Theoretical study of pyrazolate-bridged dinuclear platinum(II) complexes: Interesting potential energy curve of the lowest energy triplet excited state and phosphorescence spectra

摘要

Four kinds of 3,5-dialqpyrazolate(R(2)pz)-bridged dinuclear platinum(II) complexes [Pt-2(mu-R(2)PZ)(2)(dfPPY)(2)] (dfppy = 2-(2,4-difluorophenyl)pyridine; R(2)PZ = pyrazolate in 1, 3,5-dimethylpyrazolate in 2, 3-methyl-5-tert-butylpyrazolate in 3, and 3,5-bis(tert-butyl)pyrazolate in 4) were theoretically investigated by the DFT(B3PW91) method. The Stokes shift of their phosphorescence spectra was discussed on the basis of the potential energy curve (PEC) of the lowest energy triplet excited state (T-1). This PEC significantly depends on the bulkiness of substituents on pz. In 1 and 2, bearing small substituents on pz, one local minimum is present in the T, state besides a global minimum. The local minimum geometry is similar to the So-equilibrium one. The T, state at this local minimum is characterized as the,pi,pi* excited state in dfppy, where the d pi orbital of Pt participates in this excited state through an antibonding interaction with the pi orbital of dfppy; in other words, this triplet excited state is assigned as the mixture of the ligand-centered pi,pi* excited and metal-to-ligand charge transfer excited state ((LC)-L-3/MLCT). The geometry of the T-1-global minimum is considerably different from the So-equilibrium one. The T, state at the global minimum is characterized as the triplet metal-metal-to-ligand charge transfer ((MMLCT)-M-3) excited state, which is formed by the one-electron excitation from the d sigma-d sigma antibonding orbital to the pi* orbital of dfppy. Because of the presence of the local minimum, the geometry change in the T, state is suppressed in polystyrene at room temperature (FIT) and frozen 2-methyltetrahydrofuran (2-MeTHF) at 77 K. As a result, the energy of phosphorescence is almost the same in these solvents. In fluid 2-MeTHF at FIT, on the other hand, the geometry of the T, state easily reaches the T-1-global minimum. Because the T-1-global minimum geometry is considerably different from the So-equilibrium one, the phosphorescence occurs at considerably low energy. These are the reasons why the Stokes shift is very large in fluid 2-MeTHF but small in polystyrene and frozen 2-MeTHF. In 3 and 4, bearing bulky tert-butyl substituents on pz, only the T-1-global minimum is present but the local minimum is not. The electronic structure of this T-1-global minimum is assigned as the (MMLCT)-M-3 excited state like 1 and 2. Though frozen 2-MeTHF suppresses the geometry change of 3 and 4 in the T, state, their geometries moderately change in polystyrene because of the absence of the T-1-local minimum. As a result, the energy of phosphorescence is moderately lower in polystyrene than in frozen 2-MeTHF. The T-1-global minimum geometry is much different from the So-equilibrium one in 3 but moderately different in 4, which is interpreted in terms of the symmetries of these complexes and the steric repulsion between the tert-butyl group on pz and dfppy. Thus, the energy of phosphorescence of 3 is much lower in fluid 2-MeTHF than in frozen 2-MeTHF like 1 and 2 but that of 4 is moderately lower; in other words, the Stokes shift in fluid 2-MeTHF is small only in 4.