Light-induced reversible switching of generation and extinction of an organic radical anion.

Radicals play a crucial role across various domains, ranging from serving as catalysts in chemical reactions to materials for spintronic applications. Currently, a major challenge for the chemists is the development of the next generation of organic radicals controllable by photons. To tackle this challenge, here we introduce a dyad system that combines a dimethyldihydropyrene (DHP) photochromic unit with a naphthalene diimide (NDI) moiety. This system forms a stable organic NDI-based radical-anion upon exposure to light in a solvent containing a small amount of an amine that acts as an electron donor. The radical anion formation has also been demonstrated with a chemical reductant. The photoisomerization of this photochromic system converts it into a less-conjugated and less-electron-rich form, affecting the generation of the radical as well as its stability. Consequently, light-induced isomerization effectively quenches the radical. Thus, the formation and existence of the radical can be adjusted by manipulating the photoisomerization of the photochromic unit under diverse light sources. Additionally, the system exhibits significant differences in emission in the radical and the closed-shell state, thereby offering a dual readout of the state of the molecule.

Cyano disubstituted tetrabenzoindeno[2,1-a]fluorene: open-shell or closed-shell?

Organic diradicaloids have lately emerged as potential spintronic materials. We report the unprecedented synthesis of a near-IR absorbing indeno[2,1-a]fluorene derivative that displays remarkably low LUMO (-4.15 eV) and a small HOMO-LUMO gap (0.85 eV). NMR/EPR studies indicated its open-shell diradical property, which was supported by DFT calculations while suggesting a 30% diradical character and a small singlet (S)-triplet (T) gap (-2.52 kcal mol-1). A large bond length alternation of the as-indacene core for its single-crystals indicated a quinoidal contribution with greater antiaromaticity, which is in line with the small diradical character despite showing a small S-T gap.

Stable Diradical on the Dimethyldihydropyrene Scaffold.

The diradical character in a molecular architecture can be customized primarily in two ways: first, by employing a quinoidal pro-aromatic system with net energy gained by aromatization that compensates for the energy required to generate the diradical species and, second, by employing an antiaromatic system having easily accessible triplet states that impart a diradical character. We have chosen a 14π aromatic framework, Boekelheide's dimethyldihydropyrene, and perturbed its aromaticity through the construction of its quinoidal form. The perturbed aromaticity was evident from the bond alteration in the X-ray diffraction structure, 1H nuclear magnetic resonance chemical shifts, and quantum chemical calculations. The aromaticity was restored as the system underwent a transition to the biradical structure centered on two exocyclic carbons. In addition, upon photoexcitation and without using an external reducing reagent, the diradical could be converted to a radical anion and dianion form easily when dimethylformamide was used as a solvent.

Detection of Sialic Acid and Imaging of Cell-Surface Glycan Using a Fluorescence-SERS Dual Probe.

Sialic acid (SA) is an acidic monosaccharide present in the human brain and body fluids in the form of N-acetylneuraminic acid. It is also a well-known cancer biomarker. For decades, it has remained a challenging task to design synthetic receptors for SA. However, mainly because of the interference from other sugars with the receptors, it was challenging to differentiate SA from other sugars. Here, we report the development of a two-component aggregation-induced emissive (AIE) probes that can interact with SA and other saccharides via noncovalent interactions with unique emission fingerprints. Analysis of the output signals enabled the reliable detection and clear discrimination of SA in the presence of other saccharides with high accuracy. Further, its potential application in cellular glycan mapping has been explored by fluorescence imaging and surface-enhanced Raman scattering with MDA-MB-231 breast cancer cells.

Highly Selective and Quantitative Point-of-Care Diagnostic Method for Adrenaline.

Adrenaline, also known as epinephrine, is a neurotransmitter/hormone that is an important target in diagnostics. Development of an effective method for detecting it in the presence of other neurotransmitters is a challenging task. The electrochemical and fluorescent techniques commonly used have low selectivity in distinguishing among catecholamines. Herein, a small-molecule organic probe with an activated furfural moiety is reported to exploit the nucleophilicity of epinephrine to generate a bright-colored donor-acceptor Stenhouse adduct. Among nine common neurotransmitters or their analogues, only epinephrine was found to generate a unique colour change discernible with the naked eye, whereas the other ones remain unaffected. Under various in-field detection conditions, including solution, droplet, and paper strip-based detection, the colour change were also noticeable. The low detection limit of 1.37 nM and a limit of quantitation of 4.37 nM were achieved with simple UV/Vis methods in addition to the sub-ppm level sensing under visual conditions with naked eyes. The probe could be used for practical colorimetric measurements as a point-of-care tool without any complex and expensive machinery, making this approach accessible to all. In addition, using a simple smartphone, the determination of epinephrine concentrations is possible by using machine-learning techniques.

Mimicking the Energy Funnel of the Photosynthetic Unit Using a Dendrimer-Dye Supramolecular Assembly.

Photosynthesis involves light-harvesting complexes where an array of antenna pigment channels the absorbed solar energy to the reaction centre of a photosystem. This work reports a supramolecular dendrimer-dye assembly that mimics the natural light-harvesting mechanism. A dendrimeric molecule based on two-fluorophores has been constructed with three coumarin units at the end of three long arms and a 7-diethylaminocoumarin unit at the interior. The molecule self-aggregates in water into spherical micelles, which can encapsulate a rose-bengal dye (RB). On excitation, peripheral coumarin units shuttled the energy to the loaded RB dye reaction center via a two-step cascade resonance energy transfer (RET). The energy absorbed in the periphery is funnelled efficiently, resulting in a strong emission from the dye that resembles an energy funnel. The energy transfer cascade has been studied with both steady-state and time-resolved fluorescence spectroscopy. Molecular dynamics simulations of the self-assembled aggregates in water were also in agreement with the experimental observations.

