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.

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.

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 .

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.

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.

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).

Utility of Squaraine Dyes for Dye-Sensitized Photocatalysis on Water or Carbon Dioxide Reduction.

Red light-sensitized squaraine (SQ) dyes were developed and incorporated into dye-sensitized catalysts (DSCs) with the formula of SQ/TiO2/Cat, and their efficacies were evaluated in terms of performance on either water or carbon dioxide reduction. Pt nanoparticles or fac-[Re(4,4'-bis-(diethoxyphosphorylmethyl)-2,2'-bipyridine)(CO)3Cl] were used as each catalytic center within the DSC frame of SQ/TiO2/Pt (Type I) or SQ/TiO2/Re(I) (Type II). In order to convey the potential utility of SQ in low energy sensitization, the following catalytic reductions were carried out under selective lower energy irradiation (>500 nm). Type I and II showed different catalytic performances, primarily due to the choice of solvent for each catalytic condition: hydrogenation was carried out in H2O, but CO2 reduction in dimethylformamide (DMF), and SQ was more stable in aqueous acid conditions for hydrogen generation than CO2 reduction in DMF. A suspension of Type I in 3 mL water containing 0.1 M ascorbic acid (pH = 2.66) resulted in efficient photocatalytic hydrogen evolution, producing 37 µmol of H2 for 4 h. However, in photocatalysis of Type II (SQ/TiO2/Re(I)) in 3 mL DMF containing 0.1 M 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole, the TiO2-bound SQ dyes were not capable of working as a low energy sensitizer because SQ was susceptible to dye decomposition in nucleophilic DMF conditions, resulting in DSC deactivation for the CO2 reduction. Even with the limitation of solvent, the DSC conditions for the utility of SQ have been established: the anchoring group effect of SQ with either phosphonic acid or carboxylic acid onto the TiO2 surface; energy alignment of SQ with the flat band potentials (E fb) of TiO2 semiconductors and the reduction power of electron donors; and the wavelength range of the light source used, particularly when >500 nm.

Organometallic Iridium(III) Complex Sensitized Ternary Hybrid Photocatalyst for CO2 to CO Conversion.

A series of heteroleptic iridium(III) complexes functionalized with two phosphonic acid (-PO3 H2 ) groups (dfppy IrP, ppy IrP, btp IrP, and piq IrP) were prepared and anchored onto rhenium(I) catalyst (ReP)-loaded TiO2 particles (TiO2 /ReP) to build up a new IrP-sensitized TiO2 photocatalyst system (IrP/TiO2 /ReP). The photosensitizing behavior of the IrP series was examined within the IrP/TiO2 /ReP platform for the photocatalytic conversion of CO2 into CO. The four IrP-based ternary hybrids showed increased conversion activity and durability than that of the corresponding homo- (IrP+ReP) and heterogeneous (IrP+TiO2 /ReP) mixed systems. Among the four IrP/TiO2 /ReP photocatalysts, the low-energy-light (>500 nm) activated piq IrP immobilized ternary system (piq IrP/TiO2 /ReP) exhibited the most durable conversion activity, giving a turnover number of ≥730 for 170 h. A similar kinetic feature observed through time-resolved photoluminescence measurements of both btp IrP/TiO2 and TiO2 -free btp IrP films suggests that the net electron flow in the ternary hybrid proceeds dominantly through a reductive quenching mechanism, unlike the oxidative quenching route of typical dye/TiO2 -based photolysis.

Solid-State Photochromism by Molecular Assembly of Bis-o-carboranyl Siloles.

A new type of solid-state photochromism was observed in an AB2 -type molecular assembly comprising a central silole and two peripheral o-carborane units, and in this assembly, depending on the assembling positions of those units at the adjoining benzene ring, two different regioisomers were formed: Si-m-Cb and Si-p-Cb. Each isomer showed different solid-state photochromism depending on its solid-state molecular conformation and was either in the crystalline or amorphous state. The crystals of each meta- or para-isomer, CSi-m-Cb or CSi-p-Cb , showed yellow or blue emission, and mechanically grinding those crystals into amorphous powders of ASi-m-Cb and ASi-p-Cb , switched their emissions to blue and yellow, respectively. Photophysical studies revealed that the electronic interaction between silole and o-carborane units determined the emission color. The crystal and DFT-optimized structures each account for the crystalline and amorphous structures, respectively, and are correlated well with the electronic interactions in the molecular assembly in the solid state, thus enabling the prediction of the solid-state molecular conformational change.

