Interactions between lumophores have a critical influence on the photophysical properties of conjugated polymers. We synthesized a new series of light-harvesting polymers (poly-DSBs, I-IV) of dialkyloxy- or dialkyl-substituted distyrylbenzene (the substituents being methoxy, 2-ethylhexyloxy, and cyclohexyl) with short aliphatic linkage (methylene or ethylene) and examined the effects of interactions between lumophores and of chemical structures on the absorption, emission, and excitation spectra. The proximity between distyrylbenzene lumophores was shown to be critical to the interactions between lumophores and to the energy-transfer processes. In concentrated solutions and solid films, intermolecular aggregates exist resulting from different extents of interactions between lumophores and are found to involve at least three species: loose, compact, and the most aligned aggregates as observed by photoluminescence and excitation spectroscopies. We also found, for the first time, sequential energy transfer from individual lumophores to the most compact, aligned aggregates via the looser intermolecular aggregates, as observed directly by time-resolved fluorescence spectroscopy. Such a process mimics energy transfer in photosynthesis units and is so efficient such that the fluorescence color can be red-shifted drastically by the presence of comparatively few aggregates and that the light evolved from concentrated solutions and films of poly-DSBs I-IV is entirely or almost the aggregation emission. Although the sequential energy-transfer process in fully conjugated electro-/photoluminescent polymers due to inhomogenity other than distributed conjugation lengths has never been directly observed at room temperature, we suggest that events similar to those observed in poly-DSBs in conjugated polymers could occur but on a much shorter time scale, i.e., a few picoseconds.

Structural characteristics of optical quality poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH−PPV) films drop-cast from toluene solutions have been examined by use of wide-angle (WAXS) and small-angle X-ray scattering (SAXS) as well as field-emission scanning electron microscopy. For the as-cast film SAXS and WAXS patterns clearly suggest existence of nanodomains of mesomorphic order. Under grazing incidence, both SAXS and WAXS patterns of the as-cast film exhibit impressive uniaxial symmetry with strong scattering along the film normal; however, the corresponding normal-incidence patterns are circularly isotropic, implying in-plane random orientation. A structural model consisting of preferentially uniaxial-oriented nanodomains and disordered matrix is therefore proposed for the as-cast film. Annealing at 210 °C results in disorientation of nanodomains:  this decrease in structural asymmetry upon heat treatment correlates well with concomitant changes in charge transport behavior determined from time-of-flight measurements, manifesting dramatically decreased charge mobility as well as transition from nondispersive to dispersive modes.

Structural evolution and morphological development in films of poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, drop-cast from toluene solutions) upon isothermal heat treatment at elevated temperatures were studied by means of a combination of differential scanning calorimetry, polarized light microscopy, X-ray diffraction, transmission electron microscopy, ultraviolet−visible spectroscopy, and photoluminescence spectroscopy. Results indicated that MEH-PPV is mesomorphic in nature (optically biaxial, showing nematic-like texture under cross-polarization), with glass transition temperature T g = ca. 80 °C and isotropization temperature T i = ca. 290 °C. Upon short-term (i.e., 5 min) heat treatment at elevated temperatures (T a) below T i, MEH-PPV chains stack into boardlike entities (ca. 1.6 nm in thickness and ca. 0.4 nm in interbackbone spacing) within beadlike domains ca. 10−20 nm in size, which in turn aggregate transversely into wormlike features ca. 200 nm in length. Shearing at an elevated temperature results in disintegration of the wormlike agglomerates, leaving the beadlike domains arrayed into wavy lines transverse to the shear direction. Accompanying the morphological changes, ultraviolet−visible light absorption and photoexcited emission spectra vary systematically with improvement or disruption of the mesomorphic order.