Encapsulated Lanthanides as Luminescent Materials

Abstract

Dedicated to Professor Roald Hoffmann on the occasion of his 65th birthdayMaterials which embed organic dyes, rare earth ions, complexes, or quantum dots in a matrix with specifically tailored chemical and optical properties provide a challenging approach to novel chemical and optical applications. These materials have the potential to be used in microoptics, optoelectronics, laser materials, solar cells, sensors, battery electrodes, and photocatalysis. In this article we focus on lanthanides encapsulated in zeolites, glass films derived from sol ± gel processes, and semiconductors.The research work on the unique luminescent properties of rare earth elements hosted in different matrixes is strongly motivated by their technological importance in optoelectronic devices. [1] The materials emit over the entire spectral range of interest: near infrared (NIR; Nd 3 , Er 3 ), red (Eu 3 , Pr 3 , Sm 3 ), green (Er 3 , Tb 3 ), and blue (Tm 3 , Ce 3 ). Their optical transitions involve 4f orbitals, which are well shielded from their chemical environment by 5s 2 and 5p 6 electrons. The f-f transitions are parity forbidden and, as a result, the absorption coefficients are very low and the emissive rates are slow, which results in long-lived and linelike emission bands. As a consequence, direct excitation of the lanthanide ions is unfavorable. The comparatively fast thermal relaxation of the excitation energy is a problem when using lanthanide ions for luminescence. This nonradiative relaxation may occur by interaction of the electronic levels of the lanthanide ion with suitable vibrational modes of the environment. [2] The efficiency of these processes depends on the energy gap between the ground and excited states as well as the vibrational energy of the oscillators. For example, when solvents containing OH groups are coordinated to lanthanide ions, efficient nonradiative deactivations take place through vibronic coupling with the vibrational states of the OH oscillators. Replacement of the OH oscillators by low-frequency OD oscillators, diminishes the vibronic deactivation pathway. [3] Different ways to overcome the difficulties of low absorptivity and thermal relaxation have been used. We show the apparently most important ones in Figure 1: a) matrix excitation followed by energy transfer to the lanthanide ion, b) ligand !metal charge transfer followed by lanthanide f-f emission, and c) ligand-centered absorption followed by energy transfer to the lanthanide ion.We first discuss the use of coordinating ligands as sensitizers. After absorption of light by the ligands, the electronic excitation energy is transferred and results in a luminescence of the lanthanide ion (see Figure 1 c). A possibility, given by Vˆgtle, Balzani, and co-workers, is to use a specially designed dendrimer which is able to play the role of the ligand for the lanthanide ions but which is also capable of working as an [*] Prof. Figure 1. Three paths to efficient lanthanide luminescence (ET energy transfer; REE rare earth emission; LMCT ligand !metal ch...

Photonic and nanobiophotonic properties of luminescent lanthanide-doped hybrid organic–inorganic materials

“…[7][8][9] During the past twenty years, optical studies performed on organic-inorganic nanocomposites have evolved towards different objectives: investigating the fundamental spectroscopy of lanthanide ions in the sol-gel environment, study of energy transfer between lanthanide ions and chelates (or dyes) in solid matrices, using luminescent molecules as probes for sol-gel processing, and finally developing materials with specific optical properties based on the properties of organic or inorganic chromophores. [10][11][12][13][14][15][16][17] But in the past years, research of these luminescent materials has also been focused on bioanalytical and bio-medical applications. In this review, the different synthetic strategies to prepare lanthanide-doped hybrid materials, as well as some basic considerations involving the efficiency and sensitization of lanthanide luminescence, are discussed.…”

Organically Structured Carbon Nanotubes for Fluorescence

“…As is known to all of us that the rare-earth compounds are often used to manufacture laser materials, optoelectronic devices and fluorescence probes [95,96] . Chinese scientists designed and synthesized the SWNTs covalently modified by europium (Eu 3+ ) complex [97] [ Figure 30].…”

Photoluminescence study of terbium-exchanged ultrastable Y zeolites: Number of species, photoluminescence decays, and decay-associated spectra

“…k nonrad is proportional to the number of OH groups contained in the inner coordination shell of the terbium ion 27 while k nonrad Ј may include resonant ion-ion interactions located in close proximity or/and nonradiative energy transfer to host defects/impurities. Zeolite framework exhibits only low energy phonons, 8 which are not expected to significantly contribute to the nonradiative deactivation of Tb 3+ PL. Further, quenching of 5 D 4 related emission due to ion-ion interactions can be excluded given the low Tb 3+ concentration and lack of energy resonances involving this metastable level.…”

“…It is acknowledged that the quantum PL efficiency obtained with lanthanides in zeolites is strongly affected by the nonradiative relaxation via the vibrational coupling between the electronic levels of the lanthanide ion with the -OH groups inherently present in a solgel matrix. 8 Albeit the PL properties can be improved following a thermal treatment at high temperatures or calcination they are subject to full or partial restoration to those measured in the hydrated state upon exposure to the moist atmosphere. 5,6 To decrease the adverse effects of water on the PL properties of the lanthanides-exchanged zeolites, the surface of zeolites can be made hydrophobic by the silylation technique.…”