Fig. 3: Magnetic resonance studies of hyperfine couplings in a BF2 film.
a A schematic demonstrating the ability of the HFI to mediate spin-mixing processes between the MS = 0 inter-1CT and the MS = −1,0,+1 inter-3CT sublevels in the absence of an external magnetic field. b Under an applied external magnetic field B0, the Zeeman interaction energetically forbids HFI-induced transitions between the inter-3CT+ and inter-3CT- and the inter-1CT0. Spin-pumping MS = ± 1 transitions can then occur between the inter-3CT0 and the inter-3CT+ and inter-3CT- sublevels; this reduces the inter-3CT population that can ultimately couple to the emissive intra-1CT manifold via the inter-1CT state, resulting in a decreased PL yield from the sample and the sharp, negative PLDMR signal seen at ~333 mT. c When the magnetic field of the applied microwaves B1 is perpendicular to B0 and becomes larger than the local hyperfine field BHF, spin-locking occurs. This reduces the rate of the HFI-mediated inter-1CT0 to inter-3CT0, transitions, locking the inter-CT state spin-population in the initially generated inter-1CT0. Spin-locking manifests as the formation of a characteristic W-shaped peak in the PLDMR as B1 is increased. d The PLDMR response of a neat BF2 film at 293 K with 405 nm excitation (30 mW). e The calculated HFI-ISC rate in a model BF2 dimer as a function of the dephasing rate, γ, and the electron exchange energy, J. The dashed white lines represent the experimental dephasing times in BF2 (upper bound = 40 ns, lower bound = 1 μs), as estimated from the TA measurements of BF2 at 10 wt% in CBP film (see SI for details). The left axis shows the electron-hole separation in the inter-CT state corresponding to each value of J.
