A new family of luminescent [Pt(pbt)₂(C₆F₅)L] (n = 1, 0) complexes: synthesis, optical and cytotoxic studies

Given the widely recognized bioactivity of 2-arylbenzothiazoles against tumor cells, we have designed a new family of luminescent heteroleptic pentafluorophenyl-bis(2-phenylbenzothiazolyl) Pt^IV derivatives, fac-[Pt(pbt)₂(C₆F₅)L] (n = 1, 0) [L = 4-Mepy 1, 4-pyridylbenzothiazole (pybt) 2, 4,4'-bipyridine (4,4'-bpy) 3, 1,2-bis-(4-pyridyl)ethylene (bpe) 4 (E/Z ratio: 90/10), 1,4-bis-(pyridyl)butadiyne (bpyb) 5, trifluoroacetate (^-OCOCF₃) 6] and a dinuclear complex [{Pt(pbt)₂(C₆F₅)}₂(μ-bpyb)](PF₆)₂7, in which the trans ligand to the metalated C-(pbt) was varied to modify the optical properties and lipophilicity. Their photophysical properties were systematically studied through experimental and theoretical investigations, which were strongly dependent on the identity of the N-bonded ligand. Thus, complexes 1, 3 and 6 display, in different media, emission from the triplet excited states of primarily intraligand ³ILCT nature localized on the pbt ligand, while the emissions of 2, 5 and 7 were ascribed to a mixture of close ³IL'(N donor)/³ILCT(pbt) excited states, as supported by lifetime measurements and theoretical calculations. Irradiation of the initial E/Z mixture of 4 (15 min) led to a steady state composed of roughly 1 : 1.15 (E : Z) and this complex was not emissive at room temperature due to an enhanced intramolecular E to Z isomerization process of the 1,2-bis-(4-pyridyl)ethylene ligand. Complexes 1-3 and 6 showed excellent quantum yields for the generation of singlet oxygen in aerated MeCN solution with the values of ϕ(¹O₂) ranging from 0.66 to 0.86 using phenalenone as a reference. Cationic complexes 1-3 exhibited remarkable efficacy in the nanomolar range against A549 (lung carcinoma) and HeLa (cervix carcinoma) cell lines with notable selectivity relative to the non-tumorigenic BEAS-2B (bronchial epithelium) cells. In the A549 cell line, the neutral complex 6 showed low cytotoxicity (IC₅₀: 29.40 μM) and high photocytotoxicity (IC₅₀: 5.75) when cells were irradiated with blue light for 15 min. These complexes do not show evidence of DNA interaction.