Photophysical properties of structural isomers of homoleptic Ir-complexes derived from xylenyl-substituted N-heterocyclic carbene ligands.

The phosphorescence properties of fac-Ir(pmp)3, mer-Ir(pmp)3, fac-Ir(dmpmp)3 and mer-Ir(dmpmp)3 (where pmp = 3-methyl-1-phenyl-2,3-dihydro-1H-imidazo[4,5-b]pyridine and dmpmp = 1-(2',6'-dimethylbiphenyl-2-yl)-3-methyl-2,3-dihydro-1H-imidazo[4,5-b]pyridine) in CH2Cl2 were investigated. At 77 K, the fac-isomers showed blue emission with a vibronic structure, while the mer-isomers showed less structured emissions. At 300 K, all complexes showed broad and markedly red-shifted emission spectra compared to those at 77 K. The quantum yields of the Ir(dmpmp)3 isomers were very low, and their emission lifetimes were very short compared to those of Ir(pmp)3. In order to understand the large differences between the photodynamic properties of Ir(pmp)3 and Ir(dmpmp)3, we performed femtosecond time-resolved transient absorption (TA) spectroscopic measurements. The TA spectra of Ir(dmpmp)3 were almost the same as those of Ir(pmp)3 at a short delay time. However, Ir(dmpmp)3 showed a new broad TA band at around 720 nm with increasing delay time. The rise time of this band was ca. 10 ps for both isomers, and this may be attributed to the geometrical change in the excited state, which is associated with the steric hindrance of the bulky dimethylphenyl substituent. Actually, Ir(dmpmp)3 showed a strong rigidochromic shift in the emission spectra with varying temperature. To understand the molecular orbitals and the energy levels, theoretical calculations were performed using density functional theory. As a result, structural displacement takes place accompanied by the fast migration of localization of excited states via intraligand charge transfer.

Influence of bulky substituents on the photophysical properties of homoleptic iridium(iii) complexes.

A new series of homoleptic cyclometalated iridium(iii) complexes based on a phenylpyridine (ppy) ligand containing bulky substituents have been synthesized and characterized. The phosphorescence behavior of the Ir complexes is investigated by steady-state and time-resolved emission spectroscopic techniques. Comparison of the results with those of the reference Ir(ppy)3 reveals that the emission color and photophysical properties of other Ir complexes are influenced by the electron-donating groups (-CH3 and phenyl derivatives) attached to the ppy ligand. In particular, systematic red-shifts are observed by increasing the electron-donating ability. The emission spectrum of Ir(Me-ppy)3, having a small electron-donating -CH3 group, is red-shifted; however, the emission quantum yield is low and the nonradiative decay constant is large. On the other hand, although bulky phenyl derivative-adducts (Ir(Ph-ppy)3, Ir(MePh-ppy)3, and Ir(diMePh-ppy)3) also exhibit red-shifted emission, their kinetic and photophysical behaviors are more optimal than those of Ir(Me-ppy)3, whose behavior does not follow the energy gap law. This deviation may be attributed to the orthogonal structure associated with the steric hindrance of bulky substituents. The molecular structure, molecular orbitals in singlet/triplet manifolds, and energy band gap are verified by density functional theory calculations.

Photoinduced electron and hole transfers in carbazole dendrimers with heteroleptic Ir-complex cores.

