The pyridazine-tetrathiafulvalene conjugates: Synthesis, photophysical, and electrochemical properties

Article abstract of DOI:10.1016/j.tet.2011.12.034

To study the electronic interactions in donor-acceptor (D-A) conjugates as a precursor of optoelectronic materials, a series monopyridazine-annulated tetrathiafulvalenes and a bispyridazine-annulated tetrathiafluvalene were synthesized by a condensation r

Full text of DOI:10.1016/j.tet.2011.12.034

Tetrahedron 68 (2012) 1782e1789
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
The pyridazineetetrathiafulvalene conjugates: synthesis, photophysical,
and electrochemical properties
,
a,
*
Ningjuan Zheng a, Bao Li b, Changwei Ma a, Tie Chen a, Yuhe Kan c , Bingzhu Yin
*
a Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules, Yanbian University, Ministry of Education, Yanji, Jilin 133002, PR China
b State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
c Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Teachers College, Huaiyin 223300, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 October 2011
Received in revised form 8 December 2011
Accepted 13 December 2011
Available online 16 December 2011
To study the electronic interactions in donoreacceptor (DeA) conjugates as a precursor of optoelectronic
materials, a series monopyridazine-annulated tetrathiafulvalenes and a bispyridazine-annulated tetra-
thiafluvalene were synthesized by
a
condensation reaction of 2,3-dimethoxycarbonyl-6,7-
dibutylthiotetrathiafulvalene or/and 2,3,6,7-tetramethoxycarbonyltetrathiafulvalene with hydrazine hy-
drate and structurally characterized by conventional chemical and physical methods. Their electronic
properties have been studied experimentally by the combination of electrochemistry and UVevis
spectroscopy. All of monopyridazine-tetrathiafulvalene conjugates 7e13 show intramolecular charge
transfer interaction in ground states, which is rationalized on the basis of density functional theory. Their
HOMO energy levels and Eogpt values were estimated to be ꢀ4.88 to ꢀ5.07 eV from cyclic voltammetry
and 2.43e2.79 eV from the absorption spectra, respectively. The X-ray crystallographic analyses of the
pyridazineetetrathiafulvalene conjugates 7, 11e13 are also reported.
Keywords:
Tetrathiafulvalene
Pyridazine
Donoreacceptor conjugate
Charge transfer
Photophysical property
Electrochemical behavior
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
rings to the TTF skeleton was effective to enhance the in-
termolecular interaction, which was also useful to enhance the
Among the various candidates of organic materials for opto-
electronic applications,1 tetrathiafulvalene (TTF) is most attractive
due to its excellent electron-donating properties. The optoelec-
tronic properties of these materials are clearly dependent on both
the molecular structure and architecture in the solid state and so
the TTF skeleton and peripheral substitutes has been extensively
modified in order to enhance dimensionality of the materials and/
or to achieve a suitable solid-state organization.2 Additionally, be-
cause of the strong electron-donating property of TTF, its de-
rivatives are usually sensitive to air and light, which is one of the
main drawbacks of TTF for practical applications. In order to over-
come this barrier, heterocycle has been annulated to decrease its
HOMO energy level, which suitable for P-type organic semi-
conductors.3 A variety of donor molecules have been synthesized in
which the TTF core is annelated to benzenoid, furan, thiophene,
selenophene, pyrimidine or pyrazine units; all of these compounds
have oxidation potentials appreciably higher than that of TTF
itself.3,4 Among the heterocyclic-fused TTF donors, the pyrazine-
annulated TTF derivatives showed the excellent P-type FET (field
effect transistor) performances in thin films. Annulation of pyrazine
stability of the FET device to oxygen.3 Another heterocyclic-fused
TTFs, the pyrimidine-fused TTF derivatives have a unique ability
to form unusual stable cation radical intermolecular salts (beta-
ines). These cation radical salts and betaines in pressed pellets
showed an unexpectedly low resistivity and semiconductor be-
havior with low values of activation enegy.4i,j As early as in 1983,
a bispyridazinoTTF has been synthesized by condensation of
tetraformyl-TTF with excess hydrazine hydrate and its
p-donor
ability has also been reported.5 Surprisingly, to the best of our
knowledge, there is no any report on exploring the optoelectronic
applications of pyridazine-annulated TTFs in detail. It is probably
due to the poor solubility in organic solvents and an absence of
available substitutes. Keeping this information in mind, we annu-
lated a pyridazine ring to one side of TTF skeleton to synthesize
a series of monopyridazinoTTF derivatives as shown in Scheme 1.
Most of the TTF derivatives are soluble in chloroform, methylene
chloride, ethyl acetate, tetrahydrofuran, dimethyl sulfoxide, and
acetone and slightly soluble in acetonitrile except for the keto-
forms 7 and 5. In the present work, we describe the synthesis,
photophysical and electrochemical properties of the monopyr-
idazinoTTF derivatives. The X-ray crystallographic analyses
of 7, 11e13, together with preliminary formation of a charge
transfer complex between the donor 13 with nitrosonium
* Corresponding authors. Tel.: þ86 433 2732298; fax: þ86 433 2732456; e-mail
address: zqcong@ybu.edu.cn (B. Yin).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.12.034

N. Zheng et al. / Tetrahedron 68 (2012) 1782e1789
1783
hexafluorophosphate (NOPF6), are also reported. Such systems are
of prime interest due to the excellent solubility in organic solvents
and diversified functionalization.
