Page 355 - Organic Electronics in Sensors and Biotechnology

332 Chapter Nine

aggregation of polymer chains. The optical transitions of conjugated
polymers are believed to occur on different parts of the same polymer
chain, intrachain events, or between adjacent polymer chains, inter-
chain events. The intrachain events are mainly dependent on the con-
formation of the polymer backbone, and the interchain processes
occur as nearby polymer chains come in contact with each other, lead-
ing to stacking of the aromatic ring systems, such as the thiophene
rings. Polythiophenes have been shown to exhibit a variety of optical
15
transitions upon external stimuli such as solvents, heat, 15–18 ions, 3, 19, 20
or proteins, 21–24 leading to the design of a variety of sensory devices.
Such effects have been termed solvatochromism, thermochromism, iono-
chromism, or biochromism, respectively. As there is a strong correlation
between the electronic structure and the backbone conformation in
conjugated polymers, any change in the main chain conformation
will lead to an alteration of the effective conjugation length, coupled
with a shift of the absorption in the UV-visible range. 25–28 A coil-to-rod
(nonplanar-to-planar) conformation transition of the polymer back-
bone is observed as a red shift of the absorption maximum, due to an
increased effective π conjugation length of the polymer backbone. 15–18
Studies on different polythiophene derivatives have shown that the
conformational changes of the main chain, planar to twisted non-
planar, could be affected by order-disorder transitions of the side
chains. 16–18, 29–31 This phenomenon has also been confirmed by theo-
retical investigations. 32, 33
The highly conjugated planar form of the polymer backbone is
33
also assumed to be coupled with intermolecular aggregates. Hence,
a red shift in absorption might also be associated with π aggregation
between polymer chains which is seen as a distinct shoulder at longer
wavelength in the UV-vis spectra. 34–36 This phenomenon might occur
independently of the planarization of the backbone and should be
distinguished from the red shift owing to planarization alone. 34, 35
However, studies performed on well-defined oligothiophene model
compounds have shown that the structurally induced chromic effects
are mainly due to conformational changes of the backbone instead of
interchain interactions. 37–39 To visualize the optical transitions, the
absorption spectra of a zwitter-ionic polythiophene derivative, POWT
(Fig. 9.1), in different buffer systems are shown in Fig. 9.2a. At pH 5,
36
where most of the side chains are zwitter-ionic (both negatively and
positively charged), the polymer backbone is in a nonplanar helical
conformation, associated with a blue-shifted absorption spectrum.
When the pH is increased or decreased, the backbone of the conju-
gated polymer adopts a more planar conformation, seen as a red shift
of the absorption maximum. A closer look at the spectra for POWT in
pH 2 and pH 8 buffer solutions (Fig. 9.2a) reveals an interesting obser-
vation. The absorption maximum is similar, but the spectrum recorded
for POWT in the pH 8 buffer solution has a shoulder in the region at