Charge-transfer complex formations of tetracyanoquinone (cyanil) and aromatic electron donors

Article abstract of DOI:10.1002/poc.3485

Single-electron oxidants are the primary reagents for investigations of the new oxidants and the development of electron-accepting materials for application in optoelectronics. Quinones are the well-known class of the neutral single-electron oxidants. Here, we present the properties of the strongest neutral electron acceptor of this class tetracyanoquinone (cyanil) and investigate its electron-accepting strength by analyzing the charge-transfer complex formations with the aromatic donor molecules. Charge-transfer complexes of tetracyanoquinone with aromatic electron donors are characterized spectroscopically in solution and isolated as the single crystals.

Full text of DOI:10.1002/poc.3485

Research article
Received: 01 August 2015,
Accepted: 06 August 2015,
Published online in Wiley Online Library: 11 September 2015
(wileyonlinelibrary.com) DOI: 10.1002/poc.3485
Charge-transfer complex formations of
tetracyanoquinone (cyanil) and aromatic
electron donors
Almaz S. Jalilova*, Jianjiang Lua and Jay K. Kochia†
Single-electron oxidants are the primary reagents for investigations of the new oxidants and the development of
electron-accepting materials for application in optoelectronics. Quinones are the well-known class of the neutral
single-electron oxidants. Here, we present the properties of the strongest neutral electron acceptor of this class
tetracyanoquinone (cyanil) and investigate its electron-accepting strength by analyzing the charge-transfer complex for-
mations with the aromatic donor molecules. Charge-transfer complexes of tetracyanoquinone with aromatic electron
donors are characterized spectroscopically in solution and isolated as the single crystals. Copyright © 2015 John Wiley
& Sons, Ltd.
Keywords: acceptor; charge-transfer complex; oxidant; p-cyanil
TCQ has a higher symmetry than DDQ, which could also be
INTRODUCTION
used in organic optoelectronics as a strong electron accep-
tor.[21] TCQ was synthesized and characterized earlier;[2–24]
however, most of the earlier studies were focused on the re-
duced forms of the TCQ as hydroquinone H2TCQ, radical an-
Since the first seminal work by Mulliken[1] on the nature of
charge-transfer (CT) complexes and realizing their potential
as the conducting organic materials,[2] CT complexes have
witnessed continues interest in the development of novel
organic electronic materials.[3–6] CT complexes consist of an
electron donors and acceptors, which with the increase in
electron-donating or electron-accepting ability of the compo-
nents, CT complexes could be extended to the electron-
transfer (ET) salts.[7] While the development of wide range
of electron-donating materials starting from the molecular
electron donors as tetrathiafulvalene (TTF)[8] all the way to
the electron-rich π-conjugated polymeric materials has been
an active part of the research in organic electronics,[9] the de-
velopment of the electron-accepting counterparts is still rela-
tively scarce.[10] Electron-deficient alkenes and quinones are
among the common molecular electron acceptors (Chart 1).[11]
[TTF˙+, TCNQ˙ꢀ] ET salt was the first molecule-based electron
conductor and superconductor.[12,13] Miller and coworkers
have demonstrated the room temperature ferromagnets of
ET salts between organometallic compounds and
tetracyanoethylene.[14–16]
On the other hand, electron-deficient quinones, particu-
larly 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), which
is also known as a strong hydrogen acceptor (H+ + eꢀ) used
as an oxidant for good donors and allylic arenes such as ste-
roids.[7] Recently, it has been used in oxidative carbon–
carbon bond forming reactions via carbon–hydrogen bond
activation of organic substrates.[18–20] Often, reactions with
DDQ require high-temperature regimes, which is due to the
relatively low reduction potential of DDQ. From these per-
spectives, it was interesting for us to see the properties of
better oxidant from the same class of compounds (qui-
nones), tetracyanoquinone (cyanil, TCQ chart 1). In addition,
ionic TCQꢀ, and dianionic form TCQ2ꢀ
.
