Tuning the electronic properties of nonplanar exTTF-based pusha-pull chromophores by aryl substitution

Article abstract of DOI:10.1021/jo302047m

A new family of π-extended tetrathiafulvalene (exTTF) donora-acceptor chromophores has been synthesized by [2 + 2] cycloaddition of TCNE with exTTF-substituted alkynes and subsequent cycloreversion. X-ray data and theoretical calculations, performed at th

Full text of DOI:10.1021/jo302047m

Article
pubs.acs.org/joc
Tuning the Electronic Properties of Nonplanar exTTF-Based Push−
Pull Chromophores by Aryl Substitution
†
†
‡
§,∥
Raul García, M Angeles Herranz, M Rosario Torres, Pierre-Antoine Bouit, Juan Luis Delgado,§
a
a
́
́
Joaquín Calbo,⊥ Pedro M. Viruela,⊥ Enrique Ortí,*,⊥ and Nazario Martín*,†,§
†Departamento de Química Organ
‡Centro de Asistencia a la Investigacion
28040 Madrid, Spain
́
ica, Facultad de Química, Universidad Complutense de Madrid, 28040 Madrid, Spain
de Rayos-X, Facultad de Ciencias Químicas, Universidad Complutense, Ciudad Universitaria,
́
§IMDEA-Nanociencia, Facultad de Ciencias, Universidad Auton
́
oma, Cantoblanco, 28049 Madrid, Spain
⊥Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Spain
S
* Supporting Information
ABSTRACT: A new family of π-extended tetrathiafulvalene (exTTF) donor−acceptor
chromophores has been synthesized by [2 + 2] cycloaddition of TCNE with exTTF-
substituted alkynes and subsequent cycloreversion. X-ray data and theoretical calculations,
performed at the B3LYP/6-31G** level, show that the new chromophores exhibit highly
distorted nonplanar molecular structures with largely twisted 1,1,4,4-tetracyanobuta-1,3-
diene (TCBD) units. The electronic and optical properties, investigated by UV/vis
spectroscopy and electrochemical measurements, are significantly modified when the
TCBD acceptor unit is substituted with a donor phenyl group, which increases the twisting
of the TCBD units and reduces the conjugation between the two dicyanovinyl subunits.
The introduction of phenyl substituents hampers the oxidation and reduction processes
and, at the same time, largely increases the optical band gap. An effective electronic
communication between the donor and acceptor units, although limited by the distorted
molecular geometry, is evidenced both in the ground and in the excited electronic states.
The electronic absorption spectra are characterized by low- to medium-intense charge-transfer bands that extend to the near-
infrared.
planar. In contrast, a scarce number of nonplanar donor−
acceptor chromophores have been reported so far,6 and only
recently has the relationship existing between π-conjugation
and the electronic properties in these push−pull molecules
been systematically investigated.7
INTRODUCTION
■
The design and synthesis of novel charge-transfer (CT)
chromophores with a control on the light-harvesting properties
in the visible and near-infrared regions of the electronic spectra
is currently a very active research area for a number of
technological applications, particularly in fields such as
nonlinear optics and photonics,1 organic (plastic) photo-
voltaics,2 dye-sensitized solar cells,3 and more recently, small-
molecule photovoltaic cells.4 In this context, the electronic
structure and geometrical features of a molecular electron-
donor (D) unit can be strongly modified by the presence of a
covalently connected electron-acceptor (A) group, which
determines the appearance of new intramolecular charge-
transfer (CT) bands in the electronic spectra of the D−A
molecular entity.
A large variety of donor systems, involving molecular,
oligomeric, and polymeric species such triarylamines, porphyr-
ins, tetrathiafulvalene derivatives, oligophenylenvinylenes, and
oligo- and polythiophenes, just to name a few, have been
combined with different electron-acceptor moieties.5 In order
to get an efficient electronic communication between the donor
and acceptor moieties, planarity has been recognized to be an
important requirement. Therefore, most of the D-spacer-A
molecules reported so far are geometrically planar or almost
The singular electronic and geometrical features of π-
extended tetrathiafulvalene (exTTF) have made this molecule
one of the most useful and versatile electron donors, and
therefore, exTTF derivatives have been widely used in the
preparation of photo- and electroactive donor−acceptor dyads
and triads,8 molecular wires,9 and materials for second- and
third-order nonlinear optics,10 as a building block in supra-
molecular chemistry,11 and, more recently, for the realization of
dye-sensitized solar cells.12 In addition, exTTFs are pro-
aromatic units,13 with a characteristic butterfly shape distorted
geometry14 that, upon oxidation, experience a dramatic
geometrical change resulting in a gain of aromaticity and
planarity affording stable oxidized species.15 Thus, the exTTF
fragment looks particularly promising for modulating the
conjugation between donor and acceptor units and controlling
the electronic properties of new push−pull chromophores.
Received: September 18, 2012
Published: November 6, 2012
© 2012 American Chemical Society
10707
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
Diederich and co-workers have recently shown that alkynes
substituted with suitable electron-donating groups readily
undergo a [2 + 2] cycloaddition with powerful electron
acceptors such as tetracyanoethylene (TCNE) and 7,7,8,8-
tetracyano-p-quinodimethane (TCNQ),16,17 followed by a
retro-electrocyclization, to give nonplanar push−pull chromo-
phores such as those depicted in Figure 1. This general
Scheme 1. Synthesis of exTTF-TCBD Derivatives 7 and 8
Figure 1. Representative examples of push−pull chromophores
synthesized by cycloaddition−retro-electrocyclization processes be-
tween donor-activated alkynes and TCNE or TCNQ.
Scheme 2. Formation of the Doubly TCBD-Functionalized
exTTF Derivative 12 by Sonogashira Coupling Followed by
a [2 + 2] Cycloaddition−cycloreversion Reaction with
TCNE
procedure, besides fulfilling all the requirements for a “click
reaction” (atom economy, high yields, ambient conditions),
provides easy access to functional materials, which, in principle,
could feature enhanced physical properties such as better
solubility, dispersibility, and sublimation capability when
compared with their planar analogues. These enhanced
properties are desirable for the easier preparation and
processing of optoelectronic devices.18
In this work, we focus our attention on a new family of D-π-
A chromophores in which exTTF and 1,1,4,4-tetracyanobuta-
1,3-diene (TCBD) are covalently connected by using the
electronically controlled [2 + 2] cycloaddition of TCNE with
exTTF-substituted acetylenes, followed by retro-electrocycliza-
tion of the initially formed cyclobutenes (Schemes 1 and 2).
Importantly, although the synthetic methodology is similar to
that previously reported, the strong electron donor character
and particularly the highly distorted geometry out of the
planarity of curved exTTF should have a strong impact on the
new push−pull chromophores. In this regard, the detailed
electrochemical and optical characterization of compounds 7, 8,
and 12 reveals the important effect that the acceptor moieties
attached to the exTTF core have on the structure and optical
properties of these chromophores. Density functional theory
(DFT) calculations have been carried out to gain additional
insight into the structural and electronic properties of these
novel nonplanar push−pull chromophores. Theoretical pre-
dictions have been confirmed by experimental X-ray analysis of
compound 12 bearing two acceptor units covalently connected
to the central exTTF core.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis of the
push−pull chromophores 7 and 8 (Scheme 1) started with a
Sonogashira coupling reaction of 2-iodo-9,10-bis(1,3-dithiol-2-
ylidene)-9,10-dihydroanthracene (4)19 with ethynyl(trimethyl)-
silane to obtain compound 5,20 or with phenylacetylene, for the
10708
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
Scheme 3. Proposed Mechanism for the Reaction of exTTF-Based Alkynes with TCNE
synthesis of 6, in the presence of tetrakis(triphenylphosphine)-
palladium(0) [Pd(PPh3)4], copper iodide, and diisopropyl-
amine or triethylamine, followed by desilylation with K2CO3 for
compound 5. exTTFs 5 and 6 were obtained with a good
overall yield (60−70%). The subsequent reaction of these
exTTF-based alkynes with TCNE led to the target molecules
after purification by column chromatography. Initial efforts to
convert alkyne 6 to exTTF-TCBD 8 in refluxing benzene were
unsuccessful. The reaction proceeds, however, when 1,2-
dichloroethane was used as solvent. The fact that the starting
alkyne 6 is sterically more hindered compared to 5 results in
obtaining chomophore 7 in a higher yield (70%) than
chromophore 8 (35%).
