Herein we demonstrate an alternative route to make Eu³⁺-exchanged zeolite L crystals (Eu³⁺/ZL) highly luminescent by modification with silylated β-diketone. The modified crystals show strong red emission under UV-light irradiation due to the effective energy transfer from the grafted molecules to the Eu³⁺ ions. The obtained materials were characterized with FT-IR, SEM, XRD and elemental analysis. FTIR and elemental analysis confirm that the amount of grafted molecules decreases with increasing size of the ZL crystals. Luminescence spectroscopy was used to study the luminescence properties of the modified Eu³⁺/ZL crystals. The formation of europium(III) diketonate complexes and energy transfer from TTA to Eu³⁺ ions have been confirmed by luminescence spectroscopy. The Eu^{3+ 5}D₀ radiative (k_r) probabilities and the ⁵D₀ quantum efficiency were estimated based on the emission spectra and lifetimes. The studies showed that the luminescence performances of the modified ZL crystals can be significantly affected by the crystal size, e.g., photoluminescence emission intensity, ⁵D₀ lifetime, ⁵D₀ radiative (k_r) probabilities and the ⁵D₀ quantum efficiency (q) decreases as the size of the ZL crystals increases.