Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids

We report the growth and photophysical characterization of two polar hybrid lead halide phases, methylenedianiline lead iodide and bromide, (MDA)Pb2I6 and (MDA)Pb2Br6, respectively. The phases crystallize in noncentrosymmetric space group Fdd2, which produces a highly oriented molecular dipole moment that gives rise to second harmonic generation (SHG) upon excitation at 1064 nm. While both compositions are isostructural, the size dependence of the SHG signal suggests that the bromide exhibits a stronger phase-matching response whereas the iodide exhibits a significantly weaker non-phase-matching signal. Similarly, fluorescence from (MDA)Pb2Br6 is observed around 630 nm below 75 K whereas only very weak luminescence from (MDA)Pb2I6 can be seen. We attribute the contrasting optical properties to differences in the character of the halide sublattice and postulate that the increased polarizability of the iodide ions acts to screen the local dipole moment, effectively reducing the local electric field in the crystals

Luminescent Tris(8-hydroxyquinolates) of Bismuth(III)

Luminescent homoleptic bismuth(III) complexes have been synthesized by adding several functionalized 8-hydroxyquinolate ligands to bismuth(III) chloride in a 3:1 mole ratio in either ethanol or tetrahydrofuran (THF) solvent. These complexes have been characterized by single-crystal X-ray diffraction (XRD) analysis, UV-vis spectroscopy, fluorescence spectroscopy, and density functional theory (DFT) calculations to determine their structures and photophysical properties. Reversible dimerization of the mononuclear tris(hydroxyquinolate) complexes was observed in solution and quantified using UV-vis spectroscopy. The fluorescence spectra show a blue shift for the monomer compared with homoleptic aluminum(III) hydroxyquinolate compounds. Four dimeric compounds and one monomeric isomer were characterized structurally. The bismuth(III) centers in the dimers are bridged by two oxygen atoms from the substituted hydroxyquinolate ligands. The more sterically hindered quinolate complex, tris(2-(diethoxymethyl)-8-quinolinato)bismuth, crystallizes as a monomer. The complexes all exhibit low-lying absorption and emission spectral features attributable to transitions between the HOMO (π orbital localized on the quinolate phenoxide ring) and LUMO (π* orbital localized on the quinolate pyridyl ring). Excitation and emission spectra show a concentration dependence in solution that suggests that a monomer-dimer equilibrium occurs. Electronic structure DFT calculations support trends seen in the experimental results with a HOMO-LUMO gap of 2.156 eV calculated for the monomer that is significantly larger than those for the dimers (1.772 and 1.915 eV). The close face to face approach of two quinolate rings in the dimer destabilizes the uppermost occupied quinolate π orbitals, which reduces the HOMO-LUMO gap and results in longer wavelength absorption and emission spectral features than in the monomer form

Photophysical properties and OLEDs performances of blue Pt(II) and broad-band fac/mer Ir(III) cyclometallated complexes carrying 1-azatriphenylene ligand

Luminescent cyclometalated Iridium(III) complexes have been shown to be excellent candidates in organic light emitting diodes (OLEDs) because of their superior photophysical properties. For general lighting applications of OLEDs, broad band luminescent materials would be beneficial, to simplify device architectures, since white light can be produced using only two emitters: one blue and one broadband (yellow-red). The photophysical properties of fac/mer Ir((C^N)3 and Pt(C^N)(dpm) [where C^N is the rigid 1-azatriphenylene or dibenzo[f,h]quinoline, DBQ)] will be described. Mer-Ir(DBQ)3 shows broad featureless emission in solution between 500-750 nm (\uf06cmax,em= 586nm, QE= 0.5), whereas fac-Ir(DBQ)3 has narrow featureless emission between 475-700nm (\uf06cmax,em= 535nm, QE= 0.5). The rigid structure of DBQ prevents mer to fac isomerization processes that occur in other mer-Ir((C^N)3 complexes. Fac and mer Ir(DBQ)3 have similar emission profiles as fac and mer Ir(ppy)3 (ppy = 2-phenylpyridine). These results are interesting considering that DBQ has a larger \uf070-system than ppy. Theoretical calculations has been carried out to elucidate the experimental results. OLED devices, displaying broad emissions and 12% external quantum efficiency, have been prepared using fac/mer Ir(DBQ)3 mixtures

Tunable dual\u2010emission from cyclometallated Ir(III) Complexes carrying emissive aryl\u2010isocyanide ancillary ligands

