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Emitter orientation will play a major role in future applications of organic light-emitting diodes due to its strong impact on the efficiency of the devices. Up to now, determining the orientation of transition dipole moments required elaborate angular-dependent measurements of the light emission pattern. In this paper, we present a simplified and straightforward method to extract the emitter orientation from external quantum efficiency measurements. We demonstrate the validity of the method on three different dye-doped emitting systems. [ABSTRACT FROM AUTHOR]

A detailed examination was performed on photo-physical properties of phosphorescent cyclometalated (CN)Pt-(OO) complexes (ppy)Pt(dpm) (l), (ppy)Pt(acac) (l'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (CN = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]-quinoline (dbq); OO = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cydometalation were determined by comparison with X-ray data from model CAN ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (Eo-o = 479 nm, 1; Eo-o = 495 nm, 2; Eo-o = 470 nm, 3) with disparate radiative rate constants (kT,. = 1.4 × 105 s-1, 1; kT, = 0.10 × 105 s-1, 2; kT, = 2.6 × 105 s-1, 3). Zero-field splittings of the triplet states (ΔEIII-1, = 11.5 cm-1, 1'; ΔEIII-1 < 2 cm-1, 2; ΔEIII-1 = 46.5 cm-1, 3) were determined using high resolution spectra recorded in Shpol'skii matrices. The fact that the Eo-o energies do not correspond to the extent of πconjugation in the aromatic CN ligand is rationalized on the basis of structural distortions that occur upon cydometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the CN ligands in the triplet state alter the spin--orbit coupling in the complexes. [ABSTRACT FROM AUTHOR]

This study highlights the potential benefits of using terdentate over bidentate ligands in the construction of organometallic complexes as organic light-emitting diode (OLED) emitters offering better color purity, and explores in detail the molecular origins of the differences between the two. A pair of closely related platinum(II) complexes has been selected, incorporating a bidentate and a terdentate cyclometallating ligand, respectively, namely, Pt(4,6-dFppy)(acac) (1) {4,6-dFppy = 2-(4,6-difluorophenyl)pyridine metalated at C2 of the phenyl ring} and Pt(4,6-dFdpyb)Cl (2) {4,6-dFdpyb = 4,6-difluoro-1,3-di(2-pyridyl)benzene, metalated at C2 of the phenyl ring}. The emission properties over the range of temperatures from 1.2 to 300 K have been investigated, including optical high-resolution studies. The results reveal a detailed insight into the electronic and vibronic structures of the two compounds. In particular, the Huang-Rhys parameter S that serves to quantify the degree of molecular distortion in the excited state with respect to the ground state, though small in both cases, is smaller by a factor of 2 for the terdentate than the bidentate complex (S ≈ 0.1 and ≈0.2, respectively). The smaller value for the former reflects the greater degree of rigidity induced by the terdentate ligand, leading to a lesser contribution of intraligand Franck-Condon vibrational modes in the green spectral range of the emission spectra. Consequently, an enhanced color purity with respect to blue light emission results. The high rigidity and the short Pt-C bond in Pt(4,6-dFdpyb)Cl also serve to disfavor nonradiative decay pathways, including those involving higher-lying dd* states. These effects account for the greatly superior luminescence quantum yield of the terdentate complex in fluid solution, amounting to ψPL = 80% versus only 2% found for the bidentate complex. [ABSTRACT FROM AUTHOR]

This work descnbes the synthesis, crystal structure, and detailed photophysical studies of [n-Bu4N][Pt(4,6-dFppy)- (CN)2] (n-Bu = n-butyl, 4,6-dFppy = (4',6'-difluorophenyl)pyridinate). The material can easily be prepared in high yield and purity by the reaction of [Pt(4,6-dFppy)(H-4,6-dFppy)Cl], [n-Bu4N]Cl, and KCN in CH2CI2. Because of the bulky counterion jn-Bu4N], Pt-Pt interactions, which frequently lead to aggregate formation, are suppressed in the solid state. Thus, monomer emission is observed. The phosphorescence quantum yield of the neat powder amounts to ΦPL = 60% at ambient temperature and decays with 19 its. In tetrahydrofuran (THF) solution, on the other hand, the emission decay time is with 0.26 μs distinctly shorter, and the quantum yield is very low. By means of emission decay rime studies in frozen THF and investigabons of the highly resolved single crystal emission at 1.2 K, we can assign the emitting T1 state of the compound as being largely of ligand centered (3LC, 3ππ) character. The observed differences of the emission properties of the neat powder compared to the fluid solution are rationalized with an energy stabilization of quenching dd* states in solution because of molecular distortions and/or bond elongations. [ABSTRACT FROM AUTHOR]

