Synthesis and Characterization of Blue Phosphorescent NHC-Ir(III) Complexes with Annulated Heterocyclic 1,2,4-Triazolophenanthridine Derivatives for Highly Efficient PhOLEDs

Efficient tris-bidentate Ir­(III) phosphorescent dopants were prepared using a series of 1,2,4-triazolo­[4,3-f]­phenanthridine (tzp) moieties modified with aryl substituents (phenyl, tolyl, and xylenyl) as the main phenylimidazole-based N-heterocyclic carbene (NHC) chelates (C∧C:). According to the degree of the bulkiness of the aryl substituent and the ligation mode, the five prepared Ir­(tzpC∧C:) complexes include four homoleptic NHC-Ir­(III) complexes, fac-Ir­(tzpPh)3, fac-/mer-Ir­(tzpTol)3, and mer-Ir­(tzpXyl)3, and one heteroleptic NHC-Ir­(III) complex, cis-Ir­(tzpPh)2(tzpPh)′, in which the phenyl moiety of one tzpPh ligand is abnormally ligated to the Ir metal center, unlike other tzp ligands. The Ir­(III) complexes ligated by carbene ligands (tzpC∧C:) exhibited highly efficient emissions in the solid state (Φem = 23.2–54.0%). Electrochemical and theoretical studies revealed that the excited-state properties of these NHC-Ir­(III) complexes are variable on the extent of planarity and π-conjugation of the tzpC∧C: chelating ligand. Due to its enhanced rigidity and low excited-state energy, a result of abnormal tzpPh ligand ligation, the heteroleptic cis-Ir­(tzpPh)2(tzpPh)′ exhibited the most efficient emission properties in solution (Φem = 21.4%) and solid (Φem = 54.0%) media. Of the devices fabricated with Ir­(tzpC∧C:)3 complexes as emitters, that doped with cis-Ir­(tzpPh)2(tzpPh)′ exhibited superior electroluminescence efficiencies (external quantum efficiency (EQE) of 16.3%, current efficiency of 27.6 cd A–1, and power efficiency of 22.1 lm W–1) and CIE coordinates of [0.17,0.26], which are superior to those of other Ir­(tzpC∧C:)3 complexes and Ir­(dmp)3 (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo­[1,2-f]­phenanthridine). This study provides insight into the molecular-level engineering of Ir­(III) dopant materials for improving the emission efficiencies of phosphorescent OLEDs

Synthesis and Characterization of Blue Phosphorescent NHC-Ir(III) Complexes with Annulated Heterocyclic 1,2,4-Triazolophenanthridine Derivatives for Highly Efficient PhOLEDs

Efficient tris-bidentate Ir­(III) phosphorescent dopants were prepared using a series of 1,2,4-triazolo­[4,3-f]­phenanthridine (tzp) moieties modified with aryl substituents (phenyl, tolyl, and xylenyl) as the main phenylimidazole-based N-heterocyclic carbene (NHC) chelates (C∧C:). According to the degree of the bulkiness of the aryl substituent and the ligation mode, the five prepared Ir­(tzpC∧C:) complexes include four homoleptic NHC-Ir­(III) complexes, fac-Ir­(tzpPh)3, fac-/mer-Ir­(tzpTol)3, and mer-Ir­(tzpXyl)3, and one heteroleptic NHC-Ir­(III) complex, cis-Ir­(tzpPh)2(tzpPh)′, in which the phenyl moiety of one tzpPh ligand is abnormally ligated to the Ir metal center, unlike other tzp ligands. The Ir­(III) complexes ligated by carbene ligands (tzpC∧C:) exhibited highly efficient emissions in the solid state (Φem = 23.2–54.0%). Electrochemical and theoretical studies revealed that the excited-state properties of these NHC-Ir­(III) complexes are variable on the extent of planarity and π-conjugation of the tzpC∧C: chelating ligand. Due to its enhanced rigidity and low excited-state energy, a result of abnormal tzpPh ligand ligation, the heteroleptic cis-Ir­(tzpPh)2(tzpPh)′ exhibited the most efficient emission properties in solution (Φem = 21.4%) and solid (Φem = 54.0%) media. Of the devices fabricated with Ir­(tzpC∧C:)3 complexes as emitters, that doped with cis-Ir­(tzpPh)2(tzpPh)′ exhibited superior electroluminescence efficiencies (external quantum efficiency (EQE) of 16.3%, current efficiency of 27.6 cd A–1, and power efficiency of 22.1 lm W–1) and CIE coordinates of [0.17,0.26], which are superior to those of other Ir­(tzpC∧C:)3 complexes and Ir­(dmp)3 (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo­[1,2-f]­phenanthridine). This study provides insight into the molecular-level engineering of Ir­(III) dopant materials for improving the emission efficiencies of phosphorescent OLEDs

