Electron acceptors of the fluorene series: Part 12. 9-(Metalloceneylidene)nitrofluorene derivatives of Fc–NF, NF–Fc–NF, and NF–Rc–NF types, and the vinylogues Fc–π–NF: synthesis, characterisation, intramolecular charge transfer, redox properties and X-ray structures for three fluorene–ferrocene derivatives