Scheme 2 outlines the synthesis of chromophore 12, in
which the initial Sonogashira coupling reaction of 9 with
phenylacetylene in the presence of Pd(PPh3)4, CuI, and
triethylamine afforded exTTF 10 in a good overall yield
(65%). When derivative 10, which bears two acetylenic
moieties, was treated with a large excess (ca. 10 mmol) of
TCNE, the expected bisadduct was formed, but the reaction did
not proceed to completion. Monoadduct 11 was therefore
obtained from the reaction, and the subsequent deactivation of
the second alkyne unit makes the second addition very slow.21
The doubly functionalized derivative 12 was only isolated in
35% yield after 5 days of reaction.
Based on the experimental outcome of the reactions and on
the X-ray and theoretical structures discussed below, it is
reasonable to assume that the mechanism for the reaction of
exTTF-alkynes 5, 6, and 10 with TCNE consists of the four
basic steps depicted in Scheme 3. From starting materials, the
addition of one of the π-bonds of the alkyne to the electrophilic
carbon situated between the two cyano groups produces a
zwitterionic high-energy intermediate (step A). Subsequently,
the ring-closed product is obtained through bond formation
between the ionized carbons (step B). The thermally induced
ring-opening cycloreversion step breaks the cyclobutene to give
initially the s-cis product (step C) that eventually should
isomerize to the s-trans conformer (step D).
solvated media.22 A similar mechanism to the one proposed
here was found to be occurring. In particular, substitution at the
dicyanoethene lowered the energy of the ring-opening step by
stabilizing the zwitterionic character of the transition state, with
the net effect of making the first step rate-limiting when
additional electro-accepting substituents are present in
dicyanoethene.
In the case of exTTF-based alkynes, the first step seems to be
the rate-limiting process by two additional reasons: (i) solvent
effects: as previously discussed, reactions did not work at all in
benzene, but when more polar solvents, that decrease the
energy of this first transition state, were used (i.e., 1,2-
dichloroethane), the reaction proceeds with good yields, and
(ii) steric hindrance: the exTTF unit is a bulky substituent in
the alkynes. In addition, when TCNE was replaced by TCNQ,
the reaction does not occur with any of the exTTF-alkynes in
solvents of different polarity, which could be accounted for by
the added steric hindrance of TCNQ versus TCNE.
The synthesized exTTF-TCBD derivatives are colored and
stable solids that can be stored for months in the laboratory
without decomposition. They are also readily soluble in
common organic solvents due to their sterically hindered
structure that might reduce chromophore aggregation. The
structural assignment of compounds is based on analytical and
1
spectroscopic techniques (i.e., UV−vis, FTIR, H NMR, 13C
NMR, and HRMS). In particular, compounds 7, 8, 11, and 12
show the presence of strong cyano stretching vibrations at
1
∼2200 cm−1 in the FTIR spectrum. H and 13C NMR spectra
showed the expected signals for exTTF and TCBD moieties:
the signals of the two 1,3-dithiole rings in the region of δ 6.6−
1
6.3 of the H NMR and the sp carbon atoms of the cyano
groups at δ 114−110 in the 13C NMR. For additional details,
see the Experimental Section and Supporting Information.
Crystal and Molecular Structures. Crystals of 12 suitable
for crystallographic analysis were grown by slow evaporation of
a cooled hexane/CH2Cl2 1/1 solution. Molecules in the crystal
structure of 12, a mixed solvate with two molecules of CH2Cl2,
arrange in a packing motif of pseudodimers with mutually
recognizing bis(dithiole) anthraquinone moieties (Figure 2).
Each dimer in the unit cell is constituted by two non-
The entire reaction profile for the transformation of
dimethylanilinoacetylene with 1,1-dicyanoethene was com-
puted by Diederich and co-workers in the gas phase and in
10709
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
Figure 2. (a) ORTEP representation of the molecular structure of compound 12 with ellipsoids plotted at 20% probability level; H atoms omitted.
(b) Crystal structure of 12·CH2Cl2 showing the packing along the crystallographic “c” axis.
superimposable molecular enantiomers. Unfortunately, we were
unable to produce single crystals of 7 and 8.
TCBD unit. However, as shown in Scheme 3, the reaction
mechanism provides the s-cis conformer after the thermal
opening of the cyclobutene reaction intermediate, and due to
the steric hindrance between the donor and acceptor units, the
structure cannot evolve to the more stable s-trans conformation.
This is in accord with the twisted s-cis dispositions that the
TCBD units show in the crystal structure of compound 12
(Figure 2a). Figure 3, therefore, displays the minimum energy
Considerable nonplanarity is observed for the whole
molecular structure of compound 12 and, especially, for the
TCBD and exTTF moieties (see Figure 2 and Figure S5 and
Table S1 in the Supporting Information for crystallographic
parameters). The two dicyanovinyl units forming the TCBD
moiety are almost orthogonal and define a dihedral angle
(C19−C11−C12−C22) of −78.2 0.7°, which compares with
those previously reported for similar π-deconjugated D−A
systems.7,16 The s-cis-type conformation obtained for the
TCBD units reflects the sterical role played by the exTTF
moiety. This moiety makes the rotation around the single
C11−C12 bond more difficult, and the s-cis conformer initially
obtained from the ring-opening cycloreversion cannot evolve to
the, in principle, more stable s-trans conformer (Scheme 3).
The central ring of the anthracene unit is folded along the C4−
C4′ axis into a boat conformation, and the exTTF moiety
adopts a saddle-like structure in which the lateral benzene rings
point upward and the dithiole rings point downward. This
conformation is consistent with those observed from the crystal
structures of different exTTF derivatives.23
The X-ray bond lengths measured for 12 (Table S2,
Supporting Information) are of the expected order of
magnitude for C−C bonds in conjugated or aromatic structures
(1.40 0.08 Å), for C−S bonds (1.737 0.009 Å), and for C−
Figure 3. Mimimum energy s-cis-type conformations calculated for 7
and 8 at the B3LYP/6-31G** level. (a) Side view showing the saddle-
like shape of the exTTF unit. (b) Top view.
N bonds (1.13
0.02 Å). The shortest bond lengths
conformations calculated for 7 and 8 with an s-cis-type
orientation of the TCBD units (see Figure S8 for specific
details about the optimized geometries). The conformation
predicted for compound 12 has a C2 symmetry axis passing
through the center of the anthracene unit and is identical to the
X-ray structure depicted in Figures 2a and S5 (Supporting
Information).
As expected, the exTTF moiety has the typical saddle-like
shape in all three compounds. The value calculated for the
angle between the planes defined by the lateral benzene rings of
correspond to the C1−C2 bonds of the dithiole rings (1.295
0.011 Å) and to the exocyclic C3−C4 bonds linking the
dithiole rings to the central anthracene unit (1.356 0.008 Å).
The molecular structures of cromophores 7, 8, and 12 were
theoretically optimized by using DFT calculations at the
B3LYP/6-31G** level. An exhaustive conformational study was
first performed for compounds 7 and 8 (see Figures S6 and S7
in the Supporting Information). In both cases, the most stable
conformers correspond to twisted s-trans orientations of the
10710
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
the anthracene unit has a similar value for the three compounds
(7: 138.7°; 8: 136.8°; 12: 136.2°) and slightly underestimates
the X-ray value obtained for 12 (142.6°) due to the packing
interactions. The acceptor TCBD moieties are not coplanar
with the benzene ring of the anthracene unit to which they are
attached owing to the steric hindrance. The two dicyanovinyl
halves of the TCBD units point upward in the opposite
direction of the exTTF folding (Figure 3) and form a dihedral
angle (C19−C11−C12−C22) of −54.2° for 7 that increases to
−74.2° for 8 and −74.1° for 12 owing to the presence of the
pendant phenyl substituents. The latter value is in good
correlation with that observed from X-ray data (−78.2 0.7°).
The large twisting of the TCBD units breaks the conjugation
between the two dicyanovinyl groups, and the C11−C12 bond
lengthens to 1.473 Å for 7 and to 1.502 Å for 8 and 12 (X-ray
exTTF24 at gradually increasing positively shifted potentials
(+307 mV for 7, +346 mV for 8, and +419 mV for 12) and, in
all cases, at higher potential values than the reference exTTF
compound (+244 mV). Oxidation becomes more difficult due
to the strong tetracyanobutadiene acceptor groups and, in the
case of derivatives 8 and 12, due to the more distorted
geometry induced by the phenyl substitution.
