Efficient light harvesting by sequential energy transfer across aggregatesin polymers of finite conjugational segments with short aliphatic linkages

Interactions between lumophores have a critical influence on the photophysi cal properties of conjugated polymers. We synthesized a new series of light -harvesting polymers (poly-DSBs, I-IV) of dialkyloxy- or dialkyl-substitute d distyrylbenzene (the substituents being methoxy, 2-ethylhexyloxy, and cyc lohexyl) with short aliphatic linkage (methylene or ethylene) and examined the effects of interactions between lumophores and of chemical structures o n the absorption, emission, and excitation spectra. The proximity between d istyrylbenzene lumophores was shown to be critical to the interactions betw een lumophores and to the energy-transfer processes. In concentrated soluti ons and solid films, intermolecular aggregates exist resulting from differe nt extents of interactions between lumophores and are found to involve at l east three species: loose, compact, and the most aligned aggregates as obse rved by photoluminescence and excitation spectroscopies. We also found, for the first time, sequential energy transfer from individual lumophores to t he most compact, aligned aggregates via the looser intermolecular aggregate s, 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 pres ence of comparatively few aggregates and that the light evolved from concen trated solutions and films of poly-DSBs I-IV is entirely or almost the aggr egation emission. Although the sequential energy-transfer process in fully conjugated electro-/photoluminescent polymers due to inhomogenity other tha n 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.