Fluorescence of Organic Molecules

Often the 0-0 transitions of absorption and emission do not coincide, and a 0-0 gap results. This is referred to as anomalous Stokes shift and is due 

The relationship between the quantum yield of fluorescence and molecular structure is determined to a large extent by the structural dependence of the competing photophysical and photochemical processes. Thus, for most rigid aromatic compounds fluorescence is easy to observe, with quantum yields in the range 1 0.01. This may be explained by the fact 

A fundamental factor that determines the fluorescence quantum yield is the nature of the lowest excited singlet state—that is to say, the magnitude of the transition moment between S and S,. If the So- S, transition is symmetry forbidden, as in benzene, A , is small compared to com- 

Most compounds whose lowest excited singlet state is an (n,jr ) state exhibit only very weak fluorescence. The reason is that due to spin-orbit coupling, intersystem crossing into an energetically lower triplet state is particularly efficient. (Cf. Section 5.2.2.) Heavy atoms in the molecule (e.g., bromonaphthalene) or in the solvent (e.g., methyl iodide) may favor intersystem crossing among states to such an extent that fluorescence 

1990 for leading references) thus involves no special mechanism of nonra-diative decay. 

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The fluorescence of organic molecules and ions in solution is a photoluminescent process that decreases extremely rapidly when the excitation ceases, in contrast to phosphorescence, which latter has a much slower decrease and is seldom used for analysis. Finally, chemiluminescence, the emission of light during a chemical reaction, is involved in some analytical applications. 

However, the fluorescence of organic molecules is not only characterised by the intensity and the emission spectram, it also has a characteristic lifetime. The lifetime can be used as an additional parameter to separate the emission of different fluorophores, to probe ion concentrations and binding states in cells, and to investigate interactions between proteins by fluorescence resonance energy transfer. 

Levy D., Avnir D. Room temperature phosphorescence and delayed fluorescence of organic molecules trapped in silica sol-gel glasses. J. Photochem. Photobiol. A Chem. 1991 57 41-... 

The intensity, lifetime of the excited state, and polarization of the fluorescence of organic molecules in solution point to electric dipole radiation. Direct proof of this was obtained for fluorescein by Selenyi (1911, 1939) using his wide angle interference method and has been recently confirmed by Freed and Weissman (1941). 

Pu L. Fluorescence of organic molecules in chiral recognition. Chem. Rev. 2004 104 1687-1716. 

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. 

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. 

Module 1, Determination of Chemical and Structural Information on the Sample. The task of Module 1 is to provide non-chromato-graphic data for analytes prior to specification of the chromatographic method. Data bases have been developed for pK values of organic molecules, isoelectric points of proteins, and fluorescence spectral properties of organic molecules. 

Expansion of the data bases in Module 1 to include spectroscopic and electrochemical data to be used by the detector selection rules of Module 3. (This would include UV absorbance spectral properties of organic molecules, fluorescence quenching and activating properties of solvent environments, and electro-... 

Fluorescence detectors can be more sensitive, but have a much narrower applicability. Only a small proportion of organic molecules exhibit natural fluorescence. One may choose to derivatise samples with a fluorescent or fluoro-genic agent, but this adds to the complexity of the analysis and the validation required. 

However, before the 1990s a significant number of papers on 2PA spectroscopy had already been published. Many of these are worth mentioning, not only because they laid the foundations for the field, but also because they addressed interesting spectroscopic issues for a variety of organic molecules. Indeed, at that time, 2PA was mainly used as a spectroscopic tool, in combination with the more traditional IPA and fluorescence approaches. 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

In organic ECL reactions, the luminescent species are generally derivatives of polyaromatic hydrocarbons where A and B in Eqs. (1) through (4) can be either the same species (leading to self-annihilation) or two different PAHs with either being the analyte (mixed system). Some examples of both self-annihilation and mixed system ECL reactions of organic molecules are listed in Tables 1 and 2. One well-studied example is the self-annihilation reaction between the anion and cation radicals of 9,10-diphenylanthracene (DPA) via an S-route in acetonitrile resulting in blue fluorescence characteristic of DPA [17] ... 

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