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 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.本研究合成了11个trans-(NHC)₂Pt(CC-Ar)₂型配合物（其中NHC为N-杂环卡宾，N-heterocyclic carbene），并对其光物理性质进行了表征。该系列配合物展现出中等效率的深蓝至绿色磷光，其发射源自主要定域于芳基乙炔基配体（CC-Ar）的三重激发态。发射能量随CC-Ar上的取代基变化，当Ar为4-吡啶基时发射能量最高。该系列配合物的发射量子效率与发射寿命均随发射能量（Eem）升高而降低，该现象源于非辐射衰变速率（knr）随Eem增大而提升。对3种配合物的温度依赖性发射寿命研究表明，非辐射衰变过程的活化能约为1000 cm⁻¹，该热激活衰变过程被归因于与非发射性金属中心(d-d)激发态发生势能面交叉。在低温条件下，可观测到两种不同的发射振动序列。密度泛函理论（DFT, Density functional theory）计算结果表明，三重态能量随芳基乙炔基环相对于PtC₄单元（其中C为与Pt键合的碳原子）所在平面的扭转角变化。该多重发射现象被归因于芳基乙炔基环扭转角不同的配合物的发射。超快瞬态吸收光谱（Ultrafast transient absorption spectroscopy）测试显示存在约5 ps的快速弛豫过程，该过程也可能源于三重激发态下芳基乙炔基环的扭转弛豫。