The cyclic voltammograms of compounds 7 and 8 feature
two well-resolved reversible reduction couples with similar
currents for each peak (1e− each) centered on the
tetracyanobutadiene acceptors. In the case of compound 12,
the peak current increases significantly due to the presence of
two acceptor units (each reduction step implies 2e−). The
cathodic shifts, compared to TCNE, observed in the first
reduction potential originate from π-conjugation of the
acceptor to the exTTF unit. The reduction processes are also
strongly influenced by the introduction of phenyl substituents
that increase the twisting of the TCBD unit and induce an
additional loss of conjugation between the two acceptor halves
of the TCBD unit. A smaller effect is also visible in the second
reduction potentials. The difference between the first and
second reduction potentials is larger for the exTTF-TCBD
derivative 7 (746 mV), for which the two dicyanovinyl units are
less twisted. In contrast, substitution of TCBD with phenyl
groups, despite it induces a larger twisting between the two
acceptor subunits, determines that the first and second
reduction potentials become closer for both 8 (400 mV) and
12 (457 mV).
value: 1.518
0.008 Å). As discussed below, the limited π-
conjugation of the TCBD unit largely influences the electronic
and optical properties. The introduction of the phenyl
substituent also has a significant effect on these properties.
Electrochemical Properties. Cyclic voltammetry (CV)
and differential pulse voltammetry (DPV, see Figure S1,
Supporting Information) experiments were carried out for 7, 8,
and 12 and compared to exTTF and TCNE as reference
compounds. Figure 4 displays the cyclic voltammograms
recorded in THF for 7, 8, and 12, and Table 1 summarizes
the values of the redox potentials.
To control the performance of organic electronic devices,
molecules with energetically low HOMO−LUMO energy gaps
are of primary importance.25 In this sense, compounds 7, 8, and
12 present small electrochemical gaps (∼0.65−0.94 V), which
can be easily tuned by the substituents incorporated in the
TCBD unit.
Electronic Structure. Oxidized and Reduced Species.
Figure 5 shows the atomic orbital (AO) composition of the
highest occupied (HOMO−1 and HOMO) and lowest
unoccupied (LUMO, LUMO+1, and LUMO+2) molecular
orbitals of 8. The HOMO and HOMO-1 are localized on the
electron-donor exTTF unit, whereas the LUMO and LUMO+1
mainly spread over the acceptor TCBD moiety with some
contribution from the anthracene skeleton. The pendant phenyl
group also shows a small but important contribution to the
LUMO and LUMO+1. The LUMO+2 is again localized on the
exTTF moiety. The topology of the frontier MOs of
chomophores 7 and 12 is equivalent to that described for 8.
For 7, the contribution of the phenyl group does not exist and
for 12 the LUMO and LUMO+1 are doubled due to the
second TCBD group. The electronic communication between
the donor and acceptor units is evidenced by the charge
Figure 4. Cyclic voltammograms of compounds 7, 8, and 12 recorded
in THF at a scan rate of 0.1 V s−1, with 0.1 M TBAPF6 as supporting
electrolyte.
The exTTF-TCBD derivatives 7, 8, and 12 exhibit the
characteristic two-electron quasi-reversible oxidation of
Table 1. Cyclic Voltammetry Redox Potentials (THF, 0.1 M TBAPF6) and Electrochemical and Optical Energy Gaps (Eg)
b
b
b
c
d
e
f
Epa (mV)
E1/2
(mV)
E1/2
(mV)
ΔEred (mV)
Eg(elect) (V)
onset (nm)
Eg(opt) (eV)
ox
red,1
red,2
a
exTTF
+244
TCNE
−240
−348
−540
−524
−1112
−1094
−940
1088
746
400
457
7
+307
+346
+419
0.65
0.87
0.94
1322
929
0.93
1.33
1.34
8
12
−981
925
a
b
exTTF = 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-yilidene]-1,3-dithiole. Potentials vs Ag/AgNO3. Working electrode: glassy carbon; counter
electrode: Pt; reference electrode: Ag/AgNO3. Scan rate: 0.1 V s−1. E1/2 = (Epa + Epc)/2, where Epc and Epa are cathodic and anodic peak potentials,
c
d
e
f
respectively. ΔE = E1/2
− E1/2
.
Eg(elect) = Epa − E1/2red,1. Absorption onsets. Optical energy gap.
red
red,1
red,2
ox
10711
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
passing from 7 (−5.22 eV) to 8 (−5.14 eV) that is not in good
correspondence with the gradual increase of the oxidation
potential along the series 7, 8, and 12. The reason for this
apparent discrepancy between experiment and theory is that
Koopmans’ theorem, which suggests that the higher the
HOMO energy the easier the oxidation of the molecule, is a
one-electron approach and does not apply to oxidation
processes that involve two electrons as it is the case for
exTTF derivatives.8e,15c
To further investigate the oxidized species, B3LYP/6-31G**
calculations were performed for the dication species. The
energy required to generate the dication augments along the
series exTTF (9.81 eV), 7 (9.97 eV), 8 (10.07 eV), and 12
(10.32 eV), which is in agreement with the gradual increase
recorded for the oxidation potential along this series. As
depicted in Figure 7b, the minimum energy structure calculated
Figure 5. Electron density contours (0.03 e bohr−3) calculated for the
HOMOs and LUMOs of 8 at the B3LYP/6-31G** level.
transfer that takes place from the exTTF moiety to the TCBD
unit. The latter supports a total charge of −0.21e for 7 that
increases to −0.43e for 8 due to the additional charge transfer
from the donor phenyl group. Attending to the nature of the
HOMO and the LUMO, one can ensure that the lowest energy
excitations in all three compounds will have a large charge-
transfer character.
Figure 6 compares the molecular orbital energy distribution
calculated for the exTTF molecule and for compounds 7, 8, and
Figure 7. Minimum energy molecular structures calculated for (a) 8,
(b) 82+, and (c) 82−.
for 82+ corresponds to a conformation in which the TCBD is
largely twisted (C19−C11−C12−C22: −89.8°) and the exTTF
unit adopts an orthogonal conformation similar to those
previously found for exTTF dications both theoretically and
experimentally.15,26 Upon oxidation, the exocyclic C3C4
bonds that connect the dithiole rings to the anthracene unit
elongate from 1.367 Å in neutral 8 to 1.485 Å in charged 82+
(averaged values) This lengthening allows for the rotation of
the dithiole rings to minimize the steric interactions, and, as a
consequence, the anthracene unit becomes planar and the
dithiole rings lie perpendicular to the anthracene plane. For 82+,
the exTTF moiety supports a total charge of +2.03e (+0.23e for
neutral 8) that is mainly concentrated on the dithiole rings
(+1.43e). Similar structures are found for 72+ and 122+.
Concerning the reduced species, the energy calculated for the
LUMO correctly reproduces the experimental trend observed
for the first reduction potential. The LUMO energy increases in
passing from 7 (−3.71 eV) to 8 (−3.25 eV) in agreement with
the more negative potential measured for 8 (Table 1). This
destabilization is ascribed to the attachment of the donor
phenyl group to the TCDB unit that increases the twisting of
the two C(CN)2 subunits and thereby decreases the π-
conjugation between them. In contrast, the LUMO+1 decreases
in energy in passing from 7 (−2.41 eV) to 8 (−2.76 eV), and
Figure 6. Energy diagram showing the energy values calculated for the
highest occupied and lowest unoccupied molecular orbitals of exTTF,
7, 8, and 12. H and L denote HOMO and LUMO, respectively.
12. The introduction of the acceptor TCBD group causes an
energy lowering of the exTTF molecular orbitals that is more
pronounced for 12 owing to attachment of the second TCBD
group and provokes the appearance of low energy virtual
orbitals (LUMO and LUMO+1 for 7 and 8, LUMO to LUMO
+3 for 12) that are localized on the acceptor unit (see Figure
5). The stabilization of the HOMO justifies the higher
oxidation potentials measured for compounds 7 (+0.307 V),
8 (+0.346 V), and 12 (+0.419 V) compared to exTTF (+0.244
V). However, the attachment of the donor phenyl group to the
TCBD moiety produces an increase in the HOMO energy on
10712
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
the energy gap between the LUMO and LUMO+1 (LUMO+2
for 12) significantly narrows (see Figure 6). Chromophore 7
presents a small HOMO−LUMO energy gap (1.51 eV) that
widens for 8 (1.89 eV) and 12 (2.12 eV) in good agreement
with the experimental trend measured for the electrochemical
gap (Table 1).
the time-dependent DFT (TD-DFT) approach and the
B3LYP/6-31G**-optimized ground-state geometries. Table 2
Table 2. Lowest Energy Singlet Excited States Calculated for
7, 8, and 12 Using the TD-DFT Approach. Vertical
Excitation Energies (E), Oscillator Strengths (f), Dominant
Monoexcitations with Contributions (within Parentheses)
Greater than 35%, and Description of the Excited State Are
Summarized
The molecular geometries of the anion and dianion species
were also optimized at the B3LYP/6-31G** level. Figure 7c
displays the minimum energy structure calculated for 82−. The
introduction of the extra electrons reduces the twisting of the
TCBD unit (C19−C11−C12−C22 dihedral angle) from
−74.2° in neutral 8 to −33.5° in 8•− and −27.2° in 82− due
to the shortening of the central C11−C12 bond (8: 1.502 Å;
8•−: 1.433 Å; 82−: 1.421 Å). The extra electrons mainly enter
the TCBD unit that accumulates a charge of −1.43e for 82−, but
they are also placed in the adjacent phenyl rings due to their
participation in the LUMO (see Figure 5). The pendant phenyl
ring plays an important role in stabilizing the negative charge
because it accommodates 0.33e in passing from 8 to 82−. This
extra stabilization explains the positive shift of the second
reduction potential in passing from 7 (−1.094 eV), for which
the phenyl substituent is not present, to 8 (−0.940 eV) and 12
(−0.981 eV), and justifies the reduction of the difference
between the first and second reduction potentials (Table 1).
