Free-Standing Blatter Radical Polymer Thin Films: Precise Nanometer Thickness Tuning and Redox-Optical Properties

Polymerisation of air-stable Blatter radicals into processable/free-standing thin films while retaining their redox properties is crucial for applications in electrochemical devices such as redox flow batteries, metal-ion batteries or for electrocatalysis. Here, we elucidate the electropolymerization-assisted polymerisation of carbazolyl Blatter radicals into thin films with nanometer-precision thickness control, as well as into free-standing thin films with the retained redox characteristics of Blatter radicals integrated within. Utilising the high chemical stability of Blatter radicals, terminal carbazole units were introduced on the 1,2,4-benzotriazinyl unit via Pd(0)-catalysed C-C coupling reaction, yielding new carbazolyl monomers Cz3N• and Cz2N•. Application of an oxidative potential of 1.5 V to monomers and varying the number of scans, enabled formation of polymer thin films, polyCz3N• and polyCz2N•, with a desired nanometer thickness (30-500 nm). Both monomers and polymers showed characteristic electron paramagnetic resonance (EPR) spectra and high redox reversibility associated with the Blatter radical. Density functional theory (DFT) indicated spin delocalisation on the 1,2,4-benzotriazinyl units and supported the experimental trends of optical properties (λ > 500 nm), band gaps of Blatter radical monomers (2.33-2.48 eV) and polymer thin films (2.30-2.39 eV). Driven by strong π-π stacking interactions between 1,2,4-benzotriazinyl units in solid-state interactions, Cz3N• showed strong antiferromagnetic interactions with 2J/kB value of −53.16 K and a singlet–triplet energy gap of −0.1 kcal mol−1.