Dual emissions from ruthenium(II) complexes having 4-arylethynyl-1,10-phenanthroline at low temperature

A [Ru(phen)3](2+) complex (phen = 1,10-phenanthroline) having an arylethynyl group at the 4-position of one of the three phen ligands (aryl: (dimesityl)borylduryl group = [RuBE](2+) or duryl group = [RuDE](2+)) showed dual emissions at low temperature in propylene carbonate (PC). The shorter emission lifetime components (τ(em)(s) ≈ 13 μs) that originated from the lowest-energy excited triplet states (T1) of the complexes were almost independent of temperature (T = 77-320 K), while the longer emission lifetime components (τ(em)(l)), as the T2 emissions appeared below the fluid-to-glass transition temperature (Tg ≈ 220 K) in PC, were almost constant at 27 μs in the range of T = 77-220 K. The τ(em)(s) components of the complexes were assigned to the emissions from the metal-to-ligand charge transfer (MLCT) excited states possessing relatively large ligand-centered (LC) excited-state characters (T1(MLCT/LC)), while the τ(em)(l) components were shown to be originating from the T2(MLCT) excited states. The T2(MLCT) states of the complexes became nonemissive above Tg in PC due to fast nonradiative decay through solvent reorganization around the T2(MLCT) excited states. The photophysical properties of the complexes were also shown to be characterized by the presence of the arylethynyl units at the 4-positions of the phen ligands.