Synthesis and Aggregation of a Conjugated Helical Molecule

Described are an improved method to synthesize helicenebisquinone 1 and an efficient method to resolve its enantiomers and those of other helicenebisquinones by separating derived camphanate esters. The union of p -benzoquinone and the bis(triisopropylsilylenol ether) of a 2,7-diacetyl-4,5-dialkoxynaphthalene gives the helicene skeleton in 56% yield. Molecules of nonracemic 1 aggregate when their dodecane solutions are concentrated to 5 × 10 - 4 M or more. Chloroform dissociates the aggregates. Aggregation is characterized by enhanced circular dichroisms and g values, red-shifted UV−vis absorptions, increased specific rotations, increased light scattering at an absorption frequency, and, as reported previously, fluorescence emissions that shift to the red, 1 H NMR resonances that shift upfield, and organization of the pure material into fibers. The CD and UV−vis absorption spectra of spin-coated films are similar to those of concentrated solutions in dodecane, but the g values are higher. The specific rotation of a cast film, α D, is 158 000 (deg cm 2 )/10 g. Unlike the nonracemic material, the racemic in bulk does not form visible fibers and is too insoluble to give concentrated solutions in dodecane. Also nonracemic helicene 3 does not show the characteristics of aggregation either in solution or in bulk.