Synthesis, self-assembly, and nonlinear optical properties of conjugated helical metal phthalocyanine derivatives

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 Angstrom. 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.