Planar, Twisted, or Curved Extended Tetrathiafulvalenes with Polycyclic Aromatic Hydrocarbon Cores Featuring Five-Membered Rings

Abstract

Tetrathiafulvalene (TTF) and polycyclic aromatic hydrocarbons (PAHs) represent two distinct classes of redox-active chromophores. By merging these two classes in so-called extended tetrathiafulvalenes, new physico-chemical characteristics emerge. Here in this study, we summarize the properties of such PAH-extended TTFs for which the PAH core incorporates five-membered carbo-cyclic rings together with six-, seven-, and/or eight-membered rings. A key structural motif of these molecules is the planar 2-(1H-inden-1-ylidene)-1,3-dithiole unit. Fusing two such units directly together or via various one- or two-dimensional PAH scaffolds has during the past decade provided a large selection of planar, twisted, or curved extended TTFs. Strong associations of radical cations are in some cases obtained, corresponding to redox-controlled self-assembly in solution, and these associations have allowed for the ready generation of semi-conducting salts by electrocrystallization. Some molecules exhibit remarkable multiredox behavior and can reversibly reach high cationic states, and, in some cases, also anionic states. Several structural examples also reveal how closed-shell versus open-shell characters of the dication states can be controlled by the nature of the PAH core, particularly by the number of Clar sextets within the core.