Efficient light harvesting by sequential energy transfer across aggregates in polymers of finite conjugational...

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.