Synthesis, reactivity, and electronic properties of 6,6-dicyanofulvenes

Article abstract of DOI:10.1021/ol102384k

A series of 6,6-dicyanofulvene derivatives are synthesized starting from masked, dimeric, or monomeric cyclopentadienones. The reactivities of 6,6-dicyanofulvenes relative to their parent cyclopentadienones are discussed. 6,6-Dicyanofulvenes are capable o

Full text of DOI:10.1021/ol102384k

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5302-5305
Synthesis, Reactivity, and Electronic
Properties of 6,6-Dicyanofulvenes
Trisha L. Andrew, Jason R. Cox, and Timothy M. Swager*
Massachusetts Institute of Technology, Department of Chemistry, 77 Massachusetts
AVenue, Cambridge, Massachusetts 02139, United States
tswager@mit.edu
Received October 3, 2010
ABSTRACT
A series of 6,6-dicyanofulvene derivatives are synthesized starting from masked, dimeric, or monomeric cyclopentadienones. The reactivities
of 6,6-dicyanofulvenes relative to their parent cyclopentadienones are discussed. 6,6-Dicyanofulvenes are capable of undergoing two consecutive,
reversible, one-electron reductions and are presented as potential n-type small molecules.
During the past decade, intense research efforts have yielded
numerous optimized, air-stable, p-type (hole-transporting)
organic semiconductors with conductivities and hole mobili-
ties that are competitive with inorganic analogues.1 Recently,
several examples of n-type (electron-conducting) organic
materials have been utilized in thin-film organic field-effect
transistors (OFETs) and photovoltaic cells,2 but the electronic
and solid-state packing properties, air stability, and process-
ability of n-type semiconductors are far from being as
optimized as their p-type counterparts.3
The discovery of new molecular building blocks for n-type
materials remains germane, as the diversity in the chemical
structures of proven electron conductors does not match the
large population of known organic hole conductors. To date,
polymeric/oligomeric/small molecule oxadiazoles,4 ben-
zothiadiazoles,5 arylene imides,6 perfluorinated7 or perfluo-
roalkyl-containing arenes,8 fullerenes,9 pyridiniums,10 and
dicyanovinylene-11 or tricyanovinylene-substituted arenes12
have been demonstrated as suitable n-type materials. Notably,
nitrile incorporation is an interesting design strategy, as it
both results in desirable electronic properties (low LUMO
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10.1021/ol102384k  2010 American Chemical Society
Published on Web 10/29/2010

