Fluorescent organic molecules

As mentioned in the introductory part of this section, quantum dots exhibit quite complex non-radiative relaxation dynamics. The non-radiative decay is not reproduced by a single exponential function, in contrast to triplet states of fluorescent organic molecules that exhibit monophasic exponential decay. In order to quantitatively analyze fluorescence correlation signals of quantum dots including such complex non-radiative decay, we adopted a fluorescence autocorrelation function including the decay component of a stretched exponential as represented by Eq. (8.11). 

Dye laser A CW or pulsed source of coherent radiation in which the active medium is usually a solution of a fluorescent organic molecule (the dye) pumped with a suitable pump laser or with a flash lamp. These lasers can be tuned over a large part of the fluorescence band of the dye. 

FIGURE 6-24 Partial energy-level diagrams for a fluorescent organic molecule. 

Sciacca B, Frascella F, Venturello A, Rivolo P, Descrovi E, Giorgis F, Geobaldo F (2009) Doubly resonant porous silicon microcavities for enhanced detection of fluorescent organic molecules. 

Enantioselectivity of fluorescent organic molecules were reviewed several years ago by Pu [19] therefore this review will mainly discuss recent developments and applications of enantioselective luminescence sensing. 

As can be seen, the efficiency of LSCs obtained by the incorporation of organic colorants into a transparent matrix was low. The main factor responsible for a decrease in luminescence that is especially important in large plates is the overlap between absorption and emission of fluorescent organic molecules. Therefore, in order to overcome this difficulty, there is a need to use systems in which the absorption and emission arise from different levels. This can be achieved by using either proton transfer compounds [44,45] or the specific properties of luminescence of lanthanide complexes [46-49]. 

The strongly exothermic transfer of electrons between fluorescent organic molecules represents one of the most general mechanisms in chemiluminescence [50, 51]. It can be found in electroluminescence, radical ion annihilation and peroxide decomposition. The basic concept was introduced by Hercules [39] following the general theory of Marcus [52]. The reaction co-ordinate can be roughly indicated by the potential energy curves shown. 

Figure Bl.1.3. State energy diagram for a typical organic molecule. Solid arrows show radiative transitions A absorption, F fluorescence, P phosphorescence. Dotted arrows non-radiative transitions.

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

The quantum efficiency of fluorescence of a molecule is decided by the relative rates of fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the present time it has proved impossible to predict these relative rates. Thus, whilst it is now possible to calculate theoretically the wavelengths of maximum absorption and of maximum fluorescence of an organic molecule, it remains impossible to predict which molecular structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical knowledge plus the instinct of the research chemist. 

Electronic transitions in a solute take place very fast, i.e., almost immediately in comparison with the movement of the molecules as a whole and vibrations of atoms in organic molecules. Hence, absorption and fluorescence can be denoted in Fig. 5 by vertical arrows, in accordance with Franck-Condon principle. Both these processes are separated by relaxations, which are intermolecular rearrangements of the solute-solvent system after the excitation. 

The inhomogeneous broadening effect will be apparent in practically all cases, and the character of this broadening may be both stationary (in rigid solutions or when time of relaxation xr is less than lifetime of fluorescence x, or xr>t) and dynamic in nature. Inhomogeneous broadening affects all spectral characteristics of organic molecules in solutions. 

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