Luminescence Photoluminescence

Luminescence Photoluminescence of rare earth oxide nanocrystals is one of their most significant properties of interest for applications. The photoluminescence features of rare earth doped nanomaterials have been reviewed by Liu and Chen in an earlier chapter (Liu and Chen, 2007b). Therefore, we only review some related aspects, such as the major variations in the luminescence features of nanophased rare earth oxides, the application of rare earth doped nanocrystals as multicolor phosphors and as bioimaging probes. 

Define the following terms fluorescence, phosphorescence, luminescence, photoluminescence, and chemiluminescence. 

Luminescence The light emitted by something in response to a stimulus. Specific names are used for different stimuli, e.g., thermoluminescence, optically-stimulated luminescence, photoluminescence, triboluminescence etc. 

Keywords Luminescence Photoluminescence Chemiluminescence Electrochemiluminescence Triplet-triplet annihilation Light-emitting reaction Excimers/exciplexes... 

Nonradiative Decay. To have technical importance, a luminescent material should have a high efficiency for conversion of the excitation to visible light. Photoluminescent phosphors for use in fluorescent lamps usually have a quantum efficiency of greater than 0.75. AH the exciting quanta would be reemitted as visible light if there were no nonradiative losses. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

The electroluminescence spectra of the single-layer devices are depicted in Figure 16-40. For all these OPV5s, EL spectra coincided with the solid-state photoluminescence spectra, indicating that the same excited states are involved in both PL and EL. The broad luminescence spectrum for Ooct-OPV5-CN" is attributed to excimer emission (Section 16.3.1.4). 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

Luminescence measurements on proteins occupy a large part of the biochemical literature. In what surely was one of the earliest scientific reports of protein photoluminescence uncomplicated by concurrent insect or microorganism luminescence, Beccari (64), in 1746, detected a visible blue phosphorescence from chilled hands when they were brought into a dark room after exposure to sunlight. Stokes (10) remarked that the dark (ultraviolet) portion of the solar spectrum was most efficient in generating fluorescent emission and identified fluorescence from animal matter in 1852. In general, intrinsic protein fluorescence predominantly occurs between 300 nm and 400 nm and is very difficult to detect visually. The first... 

Several books and symposium proceedings on luminescence standards and measurements have been published in the last several years, including "Advances in Standards and Methodology in Spectrophotometry" (i), "Measurement of Fhotolumlnescence" (2), "Standards in Fluorescence Spectrometry" (J), and "Modern Fluorescence Spectroscopy" (Volumes 1-4) (4). These books, the references within them, and the classic in the field, "Photoluminescence of Solutions" by C.A. Parker (5), provide the researcher with extensive information about luminescence standards and measurements. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Strickert HH, Tong JR, EUis AB (1982) Luminescent photoelectrochemical cells. 6. Spatial aspects of the photoluminescence and electroluminescence of cadmium selenide electrodes. J Am Chem Soc 104 581-588... 

Assefa, Z Forward, J.M., Grant T.A., Staples, R.J., Hanson, B.E., Mohamed, A.A. and Fackler, J.P. Jr (2003) Three-coordinate, luminescent, water-soluble gold(l) phosphine complexes structural characterization and photoluminescence properties in aqueous solution. Inorganica Chimica Acta, 352, 31 5. 

It seems feasible that the eel occurs according to the usual mechanism. Reduction and oxidation of the complex is followed by the annihilation and luminescence. However, there must be an efficient competition by other processes since the eel intensity is rather low compared to the photoluminescence. As indicated by CV measurements the reduction at. - -0.7 V and -1.6 V and... 

---