Luminescent Organic\nTriplet Diradicals as Optically\nAddressable Molecular Qubits

Optical-spin interfaces that enable the photoinitialization,\ncoherent\nmicrowave manipulation, and optical read-out of ground state spins\nhave been studied extensively in solid-state defects such as diamond\nnitrogen vacancy (NV) centers and are promising for quantum information\nscience applications. Molecular quantum bits (qubits) offer many advantages\nover solid-state spin centers through synthetic control of their optical\nand spin properties and their scalability into well-defined multiqubit\narrays. In this work, we report an optical-spin interface in an organic\nmolecular qubit consisting of two luminescent tris(2,4,6-trichlorophenyl)methyl\n(TTM) radicals connected via the <i>meta</i>-positions of\na phenyl linker. The triplet ground state of this system can be photoinitialized\nin its |<i>T</i><sub>0</sub>⟩ state by shelving triplet\npopulations as singlets through spin-selective excited-state intersystem\ncrossing with 80% selectivity from |<i>T</i><sub>+</sub>⟩ and |<i>T</i><sub>–</sub>⟩. The\nfluorescence intensity in the triplet manifold is determined by the\nground-state polarization, and we show successful optical read-out\nof the ground-state spin following microwave manipulations by fluorescence-detected\nmagnetic resonance spectroscopy. At 85 K, the lifetime of the polarized\nground state is 45 ± 3 μs, and the ground state phase memory\ntime is <i>T</i><sub>m</sub> = 5.9 ± 0.1 μs,\nwhich increases to 26.8 ± 1.6 μs at 5 K. These results\nshow that luminescent diradicals with triplet ground states can serve\nas optically addressable molecular qubits with long spin coherence\ntimes, which marks an important step toward the rational design of\nspin-optical interfaces in organic materials.