Optical and Electrochemical Band Gaps in Mono‑, Oligo‑, and Polymeric Systems: A Critical Reassessment

The concept of a band gap was originally developed to describe periodic, perfect single-crystal materials in solid-state physics and subsequently applied to crystalline and amorphous monomeric, oligomeric, and polymeric solid materials. Furthermore, applications in organic electronics and electrochemistry have been discussed and studied with a focus on intrinsically conducting polymer ICPs and crystalline molecular semiconductors. A summary of this background and a molecular system description (of both single molecules and crystalline molecular solids) are included. This is followed by an inspection of band gap descriptions for oligomeric and polymeric systems. In particular, apparent discrepancies between band gap energies for intrinsically conducting polymers and organic semiconductors obtained using optical spectroscopy and electrochemical measurements have puzzled researchers for years. These contradictory data have, at times, been reported without any explanation or reconciliation. Similar disappointing results have been reported for molecular systems when a band model for the HOMO–LUMO gap was considered. In contrast, data pertaining to monomeric, oligomeric, and polymeric systems confirm that either positive or no significant correlations exist.