Lamp versus Laser Photolysis of a Bichromophoric Dichloroalkane:  Chemical Evidence for the Two-Photon Generation of the 1,5-Diphenylpentanediyl Biradical

Abstract

Low intensity (lamp) photolysis of 1,5-dichloro-1,5-diphenylpentane (1) leads to the formation of the 1-chloro-1,5-diphenylpentyl radical (7) through C-Cl bond cleavage. Radical 7 leads to the final products through typical free radical reactions. No cyclopentanes are formed under low intensity conditions. In contrast, high intensity laser irradiation leads to C-Cl photocleavage of radical 7 to yield the 1,5-diphenylpentanediyl biradical (11), which results in the formation of isomeric cis- and trans-1,2-diphenylcyclopentanes; the behavior of these biradicals agrees well with that observed when their precursor is 2,6-diphenylcyclohexanone. Two-color two-laser experiments suggest that both singlet and triplet biradicals are formed, even if only the latter are detectable with nanosecond techniques.

Laser Flash, Laser-Drop, and Lamp Photolysis of 1,3-Dichloro-1,3-diphenylpropane. One- versus Two-Photon Reaction Pathways

“…No disproportionation products were detected, due to efficient hydrogen abstraction from the solvent by chlorine, , which yields cyclohexyl radicals. Similar behavior has been earlier observed in the photochemistry of 1,5-dichloro-1,5-diphenylpentane …”

“…The photochemistry of dihalides in which the C−X bonds are separated by two or more carbon atoms is a subject of recent interest. − We have demonstrated the intermediacy of a hypervalent iodine radical in the photolysis of 1,5-diodo-1,5-diphenylpentane in cyclohexane, while in the case of the dichloro analogue the typical benzyl radical is formed . By contrast, we and others have demonstrated that excitation of dihalides with high-intensity light can lead to difunctional intermediates, biradicals, from which a completely different set of products can be formed.…”

Combinatorial discovery of two-photon photoremovable protecting groups

“…Therefore, a UV-excited two-photon photoremovable protecting group would be highly desirable. Few examples of UV two-photon reactions in organic chemistry are known (38), such as a photochromic system (39). An elementary chemical reaction that involves bond formation or cleavage and is nonlinear in photon dose is very rare, limiting the spatial resolution that can be achieved in molecular lithography and calling for the further development of novel nonlinear photochemical processes and groups.…”

One- vs Two-Photon Processes in the Photochemistry of 1,n-Dihaloalkanes

“…Laser flash photolysis experiments have shown that these chloroalkyl radicals have the characteristic features of benzylic radicals: absorption with λ max at 320 nm, lifetimes in the microsecond time scale, and quenching by oxygen at close to the diffusion-controlled limit , (for the 4-chloro-1,4-diphenylbutyl radical, see Figure ). Two-laser two-color techniques have been used to demonstrate that n -chloro-1, n -diphenylalkyl radicals photolyze to 1, n -diphenylalkanediyl biradicals when irradiated at a wavelength matching their optical absorption.…”