Nail M. Shavaleev

Benzothiazole- and Benzoxazole-Substituted Pyridine-2-Carboxylates as Efficient Sensitizers of Europium Luminescence

A facile synthesis of benzothiazole- and benzoxazole-substituted pyridine-2-carboxylic acids has been developed. These ligands form mononuclear nine-coordinate complexes [Ln(kappa(3)-ligand)(2)(kappa(1)-ligand)(H(2)O)(2)] with light and heavy trivalent lanthanides, as established from the X-ray analysis of 11 complexes. A crystal structure of a minor product, the anhydrous nine-coordinate complex [Eu(kappa(3)-L)(3)], has also been determined. Photophysical studies of gadolinium chelates indicate that the triplet states of the new ligands are located at 20400-21400 cm(-1). The ligands are good sensitizers of the europium luminescence with ligand-to-metal energy transfer efficiency in the range 60-100%. The overall quantum yields of the europium emission are substantial, 12-14% in the solid state, and increase to 29-39% upon replacement of two metal-coordinated water molecules with dimethylsulfoxide in solution. The luminescence of near-infrared emitting lanthanides is also sensitized, but quantum yields are much smaller, reaching 0.17% for neodymium and 1.25% for ytterbium in DMSO, while energy transfer efficiencies for these two ions are below 50%.

Surprisingly Bright Near-Infrared Luminescence and Short Radiative Lifetimes of Ytterbium in Hetero-Binuclear Yb-Na Chelates

New heterobinuclear lanthanide complexes with benzoxazole-substituted 8-hydroxyquinolines, [Ln(ligand)(2)(mu-ligand)(2)Na] (Ln: Yb, Lu), have been prepared and their structure established by X-ray crystallography, (1)H NMR spectroscopy, and photophysical studies. The complexes display efficient ligand-sensitized near-infrared luminescence of ytterbium at 925-1075 nm with lifetimes and quantum yields as high as 22 micros and 3.7%, in the solid state, and 20 micros and 2.6% in CH(2)Cl(2) solution, respectively. These quantum yields are the highest reported to date for ytterbium complexes with organic ligands containing C-H bonds. A long-wavelength and intense intraligand charge-transfer transition (lambda(max) = 446-456 nm; epsilon approximately 1.2 x 10(4) M(-1) cm(-1)) allows for the excitation of infrared luminescence with visible light up to 600 nm. Remarkable features of these complexes include (i) quantitative ligand-to-Yb(III) energy transfer resulting in high overall efficiency of the ytterbium luminescence, (ii) unusually short radiative lifetime of the Yb(III) ion, 706-745 micros for solutions in CH(2)Cl(2), calculated from the f-f absorption spectra, and 513-635 micros estimated for solid state samples from quantum yield and lifetime data, and (iii) the unexpected large influence of second-sphere composition on the radiative lifetime of ytterbium.

N-Aryl Chromophore Ligands for Bright Europium Luminescence

Sterically hindered N-aryl-benzimidazole pyridine-2-carboxylic acids (aryl = phenyl, 4-biphenyl, 2-naphthyl) readily form homoleptic, neutral, nine-coordinate europium complexes which display efficient sensitized luminescence in solid state and in dichloromethane solution with quantum yields reaching 59% and have monoexponential and nearly temperature-independent lifetimes as long as 2.7 ms. The ligand-centered absorption band with a maximum at 321-342 nm and intensity (50-56) x 10(3) M(-1)cm(-1) ensures efficient harvesting of excitation light by the complexes. Variation of N-aryl chromophore enhances the ligand absorption at 250-350 nm without changing its triplet state energy which amounts to (19.2-21.3) x 10(3) cm(-1). Photophysical properties of europium complexes benefit from adequate protection of the metal by the ligands against non-radiative deactivation and efficient ligand-to-metal energy transfer exceeding 70%. A correlation is observed between the sensitized luminescence quantum yields of europium and the ligand triplet state energy; in certain cases it points to the presence of a second-sphere quenching of Eu(III) by co-crystallized water in the solid state.

Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

A series of tridentate benzimidazole-substituted pyridine-2-carboxylic acids have been prepared with a halogen, methyl or alkoxy group in the 6-position of the benzimidazole ring, which additionally contains a solubilising N-alkyl chain. The ligands form neutral homoleptic nine-coordinate lanthanum, europium and terbium complexes as established from X-ray crystallographic analysis of eight structures. The coordination polyhedron around the lanthanide ion is close to a tricapped trigonal prism with ligands arranged in an up-up-down fashion. The coordinated ligands serve as light-harvesting chromophores in the complexes with absorption maxima in the range 321-341 nm (epsilon=(4.9-6.0)x10(4) M(-1) cm(-1)) and triplet-state energies between 21 300 and 18 800 cm(-1); the largest redshifts occur for bromine and electron-donor alkoxy substituents. The ligands efficiently sensitise europium luminescence with overall quantum yields (Q(L)(Eu)) and observed lifetimes (tau(obs)) reaching 71 % and 3.00 ms, respectively, in the solid state and 52 % and 2.81 ms, respectively, in CH(2)Cl(2) at room temperature. The radiative lifetimes of the Eu((5)D(0)) level amount to tau(rad)=3.6-4.6 ms and the sensitisation efficiency eta(sens)=Q(L)(Eu)(tau(rad)/tau(obs)) is close to unity for most of the complexes in the solid state and equal to approximately 80 % in solution. The photophysical parameters of the complexes correlate with the triplet energy of the ligands, which in turn is determined by the nature of the benzimidazole substituent. Facile modification of the ligands makes them promising for the development of brightly emissive europium-containing materials.

