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

Triplet acyl–alkyl radical pairs generated by pulsed laser excitation within the constraints of their nanocrystalline ketone precursors were recently introduced as a potential platform for the robust and repeated instantiation of spin qubit pairs for applications in quantum information science. Here, we report the transient spectroscopy of a series of nanocrystalline trityl–alkyl and trityl–aryl ketones capable of generating correlated triplet radical pairs with persistent triphenylmethyl radicals forced to remain within bonding distances of highly reactive acyl radicals. Whereas triplet trityl–acyl radical pairs decay by competing product-forming decarbonylation and intersystem crossing, triplet trityl–benzoyl radical pairs have lifetimes of up to ca. 4 ms and exclusively regenerate the starting ketone. We propose that these long lifetimes are the result of the short inter-radical distances and the colinear orientation of the two singly occupied orbitals, which are expected to result in large singlet–triplet energy gaps, large zero-field splitting parameters, and a poor geometry for spin-obit coupling. Ketones generating trityl–benzoyl radical pairs demonstrate promising performance along multiple dimensions that are crucial for quantum information science.