dithiole-2-one in net triethyl phosphate to give an unsymmetrical
diester derivative of TTF (6). Fortunately, the diester 6 reacted with
hydrazine hydrate in refluxing ethanol to give a soluble keto-form 7
O
COOCH3
COOCH3
COOCH3
CH3OOC
CH3OOC
S
S
S
S
S
S
NH2NH2-H2O
S
S
P(OEt)3
120 o
NH
NH
S
S
C
C2H5OH
reflux
COOCH3
S
3
1
4
O
SBu
2
NH2NH2-H2O
C2H5OH, reflux
P(OEt)3
120 o
S
C
S
SBu
O
O
COOMe
S
S
S
BuS
BuS
S
S
HN
HN
NH
NH
S
S
S
COOMe
O
O
6
5
NH2NH2-H2O
C2H5OH, reflux
O
O
OH
BuS
BuS
BuS
BuS
BuS
BuS
S
S
S
S
S
S
S
S
S
S
S
NH
NH
NH
N
N
N
S
O
OH
OH
keto-form
enol-form
keto-enol form
7
excess CH3COCl
excess TEA
DCM
excess CH3I
excess K2CO3
DMF
1 equiv. CH3I
1 equiv. K2CO3
DMF
POCl3
reflux
OCH3
O
O
Cl
Cl
BuS
BuS
BuS
S
BuS
BuS
BuS
BuS
CH3
S
S
S
S
S
S
S
S
S
S
S
S
S
N
N
N
NH
N
N
N
NH
S
S
BuS
8
O
9
OCH3
10
OCOCH3
11
NaH
DMF
CH3ONa
MeOH-THF
reflux
PhOH
OCH3
OPh
BuS
BuS
BuS
BuS
S
S
S
S
S
N
N
N
N
S
S
S
12
13
OCH3
OPh
Scheme 1. Synthesis route to the pyridazinoTTF derivatives.
2. Results and discussion
in a moderate yield. Upon addition of 1 equiv methyl iodide, 7 was
subjected to the methylation to afford a ketoeenol form mono-
methyl ether 8 in a good yield. As we expected, the reaction of keto-
form 7 with an excess methyl iodide in DMF at room temperature
leads to an N,O-dimethyl ketoeenol form derivatives 9 in high yield.
Unexpectedly, 7 reacted with an excess acetyl chloride in the
presence of triethylamine to give only an O-monoacyl ketoeenol
form derivatives 10. The key intermediate dichloropyridazinoTTF 11
was easily synthesized starting from 7 with refluxing POCl3 in 90%
yield. The O,O-dimethyl enol-form derivatives 12 was prepared by
nucleophilic substitution of 11 with sodium methoxide in a mixture
of MeOH and THF (2:1, v/v) in 48% yield. Similarly, the condensation
reaction of 11 with phenoxide in DMF gives rise to an O,O-diphe-
nolated enol-form derivatives 13 in a good yield. The data of the
elemental and MALDI-TOF mass analyses are consistent with the
constituents of the proposed pyridazinoTTF derivatives 5e13 (see
Supplementary data). Theoretically, compound 7 is likely to exist as
an equilibrium of three tautomeric forms as shown in Scheme 1.
Both 1H and 13C NMR spectra indicate that compounds 7, 11e13 all
exist in the C2-symmetrical structures, in which only the FT-IR
spectra of 7 exhibits a strong absorption peak of carbonyl of
2.1. Synthesizes and crystal structure analysis
The standard method of synthesis of the pyridazine ring is the
action of hydrazine on 1,4-dicarbonyl compounds or their equiva-
lent.6 We once attempted to obtain the pyridazine derivative (3) by
the reaction of dimethyl 2-thioxo-1,3-dithiole-4,5-dicarboxylate (1)
with hydrazine hydrate, but the experiment is unsuccessful
(Scheme 1). Self-coupling reaction of compound 1 in net triethyl
phosphite gave 2,3,6,7-tetramethoxycarbonyl TTF (4), then 4 con-
denses with hydrazine hydrate to give a bispyridazine-annulated
TTF 5 in good yield. Molecular structure of the bispyridazine-
annulated TTF 5 has been presumed by mass and IR spectral
studies. The mass spectrum of compound 5 displays the parent ion
peak at m/z 371.0, corresponding to [MꢀH]ꢀ (370.9) of 5 (Fig. S1).
The FT-IR spectrum of 5 moreover exhibits only a strong absorption
peak of carbonyl at 1658 cmꢀ1 and a weak absorption peak of amide
at 3182 cmꢀ1 (Fig. S2a). Unfortunately the structure and properties
of compound 5 are impossible to be fully investigated owing to its
extreme insolubility. Cross-coupling of 1 with 4.5-dibutylthio-1,3-

1784
N. Zheng et al. / Tetrahedron 68 (2012) 1782e1789
amide at 1647 cmꢀ1 (Fig. S2b, feh). Therefore 7 could be assigned as
the keto-form, while 11e13 could be assigned as the enol-form
derivatives, respectively. In contrast, compounds 8e10 showed
the non C2-symmetrical structures in both 1H and 13C NMR spectra.
