Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity.

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO2 Composite for Efficient and Sustainable Photoinduced Interfacial Electron Transfer.

Owing to their superior stability compared to those of conventional molecular dyes, as well as their high UV-visible absorption capacity, which can be tuned to cover the majority of the solar spectrum through size adjustment, quantum dot (QD)/TiO2 composites are being actively investigated as photosensitizing components for diverse solar energy conversion systems. However, the conversion efficiencies and durabilities of QD/TiO2-based solar cells and photocatalytic systems are still inferior to those of conventional systems that employ organic/inorganic components as photosensitizers. This is because of the poor adsorption of QDs onto the TiO2 surface, resulting in insufficient interfacial interactions between the two. The mechanism underlying QD adsorption on the TiO2 surface and its relationship to the photosensitization process remain unclear. In this study, we established that the surface characteristics of the TiO2 semiconductor and the QDs (i.e., surface defects of the metal oxide and the surface structure of the QD core) directly affect the QD adsorption capacity by TiO2 and the interfacial interactions between the QDs and TiO2, which relates to the photosensitization process from the photoexcited QDs to TiO2 (QD* → TiO2). The interfacial interaction between the QDs and TiO2 is maximized when the shape/thickness-modulated triangular QDs are composited with defect-rich anatase TiO2. Comprehensive investigations through photodynamic analyses and surface evaluation using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photocatalysis experiments collectively validate that tuning the surface properties of QDs and modulating the TiO2 defect concentration can synergistically amplify the interfacial interaction between the QDs and TiO2. This augmentation markedly improved the efficiency of photoinduced electron transfer from the photoexcited QDs to TiO2, resulting in significantly increased photocatalytic activity of the QD/TiO2 composite. This study provides the first in-depth characterization of the physical adhesion of QDs dispersed on a heterogeneous metal-oxide surface. Furthermore, the prepared QD/TiO2 composite exhibits exceptional adsorption stability, resisting QD detachment from the TiO2 surface over a wide pH range (pH = 2-12) in aqueous media as well as in nonaqueous solvents during two months of immersion. These findings can aid the development of practical QD-sensitized solar energy conversion systems that require the long-term stability of the photosensitizing unit.

Facile microfluidic synthesis of monodispersed size-controllable quantum dot (QD) microbeads using custom developed QD photoresist.

Fluorescent microbeads (MBs) are widely used as next-generation biosensors for the detection of target chemicals at highly sensitive concentrations, and for imaging and tracking in vitro and in vivo. However, most known methods for producing fluorescent MBs require complicated multistep processes that result in low production rates. In this study, we report a method for fabricating micrometer-sized quantum dot microbeads (QD-MBs) using a microfluidic chip and specially designed QD photoresist (QD-PR). This on-demand lab-on-a-chip method yielded monodispersed QD-MBs ranging from 1.89 to 33 µm with a coefficient of variation of less than 10%. The size distribution of the fabricated QD-MBs was Gaussian with a peak around the mean diameter and a spread of sizes around the peak. Compared with nanoscale QDs, the fabricated QD-MBs showed no emission loss. The full-width at half-maximum of the emission peak of the QD-MBs was smaller than that of the colloidal QDs, indicating a more uniform distribution and a higher density of QDs within the MB structure. In addition, we investigated the microfluidic flow regime that yielded the most uniform and controllable QD-MB. The MBs in the dripping regime were spherical and monodisperse, with an excellent particle size distribution. In this study, we present a simple and effective strategy for producing QD-MBs with controllable sizes, which can be crucial in diverse fields such as biosensing, drug delivery, and imaging.

Amplified Triplet Emission of Organic Periphery Groups by Exothermic Triplet-Triplet Energy Transfer from the 3MLCT State of an Ir(pmi)3 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.

Accumulative Charge Separation in a Modular Quaterpyridine Bridging Ligand Platform and Multielectron Transfer Photocatalysis of π-Linked Dinuclear Ir(III)-Re(I) Complex for CO2 Reduction.

