BORAZANs: Tunable Fluorophores Based on 2-(Pyrazolyl)aniline Chelates of Diphenylboron

The reaction between 2-pyrazolyl-4-X-anilines, H(pzAnX), (X = para-OMe (L1), Me (L2), H (L3), Cl
(L4), CO2Et (L5), CF3 (L6), CN (L7)) and triphenylboron in boiling toluene affords the respective,
highly emissive N,N‘-boron chelate complexes, BPh2(pzAnX) (X = para-OMe (1), Me (2), H (3), Cl (4),
CO2Et (5), CF3 (6), CN (7)) in high yield. The structural, electrochemical, and photophysical properties
of the new boron complexes can be fine-tuned by varying the electron-withdrawing or -donating power
of the para-aniline substituent (delineated by the substituent's Hammett parameter). Those complexes
with electron-withdrawing para-aniline substituents such as CO2Et (5), CF3 (6), and CN (7) have more
planar chelate rings, more ‘quinoidal' disortion in the aniline rings, greater chemical stability, higher
oxidation potentials, and more intense (φF = 0.81 for 7 in toluene), higher-energy (blue) fluorescent
emission compared to those with electron-donating substituents. Thus, for 1 the oxidation potential is
0.53 V versus Ag/AgCl (compared to 1.12 V for 7), and the emission is tuned to the yellow-green but
at an expense in terms of lower quantum yields (φF = 0.07 for 1 in toluene) and increased chemical
reactivity. Density functional calculations (B3LYP/6-31G*) on PM3 energy-minimized structures of the
ligands and boron complexes reproduced experimentally observed data and trends and provided further
insight into the nature of the electronic transitions.