Synthesis and optical behaviour of monodispersed oligo(fluorenylidene)s

The synthesis of the first series of monodispersed oligo(fluorenylidene)s is reported. The synthetic sequence envisages as key steps the obtainment (a) of a bis-pinacolboronate fluorenylidene dimer by a Ni(cod)2 promoted homocoupling of the suitable 7-bromo-fluorenylidene boronic ester and (b) of the bis-pinacolboronate fluorenylidene trimer and tetramer by a Ni(cod)2 promoted coupling of a 7-bromo-fluorenylidene boronic ester with 0.5 equiv of the corresponding 2,7-dibromofluorenylidene derivative. The optical properties of the obtained oligomers were studied. In the case of OF3 the optical behaviour was compared to that of a suitably synthesized fluorenone-containing model compound.