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

Abstract

Reaction of ferrocenecarboxaldehyde 13 and its vinylogue, ( E, E)-1-ferrocenyl-4-formyl-1,3-butadiene ( 16), with nitrofluorenes as CH-acids, results in push–pull compounds of the type Fc–π–fluorene ( 8a– g, 12a– e). Similar reaction with bifunctional ferrocene and ruthenocene dicarboxaldehydes results, depending on the fluorene structure, in the products of mono- or di-condensation, OHCFcCHfluorene ( 11d, e) or fluoreneCHFcCHfluorene and fluoreneCHRcCHfluorene ( 9a– c, 10a– c). Intramolecular charge transfer (ICT) in compounds 8 results in lowering the rotation barrier around the CHfluorene double bond and easy E–Z isomerisation in solution. Cyclic voltammetry (CV) experiments show a reversible single-electron oxidation of FcCHfluorenes ( 8) yielding the cation and two reversible single-electron reduction waves yielding the radical anion and dianion (for 8a– e) which merge into one two-electron reduction wave for 8f– h. ICT was also manifested in the electron absorption spectra of 8– 12, and energies of ICT (as well as reduction potentials in CV) were found to display excellent correlation ( r≥0.99) with Hammett-type substituents constants ( σ p −) in the fluorene moiety. Compounds 8– 12 show solvatochromism [ 8a: λ ICT=604.5 nm (acetonitrile), 622.5 nm (1,2-dichloroethane)], with, however, no quantitative regularities for 10 solvents of different polarity. Bathochromic shifts of 40–83 nm and an increase in the intensity of ICT bands were observed with lengthening of the π-bridge between ferrocene and fluorene moieties ( 8→ 12) whereas substitution of the ferrocene unit in 9 by the ruthenocene unit ( 10) resulted to a hypsochromic shift of ca. 100 nm due to decreasing donor ability of the latter metallocene fragment. Acceptor 8a was found to sensitise the photoconductivity of poly- N-(2,3-epoxypropyl)carbazole showing moderate holographic response of the materials. Molecular and crystal structures for ferrocene derivatives 8a, 8g, and 11d were determined by single-crystal X-ray diffraction. Tetranitro derivative 8a shows substantial distortion, caused by steric repulsion between the nitro groups in positions 4 and 5, which precludes the formation of stacks, and pairs of fluorene moieties contact face-to-face (interplanar distance 3.8 Å). In di- and trinitro derivatives 8g and 11d the fluorene moieties are more planar and their crystal packing motifs are similar: fluorene moieties form good stacks, parallel to the x-axis in 8g and the y-axis in 11d, with interplanar separations between fluorene moieties of 3.35–3.36 Å for 8g and alternate separations of 3.52 and 3.55 Å for 11d.