Aluminum Photosensitizers with Long-lived Triplet States on Trial: Synthesis, Crystal Structures, Photophysical and Photobiological Properties of Tris(dipyrrinato)aluminum(III) Complexes

Metal coordination compounds are currently a focus of research in developing new photosensitizers for materials and medicinal applications. As an abundant element in the earth’s crust aluminum is a suitable target element. However, only limited studies are available on its use in photoactive systems. We now report the facile preparation of a library of homoleptic tris(dipyrrinato)aluminum(III) [AL(DIPY)3] complexes. The majority of complexes was characterized by single crystal X-ray analysis and their photophysical properties upon photoexcitation and their tendency to react with the molecular oxygen of the microenvironment and generate singlet oxygen - in polar and non-polar environment was investigated. These studies are complemented by density functional theory (DFT) calculations to assess the possible electronic distribution on the frontier molecular orbitals within the complexes. As a result of charge transfer states, long-lived triplet excited states were formed and allowed for singlet oxygen generation. An initial screening of the AL(DIPY)3 complexes via in vitro phototoxicity studies against a mouse colon carcinoma cell line (CT26) was promising as these complexes were able to trigger cell death upon irradiation at safe nanomolar and micromolar concentrations. The results highlight the potential of aluminum dipyrrin complexes as a broadly applicable class of photosensitizers.