Electroluminescence Stability of Organic Light-Emitting Devices Utilizing a Nondoped Pt-Based Emission Layer

We study the effects of using an emitting material (Pt­(II) bis­(3-(trifluoromethyl)-5-(2-pyridyl)­pyrazolatePt­(fppz)₂) characterized by a preferred horizontal dipole alignment and a nearly unitary quantum yield regardless of concentration on the lifetime of organic light-emitting devices (OLEDs). Using such a material as a dopant in increasingly higher concentrations is found to lead to an increase in device stability, a trend that is different from that commonly observed with conventional OLED guests. The results are consistent with the newly discovered exciton–polaron-induced aggregation degradation mechanism of OLED materials. When this emitter is used as a neat emission layer, the material is already in a highly aggregated state, and the device is no longer affected by exciton–polaron interactions. The results demonstrate the potential stability benefits of using such materials in OLEDs

Phosphorescent PtAu₂ Complexes with Differently Positioned Carbazole–Acetylide Ligands for Solution-Processed Organic Light-Emitting Diodes with External Quantum Efficiencies of over 20%

The utilization of phosphorescent metal cluster complexes as new types of emitting materials in organic light-emitting diodes (OLEDs) is becoming an alternative and viable approach for achieving high-efficiency electroluminescence. We report herein the design of cationic PtAu₂ cluster complexes with differently positioned 9-phenylcarbazole–acetylides to serve as phosphorescent emitters in OLEDs. The rigid structures of PtAu₂ complexes cause intense phosphorescence with quantum yields of over 85%, which originates from ³[π­(phenylcarbazole–acetylide) → π*­(dpmp)] ligand-to-ligand and ³[π­(phenylcarbazole–acetylide) → p/s­(PtAu₂)] ligand-to-metal charge-transfer triplet excited states. When 8 wt % PtAu₂ is doped to blended host materials of TCTA and OXD-7 (2:1 weight ratio) as light-emitting layers, solution-processed OLEDs give a current efficiency of 78.2 cd A^–1 and an external quantum efficiency (EQE) of 21.5% at a practical luminance of 1029 cd m^–2 with a slow efficiency roll-off upon increasing luminance. This represents the best device performance and the highest efficiency recorded at practical luminance for solution-processed OLEDs

Bis-tridentate Ir(III) Phosphors and Blue Hyperphosphorescence with Suppressed Efficiency Roll-Off at High Brightness

Narrowband blue emitters are indispensable in achieving ultrahigh-definition OLED displays that satisfy the stringent BT 2020 standard. Hereby, a series of bis-tridentate Ir(III) complexes bearing electron-deficient imidazo[4,5-b]pyridin-2-ylidene carbene coordination fragments and 2,6-diaryloxy pyridine ancillary groups were designed and synthesized. They exhibited deep blue emission with quantum yields of up to 89% and a radiative lifetime of 0.71 μs in the DPEPO host matrix, indicating both the high efficiency and excellent energy transfer process from the host to dopant. The OLED based on Irtb1 showed an emission at 468 nm with a maximum external quantum efficiency (EQE) of 22.7%. Moreover, the hyper-OLED with Irtb1 as a sensitizer for transferring energy to terminal emitter v-DABNA exhibited a narrowband blue emission at 472 nm and full width at half-maximum (FWHM) of 24 nm, a maximum EQE of 23.5%, and EQEs of 19.7, 16.1, and 12.9% at a practical brightness of 100, 1000, and 5000 cd/m2, respectively

Theoretical Study of N749 Dyes Anchoring on the (TiO₂)₂₈ Surface in DSSCs and Their Electronic Absorption Properties

We have performed calculations on the panchromatic N749 dyes adsorbed on the (TiO₂)₂₈ surface. N749 is a prototypical form of Ru­(II) complexes for dye sensitized solar cells (DSSCs), which possesses a terpyridine tridentate ligand bearing four different protonation states (0, 1, 2, or 3 carboxylic protons). Depending on the type of proton bonding interaction (protonated and deprotonated), seven N749/(TiO₂)₂₈ surface models (N749-0H/(TiO₂)₂₈, N749-1H-P/(TiO₂)₂₈, N749-1H-DP/(TiO₂)₂₈, N749-2H-P/(TiO₂)₂₈, N749-2H-DP/(TiO₂)₂₈, N749-3H-P/(TiO₂)₂₈, and N749-3H-DP/(TiO₂)₂₈) have been applied in this study for the geometry optimization, frontier molecular orbital level diagrams, and calculated absorption spectra. The moderate surface area of the (TiO₂)₂₈ cluster is suitable for N749 dyes adsorbing behaviors so that all calculations can be performed using the Gaussian 09 program package. We have carefully examined these seven N749/(TiO₂)₂₈ assemblies that could influence the DSSC device performance. The calculated absorption spectra of these seven various N749/(TiO₂)₂₈ models are in good agreement with the experimental results by Hagfeldt et al. (<i>J</i>. <i>Phys</i>. <i>Chem</i>. <i>B</i> <b>2002</b>, <i>106</i>, 12693–12704) with onset ranging from the visible to near-IR region. The combination of the adsorption energy onto TiO₂ and calculated absorption spectra (cf. the experimental results) concludes that the deprotonated dyes constitute the most favorable and dominant structure in the DSSC devices. The frontier molecular orbital graphs indicate that the electron charge distributions of all HOMOs are located at the N749 dyes, while LUMOs are localized at the (TiO₂)₂₈ surface or delocalized at the interfacial regions of N749/(TiO₂)₂₈. The corresponding transitions are thus more like a type of optical electron transfer, injecting the electron to the interfacial TiO₂

