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

Abstract

We present the synthesis and in-depth photophysical analysis of a di-Pt(ii) complex with a ditopic bis-NCN 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 (kSr), a smaller singlet-triplet energy gap (ΔEST) and a longer phosphorescence decay lifetime (τPH). 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, λel = 805 nm, in a solution-processed OLED device with EQEmax = 0.51%. We believe this is the highest efficiency reported to date for an excimer Pt(ii) emitter with λel > 800 nm in a solution-processed OLED device.