Porphyrins pair up

Charged porphyrins self-assemble via ionic interactions into binary clover-like architectures that show promise as electronically tunable materials.

Credit: © 2010 ACS

Porphyrins — macrocycles that can accommodate a metal ion in their centre through coordination to four nitrogen atoms — easily assemble into a variety of supramolecular architectures. In a collaboration between several institutions in the USA and Portugal, a team of researchers led by John Shelnutt at Sandia National Laboratories have now observed the ionic co-assembly of sulfonate and pyridinium porphyrins into binary dendritic architectures that resemble four-leaf clovers¹.

The metal atoms that occupy the central cavity of porphyrins confer on them particular electronic properties: a macrocycle hosting a zinc(II) atom is the electron donor, whereas one hosting a tin(IV) atom is the electron acceptor. Despite this, the morphology of the self-assembled architectures was found to be largely independent of the metals. Rather, it depends on the temperature and the ionic strength of the reaction solution — both factors that modify the diffusion rates of the porphyrins, and thus the rate of the self-assembly process.

Four binary anionic–cationic combinations (Sn/Zn, Zn/Sn, Sn/Sn and Zn/Zn) all formed closely related structures. X-ray powder diffraction measurements revealed that they also adopted similar crystalline packing, regardless of which metal occupied the central position of which porphyrin. This shows that the ionic interactions between the porphyrins, rather than those arising from the metals, mainly drive the self-assembly process. There is therefore potential to tune the electronic properties of the materials without altering their morphology. Indeed, Shelnutt and colleagues found the Zn/Sn binary solid behaved as a photoconductor, whereas the Sn/Zn analogue was an insulator.