InP-Quantum Dot Surface-Modified TiO₂ Catalysts for Sustainable Photochemical Carbon Dioxide Reduction

In this study, an InP-cored quantum dot (InP-QD) material was prepared and physically immobilized on TiO2 particles functionalized with an archetypical reduction catalyst, (4,4-Y2-bpy)­ReI(CO)3Cl (ReP, Y = CH2PO­(OH)2), to form a new type of InP quantum dot-sensitized hybrid photocatalyst (InP-QD/TiO2/ReP) and evaluated as a lower-energy photosensitizer in this hybrid system. It was found that the TiO2 heterogenization of the InP-QD material promotes the photoexcited electron transfer process from the photoexcited InP-QD* to the inorganic TiO2 solid with rapid electron injection (by ∼25 ps) through oxidative quenching, resulting in efficient charge separation at the InP-QD/TiO2 interface. With such an effective photosensitization, the stabilization of the structurally vulnerable InP-cored QDs by TiO2 heterogenization resulted in highly efficient and durable photochemical CO2-to-CO conversion of the InP-QD/TiO2/ReP hybrid in a 10 times-repeated photolysis, giving a turnover number of ∼51,000 over a 420 h period without any damage to the InP-QD photosensitizer. The stability of TiO2-bound InP-QDs was confirmed by the comparative analysis of their photophysical and chemical structures before and after long-term photoreaction. This catalytic performance is the highest reported for QD-sensitized photocatalytic CO2 conversion systems using sacrificial organic electron donors. This study provides useful design guidelines for photocatalysts using QD materials as photosensitizing components

Efficient Photosensitization of π‑Extended Oligomeric Porphyrin Pigments through TiO₂‑Mediated Outer-Sphere Electron Transfer in Photochemical CO₂ Reduction Systems

This study describes an effective approach for addressing the challenges associated with low-lying excited states in π-extended porphyrin oligomeric dyes ([ZnP]-[ZnP] (D-ZnP) and [ZnP]-[ZnP]-[ZnP] (T-ZnP), where ZnP = Zn(II)-porphyrin unit), which create an energy barrier in the electron transfer (ET) process from the oligomeric dye to the catalytic center and hinder the maximal utilization of the superb visible-light-harvesting ability of oligomeric porphyrin arrays in dye-sensitized photocatalysis (DSPC). The outer-sphere ET from the porphyrin array dyes to TiO2, initiated by a shallow trapping site with a low-energy level within the bulk TiO2 semiconductor (positioned 0.3–1.0 eV below the conduction band (CB) of TiO2), efficiently mitigated the endergonic ET process from the porphyrin array dye to the catalytic center. In the photolysis experiments of the TiO2-mediated hybrid system (dye + TiO2/Re(I) catalyst), the D-ZnP and T-ZnP-sensitized photocatalytic systems exhibited superior CO2 conversion activities (turnover number (TON)CO = 1939 for D-ZnP and 1757 for T-ZnP) compared with the monomeric Zn(II) porphyrin dye (TONCO = 837 for M-ZnP). This enhancement is attributed to the alleviation of the endothermic ET process achieved through TiO2 mediation and the excellent light-harvesting capacity facilitated by π-extension. In contrast, the photolysis results for the mixed homogeneous system (dye + Re(I) catalyst) indicated relatively lower conversion performances of the D/T-ZnP-sensitized homogeneous systems due to the uphill photoinduced ET between components caused by the low-lying lowest unoccupied molecular orbital (LUMO) levels of porphyrin array dyes (Ered11/2 = −1.30, −1.18, and −1.14 V (vs SCE) for M-ZnP, D-ZnP, and T-ZnP, respectively). The proposed strategy provides a useful tool for achieving efficient photosensitization of long π-conjugated dye arrays in various photocatalytic systems

Synthesis and Characterization of Blue Phosphorescent NHC-Ir(III) Complexes with Annulated Heterocyclic 1,2,4-Triazolophenanthridine Derivatives for Highly Efficient PhOLEDs

