Synthesis, Characterization and Photobiological Studies of Ru(II) Dyads Derived from α-Oligothiophene Derivatives of 1,10-Phenanthroline

Three new bis(2,2′-bipyridine)-heteroleptic Ru(II) dyads incorporating thienyl groups (n = 1–3, compounds 1, 2 and 3, respectively) appended to 1,10-phenanthroline were synthesized and characterized to investigate the impact of n on the photophysical and photobiological properties within the series. All three complexes showed unstructured emission near 618 nm from a triplet metal-to-ligand charge transfer ( ³ MLCT) state with a lifetime (τ _em ) of approximately 1 μs. Transient absorption measurements revealed an additional excited state that was nonemissive and long-lived (τ _TA  = 43 μs for 2 and 27 μs for 3), assigned as a triplet intraligand ( ³ IL) state that was accessible only in 2 and 3. All three complexes were strong singlet oxygen ( ¹ O ₂ ) sensitizers, with quantum yields (Φ _∆ ) for 2 and 3 being the largest (74–78%), and all three were photocytotoxic to cancer cells with visible light activation in the order: 3 > 2 > 1. Cell-free DNA photodamage followed the same trend, where potency increased with decreasing ³ IL energy. Compounds 2 and 3 also showed in vitro photobiological effects with red light (625 nm), where their molar absorptivities were <100 m ⁻¹  cm ⁻¹ . These findings highlight that Ru(II) dyads derived from α-oligothiophenes directly appended to 1,10-phenanthroline—namely 2 and 3—possess low-lying ³ IL states that are highly photosensitizing, and they may therefore be of interest for photobiological applications such as photodynamic therapy (PDT).