A Theoretical Study of the N to O Linkage Photoisomerization Efficiency in a Series of Ruthenium Mononitrosyl Complexes.

Ruthenium nitrosyl complexes are fascinating versatile photoactive molecules that can either undergo NO linkage photoisomerization or NO photorelease. The photochromic response of three ruthenium mononitrosyl complexes, trans-[RuCl(NO)(py)4]2+, trans-[RuBr(NO)(py)4]2+, and trans-(Cl,Cl)[RuCl2(NO)(tpy)]⁺, has been investigated using density functional theory and time-dependent density functional theory. The N to O photoisomerization pathways and absorption properties of the various stable and metastable species have been computed, providing a simple rationalization of the photoconversion trend in this series of complexes. The dramatic decrease of the N to O photoisomerization efficiency going from the first to the last complex is mainly attributed to an increase of the photoproduct absorption at the irradiation wavelength, rather than a change in the photoisomerization pathways.

Is photoisomerization required for NO photorelease in ruthenium nitrosyl complexes?

The factors that explain the competition between intramolecular NO linkage photoisomerization and NO photorelease in five ruthenium nitrosyl complexes were investigated. By applying DFT-based methods, it was possible to characterize the ground states and lowest triplet potential energy surfaces of these species, and to establish that both photoisomerization and photorelease processes can occur in the lowest triplet state of each species. This work highlights the crucial role of the sideways-bonded isomer, a metastable state also known as the MS2 isomer, in the photochemical loss of NO, while the results obtained also indicate that the population of the triplet state of this isomer is compulsory for both processes and show how photoisomerization and photorelease interfere. Graphical Abstract Illustration of the crucial role of the 3MS2 state in the photoreactivities of ruthenium nitrosyl complexes.

Computational analysis of the nature and strength of the supramolecular contacts involved in the binding of chloride anions by imidazolium-based cyclic receptors.

The supramolecular bonding contacts driving the recognition of chloride ions by macrocyclic imidazolium-based receptors have been investigated by density functional theory calculations, both in vacuo and in solution (DMSO). This computational study reveals that the most stable host-guest complexes in vacuo and solution are different. While the anion interacts by means of two C-H···Cl(-) hydrogen bonds with the host molecule in vacuo (in a similar manner to that observed in the published single-crystal X-ray structure of the Cl-host complex), four C-H···Cl(-) contacts are clearly present in solution, as observed experimentally by earlier studies. In addition, the computed optimal Cl-host complex in solution confirms that the cavity of the host macrocycle, formed by four aromatic rings, does not includes the anions, which are located outside the cyclic receptor with which they interact through two CPh-H···Cl(-) hydrogen bonds and two unconventional (CIm-H)(+)···X(-) interactions.