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

This paper describes the synthesis and physicochemical characterization of novel metal-free phthalocyanines Pc(SO 2 R) 8, bearing eight peripheral alkylsulfonyl substituents. The key synthetic step is the efficient eightfold oxidation of sulfanyl (SR) into sulfonyl (SO 2 R) functions in the precursor Pc(SR) 8 . Quantum-chemical calculations show a considerable stabilization of the HOMO and LUMO levels of Pc(SO 2 R) 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(SO 2 R) 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(SO 2 R) 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(SO 2 R) 8 series: swallow-tail alkyl chains resulted in higher order in columnar mesophases compared to the linear analogues.