Four sterically distorted quaterpyridyl (qpy) ligand-bridged Ir(III)-Re(I) heterometallic complexes (Ir-qpymm-Re, Ir-qpymp-Re, Ir-qpypm-Re, and Ir-qpypp-Re), in which the position of the coupling pyridine unit of the two 2,2'-bipyridine ligands was varied (meta (m)- or para (p)-position), pypyx-pyxpy (x = m and m, qpymm; x = m and p, qpymp; x = p and m, qpypm; x = p and p, qpypp), were prepared, along with the fully π-conjugated Ir(III)-[π linker]-Re(I) complexes (π linker = 2,2'-bipyrimidine (bpm), Ir-bpm-Re; π linker = 2,5-di(pyridin-2-yl)pyrazine (dpp), Ir-dpp-Re) to elucidate the electron mediating and accumulative charge separation properties of the bridging π-linker in a bimetallic system (photosensitizer-π linker-catalytic center). From the photophysical and electrochemical studies, it was found that the quaterpyridyl (qpy) bridging ligand (BL), in which the two planar Ir/Re metalated bipyridine (bpy) ligands were connected but slightly canted relative to each other, linking the heteroleptic Ir(III) photosensitizer, [(piqC^N)2IrIII(bpy)]+, and catalytic Re(I) complex, (bpy)ReI(CO)3Cl, minimized the energy lowering of the qpy BL, which hampers the forward photoinduced electron transfer (PET) process from [(piqC^N)2IrIII(N^N)]+ to (N^N)ReI(CO)3Cl (Ered1 = -(0.85-0.93) V and Ered2 = -(1.15-1.30) V vs SCE). This result contrasts with the fully π-delocalized bimetallic systems (Ir-bpm-Re and Ir-dpp-Re) that show a significant energy reduction due to the considerable π-extension and deshielding effect caused by the neighboring Lewis acidic metals (Ir and Re) on the electrochemical scale (Ered1 = -0.37 V and Ered2 = -1.02 and -0.99 V vs SCE). Based on a series of anion absorption studies and spectroelectrochemical (SEC) analyses, all Ir(III)-BL-Re(I) bimetallic complexes were found to exist as dianionic form (Ir(III)-[BL]2--Re(I)) after a fast reductive-quenching process in the presence of excess electron donor. In the photolysis experiment, the four Ir-qpy-Re complexes displayed the reasonable photochemical CO2-to-CO conversion activities (TON of 366-588 for 19 h) owing to the moderated electronic coupling between two functional Ir(III) and Re(I) centers through the slightly distorted qpy ligand, whereas Ir-bpm-Re and Ir-dpp-Re displayed negligible performances as a result of the strong electronic coupling via π-conjugation between the two functional components resulting in the energetic constraints for PET and an unwanted side reactions competing with the forward processes. These results confirm that the qpy unit can be utilized as an efficient BL platform in π-linked bimetallic systems.

Dynamics of Photoinduced Intramolecular and Intermolecular Electron Transfers in Ligand-Conjugated Ir(III)-Re(I) Photocatalysts.

We report the electron transfer (ET) dynamics in a series of Ir(III)-Re(I) photocatalysts where two bipyridyl ligands of Ir and Re moieties are conjugated at the meta (m)- or para (p)-position of each side. Femtosecond transient absorption (TA) measurements identify the intramolecular ET (IET) dynamics from the Ir to Re moiety, followed by the formation of one-electron-reduced species (OERS) via the intermolecular ET with a sacrificial electron donor (SED). The IET rate depends on the bridging ligand (BL) structures (∼25 ps for BLmm/mp vs ∼68 ps for BLpm/pp), while the OERS formation happens on an even slower time scale (∼1.4 ns). Connecting the Re moiety at the meta-position of the bipyridyl of the Ir moiety can restrict the rotation around a covalent bond between two bipyridyl ligands by steric hindrances and facilitate the IET process. This highlights the importance of BL structures on the ET dynamics in photocatalysts.

