Charge-Localized Retention and Long-Term Memory Enabled by Cooperating Sterically Confined Molecular Crystallization with Spiro[fluorene-9,9'-xanthene]-Based Csp3-Hindrance.

Charge localization of memory materials plays a crucial role in the endurance and retention ability of organic nonvolatile memory, which is completely opposite from the charge delocalization of high-mobility materials. However, charge transfer of both though-space and through-bond based on molecular design principles still faces challenges. Herein, a nonplanar wide-bandgap semiconductor with Csp3-hindrance (DOCH3-DDPA-SFX) has been designed and synthesized. An effective crystallization effect of self-assembled two-dimensional nanosheets on charge trapping dynamics and kinetics is visualized by Kelvin probe force microscopy (KPFM). The trapped charges are localized completely on a single nanosheet, which has better charge trapping and retention properties than an amorphous film. Meanwhile, crystallization also greatly improves structure stability. Combining DFT theoretical calculations, the mechanisms of localization and long-term retention are discussed. The steric crystallization effects on the charge localization will guide the effective design of single-component semiconducting charge-memory materials by molecular assembly and aggregate control for high-performance organic memory.

17O solid-state NMR study on exposed facets of ZnO nanorods with different aspect ratios.

The physical and chemical properties of metal oxide nanocrystals are closely related to their exposed facets, so the study on facet structures is helpful to develop facet/morphology-property relationships and rationally design nanostructures with desired properties. In this study, wurtzite ZnO nanorods with different aspect ratios were prepared by controlling the Zn2+/OH- ratio, temperature and time in hydrothermal processes. An 17O solid-state NMR study was performed on these nanorods, after surface 17O labeling, to explore the relationship of the 17O NMR signals with the local surface structure of different exposed facets, i.e., nonpolar (101̄0) and polar (0002) facets. It is observed that, one of the signals, the sharp component of a peak at -18.8 ppm, comprises the contribution from the oxygen ions on the polar (0002) facets, in addition to that from nonpolar (101̄0) facets, which is confirmed by 17O NMR spectra of ZnO nanorods with controlled aspect ratios and different thermal treatment conditions. This is important for accurately interpreting the 17O NMR signal of ZnO-containing materials, especially when studying the facet-related mechanisms. The method applied here can also be extended to study the facet-dependent properties of other faceted oxide nanocrystals.

Regulating and Predicting the Polyhedral Crystal Morphology in Spirofluorene Molecular Systems.

Crystallization of organic steric molecules often leads to multiple polyhedral crystal morphologies. However, the relationships among the molecular structure, supramolecular interaction, aggregation mode and crystal morphology are still unclear. In this work, we elaborate two model crystals formed by spiro[fluorene-9,9'-xanthene] (SFX) and spiro[cyclopenta[1,2-b : 5,4-b']dipyridine-5,9'-xanthene] (SDAFX) to demonstrate the feasibility of morphology prediction by periodic bond chain (PBC) theory based on interaction energy (IE) values in terms of single point energy. With non-directional van der Waals forces, only one PBC direction is found in SFX crystal, leading to the irregular 1D rod-like structure. Compared with SFX, the extra N heteroatoms in SDAFX can bring additional hydrogen bonds and some other interactions into the bulky molecular skeletons, inducing 3-dimensionally oriented PBCs to form the explicit F-face network in SDAFX which leads to the final octahedral structure. A simple and accurate method has been provided to quantify PBC vector on the supramolecular level in the organic molecular system, and the PBC theory has also been further demonstrated and developed in the morphology prediction of organic spiro-molecules.

Hierarchical Nanoarchitectonics of Ultrathin 2D Organic Nanosheets for Aqueous Processed Electroluminescent Devices.

