We have monitored the impacts of an increment in the alkyl chain length of the imidazolium-based tetrafluoroborate ionic liquids on the local deuteroxyl probe modes of interest. For this study, we have taken 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIm][BF4], 1-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF4], 1-octyl-3-methylimidazolium tetrafluoroborate [OMIm][BF4], and 1-decyl-3-methylimidazolium tetrafluoroborate [DMIm][BF4] ionic liquid solutions with 5% HOD in H2O as the vibrational reporter of the associated ultrafast system dynamics. Classical molecular dynamics (MD) simulations were employed to determine molecular structure and dynamic properties, while the spectral profiles were derived by applying the wavelet analysis of classical trajectories. Spatial distribution functions reveal the heterogeneity within the molecular structures of the ionic liquids (ILs) with varying alkyl chain lengths. The intense position of the spectral peak, the frequency corresponding to the shoulder peak, and the spectral linewidth of the O-D stretch distribution are not influenced by the increment in the cationic chain length. In addition, the ionic liquid (IL) [BMIm][BF4] exhibits a notable trend; the dynamic timescales are longer than the other studied systems. Therefore, we have performed the Voronoi decomposition analysis of the ionic and the polar-apolar domains, symmetrically increasing the length of alkyl chains on the IL cations. Domain analysis reveals structural microheterogeneity; the anions form discrete domains, and the ionic liquid constituting cations form continuous domains irrespective of the alkyl chain length on the imidazolium cations. Therefore, this computational ultrafast spectroscopy study aids in forming a molecular-level picture of the ionic liquid cations and anions in the liquid phase, providing a detailed interpretation of the spectral properties of the probe stretching vibrations. © 2022 American Chemical Society. All rights reserved.