Excited-state orbital angular momentum enables all-optical molecular spin coherence

Abstract

Paramagnetic molecules are promising quantum sensors with dimensions and environmental compatibility inaccessible to solid-state defects. Realizing this promise, however, requires optical methods for initializing and reading out coherent spin dynamics. Ultrafast pump-probe polarization spectroscopy provides such a route, but previous demonstrations have relied on high-symmetry complexes in which ground-state orbital angular momentum enables spin-photon coupling. Here we demonstrate the first ultrafast spin coherence measurements on non-octahedral molecules and show that excited-state orbital angular momentum can instead provide the optical interface in axial tungsten(V)-oxo complexes. Circularly polarized excitation generates room-temperature spin coherence that persists for several nanoseconds, enabling time-domain optical detection of electron paramagnetic resonance (EPR) spectra, including g value anisotropy in a polymer matrix, and solution-phase DC magnetic field detection down to 5 mT. This work establishes a route to ultrafast, all-optical spin spectroscopy and quantum sensing in lower-symmetry, chemically tunable coordination complexes.