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

Molecules in which the cores of copper and nickel octaazaphthalocyanines ar e fused to four nonracemic [7]helicenes are constructed. CD and UV-vis abso rption spectroscopy show that these compounds aggregate when dissolved in 7 5% EtOH-25% CHCl3, but not in 60% EtOH-40% CHCl3. The UV-vis absorption spe ctra of films of the nickel phthalocyanine are similar to those of solution s of the aggregated molecules, suggesting that the structures of the aggreg ates in the neat samples and in solution are similar. A calculation based o n 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 A ngstrom. 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 t he stacking axis parallel to the water. Although the molecules are remarkab ly 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 chi ral.