yields (Scheme 1). Under harsher conditions, by heating to
120 uC in 1,1,2,2-tetrachloroethane, TCNQ underwent a similar
transformation with 6, yielding the cyclohexa-2,5-diene-1,4-diyli-
dene-expanded PCBD, 4, in 27% yield.
Stable strong electron acceptors, such as tetracyanoethene
(TCNE)1 and 7,7,8,8-tetracyanoquinodimethane (TCNQ),2 are in
high demand.3,4 Strong acceptors are of interest as dopants in the
fabrication of optical light-emitting diodes (OLEDs) and solar
cells.5 In recent years, we have introduced a series of potent
acceptors, such as cyanoethynylethenes (CEEs),6 1,1,4,4-tetracya-
nobuta-1,3-dienes (TCBDs)7 and TCNQ cycloadducts.8 When
conjugated to strong donors, the resulting push–pull chromo-
phores undergo efficient intermolecular charge transfer (CT)
interactions and feature high third-order optical non-linearities.9
However, electrochemical measurements by cyclic voltammetry
(CV) and rotating disk voltammetry (RDV) indicated that their
electron-accepting power was inferior to that of TCNE or
TCNQ.10 Here, we report the synthesis of the first 1,1,2,4,4-
pentacyanobuta-1,3-dienes (PCBDs), 1–3, and the cyclohexa-2,5-
diene-1,4-diylidene-expanded derivative, 4, potent organic
acceptors that rival the benchmark compounds TCNE and
TCNQ in their propensity for electron uptake.
These PCBDs are presumed to come about by initial bond
formation between the alkyne C-atom in the b-position to the
anilino donor substituent and the b-C-atom of the CLC(CN)2
moieties in TCNE or TCNQ, giving a resonance-stabilised
zwitterionic or diradical intermediate. While these transformations
involve the HOMO of the cyanoalkynes and the LUMO of the
acceptors, the concerted [2 + 2] cycloaddition mechanism is
symmetry-forbidden. The recombination of charges or unpaired
electrons occurs to form the cyclobutene, which subsequently
undergoes electrocyclic ring opening, leading to the final
butadiene. The intermediate cyclobutene could be isolated in
certain cases of TCNE-additions to metal-coordinated alkynes.11b,c
The reaction of the alkyne with TCNE or TCNQ depends on the
relative HOMO and LUMO energies, and on the frontier
molecular orbital coefficients at the b-positions. Substitution of a
While the [2 + 2] cycloaddition of TCNE with electron-rich
metal acetylides, followed by retro-electrocyclisation to give
organometallic TCDB derivatives, was described as early as
1981,11 the corresponding reaction with electron-rich, organodo-
nor-substituted alkynes has only been systematically explored in
recent years.7 The resulting CT chromophores displayed high
third-order optical non-linearities and exceptional electron storage
capacities.12 In these studies, it was observed that TCNE reacted
only with alkynes substituted with two electron-donating groups
(EDG) or one EDG and one neutral substituent (such as phenyl
or trialkylsilyl).7,12 Reactions of electronically-confused alkynes
aLaboratorium fu¨r Organische Chemie, ETH Zu¨rich, Ho¨nggerberg,
HCI, CH-8093 Zu¨rich, Switzerland.
E-mail: diederich@org.chem.ethz.ch; Fax: +41 44 632 1109
bLaboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
Institut de Chimie-LC3-UMR 7177, CNRS, Universite´ Louis Pasteur, 4,
rue Blaise Pascal, F-67000 Strasbourg, France
{ Electronic supplementary information (ESI) available: Characterisation
of the new compounds, electrochemical data and mechanistic analysis. See
DOI: 10.1039/b714731g
Scheme 1 Synthesis of new anilino donor-substituted acceptors. (i)
1.2 equiv. TCNE, THF, 20 uC, 12 h to 2 d, 60% (1), 97% (2), 98% (3); (ii)
1.0 equiv. TCNQ, 1,1,2,2-tetrachloroethane, 120 uC, 12 h, 27%.