Efficient tris-bidentate Ir­(III) phosphorescent dopants were prepared using a series of 1,2,4-triazolo­[4,3-f]­phenanthridine (tzp) moieties modified with aryl substituents (phenyl, tolyl, and xylenyl) as the main phenylimidazole-based N-heterocyclic carbene (NHC) chelates (C∧C:). According to the degree of the bulkiness of the aryl substituent and the ligation mode, the five prepared Ir­(tzpC∧C:) complexes include four homoleptic NHC-Ir­(III) complexes, fac-Ir­(tzpPh)3, fac-/mer-Ir­(tzpTol)3, and mer-Ir­(tzpXyl)3, and one heteroleptic NHC-Ir­(III) complex, cis-Ir­(tzpPh)2(tzpPh)′, in which the phenyl moiety of one tzpPh ligand is abnormally ligated to the Ir metal center, unlike other tzp ligands. The Ir­(III) complexes ligated by carbene ligands (tzpC∧C:) exhibited highly efficient emissions in the solid state (Φem = 23.2–54.0%). Electrochemical and theoretical studies revealed that the excited-state properties of these NHC-Ir­(III) complexes are variable on the extent of planarity and π-conjugation of the tzpC∧C: chelating ligand. Due to its enhanced rigidity and low excited-state energy, a result of abnormal tzpPh ligand ligation, the heteroleptic cis-Ir­(tzpPh)2(tzpPh)′ exhibited the most efficient emission properties in solution (Φem = 21.4%) and solid (Φem = 54.0%) media. Of the devices fabricated with Ir­(tzpC∧C:)3 complexes as emitters, that doped with cis-Ir­(tzpPh)2(tzpPh)′ exhibited superior electroluminescence efficiencies (external quantum efficiency (EQE) of 16.3%, current efficiency of 27.6 cd A–1, and power efficiency of 22.1 lm W–1) and CIE coordinates of [0.17,0.26], which are superior to those of other Ir­(tzpC∧C:)3 complexes and Ir­(dmp)3 (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo­[1,2-f]­phenanthridine). This study provides insight into the molecular-level engineering of Ir­(III) dopant materials for improving the emission efficiencies of phosphorescent OLEDs

Amplified Triplet Emission of Organic Periphery Groups by Exothermic Triplet–Triplet Energy Transfer from the ³MLCT State of an <b>Ir(pmi)₃</b> Core Complex to the ³LC State of Geometrically Confined Carbazole/Naphthyl Tethers

To investigate the excited-state properties of metal–organic bichromophores, including energy transfer mechanisms, a series of new homoleptic N-heterocyclic carbene (NHC)-based iridium­(III) complexes were prepared by incorporating a peripheral naphthalene (Np) (Ir­(Nppmi)3: fac-/mer-Ir­(1-Nppmi)3 and fac-/mer-Ir­(2-Nppmi)3) or carbazole (Cz) (Ir­(Czpmi)3: fac-/mer-Ir­(o-Czpmi)3, fac-/mer-Ir­(m-Czpmi)3, and fac-/mer-Ir­(p-Czpmi)3) unit to the phenyl moiety of the phenylimidazole (pmi) ligand. Through a series of photophysical analyses and femtosecond time-resolved absorption (fs-TA) spectroscopy, it was discovered that the phosphorescence of the Ir core, (Ir­(pmi)3), was considerably quenched, while intense phosphorescence peaks arising from the excited triplet Np (3Np*)/Cz (3Cz*) species were primarily observed at room temperature (r.t.) and low temperature. Such amplified phosphorescence of the tethered organic Np and Cz units originated from triplet–triplet energy transfer (TTET) from the high-lying metal-to-ligand charge transfer (3MLCT) state of the Ir­(pmi)3 core to the ligand-centered triplet state (3LC) of the peripheral Np and Cz units. This result indicates that the exothermic intramolecular energy transfer (IET) in the excited triplet state realizes the efficient phosphorescent emission of geometrically confined organic tethers

Secondary Coordination Effect on Monobipyridyl Ru(II) Catalysts in Photochemical CO₂ Reduction: Effective Proton Shuttle of Pendant Brønsted Acid/Base Sites (OH and N(CH₃)₂) and Its Mechanistic Investigation

