Photodissociation dynamics of chlorobenzene and 4‐fluoroiodobenzene in CCl 4 probed using time‐resolved infrared spectroscopy

Dynamics of Irreversible NO Release from Photoexcited Molsidomine

Molsidomine (SIN-10), an orally administered NO-delivery drug for vasodilation, cannot be used to alleviate hypertensive crisis because it releases NO at a slow rate. SIN-10 may be used to treat sudden cardiac abnormalities if the rapid and immediate release of NO is achieved via photoactivation. The photodissociation dynamics associated with the NO release process from SIN-10 in CHCl₃ was investigated using time-resolved infrared spectroscopy. Approximately 41% of photoexcited SIN-10 at 360 nm decomposed into CO₂, CH₂CH₃ radical, and the remaining radical fragment [SIN-1A(-H)] with a time constant of 43 ps. All SIN-1A(-H) released NO spontaneously with a time constant of 68 ns, becoming N-morpholino-aminoacetonitrile, resulting in 41% for the quantum yield of immediate NO release from SIN-10. The results obtained can be used to realize the quantitative control of the NO administration at a specific time, and SIN-10 can be potentially used to address the phenomenon of hypertensive crisis.

Charge-Separated Reactive Intermediates from the UV Photodissociation of Chlorobenzene in Solution

Although ultraviolet (UV)-induced photochemical cleavage of carbon–halogen bonds in gaseous halocarbons is mostly homolytic, the photolysis of chlorobenzene in solution has been proposed to produce a phenyl cation, c-C₆H₅⁺, which is a highly reactive intermediate of potential use in chemical synthesis and N₂ activation. Any evidence for such a route to phenyl cations is indirect, with uncertainty remaining about the possible mechanism. Here, ultrafast transient absorption spectroscopy of UV-excited (λ = 240 and 270 nm) chlorobenzene solutions in fluorinated (perfluorohexane) and protic (ethanol and 2,2,2-trifluoroethanol) solvents reveals a broad electronic absorption band centered at 540 nm that is assigned to an isomer of chlorobenzene with both charge-separated and triplet-spin carbene character. This spectroscopic feature is weaker, or absent, when experiments are conducted in cyclohexane. The intermediate isomer of chlorobenzene has a solvent-dependent lifetime of 30–110 ps, determined by reaction with the solvent or quenching to a lower-lying singlet state. Evidence is presented for dissociation to ortho-benzyne, but the intermediate could also be a precursor to phenyl cation formation.