Since the discovery of the semiconducting properties of π-conjugated organic oligomers
and polymers, many efforts have been devoted to generating structural features that
bring about new and outstanding properties for optoelectronics, spintronics and
magnetic devices, non-linear optics, or singlet fission processes. In this sense, the play
between the resonance and π-electron delocalization phenomena in π-conjugated
organic materials is the origin of their semiconducting behavior. For this reason,
establishing not only the electron delocalization mechanism and its extension, but also
which factors disturb the π-electron density is of utmost importance to enhance the
proper performance of the electronic devices and develop ad hoc synthesis for desired
application.