The objective of this work is to investigate the electron-
accepting strength of TCQ by studying its CT complex forma-
tion with the various arenes that differs structurally and in
their ionization potential. In particular, isolation of single crys-
tals and spectral characterization of the CT complexes will be
examined.
RESULTS AND DISCUSSION
Spectroscopy of the charge-transfer complexes of
tetracyanoquinone with aromatic donors
Addition of TCQ to a solution of organic donor molecules in di-
chloromethane results in the appearance of an absorption band
in a visible region of the spectrum. Figure 1 demonstrates the
bathochromic shift of the new band appeared upon mixing of
TCQ with the selected organic donor molecules. Bicyclic aromatic
molecules such as naphthalene and 2,6-dimethylnaphthalene to-
gether with para-dimethoxyphenyl derivatives resulted in double
absorption peaks. We correlate the lowest energy absorption
* Correspondence to: Almaz S. Jalilov, Department of Chemistry, Rice University,
6100 Main St, Houston, TX 77005, USA.
E-mail: almaz.jalilov@gmail.com
†
Deceased, August 9, 2008.
a A. S. Jalilov, J. Lu, J. K. Kochi
Department of Chemistry, University of Houston, 4800 Calhoun, Houston, TX
77204, USA
J. Phys. Org. Chem. 2016, 29 35–41
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A. S. JALILOV, J. LU AND J. K. KOCHI
donors to the dichloromethane solution of the DDQ
afforded a similar appearance of the CT bands,
which are stable at room temperature. Table 1
shows the comparison of the redox potentials of
the selected electron-donors and the quinones as
electron acceptors. Another evidence for the CT ab-
sorptions of these lowest energy bands is presented
in Fig. 2, where a clear linear relationship of the
transition energies as a function of oxidation poten-
tials of the organic donor molecules is known as
Chart 1. Typical molecular acceptors.
a Mulliken correlation.[25]
Recently, Kochi’s group has shown that such
complexes are critical precursors in intermolecular ETs,[26–30]
that is,
KCT
⇄
KET
⇄
diff
⇄
D þ A
½D; Aꢁ
½Dꢂþ; Aꢂꢀꢁ
Dþþ Aꢂꢀ
(1)
Mostly, in CT complexes, the oxidation potential of the
donor is considerably higher than the reduction potential of
the acceptor, which causes the thermal activation of the donor
to be energetically unfavorable. TCQ has the most positive
(lowest) reported reduction potential of all known quinones
with Eredo = 0.94 V vs. Saturated Calomel Electrode (SCE) (DDQ,
Eredo = 0.52 V vs. SCE).
Identification of isergonic donor/acceptor dyads is critical in
order to fully understand the ET reactivity of the cyanil towards
organic donors. Table 1 summarizes some potential electron do-
nors that have low oxidation potentials (Eoox). Selection of the
electron donors where the ET to cyanil is in the isergonic limit
(ΔGoET = F(Eoox ꢀ Eo ~0) is crucial for the driving force of the
red
ET step form CT salts.
Figure 1. Bathochromic shift of the low energy band (charge transfer)
Despite the variety of the known CT complexes, those that are
close to the isergonic limit are interesting because of the tran-
sient nature of them and high-CT components present.[28,31] In
the series of the CT complexes with cyanil, those that are tran-
sient in nature can react through the ET step (Eqn 1) to form
the hydrogen abstraction adducts as it has been shown for
DDQ (Chart 2).[32]
The CT state between neutral TCQ and its single-electron-
reduced radical anionic state (TCQ•–), for which ΔGoET = 0, is
also known as charge-resonance state.[33,34] Self-exchange
with increasing donor strength in dichloromethane
band with the CT band of the electron donor–acceptor molecular
complex.
