The nature of interchain excitations in conjugated polymers: Spatially-varying interfacial solvatochromism of annealed MEH-PPV films studied by near-field scanning optical microscopy (NSOM)

The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is "excimer-like", exhibiting an similar to4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 mum in diameter), however, the interchain excited state exhibits a large (similar to9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.