Absorbance-difference diagram

Fig. 5.6. Linear absorbance difference diagram for the photoisomerisation of azobcnzene,...

Fig. 5.7. Absorbance difference diagram for the photo-consecutive reaction of (r(Onj-stilbene...

In Fig. 5.16 the absorbance difference diagram is given as an example for a substituted anthraquinone. Fig. 5.17 demonstrates the absorbance-time curves. All wavelengths have a linear relationship. In Table 5.1 the result of an evaluation according to eq. (5.93) is given for the same compounds. The 1-chloro-substituted compounds demonstrated a comparable small differential yield in neutral methanol. 

For the measurements the apparatus given in Fig. 4.12 was used which allows the examination of photochemical with superimposed thermal reactions at room temperature and as low as 240 K. The absorbance difference diagram is given in Fig. 5.35. 

Furthermore absorbance difference diagrams can be plotted as demonstrated in Fig. 5.7. Whereas the additional information of this type of diagram is small, absorbance difference quotient diagrams can be used to decide whether the number of linear independent reactions amounts to 2 or more as given in Fig. 5.11. 

Fig. 5.11 presents an example for such a diagram according to eq. (5.73). It is called an absorbance difference quotient diagram abbreviated in the following as EDQ-diagram. The figure demonstrates the photoreaction of stilbene in perfluoromethylcyclohexane by irradiation at the wavelength 313 nm. 

Different types of diagrams influenced by differences in fluorophore properties as well as diagrams of high order in comparison to absorbance quotient diagrams are discussed in detail elsewhere [92]. 

A block diagram helps us to visualize the thermodynamic processes. The liquid refrigerant evaporates at constant temperature as it absorbs heat from the contents of the refrigerator, which are at a different constant temperature. 

A second kind of electronic defect involves the electron. Let us suppose that the second plane of the cubic lattice has a vacancy instead of a substitutional impurity of differing valency. This makes it possible for the lattice to capture and localize an extraneous electron at the vacancy site. This is shown in the following diagram. The captured electron then endows the solid structure with special optical properties since it ean absorb photon energy. The strueture thus becomes optically active. That is, it absorbs light within a well-defined band and is called a "color-center" since it imparts a specific color to the crystal. 

Fluorescence is a process that occurs after excitation of a molecule with light. It involves transitions of the outermost electrons between different electronic states of the molecule, resulting in emission of a photon of lower energy than the previously absorbed photon. This is represented in the Jablonski diagram (see Fig. 6.1). As every molecule has different energy levels, the fluorescent properties vary from one fluorophore to the other. The main characteristics of a fluorescent dye are absorption and emission wavelengths, extinction... 

The activation energy is the amount of energy that the reactants must absorb from the system in order to react. In the reaction diagram, the reactants begin at A. The reactants must absorb the energy from A to B in order to form the activated complex. The energy necessary to achieve this activated complex is the distance from A to B in the diagram and is mathematically the difference (B - A). 

FIGURE I Diagram of a capillary filled with an UV-absorbing BGE. A non-absorbing analyte will displace the UV-absorbing probe, resulting in a negative peak due to displacement of the probe (A). When the mobilities of the analyte and the probe are different, a non-symmetric peak will be observed (B). 

An infrared spectrum can be obtained for a sample of an organic compound regardless of its physical state (solid, liquid, gas or dissolved in a solvent). Infrared radiation is passed through the sample in the spectrometer. Some wavelengths are absorbed, causing bond vibrations within the molecules. The transmitted radiation then passes to a detector where the intensity at different wavelengths is measured. An Infrared spectrum, like that shown in the diagram, is obtained. 

---