Furthermore, in CDCl3 the 1H NMR spectra of 8 and 10 consists of
Table 1
Crystal data and structure refinement of complexes 7, 11e13
Compd
7
11
12
13
Formula weight
975.44
293(2)
0.71073
Triclinic
Pꢀ1
501.58
296(2)
0.71073
Triclinic
Pꢀ1
492.75
296(2)
0.71073
Orthorhombic
Pbcn
19.376(3)
10.578(6)
23.124(6)
90
90
90
4739.0(3)
8
1.381
616.88
296(2)
0.71073
Triclinic
Pꢀ1
Temperature (K)
ꢀ
l
(A)
a singlet peak at
d 3.89 and 2.34 ppm, respectively, which are
Crystal system
Space group
characteristic for the methyls in the methoxy group and acetyl
group, respectively. The 1H NMR spectrum of 9 shows two methyl
peaks at 3.87 and 3.65 ppm, which could be assigned as an O-
methyl group and a N-methyl group, respectively. Of them, the FT-
IR spectra of 8 and 9 exhibit only one strong absorption peak of
amide carbonyl at 1653 and 1647 cmꢀ1, respectively, while 10 ex-
hibits an absorption peak of amide carbonyl at 1651 cmꢀ1 and an
absorption peak of ester carbonyl at 1778 cmꢀ1 (Fig. S2cee). In view
of the facts mentioned above, the compounds 8e10 could be
assigned as O-methyl, O,N-dimethyl, and O-acetyl substituted
ketoeenol form derivatives, respectively.
Crystals suitable for an X-ray diffraction study have been ob-
tained for compounds 7, 11e13 through recrystalization from DMF
for 7 and from hexane for 11e13. Their molecular structures and
crystal data are shown in Fig. 1 and Table 1. The asymmetric unit of
compound 7 is composed of two molecules (7A and 7B) and one
ethanol molecule (Fig. 1a). The rigid components of both 7A and 7B
exhibit ‘boat’ conformations as often observed with neutral TTF. For
7A, the dihedral angle of two parts (one is composed of N1, N2, C1 to
C4, O1, O2, S1, and S2, and the other one is composed of S3 to S6, C7,
and C8) is 33.77 (19)ꢁ. For 7B, the dihedral of two parts is 22.45 (13)ꢁ
(one is composed of N3, N4 C17 to C20, O3 O4, S7, and S8, and the
other is composed of S9 to S12, C23, and C24). Furthermore, in the
same asymmetric unit, the two ‘boat’ conformation parts arrange in
the style of back-to-back. Another difference of 7A and 7B molecules
is the extension direction of butyls. For 7A, the two butyls extend to
ꢀ
a (A)
10.383(2)
12.607(3)
18.296(4)
93.64(3)
101.43(3)
110.38(3)
2177.5(8)
2
7.671(6)
11.215(8)
13.310(11)
86.10(3)
79.57(3)
79.27(2)
1105.9(2)
2
9.169(2)
10.852(2)
15.292(3)
96.86(3)
94.78(3)
91.28(3)
1504.6(5)
2
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(deg)
(deg)
(deg)
3
ꢀ
Volume (A )
Z
Dcalcd (g/cm3)
Abs coeff. (mmꢀ1
Rint
1.488
0.647
0.0210
1012
1.126
0.0601
0.1863
0.844
ꢀ0.648
1.506
0.865
0.0476
516
1.051
0.0507
0.1254
0.453
ꢀ0.489
1.362
0.483
0.0229
644
1.112
0.0477
0.1292
0.686
ꢀ0.396
)
0.594
0.0381
2064
F(000)
GOF on F2
1.067
R [I>2
s
(I)]a
0.0522
0.1580
0.560
ꢀ0.289
Rw (all data)b
3
ꢀ
3
ꢀ
(Residues)max(e/A )
(Residues)min(e/A )
a
R ¼ jjFoj ꢀ jFcjj=jFoj.
b
Rw¼[w(Fo2ꢀF2c)2/w(Fo2)2]1/2
.
the two side of the plane formed by S3eS6, while for 7B, the two
butyls nearly lie on the same plane with one formed by S9 to S12. In
the crystal structure of 7, both 7A and 7B are in diketone form, which
can also been confirmed by NMR and IR. For 7A, the distances of C1,
O2, and C4, O1 are 1.227 (3) A and 1.198 (3) A, respectively, showing
the existence of C]O. In addition, the bond lengths of C1eN2,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
N2eN1, and N1eC4 are 1.254 (4) A, 1.317 (3) A, and 1.320 (3) A,
Fig. 1. Molecular structures of pyrazinoTTFs 7 (a), 11 (b), 12 (c), and 13 (d).

N. Zheng et al. / Tetrahedron 68 (2012) 1782e1789
1785
respectively, showing the single CeN bonds and NeN bonds. The
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
bond lengths of 7B are similar with one of 7A. In the crystal structure
of compound 7A, the intermolecular NeH/O (N3eH3/O2_$1 $1:
x, ꢀ1þy, z; N4eH4/O1_$2 $2:1þx,1þy, z) hydrogen bonds with the
7
8
9
10
11
12
13
ꢀ
ꢀ
distances of H/O of 2.461 (6) A and 1.890 (5) A link the molecules to
form an infinite chain along [001] direction. Different with com-
pound 7, there is only one molecule in the asymmetric unit for
compounds 11e13. In addition, except for butyls, all the other non-
hydrogen atoms nearly lie on the same plan and all butyls in the
three compounds lie on the same side of the plan. Due tothe absence
of C]O in the six-membered heterocycle, there are some differ-
ences for neighboring CeN and NeN bond lengths comparing with
complex 7. For compound 11, the CeN and NeN bond lengths are
ꢀ
ꢀ
ꢀ
1.304 (5) A,1.314 (5) A, and 1.336 (4) A, respectively, which are longer
than one in compound 7. For compound 12, the CeO bond lengths
are 1.346 (4) A and 1.348 (4) A, which are much longer than one of
300
350
400
450
500
550
ꢀ
ꢀ
λ / nm
ꢀ
ꢀ
compound 7 with the distance of CeO of 1.227 (3) A and 1.198 (3) A,
showing the single bonds between C and O atoms. For compound 13,
1.2
(b)
ꢀ
ꢀ
the CeO bond lengths are 1.359 (3) A and 1.366 (3) A, which is
comparable with one of compound 12, also showing the existence of
single bond between C and O atoms. In compound 13, two benzene
rings exhibit different dihedral angles with TTF core plan, one is
53.44 (11)ꢁ and the other is 84.05 (16)ꢁ. For all of compounds 11e13,
0.0 eq.
