Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 137
ured by transient absorption spectroscopy [3]), compared with a value of
about 1 µs exhibitedby[Ru(bpy)3] 2+ under the same conditions [1]. Such
a short excited-state lifetime is very disappointing, as [Ru(terpy)2] 2+ has
some advantage over [Ru(bpy)3] 2+ from a structural point of view. Whereas
[Ru(bpy)3] 2+ can exist as a mixture of Λ and ∆ isomers, and the isomer
problem can become even more complicated for polynuclear species based on
“asymmetric” bidentate ligands such as 2,3-bis(2 ′ -pyridyl)pyrazine (2,3-dpp),
[Ru(bpy)3] 2+ is achiral. Moreover, by taking advantage of para substituents
on the central pyridine of the terpy ligand, [Ru(terpy)2] 2+ can give rise to
supramolecular architectures perfectly characterized from a structural viewpoint,
in particular to multinuclear one-dimensional (“wire”-like) species.
The reason for the poor photophysical properties of Ru(II) complexes with
tridentate polypyridine ligands at room temperature, compared to Ru(II)
species with bidentate chelating polypyridine, stems from the bite angle of
the tridentate ligand that leads to a weaker ligand field strength and thus to
lower-energy MC states as compared to Ru(II) complexes of bpy. The thermally
activated process from the potentially emitting 3 MLCT state to the
higher-lying 3 MC state is therefore more efficient in [Ru(terpy)2] 2+ and its
derivatives and leads to fast deactivation of the excited state by nonradiative
processes [1, 3, 4], although terpy-type Ru complexes are inherently more
photostable than bpy-type ones because of a stronger chelating effect.
Much effort has been devoted to the design and synthesis of tridentate
polypyridine ligands, leading to Ru(II) complexes with improved photophysical
properties [3, 78, 92, 174–181]. For example, the use of ligands containing
electron-withdrawing and -donor substituents on tpy increases the
gap between the 3 MLCT and the 3 MC states [174]. An increase in such
an energy gap has also been obtained by the use of cyclometallating ligands
[177]. Unavoidably, the stabilization of 3 MLCT states causes an increase
of the rate constant for radiationless decay to the ground state. This latter
effect can be balanced by extension of the π ∗ orbital by appropriate substituents,
which increases the delocalization of the acceptor ligand of the
MLCT excited state leading to a smaller Franck–Condon factor for nonradiative
decay [78, 175, 176, 178, 179, 182–188]. In this regard, species based on
ethynyl-substituted terpy ligands feature particularly interesting photophysical
properties [175, 176, 182]. Various approaches to improve the photophysical
properties of Ru(II) complexes with tridentate polypyridine ligands have
been reviewed [175, 182].
The bis-tridentate Ru(II) polypyridine complex with the best photophysical
properties reported up to now is probably the species 1, based on
the 2,6-bis(8 ′ -quinolinyl)pyridine ligand [189]. This species exhibits 3 MLCT
emission with a maximum at 700 nm, with a lifetime of 3.0 µs and a quantum
yield of 0.02 in deoxygenated methanol–ethanol solution at room temperature.
The emission maximum blue-shifts to 673 nm at 77 Kinthesame
solvent mixture, exhibiting a luminescence lifetime of 8.5 µs and a quantum