The synthesis and photophysical characterization of a series of (N,C2‘-(2-para-tolylpyridyl))2Ir(LL‘) (tpy)2Ir(LL‘) (LL‘ = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, ...diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of (dfppy)2Ir(LL‘) (dfppy = N,C2‘-2-(4‘,6‘-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)2Ir(CN)2 - and (ppy)2Ir(NCS)2 - (ppy = N,C2‘-2-phenylpyridyl). The (tpy)2Ir(PPh2CH2)2BPh2 and (tpy)2Ir(CN-t-Bu)2(CF3SO3) complexes have been structurally characterized by X-ray crystallography. The Ir−Caryl bond lengths in (tpy)2Ir(CN-t-Bu)2 + (2.047(5) and 2.072(5) Å) and (tpy)2Ir(PPh2CH2)2BPh2 (2.047(9) and 2.057(9) Å) are longer than their counterparts in (tpy)2Ir(acac) (1.982(6) and 1.985(7) Å). Density functional theory calculations carried out on (ppy)2Ir(CN-Me)2 + show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-π and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-π orbitals. Electrochemical analysis of the (tpy)2Ir(LL‘) complexes shows that the reduction potentials are largely unaffected by variation in the ancillary ligand, whereas the oxidation potentials vary over a much wider range (as much as 400 mV between two different LL‘ ligands). Spectroscopic analysis of the cyclometalated Ir complexes reveals that the lowest energy excited state (T1) is a triplet ligand-centered state (3LC) on the cyclometalating ligand admixed with 1MLCT (MLCT = metal-to-ligand charge-transfer) character. The different ancillary ligands alter the 1MLCT state energy mainly by changing the HOMO energy. Destabilization of the 1MLCT state results in less 1MLCT character mixed into the T1 state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in ln(k nr) (k nr = nonradiative decay rate). Decreased 1MLCT character in the T1 state also increases the Huang−Rhys factors in the emission spectra, decreases the extinction coefficient of the T1 transition, and consequently decreases the radiative decay rates (k r). Overall, the luminescence quantum yields decline with increasing emission energies. A linear dependence of the radiative decay rate (k r) or extinction coefficient (ε) on (1/ΔE)2 has been demonstrated, where ΔE is the energy difference between the 1MLCT and 3LC transitions. A value of 200 cm-1 for the spin−orbital coupling matrix element 〈3LC|H SO |1MLCT〉 of the (tpy)2Ir(LL‘) complexes can be deduced from this linear relationship. The (fppy)2Ir(LL‘) complexes with corresponding ancillary ligands display similar trends in excited-state properties.
Two-coordinate carbene-MI-amide (cMa, MI = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast ...radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (ΦPL > 0.95) and fast radiative rates (k r = 4 × 106 s–1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole–electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔE ST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.
To use the "apparent diffusion coefficient" (Dapp) as a quantitative imaging parameter, well-suited test fluids are essential. In this study, the previously proposed aqueous solutions of ...polyvinylpyrrolidone (PVP) were examined and temperature calibrations were obtained. For example, at a temperature of 20°C, Dapp ranged from 1.594 (95% CI: 1.593, 1.595) μm2/ms to 0.3326 (95% CI: 0. 3304, 0.3348) μm2/ms for PVP-concentrations ranging from 10% (w/w) to 50% (w/w) using K30 polymer lengths. The temperature dependence of Dapp was found to be so strong that a negligence seems not advisable. The temperature dependence is descriptively modelled by an exponential function exp(c2 (T - 20°C)) and the determined c2 values are reported, which can be used for temperature calibration. For example, we find the value 0.02952 K-1 for 30% (w/w) PVP-concentration and K30 polymer length. In general, aqueous PVP solutions were found to be suitable to produce easily applicable and reliable Dapp-phantoms.
