Bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane as Electron-Transport Material for Deep Blue Phosphorescent OLEDs

Bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane (<b>SiTAZ</b>) was designed and synthesized as an electron-transporting material for deep blue phosphorescent organic light-emitting devices (PHOLEDs). Introducing a Si atom between two 3,4,5-triphenyl-1,2,4-triazole molecules, a high triplet energy of 2.84 eV, high glass transition temperature of 115 °C, and high electron mobility of 6.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ were achieved. By employing <b>SiTAZ</b> as a hole-blocking and electron-transporting material of iridium(III)[bis(4,6-difluorophenyl)pyridinato-<i>N</i>,C²′]tetrakis(1-pyrazolyl)borate (FIr6)-based deep blue phosphorescent OLEDs, a maximum external quantum efficiency (EQE) of 15.5%, an EQE of 13.8% at high luminance of 1000 cd m⁻², and deep blue color coordinates of (0.16, 0.22) were achieved. The reduced efficiency roll-off at high luminance was attributed to the high triplet energy of the <b>SiTAZ</b>

Photoinduced Electron Transfer in a BODIPY-<i>ortho</i>-Carborane Dyad Investigated by Time-Resolved Transient Absorption Spectroscopy

We report the results of photoinduced electron transfer (PET) in a novel dyad, in which a boron dipyrromethene (BODIPY) dye is covalently linked to <i>o</i>-carborane (<i>o</i>-Cb). In this dyad, BODIPY and <i>o</i>-Cb act as electron donor and acceptor, respectively. PET dynamics were investigated using a femtosecond time-resolved transient absorption spectroscopic method. The free energy dependence of PET in the S₁ and S₂ states was examined on the basis of Marcus theory. PET in the S₁ state occurs in the Marcus normal region. Rates are strongly influenced by the driving force (−Δ<i>G</i>), which is controlled by solvent polarity; thus, PET in the S₁ state is faster in polar solvents than in nonpolar ones. However, PET does not occur from the higher energy S₂ state despite large endothermic Δ<i>G</i> values, because deactivation via internal conversion is much faster than PET

Excited-State Modification of Phenylimidazole-Based Cyclometalated Ir(III) Complexes through Secondary Bulky Aryl Substitution and Inductive Modification Enhances the Blue Emission Efficiency in Phosphorescent OLEDs

To elucidate the key parameters governing the emission properties of phenylimidazole (pim)-based Ir(III) emitters, including their electronic structure and the bulky aryl substitution effect, a series of pim-based iridium(III) complexes (Ir(Rpim-X)3, Rpim-X = 1-R-2-(X-phenyl)-1H-imidazole) bearing secondary pendants of increasing bulkiness [R = methyl (Me), phenyl (Ph), terphenyl (TPh), or 4-isopropyl terphenyl (ITPh)] and three different primary pim ligands (X = F, F2, and CN) were designed and synthesized. Based on photophysical and electrochemical analyses, it was found that the excited state properties are highly dependent on the bulkiness of the secondary substituent and the inductive nature of the primary pim ligand. The incorporation of bulky TPh/ITPh substituents in the second coordination sphere significantly enhanced the emission efficiencies in the solid state (ΦPL = 72.1–84.9%) compared to those of the methyl- or phenyl-substituted Ir(III) complexes (ΦPL = 30.4% for Ir(Mepim)3 and 63.7% for Ir(Phpim)3). Further modification of the secondary aryl substituent (Ir(TPhpim)3 → Ir(ITPhpim)3) through the incorporation of an isopropyl group and F substitution on the primary pim ligand (Ir(TPh/ITPhpim)3 → Ir(TPh/ITPhpim-F/F2)3) resulted in a slight decrease in the LUMO and a significant decrease in the HOMO energy levels, respectively; these energy level adjustments consequently amplified emission blue shifts, thereby enabling efficient blue electroluminescence in phosphorescent organic light-emitting diodes. Theoretical calculations revealed that the excited-state properties of pim-based Ir(III) complexes can be modulated by the nature of the peripheral substituent and the presence of an EWG substituent. Among the fabricated blue-emitting TPh/ITPh-substituted Ir(III) complexes, Ir(ITPhpim-F)3, Ir(TPhpim-F2)3, and Ir(ITPhpim-F2)3 were tested as blue-emitting dopants for blue phosphorescent OLEDs owing to their high solid radiative quantum yields (ΦPL = 75.9–84.9%). The Ir(ITPhpim-F)3-doped multilayer device displayed the best performance with a maximum external quantum efficiency of 21.0%, a maximum current efficiency of 43.6 cd/A, and CIE coordinates of 0.18 and 0.31

