Formation and Direct Detection of Non-Conjugated Triplet 1,2-Biradical from β,γ-Vinylarylketone

Abstract

Laser flash photolysis of 2-methyl-1-phenylbut-3-en-1-one (1) conducted at irradiation wavelengths of 266 and 308 nm results in the formation of triplet 1,2-biradical 2 that has λmax at 370 and 480 nm. Biradical 2 is formed with a rate constant of 1.1 × 107 s–1 and decays with a rate constant of 2.3 × 105 s–1. Isoprene-quenching studies support the notion that biradical 2 is formed by energy transfer from the triplet-excited state of the ketone chromophore of 1. Density functional theory calculations were used to verify the characterization of triplet biradical 2 and validate the mechanism for its formation. Thus, it has been demonstrated that intramolecular sensitization of simple alkenes can be used to form triplet 1,2-biradicals with the two radical centres localized on the adjacent carbon atoms.

Comparison of the Photochemistry of Acyclic and Cyclic 4-(4-Methoxy-phenyl)-4-oxo-but-2-enoate Ester Derivatives

“…Irradiation of ketone 3 , with a deuterium label in the β-position, yields the corresponding cis-isomer through the formation of a twisted triplet biradical 3BR . Laser flash photolysis results show that both 3BR and 4BR have lifetimes of a few microseconds. , A comparison of the calculated spin densities shows similarities for 1BR and 3BR (Scheme ), with the majority of the spin density located on the Cα and Cβ carbon atoms and a small amount located at the ketone and the ester carbonyl O atoms. In contrast, in 4BR , all the spin density is located on the Cα and Cβ carbon atoms because the vinylic bond is not conjugated.…”

“…Furthermore, the calculated rotational barrier of the vinylic bond in 4BR is ∼16 kcal/mol, which is significantly larger than the corresponding rotational barriers for 1BR and 2BR. 10,11 As the vinylic bond in ketone 1 is conjugated at both terminals, the calculated rotational barrier around the vinylic bond from T 1 of 1 to 1BR is smaller because both the radical centers are stabilized by conjugation. Thus, we theorize that the flexibility of 1BR allows it to rotate into a conformer that is favorable for intersystem crossing to its S 0 .…”

“…More recently, Ranaweera et al and Sriyarathne et al demonstrated that the cis−trans isomerization of simple alkenes with built-in acetophenone triplet sensitizers also takes place through triplet 1,2biradicals. 10,11 In this paper, we report the photochemistry of vinyl ketones 1 and 2, in which the vinylic bond is conjugated to an ester moiety and a p-methoxyacetophenone chromophore that serves as a built-in triplet sensitizer. In addition, the vinylic bond of ketone 2 is part of a ring structure, therefore making it possible to determine the effect of conjugation on cis−trans isomerization of simple acyclic and cyclic alkenes.…”

“…Thus, they theorized that intersystem crossing occurs where the two radical centers are orientated ∼45° apart, where spin–orbit coupling is more significant. More recently, Ranaweera et al and Sriyarathne et al demonstrated that the cis–trans isomerization of simple alkenes with built-in acetophenone triplet sensitizers also takes place through triplet 1,2-biradicals. , …”

International Year of Light and Light-Based Technologies

“…[12] The formation and direct detection of a non-conjugated triplet 1,2-biradical is furthermore described by Anna Gudmundsdottir and her group. [14] Rosalie Hocking and co-workers describe the structural effects of sodium birnessite-based films to improve solar fuel technology. [15] George Vamvounis and Nicholas Sandery explore the use of photochromic compounds as colorimetric sensors for trinitrotoluene-based explosives.…”