Utilizing a Pyrazine-Containing Aggregation-Induced Emission Luminogen as an Efficient Photosensitizer for Imaging-Guided Two-Photon Photodynamic Therapy

Aggregation-induced emission (AIE)-Based nanocomposites for intracellular biological process monitoring and photodynamic therapy

As a non-invasive visualization technique, photoluminescence imaging (PLI) has found its huge value in many biological applications associated with intracellular process monitoring and early and accurate diagnosis of diseases. PLI can also be combined with therapeutics to build imaging-guided theragnostic platforms for achieving early and precise treatment of diseases. Photodynamic therapy (PDT) as a quintessential phototheranostics technology has gained great benefits from the combination with PLI. Recently, aggregation-induced emission (AIE)-active materials have emerged as one of the most promising bioimaging and phototheranostic agents. Most of AIEgens, however, need to be chemically engineered to form versatile nanocomposites with improved their photophysical property, photochemical activity, biocompatibility, etc. In this review, we focus on three categories of AIE-active nanocomposites and highlight their application progresses in the intracellular biological process monitoring and PLI-guided PDT. We hope this review can guide further development of AIE-active nanocomposites and promote their practical applications for monitoring intracellular biological processes and imaging-guided PDT.

Zinc (II) and AIEgens: The "Clip Approach" for a Novel Fluorophore Family. A Review

Aggregation-induced emission (AIE) compounds display a photophysical phenomenon in which the aggregate state exhibits stronger emission than the isolated units. The common term of "AIEgens" was coined to describe compounds undergoing the AIE effect. Due to the recent interest in AIEgens, the search for novel hybrid organic-inorganic compounds with unique luminescence properties in the aggregate phase is a relevant goal. In this perspective, the abundant, inexpensive, and nontoxic d10 zinc cation offers unique opportunities for building AIE active fluorophores, sensing probes, and bioimaging tools. Considering the novelty of the topic, relevant examples collected in the last 5 years (2016-2021) through scientific production can be considered fully representative of the state-of-the-art. Starting from the simple phenomenological approach and considering different typological and chemical units and structures, we focused on zinc-based AIEgens offering synthetic novelty, research completeness, and relevant applications. A special section was devoted to Zn(II)-based AIEgens for living cell imaging as the novel technological frontier in biology and medicine.

Recent Advances in Two-Photon AIEgens and Their Application in Biological Systems

Two-photon fluorescence imaging technology has the advantages of high light stability, little light damage, and high spatiotemporal resolution, which make it a powerful biological analysis method. However, due to the high concentration or aggregation state of traditional organic light-emitting molecules, the fluorescence intensity is easily reduced or disappears completely, and is not conducive to optimal application. The concept of aggregation-induced emission (AIE) provides a solution to the problem of aggregation-induced luminescence quenching (ACQ), and realizes the high fluorescence quantum yield of luminescent molecules in the aggregation state. In addition, two-photon absorption properties can readily be improved just by increasing the loading content of AIE fluorogen (AIEgen). Therefore, the design and preparation of two-photon fluorescence probes based on AIEgen to achieve high-efficiency fluorescence imaging in vitro/in vivo has become a major research hotspot. This review aims to summarize representative two-photon AIEgens based on triphenylamine, tetraphenylethene, quinoline, naphthalene and other new structures from the past five years, and discuss their great potential in bioimaging applications.

Fluorescent AIE-Active Materials for Two-Photon Bioimaging Applications

Fluorescence imaging has been widely used as a powerful tool for in situ and real-time visualization of important analytes and biological events in live samples with remarkably high selectivity, sensitivity, and spatial resolution. Compared with one-photon fluorescence imaging, two-photon fluorescence imaging exhibits predominant advantages of minimal photodamage to samples, deep tissue penetration, and outstanding resolution. Recently, the aggregation-induced emission (AIE) materials have become a preferred choice in two-photon fluorescence biological imaging because of its unique bright fluorescence in solid and aggregate states and strong resistance to photobleaching. In this review, we will exclusively summarize the applications of AIE-active materials in two-photon fluorescence imaging with some representative examples from four aspects: fluorescence detection, in vitro cell imaging, ex vivo tissue imaging, and in vivo vascular imaging. In addition, the current challenges and future development directions of AIE-active materials for two-photon bioimaging are briefly discussed.

