Aggregation-induced emission enhancement in organic ion pairs

Effect of counter-anions on the aggregation of Thioflavin-T

The aggregation of small molecules in aqueous solution is known to be influenced by the ionic strength of the medium; however, the role played by the identity of salt in the phenomenon of small molecule aggregation is rarely investigated. In the present contribution, we have investigated the effect of counter-anions on the aggregation of a popular cationic amyloid sensing probe, Thioflavin-T (ThT), by taking six different anions, viz. chloride, bromide, acetate, iodide, tetrafluoroborate, and perchlorate. Our results clearly indicate that it is not the ionic strength of the medium which solely controls aggregation of small molecules but distinct ions behave distinctly with regard to the organization. In fact, distinct ion effects play a major role in the salt induced organization of fluorophores. Using detailed steady-state emission, time-resolved emission, and ground-state absorption measurements, the optical properties of salt induced aggregates of ThT have been characterized. We have rationalized our observations on the basis of the theory of matching water affinity, which suggests that the matching free hydration energy is a critical aspect for the formation of contact ion pairs, which eventually results in aggregation. In brief, a larger sized anion, perchlorate, has a lower free energy of hydration and forms a suitable contact ion pair, with a larger organic cation, ThT, having weaker hydration. This contact ion-pair formation subsequently leads to the formation of an aggregate assembly which is found to be emissive in nature. Therefore, it is possible to induce aggregation of ThT by selecting the right counterion with the appropriate size, which may help us to evaluate the false positive signals when high ionic strength and specific counterions are present in the sensing matrix.

Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

Tunable electron-accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π-conjugated motifs, define the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π-extended molecules functionalized with P-cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation-anion interactions, which can drastically change physical behavior of these two-component systems.

Sulfono-γ-AApeptides as Helical Mimetics: Crystal Structures and Applications

Foldamers have defined and predictable structures, improved resistance to proteolytic degradation, enhanced chemical diversity, and are versatile in their mimicry of biological molecules, making them promising candidates in biomedical and material applications. However, as natural macromolecules exhibit endless folding structures and functions, the exploration of the applications of foldamers remains crucial. As such, it is imperative to continue to discover unnatural foldameric architectures with new frameworks and molecular scaffolds. To this end, we recently developed a new class of peptidomimetics termed ″γ-AApeptides", oligomers of γ-substituted-N-acylated-N-aminoethyl amino acids, which are inspired by the chiral peptide nucleic acid backbone. To date γ-AApeptides have been shown to be resistant to proteolytic degradation and possess limitless potential to introduce chemically diverse functional groups, demonstrating promise in biomedical and material sciences. However, the structures of γ-AApeptides were initially unknown, rendering their rational design for the mimicry of a protein helical domain impossible in the beginning, which limited their potential development. To our delight, in the past few years, we have obtained a series of crystal structures of helical sulfono-γ-AApeptides, a subclass of γ-AApeptides. The single-crystal X-ray crystallography indicates that sulfono-γ-AApeptides fold into unprecedented and well-defined helices with unique helical parameters. On the basis of the well-established size, shape, and folding conformation, the design of sulfono-γ-AApeptide-based foldamers opens a new avenue for the development of alternative unnatural peptidomimetics for their potential applications in chemistry, biology, medicine, materials science, and so on.In this Account, we will outline our journey on sulfono-γ-AApeptides and their application as helical mimetics. We will first briefly introduce the design and synthetic strategy of sulfono-γ-AApeptides and then describe the crystal structures of helical sulfono-γ-AApeptides, including left-handed homogeneous sulfono-γ-AApeptides, right-handed 1:1 α/sulfono-γ-AA peptide hybrids, and right-handed 2:1 α/sulfono-γ-AA peptide hybrids. After that, we will illustrate the potential of helical sulfono-γ-AApeptides for biological applications such as the disruption of medicinally relevant protein-protein interactions (PPIs) of BCL9-β-catenin and p53-MDM2/MDMX as well as the mimicry of glucagon-like peptide 1 (GLP-1). In addition, we also exemplify their potential application in material science. We expect that this Account will shed light on the structure-based design and function of helical sulfono-γ-AApeptides, which can provide a new and alternative way to explore and generate novel foldamers with distinctive structural and functional properties.