4898 | Chem. Commun., 2007, 4898–4900
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para-anilino-substituted alkyne with an EWG, such as CN, would
tend to lower the HOMO energy. The new HOMO, being further
in energy from the LUMO of TCNE or TCNQ, would have a
diminished interaction. The presence of the cyano group may also
reduce the coefficient of the HOMO orbital at the b-position to the
anilino donor and negatively attenuate its reactivity with TCNE or
TCNQ.13 The ready accessibility of the PCBDs, as described in
Scheme 1, is thus unexpected.
upon moving to 3 can be accounted for by the two additional
s-donating alkyl substituents and the enforced amine rigidity.14
Electronic transition analysis using time-dependent density
functional theory at the TD-B3LYP/6-31g(d)//B3LYP/6-31g(d)
level for compound 1 predicts three CT absorptions at 368, 383
and 650 nm, with oscillator strengths (f) of 0.43, 0.02 and 0.02,
respectively (ESI{). The CT band at 368 nm refers to an excitation
from the HOMO to the LUMO + 1, also with transitions from the
HOMO-3, HOMO-2 and HOMO-1 to the LUMO, which are less
significant. The band at 383 nm is composed of a transition from
HOMO-1 to LUMO, with a minor one from HOMO to LUMO
+ 1. The absorption at 650 nm is predominantly a transition from
HOMO to LUMO, with some HOMO to LUMO + 1. The major
transitions distribute the electron density from the anilino to the
pentacyanobutadienyl moiety (relevant MOs shown in ESI{).
An additional shift of the CT band to lower energy is observed
in the UV/vis spectrum of TCNQ adduct 4 (Fig. 1(b)), with the
maximum in CH2Cl2 observed at lmax = 859 nm (1.44 eV; e =
17700 dm3 mol21 cm21). This band tails far into the near infrared,
and the end-absorption is observed near 1300 nm (0.95 eV). This
low optical gap is remarkable for a small chromophore, such as 4.
The CT character of the longest wavelength absorption band was
confirmed in a protonation experiment. When a solution of 4 in
CH2Cl2 was acidified by trifluoroacetic acid (TFA) (Fig. 1(b)), the
band at lmax = 859 nm nearly completely disappeared. However,
neutralisation with K2CO3 did not quantitatively regenerate the
original spectrum, as strongly electrophilic 4 decomposes rapidly in
the presence of a base.
Calculations at the MP4(SDQ/6-311 + g(d,p)//B3LYP/6-311 +
g(d,p) level using the Gaussian 03 program (see the ESI{) for
anilinoacetylene (An–CMCH) (An = 4-anilino), anilinocyanoace-
tylene (An–CMC–CMN) (5) and TCNE confirm the unexpected
reactivity of 5 (Scheme 2). The HOMO energy (p) of An–CMCH is
0.5 eV higher than that of An–CMC–CMN. Thus, 5 is predicted,
and confirmed experimentally, to be less reactive against TCNE
than the simple donor-substituted alkyne. While the reaction with
the latter is nearly instantaneous at room temperature, giving
quantitative yields,7,12 the transformations of 5–7 require much
longer stirring (12–48 h) at this temperature to proceed in good
yield. The influence of the cyano group on the HOMO coefficient
at its position of attachment, the putative initial bond-forming site,
is negligible; both An–CMCH and An–CMC–CMN have coefficients
of 20.10. The effect on the coefficients at the adjacent alkyne
carbon atoms is only minor (20.04 (5) and 20.05 (An–CMCH)).
The new push–pull systems are green (1–3) or black (4) metallic
solids stable under laboratory conditions at ambient temperature
for months (see ESI{). The regioselectivity observed in the addition
of TCNQ to the terminal dicyanovinyl moiety is in agreement with
previous findings.8 The UV/vis spectra of the PCBDs recorded in
CH2Cl2 display two intramolecular CT bands (Figure 1(a)): one
intense and one weak one of lower energy, with their maxima
shifting from lmax = 406 nm (3.05 eV; e = 21000 dm3 mol21 cm21
and lmax = 542 nm (2.28 eV; e = 3200 dm3 mol21 cm21) for 1, to
)
l
max = 450 nm (2.76 eV; e = 30000 dm3 mol21 cm21) and lmax
643 nm (1.93 eV; e = 3200 dm3 mol21 cm21) for 2, and to lmax
=
=
483 nm (2.56 eV; e = 36000 dm3 mol21 cm21) and lmax = 731 nm
(1.70 eV; e = 3000 dm3 mol21 cm21) for 3. The substantial red shift
of 0.35 eV upon passing from 1 to 2 reflects the increase in donor
strength upon N-dialkylation, while the additional shift of 0.23 eV
Scheme 2 Representations of the relevant parts of the p and p*
molecular orbitals of An–CMCH, TCNE and An–CMC–CMN (5).
MP4(SDQ/6-311 + g(d,p)//B3LYP/6-311 + g(d,p) MO coefficients are
shown below the corresponding p-orbitals. The MO energy (eV) at this
level (ZPE-corrected) is also given.
