Electron acceptors of the fluorene series. Part 13. 9-(5-Nitrofuran-2-ylidene)- and 9-(5-nitro-2-thienylidene)-2,4,5,7-tetranitrofluorenes: novel π-extended electron acceptors. Synthesis, cyclic voltammetry and X-ray crystal structures for the acceptor and its 4,5-dimethyltetrathiafulvalene complex, and a theoretical study†