Intramolecular Charge‐Transfer Dopants Enable Isolated Triplet Excitons as Spin Qutrits in a Single Crystal

Organic molecular crystals enable spin alignment of triplet excitons over macroscopic distances, offering a molecular route to solid-state quantum technologies. In typical crystals, however, dense packing promotes spin decoherence through dipolar coupling and exciton hopping. Although dilution doping can mitigate these effects, reported examples remain scarce because dopants must satisfy stringent structural and energetic constraints. Here, we broaden the design space for doped organic crystals by introducing a strategy that employs a host-derived dopant with intramolecular charge-transfer (ICT) character that maintains structural compatibility with the host lattice, while its ICT character lowers the triplet energy to localize triplet excitons. Ultrafast transient absorption spectroscopy confirms triplet formation, while time-resolved electron paramagnetic resonance (TREPR) spectroscopy reveals that these oriented triplets possess selectively addressable spin sublevel transitions. Additionally, pulse-EPR measurements yield a phase memory time (T_m) of 7.1 µs at 10 K and 3.5 µs at 85 K, enabled by reduced electron-electron dipolar coupling and suppressed exciton hopping. Temperature-dependent studies further show that coherence is ultimately limited by nuclear spin flip-flops at low temperatures and spin-phonon coupling at higher temperatures. These findings demonstrate that ordered triplet excitons produced by an ICT dopant in single crystals promote spin coherence at elevated temperatures.