Aggregation-Induced Emissive and Circularly Polarized Homogeneous Sulfono-γ-AApeptide Foldamers

Through our continuous effort in developing a new class of foldamers, we have both designed and synthesized homogenous sulfono-γ-AApeptides using tetraphenylethylene (TPE) moieties attached to the backbone as luminogenic sidechains. Based on previous crystal structures, we have found that these foldamers adopted a left-handed 414-helix. Due to the constraint of the helical scaffold, the rotation of the TPE moieties were restricted, leading to fluorescent emissive properties with high quantum yields not only at the aggregate state but also in solution. Investigation of the relationship between the structure and fluorescence behavior reveals that emission was induced by the combined effect of the aggregation-induced emission (AIE) and the rotated restriction from the backbone. Furthermore, as the packing mode of the luminogens could be precisely adjusted by the helical backbone, these foldamers were found to be circularly polarizable with relatively large luminescence dissymmetry factor (g lum). Interestingly, possessing cationic amphipathic structures similar to that of host-defense peptides (HDPs), these sulfono-γ-AApeptides were able to inhibit the growth of Gram-positive bacteria methicillin-resistant S. aureus (MRSA) through membrane interactions.

A New Kind of Nonconventional Luminogen Based on Aliphatic Polyhydroxyurethane and Its Potential Application in Ink-Free Anticounterfeiting Printing

Organic luminogens have extensive applications due to their unique photophysical properties. In recent years, nonconjugated organic luminogens, in contrast to traditional conjugated luminogens, have gained much attention because of their facile preparation, environmental friendliness, and biocompatibility. In this study, a new kind of nonconventional luminogen based on dynamic covalent cross-linked polyhydroxyurethane is reported for the first time. The new luminogen not only exhibits intrinsic strong fluorescent emission in the solid state but also possesses high mechanical properties along with good shape memory and self-healing properties. In addition, the new luminogens are synthesized from aliphatic polyfunctional cyclic carbonate and amines via a much more straightforward method, avoiding the use of toxic isocyanates. Investigations indicated that the intrinsic luminescence of the resultant luminogens was induced by the cross-linking of polymer chains and could be well tuned by controlling the degree of cross-linking. By taking advantage of the unique characteristics of the resultant polymer luminogens, we further developed a facile method, named "light-mediated ink-free screen printing", for anticounterfeiting paper fabrication. Different from traditional ink-based printing technology, the new method used UV-light instead of expensive security ink to encode anticounterfeiting information on natural cellulose paper. The anticounterfeiting information is stable under various wet conditions, showing promising applications in the fast-growing counterfeiting of pharmaceuticals, packaging, and the food industry.

Helical Sulfono-γ-AApeptides with Aggregation-Induced Emission and Circularly Polarized Luminescence

Aggregation-induced emission (AIE) was intensively studied because of packing of small molecules and polymers; however, mid-molecular-weight (1000-3000) molecular scaffold containing a precise number of AIE luminogens is rare. Herein, we report the investigation of three tetraphenylethylene (TPE)-modified sulfono-γ-AApeptides in which multiple TPE moieties are conjugated to the chiral right-handed helical peptidomimetic backbone as functional side chains. The crystal structure of the TPE-α/sulfono-γ-AA peptide 1 demonstrates that because of the rigid helical scaffold of the TPE-α/sulfono-γ-AA peptides, the intramolecular rotations of the TPE with short linker are restricted, therefore leading to the boosted fluorescent emission in solution. Peptides 2 and 3 exhibit aggregation-induced emission enhancement (AIEE), possibly because of the combination of both AIE and rotation restriction. Moreover, because of their preoriented assembly induced by the right-handed helical scaffold, these emissive chiral luminogens show effective circularly polarized luminescence signals with high dissymmetry factor glum. Finally, the amphiphilic nature of TPE-α/sulfono-γ-AA peptides could enable them to penetrate the bacterial membranes and exhibit strong fluorescence. Their antimicrobial activity and labeling-free character could further augment their potential applications in both materials and biomedical sciences.

Application of a Novel "Turn-on" Fluorescent Material to the Detection of Aluminum Ion in Blood Serum

A novel "turn-on" fluorescent bioprobe, 1,2,3,4,5-penta(4-carboxyphenyl)pyrrole sodium salt (PPPNa), with aggregation-enhanced emission characteristics was synthesized for the in situ quantitative detection of Al³⁺ in serum. It exhibited a high selectivity to Al³⁺ in both simulated serum and fetal calf serum with no interferences from other metal ions or serum components observed and no isolation required. A weak interaction between PPPNa and serum albumin was found, which caused no interference, but enhanced fluorescence response of PPPNa to Al³⁺ and improved detection sensitivity. The limit of detection was determined to be 1.50 μmol/L Al³⁺ in phosphate-buffered saline solution containing 33 μg/mL bovine serum albumin (BSA) and decreased to 0.98 μmol/L as BSA concentration increased to 100 μg/mL. The fluorescence "turn-on" mechanism of the PPPNa probe to detect Al³⁺ was proposed. A bidentate complex is formed between the carboxy group of PPPNa and Al³⁺, causing the photoluminescence (PL) emission enhancement by aggregation. BSA chains further strengthen the stacking compactness of the aggregates of PPPNa and Al³⁺ and consequently enhance the PL emission of PPPNa by further promoting the restriction of intramolecular rotation of the phenyl ring. Its application to the in situ Al³⁺ was successfully demonstrated with HeLa cells and NIH 3T3 cells. The low cytotoxicity and highly selective response of PPPNa to Al³⁺ endow its great potentials to in vivo detecting and imaging of Al³⁺ as well as an absorbent of Al³⁺.

