Spin-orbit coupling routes and OLED performance: studies of blue-light emitting Ir(III) and Pt(II) complexes

Emissive or Nonemissive? A Theoretical Analysis of the Phosphorescence Efficiencies of Cyclometalated Platinum(II) Complexes

“…As such, < T a m j H SOC j S n > will be nonvanishing only when both the coupling singlet and triplet states have an MLCT character, provided that the 1 MLCT and 3 MLCT transitions have 1) the same unoccupied ligand p* orbital; 2) different occupied Pt d orbitals; and 3) the same symmetry of spin-orbit wavefunctions, since the H SOC operator is totally symmetric. [20,21,32] Thus, we are only interested in q j = Pt d occupied orbitals. Putting Equation (6) and (7) into < T a m j H SOC j S n > , the integrals reduce to < a 3MLCT c 3 d (dp*) j H SOC j a 1MLCT c MLCT transitions of the spin-orbit-coupled T m and S n excited states, respectively).…”

“…Recently, Yersin and coworkers proposed that closer proximity of the occupied d orbitals in quasi-octahedral transition metal complexes than in quasi-square-planar transition metal complexes is likely the reason why the former are usually better emitters than the latter. [20,21] In this work, we attempt to provide a theoretical basis to account for the luminescence properties of a family of square-planar platinum(II) complexes 1-5 with tridentate cyclometalated ligands, which have received widespread interest in recent years because 1) they are strongly emissive when the platinum(II) ion is coordinated to appropriate auxiliary ligands; [15,16,22] 2) their emission intensities are sensitive to their local environments; [17,[23][24][25] 3) their open square-planar geometries allow intermolecular interactions, which in turn significantly change the emission intensities and energies; [19,26] and 4) they have demonstrated usefulness in optoelectronic device applications such as high performance white-light and near-infrared organic lightemitting diodes, as well as luminescent probes for signaling studies. [19,[27][28][29][30][31] …”

“…[37] d-orbital splittings: It has often been stated that the d-orbital splittings are critical in determining the phosphorescence efficiency of a transition metal complex. [20,21] This has been attributed to two different effects. On the one hand, large splitting between the occupied and unoccupied d orbitals (hereafter denoted d and d*, respectively) is desirable, so that the MC dd excited states are thermally inaccessible for emission quenching.…”

“…On the one hand, large splitting between the occupied and unoccupied d orbitals (hereafter denoted d and d*, respectively) is desirable, so that the MC dd excited states are thermally inaccessible for emission quenching. [14,21] On the other hand, strong SOC between the S n and T m excited states requires these two states to be close in energy [20,21] [or the energy ratio c n between these two states is preferably unity, see Eq. (3)].…”

“…In order to have nonvanishing SOC matrix elements, the 1,3 MLCT configurations contributing to the spin-orbit-coupled S n and T m excited states must involve different d orbitals (as mentioned in the Theoretical Background section). As the energy separation between the spin-orbit-coupled S n and T m excited states has been correlated to the energy difference between the occupied d orbitals, [20,21] it has been reasoned that the closer the different occupied d orbitals are (the energy difference between two different occupied d orbitals is denoted Ddd occ ), the larger the H SOC matrix element and the faster the radiative decay rate constant k r will be, according to Equation (3). Indeed, the energetically closer proximity of the occupied d orbitals in the quasi-octahedral d 6 Ir III complexes (the t 2g orbitals) compared with the quasisquare planar d 8 Pt II complexes (d s and d p orbitals) has been put forward as the reason why the former are usually more intense emitters than the latter.…”