1.0
0.8
0.6
0.4
0.2
0.0
0.5 eq.
1.0 eq.
1.5 eq.
2.0 eq.
2.5 eq.
3.0 eq.
there are no obvious hydrogen bonds or
crystal structure.
pep interactions in the
2.2. Photophysical and electrochemical properties
To fit the energetic scheme of the organic optoelectronic ma-
terials, it is necessary to determine the energy levels of the highest
occupied molecular orbital (HOMO) and lowest unoccupied mo-
lecular orbital (LUMO) and the width of the gap between the bands
of each component through electrochemistry.7 The energy gap
between the bands can moreover be easily determined with the
absorption spectroscopy.8
300
350
400
450
500
550
λ / nm
Fig. 2. (a) UVevis absorption spectra in 5ꢂ10ꢀ5
M CHCl3 solution of the pyr-
idazinoTTFs 7e13. (b) Variation of the UVevis absorption spectra of 13 in 5ꢂ10ꢀ5
M
The absorption spectra of 7e13 dissolved in CHCl3 (5ꢂ10ꢀ5 M)
are presented in Fig. 2a. They show a very weak and broad absorp-
tion band, whose highest point was located around 420 nm and an
intense absorption band centered around 320 nm, respectively. The
absorption bands centered around 420 nm result from an ICT
transition from the TTF unit to the substituted pyridazine moieties.
In the UV region, the strong absorption bands are characteristic for
CHCl3 solution upon successive addition of aliquots of NOPF6.
Table 2
UVevis absorption spectroscopy data, Eogpt, redox potentials (vs Ag/AgCl), and HOMO
energies obtained from cyclic voltammetry for pyridazinoTTF 7e13
Compd lmax,
l10%,
Eogpt/eVa E11/2/V
E E E
onset/ec/V HOMO/eVb LUMO/eVc
UV/nm max/nm
pep* transitions located on both, the TTF and the substituted pyr-
7e
8
9
10
11
12
13
387
430
432
443
432
425
410
458
520
518
545
526
501
491
2.79
2.45
2.46
2.35
2.43
2.55
2.60
0.743d 0.643
0.537 0.465
0.525 0.465
0.628 0.559
0.734 0.671
0.703 0.645
0.590 0.524
ꢀ5.03
ꢀ4.88
ꢀ4.88
ꢀ4.96
ꢀ5.07
ꢀ5.05
ꢀ4.92
ꢀ2.24
ꢀ2.43
ꢀ2.42
ꢀ2.61
ꢀ2.64
ꢀ2.45
ꢀ2.36
idazine subunits.9 The absorption maxima, lmaxUV, the values of 10%
of the absorption maximum taken from the lower energy side, l10%
max, and the optical band gap energies, Eogpt, are given in Table 2. As
can seen in Table 2, the optical band gap energy, Eogpt, of pyr-
idazinoTTFs 7e13 was estimated to be about 2.43e2.79 eV.8 It is
surprising that the number and kind of the substituting groups at
a
Optical band gap according UVevis absorption (onset method). Egopt
(10%)
pyridazine ring do not nearly influence the wavelength of
pep*
¼hc/l10%max [Ref. 8].
b
transition, but influence slightly the ICT transition and the optical
Energy level of highest occupied molecular orbital according cyclovoltammetry.
band gap energies, Eogpt, of pyridazinoTTFs.
E
HOMO/ELUMO¼[ꢀ(Eonsetꢀ0.45) ꢀ4.8] eV, where the value 0.45 V for ferrocene versus
Ag/Agþ and 4.8 eV the energy level of ferrocene below the vacuum [Ref. 8].
To investigate the electron-donating properties of synthesized
pyridazinoTTFs, chemical oxidation of 13 in CHCl3 solution
(5ꢂ10ꢀ5 M) was conducted with an excess TCNQ in CHCl3 solution
(Fig. S3). But no CT band was observed in the 400e1000 nm region,
which might be attributed to the presence of the electron-
withdrawing pyridazine ring. Whereas, upon 0.5 equiv of NOPF6
to a CHCl3 solution of 13 (5ꢂ10ꢀ5 M) resulted in the decreasing of
the absorption band around 319 nm, and the concomitant emer-
gence of new absorption bands characteristic for the formation of
the cation-radical species around 380 nm (Fig. 2b). Upon successive
addition of aliquots of NOPF6, the new absorption band at 380 nm
progressively increases and red-shifts to 402 nm, while the main
band at 319 nm continuously decreases, but no further changes
occurred after the addition of 1.5 equiv of NOPF6.
c
The LUMO levels were estimated from the HOMOeLUMO energy gaps, which
were obtained from the absorption onset since their reduction potential could not
be observed.
d
0.743 is the E1pa value because of the first redox of 7 is irreversible.
