Blue Phosphorescent trans-N-Heterocyclic Carbene Platinum Acetylides. Dependence on Energy Gap and Conformation

A series of 11 complexes of the type trans-(NHC)2Pt(CC-Ar)2 (where NHC = N-heterocyclic carbene) have been synthesized and their photophysics characterized. The complexes display moderately efficient deep blue to green phosphorescence from a triplet excited state that is localized mainly in the aryl acetylide ligand (CC-Ar). The emission energy varies with the substituent on CC-Ar, with the highest energy emission for Ar = 4-pyridyl. The emission quantum efficiency and lifetime for the series decreases with increasing emission energy (Eem), and the effect is identified as arising from an increase in the nonradiative decay rate (knr) with Eem. Temperature-dependent emission lifetime studies for three complexes give activation energies for the nonradiative decay process ∼1000 cm–1, and the thermally activated decay process is attributed to crossing to a nonemissive metal-centered (d–d) excited state. At a low temperature, two different emission progressions are observed. Density functional theory calculatio...