Electron accepters of the fluorene series. 9. 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-pi-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 (lambda(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 lambda(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 sigma(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 reversible arrow 5g(.+) + [5g-H](.).