Photophysical Identification of Three Kinds of Low-Energy Green Band Defects in Wide-Bandgap Polyfluorenes.

Blue-light-emitting semiconductors based on polyfluorenes often exhibit an undesired green emission band. In this report, three well-defined oligofluorenes corresponding to three types of "defects" attributed to aggregation, keto formation, and chain entanglement, respectively, are systemically investigated to unveil the origins of the green emission band in fluorene-based materials. First, the optical properties of defect molecules in different states are studied. The defect associated with aggregation is absent in dilute solutions and in films doped at 0.01 wt % with poly(methyl methacrylate). Second, the dependence of the emission spectra on the solvent was monitored to compare the effects of the "keto-" and "chain-entanglement defect" molecules. The green emission of keto defects exhibited a strong dependence on solvent polarity, whereas this cannot be observed in case of chain-entanglement defect. Third, energy transfer between poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]- co-[5-(octyloxy)-9,9-diphenyl-fluoren-2,7-diyl] and the keto or chain-entanglement defect molecules is illustrated. Compared to those of the chain-entanglement defect, the spectra of the keto defect molecule (1:10-3) show signs of defect emission at lower proportions. These investigations not only provide insight into the photophysics of oligofluorenes but also supply a new strategy to explore defects in semiconductor polymers, which will aid in the development of effective approaches to obtain stable, pure blue organic light-emitting diodes based on polyfluorenes.

Photophysical and Fluorescence Anisotropic Behavior of Polyfluorene ß-Conformation Films.

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (ß-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene ß-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) ß-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, ß-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to ß-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF ß-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.

Synthesis and Crystal Structure of Highly Strained [4]Cyclofluorene: Green-Emitting Fluorophore.

[4]Cyclo-9,9-dipropyl-2,7-fluorene ([4]CF) with the strain energy of 79.8 kcal/mol is synthesized in high quantum yield. Impressively, hoop-shaped [4]CF exhibits a green fluorescence emission around 512 nm offering a new explanation for the green band (g-band) in polyfluorenes. The solution-processed [4]CF-based organic light emitting diode (OLED) has also been fabricated with the a stronger green band emission. Strained semiconductors offer a promising approach to fabricating multifunctional optoelectronic materials in organic electronics and biomedicine.