Antenna-Lanthanide Complexes: A Growing Technology-Driven Research

Luminescent and stable lanthanide ions (Ln3+) complexes are of great interest because of their unique photophysical properties especially with respect to the generation and amplification of light. The emission properties of these complexes are notable and cover an exceptionally wide spectral range: near infrared (Yb3+, Nd3+, Er3+), orange (Sm3+), red (Eu3+), yellow (Dy3+), green (Tb3+) and blue (Tm3+). In our technology driven lives lanthanide complexes are ubiquitous. Indeed they find application as active components in many different kind of advanced materials and devices such as: diagnostic tools, sensors, optical fibers, lasers and amplifiers, electroluminescent and magnetic molecular materials among others. Unfortunately due to the very low absorption coefficients (1 < ? < 10 M-1 cm-1) the emissive states of the metal ions cannot be efficiently populated by direct excitation. A suitable way to overcome this problem is to complex the Ln3+ ion with a ligand containing a highly absorbing chromophore which promotes the metal centred (MC) emission through a sensitization process. In this process the excitation energy absorbed by the chromophore is transferred to the metal core thus efficiently populating the emissive states of the latter and finally the metal centred light emission is obtained (antenna effect). This review will focus on the metal centred emission properties of Ln3+ complexes either in solution or anchored into inorganic matrices. Particular attention will be paid to: (i) the description of the photophysical properties of Ln3+ ions that are relevant for the optimization of the sensitized metal centred light emission; (ii) the coordination properties of Ln3+ ions and the parameters ruling the design of ligands affording complexes with high kinetic and thermodynamic stabilities together with a complete shielding of the coordinated metal ion; (iii) the introduction of suitable reactive groups that allow the covalent insertion of the Ln3+ complex into inorganic and/or organic matrices to afford emissive materials and devices. Some examples of applications of luminescent Ln3+ complexes will be selected from the recent literature and critically reviewed.