Molecular rare-earth materials are recently considered a new material platform for rare-earth-based quantum photonic applications. The natural processability of organic materials gives the ease of developing scalar integrable quantum photonic devices through various mature thin-film growth technologies. Meanwhile, the new era is under development with efforts on material discovery and insightful understanding of the correlation between quantum optical properties and molecular structures. Organic components perform as a host, directly coordinating rare-earth ions that can hold long optical coherence time at technologically important communication wavelengths. Phonon, due to the vibrational bonds of organics, is expected to be a dephasing route to optical coherence. This talk presents the studies of the correlation between organic vibrational modes and electronic states of erbium ions, 1.5 µm emitters, by optically characterising halogenated and hydrogenated organic erbium molecules at low temperatures. The study suggests the potential of molecular engineering in harnessing phonon-related decoherence mechanisms at the single molecular level.