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.

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.

Clinical Features of Hepatitis C Virus-related Acute-on-chronic Liver Failure in a Korean Population.

Background/Aims: Acute-on-chronic liver failure (ACLF) is a widely recognized concept in which acute decompensation (AD) in patients with cirrhosis results in organ failure and high short-term mortality. On the other hand, few studies reflecting the various etiologies of cirrhosis are available. This study examined the clinical features of patients with hepatitis C virus (HCV)-related ACLF. Methods: Between January 2005 and December 2018, 109 HCV-related cirrhosis patients hospitalized for AD (ascites, hepatic encephalopathy, gastrointestinal hemorrhage, and bacterial infection) were enrolled for ACLF defined by the European Association for the Study of the Liver (EASL). Results: ACLF developed in 35 patients (32.1%) on admission. Eight, eight, and 19 patients had ACLF grades 1, 2, and 3, respectively. The 28-day and 90-day mortality rates were very low (2.7% and 5.4%, respectively) in patients without ACLF and very high (60.0% and 74.3%, respectively) in those with ACLF. In patients with HCV-related ACLF, compared to previous studies on hepatitis B virus-related ACLF and alcohol-related ACLF, the prevalence of liver failure was very low (17.1%), whereas that of kidney failure was very high (71.4%). Compared with all other prognostic scores, the Chronic liver failure Consortium Organ Failure score predicted the 90-day mortality most accurately, with an area under the receiver operator characteristic of 0.921. Conclusions: HCV-related ACLF has unique clinical characteristics distinct from hepatitis B virus-related and alcohol-related ACLF. ACLF defined by EASL can be useful for predicting the short-term mortality in HCV-related cirrhosis.

Quantum dot photolithography using a quantum dot photoresist composed of an organic-inorganic hybrid coating layer.

Quantum dots (QDs) have emerged as an important class of materials for diverse applications such as solid-state lighting, energy conversion, displays, biomedicine, and plasmonics due to their excellent photonic properties and durability. Soft lithography, inkjet printing, nanoimprinting, and polymer deep-pen lithography are primary lithography techniques employed to implement micro-patterns with QDs, however, there are limited reports on QD photolithography using conventional photolithography processes suitable for mass production. This study reports a QD photolithography technique using a custom-developed QD photoresist made of an organic-inorganic hybrid coating layer. Using this QD photoresist, various QD micro-patterns, including red or green micro lines, RGB color filters for smartphone displays at 340 ppi, and atypical micro logo patterns of the Korea University, were successfully fabricated. Furthermore, various process parameters were developed for the QD photolithography with this custom QD photoresist, and the optical properties of the QD films were also investigated. To demonstrate its applicability in contemporary smartphone displays, the color coordinates of the QD films were compared to those of the BT.2020 standard. The chromaticity of the QD photoresist in CIE 1931 color space covered 98.7% of the NTSC (1987) area while providing more expansive color space. Overall, the QD photoresist and its photolithography techniques reported in this study hold great promise in various fields of QD-based applications, including bio-labeling, optical detectors, and solar cells.

Tuning the Photophysical Properties of Homoleptic Tris-Cyclometalated Ir(III) Complexes by Facile Modification of the Imidazo-Phenanthridine and Their Application to Phosphorescent Organic Light-Emitting Diodes.

