Energy Transfer in Nanotube‐Perylene Complexes

A functional surfactant designed to solubilize and individualize nanotubes efficiently in aqueous media and to form energy transfer complexes with the carbon nanotubes through π–π stacking is presented. Upon excitation of the adsorbed perylene unit an emission from the nanotubes is observed that indicates a successful excitation transfer. The efficiency of the indirect excitation via the adsorbant is of the same order of magnitude as the direct excitation of the nanotubes. Under optimized preparation conditions the pH independent perylene‐imido‐diester compound isolates and solubilizes carbon nanotubes in biocompatible aqueous environments without additional surfactants. The resulting solutions are stable over many months.     

Electronic Properties and Structure of Assemblies of CdSe Nanocrystal Quantum Dots and Ru‐Polypyridine Complexes Probed by Steady State and Time‐Resolved Photoluminescence

Chemical and electronic interactions between CdSe nanocrystal quantum dots (NQDs) and Ru‐polypyridine complexes are studied in solution. It is shown that photoluminescence (PL) can be used to effectively monitor the formation of NQD‐complex assemblies in real time. It is also shown that with the aid of Langmuir isotherm modeling, the PL studies can be used to quantitatively characterize the composition of the assemblies and the strength of electronic interactions between their components. The approach demonstrated here is general and can be applied to other systems that combine semiconductor NQDs and appropriately functionalized organometallic or organic molecules interacting with NQDs via energy transfer, charge transfer, or other mechanisms leading to quenching of NQD emission.     

Cationic Polyfluorenes with Phosphorescent Iridium(III) Complexes for Time‐Resolved Luminescent Biosensing and Fluorescence Lifetime Imaging

The application of a time‐resolved photoluminescence technique and fluorescence lifetime imaging microscopy for biosensing and bioimaging based on phosphorescent conjugated polyelectrolytes (PCPEs) containing Ir(III) complexes and polyfluorene units is reported. The specially designed PCPEs form 50 nm nanoparticles with blue fluorescence in aqueous solutions. Electrostatic interaction between the nanoparticles and heparin improves the energy transfer between the polyfluorene units to Ir(III) complex, which lights up the red signal for naked‐eye sensing. Good selectivity has been demonstrated for heparin sensing in aqueous solution and serum with quantification ranges of 0–70 μM and 0–5 μM, respectively. The signal‐to‐noise ratio can be further improved through time‐resolved emission spectra, especially when the detection is conducted in complicated environment, e.g., in the presence of fluorescent dyes. In addition to heparin sensing, the PCPEs have also been used for specific labeling of live KB cell membrane with high contrast using both confocal fluorescent cellular imaging and fluorescence lifetime imaging microscopies. This study provides a new perspective for designing promising CPEs for biosensing and bioimaging applications.     

White Electroluminescence by Supramolecular Control of Energy Transfer in Blends of Organic‐Soluble Encapsulated Polyfluorenes

Here, it is demonstrated that energy transfer in a blend of semiconducting polymers can be strongly reduced by non‐covalent encapsulation of one constituent, ensured by threading of the conjugated strands into functionalized cyclodextrins. Such macrocycles control the minimum intermolecular distance of chromophores with similar alignment, at the nanoscale, and therefore the relevant energy transfer rates, thus enabling fabrication of white‐light‐emitting diodes (CIE coordinates: x = 0.282, y = 0.336). In particular, white electroluminescence in a binary blend of a blue‐emitting, organic‐soluble rotaxane based on a polyfluorene derivative and the green‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole ( F8BT ) is achieved. Morphological and structural analyses by atomic force microscopy, fluorescence mapping, µ‐Raman, and fluorescence lifetime microscopy are used to complement optical and electroluminescence characterization, and to enable a deeper insight into the properties of the novel blend.     

Reversible Fluorescent On–Off Recording in a Highly Transparent Polymeric Material Utilizing Fluorescent Resonance Energy Transfer (FRET) Induced by Heat Treatment

One productive technique for ultrahigh resolution readout of tiny regions is the measurement of the fluorescence signal of materials. A transparent polymeric materials whose fluorescence quantum yield is changed and recorded by thermally controlling the aggregation of fluoran dyes and developers with long alkyl chains has been developed. The recording medium can be fabricated easily by casting or coating recording materials. Fluorescence is observed after annealing at 363 K for about twelve seconds and then cooling to room temperature (RT), and quenched by annealing at 423 K for a few seconds and then quenching to RT. Nondestructive readout by excitation light with a fluorescent contrast of above 10 is achieved using red, green, and blue fluorescent dyes. Fluorescence on–off switching is induced by fluorescent resonance energy transfer (FRET) from a fluorescent dye to a colored fluoran dye in the recording material. Fluorescence was uniformly quenched in the visible region after erasing. Since the recording materials allow the penetration of laser light due to the presence of crystals smaller than the wavelength range of visible light in both the emission and quenching states, nondestructive readout of the fluorescent signal by two‐photon absorption is accomplished. This work provides an important stepping‐stone for achieving rewritable‐type near‐field optical storage or multilayer recording.     

