Spectroscopic and Computational Investigations of the Temperature-Dependent Emission Behavior of [Re(CNx)₅Cl] and [Re(CNx)₆]⁺ Complexes

[Re(CNx)5Cl] and [Re(CNx)6](PF6) complexes form with the isocyanide ligand, 2,6-dimethylphenylisocyanide (CNx). [Re(CNx)5Cl] crystallizes in the space group P(2)1/c with a Re−Cl bond length of 2.5278(16) Å, a Re−C bond length trans to Cl of 1.937(6) Å, and Re−C bond lengths in the equatorial plane ranging from 1.998(6) to 2.034(6) Å. The complexes are highly emissive at 77 K with emission lifetimes of 5.1 and 4.6 μs, respectively, but are nonemissive at room temperature. Density functional theory (DFT) geometry optimizations of the ground and the triplet metal-ligand-to-ligand charge transfer (3MLLCT) states of the complexes in the gas phase place the energy of the 3MLLCT state for [Re(CNx)5Cl] 200 cm-1 higher than the experimental emission peak at 77 K. The energies of the singlet excited states of the two complexes calculated in ethanol using time-dependent density functional theory (TDDFT) and conductor-like polarizable continuum model (CPCM) deviate by less than 600 cm-1 from the corresponding UV−vis peaks in the same solvent. The TDDFT/CPCM calculation confirms the existence of 3d−d states that provide a nonradiative relaxation pathway accounting for loss of emission from the emitting state at room temperature. Calculations also reveal a singlet to triplet excitation at 35 700 cm-1 near the absorption of the CNx ligand at 35 200 cm-1 and a 3π→π* state of CNx 200 cm-1 higher than the experimental phosphorescence energy.[Re(CNx)₅Cl]与[Re(CNx)₆](PF₆)配合物由异氰配体2,6-二甲基苯基异氰（2,6-dimethylphenylisocyanide，缩写CNx）合成得到。[Re(CNx)₅Cl]以空间群P2₁/c结晶，其Re−Cl键长为2.5278(16) Å，与Cl原子对位的Re−C键长为1.937(6) Å，赤道平面内的Re−C键长范围为1.998(6) 至 2.034(6) Å。这两种配合物在77 K下均具有强发光特性，发射寿命分别为5.1 μs和4.6 μs，但在室温下无发射。密度泛函理论（Density Functional Theory, DFT）对气相中两种配合物的基态与三线态金属-配体到配体电荷转移（³MLLCT）态进行几何优化后发现，[Re(CNx)₅Cl]的³MLLCT态能量比其77 K下的实验发射峰高200 cm⁻¹。采用含时密度泛函理论（time-dependent density functional theory, TDDFT）结合导体极化连续介质模型（conductor-like polarizable continuum model, CPCM）在乙醇溶剂中计算得到的两种配合物的单重激发态能量，与同一溶剂中的对应紫外-可见（UV−vis）吸收峰偏差小于600 cm⁻¹。TDDFT/CPCM计算证实了3d−d态的存在，该态提供了非辐射弛豫路径，可解释室温下发射态的发光猝灭现象。计算还揭示了在35700 cm⁻¹处存在单重态到三重态的激发，该位置接近CNx配体在35200 cm⁻¹处的吸收；同时CNx配体的3π→π*态能量比实验测得的磷光能量高200 cm⁻¹。