Functionalized ferrocenes and ferroceniums: synthesis, crystal structures and electrochemical properties based on carbazole/phenothiazine-ferrocene conjugated molecules

Four new D-pi-D (or D-pi-D-pi-D) complexes 9-ethyl-3-E-((1-ferrocenyl)vinyl)-carbazole (1), 9-ethyl-3,6-E,E-((1,1-diferrocenyl)vinyl)-carbazole(2), 10-ethyl-3-E-((1-ferrocenyl)vinyl)-phenothiazine (3), 10-ethyl-3,7-E,E-((1,1-diferrocenyl)vinyl)-phenothiazine (4), have been obtained by solid-phase Wittig reactions and fully characterized. The four complexes were treated with iodine leading to four corresponding [D-pi-A](+) I-3(-) ferrocenium triiodides 1(+)I(3)(-) (5), 2(+)I(3)(-) (6), 3(+)I(3)(-) (7) and 4(+)I(3)(-) (8), respectively. The results of single crystal X-ray diffraction analysis show that 1 and 2 display better coplanarity between the Fc and carbazole subunits than that between the phenothiazine and ferrocene moieties, which leads to a better pi-electron delocalization. Both the poor pi-electron in [ D-pi-A](+) ferroceniums and the heavy iodine atom have a negative effect on the electrochemical properties. The complexes 1-8 undergo multi-step redox processes and show lower oxidation potentials compared with those of ferrocene. Density functional theory (DFT) calculations are performed and the experimental redox properties of the complexes are studied. The eight complexes show MLCT and pi-pi* transitions in the UV-visible range in solution, which have been verified by TD-DFT theoretical calculations. In all cases the highest-lying occupied molecular orbitals of the eight complexes are mainly localized on the Fe d orbitals and show a greater Fe proportion than that of alkyl in the complexes, whereas the LUMOs and LUMO+1 are mainly from the p orbitals of the conjugated vinyl carbazole or phenothiazine unit.