Synthesis and self-assembly of functionalized hexa-peri-hexabenzocoronenes

Monolayers of hexa-alkyl substituted derivatives of hexa-peri-hexabenzocoro nene (HBC) 1b have previously been investigated by scanning tunneling micro scopy (STM) and scanning tunneling spectroscopy (STS). It is expected that different functional groups (electron donating or withdrawing) connected to the aromatic core will influence the packing pattern and possibly the curr ent-voltage characteristics as well. In order to provide suitable model sys tems, a new synthetic approach to synthesize functionalized HBC derivatives has been developed. This was accomplished by [4+2]-cycloaddition of suitab ly bromo-substituted diphenylacetylenes and 2,3,4,5-tetraarylcyclopenta-2,4 -dien-1-ones followed by an oxidative cyclodehydrogenation with iron(III) c hloride/nitromethane. Using this strategy three different substitution patt erns were synthesized: 2-bromo-5,8,11,14,17-pentadodecylhexa-peri-hexabenzo coronene (2a), 2,5-dibromo-8,11,14,17-pentadodecylhexa-peri-hexabenzocorone ne (2b), and 2,11-dibromo-5,8,14,17-pentadodecylhexa-peri-hexabenzocoronene (2c). These bromo-substituted HBC derivatives were subjected to palladium catalyzed coupling reactions to give donor (alkoxy, amino) as well as accep tor (ester, cyano) substituted derivatives. The self-assembly of these new HBC derivatives was studied in the bulk as well as at an interface. DSC, op tical microscopy, and X-ray diffraction revealed the existence of columnar mesophases. The bulk structure in the mesophase is largely insensitive to c hanges of the substitution pattern; however, in situ scanning tunneling mic roscopy at the solid-fluid interface between an organic solution of the HBC derivative and highly oriented pyrolytic graphite reveals different packin g patterns of the first adsorbed monolayer.