Morphology-dependent energy transfer dynamics in fluorene-based amphiphile nanoparticles

NIR-II Fluorescence Imaging for In Vivo Quantitative Analysis

In vivo real-time qualitative and quantitative analysis is essential for the diagnosis and treatment of diseases such as tumors. Near-infrared-II (NIR-II, 1000-1700 nm) bioimaging is an emerging visualization modality based on fluorescent materials. The advantages of NIR-II region fluorescent materials in terms of reduced photon scattering and low tissue autofluorescence enable NIR-II bioimaging with high resolution and increasing depth of tissue penetration, and thus have great potential for in vivo qualitative and quantitative analysis. In this review, we first summarize recent advances in NIR-II imaging, including fluorescent probe selection, quantitative analysis strategies, and imaging. Then, we describe in detail representative applications to illustrate how NIR-II fluorescence imaging has become an important tool for in vivo quantitative analysis. Finally, we describe the future possibilities and challenges of NIR-II fluorescence imaging.

Low-Dimensional Organic Crystals: From Precise Synthesis to Advanced Applications

Low-dimensional organic crystals (LOCs) have attracted increasing attention recently for their potential applications in miniaturized optoelectronics and integrated photonics. Such applications are possible owing to their tunable physicochemical properties and excellent charge/photon transport features. As a result, the precise synthesis of LOCs has been examined in terms of morphology modulation, large-area pattern arrays, and complex architectures, and this has led to a series of appealing structure-dependent properties for future optoelectronic applications. This review summarizes the recent advances in the precise synthesis of LOCs in addition to discussing their structure-property relationships in the context of optoelectronic applications. It also presents the current challenges related to organic crystals with specific structures and desired performances, and the outlook regarding their use in next-generation integrated optoelectronic applications.

Recent Developments in Semiconducting Polymer Dots for Analytical Detection and NIR-II Fluorescence Imaging

In recent years, semiconducting polymer dots (Pdots) have attracted enormous attention in applications from fundamental analytical detection to advanced deep-tissue bioimaging due to their ultrahigh fluorescence brightness with excellent photostability and minimal cytotoxicity. Pdots have therefore been widely adopted for a variety types of molecular sensing for analytical detection. More importantly, the recent development of Pdots for use in the optical window between 1000 and 1700 nm, popularly known as the "second near-infrared window" (NIR-II), has emerged as a class of optical transparent imaging technology in the living body. The advantages of the NIR-II region over the traditional NIR-I (700-900 nm) window in fluorescence imaging originate from the reduced autofluorescence, minimal absorption and scattering of light, and improved penetration depths to yield high spatiotemporal images for biological tissues. Herein, we discuss and summarize the recent developments of Pdots employed for analytical detection and NIR-II fluorescence imaging. Starting with their preparation, the recent developments for targeting various analytes are then highlighted. After that, the importance of and latest progress in NIR-II fluorescence imaging using Pdots are reported. Finally, perspectives and challenges associated with the emergence of Pdots in different fields are given.

Fluorene benzothiadiazole co-oligomer based aqueous self-assembled nanoparticles

Self-assembled π-conjugated nanoparticles with tunable optical characteristics are appealing for sensing and imaging applications due to their intrinsic fluorescence, supramolecular organization and dynamics.

Synthesis-driven, structure-dependent optical behavior in phase-tunable NaYF4:Yb,Er-based motifs and associated heterostructures

Understanding the key parameters necessary for generating uniform Er,Yb co-activated NaYF₄ possessing various selected phases (i.e. cubic or hexagonal) represents an important chemical strategy towards tailoring optical behavior in these systems. Herein, we report on a straightforward hydrothermal synthesis in which the separate effects of reaction temperature, reaction time, and precursor stoichiometry in the absence of any surfactant were independently investigated. Interestingly, the presence and the concentration of NH₄OH appear to be the most critical determinants of the phase and morphology. For example, with NH₄OH as an additive, we have observed the formation of novel hierarchical nanowire bundles which possess overall lengths of ∼5 μm and widths of ∼1.5 μm but are composed of constituent component sub-units of long, ultrathin (∼5 nm) nanowires. These motifs have yet to be reported as distinctive morphological manifestations of fluoride materials. The optical properties of as-generated structures have also been carefully analyzed. Specifically, we have observed tunable, structure-dependent energy transfer behavior associated with the formation of a unique class of NaYF₄-CdSe quantum dot (QD) heterostructures, incorporating zero-dimensional (0D), one-dimensional (1D), and three-dimensional (3D) NaYF₄ structures. Our results have demonstrated the key roles of the intrinsic morphology-specific physical surface area and porosity as factors in governing the resulting opto-electronic behavior. Specifically, the trend in energy transfer efficiency correlates well with the corresponding QD loading within these heterostructures, thereby implying that the efficiency of FRET appears to be directly affected by the amount of QDs immobilized onto the external surfaces of the underlying fluoride host materials.

