Photophyical properties of ligand localized excited state in ruthenium(II) polypyridyl complexes: a combined effect of electron donor-acceptor ligand

Abstract

We have synthesized ruthenium(II) polypyridyl complexes (1) Ru(II)(bpy)₂(L₁), (2) Ru(II)(bpy)₂(L₂) and (3) Ru(II)(bpy)(L₁)(L₂), where bpy = 2,2'-bipyridyl, L₁ = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L₂ = 4-(N,N-dimethylamino-phenyl)-(2,2'-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the presence of electron donating -NMe₂ functionality in L₂ and electron withdrawing catechol fragment in L₁ ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is attributed to quenching of the ³MLCT state by the low-emissive ³LLCT state. The emission due to ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a new luminescence band which appeared in the 670-760 nm region. The LLCT excited state of complex 3 is populated either via direct photoexcitation in the LLCT absorption band (350-700 nm) or through internal conversion from the photoexcited ³MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the ³MLCT state in a time resolved emission study. The internal conversion to LLCT and ILCT excited states are observed to be as fast as ~200 ps and ~700 ps for complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand localized excited state of newly synthesized heteroleptic ruthenium(II) polypyridyl complexes.