Protonation of Polyaniline in Hexafluoro-2-propanol. Spectroscopic Investigation

Abstract

Three principal forms of polyaniline, leucoemeraldine (LEB), emeraldine (EB), and pernigraniline (PNB), have been prepared, and their spectral properties have been studied in a newly discovered polyaniline solvent, hexafluoro-2-propanol (HFIP). In HFIP a significant hypsochromic shift of the absorption due to main electronic transitions is observed with respect to the analogous transitions in the spectra recorded for N-methylpyrrolidinone (NMP). In particular, the π to π * transition is shifted to 308 nm in LEB and to 305 nm in EB and PNB and the benzoid to quinoid ring excitonic transition is shifted to 502 nm in EB and PNB. In EB and PNB spectra, an additional peak at 800 nm is observed, which can be ascribed to the protonation of these forms of polyaniline by the highly acidic solvent. EB has been protonated in HFIP solution with three types of protonating agents: camphorsulfonic acid (CSA), phenylphosphonic acid (PPA), and poly(hexamethylene phosphate) (PHMeP). Protonation in HFIP facilitates the formation of the extended coil conformation of emeraldine similarly to the protonation in m-cresol. The extended coil conformation favors in turn the delocalization of polarons. The degree of polaron delocalization, as judged from UV-vis-NIR spectra, depends strongly on the protonation agent, the highest delocalization being observed for EB(PPA)y.