energies)13 and is capable of transforming a p-type organic
materials. In this contribution, we explore the synthesis of
DCFs, their chemical stabilities, and their optical and
electrochemical properties.
system into an electron-accepting material.14
Cyclopentadienones (CPDs) and fulvenes have received little
attention as n-type building blocks and have only been
considered indirectly, as structural subunits in indenofluorenone
and indenofluorene bis(dicyanovinylene) chromophores.11a,b
Wudl et al. reported a donor-acceptor polymer containing
a dithienylcyclopentadienone moiety in its repeat unit, where
the CPD served as the “acceptor” component;15 however,
this polymer displayed complex electrochemistry and did not
demonstrate n-type behavior in an OFET. The paucity of
CPD- or fulvene-based building blocks could also be
attributed to the reactivity and instability of these classes of
molecules: many CPDs are unisolable in their monomeric
form16 (see Figure 1A), and the electrochemical stability of
fulvenes is suspect.17
Informed by the instability of monomeric cyclopentadi-
enone, we first synthesized a diphenyl derivative of 6,6-
dicyanofulvene based on the hypothesis that two phenyl
substituents would provide enough steric protection from
dimerization. 2,3-Diphenyl-6,6-dicyanofulvene (DCF1a) was
synthesized in two steps from the 4-hydroxy-2-cyclopenten-
1-one derivative 1 (Scheme 1), which was prepared by an
aldol condendation between acetone and benzil.18 Acid-
catalyzed dehydration of 1 generates the corresponding
cyclopentadienone 2a, which either irreversibly undergoes
a [4 + 2] homodimerization16 or can be trapped by suitable
dienophiles to generate 1,2-diphenylbenzene derivatives after
decarbonylation.18 To avoid such undesirable side reactions,
only transformations of the masked cyclopentadienone 1 were
pursued. The 1,2-addition of malononitrile to compound 1
proceeded smoothly to yield 3. We initially aniticipated the
occurrence of a number of detrimental rearrangement reac-
tions competing with the 1,2-addition of malononitrile;
however, as supported by the crystal structure of 3 (Figure
2A), the desired product was the sole isolated compound.
Dehydration of 3 by a catalytic amount of PTSA at elevated
temperatures quantitatively generated a dark brown solution
of DCF1a; however, this dicyanofulvene was found to
dimerize at room temperature to DCF1b. As seen in the
crystal structure of dimer DCF1b (Figure 2B), the endo
adduct was the major isolated product.
Figure 1. (A) Structures of the unisolable cyclopentadienone, CPD,
and its isolable dimer, (CPD)2. (B) Structure of 6,6-dicyanofulvene
(DCF), depicting the numbering convention for a fulvene skeleton.
We propose, instead, the use of 6,6-dicyanofulvenes
(DCFs, Figure 1B) as building blocks for electron-transport
Scheme 1. Synthesis of Select 6,6-Dicyanofulvene Derivatives
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Dimerization was reversible and was accompanied by
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Org. Lett., Vol. 12, No. 22, 2010
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dicyanofulvene analogues of tetracyclone and 6. Neverthe-
less, the use of a strong Lewis acid, such as BF3·OEt2 or
TiCl4, in combination with pyridine afforded the desired
cyanofulvenes DCF3 and DCF4 (Scheme 2). In these
instances, the progress of the reaction can be witnessed
optically: deep purple tetracyclone forms pea green DCF3,
while navy blue 6 forms emerald green DCF4.
Scheme 2. Synthesis of 1,4-Diphenyl-6,6-dicyanofulvenes
Figure 2. Crystal structures of (A) 3 and (B) DCF1b.
solution. The monomeric fulvene could be recovered by
heating DCF1b to 80 °C, and we were able to cycle between
monomeric and dimeric forms multiple times with little to
no decomposition noticeable by 1H NMR spectroscopy. This
observation is in contrast to the behavior of 2a, for which
dimerization is an irreversible phenomenon (heating results
in decarbonylation).16 Moreover, the tendency of DCF1a to
undergo [4 + 2] dimerization is not shared by other 6,6-
disubstituted fulvenes, such as a 6,6-dimethylfulvene.17
In an attempt to isolate a monomeric 6,6-dicyanofulvene,
two methyl substituents were introduced at the 2- and
5-positions of the starting hydroxy-2-cyclopenten-1-one.
Compound 4 was obtained from the aldol condensation of
3-pentanone with benzil and was subsequently dehydrated
to yield the cyclopentadienone dimer 5. Compound 5 is a
member of a subclass of cyclopentadienones that can be
reversibly cracked to their monomeric forms at elevated
temperatures, unlike the parent CPD.16 Therefore, DCF2 was
directly accessed from 5 by conducting the 1,2-addition of
malononitrile at 80 °C. Notably, dark purple, crystalline
DCF2 was isolated in its monomeric form (see Figure 3A),
while the corresponding cyclopentadienone dimerizes at
room temperature.
The crystal structure of DCF3 is shown in Figure 3B. Both
DCF2 and DCF3 do not display edge-to-face packing in the
solid state, preferring instead weak π-π interactions between
fulvene cores (see Supporting Information, Figures S1 and
S2).
Cyclic voltammograms (Figure 4) of all monomeric
dicyanofulvenes were recorded to judge their suitability as
electron-transport materials in electronic devices. The elec-
trochemical properties of DCF2-4 are summarized in Table
1. In general, the dicyanofulvenes displayed two distinct,
reversible one-electron reduction peaks. The first one-electron
reduction for DCF3 and DCF4, which contain two phenyl
substituents in the 1,4-positions, occurred at slightly less
negative potentials than in DCFs with alkyl substituents in
the 1,4-positions.
Table 1. Optical and Electrochemical Properties of DCF2-4
Ered/V
vs
SCE
EAa/
eV
compd
λmax/nm (log ε)
DCF2
DCF3
tetra-
cyclone
DCF4
6
275 (4.2), 370 (4.1), 555 (2.5)
270 (4.2), 387 (4.1), 543 (2.6)
265 (4.3), 338 (3.8), 509 (3.1)
-0.57, -1.24
-0.45, -1.04
-1.06
3.83
3.95
-
333 (4.7), 638 (2.6)
-0.43, -0.85
-0.95
3.97
-
334 (4.2), 381 (3.9), 580 (3.1)
a EA ) Ered (V vs SCE) + 4.4 eV.
Figure 3. Crystal structures of (A) DCF2 and (B) DCF3.
It was found that introduction of phenyl substituents at
the 1,4-positions of the fulvene skeleton hindered the 1,2-
addition of malononitrile, and therefore, the reaction condi-
tions used in the synthesis of DCF1-2 failed to generate the
For cases where the parent CPDs are monomeric, such as
with DCF3-4, it was possible to evaluate the effects of the
dicyanovinylene moiety on the electronic properties of select
cyclopentadiene systems. As previously reported, cyclic
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Org. Lett., Vol. 12, No. 22, 2010

distinct, one-electron reduction peaks. Additionally, the
formal reduction potentials of DCFs are globally lower than
those of the corresponding CPDs.
The electron affinities (EA) of DCF2-4 were calculated
from their formal reduction potentials using the formula EA
) Ered + 4.4 eV10 and are listed in Table 1. With average
electron affinities of 3.9 eV, DCFs are comparable to PCBM
(EA 4.0 eV) as electron acceptors.
In conclusion, a series of substituted 6,6-dicyanofulvenes
were synthesized starting from masked, dimeric, or mono-
meric cyclopentadienones. DCFs lacking sufficient steric bulk
around the fulvene core tend to reversibly undergo a [4 +
2] dimerization. In addition to being highly crystalline, DCFs
are darkly colored compounds due to the presence of
electronic transitions in the visible region of the electromag-
netic spectrum. DCFs display two distinct, reversible one-
electron reductions by cyclic voltammetry. Based on their
high crystallinity and suitable electron affinities, and buoyed
by their relatively cheap and straightforward synthesis, DCFs
are interesting candidates for organic electron-transport
materials.
Figure 4. Cyclic voltammograms of DCF2-4 recorded in 0.1 M
TBAPF6 in CH2Cl2 using a Pt button electrode at a scan rate of
100 mV/s. Voltage values are reported vs standard calomel electrode
(SCE).
Acknowledgment. T.L.A. wishes to thank the Chesonis
Family Foundation for a graduate fellowship. J.R.C. was
supported by the MIT Institute for Solider Nanotechnologies
and the Air Force Office of Scientific Research.
voltammograms of CPDs generally display a single two-
electron reduction peak19 (see Supporting Information, Figure
S3), and the intermediate radical anion product of one-
electron reduction is observed only in select cases.15 In
contrast, the intermediate radical anions of DCFs are stable,
and therefore, cyclic voltammograms of DCFs display two
Supporting Information Available: Experimental pro-
cedures, spectral characterization data, and crystallographic
information. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) Fox, M. A.; Campbell, K.; Maier, G.; Franz, L. H. J. Org. Chem.
1983, 48, 1762.
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