David J. Giesen

E-Type Delayed Fluorescence of a Phosphine-Supported Cu2(μ-NAr2)2 Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs∥

A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S(1)-T(1) energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.

Interfacial chemistry of Alq3 and LiF with reactive metals

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure.

The MIDI! basis set for quantum mechanical calculations of molecular geometries and partial charges

We present a series of calculations designed to identify an economical basis set for geometry optimizations and partial charge calculations on medium-size molecules, including neutrals, cations, and anions, with special emphasis on functional groups that are important for biomolecules and drug design. A new combination of valence basis functions and polarization functions, called the MIDI! basis set, is identified as a good compromise of speed and accuracy, yielding excellent geometries and charge balances at a cost that is as affordable as possible for large molecules. The basis set is optimized for molecules containing H, C, N, O, F, P, S, and Cl. Although much smaller than the popular 6-31G* basis set, in direct comparisons it yields more accurate geometries and charges as judged by comparison to MP2/cc-pVDZ calculations.

Class IV charge models: A new semiempirical approach in quantum chemistry

SummaryWe propose a new criterion for defining partial charges on atoms in molecules, namely that physical observables calculated from those partial charges should be as accurate as possible. We also propose a method to obtain such charges based on a mapping from approximate electronic wave functions. The method is illustrated by parameterizing two new charge models called AM1-CM1A and PM3-CM1P, based on experimental dipole moments and, respectively, on AM1 and PM3 semiempirical electronic wave functions. These charge models yield rms errors of 0.30 and 0.26 D, respectively, in the dipole moments of a set of 195 neutral molecules consisting of 103 molecules containing H, C, N and O, covering variations of multiple common organic functional groups, 68 fluorides, chlorides, bromides and iodides, 15 compounds containing H, C, Si or S, and 9 compounds containing C-S-O or C-N-O linkages. In addition, partial charges computed with this method agree extremely well with high-level ab initio calculations for both neutral compounds and ions. The CM1 charge models provide a more accurate point charge representation of the dipole moment than provided by most previously available partial charges, and they are far less expensive to compute.

Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces

We have investigated the interfaces of aluminum on tris-(8-hydroxyquinoline) aluminum (Alq3) and aluminum on LiF/Alq3, using x-ray and ultraviolet photoemission spectroscopy (UPS). Aluminum appears to react destructively with Alq3 causing significant modification of the oxygen, nitrogen, and aluminum spectra. The well-defined UPS spectrum of Alq3 is quickly destroyed by very low coverages of aluminum. With only a 5 A layer of LiF on the Alq3, the reaction with aluminum is significantly suppressed. The Alq3 molecular orbital features in the UPS shift to higher binding energy but remain easily recognizable. In addition, a well-defined gap-state forms which is significantly different from that produced without LiF. Both the core-level spectra and the gap-state suggest that the Alq3 anion is formed in the presence of aluminum and LiF.

High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host

We report high-efficiency, low-voltage phosphorescent green and blue organic light-emitting diode (PHOLED) devices using mixed-host materials in the light-emitting layer (LEL) and various combinations of electron-injecting and electron-transporting layers. The low voltage does not rely on doping of the charge-transport layers. The mixed LEL architecture offers significantly improved efficiency and voltage compared to conventional PHOLEDs with neat hosts, in part by loosening the connection between the electrical band gap and the triplet energy. Bulk recombination in the LEL occurs within ∼10 nm of the interface with an electron-blocking layer. A “hole-blocking layer” need not have hole- or triplet-exciton-blocking properties. Optical microcavity effects on the spectrum and efficiency were used to locate the recombination zone. The effect of layer thickness on drive voltage was used to determine the voltage budget of a typical device. The behavior of undoped devices was investigated, and the electrolumines...

Highly efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes

We demonstrate highly efficient white and nonwhite hybrid organic light-emitting diodes (OLEDs) in which singlet and triplet excited states, generated in the recombination zone, are utilized by fluorescence and phosphorescence, respectively. The excited states are formed at a blue fluorescent light-emitting layer (LEL), and the triplets diffuse through a spacer layer to one or more phosphorescent LEL(s). A key feature enabling the triplet diffusion in such OLEDs is the use of a blue fluorescent emitter with triplet energy above, or not much below, that of the fluorescent host. Additional material properties required for triplet harvesting are outlined. At 1000 cd/m2 a blue and yellow harvesting OLED shows 13.6% external quantum efficiency, 3.8 V, 30.1 lm/W, and color characteristics suitable for display application. High-efficiency harvesting R+G+B white, and B+G and B+R nonwhite OLEDs are also demonstrated. The triplet-harvesting mechanism was verified in all devices by physical methods including spectra...

DENSITY FUNCTIONAL THEORY : EXCITED STATES AND SPIN ANNIHILATION

Abstract A spin-annihilation technique for a single determinant formed from Kohn-Sham orbitals is described for the calculation of excited state singlet energies. This procedure accurately calculates the 1 1 B u ← 1 1 A g vertical transition for s-trans-1,3-butadiene using the Becke-Lee-Yang Parr (BLYP) functional. For this system, extrapolating from triplet and 50:50 Bu state energies gives poor results at either the UHF or BLYP levels; spin-annihilated Hartree-Fock theory is similarly unsuccessful.

A hybrid quantum mechanical and empirical model for the prediction of isotropic 13C shielding constants of organic molecules

A method is presented that combines quantum mechanical shift calculations with empirical corrections to yield isotropic 13C nuclear magnetic resonance (NMR) shifts for organic molecules in good agreement with experiment. A comparison is made between shifts calculated using Hartree–Fock (HF), Moller–Plesset perturbation theory (MP2), and density functional theory (DFT). The absolute shifts calculated by these methods are translated into shifts relative to tetramethylsilane (TMS) using a simple empirical formula with parameters determined over a set of 37 small organic compounds. It is shown that DFT calculations using small basis sets correlate with experiments well enough that the empirical correction allows experimental shifts to be reproduced to within an RMS error of 4–5 parts per million (ppm). Carbons attached to chlorine, bromine, and iodine are treated with the same empirical corrections but with parameters of different values because of the lack of spin orbit corrections in the calculations; however, these carbons are predicted as accurately as other carbons in the data set. Two models are presented; one is applicable to very large molecules. The empirical corrections developed for these models can be used to predict shifts in a wide variety of organic molecules. One of the models is applied to a moderately sized dye molecule that contains an intramolecular hydrogen bond to demonstrate the utility of using an inexpensive quantum mechanics-based method over an empirical fragment-based method.

Coumarin-Based, Electron-Trapping Iridium Complexes as Highly Efficient and Stable Phosphorescent Emitters for Organic Light-Emitting Diodes

A new class of coumarin-based iridium tris-cyclometalated complexes has been developed. These complexes are highly emissive, with emission colors ranging from green to orange-red. Besides modification of ligand structures, color tuning was realized by incorporation of ligands with different electrochemical properties in a heteroleptic structure. The organic light-emitting diodes (OLEDs) using these compounds as emissive dopants are highly efficient and stable. Unlike other Ir(III) phosphorescent dopants, these coumarin-based Ir(III) dopants can effectively trap and transport electrons in the emissive layer.