Fluorescence and other de-excitation processes of excited molecules

Fluorescence and other de-excitation processes of excited molecules 

11) Perrin F. (1929) Doctoral thesis, Paris Annales de Physique 12, 2252-4. 

12) Nickel B. (1996) Pioneers in Photochemistry. From the Perrin Diagram to the Jablonski 

1907 E. L. Nichols and Mirror symmetry between absorption and fluorescence 

1920 F. Weigert Discovery of the polarization of the fluorescence emitted by dye solutions 

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Chapter 3 is devoted to the characteristics of fluorescence emission. Special attention is paid to the different ways of de-excitation of an excited molecule, with emphasis on the time-scales relevant to the photophysical processes - but without considering, at this stage, the possible interactions with other molecules in the excited state. Then, the characteristics of fluorescence (fluorescence quantum yield, lifetime, emission and excitation spectra, Stokes shift) are defined. 

The de-excitation path available to conjugated organic molecules is controlled by quantum-mechanical rules which are complex. Some molecules will relax spontaneously, other will not (within a reasonable time) without assistance from another material/mechanism. The presence of Oxygen is a special case. Resonant conjugated molecules with two Oxygen atoms will not fluoresce and there only means of de-excitation is by means of a direct transition that is not allowed because of the presence of the triplet state. The nonresonant conjugates normally de-excite thermally via a two-step process. 

Molecules in the fundamental state absorb light with an intensity equal to I and reach an excited state Sn. Then, different competitive processes, including fluorescence, will compete with each other to de-excite the molecule. The rate constant (k) of the excited state is the sum of the kinetic constants of the competitive processes ... 

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