A series of inorganic–organic hybrid double-tartaric bridging mono-lanthanide substituted phosphotungstates with a general formula of [N(CH₃)₄]₆K₃H₇[Ln(C₄H₂O₆)(α-PW₁₁O₃₉)]₂·27H₂O (Ln = Dy for 1Dy, Er for 2Er, and Yb for 3Yb) have been synthesized by the reactions of K₁₄[P₂W₁₉O₆₉(H₂O)]·24H₂O and LnCl₃·6H₂O in the presence of tartaric acid and tetramethyl ammonium counterions in an aqueous medium. Crystal structure analyses reveal that 1Dy–3Yb are isomorphic and are all built from two mono-lanthanide substituted Keggin polyoxoanion fragments joined together by two tartaric ligands. 1Dy–3Yb exhibit reversible photochromism and they can change from white (for 1Dy and 3Yb) or pink (for 2Er) to blue upon UV irradiation and then their colors gradually recover in the dark for about five days. The solid-state photoluminescence spectra of 1Dy–3Yb display their characteristic emissions of lanthanide components based on their 4f–4f transitions. Moreover, the fluorescence of 1Dy displays an effectively switchable behaviour when exposed to UV light, the intensity of the emission band at 573 nm gradually weakens as the irradiation time increases and the intensity drops to 2.4% of the original intensity after 30 minutes. It is noteworthy that the frequency dependence of the ac-susceptibility was found for 1Dy, suggesting a typical single-molecule magnet (SMM) behavior with an energy barrier of 20 K.