The ability to control the excited-state properties of luminescent transition metal complexes is essential for sensing applications, optoelectronics and artificial photosynthesis. Luminescent cyclometalated iridium(III) complexes have been shown to be excellent candidates in these fields because of their superior photophysical properties such as high quantum efficiency and short excited state lifetimes. The emission color of these complexes is typically determined by the ligand with the lowest triplet energy. In this presentation we report a family of iridium(III) complexes of formula (ppy)2Ir(CN-R)(X) (R= t-Butyl; 1-naphtyl; 2-naphtyl. X= CN, Cl) with strategically matched ligands that have closely spaced triplet energies. The complexes undergo intramolecular excitation transfer between both the cyclometallated 3MLCT (ppy-based) state and the long-lived, lower energy 3\uf070-\uf070* state of the ancillary CN-R (R\uf0b9t-Bu). For example, compound 1 (R= 2-naphtyl; X= CN) emits at room temperature in fluid solution (\uf046=0.15; \uf074=0.7ms) from both the 3MLCT state (\uf06cmax.=452nm) and the 3\uf070-\uf070* state of 2-isocyano-naphtalene (\uf06cmax=488nm). The emission profile of 1 is unchanged when doped in a PMMA film, however, the efficiency increases 5-fold (\uf046=0.83; \uf074=2.5ms) due to strong suppression of the non-radiative decay rates. In conclusion, this luminescent (ppy)2Ir(CN-R)(X) complexes display efficient dual-emission due to energy shuttling between the different chromophores

Evidence for enhanced dipolar interactions between Pt centers in binuclear phosphorescent complexes

a b s t r a c t Transient studies are used to examine the radiative decay dynamics in a series of phosphorescent platinum binuclear complexes. The complexes studied consist of square planar (2-(4 0 ,6 0 -difluorophenyl)pyridinato-N,C 2 0 )Pt units bridged by either pyrazole or thiopyridine ligands. We observe an increase in radiative lifetime as temperature is reduced from 300 K to 4 K when the binuclear complexes, named 1, 2 and 3 with Pt-Pt spacings 3.19 Å, 3.05 Å, and 2.83 Å, respectively, are doped into a p-bis(triphenylsilyly)benzene wide energy gap host. The lifetimes for 1, 2 and 3 are s = 6.3 ± 0.1 ls, s = 2.3 ± 0.1 ls, and s = 2.0 ± 0.1 ls at T = 295 K, respectively. At T = 4 K, those values increase to s = 8.6 ± 0.1 ls, s = 14.4 ± 0.1 ls, and s = 17.0 ± 0.1 ls, suggesting that the neighboring heavy metal centers in compounds 2 and 3 have significant orbital overlap. A three-level zero-field splitting model yields the lowest triplet energy splittings of 28 ± 3 cm À1 , 142 ± 9 cm À1 , and 113 ± 10 cm À1 for compounds 1, 2 and 3, respectively

Spin-orbit coupling routes and OLED performance - Studies of blue light emitting Ir(III) and Pt(II) complexes

In this study, detailed spectroscopic investigations of the blue emitting compounds Ir(4,6-dFppy)₂(pic) and Pt(4,6-dFppy)(acac) are presented. Due to spin-orbit coupling (SOC) of the emitting triplet state with higher lying singlet states both complexes show an intense phosphorescence and are utilized as emitters in organic light emitting diodes (OLEDs). Distinct differences with respect to important photophysical properties are found for the two compounds. For example, the (distorted) octahedral Ir(4,6-dFppy)₂(pic) complex exhibits a shorter emission decay time and shows a larger zero-field splitting (ZFS) than the (distorted) square planar Pt(4,6-dFppy)(acac) complex (τ(Ir) = 0.4 μs and τ(Pt) = 3.6 μs of the respective shortest-lifed triplet substate; ΔE(ZFS, Ir) = 67 cm⁻¹, ΔE(ZFS, Pt) = 8 cm⁻¹). This behaviour is connected with the extent of metal-to-ligand charge transfer (MLCT, dπ*) character in the emitting triplet state. High MLCT character usually results in a high emission decay rate and indicates a good suitability as OLED emitter material. Of crucial importance in this respect is the effectiveness of SOC. In this study it is shown that the SOC routes depend on the coordination geometry of the emitter compound. In particular, the couplings can be more effective in (distorted) octahedral than in (distorted) square planar compounds. Hence, the photophysical differences of Ir(4,6-dFppy)₂(pic) compared to Pt(4,6-dFppy)(acac) can be rationalized. Moreover, this investigation shows that the analysis of SOC paths provides general guidelines for the design of efficient emitters for OLED applications

A Molecular Boron Cluster-Based Chromophore with Dual Emission

Bromination of the luminescent borane, anti-B18H22, via electrophilic substitution using AlCl3 and Br2 yields the monosubstituted derivative 4-Br-anti-B18H21 as an air-stable crystalline solid. In contrast to the unsubstituted parent compound, 4-Br-anti-B18H21 product possesses dual emission upon excitation with UV light and exhibits fluorescence at 410 nm and phosphorescence at 503 nm, with Фtotal = 0.07 in oxygen-free cyclohexane. Increased oxygen content in cyclohexane solution quenches the phosphorescence signal. The fluorescent signal intensity remains unaffected by oxygen, suggesting that this molecule could be used as a ratiometric oxygen probe