Precise Molecular Design for High-Performance Luminogens with Aggregation-Induced Emission

Precise design of fluorescent molecules with desired properties has enabled the rapid development of many research fields. Among the different types of optically active materials, luminogens with aggregation-induced emission (AIEgens) have attracted significant interest over the past two decades. The negligible luminescence of AIEgens as a molecular species and high brightness in aggregate states distinguish them from conventional fluorescent dyes, which has galvanized efforts to bring AIEgens to a wide array of multidisciplinary applications. Herein, the useful principles and emerging structure-property relationships for precise molecular design toward AIEgens with desirable properties using concrete examples are revealed. The cutting-edge applications of AIEgens and their excellent performance in enabling new research directions in biomedical theranostics, optoelectronic devices, stimuli-responsive smart materials, and visualization of physical processes are also highlighted.

AIEE Active Nanoassemblies of Pyrazine Based Organic Photosensitizers as Efficient Metal-Free Supramolecular Photoredox Catalytic Systems

Pyrazine derivatives DIPY, TETPY and CNDIPY have been designed and synthesized which form fluorescent supramolecular assemblies in mixed aqueous media due to their AIEE and ICT characteristics. Among all the derivatives, the assemblies of TETPY and CNDIPY show strong absorption in the visible region with high absorption coefficients, low HOMO-LUMO gap, and high photostability. Further, the supramolecular nanoassemblies of TETPY and CNDIPY show excellent potential to generate reactive oxygen species (ROS) under the visible light irradiation. Owing to their strong absorption in the visible region and ROS generation ability, the supramolecular nanoassemblies of TETPY and CNDIPY act as efficient photoredox catalytic systems for carrying out (a) oxidative amidation of aromatic aldehydes (b) hydroxylation of boronic acid and (c) oxidative homocoupling of benzylamines under mild conditions such as aqueous media, aerial environment, and natural sunlight as a source of irradiation. All the mechanistic investigations suggest the participation of in-situ generated ROS in the organic transformations upon light irradiation.

An oxygen self-sufficient NIR-responsive nanosystem for enhanced PDT and chemotherapy against hypoxic tumors

The efficacy of photodynamic therapy and chemotherapy is largely limited by oxygen deficiency in the hypoxic tumor microenvironment. To solve these problems, we fabricated a novel NIR-responsive nanosystem which could co-deliver oxygen and anticancer drug DOX. An oxygen self-sufficient amphiphile (F-IR780-PEG) was first synthesized and subsequently utilized to load anticancer drug DOX to form nanoparticles (F/DOX nanoparticles). Due to the high oxygen capacity of such nanoparticles, the hypoxic tumor microenvironment was greatly modulated after these nanoparticles reached the tumor region, and the results revealed that hypoxia-inducible factor α (HIF-1α) was down-regulated and the expression of P-glycoprotein (P-gp) was then reduced, which were in favor of chemotherapy. Under light irradiation at 808 nm, IR780 could efficiently produce singlet oxygen to damage cancer cells by photodynamic therapy (PDT). Simultaneously, the IR780 linkage could be cleaved by singlet oxygen generated by itself and resulted in DOX release, which further caused cell damage by chemotherapy. With the combination of PDT and chemotherapy, F/DOX nanoparticles showed remarkable therapeutic efficacy under in vitro and in vivo conditions. Furthermore, the F/DOX nanoparticles are favorable for imaging-guided tumor therapy due to the inherent fluorescence properties of IR780. We thus believe that the synergistic treatment described here leads to an ideal therapeutic approach to hypoxic tumors.

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly-efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through-space charge transfer (TSCT) and multi-resonance induced TADF (MR-TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.