Optical Reporting by Conjugated Polymers via Conformational Changes - Advanced Fluorescence Reporters in Chemistry and Biology - page 393

Fig. 3 Regioregular poly
[3-(2-methyl-1-butoxy)-
4-methylthiophene]
(PMBMT) synthesized
via oxidative
electropolymerization

O

S

n

Fig. 4 Temperature
dependant UV-vis absorption
spectra of a PMBMT thin
film [ 1 ]

5-(-2 0 ethylehexyloxy)- p -phenylene-vinylene] (MEH-PPV) spun from solution in
either toluene or chloroform [ 2 ]. The polar MEH-PPV polymer in nonpolar toluene
tends to stack as a coil to reduce solvent interactions whereas in chloroform it takes
on a more extended chain formation. They demonstrated that the films cast from
toluene exhibited intermittent fluorescence with rapid intervals of abruptly changing
intensity. This behavior was attributable to defects quenching excitations along the
chain (e.g.,

p

-stacking between adjacent coils). Pi-stacking and intra-chain emissions
could account for the lower fluorescence yields and the red-spectral shift when
compared with the films cast from chloroform. Various studies conducted on
the topic of conformation and chromic transitions agree upon the notion that confor-
mational changes within the polymer backbone result in the alteration of the optical
properties of CPs [ 2 - 8 ]. These chromic changes have been used to create sensing
systems based on the conformation of the polymer and its associated optical changes.

2.2 pH Induced Chromic Transitions

Optical transitions of CPs can also be induced by switching the net charge of the
polymer side chain. By using a zwitter-ionic polythiophene derivative, POWT,
Nilsson and co-workers were able to induce different chromic transitions by