ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The temperature-dependent optical spectra of the major insulating forms of polyaniline—the fully reduced leucoemeraldine base, half-oxidized emeraldine base (EB), and fully oxidized pernigraniline base (PNB)—and of two oligomeric model compounds for polyaniline are reported. All three forms of the polymer possess a ∼3.8 eV optical absorption assigned to a transition from the valence band to a narrow π* conduction band; in all cases this transition shifts to higher energy and weakens with increasing temperature, T. The oxidized forms also exhibit transitions at ∼2 eV; in EB, this absorption feature has previously been assigned to a "molecular exciton'' and is found to increase in energy with increasing T. In PNB, the observed ∼2.3 eV transition is correlated with the expectation of a Peierls dimerization gap in this half-filled band system; in contrast to the ∼2 eV transition in EB, this optical transition energy decreases with increasing T in PNB. These observations are for the most part understood by a simple model which accounts for the steric repulsion between the constituent phenyl rings of the polymer chain. The dependence of the steric potential on ring angle is anharmonic, giving rise to thermal expansion of the average phenyl ring-torsion angles and a concomitant reduction in the bandwidth and average energy of the valence band; the observed thermochromic behavior is generally consistent with these changes in electronic structure. The potential role of other conformational changes—rod-to-coil transitions, order–disorder transitions, bond-length order changes, and structural isomerizations—is discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.462762
Permalink