Optical Properties. The UV/vis spectra of the exTTF-
TCBD derivatives 7, 8, and 12 were recorded in diluted
dichloromethane solutions and display the well-known
absorptions of exTTF in the 400−440 nm visible region
(Figure 8).15,27 Additionally, new absorption features are
a
b
c
state
E (eV)
f
monoexcitations
description
7
S1
S2
S3
S4
S5
S6
1.13 (1097)
1.70 (729)
2.38 (521)
2.88 (430)
2.93 (423)
3.11 (399)
0.046
0.034
0.095
0.033
0.150
0.028
H → L (98)
CT
H−1 → L (98)
H → L+1 (95)
H−2 → L (86)
H−1 → L+1 (80)
H−3 → L (50)
H → L+2 (35)
H−3 → L (42)
H → L+2 (48)
H → L (98)
CT
CT
CT
CT
CT
exTTF
CT
S7
3.14 (395)
0.117
exTTF
CT
8
S1
S2
S3
S4
S5
S1
S2
S3
S4
S5
S6
S7
S8
S9
1.48 (838)
1.96 (634)
2.06 (602)
2.53 (490)
3.08 (403)
1.71 (725)
1.71 (725)
2.17 (571)
2.17 (571)
2.27 (545)
2.27 (545)
2.69 (461)
2.76 (450)
3.19 (389)
0.038
0.042
0.044
0.093
0.212
0.017
0.062
0.016
0.091
0.019
0.069
0.033
0.148
0.149
H → L+1 (95)
H−1 → L (95)
H−1 → L+1 (98)
H → L+2 (92)
H → L (98)
CT
CT
CT
exTTF
CT
12
H → L+1 (98)
H → L+3 (98)
H → L+2 (95)
H−1 → L+1 (98)
H−1 → L (95)
H−1 → L+2 (96)
H−1 → L+3 (98)
CT
CT
CT
CT
CT
CT
CT
H → L+4 (95)
exTTF
a
b
Wavelengths (in nm) are given within parentheses. H and L denote
c
HOMO and LUMO, respectively. CT indicates a charge-transfer
exTTF → TCBD transition. exTTF denotes an exTTF-centered
electronic transition.
summarizes the excited states with energies below 3.20 eV
(wavelengths above 387 nm), which give rise to the absorption
bands in the visible and near-infrared parts of the spectra. For
compound 8, TD-DFT calculations predict four states above
490 nm that result from electron excitations from the HOMO
and HOMO−1, localized on the exTTF moiety, to the LUMO
and LUMO+1, localized on the TCBD unit (see Figure 5). The
four states imply an electron density transfer from the donor
exTTF moiety to the acceptor TCBD unit and are, therefore, of
CT nature. The S2 and S3 states are calculated close in energy at
1.96 eV (634 nm) and 2.06 eV (602 nm) with a total oscillator
strength (f) of 0.086 and account for the broad absorption
band centered at 656 nm. The S4 state (2.53 eV, 490 nm) is
more intense (f = 0.093) and correlates with the narrow band
peaking at 491 nm. The S1 state (HOMO→LUMO excitation)
is calculated in the near-infrared (1.48 eV, 838 nm) and can be
involved in the tail of the lowest energy band. The absorption
band at 421 nm originates in the transition to the S5 state
(HOMO→LUMO+2) computed at 3.08 eV (403 nm) that is
more intense (f = 0.212) and fully corresponds to the exTTF
excitation (see Figure 5). Calculations therefore support that
the first two bands centered around 650 and 490 nm
correspond to CT bands.
Figure 8. UV/vis spectra of exTTF-TCBD derivatives 7, 8, and 12 in
CH2Cl2 (1 × 10−5 M).
observed in the red part of the spectrum (starting from 500
nm) with absorption onsets recorded in the NIR region (925−
1325 nm). Low-energy optical gaps of 0.94−1.34 eV are
therefore obtained in good linear correlation with the
electrochemical gaps calculated from redox potentials (see
Table 1). These low energy absorptions present low molar
extinction coefficients (below 5000 M−1 cm−1) and are assigned
to CT transitions involving the donor exTTF and acceptor
TCBD moieties. They exhibit a slight solvatochromism without
any clear trend depending on solvent polarity (see Figures S2−
S4, Supporting Information, for details).
To investigate the nature of the absorption bands observed in
the electronic spectra, vertical transitions to the lowest energy
singlet excited states (Sn) were calculated for 7, 8, and 12 using
10713
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
The distribution of the excited states for compound 12 is
identical to that discussed for 8, but now there are eight CT
states (S1 to S8) due to the presence of two TCBD units. The
CT states appear by pairs and are calculated at higher energies
than for 8 because of the higher energy gap between the
HOMO/HOMO−1 and the LUMO to LUMO+3 orbitals (see
Figure 6). The broad band observed in the visible region (450−
800 nm) for 12 (Figure 8), which presents maximum
absorption at 502 nm and a shoulder around 600 nm, is
therefore assigned to CT electronic transitions. The excitation
of the exTTF moiety is calculated at higher energies (state S9,
3.19 eV, 389 nm) and is immersed under the low energy side of
the broad and intense band centered around 300 nm (Figure
8).
For compound 7, the absence of the donor phenyl group
attached to the TCBD unit determines that the HOMO−
LUMO energy gap has a value of 1.51 eV significantly smaller
than for chromophores 8 and 12 (Figure 6). As a consequence,
the HOMO→LUMO (state S1) and HOMO−1→LUMO (S2)
excitations are calculated at very low energies (1.13 eV (1097
nm) and 1.70 eV (729 nm), respectively). This explains the
long tail observed for the lowest-energy CT band of compound
7 that extends to 1325 nm. States S3 to S5 are also of CT nature
and contribute to this band (S3) and to the band peaking at 438
nm (S4 and S5). The excitation of the exTTF moiety is
predicted around 390 nm mixed with other CT excitations
(Table 2, states S6 and S7).
Calculations therefore predict that the absorption bands
observed in the visible region for chromophores 7, 8, and 12
are mostly due to photoinduced charge-transfer transitions
between the donor exTTF moiety and the acceptor TCBD
unit. The intensity of these bands is relatively high because of
the non-negligible overlap between the molecular orbitals of the
exTTF moiety (HOMO and HOMO−1) and those of the
TCBD unit (LUMO and LUMO+1) (Figure 5). Therefore,
despite the highly distorted nonplanar structures exhibited by
chromophores 7, 8, and 12, an effective electronic communi-
cation takes place between the donor and acceptor units.
made more difficult both by the conjugation with the strong
tetracyanobutadiene acceptor and by the additional sterical
hindering induced by the phenyl ring. The reduction processes
are strongly influenced by the introduction of phenyl
substituents because they augment the twisting of the TCBD
unit and provoke an additional loss of conjugation between the
two acceptor dicyanovinyl halves of the TCBD unit.
Theoretical calculations carried out by using the DFT
approach (B3LYP/6-31G**) forcefully support the exper-
imental findings. The HOMO and HOMO−1 are localized on
the exTTF moiety, and the introduction of the TCBD units
determines the appearance of low-lying LUMO and LUMO+1
orbitals associated to these units. The electronic communica-
tion between the electron-donor and electron-acceptor frag-
ments is supported both by the charge transfer that takes place
between them for the neutral molecules and by their
participation in the reduction and oxidation processes. The
calculations performed for the dication, anion, and dianion
species evidence the mutual influence of the donor and
acceptor units and explain the experimental trends observed for
the oxidation and reduction potentials.
