Expanding the chemical structure space of opto-electronic molecular materials: Unprecedented push-pull chromophores by reaction of a donor-substituted tetracyanofulvene with electron-rich alkynes

Article abstract of DOI:10.1021/ja312084s

The reaction of a 3,5-bis(N,N-dimethylanilino)-substituted 2,4,6,6-tetracyanopentafulvene (TCPF) with mono- and bis(N,N-dimethylanilino) acetylene provides facile access to push-pull chromophores with diverse new scaffolds. The starting TCPF reacts with b

Full text of DOI:10.1021/ja312084s

Article
pubs.acs.org/JACS
Expanding the Chemical Structure Space of Opto-Electronic
Molecular Materials: Unprecedented Push−Pull Chromophores by
Reaction of a Donor-Substituted Tetracyanofulvene with Electron-
Rich Alkynes
Govindasamy Jayamurugan,† Oliver Dumele,† Jean-Paul Gisselbrecht,‡ Corinne Boudon,‡
W. Bernd Schweizer,† Bruno Bernet,† and Francois Diederich*,†
̧
†Laboratorium fur Organische Chemie, ETH Zurich, Honggerberg, HCI 8093 Zurich, Switzerland
̈
̈
‡Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Institut de Chimie-UMR 7177, CNRS, Universite
́
de
Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
S
* Supporting Information
ABSTRACT: The reaction of a 3,5-bis(N,N-dimethylanilino)-
substituted 2,4,6,6-tetracyanopentafulvene (TCPF) with
mono- and bis(N,N-dimethylanilino)acetylene provides facile
access to push−pull chromophores with diverse new scaffolds.
The starting TCPF reacts with bis(N,N-dimethylanilino)-
acetylene in a formal [2+2] cycloaddition at the exocyclic
double bond, followed by retroelectrocyclization, to yield an
ethenylene-extended push−pull pentafulvene. The trans-
formation with 4-ethynyl-N,N-dimethylaniline also yields a
similar extended pentafulvene as well as two other products
that required X-ray analysis for their structure elucidation. One
features an 8,8-dicyanoheptafulvene core formed by formal [2+2] cycloaddition, followed by ring opening via fragmentation. The
second is a chiral cyclobutenylated tetrahydropentalene, resulting from a cascade of formal [6+2] and [2+2] cycloadditions. All
new nonplanar push−pull chromophores display amphoteric redox behavior with both strong electron-donating and -accepting
potency. Notably, the N,N-dimethylanilino-substituted extended pentafulvenes show remarkably low oxidation potentials (0.27/
0.28 V vs Fc/Fc+ reference) that are lower than those for N,N-dimethylaniline itself. The push−pull-substituted extended
pentafulvenes feature intense electronic absorption bands, extending over the entire visible spectral range into the near infrared,
and low highest occupied molecular orbital−lowest unoccupied molecular orbital gaps. These properties, together with high
thermal stability and good solubility, suggest the potential use of the new chromophores as advanced materials in molecular
electronics devices.
were exploited as n-type additives in solar cells,8 whereas the
electronic properties of perphenylated and peralkynylated
derivatives were investigated by electronic paramagnetic
resonance spectroscopy of their radical anions.9
INTRODUCTION
■
The performance of organic opto-electronic materials1,2 relies
on the ability to efficiently transfer electrons from a high-lying
highest occupied molecular orbital (HOMO, an electron
“donor”) to a low-lying lowest unoccupied molecular orbital
(LUMO, an electron “acceptor”). A low HOMO−LUMO gap
is characteristic of conjugated π-chromophores substituted with
strong electron donors (D) and acceptors (A), and such
“push−pull” systems have been extensively investigated.3
Additional interest in D−A chromophores arises from their
electrochemically amphoteric behavior leading to a range of
readily accessible redox states.4
Fulvenes make up an interesting class of non-benzenoid
chromophores5 that in recent years have been decorated with
donor and acceptor functionalities to tune the HOMO−
LUMO gap.6 Substituted 6,6-dicyanopentafulvenes and 8,8-
dicyanoheptafulvenes were found to show promising optical
nonlinearities.7 Recently, phenylated 6,6-dicyanopentafulvenes
The formal [2+2] cycloaddition (CA) of donor-substituted
alkynes to electron-deficient olefins, followed by retroelec-
trocyclization (RE) (CA−RE reaction), is a highly efficient,
broadly applicable method3e,g,10 for accessing stable, soluble,
nonplanar push−pull chromophores with intense intramolec-
ular charge-transfer absorptions (ICT) and high third-order
optical nonlinearities. Some of these compounds have already
found application in opto-electronic devices.11 During inves-
tigations aimed at further extending the scope of this
transformation, we serendipitously prepared 3,5-bis[(4-
dimethylamino)phenyl]-substituted 2,4,6,6-tetracyanopentaful-
Received: December 11, 2012
© XXXX American Chemical Society
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Scheme 1. A Possible Mechanism for the (Acidic) SiO2/MeCN-Promoted Rearrangement of Pentacyanohexa-1,3,5-triene 2 via
a
TCPF 1 to Tetracyanobenzene 3
a
The colors correspond to those of the solutions.
