Novel ter(9,9-diarylfluorene)s were synthesized by a Suzuki-coupling reaction of 2-bromofluorene (1) and 2,7-fluorenediboronic ester derivatives (3) with high isolated yields (63-86%). The X-ray structure analysis of ter(9,9'-spirobifluorene) (4aa) revealed that the conjugated chromophore adopts a helical conformation. This conformation effectively releases the steric interaction between the fluorene moieties and prevents inter-chromophore interactions. The introduction of aryl groups at the C9 position of fluorene was highly beneficial to the thermal and morphological stability of these oligomers. These terfluorenes exhibit intense blue fluorescence with excellent quantum yields both in solution ( approximately 100%) and in solid state (66-90%), and possess interesting reversible redox properties. Highly efficient blue light-emitting OLED devices were fabricated using 4aa and 4cc as emitters as well as hole transporters. The devices exhibit low turn-on voltage ( approximately 3 V) and high EL external quantum efficiency (2.5-3%).
The external quantum efficiency of solar cells can be improved by using texturing pyramid- and honeycomb-like structures with minimum reflection. In this study, we investigated the reflection properties of texturing structures through rigorous coupled-wave analysis and the three-dimensional finite-difference time domains (FDTD) method to analyze close-packed texturing structures. We also demonstrate a simple method-combining sub-wavelength-scale monolayer and bilayer polystyrene spheres with a one-step reactive ion etching process-to fabricate optimized pyramid- and honeycomb-shaped antireflection structures, respectively. Thus, sub-wavelength pyramidal and honeycomb-like structures displaying low reflectance were obtained readily without the need for any lithography equipment.
Ter(9,9‐diarylfluorene)s (TDAFs) exhibit many intriguing properties promising for blue light‐emitting devices, such as high thin‐film photoluminescence quantum yields (∼ 90 %), high glass‐transition temperatures (> 200 °C), and an unusual ambipolar carrier‐transport capability. Successful implementation of a double‐ heterostructure device configuration that provides effective confinement of both carriers and excitons in TDAFs results in an electroluminescence performance (5.3 % external quantum efficiency) promising for application in blue‐emitting devices.
Nondispersive ambipolar carrier transport with comparably high electron and hole mobilities for amorphous molecular solids that are composed of only a single type of chromophores was observed for the first time in amorphous ter(9,9-diarylfluorene)s. High hole and electron mobilities over 10-3 cm2/(V.s) can be achieved with these terfluorenes. In particular, the electron mobility observed represents the highest ever reported for amorphous molecular solids.
The 77 K emission
spectra of cyclometalated ruthenium(II)-2,2′-bipyridine
(CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy
complexes with am(m)ine or diimmine ancillary ligands, and density
functional theory (DFT) modeling confirms that the lowest energy triplet
metal to ligand charge transfer (3MLCT) excited states
of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic
configurations. However, the phosphorescence decay efficiencies of
CM-Ru-bpy excited states are about twice those of the conventional
Ru-bpy analogues. In contrast to the similar 3MLCT excited
state electronic configurations of the two classes of complexes, the
CM-Ru-bpy chromophores have much broader visible region MLCT absorptions
resulting from several overlapping transitions, even at 87 K. The
emitting excited-state emission efficiencies depend on spin–orbit
coupling (SOC) mediated intensity stealing from singlet excited states,
and this work explores the relationship between the phosphorescence
efficiency and visible region absorption spectra of Ru-bpy 3MLCT excited states in the weak SOC limit. The intrinsic 3MLCT emission efficiency, ιem, depends on mixing
with singlet excited states whose RuIII-dπ-orbital
angular momenta differ from that of the emitting state. DFT modeling
of the 1MLCT excited-state electronic configurations that
contribute significantly to the lowest energy absorption bands have
RuIII-dπ orbitals that differ from those of their
emitting 3MLCT excited states. This leads to a very close
relationship between ιem and the lowest energy MLCT
band absorptivities in Ru-bpy chromophores. Thus, the larger number
of 1MLCT transitions that contribute to the lowest energy
absorption bands accounts for the enhanced phosphorescence efficiency
of Ru-bpy complexes with cyclometalated ancillary ligands.
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