Donor-acceptor thieno[2,3-b]thiophene systems: Synthesis and structural study of 3-anisyl-4-pyridyl(pyridinium) thieno[2,3-b]thiophenes

Article abstract of DOI:10.1016/j.molstruc.2003.10.026

Synthesis of a new class of cofacially oriented neutral donor-acceptor thienothiophene, namely 3-anisyl-4-pyridyl thieno[2,3-b]thiophenes 7 and its ionic analog 8 has been described to probe the presence of through-space charge transfer interaction. However, structural studies by ¹H NMR and UV-visible data indicate at best rather weak charge transfer interaction. The single crystal X-ray analysis of the ionic donor-acceptor thienothiophene 8 reveals many interesting features, i.e. (i) the donor and acceptor rings are not parallel to each other (ii) the interchromophoric distance is much larger (4.08?) than the van der Waals distance and (iii) the torsional angle between the pyridinium ring and the thienothiophene is unusually small, being only 38°. These structural features preclude effective through-space charge transfer interaction in 8. Conformational analysis of a model system, 3,4-diphenyl thienothiophene indicated that conformations with reduced torsion between the phenyl rings and the thienothiophene plane are possible to maintain a balance between conjugation and steric congestion and the calculated low energy barrier between the high and low energy conformations of the model system suggest the possibility of aryl ring rotations in peri disubstituted thienothiophenes.

Full text of DOI:10.1016/j.molstruc.2003.10.026

Journal of Molecular Structure 689 (2004) 107–113
www.elsevier.com/locate/molstruc
Donor–acceptor thieno[2,3-b]thiophene systems: synthesis and structural
study of 3-anisyl-4-pyridyl(pyridinium) thieno[2,3-b]thiophenes
a,
a
a
*
Sabir H. Mashraqui , Mohamed Ashraf , Harini Hariharasubrahmanian ,
Richard M. Kelloggb, A. Meetsmab
aDepartment of Chemistry, University of Mumbai, Vidyanagari, Santacruz (E), Mumbai 400 098, India
bDepartment of Chemistry, University of Groningen, Nijenborg 19, Groningen 9747 AG, The Netherlands
Received 25 September 2003; revised 28 October 2003; accepted 30 October 2003
Abstract
Synthesis of a new class of cofacially oriented neutral donor–acceptor thienothiophene, namely 3-anisyl-4-pyridyl thieno[2,3-b ]thio-
phenes 7 and its ionic analog 8 has been described to probe the presence of through-space charge transfer interaction. However, structural
1
studies by H NMR and UV–visible data indicate at best rather weak charge transfer interaction. The single crystal X-ray analysis of the
ionic donor–acceptor thienothiophene 8 reveals many interesting features, i.e. (i) the donor and acceptor rings are not parallel to each other
˚
(ii) the interchromophoric distance is much larger (4.08 A) than the van der Waals distance and (iii) the torsional angle between the
pyridinium ring and the thienothiophene is unusually small, being only 388. These structural features preclude effective through-space charge
transfer interaction in 8. Conformational analysis of a model system, 3,4-diphenyl thienothiophene indicated that conformations with
reduced torsion between the phenyl rings and the thienothiophene plane are possible to maintain a balance between conjugation and steric
congestion and the calculated low energy barrier between the high and low energy conformations of the model system suggest the possibility
of aryl ring rotations in peri disubstituted thienothiophenes.
q 2003 Published by Elsevier B.V.
Keywords: Donor–acceptor thienothiophenes; Synthesis; Charge transfer interaction; Spectral data; Single crystal X-ray
1. Introduction
through-space p–p and polar interactions between the
closely spaced aryl/heteroaryl groups [6].
