The condensation reactions of 2-formylpyrrole (1) or 2-formylphenanthro[9,10-c]pyrrole (2) with various aliphatic amines afforded the corresponding 2-iminopyrrole ligand precursors 3‒10, which, upon stoichiometric reaction with BPh3, led to the new mononuclear boron chelate compounds Ph2B[NC4H3C(H)=N-R] (R = Me 11; iPr 12; tBu 13; nOct 14; Cy 15; Adam 16), and Ph2B(NC16H9C(H)=N-R) (R = Me 17; Adam 13), respectively. Boron complexes 11‒16, containing a simple 2-(N-alkylformimino)pyrrolyl ligand, are violet emitters and showed relatively modest fluorescence quantum efficiencies in solution (10% ‒ 16%), whereas complexes 17 and 18, bearing the -extended 2-(N-alkylformimino)phenanthro[9,10-c]pyrrolyl ligand, are blue emitters presenting enhanced quantum efficiencies of 35% and 43%, respectively, in THF solution. DFT and TDDFT calculations were in good agreement with experimental results, showing that π systems (pyrrolyl and phenanthropyrrolyl in this case) have a strong influence on the observed optical properties by changing the nature of the low energy transitions. Non-doped single-layer light-emitting diodes (OLEDs) were fabricated with complexes 11‒18, deposited essentially by spin coating, those of complexes 17 and 18 revealing maximum luminances of 69 and 88 cd m-2, respectively.