Molecular Single-Photon Emitters across Chemical Space: Substrate Effects and Design Principles

Abstract Polycyclic aromatic hydrocarbons (PAHs) on hexagonal boron nitride (hBN) are an emerging molecular platform for single-photon emitters, but predictive design strategies across chemical space have been lacking. Here we establish two general principles governing PAH/hBN emitters. The zero-phonon-line energy is molecule-controlled and robust to the substrate environment, whereas the Huang–Rhys (HR) factor is environment-governed and exhibits two distinct regimes. Pristine hBN modestly enhances HR through soft interfacial modes. In contrast, the NB antisite defect, introduced to immobilize the molecules, produces a molecule-specific HR response that cannot be predicted from gas-phase or pristine-hBN data alone. Strikingly, the defect can either partially restore the HR factor toward its intrinsic gas-phase value or amplify it by up to 3.7×, depending on the molecule. This positions defect engineering as a tunable design knob for vibronic coupling in molecule/hBN emitters. Combining high-throughput hybrid density functional theory (DFT) screening of 662 PAHs identifies 47 gas-phase candidates with low HR factors and high radiative rates spanning the near-infrared to ultraviolet. Among five representative candidates explicitly tested on NB@hBN, two retain HR factors below unity in gas-phase, pristine hBN, and defect-bound environments. However, no PAH simultaneously exhibits favorable single-photon-emitter properties and an excited-state singlet–triplet splitting below 0.5 eV, suggesting that PAH-based spin–photon interfaces will require further molecular design. All data are released through the open MolExDB platform.