Symmetry-Enabled Optical Spin Initialization of Luminescent Organic Radical Doublet States.

Optical-spin interfaces that enable the photoinitialization, coherent microwave manipulation, and optical readout of ground-state spins are promising for emerging quantum technologies. Molecular optical-spin interfaces offer advantages over solid-state defects through synthetic control of their optical and spin properties. Optical initialization of these systems relies on spin-selective intersystem crossing between electronic states of different spin multiplicity. In this work, we demonstrate experimentally and theoretically that coherent excited-state evolutions enable optical spin polarization of luminescent tris(2,4,6-trichlorophenyl)methyl (TTM) monoradicals without the need for intersystem crossing. Inspired by the alignment-to-orientation conversion (AOC) phenomenon in atomic physics, we find that the doubly degenerate first excited state of D3-symmetric TTM possesses a pseudo-orbital angular momentum that couples to the electron spin through in-state spin-orbit coupling, resulting in spin-dependent excited-state dynamics following photoexcitation. These coherent dynamics produce differential decay pathways to the ground state that generate persistent spin polarization in an applied magnetic field. Time-resolved electron paramagnetic resonance spectroscopy confirms photoinduced ground-state spin polarization in TTM and its symmetry-preserving derivatives, while lower-symmetry analogues exhibit no polarization, consistent with a loss of pseudo-orbital angular momentum. These results establish a fundamentally new paradigm for optical spin initialization in organic radicals based on symmetry-enabled coherent dynamics and lay the foundation for establishing optical-spin interfaces in organic monoradicals.