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Tunable Fluorophores Based on 2‐( N ‐Arylimino)pyrrolyl Chelates of Diphenylboron: Synthesis, Structure, Photophysical Characterization, and Application in OLEDs
Author(s) -
Suresh D.,
Lopes Patrícia S.,
Ferreira Bruno,
Figueira Cláudia A.,
Gomes Clara S. B.,
Gomes Pedro T.,
Di Paolo Roberto E.,
Maçanita António L.,
Duarte M. Teresa,
Charas Ana,
Morgado Jorge,
Calhorda Maria José
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201303607
Subject(s) - chemistry , aryl , aniline , toluene , medicinal chemistry , chelation , fluorescence , oled , boron , stereochemistry , crystallography , organic chemistry , alkyl , physics , layer (electronics) , quantum mechanics
Reactions of 2‐( N ‐arylimino)pyrroles (HNC 4 H 3 C(H)N‐Ar) with triphenylboron (BPh 3 ) in boiling toluene afford the respective highly emissive N , N ′‐boron chelate complexes, [BPh 2 {κ 2 N , N′ ‐NC 4 H 3 C(H)N‐Ar}] (Ar=C 6 H 5 ( 12 ), 2,6‐Me 2 ‐C 6 H 3 ( 13 ), 2,6‐ i Pr 2 ‐C 6 H 3 ( 14 ), 4‐OMe‐C 6 H 4 ( 15 ), 3,4‐Me 2 ‐C 6 H 3 ( 16 ), 4‐F‐C 6 H 4 ( 17 ), 4‐NO 2 ‐C 6 H 4 ( 18 ), 4‐CN‐C 6 H 4 ( 19 ), 3,4,5‐F 3 ‐C 6 H 2 ( 20 ), and C 6 F 5 ( 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., ϕ f =0.71 for Ar=4‐CN‐C 6 H 4 ( 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 ( ϕ f =0.13 and 0.14 for Ar=4‐OMe‐C 6 H 4 ( 15 ) and 3,4‐Me 2 ‐C 6 H 3 ( 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 ( λ max =410–465 nm) and a significant decrease in quantum yield ( ϕ f =0.005, 0.023, and 0.20 for Ar=2,6‐Me 2 ‐C 6 H 3 ( 13 ), 2,6‐ i Pr 2 ‐C 6 H 3 ( 14 ), and C 6 F 5 ( 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.