Efficient Light Harvesting and Energy Transfer in a Red Phosphorescent Iridium Dendrimer

A series of red phosphorescent iridium dendrimers of the type [Ir­(btp)₂(pic-PC_<i>n</i>)] (<b>Ir-G</b>_{<b><i>n</i></b>}; <i>n</i> = 0, 1, 2, and 3) with two 2-(benzo­[<i>b</i>]­thiophen-2-yl)­pyridines (btp) and 3-hydroxypicolinate (pic) as the cyclometalating and ancillary ligands were prepared in good yields. Dendritic generation was grown at the 3 position of the pic ligand with 4-(9<i>H</i>-carbazolyl)­phenyl dendrons connected to 3,5-bis­(methyleneoxy)­benzyloxy branches (PC_<i>n</i>; <i>n</i> = 0, 2, 4, and 8). The harvesting photons on the PC_<i>n</i> dendrons followed by efficient energy transfer to the iridium center resulted in high red emissions at ∼600 nm by metal-to-ligand charge transfer. The intensity of the phosphorescence gradually increased with increasing dendrimer generation. Steady-state and time-resolved spectroscopy were used to investigate the energy-transfer mechanism. On the basis of the fluorescence quenching rate constants of the PC_<i>n</i> dendrons, the energy-transfer efficiencies for <b>Ir-G</b>_<b>1</b>, <b>Ir-G</b>_<b>2</b>, and <b>Ir-G</b>_<b>3</b> were 99, 98, and 96%, respectively. The energy-transfer efficiency for higher-generation dendrimers decreased slightly because of the longer distance between the PC dendrons and the core iridium­(III) complex, indicating that energy transfer in <b>Ir-G</b>_{<b><i>n</i></b>} is a Förster-type energy transfer. Finally, the light-harvesting efficiencies for <b>Ir-G</b>_<b>1</b>, <b>Ir-G</b>_<b>2</b>, and <b>Ir-G</b>_<b>3</b> were determined to be 162, 223, and 334%, respectively

A Detailed Evaluation for the Nonradiative Processes in Highly Phosphorescent Iridium(III) Complexes

To understand the intrinsic nature of nonradiative processes in heteroleptic cyclometalated Ir­(III) complexes, highly phosphorescent Ir³⁺ complexes containing 2-(3-sulfonylfluorophenyl)­pyridine (ppySO₂F) as the cyclometalated ligand were newly synthesized. Three ancillary ligands, acetylacetonate (acac), picolinate (pic), and <i>tetrakis</i>-pyrazolyl borate (bor), were employed for the preparation of the Ir­(III) complexes [Ir­(ppySO₂F)₂(acac)] (<b>Ir-acac</b>), [Ir­(ppySO₂F)₂(pic)] (<b>Ir-pic</b>), and [Ir­(ppySO₂F)₂(bor)] (<b>Ir-bor</b>). The molecular structures were characterized by X-ray crystallography. Blue phosphorescence maxima were observed at 458, 467, and 478 nm for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively, at 77 K, and the corresponding emission quantum yields were determined to be 0.79, 0.80, and 0.98 in anaerobic CH₂Cl₂ at 300 K. Additionally, the phosphorescence decay times were measured to be 3.58, 1.94, and 1.44<i>μ</i>s for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively. No temperature dependence was observed for the emission lifetimes in 298–338 K. These results indicate that there is no activation barrier to crossing to a nonradiative state like metal-centered (MC, d–d) state. The radiative rate constants (<i>k</i>_r) are within a narrow range of 3.0–5.5 × 10^–5 s^–1. However, the nonradiative rate constants (<i>k</i>_nr) are within a wide range of 14.2–0.52 × 10^–4 s^–1. The <i>k</i>_nr values showed exponetial correlation with the energy gap. We carried out <i>ab</i> <i>initio</i> calculations to evaluate the energy states and their corresponding orbitals. The nonemissive MC states lie at higher energies than the emissive metal-to-ligand charge transfer (MLCT) state, and hence, the MC states can be excluded from the nonradiative pathway

Carborane-Based Optoelectronically Active Organic Molecules: Wide Band Gap Host Materials for Blue Phosphorescence

