Enhancement in the luminescence of lanthanide metal complexes via the addition of fluorescent carbon dots

Highly fluorescent Lanthanide metal complexes have shown great potential application in the optics area. However, the insufficient luminescence of lanthanide complexes remains the potential bottleneck. Earlier, on the addition of an organic light-harvesting molecule, an enhancement in the fluorescent quantum yield of the lanthanide complexes was perceived, although it was inadequate for practical applications. We have synthesized the complex Tb^III₂(L)₂(NO₃)₂(dmf)₂, using a ligand obtained by 1 : 1 condensation of Salicylaldehyde and 2-(2-Aminoethoxy)ethanol, H₂L. Photoluminescence study revealed pure green light emission from the Tb^IIIL-complex with the QY of ∼3.37%. The fluorescent emission was further enhanced by combining Tb^IIIL with N, S-doped carbon dots (CDs), derived from sulphilinamide and citric acid: almost six-fold increase in photoluminescence quantum yield from 3.37% to 21%. The enhancement was rationalized by the energy harvesting capacity of the CDs with consecutive transfer of the energy to the excited level of the luminescent Tb^III metal centres. This phenomenon has been reversed on the addition of luminescent Tb^IIIL to the fluorescent CD solution, supporting indeed an energy transfer from CDs → Tb^IIIL. Herein, a simple and easy strategy was conveyed to improve the quantum yield of the lanthanide complexes on incorporation of CDs to be implemented for optical, sensing and anti-counterfeiting applications.