Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 362

electron to the ground state. The formation and fate of excitons are affected by the
extent of polarization and conjugation of the polymer. Greater polarizability of the
polymer increases the likelihood of free-carrier generation, which also reduces
emission [ 1 ].
In “real world” applications there are many factors affecting the fate of the
polymer excited state and hence its emission. Conjugated polymers consist of
different length segments of conjugated bonds with twists or other defects separat-
ing the segments. Such polymers can therefore be viewed as a collection of
structurally related fluorophores strung together. There have been extensive spec-
troscopic studies of MEH-PPV examining their electronic states, driven primarily
by interest in the optoelectronic properties of MEH-PPV and its use in light-
emitting organic materials [ 34 - 37 ]. Exciton travel in these polymers is portrayed
as the exciton hopping along chromophore segments of 5-10 monomer units,
generally moving down a “funnel” of segments of decreasing energy, eventually
arriving at a low-energy segment which acts as an exciton trap and leads to emission
[ 34 ]. As the polymer chain coils or bends back on itself, the conjugated segments
may align allowing transfer between noncontiguous chain segments; this has been
referred to as interchain transfer, despite the segments being part of same chain.
Polymer chains can also aggregate in solution and are adjacent in solid materials,
again affecting the exciton migration and lifetime by allowing transfer between two
chains. Interchain transfer has been shown to be faster than exciton travel down
adjacent chain segments [ 38 ].
To act as fluorescent transducers in sensing, the emission of conjugated poly-
mers must be affected by external environmental triggers. An emissive polymer
may be quenched (turn-off), a nonemissive polymer may become emissive (turn-on),
energy from the polymer excited state may be transferred to another fluorophore,
which then emits, or the polymer emission itself may shift wavelength. The target
can interact with pendant groups off the backbone through electrostatics, hydrogen
bonding and host-guest interactions, or by participating in a chemical reaction. The
target may interact with the backbone directly if binding groups (for example,
bipyridine) are incorporated in the backbone. These interactions may alter the
valence energy leading to the formation or the removal of an exciton trap, cause
conformational changes to the backbone thus increasing or decreasing the conjuga-
tion length, and/or cause changes in the polymer aggregation state.
4 Material Forms
Development of a commercial sensor or assay requires incorporation of the sensing
elements into materials appropriate for device manufacture. Some assay applica-
tions use solution-phase materials only; this is more common for drug discovery
assays and diagnostic testing than in environmental sensing. Polymers are less
soluble than small molecules; however, with appropriate choice of side chains