Aggregation‐Induced and Polymorphism‐Dependent Thermally Activated Delayed Fluorescence (TADF) Characteristics of an Oligothiophene: Applications in Time‐Dependent Live Cell Multicolour Imaging

Polymorphism Dependent Cytotoxicity, Cellular Uptake, and Live Cell Imaging Studies on Napthalimide-Vinyl-Phenothiazine Conjugate

Polymorphism-dependent cytotoxicity and cellular uptake of drug molecules have been studied for the past two decades. However, the visualization of polymorph-dependent cellular uptake and cytotoxicity using microscopy imaging techniques has not yet been reported. The luminescent polymorph is an ideal candidate to validate the above hypothesis. Herein, we report the polymorph-dependent cellular uptake, cytotoxicity, and bio-imaging functions of polymorphs 1Y and 1R of a naphthalimide-phenothiazine dyad. These polymorphs show different luminescence colors in the solid state and exhibit aggregation-induced enhanced emission (AIEE) in the DMSO-Water mixture. Bioimaging, cytotoxicity assay, and fluorescence-activated cell sorting (FACS) studies revealed that these polymorphs show different levels of cytotoxicity, cellular uptake, localization, and imaging potential. Detailed photophysical, morphological, and biological studies revealed that the difference in molecular conformation in these polymorphs enables them to form aggregates of different sizes and morphology, which leads to the differential uptake of these into the cells and consequently shows different cytotoxicity and imaging potentials.

Triarylborane-triphenylamine based luminophore for the mitochondria targeted live cell imaging and colorimetric detection of aqueous fluoride

Bioimaging of subcellular organelles such as mitochondria is crucial for detecting physiological abnormalities induced by fluctuations in the levels of various analytes. Herein, we report the design and synthesis of two novel water-soluble cationic Lewis acid triarylborane-triarylamine conjugates 1 and 2. The optical characteristics of 1 and 2 and their precursor compounds BTPA-NMe2 and BTPA-2NMe2 were evaluated, which show similar absorption and fluorescence spectra, with 1 and 2 exhibiting higher quantum yields of 0.73 and 0.64, respectively, than those of the precursors BTPA-NMe2 and BTPA-2NMe2, indicating the partial disruption of the ICT process and the activation of alternative emission bands in 1 and 2. The live cell imaging ability of compound 2 was examined in HeLa cells using a confocal microscope. Moreover, mitochondrial internalisation using compound 2 was effective and it was found to have high photostability under UV light conditions. Furthermore, compound 2 demonstrated an evident colorimetric response with a colour change to dark yellow in aqueous environments, indicating that it could be used for anion sensing. The spectral changes were observed in UV-visible and fluorescence titration experiments, which were strongly supported by DFT calculations. In short, compound 2 synthesized by us can be exclusively utilized for the selective localization of mitochondria with less cytotoxicity and shows excellent colorimetric response to aqueous inorganic fluoride at levels as low as 0.1 ppm with high selectivity.

Time-Resolved Luminescent Sensing and Imaging for Enzyme Catalytic Activity Based on Responsive Probes

Enzymes, as a kind of biomacromolecules, play an important role in many physiological processes and relate directly to various diseases. Developing an efficient detection method for enzyme activity is important to achieve early diagnosis of enzyme-relevant diseases and high throughput screening of potential enzyme-relevant drugs. Time-resolved luminescence assay provide a high accuracy and signal-to-noise ratios detection methods for enzyme activity, which has been widely used in high throughput screening of enzyme-relevant drugs and diagnosis of enzyme-relevant diseases. Inspired by these advantages, various responsive probes based on metal complexes and metal-free organic compounds have been developed for time-resolved bioimaging and biosensing of enzyme activity owing to their long luminescence lifetimes, high quantum yields and photostability. In this review, we comprehensively reviewed metal complex- and metal-free organic compound-based responsive probes applied to detect enzyme activity through time-resolved imaging, including their design strategies and sensing principles. Current challenges and future prospects in this rapidly growing field are also discussed.

