Spectro-electrochemical Studies on [Ru(TAP)2(dppz)]2+-Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides

Nitrile Substituents at the Conjugated Dipyridophenazine Moiety as Infrared Redox Markers in Electrochemically Reduced Heteroleptic Ru(II) Polypyridyl Complexes

Ruthenium(II) complexes [Ru(tap)2(NN)]2+ (tap = 1,4,5,8-tetraazaphenanthrene, NN = 11-cyano-dipyrido[3,2-a:2',3'-c]phenazine (11-CN-dppz) and 11,12-dicyano-dipyrido[3,2-a:2',3'-c]phenazine (11,12-CN-dppz)) feature the C≡N groups as infrared (IR)-active redox markers. They were studied by cyclic voltammetry, UV-vis, and IR spectroelectrochemistry (SEC), and density functional theory calculations to assign the four 1e- reduction waves R1-R4 observed in dichloromethane. Generally, the NN ligands are reduced first (R1). For [Ru(tap)2(11,12-CN-dppz)]2+, R1 is sufficiently separated from R2 and delocalized over both tap ligands. Accordingly, IR SEC conducted at R1 shows a large red shift of the νs,as(C≡N) modes by -18/-28 cm-1, accompanied by a 4-fold enhancement of the νs(C≡N) intensity, comparably with reference data for free 11,12-CN-dppz. The first tap-based reduction of spin-doublet [Ru(tap)2(11,12-CN-dppz)]+ to spin-triplet [Ru(tap)2(11,12-CN-dppz)] at R2 decreased ν(C≡N) by merely -2 cm-1, while the intensity enhancement reached an overall factor of 8. Comparably, a red shift of ν(C≡N) by -27 cm-1 resulted from the 1e- reduction of [Ru(tap)2(11-CN-dppz)]2+ at R1 (poorly resolved from R2), and the intensity enhancement was roughly 3-fold. Concomitant 1e- reductions of the tap ligands (R2 and R3) caused only minor ν(C≡N) shifts of -3 cm-1 and increased the absorbance by overall factors of 6.5 and 8, respectively.

Photophysical Properties and DNA Binding of Two Intercalating Osmium Polypyridyl Complexes Showing Light-Switch Effects

The synthesis and photophysical characterization of two osmium(II) polypyridyl complexes, [Os(TAP)₂dppz]²⁺ (1) and [Os(TAP)₂dppp2]²⁺ (2) containing dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppp2 (pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline) intercalating ligands and TAP (1,4,5,8-tetraazaphenanthrene) ancillary ligands, are reported. The complexes exhibit complex electrochemistry with five distinct reductive redox couples, the first of which is assigned to a TAP-based process. The complexes emit in the near-IR (1 at 761 nm and 2 at 740 nm) with lifetimes of >35 ns with a low quantum yield of luminescence in aqueous solution (∼0.25%). The Δ and Λ enantiomers of 1 and 2 are found to bind to natural DNA and with AT and GC oligodeoxynucleotides with high affinities. In the presence of natural DNA, the visible absorption spectra are found to display significant hypochromic shifts, which is strongly evident for the ligand-centered π–π* dppp2 transition at 355 nm, which undergoes 46% hypochromism. The emission of both complexes increases upon DNA binding, which is observed to be sensitive to the Δ or Λ enantiomer and the DNA composition. A striking result is the sensitivity of Λ-2 to the presence of AT DNA, where a 6-fold enhancement of luminescence is observed and reflects the nature of the binding for the enantiomer and the protection from solution. Thermal denaturation studies show that both complexes are found to stabilize natural DNA. Finally, cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Osmium(II) polypyridyl complexes comprising extended dipyrido[3,2-a:2′,3′-c]phenazine (1) and pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (2) intercalating ligands are shown to be effective DNA binders accompanied by enhanced near-IR emission. The emission response to B-DNA is found to be sensitive to the enantiomer and the composition of DNA, with greater emission observed for AT-rich sequences. Thermal denaturation studies show that both complexes stabilize natural DNA. Cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Evaluation of 1 H-NMR Spectroscopy-Based Quantification Methods of the Supramolecular Aggregation of a Molecular Photosensitizer

The supramolecular dimerization of a ruthenium polypyridyl precursor of a well-developed family of hydrogen evolving photocatalysts via π-π-interactions of the polyheteroaromatic bridging ligand was quantified with concentration dependent ¹ H-NMR-spectroscopy. The data sets were analyzed with different calculation and fit methods. A comparison between the results of direct calculation, linear and nonlinear approaches showed that the application of a global nonlinear fit procedure yields the best results. The presented methods are also applicable for dimerization processes in solution of other molecular moieties.