vene (TCPF) 1 (Scheme 1), a stable, high-melting solid
featuring an intense, bathochromically shifted ICT band (λmax
pentacyanohexatriene 2 changed upon treatment with
moisturized SiO2 in CH2Cl2 to a maroon-colored solution
after 45 min. Filtration after 4 h with a 2:1 CH2Cl2/Me2CO
mixture generated a purple filtrate, which adopted the green
color of TCPF 1 upon dilution with CH2Cl2 and standing for
∼5 h. This suggests an intermediate purple-colored complex B
(Scheme 1), an addition product to acetone. In contrast, a
brownish-maroon filtrate was obtained when the suspension
was filtered with MeCN, and after 10 min, the red color
indicated a complete transformation to 3. These observations
were confirmed by LC−MS, showing full consumption of 2 and
formation of either 1 or 3 (Figure 1SI of the Supporting
Information).
TCPF 1 is stable in a CH2Cl2 solution but is completely
transformed to hexasubstituted benzene 3 under the conditions
for the preparation of 3 from 2, suggesting 1 is a direct
precursor of 3 (Scheme 1). A zwitterionic resonance structure
of 1, due to D−A conjugation, allows the formation of adducts
to polar solvents containing electrophilic centers, such as B and
C with acetone and acetonitrile, respectively. Similarly, 1 may
be reversibly protonated at C(6) (intermediate A) by the acidic
wet SiO2. Adducts A and B revert to 1 upon dilution with
CH2Cl2. MeCN adduct C, on the other hand, undergoes an
irreversible 1,2-CN migration from C(6) to C(1) generating
intermediate D, a carbocation stabilized by two π-substituents.
Subsequent ring expansion of intermediate D to E, followed by
elimination of MeCN, leads to hexasubstituted benzene 3.
Interestingly, Shoji et al. recently reported the isolation of a 3,5-
bis-azulenyl-substituted 2,4,6,6-tetracyanopentafulvene by SiO2
chromatography with a CH2Cl2/EtOAc mixture as the eluent.13
They did not report a 1,2-CN rearrangement, probably because
of the weaker electron-donating ability of the azulenyl versus
the DMA substituents in 1. To the best of our knowledge, such
=
782 nm; ε = 27500 M−1 cm−1) expanding from 600 to ∼1100
nm in CH2Cl2.12 We subsequently became interested in
exploring the reactivity of TCPF 1 in the CA−RE reaction
and report here its transformation with activated, N,N-
dimethylanilino (DMA)-substituted alkynes. These reactions
led to the formation of a variety of unusual ICT chromophores,
thereby expanding the chemical structure space for push−pull
systems featuring opto-electronic properties of interest for
potential device applications, such as in organic solar cells.8b
RESULTS AND DISCUSSION
■
Preparation of TCPF 1 and Its SiO2-Promoted
Rearrangement. The interesting opto-electronic properties
of 1 mandated the development of an improved synthetic
protocol starting from (E/Z)-1,1,3,6,6-pentacyanohexa-1,3,5-
triene 2 (Scheme 1).12 The latter compound is available in one
step by CA−RE reaction of 3-DMA-1,1,2,4,4-pentacyanobuta-
1,3-diene with 4-ethynyl-N,N-dimethylaniline. Treatment of 2
with acidic wet SiO2 [10% (w/w) H2O] in CH2Cl2 for 4 h,
followed by filtration with a 2:1 CH2Cl2/Me2CO mixture and
column chromatography, afforded pure TCPF 1 in 70% yield.
