Fluorine-free, blue-emitting cationic iridium complexes with a phenyl-triazole type cyclometalating ligand: Synthesis, characterizations and their use for efficient organic light-emitting diodes

On the importance of equatorial metal-centered excited states in the photophysics of cyclometallated Ir(iii) complexes

“…Regarding their energy positions, it is important to stress that in all the cases reported in the literature, the 3 MC eq state appears to be lower in energy than the 3 MC ax one, consequently making its involvement in principle more relevant than that of the 3 MC ax state. [18][19][20][21][22][23][24] As a final remark, similar MC states were also found while studying the photorelease of N^N ligands in some complexes of the [Ru(N^N) 3 ] 2+ family. [25][26][27][28][29] In the present work, the photophysical properties of [Ir ( ppy) 2 (bpy)] + , [Ir( ppy) 2 ( pbpy)] + and [Ir( ppy) 2 (dpbpy)] + (hereafter complexes 1, 2 and 3, respectively, see Fig.…”

“…Regarding their energy positions, it is important to stress that in all the cases reported in the literature, the 3 MC eq state appears to be lower in energy than the 3 MC ax one, consequently making its involvement in principle more relevant than that of the 3 MC ax state. 18–24 As a final remark, similar MC states were also found while studying the photorelease of N^N ligands in some complexes of the [Ru(N^N) 3 ] 2+ family. 25–29…”

Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices

“…As an archetypal complex, [Ir­(ppy) 2 (bpy)]­PF 6 emits orange-red light (Hppy and bpy are 2-phenylpyridine and 2,2′-bipyridine, respectively). , So far, different approaches have been employed to develop blue emissive complexes, which is usually achieved by widening the energy gap between the LUMO and HOMO orbitals (LUMO and HOMO are the lowest unoccupied molecular orbital and the highest occupied molecular orbital, respectively). ,, Electron-deficient substituents like −F or −CF 3 are substituted onto the phenyl moiety of ppy for a stabilization of the HOMO, , and electron-rich substituents like −N­(CH 3 ) 2 are anchored to bpy for a destabilization of the LUMO . The introduction of sp 2 -hybridized C–F bonds through fluorine substitution onto the phenyl ring of a C ∧ N ligand has been widely utilized for shifting the emission toward blue, but the expense is that both electrochemical and thermal stabilities of the complexes are decreased. , C ∧ N ligands without fluorine substitutions are thus highly preferred for the construction of blue emissive complexes. ,− It has been shown that the LUMO can be significantly destabilized by the employment of A ∧ A ligands with a highly electron-rich character like 2-(1 H -pyrazol-1-yl)­pyridine or unconjugated A ∧ A ligands like methylene-linked pyrazole or N -heterocyclic carbene, ,,− leading to remarkable blue shifts in the emission. In such cases, the emission originates from the mixed 3 π–π* (C ∧ N-centered)/ 3 MLCT (Ir → C ∧ N) state (MLCT is short for metal-to-ligand charge transfer), and as a consequence, the 3 π–π* energy of the C ∧ N ligand essentially determines the emission energy. , The combination of fluorine-free C ∧ N ligands featuring high 3 π–π* energy and appropriate unconjugated/electron-rich A ∧ A ligands has been regarded as an effective strategy to prepare blue emissive cationic Ir­(III) complexes. ,,, With this molecular design principle, fluorine-free deep blue emissive Ir­(III) complexes were prepared by using high-triplet-energy C ∧ N ligands like 5-phenyl-1 H -1,2,4-triazole-type ligands and electron-rich A ∧ A ligands like 4-dimethylamino-2-(1 H -pyrazol-1-yl)­pyridine (dmapzpy) .…”

“…The introduction of sp 2 -hybridized C–F bonds through fluorine substitution onto the phenyl ring of a C ∧ N ligand has been widely utilized for shifting the emission toward blue, but the expense is that both electrochemical and thermal stabilities of the complexes are decreased. , C ∧ N ligands without fluorine substitutions are thus highly preferred for the construction of blue emissive complexes. ,− It has been shown that the LUMO can be significantly destabilized by the employment of A ∧ A ligands with a highly electron-rich character like 2-(1 H -pyrazol-1-yl)­pyridine or unconjugated A ∧ A ligands like methylene-linked pyrazole or N -heterocyclic carbene, ,,− leading to remarkable blue shifts in the emission. In such cases, the emission originates from the mixed 3 π–π* (C ∧ N-centered)/ 3 MLCT (Ir → C ∧ N) state (MLCT is short for metal-to-ligand charge transfer), and as a consequence, the 3 π–π* energy of the C ∧ N ligand essentially determines the emission energy. , The combination of fluorine-free C ∧ N ligands featuring high 3 π–π* energy and appropriate unconjugated/electron-rich A ∧ A ligands has been regarded as an effective strategy to prepare blue emissive cationic Ir­(III) complexes. ,,, With this molecular design principle, fluorine-free deep blue emissive Ir­(III) complexes were prepared by using high-triplet-energy C ∧ N ligands like 5-phenyl-1 H -1,2,4-triazole-type ligands and electron-rich A ∧ A ligands like 4-dimethylamino-2-(1 H -pyrazol-1-yl)­pyridine (dmapzpy) . However, these blue emissive complexes exhibit low luminescence efficiencies (∼0.2 in doped films) because the emissive triplet states energetically approach nonradiative “dark” states like metal-centered states ( 3 MC) .…”

Aqueous emissive cyclometalated iridium photoreductants: synthesis, computational analysis and the photocatalytic reduction of 4-nitrophenol

“…Luminescent cyclometallated iridium complexes have received great attention in the last few decades because of their exciting photophysical property and enthralling applications in different fields of science and technology. [1][2][3][4][5] Such systems display properties such as high photostability, high quantum yield, and longer emission lifetime due to intersystem crossing (ISC), [6][7][8][9] resulting in their application as photocatalysts for organic transformation, [10][11][12][13][14][15][16][17][18][19][20] materials for organic light-emitting diodes (OLEDs), 16,21,22 light-emitting electrochemical cells (LEECs), 23 theranostic and bioimaging agents, [24][25][26][27][28][29][30][31][32][33] etc. The majority of the developed cyclometallated iridium systems are readily accessible in organic solvents.…”