Ultrathin 2D organic nanosheets (2DONs) with high mobility have received tremendous attention due to thickness of few molecular layers. However, ultrathin 2DONs with high luminescence efficiency and flexibility simultaneously are rarely reported. Here, the ultrathin 2DONs (thickness: 19 nm) through the modulation of tighter molecular packing (distance: ≈3.31 Å) achievable from the incorporation of methoxyl and dipenylamine (DPA) groups into 3D spirofluorenexanthene (SFX) building blocks is successfully prepared. Even with closer molecular stacking, ultrathin 2DONs still enable the suppression of aggregation quenching to exhibit higher quantum yields of blue emission (ΦF  = 48%) than that on amorphous film (ΦF  = 20%), and show amplified spontaneous emission (ASE) with a mediate threshold (332 mW cm-2 ). Further, through drop-casting method, the ultrathin 2DONs are self-organized into large-scale flexible 2DONs films (1.5 × 1.5 cm) with the low hardness (H: 0.008 Gpa) and low Young's modulus (Er : 0.63 Gpa). Impressively, the large-scale 2DONs film can realize electroluminescence performances with a maximum luminance (445 cd m-2 ) and low turn on voltage (3.7 V). These ultrathin 2DONs provide a new avenue for the realization of flexible electrically pumping lasers and intelligent quantum tunneling systems.

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative.

The exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet-triplet energy splitting (ΔEST) and doping control. However, exciplex-TADF is still faced with the problems of low external quantum efficiency (ηext) and unclear effect of structure modification in electron acceptors. Herein, we provide a steric hindrance increase strategy to obtain high-efficiency exciplex emissions. Through introducing a 9-phenylfluorene group into N-ethylcarbazole of the dicyano-substituted 9-phenylfluorene, an electron acceptor material with increased steric hindrance is obtained, which helps the exciplex harvest a larger driving force and higher emission efficiencies. Encouragingly, the obtained OLED displays a maximum ηext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A-1 and power efficiency of 93.7 lm W-1. It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex emissions.

A Bio-Inspired Molecular Design Strategy toward 2D Organic Semiconductor Crystals with Superior Integrated Optoelectronic Properties.

Inspired by the 2D bilayer lipid membranes in nature, a unique supramolecular "push-pull" synergetic strategy toward self-assembled 2D organic crystals (2DOCs) is proposed in this work, which can effectively suppress the interlayer 3D stacking while maintaining the assembly of the intralayer for 2D growth. For this purpose, a model molecule PF-Py consisting of a planar supramolecular "attractor" and a nonplanar steric "repellor" is designed for the solution self-assembly process. Well-defined 2DOCs including crystal nanosheets and millimeter-sized crystal films with layered amphiphile-like packing are obtained, which is analogical to the cell membranes of living organisms. Thanks to the special packing mode, the 2DOCs have fascinating integrated photoelectric property, with high mobility of 7.8 × 10-2 cm2 V-1 s-1 , high crystalline state photoluminescence quantum yield of 55%, and superior deep-blue laser characteristics with a low threshold of 5.51 µJ cm-2 . This supramolecular synergetic strategy advances the design of 2D organic semiconductor crystals for high performance optoelectronics.

Supramolecular Non-Helical One-Dimensional Channels and Microtubes Assembled from Enantiomers of Difluorenol.

The design and assembly of photoelectro-active molecular channel structures is of great importance because of their advantages in charge mobility, photo-induced electron transfer, proton conduction, and exciton transport. Herein, we report the use of racemic 9,9'-diphenyl-[2,2'-bifluorene]-9,9'-diol (DPFOH) enantiomers to produce non-helical 1D channel structures. Although the individual molecule does not present any molecular symmetry, two pairs of racemic DPFOH enantiomers can form a C2 -symmetric closed loop via the stereoscopic herringbone assembly. Thanks to the special symmetry derived from the enantiomer pairs, the multiple supramolecular interactions, and the padding from solvent molecules, this conventionally unstable topological structure is achieved. The etching of solvent in 1D channels leads to the formation of microtubes, which exhibit a significant lithium-ion conductivity of 1.77×10-4  S cm, indicating the potential research value of this novel 1D channel structure.

Enhanced emission in organic nanocrystals via asymmetrical design of spirocyclic aromatic hydrocarbons.

Two spirocyclic aromatic hydrocarbon derivatives were prepared to clarify the molecular geometry effects on the regulation of the crystalline morphologies and photophysical behaviors of organic nanocrystals. Due to the different structural symmetry of a spiro-center, distinguishing nanocrystal morphologies with unique crystallization-enhanced/quenched emission was achieved.

The Dimensionality Transition between Three-Dimensional and Two-Dimensional Organic Microcrystals: Specific Symmetry and Selective Adhesion.

