Electron Acceptors of the Fluorene Series. 9.1 Derivatives of 9-(1,2-Dithiol-3-ylidene)-, 9-(1,3-Dithiol-2-ylidene)-, and 9-(1,3-Selenathiol-2-ylidene)fluorenes: Synthesis, Intramolecular Charge Transfer, and Redox Properties

The synthesis and physical properties of four series of novel fluorene push−pull compounds ( 4 − 7 ) of the D−π−A type with intramolecular charge transfer (from 1,3- and 1,2-dithiole and 1,3-selenathiole donor (D) moieties) are described. The nature of the heteroatom (S or Se) in the donor fragment has no effect on the maxima of intramolecular absorption bands (λ ICT ), whereas a change of position of the heteroatoms in the dithiole moiety from 1,3 to 1,2 leads to a substantial bathochromic shift in λ ICT . Solvatochromism, thermochromism, and negative halochromism in these compounds are demonstrated. Cyclic voltammograms of 4 − 7 exhibit two, separate, single-electron reversible redox waves, the potentials of which (as well as the ICT energies in the molecules) are quantitatively described by σ p - constants of the substituents in the fluorene ring. Compounds 4 − 7 exhibit reversible salt formation in sulfuric acid, and for compound 5g in diluted sulfuric acid an additional absorption in the near-IR region has been observed that we attribute to radical species formation from the equilibrium [ 5g -H] + + 5g ⇄ 5g • + + [ 5g -H] • .