Nature repeatedly demonstrates exquisite control over the molecular forces that underpin the very existence of life. Inspired by nature, the synthetic introduction of specific molecular interactions to guide the association of matter is an overarching theme in current nanoscale materials and device design. Herein we report the self-assembly of porphyrinic arrays and clusters. Specifically, we demonstrate hierarchical solution phase organization of self-assembled nonameric porphyrin arrays into columnar stacks that are ~ 6.2 nm in diameter and 0.4 nm to ~ 10 nm tall. These heights correspond to stacks of 1 to ~ 22 nonamers, and represent the self-organization of 21 to ~ 460 particles, respectively, of four different chemical types. The size of the aggregates is pre-determined by the choice of appended alkyl group, solvent, temperature, porphyrin metallation, and an understanding of hierarchical organization kinetics. Moreover, these materials can be deposited onto a variety of surfaces with high structural fidelity. AFM and UV-visible analysis shows that once deposited on surfaces these structures are stable in air for more than a year at room temperature, and retain their photophysical properties.
Herein we present data for the hierarchical self-assembly of the free base porphyrins, and note that the metallated porphyrin nonameric arrays exhibit similar organizational properties. In this case each porphyrin in the nonameric array coordinates the same metal, such as Ni(II), Co(II), or Zn(II). This feat is accomplished by taking advantage of the highly selective porphyrin complexation kinetics and thermodynamics for different metals and allows for additional tunability of the aggregate optoelectronic and optomagnetic properties (in the case of Ni or Co). In a second, hierarchical self-assembly process, non-specific intermolecular interactions can be exploited to form nanoscaled three-dimensional aggregates of these supramolecular porphyrin arrays. In solution, the size of the nanoscaled aggregate can be directed by fine-tuning the properties of the component macrocycles, by choice of metalloporphyrin, and the kinetics of the secondary self-assembly process. As precursors to device formation, nanoscale structures of the porphyrin arrays and aggregates of controlled size may be deposited on surfaces. AFM and STM of these materials show that the choice of surface (Au, mica, glass etc.) may be utilized to modulate the aggregate size, and thus its photophysical properties. Studies of conductivity and magnetic properties of the arrays are being conducted via STM, conducting AFM and magnetic force microscopy (MFM). Once on the surface the materials are extremely robust and may be further organized via nanopatterning and selective adsorption. These compounds also represent a unique class of self-organizing materials that should prove to be valuable as testbeds for a wide range of potential molecular scale devices.
Drain CM, Batteas JD, Flynn GW, Milic T, Chi N, Yablon DG, Sommers H. PNAS 2002; 99: 6498-6502.
Milic, T, Chi, N, Yablon, DG, Flynn, GW, Batteas, JD, Drain, CM. Angewandte Chemie 2002; 41 (in press).