Enhanced Thermally Activated Delayed Fluorescence in New Fluorescein Derivatives By Introducing Aromatic Carbonyl Groups

Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor-Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

The photophysical processes involved in the electron donor-acceptor thermally activated delayed fluorescence (TADF) emitters are complicated and controversial. The recent consensus is that at least three states are involved, i. e. the singlet charge transfer state (¹ CT), the triplet localized excited state (³ LE) and the triplet CT state (³ CT). It is clear the very often used steady state and time-resolved luminescence spectroscopic methods are unable to present direct evidence for the dark states, i. e. the ³ LE and ³ CT states, as well as the interconversion of these states. Concerning this aspect, the femtosecond-nanosecond transient absorption spectroscopic methods are in particular interests. Both the emissive state and the dark state can be detected in these spectra, and interconversion of the states involved in TADF process can be also revealed. This review article focuses on the recent development of using the transient absorption spectra to study the photophysics of the TADF emitters.

Naphthofluorescein-based organic nanoparticles with superior stability for near-infrared photothermal therapy

Photothermal agents (PTAs) based on organic small molecules with near-infrared (NIR) absorption (700-900 nm) have attracted increasing attention in cancer photothermal therapy (PTT). However, NIR organic PTAs often suffer from poor stability. Fluorescein and its derivatives have been widely used in biological imaging and sensing due to their minimal cytotoxicity. But fluorescein and its derivatives have not been used in PTT because most of them don't have NIR absorption. In this work, two NIR naphthofluorescein derivatives, namely NFOM-1 and NFOM-2, were synthesized. In contrast to NFOM-1, NFOM-2 possesses an intramolecular hydrogen bonding network, which extends the absorption to the NIR region and significantly improves the photostability. NFOM-2 was encapsulated into an amphiphilic polymer (DSPE-mPEG2000) to obtain NFOMNPs as PTAs. Compared to the organic molecule NFOM-2, the absorption of NFOMNPs is broadened and further red-shifted to fit an 808 nm light source. Moreover, NFOMNPs exhibit good photothermal conversion efficiency (PCE, 40.4%, 808 nm, 1.0 W cm^-2), remarkable photostability and physiological stability, and significant PTT efficacy in vitro and in vivo was achieved. In other words, this study provides an intramolecular hydrogen bond network strategy and a fluorescein-based molecular platform to construct ultra-stable PTAs for efficient NIR PTT.

Thermally Activated Delayed Fluorescence Material: An Emerging Class of Metal-Free Luminophores for Biomedical Applications

The development of simple, efficient, and biocompatible organic luminescent molecules is of great significance to the clinical transformation of biomaterials. In recent years, purely organic thermally activated delayed fluorescence (TADF) materials with an extremely small single-triplet energy gap (ΔEST ) have been considered as the most promising new-generation electroluminescence emitters, which is an enormous breakthrough in organic optoelectronics. By merits of the unique photophysical properties, high structure flexibility, and reduced health risks, such metal-free TADF luminophores have attracted tremendous attention in biomedical fields, including conventional fluorescence imaging, time-resolved imaging and sensing, and photodynamic therapy. However, there is currently no systematic summary of the TADF materials for biomedical applications, which is presented in this review. Besides a brief introduction of the major developments of TADF material, the typical TADF mechanisms and fundamental principles on design strategies of TADF molecules and nanomaterials are subsequently described. Importantly, a specific emphasis is placed on the discussion of TADF materials for various biomedical applications. Finally, the authors make a forecast of the remaining challenges and future developments. This review provides insightful perspectives and clear prospects towards the rapid development of TADF materials in biomedicine, which will be highly valuable to exploit new luminescent materials.