White Light Luminescence from a Homo-conjugated Molecule with Thermally Activated Delayed Fluorescence

Luminophores with tunable emission properties are appealing due to various applications. Among those properties, thermally activated delayed fluorescence (TADF) has been attracting enormous research interests. Herein, we synthesized a 9,9'-spirobifluorene based homo-conjugated molecule 1, which connects a diphenylamino moiety as electron donor and a naphthalimide group as electron acceptor via 2,2'-positions of spirofluorene. Compound 1 displays dual emission behaviour with both blue and orange fluorescence. The one orange fluorescence around 555 nmshows sensitivity to oxygen and a prolonged lifetime of 284 ns in degassed toluene. Such characteristics imply TADF nature for this emission from a charge-transfer excited state. The other emission at 440 nm with blue colour displayed resistance to oxygen quenching and a normal fluorescence lifetime of 1.5 ns. Compared with control molecule, this emission band is assigned as conventional fluorescence from a localized excited state. In addition, dual emission property allows molecule 1 to be modulated to emit white photoluminescence in thin film with a CIE color coordinate of (0.25, 0.33).

Highly Efficient Aggregation-Induced Red-Emissive Organic Thermally Activated Delayed Fluorescence Materials with Prolonged Fluorescence Lifetime for Time-Resolved Luminescence Bioimaging

Organic thermally activated delayed fluorescence (TADF) materials are emerging as potential candidates for time-resolved fluorescence imaging in biological systems. However, the development of purely organic TADF materials with bright aggregated-state emissions in the red/near-infrared (NIR) region remains challenging. Here, we report three donor-acceptor-type TADF molecules as promising candidates for time-resolved fluorescence imaging, which are engineered by direct connection of electron-donating moieties (phenoxazine or phenothiazine) and an electron-acceptor 1,8-naphthalimide (NI). Theoretically and experimentally, we elucidate that three TADF materials possessed remarkably small ΔE_ST to promote the occurrence of reverse intersystem crossing (RISC). Moreover, they all exhibit aggregation-induced red emissions and long delayed fluorescence lifetimes without the influence of molecular oxygen. More importantly, these long-lived and biocompatible TADF materials, especially the phenoxazine-substituted NI fluorophores, show great potential for high-contrast fluorescence lifetime imaging in living cells. This study provides further a molecular design strategy for purely organic TADF materials and expands the versatile biological application of long-lived fluorescence research in time-resolved luminescence imaging.

Lighting Silver(I) Complexes for Solution-Processed Organic Light-Emitting Diodes and Biological Applications via Thermally Activated Delayed Fluorescence

Luminescent coinage metal complexes have shown promising applications as electroluminescent emitters, photocatalysts/photosensitizers, and bioimaging/theranostic agents, rendering them attractive alternatives to transition metal complexes based on iridium, ruthenium, and platinum that have extremely low earth abundance. In comparison to the widely studied Au(I) and Cu(I) complexes, Ag(I) complexes have seldom been explored in this field because of their inferior emission properties. Herein, we report a novel series of [Ag(N^N)(P^P)]PF₆ complexes exhibiting highly efficient thermally activated delayed fluorescence by using easily accessible neutral diamine ligands and commercially available ancillary diphosphine chelates. The photoluminescence quantum yields (PLQYs) of the Ag(I) emitters are ≤0.62 in doped films. The high PLQY with a large delayed fluorescence ratio enabled the fabrication of solution-processed organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 8.76%, among the highest values for Ag(I) emitter-based OLEDs. With superior emission properties and an excited state lifetime in the microsecond regime, together with its potent cytotoxicity, the selected Ag(I) complex has been used for simultaneous cell imaging and anticancer treatment in human liver carcinoma HepG2 cells, revealing the potential of luminescent Ag(I) complexes for biological applications such as theranostics.

Delayed Fluorescence, Room Temperature Phosphorescence, and Mechanofluorochromic Naphthalimides: Differential Imaging of Normoxia and Hypoxia Live Cancer Cells

We study the effect of molecular conformation on the electronic coupling between the donor amines and acceptor 1,8-naphthalimide (NPI) in a series of D-A systems 1-4 (A = NPI; D = phenothiazine, phenoxazine, carbazole, diphenylamine, respectively, for 1, 2, 3, and 4). Weakly coupled systems show dual emission in the solution state, while strongly coupled systems show single emission bands. The energy of transitions and photoluminescence (PL) quantum yield are sensitive to the molecular conformation and donor strength. These compounds show delayed emission in the solutions and aggregated state and phosphorescence in the solid state. Compounds 3 and 4 with weak donors exhibit intermolecular slipped π···π interactions in the solid state and consequently exhibit dual (intra- and inter-) phosphorescence at low temperature. Steady state and time-resolved PL studies at variable temperature together with computational and crystal structure analysis were used to rationalize the optical properties of these compounds. The delayed emission of these compounds is sensitive to molecular oxygen; accordingly, these molecules are utilized for differential imaging of normoxia and hypoxia cancer cells.