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)vN-2,6-Ar] (Ar = 2,6-iPr2C6H3 9; C6H5 10),
Ph2B[(NC8H5C(H)vN)2-1,4-C6H4]BPh2 11, Ph2B(NC16H9C(H)vN-Ar) (Ar = 2,6-iPr2C6H3 12; C6H5 13), and
Ph2B[(NC16H9C(H)vN)2-1,4-C6H4]BPh2 14, 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-iPr2C6H3 N-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.