Efficient sensitization of europium, ytterbium, and neodymium functionalized tris-dipicolinate lanthanide complexes through tunable charge-transfer excited states

A series of push-pull donor-pi-conjugated dipicolinic acid ligands and related tris-dipicolinate europium and lutetium complexes have been prepared. The ligands present broad absorption and emission transitions in the visible spectral range unambiguously assigned to charge-transfer transitions (CT) by means of time-dependent density functional theory calculations. The photophysical properties (absorption, emission, luminescence quantum yield, and lifetime) of the corresponding europium complexes were thoroughly investigated. Solvatochromism and temperature effects clearly confirm that Eu(III) sensitization directly occurs from the ligand CT state. In addition, modulation of the energy of the CT donating state by changing the nature of the donor fragment allows the optimal energy of the antennae for europium sensitization to be determined, and this optimal energy was found to be close to the (5)D 1 accepting state. Finally, this CT sensitization process has been successfully extended to near-infrared emitters (neodymium and ytterbium).     

Blending Gelators to Tune Gel Structure and Probe Anion‐Induced Disassembly

Blending different low molecular weight gelators (LMWGs) provides a convenient route to tune the properties of a gel and incorporate functionalities such as fluorescence. Blending a series of gelators having a common bis‐urea motif, and functionalised with different amino acid‐derived end‐groups and differing length alkylene spacers is reported. Fluorescent gelators incorporating 1‐ and 2‐pyrenyl moieties provide a probe of the mixed systems alongside structural and morphological data from powder diffraction and electron microscopy. Characterisation of the individual gelators reveals that although the expected α‐urea tape motif is preserved, there is considerable variation in the gelation properties, molecular packing, fibre morphology and rheological behaviour. Mixing of the gelators revealed examples in which: 1) the gels formed separate, orthogonal networks maintaining their own packing and morphology, 2) the gels blended together into a single network, either adopting the packing and morphology of one gelator, or 3) a new structure not seen for either of the gelators individually was created. The strong binding of the urea functionalities to anions was exploited as a means of breaking down the gel structure, and the use of fluorescent gel blends provides new insights into anion‐mediated gel dissolution.     

Tris-cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands: synthesis, redox and photophysical properties, and electrophosphorescent light-emitting diodes

Using ligands synthesized by Suzuki cross-coupling methodology, new phosphorescent homoleptic tris-cyclometalated complexes have been obtained, namely fac-[Ir(Cz-2-Fl(n)Py)(3)] (1 d-f) and fac-[Ir(Cz-3-Fl(n)Py)(3)] (2 d-f), which are solution-processible triplet emitters (Cz denotes N-hexylcarbazole, n is the number of 9,9'-dihexylfluorene (Fl) units (n=0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2- and 2,7-positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2- or 3-positions, in series 1 and 2, respectively. The oxidation potential of 1 d studied by cyclic voltammetry ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.14 V, versus Ag/AgNO(3), CH(2)Cl(2)) is less positive (i.e. raised HOMO level) compared to that of the isomer 2 d ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.30 V), where the Cz-nitrogen is meta to the Ir center. Ligand-centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi-reversibility, for example, for 2 f {E{{{\rm ox}\hfill \atop {\rm pa}\hfill}}} =0.45 (1e), 0.95 (3e), 1.24 V (3e). Striking differences are seen in the solution-state photophysical data between complexes [Ir(Cz-2-Py)(3)] (1 d) and [Ir(Cz-3-Py)(3)] (2 d), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue-shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin-coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes 1 d or 2 d as dopants. Turn-on voltages are low (3-4 V). With 1 d orange light is emitted at lambda(max)=590 nm with an EQE of 1.3 % (at 7.5 mA cm(-2)) and an emission intensity (luminance) of 4354 cd m(-2) (at 267 mA m(-2)). The green emission from 2 d devices (lambda(max)=500 nm) is due to the reduced electron-donating ability of the carbazole unit in 2 d. Recording the EL spectra of the 1 d device at 6 V (current density, 100 mA cm(-2)) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.     

Aqueous solutions of transition metal containing micelles

Incorporation of d- or f-block metals into ligand systems that renders a metal complex surface-active or drives its partitioning into surfactant phases enables the localisation of chemical functionality at interfaces. This article discusses a number of fundamental aspects of these interesting materials and examines potential applications.     

The Synthesis and One‐ and Two‐Photon Optical Properties of Dipolar,Quadrupolar and Octupolar Donor–Acceptor Molecules Containing Dimesitylboryl Groups