To explore the excited-state electronic structure of the blue-emitting Ir(dmp)3 dopant material (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine), which is notable for durable blue phosphorescent organic light-emitting diode (PhOLED), a series of homoleptic dmp-based Ir(III) complexes (DMP-R, tris[3-(2,6-dimethylphenyl)-7-R-imidazo[1,2-f]phenanthridin-12-yl-κC 12,κN 1]iridium, R = H, CH3, F, and CF3) were prepared by introducing an electron-donating group (EDG; -CH3) or an electron-withdrawing group (EWG; -F and -CF3) at the 7-position of the imidazo-phenanthridine ligand. The photophysical analysis demonstrated that the alteration from EDG to EWGs led to redshifted structureless emission profiles, which were correlated with variations in the 3MLCT/3ILCT ratio in the T1 excited state. From electrochemical studies and density functional theory calculations, it turned out that the excited-state nature of the dmp-based Ir(III) complexes was significantly affected by the inductive effect of the 7-substituent of the cyclometalating dmp ligand. As a result of the lowest unoccupied molecular orbital energy stabilization by the EWGs that suppressed the non-radiative pathway from the emissive triplet excited state to the 3 d-d state, the F- and CF3-modified Ir(dmp)3 complexes (DMP-F and DMP-CF 3 ) showed quantum yields of 27-30% in the solution state, which were at least 4- or 5-fold higher than those shown by DMP-H and DMP-CH 3 . A PhOLED device based on DMP-CF 3 [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct 3MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP-CH 3 .

Acute hepatitis E virus superinfection increases mortality in patients with cirrhosis.

BACKGROUND: Although acute hepatitis E is not fatal in healthy individuals, it is unclear whether hepatitis E superinfection increases the mortality in patients with pre-existing liver disease. Thus, we investigated the prognosis of patients with acute hepatitis E according to their cirrhosis diagnosis, and the prognosis according to the development of acute-on-chronic liver failure (ACLF) in patients with cirrhosis and chronic liver disease (CLD). METHODS: This study included 74 consecutive patients who were diagnosed with acute viral hepatitis E between January 2007 and December 2019. Of them, 39 patients without CLD, 13 patients with non-cirrhotic CLD, and 22 patients with cirrhotic CLD were analyzed. RESULTS: Among the 74 patients with HEV infection, 7 (9.5%) died within 180 days: 5 with underlying cirrhosis (71.4%) and 2 without cirrhosis (28.6%). The 180-day mortality was significant higher for patients with cirrhosis than for patients without cirrhosis (22.7% vs. 3.8%, P = 0.013). The age- and sex-adjusted proportional-hazard model revealed an approximately eightfold increase in the 180-day mortality risk in patients with cirrhosis compared to patients without cirrhosis. In addition, development of hepatitis E virus-related ACLF due to acute liver function deterioration in patients with pre-existing CLD or cirrhosis worsened the 180-day mortality rate. CONCLUSIONS: Our findings suggest that the acute hepatitis E mortality rate was low in healthy individuals but higher in patients with cirrhosis, and especially high in those with ACLF.

Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives.

Photocatalytic systems for CO2 reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO2 conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO2. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs•-) with TiO2 compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs•- formed at adjacent TiO2 without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.

Inorganometallic Photocatalyst for CO2 Reduction.

