Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 367

Quenching mechanisms are discussed in detail by Lakowicz in Principles of
Fluorescence Spectroscopy . Interaction of the fluorophore and the quencher leads
to nonradiative return to the ground state of the fluorophore via several different
mechanisms [ 6 , 63 ]. A quencher such as oxygen may induce the singlet excited
state of the fluorophore to undergo intersystem crossing to the triplet state, which
then undergoes nonradiative decay. The excited state of a fluorophore can exchange
pairs of electrons with a quencher in close proximity (allowing MO overlap) in a
Dexter exchange, dissipating the excited state as heat. Alternatively, the quencher
and fluorophore may undergo photoinduced electron transfer (PET) where an
electron is transferred either from the quencher to the excited state of the fluor-
ophore or from the excited fluorophore to the quencher. Energy transfer from the
excited state of the fluorophore to a chromophore can also occur via a F¨rster
mechanism, as discussed below in Sect. 5.3 ; if the acceptor is not emissive, this
leads to quenching of the system.
For small molecules, quenching often occurs through collisions between the
quencher and the excited fluorophore (dynamic quenching). Conjugated polymers
usually do not have excited states with sufficiently long lifetimes to allow dynamic
quenching; however, quenching can occur through binding of the quencher (before
excitation) creating a nonemissive species (static quenching). The Stern-Volmer
equation is used to determine the efficiency of the quenching and can be expressed
in terms of relative emission or as changes in lifetime. In dynamic quenching, a
reduction in fluorescence lifetime is seen as the interaction with the quencher
happens during the excited state lifetime cutting it short. In static quenching, the
fluorophore/quencher complex is nonemissive and therefore while a reduction in
emission is seen, there is no change in lifetime. The Stern-Volmer static quenching
equation is:

F 0 = F ¼ 1 þ K SV Q ½

where F 0 is the emission without the quencher present, F is the emission with
the quencher, [ Q ] is the concentration of the quencher, and K SV is the static
Stern-Volmer constant. With perfect Stern-Volmer static quenching behavior, a
plot of [ Q ] vs. F 0 / F yields a straight line with a slope of K SV . The more effectively
the emission of the fluorophore population is quenched, the higher K SV will be.
Needless to say, real-life Stern-Volmer plots for quenching of conjugated
polymers often show deviations from linearity over some portion of the [ Q ]
range. Upward curvature (Fig. 7 , blue line) can be caused by the aggregation of
the polymer. Aggregation can enhance quenching in two ways: through extending
the exciton path so that it can sample more binding sites, and by stacking and self-
quenching of polymer chains. Upward curvature has also been attributed to the
sphere of action mechanism, which proposes that there is a volume around a
fluorophore where quenching will always occur [ 64 ]. As quencher concentration
increases, the probability that this volume will have a quencher in it goes up, adding
to the observed quenching. Swager and co-workers in their 2007 review argue,
however, that upward curvature seen in conjugated polymer systems is due to