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.

The synthesis, characterization, and fluorescence spectroscopy of E(CH) complexes (E = P, As, Sb, and Bi), as well as OP(CH), OAs(CH), and SP(CH), is reported. These compounds exhibit fluorescence quantum yields that span two orders of magnitude, from well below 1% to ca. 14%.

A series of novel bimetallic Ti IV amine bis(phenolate) complexes was synthesised and fully characterised. X‐ray crystallography studies revealed distorted octahedral geometries around the Ti centres with single or double oxo‐bridges connecting the two metals. These robust, air‐ and moisture‐stable complexes were employed as photosensitisers generating singlet oxygen following irradiation with visible light (420 nm) LED module in a commercial flow reactor. All five complexes showed high activity in the photo‐oxygenation of α‐terpinene and achieved complete conversion to ascaridole in four hours at ambient temperature. The excellent selectivity of these photosensitisers towards ascaridole (vs. transformation to p ‐cymene) was demonstrated with control experiments using a traditional TiO 2 catalyst. Further comparative studies employing the free pro‐ligands as well as a monometallic analogue highlighted the importance of the ‘TiO 2 ‐like’ moiety in the polymetallic catalysts. Computational studies were used to determine the nature of the ligand to metal charge transfer (LMCT) states and singlet–triplet gaps for each complex, the calculated trends in the UV‐vis absorption spectra across the series agreed well with the experimental results.

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