It was observed that some of the CT complexes of TCQ with
donor molecules with more negative oxidation potentials are
not stable at room temperature. In Fig. 1, those are given in
red colors.
Absorption bands of the transient CT complexes were mea-
sured at lower temperatures. Addition of the same organic
Table 1. Formal potentials of some donors and acceptors
Eo
EDonor ꢀ Ered
(V)
Electron acceptor
Eored (V versus SCE)a
TCQ
Electron donor
ox
(V versus SCE)a
ox
DMMB
DMT
1.10
0.16
0.22
DCQ
0.32
1.16
DIQ
0.51
(Continues)
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TETRACYANOQUIONONE AS ACCEPTOR
Table 1. (Continued)
TCQ
Electron donor
Eo
EDonor ꢀ Ered
(V)
Electron acceptor
Eo (V versus SCE)a
ox
(V versus SCE)a
ox
red
0.54
DMB
1.36
0.42
0.42
DBQ
0.57
0.94
1.36
1.54
DDQ
TCQ
26Napth
0.60
Naph
DMMB, dimethoxydimethylbenzene; DMT, dimethoxytoluene; DMB, dimethoxybenzene; DCQ, 2,3-dicyanoquinone; DIQ, 2,3-diiodo-
5,6-dicyano-p-benzoquinone; DBQ, 2,3-dibromo5,6-dicyano-p-benzoquinone; DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; TCQ,
tetracyanoquinone.
aIn CH2Cl2 at 22 °C in the presence of supporting electrolyte 0.1-M tetrabutylammonium hexafluorophosphate (scan rate= 100 mV/s).
of an electron acceptor (TCQ) with its paramagnetic radical
anion counterpart (TCQ•–) is shown in Eqn 2.
TCQ þ TCQꢂꢀ
⇄
½TCQ; TCQꢁꢂꢀ
⇄
TCQꢂꢀþ TCQ
(2)
Charge-resonance bands corresponding to the mixed-valence
complex of TCQ with its single-electron-reduced radical anionic
counterpart (TCQ•–) appear at 1165 nm (8584 cmꢀ1) in dichloro-
methane. Despite the considerable work performed on mixed-
valence complexes of TCQ and its radical anion in different sol-
vents, detection of the charge-resonance bands in any other po-
lar or protic solvents has failed. This appears to be because of
the low stability of the neutral cyanil in polar and protic sol-
vents, which are difficult to purify and dry to a reasonable ex-
tent. In the Mulliken plot (Fig. 2), the radical anion of the
starting acceptor appears in far left corner.
Charge-transfer interactions of TCQ with the four organic do-
nor molecules (napthalene, dimethoxybenzene (DMB),
dimethoxytoluene (DMT), and dimethoxydimethylbenzene
(DMMB)) have the lowest oxidation potentials among the stud-
ied electron donors, which will be studied in more detail. Mixing
of dichloromethane solutions of each of the donor molecules
with a dichloromethane solution of an equimolar concentration
of TCQ resulted in an immediate appearance of the new band
in near-infrared (NIR) region of the spectra. As already men-
tioned previously, upon mixing of the DMT and DMMB solutions
with TCQ solutions at room temperature,
Figure 2. Mulliken correlation of the charge-transfer absorption bands
(hvCT) of the complexes of cyanil (tetracyanoquinone) with oxidation po-
tentials (Eoox) of various organic donors in dichloromethane
the new bands were not observed. How-
ever, mixing of the same solutions at
lower tempartures (–60 °C for DMT and –
80 °C for DMMB) resulted in new
Gaussian-shaped absorption bands at
Chart 2. Illustration of chemical reaction of arenes with TCQ through C-H activation of methyl group.
860 and 880 nm, respectively. Analogous
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A. S. JALILOV, J. LU AND J. K. KOCHI
experiments of the same donor molecules with weaker electron
acceptor as DDQ resulted in an appearance of the stable new ab-
sorption bands in the NIR region of the optical spectra. The Mul-
liken correlation of the CT absorption bands of DMT with various
electron acceptors in dichloromethane is shown in Fig. 3.
