Tuning the Electronic Properties of Acetylenic Fluorenes by Phosphaalkene Incorporation

Abstract

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.

Golden Age of Fluorenylidene Phosphaalkenes–Synthesis, Structures, and Optical Properties of Heteroaromatic Derivatives and Their Gold Complexes

“…In order to investigate this reactivity, we reacted 1 with [Pd(PPh 3 ) 4 ] and excess triphenylphosphine under microwave heating in THF. From the crude reaction mixture, we identified residual starting material 1 (δ 31 P = 272.7 ppm), a mixture of the two monobromo derivatives (7-bromo-fluorenylidene phosphaalkene and 2-bromo-fluorenylidene phosphaalkene, δ 31 P = 268.9 and 264.0 ppm, respectively; see SI Figure 19 ), 30 and a phosphaalkene species in which the 31 P peak had shifted to lower frequencies (δ 31 P = 255.2 ppm). Chromatographic workup allowed us to identify this compound as the 7-diphenylphosphine oxide-2-bromo-fluorenyl phosphaalkene 8 .…”

“…Notably, the steric demand of the Mes* substituent also has a remarkable effect on the reactivity and electronic situation of the two sides of the molecule, as reflected in the low and high NMR frequencies of protons in the 1- and 8-positions of such compounds, respectively. 30 , 31 …”

Organophosphorus Compounds in Organic Electronics

“…Notably, the diacetylenic system 3,6‐ 65 a shows involvement of the P=C π‐orbitals in the HOMO, while in its 2,7‐substituted analogue this exocyclic double bond is not part of the HOMO. The major impact of phosphorus substitution in these systems is the significant stabilization of the LUMO levels, as confirmed by cyclic voltammetry and UV/Vis spectroscopy …”

“…The major impact of phosphorus substitution in these systems is the significant stabilization of the LUMO levels,a sc onfirmed by cyclic voltammetry and UV/Vis spectroscopy. [112] Already in 1995, Niecke reported the first biradical based on aP 2 C 2 core prepared from the phosphaalkene Mes*ÀP=CClLi. Crystallographic analysiso ft he deep red crystalsc onfirmed the planar nature of the four-membered cycle.…”

Synthesis and Characterization of Cyclopentadithiophene Heterofulvenes: Design Tools for Light‐Activated Processes

“…Dibenzofulvenoids with bridgehead E=P and As substituents. ( E ) . Heterofulvenoid CPDT derivatives explored in this work: AsA , PA , and PA‐AuCl .…”