The purpose of the present study was to investigate the influence of π-expansive cyclometalating ligands on the photophysical and photobiological properties of organometallic Ru(II) compounds. Four compounds with increasing π conjugation on the cyclometalating ligand were prepared, and their structures were confirmed by HPLC, 1D and 2D (1)H NMR, and mass spectrometry. The properties of these compounds differed substantially from their Ru(II) polypyridyl counterparts. Namely, they were characterized by red-shifted absorption, very weak to no room temperature phosphorescence, extremely short phosphorescence state lifetimes (<10 ns), low singlet oxygen quantum yields (0.5-8%), and efficient ligand-centered fluorescence. Three of the metal complexes were very cytotoxic to cancer cells in the dark (EC50 values = 1-2 μM), in agreement with what has traditionally been observed for Ru(II) compounds derived from small C^N ligands. Surprisingly, the complex derived from the most π-expansive cyclometalating ligand exhibited no cytotoxicity in the dark (EC50 > 300 μM) but was phototoxic to cells in the nanomolar regime. Exceptionally large phototherapeutic margins, exceeding 3 orders of magnitude in some cases, were accompanied by bright ligand-centered intracellular fluorescence in cancer cells. Thus, Ru(II) organometallic systems derived from π-expansive cyclometalating ligands, such 4,9,16-triazadibenzo[a,c]napthacene (pbpn), represent the first class of potent light-responsive Ru(II) cyclometalating agents with theranostic potential.

Combining the best attributes of organic photosensitizers with those of coordination complexes is an elegant way to achieve prolonged excited state lifetimes in Ru(II) dyads. Not only do their reduced radiative and nonradiative rates provide ample time for photosensitization of reactive oxygen species at low oxygen tension but they also harness the unique properties of (3)IL states that can act as discrete units or in concert with (3)MLCT states. The imidazo[4,5-f][1,10]phenanthroline framework provides a convenient tether for linking π-expansive ligands such as pyrene to a Ru(II) scaffold, and the stabilizing coligands can fine-tune the chemical and biological properties of these bichromophoric systems. The resulting dyads described in this study exhibited nanomolar light cytotoxicities against cancer cells with photocytotoxicity indices exceeding 400 for some coligands employed. This potency extended to bacteria, where concentrations as low as 10 nM destroyed 75% of a bacterial population. Notably, these dyads remained extremely active against biofilm with light photocytotoxicities against these more resistant bacterial populations in the 10-100 nM regime. The results from this study demonstrate the versatility of these highly potent photosensitizers in destroying both cancer and bacterial cells and expand the scope of compounds that utilize low-lying (3)IL states for photobiological applications.

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The photophysics of a series of Ru(II) dyads based on the 2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]-phenanthroline ligand was investigated. The ability of these metal complexes to intercalate DNA and induce cell death upon photoactivation makes them attractive photosensitizers for a range of photobiological applications, including photodynamic therapy. In the present study, time-resolved transient absorption and emission spectroscopy were used to interrogate the photoinduced processes that follow metal-to-ligand charge transfer excitation of the complexes in solution. It was found that energy transfer to pyrene-localized intraligand triplet states, facilitated by torsional motion of the pyrene moiety relative to the imidazo[4,5-f][1,10]phenanthroline ligand, was an important relaxation pathway governing the photophysical dynamics in this class of compounds. Biphasic decay kinetics were assigned to spontaneous (pre-equilibrium) and delayed emission, arising from an equilibrium established between (3)MLCT and (3)IL states. TDDFT calculations supported these interpretations.

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 ( MLCT) state with a lifetime (τ ) of approximately 1 μs. Transient absorption measurements revealed an additional excited state that was nonemissive and long-lived (τ = 43 μs for 2 and 27 μs for 3), assigned as a triplet intraligand ( IL) state that was accessible only in 2 and 3. All three complexes were strong singlet oxygen ( O ) 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 IL energy. Compounds 2 and 3 also showed in vitro photobiological effects with red light (625 nm), where their molar absorptivities were<100 m cm . These findings highlight that Ru(II) dyads derived from α-oligothiophenes directly appended to 1,10-phenanthroline-namely 2 and 3-possess low-lying IL states that are highly photosensitizing, and they may therefore be of interest for photobiological applications such as photodynamic therapy (PDT).