Photodynamic therapy (PDT) exploits light-activated compounds for therapeutic use. It relies on a photosensitiser (PS) that is inactive in the absence of light. When irradiated, the PS absorbs light and is promoted to a higher energy, “excited” state (PS^∗), which is either toxic to cells in itself or triggers formation of other species which are toxic to cells, and hence particular wavelengths of light can be used to induce light-dependent cell killing. In PDT occurring via the so-called type I and type II mechanisms, the PS^∗ engages in energy transfer to dioxygen present in cells and tissues. This process generates highly reactive singlet oxygen (¹O₂) and/or other reactive oxygen species (ROS), which in turn cause damage in the immediate vicinity of the irradiation and ultimately can lead to cell death. Whilst the main focus of research for the last 50 years has been on organic molecules or porphyrins as sensitisers, there is now emerging interest in extending the use to transition metal (TM) complexes, which can display intense absorptions in the visible region, and many also possess high two-photon absorption cross-sections, enabling two-photon excitation with NIR light. As with any other type of photosensitiser, the issues to consider whilst designing a TM complex as a photosensitiser include cell permeability, efficient absorption of NIR light for deeper penetration, preferential affinity to cancer cells over healthy cells, targeted intracellular localisation and lack of side effects. This review summarises recent developments involving photosensitisers containing Ru(II), Os(II), Pt, Ir(III), and Re(I) ions, and the approaches used to address the above requirements. Several remarkable recent advances made in this area, including the first clinical trial of a metal complex as a photosensitiser, indicate the bright future of this class of compounds in PDT.

光动力疗法（PDT）将光活化的化合物用于治疗用途。它依赖于在没有光照的情况下不活跃的光敏剂（PS）。当被照射时，PS吸收光并被提升为更高的能量，即“激发”态（PS ^∗），该态对细胞本身是有毒的或触发了对细胞有毒的其他物种的形成，因此特定波长的光可用于诱导光依赖性细胞杀伤。在通过所谓的I型和II型机制发生的PDT中，PS ^∗参与能量转移至细胞和组织中存在的双氧。此过程产生高反应性单线态氧（¹ O ₂）和/或其他活性氧（ROS），继而在紧邻辐照的地方造成损害，并最终导致细胞死亡。尽管过去50年的研究重点一直放在有机分子或卟啉作为敏化剂上，但现在人们开始将其用途扩展到过渡金属（TM）络合物，这种络合物可以在可见光区域显示出强烈的吸收性，并且许多人对此产生了兴趣。它还具有较高的双光子吸收截面，从而可以用近红外光激发双光子。与任何其他类型的光敏剂一样，在将TM复合物设计为光敏剂时要考虑的问题包括细胞渗透性，有效吸收NIR光以更深层渗透，对癌细胞的优先于健康细胞的亲和力，靶向的细胞内定位以及无副作用。这篇综述总结了包含Ru（II），Os（II），Pt，Ir（III）和Re（I）离子的光敏剂的最新进展，以及解决上述要求的方法。该领域最近取得的一些显着进展，包括首次将金属配合物用作光敏剂的临床试验表明，该类化合物在PDT中的光明前景。