While the incorporation of pendant Brønsted acid/base sites in the secondary coordination sphere is a promising and effective strategy to increase the catalytic performance and product selectivity in organometallic catalysis for CO2 reduction, the control of product selectivity still faces a great challenge. Herein, we report two new trans(Cl)-[Ru­(6-X-bpy)­(CO)2Cl2] complexes functionalized with a saturated ethylene-linked functional group (bpy = 2,2′-bipyridine; X = −(CH2)2–OH or −(CH2)2–N­(CH3)2) at the ortho(6)-position of bpy ligand, which are named Ru-bpyOH and Ru-bpydiMeN, respectively. In the series of photolysis experiments, compared to nontethered case, the asymmetric attachment of tethering ligand to the bpy ligand led to less efficient but more selective formate production with inactivation of CO2-to-CO conversion route during photoreaction. From a series of in situ FTIR analyses, it was found that the Ru–formate intermediates are stabilized by a highly probable hydrogen bonding between pendent proton donors (−diMeN+H or −OH) and the oxygen atom of metal-bound formate (RuI–OCHO···H–E–(CH2)2–, E = O or diMeN+). Under such conformation, the liberation of formate from the stabilized RuI–formate becomes less efficient compared to the nontethered case, consequently lowering the CO2-to-formate conversion activities during photoreaction. At the same time, such stabilization of Ru–formate species prevents the dehydration reaction route (η1-OCHO → η1-COOH on Ru metal) which leads toward the generation of Ru–CO species (key intermediate for CO production), eventually leading to the reduction of CO2-to-CO conversion activity

Sustainable Carbon Dioxide Reduction of the P3HT Polymer-Sensitized TiO₂/Re(I) Photocatalyst

In this study, a p-type π-conjugated polymer chain, poly(3-hexylthiophene-2,5-diyl) (P3HT), was physically adsorbed onto n-type TiO2 nanoparticles functionalized with a molecular CO2 reduction catalyst, (4,4-Y2-bpy)ReI(CO)3Cl (ReP, Y = CH2PO(OH)2), to generate a new type of P3HT-heterogenized hybrid system (P3HT/TiO2/ReP), and its photosensitizing properties were assessed in a heteroternary system for photochemical CO2 reduction. We found that P3HT immobilization on TiO2 facilitated photoinduced electron transfer (PET) from photoactivated P3HT* to the n-type TiO2 semiconductor via rapid interfacial electron injection (∼65 ps) at the P3HT and TiO2 surface interface (P3HT* → TiO2). With such effective charge separation, the heterogenization of P3HT onto TiO2 resulted in a steady electron supply toward the co-adsorbed Re(I) catalyst, attaining durable catalytic activity with a turnover number (TON) of ∼5300 over an extended time period of 655 h over five consecutive photoreactions, without deformation of the adsorbed P3HT polymer. The long-period structural stability of TiO2-adsorbed P3HT was verified based on a comparative analysis of its photophysical properties before and after 655 h of photolysis. To our knowledge, this conversion activity is the highest reported so far for polymer-sensitized photochemical CO2 reduction systems. This investigation provides insights and design guidelines for photocatalytic systems that utilize organic photoactive polymers as photosensitizing units

Highly Selective and Durable Photochemical CO₂ Reduction by Molecular Mn(I) Catalyst Fixed on a Particular Dye-Sensitized TiO₂ Platform

A Mn­(I)-based hybrid system (OrgD-|TiO2|-MnP) for photocatalytic CO2 reduction is designed to be a coassembly of Mn­(4,4′-Y2-bpy)­(CO)3Br (MnP; Y = CH2PO­(OH)2) and (E)-3-[5-(4-(diphenylamino)­phenyl)-2,2′-bithiophen-2′-yl]-2-cyanoacrylic acid (OrgD) on TiO2 semiconductor particles. The OrgD-|TiO2|-MnP hybrid reveals persistent photocatalytic behavior, giving high turnover numbers and good product selectivity (HCOO– versus CO). As a typical run, visible-light irradiation of the hybrid catalyst in the presence of 0.1 M electron donor (ED) and 0.001 M LiClO4 persistently produced HCOO– with a >99% selectivity accompanied by a trace amount of CO; the turnover number (TONformate) reached ∼250 after 23 h of irradiation. The product selectivity (HCOO–/CO) was found to be controlled by changing the loading amount of MnP on the TiO2 surface. In situ FTIR analysis of the hybrid during photocatalysis revealed that, at low Mn concentration, the Mn–H monomeric mechanism associated with HCOO– formation is dominant, whereas at high Mn concentration, CO is formed via a Mn–Mn dimer mechanism