Porous molecular crystals (PMCs) should function as new-generation functional porous materials, but the selective crystallisation of PMCs is still difficult. Herein we demonstrate that the liquid–liquid interface between the MeOH/H₂O mixture and alkanes promotes the crystallisation of a Pt(II)-based PMC, rather than the nonporous form. This new crystallisation method allows control of not only the porosity but also the luminescence of the Pt(II) complex crystal.