Tunable fluorophores based on 2-(N-arylimino)pyrrolyl chelates of diphenylboron: synthesis, structure, photophysical characterization, and application in OLEDs

Abstract

Reactions of 2-(N-arylimino)pyrroles (HNC4H3C(H)=N-Ar) with triphenylboron (BPh3) in boiling toluene afford the respective highly emissive N,N'-boron chelate complexes, [BPh2 {κ(2)N,N'-NC4H3C(H)=N-Ar}] (Ar=C6H5 (12), 2,6-Me2-C6H3 (13), 2,6-iPr2-C6H3 (14), 4-OMe-C6H4 (15), 3,4-Me2-C6 H3 (16), 4-F-C6H4 (17), 4-NO2-C6H4 (18), 4-CN-C6H4 (19), 3,4,5-F3-C6H2 (20), and C6F5 (21)) in moderate to high yields. The photophysical properties of these new boron complexes largely depend on the substituents present on the aryl rings of their N-arylimino moieties. The complexes bearing electron-withdrawing aniline substituents 17-20 show more intense (e.g., ϕf =0.71 for Ar=4-CN-C6H4 (19) in THF), higher-energy (blue) fluorescent emission compared to those bearing electron-donating substituents, for which the emission is redshifted at the expense of lower quantum yields (ϕf=0.13 and 0.14 for Ar=4-OMe-C6H4 (15) and 3,4-Me2-C6H3 (16), respectively, in THF). The presence of substituents bulkier than a hydrogen atom at the 2,6-positions of the aryl groups strongly restricts rotation of this moiety towards coplanarity with the iminopyrrolyl ligand framework, inducing a shift in the emission to the violet region (λmax =410-465 nm) and a significant decrease in quantum yield (ϕf=0.005, 0.023, and 0.20 for Ar=2,6-Me2-C6H3 (13), 2,6-iPr2-C6H3 (14), and C6F5 (21), respectively, in THF), even when electron-withdrawing groups are also present. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have indicated that the excited singlet state has a planar aryliminopyrrolyl ligand, except when prevented by steric hindrance (ortho substituents). Calculated absorption maxima reproduce the experimental values, but the error is higher for the emission wavelengths. Organic light-emitting diodes (OLEDs) have been fabricated with the new boron complexes, with luminances of the order of 3000 cd m(-2) being achieved for a green-emitting device.

Keywords: N-iminopyrrolyl ligands; OLEDs; boranes; density functional calculations; fluorescence.