A red-emitting cationic hyperbranched polymer: facile synthesis, aggregation-enhanced emission, large Stokes shift, polarity-insensitive fluorescence and application in cell imaging

A cationic hyperbranched polymer containing TPE units demonstrates bright and stable red-emission with AEE-characteristics and good performance in cell imaging.

A comparative study of nine berberine salts in the solid state: optimization of the photoluminescence and self-association properties through the choice of the anion

Counter-ions regulate the photoluminescence and self-association properties of berberine salts.

Quantitation of Albumin in Serum Using "Turn-on" Fluorescent Probe with Aggregation-Enhanced Emission Characteristics

An aggregation-enhanced emission active luminogen named as sodium 4,4'4″-(3,4-diphenyl-1H-pyrrole-1,2,5-triyl)tribenzoate (DP-TPPNa) with propeller construction was synthesized and developed as a "turn on" fluorescent probe for in situ quantitation of albumin in blood serum. The DP-TPPNa fluorescence intensity was linearly correlated with the concentration of two serum albumins, bovine serum albumin (BSA) and human serum albumin (HSA), in pure PBS buffer in the ranges of 2.18-70 and 1.68-100 μg/mL, respectively. The detection limits were as low as 2.18 μg/mL for BSA and 1.68 μg/mL for HSA. The response time of fluorescence to serum albumin (SA) was very short (below 6 s), which achieved real-time detection. It also showed high selectivity to SA because other components in serum barely interfere with the detection of DP-TPPNa to SA, enabling in situ quantitative detection of SA without isolation from serum. DP-TPPNa was successfully applied for the quantitative detection of BSA in fetal bovine serum. The mechanism of fluorescent turn-on behavior was elucidated utilizing an unfolding process induced by guanidine hydrochloride, which revealed a capture process via selective hydrophobic interaction and hydrogen bonding between luminogen and SA.

Effect of ionic interaction on the mechanochromic properties of pyridinium modified tetraphenylethene

Introducing ionic interaction into a soft tetraphenylethene-based AIE-molecule furnished the compound with robust mechanochromic effects while exhibiting aggregation-induced emission enhancement properties.

How is the AIE mechanism profoundly changed in an ESIPT family: the novel introduction of a tetraphenylethene group onto (Z)-3-(quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2(1H)-one

The first example showing that the AIE mechanism can be completely changed from restriction of TICT to RIR by the introduction of a tetraphenylethene group onto a molecule with ESIPT character is reported herein.

Forcing aggregation of cyanine dyes with salts: a fine line between dimers and higher ordered aggregates

It is uncommon to read about cyanine dyes in the literature and not have their aggregation discussed. They are of high interest considering their propensity to undergo self-organization in aqueous solution, leading to interesting photophysical properties resulting from the formation of their dimers and higher ordered aggregates. Currently, the study of their aggregation is in high demand due to their diverse application range including dye-sensitized solar cells. However, their aggregation in high salt solutions is under studied, and the effect on aggregation in congruence with high ionic strength is often overlooked. In a previous study, our group established the role of specific ion effects and in particular the necessity of matching water affinity to induce aggregation of a cationic cyanine dye, thiazole orange. In order to advance the understanding of this topic, we present in this article the diverse aggregation of cyanine dyes, as a single monovalent salt can cause different aggregation responses in a variety of these dyes. We established via absorption spectroscopy combined with chemometric analyses that the inherent monomer-dimer equilibrium of a dye depends on its geometry. More interestingly, experimental data coupled with DFT calculations reveal that not only the geometry of a dye but also its charge location plays a role in the aggregate morphology formed by the interaction of a cationic cyanine dye and an anion. It is thought that contact ion pair formation and effective charge screening generated within that ion pair are responsible for aggregates with a greater order.

Fluorescence sensing of arsenate at nanomolar level in a greener way: naphthalene based probe for living cell imaging

Naphthalene-salisaldehyde conjugate (NAPSAL) is established as a novel arsenate (H2AsO4(-)) selective 'turn-on' fluorescence probe. It can detect as low as 5 × 10(-9) M H2AsO4(-) in HEPES buffered EtOH : water (0.1 M, 1 : 9, v/v, pH 7.4). Trace level H2AsO4(-) in drinking water samples is measured using standard addition method. Intracellular arsenate in Candida albicans, grown in arsenic contaminated water of Purbasthali has successfully been detected under fluorescence microscope.

