Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 368

Fig. 7 Static Stern-Volmer
plots. The black line is the
ideal Stern-Volmer response,
the colored lines illustrate
deviations from linearity

F0/F

[Q]

aggregation effects and not sphere of action [ 3 ]. In addition, contributions from
dynamic quenching mechanisms will lead to upward curvature of the static quench-
ing Stern-Volmer plot; studies of excited state lifetimes can determine whether
dynamic quenching is occurring as well as static quenching. Saturation of the
quenching response (Fig. 7 , red line) is seen with a leveling of the curve at higher
[ Q ] concentrations. Occasionally, the Stern-Volmer plot has a downward curvature
(Fig. 7 , green line) showing a reduction in quenching with higher quencher con-
centrations. This can be attributed to the quencher causing a change in polymer
conformation or aggregation state that reduces quencher access to the fluorophore
or decreases the exciton sampling of quenching sites.
Many conjugated polymer molecular wire sensors exhibit what Whitten and
co-workers have called “super-quenching” [ 12 , 13 ]; binding of one target creates an
exciton trap which quenches an entire polymer (in theory), or more realistically,
quenches an extended segment of the polymer over which an exciton can travel.
The term “super-quenching” is, however, somewhat misleading. A conjugated
polymer has many more capture groups for the quenching target per “fluorophore”
than a small molecule. In a small molecule, the excited state is localized, and
usually there can only be one or two groups to capture the target on each molecule,
whereas in the conjugated polymer the exciton can travel up and down the back-
bone and sample many binding sites. If any binding site is occupied, it can destroy
the exciton and quench the emission of the whole conjugated segment (Fig. 8 ).
Enhanced sensitivity therefore depends on the polymer having many binding sites,
often one or more site per repeat unit combined with extensive exciton delocaliza-
tion. Polymers that do not have extensive exciton delocalization do not show the
enhanced sensitivity to the quenching target [ 3 ]. Sensitivity to quenchers can be
increased by altering the polymer structure to increase exciton lifetime [ 65 ],
allowing it to sample more binding sites [ 66 , 67 ], and by choosing polymers with
higher fluorescence efficiency.
It should also be noted that comparison of quenching efficiency of conjugated
polymers with small-molecule fluorophores is complicated and oversimplification
can lead to misleading claims of exceptional sensitivity enhancements. In their
2007 review, Swager and co-workers discuss cases where the charge state of
the small molecule used for comparison differed fundamentally from the polymer
and exceptional quenching efficiencies were inappropriately attributed to super-
quenching alone [ 3 ]. In addition, a polymer will create a different local