Application of Discrete First-Row Transition-Metal Complexes as Photosensitisers

This Minireview summarises and critically evaluates recent advances in the utilisation of discrete first-row transition-metal (TM) complexes as photosensitisers. Whilst many compounds absorb light, TM complexes are generally more desirable for photochemical applications, as they usually exhibit strong absorption of visible light, making them ideally suited to exploiting the sun as a freely available light source. Due to their outstanding activities, precious metals, such as iridium and ruthenium, are currently still at the forefront of photochemistry research. However, they also bear disadvantages with respect to abundance, cost and toxicity. Therefore, it is desirable to move to more abundant and less expensive systems that retain good photosensitising abilities. This Minireview will focus on first-row transition-metals, specifically titanium, copper, iron, and zinc, which have become the focus of increased attention over recent years as potential replacements for noble metals as photosensitisers. Their structure - activity relationships are explored and challenges in designing the ligands and complexes are discussed.