Luminescent guest encapsulated metal-organic framework composites for photonic sensing technology

Luminescent metal-organic frameworks (LMOFs) are promising hybrid materials and are considered ideal for fabricating sensors and optoelectronics. A unique direction of LMOFs research is to harness MOFs as ‘hosts’ to encapsulate various luminescent ‘guest’ (LG) molecules for producing LG@MOF systems. However, feasible design ideas and in-depth mechanism analyses are scarce, compromising their practical engineering potential. This section, combined with the structure, synthesis, types, and applications of LG@MOF systems, is described in detail in Chapter 1 and 2. In Chapter 3, 4, and 5, three new LG@MOF systems have been designed and prepared. The results demonstrated that effective wavelength-tuning (~ tens of nanometres) or turn-on (~ tenfold) type sensing materials could be readily synthesised even with common fluorescent materials (e.g., rhodamine B, tetraphenylethylene, perylene). Different polymers have been combined with the obtained LG@MOF systems to enhance stability and processability. Detailed photophysical characterisations have been conducted to investigate the obtained LG@MOF systems. By analysing the excitation, emission, lifetime, and quantum yield, three novel sensing mechanisms have been proposed: (i) Guest aggregates of different sizes are protected differently by the MOF, thus, generating sensing behaviours. Moreover, the configuration of the guest aggregates may change under stimuli leading to further sensing performance (Chapter 3); (ii) MOF can provide a strong caging effect to aggregation-induced emission (AIE) materials resulting in turn-on type mechanochromism (Chapter 4); (iii) The lattice of MOF can influence the electron-phonon coupling of guest excimers causing distinct luminescent properties (Chapter 5). Overall, this work helps address the pain points of LG@MOFs research and illustrates the possibility for large scale production, which may inspire more research into high-performance fluorescent sensors. These are summarised in Chapter 6, along with some specific examples of future development.