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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...

Thin silver chloride layers evolve oxygen under UV/vis illumination in aqueous solution under appropriate conditions. AgCl deposited on a conducting support photocatalyzes the oxidation of water to O2 in the presence of a small excess of silver ions in solution. The light sensitivity in the visible part of the spectrum is due to self-sensitization caused by reduced silver species. Anodic polarization reoxidizes the reduced silver species. Considerable improvement of sensitivity has been observed with AgBr sensitized AgCl photoanodes. To test its water splitting capability, the AgCl photoanode was combined with hydrogen-producing semiconductors, such as a platinized silicon solar cell and platinized p-GaInP2. AgCl layers were employed in the anodic part and the H2 evolving semiconductors in the cathodic part of a photoelectrochemical cell for light-assisted water splitting. The AgCl electrodes were characterized with surface photovoltage spectroscopy (SPS), which identified the transition from the valence band to silver cluster levels and by in-situ UV/vis diffuse reflectance spectroscopy, which detects the reduced and reoxidized silver species.

To halt the buildup of carbon dioxide (CO2), the main greenhouse gas, the development of an alternative energy source to fossil fuels becomes more and more important. Hydrogen (H2) has the potential to meet the requirements as a clean non-fossil fuel in the future, if it can be produced using our primary source of energy, the sun, and stored and transported safely. The present article will give a short review on different approaches on semiconductor-based photoelectrochemical water splitting into hydrogen and oxygen (O2) using sunlight as the energy input, also generally described as photoelectrolysis.