The podand-type ligand L, based on a tertiary amine substituted by three pyridyl-6-phosphonic acid functions, forms hydrated complexes with Ln³⁺ cations. The luminescence properties of the YbL complex were studied in D₂O as a function of the pD and temperature. In basic conditions, increases in the luminescence quantum yield and the excited state lifetime of the Yb centered emission associated with the ²F_5/2 → ²F_7/2 transition were observed and attributed to a change in the hydration number from two water molecules in the first coordination sphere of Yb at acidic pH to a single one in basic conditions. Upon the addition of TbCl₃ salts to a solution containing the YbL complex in D₂O, heteropolynuclear Yb/Tb species formed, and excitation of the Yb at 980 nm resulted in the observation of the typical visible emission of Tb as a result of a cooperative upconversion (UC) photosensitization process. The UC was further evidenced by the quadratic dependence of the UC emission as a function of the laser power density.