Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 378

A related approach to quencher displacement for turn-on sensing is quencher
modification. A simple approach is to design a system where the quencher and the
polymer interact electrostatically causing the target to change the charge of the
quencher and thus removing the attraction between the quencher and the polymer.
An illustration of this approach was developed by Schanze, Lackowicz, and
co-workers, who used sulfonated PPE complexed with a boronic acid functiona-
lized methyl viologen analog for monosaccharide detection [ 96 ]. When sugars
reacted with the boronic acid, they neutralized the quencher and it diffused away,
turning-on the polymer emission. Another approach is to chemically modify the
quencher. For example, Thomas and Swager developed reversible hydrazine turn-
on sensing materials by preparing emissive PAE films [ 97 ] with alternating bis
(alkylamino)phenylene and pentiptycene units. Upon exposure to hydrazine vapor,
the emission increased. The authors hypothesized that the hydrazine reduced
exciton traps formed from the oxidation of the polymer. Photobleaching before
hydrazine exposure, to increase the number of exciton traps, increased the sensitiv-
ity of the material. Another example is the work of Lippard and co-workers who
developed a selective NO sensor from a PPV with bipyridine units in the backbone
[ 98 ]. The polymer was exposed to Cu(II) which quenched its emission. NO reacted
with Cu(II) to form Cu(I); Cu(I) also quenched the polymer emission but not as
strongly as Cu(II), so the overall emission rose.

5.2.2 “Turn-On” From Change in Conjugation Length

The discussion so far has focused on emission changes arising from the introduc-
tion, removal or modification of species that quench the polymer through some
form of energy transfer. Changes in conjugation length of the polymer backbone
affect both the absorbance and emission spectra; increases in conjugation lead to
redshifts in absorbance, while the effect of conjugation length/exciton delocaliza-
tion on overall emission is a more complicated issue. Greater delocalization
(conjugation) generally reduces the energy gap which leads to an increase of
nonradiative pathways, shortening the excited state lifetime and reducing the
emission. For some classes of polymers, such as PDA and PT, the polymer's
emission decreases or is eliminated at longer conjugation lengths, while the emis-
sion increases with backbone twisting that breaks conjugation. Changes in the
conjugation length of PPVs and PAEs are also known [ 99 ] but are less commonly
reported, possibly because of the relative rigidity of their backbones. For PPV
polymers, with relatively short conjugation lengths and exciton delocalization over
approximately 5-10 units [ 34 , 37 ], increases in conjugation length have been
reported to lead to increases in emission [ 52 , 100 , 101 ].
Leclerc, Ho, and co-workers have developed many sensing systems for different
targets using cationic PT and exploiting the differences in emission and color
between the extended conjugation form and the coiled form. The basis of these
assays is that the polymer microstructure and conformation change as it interacts
with the target. For example, in DNA hybridization assays, imidazolium-PT (see