In analogy to 1,8-diaryl naphthalenes, we envisaged that
thieno[2,3-bthiophene framework I, a 10p analog of
naphthalene could also be exploited to design potentially
interesting intramolecular c.t. systems by placing donor and
acceptor at its C3 and C4 ( peri) positions to create
structures of the type II (Chart 1). Based on molecular
modelling and analogy to 1,8-diarylnaphthalene systems,
we expected the donor and acceptor substituents to occupy
near parallel orientation to each other, but orthogonal with
respect to the thienothiophene plane in order to minimize
steric congestion. We presumed that such geometry would
be well suited to exhibit through-space c.t. interaction in
systems of type II. Thus, in pursuit of the above goal, we
chose our target as the compounds 7/8 having p-methox-
yphenyl as the donor and pyridine/pyridinium rings as
the acceptor substituents attached to the peri positions of the
thienothiophene framework.
Charge transfer (c.t.) interaction has been recognized as a
key molecular interaction in numerous technological
applications such as in developing conducting materials,
photonic devices, dyes, sensors and selective chromato-
graphic materials [1]. In addition, the c.t. phenomena are
also widely implicated in reaction mechanisms, host–guest
interaction, crystal packing and self-organized supramole-
cular systems [2]. In recent years, several molecular designs
which include cyclophanes [3] (A), trypticene analogs [4]
(B) and 1,8-diarylnaphthalenes [5] (C) have been developed
to entice through-space c.t. interaction by placing donor and
acceptor in close proximity to each other (Chart 1). In the
case of 1,8-diaryl naphthalenes, several elegant studies
have been carried out to demonstrate the presence of
*
Corresponding author. Tel.: þ912226526119; fax: þ912226528547.
E-mail address: sh_mashraqui@yahoo.com (S.H. Mashraqui).
0022-2860/$ - see front matter q 2003 Published by Elsevier B.V.
doi:10.1016/j.molstruc.2003.10.026

108
S.H. Mashraqui et al. / Journal of Molecular Structure 689 (2004) 107–113
Chart 1. Some novel charge transfer systems.
2. Experimental
30 mmol) in dry DMF (20 ml) was added at room
temperature to a vigorously stirred mixture of dry DMF
(20 ml) containing anhyd K2CO3 (3.0 g) and a catalytic
amount of tetrabutylammonium bromide (100 mg). The
addition was completed during 30 min and then ethyl
bromoacetate (3.3 ml, 30 mmol) was added dropwise during
30 min. The reaction mixture was stirred for 24 h, diluted
with cold 5% cold aq NaOH solution and extracted with
diethyl ether. After water washing, the organic extract was
dried over anhyd Na2SO4. Removal of the solvent gave a red
oil which was subjected to silica gel column chromato-
graEberhardt, Hans (eluant ethyl acetate–petroleum ether
30:70) to afford 4 (60%) as an unstable light yellow oil. IR
(oil film, n cm21): 2830, 1720, 1685, 1620, 1470, 1383,
1185, 1124, 1040, 955. The crude 4 was immediately used
as such for the next reaction.
The chemicals and solvents for synthesis and column
chromatography were purchased from S/D Fine Chemicals
and Merck (India) and used as received. Melting points
(uncorrected) were determined on a Gallenkamp melting
point apparatus. IR spectra were recorded on a Shimadzu
FTIR-420 spectrophotometer. 1H NMR spectra were
recorded on a Bruker-AMX-500 spectrometer with TMS
as an internal standard. Mass Spectra were recorded on
GCMS-QP 5050A Shimadzu spectrophotometer and UV–
visible spectra were taken on Shimadzu UV–visible
spectrophotometer UV-2100.
1-(4-Methoxyphenyl)3-(3-pyridyl)propane1,3-dione (3).
