Metal-Free Phthalocyanines Bearing Eight Alkylsulfonyl Substituents: Design, Synthesis, Electronic Structure, and Mesomorphism of New Electron-Deficient Mesogens

Abstract

This paper describes the synthesis and physicochemical characterization of novel metal-free phthalocyanines Pc(SO2R)8, bearing eight peripheral alkylsulfonyl substituents. The key synthetic step is the efficient eightfold oxidation of sulfanyl (SR) into sulfonyl (SO2R) functions in the precursor Pc(SR)8. Quantum-chemical calculations show a considerable stabilization of the HOMO and LUMO levels of Pc(SO2R)8 as a result of the presence of the strongly electron-withdrawing groups, while the HOMO−LUMO gap remains nearly unaffected. The redox behavior and electronic spectra of Pc(SO2R)8 were investigated by cyclic voltammetry and electronic absorption spectroscopy, respectively, to confirm the theoretical predictions. Particularly noteworthy is the first reduction potential of −0.14 V vs the saturated calomel electrode (SCE), which falls within the empirically established air-stability window for n-type semiconductors. Supramolecular structures of mesophases of Pc(SO2R)8 and their synthetic precursors Pc(SR)8 were studied in detail with the aid of powder X-ray diffraction in combination with differential scanning calorimetry and polarized light optical microscopy. As it was demonstrated earlier for many different discotic mesogens, installation of either linear or swallow-tail branched peripheral alkyl substituents plays an important role in the formation of mesophases. However, an unusual behavior was found for the Pc(SO2R)8 series: swallow-tail alkyl chains resulted in higher order in columnar mesophases compared to the linear analogues.