Strongly Entangled Triplet Acyl-Alkyl Radical Pairs in Crystals of Photostable Diphenylmethyl Adamantyl Ketones

Radical pairs generated in crystalline solids by bond cleavage reactions of triplet ketones offer the unique opportunity to explore a frontier of spin dynamics where rigid radicals are highly entangled as the result of short inter-radical distances, large singlet-triplet energy gaps (ΔE_ST), and limited spin-lattice relaxation mechanisms. Here we report the pulsed laser generation and detection of strongly entangled triplet acyl-alkyl radical pairs generated in nanocrystalline suspensions of 1,1-diphenylmethyl 2-ketones with various 3-admantyl substituents. The sought-after triplet acyl-alkyl radical pairs could be studied for the first time in the solid state by taking advantage of the efficient triplet excited state α-cleavage reactions of 1,1-diphenylmethyl ketones and the slow rate of CO loss from the acyl radicals, which would have to generate highly unstable phenyl and primary alkyl radicals or relatively unstable secondary and tertiary alkyl radicals. With the loss of CO prevented, the lifetime of the triplet acyl-alkyl radical pair intermediates is determined by intersystem crossing to the singlet radical pair state, which is followed by immediate bond formation to the ground state starting ketone. Experimental results revealed biexponential kinetics with long-lived components that account for ca. 87-92% of the transient population and lifetimes that extend to the range of 53-63 μs, the longest reported so far for this type of radical pair. Structural information inferred from the starting ketone will make it possible to analyze the affects of proximity and orientation of the singly occupied orbitals and potentially help set a path for the use of triplet radical pairs as qubits in quantum information technologies.

A photochemical mechanism for homochirogenesis

Theoretical analysis of one-step and multiple-step photoreactions initiated with circularly polarized light shows that the enantiomeric excess of a chiral reactant approaches +/- 1 as the amount of unreacted reactant approaches 0. The final product never has a large enantiomeric excess at any stage of its formation and slowly decreases to 0 at the completion of the reaction. For multiple-step reactions the behavior of the intermediate photoproducts is much more interesting. During certain stages of the overall reaction both the size of the enantiomer excess and the amount of a given intermediate photoproduct are large. Furthermore, the sign of the enantiomeric excess of an intermediate may change during the course of the reaction. Multiple-step photoreactions initiated with circularly polarized light may be a method by which the exogenous and endogenous synthesis of optically active molecules occurred in the prebiotic universe.

Photoreaction of a racemate proceeding through radical or ion pairs. The effect of circularly polarized light on the enantiomeric excess of recovered reactant

A simple theory has been developed to explain how circularly polarized light will affect the outcome of a photoreaction of a racemic substrate proceeding through ion pairs or radical pairs. One can calculate the enantiomeric excess and amount of recovered substrate at any stage of the reaction.