ABSTRACT
Orienting light-emitting molecules relative to the substrate is an effective method to enhance the optical outcoupling of organic light-emitting devices. Platinum(II) phosphorescent complexes enable facile control of the molecular alignment due to their planar structures. Here, the orientation of Pt(II) complexes during the growth of emissive layers is controlled by two different methods: modifying the molecular structure and using structural templating. Molecules whose structures are modified by adjusting the diketonate ligand of the Pt complex, dibenzo-(f,h)quinoxaline Pt dipivaloylmethane, (dbx)Pt(dpm), show an ≈20% increased fraction of horizontally aligned transition dipole moments compared to (dbx)Pt(dpm) doped into a 4,4'-bis(N-carbazolyl)-1,1'-biphenyl, CBP, host. Alternatively, a template composed of highly ordered 3,4,9,10-perylenetetracarboxylic dianhydride monolayers is predeposited to drive the alignment of a subsequently deposited emissive layer comprising (2,3,7,8,12,13,17,18-octaethyl)-21H,23H-porphyrinplatinum(II) doped into triindolotriazine. This results in a 60% increase in horizontally aligned transition dipole moments compared to the film deposited in the absence of the template. The findings provide a systematic route for controlling molecular alignment during layer growth, and ultimately to increase the optical outcoupling in organic light-emitting diodes.
ABSTRACT
Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more-earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.
ABSTRACT
Understanding molecular interactions with monolayers and bilayers of graphene and its derivatized forms is very important because of their fundamental role in gas sensing and separation, gas storage, catalysis, etc. Herein, motivated by the recent realization of graphene-based sensors for the detection of single gas molecules, we use density functional theory to study the noncovalent interactions of molecules and molecular clusters with graphene, graphene oxide, and graphane, which are represented by coronene-based molecular model systems, C24H12 (coronene), C24OH12 (coroepoxide), and C24H36 (perhydrocoronene), respectively. The objective is to understand the structural and energetic changes that occur as a result of adsorption on monolayers and intercalation within bilayers. To begin with, the interactions of coronene, coroepoxide, and perhydrocoronene with a variety of small molecules like HF, HCl, HBr, H2O, H2S, NH3, and CH4 are studied. Subsequently, the binding of coronene and coroepoxide substrates with molecular clusters of HF, H2O, and NH3 is studied to understand the strength of adsorption on the substrates and the effect of substrates on hydrogen-bonding interactions within the molecular clusters. Further, bilayers of the model systems, namely, coronene-coronene, coronene-coroepoxide, and two configurations of coroepoxide-coroepoxide (one in which the oxygen atoms are facing each other and the other in which they do not face each other) are generated. The energetics for the nanoscale confinement or intercalation of the clusters within the bilayers along with the impact of the intercalation on the intermolecular hydrogen-bonding interactions are investigated. Our coronene-based model systems can provide a simple way of describing the rather complex events that occur in representative regions of graphene-based heterogeneous substrates.
