Photoluminescence principles

In principle, optical chemosensors make use of optical techniques to provide analytical information. The most extensively exploited techniques in this regard are optical absorption and photoluminescence. Moreover, sensors based on surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) have recently been devised. 

In an earlier review of the applications of photoluminescence (PL) techniques to the characterization of adsorption, catalysis, and photocatalysis (Anpo and Che, 1999), we addressed the basic principles of PL and the importance of PL measurements for understanding of (photo)catalytic processes. This chapter describes more recent developments and focuses on investigations of catalysts in the working state, with an emphasis on the role of local structure on photocatalytic reactions determined with PL and related techniques. 

The latest review of photochemical studies of zeolites was that of Pott and Stork (1 ). The general principles of photoluminescence as regards inorganic ions in host lattices are discussed and will not be described here. The review of Pott and Stork concerns many different oxide catalysts such as alumina, silica and zeolites. The zeolite work mainly deals with phosphorescence of inorganic ions such as Fe , Mn " ", and Eu in zeolites A, Y and mordenite. 

As noted above, observations of large enhancements of the photoluminescence are insufficient to guarantee utility for application of plasmon-enhanced emission in OLEDs where the excited state is not photogenerated. In principle, increases in photoluminescence observed exfierimentally could be completely due to absorption enhancement. Even observation of reduced excited state lifetimes in conjunction with increased emission is insufficient to prove radiative rate enhancement since the lifetime reduction could be due to excited state quenching by the metallic surface and compensated by large absorption enhancements. 

This review deals with the applications of photolurainescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis, After a short introduction, the review presents the basic principles of photolumines-cence spectrosajpy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following coordination chemistry of ions at the surface of alkaline-earth and zirconium oxides, energy and electron transfer processes, photoluminesccncc and local structure of grafted vanadium oxide, and photoluniinescence of various oxide-... 

Photoluminescence can be defined as the radiation emitted from a molecule or a solid which, after it has absorbed energy from an external source and been transferred into an electronic excited state, returns to its ground electronic state. Although it can be said that photoluniinescence consists of both fluorescence and phosphorescence, the distinction between the two is generally phenomenological, but, as shown later, a more theoretical distinction can be proposed. This review is presented in the context of photoluminescence as an analytical tool, the basic principles of which can... 

The principles of photoluminescence applied to solid oxide surfaces can be most easily understood by assuming some simplifications. For example, we can start by considering the Morse potential energy curves (Fig. 1) related to an ion pair such as M-+0-, taken as a harmonic oscillator to represent an oxide, typically an alkaline earth oxide. The absorption of light close to the fundamental absorption edge of this oxide leads to the excitation of an electron in the oxide ion followed by a charge-transfer process to create an exciton (an electron-hole pair), which is essentially... 

If the internuclear equilibrium distance of the excited electronic state (r s) shifts by the value A from the internuclear equilibrium distance of the ground state ( e). the Franck-Condon principle allows transitions to many excited vibrational levels. The shapes of the harmonic potentials also have an effect on the magnitude of the Franck-Condon integral. In this case, the theoretical intensities have been calculated as a function of A and B. The parameters B and A were varied until the theoretical intensities showed the closest match to the experimental intensities. In Fig. 21, the best fit for the progression obtained from the photoluminescence spectrum for the anchored vanadium oxidc/Si02 catalyst and theoretical Franck-Condon analysis is represented (725). 

If the lowest energy elementary excitations are bound singlet and triplet excitons, the theoretical maximum EL quantum efficiency is 25% of the photoluminescence (PL) quantum efficiency. On the contrary, if separated electron and hole polarons are the elementary excitations, the maximum theoretical EL efficiency can approach unity. Thus, a quantitative comparison of the quantum efficiencies for electroluminescence and photoluminescence can, in principle, provide fundamental information on the nature of the excited states. 

Self-assembly principles of the formation of multiporphyrin arrays are extended to anchor the porphyrin triads on semiconductor CdSe/ZnS quantum dot (QD) surface. Comparing with individual counterparts (QD, pyridylsubstituted porphyrin H2P(p-Pyr)4, and Zn-octaethylporphyrin chemical dimer (ZnOEP Ph), the formation of heterocomposites QD-porphyrin triad results in the specific quenching of QD photoluminescence, accompanied by the dimer fluorescence strong quenching (Tsd 1-7 ps due to energy and/or electron transfer) and the noticeable decease of the extra-ligand H2P(p-Pyr)4 fluorescence efficiency by 1.5-2 times via hole transfer H2P—>dimer. 

Photoluminescence results from the radiative emission of photon absorptiongenerated excited states. In principle, both intramolecular and intermolecular excited states can be generated with singlet and triplet configurations by photon absorption in organic semiconducting materials (Scheme 4.2). 

Fig. 1.2 Schematic diagrams showing the general principles of a photoluminescence, b chemiluminescence, and c electrogenerated Chemiluminescence (Reprinted with permission from Ref. [1]. Copyright 2008 American Chemical Society)...

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