The absorbance of the new Gaussian bands rose progres-
sively as a function of the donor concentration, as shown in
Fig. 4 for [DMT, TCQ]. Linear correlation of the calculated ab-
sorbance and the experimental absorbance (inset, Fig. 4) with
correlation coefficients of 0.98 and the proportionality con-
stants of 1.0 for both [DMT, TCQ] confirmed the 1:1 stoichiom-
etry for the formation of the donor/acceptor complex
according to Eqn. 3:
Quantitative analysis of the concentration dependence led to
the formation constants of the CT complex formation as summa-
rized in Table 2. Determinations of the CT complex formation
constants for the DMT and DMMB pairs with TCQ at low temper-
atures resulted in high-equilibrium constants.
Lowering the temperature of the solutions resulted in a sig-
nificant increase of the absorbance of the CT bands in NIR re-
gion. Increase of the absorbance with lowering the
temperature of the solution indicates a shift of the equilibrium
to the right in Eqn 3. The linear dependence of lnKCT with the
inverse temperature afforded the thermodynamic parameters
for the [DMT, TCQ] and [DMB, TCQ] complex formations as
presented in Table 2. The temperature dependence of the
spectrum of the dichloromethane solution containing equimo-
lar concentrations of DMT and TCQ (3.4 mM each) show an in-
crease of the 860-nm band upon lowering the temperature
(Fig. 5).
KCT
⇄
DMT þ TCQ
½DMT; TCQꢁ
(3)
X-ray crystallographic characterization of charge-transfer
complexes of tetracyanoquinone with aromatic donors
Colored solutions of the CT complexes in dichloromethane
were subjected to slow cooling and diffusion of the hexane
through solution, which resulted in the precipitation of the
crystalline solids. X-ray crystallographic analysis of the follow-
ing crystals gives infinite π-stacked dimer pairs of 2:1
donor/acceptor complexes of toluene and mesitylene as do-
nors and cyanil as an acceptor (Fig. 6). X-ray analysis revealed
the presence of the infinite …DDADDA… stacks. Both of the
complexes show close contacts between donor molecules
and the acceptor cyanil with rπ ~3.17 Å in both cases and lon-
ger distances between stacks of ~3.5 Å.
Similar X-ray crystallographic analysis of the crystalline
donor/acceptor complexes of naphthalene and DMB with
TCQ gives infinite 1:1 complexes as can be seen in Fig. 7. In
this case, crystallographic analysis revealed the presence of
the infinite …DADADA… stacks. Significant difference in the
distances between π stacks is observed for these series of
the CT complexes, which are closer to the isergonic region of
the donor/acceptor dyads. π–π distances for [DMB, TCQ] and
[Naph, TCQ] are rπ ~2.86 Å and rπ ~3.20 Å, respectively (Fig. 7).
Analysis of the average bond lengths (Å) within TCQ moieties
in the various CT complexes in comparison with those in neutral
and anion radical of TCQ allowed to evaluate the degree of CT in
the following CT complexes. Table 3 summarizes the bond
lengths of the TCQ motifs in the complexes, as well as the neutral
and anion radical moieties.
