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

Abstract

Figure 0: TOC Entry -Synopsis: A series of 17 platinum (II) and iridium (III) complexes have been investigated theoretically and experimentally to elucidate the charge-transfer character in emission from the lowest triplet state. Time-dependent density functional theory is found to be surprisingly accurate for metal-to-ligand transitions. ABSTRACTLight 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.

A new class of deep-blue emitting Cu(i) compounds – effects of counter ions on the emission behavior

“…For example, at ambient temperature the emission of [Cu(PPh 3 )tpym]PF 6 (1) peaks at 466 nm and the emission quantum yield amounts to 43 %. However, if this compound is doped into a polymer PMMA matrix the emission is 15 slightly red-shifted to 470 nm, but the quantum yield is drastically reduced to 7 %. In fluid dichloromethane solution, the compound is not emissive.…”

“…In addition, also the absorption spectra of the tpym and the PPh 3 ligands are shown. 15 All spectra were recorded under ambient conditions for the compounds dissolved in dichloromethane, except for PPh 3 for which the absorption was measured in acetonitrile 93 . The absorption spectra of compounds 1 -3 show similar spectral shapes.…”

“…Interestingly, for the powder samples with different counter anions one finds distinct differences in the emission properties. The spectra of the three compounds are clearly shifted 15 relative to each other with emission maxima lying at λ max (1) = 466 nm, λ max (2) = 449 nm, and λ max (3) = 452 nm ( Figure 5). Also, the emission quantum yields vary distinctly amounting to Φ PL (1) = 43 %, Φ PL (2) = 19 %, and Φ PL (3) = 7 %, respectively, at ambient temperature.…”

“…Vibrational frequency calculations confirm that all three optimized structures are minima on the potential energy surface. The data collection for the single crystal structure determinations was performed on a BRUKER D8 QUEST diffractometer by the X- 15 ray service department of the faculty of chemistry, University of Marburg. The D8-QUEST is equipped with a Mo-K α X-ray micro source (INCOTEC), a fixed chi goniometer and a PHOTON 100 CMOS detector.…”

“…14, 31-36, 38, 40, 43, 45, 54, 56-80 However, so far photophysical investigations were mainly focused on mononuclear complexes 10 with two bidentate ligands, 32,33,37,40,[70][71][72][77][78][79][80][81][82][83] mononuclear complexes with one mono and one bidentate ligand, 34,38,43,[63][64][65] and on dinuclear complexes in which the two copper centers are bridged by halides. 14,35,36,56,[59][60][61][62] To our knowledge only very few Cu(I) complexes with tripodal ligands have been studied with 15 regard to their photophysical properties. 54 35 prepared in a two-step synthesis.…”