Soluble Synthetic Multiporphyrin Arrays. 3. Static Spectroscopic and Electrochemical Probes of Electronic Communication

Abstract

A comprehensive electrochemical (cyclic and square-wave voltammetry, coulometry) and static spectroscopic (absorption, resonance Raman (RR), electron paramagnetic resonance (EPR)) study is reported for a series of dimeric and trimeric porphyrin-based arrays. All the arrays consist of tetraarylporphyrins linked via ethyne groups at the p-positions of the aryl rings. The complexes investigated include zinc-free base and bis-zinc dimers which contain varying degees of torsional constraint between the porphyrin rings and the aryl group of the linker, and linear and right-angle trimers in which two zinc porphyrins are bridged by either a zinc or free base porphyrin. The spectroscopic studies were performed on singly and multiply oxidized complexes as well as the natural species. The electrochemical and spectral properties of the arrays indicate that the electronic communication between the macrocycles is relatively weak in the ground and excited electronic states. This communication is through-bond, rather than through-space, and is mediated by the diarylethyne linker. The EPR spectra of the oxidized arrays exhibit complex temperature-dependent signatures that reflect hole/electron hopping and/or spin exchange interactions in the ground electronic state. Exchange interactions in the multiply oxidized arrays are significant (probably 1000 MHz pr greater) in both liquid and frozen solutions and, in certain cases,more » are enhanced upon solvent freezing. 45 refs., 12 figs., 3 tabs.« less