solution of 4-methoxyacetophenone 1 (19.5 g,
A
130 mmol) and ethyl nicotinate 2 (18.0 g, 130 mmol) in
dry DMF (50 ml) was added dropwise during 30 min to a
stirred suspension of sodium hydride (7.0 g, 260 mmol) in
dry DMF (150 ml) at 0 8C. The reaction mixture was
stirred further at this temperature for 4 h and then allowed
to warm-up to room temperature during next 2 h. The
contents of the reaction were poured into cold water
(1.5 l) and neutralized with acetic acid. The precipitated
yellow solid was filtered under suction, washed well with
water and air-dried. Crystallization from ethanol gave
3- (4- Methoxyphenyl) 4(3- pyridyl) thieno[2,3-b]thio-
phene-2,5-diethylester (5). The crude 4 (1.53 g, 3 mmol,
purity ca. 90%) was dissolved in absolute ethanol (5 ml) and
added dropwise at 0 8C during 10 min to a stirred solution of
sodium ethoxide (prepared from of 230 mg Na) in absolute
ethanol (20 ml). The reaction was stirred at this temperature
for 30 min and then poured over crushed ice. A yellowish
brown solid which seperated out was filtered and washed
thoroughly with water. The crude solid was purified by
repeated crystallization from ethy acetate–petroleum ether
(1:3) to provide 5 as a light yellow solid, mp 115–118 8C in
50% yield. IR (KBr, n cm21) 2905, 1703, 1680, 1605, 1580,
1504, 1460, 1410, 1382, 1220, 1165, 840. 1H NMR
(200 MHz, CDCl3) d 1.02 (3H, t), 1.05 (3H, t,
J ¼ 7:5 Hz), 4.1–4.2 (4H, two q, J ¼ 7:5 Hz), 6.40 (2H,
d, J ¼ 8 Hz), 6.66 (2H, d, J ¼ 8 Hz), 6.87 (1H, m), 7.17
(1H, d, J ¼ 6:5 Hz), 8.1 (1H, s), 8.27 (1H, d, J ¼ 6:5 Hz).
19.5 g (66%) of 3, mp 133–135 8C. IR (KBr, n cm21
)
2903, 1620, 1600, 1542, 1421, 1380, 1203, 1075, 860,
1
802; H NMR (60 MHz, CDCl3) d 3.9 (3H, s), 6.5 (2H, s),
7.05–8.80 (8H, m), 9.20 (1H, s). Anal. calcd for
C15H13NO3: C, 70.58%; H, 5.09%; N, 5.49%; found: C,
70.83; H, 4.88; N, 5.27.
Ketene thioacetal 4. A solution of diketone 3 (3.75 g,
15 mmol) and freshly distilled carbon disulfide (2.03 ml,

S.H. Mashraqui et al. / Journal of Molecular Structure 689 (2004) 107–113
109
Anal. Calcd for C24H21NO5S2; C, 61.67%; H, 4.50%; N,
2.99%; S, 13.70%. Found. C, 61.93; H, 4.34; N, 2.95; S,
13.46.
Table 1
1H NMR spectral assignments for 7 and 8
Preparation of the diacid 6. The diester 5 (1.0 g,
2.27 mmol) was dissolved in 20 ml of DMSO containing
10% aq KOH (10 ml) and the reaction mixture heated at
100 8C for 10 h. The reaction was cooled, diluted with water
and acidified with conc. HCl to ca. pH 3. The precipitated
diacid 6 was obtained as a light brown solid in 85% yield, mp
235–240 8C. IR (KBr, n cm21) 3400, 3060, 1683, 1622,
1580, 1507, 1485, 1341, 1300, 1218, 1155, 1103, 998, 880.
Anal. Calcd for C20H13NO5S2; C, 58.39%; H, 3.16%;
N, 3.40%; S, 15.57%. Found. C, 58.55; H, 3.35; N, 3.22;
S, 15.38.
Type of protons Chemical Shifts in d
Dd
7
8
3-Anisyl-4-pyridyl thieno[2,3-b]thienophene (7). The
diacid 6 (400 mg, 0.97 mmol) was taken in a Slenck
tube and heated for 10 min in a silicone oil-bath kept at
240–250 8C under N2 atmosphere. The reaction mixture,
which had turned dark brown was cooled to room
temperature, extracted with chloroform and filtered to
remove a small amount of insoluble residue. The filtrate
was concentrated and the crude product purified by SiO2
column chromatography using petroleum ether as an eluant
to provide the required product 7 as a pale yellow solid, mp
77–79 8C in 32% yield. IR (KBr, n cm21) 2980, 1605,
1403, 1350, 1178, 1112, 1003, 920, 840, 821. For 1H NMR
(200 MHz, DMSO-d6) see Table 1. MS (EI, 70 eV, m/z (%))
323 (Mþ, 100), 293 (45), 279 (17), 246 (11), 149 (15), 132
(9). UV–visible (CH3CN, l; nm ð1maxÞ 236.5 (20,000), 277
(9000)). Anal. calcd for C18H13NOS2: C, 16.87%; H,
4.02%; N, 4.33%; S, 19.81%. Found. C, 16.91; H, 3.88;
N, 4.56; S, 20.13.