A three-dimensional-two-dimensional dimensionality transition of organic microcrystals was performed via elongated octahedrons, oblique octahedrons, diamond-like particles, and parallelograms using nonplanar 2,7-di(9H-carbazol-9-yl)spiro[fluorene-9,9'-xanthene] molecules. The specific crystal symmetry and selective adhesion of P123 on the {11-1}s facet are supposed to induce the morphological transition.

Halogen bonding in the co-crystallization of potentially ditopic diiodotetrafluorobenzene: a powerful tool for constructing multicomponent supramolecular assemblies.

Halogen bonding is emerging as a significant driving force for supramolecular self-assembly and has aroused great interest during the last two decades. Among the various halogen-bonding donors, we take notice of the ability of 1,4-diiodotetrafluorobenzene (1,4-DITFB) to co-crystallize with diverse halogen-bonding acceptors in the range from neutral Lewis bases (nitrogen-containing compounds, N-oxides, chalcogenides, aromatic hydrocarbons and organometallic complexes) to anions (halide ions, thio/selenocyanate ions and tetrahedral oxyanions), leading to a great variety of supramolecular architectures such as discrete assemblies, 1D infinite chains and 2D/3D networks. Some of them act as promising functional materials (e.g. fluorescence, phosphorescence, optical waveguide, laser, non-linear optics, dielectric and magnetism) and soft materials (e.g. liquid crystal and supramolecular gel). Here we focus on the supramolecular structures of multicomponent complexes and their related physicochemical properties, highlight representative examples and show clearly the main directions that remain to be developed and improved in this area. From the point of view of crystal engineering and supramolecular chemistry, the complexes summarized here should give helpful information for further design and investigation of the elusive category of halogen-bonding supramolecular functional materials.

Superelectrophilic-Initiated C-H Functionalization at the ß-Position of Thiophenes: A One-Pot Synthesis of trans-Stereospecific Saddle-Shaped Cyclic Compounds.

Superelectrophilic-initiated direct C-H functionalization of thiophenes at the ß-position was developed. A series of trans-stereospecific [2,1-a]-IF-thiophene-fused cyclic compounds (4) with saddle-shaped structure were prepared in 17-30% yields through a one-pot superelectrophilic Friedel-Crafts reaction of dihydroindenofluorene with thiophene derivatives. From the crystal packing analyses of 4a, its skeleton shows both strong intermolecular π-π stacking and C-H···π stacking. Furthermore, the ring-dependent photophysical properties of 4 were confirmed by UV-vis absorption and photoluminescence spectroscopy as well as through the study of their fluorescence quantum yield.

A convenient one-pot nanosynthesis of a C(sp2)-C(sp3)-linked 3D grid via an 'A2 + B3' approach.

Fluorene-based 3D-grid-FTPA was synthesised with a total yield of 55% via the one-pot formation of six C(sp2)-C(sp3) bonds through a BF3·Et2O-mediated Friedel-Crafts reaction of A2-type bifluorene tertiary alcohol (BIOH) and two B3-type triphenylamines. At the same time, Un-grid-FTPA (2.7%) and 2D-grid-FTPA (5.6%) were obtained as by-products from this synthesis method. In addition, the effect of stereoisomers of BIOH was evaluated to demonstrate that Rac-BIOH is a better A2-type building block to prepare 3D-grid-FTPA in a relatively high yield. Furthermore, 3D-grid-FTPA showed excellent chemical, thermal, and photo-stabilities.

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.

Supramolecular steric hindrance effect on morphologies and photophysical behaviors of spirocyclic aromatic hydrocarbon nanocrystals.

Three pyrene-based spirocyclic aromatic hydrocarbons (Py-SAHs) were prepared to clarify the roles of molecular segments in regulating the morphologies and photophysical properties of organic microcrystals. Due to the different supramolecular steric hindrance (SSH) effect between bulky groups and pyrene rings, distinct nanocrystal morphologies with unique photoluminescence properties were realized.

An eco-friendly water-assisted polyol method to enhance the aspect ratio of silver nanowires.

With water as an eco-friendly heterogeneous nucleation accelerator, silver nanowires (Ag NWs) have been successfully prepared with a high aspect ratio (>1600). The Ag NW-based film exhibits a low sheet resistance of 8.1 Ω sq-1 with a transparency of 81.9% at 550 nm, showing the potential application of electrode materials in polymer solar cells.

4,5-Diazafluorene-Based Donor-Acceptor Small Molecules as Charge Trapping Elements for Tunable Nonvolatile Organic Transistor Memory.

