Platinum(II) 2,4-di(2'-pyridyl)-6-(p-tolyl)-1,3,5-triazine complexes:: Synthesis and photophysics

Five platinum(II) complexes based on the 2,4-di(2'-pyridyl)-6-(p-tolyl)-1,3,5-triazine ligand (1-5) have been synthesized and characterized. The effects of the triazine-containing terdentate ligand on the photophysics and nonlinear transmission have been explored. The influence of the monodentate ligand, i.e. chloride, pentynyl, and phenylacetylide, and counteranion on the photophysical and nonlinear transmission characteristics of these complexes has also been systematically investigated. It is found that, due to the electron-deficient nature of the triazine ring and the extension of the pi system between the terdentate ligand and the 6-(p-tolyl) substituent, the metal-to-ligand charge transfer (MLCT) bands in both the UV-vis spectra ((1)MLCT) and the emission spectra ((3)MLCT) red-shift in comparison to those of the corresponding platinum(II) complexes with 2,2':6',2 ''-terpyridyl ligands. However, both the emission efficiency from the (3)MLCT state and the emission lifetime are significantly reduced. The different monodentate ligands cause a decrease of the energy of the (1)MLCT band, going down from the complex with a chloride ligand (1 and 2) to 3 and 4 with a pentynyl ligand to 5 with a phenylacetylide ligand. The chloride-containing complex (2) only shows a terdentate ligand (1)pi,pi* originated emission at 460 nm, while the pentynyl-containing complexes (3 and 4) exhibit an (3)MLCT-based emission at similar to 615 nm with a lifetime of approximately 180 ns. In contrast, complex 5 with the phenylacetylide ligand emits at both 460 and 622 nm. Among these five complexes, only 3 and 4 exhibit a moderate triplet transient absorption, with a positive band extending from 510 to 820 nm. The lifetimes of the transients are approximately 200 ns, which is tentatively assigned to the (3)MLCT excited state. Complexes 3-5 display a weak nonlinear transmission that is affected by the monodentate ligand and the counteranions. In addition, it is revealed that the counteranion has a negligible effect on the (1)MLCT state, whereas its effect on the (3)MLCT state is drastic, especially on the triplet excited-state absorption coefficient, the quantum yield of the triplet state formation, the quantum yield of the (3)MLCT emission, and the nonlinear transmission, as exemplified by complexes 3 and 4.