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on the chemical sensors [20
22]. There appears, however, to be few
review articles that deals exclusively with the functional properties
of metallacyclic supramolecules, particularly with respect to their
sensing/recognition capability [23
−
−
25]. In this chapter, we highlight
the recent advances made in the photophysical and photochemical
properties of functional metallacyclic supramolecules and in
particular focus on their potential applications with respect to guest
encapsulation and chemical sensing.
10.2
Photophysical and Photochemical
Properties of Metal-Directed Macrocyclic
Complexes Incorporating Diimine Rhenium
Tricarbonyl Moieties
The rhenium(I)-based compounds offer several advantages for
elucidating the various excited-state properties of organometallic
complexes. As pointed out by Vogler and Kunkley, the photophysics
and photochemistry of rhenium complexes are rich, spanning eight
oxidation states from formal rhenium(0) (for example, Re
(CO)
)
2
10
to formal rhenium(VII) (for example MeReO
) [26]. In general, the
excited-state properties of diimine rhenium(I) tricarbonyl complexes
primarily occur through their lowest triplet excited states, owing
to rapid vibrational relaxation and intersystem crossing from the
upper singlet manifolds [27
3
29]. Thus, the nature of the lowest-
energy-acceptor ligands (either diimine ligands or bridging ligands)
plays a decisive role in determining the ultimate photophysical and/
or photochemical properties. Various excited states are generated,
depending on the relative energy levels of the metal and ligand
orbitals, as well as the extent of interaction between them. Many
mononuclear diimine rhenium(I) tricarbonyl complexes are highly
emissive (
−
F
em
−
= 0.001
0.5) and feature relatively long emission
lifetimes (10 ns to 1
s) in solution, owing to the existence of lowest
energy triplet metal-to-ligand charge transfer (MLCT) excited states
[27
µ
34]. In these cases, the decay of the lowest-lying MLCT-emitting
states is often primarily determined by an energy gap law effect
[30,31]. Another prominent feature of these complexes is the large
hypsochromic shift of their emission maxima when going from a fluid
environment to a rigid medium and is described as “luminescence
−
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