title: Photophysical 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)(L(1)), (2) Ru(II)(bpy)(2)(L(2)) and (3) Ru(II)(bpy)(L(1))(L(2)), where bpy = 2,2'-bipyridyl, L(1) = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L(2) = 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 -NMe(2) functionality in L(2) and electron withdrawing catechol fragment in L(1) 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 (3)MLCT state by the low-emissive (3)LLCT 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 (3)MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the (3)MLCT 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.
authors.affiliation: Radiation and Photo Chemistry Division, Bhabha Atomic Research Center, Mumbai, India.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Verma
authors.forename: Prasenjit
authors.initials: P
authors.lastname: Kar
authors.forename: Amitava
authors.initials: A
authors.lastname: Das
authors.forename: Hirendra Nath
authors.initials: HN
authors.lastname: Ghosh
mesh_terms: D004563:Electrochemistry; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D055668:Photochemical Processes; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D013237:Stereoisomerism; D013696:Temperature; D013997:Time Factors
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D011725:Pyridines; D012428:Ruthenium