Luminescent Di- and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis, Characterization, and Application in OLEDs.

New bis- and tris(iminopyrrole)-functionalized linear (1,2-(HNC4H3-C(H)N)2-C6H4 (2), 1,3-(HNC4H3-C(H)N)2-C6H4 (3), 1,4-(HNC4H3-C(H)N)2-C6H4 (4), 4,4′-(HNC4H3-C(H)N)2-(C6H4-C6H4) (5), 1,5-(HNC4H3C-(H)N)2-C10H6 (6), 2,6-(HNC4H3C-(H)N)2-C10H6 (7), 2,6-(HNC4H3C-(H)N)2-C14H8 (8)) and star-shaped (1,3,5-(HNC4H3-C(H)N-1,4-C6H4)3-C6H3 (9)) π-conjugated molecules were synthesized by the condensation reactions of 2-formylpyrrole (1) with several aromatic di- and triamines. The corresponding linear diboron chelate complexes (Ph2B[1,3-bis(iminopyrrolyl)-phenyl]BPh2 (10), Ph2B[1,4-bis(iminopyrrolyl)-phenyl]BPh2 (11), Ph2B[4,4′-bis(iminopyrrolyl)-biphenyl]BPh2 (12), Ph2B[1,5-bis(iminopyrrolyl)-naphthyl]BPh2 (13), Ph2B[2,6-bis(iminopyrrolyl)-naphthyl]BPh2 (14), Ph2B[2,6-bis(iminopyrrolyl)-anthracenyl]BPh2 (15)) and the star-shaped triboron complex ([4′,4′′,4′′′-tris(iminopyrrolyl)-1,3,5-triphenylbenzene](BPh2)3 (16)) were obtained in moderate to good yields, by the treatment of 3–9 with B(C6H5)3. The ligand precursors are non-emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π-conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time-dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13, in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples (11, 12, 14, and 15), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π-system, to afford green to yellow emissions. Organic light-emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m−2, for single layer devices, increasing to 4400 cd m−2 when a hole-transporting layer is used.