Tuning the Electronic Properties of Acetylenic Fluorenes by Phosphaalkene Incorporation

Versatile synthetic protocols for 2,7‐ and 3,6‐diacetylenic fluorene‐9‐ylidene phosphanes (F9Ps) were developed. Protodesilylation of trimethylsilyl‐protected acetylenic F9Ps affords terminal acetylenes that can be employed in Sonogashira and Glaser‐type C?C coupling reactions to give thienyl‐decorated and butadiyne‐bridged fluorene‐9‐ylidene phosphanes, respectively. As evidenced by UV/Vis spectroscopy and cyclic voltammetry and corroborated by ab initio calculations, the presence of the P center in the F9Ps induces a significantly reduced HOMO–LUMO splitting that originates from stabilization of the LUMO levels. Variation of the acetylene substitution pattern is an additional tool to influence the optical and electronic properties. Whereas 3,6‐disubstituted F9Ps have strong absorptions around 400 nm, mainly due to π–π* transitions, 2,7‐diacetylenic F9Ps exhibit longest‐wavelength absorptions that have significant charge‐transfer character with an onset around 520 nm.