The CV and DPV measurement of compound 7 was performed in dry DMSO.
e
Cyclic voltammetry (CV) and/or differential pulse voltammetry
(DPV) are easy and effective methods to measure redox potentials
and to simultaneously evaluate both the HOMO and LUMO energy
levels and the band gap energy, Eegc, of an organic molecule. To
evaluate the electron-donating properties and the HOMO energy
levels of pyridazinoTTFs, the CV (Table 2 and Fig. 3) and DPV
(Fig. S4) measurements were performed in a mixture of dry
dichloromethane and acetonitrile (1:1) at room temperature. As

1786
N. Zheng et al. / Tetrahedron 68 (2012) 1782e1789
ability of the pyridazine part may provide low HOMO energy levels
and large energy gaps to improve its air/light stability,13 which
suggest that the pyridazine-TTF conjugates are promising hole
transporting organic materials.
To investigate the nature of the electronic transitions that give
rise to the absorption bands observed in the electronic spectra, the
lowest-energy singlet excited states were calculated for compound
12 using the time-dependent DFT (TDDFT) approach (see
Supplementary data). The results of the TD-DFT calculations per-
formed with the B3LYP, PBE0, BH and HLYP, and MPWPW91 func-
tionals in chloroform solvent are shown in Fig. 4. The excitation
energies, oscillator strengths, and transition characters are listed in
Table 3.
Exp.
MPWPW91/TZVP
BHandHLYP/TZVP
PBE0/TZVP
B3LYP/TZVP
300
400
Wavelength/nm
500
600
Fig. 4. Comparision between experimental (dot) and simulated electronic absorption
spectra of compound 12 at MPWPW91/TZVP level in chloroform solvent.
Fig. 3. Cyclic voltammograms of 8e10, 12e13 (a) and 7, 11 (b) in CH2Cl2/MeCN (1:1)
(1ꢂ10ꢀ3 M). Inset: differential pulse voltammogram of compound 7 (bottom) and 11
(top) in CH2Cl2/MeCN (1:1) (1ꢂ10ꢀ3 M).
Table 3
MPWPW91/TZVP calculated absorption wavelength (
(DE, in eV), oscillator strength (f ), and major transition contribution for 12 in
chloroform
l
, in nm), transition energies
anticipated from its structure, all of monopyridazinoTTFs exhibit
two reversible one-electron redox couples with the exception of
the non-substituted pyrazinoTTF 7 and the dichloro-substituted
pyrazinoTTF 11, which are associated with the successive oxida-
State
lcal
DE/eV
f
Transition nature
tion of the TTF unit to TTFþ and TTFþ2, respectively (Fig. 3). In
ꢃ
S1
S2
S3
S7
S12
534.0
491.0
425.1
350.4
333.4
2.32
2.53
2.92
3.54
3.72
0.0116
0.0011
0.0279
0.1121
0.1873
HOMO/LUMO (99%)
comparison to TTF (E11/2¼0.34 V, E12/2¼0.73 V),10 their oxidation
potentials are positively shifted by 190e400 mV owing to the an-
nulation of electron-withdrawing pyridazine ring. Compounds 8
and 9 exhibit typical two reversible oxidation waves with half-wave
potentials (E1/2) at w0.537 and 0.879 V, while compounds 10, 12,
and 13 showed split second redox couples, with E21/2 at w0.999 V
and w1.208 V, which probably is caused by adsorption phenomena
on the electrode11 (Fig. 3a). In contrast, CV analysis of 7 displays two
irreversible oxidation waves at E1pa¼0.662, Ep2a¼1.144 V (Fig. 3b).
Compound 11 containing two chlorine atoms displays a compli-
cated cyclic voltammogram, which showed four irreversible one-
electron oxidations at 0.799, 1.031, 1.199, and 1.379 V, respectively
(Fig. 3b). Obviously, the presence of the different substituents at the
pyridazine ring has a strong influence on the electron-donating
ability of TTF. It is noticeable that introduction of chlorine atom
probably induced the degradation of pyridazinoTTF system. The
redox potentials, E11/2/V, the onset potentials, Eonset/ec/V, and HOMO
energies, EHOMO/eV, obtained from cyclic voltammetry for Pyr-
idazinoTTF 7e13 are listed in Table 2. As shown in Table 2, their
HOMO levels were estimated from the onset of oxidation potentials
to be between ꢀ4.88 and ꢀ5.07 eV. HOMO levels of good P-type
organic semiconductors are known typically to be in the range
of ꢀ4.9 to ꢀ5.5 eV.12 Clearly, such a combination of multi-redox
property of the TTF moiety and strong electron-withdrawing
HOMO/LUMOþ1 (100%)
HOMO/LUMOþ2 (95%)
HOMO/LUMOþ3 (27%)
HOMO/LUMOþ4 (61%)
HOMO/LUMOþ3 (38%)
HOMO/LUMOþ4 (20%)
HOMO/LUMOþ6 (32%)
MPWPW91 provides transition wavelengths and band shape in
better agreement with experimental data than other functionals.
