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

Abstract

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 interradical 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.

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere

“…If the resulting alkyl radical recombines with the second alkoxy radical, they form an ester. The overall proposed mechanism is analogous to a photochemical process recently characterized in crystalline solids, where a triplet radical pair produced by laser photolysis may undergo CO loss, followed by ISC and recombination …”

Exceptionally Long Lifetimes of Strongly Entangled Acyl–Trityl Radical Pairs Photochemically Generated in Crystalline Trityl Ketones

“…Crystalline solids offer opportunities to study previously unchartered regions of chemical dynamics 1,2 and to carry out highly selective reactions. 3,4 Our recent report on the reversible formation of triplet acyl−alkyl radical pairs generated by photochemical excitation of nanocrystalline diphenylmethyladamantyl ketones (Figure 1a, R = adamantyl, R 1 = R 2 = Ph, R 3 = H) 5 suggested a new platform for the repeated generation of qubit pairs with potential applications in quantum information science. 6−8 We confirmed that triplet radical pairs generated by the cleavage of a single bond, and forced to remain nearly static within a bond distance, are strongly correlated as they lack efficient mechanisms for intersystem crossing.…”

“…9 Using laser flash photolysis with nanocrystalline samples suspended in water, 10 we showed that triplet acyladamantyl-diphenylmethyl radical pairs ( 3 RP-A) have lifetimes in the range of ca. 50−60 μs, 5 which are one to four orders magnitude longer than analogous acyl−alkyl biradicals held close together by short alkyl chains in solution. 11−13 Recognizing that longer lifetimes will make it possible to execute multiple logical operations by quantum spin-state manipulation, 14 we sought to investigate a series of triphenylmethyl-substituted ketones that may lead to longerlived triplet pairs by taking advantage of the persistent nature of the triphenylmethyl (trityl) radical (Figure 1a, R 1 = R 2 = R 3 = Ph).…”

“…Focusing our analysis on the long-lived components, we can see that a lifetime of 4.22 ms for the acyl−alkyl radical pairs in the case of K4 is a remarkable result, as it surpasses the previous record of 63 μs, previously reported by our group by almost two orders of magnitude. 5 Taking K4 as the parent ketone, the effects of aryl substituents cause lifetime variations that range from negligible up to ca. 62%.…”

“…Using laser flash photolysis with nanocrystalline samples suspended in water, we showed that triplet acyladamantyl-diphenylmethyl radical pairs ( 3 RP-A ) have lifetimes in the range of ca. 50–60 μs, which are one to four orders magnitude longer than analogous acyl–alkyl biradicals held close together by short alkyl chains in solution. − …”