For reproducible high yields of 1, the addition of water to SiO2
is essential, as exposure of 2 to untreated SiO2 yielded
substantial amounts of the red-colored tetracyanobenzene 3
(for X-ray, see Figure 3SI of the Supporting Information).
Independent of the treatment of SiO2 with water, hexasub-
stituted benzene 3 was the exclusive product if MeCN was used
instead of a 2:1 CH2Cl2/Me2CO mixture for the filtration
described above.
We monitored these transformations more carefully, with the
following findings. The dark green-colored solution of
B
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a
Scheme 2. Transformations of TCPF 1 with Electron-Rich Alkynes
a
Reagents and conditions: (i) for details see Table 1; (ii) 1:1 (CH2Cl)2/MeCN, 175 °C, microwave, 1 h, 85%; (iii) 1:1 (CH2Cl)2/MeCN, 175 °C,
microwave, 1 h, no reaction.
a thermal rearrangement of 6,6-dicyanopentafulvenes is
unprecedented. However, in theoretical studies of the thermal
isomerization of unsubstituted pentafulvene to benzene, an
intermediate “prefulvene”, a bicyclo[3.1.0]hex-3-ene 2,6-
diradical, has been postulated.14
Transformations of TCPF 1 with Electron-Rich
Alkynes. The reaction of 1 in CH2Cl2 or MeCN with
electron-rich alkynes was investigated. The transformation with
4-ethynyl-N,N-dimethylaniline (4) gave three new products in
an 88% combined yield: ring expanded heptafulvene 5,
cyclobutene-fused tetrahydropentalene ( )-6, and extended
pentafulvene 7 (Scheme 2). The isolation of all products (see
1, even under forcing conditions (up to 175 °C and microwave
irradiation). Apparently, steric hindrance renders the attack of 8
more difficult and even prevents the reaction of 10 entirely.
Structural Characterization. The structures of 5, ( )-6, 7,
and 9 were evidenced by single-crystal X-ray analysis (Figure
1). Crystals suitable for X-ray analysis were obtained for 5,
( )-6, and 7 by slow diffusion of pentane into CH2Cl2 or
CHCl3 solutions and for 9 by slow diffusion of pentane into a
toluene solution. Single crystals of heptafulvene 5 contain three
distinct molecules in the unit cell (Figure 4SI of the Supporting
Information), differing mainly in the orientation of the DMA
units. The cycloheptatriene ring of 5 adopts a boat-shaped
conformation (a 4,5,1B conformation according to Stoddart’s
notation for carbohydrates15 in which atoms 4, 5, and 1 are
above the reference plane). The double and single bonds in the
heptafulvene ring are fully localized, showing that 5 does not
display aromatic character (for bond lengths, see Figure 4SI of
the Supporting Information).
The structure of ( )-6 shows an enantiomeric pair in the
unit cell. The DMA moiety at the ring junction and the
annulated cyclobutene ring are in exo positions (Figure 1). The
X-ray crystal structure of the extended pentafulvenes 7 and 9
shows Z- and E-configurations, respectively, at the exocyclic
double bond. The Z-configuration of 7 is kept upon
introduction of the DMA substituent at C(7), leading to 9
(note the change in the Cahn−Ingold−Prelog sequence upon
introduction of the additional DMA substituent of 9). The E-
diastereoisomer of 7 is disfavored because of allylic A(1,3)
strain.16 Because 9 exists as an E/Z mixture in solution (see
below), 10 different crystals of 9, obtained from the same batch,
were analyzed by X-ray diffraction. All single crystals analyzed
had the same unit cell dimensions, suggesting that only the E-
isomer had crystallized. Furthermore, the 1H NMR spectrum of
crystals of 9 recorded at −20 °C in CD2Cl2 showed exclusively
signals of the E-isomer (Figure 18SI of the Supporting
Information). When the sample was warmed to 20 °C, signals
for a 3:1 E/Z mixture were observed (Figure 18SI of the
Supporting Information), indicating that E/Z-isomerization in
solution is facile at room temperature.
