Aggregation-induced phosphorescence enhancement in deep-red and near-infrared emissive iridium(iii) complexes for solution-processable OLEDs

Abstract

AIPE active deep-red and near infrared iridium(iii) complexes were developed using electron withdrawing substituents and effectively utilized them in solution processable PhOLEDs.

C1‐Symmetric [Ir(C^N1)(C^N2)(O^O)]‐Tris‐Heteroleptic Iridium(III)‐Complexes with the Preferentially Horizontal Orientation for High‐Performance Near‐Infrared Organic Light‐Emitting Diodes

“…Suzuki coupling [ 10c ] of cost‐effective 2‐Cl‐isoquinoline (instead of 2‐Br‐isoquinoline [ 9e ] ) with benzo[ b ]thien‐2‐yl boronic acid gave the HC^N 1 main ligand Hiqbt in 73% yield. Also as shown in Scheme 1, straightforward metalation [ 15 ] in the 2‐ethoxyethanol/H 2 O (v/v = 3:1) mixed‐solvent of the Hiqbt and one of the HC^N 2 ligands ( HdFppy , Hppy , and Hdpqx ) in an equimolar ratio with IrCl 3 ·3H 2 O, gave rise to the corresponding in situ formed µ ‐chloro‐bridged dimer intermediate [Ir(C^N 1 )(C^N 2 )( µ ‐Cl)] 2 , which was not isolated from the inevitable interferents of [Ir(C^N 1 ) 2 ( µ ‐Cl)] 2 and [Ir(C^N 2 ) 2 ( µ ‐Cl)] 2 .…”

“…To date, cyclometalated Ir(III)‐complexes, possessing neutral ([Ir(C^N) 3 ]‐homoleptic [ 8 ] or [Ir(C^N) 2 (L^X)]‐heteroleptic; L^X = O^O [ 9 ] or N^O [ 10 ] ) and cationic ([Ir(C^N) 2 (N^N)] + ) [ 11 ] forms and acting as both the chromophore and the charge‐transport medium, were demonstrated for reliable NIR‐OLEDs. Apparently, in order to narrow the HOMO‐LUMO (HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital) gap of a specific Ir(III)‐complex toward the restrictive NIR emission, the use of π‐conjugation expansion [ 8–11 ] of the C^N‐cyclometalated main ligand, especially with electron‐rich substituents, is generally preferred which can effectively destabilize the HOMO energy level. Considering the facile aggregation‐caused quenching (ACQ) [ 12 ] effect arising from the large π‐conjugation of the C^N‐cyclometalated main ligand, modification of the HL^X ancillary ligand seems to be an accessible alternative way to stabilize the LUMO energy level of the [Ir(C^N) 2 (L^X)]‐heteroleptic [ 9–11 ] Ir(III)‐complex.…”

“…Apparently, in order to narrow the HOMO‐LUMO (HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital) gap of a specific Ir(III)‐complex toward the restrictive NIR emission, the use of π‐conjugation expansion [ 8–11 ] of the C^N‐cyclometalated main ligand, especially with electron‐rich substituents, is generally preferred which can effectively destabilize the HOMO energy level. Considering the facile aggregation‐caused quenching (ACQ) [ 12 ] effect arising from the large π‐conjugation of the C^N‐cyclometalated main ligand, modification of the HL^X ancillary ligand seems to be an accessible alternative way to stabilize the LUMO energy level of the [Ir(C^N) 2 (L^X)]‐heteroleptic [ 9–11 ] Ir(III)‐complex. However, its exclusively reduced HOMO‐LUMO band‐gap for NIR luminescence does not have an universal effect, [ 13 ] and even the undesirable hypsochromic shift [ 13 ] beyond the NIR regime results from the simultaneous stabilization of the HOMO energy level.…”

“…Convincingly, these two factors, which are key to reliable NIR‐OLEDs, are highly associated with the specific excited state situations of the Ir(III)‐complexes (such as energy‐level and electron/charge transfer (CT) among the 3 MLCT/ 3 LC (LC = ligand‐centered; MLCT = metal‐to‐ligand charge transfer) of the 1 T state, etc.). [ 14 ] In light of the transformation [ 15 ] of the 1 T nature in the C 1 ‐symmetric tris ‐heteroleptic Ir(III)‐complex composed of an Ir(III) ion and three different ligands, it is of notable interest on expanding that novel molecule‐engineered strategy to Ir(III)‐complex‐based NIR‐emitters, which should fill in the blank after previously reported NIR‐emissive [Ir(C^N) 3 ]‐ [ 8 ] and [Ir(C^N) 2 (L^X)]‐heteroleptic [ 9–11 ] Ir(III)‐complexes. Indeed, for C 1 ‐symmetric tris ‐heteroleptic Ir(III)‐complexes, their evident advantage lies in the potentially enriched inventory ([Ir(C^N 1 )(C^N 2 )(L^X)] (L^X = O^O or N^N), [ 15,16 ] [Ir(C^N 1 )(C^N 2 )(C^N 3 )], [ 17 ] [Ir(C^N 1 )(C^C 2 )(C^N 3 )], [ 18 ] [Ir(C^N 1 )(N^N 1 )(N^N 2 )], [ 19 ] etc.)…”

“…compared to conventional [Ir(C^N) 3 ]‐ [ 8 ] and [Ir(C^N) 2 (L^X)]‐counterparts. [ 9–11 ] Meanwhile, the breaking of the symmetry of molecular structures can help increasing the solubility of resultant Ir(III)‐complexes. Moreover, arising from the C 1 ‐symmetric tris ‐heteroleptic spatial configuration, the emitting dipole orientation (EDO) is rearranged.…”