The optical properties of compounds 7, 8, and 12 are
characterized by low- to medium-intense absorption bands that
completely cover the visible region and extend to the near-
infrared. Theoretical calculations show that these bands are due
to charge-transfer electronic transitions between the donor
exTTF moiety and the acceptor TCBD units. The intensity of
these CT bands is relatively high because, despite the highly
distorted nonplanar structures exhibited by chromophores 7, 8,
and 12, an effective electronic communication takes place
between the donor and acceptor units.
In summary, the new donor−acceptor chromophores 7, 8,
and 12 exhibit interesting geometrical and electronic properties
which can be tuned by the presence of an aryl group on the
TCBD moiety. This strategy paves the way to the synthesis of
more sophisticated push−pull exTTF derivatives where the
substituent or functional group present on the starting alkyne
determine the planarity, and hence the electronic properties, of
the final chromophore.
CONCLUSIONS
■
We have explored the strategy recently developed by Diederich
and co-workers for the preparation of nonplanar push−pull-
substituted buta-1,3-dienes, in combination with exTTF units.
This methodology, which consists of a [2 + 2] cycloaddition of
tetracyanoethene (TCNE) with electron-donor substituted
alkynes and subsequent cycloreversion, has efficiently provided
a new family of highly distorted exTTF-based push−pull
chromophores. In addition, we have carried out an
experimental and computational study on their electrochemical
and optical absorption characteristics, which reveals the strong
impact that aryl substitution plays in modulating the
optoelectronic properties of the new chromophores.
Based on the X-ray crystal structure obtained for 12, we were
able to validate the mechanistic pathway of the reaction, which
consists of a thermal conrotatory opening of the cyclobutene
reaction intermediate that leads to the s-cis conformer. The
largely twisted s-cis-type conformation adopted by the TCBD
units in the crystal supports this mechanism and corroborates
the prohibited interconversion of this conformer to the more
stable s-trans owing to the steric hindrance between the donor
and acceptor units.
EXPERIMENTAL SECTION
■
General Methods. All solvents were dried according to standard
procedures. Reagents were used as purchased. 2-Iodo-9,10-bis(1,3-
dithiol-2-ylidene)-9,10-dihydroanthracene (4),19 2-ethynyl-9,10-bis-
(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (5),20 and 2,6-diiodo-
9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (9)20 were pre-
pared using described procedures. All air-sensitive reactions were
carried out under argon atmosphere. Flash chromatography was
performed using silica gel (230−240 mesh). Analytical thin-layer
chromatography (TLC) was performed using aluminum-coated plates.
NMR spectra were recorded on 300, 500, or 700 MHz spectrometers
at 298 K using partially deuterated solvents as internal standards.
Coupling constants (J) are denoted in Hz and chemical shifts (δ) in
ppm. Multiplicities are denoted as follows: s = singlet, d = doublet, t =
triplet, m = multiplet, br = broad. FT-IR spectra were recorded on a
spectrometer suited with an ATR device. UV−vis spectra were
recorded in CH2Cl2 solutions, with concentrations around 0.2 mM.
The exact molecular weight of the new compounds was obtained by
electrospray ionization mass spectra (ESI-MS) and matrix-assisted
laser desorption ionization (coupled to a Time-Of-Flight analyzer)
experiments (MALDI-TOF).
Electrochemistry. Electrochemical measurements were performed
at room temperature in a potentiostate/galvanostate equipped with a
home-built one-compartment cell with a three-electrode configuration,
containing 0.1 M tetrabutylammonium hexafluorophosphate
Phenyl substitution of the acceptor TCBD units affects both
the oxidation and reduction processes. Oxidation of exTTF is
10714
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
1
(TBAPF6) as supporting electrolyte. A glassy carbon (GCE) was used
as the working electrode, a platinum wire as the counter electrode, and
a Ag/AgNO3 nonaqueous electrode was used as reference. Prior to
each voltammetric measurement the cell was degassed under an argon
atmosphere by ca. 20 min. The solvent, THF, was freshly distilled from
Na. The electrochemical measurements were performed using a
concentration of approximately 0.2 mM of the corresponding
compound.
2-(Phenylethynyl)-9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihy-
droanthracene (6). To a solution of 4 (100 mg, 0.2 mmol) in dry
THF (15 mL) under argon atmosphere were added Pd(PPh3)4 (12
mg, 0.01 mmol) and CuI (2 mg, 0.01 mmol) and subsequently
phenylacetylene (0.02 mL, 0.2 mmol) and triethylamine (0.13 mL).
The mixture was allowed to stir overnight and then was washed with
NH4Cl, H2O, and brine. Afterward, the solvent was evaporated and the
crude product purified by silica gel flash column chromatography
(silica gel, hexane/dichloromethane 9:1 to 7:3) to afford product 6 as
an orange solid (70% yield): Mp: 233−235 °C. 1H NMR (CDCl3, 300
MHz): δ = 7.86 (d, 1H, J = 1.7 Hz), 7.71 (m, 2H), 7.57 (m, 2H), 7.46
(m, 2H), 7.36 (m, 3H), 7.31 (m, 2H), 6.33 (s, 4H) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 135.6, 135.5, 134.5, 134.3, 134.2, 132.5, 130.6,
128.1, 127.3, 127.2, 126.9, 125.1, 125.1, 124.9, 124.0, 123.9, 122.4,
120.7, 120.5, 120.3, 119.6, 116.3, 116.2, 116.2, 88.7, 88.6 ppm. FTIR
(KBr): ν = 2923, 2854, 1716, 1597, 1546, 1510, 1455, 1407, 1266,
1153, 1094, 1056, 833, 800, 755, 643 cm−1. UV/vis (CH2Cl2): λmax
(log ε) = 376 (4.19), 424 (4.22), 440 (4.26) nm. HRMS (MALDI-
TOF): calcd for [C28H16S4]+ 480.0129, found 480.0110.
2,6-Diphenylethynyl-9,10-bis(1,3-dithiol-2-ylidene)-9,10-di-
hydroanthracene (10). To a solution of 9 (250 mg, 0.4 mmol) in
dry THF (25 mL) under argon atmosphere were added Pd(PPh3)4
(46 mg, 0.04 mmol) and copper iodide (8 mg, 0.04 mmol) and
subsequently phenylacetylene (0.13 mL, 1.2 mmol) and triethylamine
(0.3 mL). The mixture was allowed to stir overnight and then was
washed with NH4Cl, H2O, and brine. Afterward, the solvent was
evaporated and the crude product purified by silica gel flash column
chromatography (silica gel, hexane/dichloromethane 9:1 to 7:3) to
afford 10 as an orange solid (65% yield). Mp: 278−282 °C. 1H NMR
(CDCl3, 300 MHz): δ = 7.87 (d, 2H, J = 1.50 Hz), 7.69 (d, 2H, J =
8.03 Hz), 7.56 (m, 4H), 7.46 (dd, 2H, J1 = 8.03 Hz, J2 = 1.50 Hz), 7.37
(m, 6H), 6.36, (s, 4H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 136.6,
134.4, 134.2, 130.7, 128.2, 127.3, 127.2, 126.8, 123.9, 122.3, 119.7,
116.4, 88.6, 88.5 ppm. FTIR (KBr): ν = 2924, 2854, 1737, 1599, 1547,
1499, 1461, 1401, 1263, 1220, 769 cm−1. UV/vis (CH2Cl2): λmax (log
ε) = 389 (4.17), 436 (4.25), 454 (4.31) nm. HRMS (MALDI-TOF):
calcd for [C36H20S4]+ 580.0442, found 580.0428.
product 8 as a dark blue solid (35% yield). Mp > 300 °C. H NMR
(CDCl3, 300 MHz): δ = 8.02 (dd, 1H, J1 = 8.5 Hz, J2 = 2.3 Hz), 7.93
(d, 1H, J = 8.5 Hz), 7.76 (m, 3H), 7.70 (m, 2H), 7.59 (m, 3H), 7.35
(m, 2H), 6.47 (d, 2H, J = 2.1 Hz), 6.38 (d, 1H, J = 6.5 Hz), 6.33 (d,
1H, J = 6.5 Hz) ppm. 13C NMR (CDCl3, 75 MHz): δ = 168.1, 165.7,
143.5, 143.2, 142.4, 139.5, 137.4, 135.1, 135.0, 135.0, 131.6, 130.4,
130.0, 128.8, 128.0, 127.4, 127.1, 127.0, 126.6, 126.5, 125.5, 125.3,
120.9, 120.1, 118.6, 118.3, 118.2, 117.0, 112.6, 112.2, 112.1, 111.6,
88.0, 84.7 ppm. FTIR (KBr): ν = 2923, 2854, 2201, 1725, 1590, 1504,
1462, 1366, 1261, 1155, 805, 725, 554 cm−1. UV/vis (CH2Cl2): λmax
(log ε) = 421 (3.75), 491 (3.56), 656 (3.12) nm. HRMS (MALDI-
TOF): calcd for [C34H16N4S4]+ 608.0252, found 608.0260.
