| Abstract |
A homologous series of p-phenylenevinylene oligomers (PPV) (n = 3-5) with terminal dialkylamino groups have been synthesized, and characterized. The photophysical and solvatochromic properties of oligomers with three, four, and five phenyl groups were investigated in solution through steady-state and time-resolved fluorescence. The red-shift seen in absorption and photoluminescence (PL) spectra on increasing chain length is consistent with an extension of the conjugation length. Decreasing solvent polarity leads to small shifts of the absorption spectra and large blue-shifts of the PL spectra. Time-resolved emission on these oligomers allows separation of conformational and solvent contributions to their photophysical behavior. Global analysis of fluorescence decays collected at different emission wavelengths shows biexponential behavior in toluene at room temperature. The longer time appears as a decay component at all emission wavelengths and decreases from 949 to 850 ps with increasing chain length. The shorter time component (19-26 ps) appears as a decay at the onset of the emission spectrum and as a rise time at longer wavelengths. With PPV oligomers substituted on the phenyl rings, similar kinetic characteristics have been attributed to conformational relaxation of the initially excited oligomer into a more planar conjugation. However, the solvent and temperature dependence exclude this hypothesis. Instead, this component is assigned to specific interaction between the oligomer main chain and the solvent. Information on backbone conformational behavior has been obtained by temperature dependence of multinuclear NMR studies coupled with molecular dynamics simulations, Moller-Plesset 2, and density functional theory (DFT) calculations. The major result is that conformational relaxation in the excited state can be decoupled from solvent relaxation. Since conformational relaxation enhances nonradiative decay, control of this is expected to improve photoluminescence yields in these systems. |
