Fluorescent materials are one of the most promising candidates in bioimaging and therapeutic application. In particular, fluorophores with long emission in the red/near-infrared (NIR) region are nowadays gained a lot of attention in biological applications, owing to minimizing interference of optical absorption and photodamage to biological samples, reducing light scattering and avoiding autofluorescence of physiological environments. Although many types of red/NIR-emissive fluorophores have been commercialized, they generally suffer from a common photophysical phenomenon notoriously named as aggregation - caused quenching (ACQ), which emit strongly in solution phase but experience emission quenching upon aggregates formation in physiological environments. This would greatly impede the...[ Read more ]

Fluorescent materials are one of the most promising candidates in bioimaging and therapeutic application. In particular, fluorophores with long emission in the red/near-infrared (NIR) region are nowadays gained a lot of attention in biological applications, owing to minimizing interference of optical absorption and photodamage to biological samples, reducing light scattering and avoiding autofluorescence of physiological environments. Although many types of red/NIR-emissive fluorophores have been commercialized, they generally suffer from a common photophysical phenomenon notoriously named as aggregation - caused quenching (ACQ), which emit strongly in solution phase but experience emission quenching upon aggregates formation in physiological environments. This would greatly impede the practical applications in the area of biomedical research. In 2001, a phenomenon antithetic to the common ACQ effect was observed and reported by Tang et al. In that, the pentaphenylsilole was almost nonluminescent when molecularly dissolved, while it exhibited highly emissive at aggregated state. Since then, a range of molecules with similar fluorescence property were synthesized and coined as 'aggregation-induced emission' (AIE) molecules. These AIE-active materials have attracted much research interest and have been identified as a novel class of luminogens to develop fluorescent turn-on biosensors with superior sensitivity.

Considering the great significances of both AIE and red/NIR - emission, red/NIR fluorescent materials possessing the AIE characteristics as well as specific targeting capability are much-sought-after for bioimaging and therapeutic applications due to their deep penetration depth and high resolution. In these works, we aim to further explore new red/NIR functional AIE-active fluorogens (AIEgens) for biological application.

The main objective of this thesis is to design and synthesize explore new red/NIR functional AIEgens for biological application. They possess good solubility, photostability and high quantum yields for bioimaging by developing facile and powerful synthetic routes with high yields. To achieve this goal, the synthesized AIEgens were firstly characterized using various techniques, such as NMR, HRMS, X-ray crystallography, DLS, DFT theoretical calculations, etc. Then, the photophysical properties were evaluated from UV, PL, QY and lifetime. The potential applications of the obtained AlIEgens in various fields were also explored.

In this thesis, four small molecular AIE systems including positively charged molecules, supramolecular assemblies, amphiphilic molecules, and neutral molecules have been developed. Guided by the RIM and TICT mechanism, they exhibited bright emission with emission wavelength located at red or NIR region. Furthermore, they generally possess large Stock shifts, good chemical stability and thermal stability, specific targeting ability both in vitro and in vivo, which can be used for bioimaging as well as phototheranostics such as PDT. Besides, an interesting project on the extension application of fluorescent AIE probes for optoelectronic devices was accomplished.