Doubly TCBD Functionalization: General Method. 100 mg
(0.17 mmol) of 10 were dissolved in 30 mL of 1,2-dichloroethane, and
the solution was heated to 80 °C. A 240 mg (1.9 mmol) portion of
TCNE was then added, and the reaction was allowed to stir for 5 days.
Then, the solvent was removed under vacuum, and the crude products
were separated and purified by column chromatography (silica gel,
dichloromethane).
2-(3-Phenylbuta-1,3-diene-1,1,4,4-tetracyano)-6-(phenyle-
thynyl)-9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene
1
(11). Dark blue solid (9 mg, 8% yield). Mp > 300 °C. H NMR
(CDCl3, 300 MHz): δ = 8.02 (dd, 1H, J1 = 8.7 Hz, J2 = 2.1 Hz), 7.94
(d, 1H, J = 8.7 Hz), 7.91 (d, 1H, J = 1.3 Hz), 7.77 (dd, 2H, J1 = 8.2 Hz,
J2 = 1.5 Hz), 7.71 (d, 1H, J = 7.5 Hz), 7.68 (d, 1H, J = 7.5 Hz). 7.59
(m, 4H), 7.50 (dd, 1H, J1 = 8.0 Hz, J2 = 1.5 Hz), 7.39 (m, 4H), 6.50 (s,
2H), 6.42 (d, 1H, J = 6.7 Hz), 6.36 (d, 1H, J = 6.7 Hz) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 168.0, 165.6, 144.2, 142.2, 140.5, 137.3, 135.2,
135.1, 134.9, 132.2, 132.1, 132.0, 131.6, 130.5, 130.1, 129.9, 128.9,
128.9, 128.8, 128.8, 128.1, 128.1, 127.5, 126.6, 126.5, 125.5, 123.5,
121.8, 120.0, 119.6, 118.7, 118.5, 118.3, 117.2, 112.6, 112.2, 112.1,
111.6, 90.6, 89.7, 88.0, 84.8 ppm. FTIR (KBr): ν = 2955, 2910, 2840,
2190, 1670, 1650, 1500, 1383, 1375, 1340, 1180, 990, 965, 606 cm−1.
UV/vis (CH2Cl2): λmax (log ε) = 805 (2.28), 439 (3.23), 368 (3.77),
305 (3.68) nm. HRMS (MALDI-TOF): calcd for [C42H20N4S4]+
708.0565, found 708.0565.
2,6-Bis(3-phenylbuta-1,3-diene-1,1,4,4-tetracyano)-9,10-bis-
(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (12). Dark blue
solid (45 mg, 35% yield). Mp > 300 °C. 1H NMR (CDCl3, 500 MHz):
δ = 7.95 (m, 4H), 7.84 (d, 2H, J = 1.9 Hz), 7.77 (m, 4H), 7.70 (m,
2H), 7.61 (m, 4H), 6.54 (d, 2H, J = 6.8 Hz), 6.48 (d, 2H, J = 6.8 Hz)
ppm. 13C NMR (CDCl3, 125 MHz): δ = 167.9, 165.5, 146.2, 141.6,
136.8, 135.3, 131.5, 130.6, 129.9, 128.7, 127.9, 126.7, 126.3, 119.1,
118.4, 118.2, 112.4, 112.1, 111.9, 111.6, 88.0, 85.7 ppm. FTIR (KBr): ν
= 2958, 2922, 2854, 2208, 1735, 1505, 1383, 1261, 1212, 1185, 1099,
1026, 805, 534 cm−1. UV/vis (CH2Cl2): λmax (log ε) = 502 (3.79), 606
(3.62) nm. HRMS (MALDI-TOF): calcd for [C48H20N8S4]+
836.0694, found 836.0652.
Computational Details. DFT calculations were carried out using
the Gaussian 03 program package.28Geometry optimizations of both
the neutral molecules and the dication, anion, and dianion species
were performed with Becke’s three-parameter B3LYP exchange-
functional29 and the 6-31G** basis set.30 Anions of 7 and 8 were
calculated as open-shell doublet systems using the unrestricted
UB3LYP approach. Dications of 7, 8, and 12 and dianions of 7 and
8 were treated as closed-shell systems. Both neutral and charged
species were also optimized in the presence of the solvent (THF, ε =
7.58) within the SCRF (self-consistent reaction field) theory using the
polarized continuum model (PCM)31 approach to model the
interaction with the solvent. The PCM model considers the solvent
as a continuous medium with a dielectric constant ε, and represents
the solute by means of a cavity built with a number of interlaced
spheres.32 The solvent has a small influence on the optimized
molecular geometries and all geometrical parameters quoted in the text
correspond to values obtained in gas phase. The energies required to
generate the dications were calculated as the difference between the
total energies of the dication and the neutral molecule optimized in
THF solution. Molecular orbitals were plotted using Chemcraft 1.6.33
Vertical electronic transition energies were computed at the
B3LYP/6-31G** level using the TDDFT approach34 and the
2-(Buta-1,3-diene-1,1,4,4-tetracyano)-9,10-bis(1,3-dithiol-2-
ylidene)-9,10-dihydroanthracene (7). A mixture of 5 (100 mg,
0.24 mmol) and TCNE (31 mg, 0.26 mmol) dissolved in 1,2
dichloroethane (25 mL) was refluxed overnight under argon
atmosphere. The solvent was evaporated, and the crude was purified
by column chromatography on silica gel using dichloromethane as
eluent to afford 7 as a dark green solid (95 mg, 70% yield). Mp > 300
1
°C. H NMR (CDCl3, 700 MHz): δ = 8.08 (s, 1H), 7.94 (d, 1H, J =
8.1 Hz), 7.76 (m, 2H), 7.66 (d, 1H, J = 1.8 Hz), 7.48 (dd, 1H, J1 = 8.1
Hz, J2 = 1.8 Hz), 7.36 (d, 1H, J = 5.7 Hz), 7.35 (d, 1H, J = 5.7 Hz),
6.41 (m, 4H) ppm. 13C NMR (THF-d8, 175 MHz): δ = 162.8, 155.6,
142.4, 141.6, 140.2, 138.1, 136.3, 136.3, 129.8, 128.4, 127.3, 127.3,
127.1, 126.8, 126.2, 126.1, 121.6, 121.1, 119.2, 119.1, 119.0, 118.4,
113.6, 113.3, 112.8, 110.6, 98.3, 91.0 ppm. FTIR (KBr): ν = 2961,
2923, 2853, 2225, 1594, 1543, 1501, 1451, 1335, 1093, 1022, 801, 756
cm−1. UV/vis (CH2Cl2): λmax (log ε) = 378 (4.21), 438 (4.24), 636
(3.25) nm. HRMS (MALDI-TOF): calcd for [C28H12N4S4]+
531.9939, found 531.9918.
2-(3-Phenylbuta-1,3-diene-1,1,4,4-tetracyano)-9,10-bis(1,3-
dithiol-2-ylidene)-9,10 dihydroanthracene (8). To a solution of
50 mg (0.1 mmol) of 6 in 1,2-dichloroethane (20 mL) heated to 80 °C
was added 73 mg (0.56 mmol) of TCNE under argon atmosphere.
The mixture was allowed to stir for 3 days, and then the solvent was
evaporated under reduced pressure and the crude was purified by
column chromatography using dichloromethane as eluent, to afford
10715
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
optimized ground-state molecular geometries. The vertical excitation
energies were also calculated using the hybrid PBE0 functional,35
which provides a similar description (energies and nature) of the
lowest-energy excited states. For instance, the two first states of
compound 7 were calculated at 1.28 eV (f = 0.050) and 1.88 eV (f =
0.039), respectively, slightly higher in energy than those predicted at
the B3LYP level (see Table 2). However, both the B3LYP and the
PBE0 functionals sometimes underestimate the energy of the CT
excited states. This shortcoming of standard global hybrid functionals
has been reported for donor−acceptor compounds for which there is a
negligible overlap between the molecular orbitals involved in the
excitation.36 To solve the problem, the use of a long-range corrected
functional as, e.g., the CAM-B3LYP functional,37 is recommended.