H(a)
7.52 (s, 1H)
7.72 (s, 3H)
8.27 (s, 1H)
7.64 (s, 1H)
8.05 (s, 3H)
8.67 (s, 1H)
0.12
0.33
0.40
H(b)
H(20)
H(40)
H(50)
H(60)
H(2)
7.34 (d, 1H, J ¼ 8 Hz) 8.22 (d, 1H, J ¼ 8 Hz) 0.88
7.08 (t, 1H, J ¼ 8 Hz) 7.88 (t, 1H, J ¼ 8 Hz) 0.80
8.37 (d, 1H, J ¼ 5 Hz) 8.82 (d, 1H, J ¼ 8 Hz) 0.45
6.63 (d, 1H, J ¼ 8 Hz) 6.75 (d, 1H, J ¼ 8 Hz) 0.12
6.92 (d, 1H, J ¼ 8 Hz) 7.00 (d, 1H, J ¼ 8 Hz) 0.08
H(3)
O–CH3
N–CH3
3.70 (s, 3H)
–
3.77 (s, 3H)
4.05 (s, 3H)
0.07
3. Results and discussion
The synthetic strategy towards the target structures 7 and
8 (Scheme 1) entail as the key step the double Dieckman
cyclization [7] on the oxo-ketene dithioacetals intermediate
4. To start with, we prepared the known diketone 3 more
conveniently by condensing p-methoxyacetophenone 1 with
nicotinate ester 2 using NaH as the base instead of NaNH2
reported in the Patent [8]. The condensation of 3 with CS2
under anhydrous K2CO3/DMF conditions followed by
treatment with two equivalents of ethyl bromoacetate
provided ketene dithioacetal 4 as an oil. The treatment of
4 with sodium ethoxide in absolute alcohol led to a
spontaneous cyclization to afford the desired thienothio-
phene diester 5 in 50% yield as a yellow crystalline solid,
mp 115–118 8C. Base catalyzed hydrolysis of 5 (KOH in aq
DMSO) afforded upon acidification diacid 6 in excellent
yield. Finally, thermal decarboxylation of 6 followed by
SiO2 chromatographic purification of the crude gave the
target thienothiophene 7 in 32% yield, mp 77–79 8C, Mþ at
m=z 323. To enhance the acceptor character of the pyridyl
ring, the compound 7 was quarternized with CH3I to obtain
N-methyl pyridinium salt 8. The latter was expected to
exhibit c.t. interaction which could be evaluated in
comparison to the neutral 7 as the reference standard.
Preparation of the N-methyl pyridinium salt 8. The
thieno[2,3-b ]thiophene 7 (50.0 mg) was dissolved in dry
acetonitrile (2.0 ml) and freshly distilled CH3I (0.35 ml)
was added. The reaction mixture was kept at room
temperature for 48 h whereby yellow crystals were
deposited. The solvent was removed by decantation, the
crystals washed with little cold acetonitrile and then dried
over KOH under vacuum to give 8 in almost quantitative
yield, mp 210–215 8C (dec). IR (KBr, n cm21) 3007, 1600,
1433, 1378, 1330, 1176, 1121, 1020, 929, 843, 821. For 1H
NMR (200 MHz, DMSO-d6) see Table 1. UV–visible
(CH3CN, l; nm ð1maxÞ 244 (42,000), 285 (11,000)).
Anal. calcd for C19H16NOS2I: C, 49.03; H, 3.44; N, 3.01;
S, 13.76; I, 27.31. Found. C, 49.34; H, 3.22; N, 2.87;
S, 13.98; I, 27.66.
