An investigation into the photochemistry of, and the electrochemically induced CO-loss from, [(CO)5MC(OMe)Me](M = Cr or W) using low-temperature matrix isolation, picosecond infrared spectroscopy, cyclic voltammetry, and time-dependent density functional theory

Photo-activated CO-release in the amino tungsten Fischer carbene complex, [(CO)5WC(NC4H8)Me], picosecond time resolved infrared spectroscopy, time-dependent density functional theory, and an antimicrobial study

Picosecond time-resolved infrared spectroscopy was used to probe the photo-induced early state dynamics preceding CO loss in the Fischer carbene complex, [(CO)₅WC(NC₄H₈)CH₃]. Time-dependent density functional theory calculations were employed to help in understanding the photochemical and photophysical processes leading to CO-loss. Electrochemical initiated CO release was quantified using gas chromatography. The potential of [(CO)₅WC(NC₄H₈)CH₃], as an antimicrobial agent under irradiation conditions was studied using a Staphylococcus aureus strain.

Insight into the mechanism of CO-release from trypto-CORM using ultra-fast spectroscopy and computational chemistry

Photolysis of trypto-CORM results in ultra-fast CO-dissociation and formation of a 16-e triplet followed by solvation.

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)5MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes

The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)₅M=C(Me)(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Møller-Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play a decisive role in these photoisomerizations, are determined. The former is applied to the chromium and molybdenum systems because their photoproducts are essentially from the singlet excited states. The latter is applied to the tungsten complex because its photoproducts are formed from a low-lying triplet excited state. Two reaction pathways are examined in this work: photocarbonylation (path I) and CO-photoextrusion (path II). The model studies strongly indicate that in the photochemistry of Cr and Mo Fischer carbene systems, the formation of metallaketene intermediates may occur at higher excitation wavenumbers, whereas the five-coordinated complexes that are attached by a solvent molecule are obtained at lower excitation wavenumbers. However, in the W analogue, because the activation barriers for path I are greater than that for path II and path I has more reaction steps than path II, the quantum yields for the metallaketene intermediate should be smaller than those for the five-coordinated species, which is also attached by a solvent molecule. These theoretical studies also suggest that the conical intersection and the spin crossover mechanisms that are identified in this work explain the process well and support the experimental observations.

Mechanistic Study for the Photochemical Reactions of d6 M(CO)5(CS) (M = Cr, Mo, and W) Complexes

The mechanisms of photoextrusion reactions are determined theoretically for the model system of six-coordinated M(CO)₅(CS) (M = Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD basis set. Three types of elimination reaction pathways (i.e., path I, path II, and path III for axial CO extrusion, equatorial CO extrusion, and CS ligand extrusion, respectively) are considered in this study. Theoretical findings show that the photoextrusion mechanism for Cr and Mo complexes proceeds as follows: M-S₀-Rea + hν → M-S₁-FC → M-CI → M-Pro + CO. This study shows that when the reactant, M(CO)₅(CS) (M-S₀-Rea), is photoirradiated by UV light, it is excited vertically to many low-lying singlet excited states. It then relaxes to the first singlet excited state from the Franck-Condon point (M-S₁-FC). After passing through a conical intersection point (M-CI), this species eliminates a CO group to yield a five-coordinated product, M(CO)₄(CS) (M-S₀-Pro). However, for the W analogue, the photolysis mechanism is represented as W-S₀-Rea + hν → W-T₁-Min → W-T₁-TS → W-T₁/S₀ → W-S₀-Pro + CO. That is to say, when the reactant, W(CO)₅(CS) (W-S₀-Rea), absorbs UV light, it is excited to its several low-lying excited states by a vertical excitation. This species may then return to an intermediate at the first triplet excited state (W-T₁-Min) by means of intersystem crossings or conical intersections. After passing through a triplet transition state (W-T₁-TS) and a subsequent intersystem crossing (W-T₁/S₀), this molecule finally loses a CO ligand to produce a photoproduct at the ground singlet state (W-S₀-Pro). In other words, conical intersections and intersystem crossings play a decisive role in these photoextrusion reactions for M(CO)₅(CS). Theoretical evidence from a kinetic viewpoint strongly supports the theory that the photolysis of M(CO)₅(CS) only produces CO-loss photoproducts rather than the CS-loss photoproduct. Theoretical analysis using the results of this study allows a good interpretation of the available experimental observations.

Direct Observation of a Dark State in the Photocycle of a Light-Driven Molecular Motor

Controlling the excited-state properties of light driven molecular machines is crucial to achieving high efficiency and directed functionality. A key challenge in achieving control lies in unravelling the complex photodynamics and especially in identifying the role played by dark states. Here we use the structure sensitivity and high time resolution of UV-pump/IR-probe spectroscopy to build a detailed and comprehensive model of the structural evolution of light driven molecular rotors. The photodynamics of these chiral overcrowded alkene derivatives are determined by two close-lying excited electronic states. The potential energy landscape of these “bright” and “dark” states gives rise to a broad excited-state electronic absorption band over the entire mid-IR range that is probed for the first time and modeled by quantum mechanical calculations. The transient IR vibrational fingerprints observed in our studies allow for an unambiguous identification of the identity of the “dark” electronic excited state from which the photon’s energy is converted into motion, and thereby pave the way for tuning the quantum yield of future molecular rotors based on this structural motif.