Biology Reference
In-Depth Information
10.3
Optical Sensing of Guest Molecules
by Chromogenic and/or Luminescent
Rhenium-Based Metallacyclic
Supramolecules
Introduction of chromogenic or luminescent properties is especially
interesting in metallacyclic supramolecular assemblies since that
can be used as an alternative to conventional NMR spectroscopy
and electrochemical detection of guest inclusion and is much more
sensitive to subtle changes in the geometry and medium environment
[63
−
66]. In addition, photoluminescence allows examination of
excited-state behavior of the host
−
guest complexation owing to
the changes in molecular structure, proton transfer, charge transfer,
and isomerization. Luminescent compounds with internal cavities
have been demonstrated as having potential applications in sensing
devices [23
69]. The metal-coordination directed assembly
such as rhenium squares
−
25,63
−
are emissive both in solution
and solid state with the luminescence originating from the triplet
MLCT transition, and their square cavity sizes range from 3 Å (
3a, 3b,
and
3f
3a
) to
14 Å (
43]. One general and particular interesting feature in
these Re(I)-based directed assembly cyclophanes is their tendency
to form one-dimensional open channels not only in the macroscopic
single crystals but also in the microcrystalline thin films [41,67].
As shown in several examples in this chapter, the induction of
nanometer-scale porosity and permeability could provide significant
implications in terms of selective molecular sensing and related
applications.
In an extension of similar luminescent Re(I)-based cyclophanes
reported by Sun and Lees [41], square
3f
) [39
−
was shown to be an
effective probe for molecular sensing of nitroaromatics
3d
surrogating
trinitrotoluene (TNT), an explosive. A series of nitro-substituted
aromatic compounds have been found to effectively quench the thin-
film luminescence of molecular square
−
3d
but exhibit little effect on
. The quenching phenomenon
was attributed to the porosity that exists in the film of square
its corresponding corner molecule
15
,
which provides cavities for binding the quencher molecules. The
heterometallic molecular square,
3d
Cl and
Pd(dppp) (dppp = 1,3-bis(diphenylphosphino)propane) corners
and with bridging 4,4
6,
with alternating Re(CO)
3
′
-bipyridine reported by Hupp
et al.
showed
emission from the
3
MLCT state at 610 nm in acetone solution [49]. It
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