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Tunable Fluorophores Based on 2‐(N‐Arylimino)pyrrolyl Chelates of Diphenylboron: Synthesis, Structure, Photophysical Characterization, and Application in OLEDs

Suresh, D. ; Lopes, P. S. ; Ferreira, B. F. ; Figueira, C. A. F. ; Gomes, C. S. B. ; Gomes, P. ; Paolo, R. ; Maçanita, A. ; Charas, A. ; Morgado, J.

Chemistry A. European Journal Vol. 20, Nº 14, pp. 4126 - 4140, April, 2014.

ISSN (print): 0947-6539
ISSN (online): 0947-6539

Journal Impact Factor: 5,454 (in 2008)

Digital Object Identifier: 10.1002/chem.201303607

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{κ2N,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‐C6H3 (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.

Fluorescent­ N,N′‐boron chelate complexes: Mononuclear boron complexes of 2‐(N‐arylimino)pyrrolyl emit violet to bluish‐green colors in solution (see figure, ITO=indium tin oxide, PEDOT:PSS=poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)), depending on the substituents on the N‐aryl group. Organic light‐emitting diodes have been successfully fabricated with the new boron complexes, achieving luminances of the order of 3000 cd m−2.