Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 380

with a target. Changes in backbone conformation and polymer aggregation can
cause shifts in emission wavelength as well as changes in emission intensity; both
aggregation and increases in conjugation lead to redshifts in fluorescence. In the
cases where the polymer is emissive pre- and post-target interaction, the shifts and
ratios of peak intensities can be used to gain more information and to distinguish
between possible targets. For example, the influential 1997 paper by Wang and
Wasielewski in the area of metal ion sensing described a PPV-bipyridine copoly-
mer that had redshifted emission in the presence of ions that flattened the bipyridine
units increasing conjugation, blueshifted emission with ions that twisted the back-
bone upon binding reducing conjugation, and quenched emission for a third set
of ions [ 77 ].
Resonance energy transfer (RET) from the excited state of a donor fluorophore/
chromophore to an acceptor fluorophore/chromophore, is a well-known phenome-
non in small-molecule fluorescence that has analogs in the field of emissive
conjugated polymer sensing. RET between small molecules is generally described
by the F¨rster (FRET) mechanism. The F¨rster mechanism involves point dipole to
point dipole coupling of the excited state of the donor to the ground state of an
acceptor leading to the donor returning to the ground state and the acceptor being
excited [ 63 ]; this transfer has a 1/ r 6 distance dependence and is referred to as
“through space” energy transfer. If the acceptor molecule is not fluorescent, this
energy transfer will lead to quenching as seen above, but if the acceptor is a
fluorophore, it can emit at a lower-energy (redshifted) wavelength compared to
the donor emission. This leads to an increase in the Stokes shift of the sensing
system and opportunities for “multi-color” and ratiometric emissive-based sensing.
The extent of spectral overlap between the emission spectrum of the donor and the
absorbance spectrum of the acceptor is often used as a quick guide as to whether
FRET will occur. An in-depth discussion of the factors affecting FRET transfer and
competition from PET is presented in [ 113 ]. It should be noted that though the
F ¨ rster mechanism is frequently invoked to describe conjugated polymer to small-
molecule fluorophore energy transfer, the mechanism does not always predict the
behavior accurately as the chromophoric segments of conjugated polymers are
frequently too large to be convincingly modeled as point dipoles [ 112 , 114 , 115 ].
Energy can also be transferred from donor to acceptor via the Dexter mechanism,
described in Sect. 5.1 . The Dexter mechanism involves direct orbital overlap and
operates at close distances, while the F¨rster mechanism can operate at either
relatively far or very close distances.
FRET is a good technique for sensing schemes where the target affects the
distance between the donor and acceptor. The simplest approach is to label a target
that then binds to the polymer; this is an appropriate approach for competition
assays but not for sensing situations where target modification is not practical.
Other design schemes that deploy FRET include: (1) the target reducing the
donor-acceptor distance to “turn-on” FRET, (2) the target displacing a labeled
species and thus “turning-off” FRET, or (3) the target altering the macromolecular
structure of the polymer thus turning it into a donor (or acceptor) by changing its
excited state behavior. FRET-based quenching-sensing schemes have been used in