Charge-transfer excited states in phosphorescent organo-transition metal compounds: a difficult case for time dependent density functional theory?

Light emitting organo-transition metal complexes have found widespread use in the past. The computational modelling of such compounds is often based on time-dependent density functional theory (TDDFT), which enjoys popularity due to its numerical efficiency and simple black-box character. It is well known, however, that TDDFT notoriously underestimates energies of charge-transfer excited states which are prominent in phosphorescent metal–organic compounds. In this study, we investigate whether TDDFT is providing a reliable description of the electronic properties in these systems. To this end, we compute 0–0 triplet state energies for a series of 17 pseudo-square planar platinum(II) and pseudo-octahedral iridium(III) complexes that are known to feature quite different localization characteristics ranging from ligand-centered (LC) to metal-to-ligand charge transfer (MLCT) transitions. The calculations are performed with conventional semi-local and hybrid functionals as well as with optimally tuned range-separated functionals that were recently shown to overcome the charge transfer problem in TDDFT. We compare our results against low temperature experimental data and propose a criterion to classify excited states based on wave function localization. In addition, singlet absorption energies and singlet–triplet splittings are evaluated for a subset of the compounds and are also validated against experimental data. Our results indicate that for the investigated complexes charge-transfer is much less pronounced than previously believed

Coronary arterial fistulas

ABSTRACT: A coronary arterial fistula is a connection between one or more of the coronary arteries and a cardiac chamber or great vessel. This is a rare defect and usually occurs in isolation. Its exact incidence is unknown. The majority of these fistulas are congenital in origin although they may occasionally be detected after cardiac surgery. They do not usually cause symptoms or complications in the first two decades, especially when small. After this age, the frequency of both symptoms and complications increases. Complications include 'steal' from the adjacent myocardium, thrombosis and embolism, cardiac failure, atrial fibrillation, rupture, endocarditis/endarteritis and arrhythmias. Thrombosis within the fistula is rare but may cause acute myocardial infarction, paroxysmal atrial fibrillation and ventricular arrhythmias. Spontaneous rupture of the aneurysmal fistula causing haemopericardium has also been reported. The main differential diagnosis is patent arterial duct, although other congenital arteriovenous shunts need to be excluded. Whilst two-dimensional echocardiography helps to differentiate between the different shunts, coronary angiography is the main diagnostic tool for the delineation of the anatomy. Surgery was the traditional method of treatment but nowadays catheter closure is recommended using a variety of closure devices, such as coils, or other devices. With the catheter technique, the results are excellent with infrequent complications. DISEASE NAME AND SYNONYMS: Coronary arterial fistulas Coronary arterial fistulas or malformation

The triplet state of fac-Ir(ppy)3

The emitting triplet state of fac-Ir(ppy)₃ (fac-tris(2-phenylpyridine)iridium) is studied for the first time on the basis of highly resolved optical spectra in the rande of the electronic 0-0 transitions. For the compound dissolved in CH₂Cl₂ and cooled to cryogenic temperatures, three 0-0 transitions corresponding to the triplet substates I, II, and III are identified. They lie at 19693 cm⁻¹ (507.79 nm, I ―› 0), 19712 cm⁻¹ (507.31 nm, II ―› 0), and 19863 cm⁻¹ (503.45 nm, III ―› 0). From the large total zero-field splitting (ZFS) of 170 cm⁻¹, the assignment of the emitting triplet term as a ³MLCT state (metal-to-ligand charge transfer) is substantiated, and it is seen that spin-orbit couplings to higher lying ¹,³MLCT states are very effective. Moreover, the studies provide emission decay times for the three individual substates of τ(I) = 116 μs, τ(II) = 6.4 μs, and τ(III) = 200 ns. Further, group-theoretical considerations and investigations under application of high magnetic fields up to B = 12 T allow us to conclude that all three substates are non degenerate and that the symmetry of the complex in the CH₂Cl₂ matrix cage is lower than C₃. It follows that the triplet parent term is of ³A character. Studies of the emission decay time and photoluminescence quantum yield, φ(PL) of Ir(ppy)₃ in poly(methylmethacylate) (PMMA) in the temperature range of 1.5 =< T =< 370 K reveal average and individual radiative and nonradiative decay rates and quantum yields of the substates. In the range 80 =< T =< 370 K, φ(PL) is as high as almost 100 %. The quantum yield φ(PL) drops to ~88 % when cooled to T = 1.5 K. The investigations show further that the emission properties of Ir(ppy)₃ depend distinctly on the complex's environment or the matrix cage according to distinct charges of spin-orbit coupling effectiveness. These issues also have consequences for optimizations of the material's properties if applied as an organic light-emitting diode (OLED) emitter

