Abstract
<jats:p>Control of spin dynamics at the single-molecule level is a major goal of emerging molecular platforms for quantum information sciences. Substantial control could be provided by shifting the energetics of the lowest lying triplet state above the first excited singlet, limiting spontaneous population of the triplet manifold. Such molecules with a negative singlet-triplet gap (STG), breaking Hund’s rule, are generally thought of as weak optical absorbers unsuited for single-molecule investigation. Here, we show that individual molecules of 1,3,4,6,9b-pentaazaphenalene (5AP) are conspicuously visible via fluorescence microscopy at room and cryogenic temperatures. At 1.6K, choice of excitation above or below the STG exerts control over triplet population dynamics in single 5AP molecules, as evidenced by starkly different photon bunching behavior. Ultrafast spectroscopy identifies a long-lived excited state as a key intermediate in populating the triplet. 5AP is rendered a substantially stronger photon absorber and emitter by asymmetric interaction with the silica surface.</jats:p>