Two series of related donor–acceptor conjugated dipolar, pseudo‐quadrupolar (V‐shaped) and octupolar molecular systems based on the p‐dimesitylborylphenylethynylaniline core, namely, 4‐(4‐dimesitylborylphenylethynyl)‐N,N‐dimethylaniline, 4‐[4‐(4‐dimesitylborylphenylethynyl)phenylethynyl]‐N,N‐dimethylaniline, 3,6‐bis(4‐dimesitylborylphenylethynyl)‐N‐n‐butylcarbazole and tris[4‐(4‐dimesitylborylphenylethynyl)phenyl]amine, and on the E‐p‐dimesitylborylethenylaniline motif, namely, E‐4‐dimesitylborylethenyl‐N,N‐di(4‐tolyl)aniline, 3,6‐bis(E‐dimesitylborylethenyl)‐N‐n‐butylcarbazole and tris(E‐4‐dimesitylborylethenylphenyl)amine have been synthesised by palladium‐catalyzed cross‐coupling and hydroboration routes, respectively. Their absorption and emission maxima, fluorescence lifetimes and quantum yields have been obtained and their two‐photon absorption (TPA) spectra and TPA cross‐sections have been examined. Of these systems, the octupolar compound tris(E‐4‐dimesitylborylethenylphenyl)amine has been shown to exhibit the largest TPA cross‐section among the two series of approximately 1000 GM at 740 nm. Its TPA performance is comparable to those of other triphenylamine‐based octupoles of similar size. The combination of such large TPA cross‐sections and high emission quantum yields, up to 0.94, make these systems attractive for applications involving two‐photon excited fluorescence (TPEF).     

Colourimetric Carboxylate Anion Sensors Derived from Viologen‐Based Receptors

A series of tri‐ and tetrapodal viologen‐based anion receptors showing a colourimetric response to carboxylates, such as acetate, have been synthesised. Alteration of the anion binding sites allows for binding site competition within a receptor. This results in a delayed colourimetric response for urea derivatives compared with pyridinium systems because the anions are initially bound to the periphery of the receptor, away from the viologen unit. DFT calculations and experimental measurements allow the colour change to be assigned to an anion–receptor charge‐transfer process, facilitated by the exceptionally low reduction potential of the cationic host compounds. Evidence for electron transfer to give the viologen radical cation is also seen in some cases.     

Thermally induced defluorination during a mer to fac transformation of a blue-green phosphorescent cyclometalated iridium(III) complex

The new homoleptic tris-cyclometalated [Ir(C^N)(3)] complexes mer-8, fac-8, and fac-9 incorporating γ-carboline ligands are reported. Reaction of 3-(2,4-difluorophenyl)-5-(2-ethylhexyl)-pyrido[4,3-b]indole 6 with iridium(III) chloride under standard cyclometalating conditions gave the homoleptic complex mer-8 in 63% yield. The X-ray crystal structure of mer-8 is described. The Ir-C and Ir-N bonds show the expected bond length alternations for the differing trans influence of phenyl and pyridyl ligands. mer-8 quantitatively isomerized to fac-8 upon irradiation with UV light. However, heating mer-8 at 290 °C in glycerol led to an unusual regioselective loss of one fluorine atom from each of the ligands, yielding fac-9 in 58% yield. fac-8 is thermally very stable: no decomposition was observed when fac-8 was heated in glycerol at 290 °C for 48 h. The γ-carboline system of fac-8 enhances thermal stability compared to the pyridyl analogue fac-Ir(46dfppy)(3)10, which decomposes extensively upon being heated in glycerol at 290 °C for 2 h. Complexes mer-8, fac-8, and fac-9 are emitters of blue-green light (λ(max)(em) = 477, 476, and 494 nm, respectively). The triplet lifetimes for fac-8 and fac-9 are ~4.5 μs at room temperature; solution Φ(PL) values are 0.31 and 0.22, respectively.     

Anomalous reversal of C-h and C-d quenching efficiencies in luminescent praseodymium cryptates

A series of selectively deuterated praseodymium cryptates has been synthesized. Their luminescence lifetimes in solution range from 150 to 595 ns for the (1)D(2)?→?(3)F(4) transition. Global fitting of the nonradiative deactivation rate differences of the isotopologic C-(H/D) oscillators revealed that aromatic C-D overtones anomalously quench the luminescence more than C-H vibrations. This is explained by the dominance of Franck-Condon overlap factors that greatly favor C-D oscillators, which are in almost ideal resonance with the relevant energy gap (1)D(2)-(1)G(4) of praseodymium.     

Enhancement of second harmonic generation efficiency: Cs(I)-doped tris(thiourea)zinc(II) sulphate crystals

The effect of dopant cesium (Cs(I)) over a concentration range from 1 to 10 mol% on the growth process, morphology, thermal and optical properties of tri(thiourea)zinc(II) sulfate (ZTS) single crystals grown by slow evaporation solution growth technique has been investigated. Incorporation of Cs(I) into the crystal lattice was well confirmed by energy dispersive X-ray spectroscopy (EDS). The lattice parameters of the as-grown crystals were obtained by single crystal X-ray diffraction analysis. The reduction in the intensities observed in powder X-ray diffraction patterns of doped specimen and slight shifts in vibrational frequencies in fourier transform infrared spectra (FT-IR) indicate the lattice stress as a result of doping. Thermal studies reveal the purity of the material and no decomposition is observed up to the melting point. High transmittance is observed in the visible region and the cut-off λ is ～280 nm. The surface morphology of the as-grown specimens was studied by scanning electron microscopy (SEM). The second harmonic generation (SHG) efficiency of the host crystal is enhanced greatly in the presence of high concentrations of the dopant.