Hydrophobicity determines the fate of self-assembled fluorescent nanoparticles in cells

The fate of small molecule nanoparticles (SMNPs) composed of self-assembling intrinsically fluorescent π-conjugated oligomers was studied in cells as a function of side-chain hydrophobicity. While the hydrophobic SMNPs remained intact upon cellular uptake, the more hydrophilic SMNPs disassembled and dispersed throughout the cytosol.

Air-Stable Pd Catalytic Systems for Sequential One-Pot Synthesis of Challenging Unsymmetrical Aminoaromatics

The selective functionalization of dibromoaromatic scaffolds using air-stable palladium catalytic systems was carried out. This methodology involved rapid mono and diselective Buchwald-Hartwig aminations via microwave irradiation. The conditions were optimized to couple sequentially different moieties in one pot. Couplings with a wide scope of amines allowed accessing a new library of symmetrical and unsymmetrical derivatives (35 examples). Using this versatile method, a near-IR push-pull sensor was prepared installing the electron-donating and -withdrawing groups through a multicomponent reaction. These conditions revealed to be gram-scalable and adaptable to various groups; hence, promoting facile use in synthetic chemistry.

A carbazole–fluorene molecular hybrid for quantitative detection of TNT using a combined fluorescence and quartz crystal microbalance method

Self-assembled fluorescent rods and nanoparticles prepared from a carbazole–fluorene molecular hybrid have been used for the sensing of TNT.

Triple helix formation in amphiphilic discotics: demystifying solvent effects in supramolecular self-assembly

A set of chiral, amphiphilic, self-assembling discotic molecules based on the 3,3'-bis(acylamino)-2,2'-bipyridine-substituted benzene-1,3,5-tricarboxamide motif (BiPy-BTA) was prepared. Amphiphilicity was induced into the discotic molecules by an asymmetrical distribution of alkyl and oligo(ethylene oxide) groups in the periphery of the molecules. Small-angle X-ray scattering, cryogenic transmission electron microscopy, and circular dichroism spectroscopy measurements were performed on the discotic amphiphiles in mixtures of water and alcohol at temperatures between 0 °C an 90 °C. The combined results show that these amphiphilic discotic molecules self-assemble into supramolecular fibers consisting of either one or three discotic molecules in the fiber cross-section and that the presence of water induces the bundling of the supramolecular fibers. The rich phase behavior observed for these molecules proves to be intimately connected to the mixing thermodynamics of the water-alcohol mixtures.

Self-assembled fluorescent organic nanoparticles for live-cell imaging

Fluorescent, cell-permeable, organic nanoparticles based on self-assembled π-conjugated oligomers with high absorption cross-sections and high quantum yields have been developed. The nanoparticles are generated with a tuneable density of amino groups for charge-mediated cellular uptake by a straightforward self-assembly protocol, which allows for control over size and toxicity. The results show that a single amino group per ten oligomers is sufficient to achieve cellular uptake. The non-toxic nanoparticles are suitable for both one- and two-photon cellular imaging and flow cytometry, and undergo very efficient cellular uptake.

Fluorescent π-conjugated polymer dots versus self-assembled small-molecule nanoparticles: what's the difference?

Fluorescent nanoparticles based on π-conjugated small molecules and polymers are two different classes of π-conjugated systems that have attracted much interest. To date, both emerging classes have only been studied separately and showed no clear differences in their properties. Herein these nanoparticles are compared on the basis of a fluorene co-polymer and its corresponding small molecule. Both systems formed nanoparticles with the same diameter, whereas the fluorescence properties clearly differed. In case of the polymer the fluorescence diminished, whereas for the small molecules the fluorescence increased. In addition, the capability of encapsulation and release of a hydrophobic dye from the fluorescent nanoparticles was studied. For the polymer system, encapsulation was highly efficient and no release was observed, whereas for the small molecule system encapsulation was less efficient and release of the dye was observed. These studies show a clear difference between small molecules and polymers which has important implications for the design of fluorescent nanoparticles.