Three diazafluorene derivatives triphenylamine (TPA)(PDAF) n (n = 1, 2, 3) serving as small molecular elements are designed and synthesized via concentrated sulfuric acid mediated Friedel-Crafts reaction. With highly nonplanar topological configuration, TPA(PDAF)3 shows weaker intermolecular interaction in the solid states and thus exhibits single nanomolecular behavior, which is crucial for charge stored and retained in an organic field-effect transistor (OFET) memory device. Furthermore, diazafluorene derivatives possess a completely separate highest occupied molecular orbital/lowest unoccupied molecular orbital, which offers ideal hole and electron trapping sites. As charge storage elements, triphenylamine groups provide the hole trapping sites, while diazafluorene units provide the electron trapping sites and act as a hole blocking group to restrain the leakage of stored holes trapped in triphenylamine. The pentacene-based OFET memory device with solution-processing TPA(PDAF)3 shows a good hole-trapping ability, high hole trapping density (4.55 × 1012 cm-2), fast trapping speed (<20 ms), a large memory window (89 V), and a tunable ambipolar memory behavior. The optimized device shows a large ON/OFF current ratio (2.85 × 107), good charge retention (>104 s), and reliable endurance properties. This study suggests that diazafluorene based donor-acceptor small molecular elements have great promise for high-performance OFET memory.

Variable segment roles: modulation of the packing modes, nanocrystal morphologies and optical emissions.

Three isomers were prepared by covalently grafting carbazole (Cz) onto spiro[fluorene-9,9'-xanthene] (SFX) at different positions. Due to the complicated and variable roles of molecular segments, an evolution of the corresponding molecular packing mode was realized, accompanied by the change of nanocrystal morphology and photoluminescence properties.

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.

SMART Design of a Bulk-Capped Supramolecular Segment for the Assembly into Organic Interdigital Lipid Bilayer-Like (ILB) Nanosheets.

Rational molecular design for the organic nanocrystal morphology still remains a challenge due to the structural diversity and complicated weak intermolecular interactions. In this work, a typical attractor-repulsor molecule N,N-diphenyl-4-(9-phenyl-fluoren-9-yl) phenylamine (TPA-PF) is designed to explore a general assembly strategy for 2D nanocrystals. Via an interdigital lipid bilayer-like (ILB) molecular packing mode, large-sized lamellar 2D nanosheets are obtained with a length:width:thickness ratio as ≈2500:1000:1. The d-spacing of the largest (001) plane is 1.32 nm, which equals to the thickness of a single interdigital stacking layer. The synergetic effect of the attractive supramolecular segment (TPA) and the repulsive bulky group (PF) is supposed to be the critical factor for the ILB packing that leads to the 2D structures. The attractor-repulsor molecule design is expected to be an effective strategy for the growth of 2D nanocrystals based on small organic molecules.

Detection of trapped charges in the blend films of polystyrene/SFDBAO electrets by electrostatic and Kelvin probe force microscopy.

The charge trapping properties of the blend of polystyrene (PS) and a sterically hindered organic semiconductor SFDBAO (spiro[fluorene-9,7-dibenzo[c,h]acridin-5-one]) are investigated by electrostatic and Kelvin probe force microscopy (EFM and KPFM). EFM signals of trapped charge spots injected with controllable tip biases, which are recorded with different dissipation times t, the percent of SFDBAO in blends, and the scanning tip bias, have been measured. By the quantitative analysis, the excellent trapped charge density of PS/SFDBAO blend films for the holes (∼×10(-5) C m(-2)) is much higher than that of the SFDBAO film (∼×10(-6) C m(-2)) and the PS film (∼×10(-7) C m(-2)). However, the trapped charge density of electrons (∼×10(-7) C m(-2)) has the same order magnitude for SFDBAO, PS and the blend films. The results indicate that the blend of PS and SFDBAO enhances the high-density storage and retention abilities of the holes to a larger extent, but the endurance improvement of the electrons is not that obvious. By the KPFM measurement, we further verify the different diffusion rates of the trapped holes and electrons in the PS/SFDBAO blend films, and discuss the possible physical mechanism. The qualitative and quantitative determination of charge trapping properties in this work can be very useful for the characterization of PS/SFDBAO based charge trapping memory devices.