The discovery of the semiconducting properties of π-conjugated organic oligomers
and polymers has been a turning point in the development of new electronic devices
and transformed the study of these systems in an emerging research field. Since then,
many efforts have been devoted to generate structural features that bring about new
and outstanding properties for optoelectronics, spintronics and magnetic devices, nonlinear optics, or singlet fission processes. Since the behavior of the π-electrons
determines the properties of these molecules, establishing not only the electron
delocalization mechanism and its extension, but also which factors disturb the πelectron density is of utmost importance to enhance the proper performance of these
materials and develop ad hoc synthesis for desired application. In this context, the coexistence of alternative π-electron delocalization frameworks with the main linearly
conjugated sequence must be considered as the existence of contributing resonance
structures can modify significantly the optical, electronic and molecular properties of
the system under study.
In this communication, three different policonjugation patterns are addressed and
their influence on the π-systems is revealed through electronic and vibrational
spectroscopies. Through-bond π-electron delocalization is exemplified by molecules
with cross-conjugated and parallelly-conjugated frameworks. For the former, the two πconjugated pathways compete for the π-electron density in the common sections of the
molecule. Conversely, parallel π-conjugated sequences do not share any fragment. On
the other hand, through-space π-conjugation is demonstrated in spiro molecules, in
which a proper spatial configuration allows the interaction between π-conjugated
moieties disconnected by an insulating atom.
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