The role of dinuclearity in promoting thermally activated delayed fluorescence (TADF) in cyclometallated, N^C^N-coordinated platinum(ii) complexes

Abstract

We present the synthesis and in-depth photophysical analysis of a di-Pt(II) complex with a ditopic bis-N^C^N ligand. The complex exhibits a dual luminescent behaviour by emitting simultaneously delayed fluorescence and phosphorescence. By comparing with the mono-Pt(II) analogue, we demonstrate that thermally activated delayed fluorescence (TADF) is turned on in the di-Pt(II) complex due to the occurrence of three main differences relative to the mono-Pt(II) analogue: a larger singlet radiative rate constant (k^S_r), a smaller singlet–triplet energy gap (ΔE_ST) and a longer phosphorescence decay lifetime (τ_PH). We observe similar trends among other di-Pt(II) complexes and conclude that bimetallic structures promote conditions favourable for TADF to occur. The diplatinum(II) complex also shows a long wavelength-emissive excimer which yields near infrared electroluminescence, λ_el = 805 nm, in a solution-processed OLED device with EQE_max = 0.51%. We believe this is the highest efficiency reported to date for an excimer Pt(II) emitter with λ_el > 800 nm in a solution-processed OLED device.