Spectroscopy, luminescence

Principles and Characteristics The term luminescence describes the radiative evolution of energy other than blackbody radiation which may accompany the decay of a population of electronically excited chro-mophores as it relaxes to that of the thermally equilibrated ground state of the system. The frequency of the 

108 times longer than fluorescence). Chemiluminescence and bioluminescence (such as emission of VIS light by fireflies and glow-worms) are not considered to be fluorescence. In both phenomena, there is no transfer of light energy as in fluorescence, and the emission of light is only the result of a chemical reaction. 

Fluorimetric methods of analysis make use of the natural fluorescence of the analyte, the formation of a fluorescent derivative or the quenching of the fluorescence of a suitable compound by the analyte. Fluorescence cannot occur unless there is light absorption, so that all fluorescent molecules absorb, but the reverse is not true only a small fraction of all absorbing compounds exhibits fluorescence. The types of molecule most likely to show useful fluorescence are those with delocalised ji-orbital systems. Often, the more rigid the molecule the stronger the fluorescence intensity. Naturally fluorescent compounds include Vitamin A, E (tocopherol). 

Instrumentation for fluorescence spectroscopy has been reviewed [8]. For standards in fluorescence spectroscopy, see Miller [138]. Fluorescence detection in HPLC has recently been reviewed [137], Phosphorescence detection of polymer/additive extracts is not being practised. 

Fluorescence is much more widely used for analysis than phosphorescence. Yet, the use of fluorescent detectors is limited to the restricted set of additives with fluorescent properties. Fluorescence detection is highly recommended for food analysis (e.g. vitamins), bioscience applications, and environmental analysis. As to poly-mer/additive analysis fluorescence and phosphorescence analysis of UV absorbers, optical brighteners, phenolic and aromatic amine antioxidants are most recurrent [25] with an extensive listing for 29 UVAs and AOs in an organic solvent medium at r.t. and 77 K by Kirkbright et al. [149]. 

Instrumentation. A cell design employing reticulated vitreous carbon as the working electrode material that enables both UV-Vis absorption and luminescence measurements has been described [47]. A thin-layer cell with a platinum working electrode has been developed [69]. The luminescence of the electrooxidation products of o-tolidine as a function of electrode potential was studied. A simplified flow cell design has been reported [70]. Luminescence spectra and fluorescence intensity for various aromatic compounds and their electrochemical and photochemical reaction products were observed as a function of flow rate, current and time after the potential step. In the latter study the electrooxidation of p-phenylenediamine (PPD) was examined. The cyclic voltammogram showed two oxidation peaks the first one is assumed to be caused by the formation of the radical cation according to 

Upon excitation with light of about X = 245 nm, a luminescence peak of the parent compound at A. = 391 nm (tface (a) in Fig. 5.13) was observed. During electrooxidation a different spectrum with a peak around X = 342 nm (trace (b) in Fig. 5.13) was seen. The latter spectrum was initially assigned to the radical cation, but no agreement between experimental data and this assumption could be reached. In a second step it was assigned to the dication formed according to 

This assumption was discarded when experiments with an electrode potential beyond the second voltammetric oxidation peak where the dication is generated 

The photoelectrooxidation of bis(benzylidene)acenaphthene has been studied by Compton et al. [71] using the cell described above [71] a highly fluorescent product was identified. 

Photoluminescence data of numerous semiconductors in contact with various electrolyte solutions have been reviewed [69]. 

A discussion on steady state fluorescent monitoring necessitates a distinction between spectroscopic and photometric measurements. The former involves a grating-based spectrofluorometer where full spectrum excitation and emission multivariate spectra are acquired. In contrast a filter photometer involves optical elements (e.g., optical Alters) to isolate excitation and emission bands thereby resulting in a univariate output emission response. 

Solid and solution phase fluorescent spectra at room temperature exhibit relatively broad, often mostly featureless excitation (absorption) and emission spectra, particularly when compared to mid- and far-infrared spectroscopies. These spectra are often mirror images of each other but there are several exceptions as a result of either disparate molecular geometries between the ground and excited states or when the fluor is an excimer.  

Fluorescent spectra are further characterized by the Stokes shift ((5), which is the differential between excitation and emission peak maxima (5 = - Aen() and range from lOnm to 150nm. The Stokes shift and 

---

All forms of spectroscopy require a source of energy. In absorption and scattering spectroscopy this energy is supplied by photons. Emission and luminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a less stable, higher energy state. 

