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 T-1 states from the (MLCT)-M-3, hybridized (3)(MLCT/LC) to (LC)-L-3 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 T-1. Moreover, the solution-processed device of Pt(czpyOczpy-OMe), featuring a (LC)-L-3 transition, shows the best color purity of blue light. Compared to the device of Pt(czpyOczpy) with (MLCT)-M-3 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/m(2). Therefore, this work gives a mechanistic interpretation of the phosphorescent properties of tetradentate Pt(II) complexes derived from the manageable lowest triplet excited states.