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

Abstract

Two novel polynitrofluorene acceptors π-extended with 5-nitrofuran and 5-nitrothiophene moieties (2 and 3), have been synthesised. Cyclic voltammetry 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 geometry of reduced species) is electrochemically irreversible and re-oxidation of the derived radical anions is observed at +0.05 and −0.04 V, for 2 and 3, respectively. Electronic absorption spectroscopy confirms charge transfer complexation of the studied acceptor with 4,5-dimethyltetrathiafulvalene (Me₂TTF) in solution. The X-ray crystal structure of the 1 ∶ 1 charge transfer complex (CTC) of acceptor 2 with Me₂TTF shows mixed A⋯D⋯A⋯D stacking, and its comparison with the structure of the acceptor itself indicates a neutral ground state of this CTC. Theoretical HF/6-31G(d) calculations for the acceptor 2 and its radical anion and dianion states are in agreement with this conclusion. Intramolecular hydrogen bonding in 2 between the oxygen of the furan ring and C(8)-H hydrogen of the fluorene moiety is observed in both solution (¹H NMR) and solid state (X-ray) and also supported by theoretical calculations