Trans–Cis Isomerization of Vinylketones through Triplet 1,2-Biradicals

Abstract

The irradiation of trans-vinylketones 1a-c yields the corresponding cis isomers 2a-c. Laser flash photolysis of 1a and 1b with 308 and 355 nm lasers results in their triplet ketones (T1K of 1), which rearrange to form triplet 1,2-biradicals 3a and 3b, respectively, whereas irradiation with a 266 nm laser produces their cis-isomers through singlet reactivity. Time-resolved IR spectroscopy of 1a with 266 nm irradiation confirmed that 2a is formed within the laser pulse. In comparison, laser flash photolysis of 1c with a 308 nm laser showed only the formation of 2c through singlet reactivity. At cryogenic temperatures, the irradiation of 1 also resulted in 2. DFT calculations were used to aid in the characterization of the excited states and biradicals involved in the cis-trans isomerization and to support the mechanism for the cis-trans isomerization on the triplet surface.

Flavin Photocatalysts for Visible‐Light [2+2] Cycloadditions: Structure, Reactivity and Reaction Mechanism

“…[20b] The [2+ +2] cyclisation was not observed in the case of these simple alkenes, which can be explained by the very short lifetimeo fthes tyrene triplet excited state disfavouring the intermolecular process. [28] These indirect evidences supportedb yt he redox potentials clearlys how that both 2e and 3a cannot participate in either oxidative or reductivee lectron-transfer processes. Additionally, no effect of the substrates or their models on the transient absorptions pectra of 2e on the picosecond time scale was ob- www.chemcatchem.org served.…”

Photolysis of acetophenone derivatives with α‐cyclopropyl substituents

“…Phosphorescence measurements were carried out at 77 K with 300-nm irradiation. [6] Electron Spin Resonance spectroscopy Steady-state ESR measurements were taken at 10 K and at 80 K in ethanol glass in a Brucker ESR E500 spectrometer with an INDI-10 YAG laser source with 355-nm irradiation and 80-mW laser beam.…”

“…It has been theorized that intersystem crossing in short-chained biradicals, such as 1,2-, 1,3-and 1,4-biradicals, is facilitated by spin-orbit coupling. [5][6][7][8][9] In addition, the energy gap between the triplet and singlet surfaces affects intersystem crossing and becomes more efficient as the gap decreases. Because the energy gap and spin-orbit coupling are affected by biradical conformation, control of the reactivity of flexible biradicals is difficult.…”

“…However, there are several limitations on the use of triplet biradicals for bond formation, such as selective biradical formation and control of intersystem crossing to yield selective product formation. It has been theorized that intersystem crossing in short‐chained biradicals, such as 1,2‐, 1,3‐ and 1,4‐biradicals, is facilitated by spin–orbit coupling . In addition, the energy gap between the triplet and singlet surfaces affects intersystem crossing and becomes more efficient as the gap decreases.…”

Solvent controlling excited state proton transfer reaction in quinoline/isoquinoline‐pyrazole isomer QP‐I: A theoretical study

“…As one of the fastest and quite complex reactions in nature, the excited state intramolecular or intermolecular proton transfer (ESIPT) is considered to be one of the most fundamental and important processes in chemistry, biology, and materials . ESIPT reactions involving photoacid molecules are spectacular examples of how the interaction with electromagnetic radiation can change the properties of matter.…”