Boron complexes of aromatic ring fused iminopyrrolyl ligands: synthesis, structure, and luminescence properties

The condensation reactions of 2-formylindole (1) or 2-formylphenanthro[9,10-c]pyrrole (2) with various aromatic amines afforded the corresponding phenyl or phenanthrene ring fused mono-/bis-iminopyrrole ligand precursors 3-8, which, upon reaction with BPh₃ in an appropriate molar ratio, led to the new mono- and diboron chelate compounds Ph₂B[NC₈H₅C(H)[double bond, length as m-dash]N-2,6-Ar] (Ar = 2,6-iPr₂C₆H₃9; C₆H₅10), Ph₂B[(NC₈H₅C(H)[double bond, length as m-dash]N)₂-1,4-C₆H₄]BPh₂11, Ph₂B(NC₁₆H₉C(H)[double bond, length as m-dash]N-Ar) (Ar = 2,6-iPr₂C₆H₃12; C₆H₅13), and Ph₂B[(NC₁₆H₉C(H)[double bond, length as m-dash]N)₂-1,4-C₆H₄]BPh₂14, respectively. Boron complexes 12-14, containing a phenanthrene fragment fused to the pyrrolyl C3-C4 bond, are highly fluorescent in solution, with quantum efficiencies of 37%, 61% and 58% (in THF), respectively, their emission colours ranging from blue to orange depending on the extension of π-conjugation. Complexes 9-11, containing a benzene fragment fused to the pyrrolyl C4-C5 bond, are much weaker emitters, exhibiting quantum efficiencies of 10%, 7% and 6%, respectively. DFT and TDDFT calculations showed that 2,6-iPr₂C₆H₃N-substituents or, to a smaller extent, the indolyl group prevent a planar geometry of the ligand in the excited state and reveal the existence of a low energy weak band in all the indolyl complexes, which is responsible for the different optical properties. Non-doped single-layer light-emitting diodes (OLEDs) were fabricated with complexes 9-14, deposited by spin coating, that of complex 13 revealing a maximum luminance of 198 cd m^-2.