Frequency-Selective Photobleaching as a Route to Chromatic Control in Supramolecular OLED Devices.

We report a series of molecules that spontaneously self-organize into small electroluminescent domains of sub-micrometer dimensions when dissolved in tetrahydrofuran. The self-assembled spherical aggregates have an average diameter of 300 nm and exhibit efficient energy transfer from the blue to the green or red component. The aggregates can be chromatically addressed or patterned by selective bleaching of the energy-acceptor component using a laser source. This allows the fabrication of electroluminescence devices by directly photopatterning the active layer without the need of additional steps. Submicron features (700 nm) can be achieved using a collimated light source.

Green Synthesis of InP/ZnS Core/Shell Quantum Dots for Application in Heavy-Metal-Free Light-Emitting Diodes.

Quantum dot light-emitting diodes (QD-LEDs) have been considered as potential display technologies with the characterizations of high color purity, flexibility, transparency, and cost efficiency. For the practical applications, the development of heavy-metal-free QD-LEDs from environment-friendly materials is the most important issue to reduce the impacts on human health and environmental pollution. In this work, heavy-metal-free InP/ZnS core/shell QDs with different fluorescence were prepared by green synthesis method with low cost, safe, and environment-friendly precursors. The InP/ZnS core/shell QDs with maximum fluorescence peak at ~ 530 nm, superior fluorescence quantum yield of 60.1%, and full width at half maximum of 55 nm were applied as an emission layer to fabricate multilayered QD-LEDs. The multilayered InP/ZnS core/shell QD-LEDs showed the turn-on voltage at ~ 5 V, the highest luminance (160 cd/m2) at 12 V, and the external quantum efficiency of 0.223% at 6.7 V. Overall, the multilayered InP/ZnS core/shell QD-LEDs reveal potential to be the heavy-metal-free QD-LEDs for future display applications.

Enhanced electroluminescence based on thermally activated delayed fluorescence from a carbazole-triazine derivative.

Thermally activated delayed fluorescence (TADF) properties of a dicarbazole-triazine compound, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-3,3'-bicarbazole (CzT), and its OLED characteristics were investigated. An estimated small energy gap of about 90 meV between the singlet and triplet energy states of CzT made the up-conversion of triplet excitons back to a singlet state possible. The origin of the observed delayed fluorescence has been shown to be thermally activated delayed fluorescence. An organic light emitting diode (OLED) with CzT as an emitter showed the maximum external quantum efficiency (EQE) of 6%. For comparison, another carbazole-triazine derivative of 3-(2'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-2-yl)-9-phenyl-9H-carbazole (PhCzTAZ) with a similar structure was also studied. PhCzTAZ showed a low fluorescence quantum yield with no TADF.

In situ reversible conversion of porphyrin aggregate morphology.

A porphyrin derivative possessing orthogonal self-assembly units displays in situ reversible transformation of aggregate morphology between nano-rods and hollow spheres upon exposure to different solvents.

New synthesis of zinc tetrakis(arylethynyl)porphyrins and substituent effects on their redox chemistry.

Sonogashira coupling of zinc 5,10,15,20-tetraethynylporphyrin with various phenyl iodides under mild conditions afforded good yields of the corresponding zinc porphyrins. This method is applicable to a variety of aryl iodides including meso-substituted iodoporphyrin to form a conjugated star-shaped multiporphyrin. The UV-Vis spectra show that peak broadening, red shifts, and changes in the oscillator strength of absorptions increase with the extension of pi-conjugation. In the electrochemical measurements, the first oxidation of porphyrins 4-9 occurs at potentials in the range +0.89 to +1.08 V, which are comparable to that of ZnTPP (TPP = tetraphenylporphyrin). The first reduction was observed at potentials from -0.73 to -0.89 V, which is anodically shifted by 390-550 mV as compared to that of ZnTPP, and the second reduction occurs at potentials in the range -1.12 to -1.33 V. The para-substituted tetrakis(phenylethynyl)porphyrins show substituent effects on their redox chemistry and exhibit only slight substituent effects in their emission and absorption maxima.

Synthesis, structure, and optical and electrochemical properties of star-shaped porphyrin-triarylamine conjugates.

A series of novel star-shaped porphyrin-triarylamine conjugates were synthesized by palladium-catalyzed cross-coupling reactions. The spectroscopic and electrochemical studies show that pi-conjugation of the porphyrin core is extended to the triarylamine moieties.

Highly conjugated multiporphyrins: synthesis, spectroscopic and electrochemical properties.

A series of meso-to-meso ethynyl-bridged multiporphyrin arrays have been synthesized using Sonogoshira palladium-catalyzed cross-coupling reactions involving the appropriate ethynylporphyrin and iodoporphyrin precursors. The absorption spectra of these multiporphyrins show splitting of the Soret bands and significant red shifts of the Q bands as compared to the combination of the corresponding components. These conjugated multiporphyrins also show red shifts in their emission spectra as the pi-conjugation is expanded. In the electrochemical measurements, the porphyrins dimer 7 shows two 1 - e- oxidations at E(1/2) = +0.63 and +0.76 V for the first electron abstraction from the two porphyrin rings, indicating electronic communication between the two porphyrin units. The porphyrin trimer 4 exhibits the first and second 1 - e- oxidations at E(1/2) = +0.68 and +0.77 V, respectively, which correspond to the two outer porphyrins. The cyclic voltammogram of pentamer 5 shows two overlapping 1 - e- couples at E(1/2) = +0.56 and +0.66 V, and one 2 - e- couple at E(1/2) = +0.86 V, for the four outer porphyrin units. These results demonstrate that in the porphyrin trimer and pentamer the individual peripheral porphyrin units are electrochemically coupled via a central porphyrin core. The UV-Vis-NIR spectra of the oxidized species of these multiporphyrins exhibit a broad intervalence charge transfer (IVCT) band in the region from 1200 to 3000 nm. The present work shows that a central porphyrin unit appended with ethynyl bridges affords strong electronic interactions between the peripheral porphyrin rings over a distance of about 15 A.