Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 52

Fig. 6 Typical PET probes (a) and representative fluorescence light-up responses toward selected
metal ions in tabulated (b) and graphical form (c; trace 1 ¼ 14, trace 2 ¼ 14-(Zn 2+ ) 2 , trace
DMA ¼ 9,10-dimethylanthracene in MeCN). Color code: coordinating atoms in blue , atoms
which take part in the complexation and show (main, in 14) PET activity in orange , fluorophore
in green . Lincoln and co-workers have demonstrated that the attachment of two dimethylamino
groups through propylene spacers to the 9,10-positions of anthracene has a more than 100-fold
weaker PET activity than the attachment through methylene spacers [ 62 ]. The blue N atoms in 14
are thus predominantly responsible for coordination. For symbols, see Fig. 3 . Quantum yield of 14
in MeCN estimated from intensity readings published in [ 61 ] and quantum yield data of the parent
compound without active PET, DMA, published in [ 63 ]. (Reprinted in part with permission from
[ 61 ]. Copyright 1988 American Chemical Society)

formalism for the energetic consideration [ 65 ]. Due to these favorable signal
amplification features, the basic architecture of PET probes did not change much
over the past 20 years. Attempts were mainly made to increase the selectivity of the
probes - as one can imagine, the polyaza crown analogs of 13 for instance do not
discriminate satisfyingly between aminophilic heavy and transition metal ions such
as Zn 2+ ,Cu 2+, and Hg 2+ [ 66 ] - because all the complexes a single probe forms with
various analytes show virtually identical spectral features [ 67 , 68 ]. Moreover, since
simple and flexible architectures such as 13 and 14 are also prone to quenching
interactions by paramagnetic or heavy-atom species - for example, Cu 2+ and Hg 2+
even quench the “switched off” state of the azacrown analogs of 13 due to the
mechanisms sketched in Sect. 2 [ 66 ] - research effort has also been directed toward
achieving fluorescence enhancement upon binding to quenchers (see below). The
third field of research activity has been the development of potent PET probes that
rely on analytically more useful chromophores than anthracene and naphthalene,
the traditional PET fluorophore “workhorses”, i.e., that emit well in the visible [ 69 ,
70 ] or the near-infrared region of the spectrum [ 71 ].
In contrast to the -conjugated probe architecture utilizing an ICT process,

the number of anion probes that rely on the fluorophore-spacer-receptor design
and an active PET process is abundant [ 72 ]. Again, anthracene and naphthalene

p