Bottom-up approach to making larger hydrocarbon molecules capable of optical cycling

Molecules capable of repeatable, narrow-band spontaneous photon scattering are prized for direct laser cooling and quantum state detection. Recently, large molecules incorporating phenyl rings have been shown to exhibit a high probability of returning to the same vibrational state after photon emission, a behaviour previously observed in small molecules, although it is not yet known if the high vibrational-mode density of even larger species will eventually compromise optical cycling. Here we systematically increase the size of hydrocarbon ligands attached to single alkaline-earth phenoxides from -H to -C₁₄H₁₉ while measuring the vibrational branching fractions of the optical transition. We find that varying the ligand size from one to more than 30 atoms does not systematically reduce the cycle closure, which remains around 90%. Theoretical extensions to larger diamondoids and diamond surfaces suggest that alkaline-earth phenoxides may maintain their desirable scattering behaviour as the system size grows further, with no indication of an upper limit.