ConspectusDuring the last few decades, the design of catalytic systems for CO2 reduction has been extensively researched and generally involves (1) traditional approaches using molecular organic/organometallic materials and heterogeneous inorganic semiconductors and (2) combinatory approaches wherein these materials are combined as needed. Recently, we have devised a number of new TiO2-mediated multicomponent hybrid systems that synergistically integrate the intrinsic merits of various materials, namely, molecular photosensitizers/catalysts and n-type TiO2 semiconductors, and lower the energetic and kinetic barriers between components. We have termed such multicomponent hybrid systems assembled from the hybridization of various organic/inorganic/organometallic units in a single platform inorganometallic photocatalysts. The multicomponent inorganometallic (MIOM) hybrid system onto which the photosensitizer and catalyst are coadsorbed efficiently eliminates the need for bulk-phase diffusion of the components and avoids the accumulation of radical intermediates that invokes a degradation pathway, in contrast to the homogeneous system, in which the free reactive species are concentrated in a confined reaction space. In particular, in energetic terms, we discovered that in nonaqueous media, the conduction band (CB) levels of reduced TiO2 (TiO2(e-)) are positioned at a higher level (in the range -1.5 to -1.9 V vs SCE). This energetic benefit of reduced TiO2 allows smooth electron transfer (ET) from injected electrons (TiO2(e-)) to the coadsorbed CO2 reduction catalyst, which requires relatively high reducing power (at least more than -1.1 V vs SCE). On the other hand, the existence of various shallow surface trapping sites and surface bands, which are 0.3-1.0 eV below the CB of TiO2, efficiently facilitates electron injection from any photosensitizer (including dyes having low excited energy levels) to TiO2 without energetic limitation. This is contrasted with most photocatalytic systems, wherein successive absorption of single high-energy photons is required to produce excited states with enough energy to fulfill photocatalytic reaction, which may allow unwanted side reactions during photocatalysis. In this Account, we present our recent research efforts toward advancing these MIOM hybrid systems for photochemical CO2 reduction and discuss their working mechanisms in detail. Basic ET processes within the MIOM system, including intervalence ET in organic/organometallic redox systems, metal-to-ligand charge transfer of organometallic complexes, and interfacial/outer-sphere charge transfer between components, were investigated by conducting serial photophysical and electrochemical analyses. Because such ET events occur primarily at the interface between the components, the efficiency of interfacial ET between the molecular components (organic/organometallic photosensitizers and molecular reduction catalysts) and the bulk inorganic solid (mainly n-type TiO2 semiconductors) has a significant influence on the overall photochemical reaction kinetics and mechanism. In some TiO2-mediated MIOM hybrids, the chemical attachment of organic or organometallic photosensitizing units onto TiO2 semiconductors efficiently eliminates the step of diffusion/collision-controlled ET between components and prevents the accumulation of reactive species (oxidatively quenched cations or reductively quenched anions) in the reaction solution, ensuring steady photosensitization over an extended reaction period. The site isolation of a single-site organometallic catalyst employing TiO2 immobilization promotes the monomeric catalytic pathway during the CO2 reduction process, resulting in enhanced product selectivity and catalytic performance, including lifetime extension. In addition, as an alternative inorganic solid scaffold, the introduction of a host porphyrin matrix (interlinked in a metal-organic framework (MOF) material) led to efficient and durable photocatalytic CO2 conversion by the new MOF-Re(I) hybrid as a result of efficient light harvesting/exciton migration in the porphyrinic MOF and rapid quenching of the photogenerated electrons by the doped Re(I) catalytic sites. Overall, the case studies presented herein provide valuable insights for the rational design of advanced multicomponent hybrid systems for artificial photosynthesis involving CO2 reduction.

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.

A Hybrid Ru(II)/TiO2 Catalyst for Steadfast Photocatalytic CO2 to CO/Formate Conversion Following a Molecular Catalytic Route.

Herein, we employed a molecular Ru(II) catalyst immobilized onto TiO2 particulates of (4,4'-Y2-bpy)RuII(CO)2Cl2 (RuP; Y = CH2PO(OH)2), as a hybrid catalyst system to secure the efficient and steady catalytic activity of a molecular bipyridyl Ru(II)-complex-based photocatalytic system for CO2 reduction. From a series of operando FTIR spectrochemical analyses, it was found that the TiO2-fixed molecular Ru(II) complex leads to efficient stabilization of the key monomeric intermediate, RuII-hydride (LRuII(H)(CO)2Cl), and suppresses the formation of polymeric Ru(II) complex (-(L(CO)2Ru-Ru(CO)2L)n-), which is a major deactivation product produced during photoreaction via the Ru-Ru dimeric route. Active promotion of the monomeric catalytic route in a hetero-binary system (IrPS + TiO2/RuP) that uses TiO2-bound Ru(II) complex as reduction catalyst led to highly increased activity as well as durability of photocatalytic behavior with respect to the homogeneous catalysis of free Ru(II) catalyst (IrPS + Ru(II) catalyst). This catalytic strategy produced maximal turnover numbers (TONs) of >4816 and >2228, respectively, for CO and HCOO- production in CO2-saturated N,N-dimethylformamide (DMF)/TEOA (16.7 vol % TEOA) solution containing a 0.1 M sacrificial electron donor.

