title: Control of the orbital delocalization and implications for molecular rectification in the radical anions of porphyrins with coplanar 90 degrees and 180 degrees beta,beta'-fused extensions.
abstract: Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" bis(quinoxalino)porphyrins in free-base form and their ZnII, CuII, NiII, and PdII derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent beta,beta'-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 delocalization into the fused rings in the pi-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 pi-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 through-molecule 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 pi-radical anion becomes confined so that one quinoxaline group is omitted from spin delocalization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.
authors.affiliation: Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA.
authors.forename: Wenbo
authors.initials: W
authors.lastname: E
authors.forename: Karl M
authors.initials: KM
authors.lastname: Kadish
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Sintic
authors.forename: Tony
authors.initials: T
authors.lastname: Khoury
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Govenlock
authors.forename: Zhongping
authors.initials: Z
authors.lastname: Ou
authors.forename: Jianguo
authors.initials: J
authors.lastname: Shao
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Reimers
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
authors.forename: Maxwell J
authors.initials: MJ
authors.lastname: Crossley