COMPARISON OF PHYSICAL AND PHOTOPHYSICAL PROPERTIES OF MONOMETALLIC AND BIMETALLIC RUTHENIUM(II) COMPLEXES CONTAINING STRUCTURALLY ALTERED DIININE LIGANDS
A. Macatangay et al., COMPARISON OF PHYSICAL AND PHOTOPHYSICAL PROPERTIES OF MONOMETALLIC AND BIMETALLIC RUTHENIUM(II) COMPLEXES CONTAINING STRUCTURALLY ALTERED DIININE LIGANDS, Inorganic chemistry, 35(23), 1996, pp. 6823-6831
The physical and photophysical properties of a series of monometallic,
[Ru(bpy)(2)(dmb)](2+), [Ru(bpy)(2)(BPY)](2+), [Ru(bpy)(Obpy)](2+) and
[Ru(bpy)(2)(Obpy)](2+), and bimetallic, [{Ru(bpy)(2)}(2)(BPY)](4+) an
d [{Ru(bpy)(2)}(2)(Obpy)](4+), complexes are examined, where bpy is 2,
2'-bipyridine, dmb is 4,4'-dimethyl-2,2'-bipyridine, BPY is 1,2-bis(4-
methyl-2,2'-bipyridin-4'-yl)ethane, and Obpy is 1,2-bis(2,2'-bipyridin
-6-yl)ethane. The complexes display metal-to-ligand charge transfer tr
ansitions in the 450 nm region, intraligand pi --> pi transitions at
energies greater than 300 nm, a reversible oxidation of the ruthenium(
II) center in the 1.25-1.40 V vs SSCE region, a series of three reduct
ions associated with each coordinated ligand commencing at -1.3 V and
ending at similar to-1.9 V, and emission from a (3)MLCT state having e
nergy maxima between 598 and 610 nm. The Ru-III/Ru-II oxidation of the
two bimetallic complexes is a single, two one-electron process. Relat
ive to [Ru(bpy)(2)(BPY)](2+), the Ru-III/Ru-II potential for [Ru(bpy)(
2)(Obpy)](2+) increases from 1.24 to 1.35 V, the room temperature emis
sion lifetime decreases from 740 to 3 ns, and the emission quantum yie
ld decreases from 0.078 to 0.000 23. Similarly, relative to [{Ru(bpy)(
2)}(2)(BPY)](4+), the Ru-III/Ru-II potential for [{Ru(bpy)(2)}(2)(Obpy
)](3+) increases from 1.28 to 1.32 V, the room temperature emission li
fetime decreases from 770 to 3 ns, and the room temperature emission q
uantum yield decreases from 0.079 to 0.000 26. Emission lifetimes meas
ured in 4:1 ethanol:methanol were temperature dependent over 90-360 K.
In the fluid environment, emission lifetimes display a biexponential
energy dependence ranging from 100 to 241 cm(-1) for the first energy
of activation and 2300-4300 cm(-1) for the second one. The smaller ene
rgy is attributed to changes in the local matrix of the chromophores a
nd the larger energy of activation to population of a higher energy dd
state. Explanations for the variations in physical properties are bas
ed on molecular mechanics calculations which reveal that the Ru-N bond
distance increases from 2.05 Angstrom (from Ru-II to bpy and BPY) to
2.08 Angstrom (from Ru-II to Obpy) and that the metal-to-metal distanc
e increases from similar to 7.5 Angstrom for [{Ru(bpy)(2)}(2)(Obpy)](4
+) to similar to 14 Angstrom for [{Ru(bpy)(2)}(2)(BPY)](4+).