Boron complexes of aromatic 5-substituted iminopyrrolyl ligands: synthesis, structure, and luminescence properties

A group of new mononuclear boron chelate compounds [BPh₂{κ²N,N'-5-R-NC₄H₂-2-C(H)[double bond, length as m-dash]N-Ar}] (R = Ar = C₆H₅7; R = C₆H₅, Ar = 2,6-iPr₂C₆H₃8; R = Anthracen-9-yl (Anthr), Ar = C₆H₅9; R = Anthr, Ar = 2,6-iPr₂C₆H₃10) were synthesized via the reaction of B(C₆H₅)₃ with the corresponding 5-substituted 2-(N-arylformimino)pyrrole ligand precursors 3-6. These complexes were prepared in order to evaluate the luminescence potential derived from the substitution of the position 5 of the pyrrolyl ring with an aromatic group. Compounds 7-10 were photophysically characterized in solution and in the solid state. The 5-phenyl-2-iminopyrrolyl-BPh₂ complexes 7 and 8 are blue emitters and have enhanced photoluminescence quantum yields in the solid state (Φ_PL) up to 0.95, whereas the 5-anthracenyl derivatives 9 and 10 have green-bluish fluorescence and a Φ_PL of 0.49 and 0.24, respectively. DFT and TDDFT studies were performed, considering the effect of solvent and dispersion, in order to show how the geometries of compounds 7-10 changed from the ground to the excited state, to assign electronic transitions, and to rationalize the observed luminescence. These materials were applied in organic light-emitting diodes (OLEDs), with various device structures, the best showing an external quantum efficiency of 2.75% together with a high luminance of 23 530 cd m^-2.