Graphene Liquid Cells Assembled through Loop‐Assisted Transfer Method and Located with Correlated Light‐Electron Microscopy

Graphene liquid cells (GLCs) for transmission electron microscopy (TEM) enable high‐resolution, real‐time imaging of dynamic processes in water. Large‐scale implementation, however, is prevented by major difficulties in reproducing GLC fabrication. Here, a high‐yield method is presented to fabricate GLCs under millimeter areas of continuous graphene, facilitating efficient GLC formation on a TEM grid. Additionally, GLCs are located on the grid using correlated light‐electron microscopy (CLEM), which reduces beam damage by limiting electron exposure time. CLEM allows the acquisition of reliable statistics and the investigation of the most common shapes of GLCs. In particular, a novel type of liquid cell is found, formed from only a single graphene sheet, greatly simplifying the fabrication process. The methods presented in this work—particularly the reproducibility and simplicity of fabrication—will enable future application of GLCs for high‐resolution dynamic imaging of biomolecular systems.     

Atmosphere‐Induced Reversible Resistivity Changes in Ca/Y‐Doped Bismuth Iron Garnet Thin Films

Bismuth iron garnet Bi₃Fe₅O₁₂ (BIG) is a multifunctional insulating oxide exhibiting remarkably the largest known Faraday rotation and linear magnetoelectric coupling. Enhancing the electrical conductivity in BIG while preserving its magnetic properties would further widen its range of potential applications in oxitronic devices. Here, a site‐selective codoping strategy in which Ca²⁺ and Y³⁺ substitute for Bi³⁺ is applied. The resulting p‐ and n‐type doped BIG films combine state‐of‐the‐art magneto‐optical properties and semiconducting behaviors above room temperature with rather low resistivity: 40 Ω cm at 450 K is achieved in an n‐type Y‐doped BIG; this is ten orders of magnitude lower than that of Y₃Fe₅O₁₂. High‐resolution electron spectromicroscopy unveils the complete dopant solubility and the charge compensation mechanisms at the local scale in p‐ and n‐type systems. Oxygen vacancies as intrinsic donors play a key role in the conduction mechanisms of these doped BIG films. On the other hand, a self‐compensation of Ca²⁺ with oxygen vacancies tends to limit the conduction in p‐type Ca/Y‐doped BIG. These results highlight the possibility of integrating n‐type and p‐type doped BIG films in spintronic structures as well as their potential use in gas sensing applications.     

ph‐Dependent Synthesis of Pepsin‐Mediated Gold Nanoclusters with Blue Green and Red Fluorescent Emission

This report demonstrates the first pH‐dependent synthesis of pepsin‐mediated gold nanoclusters (AuNCs) with blue‐, green‐, and red‐fluorescent emission from Au₅ (Au₈), Au₁₃, and Au₂₅, respectively. Pepsin is a gastric aspartic proteinase (molecular weight, 34 550 g/mol) that plays an integral role in the digestive process of vertebrates. It was found that the pH of the reaction solution was critical in determining the size of Au NCs (i.e., the number of gold atoms of AuNCs). Interestingly, enzyme function of pepsin contributes to the formation of these AuNCs. The photo‐stability of the Au₂₅ (or Au₁₃) NCs is much higher than that of Au₅NCs (i.e., Au₂₅ ～ Au₁₃ > > Au₅). The pepsin‐mediated Au₂₅NCs were also found to be useful as fluorescent sensors for the detection of Pb²⁺ ions by enhanced fluorescence and the detection of Hg²⁺ ions by fluorescence quenching. Although the detailed formation mechanisms of these AuNCs require further analysis, the synthetic route using proteinase demonstrated here is promising for preparing new types of fluorescent metal nanoclusters for application in catalysis, optics, biological labeling, and sensing.     

Time‐of‐Flight Studies of Electron‐Collection Kinetics in Polymer:Fullerene Bulk‐Heterojunction Solar Cells

The charge‐collection dynamics in poly(3‐hexylthiophene:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) bulk heterojunctions are studied in thick (>1 μm) devices using time‐of‐flight measurements and external quantum‐efficiency measurements. The devices show Schottky‐diode behavior with a large field‐free region in the device. Consequently, electron transport occurs by diffusion in the bulk of the active layer. At high applied biases where the depletion region spans the entire active layer, normal time‐of‐flight transients are observed from which the electron mobility can be determined. Here, the electron mobility follows Poole–Frenkel behavior as a function of field. At lower applied biases, where the depletion region only spans a small portion of the active layer, due to a high density of dark holes, the recombination kinetics follow a first‐order rate law with a rate constant about two orders of magnitude lower than that predicted by Langevin recombination.     