Table 1
a
ratio (%)
conditions for step i of
Scheme 2
4
entry
(equiv)
5
( )-6
32
26 (23) 41 (36) 33 (29)
24 19 57
7
1
2
3
CH2Cl2, 20 °C, 60 h
CH2Cl2, 20 °C, 24 h
CH3CN, 20 °C, 60 h
1
2
1
34
34
a
The ratios were determined by both LC−MS and 1H NMR
spectroscopy. Isolated yields are given in parentheses.
entry 2 in Scheme 2) by column chromatography on SiO2 was
not possible because of the instability of ( )-6 to SiO2. This
was overcome by using recycling preparative gel permeation
chromatography, which allowed the separation of ( )-6 (36%)
from the mixture of 5 and 7. Subsequent column chromatog-
raphy on SiO2 gave 5 (23%) and 7 (29%). The product
distribution of this reaction depended moderately on solvent
and stoichiometry. In MeCN, 7 was the preferred product
(57%), whereas in less polar CH2Cl2 and with an equimolar
amount of 4, all three products were obtained in similar
amounts. Doubling the amount of 4 favored slightly the
formation of ( )-6 (41%). Heating [e.g., for 5 min at 160 °C in
MeCN (see Figure 2SI of the Supporting Information)]
reduced the reaction time drastically but led to the formation of
additional minor side products.
Intramolecular D−A interactions, that is, the interaction
between the π-conjugated DMA donor and the cyanated
pentafulvene core, enhance the quinoid character (δr) of the
DMA group (for a comprehensive summary of all δr values, see
Table 1SI of the Supporting Information).17,18 A high δr
The reaction of 1 with bis-DMA-acetylene 8 required much
harsher conditions (175 °C for 1 h in a sealed tube) and gave
selectively the diastereoisomeric mixture 9 in 85% yield
(Scheme 2), in a 1:0, 1:1, or 0:1 (CH2Cl)2/MeCN mixture.
The tert-butylated DMA-acetylene 10 did not react with TCPF
C
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Figure 1. ORTEP plots of 5 (conformer A), ( )-6, Z-7, and E-9 (arbitrary numbering; H atoms omitted for clarity). Atomic displacement parameter
ellipsoids shown at the 50% probability level. T = 100 K. For details of the X-ray analyses, including selected bond lengths and angles, see Figures
4SI−7SI of the Supporting Information. Different DMA moieties in a molecule are marked as I−IV. The fulvene numbering is used for 5, 7, and 9 in
the theoretical part; however, the numbering in the experimental part follows the systematic IUPAC nomenclature (as proposed by the ACD
software). For the definition of the quinoid character (δr) of the DMA groups, see ref 17 and the text; θ is the torsional angle between the DMA
planes and the acceptor moieties.
correlates with a low torsional angle θ between the π-planes of
donor and acceptor moieties, as found for the DMA group at
C(3) of 1 (δrI = 0.065; θI = −13.6°).12 For example, DMA
group III (Figure 1) at C(6) in E-9 (δrIII = 0.053; θIII = 26.4°)
has a better conjugation with the pentafulvene core than DMA
group I at C(3) (δrI = 0.022; θ = −39.0°). Similar to 1, DMA
group II at C(5) of Z-7 and E-9 has a low quinoid character
(for Z-7, δrII = 0.028 and θII = 41.1°; for E-9, δrII = 0.013 and
θII = 59.6°). Further proof of the efficient D−A interactions in
both Z-7 and E-9 comes from the lengthening of the exocyclic
double bond on the cyclopentadienyl core, found to be 1.41
and 1.40 Å, respectively (Figure 1), compared to the length of
1.35 Å for a 6-aryl-substituted 2,3,4,5-tetrachloropentafulvene.19
The high quinoidal character of DMA group III at C(6) and the
reduced double bond character of the exocyclic C(1)−C(6)
bond together indicate a facile E/Z isomerization around this
bond.
Proposed Mechanism for the Formation of the New
Chromophores. The formation of 5 and ( )-6 most probably
proceeds first through attack at the C(5) position of 1 by the
terminal C atom of the acetylene (pathway A, Figure 2),
generating a stabilized zwitterion.20 The latter may complete a
formal [2+2] CA to yield a bicyclo[3.2.0]heptadiene derivative.
