Exploring excimeric interactions in mono- and dinuclear cyclometallated platinum(II) complexes for deep-red/near-infrared emission

Abstract

The work described in this thesis is concerned with the development of new molecular materials that emit deep-red or near-infrared (NIR) light upon excitation. Molecules that emit efficiently in these regions of the electromagnetic spectrum are important in a variety of security, technological, and biological applications. The focus is on platinum-containing molecules, the key role of the metal being to enhance spin-orbit coupling and thus facilitate the formally forbidden emission from triplet states that is necessary to produce efficient devices. A range of platinum(II) complexes with cyclometallating ligands have been synthesised and their photophysical properties studied. The luminescence both from isolated molecules and from species comprised of two or more molecules has been evaluated, with the aim of establishing reliable strategies for tuning the emission further into the NIR.
Chapter 2 describes complexes featuring symmetric, tridentate N^C^N-coordinating ligands and one monodentate ligand, X. Organic light-emitting diodes (OLEDs) fabricated using some of these complexes as the emissive layer display NIR electroluminescence through the formation of aggregates. The identity of X is found to influence the aggregation: the change from X = Cl to I is accompanied by a 4-fold increase in the external quantum efficiency (EQE) of the device, whilst X = SCN red-shifts the emission to λELmax = 940 nm, apparently the longest-wavelength emission reported to date for a device employing a non-porphyrinic platinum(II) complex.
The work of Chapter 3 introduces related complexes featuring asymmetrically-substituted N^C^N ligands: it establishes that the wavelength of the unimolecular emission tends to follow that of the corresponding parent symmetric complex of lowest excited-state energy. Complexes of ligands that incorporate 3-substituted isoquinoline are found to have a particularly high propensity to excimer formation, whilst 4-trifluoromethylpyridine ligands lead to a red shift in the emission, both unimolecular and excimer/aggregate.
Chapters 4 and 5 focus on the strategy of covalently linking two complexes through a rigid xanthene scaffold, with a view to facilitating their face-to-face interaction. Complexes with tridentate and bidentate ligands are discussed in the two chapters, respectively. They mostly display broad, long-wavelength emission at all concentrations in solution, likely arising from intramolecular excimers, with impressive quantum yields ranging from 0.11 to 0.61. The interactions are further favoured using a second xanthene to generate “molecular squares” which display purely excimeric emission.
Finally, in Chapter 5, a series of novel dinuclear platinum compounds is presented that feature bidentate N^C-coordinating ligands. The emission wavelengths are influenced both by structural modifications of the N^C moiety and by intramolecular excimeric interactions, the most red-shifted of which has λPLmax of 777 nm.