Small Molecules of Cyclopentadithiophene Derivatives: Effect of Sulfur Atom Position and Substituted Groups on Their UV–Abs Properties

Thiophene-based
organic semiconductors used as the active components have received
much attention. Their photoelectric properties can be easily tuned
with various substitutions at different positions on molecular structures.
Here, we synthesized series cyclopentadithiophene (CDT) derivatives
with sulfur atoms at ortho- (o-CDT), meta- (m-CDT), and para-positions (p-CDT) of the bridge carbon. These CDT
derivatives were substituted by carbonyl/dicyanomethylene at the bridge
position and/or by phenyl groups at the α position, respectively.
Due to the different conjugation extent and the variation of donor–acceptor
(D–A) interaction originating from the change of sulfur atom
position, diverse absorption spectra were observed. Especially for
dicyanomethylene substituted o-CDT with phenyl as
substitution group (DPCN-o-CDT), its absorption spectrum
covers the whole region of visible light. Combining with the electrochemical
behaviors and theoretical calculations, it was found that the sulfur
atoms mainly contribute to the molecular conjugation in these CDT
derivatives, especially for o-CDT derivatives. For
phenyl groups, they primarily act as electron donor in m-CDT derivatives, and chiefly contribute to molecular conjugation
in p-CDT derivatives, and simultaneously work as
electron donor and conjugation component in o-CDT
derivatives, respectively.