Figure 3. Mulliken correlation of the charge-transfer absorption bands
(hvCT) of the complexes of 2,6-dimethoxytoluene with reduction poten-
tials (Eored) of various organic acceptors in dichloromethane
Molecular geometries of the corresponding cyanil or the do-
nor molecules are considerably different within different CT
complexes. In particular, C=O bond length is the longest in rad-
ical anion moiety(ꢀC–O ̇) and the shortest in the neutral cyanil
moiety, mostly resembling the (C=O) carbonyl double bond. All
of the C=O bond lengths from the CT complexes are neither
shorter nor longer than those in the neutral or anion radical moi-
eties, respectively. This finding leads us to consider the CT com-
plexes as being intermediate partial CT species between neutral
and fully reduced radical anion form of the cyanil. By the analysis
of the C=O bond length differences, one can easily estimate the
transfered charge (q). For the [Naph, TCQ] complex moiety based
on the crystallographic data, it was estimated as q = 0.3 and al-
most close to zero for both [Toluene, TCQ] and [Mesitylene,
TCQ]. Similar analysis for the [DMB, TCQ] failed to give
Figure 4. Optical spectra of the solution of dimethoxytoluene (donor)
and tetracyanoquinone (acceptor) (1.0 mM) at different concentrations
of the donor, 0, 1.7, 1.8, 2.1, 2.3, 2.5, 2.7, 2.8, and 2.9 mM in dichlorometh-
ane, at ꢀ60 °C
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TETRACYANOQUIONONE AS ACCEPTOR
Table 2. Properties and thermodynamic characteristics of the charge-transfer complexes of TCQ with donors near the isergonic
region
Complex
Solvent
vCT, 103 cmꢀ1
KCT, Mꢀ1
ε
CT, 103 Mꢀ1 cmꢀ1
ΔHCT, kcal/mol
ΔSCT, cal Mꢀ1 Kꢀ1(e.u.)
c
c
[TCQ, Naph]
[TCQ, DMB]
[TCQ, DMT]
[TCQ, DMMB]
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
13.4
11.9
11.7
11.4
13
30
1154
4000
ꢀ5.3
ꢀ7.9
—
ꢀ10.9
ꢀ19.8
—
500a
550b
12,980a
14,570b
TCQ, tetracyanoquinone; DMB, dimethoxybenzene; DMT, dimethoxytoluene; DMMB, dimethoxydimethylbenzene.
aAt ꢀ60 °C.
bAt ꢀ82 °C.
cBecause of the precipitation of the charge-transfer complexes at low-temperatures, thermodynamic parameters were not
calculated.
Figure 5. Optical spectra of the dichloromethane solution of
tetracyanoquinone and dimethoxytoluene at variable temperatures (be-
tween ꢀ30 and ꢀ82 °C)
Figure 7. X-ray structure of the 1:1 charge-transfer complexes of [TCQ,
Nath] and [TCQ, DMB]. Molecular diagrams are drawn with 50% probabil-
ity ellipsoids
Table 3. Bond length characteristics of the TCQ moiety in
the neutral, radical anionic, and charge-transfer complexes
with various donor molecules
Structure
a
b
c
d
e
TCQa
1.204(3) 1.487(5) 1.338(3) 1.437(7) 1.132(2)
1.229(3) 1.463(7) 1.372(5) 1.439(7) 1.137(6)
1.207(2) 1.492(2) 1.344(1) 1.442(2) 1.133(2)
[TCQ]ꢀ.a
TCQ-Toluene
TCQ-Mesitylene 1.209(4) 1.477(3) 1.357(1) 1.413(6) 1.099(4)
TCQ-Naph
TCQ-DMB
1.212(2) 1.494(8) 1.357(4) 1.467(3) 1.090(2)
1.206(2) 1.484(2) 1.355(1) 1.450(5) 1.081(1)
Figure 6. X-ray structure of the 2:1 charge-transfer complexes of [TCQ,
Toluene] and [TCQ, Mesitylene]. Molecular diagrams are drawn with
50% probability ellipsoids
aFrom the work of Vazquez et al.[24]
J. Phys. Org. Chem. 2016, 29 35–41
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A. S. JALILOV, J. LU AND J. K. KOCHI
Table 4. Crystallographic parameters
Compound
[Toluene, TCQ]
[Mesitylene, TCQ]
[Naph, TCQ]
[DMB, TCQ]
Color
Formula
Mr
Space group
a/Å