Its cis-oriented DMA and cyano groups at the ring junctions are
ideally located for ring opening via fragmentation and following
isomerization afford the donor-substituted tetracyanoheptaful-
vene 5. Alternatively, the zwitterion may react at C(6) to
complete a formal [6+2] CA, generating the racemic
dihydropentalene 11. The more accessible of the two
cyanovinyl units in the bicyclic core in 11 subsequently
undergoes a second formal [2+2] CA with excess 4.21 The
resulting cyclobutene ( )-6 cannot undergo a RE reaction,
because a thermally allowed conrotatory ring opening would
generate a highly strained cis,trans,cis- or a trans,cis,cis-
cycloheptatriene.22−24 Indeed, a solution of ( )-6 in
(CDCl2)2 proved to be stable at 120 °C for 2 days. Pathway
B corresponds to the expected regioselective CA−RE reaction,
involving initial attack at C(6) under formation of an aromatic
cyclopentadienyl anion-containing zwitterionic intermediate,9
and leads to the ethenylene-extended pentafulvene 7.
The reactivity profile of TCPF 1 is unique as it gives
cycloaddition products at the endo- and exocyclic double bonds
of the fulvene core that have not been seen with previously
reported pentafulvenes.9,25 For instance, the ring expansion of
push−pull pentafulvene derivatives via [2+2] CA, followed by
fragmentation, has been observed only with electron-deficient
alkynes, such as dimethyl acetylenedicarboxylate, with the initial
attack presumably occurring at C(2) of the endocyclic C(2)
C(3) bond.25 6-Aminopentafulvenes26 and 5,6-dihydropenta-
lenes27 undergo intermolecular [6+2] cycloaddition, whereas
no such addition is known for 6,6-dicyanopentafulvenes.
Pentafulvenes bearing a terminal unsaturated substituent at
C(6) undergo intramolecular [6+2] cycloaddition to afford
tricyclic products.28
E/Z Isomerization of 7 and 9. The E/Z isomerization of 7
and 9 in solution was investigated in detail by one- and two-
dimensional (DQF-COSY, HSQC, and HMBC) NMR spec-
troscopy (section 6SI of the Supporting Information),
complemented by density functional theory (DFT) calculations
(section 7SI of the Supporting Information). A full account of
D
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Figure 2. Possible pathways for the regioselective formal [2+2] and [6+2] cycloaddition reactions of TCPF 1 with DMA-acetylene 4.
Figure 3. Electronic absorption spectra in CH2Cl2 at 293 K. (a) Hexasubstituted benzene 3, heptafulvene 5, and cyclobutene-annulated pentalene
( )-6. (b) Pentafulvenes 1, 7, and E/Z-9 and heptafulvene 5. Solutions of 7 and E/Z-9 were equilibrated for 24 h at 293 K prior to measurement.
this elaborate analysis can be found in the Supporting
here. In agreement with the solid state X-ray analysis, the joint
Information, and the most important findings are summarized
experimental−computational analysis revealed that compound
E
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Table 2. Electrochemical and Optical Properties of Compounds 1,12 3, 5, ( )-6, 7, and E/Z-9 Observed by Cyclic Voltammetry
cyclic voltammetry
optical
a
b
c
compd
E° (V)
ΔEp (mV)
Δ(Eox,1 − Ered,1) (V)
EA (eV)
λmax (nm) (log ε)
λend (nm) [E (eV)]
d
e
1
0.90
1.12
4.35
782 (4.4)
1100 (1.13)
0.67 (1e−)
−0.45 (1e−)
60
60
e
−0.96
0.76
e
3
5
2.08
1.78
3.48
3.47
440 (4.2)
490 (4.5)
600 (2.07)
675 (1.84)
−1.32 (1e−)
70
e
−2.21 (1e−)
ef
,
0.72
0.60 (1e−)
−1.18 (1e−)
−1.30 (1e−)
60
80
80
e
( )-6
0.82
2.88
0.94
1.16
2.46
4.13
3.92
465 (3.5)
765 (4.2)
710 (4.4)
550 (2.26)
1150 (1.07)
1000 (1.24)
e
0.64
e
0.54
e
−2.34
e
7
0.85 (1e−)
0.27 (2e−)
45
75
−0.67 (1e−)
−0.89 (1e−)
100
e
E/Z-9
0.81
0.28 (2e−)
−0.88 (1e−)
−1.05 (1e−)
70
60
80
a
E° = (Epc + Epa)/2, where Epc and Epa correspond to the cathodic and anodic peak potentials, respectively. Values measured by cyclic voltammetry
in a CH2Cl2 solution (with 0.1 M Bu4NPF6), where v = 0.1 V/s. Values reported vs Fc/Fc+. ΔEp = Epa − Epc. EA = Ered (V vs Fc+/Fc) + 4.8 eV.29
b
c
d
e
f
Taken from ref 12. Ep is the irreversible peak potential. A redissolution peak was observed on the reverse scan at 0.55 V.