MPWPW91 calculations predict that the long wavelength absorp-
tion bands observed for 12 at 425 nm is due to electronic transitions
to the first three excited singlets, S1 to S3, calculated at
2.32e2.92 eV. The S1 state results from the HOMO/LUMO exci-
tation (Fig. 5), which is assigned as charge transfer from TTF to
pyridazine moiety. The S3 state originates in the HOMO/LUMOþ2
excitation, and corresponds to an electron excitation from TTF to
sulfur atoms in dibutyl and TTF moiety. Therefore, calculations
confirm the charge transfer (CT) nature of the low-energy absorp-
tion bands of 12. For the excited states at the intense absorption
intensity is 333 nm, and mainly corresponds to S12 state electron
excitation from HOMO-2 to LUMOþ3, which are assigned as charge
transfer from pyridazine ligand to TTF. Moreover, the absorption
property of the molecule in DMSO is same to that in chloroform.

N. Zheng et al. / Tetrahedron 68 (2012) 1782e1789
1787
Fig. 5. Electron density contours (0.03 a.u.) calculated for the frontier orbital of molecule 12 at MPWPW91/TZVP level in chloroform.
3. Conclusion
(15 mL) was refluxed for 4 h. After cooling, the precipitate was
filtered off and was washed by EtOH to give 5 (60 mg, 73.7%), Mp
>290 ꢁC; IR (cmꢀ1): 3182 v (NeH), 1658 v (amide C]O); MS (ESI)
m/z: 371.0 ([MꢀH]ꢀ, 100).
In conclusion, we have synthesized a series donoreacceptor
type pyridazine-TTF conjugates in which pyridazine ring part
annulated directly to a TTF part. The studies on absorption spectra
suggest the intramolecular charge transfer interaction between TTF
and pyridazine parts in ground states, which is rationalized on the
basis of density functional theory. Their HOMO energy levels and
Eogpt values were estimated to be ꢀ4.88 to ꢀ5.07 eV from cyclic
voltammetry and 2.43e2.79 eV from the absorption spectra, re-
spectively. These results suggest that the present conjugates can be
considered as candidates for hole transporting organic materials,
such as semiconducting applications.
4.2.2. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-5,6-dihydro-[1,3]
dithiolo[4,5-d]pyridazine-4,7-dione (7). A mixture of 6 (500 mg,
1.212 mmol) and hydrazine hydrate (5 mL) in EtOH (45 mL) was
refluxed for 4 h. After cooling, the precipitate was filtered off and
was washed with EtOH. The solid was recrystallized from DMF to
give 7 as a red prisms (245.6 mg, 53%). Mp 209e210 ꢁC; 1H NMR
(300 MHz, DMSO-d6):
d
2.87 (t, J¼6.90 Hz, 4H), 1.56e1.48 (m, 4H),
1.42e1.20 (m, 4H), 0.87 (t, J¼7.20 Hz, 6H); 13C NMR (75 MHz,
DMSO-d6):
d 153.00, 136.80, 127.74, 110.56, 34.67, 31.56, 21.37,
4. Experimental section
14.62; UVevis (DMSO)
l
(nm) (
L molꢀ1 cmꢀ1): 306 (15,460, sh),
3
322 (17,420), 387 (4280); IR (cmꢀ1): 3165 v (NH), 1647 v (C]O); MS
(ESI) m/z: 462.9 ([MꢀH]ꢀ, 100); Anal. Calcd for C16H20N2O2S6: C,
41.35; H, 4.34; N, 6.03. Found C, 41.56; H, 4.30; N, 5.99.
4.1. Materials and apparatus
Commercially available compounds were used without further
purification. Solvents were dried according to standard procedures.
Compound 1,14 2,15 4,16 and 617 were synthesized according to the
reported procedures. All reactions were magnetically stirred and
monitored by thin-layer chromatography (TLC) using QingDao GF254
silica gel coated plates. UVevis spectra were recorded with a Shi-
madzu UV-2550 spectrophotometer. NMR spectra were recorded on
a Bruker AV-300 Spectrometer (300 MHz for 1H and 75 MHz for 13C),
and chemical shifts were referenced relative to tetramethylsilane
4.2.3. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-7-methoxy-[1,3]
dithiolo[4,5-d]pyridazin-4(5H)-one (8). A mixture of 7 (100 mg,
0.2152 mmol) and K2CO3 (29.74 mg, 0.22 mmol) in DMF (20 mL) was
stirred at room temperature for 30 min, then MeI (13.4 mL, 0.22 mmol)
was added. The mixture was stirred for 2 h and was monitored by TLC.
Upon completion, the solution was evaporated in vacuo to dryness.
The crude product was purified by column chromatography on silica
gel using EtOAc/petroleum ether (v/v, 1:5) as the eluant to give
a yellow solid. Recrystallization of the solid from DMF gave 8 as
a yellow powder (58.6 mg, 57%). Mp 183e184 ꢁC; 1H NMR (300 MHz,
(dH/dC¼0). IR spectra were recorded on a Shimadzu FT-IR Prestige-21
instrument (KBr pressed disc method). Cyclic voltammetric studies
were carried out on a Potentiostat/Galvanostat 273A instrument in
CH2Cl2/CH3CN (1:1, c¼1ꢂ10ꢀ3 M) and 0.1 M Bu4PF6 as the supporting
electrolyte and scan rate is 100 mV Sꢀ1. Counter and Working elec-
trodes were made of Pt and Glass-Carbon, respectively, and the ref-
erence electrode was calomel electrode (Ag/AgCl). MALDI-TOF mass
data were obtained by a Shimadzu AXIMA-CFRÔ plus mass spec-
trometry, using a 1, 8, 9-anthracenetriol (DITH) matrix.
