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

Two novel polynitrofluorene acceptors pi -extended with 5-nitrofuran and 5- nitrothiophene moieties (2 and 3); have been synthesised. Cyclic voltammetr y experiments show single electron reduction peaks at ca. -0.66-0.67 V (vs. Fc/Fc(+)); however, this process (probably due to substantial changes in g eometry of reduced species) is electrochemically irreversible and re-oxidat ion of the derived radical anions is observed at +0.05 and -0.04 V, for 2 a nd 3, respectively. Electronic absorption spectroscopy confirms charge tran sfer complexation of the studied acceptor with 4,5-dimethyltetrathiafulvale ne (Me2TTF) in solution. The X-ray crystal structure of the 1 : 1 charge tr ansfer complex (CTC) of acceptor 2 with Me2TTF shows mixed A . . .D . . .A . . .D stacking, and its comparison with the structure of the acceptor itse lf indicates a neutral ground state of this CTC. Theoretical HF/6-31 G(d) c alculations for the acceptor 2 and its radical anion and dianion states are in agreement with this conclusion. Intramolecular hydrogen bonding in 2 be tween the oxygen of the furan ring and C(8)-H hydrogen of the fluorene moie ty is observed in both solution (H-1 NMR) and solid state (X-ray) and also supported by theoretical calculations.