Nanoscale engineering of guest@host metal-organic framework materials for optoelectronic properties

This thesis describes the nano-engineering of self-assembly processes, to accomplish nanoscale metal-organic framework (MOF) systems potentially useful for optoelectronic applications. The well-known fact underlying the large crystal size formation of common MOF compounds has been re-investigated here, not only to minimise the prolonged synthesis time but also to yield facile deposition in uniform thin film formats. This thesis presents the innovative concept of high concentration reaction (HCR) and its utility for making nano MOFs. Surprisingly, this method resulted in formation of a new kind of gel-like soft materials, for which I coined the term —supraMOFs — meaning supramolecular MOF hybrid materials. A detailed study of supraMOFs focussing on their constituent nano MOF elements, stimuli-responsive behaviours, and sol-gel conversion phenomena have been systematically performed. Akin to low molecular weight gels (LMWG), mechanical properties of supraMOFs displaying storage modulus (G') > loss modulus (G") have been confirmed by rheological experiments. The use of sol-gel MOF system to attain a uniform MOF film, has been demonstrated through an example of HKUST-1 thin coating of ∼10 nm roughness and ∼1 μm thickness. The HCR concept was further extended to develop advanced functional nano MOF systems, where one-step rapid synthesis of fluorescent MOF nanosheets has been accomplished. The old but effective concept of functionalisation of MOFs using porous coordination space and external guest species was implemented here, but with a twist. Particularly, the challenge of caging larger sized guest species into smaller pore apertures of MOFs, in parallel to controlling material growth in the nanoscale regime were solved by adopting the HCR approach. Furthermore, this thesis has demonstrated new interesting possibilities employing fluorescent nano MOF system to engineer smart sensors for detecting volatile organic compounds (VOCs), and mechanical stresses via a mechanochromic luminescent MOF nanoplate system.