The crystal structure of 8. The crystal, a parallello-
piped of approximate size 0.25 £ 0.30 £ 0.30 mm3 was
used for characterization and data collection. Unit cell
parameters and orientation matrix were determined from
a least squares treatment of SET 4 setting angles of 22
reflections in the range 16.49 , 0 , 21.848. The structure
was solved by Patterson methods using the DIRDIF
program.
1
3.1. Structural study: H NMR and UV analysis of 7 and 8
The presence of intramolecular c.t. interaction involving
the overlap of the HOMO of the p-rich anisyl donor to

110
S.H. Mashraqui et al. / Journal of Molecular Structure 689 (2004) 107–113
Scheme 1. Synthesis of thienothiophene 7 and its ionic salt 8. Reagents and conditions: (i) NaH/DMF, 0 8C-RT, 6 h; (ii) CS2/Anhyd.K2CO3, RT, 1 h, then 2eq
BrCH2CO2C2H5, RT, 24 h; (iii) C2H5ONa/C2H5OH, 0 8C, 1 h; (iv) DMSO þ aq KOH, 100 8C, 10 h; (v)D; 240–250 8C, 10 min, N2 atm and (vi) CH3I/CH3CN,
RT, 48 h.
the LUMO p-deficient pyridinium ring would be expected
to diminish the electron density around the anisyl ring in the
ionic compound 8 in comparison to the neutral 7.
The reduced electron density would reflect itself in the
deshielding of the anisyl protons in 8 relative to that of the
neutral 7. Accordingly, the magnitude of Dd (the deshield-
ing coefficient) [9] could at least provide a qualitatively
indication of the c.t. interaction in pyridinium salt 8.
the shorter wave length absorption band is slightly red
shifted with marked enhancement in its intensity (lmax
244 nm; 1max 4.2 £ 104). Though, not well defined, there is
also present a longer wavelength absorption in 8, which is
relatively broad and extends up to 370 nm. However, no
new absorption band at longer wavelength arising out of
the c.t. interaction between the donor anisyl ring and
the acceptor pyridinium ring could be observed for 8.
Nevertheless, the presence of a broad band extending up to
370 nm in 8 could be indicative of a modest c.t. Earlier, we
1
The H NMR chemical shift assignments for 7 and 8
together with deshielding coefficient Dd are summarized in
Table 1. It can be seen that all the protons attached to the
pyridinium ring in 8 are downfield shifted in the range from
0.40 to 0:88d relative to 7 so also the thienothiophene
protons Ha and Hb experience downfield shift by 0.33 and
0:12d; respectively, with respect to the compound 7. These
downfield shifts are a direct consequence of the presence of
the positively charged pyridinium ring in 8. More
importantly, in context to the c.t. interaction, H2 and H3
protons of anisyl group as well as the methoxyl protons in 8
also experience downfield shift, though by a small margin of
only 0.08 and 0:12d; respectively. As stated above, these
downfield shifts form the basis for evaluating the degree of
c.t. interaction. Unfortunately, the magnitude of Dd for the
protons associated with the anisyl ring is rather small
ð0:08–0:12dÞ to indicate the presence of significant through-
space c.t. in 8 relative to 7 [10].
The UV spectrum of compound 7 was found to exhibit a
strong band at 236.5 nm (1max 2.0 £ 104) and a weak band at
277 nm (1max 9 £ 103) which tapers-off at 320 nm (Fig. 1).
In comparison to 7, in the case of corresponding ionic salt 8,
Fig. 1. UV–visible spectra of 7 (plot I) and 8 (plot II) in CHCl3.

S.H. Mashraqui et al. / Journal of Molecular Structure 689 (2004) 107–113
111
reached similar conclusion on the basis of 1H NMR spectral
analysis.