The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs

Based on a very comprehensive set of experimental data and on theoretical models, an understanding of the triplet state properties of organo-transition metal compounds is worked out. Important trends and guidelines for controlling photophysical properties are revealed. In this respect, we focus on spin–orbit coupling (SOC) and its importance for radiative as well as for nonradiative transitions between the lowest triplet state and the electronic ground state. Moreover, as is discussed on the basis of an extensive data set, summarized for the first time, the efficiency of SOC also depends on the geometry of a complex. The investigations are exemplified and supported by instructive case studies, such as efficient blue and very efficient green and red emitters. Additionally, trends being important for applications of these compounds as emitters in OLEDs are worked out. In particular, the properties of the emitters are discussed with respect to the harvesting of singlet and triplet excitons that are generated in the course of the electroluminescence process. The well-known triplet harvesting effect is compared to the recently discovered singlet harvesting effect. This latter mechanism is illustrated by use of a blue light emitting Cu(I) complex, which represents an efficient fluorescent emitter at ambient temperature. By this mechanism, 100% of the generated singlet and triplet excitons can, at least in principle, be harvested by the emitting singlet state. Potentially, this new mechanism can successfully be applied in future OLED lighting with a distinctly reduced roll-off of the efficiency

Triplet state properties of a red emitting [Pt(s-thpy)(acac)] compound

A photophysical characterization based on optical high-resolution spectra and emission decay properties at low temperatures and at high magnetic fields is carried out for [Pt(s-thpy)(acac)] (s-thpy = 5,2-bis(2-thienyl)pyridinate and acac = acetylacetonate). The electronic 0-0 transition between the lowest triplet state and the ground state lies at 16150 cm⁻¹ (619 nm). The zero-field splitting (ZFS) of T₁ is smaller than 1 cm⁻¹. Thus, the emitting excited state is mainly of ligand-centered ³LC(³ππ*) character and experiences only weak spin-orbit couplings to higher lying singlet states. The compound does not fulfill important requirements for OLED applications, but strategies for improvements are pointed out

Triplet state properties of the OLED emitter Ir(btp)₂(acac) - Characterization by site-selective spectroscopy and application of high magnetic fields

The well-known red emitting complex Ir(btp)₂(acac) (bis(2-(2'-benzothienyl)-pyridinato N,C³')iridium(acetylacetonate)), frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH₂Cl₂ matrix. The studies were carried out under variation of temperature down to 1.2 K and at magnetic fields up to B = 10 T. Highly resolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. For the preferentially investigated site (I ―› 0 at 16268 cm⁻¹), the three substates I, II, and III of the T₁ triplet state are separated by ΔE(II-I) = 2.9 cm⁻¹ and ΔE(III-I) = 25.0 cm⁻¹, respectively. ΔE(III-I) represents the total zero-field splitting (ZFS). The individual decay times of these substates are τ(I) = 150 μs, τ(II) = 58 μs, and τ(III) = 2 μs, respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for relaxation from state II to state I (spin-lattice relaxation, SLR) is as long as 22 μs at T = 1.5 K, while the thermalization between the two lower lying substates and substate III is fast. Application of a magnetic field induces Zeeman mixing of the substates of T₁, resulting in an increased splitting between the two lower lying substates from 2.9 cm⁻¹ at zero field to, for example, 6.8 cm⁻¹ at B = 10 T. Further, the decay time of the B-field perturbed lowest substate I(B) decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that the three investigated states belong to the same triplet parent term of one single site. Other sites show a similar behavior, though the values of ZFS vary between 15 and 27 cm⁻¹. Since the amount of ZFS reflects the extent of MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T₁ is a ³LC (ligand centered) state with significant admixtures of ¹,³MLCT (metal-to-ligand charge transfer) character. Interestingly, the results show that the MLCT perturbation is different for the various sites. An empirical correlation between the amount of ZFS and the compound’s potential for its use as emitter material in an OLED is presented. As a rule of thumb, a triplet emitter is considered promising for application in OLEDs, if it has a ZFS larger than about 10 cm⁻¹

Photophysical properties of Re(pbt)(CO)₄ studied by high resolution spectroscopy

Photophysical properties of Re(pbt)(CO)₄ are investigated at cryogenic temperatures and high magnetic fields. The highly resolved spectra show that the zero field splitting of the lowest triplet T₁ into three substates is smaller than 2 cm⁻¹. With this result, the T₁ state can be classified as only slightly MLCT-perturbed ³LC (³ππ*) state. Consistently, spin-lattice relaxation times are slow at T = 1.2 K and emission decay times with τ(I) = 960 μs, τ(II) = 320 μs, and τ(III) = 24 μs are long due to only small singlet admixtures. The vibrational satellite structures observed reflect different vibrational deactivation mechanisms and reveal similar geometries of the emitting triplet and the ground state