Iridium(III) Complexes Bearing Tridentate Chromophoric Chelate: Phosphorescence Fine-Tuned by Phosphine and Hydride Ancillary

Functional 2-pyrazolyl-6-phenylpyridine chelatesnamely, (pzpyph^Bu)­H₂ and (pzpyph^CF₃)­H₂ and phosphinesare successfully employed in the preparation of emissive Ir­(III) metal complexes, for which the reaction with phosphine such as PPh₃, PPh₂Me, and PPh₂(CH₂Ph) afford corresponding Ir­(III) complexes [Ir­(pzpyph^Bu)­(PPh₃)₂H] (<b>1a</b>), [Ir­(pzpyph^CF₃)­(PPh₂R)₂H] (<b>2a</b>–<b>2c</b>), R = Ph, Me, CH₂Ph, which also show an equatorial coordinated hydride. In contrast, treatment with 1,2-bis­(diphenylphosphino)­benzene (dppb) and 1,2-bis­(diphenylphosphino)­ethane (dppe) yields the isomeric products [Ir­(pzpyph^Bu)­(dppb)­H] (<b>3a</b>) and [Ir­(pzpyph^Bu)­(dppe)­H] (<b>3b</b>), for which the distinctive, axial hydride undergoes rapid chlorination, forming chlorinated complexes [Ir­(pzpyph^Bu)­(dppb)­Cl] (<b>4a</b>) and [Ir­(pzpyph^Bu)­(dppe)­Cl] (<b>4b</b>), respectively. On the other hand, upon extensive heating of <b>2c</b>, one of its coordinated PPh₂(CH₂Ph) exhibits benzyl cyclometalation and hydride elimination to afford [Ir­(pzpyph^CF₃)­(PPh₂R)­(PPh₂R′)] (<b>5c</b> and <b>6c</b>) R = CH₂Ph and R′ = CH₂(<i>o</i>-C₆H₄) as the kinetic and thermodynamic products, respectively. Their structural, photophysical, and electrochemical properties are examined and further affirmed by the computational approaches

Harvesting Highly Electronically Excited Energy to Triplet Manifolds: State-Dependent Intersystem Crossing Rate in Os(II) and Ag(I) Complexes

A series of newly synthesized Os­(II) and Ag­(I) complexes exhibit remarkable ratiometric changes of intensity for phosphorescence versus fluorescence that are excitation wavelength dependent. This phenomenon is in stark contrast to what is commonly observed in condensed phase photophysics. While the singlet to triplet intersystem crossing (ISC) for the titled complexes is anomalously slow, approaching several hundred picoseconds in the lowest electronic excited state (S₁ → T₁), higher electronic excitation leads to a much accelerated rate of ISC (10¹¹–10¹² s^–1), which is competitive with internal conversion and/or vibrational relaxation, as commonly observed in heavy transition metal complexes. The mechanism is rationalized by negligible metal d orbital contribution in the S₁ state for the titled complexes. Conversely, significant ligand-to-metal charge transfer character in higher-lying excited states greatly enhances spin–orbit coupling and hence the ISC rate. The net result is to harvest high electronically excited energy toward triplet states, enhancing the phosphorescence

Mechanistic Investigation of Improved Syntheses of Iridium(III)-Based OLED Phosphors

Treatment of [IrCl₃(tht)₃] (tht = tetrahydrothiophene) with a stoichiometric amount of PPh₃ gave the monosubstitution product [Ir­(tht)₂(PPh₃)­Cl₃] (<b>5</b>), whose synthesis, particularly that leading to the effective preparation of OLED phosphors, was studied and optimized to achieve the best product yields. Thus, the independent treatment of <b>5</b> with 2,4-difluorophenylpyridine (dfppyH) or with variable amounts of benzyldiphenylphosphine (bdpH) gave rise to the formation of the cyclometalation products [Ir­(dfppy)­(tht)­(PPh₃)­Cl₂] (<b>7</b>), [Ir­(bdp)­(bdpH)­(tht)­Cl₂] (<b>8</b>), and [Ir­(bdp)­(PPh₃)­(tht)­Cl₂] (<b>10</b>), depending on the stoichiometry and conditions employed. Upon further treatment with 5-pyridyl-3-trifluoromethyl-1<i>H</i>-pyrazole (fppzH), these Ir­(III) complexes <b>7</b>, <b>8</b>, and <b>10</b> were capable of yielding the phosphors [Ir­(dfppy)­(fppz)₂] (<b>1</b>), [Ir­(bdp)₂(fppz)] (<b>4</b>), and [Ir­(bdp)­(fppz)₂] (<b>2</b>), respectively. The general mechanism en route to their formation was studied and discussed