Brown blue
C17H8N4O2
300.27
C2/c
Plum
C19H12N4O2
328.32
P-1
8.22(3)
8.98(4)
9.90(2)
102.35(14)
106.99(19)
113.54(10)
593(3)
2
Brown
C20H8N4O2
336.30
P-1
6.96(3)
7.78(2)
8.24(2)
111.70(13)
102.44(17)
94.62(19)
398(2)
1
Green
C18H10N4O4
346.30
P-1
7.292(4)
7.412(4)
8.571(5)
75.988(12)
66.966(11)
77.099(12)
409.4(4)
1
8.559(4)
12.895(5)
18.697(9)
90.00
90.731(8)
90.00
2063.4(16)
8
1.263
7601
b/Å
c/Å
α/o
β/o
γ/o
V/Å3
Z
ρ/g cmꢀ3
Refls
1.256
7428
1.402
5159
1.405
6224
Indpndt
Obsd (F > 4σ)
R1 (F > 4σ)
wR2
2985
1836
0.0553
0.1298
3463
2175
0.0673
0.1701
2253
1212
0.0553
0.1163
2400
1342
0.0638
0.1444
considerable correlation with the other three CT complexes. This
could be understandable with a close look to the [DMB, TCQ]
complex, which differs with its closer π–π distances and lack of
planarity of the p-methoxyphenyl moiety.
Details of the crystallographic analysis are summarized in Table 4.
Unfortunately, despite the relative persistency and observa-
tion of the CT absorption bands of [DMT, TCQ] and [DMMB,
TCQ] complexes at low temperatures, we were unable to grow
single crystals of them that were suitable for X-ray crystallo-
graphic analysis.
synthesized according to the reported procedures. Single crystals of the 1:1
[TCQ, DMB] and [TCQ, DMB] and 1:2 [TCQ, Toluene] and [TCQ, Mesitylene]
complexes were prepared by slow diffusion of hexane into the dichloro-
methane solutions containing the TCQ acceptor and a large excess of
DMB donor at ꢀ60 °C.
Electronic absorption spectra were recorded on a UV–Vis-NIR (200–
3000 nm) spectrometer in Teflon-capped quartz cuvettes under an argon
atmosphere. The measurement of the newly formed NIR bands was car-
ried out by addition of the donor to the solution of acceptor. Low-
temperature measurements were carried out with the aid of a Dewar
equipped with quartz windows. Formation of the transient CT complexes
of various donor/acceptor dyads was studied under an argon atmo-
sphere in dichloromethane. The quantitative analysis of the NIR intensi-
ties was carried out with the aid of Benesi–Hildebrand as described
earlier.[28,31,37]
CONCLUSIONS
Interaction of the strongest neutral organic π-electron acceptor
TCQ with various aromatic electron donor molecules was inves-
tigated. Single crystals of four CT complexes of TCQ with aro-
matic donor molecules were isolated and analyzed by X-ray
crystallography. Two types of π-associations were observed in
solid state, one with the infinite …DDADDA… stacks with the
weaker donor molecules and one with the infinite …DADADA…
stacks with the stronger donor molecules, which potentially
Electrochemical measurements (cyclic voltammetry) were performed
on
a
potentiostat in dichloromethane in the presence of
tetrabutylammonium hexafluorophosphate as the supporting electrolyte
and ferrocene with E° = 0.475 V vs SCE as the internal standard, as de-
scribed previously.(33,36b)
could be extended to …DD+AꢀDD+Aꢀ… and
…
Acknowledgements
D+AꢀD+AꢀD+Aꢀ… CT salts, respectively. Transient CT complexes
of TCQ with strong aromatic donor molecules at the isergonic
limit were observed spectroscopically at low temperatures, and
thermodynamic parameters for π-associations were estimated
experimentally.
A. S. J. thanks Prof. Stephen F. Nelsen and Prof. Sergiy V. Rosokha
for many helpful discussions. This work was financially supported
by Robert A. Welch Foundation.
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