Detailed photophysical studes of the emitting triplet state of the highly phosphorescent compound Pt(dpyb)Cl based on high-resolution optical spectroscopy at cryogenic temperatures are presented (dpyb n NC2N-onordinated 1,3-di(pyrtdylbenzene)). The results reveal a total zero-field splitting of the emitting triplet state T1 of 10 cm-1 and relatively short individual decay times for the two higher lying T1 substates II and Ill, while the decay time of the lowest substate I a distinctly longer. Further evidence for the assignment of the T1 substates is gained by emission measurements under high magnetic fields. Distinct differences are observed in the vibrational satellite structures of the emissions from the substates I and II, which are dominated by Herzberg-Teller and Franck-Condon activity, respectively. At T=1.2 K, the individual spectra of these two substates can be separated by time-resolved spectroscopy. For the most prominent Frarick-Condon active modes, Huang-Rhys parameters of S ≈ 0.1 can be determined, which are characteristic of very small geometry rearrangements between the singlet ground state and the triplet state T1. The similar geometries are ascribed to the high rigidity of the Pt(NCN) system which, unlike complexes incorporating bidentate phenytpyridine-type ligands and exhibiting similar metal-to-ligand charge transfer admixtures, cannot readily distort from planarity. The results provide new insight into strategies for optimizing the performance of platinum-based emitters for applications such as organic light-emitting diode (OLED) technology and imaging. [ABSTRACT FROM AUTHOR]

The sky-blue emitting compound lr(4,6-dFppy)[sub2](pic) (iridium(lll)bis[2-(4',6'-difluorophenyl)pyridinato-N,C[sup2']]-picolinate), commonly referred to as Flrpic and representing a well-known emitter material for organic light emitting diodes (OLEDs), has been investigated in detail by optical spectroscopy. Studies at temperatures from T = 1.5 K to T = 300 K were carried out in CH[sub2]CI[sub2] and tetrahydrofuran (THF). In CH[sub2]CI[sub2], two discrete sites were observed at cryogenic temperatures and studied by site-selective, high-resolution spectroscopy. The investigations reveal that the molecules located at the two sites exhibit distinctly different photophysical properties. For example, the three substates I, II, and Ill of the emitting triplet state T[sub1] of the low-energy site A show a distinctly larger zero-field splitting (ZFS) and exhibit shorter individual decay times than observed for the high-energy site B. The vibrational satellite structures in the emission spectra of the substates I(A) and I(B) exhibit clear differences in the ranges of metal-ligand (M-L) vibrations. For the compound studied in a polycrystalline THF host, giving only strongly inhomogeneously broadened spectra, the ZFS parameters and substate decay times vary in a similar range as observed for the two discrete sites in the CH[sub2]CI[sub2] matrix. Thus, the amount of ZFS, the emission decay times, and also the intensities of the M—L vibrational satellites are affected by the matrix cage, that is, the host environment of the emitting complex. These properties are discussed with respect to variations of spin-orbit coupling routes. In particular, changes of d-orbital admixtures, that is, differences of the metal-to-ligand charge transfer (MLCT) character in the emitting triplet, play an important role. The matrix effects are expected to be also of importance for Flrpic and other lr(lll) compounds when applied as emitters in amorphous OLED matrixes. [ABSTRACT FROM AUTHOR]

Irradiation into the dye-based absorption band of complexes (tBu2bipy)Pt(SR)2and (tBu2bipy)Pt(OR)2where R denotes a coumarine-based thiolate and alkoxolate substituent populates the same excited triplet state as is obtained by excitation into the much weaker (RX)2Pt→tBu2bipy (X = O, S) charge-transfer band. This paves the way toward more efficient photosensitizers. [ABSTRACT FROM AUTHOR]

