Chemistry Reference
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cyanines in general and, depending on their substitution pattern, also asymmetric
cyanines) present resonant dyes. Typical for these fluorophores are slightly
structured, comparatively narrow absorption and emission bands, which often mirror
each other, and a small, almost solvent polarity-insensitive Stokes shift (Fig.
1d
)as
well as high molar absorption coefficients. For example for the best cyanine dyes,
e
M
10
5
M
1
cm
1
can be found. Commonly associated with a small
Stokes shift are high fluorescence quantum yields for dyes with rigid structures
emitting in the visible region (
values of 2-3
F
F
values of 0.80-1, e.g., rhodamines, fluoresceins,
and BODIPY dyes) and, in the case of near-infrared (NIR) chromophores, moderate
F
F
values of 0.1-0.2 (Table
1
). The small Stokes shift of these chromophores results
in a considerable spectral overlap between absorption and emission, that can be
disadvantageous for certain applications (see, e.g., Sects.
3.4
and
3.5
). CT dyes
such as coumarins or dansyl fluorophores are characterized by well-separated,
broader, and structureless absorption and emission bands at least in polar solvents
and a larger Stokes shift (Fig.
1f
). The molar absorption coefficients of CT dyes, and
in most cases, also their fluorescence quantum yields, are generally smaller than those
of dyes with a resonant emission. CT dyes show a strong polarity dependence of their
spectroscopic properties (e.g., spectral position and shape of the absorption and
emission bands, Stokes shift, and fluorescence quantum yield). Moreover, in the
majority of cases, NIR absorbing and emitting CT dyes reveal only low fluorescence
quantum yields, especially in polar and protic solvents. The spectroscopic properties
of resonant and CT dyes can be fine-tuned by elaborate design strategies if the
structure-property relationship is known for the respective dye class. Selection within
large synthetic chromophore library becomes popular. The chapter of Kim and Park
within these series [
152
] addresses the comparison of rational design and library
selection approaches.
2.1.3 Metal Ligand Complexes
The most prominent metal ligand complexes used in bioanalytics and life sciences are
ruthenium(II) complexes with ligands such as bipyridyl- or 1,10-phenenthroline
derivatives [
8
,
9
] followed by platinum(II) and palladium(II) porphyrins [
51
]. Ru(II)
coordination compounds absorb energy in the visible region of the spectrum (typically
excitable at, e.g., 488 nm) or in the NIR depending on the ligand [
52
]populatinga
metal-to-ligand charge transfer (
1
MLCT) state. Subsequent intersystem crossing
leads to quantitative population of the
3
MLCT state, which can be deactivated via
luminescence, nonradiative decay, or via population of a nonemissive metal- or
ligand-centered state. The most characteristic spectroscopic features of this class of
fluorescent reporters are broad, well-separated absorption and emission bands, mod-
erate luminescence quantum yields, and comparatively long emission lifetimes in the
order of a few 10 ns up to several hundred nanoseconds due to the forbidden nature of
the electronic transitions involved [
53
]. Platinum (II) and palladium(II) porphyrins,
that present, e.g., viable oxygen sensors, as well as other coordination compounds
such as iridium(II) complexes are not further detailed here. The spectral features of
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