For more information on optical luminescence spectroscopy, see the following sources. 

Winefordner, J. D. Schulman, S. G. O Haver, T. G. Luminescence Spectroscopy in Analytical Chemistry. Wiley-lnterscience New York, 1969. 

S G Schulman, Molecular Luminescence Spectroscopy, Wiley, New York, 1985... 

Ivanov A. A., Kamalov V. F., Koroteev N. I., Orlov R. Yu. Nonlinear and luminescence spectroscopy of vibrationally and electronically excited... 

The HS2 radical was detected by its infrared absorption spectrum and the S2 molecule by luminescence spectroscopy. In addition, infrared bands assigned to dimers of disulfane molecules were observed at higher H2S2 concentrations. The HS2- radicals may further be split into hydrogen atoms and S2 molecules during the photolysis since the concentration of HS2- first increases and then decreases while that of S2 steadily increases. No evidence for the thiosulfoxide H2S=S was found, and the probably formed HS- radicals are assumed to be unable to leave their cage in the matrix and either recombine to H2S2 or form H2+S2 [69]. 

Chen, R.F. Scott C.H. In advances in Luminescence Spectroscopy Cline-Love, L. 

Molecular Luminescence Spectroscopy Methods and Applications in three parts). Edited by Stephen G. Schulman... 

Note that a similar situation arises in the study of heterogeneous deactivation of electron-excited molecules of N2. Thus, an opinion expressed by Clark et al. [152] states that the coefficients of heterogeneous deactivation of N2(A S, v = 0.1) for all surfaces are close to unity. On the other hand, Vidaud with his coworkers [59, 153] have obtained 3 10 2 and (1.8 + 1.2) 10 values for these coefficients shown by platinum and Pyrex, respectively. Tabachnik and Shub [154] investigated heterogeneous decay of NaC A SJJ ) molecules on a quartz surface by the method of bulk-luminescence spectroscopy. The authors carried out a series of experiments within a broad (about four orders of magnitude) range of active particle concentrations and arrived at a conclusion that at a concentration of N2( A 2 ) in excess of 10 mole/cm , the... 

Retzik, M. and Froehlich, P., Extending the capability of luminescence spectroscopy with a rapid-scanning fluorescence spectrophotometer, Am. Lab., March, 68, 1992. 

Table 5.12 shows the main features of luminescence spectroscopy. The much higher sensitivity and specificity of luminescence techniques compared to absorption techniques is an obvious advantage for excitation spectra. In solution studies, pg ml. 1 levels can often be determined, as compared to p,gmL-1 levels in absorption spectroscopy. The greater sensitivity of luminescence techniques stems from the fact that the... 

S.G. Schulman (ed.), Molecular Luminescence Spectroscopy, Methods and Applications, John Wiley Sons, Inc., New York, NY, Parts 1-3 (1985, 1988, 1993). 

The electronic structure of /nmv-[NiCl2(I I20)4]-21LO has been investigated in detail, both by calculations and by absorption and luminescence spectroscopy.1138... 

Luminescence spectroscopy, Ed. M D Lumb (New York Academic Press, 1978). 

The most commonly used methods for characterization of ruthenium sensitizers are elemental analysis, NMR, IR, Raman, UV-vis, and luminescence spectroscopy and cyclic voltammetry, HPLC, and X-ray crystallography. 

Luminescence spectroscopy is one of the most sensitive techniques for identification of impurities in dyes. The most commonly observed impurities in to-bipyridyl complexes of the type [RuL2X2] are the homoleptic tris-bipyridyl species [RuL3]2+. Since the emission quantum yields of the [RuL3]2+ complexes are significantly higher than those of the [RuL2X2] complexes, one can identify the homoleptic impurities at a level of less than 1%. This does depend, however, on the relative quantum yields, and position of the emission spectral maxima, for the complexes and impurities involved. 

Nelly R.N., Schulman S.G., Proton-Transfer Kinetics of Electronically Excited Acids and Bases, in Molecular Luminescence Spectroscopy Methods and Applications, part 2, Schulman S.G. (ed.), Wiley-Interscience, New York, 1988 pp 461-510. 

---