We synthesised carbazole (Cz) dendrimers with heteroleptic Ir-complex cores. Upon excitation of the carbazole (Cz) dendrons, the phosphorescence of the core Ir(iii) complex was quenched due to the photoinduced electron transfer (PET) process. The PET dynamics of the excited Cz-dendrons were investigated using the femtosecond time-resolved transient absorption technique. A broad transient absorption (TA) band attributed to the S1-Sn transition of the 1Cz*-dendron was observed at around 630 nm in the first generation Cz-dendrimer (G1). This TA band in the second-generation dendrimer (G2) decayed with a longer lifetime of 55.5 ps compared to that of G1 (9.8 ps), because G2 has a larger distance between the Cz-dendron and Ir-complex core than that of G1. The decay time of the free 1Cz*-dendron was 6.3 ns, and thus, the reduced decay time in Gn corresponds to the PET dynamics. As a result of the PET process, the Cz cationic radical species (Cz˙+) was observed at around 780 nm. Interestingly, when the core Ir-complex in the dendrimer was excited with a 400 nm pulse selectively, the TA band of Cz˙+ was also detected at around 780 nm. This may be due to the photoinduced hole transfer (PHT) from the highest occupied molecular orbital (HOMO) energy state of Cz to the lowest singly occupied molecular orbital (LSOMO) energy state of the excited Ir-complex. The oxidation potential of Cz is lower than that of the Ir-complex, indicating that the HOMO of the Cz-dendron is located at a higher energy state than that of the Ir-complex. To investigate the relative order of the energy states and their orbital shapes, we performed theoretical calculations using density functional theory. The TA spectra were globally deconvoluted to generate the decay-associated spectra (DAS), from which the species-associated spectra (SAS) were calculated. The SAS can distinguish the individual intermediate species participating in the PET and PHT processes. The analysed rate constants of SAS were consistent with the results determined by the TA decays.

Excitation spectroscopic and synchronous fluorescence spectroscopic analysis of the origin of aggregation-induced emission in N,N-diphenyl-1-naphthylamine-o-carborane derivatives.

We have synthesised mono-(NpCb) and bis-[(N,N-phenyl-1-naphthylamino)benzo]-o-caboranes (NpCbNp), which show anomalously intense aggregation-induced emission (AIE) at long wavelengths and monomer emission at short wavelengths. The actual concentration of the aggregator in intense AIE is very low, so absorption spectroscopy is unsuitable for detecting small changes in the absorbance. Hence, to understand the aggregation pattern, we employ excitation spectroscopy, since this method has excellent sensitivity in compliance with the emission intensity. Moreover, we carried out synchronous fluorescence spectroscopic measurements to confirm that the aggregator is different from the monomeric species. The excitation spectrum shows distinguishable differences between the AIE and the normal emission. For the triad NpCbNp, the excitation spectrum for the AIE is located at a shorter wavelength than that for the monomeric emission spectrum, which means that the AIE is attributed to the H-type aggregator. On the other hand, for the dyad NpCb, the excitation spectrum for the AIE is observed at an identical wavelength as that for the monomeric species, which indicates that the aggregator is of the oblique type.

Photoinduced Electron Transfer in a BODIPY- ortho-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 o-carborane ( o-Cb). In this dyad, BODIPY and o-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 G), 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 G values, because deactivation via internal conversion is much faster than PET.

Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO2 by Re(I)-Complex-Anchored TiO2 Hybrid Catalyst.

A series of cationic Ir(III) complexes ([Ir(btp)2(bpy-X2)]+ (Ir-X+: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X2 = 4,4'-X2-2,2'-bipyridine (X = OMe, tBu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO2 reduction to CO using a hybrid catalyst (TiO2/ReP) prepared by anchoring of Re(4,4'-Y2-bpy)(CO)3Cl (ReP; Y = CH2PO(OH)2) on TiO2 particles. Irradiation of a solution containing Ir-X+, TiO2/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO2 to CO with efficiencies in the order X = OMe > tBu ≈ Me > H; Ir-CN+ has no photosensitization effect. A notable observation is that Ir-tBu+ and Ir-Me+ are less efficient than Ir-OMe+ at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X+ sensitizers are commonly superior compared to Ru(bpy)32+, a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe+ and TiO2/ReP is much more efficient than a homogeneous-solution system using Ir-OMe+ and Re(4,4'-Y'2-bpy)(CO)3Cl (Y' = CH2PO(OEt)2). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.

Time-resolved spectroscopic analysis of the light-energy harvesting mechanism in carbazole-dendrimers with a blue-phosphorescent Ir-complex core.