Carborane-based host materials were prepared to fabricate deep blue phosphorescence organic light-emitting diodes (PHOLEDs), which constituted three distinctive geometrical structures stemming from the corresponding three different isomeric forms of carboranes, namely, <i>ortho</i>-, <i>meta</i>-, and <i>para</i>-carboranes. These materials consist of two carbazolyl phenyl (CzPh) groups as photoactive units on each side of the carborane carbons to be bis­[4-(<i>N</i>-carbazolyl)­phenyl]­carboranes, <b><i>o</i>-Cb</b>, <b><i>m</i>-Cb</b>, and <b><i>p</i>-Cb</b>. To elaborate on the role of the carboranes, comparative analogous benzene series (<b><i>o</i>-Bz</b>, <b><i>m</i>-Bz</b>, and <b><i>p</i>-Bz</b>) were prepared, and their photophysical properties were compared to show that advantageous photophysical properties were originated from the carborane structures: high triplet energy. Unlike <b><i>m</i>-Bz</b> and <b><i>p</i>-Bz</b>, carborane-based <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b> showed an unconjugated nature between two CzPh units, which is essential for the blue phosphorescent materials. Also, the carborane hosts showed high glass transition temperatures (<i>T</i>_g) of 132 and 164 °C for <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>, respectively. Albeit <b><i>p</i>-Cb</b> exhibited slightly lower hole mobility when compared to <b><i>p</i>-Bz</b>, it still lies at the high end hole mobility with a value of 1.1 × 10^–3 cm²/(V s) at an electric field of 5 × 10⁵ V/cm. Density functional theory (DFT) calculations revealed that triplet wave functions were effectively confined and mostly located at either side of the carbazolyl units for <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>. Low-temperature PL spectra indeed provided unequivocal data with higher triplet energy (<i>T</i>₁) of 3.1 eV for both <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>. <b><i>p</i>-Cb</b> was successfully used as a host in deep blue PHOLEDs to provide a high external quantum efficiency of 15.3% and commission internationale de l’elcairage (CIE) coordinates of (0.15, 0.24)

Carborane-Based Optoelectronically Active Organic Molecules: Wide Band Gap Host Materials for Blue Phosphorescence

Carborane-based host materials were prepared to fabricate deep blue phosphorescence organic light-emitting diodes (PHOLEDs), which constituted three distinctive geometrical structures stemming from the corresponding three different isomeric forms of carboranes, namely, <i>ortho</i>-, <i>meta</i>-, and <i>para</i>-carboranes. These materials consist of two carbazolyl phenyl (CzPh) groups as photoactive units on each side of the carborane carbons to be bis­[4-(<i>N</i>-carbazolyl)­phenyl]­carboranes, <b><i>o</i>-Cb</b>, <b><i>m</i>-Cb</b>, and <b><i>p</i>-Cb</b>. To elaborate on the role of the carboranes, comparative analogous benzene series (<b><i>o</i>-Bz</b>, <b><i>m</i>-Bz</b>, and <b><i>p</i>-Bz</b>) were prepared, and their photophysical properties were compared to show that advantageous photophysical properties were originated from the carborane structures: high triplet energy. Unlike <b><i>m</i>-Bz</b> and <b><i>p</i>-Bz</b>, carborane-based <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b> showed an unconjugated nature between two CzPh units, which is essential for the blue phosphorescent materials. Also, the carborane hosts showed high glass transition temperatures (<i>T</i>_g) of 132 and 164 °C for <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>, respectively. Albeit <b><i>p</i>-Cb</b> exhibited slightly lower hole mobility when compared to <b><i>p</i>-Bz</b>, it still lies at the high end hole mobility with a value of 1.1 × 10^–3 cm²/(V s) at an electric field of 5 × 10⁵ V/cm. Density functional theory (DFT) calculations revealed that triplet wave functions were effectively confined and mostly located at either side of the carbazolyl units for <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>. Low-temperature PL spectra indeed provided unequivocal data with higher triplet energy (<i>T</i>₁) of 3.1 eV for both <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>. <b><i>p</i>-Cb</b> was successfully used as a host in deep blue PHOLEDs to provide a high external quantum efficiency of 15.3% and commission internationale de l’elcairage (CIE) coordinates of (0.15, 0.24)

Highly Robust Hybrid Photocatalyst for Carbon Dioxide Reduction: Tuning and Optimization of Catalytic Activities of Dye/TiO₂/Re(I) Organic–Inorganic Ternary Systems

