1
572
O. Paz-Tal Le6i et al. / Tetrahedron Letters 42 (2001) 1571–1573
Figure 1. ORTEP drawing of 7 at the 50% probability level. C1ꢀC9 1.332(7), C2ꢀC3 1.468(7), C2ꢀC8 1.322(8), S2ꢀC8 1.705(6),
S3ꢀC10 1.747(5), C10ꢀC13 1.337(8) A. The torsion angle between the pyridine and the dithiole rings is 25.9°.
,
problems owing to undesired donor–acceptor interac-
tions either at stages preceding the chemical linking or
during isolation of the final product.
3, 4 and 7 were determined by X-ray diffraction
7
measurements of single crystals. The crystal structure
2
of 7 (Fig. 1) shows no intermolecular interactions,
although the stack is segregated along a diagonal axis.
The interplanar distances between the planes of pyri-
,
dine–pyridine and TTF–TTF are longer than 5.7 A.
One of the strongest acceptors used for linking with the
TTF moiety, by a unique approach, is the N-alkyl
+
3
pyridinium cation in a TTF-sigma spacer-Py system.
+
Derivatives involving both TTF and Py moieties are
excellent models for studying ICT versus ET processes
since the formation of the strong accepting moiety
occurs on the very last stage of the synthesis, and under
mild and controllable conditions.
Alkylation of 7 is facile using 3–4-fold excess of methyl
iodide at room temperature in acetonitrile or acetone,
from which product 8 precipitates as black crystals. Salt
8 exhibits an ICT absorption band at umax=572 nm in
ethanol, which does not correspond to any of the TTF
or pyridinium moieties, and obeys the Beer–Lambert
law. The position of this band is solvent dependent: a
blue shift in DMSO solutions (540 nm) and a red shift
in methylene chloride (618 nm) were observed. Two
reversible oxidation waves were observed for this
compound at higher potentials (E1/2(1)=0.58 V and
E1/2(2)=0.81 V (versus Ag/AgCl)) in agreement with
the electron accepting nature of the substituent. Photo-
chemical properties of 8 are similar to the analog
+
Recently a TTF-p-spacer-Py system with an ethenyl p
4
spacer was prepared. In the present communication we
report a simple synthetic route for the synthesis of a
D–A system in which the TTF moiety is directly linked
to the pyridinium cation moiety, without any spacer,
according to Scheme 1. Attempts to apply the known
procedure described for the synthesis of aryl substituted
1
,3-dithiole-2-thiones from the corresponding 1,3-dithi-
5
8
ole-2-(N-diethyl)immonium salt for the key precursor,
-(4%-pyridyl)-1,3-dithiole-2-thione 5, have been unsuc-
involving the trimethyl TTF donor unit and will be
4
discussed in detail elsewhere.
cessful. However, the cyclization of 3 in concentrated
sulfuric acid afforded 1,3-dithiole-2-one derivative 4 in
7
0% yield. Derivative 5 can be obtained in 65–70%
Acknowledgements
yields upon treatment with phosphorus sulfide. Al-
though the coupling between 5 and 4,5-dimethylthio-
6
1
,3-dithiole-2-thione with triethyl phosphite gives the
This work was partially supported by a grant from the
Ministry of Science, Israel.
unsymmetrical TTF derivative 7 in low yields along
with symmetrical derivatives, chromatographic separa-
tion is simple due to the large difference in R of the
f
products. Long chain substituted alkylthio substituted
derivatives can also be easily prepared according to the
above protocol (for instance, the 2,3-dihexadecylthio
derivative of 7 in 12% yield).
References
1. (a) Bryce, M. R. Adv. Mater. 1999, 11, 11; (b) Tsiperman,
E.; Regev, T.; Becker, J. Y.; Bernstein, J.; Ellern, A.;
Khodorkovsky, V.; Shames, A.; Shapiro, L. J. Chem. Soc.,
Chem. Commun. 1999, 1125.
2. (a) Bryce, M. R. Adv. Mater. 2000, 10, 589; (b) Panetta, C.
A.; Baghdadchi, J.; Metzger, R. M. Mol. Cryst. Liq. Cryst.
1984, 107, 103.
Cyclic voltammetry of derivative 7 in acetonitrile
exhibits two reversible oxidation waves, E1/2(1)=0.49 V
and E1/2(2)=0.76 V (versus Ag/AgCl), almost identical
to the values obtained for dimethylthio-TTF (E1/2(1)=
0.45 V; E1/2(2)=0.76 V). The structures of derivatives