In order to investigate the light-energy harvesting mechanism, a series of dendrimers with a heteroleptic iridium(iii) complex core, [Ir(dmb)2(pic-Czn)] (Gn: n = 1, 2, and 3), with 2,6-difluoro-3-(4-methylpyridin-2-yl)benzonitrile (dmb) as the cyclometallating ligand and 3-hydroxypicolinate (pic) as the ancillary ligand, connected to carbazole-based dendrons (Czn: n = 2, 4, and 8) was synthesized. The Ir centred complex [Ir(dmb)2(pic-OCH3), G0] shows a blue emission at <500 nm, which is assigned to metal-to-ligand charge-transfer (3MLCT) phosphorescence. This phosphorescence was enhanced with increasing generations due to the increase in the total absorbance of the Cz-dendron. The light-harvesting efficiencies determined by various methods were approximately 160 (G1), 220 (G2), and 330% (G3). The energy transfer efficiencies for G1-G3 from the peripheral Cz-dendron to the Ir-core complex were above 97%, determined using the reduction in the lifetime of the excited 1Cz*-dendrons. G1-G3 showed a transient absorption (TA) band at 600 nm, which was attributed to the Sn ← S1 transition of the Cz-dendrons. The fast decay of these TA bands was consistent with the fast emission decay times. The time-resolved TA band correlated with the core Ir-complex was observed at 500 nm, though it overlapped and interfered with the intense TA band of the Cz-dendrons. Therefore, we attempted a global analysis by singular value decomposition (SVD). The determination of the absorption spectra of the individual species participating in the energy transfer process by SVD analysis can distinguish between different mechanistic models. The analysed rate constants were consistent with the results determined by the emission decays.

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)-Platinum(II) and Iridium(III)-Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine.

We investigated the electrochemical and excited-state properties of 2,3-bis(2-pyridyl)pyrazine (dpp)-bridged bimetallic complexes, (L)2Ir-dpp-PtCl [1, L = 2-(4',6'-difluorophenyl)pyridinato-N,C2 (dfppy); 2, L = 2-phenylpyridinato-N,C2 (ppy)] and [(L)2Ir]2(dpp) [3, L = dfppy; 4, L = ppy] compared to monometallic complexes, (L)2Ir-dpp (5, L = dfppy; 6, L = ppy) and dpp-PtCl (dpp-PtIICl2; 7). The single-crystal X-ray crystallographic structures of 1, 3, 5, and 6 showed that 1 and 3 have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in 5 and 6 is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir-Pt and Ir-Ir bimetallic complexes (1-4) could be explained by assuming the involvement of crossing to platinum- and iridium-based d-d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d-d states from the emissive triplet metal-to-ligand charge-transfer (3MLCT) state, followed by cleavage of the dpp-Pt and (L)2Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based 3MLCT to the Ir d-d or Pt d-d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d-d or Pt d-d) with a significant dependency on the relative 3MLCT levels of the (L)2Ir-dpp component.

The effect of interligand energy transfer on the emission spectra of heteroleptic Ir complexes.

In order to understand the causes of the emission shape and colour changes of heteroleptic Ir3+ complexes containing 2-(2,4-difluorophenyl)pyridine (dfppy) as the main ligands, we introduced two types of ancillary ligands: (1) non-luminescent ancillary ligands, namely tetrakis(pyrazolyl)borate (bor) and picolinate (pic), which were employed for the preparation of Ir(dfppy)2(bor) and Ir(dfppy)2(pic), respectively, and (2) luminescent ancillary ligands, namely 1,10-phenanthroline (phen), bipyridine (bpy), and 2,3-dipyridylpyrazine (dpp), which were employed for the preparation of Ir(dfppy)2(phen), Ir(dfppy)2(bpy), and Ir(dfppy)2(dpp), respectively. In a glassy matrix at 77 K, the Ir complexes showed well-structured emission spectra, except Ir(dfppy)2(dpp). The vibronic structures in the emission spectra of Ir(dfppy)2(bor) and Ir(dfppy)2(pic) were maintained even at 300 K. However, Ir(dfppy)2(phen), Ir(dfppy)2(bpy), and Ir(dfppy)2(dpp) showed markedly red-shifted and broad emission spectra. The anomalous rigidochromism was attributed to an interligand energy transfer (ILET), and showed a strong temperature dependence. The excited states of dfppy are higher than those of phen, bpy, and dpp; thus, ILET occurs from dfppy to the other ligands lying in lower energy states. The ILET dynamics were probed directly using femtosecond transient absorption (TA) spectroscopy after the excitation of dfppy. As the time delay increased, the intensity of the TA band of dfppy decreased, while those of the bands related to the phen, bpy, and dpp ancillary ligands increased. On the other hand, no changes in the TA spectra were observed for Ir(dfppy)2(bor) and Ir(dfppy)2(pic). The TA spectral behaviours can be explained in terms of the relative ordering of the emissive states for cyclometalating and ancillary ligands.