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

The charge carrier dynamics of epitaxial hematite films is studied by time‐resolved microwave (TRMC) and time‐resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20–50 ns and 1–2 μs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.     

Following Chemical Charge Trapping in Pentacene Thin Films by Selective Impurity Doping and Wavelength‐Resolved Electric Force Microscopy

Charge trapping is one of several factors that limit the performance of organic electronic materials, yet even in pentacene, a prototypical small‐molecule semiconductor, the precise chemical nature of charge trapping remains poorly understood. Here the effects of three chemical trap‐precursor candidates are examined by layering thin‐film pentacene transistors with different pentacene defect species. The resulting charge trapping is studied in each device via scanning‐probe electric force microscopy coupled with variable‐wavelength sample illumination. Firstly, it is found that layering with pentacen‐6(13H)‐one (PHO) readily produces uniform charge trapping everywhere in the transistor channel, as expected for an active blanket‐deposited trap‐precursor. However, layering with 6,13‐dihydropentacene (DHP) produces fewer, more‐isolated traps, closely resembling the surface potential distribution in pristine pentacene thin films. Secondly, the rates of trap‐clearing versus illuminating wavelength (trap‐clearing spectra) are measured, revealing enhanced trap‐clearing rates at wavelengths assigned to the absorption of either pentacene or the charged trap species. The trap‐clearing spectrum for the PHO‐layered sample closely resembles the spectrum obtained from pentacene aged in a working transistor, while the trap‐clearing spectrum for the DHP‐layered sample resembles the spectrum observed in pristine pentacene. We conclude that PHO competently creates traps in pentacene that match the expected trap‐clearing spectrum for degraded pentacene, while DHP does not, and that the chemical trap species in aged pentacene is very likely PHO⁺.     

A Triphenylamine Dye Model for the Study of Intramolecular Energy Transfer and Charge Transfer in Dye‐Sensitized Solar Cells

A novel dye ( 2TPA‐R ), containing two triphenylamine (TPA) units connected by a vinyl group and rhodanine‐3‐acetic acid as the electron acceptor, is designed and synthesized successfully to reveal the working principles of organic dye in dye‐sensitized solar cells (DSSCs). 2TPA and TPA‐R , which consist of two TPA units connected by vinyl and a TPA unit linked with rhodanine‐3‐acetic acid, respectively, are also synthesized as references to study the intramolecular energy transfer (E_nT) and charge transfer (ICT) processes of 2TPA‐R in CH₂Cl₂ solution and on a TiO₂ surface. The results suggest that the intramolecular E_nT and ICT processes show a positive effect on the performance of DSSCs. However, the flexible structure and less‐adsorbed amount of dye on TiO₂ may make it difficult to improve the efficiency of DSSCs. This study on intramolecular E_nT and ICT processes acts as a guide for the design and synthesis of efficient organic dyes in the future.     

Free‐Standing and Eco‐Friendly Polyaniline Thin Films for Multifunctional Sensing of Physical and Chemical Stimuli

Multifunctional flexible sensors that are sensitive to different physical and chemical stimuli but remain unaffected by any mechanical deformation and/or changes still present a challenge in the implementation of flexible devices in real‐world conditions. This challenge is greatly intensified by the need for an eco‐friendly fabrication technique suitable for mass production. A new eco‐friendly and scalable fabrication approach is reported for obtaining thin and transparent multifunctional sensors with regulated electrical conductivity and tunable band‐gap. A thin (≈190 nm thickness) freestanding sensing film with up to 4 inch diameter is demonstrated. Integration of the freestanding films with different substrates, such as polyethylene terephthalate substrates, silk textile, commercial polyethylene thin film, and human skin, is also described. These multifunctional sensors can detect and distinguish between different stimuli, including pressure, temperature, and volatile organic compounds. All the sensing properties explored are stable under different bending/strain states.     

Interlayer‐Sensitized Linear and Nonlinear Photoluminescence of Quasi‐2D Hybrid Perovskites Using Aggregation‐Induced Enhanced Emission Active Organic Cation Layers

A concept of interlayer‐sensitized photoluminescence (PL) of quasi‐2D hybrid perovskite (PVK) with a π‐conjugated optically interacting organic cation layer is introduced and demonstrated. A rod‐shaped aggregation‐induced enhanced emission (AIEE) organic cation (BPCSA+), well fitted into the lattice size of 2D PVK layers, is designed and synthesized to prolong the exciton lifetime in a condensed layer assembly in the PVK. The BPCSA+ promotes the PL of this hybrid PVK up to 10‐folds from that of a non‐π‐conjugated organic cation (OA) 2D PVK. Notably, different from PL of OA 2D PVK, the increased PL intensity of BPCSA 2D PVKs with an increase of the BPCSA ratio in the PVK indicates a critical photon‐harvesting contribution of BPCSA. The films of BPCSA 2D PVKs are incredibly stable in ambient environments for more than 4 months and even upon direct contact with water. Additionally, due to the strong two‐photon absorption property of BPCSA, the BPCSA 2D PVK displays superior emission properties upon two‐photon excitation with a short wavelength IR laser. Thus, the AIEE sensitization system for quasi‐2D PVK hybrid system can make a drastic improvement in performance as well as in the stability of the PVK emitter and PVK based nonlinear optical devices.     