Adenine Radical Cation Formation by a Ligand-Centered Excited State of an Intercalated Chromium Polypyridyl Complex Leads to Enhanced DNA Photo-oxidation

Assessment of the DNA photo-oxidation and synthetic photocatalytic activity of chromium polypyridyl complexes is dominated by consideration of their long-lived metal-centered excited states. Here we report the participation of the excited states of [Cr(TMP)₂dppz]³⁺ (1) (TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) in DNA photoreactions. The interactions of enantiomers of 1 with natural DNA or with oligodeoxynucleotides with varying AT content (0–100%) have been studied by steady state UV/visible absorption and luminescence spectroscopic methods, and the emission of 1 is found to be quenched in all systems. The time-resolved infrared (TRIR) and visible absorption spectra (TA) of 1 following excitation in the region between 350 to 400 nm reveal the presence of relatively long-lived dppz-centered states which eventually yield the emissive metal-centered state. The dppz-localized states are fully quenched when bound by GC base pairs and partially so in the presence of an AT base-pair system to generate purine radical cations. The sensitized formation of the adenine radical cation species (A^•+T) is identified by assigning the TRIR spectra with help of DFT calculations. In natural DNA and oligodeoxynucleotides containing a mixture of AT and GC of base pairs, the observed time-resolved spectra are consistent with eventual photo-oxidation occurring predominantly at guanine through hole migration between base pairs. The combined targeting of purines leads to enhanced photo-oxidation of guanine. These results show that DNA photo-oxidation by the intercalated 1, which locates the dppz in contact with the target purines, is dominated by the LC centered excited state. This work has implications for future phototherapeutics and photocatalysis.

New Insights on the Interaction of Phenanthroline Based Ligands and Metal Complexes and Polyoxometalates with Duplex DNA and G-Quadruplexes

This work provides new insights from our team regarding advances in targeting canonical and non-canonical nucleic acid structures. This modality of medical treatment is used as a form of molecular medicine specifically against the growth of cancer cells. Nevertheless, because of increasing concerns about bacterial antibiotic resistance, this medical strategy is also being explored in this field. Up to three strategies for the use of DNA as target have been studied in our research lines during the last few years: (1) the intercalation of phenanthroline derivatives with duplex DNA; (2) the interaction of metal complexes containing phenanthroline with G-quadruplexes; and (3) the activity of Mo polyoxometalates and other Mo-oxo species as artificial phosphoesterases to catalyze the hydrolysis of phosphoester bonds in DNA. We demonstrate some promising computational results concerning the favorable interaction of these small molecules with DNA that could correspond to cytotoxic effects against tumoral cells and microorganisms. Therefore, our results open the door for the pharmaceutical and medical applications of the compounds we propose.

Two RuII Linkage Isomers with Distinctly Different Charge Transfer Photophysics

The ligand PHEHAT (PHEHAT = 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene) presents a structural asymmetry that has a dramatic influence on the photophysical properties depending on the chelation site of the metal ion in the linkage isomers. While [Ru^II(phen)₂HATPHE]²⁺ behaves classically, like [Ru^II(bpy)₃]²⁺, [Ru^II(phen)₂PHEHAT]²⁺ exhibits an unusual behavior. It appears that this complex has two ³MLCT bright states, the lower one being weakly emissive or nonemissive depending on the solvent and temperature. Different photophysical techniques involving a wide range of various temperatures and timescales are essential to analyze this difference. A full photophysical scheme is proposed based on experimental data and density functional theory calculations. While previous studies focused on high temperatures and longer timescale emission, we explore the complexes at very low temperatures and very short times in order to obtain a more complete picture of the intriguing photophysical behavior of these complexes.

Understanding the factors controlling the photo-oxidation of natural DNA by enantiomerically pure intercalating ruthenium polypyridyl complexes through TA/TRIR studies with polydeoxynucleotides and mixed sequence oligodeoxynucleotides

Ruthenium polypyridyl complexes which can sensitise the photo-oxidation of nucleic acids and other biological molecules show potential for photo-therapeutic applications. In this article a combination of transient visible absorption (TrA) and time-resolved infra-red (TRIR) spectroscopy are used to compare the photo-oxidation of guanine by the enantiomers of [Ru(TAP)₂(dppz)]²⁺ in both polymeric {poly(dG-dC), poly(dA-dT) and natural DNA} and small mixed-sequence duplex-forming oligodeoxynucleotides. The products of electron transfer are readily monitored by the appearance of a characteristic TRIR band centred at ca. 1700 cm⁻¹ for the guanine radical cation and a band centered at ca. 515 nm in the TrA for the reduced ruthenium complex. It is found that efficient electron transfer requires that the complex be intercalated at a G-C base-pair containing site. Significantly, changes in the nucleobase vibrations of the TRIR spectra induced by the bound excited state before electron transfer takes place are used to identify preferred intercalation sites in mixed-sequence oligodeoxynucleotides and natural DNA. Interestingly, with natural DNA, while it is found that quenching is inefficient in the picosecond range, a slower electron transfer process occurs, which is not found with the mixed-sequence duplex-forming oligodeoxynucleotides studied.

Efficient electron transfer requires the complex to be intercalated at a G-C base-pair. Identification of preferred intercalation sites is achieved by TRIR monitoring of the nucleobase vibrations before electron transfer.