Molecular motion and performance enhancement of BORAZAN fluorescent dyes

The preparation of three 2,6-dipyrazolyl-4-X-anilines, H(pz ₂An^X) (X = p-CF₃, Cl, tBu) using CuI-catalyzed amination is described. Subsequent reactions of H(pz₂An^X) with triphenylboron proceeds with benzene elimination to give the corresponding Ph₂B(pz₂An^X) compounds in high yields. The Ph₂B(pz₂An^X) are more highly emissive in the solid state than the previously reported BORAZAN fluorophores, Ph ₂B(pzAn^X), their monopyrazolyl counterparts. As with the Ph₂B(pzAn^X), the color of emission in Ph ₂B(pz₂An^X) can be tuned simply by varying the para-aniline substituent where the emission of Ph₂B(pz ₂An^X) is red-shifted relative to the corresponding Ph ₂B(pzAn^X) 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(pz ₂An^X), 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.