Light conversion processes in lanthanide-based molecular materials

This work is mainly focused on the investigation and processing of luminescent molecular complexes of lanthanide ions (Ln), for the development of efficient near infrared (NIR)-emitting materials. In the light of a detailed understanding of the chemical/photophysical properties of the special class of lanthanide quinolinolato complexes, as prototype molecular materials for NIR applications, two main aspects have been taken into consideration for a progress in this field: i) (multi-) functional NIR-emissive heterolanthanide complexes and ii) lanthanide-doped sol-gel glasses as NIR-emissive optical materials. Heterometallic assemblies, containing two or more different lanthanide cations as carriers of distinctive functionalities, represent promising materials where different physical properties can coexist or be in interplay thanks to intermetallic communication. A thorough discussion on heterolanthanide assemblies, pointing out the general strategies to achieve the desired (multi-) functionalities in these systems, is provided herein. In this framework, heterolanthanide Er/Yb systems have attracted significant interest for Er3+ sensitization through resonance energy transfer (RET) from Yb3+ leading to enhancement of Er3+ NIR emission at 1.5 μm in glass host Yb-Er co-doped optical fiber amplifiers. A strategy to achieve composition control and homogeneity in such materials resorts to the encapsulation of Er and Yb in the same molecule to afford intermetallic communication between lanthanide ions sitting at short distance in a discrete polynuclear architecture. Fully efficient Yb-to-.Er RET at molecular level is firstly demonstrated in the mixed-lanthanide Yb2ErQ9 complex (Q = quinolinol ligand), and, as an extension of these studies, the investigation of the full photocycle leading to narrow-band NIR emission in a series of heterolanthanide ErxYb3-x quinolinolato compounds, on variation of metal composition and molecular stoichiometry, has been performed. Moreover, the first example of a tri-lanthanide discrete polynuclear molecule containing three NIR-emissive lanthanide ions: Nd3+, Yb3+ and Er3+, is presented. The NdYbErQ9 complex was investigated as a potential broad-band NIR emitter where the intensities of the three different main emissions from each lanthanide ion can be finely tuned on varying metal composition. The investigation of the processing potential of luminescent lanthanide complexes through doping into inorganic or polymeric host matrixes is of crucial interest for the development of optical devices (amplifiers, optical waveguides, OLEDs, etc.). To this purpose, studies on the incorporation of an Yb quinolinolate complex into a silica sol-gel glass, with a thorough account of the photophysical properties of this hybrid material, have been performed. Promising results on silica thin films doped with heterolanthanide Er/Yb quinolinolates are also presented as a molecular approach to achieve controlled light conversion in NIR-emissive optical materials.