Highly Luminescent Homoleptic Europium Chelates

The need for efficient and photostable lanthanide luminescent materials is dramatically increasing, in particular with respect to their growing application in lighting devices and biosciences. To this end, we have developed a facile synthesis of benzimidazole-substituted pyridine-2-carboxylic acids that efficiently sensitize europium luminescence in homoleptic neutral nine-coordinate complexes with overall quantum yields of 56-61% and lifetimes of 2.1-2.6 ms in the solid state at ambient conditions. The complexes reported here are potential synthons for the design of a variety of luminescent materials.

A simple, general synthesis of mixed d–f complexes containing both {Re(CO)3Cl(diimine)} and lanthanide-tris(β-diketonate) luminophores linked by bis-diimine bridging ligands

Reaction of the mononuclear ‘complex ligands’ [Re(CO)3Cl(L)] [L = 2,2′-bipyrimidine (bpym) or 2,3-bis(2-pyridyl)pyrazine (bppz)] with a Ln(β-diketonate)3 unit affords the heterodinuclear complexes [Re(CO)3Cl(μ-bpym)Ln(fod)3] and [Re(CO)3Cl(μ-bppz)Ln(tta)3]; one member of each series has been structurally characterised.

Efficient orange light-emitting electrochemical cells

We report the first bis-cyclometalated cationic iridium(III) complex with N-aryl-substituted 1H-imidazo[4,5-f][1,10]phenanthroline. The complex emits yellow-orange phosphorescence with a maximum at 583 nm, a quantum yield of 43%, and an excited-state lifetime of 910 ns in argon-saturated dichloromethane. Optimized orange light-emitting electrochemical cells with the new Ir(III) complex exhibit fast turn-on, a peak luminance of 684 cd m−2 and a peak efficacy of 6.5 cd A−1; in 850 h of continuous operation their luminance and efficacy decrease only by 20%.

Syntheses and structures of mononuclear {Re(CO)3Cl(NN)} 'complex ligands' with a pendant imino /pyridine binding site, and preparation of some heterodinuclear Re(I) /lanthanide(III) complexes

Abstract A series of potentially dinucleating ligands, each with two bidentate imino–pyridine compartments, has been used to prepare the mononuclear complexes [Re(CO)3Cl(Ln)] (n=1–4) which each have a vacant binding site and so constitute ‘complex ligands’ for the stepwise synthesis of heterodinuclear complexes. Three of the complexes (with L1, L2 and L4) have been structurally characterised. The mononuclear complex [Re(CO)3Cl(L5)], arising from decomposition of [Re(CO)3Cl(L3)] by hydrolysis of the non-coordinated imine unit, was also isolated and structurally characterised. Luminescence studies show that these compounds are only very weak emitters. The ‘complex ligand’ [Re(CO)3Cl(L1)] has been reacted with [Ln(hfac)3]·2H2O (Ln=Yb, Er) to prepare the heterodinuclear complexes [Re(CO)3Cl(μ-L1)Ln(hfac)3] in which the {Ln(hfac)}3 unit is attached to the second bidentate imino–pyridine site of [Re(CO)3Cl(L1)]. The crystal structures of these show that the bridging ligand has to twist to accommodate the two bulky metal fragments, and that the lanthanide units have an 8-coordinate square-antiprismatic geometry.

Pulsed-current versus constant-voltage light-emitting electrochemical cells with trifluoromethyl-substituted cationic iridium(III) complexes

We report on five cationic iridium(III) complexes with cyclometalating 2-(3′-trifluoromethylphenyl)pyridine and a diimine, [(C⁁N)2Ir(N⁁N)](PF6), N⁁N = 4,4′-R2-2,2′-dipyridyl or 4,7-R2-1,10-phenanthroline (R = H, Me, tert-Bu, Ph), and characterize three of them by crystal structure analysis. The complexes undergo oxidation of the Ir–aryl fragment at 1.13–1.16 V (against ferrocene couple) and reduction of the N⁁N ligand at −1.66 V to −1.86 V, and have a redox gap of 2.84–2.99 V. The complexes exhibit bluish-green to green-yellow phosphorescence in an argon-saturated dichloromethane solution at room temperature with a maximum at 486–520 nm, quantum yield of 61–67%, and an excited-state lifetime of 1.2–4.3 μs. In two-layer spin-coated light-emitting electrochemical cells (LEC) operated at a constant-voltage (4 V) or a pulsed-current (100 A m−2 per pulse; block wave, 1000 Hz; 50% duty), the complexes exhibit green-yellow electroluminescence with a maximum at 547–556 nm. The luminance and efficiency of LEC do not level off after peaking but decay; for example, the luminance of the devices after reaching the peak of 195–1094 cd m−2 halves in 9–580 min. The best of the new LEC runs under pulsed-current driving and exhibits peak efficiencies of 16.8 cd A−1 and 7.9 lm W−1 and an EQE of 5.4% at a luminance of ≥834 cd m−2. We find that the pulsed-current LEC offer the following advantages over the constant-voltage LEC: lower current, higher stability, faster turn-on, and higher efficiency at higher luminance.

Host–guest blue light-emitting electrochemical cells

Carbazole, a commonly used hole-transporter for organic electronics, has been modified with an imidazolium cation and a hexafluorophosphate counter-anion to give an ionic hole-transporter. It has been applied as one of the hosts in a host–guest blue light-emitting electrochemical cell (LEC) with the neutral blue emitter FIrPic. We have obtained efficient and bright blue LECs with an electroluminescence maximum at 474 nm and efficacy of 5 cd A−1 at a luminance of 420 cd m−2, thereby demonstrating the potential of the ionic organic charge-transporters and of the host–guest architecture for LECs.