Aggregation induced bright organic luminogens: Design strategies, advanced bio-imaging and theranostic applications.

The concept of aggregation-induced emission (AIE) in purely organic luminescent molecules has drawn wide attention in the last two decades. Despite the many challenges, AIE-probes have opened versatile opportunities in many research fields. In particular, the emerging functional properties of room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) have boosted the unique features of AIE luminogens (AIEgens). Thus, these luminescent materials extended the utility in sensing, imaging, optoelectronics and theranostic applications in biological field over the conventional fluorescent probe. Unlike the sensitivity of triplet state by oxygen and moisture, the long-lived phosphorescence and delayed fluorescence resulting from the enhanced intersystem crossing (ISC) and reverse intersystem crossing (RISC) from excited triplet state (T1) to excited singlet state (S1) in these luminophores gives rise to long lifetimes ranging from nanoseconds to milliseconds even up to seconds. As compared to traditional fluorescence molecules advanced AIE probes show high contrast imaging and deeper penetration depth, which have been demonstrated through near infrared I and II (NIR-I & NIR-II) fluorescence imaging, room temperature after-glow imaging and photoacoustic imaging. This chapter highlights the recent developments and principle of the efficient design of AIE probe with multi-functional properties evolved with new strategies for translational applications via fluorescence imaging, photoacoustic imaging and image-guided photodynamic/photothermal therapy (PDT/PTT) including future opportunities for AIEgens to advance the overall biomedical field.

Recent Developments on Multi-Functional Metal-Free Mechanochromic Luminescence and Thermally Activated Delayed Fluorescence Organic Materials.

Metal-free organic compounds with highly ordered π-conjugated twisted skeletons are capable of generating brilliant multi-colored light. Additionally, the co-existence of numerous other multi-functional properties have endowed them with the potential to be a promising class of materials for several electronic and photonic applications and next-generation advanced luminescent material-based devices. This review highlights the recent developments made in this fascinating class of multi-property encompassing materials, involving a highly twisted donor-acceptor based single molecular platform with synchronized photophysical behavior such as thermally activated delayed fluorescence (TADF), mechanoresponsive (MR), room-temperature phosphorescence (RTP), and aggregation induced emission (AIE) with associated unique and inherently manifested structure-property relationship investigations. Furthermore, a brief summary of the optoelectronic behavior of TADF materials are also presented by correlating their performances in the organic light-emitting diodes (OLEDs) and corresponding EL devices. In addition to mechanochromic luminescence (MCL) with TADF behavior, new types of emitters are also being developed, with tunable color changes such as blue-green, yellow-orange, yellow-red, etc., with some emitters crossing the entire visible span to produce white OLEDs. These developments have enriched the library of fascinating organic materials in addition to providing new directions of multifunctional material design for solutions processed OLED and several other advanced devices.