7 is almost exclusively present (>98:2) in the Z-configuration in
CD2Cl2 and in (CDCl2)2 in the temperature range from −50 to
100 °C. On the other hand, compound 9 exists at room
temperature as a mixture of E- and Z-diastereoisomers with an
E:Z ratio of 3:1 in CD2Cl2 and 1:1 in CD3CN. This is in
contrast to the solid-state analysis, which provided X-ray
crystals of only the E-isomer of 9. As stated previously, crystals
of E-9 dissolved at −20 °C showed only 1H NMR signals of the
E-isomer; increasing the temperature to 20 °C led to a partial
isomerization (Figure 18SI of the Supporting Information).
The described NMR analysis allowed us to unambiguously
assign the resonances and attachment points of all three DMA
groups in Z-7, and of all four DMA groups in E- and Z-9.
Electronic Absorption Spectroscopy. Tetracyanoben-
zene 3, heptafulvene 5, and the extended pentafulvenes 7 and
E/Z-9 are dark metallic-shiny solids that are stable at ambient
temperature under air and soluble in common organic solvents
such as CH2Cl2, toluene, acetone, and acetonitrile. The
electronic absorption spectra (CH2Cl2) of the penta- and
heptafulvenes feature intense, bathochromically shifted ICT
transitions, as shown in Figure 3.
The ICT band of heptafulvene 5 (Figure 3a) is intense but
substantially hypsochromically shifted [λmax = 490 nm (2.53
eV); ε = 29000 M−1 cm−1] compared to the corresponding
bands in the spectra of the pentafulvene derivatives. We ascribe
this hypsochromic shift largely to the reduced planarity of the
central core in the 8,8-dicyanoheptafulvene, which reduces its
electron accepting ability. The spectra of the extended
pentafulvenes are quite exciting, as they show broad, intense
ICT bands spanning from ∼500 nm over nearly the entire
visible spectral region into the near infrared (NIR). Extended
pentafulvene 7 features two ICT transitions at λmax = 680 nm
(1.82 eV; ε = 15000 M−1 cm−1) and 760 nm (1.63 eV; ε =
15200 M−1 cm−1), whereas the bands of E/Z-9 appear at λmax
=
600 nm (2.07 eV; ε = 20000 M−1 cm−1) and 700 nm (1.77 eV;
ε = 20000 M−1 cm−1), respectively. In contrast, TCPF 1
features a single, more narrow transition (1.59 eV; λmax = 782
nm, and ε = 27500 M−1 cm−1).12 The appearance of a second
ICT band in the spectra of 7 and E/Z-9 reflects the presence of
the additional DMA donors at C(6) that can undergo push−
pull interactions with the electron-accepting, 2,4-dicyanopenta-
fulvene core.
Acid−base titration with CF3COOH and Et3N gave further
insight into the nature of the ICT interactions in the new
compounds (see Figures 30SI−34SI of the Supporting
Information). Treatment of the solution of tetracyanobenzene
3 with CF3COOH led to a change in color from red to
colorless and a complete disappearance of the intense CT band
at 440 nm. Similarly, treatment of heptafulvene 5 with
CF3COOH led to a change in color from maroonish red to
colorless and again a complete disappearance of the intense CT
band at 490 nm. Neutralization with Et3N regenerated the
original spectra of 3 and 5 completely, with a slight decrease in
the absorbance due to dilution. When a solution of 7 or E/Z-9
was acidified with CF3COOH, the color changed from bluish
green to green and the bands at longer wavelengths (for 7, λmax
= 760 nm; for E/Z-9, λmax = 700 nm) remained intact, whereas
the bands at shorter wavelengths (for 7, λmax = 680 nm; for E/
Z-9, λmax = 600 nm) completely disappeared. Neutralization
with Et3N regenerated the original absorption profile almost
quantitatively as well as the color of the solution. Protonation
in 7 and E/Z-9 occurs at the most basic DMA moiety, which
presumably is attached to the exocyclic allylidene moiety, and
therefore, the ICT band involving these donor groups
F
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disappears in the presence of acid. This leaves a spectral trace
(Figures 33SI and 34SI of the Supporting Information) that
resembles much more the spectrum of TCPF 1 that lacks these
additional DMA moieties. We therefore conclude that the
lower-energy ICT band in 7 and E/Z-9 arises from the transfer
of charge from DMA moieties on the cyclopentadienyl ring to
the electron-accepting extended pentafulvene core, whereas the
higher-energy ICT band originates from charge transfer
involving the DMA donor at C(6) on the exocyclic allylidene
fragment.
