Bonding, charge transfer and aggregation of luminescent Platinum complexes at metallic interfaces

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The realization of cheap, printable optoelectronic devices on flexible substrates requires a fundamental understanding of molecular aggregation and interactions with interfaces. Pt(II) complexes constitute a class of triplet emitters exhibiting a square planar coordination geometry that favors the coupling of d-orbitals in phosphorescent aggregates, resulting in tunable colors for organic light-emitting diodes (OLEDs). We have recently developed a strategy to obtain flat Pt(II) complexes that can be sublimed and deposited on metal substrates forming 2D monolayers. With the aid of scanning tunneling microscopy (STM) and spectroscopy (STS) as well as density functional theory (DFT), we explored structural and electronic properties at the Au(111) interface with submolecular resolution. Chemical substitution at defined positions enabled the realization of deep-blue phosphors. We will now use tetradentate luminophores providing the ultimate stability required for optoelectronic devices, irrespective of the processing method. The phosphorescent aggregates will be tuned from blue to red by finding the sub- and intermolecular electronic set-screws, i.e. suitable ligands and substituents. Intermolecular interactions will be controlled by varying the electron density at the metal center and the bulk of the substituents. Chemical synthesis and integral spectroscopic studies will be guided by a range of static and dynamic DFT-based methods for the prediction of stabilities and excited state properties. The formation and stability of aggregates as well as their luminescent properties will be investigated as a function of temperature in solution and for different substrates by ab inito simulation. The best candidates will be used for the realization of solution-processed and vapor-deposited OLEDs. When blue-green emitters stack into red clusters, white OLEDs can be realized by adjusting the degree of aggregation. As charge injection can occur via direct coupling of d-orbitals from the first layer with the electrodes, simplified devices can be envisaged, provided that layers of polychromatic phosphorescent aggregates are formed and directly electrically excited on the surface of electrodes. Thus, mono-, bi- and multilayers of stacked complexes will be investigated regarding their electronic properties, conductivity and (electro-)luminescence at conductive interfaces. Employing STM and STS, local coupling of the planar emitters with the underlying metallic surface via protruding orbital lobes will be correlated with charge injection phenomena towards simplified OLED arrays. STM-induced electroluminescence and photomapping with spatial, spectral and temporal resolution will enable a detailed understanding of the individual complexes in aggregates and in multilayers.