Electrophilic aromatic substitution in triphenylene discotics: Synthesis of alkoxynitrotriphenylenes

Abstract

Functionalization of triphenylene-based discotic liquid crystals by direct nitration of hexaalkoxytriphenylene has very recently been reported to produce exclusively the mononitrohexaalkoxytriphenylene. W e have found that nitration of such discotics is not restricted to only one ring of triphenylene, but all the three rings can be nitrated successively to produce mononitro-, dinitro- and trinitro-hexaalkoxytriphenylenes. All the mononitro derivatives show a very broad mesophase range. While the dinitrohexabutyloxytriphenylene is non-liquid crystalline, its higher homologues are room temperature liquid crystals. Photomicroscopic pictures of these liquid crystalline materials show classical textures of an ordered hexagonal columnar phase. X-ray diffraction studies of two representative compounds, 4a and 4b, confirmed the above conclusions. None of the trinitro derivatives are liquid crystalline. Nitrotriphenylenes are valuable precursors to several other triphenylene derivatives like amino, acyl, alkylamino, diazo, etc. Mononitration of 2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylenes gives the 1-nitro-2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylenes. These double functionalized triphenylene derivatives are extremely important precursors as the functional group such as nitro, amino, azo, etc. can be utilized to modify the electronic nature of the core and at the same time the hydroxy functional group may be converted to a polymerizable group. Thus, processable oligomers and polymers can be synthesized.