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 (E_em), and the effect is identified as arising from an increase in the nonradiative decay rate (k_nr) with E_em. 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 calculations suggest that the triplet energy varies with the torsion of the aryl acetylide rings relative to the plane defined by the PtC₄ unit (where C = the carbon atoms bonded to Pt). The multiple emission is ascribed to emission from complexes differing with respect to the aryl acetylide ring torsion. Ultrafast transient absorption spectroscopy reveals a fast relaxation (∼5 ps) that may also be due to aryl acetylide ring torsional relaxation in the triplet excited state.