Table 2
Summary of crystallographic data and refinement details
Empirical formula
Formula mass
[C19H16NOS2]þ·I2
465.38
3.2. X-ray crystal structural analysis of pyridinium salt 8
Crystal color and habit
Crystal size (nm3)
Crystal system
˚
a (A)
˚
b (A)
Yellow, block
0.25 £ 0.30 £ 0.30
Monoclinic
8.816(1)
14.411(1)
15.174(1)
106.685(7)
1846.7(3)
4.0
In order to understand the underlying reasons for the
weak through-space c.t. interaction in 8, we carried out its
single crystal X-ray analysis [11]. The ORTEP diagram of
the X-ray structure of 8 is shown in Fig. 2. The
crystallographic data are summarized in Table 2. Several
interesting structural features become readily apparent from
the X-ray structural analysis. The torsional angle f; of the
anisyl ring (involving C(4), C(3), C(7) and C(12)) is found
to be 50.98 whereas for the pyridinium ring, the f
encompassing C(4), C(5), C(13) and C(14) is only 38.18.
The torsional angle, being a measure of deviation away from
coplanarity, indicates the anisyl ring to be more out of plane
than the pyridinium ring (50.98 vs 38.18) with respect to the
plane of thienothiophene framework. Consequently, there
exists reasonably good possibility for appreciable through-
bond overlap between the p-deficient pyridinium ring and
the p-rich thienothiophene ring. Indeed, this energy
stabilizing interaction could well be the driving force for
the much reduced torsion between the pyridinium ring and
thienothiophene ring compared to the torsion of the p rich
anisyl ring.
˚
c (A)
b (8)
3
˚
V (A )
Z
rcalcd (g/cm3)
1.674
Fð000Þ
920
m (cm21
2umax (8)
No. of reflections
)
0.71 (Mo Ka)
55.0 ðI . 3:0sÞ
4039
Rint
0.025
No. of parameters
341
R
0.043
Rw
0.094
GOF
1.312
˚
(i.e. C(5)–C(13)) is 1.48 A compared to that between
between anisyl and thienothiophene ring, C(3)–C(7)
˚
distance being 1.491 A. The shorter C–C bond between
the pyridinium ring and the thienothiophene ring further
supports the existence of through-bond orbital overlap
between these two rings. The bond angle C(4)–C(3)–C(7)
between the anisyl and thienothiophene rings is found to be
124.58 whereas the bond angle C(4)–C(5)–C(13) between
the pyridinium and the thienothiophene rings is 127.18 (the
ideal sp2 angle is 1208). Consequently, the anisyl and
pyridinium rings are not parallel but tilted away from each
other by an angle of 11.68. In addition to this tilt, the van der
Another important structural feature born out from the
X-ray crystal structure of 8 is the shorter C–C bond distance
between the thienothiophene and pyridinium rings
˚
Waals distance of 4.08 A between the anisyl ring and the
pyridinium ring is found to be much too large (for the
cofacial 1,8-diaryl naphthalenes, the separation between
˚
aryl rings is 3.48 A) [5] to allow for effective intramolecular
c.t. to occur in 8.
3.3. Energy minimization on model 3,4-diphenyl thieno[2,
3-b ]thiophene
In order to shed light on the conformational aspect of peri
substituted thienothiophenes, we have also carried
out energy minimization on a model system, namely
3,4-diphenyl thieno[2,3-b]thiophene (9) using PM3 Ham-
iltonian within the MOPAC 93 program [12]. The heat of
formation DHf in kcal/mol was calculated by keeping one
phenyl ring stationary while rotating the other phenyl ring
about the thienothiophene-phenyl bond in steps of 58 each.
Fig. 3 shows the plot of change in DHf with change in the
torsional angle. The lowest energy structure (105.74 kcal/
mol) corresponds to a dihedral angle of 858 (structure 9a)
which gives a near parallel orientation of the two aryl rings
Fig. 2. ORTEP plot of the crystal structure of 8. Selected (i) interatomic
distances (A): S(1)–C(1), 1.709; S(1)–C(6), 1.728; S(2)–C(1), 1.722;
S(2)–C(2), 1.734; C(2)–C(3), 1.357; C(5)–C(6), 1.358; C(3)–C(7),
1.491; C(5)–C(13), 1.480; (ii) bond angles (8): S(1)–C(1)–S(2), 133.8;
C(4)–C(3)–C(7), 124.5; C(3)–C(4)–C(5), 137.2; C(4)–C(5)–C(13),
127.1 and (iii) torsion angle (8): C(4)–C(3)–C(7)–C(12), 250.9;
C(4)–C(5)–C(13)–C(14), 238.1; S(2)–C(1)–C(4)–C(5), 3.8.

