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

Abstract

Poly(benzimidazole–benzophenanthroline) (BBL) is a ladder‐type conjugated polymer showing remarkable charge transport properties. Upon doping it displays various conductive regimes, leading to two insulator‐to‐conductor transitions. Such transitions are never fully characterized, limiting understanding of its charged states. Open issues are: i) the electron/hole polaron relaxations, ii) the structure–function relationships of multiple redox states and their connection with the conductive regimes, and iii) the role of protonation. Such knowledge‐gaps are tackled via a comprehensive computational investigation of multiple redox species. Polarons show polyradicaloid character, as revealed by combining broken‐symmetry density functional theory, fragment orbital density, and multireference analysis. Electron/hole polaron relaxations occur on the polymer chain, the former localizing on the benzophenanthroline moieties, the latter on the benzimidazole units. Modeling of multiple charged species, up to one electron per repeat unit (1 eru), reveals a complex scenario of quasidegenerate states each featuring different spin multiplicity. Four redox states are responsible for the BBL insulator‐to‐conductor transitions. The two high conductive states refer to the electron polaron (0.25 eru) and the redox species with 0.75 eru. The insulating regimes refer to the bipolaron (0.50 eru) and the redox state with 1 eru. Protonation is modeled, revealing polaron‐like features in the spectroscopic properties.

In Situ Spectroscopic and Electrical Investigations of Ladder-type Conjugated Polymers Doped with Alkali Metals

“…According to time-dependent density functional theory (TD-DFT) calculations, these two bands are mainly attributed to transitions from new gap states to unoccupied orbitals, with band II attributed to both cis and trans conformers and band III attributed only to the trans conformer, whose structures are shown in Figure S2. 25,26 At c red ≤ 50%, a new feature IV appears at approximately 3.0 eV, along with the bleaching of the neutral state absorption, in agreement with the previous observation for BBL doped with molecular or polymeric dopants. 19,20,23 This change is ascribed to a replacement of the frontier orbitals in the charged polymers.…”

“…The intensity of these two bands increases simultaneously with increasing the K content. According to time-dependent density functional theory (TD-DFT) calculations, these two bands are mainly attributed to transitions from new gap states to unoccupied orbitals, with band II attributed to both cis and trans conformers and band III attributed only to the trans conformer, whose structures are shown in Figure S2. , …”

“… 21 − 23 Density functional theory (DFT) calculations have also been carried out to explain the experimental observations by modeling the multiple charged states of the oligomer. 24 − 26 However, additional studies are needed to achieve a thorough understanding of the doping-induced properties. One severe limitation is the sparse experimental data on chemically doped BBL.…”

“…Many experimental and theoretical studies have been conducted on conjugated polymers and their corresponding oligomers, yielding rich information about the nature of the charged conjugated chains. − However, these studies are primarily based on the classical conjugated polymers with a single-stranded linkage between repeat units, such as polythiophene, polyfluorene, poly­(phenylene vinylene), and their derivatives. In comparison, ladder-type conjugated polymers exhibit lower intrachain torsional disorder and have enhanced π–π stacking due to their double-stranded structure. , Polarons on such rigid-chain conjugated polymers are attracting more and more attention due to the increasing application of the materials in organic electronics. − Previously, the spectroscopic and electrical properties of chemically and electrochemically doped poly­(benzimidazobenzophenanthroline) (BBL) have been extensively explored. − Density functional theory (DFT) calculations have also been carried out to explain the experimental observations by modeling the multiple charged states of the oligomer. − However, additional studies are needed to achieve a thorough understanding of the doping-induced properties. One severe limitation is the sparse experimental data on chemically doped BBL.…”

Screening semiconducting polymers to discover design principles for tuning charge carrier mobility

“…54 Higher mobility has also been attributed to polymer backbones which can stabilise the charge carrier polaron, where localisation of the polaron to a smaller segment on polymer chains gives an energetically narrow distribution of states that the charge carrier can occupy and allows the chain to host more charge carriers on the chain. 55,56 The sequence of monomers is significant for the electronic coupling features of the polymer. An important result from phenomenological modelling reveals the importance of the sequence architecture for donor acceptor (DA) copolymers, showing that overall perfectly alternating (AB type sequences) DA sequences and block copolymers with closer to uniform block lengths exhibit better predicted mobilities than more randomised DA sequences, and the influence of the energy gap between charge carrier states on mobility.…”