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Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6′-dhbp) or distal (4,4′-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)2Ru(n,n′-dhbp)]Cl2, or as the doubly deprotonated (O− bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7A, [(dop)2Ru(4,4′-dhbp)]Cl2, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(Do/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1–4 bearing the 6,6′-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5–8 bearing the 4,4′-dhbp ligand, the deprotonated Ru complexes (5B–8B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the 3LLCT excited state and charge transfer from the [O2-bpy]2− ligand to the N,N spectator ligand. The protonated OH bearing 4,4′-dhbp Ru complexes (5A–8A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8A, has the longest lifetime of the series at 3.45 μs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with 3O2 to yield 1O2.

Circulating tumor cells (CTCs) are known to have cancer stem cell (CSC) properties and survive physiological conditions of fluid shear stress (FSS). However, current chemotherapy screening techniques do not adequately recapitulate this FSS environment and are not predictive of a drug response. In this study, MCF7 and MDA‐MB‐231 cells under FSS are used as an in vitro model of CTCs. The effects of doxorubicin (DOX) and paclitaxel on sheared cells using WST8 assay and stemness (CD44⁺/CD24⁻) and apoptosis (Annexin V⁺/7‐AAD⁺) using flow cytometry are tested. Quantitative polymerase chain reaction is used to test gene expression. It is shown that suspension‐cultured and FSS treated MCF7 cells increase in drug resistance, especially with DOX. There is a synergistic increase in the CD44⁺/CD24⁻CSC‐like population and an increase in drug resistance‐related gene expression in MCF7 cells co‐treated with FSS and drugs. There is also a correlated increase in STAT3 and NANOG expression under FSS. To the best of the authors' knowledge, this is the first report to suggest that the increase in CSC‐like cells from FSS contributes to drug resistance via the STAT3/NANOG pathway. This increase in CTC drug resistance also highlights the importance of implementing FSS, which is unavailable in current drug screening techniques.