Non-Stereogenic Dinuclear Ir(III) Complex with a Molecular Rack Design to Afford Efficient Thermally Enhanced Red Emission.

Cyclometalated complexes containing two or more metal centers were recently shown to offer photophysical properties that are advantageous compared to their mononuclear analogues. Here we report the design, synthesis, and luminescent properties of a dinuclear Ir(III) complex formed by a ditopic N^C^N-N^C^N bridging ligand (L1) with pyrimidine as a linking heterocycle. Two dianionic C^N^C terminal ligands were employed to achieve a charge-neutral and nonstereogenic dinuclear complex 5. This complex shows a highly efficient red emission with a maximum at λem = 642 nm as measured for a toluene solution. The decay time and emission quantum yield of the complex measured for the degassed sample are τ = 1.31 µs and ΦPL = 80%, respectively, corresponding to the radiative rate of kr = 6.11·105 s-1. This rate value is approximately fourfold faster than for the green-emitting mononuclear analogue 3. Cryogenic temperature measurements show that the three substrates of the lowest triplet state T1 of 5 emit with decay times of τ(I) = 120 µs, τ(II) = 7 µs, and τ(III) = 1 µs that are much shorter compared to those of the mononuclear complex 3, which has values of τ(I) = 192 µs, τ(II) = 65.6 µs, and τ(III) = 3.6 µs. These data indicate that the spin-orbit coupling of state T1 with the singlet states is much stronger in the case of complex 5, which results in a much higher T1 → S0 emission rate. Indeed, a computational analysis suggests that in the dinuclear complex 5 the T1 state is spin-orbit coupled with twice the number of singlet states compared to that of mononuclear 3, which is a result of the electronic coupling of two coordination sites. The investigation of the temperature dependence of the emission rates of 3 and 5 shows that the room-temperature emission of both complexes is mainly contributed by a thermally populated excited state lying above the T1 state. To the best of our knowledge, complexes 3 and 5 are the first examples of Ir(III) complexes that show photophysical behavior reminiscent of thermally activated delayed fluorescence (TADF).

1,2,4-Triazines in the Synthesis of Bipyridine Bisphenolate ONNO Ligands and Their Highly Luminescent Tetradentate Pt(II) Complexes for Solution-Processable OLEDs.

This article describes a convenient method for the synthesis of ONNO-type tetradentate 6,6'-bis(2-phenoxy)-2,2'-bipyridine (bipyridine bisphenolate, BpyBph) ligands and their platinum(II) complexes. The methodology includes the synthesis of 1,2,4-triazine precursors followed by their transformation to functionalized pyridines by the Boger reaction. Two complementary routes employing 3,3'- and 5,5'-bis-triazines allow a modification of the central pyridine rings in different positions, which was exemplified by the introduction of cyclopentene rings. The new ligands were used to prepare highly luminescent ONNO-type Pt(II) complexes. The position of the cyclopentene rings significantly influences the solubility and photophysical properties of these complexes. Derivatives with closely positioned cyclopentene rings are soluble in organic solvents and proved to be the best candidate for solution-processable organic light-emitting devices (OLEDs), showing efficient single-dopant candlelight electroluminescence.