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-κC12,κN1]­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 3d–d state, the F- and CF3-modified Ir­(dmp)3 complexes (DMP–F and DMP–CF3) 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–CH3. A PhOLED device based on DMP–CF3 [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–CH3

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-κC12,κN1]­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 3d–d state, the F- and CF3-modified Ir­(dmp)3 complexes (DMP–F and DMP–CF3) 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–CH3. A PhOLED device based on DMP–CF3 [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–CH3

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-κC12,κN1]­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 3d–d state, the F- and CF3-modified Ir­(dmp)3 complexes (DMP–F and DMP–CF3) 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–CH3. A PhOLED device based on DMP–CF3 [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–CH3

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-κC12,κN1]­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 3d–d state, the F- and CF3-modified Ir­(dmp)3 complexes (DMP–F and DMP–CF3) 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–CH3. A PhOLED device based on DMP–CF3 [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–CH3

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-κC12,κN1]­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 3d–d state, the F- and CF3-modified Ir­(dmp)3 complexes (DMP–F and DMP–CF3) 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–CH3. A PhOLED device based on DMP–CF3 [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–CH3

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

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

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

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

Elucidation of Excited-State Properties of Bimetallic Ir(III)–Pt(II) Complexes with Conjugated Bridging Ligands

We investigated excited-state properties of 4′,7′-phenanthrolino-5′,6′:2,3-pyrazine (ppz)-bridged bimetallic complexes, (L)2Ir-ppz-PtCl2 (Ir-ppz-Pt) (L = 2-(4′,6′-difluorophenyl)­pyridinato-N,C2 (dfppy)), and [(L)2Ir]2(ppz) (Ir-ppz-Ir). These properties were compared to those of reported dpp bridging complexes, 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, Ir-dpp-Pt, and Ir-dpp-Ir. Excited-state properties of hetero-bimetallic Ir–Pt systems were very different from those of related Ru–Pt systems with a dpp bridging ligand; Ru-dpp-Pt displayed the spin forbidden metal-to-ligand charge-transfer emission, whereas Ir-dpp-Pt was relatively silent in emission. Steady-state photochemical studies and photodynamic studies were undertaken for each Ir or Pt center within peripheries of bridging ligands to confirm either energy or ET and at the same time its direction. Thermodynamic driving force of the electron-transfer (ET) process from Ir to Pt in the hetero-bimetallic Ir–Pt systems was predicted by the Rehm–Weller equation to be unfavorable for both Ir-ppz-Pt and Ir-dpp-Pt. However, when photoinduced ET in ground state was considered, the complexes could undergo an ET process from Pt to Ir center. Such a prediction is well manifested from selective excitation on the bridging ligand