Electrochemistry. The redox properties of the new
chromophores 3, 5, ( )-6, 7, and E/Z-9 were investigated by
cyclic voltammetry (CV) and rotating-disk voltammetry (RDV)
in CH2Cl2 (with 0.1 M Bu4NPF6, all potentials vs Fc+/Fc). The
CV data together with a comparison of optical and electro-
chemical HOMO−LUMO gaps are listed shown in Table 2,
whereas Table 7SI of the Supporting Information includes the
RDV data and Figure 36SI of the Supporting Information the
raw CV traces.
Bis-1,4-DMA-substituted tetracyanobenzene 3 shows a single
reversible one-electron reduction at −1.32 V, an irreversible
one-electron reduction at −2.21 V, and an irreversible oxidation
at 0.76 V. A comparison of the redox properties of 8,8-
dicyanoheptafulvene 5 and 6,6-dicyanopentafulvene 1 demon-
strates the inherent drastic changes in the electronic
configurations possible for fulvene-based materials when the
ring size is modified. Both pentafulvene 1 (Ered,1 = −0.45 V)12
and heptafulvene 5 (Ered,1 = −1.18 V) display two reversible
one-electron reductions. However, the electron affinity of 5 is
much lower than that of 1 (ΔEred,1 = 0.73 V). DFT calculations
suggest that the LUMOs are located on the fulvene moiety of
both 1 and 5 (Figure 29SI of the Supporting Information). The
HOMO−LUMO gap for heptafulvene 5 is quite large (1.78 V)
compared to that of the parent pentafulvene 1 (1.12 V). The
cyclic voltammogram of cyclobutene-fused tetrahydropentalene
( )-6 shows one irreversible reduction step at −2.34 V as well
as several irreversible oxidation steps at 0.54, 0.64, and 0.82 V.
The potential difference between the first oxidation and the first
reduction corresponds to 2.88 V, which is roughly in agreement
with the energetics of the intramolecular CT band of ( )-6
(λend = 550 nm, 2.26 eV).
Electrochemical investigations of the extended pentafulvenes
7 and E/Z-9 give further insight into the electronics of these
systems. The oxidation potentials of 7 and E/Z-9 are quite
similar, with a first unresolved step corresponding to one
reversible two-electron transfer at 0.27 and 0.28 V for 7 and E/
Z-9, respectively, and a second oxidation wave of 7 (0.85 V)
and E/Z-9 (0.81 V). The low first oxidation potential is quite
remarkable. N,N-Dimethylaniline itself shows an irreversible
oxidation potential at 0.44 V (in CH2Cl2, Fc+/Fc reference).30
In contrast, DMA moieties in D−A systems participating in
efficient ICT interactions show poor electron donating ability
and high oxidation potentials (Eox ≈ 0.7−1.1 V).10 The first
two-electron oxidation step in 7 and E/Z-9 is preferred
compared to the oxidation of pure N,N-dimethylaniline,
whereas the second oxidation wave is closer to the oxidation
potentials measured for the two DMA groups in 1 and other
push−pull systems (Table 2). A DFT computational study
[B3LYP/6-31(d) level of theory] of Z-7, Z-7+, and Z-72+ and E-
9, E-9+, and E-92+ proposes a solution for the remarkably facile
first two-electron oxidation of 7 and 9. According to the
calculations, the five-membered ring in the dication features
two separate π-systems in a nearly orthogonal conformation to
each other, each presumably bearing one positive charge
(Figures 25SI−28SI of the Supporting Information). One is an
allylic cation stabilized by the two DMA residues I and II on the
ring. The other is the exocyclic allylidene with the DMA
residues at position C(6) (DMA III in 7) and positions C(6)
and C(7) (DMA III and IV in E/Z-9). Such a finding obviously
requires much further experimental and computational
research, beyond this study.