Rapid Exciton Migration and Amplified Funneling Effects of Multi-Porphyrin Arrays in a Re(I)/Porphyrinic MOF Hybrid for Photocatalytic CO2 Reduction.

A porphyrinic metal-organic framework (PMOF) known as PCN-222(Zn) was chemically doped with a molecular Re(I) catalyst-bearing carboxylate anchoring group to form a new type of metal-organic framework (MOF)-Re(I) hybrid photocatalyst. The porphyrinic MOF-sensitized hybrid (PMOF/Re) was prepared with an archetypical CO2 reduction catalyst, (L)ReI(CO)3Cl (Re(I); L = 4,4'-dicarboxylic-2,2'-bipyridine), in the presence of 3 vol % water produced CO with no leveling-off tendency for 59 h to give a turnover number of ≥1893 [1070 ± 80 µmol h-1 (g MOF)-1]. The high catalytic activity arises mainly from efficient exciton migration and funneling from photoexcited porphyrin linkers to the peripheral Re(I) catalytic sites, which is in accordance with the observed fast exciton (energy) migration (≈1 ps) in highly ordered porphyrin photoreceptors and the effective funneling into Re(I) catalytic centers in the Re(I)-doped PMOF sample. Enhanced catalytic performance is convincingly supported by serial photophysical measurements including decisive Stern-Volmer interpretation.

Peripheral Ligand Effect on the Photophysical Property of Octahedral Iridium Complex: o-Aryl Substitution on the Phenyl Units of Homoleptic IrIII(C∧C)3 Complexes (C∧C = 1-Phenyl-3-methylimidazolin-2-ylidene-C,C2') for Deep Blue Phosphorescence.

To evaluate the efficacy of ortho-arylation in the second coordination sphere of octahedral iridium complex, a series of homoleptic N-heterocyclic carbene (NHC)-based Ir(C∧CR)3-type complexes were designed and prepared by introducing various substituents (R = H, Me, Ph, MePh, and diMePh) at the ortho-position of the aryl unit of the orthometalated phenyl group. In solution, an unnoticeable increase of emission quantum yields was observed within the variation of the ortho-substituent of the sterically demanding side-branch, a diMePh- group, showing the radiative quantum yield of mer-Ir(C∧CdiMePh)3 (ΦPL = 1.9%), being higher than that of the unsubstituted carbene-based mer-Ir(C∧CH)3 (ΦPL = 1.2%), due to a considerable difference in the nonradiative decay rate (knr = 65.40 × 105 s-1 for mer-Ir(C∧CdiMePh)3 vs knr = 141.1 × 105 s-1 for mer-Ir(C∧CH)3). Such a difference is attributed to the reduction of nonradiative pathway via the 3MLCT → 3MC transition by the widening gap between triplet emissive states and 3MC state, and a rigidity increase in structure by steric hindrance of bulky aryl substituent. In contrast, significant increase of emission quantum yield was observed in the films cast by spin coating, and fac-/mer-Ir(C∧CdiMePh)3 (ΦPL = 60.1/49.1%) were the most efficient ones among NHC-Ir(III) complexes, compatible with the assumption that the secondary coordination effect, i.e. a peripheral constraint, was put into action. As the substituent R increases in size on going from H, Me, Ph, MePh, to diMePh, notable structural changes in the periphery are evident, while an increase of emission quantum yields is also seen. Such a peripheral difference was under scrutiny first with X-ray structural studies, and its manifestation in photophysics was investigated along with quantum calculations that finally addressed the peripheral effect being maximized at R = diMePh. In the application of PhOLED, the mer-Ir(C∧CdiMePh)3-doped multilayer device showed highly enhanced efficiency with an external quantum efficiency (EQE) of up to 8.1%, compared to that of the mer-Ir(C∧CH)3-based device (1.2%), indicating the multiple positive effects of bulky aryl substitution of Ir(III) dopant. A deep-blue CIE chromaticity diagram (0.16, 0.09) was achieved from the device using mer-Ir(C∧CdiMePh)3 as a dopant.

