Investigations Towards the Synthesis of Hybrid Donor-Acceptor Stenhouse Adduct - Spin-Crossover Materials

Donor-Acceptor Stenhouse Adducts (DASA) are a class of photochromic molecules specifically falling into the subcategory of molecular photoswitches. Upon irradiation with visible light DASAs exhibit a photoisomerisation from a highly coloured linear form to a compact colourless form. This light-induced transformation can subsequently be reversed upon removal of the light source, with additional heating capable of increasing the rate of the reverse process. Since their discovery in 2014, the desirable photochromic properties they demonstrate have garnered significant attention. Prospective application in areas such as sensing technologies, nano-reactors, drug delivery and soft actuators have been explored to date. Spin-crossover (SCO) compounds are another class of material which exhibit a transformation between two distinct states. They are a group of transition metal complexes that possess the ability to switch between two spin states, high-spin (HS) and low-spin (LS), in response to external stimuli, most prominently changes in temperature and pressure, but also light and external magnetic fields. SCO materials have demonstrated functionality in various areas, including smart sensing devices, solid-state cooling systems and actuators. Ligand-driven lightinduced spin change (LD-LISC) is a SCO process that some specific SCO materials undergo. In these cases, the change in spin-state is driven by photoisomerisation of the ligands of the metal complex in response to irradiation with light. DASAs and SCO compounds have never been combined into a single material, the combination of their unique properties could result in a novel LD-LISC system. Such a hybrid material would take advantage of the visible light activated nature of DASA photoswitches to induce room temperature SCO activity producing new stimuli responsive smart materials with potential applications in sensing, actuators and logic systems. After a thorough survey of the DASA and SCO literature, the concentration dependent nature of DASAs was explored in Chapter 2. Five DASA molecules were synthesised using a piperazine donor moiety, three of which were newly introduced, the properties of all five were characterised using UVvis spectroscopy. It was found that these systems do not exhibit the concentration dependent photoswitching properties plaguing other DASAs. This phenomenon was investigated, showcasing the full photoswitching capabilities of piperazine-based DASAs. Additionally, through NMR spectroscopy, crystallographic analysis and computational experiments, the lack of concentration dependence observed in piperazine-based DASAs was attributed to structural features that prevent supramolecular aggregations. In the case of DASAs synthesised with piperazine and methyl piperazine, resulting from the unique ability of the terminal nitrogen to accept a proton in the zwitterionic, colourless form. Strategies to eliminate the concentration dependent properties of DASAs is of significant importance in the pursuit of DASA-SCO hybrid materials. Without overcoming this limitation, the cooperativity required in materials for successful SCO activity with favourable properties would be difficult to achieve. In Chapter 3, the first evidence of solid-state switching in a DASA molecule was presented. The solid-state activity was investigated through powder X-ray diffraction, NMR and IR spectroscopy. Crystals of a hydrate of a DASA synthesised with a piperidine donor and the barbituric acid acceptor were grown. Heating both single crystal and powder samples of this DASA hydrate over 90C would result in a transformation from the colourless cyclopentanone isomer into the coloured triene form. Illustrating that DASAs can switch between forms in the solid-state removes another barrier towards the goal of synthesising DASA-SCO hybrid materials capable of solid-state activity. Finally, Chapter 4 focuses on the attempted synthesis of hybrid DASA-SCO materials through various strategies. Multiple synthetic routes exploring the modification of four different core ligands, 1,10-phenanthroline, 2,6-di(pyrazol-1-yl)isonicotinic acid, 4-amino-1,2,4-triazole and dipicolyl amine were attempted in the pursuit of DASA-based ligands capable of metal coordination. Cocrystallization was also attempted as a method for the development of a hybrid DASA-SCO system. Evidence of a Dipicolyl amine based DASA is presented, in addition to the successful synthesis and analysis of a novel DASA with a triazole-based donor molecule. While no hybrid DASA-SCO materials were isolated, the work explores synthetic progress towards this goal. While no hybrid DASA-SCO material was synthesised, meaningful progress towards this goal was achieved. Tackling the concentration dependence of DASAs with a broadly applicable strategy in addition to the first observations of solid-state switching in DASAs represent important milestones toward the production of multifunctional hybrid systems. Significant aspects of DASAs that could have been fundamental limitations have been overcome, and important synthetic challenges were explored, from which improved synthetic strategies can be designed.