Tuning The Excited States Of Pt~Ⅱ Complexes For Their Application In Upconversion

Transition metal PtⅡ complexes are regarded as phosphorescent materials with excellent properties due to their abundant applications in OLED, oxygen sensors, photodynamic therapy, molecular sensors and solar photoelectric materials. Based on the strengths of phosphorescence such as long excited state lifetime and sensitivity to oxygen, a number of photoelectric materials and oxygen sensors are developed. In this work, to conveniently and effectively design the phosphorescent compounds, DFT/TDDFT methods were applied as assistant tools to calculate and predict the relationship between the optical properties and the electronic structure.As reported, modifications of the ligand are helpful to effectively tune the properties of Pt11complexes at the excited state. Upon the previous researches, fluorene as a linker was added between the ligands and PtⅡ center to achieve a series of PⅡ complexes. As expected, it is obvious that the absorption in the visible range was enhanced with a shift to the red-end(ε>105). Meanwhile, the triplet lifetime was found to be much longer which resulted from the3IL emission by the theoretical calculation (τPt-4=54.7μs). The complexes were used as triplet sensitizers for triplet-triplet annihilation (TTA) based upconversion and an upconversion quantum yield (φuc) up to22.4%was observed.In order to further the work, another group of PtⅡ complexes were synthesized by changing the ligands into naphthalimide and the linkers into carbazole or acetylene. Compared their photophysical and photochemical properties, the participation of fluorene and carbazole can all make the absorption bathochromic and the extinction coefficient increased. However, only fluorene as the linker can generate3IL emission to possess relatively long triplet lifetime (τPt-7=138.0μs) and high upconversion quantum yield (φuc=24.3%).Also, in this work, weakly phosphorescent or non-phosphorescent PtⅡ complexes as sensitizers for TTA based upconversion were studied. And it was found that these complexes can also be used as triplet sensitizers as long as they were easy to reach their triplet excited states and given suitable energy acceptors. This maybe broaden the scope for the research of triplet sensitizers.