Scanning‐Tunneling‐Spectroscopy‐Directed Design of Tailored Deep‐Blue Emitters

Abstract

Frontier molecular orbitals can be visualized and selectively set to achieve blue phosphorescent metal complexes. For this purpose, the HOMOs and LUMOs of tridentate Pt(II) complexes were measured using scanning tunneling microscopy and spectroscopy. The introduction of electron-accepting or -donating moieties enables independent tuning of the frontier orbital energies, and the measured HOMO-LUMO gaps are reproduced by DFT calculations. The energy gaps correlate with the measured and the calculated energies of the emissive triplet states and the experimental luminescence wavelengths. This synergetic interplay between synthesis, microscopy, and spectroscopy enabled the design and realization of a deep-blue triplet emitter. Finding and tuning the electronic "set screws" at molecular level constitutes a useful experimental method towards an in-depth understanding and rational design of optoelectronic materials with tailored excited state energies and defined frontier-orbital properties.

Correlating the Structural and Photo­physical Features of Pincer Luminophores and Monodentate Ancillary Ligands in PtII Phosphors

“…A comparison of the crystal structures reveals that Pt‐ L1 ‐P, Pt‐ L1 ‐NHC, Pt‐ L3 ‐P, and Pt‐ L4 ‐P display comparable features (Figure 1). The Pt II centers adopt distorted square‐planar geometries, and the most noticeable distortions are for the N pyrazole –Pt–N pyrazole bite angles of the tridentate ligand, which range from 157.5 to 158.5° and are comparable to the values reported previously 9c,14,15. The Pt–P bond lengths are almost invariable and within the expected range (2.242–2.253 Å).…”

“…In some cases, however, the formation of aggregates is undesirable, because it leads to a loss of color purity owing to the formation of excimers and exciplexes as bimolecular quenching processes such as triplet–triplet annihilation provide efficient nonradiative decay pathways 12. A widely accepted strategy to minimize such interactions is the functionalization of luminophores with bulky groups to prevent significant intermolecular contacts 13–15…”

Fluorination‐controlled Aggregation and Intermolecular Interactions in Pt(II) Complexes with Tetradentate Luminophores

“…However, a deeper understanding and a defined control of intermolecular interactions are still required, as stacking most frequently causes lowered solubility, undesired triplet−triplet annihilation and lowered quantum yields ,. We have recently reported on the use of bidentate and tridentate luminophores in Pt(II) complexes that are able to emit as blue or green monomeric species, due to the presence of bulky ancillary ligands preventing intermetallic interactions. On the other hand, aggregation‐induced phosphorescence was observed for planar luminophores and ancillary ligands stacking into bright aggregates in which Pt−Pt interactions play a significant role ,,.…”

“…showed that self‐assembly processes can be tracked dynamically by monitoring the luminescence of diverse aggregation states . We have also monitored intermetallic interactions by means of scanning tunneling micro(spectro)scopy, XPS and electrochemical measurements in solutions and at interfaces of (semi)conducting substrates; at metallic interfaces, hybridization of the metal centers with the substrates was observed upon 2‐D confinement and self‐assembly into ordered monolayers ,,…”

Fluorescent Heteroleptic Zirconium and Hafnium Complexes

“…Moreover, their emission properties were studied, including emission spectra and excited‐state lifetimes. The original intention for this study was to introduce heavy atoms, which are in principle capable of influencing the excited state transitions as has been demonstrated earlier for so‐called triplet‐harvesting complexes including examples such as tris(pyridylphenyl)iridium(II) Ir(ppy) 3 , osmium(II) polypyridine complexes, or a range of platinum complexes with extended heterocyclic ligands …”