Redox properties of non-alternant cyclopenta-fused polycyclic aromatic hydrocarbons: The effect of peripheral pentagon annelation

The redox properties of 23 alternant PAH and non-alternant mono- and bis-CP-PAH that contain annelated peripheral pentagons were determined using cyclic voltammetry. The results show that peripheral cyclopenta-fusion markedly enhances their electron affinity. Unexpectedly for the non-alternant PAH, a good linear correlation between the first reduction potential [E-1/2(0/-1) V vs. SCE] and their standard Huckel LUMO energy (-epsilon(LUMO)/beta) is found. This indicates that the peripheral pentagons perturb the LUMO of the original alternant PAH core in a systematic fashion. A survey of the reduction behaviour of the mono- and bis-CP-PAH reveals that upon reduction the effect of the cyclopenta-moiety on the remainder of the molecule becomes negligible. Evidence for the formation of 6pi-electron cyclopentadienide sub-structures is obtained, i.e. localization of the added electrons in the peripheral pentagons occurs.