Stabilization of free radicals in layer-by-layer nanoarchitectures containing multiple arylenediimides

Layer-by-layer thin films of arylenediimides were grown on phosphonate-primed silicon and quartz substrates by the zirconium phosphonate method. The method consists in depositing alternated organic/inorganic layers, where the organic layer was a phosphonate-substituted arylenediimide and the inorganic layer was Zr4+. Heterogeneous films with up to 50 layers, containing different imides, were produced. The building blocks were N, N'-bis(2-phosphonoethyl)pyromellitimide (PPMI), N,N'-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI) and N,N'-bis(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI). Thin films with the sequence PPDI/PNDI, PPDI/PPMI, PNDI/PPMI and PPDI/PNDI/PPMI were produced. Organized film growth was observed by UV-visible-NIR spectroscopy (quartz substrates) and ellipsometry (silicon substrates). AFM images showed that the surface of the films was smooth and uniform. The imides in the films were reduced using sodium dithionite as a reducing agent, generating stable radical anions and dianions of the imides. The following species were detected in the absorption spectra of the reduced films: PPDI-center dot, PPDI2-, PNDI-center dot, and PNDI2-. Reduction of the imides in the films was rather slow, as compared to homogeneous solutions. When the reduced films were exposed to atmospheric oxygen, reoxidation took place, which was also much slower than in homogeneous solutions. In conclusion, a mix of aromatic imide reduced species, absorbing through the whole visible range, was produced and stabilized in thin films.