Regioregular head-to-tail oligothiophene-functionalized 9,9′-spirobifluorene derivatives. 2. NMR characterization, thermal behaviors, and electrochemical properties

Abstract

Oligothiophene-functionalized 9,9′-spirobifluorene derivatives exhibit good solubility in polar organic solvents, especially compounds 5a-d to 7a-d due to the introduction of the flexible n-hexyl chain. The structures of all compounds studied are characterized and verified by 1H and 13C NMR spectra. The results not only show that these organic conjugated materials with no substituents or n-hexyl substituents are successfully synthesized through the corresponding synthetic methodologies, but also prove that chemical shifts of protons and carbons in the aromatic range change with the attachment and the increase of the thiophene ring at the 9,9′-spirobifluorene fragment. The thermal analysis results demonstrate that these compounds are quite stable, and evaporate from 300 to 700 °C. Most of these compounds exhibit a stable amorphous state in the solid state. The electrochemical properties of all compounds studied are also investigated. The results indicate that radical cation behaviors of oligothiophene-modified 9,9′-spirobifluorene derivatives are more stable than those of normal oligothiophenes, that oxidative and reductive peak potentials shift cathodically or anodically, and that the HOMO and LUMO energy levels are also tuned with the increase of the thiophene ring owing to the enhanced π-electron delocalization and the increasing conjugation length. Oxidative potentials of nonsubstituted oligothiophene-functionalized 9,9′-spirobifluorene derivatives are more sensitive to the increase of thiophene rings than those of 9,9′-spirobifluorene derivatives with n-hexyl groups. The energy levels and band gaps of all compounds studied are also calculated from the onset potentials of n-doping and p-doping and are adjusted by varying thiophene oligomers and their attachment patterns to the 9,9′-spirobifluorene ring to make them suitable for the work functions of the electrodes.