However, the CAM-B3LYP approach completely fails in reproducing
the optical spectra of 7 since: (i) the lowest energy CT states are
calculated too high in energy (for instance, S1: 2.25 eV (551 nm); S2:
2.92 (425 nm)), strongly underestimating the wavelength observed
experimentally (600−800 nm) for the CT absorption band, and (ii)
the CT states are computed to be among the most intense (f = 0.1−
0.3) electronic transitions in contradiction with the low relative
intensity of the CT band. The good performance of the B3LYP and
PBE0 in describing the CT absorption bands of compounds 7, 8, and
12 is attributed to the non-negligible overlap between the molecular
orbitals of the donor exTTF moiety and the acceptor TCBD unit (see
Figure 5).
(2) (a) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153−161.
(b) Special issue on Organic Photovoltaics: Acc. Chem. Res. 2009, 42,
1689. (c) Fischer, M. K. R.; Wenger, S.; Wang, M.; Mishra, A.;
Zakeeruddin, S. M.; Gratzel, M.; Bauerle, P. Chem. Mater. 2010, 22,
̈
̈
1836. (d) Unger, E. L.; Ripaud, E.; Leriche, P.; Cravino, A.; Roncali, J.;
Johansson, E. M. J.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2010,
114, 11659. (e) Delgado, J. L.; Bouit, P.-A.; Filippone, S.; Herranz, M.
A.; Martín, N. Chem. Commun. 2010, 46, 4853.
(3) (a) Ragoussi, M.-E.; Cid, J.-J.; Yum, J.-H.; de la Torre, G.;
DiCenso, D.; Gratzel, M.; Nazeeruddin, M. K.; Torres, T. Angew.
̈
Chem., Int. Ed. 2012, 51, 4375−4378. (b) Bouit, P.-A.; Marszalek, M.;
Humphry-Baker, R.; Viruela, R.; Ortí, E.; Zakeeruddin, S. M.; Gratzel,
̈
M.; Delgado, J. L.; Martín, N. Chem.Eur. J. 2012, 18, 11621−11629.
(c) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M.; Diau, E. W.-H.; Yeh, Ch.-Y.; Zakeeruddin, S. M.;
Gratzel, M. Science 2011, 334, 629−634.
̈
(4) (a) Xiao, X.; Wei, G.; Wang, S.; Zimmerman, J. D.; Renshaw, C.
K.; Thompson, M. E.; Forrest, S. R. Adv. Mater. 2012, 24, 1956−1960.
(b) Lin, Y.; Li, Y.; Zhan, X. Chem. Soc. Rev. 2012, 41, 4245−4272.
(c) Walker, B.; Kim, C.; Nguyen, T.-Q. Chem. Mater. 2011, 23, 470−
482.
(5) (a) Walker, B.; Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.;
Garcia, A.; Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19,
3063. (b) Mayerhoffer, U.; Deing, K.; Gruß, K.; Braunschweig, H.;
̈
Meerholz, K.; Wurthner, F. Angew. Chem., Int. Ed. 2009, 48, 8776.
̈
(c) Zhao, X.; Piliego, C.; Kim, B.; Poulsen, D. A.; Ma, B.; Unruh, D.
ASSOCIATED CONTENT
* Supporting Information
A.; Frec
́
het, J. M. J. Chem. Mater. 2010, 22, 2325. (d) Fitzner, R.;
■
S
Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Korner, C.;
Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiß, A.; Uhrich, C.;
̈
1H NMR, 13C NMR, and HRMS (MALDI-TOF) spectra for all
new compounds, DPVs, and absorption spectra in different
solvents for 7, 8, and 12, single-crystal structure of 12, and
theoretical calculations details. This material is available free of
charge via the Internet at http://pubs.acs.org.
Pfeiffer, M.; Bauerle, P. Adv. Funct. Mater. 2011, 21, 897. (e) Jia, H. P.;
̈
Ding, J.; Ran, Y. F.; Liu, S. X.; Blum, C.; Petkova, I.; Hauser, A.;
Decurtins, S. Chem Asian J. 2011, 6, 3312. (f) Zhang, J.; Wu, G.; He,
C.; Deng, D.; Li, Y. J. Mater. Chem. 2011, 21, 3768. (g) El-Khouly, M.
E.; Jaggi, M.; Schmid, B.; Blum, C.; Liu, S.-X.; Decurtins, S.; Ohkubo,
K.; Fukuzumi, S. J. Phys. Chem. C 2011, 115, 8325.
AUTHOR INFORMATION
Corresponding Author
*E-mail: enrique.orti@uv.es, nazmar@quim.ucm.es.
■
(6) (a) Roncali, J.; Leriche, P.; Cravino, A. Adv. Funct. Mater. 2007,
19, 2045. (b) Pappenfus, T. M.; Schneiderman, D. K.; Casado, J.;
Lopez Navarrete, J. T.; Ruiz Delgado, M. C.; Zotti, G.; Vercelli, B.;
́
Lovander, M. D.; Hinkle, L. M.; Bohnsack, J. N.; Mann, K. R. Chem.
Mater. 2011, 23, 823.
(7) (a) Kato, S.-I.; Diederich, F. Chem. Commun. 2010, 46, 1994.
(b) Yamada, M.; Rivera-Fuentes, P.; Schweizer, W. B.; Diederich, F.
Angew. Chem., Int. Ed. 2010, 49, 3532. (c) Jordan, M.; Kivala, M.;
Boudon, C.; Gisselbrecht, J.-P.; Schweizer, W. B.; Seiler, P.; Diederich,
F. Chem. Asian J. 2011, 6, 396. (d) Breiten, B.; Wu, Y.-L.; Jarowski, P.
D.; Gisselbrecht, J.-P.; Boudon, C.; Griesser, M.; Onitsch, C.;
Gescheidt, G.; Schweizer, W. B.; Langer, N.; Lennartz, C.;
Diederich, F. Chem. Sci 2011, 2, 88.
Present Address
∥Institut des Sciences Chimiques de Rennes, UMR 6226
́
CNRS, Universite de Rennes 1, Campus de Beaulieu, 35042
Rennes Cedex, France.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by MINECO of Spain (CTQ2011-24652,
CT2009-08790, PIB2010JP-00196, 2010C-07-25200, and Con-
solider-Ingenio CSD2007-00010), FUNMOLS (FP7-212942-
1), Generalitat Valenciana (PROMETEO/2012/053), and
CAM (MADRISOLAR-2 S2009/PPQ-1533) is acknowledged.
R.G. thanks the MECD for a FPU studentship. J.L.D.
(8) (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev. 2004,
104, 4891. (b) Martín, N.; San
Guldi, D. M. Acc. Chem. Res. 2007, 40, 1015. (c) Brunetti, F. G.;
Lopez, J. L.; Atienza, C.; Martín, N. J. Mater. Chem. 2012, 22, 4188.
́
chez, L.; Herranz, M. A.; Illescas, B.;
́
(d) Takano, Y.; Herranz, M. A.; Martín, N.; Gayathri Radhakrishnan,
S.; Guldi, D. M.; Tsuchiya, T.; Nagase, S.; Akasaka, T. J. Am. Chem.
Soc. 2010, 132, 8048. (e) Santos, J.; Illescas, B. M.; Martín, N.; Adrio,
́
acknowledges the MINECO for a Ramon y Cajal fellowship
cofinanced by the European Social Fund.
J.; Carretero, J. C.; Viruela, R.; Ortí, E.; Spanig, F.; Guldi, D. M.
̈
Chem.Eur. J. 2011, 17, 2957.
(9) (a) Guldi, D. M.; Illescas, B. M.; Atienza, C. M.; Wielopolski, M.;
Martín, N. Chem.Soc. Rev. 2009, 38, 1587. (b) Molina-Ontoria, A.;
Wielopolski, M.; Gebhardt, J.; Gouloumis, A.; Clark, T.; Guldi, D. M.;
Martín, N. J. Am. Chem. Soc. 2011, 133, 2370. (c) Wielopolski, M.;
Santos, J.; Illescas, B. M.; Ortiz, A.; Insuasty, B.; Bauer, T.; Clark, T.;
Guldi, D. M.; Martín, N. Energy Environ. Sci. 2011, 4, 765.
(10) Otero, M.; Herranz, M. A.; Seoane, C.; Martín, N.; Garín, J.;
DEDICATION
■
Dedicated to the memory of our colleague Christian Claessens.
