Structural and Photophysical Properties of Mononuclear and Dinuclear Lanthanide(III) Complexes of Multidentate Podand Ligands Based on Poly(pyrazolyl)borates

Abstract

Lanthanide(III) complexes have been prepared with [L 1 ] -[the tetradentate chelating ligand bis{3-(2-pyridyl)-pyrazolyl}dihydroborate], [L 2 ] -[the tetradentate chelating ligand bis{3-(2-pyrazinyl)pyrazolyl}dihydroborate], [L 3 ] -[the hexadentate chelating ligand bis[3-{6′-(2,2′-bipyridyl)}pyrazol-1-yl]dihydroborate], and [L 4 ] 2-[the 12-dentate compartmental ligand hexakis{3-(2-pyridyl)pyrazol-1-yl}diboran(IV)ate, which has two hexadentate tris-(pyrazolyl)borate-based cavities linked "back-to-back" by a B-B bond]. [Ln(L 1 ) 2 (NO) 3 ] are 10-coordinate with two tetradentate N-donor ligands and one bidentate nitrate. [Ln(L 2 ) 2 (NO) 3 ] have 10-coordinate structures similar to those of the [L 1 ] -complexes except that the coordinated N 1 of the pyrazine rings is not such a good donor as the pyridine rings in the [L 1 ] -complexes, leading to marked lengthening of these Ln-N bonds. [Ln(L 3 )(NO 3 ) 2 ] are also 10-coordinate from one hexadentate chelating ligand which has a pseudoequatorial coordination mode and two pseudoaxial bidentate nitrate ligands; the hexadentate ligand has a shallow helical twist to prevent steric interference between its ends. Finally [{Ln(NO 3 ) 2 (L 4 )] are dinuclear, with each metal center being 10-coordinate from a tripodal hexadentate ligand cavity and two bidentate nitrates. Five complexes were structurally characterized: [Tb(L 2 ) 2 (NO 3 )]‚dmf is monoclinic (P2 1 /c) with a ) 14.881(3) Å, b ) 15.5199(12) Å, c ) 15.845(2) Å, ) 92.387(12)°, and Z ) 4. [Gd(L 2 ) 2 (NO 3 )]‚dmf is monoclinic (P2 1 /c) with a ) 14.926(2) Å, b ) 15.465(2) Å, c ) 15.878(2) Å, ) 92.698(11)°, and Z ) 4. [Eu(L 3 )(NO 3 ) 2 ]‚dmf‚0.5Et 2 O is triclinic (P1 h) with a ) 10.020(3) Å, b ) 13.036(3) Å, c ) 14.740(3) Å, R ) 70.114(14)°, ) 71.55(2)°, γ ) 79.66(2)°, and Z ) 2. [{La(NO 3 )-(dmf) 2 } 2 (L 4 )](NO 3 ) 2 ‚dmf is orthorhombic (Pbca) with a ) 18.813(2) Å, b ) 15.241(2) Å, c ) 27.322(2), and Z ) 4. [{Gd(NO 3 ) 2 } 2 (L 4 )]‚2.4dmf is tetragonal (P4 2 /n) with a ) 16.622(6), c ) 24.19(5) Å, and Z ) 4. Detailed photophysical studies have been performed on the free ligands and their complexes with Gd(III), Eu(III), and Tb(III) in several solvents. The results show a wide range in the emission properties of the complexes which can be rationalized in terms of subtle variations in the steric and electronic properties of the ligands. In particular the dinuclear Tb(III) complex of [L 4 ] 2-has an emission quantum yield of ca. 0.5 in D 2 O and MeOD.

Synthesis and Photoluminescence Properties of Heteroleptic Europium(III) Complexes with Appended Carbazole Units

“…This suggests that although the triplet state of the carbazole unit is energetically compatible with an efficient EnT process, the lower-lying dbm level may undergo thermal back-energy transfer from the central core. [19] Furthermore, the 5 D 1 Eu III emitting state, located at approximately 18800 cm -1 , is critically close to the triplet energy level of dbm (19500 cm -1 ). A schematic representation of ligand-centred and metal-centred energy levels is depicted in Figure 4.…”

“…By contrast, Eu·dbm·phen can be selectively excited on both the phenanthroline (≈ 260 nm) and the β-diketonate (≈350 nm) moieties. In the first case, a relatively intense metal-centred (MC) luminescence was detected (Φ = 0.05), whereas the excitation at 350 nm afforded an almost 10-fold weaker emission spectrum ( luminescence intensity from Eu·dbm·phen was detected in solvents bearing -OH quenching groups, [19] such as CH 3 OH (Table 1), which suggests that the phen and dbm coordinating ligands do not provide efficient shielding of the metal core towards external quenching. Measurements in solid state matrices (6 % doped polycarbonate) were performed for Eu·dbm·phen, Eu·dbm· carb·phen and Eu·dbm·carb·bath (Table 1).…”

“…[17] However, in spite of the excellent photoluminescence (PL) properties of Eu III materials, bright-red europium-containing electroluminescent (EL) devices with satisfying performance are hardly obtainable because of the poor charge-transporting ability of the lanthanide complexes and the luminescence quenching by matrix vibrations through nonradiative pathways. [18,19] To improve the charge-transporting properties of Eu III complexes to be used in electroluminescent devices, a suitable strategy would involve doping with electron/hole-transporting molecules, such as biphenyl-p-(tert-butyl)phenyl-1,3,4-oxadiazole (PBD), N,NЈ-diphenyl-N,NЈ-bis(3-methylphenyl)-1,1Ј-biphenyl-4,4Ј-diamine (TPD) or 4,4Ј-N,NЈ-ane solution and in solid matrix show that the carbazole moiety is a better sensitizer for the metal-centred emitting states of the Eu III ion compared to the dibenzoylmethane and phenanthroline units. Furthermore, the charge-transporting properties of the carbazole moieties appear to be appealing when integrated into the emitting units.…”

Anionically Substituted 1,1′,1″‐Methylidynetris[1H‐pyrazole] Ligands for the Formation of Neutral Lanthanide Complexes in Water: Synthesis, Characterization, and Photophysical Properties

“…In this context, we envisaged that a 1,1',1''-methylidynetris[1H-pyrazole] base should be an adequate cornerstone for the construction of such a C 3 -symmetric complex. Further introduction of pyridine rings at the 3-position of the pyrazole rings are expected to lead to highly luminescent complexes, especially in the case of terbium complexes [11]. Finally, addition of a carboxylate function at the 2-position of the pyridine rings should increase the stability of the complex by introduction of three negatively charged functions leading to a potentially tris-tridentate ligand.…”

Lanthanide probes for luminescence microscopy and anion sensing

“…Trispyrazolylmethane [18] ligands and their borate analogues [19] have long been shown as interesting ligands and recent examples of their lanthanide complexes demonstrated good co-ordination properties [20]. Furthermore, the pyrazolylpyridine arms are very good chromophoric units for the antenna effect, especially in the case of terbium [20,21]. The further introduction of a carboxylate function on the pyridine rings was expected to allow for completion of the co-ordination sphere of the Ln cation in the pocket of the ligand.…”