Luminescent processes Subject

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

Mixtures of phosphine and oxygen, both above and below the explosion limits, subjected to flash photolysis show, in the spectra, the presence of PH-, OH- and PO-radicals as well as the PH2-radical Eiuiing the reaction of atomic oxygen with phosphine visible luminescence up to 3600 A and UV emission were observed, which were attributed to the partial processes ... 

Let us now briefly outline the structure of this review. The next section contains information concerning the fundamentals of the electrochemistry of semiconductors. Part III considers the theory of processes based on the effect of photoexcitation of the electron ensemble in a semiconductor, and Parts IV and V deal with the phenomena of photocorrosion and light-sensitive etching caused by those processes. Photoexcitation of reactants in a solution and the related photosensitization of semiconductors are the subjects of Part VI. Finally, Part VII considers in brief some important photoelectrochemical phenomena, such as photoelectron emission, electrogenerated luminescence, and electroreflection. Thus, our main objective is to reveal various photo-electrochemical effects occurring in semiconductors and to establish relationships among them. 

In most cases the phosphor layer is then subjected to an aluminum vapor deposition process. This increases the brightness by reflection of the backward-radiating luminescence and inhibits charge buildup in the phosphor layer. 

In the sections which follow, the principles discussed above will be used in exploring the properties of a range of platinum(II) complexes. The emphasis of the chapter will be on emission—luminescence—from Pt(II) complexes, on the features and properties of molecules that tend to favor emission over other non-radiative processes. In other words, photophysics, as opposed to photochemistry, is our main subject here, but we also consider other excited state processes in selected systems, such as electron transfer and photooxidation. 

Initially, interest for NIR emission of lanthanide ions stemmed from the development of optical libers, lasers and amplifiers for telecommunication (Kido and Okamoto, 2002 Kuriki et al., 2002) and there are a wealth of theoretical and technical papers published in this area. Up-conversion processes have also been the subject of intense investigations (Auzel, 2004). These two areas of research and development mostly deal with purely inorganic compounds or, more recently, with luminescent polymers they will not be covered in this chapter, with the exception of the latter, which will be partly described. 

The optical properties of this new family of semiconductors are the subject of Volume 21, Part B. Phenomena discussed include the absorption edge, defect states, vibrational spectra, electroreflectance and electroabsorption, Raman scattering, luminescence, photoconductivity, photoemission, relaxation processes, and metastable effects. 

Topics which have formed the subjects of reviews this year include photosubstitution reactions of transition-metal complexes, redox photochemistry of mononuclear and polynuclear" complexes in solution, excited-state electron transfer processes, transition-metal complexes as mediators in photochemical and chemiluminescence reactions, lanthanide ion luminescence in coordination chemistry, inorganic photosensitive materials," and photocatalytic systems using light-sensitive co-ordination compounds. Reviews have also appeared on the photoreduction of water.Finally, various aspects of inorganic photochemistry have been reviewed in a single issue of the Journal of Chemical Education. 

Additives, having really used concentrations, provide protective effect, though luminescence on the wave-length 420-430 nm has been already suppressed by them, so there are no reasons to connect protective effect only with the process of suppression. This means that polymer protection over shielding effect needs explanation through chemical action of additives being used. Study of this photooxidation process must be the subject of separate invesrigation. 

The project Carl gave me was to build a sensitive instrument to search for luminescence from the permanganate ion, which had been the subject of a series of experimental single crystal absorption spectral studies and theoretical studies in the laboratory [6]. The spectrometer was built, but after repeated attempts using a range of crystals, excitation conditions and temperatures, no luminescence was detected. All subsequent efforts by others have confirmed this failure [7], under laser irradiation in iodide lattices some emission has been detected, but this is derived from the manganese ion MnO, 2 produced by a photoredox process [8]. This left me without many results to show for my year s work. I made some measurements on the intensely luminescent alkali metal platinocyanides but this did not lead to any new insights. 

Topics, which have formed the subjects of reviews this year, include the luminescence kinetics of metal complexes in solution, photochemical rearrangements of co-ordination compounds, photochromic complexes of heavy metals with diphenylthiocarbazone derivatives, the photochemistry of actinides, actinide separation processes, and light-induced electron-transfer reactions in solution and organized assemblies. A discussion has also appeared on assigning excited states in inorganic photochemistry. ... 

The rapid development of lasers has led to the publication of increasing numbers of papers concerned this year with such subjects as superfluorescence and co-operative radiation processes,451 the thermodynamics of co-operative luminescence,452 saturation, collisional dephasing, and quenching of fluorescence of organic vapours in intense laser excitation studies,453 a theoretical model for fluorescence in gases subjected to continuous i.r. excitation,454 a quantum treatment of spontaneous emission from strongly driven two-level atoms,455 the development of site-selection spectroscopy,45 and measurements of relaxation times 457 using laser excitation. 

A luminescent material will only emit radiation when the excitation energy is absorbed. These absorption processes will be the subject of this chapter with stress on excitation with ultraviolet radiation. The emission process will be treated in the next chapter. 

The luminescence of metal complexes is a rapidly expanding research field. In 1970 Fleischauer and Fleischauer published the first review on this subject [1]. In the meantime the number of publications has increased considerably. While prior to 1970 isolated observations had been reported, systematic studies have now led to a fairly good understanding of the luminescence properties of metal complexes. The interest in this area is based on various circumstances. Luminescence spectroscopy is an important tool in photochemistry since it provides a deeper insight in excited state processes in general. Of course, the emission behavior of metal complexes is also rather interesting in its own right. In particular, potential applications have attracted much attention. Luminescence spec-... 

Where kp and km are the rate constant of fluorescence and non-radiative processes, respectively. The fluorescence quantum yield (Of) value in the range of 0.0 to 1.0. If the non-radiative relaxation is fast compared to fluorescence (km > k,), O will be small, that is the compound will fluoresce very little or not at all. Often different non-radiative events are limited in the solid phase, and long-lived luminescence (e.g. phosphorescence) is often studied in frozen solution or other solid phases. Quenchers make non-radiative relaxation routes more favorable and often there is a simple relation between 0 and the quencher concentration. The hest-known quencher is probably O2, which quench almost all fluorophores other quenchers only quench a limited range of fluorophores. If a molecule is subject to intramolecular quenching, O may yield information about the structure. 

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