Copper & friends: Luminescence & triazole based switching

Abstract

This thesis describes attempts towards producing a new motif for Cu(I/II) switching that incorporates CuAAC synthesised 1,2,3-triazoles. A variety of techniques is used to explore the properties of new and model ligands, their potential for Cu(I/II) switching and photoswitching. The thesis consists of five chapters, where the work presented in Chapter 2 and Chapters 3/4 was carried out concurrently. Chapter 1 introduces switching by first discussing several important biological switches. Switching in terms of this thesis is defined and synthetic switches are broadly categorised by the method with which switching is achieved, i.e. chemical, electrochemical or photophysical/photochemical. The photophysics of transition metal complexes is briefly described before an overview of Cu(I/II) switching is provided. Emphasis is placed on the pioneering work of Sauvage and co-workers, and on the HETPHEN strategy reported by Schmittel and co-workers. The previous work from these groups provided the inspiration for the research described in chapters 2-4, where the goal was to move away from polypyridyl systems to the synthetically more facile “click” triazoles while also exploring the potential for a new Cu(I/II) photoswitch. Chapter 2 outlines previous work by Crowley and co-workers on Cu(I/II) switchable 1,1'-disubstituted ferrocene complexes. CuAAC “click” chemistry is then described, its mechanism presented and its potential to be a promising alternative to the traditionally used polypyridyl systems in Cu(I/II) switching is discussed. A description of the switching systems targeted in this thesis is provided, highlighting the different components of the switch that should provide selectivity for Cu(I) or Cu(II) ions. Following this the synthesis and coordination properties of several model systems, that reflect the different components of the switches, are explored as a means of determining the efficacy of the desired switching systems. Upon confirming the model systems to behave in the expected manner, the synthesis of the switch ligands through “click” chemistry and Pd cross couplings is presented. The coordination properties of the switch ligands with Cu(I) and Cu(II) ions is explored. Followed by a series of competition experiments being performed due to the tridentate pockets within the switch ligands being hypodentate, resulting in the binding of Cu(I) ions at this site. Finally, an interesting result in which an oxo-bridged diCu(II) complex was obtained, reminiscent of hemocyanin, is discussed as well as its potential for future study. Chapter 3 explores the electrochemical and photophysical properties of a small family of 2-pyridyl-1,2,3-triazole Cu(I) complexes. The use of 2-pyridyl-1,2,3-triazole (pytri) ligands for the development of photophysically active transition metal complexes is broadly examined, before emphasis is placed on previous studies of 2-pyridyl-1,2,3-triazole Cu(I) complexes. The overarching goal of the work presented in the chapter was to attempt to develop a photoswitchable Cu(I/II) system. To achieve this a study of the properties of Cu(I) pytri complexes was undertaken. Given the parent [Cu(pytri)(diMesbpy)](PF6) complex proved to be non-emissive the effects of adding a known chromophoric substituent (TPA) and a change of ancillary ligand were examined. Chapter 4 moves away from 1,2,3-triazole ligands to look at a 6,6'-diTPA-2,2'-bipyridine (diTPAbpy) ligand as an alternative ancillary ligand in the HETPHEN strategy. Again, the overall intention was towards producing a photoswitchable Cu(I/II) system. Previous literature on analogous ligands are discussed, though surprisingly this 6,6' disubstituted ligand had not been reported despite studies on both its 4,4' and 5,5' analogues. As such the synthesis of diTPAbpy is described and its coordination chemistry with 4- and 6-coordinate transition metals explored. The compounds electronic properties are also examined. Chapter 5 gives a summary of the work presented in the thesis, while also providing several avenues for further study. These include adjustments to the switch ligands to improve selectivity, alternative chromophoric substituents for photoswitching and the possible use of ligands within the thesis to produce supramolecular architectures.