Electron transfer within C4 unsaturated spacers between terminal ferrocenyl moieties

► The ET properties of a series of (bis)ferrocenyl complexes were studied. ► The near-IR data were analyzed according to the Hush theory. ► Efficiency of the C4 unsaturated spacers increases in the order diyne < enyne < diene. ► Enyne system shows ET properties falling in between those of dienyl and diynyl ones.