CDCl3):
d
11.22 (br, 1H, NH), 3.89 (s, 3H), 2.84 (t, J¼7.26 Hz, 4H),
1.67e1.58 (m, 4H),1.51e1.39 (m, 4H), 0.94 (t, J¼7.29 Hz, 6H); 13C NMR
(75 MHz, CDCl3):
d 155.43, 148.64, 139.34, 134.29, 128.40, 127.62,
115.23,107.23, 55.11, 36.12, 31.75, 21.64,13.58; UVevis (CHCl3) l (nm)
(
L molꢀ1 cmꢀ1): 306 (16,268, sh), 322 (18,440), 430 (1640); IR
3
(cmꢀ1): 3128 v (NH), 1653 v (C]O); MS (MALDI-TOF): m/z 479.7
([MþH]þ, 100); Anal. Calcd for C17H22N2O2S6: C, 42.65; H, 4.63; N,
5.85. Found C, 42.56; H, 4.41; N, 5.80.
4.2. Synthesis
4.2.4. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-7-methoxy-5-
methyl-[1,3]dithiolo[4,5-d]pyridazin-4(5H)-one (9). A mixture of 7
(50 mg, 0.11 mmol) and K2CO3 (32.7 mg, 0.24 mmol) in DMF (10 mL)
4.2.1. Bis(3,6-dioxopyridazino[4,5-d])tetrathiafuvlalene (5). A mix-
ture of 4 (95 mg, 0.219 mmol) and hydrazine hydrate (3 mL) in EtOH

1788
N. Zheng et al. / Tetrahedron 68 (2012) 1782e1789
was stirred at room temperature for 30 min, then MeI (14.7
m
L,
6H), 2.86 (t, J¼7.2 Hz, 4H),1.65e1.58 (m, 4H),1.49e1.46 (m, 4H), 0.96
0.24 mmol) was added. The mixture was stirred and was monitored
by TLC. Upon completion, the solution was evaporated in vacuo to
dryness. The crude product was purified by column chromatography
on silica gel using EtOAc/petroleum ether (v/v, 1:5) as the eluant to
give a yellow solid. Recrystallization of the solid from hexane gave 9
asayellow powder (51.1 mg, 96%). Mp99e100 ꢁC; 1H NMR (300 MHz,
(t, J¼7.3 Hz, 6H); 13C NMR (75 MHz, CDCl3):
d
156.31, 131.05, 128.13,
(nm) (
115.25, 100.18, 55.36, 31.95, 21.83, 13.79; UVevis (CHCl3)
l
3
L molꢀ1 cmꢀ1): 305 (21,460, sh), 319 (22,500), 404 (2460); IR (cmꢀ1):
1572 v, 1466 v, 1373 v (aromatic heterocycle), 1039 (CeO); MS
(MALDI-TOF): m/z 492.1 (Mþ, 100); Anal. Calcd for C18H24N2O2S6: C,
43.87; H, 4.91; N, 5.68. Found C, 44.01; H, 4.78; N, 5.66.
CDCl3): d 3.87 (s, 3H), 3.65 (s, 3H), 2.85e2.79 (m, 4H), 1.67e1.56 (m,
4H), 1.50e1.38 (m, 4H), 0.93 (t, J¼7.5 Hz, 6H); 13C NMR (75 MHz,
4.2.8. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-4,7-diphenoxy-
[1,3]dithiolo[4,5-d]pyridazine (13). A mixture of 11 (32.2 mg,
0.06 mmol), NaH (3.4 mg, 0.14 mmol), and phenol (13.3 mg,
0.14 mmol) in DMF (5 mL) was stirred at room temperature for 1 h.
The mixture was poured into EtOAc and washed with water. The
organic layer was dried over anhydrous magnesium sulfate, and
concentrated. The crude product was purified by column chroma-
tography on silica gel using EtOAc/petroleum ether (v/v, 1:9) as the
eluant to give a yellow solid. Recrystallization of the solid from
hexane gave 13 as an orange fine needle (25.3 mg, 64%). Mp
CDCl3): d 153.79, 147.12,140.17, 132.44,128.64,127.60,114.53,108.33,
55.08, 39.10, 36.28, 31.98, 21.81, 13.76; UVevis (CHCl3)
l
(nm) (
3
L molꢀ1 cmꢀ1): 307 (18,300, sh), 323 (21,660), 432 (1840); IR (cmꢀ1):
1647 v (C]O),1578 v (C]N); MS (MALDI-TOF): m/z 491.8 (Mþ, 100);
Anal. Calcd for C18H24N2O2S6: C, 43.87; H, 4.91; N, 5.68. Found C,
44.00; H, 4.61; N, 5.72.