112
S.H. Mashraqui et al. / Journal of Molecular Structure 689 (2004) 107–113
Fig. 3. Energy minimization of model system,3,4-diphenyl-thieno[2,3-b]thiophene.
on the thienothiohene framework. On other hand, the
conformation 9b having a dihedral angle of 1908 (one
phenyl ring perpendicular to the another) occupies the
highest energy maximum, corresponding to DHf of
110.46 kcal/mol. Interestingly, conformational energy is
found to vary by no more than 1 kcal/mol on changing the
torsion angle from 858 to 458. Therefore, at least
theoretically this result implies that minimum energy
conformations of 9 with reduced torsion are possible,
presumably to achieve balance between the conjugative
tendency towards coplanarity and steric hindrance. In the
light of this observation, it appears reasonable to assume
that the tendency of the ionic 8 in comparison to the model
system 9 to tend towards coplanarity would be even more
pronounced to achieve direct p conjugation between the
acceptor pyridinium and the donor thienothiophene rings.
Further, in view of relatively large interchromophoric
the peri substituents in thienothiophenes would be
relatively less energy demanding [6] than reported for
1,8-diaryl naphthalenes.
4. Conclusion
In a quest to find molecular systems with a potential to
show intramolecular c.t. interaction, we have successfully
carried out the synthesis of novel donor acceptor thie-
nothiophene 7 and its ionic analog 8. However, the structural
analysis by high resolution 1H NMR and UV–visible
absorption spectra reveal rather weak through bond c.t.
interactions in 8. An X-ray crystal structure of 8 reveals key
structural features that work against appreciable through-
space c.t. interaction. Conformational energy minimization
carried out on a model system, 3,4-diphenyl thienothio-
phene indicated a low energy barrier to aryl ring rotation in
peri substituted thienothiophene relative to analogous 1,8-
diaryl naphthalenes. Work is currently in progress to study
conformational behavior and to evaluate nonlinear optical
properties of donor–acceptor thienothiophenes.
˚
distance (ca. 4.08 A), couple with the reduced dihedral
angle between the pyridinium and thienothiophene rings,
the presence of rather weak through-space c.t. interaction
in 8 is not entirely unexpected. In the case of 1,8-diphenyl
naphthalene, House et al. [5] have calculated free energy
of rotation of phenyl rings to be approximately 15 kcal/
mol and the distance between the cofacial rings in this
˚
molecule in the lowest energy conformation to be 3.59 A.
˚
Relatively, greater inter-ring separation of 4.08 A in 8
Acknowledgements
˚
compared to 3.59 A for 1,8-diaryl naphthalenes together
with low energy barrier (ca. 5 kcal/mol) between the
lowest and the highest energy conformations 9a and 9b
seem to suggest that the conformational ring rotation of
S.H.M. thanks CSIR, New Delhi, India for a generous
research grants and for awarding junior research fellowship
to M.A.

S.H. Mashraqui et al. / Journal of Molecular Structure 689 (2004) 107–113
113
(c) A. Bahl, W. Grahn, S. Staddler, F. Feiner, G. Bourhill, C.
Brauchle, A. Reisner, P.G. Jones, Angew. Chem. Int. Ed. Engl. 1485
(1995) 34.
References
[1] (a) R. Foster, Organic Charge Transfer Complexes, Academic Press,
London, 1969.
(d) N. Kaneta, F. Mitamura, M. Uemura, Y. Murata, K. Komatsu,
Tetrahedron Lett. 5385 (1996) 37.