Fig. 1 (a) UV/vis spectra of 1–3 in CH2Cl2 at 298 K. Inset: zoom on the
low energy CT bands. (b) UV/vis spectra of 4 in CH2Cl2 at 298 K recorded
neat, after acidification with trifluoroacetic acid (TFA) and after
neutralisation with K2CO3.
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Table 1 Electrochemical data (CV) in CH2Cl2 (+0.1 M n-Bu4NPF6)
vs. Fc+/Fc (scan rate v = 0.1 V s21
TCNQ adduct 4 undergoes two one-electron reductions at
20.27 and 20.53 V, as well as an irreversible one-electron
oxidation at +0.52 V (Table 1). Again, a large influence on the
redox properties are observed, compared to 9 (20.50 and
20.76 V), as a result of the additional CN group; both reduction
steps occur anodically-shifted by 230 mV. While the first reduction
is similar to that in TCNQ (20.25 vs. 20.27 V (4)), the second is
greatly facilitated (20.53 (4) vs. 20.81 V (TCNQ)), which can be
explained by the larger distance between the charge-hosting
dicyanovinyl and tricyanovinyl moieties in 4 and the concomitant
reduction of electrostatic repulsion. Ground state CT interactions
are much less effective in 4 than in 1–3; the anilino donor is much
more readily oxidised (Ep = +0.52 V), which means that it transfers
less electron density into the acceptor chromophore.
)
Eu/Va DEp/mVb Ep/Vc
Eu/Va DEp/mVb Ep/Vc
1
2
3
4
a
+1.14 TCNE 20.32d
21.35d
20.27 100
20.86 100
+1.00 90
20.30 90
20.85 100
+0.74 75
20.32 60
20.81 75
TCNQ 20.25d
20.81d
The full scope of the reaction between TCNE or TCNQ and
electronically confused alkynes and the optoelectronic applications
of these new advanced materials are now being investigated.
This work was supported by the ETH Research Council and the
French State (Lavoisier post-doctoral fellowship to P. R.).
8
9
+0.86e 80
20.69e 80
21.26e 90
+0.52
+0.42f
20.27 80
20.53 85
20.50f 80
20.76f 80
Eu = (Epc + Epa)/2, where Epc and Epa correspond to the cathodic
b
and anodic peak potentials, respectively. DEp = Eox 2 Ered, where
Notes and references
subscripts ‘ox’ and ‘red’ refer to the conjugated oxidation and
reduction steps, respectively. Ep = Irreversible peak potential at
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The electrochemical properties of the new chromophores were
recorded by CV and RDV (see ESI{), and compared to those of
TCNE,15 TCNQ,15 anilino-substituted TCBD 87b and TCNQ
adduct 9 (Table 1).8 All three PCBDs, 1–3, give two well-defined
reversible one-electron reduction steps, with the first electron
transfer occurring at 20.27 to 20.32 V and the second at 20.81 to
20.86 V. The electron uptakes occur on the PCBD moieties, as
suggested by molecular orbital analysis (see ESI{). The observed
one-electron oxidation step occurs on the anilino donor moiety
and becomes facilitated upon moving from 1 (irreversible
oxidation with Ep = +1.14 V) to 2 (Euox = +1.00 V) and on to 3
(Euox = +0.74 V). A good correlation exists between the optical
spectroscopic and the electrochemical data: both the optical and
electrochemical gap are reduced upon moving from 1 to 2 and on
to 3 (see ESI{). The introduction of the extra CN group upon
moving from TCBD 87b to PCBD 2 has a dramatic effect on the
redox properties. The CT interaction is much more efficient due to
the increased acceptor strength, and this is reflected in the
oxidation potential, which shifts from +0.86 V (8) to +1.00 V (2).
More importantly, the two first electron uptakes become
considerably more favoured as a result of the extra CN group in
the acceptor; TCBD 8 is reduced at 20.69 and 21.26 V, whereas
the two electron transfers in PCBD 2 occur at 20.30 and 20.85 V,
respectively. Thus, both reduction steps in 2 are greatly, and nearly
equally, facilitated, occurring at a ca. 400 mV less negative
potential.16 Gratifyingly, despite substitution with potent anilino
donors, the first reduction of 1–3 (20.27 to 20.32 V) occurs at
very similar potentials to that of TCNE (20.32 V), whereas the
second electron uptake is even more greatly facilitated (1–3: 20.81
to 20.86 V; TCNE: 21.35 V). Both the first and second reduction
potentials of 1–3 approach those of TCNQ (20.25 and 20.81 V).
It can be expected that, upon attenuation of the donor moiety, the
reduction steps become further facilitated.
4900 | Chem. Commun., 2007, 4898–4900
This journal is ß The Royal Society of Chemistry 2007