Mind the Gap

Intraband-gap states in conjugated polymers have captivated the chemistry community for the better part of five decades due to their position within the forbidden band gap, between the valence and conduction bands. In a large part, this fascination was (and still is) aided by the quest to find alternative materials that are easily processable compared to their solid-state semiconductor counterpart, which require considerable energy input. The outcome of the polymer story is well-known now, in 1977:1 “...the dream of combining the processing and mechanical properties of polymers with the electrical and optical properties of metals...” was first realized with the π-conjugated polymer, polyacetylene, or CHx.2 There was an additional “surprise” that came out of these materials—the transport properties measured were not consistent with electrons (or holes) being the charge carrier. Polarons, one of the quasiparticles that mediate the intraband-gap states, were implicated, and although being widely studied, there is still uncertainty in fundamental aspects of their nature