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.

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

“…All the remaining structural features presented by these compounds are similar to other iminopyrrole molecules reported previously 13gi,k, 14b, 18e,f, 20…”

“…Although the two dihedral angles drop to about half, they are still very high (26.18/−26.40 and 37.28/−30.73 ° in TDDFT). As observed before,14b, 15, 17 rotation of the iminopyrrolyl boron diphenyl fragments (about the N imino ‐C aryl−ipso bond) in relation to the aryl bridging groups, toward coplanarity, is a key feature of these molecules, which helps them to reach maximum π‐delocalization in the excited state.…”

“…The molecular geometries of the boron complexes may be well reproduced by DFT21 calculations (ADF program,22 BP86 functional, see the Computational details below), as was discussed in previous works in which several crystal structures were available 14b. 15, 17 As mentioned above in the description of the structure of 11⋅ C 6 H 14 , the bond lengths around boron do not vary significantly, the dihedral angle over the aromatic bridge of the iminopyrrolyl fragment being the relevant structural parameter.…”

“…2‐Formiminopyrrole ligands are an important class of bidentate chelating ligand precursors in coordination and organometallic chemistry, their complexes having a wide range of applications in various organic transformations 13. Steric and electronic tuning through the N ‐aryl substituents in the imine group is easily achieved by using a condensation reaction (see below, I )14 and the π‐conjugation length of the molecule can be readily altered by incorporating bridging aromatic spacers ( II ),14b, 15 or by fusing aromatic groups on the edges of the pyrrole ring ( III and IV ; Ar=aromatic group),14a, 16 making these ligands very versatile…”

Synthesis and Characterization of Bis(2‐iminopyrrolyl) Iron(II) Complexes by N–H Bond Activation

“…[12,13] Some iminopyrrolyl complexes have interesting photophysical properties [14] and their applications in OLED materials have been explored. [15,16] The iminopyrrolyl complexes of main group metals were found to be catalytically active for the ring-opening polymerization of lactides. [17] Based on different metals in different oxidation states, there are mono(2-iminopyrrolyl), bis(2-iminopyrrolyl), and tri(2-iminopyrrolyl) metal complexes.…”