This artic
le describes Fourier transform Raman/infrared spectroscopic studies to determine the conjugationa
lproperties of nove
l ![](/images/gifchars/beta2.gif)
le">-o
ligothiophenes, in which thiophene rings are he
lica
lly
annelated, forming a cross-conjugated
![](/images/gifchars/pi.gif)
-system. These he
licenes may be viewed as fragments of the unprecedented carbon-su
lfur (C2S)
nhe
lix, having a su
lfur-rich mo
lecu
lar periphery. The B3LYP/6-31G** vibrationa
l ana
lysis of the experimenta
lspectroscopic data for
![](/images/gifchars/beta2.gif)
le">-o
ligothiophenes with three, seven, and e
leven thiophene rings indicates se
lectiveenhancement of a
limited number of Raman scatterings. In particu
lar, the enhancement of the Raman-activeske
leta
l ![](/images/gifchars/nu.gif)
(C=C) stretching modes in the 1400-1300-cm
-1 region is re
lated to the occurrence of a vibroniccoup
ling between the highest-occupied mo
lecu
lar orbita
l and
lowest-unoccupied mo
lecu
lar orbita
l frontiermo
lecu
lar orbita
ls. Decreased dispersion of enhanced Raman scatterings and great
ly increased near degeneracyof the highest occupied MOs with the increasing number of
annelated thiophene rings suggest significante
lectron
loca
lization in
![](/images/gifchars/beta2.gif)
le">-o
ligothiophenes, simi
lar to that in cross-conjugated
![](/images/gifchars/pi.gif)
-systems.