(b) J.M. Williams, J.R. Ferraro, Introduction to Synthetic Electrical
Conductors, Academic Press, Orlando, FL, 1987.
(e) F. Cozzi, M. Cinquini, R. Annunziata, T. Dwyer, J.S. Siegel, J. Am.
Chem. Soc. 5729 (1992) 114.
(c) M.R. Wasielewski, Chem. Rev., 435, 1992, pp. 92.
(d) H.S. Nalwa, S. Miyata, Nonlinear Optics of Organic Molecules
and Polymers, CRC Press, Boca Raton, 1997.
(f) S. Cohen, M. Rabinovitz, J. Phys. Org. Chem. 435 (1993) 6.
(g) S. Lavier, J.A. Zoltewich, J. Org. Chem. 7227 (2001) 66.
[7] (a) S.H. Mashraqui, H. Hariharasubramanian, Synth. Commun. 1695
(2000) 30.
[2] (a) J.M. Lehn, Supramolecular Chemistry, Concept and Perspective,
VCH, New York, 1995.
(b) S.H. Mashraqui, H. Hariharasubramanian, S. Kumar, Synthesis
(1999) 2030.
(b) P.R. Ashton, J.F. Stoddart, D.J. Williams, Angew. Chem. Int. Ed.
Engl., 1550, 1988, pp. 27.
(c) A.M.M. El-Saghier, Bull. Chem. Soc. Jpn 2011 (1993) 66.
[8] H. Eberhardt, R. Albert, Ger. Offen., 2,105,672 (CA, 77, P151957h).
[9] B.J. Whitlock, W.H. Whitlock Jr., J. Am. Chem. Soc. 838
(1983) 105.
(c) B. Dietrich, P. Viout, J.-M. Lehn, Macrocyclic Chemistry, VCH,
Weinheim, 1995.
(d) D.O.A. Dougherty, Science, 271, 1996, pp. 163.
(e) N. Martin, P. Bando, K. Davidov, J.L. Segura, C. Seone, A.
Gonzalez, J.M. Pingarron, Synth. Met., 1721, 1993, pp. 56.
(f) J. Laxmikanth, K. Bhanuprakash, Ind. J. Chem., 114, 2000, pp. 39A.
(g) G.R. Desiraju, Angew. Chem. Int. Ed. Engl., 1485, 1995, pp. 34.
(h) J.F. Stoddart, S.A. Nepogodiev, Chem. Rev., 393, 1998, pp. 30.
[3] F. Vogtle, Cyclophane Chemistry, Wiley, Chichester, 1993.
[4] T. Nakazawa, I. Murata, J. Am. Chem. Soc. 1996 (1997) 99.
[5] (a) H.O. House, D. Koepsell, W. Jaeger, J. Org. Chem. 1003 (1972) 37.
(b) R.L. Clough, P. Mison, J.D. Roberts, J. Org. Chem. 2252 (1976) 41.
(c) H.O. House, W.J. Campbell, M. Gall, J. Org. Chem. 1815 (1970)35.
[6] (a) R. Clough, J.D. Roberts, J. Am. Chem. Soc. 1018 (1976) 98.
(b) J.A. Zoltewicz, N.M. Maier, W.M.F. Fabian, J. Org. Chem. 7018
(1996) 61.
[10] In contrast, for stacked cyclophanes and 1,8-diaryl naphthalenes, the
shielding effects on account of charge transfer interactions range
from 0.50 to 1.5 d or more. For examples, seeP.M. Keehn, S.M.
Rosenfeld, Cyclophanes, vol. I, Academic Press, New York, 1983,
and Ref. [2].
[11] X-Crystallographic data for 8 have been deposited with the Cam-
bridge Data base (Deposited Data-CCDC 191022).
[12] The MOPAC 93 program (Fujista Ltd) was used for the energy
minimization. The visualization was done using the Spartan v 4.0
(Wavefunction Inc.) on a Silicon Graphics Indigo computer running
the Irix v 5.3 OS as well as CS Chem3D Pro (Cambridge Soft Corp.)
on a Macintosh Performa 636 CD computer.