Enforcing luminescence at organic nanointerfaces: luminescence spatial confinement and amplification in molecular-based core-shell nanoparticles

Fully organic core-shell nanoparticles that promote luminescence spatial confinement and enhancement at the core-shell nanointerface are designed and prepared. These molecular-based bicomponent nanostructures give rise to very efficient directional excitation energy transfer from the shell to acceptor molecules in the core located at the core-shell nanointerface. A striking luminescence enhancement is observed with respect to the corresponding single-component nanoparticles, which is ascribed to large local electric fields generated at the nanointerface between the polarizable molecular core and shell.

Side chains control dynamics and self-sorting in fluorescent organic nanoparticles

To develop fluorescent organic nanoparticles with tailored properties for imaging and sensing, full control over the size, fluorescence, stability, dynamics, and supramolecular organization of these particles is crucial. We have designed, synthesized, and fully characterized 12 nonionic fluorene co-oligomers that formed self-assembled fluorescent nanoparticles in water. In these series of molecules, the ratio of hydrophilic ethylene glycol and hydrophobic alkyl side chains was systematically altered to investigate its role on the above-mentioned properties. The nanoparticles consisting of π-conjugated oligomers containing polar ethylene glycol side chains were less stable and larger in size, while nanoparticles self-assembled from oligomers containing nonpolar pendant chains were more stable, smaller, and generally had a higher fluorescence quantum yield. Furthermore, the dynamics of the molecules between the nanoparticles was enhanced if the number of hydrophilic side chains increased. Energy transfer studies between naphthalene and benzothiadiazole fluorene co-oligomers with the same side chains showed no exchange of molecules between the particles for the apolar molecules. For the more polar systems, the exchange of molecules between nanoparticles took place at room temperature or after annealing. Self-assembled nanoparticles consisting of π-conjugated oligomers having different side chains caused self-sorting, resulting either in the formation of domains within particles or the formation of separate nanoparticles. Our results show that we can control the stability, fluorescence, dynamics, and self-sorting properties of the nanoparticles by simply changing the nature of the side chains of the π-conjugated oligomers. These findings are not only important for the field of self-assembled nanoparticles but also for the construction of well-defined multicomponent supramolecular materials in general.

Organic solar cells: understanding the role of Förster resonance energy transfer

Organic solar cells have the potential to become a low-cost sustainable energy source. Understanding the photoconversion mechanism is key to the design of efficient organic solar cells. In this review, we discuss the processes involved in the photo-electron conversion mechanism, which may be subdivided into exciton harvesting, exciton transport, exciton dissociation, charge transport and extraction stages. In particular, we focus on the role of energy transfer as described by Förster resonance energy transfer (FRET) theory in the photoconversion mechanism. FRET plays a major role in exciton transport, harvesting and dissociation. The spectral absorption range of organic solar cells may be extended using sensitizers that efficiently transfer absorbed energy to the photoactive materials. The limitations of Förster theory to accurately calculate energy transfer rates are discussed. Energy transfer is the first step of an efficient two-step exciton dissociation process and may also be used to preferentially transport excitons to the heterointerface, where efficient exciton dissociation may occur. However, FRET also competes with charge transfer at the heterointerface turning it in a potential loss mechanism. An energy cascade comprising both energy transfer and charge transfer may aid in separating charges and is briefly discussed. Considering the extent to which the photo-electron conversion efficiency is governed by energy transfer, optimisation of this process offers the prospect of improved organic photovoltaic performance and thus aids in realising the potential of organic solar cells.

Well-defined novel fluorene-containing polymers: synthesis, fluorescent properties, and micellar nanoparticles

We report the synthesis of a new monomer, 9,9-diethylfluoren-2-yl methyl methacrylate (FMMA) and its controlled reversible addition-fragmentation chain transfer (RAFT) homopolymerization to give PFMMA with narrow polydispersity indices (PDIs). The corresponding copolymerization with 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) also gave well-defined block and random copolymers with controlled molecular weights and narrow PDIs. Their thermal behavior, UV-Vis absorption, and photoluminescent properties were studied. The PDMAEMA-b-PFMMA amphiphilic block copolymers showed self-assembly into aqueous spherical micellar nanoparticles with uniform size. The PFMMA core showed photoluminescence when carrying dichloromethane "guest" molecules and its emission was shown to be quenched after the release of the guest.