Modulation of Solid‐State Aggregation of Square‐Planar Pt(II) Based Emitters: Enabling Highly Efficient Deep‐Red/Near Infrared Electroluminescence

Abstract

The design of square‐planar Pt(II) complexes with highly efficient solid‐state near infrared (NIR) luminescence for electroluminescence is attractive but challenging. This study presents the fine‐turning of excited‐state properties and application of a series of isoquinolinyl pyrazolate Pt(II) complexes that are modulated by steric demanding substituents. It reveals that the bulky substituents do not always disfavor metallophilic Pt···Pt interactions. Instead, π–π stacking among chelates, which are fine‐tuned by the associated substituents, also exerts strong influence to the metal‐metal‐to‐ligand charge transfer (MMLCT) transition character. Theoretical calculations indicate that Pt···Pt contacts become more relevant in the trimers rather than the dimers, especially in their T1 states, associated with a change from mixed 3LC/3MLCT transition in the monomer/dimer to mixed 3LC/3MMLCT transition character in the trimer. Electroluminescence devices affording intense deep‐red/NIR emission (near 670 nm) with unprecedentedly high external quantum efficiency over 30% are demonstrated. This work provides deep insights into formation MMLCT transition of square‐planar Pt(II) complexes and efficient molecular design for deep‐red/NIR electroluminescence.

An Efficient Cyan Emission from Copper (II) Complexes with Mixed Organic Conjugate Ligands

“…To date, a variety of noble metal complexes have been developed (mainly Au, Ag, and Pt) [ 10 , 11 , 12 , 13 ] due to their chemical stability and facile synthetic procedures. However, the commercial development of noble metal ions requires further consideration because of their toxic environmental impact.…”

Extended ligand conjugation and dinuclearity as a route to efficient platinum-based near-infrared (NIR) triplet emitters and solution-processed NIR-OLEDs

“…For example, the planar geometry of d 8 Pt(II) complexes may allow the formation of emissive excimers or aggregates through face-to-face interactions: [22][23][24][25] their red-shifted emission has been shown to afford a promising strategy for obtaining efficient deep red / NIR OLEDs. [26][27][28][29] Such approaches do, however, require appropriate intermolecular interactions to be present, typically necessitating somewhat higher dopant concentrations. Unimolecular emission, yet involving two Pt(II) centres, is shown in this contribution to offer an attractive alternative.…”

Advances in solution-processed near-infrared light-emitting diodes

“…Owing to the short stacking distance in these aggregates, a strong interaction among HOMO and LUMO levels was achieved, which strongly prohibited non-radiative deactivation paths through exciton-optical phonon coupling according to the energy gap law. More recently, the same group demonstrated that delocalized excitonic aggregates can decouple the exciton band from highly vibrational ladders in the ground state, thus further enhancing the φ PL in the NIR region 40,47 . On this basis, they successfully fabricated NIR-OLEDs achieving a high EQE of 2.18% in the NIR region of 890-930 nm 40 ; added to the merits, these devices exhibited negligible efficiency roll-off for J up to 500 mA cm -2 .…”