Selective separation of gases/vapors with similar physicochemical properties involves energetically costly distillation processes. Alternative separation processes based on shape-selective molecular sieving, taking place on porous frameworks (or membranes), are less energy demanding but require an optimal balance between selectivity and diffusion kinetics (permeability). Herein, we report a rational strategy to select an optimal soft noncovalent microporous material (NPM) suitable for the low-energy separation of C6-hydrocarbons with kinetic diameters in the range of 4.3-6.3 Å. This strategy is based on a Cambridge Structural Database search of diamondoid NPMs with a low packing factor, leading to the selection of an oxidotetrazinc cluster based diamondoid NPM network named DiaMM-1 containing tetrahedral voids of 336 Å3 (tetrahedron insphere diameter of 5.8 Å accessible through 8.2 Å triangular windows) suitable for this separation. Based on this result the fluorinated analogue DiaMM-2 was designed and synthesized. DiaMM-1 and DiaMM-2 exhibit permanent porosity and high thermal stability. The optimal combination of molecular crystal softness, pore size, and decoration of pore surface of DiaMM-1,-2 leads to high adsorbate diffusivity and low adsorption energy, allowing fast separation of hexane isomers and benzene/cyclohexane mixtures at low temperature.