Single-Molecule Spectroscopy of MEH-PPV Polymer Molecules in Different Host Matrices

Fluorescence emission and excitation spectra of single MEH-PPV molecules dispersed in three different host polymers (PMMA, PS, and Zeonex) have been recorded at 1.2 K. We observed only minor effects of the host matrix on the following parameters: the ratios of single-chromophoric to multichromophoric emission, the widths of the distributions of emission maxima, and the (generally very low) fraction of emission spectra with sharp zero-phonon lines. The differences are tentatively attributed to different conformations of MEH-PPV chains, subtle variations in local chromophore-matrix interactions, and/or different distributions of conjugation lengths of emitting chromophores, respectively. Using excitation spectroscopy, in all samples we found narrow zero-phonon lines, providing an upper limit for the homogeneous line width and indicating that in each of the host polymers nearly lifetime limited widths can be reached at low temperatures. While our investigations point to some influence of the host polymer on the chain conformations, the latter seem to be of only minor importance for those photophysical properties of the chromophores of individual MEH-PPV chains studied herein.