Chiral metal-organic frameworks for photonics

Recently, there has been significant interest in the use of chiral metal-organic frameworks (MOFs) and coordination polymers (CPs) for photonics applications. The promise of these materials lies in the ability to tune their properties through judicious selection of the metal and ligand components. Additionally, the interaction of guest species with the host framework can be exploited to realise new functionalities. In this review, we outline the methods for synthesising chiral MOFs and CPs, then analyse the recent innovations in their use for various optical and photonics applications. We focus on two emerging directions in the field of MOF chemistry - circularly polarised luminescence (CPL) and chiroptical switching - as well as the latest developments in the use of these materials for second-order nonlinear optics (NLO), particularly second-harmonic generation (SHG). The current challenges encountered so far, their possible solutions, and key directions for further research are also outlined. Overall, given the results demonstrated to date, chiral MOFs and CPs show great promise for use in future technologies such as optical communication and computing, optical displays, and all-optical devices.

Reversible single crystal-to-single crystal double [2+2] cycloaddition induces multifunctional photo-mechano-electrochemical properties in framework materials

Reversible structural transformations of porous coordination frameworks in response to external stimuli such as light, electrical potential, guest inclusion or pressure, amongst others, have been the subject of intense interest for applications in sensing, switching and molecular separations. Here we report a coordination framework based on an electroactive tetrathiafulvalene exhibiting a reversible single crystal-to-single crystal double [2 + 2] photocyclisation, leading to profound differences in the electrochemical, optical and mechanical properties of the material upon light irradiation. Electrochemical and in situ spectroelectrochemical measurements, in combination with in situ light-irradiated Raman spectroscopy and atomic force microscopy, revealed the variable mechanical properties of the framework that were supported using Density Functional Theory calculations. The reversible structural transformation points towards a plethora of potential applications for coordination frameworks in photo-mechanical and photoelectrochemical devices, such as light-driven actuators and photo-valves for targeted drug delivery.

Porous coordination frameworks that undergo reversible structural transformations are promising for sensing, switching and separations. Here, the authors report an electroactive framework that exhibits a reversible single crystal-to-single crystal double [2+2] photocyclisation, leading to property changes.

Vapour-driven crystal-to-crystal transformation showing an interlocking switch of the coordination polymer chains between 1D and 3D

A rare crystal-to-crystal transformation occurs between 1D coordination polymer chains and 3D mechanically interlocked structures via reversible opening and closing of the homogeneous chains without any change in the coordination mode and composition.

Post-cycloaddition modification of a porous MOF for improved GC separation of ethanol and water

An olefin-containing porous framework undergoes [2 + 2] cycloaddition to give post-modified pore space, accompanied by turn-on luminescence upon light irradiation. The photochemical modification reinforces the supramolecular network and improves GC separation performance of ethanol and water.

A dual-stimuli responsive small molecule organic material with tunable multi-state response showing turn-on luminescence and photocoloration

A multiple responsive organic small molecular material has been developed, which exhibits dual pathway photoreactions and gives turn-on luminescence and photocoloration under different light sources.