BORAZANs: Tunable fluorophores based on 2-(pyrazolyl)aniline chelates of diphenylboron

(Figure Presented) The reaction between 2-pyrazolyl-4-X-anilines, H(pzAn^X), (X = para-OMe (L1), Me (L2), H (L3), Cl (L4), CO ₂Et (L5), CF₃ (L6), CN (L7)) and triphenylboron in boiling toluene affords the respective, highly emissive N,N′-boron chelate complexes, BPh₂(pzAn^X) (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 (φ_F = 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 (φ_F = 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.