Thermally Activated Delayed Fluorescence Enabled by Reversed Conformational Distortion for Blue Emitters

In this work, we present for the first time a general strategy via molecular reversed conformational distortion for thermally activated delayed fluorescence (TADF). A model purely organic compound named BNNIO with a common fluorophore flexibly linked to benzene by an oxygen atom is rationally designed and successfully synthesized. Moreover, the rate constant of reverse intersystem crossing reaches 2.34 × 10⁴ s^-1 as determined by transient spectroscopy. As a result, TADF emission of BNNIO is observed with a photoluminescence quantum yield of 90.72% and a lifetime of 84.76 μs at 415 nm. This universal regulation strategy undoubtedly opens a new avenue for the development of novel purely organic blue light-emitting materials.

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.

A turn-on TADF chemosensor for sulfite with a microsecond-scale luminescence lifetime

A reaction-based luminescence chemosensor was synthesized for sulfite detection based on a fluorescein derivative with thermally activated delayed fluorescence (TADF). The chemosensor exhibited a fluorescence turn-on effect on sulfite with good sensitivity and selectivity. Importantly, utilizing the long luminescence lifetime of the TADF compound, the chemosensor realized photoluminescence lifetime imaging for sulfite in living cells with the luminescence lifetime distribution mainly around 14 μs.

Enhancing Intersystem Crossing to Achieve Thermally Activated Delayed Fluorescence in a Water-Soluble Fluorescein Derivative with a Flexible Propenyl Group

It is a challenge to rationally design an organic molecule with thermally activated delayed fluorescence (TADF) due to the intrinsically spin-forbidden transition. Meanwhile, those reported TADF organic molecules have difficulty to be directly applied in the field of biological and medical imaging because they usually have no water solubility. Here, a water-soluble TADF organic molecule DCF-BXJ was developed by introducing a flexible propenyl group into the commercial traditional fluorophore DCF (2,7-dichlorofluorescein). The flexible group provides nonradiative rotational motion, which causes an efficient energy level cross between the S₁ state and the T₂ state of DCF-BXJ. Results of transient absorption spectra and theoretical calculations supported that nonradiative rotational motion of the flexible group can enhance intersystem crossing (ISC) and bring out TADF. This work provides a new mechanism explanation for TADF existing in organic molecules.

Dual-Mode Detection of Bacterial 16S Ribosomal RNA in Tissues

The specific detection of pathogens has long been recognized as a vital strategy for controlling bacterial infections. Herein, a novel hydrophilic aromatic-imide-based thermally activated delayed fluorescence (TADF) probe, AI-Cz-Neo, is designed and synthesized by the conjugation of a TADF emitter with a bacterial 16S ribosomal RNA-targeted moiety, neomycin. Biological data showed for the first time that AI-Cz-Neo could be successfully applied for the dual-mode detection of bacterial 16S rRNA using confocal fluorescence imaging and time-resolved fluorescence imaging (TRFI) in both cells and tissues. These findings greatly expand the application of TADF fluorophores in time-resolved biological imaging and provide a promising strategy for the precise and reliable diagnosis of bacterial infections based on the dual-mode imaging of bacterial 16S rRNA by fluorescence intensity and fluorescence lifetime.

Silica nanoparticles with thermally activated delayed fluorescence for live cell imaging

Thermally activated delayed fluorescence (TADF) has revolutionized the field of organic light emitting diodes owing to the possibility of harvesting non-emissive triplet states and converting them in emissive singlet states. This mechanism generates a long-lived delayed fluorescence component which can also be used in sensing oxygen concentration, measuring local temperature, or on imaging. Despite this strong potential, only recently TADF has emerged as a powerful tool to develop metal-free long-lived luminescent probes for imaging and sensing. The application of TADF molecules in aqueous and/or biological media requires specific structural features that allow complexation with biomolecules or enable emission in the aggregated state, in order to retain the delayed fluorescence that is characteristic of these compounds. Herein we demonstrate a facile method that maintains the optical properties of solvated dyes by dispersing TADF molecules in nanoparticles. TADF dye-doped silica nanoparticles are prepared using a modified fluorescein fluorophore. However, the strategy can be used with many other TADF dyes. The covalent grafting of the TADF emitter into the inorganic matrix effectively preserves and transfers the optical properties of the free dye into the luminescent nanomaterials. Importantly, the silica matrix is efficient in shielding the dye from solvent polarity effects and increases delayed fluorescence lifetime. The prepared nanoparticles are effectively internalized by human cells, even at low incubation concentrations, localizing primarily in the cytosol, enabling fluorescence microscopy imaging at low dye concentrations.

