Molecular Motion and Performance Enhancement of BORAZAN Fluorescent Dyes
European Journal of Inorganic Chemistry
The preparation of three 2,6-dipyrazolyl-4-X-anilines, H(pz2AnX) (X = p-CF3, Cl, tBu) using CuI-catalyzed amination is described. Subsequent reactions of H(pz2AnX) with triphenylboron proceeds with benzene elimination to give the corresponding Ph2B(pz2AnX) compounds in high yields. The Ph2B(pz2AnX) are more highly emissive in the solid state than the previously reported BORAZAN fluorophores, Ph2B(pzAnX), their monopyrazolyl counterparts. As with the Ph2B(pzAnX), the color of emission in Ph2B(pz2AnX) can be tuned simply by varying the para-aniline substituent where the emission of Ph2B(pz2AnX) is red-shifted relative to the corresponding Ph2B(pzAnX) derivatives. The electronic properties were studied by cyclic voltammetry and electronic absorption/emission spectroscopy as well as by density functional calculations (B3LYP/6-31G*). The di-pyrazolyl derivatives exhibit greater stability toward solvolysis and higher photoluminescent quantum yields (despite the red-shift in emission) compared to their monopyrazolyl counterparts presumably due to kinetic stabilization of the chromophore imparted by the second pyrazolyl ligand. For Ph2B(pz2AnX), evidence for intramolecular motion of the diphenylboryl moiety traversing both pyrazolyl groups was detected by variable temperature 1H NMR spectroscopy. The rate increases with increasing electron-donor abilities of the para-aniline substituent.