Importance of unpaired electrons in organic electronics

Abstract

The first observation that PBBTPD, a low bandgap, ambipolar conjugated donor-acceptor (DA) polymer based on benzobisthiadiazole (BBT), possesses an open-shell singlet ground state as well as a thermally accessible triplet state is described. Similarly, interesting electronic behavior in semiconducting organic DA oligomers based on BBT is also observed. Theoretical predictions have suggested that such behavior is due to the biradicaloid character of BBT and we provide experimental evidence indicating that these predictions are correct. Furthermore, the open shell character strengthens as the conjugation length increases, as observed in the BBTbased polymer, PBBTPD. We show that this biradicaloid structure is observed in each BBT moiety along the chain and that therefore PBBTPD is in fact a polyradicaloid. This observation will most likely aid in the development of better n-type polymeric acceptors for organic semiconductor applications.

Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

“…While most conventional conjugated polymers are closed‐shell species accommodating their π‐electrons in bonding orbitals, donor–acceptor (DA) copolymers with quinoidal character, extended π‐systems, and narrow bandgaps demonstrate open‐shell electronic structures. [ 4,26–29 ] We previously demonstrated that DA copolymers based on cyclopentadithiophene–thiadiazoloquinoxaline (CPDT–TQ) frameworks exhibit very narrow bandgaps (0.5 eV < E g < 1 eV), room‐temperature conductivities (σ RT ) of ≈10 −2 –10 −3 S cm −1 , and controlled spin multiplicities, emanating from a high degree of electronic coherence along the π‐conjugated backbone. [ 26,27 ] In these materials, narrow bandgaps increase configuration mixing, while extension of the π‐system promotes topological localization of α and β singly occupied molecular orbitals to opposite sides of the macromolecule, diminishing the covalency of the ground state and increasing the diradical character ( y ).…”

Toward Benzobis(thiadiazole)‐based Diradicaloids

“…Wudl et al first reported the experimental observation of paramagnetic activity of BBT-basedd onor-acceptor polymers and ascribed the magnetic property to the intrinsic diradical character of BBT. [11] To deep understand the origins, oligomer approach is mored esirable. The electron-deficiency also implies that BBT-based compoundswill have alow-lying HOMO and thus are stable.…”

Temperature Tunable Self‐Doping in Stable Diradicaloid Thin‐Film Devices

“…In the past few decades, the development of long‐lived open‐shell molecules has enabled a variety of advanced applications based on this type of materials . In addition to the unique spin properties, the existence of unpaired electrons in radical molecules can offer an interesting possibility for usage as dopants through charge transfer . For example, organic radical anions or cations have been shown to molecularly dope organic field‐effect transistors (OFETs), leading to improved electrical properties .…”