Theoretical study of excited states of pyrazolate- and pyridinethiolate- bridged dinuclear platinum(II) complexes: Relationship between geometries of excited states and phosphorescence spectra

摘要

Dinuclear platinum(II) complexes [Pt_2(μ-pz) _2(bpym)_2]~(2+) (1; pz = pyrazolate and bpym = 2,2′-bipyrimidine) and [Pt_2(μ-pyt)_2(ppy) _2] (2; pyt = pyridine-2-thiolate and Hppy = 2-phenylpyridine) were theoretically investigated with density functional theory (DFT) to clarify the reasons why the phosphorescence of 1 is not observed in the acetonitrile (CH_3CN) solution at room temperature (RT) but observed in the solid state at RT and why the phosphorescence of 2 is observed in both the CH _3CN solution and the solid state at RT. The S_1 and T _1 states of 1 in the CH_3CN solution are assigned as a metal-metal-to-ligand charge-transfer (MMLCT) excited state. Their geometries are C_(2v) symmetrical, in which spin-orbit interaction between the S_1 and T_1 excited states is absent because the direct product of irreducible representations of the singly occupied molecular orbitals (SOMOs) of these excited states and the orbital angular momentum (l) operator involved in the Hamiltonian for spin-orbit interaction does not belong to the a_1 representation. As a result, the S_1 → T _1 intersystem crossing hardly occurs, leading to the absence of T_1 → S_0 phosphorescence in the CH_3CN solution at RT. In the solid state, the geometry of the S_1 state does not reach the global minimum but stays in the C_1-symmetrical local minimum. This S_1 excited state is assigned as a mixture of the ligand-centered α-α~* excited state and the metal-to-ligand charge-transfer excited state. Spin-orbit interaction between the S_1 and T_1 excited states operates to induce the S _1 → T_1 intersystem crossing because the direct product of the irreducible representations of the SOMOs of these excited states and the l operator belongs to the "a" representation. As a result, T_1 → S_0 phosphorescence occurs in the solid state. In 2, the S_1 and T_1 excited states are assigned as the MMLCT excited state. Their geometries are C_2-symmetrical in both the CH_3CN solution and the solid state, in which spin-orbit interaction between the S_1 and T_1 states operates to induce the S _1 → T_1 intersystem crossing because the direct product of the irreducible representations of the SOMOs and the l operator belongs to the "a" representation. Thus, T_1 → S _0 phosphorescence occurs in both the CH_3CN solution and the solid state at RT, unlike 1.