VIS/NIR Absorption Spectra of Positively Charged Oligo(phenylenevinylene)s and Comparison with Time-Dependent Density Functional Theory Calculations

A combined experimental and theoretical study of the optical properties of positively charged unsubstituted and dialkoxy-substituted phenylenevinylene (PV) oligomers in solution is presented. Cations of PV oligomers were produced by irradiation of a solution with high-energy electron pulses. The optical absorption spectra were measured using time-resolved visible/near-infrared (VIS/NIR) spectroscopy in the range of 500-2100 nm (0.6-2.5 eV). The optical absorption spectra of positively charged PVs are compared with results from time-dependent density functional theory (TDDFT) calculations and previous semiempirical calculations. The experimental spectra of cations of partially dialkoxy-substituted PVs indicate the presence of a transition with a maximum below 0.6 eV. According to earlier semiempirical calculations, the energy of this transition exhibits an oscillating behavior as a function of the length of the oligomer. This was not observed experimentally. However, the monotonic decrease of the low-energy absorption band, as obtained from TDDFT calculations, is in agreement with the experimental findings.