The photophysical properties for a series of facial (fac) cyclometalated Ir(III) complexes (fac-Ir(C∧N)3 (C∧N = 2-phenylpyridyl (ppy), 2-(4,6-difluorophenyl)pyridyl (F2ppy), 1-phenylpyrazolyl (ppz), ...1-(2,4-difluorophenyl)pyrazolyl) (F2ppz), and 1-(2-(9,9′-dimethylfluorenyl))pyrazolyl (flz)), fac-Ir(C∧N)2(C∧N′) (C∧N = ppz or F2ppz and C∧N′ = ppy or F2ppy), and fac-Ir(C∧C′)3 (C∧C′ = 1-phenyl-3-methylbenzimidazolyl (pmb)) have been studied in dilute 2-methyltetrahydrofuran (2-MeTHF) solution in a temperature range of 77−378 K. Photoluminescent quantum yields (Φ) for the 10 compounds at room temperature vary between near zero and unity, whereas all emit with high efficiency at low temperature (77 K). The quantum yield for fac-Ir(ppy)3 (Φ = 0.97) is temperature-independent. For the other complexes, the temperature-dependent data indicates that the luminescent efficiency is primarily determined by thermal deactivation to a nonradiative state. Activation energies and rate constants for both radiative and nonradiative processes were obtained using a Boltzmann analysis of the temperature-dependent luminescent decay data. Activation energies to the nonradiative state are found to range between 1600 and 4800 cm−1. The pre-exponential factors for deactivation are large for complexes with C∧N ligands (1011−1013 s−1) and significantly smaller for fac-Ir(pmb)3 (109 s−1). The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir−N (Ir−C for fac-Ir(pmb)3) bond rupture leading to a five-coordinate species that has triplet metal-centered (3MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and 3MC states. The energy level for the 3MC state is estimated to lie between 21 700 and 24 000 cm−1 for the fac-Ir(C∧N)3 complexes and at 28 000 cm−1 for fac-Ir(pmb)3.
This study presents the synthesis and characterization of two spirobifluorenyl derivatives substituted with either triphenylmethyl (
) or triphenylsilyl (
) moieties for use as host materials in ...phosphorescent organic light-emitting diodes (PHOLED). Both molecules have similar high triplet energies and large energy gaps. Blue Ir(tpz)
and green Ir(ppy)
phosphorescent devices were fabricated using these materials as hosts. Surprisingly,
demonstrated superior charge-transporting ability compared to
, despite having similar energies for their valence orbitals. In particular,
proved to be a highly effective host for both blue and green devices, resulting in maximum efficiencies of 12.6% for the Ir(tpz)
device and 9.6% for the Ir(ppy)
device. These results highlight the benefits of appending the triphenylsilyl moiety onto host materials and underscore the importance of considering the morphology of hosts in the design of efficient PHOLEDs.
In this essay, Mark Jonas argues that there are three broadly held misconceptions of Plato's philosophy that work against his relevance for contemporary moral education. The first is that he is an ...intellectualist who is concerned only with the cognitive aspect of moral development and does not sufficiently emphasize the affective and conative aspects; the second is that he is an elitist who believes that only philosopher‐kings can attain true knowledge of virtue and it is they who should govern society; the third is that he affirms the realm of the Forms as a literal metaphysical reality and believes that for individuals to attain virtue they must access this realm through contemplation. The goal of this essay is to correct these misconceptions. The rehabilitation of Plato's reputation may enable future researchers in moral education to discover in his philosophy new avenues for exploring how best to cultivate virtues in students.
A series of bimetallic carbene-metal-amide (cMa) complexes have been prepared with bridging biscarbene ligands to serve as a model for the design of luminescent materials with large oscillator ...strengths and small energy differences between the singlet and triplet states (ΔE ST). The complexes have a general structure (R2N)Au(:carbenecarbene:)Au(NR2). The bimetallic complexes show solvation-dependent absorption and emission that is analyzed in detail. It is found that the molar absorptivity of the bimetallic complexes is correlated with the energy barrier to rotation of the metal–ligand bond. The bimetallic cMa complexes also exhibit short emission lifetimes (τ = 200–300 ns) with high photoluminescence efficiencies (ΦPL > 95%). The radiative rates of bimetallic cMa complexes are 3–4 times faster than that of the corresponding monometallic complexes. Analysis of temperature-dependent luminescence data indicates that the lifetime for the singlet state (τS1 ) of bimetallic cMa complexes is near 12 ns with a ΔE ST of 40–50 meV. The presented compounds provide a general design for cMa complexes to achieve small values for ΔE ST while retaining high radiative rates. Solution-processed organic light-emitting devices (OLEDs) made using two of the complexes as luminescent dopants show high efficiency and low roll-off at high luminance.
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