A multifunctional lanthanide-organic framework Tb−TCA (H3TCA = tricarboxytriphenylamine) comprising a triphenylamine moiety as an efficient luminescence sensitizer is synthesized by a solvothermal method and structurally characterized. Tb−TCA exhibits lanthanide-based emission (540 nm) and triphenylamine emission (435 nm) after excitation at 350 nm and works as a luminescent chemosensor towards salicylaldehyde with a sensitivity of 10 ppm under optimized conditions. Tb−TCA features a high concentration of Lewis acid Tb3+ sites and Lewis base triphenylamine sites on its internal surfaces; it thus enables both Knoevenagel and cyanosilylation reactions in a size-selective fashion. The ratiometric fluorescent response towards aldehydes (I540/I435) demonstrates that the absorption of the porous material is size selective, and the interactions between the aldehyde molecules and the Tb3+ ions play a dominant role in the absorption and activation of the aldehyde substrates. In particular, these studies provide opportunities to directly validate the sorption sites and the catalytic mechanism of the multifunctional Ln metal–organic framework material by luminescence.