Photoluminescence of MEH-PPV Brushes, Pancakes, and Free Molecules in Solutions and Dry States

Photoluminescence (PL) of a conjugated polymer MEH-PPV, poly[2-methoxy-5-(2′-ethylhexyl)oxy)-1,4-phenylenevinylene], grafted on a silicon wafer with controlled tether spacing was studied to reveal the effects of molecular conformation, chain packing, and mechanical stress. In the solvent-swollen state, the PL of the densely grafted polymer (denoted “brushes”) was blue-shifted substantially relative to the lightly grafted (denoted “pancakes”) and free polymers. As solvent evaporated, while for the brushes the changes in PL were insignificant, the PL spectra of the pancakes underwent large blue shifts and exhibited significant efficiency enhancements up to ∼175-fold. The solvent evaporation effects were attributed to molecular deformations resulting from coil contraction on the substrate, which gave rise to conjugation-disruptive kinks (blue shift) and segmental stretching (PL enhancement) in the dried molecules. Moreover, heterojunctional quenching was found significantly suppressed by the mechanical stresses. Similar behavior was observed in dried free single molecules. These results unveil the fundamental role of mechanical stresses, not only indirectly through their influence on molecular conformations, but directly via alterations of the excitonic behavior.