Sky Blue-Emitting Iridium(III) Complexes Bearing Nonplanar Tetradentate Chromophore and Bidentate Ancillary

Tetradentate chelates bearing tripodal arranged terpyridine and a functional pyrazole unit (i.e., L1-H and L2-H) were employed in preparation of Ir­(III) complexes [Ir­(L1)­Cl₂] (<b>1</b>) and [Ir­(L2)­Cl₂] (<b>2</b>); subsequent chloride-to-bipyrazolate substitution gave [Ir­(L1)­(bipz)] (<b>3</b>) and [Ir­(L2)­(bipz)] (<b>4</b>). Single-crystal X-ray structural studies on <b>1</b> and <b>3</b> showed the possession of a tetradentate chelate, whereas the remaining <i>cis</i>-sites are occupied by either dual chlorides or the bipz chelate, respectively. Sky blue organic light-emitting diode with peak efficiencies (10.1%, 19.8 cd·A^–1, and 20.4 lm·W^–1) was successfully fabricated using <b>3</b> (or <b>4</b>) as dopant emitter, highlighting the potential application of this class of Ir­(III) phosphor

4,4′,5,5′-Tetracarboxy-2,2′-bipyridine Ru(II) Sensitizers for Dye-Sensitized Solar Cells

Two Ru­(II) sensitizers TCR-1 and TCR-2 bearing four carboxy anchoring groups were prepared using 4,4′,5,5′-tetraethoxycarbonyl-2,2′-bipyridine chelate and 4-(5-hexylthien-2-yl)-2-(3-trifluoromethyl-1<i>H</i>-pyrazol-5-yl)­pyridine and 6-<i>t</i>-butyl-1-(3-trifluoromethyl-1<i>H</i>-pyrazol-5-yl)­isoquinoline, respectively. Dissolution of these sensitizers in DMF solution afforded a light green solution up to 10^–5 M, for which their color gradually turned red upon further dilution and deposition on the surface of a TiO₂ photoanode due to the spontaneous deprotonation of carboxylic acid groups. These sensitizers were characterized using electrochemical means and structural analysis time-dependent density functional theory (TDDFT) simulation and were also subjected to actual device fabrication. The as-fabricated DSC devices showed overall efficiencies η = 6.16% and 6.23% versus their 4,4′-dicarboxy counterparts TFRS-2 and TFRS-52 with higher efficiencies of 7.57% and 8.09%, using electrolyte with 0.2 M LiI additive. Their inferior efficiencies are possibly caused by the combination of blue-shifted absorption on TiO₂, inadequate dye loading, and the perpendicularly oriented central carboxy groups

The Empirical Correlation between Hydrogen Bonding Strength and Excited-State Intramolecular Proton Transfer in 2-Pyridyl Pyrazoles

A series of 2-pyridyl pyrazoles <b>1a</b> and <b>1</b>–<b>5</b> with various functional groups attached to either pyrazole or pyridyl moieties have been strategically designed and synthesized in an aim to probe the hydrogen bonding strength in the ground state versus dynamics of excited-state intramolecular proton transfer (ESIPT) reaction. The title compounds all possess a five-membered-ring (pyrazole)­N–H···N­(pyridine) intramolecular hydrogen bond, in which both the N–H bond and the electron density distribution of the pyridyl nitrogen lone-pair electrons are rather directional, so that the hydrogen bonding strength is relatively weak, which is sensitive to the perturbation of subtle chemical substitution and consequently reflected from the associated ESIPT dynamics. Various approaches such as ¹H NMR (N–H proton) to probe the hydrogen bonding strength and absorption titration to assess the acidity-basicity property were made for all the title analogues. The results, together with supplementary support provided by a computational approach, affirm that the increase of acidity (basicity) on the hydrogen bonding donor (acceptor) sites leads to an increase of hydrogen-bonding strength among the title 2-pyridyl pyrazoles. Luminescence results and the associated ESIPT dynamics further reveal an empirical correlation in that the increase of the hydrogen bonding strength leads to an increase of the rate of ESIPT for the title 2-pyridyl pyrazoles, demonstrating an interesting relationship among N–H acidity, hydrogen bonding strength, and the associated ESIPT rate