Photochemistry and Photophysics of Coordination Compounds

122 S. Campagna et al.
From the above discussion, it is clear that the excited-state properties of
a complex are related to the energy ordering of its low-energy excited states
and, particularly, to the orbital nature of its lowest excited state. The energy
positions of the MC, MLCT, and LC excited states depend on the ligand field
strength, the redox properties of metal and ligands, and intrinsic properties
of the ligands, respectively [1, 2, 6]. Thus, in a series of complexes of the same
metal ion, the energy ordering of the various excited states, and particularly
the orbital nature of the lowest excited state, can be controlled by the choice of
suitable ligands [1, 2, 5, 6]. It is therefore possible to design complexes having,
at least to a certain degree, desired properties.
For most Ru(II) polypyridine complexes, the lowest excited state is
a 3 MLCT level (or, better, a cluster [6] of closely spaced 3 MLCT levels, see
later) which undergoes relatively slow radiationless transitions and thus exhibits
relatively long lifetime and intense luminescence emission. Such a state
is obtained by promoting an electron from a metal πM orbital to a ligand π∗ L
orbital (Fig. 1). The same π∗ L orbital is usually involved in the one-electron
reduction process. For a long time it has been discussed whether in homoleptic
complexes the emitting 3 MLCT state is best described with a multichelate
ring-delocalized orbital (Fig. 4a) or a single chelate ring-localized orbital with
a small amount of interligand interaction (Fig. 4b) [20]. This problem has
been tackled with a variety of techniques on both reduced and excited complexes.
Compelling evidence for “spatially isolated” [21] redox orbitals has
been obtained from low-temperature cyclic voltammetry [22, 23], electron
spin resonance [24], electronic absorption spectra of reduced species [25, 26],
nuclear magnetic resonance [27], resonance Raman spectra [28, 29], and
time-resolved infrared spectroscopy [30]. In the last 10 years, with the coming
into play of ultrafast spectroscopic techniques, it has also been possible to
investigate the nature of the Franck–Condon state and the rate constants of
the localization/delocalization processes, as well as the interligand hopping
(sometimes called “randomization of the excitation”) in the MLCT excited
state. These issues will be discussed in more detail later.
Fig. 4 Pictorial description of the electron promoted to the π ∗ L
delocalized orbital; b single chelate ring-localized orbital
orbital: a multichelate ring