Control of the orbital derealization and implications for molecular rectification in the radical anions of porphyrins with coplanar 90° and 180° β,β′-fused extensions

Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" bis(quinoxalino)porphyrins in free-base form and their Zn^II, Cu^II, Ni^II, and Pd^II derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent β,β'-pyrrolic positions; they were synthesized from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)- porphyrin-2,3,12,13- and -2,3,7,8-tetraone, respectively, by reaction with 1,2-phenylenediamine. The degree of electron spin derealization into the fused rings in the π-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry, UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the π-radical anions show that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions are mediated through the central porphyrin unit, these interactions being sufficient to guarantee throughmolecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the π-radical anion becomes confined so that one quinoxaline group is omitted from spin derealization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.