Luminescent organoboron ladder compounds via directed electrophilic aromatic C–H borylation

Abstract

Luminescent B–N containing conjugated ladder compounds are prepared by directed electrophilic aromatic substitution. Negishi cross-coupling reaction of 2,7-dibromo-9,9-dihexylfluorene with 6-methylpyridin-2-yl(pivaloyloxy)zinc furnishes the dipyridylfluorene precursor (1) in 44% yield. Compound 1 is then subjected to electrophilic borylation with BBr₃ in the presence of iPr₂NEt as a bulky base to give the tetra-coordinate ladder-type organoboron species 2-Br. Arylation of 2-Br is accomplished by addition of Ph₂Zn or C₆F₅Cu in toluene furnishing the targeted bis(pentafluorophenyl)borane and diphenylborane species 2-Ar (Ar = Ph, C₆F₅) in 53 and 44% yield respectively. The products show a strong blue emission and undergo multi-step reversible reduction and oxidation processes. The effect of fluorination of the pendant aryl groups on the electronic structure is further examined by DFT methods.