Light has profoundly impacted modern medicine and healthcare, with numerous luminescent agents and imaging techniques currently being used to assess health and treat diseases. As an emerging concept in luminescence, aggregation-induced emission (AIE) has shown great potential in biological applications due to its advantages in terms of brightness, biocompatibility, photostability, and positive correlation with concentration. This review provides a comprehensive summary of AIE luminogens applied in imaging of biological structure and dynamic physiological processes, disease diagnosis and treatment, and detection and monitoring of specific analytes, followed by representative works. Discussions on critical issues and perspectives on future directions are also included. This review aims to stimulate the interest of researchers from different fields, including chemistry, biology, materials science, medicine, etc., thus promoting the development of AIE in the fields of life and health.

Twenty years ago, the concept of aggregation‐induced emission (AIE) was proposed, and this unique luminescent property has attracted scientific interest ever since. However, AIE denominates only the phenomenon, while the details of its underlying guiding principles remain to be elucidated. This minireview discusses the basic principles of AIE based on our previous mechanistic study of the photophysical behavior of 9,10‐bis(N,N‐dialkylamino)anthracene (BDAA) and the corresponding mechanistic analysis by quantum chemical calculations. BDAA comprises an anthracene core and small electron donors, which allows the quantum chemical aspects of AIE to be discussed. The key factor for AIE is the control over the non‐radiative decay (deactivation) pathway, which can be visualized by considering the conical intersection (CI) on a potential energy surface. Controlling the conical intersection (CI) on the potential energy surface enables the separate formation of fluorescent (CI:high) and non‐fluorescent (CI:low) molecules [control of conical intersection accessibility (CCIA)]. The novelty and originality of AIE in the field of photochemistry lies in the creation of functionality by design and in the active control over deactivation pathways. Moreover, we provide a new design strategy for AIE luminogens (AIEgens) and discuss selected examples.

A series of regioisomers of piperidylanthracenes (PA) and bis(piperidyl)anthracenes (BPA) were synthesized and their photophysical properties examined in solution, suspension, and the solid state. Aggregation-induced emission (AIE) was observed only for 1,4-BPA and 9,10-BPA, in which two piperidyl groups are substituted at the anthracene moiety in the para-position with respect to each other. Compared to previously reported AIE luminogens, these easily obtainable para-substituted BPAs, exhibited several unique and beneficial features, such as simple structures, bright solid-state fluorescence (Φ_fl = 0.49 and 0.86 for 1,4-BPA and 9,10-BPA, respectively), tunable fluorescence emission, and large Stokes shifts. Results from diffuse-reflectance and fluorescence lifetime measurements demonstrated that PAs and BPAs intrinsically possess an efficient non-radiative transition pathway in the solid state, and that 1,4-BPA and 9,10-BPA may overcome this pathway. The X-ray crystallographic analysis of 1,4-BPA revealed that undesirable interchromophoric interactions can be minimised, while TD-DFT calculations suggested that the enhanced Stokes shift of 1,4-BPA arises from severe electronic repulsion between neighbouring piperidyl moieties, which presumably results in the absence of self-absorption.

Fluorescence thermometry has emerged as a significant area of research in the field of remote temperature sensing with high accuracy. However, the development of noninvasive and reliable small organic fluorescence thermometers (FTs) remains challenging. In this study, we developed a high-sensitivity, high-resolution small organic ratiometric FT with a solvatochromic dye and analyzed its temperature response mechanism. π-Extended fluorene-based D-π-A-type small solvatochromic dyes (FπAc, FπF, and FπVC) were synthesized and characterized. A significant solvatochromic shift of >200 nm was observed between n-hexane and tetrahydrofuran (THF) for both FπF and FπVC, with emission in THF reaching the red region (701 nm, 828 nm). This substantial solvatochromic shift was accompanied by a concurrent decrease in the fluorescence quantum yield and is attributable to accelerated internal conversion. The fluorescence spectra of FπF in THF and FπVC in diethyl ether exhibited blue shifts and increased fluorescence intensities with a decrease in polarity at increased temperatures. Our observations revealed an absolute sensitivity of 21%/°C and a relative sensitivity of 3.0%/°C at the maximum. These results represent the highest sensitivity and resolution reported for single fluorophore small organic ratiometric FTs dispersed in solution. The positive temperature coefficient of the fluorescence intensity was attributed to control of the nonradiative decay pathway by solvatochromism. The temperature responsiveness of FπF could be detected in living HeLa cells by ratiometric confocal microscopy. Using the proposed strategy to develop FTs, we plan to build a library of FTs that will cover various environments of interest in both simple and complex systems.