Herein we report a detailed investigation of a highly robust hybrid system (sensitizer/TiO₂/catalyst) for the visible-light reduction of CO₂ to CO; the system comprises 5′-(4-[bis­(4-methoxy­methyl­phenyl)­amino]­phenyl-2,2′-dithiophen-5-yl)­cyano­acrylic acid as the sensitizer and (4,4′-bis­(methyl­phosphonic acid)-2,2′-bipyridine)­Re^I(CO)₃Cl as the catalyst, both of which have been anchored on three different types of TiO₂ particles (s-TiO₂, h-TiO₂, d-TiO₂). It was found that remarkable enhancements in the CO₂ conversion activity of the hybrid photocatalytic system can be achieved by addition of water or such other additives as Li⁺, Na⁺, and TEOA. The photocatalytic CO₂ reduction efficiency was enhanced by approximately 300% upon addition of 3% (v/v) H₂O, giving a turnover number of ≥570 for 30 h. A series of Mott–Schottky (MS) analyses on nanoparticle TiO₂ films demonstrated that the flat-band potential (<i>V</i>_fb) of TiO₂ in dry DMF is substantially negative but positively shifts to considerable degrees in the presence of water or Li⁺, indicating that the enhancement effects of the additives on the catalytic activity should mainly arise from optimal alignment of the TiO₂ <i>V</i>_fb with respect to the excited-state oxidation potential of the sensitizer and the reduction potential of the catalyst in our ternary system. The present results confirm that the TiO₂ semiconductor in our heterogeneous hybrid system is an essential component that can effectively work as an electron reservoir and as an electron transporting mediator to play essential roles in the persistent photocatalysis activity of the hybrid system in the selective reduction of CO₂ to CO

Rational Design, Synthesis, and Characterization of Deep Blue Phosphorescent Ir(III) Complexes Containing (4′-Substituted-2′-pyridyl)-1,2,4-triazole Ancillary Ligands

On the basis of the results of frontier orbital considerations, 4-substituted-2′-pyridyltriazoles were designed to serve as ancillary ligands in 2-phenylpyridine main ligand containing heteroleptic iridium­(III) complexes that display deep blue phosphorescence emission. The iridium­(III) complexes, <b>Ir1</b>–<b>Ir7</b>, prepared using the new ancillary ligands, were found to display structured, highly quantum efficient (Φ_p = 0.20–0.42) phosphorescence with emission maxima in the blue to deep blue 448–456 nm at room temperature. In accord with predictions based on frontier orbital considerations, the complexes were observed to have emission properties that are dependent on the electronic nature of substituents at the C-4 position of the pyridine moiety of the ancillary ligand. Importantly, placement of an electron-donating methyl group at C-4′ of the pyridine ring of the 5-(pyridine-2′-yl)-3-trifluoromethyl-1,2,4-triazole ancillary ligand leads to an iridium­(III) complex that displays a deep blue phosphorescence emission maximum at 448 nm in both the liquid and film states at room temperature. Finally, an OLED device, constructed using an Ir-complex containing the optimized ancillary ligand as the dopant, was found to emit deep blue color with a CIE of 0.15, 0.18, which is close to the perfect goal of 0.15, 0.15

Rational Design, Synthesis, and Characterization of Deep Blue Phosphorescent Ir(III) Complexes Containing (4′-Substituted-2′-pyridyl)-1,2,4-triazole Ancillary Ligands

On the basis of the results of frontier orbital considerations, 4-substituted-2′-pyridyltriazoles were designed to serve as ancillary ligands in 2-phenylpyridine main ligand containing heteroleptic iridium­(III) complexes that display deep blue phosphorescence emission. The iridium­(III) complexes, <b>Ir1</b>–<b>Ir7</b>, prepared using the new ancillary ligands, were found to display structured, highly quantum efficient (Φ_p = 0.20–0.42) phosphorescence with emission maxima in the blue to deep blue 448–456 nm at room temperature. In accord with predictions based on frontier orbital considerations, the complexes were observed to have emission properties that are dependent on the electronic nature of substituents at the C-4 position of the pyridine moiety of the ancillary ligand. Importantly, placement of an electron-donating methyl group at C-4′ of the pyridine ring of the 5-(pyridine-2′-yl)-3-trifluoromethyl-1,2,4-triazole ancillary ligand leads to an iridium­(III) complex that displays a deep blue phosphorescence emission maximum at 448 nm in both the liquid and film states at room temperature. Finally, an OLED device, constructed using an Ir-complex containing the optimized ancillary ligand as the dopant, was found to emit deep blue color with a CIE of 0.15, 0.18, which is close to the perfect goal of 0.15, 0.15