Steric effect on excimer formation in planar Pt(ii) complexes.

In order to understand the steric influence on excimer formation in square planar metal complexes, three different Pt(ii) complexes were prepared by modifying the substituents in the main ligand: Pt(ii)(dfppy)(acac) (Pt-1, where dfppy is difluorophenylpyridine, acac is acetylacetonate); the bulky triphenyl silyl (Ph3Si-) group was substituted at the pyridine moiety (Pt-2) and at the phenyl moiety (Pt-3) of the main ligand of Pt-1. The Pt-complexes showed sky-blue emission at ∼460 nm. In addition, Pt-1 and Pt-3 showed excimer emission at ∼600 nm in the concentrated solution and the solid sample. The emission lifetimes and intensities for monomeric Pt-1 and Pt-3 showed strong concentration dependence. Indeed, the lifetime of the monomer was reduced in highly concentrated solutions due to excimer formation. The intrinsic emission lifetimes were determined as 364 ns (Pt-1) and 300 ns (Pt-3) by Stern-Volmer analysis, considering the self-quenched lifetime of monomer emission. Pt-2 did not show any excimer emission in the concentrated solution or solid sample. The crystal structures of Pt-1 and Pt-3 were analysed by X-ray crystallographic measurements. The results revealed that the LUMO moiety closely overlapped with that of another Pt-complex. In this study, based on the influence of steric hindrance of the bulky Ph3Si group, we concluded that the LUMO-LUMO interaction between the pyridine moieties of the main ligand is the main factor responsible for excimer formation.

Influence of π-conjugation structural changes on intramolecular charge transfer and photoinduced electron transfer in donor-π-acceptor dyads.

The influence of π-conjugation structural changes on photoinduced electron transfer (PET) and intramolecular charge transfer (ICT) processes in π-conjugated donor (D)-acceptor (A) dyads (D-π-A) was investigated. Three types of D-π-A dyads were prepared through the modification of the structure of their π-conjugated linker, including D-π-A (1) and D-πtw-A (2) having a twisted π-conjugation, and D-π-Si-π-A (3) with a π-conjugation severed by a Si-atom. In these dyads, carbazole (Cz) and oxadiazole (Oz) moieties act as an electron donor and acceptor, respectively. The emission maxima of dyads 1 and 3 red-shifted with the increase in polarity, which could be attributed to the ICT process. The fluorescence lifetimes of dyads 1 and 3 were 2.64 and 4.29 ns in CH2Cl2, respectively. In contrast, dyad 2 showed dual emission at 350 and 470 nm in CH2Cl2. The emission of dyad 2 at 380 nm corresponded to the monomer fluorescence in the locally excited state. Moreover, the emission at 470 nm increased simultaneously with the diminishing of the fluorescence at 380 nm. This emission band can be assigned as the intramolecular exciplex emission, and showed a strong solvatochromic shift. The low emission quantum yield (<3%) of dyad 2 is due to the PET process. In dyad 2, the cationic and anionic radical species generated by the PET process were confirmed by femtosecond transient absorption (fs-TA) spectroscopy. Upon photoexcitation at 290 or 340 nm, the A or D moieties can be selectively excited. Upon excitation at 290 nm, the acceptor moiety can be excited to the 1A* state, thus the photoinduced hole transfer (PHT) takes place from 1A* to D through the HOMO levels within a few picoseconds. On the other hand, when the donor moiety is excited at 340 nm, the PET process occurs from 1D* to A. Based on the fs-TA studies, it was found that the dynamics and mechanisms for the electron (or charge) transfer were strongly affected by the variation of the π-conjugation of the linker. Herein, we can conclude that the PET and ICT processes are strongly influenced by the π-conjugation properties and their mechanisms are also affected by whether selective excitation of the donor or acceptor moiety occurs. Moreover, unit electron transfers (PET or PHT) were observed dominantly in the dyads having severed/twisted linkers in π-conjugation. However, dyad 1 possessing a well-conjugated linker showed a partial charge transfer character.