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Photophysical Properties and Photobiological Activities of Ruthenium(II) Complexes Bearing π‑Expansive Cyclometalating Ligands with Thienyl Groups

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journal contribution
posted on 06.08.2019, 16:12 by Julia McCain, Katsuya L. Colón, Patrick C. Barrett, Susan M. A. Monro, Tariq Sainuddin, John Roque III, Mitch Pinto, Huimin Yin, Colin G. Cameron, Sherri A. McFarland
A new family of cyclometalated ruthenium­(II) complexes [Ru­(N^N)2(C^N)]+ derived from the π-extended benzo­[h]­imidazo­[4,5-f]­quinolone ligand appended with thienyl groups (n = 1–4, compounds 14) was prepared and its members were characterized for their chemical, photophysical, and photobiological properties. The lipophilicities of 14, determined as octanol–water partition coefficients (log Po/w), were positive and increased with the number of thienyl units. The absorption and emission bands of the C^N compounds were red-shifted by up to 200 nm relative to the analogous Ru­(II) diimine systems. All of the complexes exhibited dual emission with the intraligand fluorescence (1IL, C^N-based) shifting to lower energies with increasing n and the metal-to-ligand charge transfer phosphorescence (3MLCT, N^N-based) remaining unchanged. Compounds 13 exhibited excited state absorption (ESA) profiles consistent with lowest-lying 3MLCT states when probed by nanosecond transient absorption (TA) spectroscopy with 532 nm excitation and had contributions from 1IL­(C^N) states with 355 nm excitation. These assignments were supported by the lifetimes observed (<10 ns for the 1IL states and around 20 ns for the 3MLCT states) as well as a noticeable ESA for 3 with 355 nm excitation that did not occur with 532 nm excitation. Compound 4 was the only member of the family with two 3MLCT emissive lifetimes (15, 110 ns), and the TA spectra collected with both 355 and 532 nm excitation was assigned to the 3IL state, which was corroborated by its 4–6 μs lifetime. The ESA for 4 had a rise time of approximately 10 ns and an initial decay of 110 ns, which suggests a possible 3MLCT–3IL excited state equilibrium that results in delayed emission from the 3MLCT state. Compound 4 was nontoxic toward human skin melanoma cells (SKMEL28) in the dark (EC50 = >300 μM); 13 were cytotoxic and yielded EC50 values between 1 and 20 μM. The photocytotoxicites with visible light ranged from 87 nM with a phototherapeutic index (PI) of 13 for 1 to approximately 1 μM (PI = >267) for 4. With red light, EC50 values varied from 270 nM (PI = 21) for 3 to 12 μM for 4 (PI = >25). The larger PIs for 4, especially with visible light, were attributed to the much lower dark cytotoxicity for this compound. Because the dark cytotoxicity contributes substantially to the observed photocytotoxicity for 13, it was not possible to assess whether the 3IL state of 4 led to a much more potent phototoxic mechanism in the absence of dark toxicity. There was no stark contrast in cellular uptake and accumulation by laser scanning confocal and differential interference contrast microscopy to explain the large differences in dark toxicities between 13 and 4. Nevertheless, the study highlights a new family of Ru­(II) C^N complexes where π-conjugation beyond a certain point results in low dark cytotoxicity with high photocytotoxicity, opposing the notion that cyclometalated Ru­(II) systems are too toxic to be phototherapeutic agents.