REFERENCES
■
(1) (a) Pron, A.; Gawrys, P.; Zagorska, M.; Djurado, D.; Demadrille,
R. Chem. Soc. Rev. 2010, 39, 2577. (b) Koos, C.; Vorreau, P.; Vallaitis,
T.; Dumon, P.; Bogaerts, W.; Baets, R.; Esembeson, B.; Biaggio, I.;
Michinobu, T.; Diederich, F.; Freude, W.; Leuthold, J. Nat. Photonics
2009, 3, 216. (c) Sumalekshmy, S.; Henary, M. M.; Siegel, N.; Lawson,
Orduna, J.; Alcala,
(11) (a) Perez, E. M.; Martín, N. Chem. Soc. Rev. 2008, 37, 1512.
(b) Huerta, E.; Isla, H.; Perez, E. M.; Bo, C.; Martín, N.; de Mendoza,
J. J. Am. Chem. Soc. 2010, 132, 5351. (c) Isla, H.; Gallego, M.; Perez, E.
́
R.; Villacampa, B. Tetrahedron 2002, 58, 7463.
́
́
P. V.; Wu, Y.; Schmidt, K.; Bred
Am. Chem. Soc. 2007, 129, 11888.
́
as, J.-L.; Perry, J. W.; Fahrni, C. J. J.
́
M.; Viruela, R.; Ortí, E.; Martín, N. J. Am. Chem. Soc. 2010, 132, 1772.
10716
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717

The Journal of Organic Chemistry
Article
(d) Grimm, B.; Santos, J.; Illescas, B. M.; Munoz, A.; Guldi, D. M.;
Martín., N. J. Am. Chem. Soc. 2010, 132, 17387. (e) Canevet, D.;
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09;
Gaussian, Inc.: Wallingford, CT, 2009.
̃
Gallego, M.; Isla, H.; de Juan, A.; Per
Soc. 2011, 133, 3184.
́
ez, E. M.; Martín, N. J. Am. Chem.
(12) Wenger, S.; Bouit, P.-A.; Chen, Q.; Teuscher, J.; Di Censo, D.;
Humphry-Baker, R.; Moser, J.-E.; Delgado, J. L.; Martín, N.;
Zakeeruddin, S. M.; Gratzel, M. J. Am. Chem. Soc. 2010, 132, 5164.
̈
(29) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(13) Andreu, R.; Galan
Lop
ez Navarrete, J. T.; Casado, J.; Garín, J. Chem.Eur. J. 2011, 17,
826.
́
, E.; Orduna, J.; Villacampa, B.; Alicante, R.;
́
(30) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; Defrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(31) (a) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027.
(b) Cramer, C. S.; Truhlar, D. G. Solvent Effects and Chemical
Reactivity; Kluwer: Dordrecht, 1996; pp 1−80.
(32) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55,
117. (b) Miertus, S.; Tomasi, J. J. Chem. Phys. 1982, 65, 239. (c) Cossi,
M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996, 255,
327. (d) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107,
3032. (e) Barone, V.; Cossi, M.; Tomasi, J. J. Comput. Chem. 1998, 19,
404. (f) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys.
2002, 117, 43.
(14) (a) Perepichka, D. F.; Bryce, M. R.; Perepichka, I. F.; Lyubchik,
S. B.; Christensen, C. A.; Godbert, N.; Batsanov, A. S.; Levillain, E.;
McInnes, E. J. L.; Zhao, J. P. J. Am. Chem. Soc. 2002, 124, 14227.
(b) Christensen, C. A.; Batsanov, A. S.; Bryce, M. R. J. Am. Chem. Soc.
2006, 128, 10484.
́
(15) (a) Martín, N.; Sanchez, L.; Seoane, C.; Ortí, E.; Viruela, P. M.;
Viruela, R. J. Org. Chem. 1998, 63, 1268. (b) Díaz, M. C.; Illescas, B.
M.; Martín, N.; Viruela, R.; Viruela, P. M.; Ortí, E.; Brede, O.;
Zilbermann, I.; Guldi, D. M. Chem.Eur. J. 2004, 10, 2067. (c) Bouit,
́
P.-A.; Villegas, C.; Delgado, J. L.; Viruela, P. M.; Pou-AmeRigo, R.;
Ortí, E.; Martín, N. Org. Lett. 2011, 13, 604.
(33) http://www.chemcraftprog.com.
(16) (a) Kato, S.-I.; Kivala, M.; Schweizer, W. B.; Boudon, C.;
Gisselbrecht, J.-P.; Diederich, F. Chem.Eur. J. 2009, 15, 8687.
(b) Breiten, B.; Jordan, M.; Taura, D.; Zalibera, M.; Griesser, M.;
Confortin, D.; Boudon, C.; Gisselbrecht, J.-P.; Schweizer, W. B.;
Gescheidt, G.; Diederich, F. J. Org. Chem. 2012, DOI: 10.1021/
jo301194y.
(34) (a) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J.
Chem. Phys. 1998, 108, 4439. (b) Jamorski, C.; Casida, M. E.; Salahub,
D. R. J. Chem. Phys. 1996, 104, 5134. (c) Petersilka, M.; Grossmann,
U. J.; Gross, E. K. U. Phys. Rev. Lett. 1996, 76, 1212.
(35) (a) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158.
(b) Ernzerhof, M.; Scuseria, G. E. J. Chem. Phys. 1999, 110, 5029.
(36) (a) Wiggins, P.; Williams, J. A. G.; Tozer, D. J. J. Chem. Phys.
2009, 131, 091101. (b) Peach, P.; Benfield, M. J. G.; Helgaker, T.;
Tozer, D. J. J. Chem. Phys. 2008, 128, 044118.
(17) Fesser, P.; Iacovita, C.; Wackerlin, C.; Vijayaraghavan, S.; Ballav,
̈
N.; Howes, K.; Gisselbrecht, J.-P.; Crobu, M.; Boudon, C.; Stohr, M.;
̈
Jung, T. A.; Diederich, F. Chem.Eur. J. 2011, 17, 5246.
(18) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. Rev. 2007,
107, 1233.
(37) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393,
51.
(19) Díaz, M. C.; Illescas, B. M.; Seoane, C.; Martín, N. J. Org. Chem.
2004, 69, 4492.
(20) Illescas, B. M.; Santos, J.; Díaz, M. C.; Martín, N.; Atienza, C.
M.; Guldi, D. M. Eur. J. Org. Chem. 2007, 5027.
(21) For an exhaustive investigation about consecutive [2 + 2]
cycloaddition−cycloreversion reactions in activated internal acetylenes,
see: Silvestri, F.; Jordan, M.; Howes, K.; Kivala, M.; Rivera-Fuentes, P.;
Boudon, C.; Gisselbrecht, J.-P.; Schweizer, W. B.; Seiler, P.; Chiu, M.;
Diederich, F. Chem.Eur. J. 2011, 17, 6088.
(22) Jarowski, P. D.; Wu, Y.-L.; Boudon, C.; Gisselbrecht, J.-P.;
Gross, M.; Schweizer, W. D.; Diederich, F. Org. Biomol. Chem. 2009, 7,
1312.
(23) (a) Bryce, M. R.; Moore, A. J.; Hasan, M.; Ashwell, G. J.; Fraser,
A. T.; Clegg, W.; Hursthouse, M. B.; Karaulov, A. I. Angew. Chem., Int.
Ed. Engl. 1990, 29, 1450. (b) Jones, A. E.; Christensen, C. A.;
Perepichka, D. F.; Batsanov, A. S.; Beeby, A.; Low, P. J.; Bryce, M. R.;
Parker, A. W. Chem.Eur. J. 2001, 7, 973. (c) Christensen, C. A.;
Batsanov, A. S.; Bryce, M. R.; Howard, J. A. K. J. Org. Chem. 2001, 66,
3313.
(24) (a) Perez, I.; Liu, S.-G.; Martín, N.; Echegoyen, L. J. Org. Chem.
́
2000, 65, 3796. (b) Herranz, M. A.; Yu, L.; Martín, N.; Echegoyen, L.
J. Org. Chem. 2003, 68, 8379.
(25) Gautier, N.; Dumur, F.; Lloveras, V.; Vidal-Gancedo, J.; Veciana,
J.; Rovira, C.; Hudhomme, P. Angew. Chem., Int. Ed. 2003, 42, 2765.
(26) Díaz, M. C.; Illescas, B. M.; Martín, N.; Perepichka, I. F.; Bryce,
M. R.; Levillain, E.; Viruela, R.; Ortí, E. Chem.Eur. J. 2006, 12, 2709.
́
(27) Guldi, D. M.; Sanchez, L.; Martín, N. J. Phys. Chem. B 2001, 105,
7139.
(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
10717
dx.doi.org/10.1021/jo302047m | J. Org. Chem. 2012, 77, 10707−10717