Ultrafast Dynamics of Cu(I) complexes for Light-Driven Antibacterial Water Purification

Water scarcity is one of the most pressing issues facing the world today. One approach to tackling this problem is to use sunlight as a source of clean and free energy to drive disinfection of water, where a photosensitiser absorbs a photon, entering an excited state. The excited photosensitiser interacts with oxygen to produce reactive oxygen species which kill bacteria in water.

The focus of this thesis is two-fold: (i) to investigate the efficiency of [Cu(PP)(NN)]BF4 complexes (NN = 1,10-phenanthroline and its derivatives, PP = 4,5-bis(diphenylphosphino)-9,9-dimethyl-xanthene and DPEphos) for antibacterial water purification and (ii) to further our understanding of the excited state dynamics and structural changes of this class of complexes.

Chapters 1 and 2 introduce the field and offer a brief outline of the experimental methods used.

Chapters 3 and 4 describe the effects of the 2,9 substituents on the phenanthroline ligand on the excited state dynamics in such [Cu(PP)(NN)]+ complexes. The excited state dynamics involves a complicated interplay between singlet and triplet states, in tetrahedral and/or flattened geometry. Contrary to the current accepted mechanism for the heteroleptic [Cu(PP)(NN)]+ complexes, preliminary evidence is presented to support the presence of ultrafast ISC in [Cu(PP)(NN)]+ occurring in <100 fs. From these results the proposed mechanism for the decay of the initially populated excited singlet “tetrahedral” state, involves ultrafast intersystem crossing into a triplet manifold, and structural changes in both the singlet and the triplet manifolds towards a pseudo-square planar geometry. The relative efficiency of each pathway has been shown to depend on the bulk of the ligands used. The lifetime of the final excited state, flattened triplet MLCT state, is also strongly dependent on the ligand bulk, varying from 280 ps to 64 ns in aerated acetonitrile solutions at r.t.

Chapter 5 presents, at the time of writing, the first (and only) example of a heteroleptic Cu(I) complex used for light-driven antibacterial water treatment. [Cu(xant)(dmp)]tfpb immobilised on silica achieves 99.9999% killing of S. aureus and E. Coli after 2 - 3 h of irradiation, respectively. This performance corresponds to the WHO classification of “highly protective” treatments and highlights the potential of these complexes for photo-driven applications in general.

Chapter 6 covers several related projects that have been initiated during the PhD, including
investigating the effects of the flexibility of the phopshine ligand and of the more electron-accepting diimines, biquinoline derivatives, on the excited state dynamics of [Cu(PP)(NN)]+ complexes.