The reduction potentials for both 7 and E/Z-9 occur in two
well-defined, reversible, one-electron reductions at −0.67 and
−0.89 V for 7 and −0.88 and −1.05 V for E/Z-9. The
difference in reduction potentials of 7 and E/Z-9 results from
the presence of one more DMA donor in E/Z-9, which also
induces a twist around the C(6)−C(7) bond of the exocyclic
allylidene moiety. This twisting reduces the level of conjugation
between the dicyanovinyl and the pentafulvene ring, resulting
in a significant cathodic shift of the reduction potentials.
The optical HOMO−LUMO gaps, determined from the λend
of the longest-wavelength absorption band, correlate well (R2 =
0.999) with the electrochemical gaps Δ(Eox,1 − Ered,1),
suggesting that the same orbitals are involved in both the
optical and electrochemical band-gaps for 3, 5, ( )-6, 7, and E/
Z-9 (Table 2). To the best of our knowledge, the small optical
band gap of 7 (0.94 V) is the smallest in pentafulvene-based
scaffolds and is quite remarkable for a small chromophore such
as 7.31 Therefore, it is worthwhile to modify the pentafulvene at
position C(6) to obtain amphoteric redox chromophores with
small HOMO−LUMO gaps.
CONCLUSIONS
■
We report an optimized synthesis of push−pull-substituted
pentafulvene 1 (TCPF), which avoids unexpected SiO2- and
solvent-promoted 1,2-CN shifts leading to 1,3-bis-DMA-
substituted tetracyanobenzene 3. A mechanism for this
unprecedented rearrangement is proposed on the basis of
experimental observations. TCPF 1 reacts in one step with
DMA-acetylene to yield a series of new nonplanar push−pull
chromophores of substantial complexity, featuring heptafulvene
5, cyclobutene-fused tetrahydropentalene ( )-6, and extended
pentafulvene cores 7 and E/Z-9. The elucidation of their
structures required X-ray analysis, but they were also
investigated and confirmed by advanced NMR spectroscopy
in solution. Mechanisms for their formation are proposed.
Interestingly, steric hindrance makes the addition of bis-DMA-
acetylene to TCPF 1 much more selective, and only the
extended pentafulvene E/Z-9, resulting from formal [2+2]
cycloaddition, followed by retroelectrocyclization (CA−RE
reaction), was isolated. While X-ray analysis of a series of
crystals revealed only the E-9 isomer in the solid state, the two
E- and Z-isomers are present in solution to different extents,
which is dependent on solvent polarity. The extended
pentafulvenes 7 and E/Z-9 feature outstanding opto-electronic
properties. The electronic absorption spectra show broad ICT
bands expanding from ∼500 nm into the NIR, with maxima
(λmax) at 680 and 760 nm (7) and 600 and 700 nm (E/Z-9).
Electrochemical investigations revealed exceptionally high
electron donating ability of these chromophores (first oxidation
waves at 0.27/0.28 V), which together with their strong
electron accepting property [first reduction waves at −0.67 V
(7) and −0.88 V (E/Z-9)] affords very small HOMO−LUMO
gaps of 0.94 eV (7) and 1.16 eV (E/Z-9). The study
demonstrates that the functionalization of the exocyclic C(6)
position in push−pull pentafulvenes provides an excellent tool
G
dx.doi.org/10.1021/ja312084s | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
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and the HOMO−LUMO gap of pentafulvene-based molecular
materials. In view of their stability, solubility, and facile
availability, adding to the beneficial opto-electronic properties,
the reported chromophores warrant future exploration for use
as organic electronic materials, such as in organic solar cells.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis and characterization of all new compounds, details of
the DFT calculations, spectroscopic and electrochemical data,
computational details, energies, Cartesian coordinates, and
crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
diederich@org.chem.ethz.ch
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(17) Quinoid character: δr = (d1−2 + d1−6 + d3−4 + d4−5)/4 − (d2−3
+
Notes
d5−6)/2.18 For benzene, δr = 0; for fully quinoid rings, δr = 0.08−0.10
Å (see Figure 1).
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
Mitchell, C.; Schurmann, M. Aust. J. Chem. 2005, 58, 119−127.
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This work was supported by ERC Advanced Grant 246637
(“OPTELOMAC”). O.D. acknowledges the Fonds der
Chemischen Industrie for a Kekule
Marc-Olivier Ebert (ETH Zurich) for helpful discussions
regarding NMR and Dr. Aaron Finke and Dr. Adam Lacy for
reading the manuscript.
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scholarship. We thank Dr.
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