Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers.

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 bipolaron. 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.

The Stabilities of N-Cl Bonds in Biocidal Materials.

N-halamine chemistry has been a research topic of considerable importance in these laboratories for over two decades. N-halamine compounds are useful in preparing biocidal materials. There are three N-Cl moieties available in cyclic N-halamine compounds: imide, amide, and amine. The stabilities toward the release of free halogen have been experimentally shown to decrease in the order amine > amide > imide. In this work, this generalization has been tested theoretically at the level of B3LYP/6-31+G(d) and using the conductor-like polarizable continuum aqueous solvation model with UAKS cavities. Excellent accord was observed between theory and experiment. It was also found that the imide and amide N-halamine stabilities on hydantoin rings could be reversed with substitution patterns at the 5 position.