The Influence of Redox-Active Linkers on the Stability and Physical Properties of a Highly Electroactive Porphyrin Nanoprism

Abstract

Redox-active metal-organic nanocages are of interest for many applications, but the development of cages with extensive redox activity is often hindered by their limited stability and solubility across multiple charge states. This report reveals that these properties can be tuned for cages with redox-active walls by incorporating additional redox activity into the linkers. In particular, new +12 charged triangular nanoprisms <b>1a</b>,<b>b</b> were formed from three electroactive tetrakis(3-pyridyl)porphyrin walls linked by six [(TMEDA)Pt]²⁺ (for <b>1a</b>) or [(2,2'-bipy)Pt]²⁺ (for <b>1b</b>) vertices, the latter of which are also electroactive. Thus, <b>1b</b> exhibits extensive redox activity, consisting of two porphyrin-centered (x3) and two 2,2'-bipy-centered (x6) reductions that provide reversible access to +12, +9, +3, 0, and -6 charge states, whereas <b>1a</b> undergoes only two, porphyrin-centered (x3) reversible reductions. Comparisons of <b>1a</b> and <b>1b</b> (and monomeric control compounds) by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry show that the redox-activity of the linkers in <b>1b</b> lowers the second reduction potential of the porphyrins by 100 mV and improves the stability and solubility of this structure under highly reducing conditions (e.g., -2.25 V vs Fc^+/0 in MeCN). These findings reveal new principles for controlling the properties of highly electroactive molecular nanostructures. Anion exchange rates (≫10³ s^-1) were also probed, showing that the narrow apertures (≤3 Å van der Waals width) of <b>1a</b>,<b>b</b> do not impede the loss/gain of PF₆^- anions during redox processes.