Charge-Transfer States Enable Spin-Selective Formation of Quartet State Qudits in Luminescent Radical-Chromophore Dyads.

Luminescent tris(2,4,6-trichlorophenyl)methyl (TTM) radicals are promising doublet emitters that have been used to generate spin-optical interfaces in molecular electron spin qudits. In particular, photoexcitation of covalent TTM-chromophore dyads can generate quartet spin states that can be detected via photoluminescence. However, the mechanism of quartet spin initialization is complicated by competing charge-transfer (CT) and energy-transfer (EnT) pathways between the radical and the chromophore. Here, we demonstrate the role of CT intermediates by engineering a covalent dyad of TTM and naphthalene-(1,4:5,8)-bis(dicarboximide) (NDI), a well-known electron acceptor chromophore. Transient absorption spectroscopy indicates that photoexcitation of TTM results in ultrafast electron transfer from 2*TTM to NDI, which kinetically outcompetes EnT. Electron paramagnetic resonance spectroscopy reveals that charge recombination proceeds via spin-orbit charge-transfer intersystem crossing to generate 3*NDI, followed by spin mixing with 2TTM to form the quartet state. This process uniquely spin-polarizes the quartet state, providing fundamental design principles to purify the initial wave function for quantum information processing.