Hydrophilic, Red-Emitting, and Thermally Activated Delayed Fluorescence Emitter for Time-Resolved Luminescence Imaging by Mitochondrion-Induced Aggregation in Living Cells

Thermally activated delayed fluorescence (TADF) materials have provided new strategies for time-resolved luminescence imaging (TRLI); however, the development of hydrophilic TADF luminophores for specific imaging in cells remains a substantial challenge. In this study, a mitochondria-induced aggregation strategy for TRLI is proposed with the design and utilization of the hydrophilic TADF luminophore ((10-(1,3-dioxo-2-phenyl-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)-9,9-dimethyl-9,10-dihydroacridin-2-yl)methyl)triphenylphosphonium bromide (NID-TPP). Using a nonconjugated linker to introduce a triphenylphosphonium (TPP+) group into the 6-(9,9-dimethylacridin-10(9H)-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (NID) TADF luminophore preserves the TADF emission of NID-TPP. NID-TPP shows clear aggregation-induced delayed fluorescence enhancement behavior, which provides a practical strategy for long-lived delayed fluorescence emission in an oxygen-containing environment. Finally, the designed mitochondrion-targeting TPP+ group in NID-TPP induces the adequate accumulation of NID-TPP and results in the first reported TADF-based time-resolved luminescence imaging and two-photon imaging of mitochondria in living cells.

Red-to-blue photon up-conversion with high efficiency based on a TADF fluorescein derivative

A new photon up-conversion system with a TADF fluorescein derivative as a photosensitizer was developed to achieve a quite large anti-Stokes shift from red to blue with a fairly high up-conversion emission quantum yield.

Achieving long-lived thermally activated delayed fluorescence in the atmospheric aqueous environment by nano-encapsulation

Fluorescent silica nanoparticles which encapsulated dye DCF-BYT with thermally activated delayed fluorescence (TADF) were fabricated by a simple synthetic method.

Organic emitter integrating aggregation-induced delayed fluorescence and room-temperature phosphorescence characteristics, and its application in time-resolved luminescence imaging

A luminophore integrating aggregation-induced delayed fluorescence and room-temperature phosphorescence for time-resolved luminescence imaging.

Thermally activated delayed fluorescence (TADF) with a substantially long lifetime furnishes a new paradigm in developing probes for time-resolved imaging. Herein, a novel TADF fluorophore, namely, PXZT, with terpyridine as the acceptor and phenoxazine (PXZ) as the donor, was rationally designed and synthesized. The new compound shows typical thermally activated delayed fluorescence, aggregation-induced emission and crystallization-induced room-temperature phosphorescence (RTP). The coordination of PXZT with a zinc ion causes the quenching of the fluorescence of PXZT due to the enhanced intramolecular charge transfer of the resulting complex ZnPXZT1. With the dissociation of the ZnPXZT1 to release PXZT and the subsequent in situ hydrophobic aggregation of the free PXZT to resist the influence of oxygen, the TADF emission of PXZT is recovered. This zinc-assisted process is successfully used for time-resolved imaging of HeLa and 3T3 cells. This work presents a simple and effective strategy for time-resolved imaging by in situ forming TADF aggregates to turn on the TADF emission.

Synthesis of thieno[2,3-c]acridine and furo[2,3-c]acridine derivatives via an iodocyclization reaction and their fluorescence properties and DFT mechanistic studies

In this paper, we report the novel synthesis of thieno[2,3-c]acridine and furo[2,3-c]acridine derivatives via intramolecular iodocyclization reaction. The thieno[2,3-c]acridine derivatives exhibited blue fluorescence in hexane.