Interaction Scheme and Temperature Behavior of Energy Transfer in a Light‐Emitting Inorganic‐Organic Composite System

Determining and controlling the inter‐component excitation conversion in light‐emitting nanocomposite materials is a key factor for predicting the composite luminescence properties and for the operation of many opto‐electronic devices. Here we present an extensive study of the inter‐component energy transfer in the composite system given by ZnO particles interacting with the conjugated polymer, poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene]. The composite emission is studied upon varying the acceptor concentration, and the system temperature in the range 50–300 K. The temperature dependence of the energy transfer rate is described by a rate model, taking into account the temperature dependence of the single components nonradiative decay rates, and a dipole–surface interaction scheme in the hybrid material. The proposed model accounts very well for the experimental observation of energy transfer and can be used to predict the temperature behavior of the emission from light‐emitting nanocomposite materials.     

Deep‐Blue Phosphorescent Ir(III) Complexes with Light‐Harvesting Functional Moieties for Efficient Blue and White PhOLEDs in Solution‐Process

The photoluminescence (PL) efficiency of emitters is a key parameter to accomplish high electroluminescent performance in phosphorescent organic light‐emitting diodes (PhOLEDs). With the aim of enhancing the PL efficiency, this study designs deep‐blue emitting heteroleptic Ir(III) complexes (tBuCN‐FIrpic, tBuCN‐FIrpic‐OXD, and tBuCN‐FIrpic‐mCP) for solution‐processed PhOLEDs by covalently attaching the light‐harvesting functional moieties (mCP‐Me or OXD‐Me) to the control Ir(III) complex, tBuCN‐FIrpic. These Ir(III) complexes show similar deep‐blue emission peaks around 453, 480 nm (298 K) and 447, 477 nm (77 K) in chloroform. tBuCN‐FIrpic‐mCP demonstrates higher light‐harvesting efficiency (142%) than tBuCN‐FIrpic‐OXD (112%), relative to that of tBuCN‐FIrpic (100%), due to an efficient intramolecular energy transfer from the mCP group to the Ir(III) complex. Accordingly, the monochromatic PhOLEDs of tBuCN‐FIrpic‐mCP show higher external quantum efficiency (EQE) of 18.2% with one of the best blue coordinates (0.14, 0.18) in solution‐processing technology. Additionally, the two‐component (deep‐blue:yellow‐orange), single emitting layer, white PhOLED of tBuCN‐FIrpic‐mCP shows a maximum EQE of 20.6% and superior color quality (color rendering index (CRI) = 78, Commission Internationale de L'Eclairage (CIE) coordinates of (0.353, 0.352)) compared with the control device containing sky‐blue:yellow‐orange emitters (CRI = 60, CIE coordinates of (0.293, 0.395)) due to the good spectral coverage by the deep‐blue emitter.     

A Ratiometric Near‐Infrared Fluorescent Probe for Quantification and Evaluation of Selenocysteine‐Protective Effects in Acute Inflammation

Selenocysteine (Sec) is a primary kind of reactive selenium species in cells whose antioxidant roles in a series of liver diseases have been featured. However, it is difficult to determine Sec in living cells and in vivo due to its high reactivity and instability. This work reports a ratiometric near‐infrared fluorescent probe (Cy‐SS) for qualitative and quantitative determination of Sec in living cells and in vivo. The probe is composed of heptamethine cyanine fluorophore, the response unit bis(2‐hydroxyethyl) disulfide, and the liver‐targeting moiety d ‐galactose. Based on a detection mechanism of selenium–sulfur exchange reaction, the concentrations of Sec in HepG2, HL‐7702 cells, and primary mouse hepatocytes is determined as 3.08 ± 0.11 × 10^?6m , 4.03 ± 0.16 × 10^?6m and 4.34 ± 0.30 × 10^?6m , respectively. The probe can selectively accumulate in liver. The ratio fluorescence signal of the probe can be employed to quantitatively analyze the fluctuation of Sec concentrations in cells and mice models of acute hepatitis. The experimental results demonstrate that Sec plays important antioxidant and anti‐inflammatory roles during inflammatory process. And the levels of intracellular Sec have a close relationship with the degree of liver inflammation. The above imaging detections make this new probe a potential candidate for the accurate diagnosis of inflammation.