Composition Control in Molecular Cluster-Aggregates: A Toolbox for Optical Output Tunability via Energy Transfer Pathways.

Composition control is a powerful tool for obtaining high-performance lanthanide (Ln) luminescent materials with adjustable optical outputs. This strategy is well-established for hierarchically structured nanoparticles, but it is rarely applied to molecular compounds due to the limited number of metal centers within a single unit. In this work, we present a series of molecular cluster-aggregates (MCAs) with an icosanuclear core {Ln2Eu2Tb16} (Ln = Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, and Yb) in which we explore composition control, akin to nanoparticles, to modulate the optical output. More specifically, we target to understand how the presence of a third LnIII doping ion would impact the well-known TbIII → EuIII energy transfer and the ratiometric optical thermometry performance based on the TbIII/EuIII pair. Photophysical properties at room and at varying temperatures were investigated. Based on experimental data and well-established intrinsic features, such as spin-orbit coupling strength and LnIII 4f energy levels' structure, we discuss the possible luminescent processes present in each MCA and provide insight into qualitative trends that can be rationally correlated throughout the series.

Controlling the Energy-Transfer Processes in a Nanosized Molecular Upconverter to Tap into Luminescence Thermometry Application.

Photon upconversion (UC) in molecular species remains a highly sought-after property with vast potential applications in many fields. Until now, a few reports on molecular upconverters are limited to demonstrating upconversion. The low UC quantum yields (QY) and nuclearities hindered the application capabilities for molecular upconverters. To overcome these limitations, we report the use of a molecular cluster-aggregate (MCA) containing 20 lanthanide ions to target YbIII -TbIII -based cooperative UC. Upconversion quantum yield value of 1.04×10-4 %, among the highest value observed for a molecular cooperative UC, was attained for the {Gd11 Tb2 Yb7 } composition. Substitution of GdIII ions for EuIII centers opens a YbIII →TbIII →EuIII energy-transfer pathway, allowing the first proof-of-concept of potential application for molecular UC. This report on upconversion-based luminescence thermometry in a molecular species endorses further development of upconversion properties of nanoscale MCAs.

Inside-Out/Outside-In Tunability in Nanosized Lanthanide-Based Molecular Cluster-Aggregates: Modulating the Luminescence Thermometry Performance via Composition Control.

Modulating the optical property of a material via structural modification is a powerful tool for obtaining the desired optical output. If a material can be tuned inside (core) and outside (outer shell), then the degree of control is greater toward application. Herein, we present a lanthanide-based nanosized molecular cluster aggregate (MCA) that allows fine-tuning of the inner core via composition control akin to nanoparticles. At the same time, the tunable outer shell enables light-harvesting properties similar to molecular systems. As such {Eu4Tb16}, {Eu3Gd5Tb12}, {Eu2Gd10Tb8}, and {Eu1Gd15Tb4} compositions were synthesized, and their photophysical properties were investigated in solution and in the solid state. Controlling the composition and spacing of the emitter ions with the optically silent GdIII ions results in a decrease in the TbIII → EuIII energy-transfer process efficiency. Consequently, ratiometric luminescence thermometry performance is fine-tuned to reach a maximum relative sensitivity of 4.17% °C-1 at 36 °C for the {Eu4Tb16} MCA. This study demonstrates that the optical properties are intrinsic to individual MCA species rather than a collective intermolecular effect. The color change observed close to room temperature for {Eu2Gd10Tb8} suggests potential applications such as multistage anticounterfeiting technology.

Room-Temperature Upconversion in a Nanosized {Ln15} Molecular Cluster-Aggregate.

The successive absorption of low-energy photons to the accumulation of the intermediate excited states leading to higher energy emission is still a challenge in molecular architectures. Contrary to low-phonon solids and nanoparticles, the rational construction of molecular systems containing an excess of donor atoms in relation to acceptor ones is far from trivial. Moreover, the vibrations caused by high-energy oscillators commonly present on coordination compounds result in serious drawbacks on molecular upconversion. To overcome these limitations, we demonstrate that upconversion can be achieved even at room temperatures through the use of molecular cluster-aggregates (MCAs). To achieve the upconverted emission, we synthesized a MCA containing 15 lanthanide ions, {Er2Yb13}, ensuring an excess of donor atoms. With the excitation on the ytterbium ion, the characteristic green and red emissions from erbium were obtained at room temperature. To prove the mechanism behind the upconversion process, four other compositions were synthesized and studied, namely, {Y13Er2}, {Y10Er5}, {Er10Yb5}, and {Y10Er1Yb4}. Upconversion quantum yield values on the order of 10-3% were obtained, values 100000 times higher than for previously reported lanthanide-based molecular upconverting systems. The presented methodology is an interesting approach to address a fine composition control and harness the upconversion properties of nanoscale molecular materials.

Lanthanide-Based Molecular Cluster-Aggregates: Optical Barcoding and White-Light Emission with Nanosized {Ln20 } Compounds.

Counterfeit goods represent a major problem to companies, governments, and customers, affecting the global economy. In order to protect the authenticity of products and documents, optical anti-counterfeit technologies have widely been employed via the use of discrete molecular species, extended metal-organic frameworks (MOFs), and nanoparticles. Herein, for the first time we demonstrate the potential use of molecular cluster-aggregates (MCA) as optical barcodes via composition and energy transfer control. The tuneable optical properties for the [Ln20 (chp)30 (CO3 )12 (NO3 )6 (H2 O)6 ], where chp- =deprotonated 6-chloro-2-pyridinol, allow the fine control of the emission colour output, resulting in high-security level optical labelling with a precise read-out. Moreover, a unique tri-doped composition of GdIII , TbIII , and EuIII led to MCAs with white-light emission. The presented methodology is a unique approach to probe the effect of composition control on the luminescent properties of nanosized molecular material.