Molecules in which the cores of copper and nickel octaazaphthalocyanines are fused to four
nonracemic [7]helicenes are constructed. CD and UV−vis absorption spectroscopy show that these compounds
aggregate when dissolved in 75% EtOH−25% CHCl3, but not in 60% EtOH−40% CHCl3. The UV−vis
absorption spectra of films of the nickel phthalocyanine are similar to those of solutions of the aggregated
molecules, suggesting that the structures of the aggregates in the neat samples and in solution are similar. A
calculation based on molecular mechanics shows that the energy is minimized when the molecules stack in a
chiral superstructure with a core-to-core distance of ca. 3.4 Å. Atomic Force Microscopic images are in accord
with this structure if on the surface of mica the stacks are isolated and perpendicular to the surface. In Langmuir
films, the molecules stack in the opposite way, with the stacking axis parallel to the water. Although the
molecules are remarkably symmetrical, their Langmuir−Blodgett films give very large second-order nonlinear
optical responses, dominated by the ca. 21 pm/V components of the susceptibility tensors that are allowed
only because the materials are chiral.
The questions considered in this paper are why, as agents for resolving helicenols, camphanate esters are particularly effective, and why, in all 19 examples studied, when the (1S)-camphanates of (P)- and (M)-helicen-1-ols are chromatographed on silica gel, the former has the lower R(f)(). Models are proposed for the favored conformations of the esters, and to support the models, evidence is provided from five X-ray diffraction analyses and four ROESY analyses supplemented by molecular mechanics calculations. The essential discovery is that, presumably to avoid a steric interaction between a methyl on the camphanate's bridge and the helicene skeleton, the O=CCO conformation is anti-periplanar in (M)-helicenol camphanates and syn-periplanar in (P)-helicenol camphanates. In the former, the lactone carbonyl points toward the helicene ring system, and in the latter, it points away.
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