Ancillary Ligand Effects on Heteroleptic IrIII Dye in Dye-Sensitized Photocatalytic CO2 Reduction: Photoaccumulation of Charges on Arylated Bipyridine Ligand and Its Control on Catalytic Performance.

Herein, we report the synthesis, and photochemical and -physical properties, as well as the catalytic performance, of a series of heteroleptic IrIII photosensitizers (IrPSs), [Ir(C^N)2 (N^NAryl )]+ , possessing ancillary ligands that are varied with aryl-substituents on bipyridyl unit [C^N=(2-pyridyl)benzo[b]thiophen-3-yl (btp); N^NAryl =4,4'-Y2 -bpy (Y=-Ph or -PhSi(Ph)3 ]. We found that the π-extension of bipyridyl ligand by aryl-substitution put bipyridyl ligand in use as an electron relay unit that performed charge accumulation before delivering to the catalytic center, greatly improving the overall CO2 -to-CO conversion activities. In a typical run, the aryl-substituted IrPS (tBu IrP-PhSi )-sensitized homogeneous systems (IrPS+ReI catalyst) gave a turnover number of 1340 (ΦCO =24.2 %) at the early stage of photolysis (<5 h). This study demonstrates that the π-character modulation on the ancillary bipyridyl ligand is critical for forthcoming catalytic performance.

Blue Phosphorescence with High Quantum Efficiency Engaging the Trifluoromethylsulfonyl Group to Iridium Phenylpyridine Complexes.

Incorporation of an electron-withdrawing -SO2CF3 substituent to cyclometalating C^N-phenylpyridine (ppy) ligand resulted in an expected blue-shifted phosphorescence in the corresponding homoleptic Ir(ppySCF3)3 complex, showing the emission of λem = 464 nm at 300 K. One of its heteroleptic derivatives, modified by a pyrazolyl borate LX ligand, Ir(ppySCF3)2(bor), exhibited further blue-shifted phosphorescence of λem = 460 nm at 300 K. Cyclic voltammograms (CVs) and density-functional theory (DFT) calculations supported the efficacy of the electron-withdrawing capability of the SO2CF3 substituent lowering HOMO energy and obtained widened bandgaps and resumed blue emissions for all of the iridium complexes studied. The homoleptic complexes of both substituents, Ir(ppySCF3)3 and Ir(ppySF)3, reached the higher quantum yields (ΦPL) of (0.89 and 0.72), respectively. Similarly, emission quantum yields (ΦPL) of the heteroleptic derivatives were reported to be (0.75, 0.83, and 0.87) for Ir(ppySCF3)2(acac), Ir(ppySCF3)2(bor), and Ir(ppySCF3)2(pic), respectively. Emission kinetics support the enhanced quantum efficiency when kr and knr values are compared between Ir(ppySCF3)3 and Ir(ppySF)3, and both values favorably contribute to attaining a higher quantum efficiency for Ir(ppySCF3)3. Among solution-processed multilayered devices having an ITO/PEDOT:PSS/TCTA:Ir dopant (10:1, w/w)/TmPyPB/Liq/Al structure, a heteroleptic dopant, Ir(ppySCF3)2(bor), exhibited better device performance, reporting an external quantum efficiency (EQE) of 1.14%, current efficiency (CE) of 2.31 cd A-1, and power efficiency (PE) of 1.21 lm W-1, together with blue chromaticity of CIEx,y = (0.16, 0.32).