Multiple Electrochemical Doping-Induced Insulator-to-Conductor Transitions Observed in the Conjugated Ladder Polymer Polybenzimidazobenzophenanthroline (BBL)

Abstract

Spectroelectrochemical techniques have been used to study doping-induced reactions in conjugated polymers. Here, we report results on reduction reactions (n-doping) of the conjugated ladder polymer polybenzimidazobenzophenanthroline (BBL). The spectra are recorded in situ during applied potential in a three-electrode spectroelectrochemical cell. The spectral and thus the structural changes during the reduction (n-doping) of the polymer film at different electrode potentials are discussed. In contrast to most of the other conjugated polymers, this polymer shows four reversible redox reactions during n-doping, corresponding to various insulating and conducting forms of BBL.

Addressing the Elusive Polaronic Nature of Multiple Redox States in a π‐Conjugated Ladder‐Type Polymer

“…Our study reconciles contradictory observations, [ 10,14,22 ] addressing fundamental questions, namely, i) what are the multiple‐charging processes observed in BBL upon doping, ii) how much charge is stored per polymer repeat unit, iii) what are the polaronic species governing the high and low conductive states during the insulator‐to‐conductor transitions, iv) how electrons and holes relax over the polymer chain, and v) what are their main spectroscopic responses.…”

“…Multiple negatively charged species cover a crucial role in understanding the variety of redox and charge transport processes occurring in BBL upon charging, [ 13,14,20b,41 ] as documented in the introduction. Upon reduction BBL shows two insulator‐to‐conductor transitions, having two conductive states (high, conductive state I: 9 × 10 3 S cm −1 , and low, conductive state II: 1.5 × 10 3 S cm −1 ) and two insulating states.…”

“…In accordance to Wilbourn and Murray, [ 13 ] they found that the electrical conductivity of BBL varied by changing the potential during electrochemical reduction, showing two insulator‐to‐conductor transitions, thus leading to two conductive states with high (conductive state I: 9 × 10 3 S cm −1 at −600 mV in 0.1 m Bu 4 NClO 4 –acetonitrile electrolyte), and low (conductive state II: 1.5 × 10 3 S cm −1 at −1000 mV) conductivity. As inferred by Sariciftci and co‐workers, [ 14 ] the two insulator‐to‐conductor transitions referred to the presence of four reversible redox reactions (in contrast to Murray, who reported only two). Such redox species (named A, B, C, D) were assigned to two conductive and two insulating states, respectively.…”

“…Such redox species (named A, B, C, D) were assigned to two conductive and two insulating states, respectively. Sariciftci and co‐workers proposed a multiple‐charging reduction scheme in which the total number of consumed electron per repeat unit (eru) was measured as one, and the four redox species were classified as follows: [ 14 ] A, eru = 0.25 , conductive state I; B, eru = 0.50, insulator state I; C, eru = 0.85, conductive state II; D, eru = 1, insulator state II. FT‐IR spectra showed induced absorption vibration (IRAV) bands tentatively assigned to different negatively charged species, however insights into the structural, vibrational, and electronic properties of such hypothesized polaronic states were missing.…”

“…We were able to provide a comprehensive understanding to the experiments, by assigning the spectroscopic features of multiple redox species underlying the two observed insulator‐to‐conductor transitions, [ 14 ] and by describing the role of protonated states in affecting the vibrational and electronic spectra of BBL. We found that the first high conductive state can be assigned to a polaron (0.25 eru), while the first insulating state is attributed to a bipolaron (0.50 eru).…”