Solvent and molecular weight effects on fluorescence emission of MEH-PPV

Here we systematically studied the excitation and the fluorescence steady-state spectroscopy of poly(2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) with three molecular weights, (M) over bar (n) = 51 kg mol(-1), (M) over bar (n) = 86 kg mol(-1) and (M) over bar (n) = 125 kg mol(-1), in two equally good solvents and several concentrations, from dilute solutions to solid-state films produced by casting. The appropriateness of the two solvents was established by comparing their solubility parameters and the solubility parameter of the MEH-PPV estimated using the Small, the Van Krevelen and the Hoy models. Thus, chloroform and toluene were chosen. Fluorescence spectra were recorded for solutions in several concentrations (10(-8) mol L-1 to 10(-4) mol L-1) and films produced by casting, showing that chloroform and toluene solvate the polymer chain differently. Diluted solutions (10(-8) mol L-1) in chloroform exhibit broader fluorescence spectra. A red-shift of the fluorescence spectra was observed for concentrations higher than 10(-6) mol L-1 that can be explained using Kasha's rule for the sandwich anti-parallel orientation of the transition moments of the two chromophores. The conformations observed in solutions are partially retained in the solid films in addition to the broader red-shift spectra attributed to aggregated forms of the macromolecular segments. Annealing of the polymer films at the glass transition temperature eliminates the more stressed conformations, erases the memory and leads to sharper fluorescence spectra.