Browsing by Subject "Nitrogen compounds"

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Open Access
Effects of perfluorination on thiophene and pyrrole oligomers
(2010) Salzner, U.
The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.58 eV results, as unsubstituted pyrroles are nonplanar and disordered in the solid state. For thiophene the band gap is slightly increased as long thiophene oligomers are almost planar. Ionization energies and electron affinities increase upon fluorination by 0.65 and 0.60 eV for polythiophene and by 0.45 and 0.90 eV for polypyrrole. Conduction band widths increase by 0.5 for polythiophene and by 0.7 eV for polypyrrole. Spectra of charged (doped) forms are almost identical to those of the parent systems. Like parent systems, fluorinated oligomers with chain lengths of more than six rings develop a third UV absorption that increases in strength and decreases in energy upon chain length increase.
Open Access
Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers
(2008) Okur, S.; Salzner, U.
Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaren. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory. © 2008 American Chemical Society.