Electroluminescence of tetradentate Pt(II) complexes: O^N^N^O versus C^N^N^O coordination

Alkylation of one of the phenolic hydroxyl groups in a silene-type tetradentate ligand changes the coordination mode from O^N^N^O to cyclometallating C^N^N^O type. The ligand was used to synthesise a new cyclometallated luminescent Pt(II) complex 2. While in solution the complex is poorly luminescent, in the solid state the emission is reinstated which allowed to evaluate the complex 2 as phosphorescent emitter in organic light emitting devices (OLEDs). 2 displays EQE = 9.1 % and maximum luminance of 9000 cd m-2 in a vacuum deposited device. We carried out comparative analysis of photo- and electroluminescence of complex 2 with O^N^N^O complex 1 and demonstrated that the similar luminescent properties of O^N^N^O and C^N^N^O complexes are rather coincidental as they display different excited state landscapes. Surprisingly, the two complexes display a dramatically different electrochemical behaviour with the O^N^N^O coordination leading to formation of a stable electropolymer, but C^N^N^O coordination fully preventing electropolymerisation.