Meta-para-linked octaaza[1(8)]cyclophanes and their polycationic states

Octaazacyclophanes, octaaza[1(8)]m,p,m,p,m,p,m,p-cyclophane (2) and octaaza[1(8)]m,p,p,p,m,p,p,p-cyclophane (3), as ring-size extended congeners of tetraaza[1(4)]m,p,m,p-cyclophane were synthesized, and the electronic states of their polycationic species were investigated by quantum chemical calculations, electrochemical measurements (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)), UV-vis-NIR spectroelectrochemical measurements, and pulsed electron spin resonance (ESR) spectroscopy. These octaazacyclophanes exhibited multiredox activities depending on different linkage patterns along the macrocyclic molecular skeletons, and both molecules were oxidizable up to their respective octacations. Spectroelectrochemical measurements demonstrated that p-phenylenediamine (PD) moieties in 2 could be converted from the semiquinoidal structure to the quinoidal sturcture with increasing oxidation number, whereas higher oxidation states of 3 did not show definite quinoidal deformation of PD moieties. A pulsed ESR spectrum gave evidence about formation of the almost pure spin-triplet state for 3(2+), whereas the high-spin states of 2(2+) and 2(4+) are virtually degenerate with the competing low-spin states even at low temperatures, probably due to the fragility of spin-coupling pathway caused by facile conformational changes.