We present a method to achieve a consistent, comprehensive efficiency analysis of fluorescent organic light-emitting diodes (OLEDs) showing non-isotropic emitter orientation and triplet-to-singlet up-conversion. Combining photoluminescence lifetime and external quantum efficiency measurements on OLEDs with varying cavity length allows for an independent determination of the radiative emitter efficiency under optical as well as electrical excitation. The difference clearly shows a significant enhancement of the singlet exciton fraction to more than 25% under electrical operation. Furthermore, the presented method does not require detailed information about the emitting system and is generally applicable for a comprehensive efficiency analysis of bottom-emitting OLEDs. [ABSTRACT FROM AUTHOR]

The complexes [Cu(I)(POP)(dmbpy)][BF4] (1) and [Cu(I)(POP)(tmbpy)][BF4] (2) (dmbpy = 4,4'-dimethyl-2,2'-bipyridyl; tmbpy = 4,4',6,6'-tetramethyl-2,2'-bipyridyl; POP = bis[2-(diphenylphosphino)-phenyl]ether) have been studied in a wide temperature range by steady-state and time-resolved emission spectroscopy in fluid solution, frozen solution, and as solid powders. Emission quantum yields of up to 74% were observed for 2 in a rigid matrix (powder), substantially higher than for 1 of around 9% under the same conditions. Importantly, it was found that the emission of 2 at ambient temperature represents a thermally activated delayed fluorescence (TADF) which renders the compound to be a good candidate for singlet harvesting in OLEDs. The role of steric constraints within the complexes, in particular their influences on the emission quantum yields, were investigated by hybrid-DFT calculations for the excited triplet state of 1 and 2 while manipulating the torsion angle between the bipyridyl and POP ligands. Both complexes showed similar flexibility within a ±10° range of the torsion angle; however, 2 appeared limited to this range, whereas 1 could be further twisted with little energy demand. It is concluded that a restricted flexibility leads to a reduction of nonradiative deactivation and thus an increase of emission quantum yield.

Phenyl-2,6-bis(oxazole) ligands have been explored for the synthesis of novel palladium(II) and platinum(II) pincer complexes. The materials were characterized by spectroscopic methods and by X-ray crystallography. Investigations of the photophysical properties revealed that the lowest triplet states of the materials are largely centred at the bis(oxazole) ligands. The platinum(II) compounds are moderately emissive in fluid solution at ambient temperature. Introduction of both strong donors and strong acceptors leads to a significant red shift of the emission. Due to the facile synthesis of bis(oxazole) based complexes with electronically tuneable oxazole moieties, these materials might be promising alternatives to the well-established phenyl-2,6-bipyridyl systems., (This journal is © The Royal Society of Chemistry 2011)

Irradiation into the dye-based absorption band of complexes ((t)Bu(2)bipy)Pt(SR)(2) and ((t)Bu(2)bipy)Pt(OR)(2) where R denotes a coumarine-based thiolate and alkoxolate substituent populates the same excited triplet state as is obtained by excitation into the much weaker (RX)(2)Pt→(t)Bu(2)bipy (X = O, S) charge-transfer band. This paves the way toward more efficient photosensitizers., (This journal is © The Royal Society of Chemistry 2011)

The syntheses, crystal structures, and detailed investigations of the photophysical properties of phosphorescent platinum(II) Schiff base complexes are presented. All of these complexes exhibit intense absorption bands with lambda(max) in the range 417-546 nm, which are assigned to states of metal-to-ligand charge-transfer ((1)MLCT) (1)[Pt(5d)-->pi*(Schiff base)] character mixed with (1)[lone pair(phenoxide)-->pi*(imine)] charge-transfer character. The platinum(II) Schiff base complexes are thermally stable, with decomposition temperatures up to 495 degrees C, and show emission lambda(max) at 541-649 nm in acetonitrile, with emission quantum yields up to 0.27. Measurements of the emission decay times in the temperature range from 130 to 1.5 K give total zero-field splitting parameters of the emitting triplet state of 14-28 cm(-1). High-performance yellow to red organic light-emitting devices (OLEDs) using these platinum(II) Schiff base complexes have been fabricated with the best efficiency up to 31 cd A(-1) and a device lifetime up to 77 000 h at 500 cd m(-2).