Density Functional Theory Interpretation of the 1H Photo-Chemically Induced Dynamic Nuclear Polarization Enhancements Characterizing Photoreduced Polyazaaromatic Ru(II) Coordination Complexes

A Silver-Based Integrated System Showing Mutually Inclusive Super Protonic Conductivity and Photoswitching Behavior

Photoinduced electricity and proton conductivity led fuel cells have emerged, inter alia, as highly promising systems for unconventional energy harvesting. Notwithstanding their individual presence with widely acclaimed results, an integrating system with mutually inclusive manifestation of both features has hitherto not been reported in the literature. To achieve this objective, our approach was to design a ligand system incorporating prerequisite features of both systems, like extended conjugation instigating photophysical activity and functional groups facilitating ionic conduction. As such, we report herein the design, synthesis, and characterization of a pyridyl-pyrazole-based silver compound that exhibits an excellent photocurrent generation and very high proton conductivity. The X-ray single-crystal structure of the Ag complex fully supports our notion, showing extensive π-π conjugated aromatic rings with a protruding free sulfonic group, facing toward solvent-filled channels with numerous supramolecular interactions. The nanoscopic silver metallogel induces semiconductive features in the system which ultimately result in photoresponse behavior in terms of photocurrent generation with an whopping photocurrent gain (I_on/I_off) of 21.2. To complete the idea of an integrated system, the proton conductivity values were also measured for both gel and crystalline states, while the former state yields a better result. The maximum proton conductivity value turns out to be 1.03 × 10^-2 S cm^-1 at 70 °C, which is higher than or comparable to those of well-known systems in the literature for proton conductivity.

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

[Ru(TAP)₂(dppz)]²⁺ (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2- a:2',3'- c]phenazine) is known to photo-oxidize guanine in DNA. Whether this oxidation proceeds by direct photoelectron transfer or by proton-coupled electron transfer is still unknown. To help distinguish between these mechanisms, spectro-electrochemical experiments have been carried out with [Ru(TAP)₂(dppz)]²⁺ in acetonitrile. The UV-vis and mid-IR spectra obtained for the one-electron reduced product were compared to those obtained by picosecond transient absorption and time-resolved infrared experiments of [Ru(TAP)₂(dppz)]²⁺ bound to guanine-containing DNA. An interesting feature of the singly reduced species is an electronic transition in the near-IR region (with λ_max at 1970 and 2820 nm). Density functional and time-dependent density functional theory simulations of the vibrational and electronic spectra of [Ru(TAP)₂(dppz)]²⁺, the reduced complex [Ru(TAP)₂(dppz)]⁺, and four isomers of [Ru(TAP)(TAPH)(dppz)]²⁺ (a possible product of proton-coupled electron transfer) were performed. Significantly, these predict absorption bands at λ > 1900 nm (attributed to a ligand-to-metal charge-transfer transition) for [Ru(TAP)₂(dppz)]⁺ but not for [Ru(TAP)(TAPH)(dppz)]²⁺. Both the UV-vis and mid-IR difference absorption spectra of the electrochemically generated singly reduced species [Ru(TAP)₂(dppz)]⁺ agree well with the transient absorption and time-resolved infrared spectra previously determined for the transient species formed by photoexcitation of [Ru(TAP)₂(dppz)]²⁺ intercalated in guanine-containing DNA. This suggests that the photochemical process in DNA proceeds by photoelectron transfer and not by a proton-coupled electron transfer process involving formation of [Ru(TAP)(TAPH)(dppz)]²⁺, as is proposed for the reaction with 5'-guanosine monophosphate. Additional infrared spectro-electrochemical measurements and density functional calculations have also been carried out on the free TAP ligand. These show that the TAP radical anion in acetonitrile also exhibits strong broad near-IR electronic absorption (λ_max at 1750 and 2360 nm).

Full assignment of the 1H NMR spectrum of heteroleptic polyazaaromatic Ru(II) complexes based on long-range interligand NOEs

Chemical equivalence in symmetrical polyazaaromatic ligands is not necessarily retained in the corresponding heteroleptic ruthenium(II) complexes, and the assignment of the (1)H NMR spectrum of these octahedral metallic compounds is therefore not straightforward. This work shows that full assignment can be achieved on the basis of long-range interligand NOE measurements.