Triplet Excited-State Engineering of Phosphorescent Pt(II) Complexes

Three Pt(II) complexes, Pt(czpyOczpy), Pt(czpyOczpy-Me), and Pt(czpyOczpy-OMe), are designed to elucidate the inherent relationship between electronically excited-state and photo- and electroluminescent properties. These complexes showed a blue-shifted phosphorescence with a narrowing spectral profile, which are interrelated with the variation of T1 states from the 3MLCT, hybridized 3(MLCT/LC) to 3LC transition. This change is ascribed to the destabilization of LUMO energy levels on the pyridinyl site, leading to more electron distribution on the carbazolide unit in T1. Moreover, the solution-processed device of Pt(czpyOczpy-OMe), featuring a 3LC transition, shows the best color purity of blue light. Compared to the device of Pt(czpyOczpy) with 3MLCT character, the device of Pt(czpyOczpy-Me) with hybridized 3(MLCT/LC) exhibits improved color purity and external quantum efficiency (10.2%) at a luminance of 1000 cd/m2. Therefore, this work gives a mechanistic interpretation of the phosphorescent properties of tetradentate Pt(II) complexes derived from the manageable lowest triplet excited states.