Yu-Yu Liu

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.

Friedel–Crafts Bottom-up Synthesis of Fluorene-Based Soluble Luminescent Organic Nanogrids

A series of fluorene-based grid molecules (so-called Grid fluorenes) have been synthesized by means of shape-supported cyclization, starting from H-shaped precursors via the alternative Friedel-Crafts reactions of fluorenols and Suzuki cross-coupling reactions with key cyclization yields up to 26%. Fluorenol approaches and nanogrids open a door to soluble one-, two-, or three-dimensional nanoporous polymers as next-generation polymer mechano-semiconductors facing a new era of consciousness.

Progress in fluorene-based wide-bandgap steric semiconductors

Molecular bulks are favorable for the thermal and morphological stability in organic wide-bandgap semiconducting polymers with potential applications in both information and energy electronics. In this review, we present our progress in the design of fluorene-based bulky semiconductors with a fractal four-element pattern. Firstly, we established one-pot methods to spirofluorenes, especially spiro[fluorene-9,9′-xanthene] (SFX) serving as the next-generation spiro-based semiconductors. Secondly, we observed the supramolecular forces at the bulky groups and discovered the supramolecular steric hindrance (SSH) effect on polymorphisms, nanocrystals as well as device performance. Thus, a synergistically molecular attractor-repulsor theory (SMART) was proposed for the control of nanocrystal morphology, thin film phase and morphology. Thirdly, the third possible type of defects has been identified to generate green band (g-band) emission in wide-bandgap semiconductors by the introduction of molecular strain design of cyclofluorene. Finally, the first bulky polydiarylfluorene with highly crystalline and β conformation was achieved by an attractor-repulsor design of tadpole-shape monomer, which offered an effective platform to fabricate stable wide-bandgap semiconducting devices. All the discoveries offer the solid basis to break through bottlenecks of organic/polymer wide-bandgap semiconductors by the improvements of overall performances.

Vertical n-type organic transistors with tri(8-hydroxyquinoline) aluminum as collector and fullerene as emitter

A vertical n-type permeable organic metal-base transistor was demonstrated. In this transistor, two kinds of n-type organic semiconductors of fullerene (C60) and tri(8-hydroxyquinoline) aluminum (Alq3) were used as emitter and collector, respectively, and the metal base was comprised of Al/Al2O3/LiF. The formation of a thin oxide film (Al2O3) on the metal aluminum (Al) surface by thermally annealing makes the base self-selected charge carrier characteristic. As a result, the device exhibits low leakage current, and the common-base and common-emitter current gains reach near 1 and 30 at less than 1 V bias, respectively.

Solution-processed diarylfluorene derivatives for violet-blue amplified spontaneous emission and electroluminescence

A series of solution-processable diarylfluorene-based materials with pendent 4-(hexyloxy carbazole)-9,9-diphenyl-9H-fluorene (MC6Cz) or 4-(octyloxy)-9,9-diphenyl-9H-fluorene (MC8) as the core and endcapped with different charge transport moieties such as phenylcarbazole (NPC) or triphenylemine (TPA) have been synthesized and investigated systemically. These materials exhibited bright fluorescence emission from violet-blue to deep blue without molecular aggregation in diluted solutions and films. Furthermore, these materials also displayed prominent virtues including superior solution-processability and amorphous characteristics. Consequently, the EL spectra of the solution-processed organic light-emitting diodes (OLEDs) are expected to be similar to the corresponding PL spectra, suggesting intramolecular photophysical behavior. MC6Cz-9-NPC as an emitting layer presented the highest maximum luminance of about 1560 cd m−2 with a lower turn on voltage of 3.5 V. In contrast, the ASE results revealed that the MC8 core-containing compounds exhibited relatively lower thresholds comparable with traditional conjugated or star type polymers, which could be promising optical gain media. We proposed a novel molecular design strategy to construct high-performance solution-processed violet-blue fluorophores.

A robust molecular unit nanogrid servicing as network nodes via molecular installing technology

Organic square nanogrid units have been designed in order to create molecular system nodes and their networks. A nanogrid that consists of four fluorenes and two carbazoles has been synthesized by molecular installing technology from an L-shaped synthon at a total yield of 21%. The fluorene-based nanogrids showed excellent thermal, electrochemical and spectral stability. Nanogrids will be one robust platform to polynodes for the exploration of molecular consciousness, intelligence and robotics.

A fiber based diamond RF B-field sensor and characterization of a small helical antenna

We present a microwave B-field scanning imaging technique using diamond micro-crystal containing nitrogen vacancy center that is attached to a fiber tip. We propose a pulsed modulation technique, enabling the implementation of a variety of pulsed quantum algorithm for state manipulation and fast readout of spin state. A detailed mapping of the magnetic B-field distribution of a helical antenna with sub-100 micron resolution is presented and compared with numerical simulations. This fiber based microwave B-field probe has the advantage of minimized invasiveness, small overall size, will boost broad interest in a variety of applications where near field distribution is essential to device characterization, to name a few, antenna radiation profiling, monolithic microwave integrated circuits failure diagnosis, electromagnetic compatibility test of microwave integrated circuits and microwave cavity field mode mapping.