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-pi-NF: synthesis, characterisation, intramolecular charge transfer, redox properties and X-

Perepichka, D.F.; Perepichka, I.F.; Popov, A.F.; Bryce, M.R.; Batsanov, A.S.; Chesney, A.; Howard, J.A.K.; Sokolov, N.I.

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-pi -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 greater than or equal to 0.99) with Hammett-type substituents constants (sigma (-)(p)) in the fluorene moiety. Compounds 8-12 show solvatochromism [8a: lambda (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 run. and an increase in the intensity of ICT bands were observed with lengthening of the pi -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 Angstrom). 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 Angstrom for 8g and alternate separations of 3.52 and 3.55 Angstrom for 11d. (C) 2001 Elsevier Science B.V. All rights reserved.

• Journal Article Type: Article
• Publication Date: 2001-12
• Journal: Journal of Organometallic Chemistry
• Print ISSN: 0022-328X
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 637
• Pages: 445-462
• Public URL: https://durham-repository.worktribe.com/output/1580375
• Publisher URL: <Go to ISI>://000172577800058