4.2.5. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-7-oxo-6,7-
dihydro-[1,3]dithiolo[4,5-d]pyridazin-4-yl acetate (10). A mixture of
7 (26.2 mg, 0.056 mmol), acetyl chloride (1 mL), and dry triethyl-
amine (0.6 mL) in dry CH2Cl2 (3 mL) was stirred at room temper-
ature for 2 h under N2. Then the solution was evaporated in vacuo to
dryness. The crude product was purified by column chromatogra-
phy on silica gel using EtOAc/petroleum ether (v/v, 1:2) as the el-
uant to give a red solid. The solid was recrystallized from
acetonitrile to give 10 as red needles (10.6 mg 37%). Mp
102e103 ꢁC; 1H NMR (300 MHz, CDCl3):
d 7.40e7.34 (m, 4H),
7.22e7.18 (m, 6H), 2.86 (t, J¼7.2 Hz, 4H), 1.70e1.60 (m, 4H),
1.53e1.41 (m, 4H), 0.95 (t, J¼7.3 Hz, 6H); 13C NMR (75 MHz, CDCl3):
d
156.84, 153.13, 132.64,129.77, 128.20, 125.71, 121.36, 116.75, 107.11,
36.35, 31.96, 21.84, 13.81; UVevis (CHCl3)
l
(nm) (
L molꢀ1 cmꢀ1):
3
308 (18,560, sh), 321 (20,000), 415 (1760); IR (cmꢀ1): 1568 v, 1489 v,
1379 (aromatic heterocycle), 1209 (CeO); MS (MALDI-TOF): m/z
617.0 ([MþH]þ, 100); Anal. Calcd for C28H28N2O2S6: C, 54.51; H,
4.57; N, 4.54. Found C, 54.52; H, 4.58; N, 4.60.
165e166 ꢁC; 1H NMR (300 MHz, CDCl3):
d 11.18 (br, 1H, NH), 2.83 (t,
J¼6.0 Hz, 4H), 2.34 (s, 3H), 1.67e1.57 (m, 4H), 1.50e1.38 (m, 4H),
0.95 (t, J¼7.20 Hz, 6H). 13C NMR (75 MHz, CDCl3):
d 167.14, 155.18,
142.04, 140.91, 137.01, 128.89, 127.53, 117.02, 105.01, 36.26, 31.86,
4.3. X-ray crystal structure determination
21.70, 20.54, 13.65; UVevis (CHCl3)
l
(nm) (
L molꢀ1 cmꢀ1): 307
3
(17,440, sh), 322 (18,640), 443 (1640); IR (cmꢀ1): 1780 v (ester C]
O), 1649 (amide C]O), 1170 v (CeO); MS (MALDI-TOF): m/z 505.8
(Mþ, 100); Anal. Calcd for C18H22N2O3S6: C, 42.66; H, 4.38; N, 5.53.
Found C, 43.00; H, 4.50; N, 5.63.
X-ray diffraction experiment was performed with a Rigaku/MSC
mercury diffractometer with graphite monochromated Mo K
a ra-
diation (l¼0.71073 E) at 293 K. Empirical absorption corrections
based on equivalent reflections were applied. The crystal structures
were solved by direct method and refined by full matrix least
squares fitting on F2 using the SHELXTL-97 software. All non-
hydrogen atoms were refined with anisotropic thermal parame-
ters. Crystallography data has been deposited to the Cambridge
Crystallography Data Centre with deposition number of
846983e846986 for 7, 11e13. Copies of this information may be
obtained free of charge from The CCDC, 12 Union road, Cam-
bridgeCB2 1EZ, UK (fax: þ44 1223 336033; e-mail: depos-
it@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
4.2.6. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-4,7-dichloro-[1,3]
dithiolo[4,5-d]pyridazine (11). A mixture of 2 (208 mg, 0.45 mmol)
in neat POCl3 (5 mL) was refluxed overnight. After cooling, the
excess POCl3 was removed under reduced pressure. The residual
mixture was poured into water and extracted by CH2Cl2, the or-
ganic layer was separated, dried over anhydrous magnesium sul-
fate, and concentrated. The crude product was purified by column
chromatography on silica gel using EtOAc/petroleum ether (v/v,
1:9) as the eluant to give a red solid. Recrystallization of the solid
from hexane gave 11 as a red needles (201.7 mg, 90%). Mp
Acknowledgements
92e93 ꢁC; 1H NMR (300 MHz, CDCl3):
d
2.84 (t, J¼7.2 Hz, 4H),
1.65e1.58 (m, 4H), 1.49e1.41 (m, 4H), 0.94 (t, J¼7.3 Hz, 6H); 13C
The authors acknowledge financial support from the National
Natural Science Foundation of China (grant No.21062022) and the
Specialized Research Fund for the Doctoral Program of Higher Ed-
ucation (Grant No. 20102201110001).
NMR (75 MHz, CDCl3):
d 146.84, 142.58, 128.34, 120.37, 101.05,
36.40, 31.92, 21.81, 13.79; UVevis (CHCl3)
l
(nm) (
L molꢀ1 cmꢀ1):
3
316 (16,420), 355 (7,080, sh), 437 (1600); IR (cmꢀ1): 1485 v, 1267 v
(aromatic heterocycle); MS (MALDI-TOF): m/z 500.1 (Mþ, 100);
Anal. Calcd for C16H18Cl2N2S6: C, 38.31; H, 3.62; N, 5.58. Found C,
39.00; H, 3.61; N, 5.56.
Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2011.12.034.
4.2.7. 2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-4,7-dimethoxy-
[1,3]dithiolo[4,5-d]pyridazine (12). Compound 11 (63.8 mg,
0.13 mmol) was added to solution of sodium (640 mg) in dry
methanol (10 mL) and THF (5 mL). The reaction mixture was
refluxed overnight. After cooling, the solvent was removed under
reduced pressure. The solid residue was extracted by EtOAc and
washed with water, the organic layer was dried over anhydrous
magnesium sulfate, and concentrated. The crude product was pu-
rified by column chromatography on silica gel using EtOAc/petro-
leum ether (v/v, 1:5) as the eluant to give an orange solid.
Recrystallization of the solid from hexane gave 12 as a red prism
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