A naphthalimide-linked new pyridylazo phenol derivative for selective sensing of cyanide ions (CN-) in sol-gel medium.

Napthalimide-linked pyridylazo derivatives 1 and 2 have been designed and synthesized. Compound 1 acts as a gelator in DMF-H2O (1 : 1, v/v). The brown gel is photostable and shows good viscoelastic properties. The value of G' is almost 10 times higher than that of G'' over the entire range of frequencies at a constant strain of 1%. The SEM image shows the presence of densely stacked flakes. In comparison, compound 2, devoid of free phenolic -OH, does not show gelation properties under identical conditions. However, the brown gel of 1 shows selective sensing of CN- ions over a series of anions involving phase change through the deprotonation mechanism. While the brown gel of 1 is selectively ruptured in the presence of CN- to the sol, compound 1 in solution shows measurable UV-vis and emission changes in the presence of CN- over the other anions and validates the visual sensing of CN-. In the test-kit application, the yellow paper strip turned into pinkish-red upon contact with CN-.

Live-cell imaging of lipid droplets using solvatochromic coumarin derivatives.

Lipid droplets (LDs), the lipid-rich intracellular organelles, serve to regulate many physiological processes and therefore attention has been attracted towards their selective detection. We report positively solvatochromic lipophilic dyes, based on the push-pull framework containing coumarin-pyridine heterocycles for selective live-cell imaging of lipid droplets (LDs) in Cos-7 and McA-RH7777 cells at ultralow concentrations (200 nM). The fluorescent probes show a remarkable increase in fluorescence intensity with time with the hydrophobic core of the lipid droplets contributing to the observed intensity enhancement. The simple structural framework, red emission, strong Stokes shift (>80 nm), and excellent biocompatibility highlight their significance as a versatile imaging tool for studying lipid droplets (LDs).

Superior Resonant Nanocavities Engineering on the Photonic Crystal-Coupled Emission Platform for the Detection of Femtomolar Iodide and Zeptomolar Cortisol.

Although luminescence spectroscopy has been a promising sensing technology with widespread applications in point-of-care diagnostics and chem-bio detection, it fundamentally suffers from low signal collection efficiency, considerable background noise, poor photostability, and intrinsic omnidirectional emission properties. In this regard, surface plasmon-coupled emission, a versatile plasmon-enhanced detection platform with >50% signal collection efficiency, high directionality, and polarization has previously been explored to amplify the limit of detection of desired analytes. However, high Ohmic loss in metal-dependent plasmonic platforms has remained an inevitable challenge. Here, we develop a hybrid nanocavity interface on a template-free and loss-less photonic crystal-coupled emission (PCCE) platform by the quintessential integration of high refractive index dielectric Nd2O3 "Huygens sources" and sharp-edged silver nanoprisms (NPrs). While efficient forward light scattering characteristics of Nd2O3 nanorods (NRs) present 460-fold emission enhancements in PCCE, the tunable localized plasmon resonances of NPrs display high electromagnetic field confinement at sharp nanotips and protrusions, boosting the enhancements 947-fold. The judicious use of silver NPr (AgNPr) metal-Nd2O3 dielectric hybrid resonances in conjugation with surface-trapped Bloch surface waves of the one-dimensional photonic crystal (1DPhC) displayed unprecedented >1300-fold enhancements. The experimental results are validated by excellent correlations with numerical calculations. The multifold hotspots generated by zero and nonzero nanogaps between the coassembly of NPrs, NRs, and 1DPhCs are used for (i) determination of hyper and hypothyroidism levels through monitoring the concentration of iodide (I-) ions and (ii) single-molecule detection (zeptomolar) of the stress hormone, cortisol, through the synthesized cortisol-rhodamine B conjugate obtained using a simple esterification reaction.

Aggregation-Induced Emission and Organogels with Chiral and Racemic Pyrene-Substituted Cyanostyrenes.

The synthesis, photophysical investigations, and organogel formation of pyrene-phenyl acrylonitriles (1-6) bearing mono-, di-, and trichiral and racemic substitutions were studied. The molecules self-assemble in water and show remarkable emission wavelength and intensity changes associated with distinct color changes. Cryo-scanning electron microscopy (Cryo-SEM) images show the formation of uniform nanoaggregates for the monosubstituted derivatives and network-like structures for di- and trisubstituted derivatives. The favorable π-π stacking ability of the coplanar pyrene ring, steric restrictions due to the cyano group, and beneficial noncovalent interactions from the citronellol moiety allow the molecules to form excellent organogels with fibrous and twisted ribbon morphology for the racemic and chiral derivatives. The organogels based on small molecules could be of high relevance for potential investigations involving soft biological matter.

A "turn-off" red-emitting fluorophore for nanomolar detection of heparin.

A simple fluorophore bearing a diethylaminocoumarin donor and a pyridinium acceptor was synthesized and utilized for the ultra-sensitive detection of heparin. The synthesized dicationic push-pull coumarin derivative emits strongly in the red-region (665 nm) and detects nanomolar concentrations (14.8 nM to 148 nM) of heparin in HEPES buffer and FBS serum solutions. The dication exhibits excellent fluorescence selectivity and sensitivity towards heparin over its analogues such as chondroitin 4-sulfate (CS), hyaluronic acid (HA) and dextran. This fluorescence assay is a convenient, sensitive method for monitoring heparin levels in biological samples. These findings were confirmed using coarse-grained Monte Carlo simulations, which provide us with a rationale for the selective binding of heparin.