Photochemistry of P,N-bidentate rhenium(i) tricarbonyl complexes: reactive species generation and potential application for antibacterial photodynamic therapy.

In this work, we describe the photoisomerization of facial rhenium(i) tricarbonyl complexes bearing P,N-bidentate pyridyl/phosphine ligands with different chelating rings and anions: RePNBr, RePNTfO, and RePNNBr, which are triggered under irradiation at 365 nm in solutions. The apparent photodegradation rate constants (k app) depend on the coordinating ability of the solvent, being lowest in acetonitrile. The k app value increases as the temperature rises, suggesting a reactive IL excited state thermally populated from the MLCT excited state involved. Using the Eyring equation, positive activation enthalpies (ΔH ≠) accompanied by high negative values for the activation entropy (ΔS ≠) were obtained. These results suggest whatever the P,N-ligand or anion, the reaction proceeds through a strongly solvated or a compact transition state, which is compatible with an associative mechanism for the photoisomerization. A 100-fold decrease in the log10 CFU value is observed for E. coli and S. aureus in irradiated solutions of the compounds, which follows the same tendency as their singlet oxygen generation quantum yield: RePNBr > RePNTfO > RePNNBr, while no antibacterial activity is observed in the darkness. This result indicates that the generation of singlet oxygen plays a key role in the antibacterial capacity of these complexes.

The Effect of Pyrazolyl Substituents on the Photophysical and Photochemical Properties of Pyrazine Derivatives.

The reaction of 2,5-dibromopyrazine with N-Lithium pyrazolate in a 1:2 ratio leads to a mixture of 2-bromo-5-(1H-pyrazol-1-yl)pyrazine (I) and 2,5-di(1H-pyrazol-1-yl)pyrazine (II). The structures of I and II are highly planar. Two absorption bands can be observed for the compounds in the UV-Vis region, having ε in the order of 104  m-1  cm-1 . TD-DFT computed results support the nature of the lower energy absorptions as πpyrazine →π*pyrazine transitions, including an additional intraligand charge transfer transition for I (πpyrazol →π*pyrazine ). Upon excitation at 280 or 320 nm, the emission of both compounds is almost not affected by solvent polarity or oxygen presence, showing two bands for I and one for II in the 350-450 nm region. Emission of II follows a mono-exponential decay, while I decays following a bi-exponential law, hypothesized from πpyrazine →π*pyrazine and πpyrazol →π*pyrazine transitions. Photodegradation of I and II follows a first-order kinetic with constants of 1.18 × 10-2  min-1 and 0.13 × 10-2  min-1 , respectively. Results suggest that photodegradation of I starts with the loose of bromide followed by intermolecular pyrazolyl subtraction and ring opening. This path is not available for II, which is reflected in its enhanced photostability.

The first ReI organometallic complex with an organoimido-polyoxometalate ligand.

The spectroscopic, electrochemical and photophysical properties of the first ReI organometallic organoimido-polyoxometalate complex [n-Bu4N][Mo6O18NC6H4-CH2-N3C2H2-Re-phen(CO)3] compared with all fragments are reported. The UV-Vis spectra are analysed using experimental and theoretical tools. In contrast to the reported studies in the literature, our results show that a new more intense band is present in the spectra of the hybrid ligand obscuring the intra-polyanion charge transfer. The electrochemical results show that the strong acceptor character of the polyoxometalate fragment is quenched by the condensation of the phenyl-triazole molecule.