Size does matter: how to control organization of organic dyes in aqueous environment using specific ion effects

Understanding the role played by external factors on the organization of molecules has the potential to contribute greatly to fundamental research and applications in fields as diverse as nanotechnology, medicine, material chemistry, etc. Countless studies involve the organization of small organic molecules in environments rich in ionic species, yet their participation in molecular organization is often overlooked. Herein, we critically assess the organization in aqueous solution of the cationic cyanine dye, thiazole orange, in the presence of different monovalent sodium salts. Our findings clearly indicate that not all ions are identical with regards to the organization of thiazole orange molecules and specific ions effects are at play. The conventional Debye and Hückel model is not sufficient to explain our results, and the participation of ionic species in molecular organization is explained in terms of the recent theory of water matching affinity. Herein, by choosing the right counterion with the appropriate size, we have shown that it is possible to either induce a simple shift in the monomer-dimer equilibrium of thiazole orange or to turn on the formation of larger organized structures.

Facile access to highly fluorescent nanofibers and microcrystals via reprecipitation of 2-phenyl-benzoxazole derivatives

2-Phenyl-benzoxazole and five derivatives bearing an alkyl or alkoxy substituent on the phenyl ring were used to prepare aqueous suspensions of particles via a solvent-exchange method. In these conditions, the methyl and methoxy derivatives spontaneously gave nanofibers, while the other compounds led to microcrystals. This shows that minor chemical changes are enough to direct the formation of a given type of particle. From a spectroscopic viewpoint, all compounds strongly emit blue light in the solid state, with spectra much broader than those registered in n-heptane and ethanol solutions. The photoluminescence quantum yields reached 38% and were slightly affected in aqueous suspension by the polarity of the environment. The molecular arrangement, deduced from X-ray analysis for the methyl and methoxy derivatives, was used to explain the fluorescence properties in the solid state. This work shows that 2-phenyl-benzoxazole derivatives are interesting candidates for applications as fluorescent nanomaterials, including in aqueous and biological media.

Luminescent charge-transfer complexes: tuning emission in binary fluorophore mixtures

Charge-transfer (CT) complexes composed of a π-electron-poor naphthalene diimide (NDI) derivative combined with a series of π-electron-rich donors were investigated. Solutions of the CT complexes are nonemissive; however, solid-state complexes and aqueous suspensions display emission that is dependent on the energy of the HOMO of the electron donor. Crystallographic analysis of a pyrene-NDI complex reveals columnar packing and a high degree of frontier molecular orbital (FMO) overlap that likely contributes to the observed optical properties. The fluorescent CT particles are utilized as imaging agents; additional luminescent CT complexes may be realized by considering FMO energies and topologies.

Spontaneous formation of fluorescent nanofibers and reticulated solid from berberine palmitate: a new example of aggregation-induced emission enhancement in organic ion pairs

The salt formed between the large aromatic berberine cation and the long-chain palmitate anion was synthesized and used to prepare aqueous suspensions of particles owing to a solvent-exchange method. Under these conditions, elongated particles were readily obtained. They were studied by transmission microscopy with polarized light, as well as by fluorescence and electron microscopy. They were shown to be probably crystallized nanofibers, which were stable in suspension. Unexpectedly, upon filtration and drying, these fibers evolved to give a reticulated solid. The fluorescence properties of the compound were analyzed in solution, in aqueous suspension and in the powder crystalline state. Interestingly, berberine palmitate is virtually not fluorescent in aqueous solution because of the quenching effect of water, but transition to the solid state was accompanied by a strong increase in fluorescence intensity. This phenomenon was explained by the original molecular arrangement in the solid state. Actually, in the crystal, the anions form a distinct layer, which limits parallel-stacking of the fluorescent cations. Moreover, the berberine cations are protected from the access of water molecules, and so no quenching effect can take place. This example confirms that the newly introduced concept of ion-pair aggregation-induced fluorescence enhancement can be extended to a variety of structures. It also shows the interest of ion pairs for preparing fluorescent nanofibers and reticulated solids using a solvent-exchange method that is particularly easy to implement.

Carbazole-based cyano-stilbene highly fluorescent microcrystals

Aggregation-induced enhanced emission (AIEE) is reported for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 μm sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue-shift in emission and enhances the intensity. X-ray crystallographic analysis revealed that key factors causing high luminescence efficiency in the crystal are a lack of strong cofacial π-π alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seem to be responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation, thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for the unusual emission blue-shift in the aggregate.