Triplet‐Triplet Annihilation Upconversion from Ru(II) Phenanthroline Complexes and 2‐Substituted Anthracene Derivatives

Two Ru(II) phenanthroline complexes (Ru-ph and Ru-py) were synthesized and their photophysical properties were systematically investigated. For Ru-py, the conjugation of pyrene at 3- and 8-positions of auxiliary ligand 1,10-phenanthroline can increase the phosphorescence quantum yield, prolong the triplet lifetime and reduce the triplet energy level. The extended triplet lifetime of Ru-py was revealed by using the density function theory calculations, which is originated from the triplet state equilibrium between the metal-to-ligand charge-transfer ((MLCT)-M-3) state and the intraligand ((IL)-I-3) state. The upconversion fluorescence of 2-substituted anthracene derivatives (DTACl, DTACN and DNACN) sensitized by Ru-ph and Ru-py in DMF was investigated under monochromatic 488 nm excitation with an ultralow power density of 40 mW/cm(2). Among them, Ru-py/DTACl presents the highest upconversion quantum yield (7.4 %). It is found that a small energy gap between the triplet states of sensitizer and emitter helps to increase upconversion quantum yield besides the high triplet-triplet annihilation efficiency and fluorescence radiation of emitter and the long triplet lifetime of sensitizer.