Impact of aromatic to quinoidal transformation on the degradation kinetics of imine-based semiconducting polymers†

Abstract

Degradable semiconducting polymers featuring acid-labile imine bonds are often investigated for use in transient electronics. However, the structure–property relationship of these polymers, particularly regarding degradation kinetics, remains underexplored. Herein, we designed and synthesized two imine-based semiconducting polymers which undergo an aromatic to quinoidal transformation upon acidification, leading to slower degradation rates compared to previously reported imine-based polymers. By utilizing a thieno[3,2-b]thiophene (TT)-inserted thienoisoindigo (TII)-dimer unit (TT-(TII-CHO)₂) and two diamines, p-phenylenediamine (PD) and 2,6-naphthalenediamine (2,6ND), we generated polymers p(TT-TII-PD) and p(TT-TII-2,6ND). The insertion of the TT unit between TII units results in high lying HOMO and low lying LUMO levels, facilitating a shift from an aromatic to quinoidal structure in the polymer backbone. Using ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy, infrared (IR) spectroscopy, and density functional theory (DFT) calculations, we investigated the influence of the quinoidal form on the degradation properties of these polymers. Notably, complete degradation of p(TT-TII-2,6ND) required over 30 days, indicating enhanced stability towards acid compared to previously reported TII-based polymers without the TT unit. Additionally, the protonated polymers demonstrated improved electrical properties compared to the pristine polymers, with field-effect transistor mobilities in the order of 10⁻² cm² V⁻¹ s⁻¹. These findings highlight the importance of quinoidal stability in modulating lifetimes and improving charge carrier transport in imine-based semiconducting polymers.