Non-radiative deactivation

Figure 2. Principles of reversible luminescence sensing using photochemical quenching processes (electron, energy or proton transfer). Dye = luminescent indicator Q = quencher species dotted arrow non-radiative deactivation processes. The luminescence intensity (and excited state lifetime) of the indicator dye decreases in the presence of the quencher. The indicator dye is typically supported onto a polymer material in contact with the sample. The quencher may he the analyte itself or a third partner species that interacts with the analyte (see text).

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

Primary energy loss pathways include radiative and non-radiative deactivation of the dye sensitizer (Process 6), recombination of the conduction band electrons by the oxidized sensitzer (Process 7), or recombination of the conduction band electrons by the the oxidized form of the redox system (Process 8). 

Beeby, A. Clarkson, I. M. Dickins, R. S. Faulkner, S. Parker, D. Royle, L. de Sousa, A. S. Williams, J. A. G. Woods, M. Non-radiative deactivation of the excited states of europium, terbium and ytterbium complexes by proximate energy-matched OH, NH and CH oscillators an improved luminescence method for establishing solution hydration states. J. Chem. Soc., Perkin Trans. 2 1999, (3), 493-503. 

In order to avoid such ambiguities, the definition of chemical species will depend on the simple concept of stability. In the absence of chemical reactions, a chemical species will last indefinitely. Thus an ion is a distinct chemical species, and an electron transfer reaction must be seen as a chemical change. However, an electronic excited state of an atom or molecule must inevitably decay back to the ground state, so the processes of excitation, emission and non-radiative deactivation are photophysical processes. 

Quenching is the non-radiative deactivation of an excited molecule M by a molecule Q (the quencher), the excited state energy eventually becoming heat energy of the surroundings (e.g. liquid solvent or solid matrix)... 

Radiative or non-radiative deactivation of Q to Q then completes the quenching process. There are two major energy transfer processes. 

For a reaction originating from the (lowest) triplet state the radiative and non-radiative deactivations of the molecule (3M ) must also be considered... 

Upper excited states are extremely short-lived. When the molecule is promoted to an excited singlet state beyond S1 the non-radiative deactivation by internal conversion is much faster than the spin-forbidden intersystem crossing to any triplet state. Therefore, the first excited singlet state is formed with near unit quantum yield. If an upper triplet state could be reached, it would also deactivate very rapidly to T1 and no singlet excited state would be formed. The extremely short lifetime of all upper excited states Sb(m>1) and Tb(w>1) means that luminescence emission and chemical reaction are, as a rule, not observed from such states. There are some exceptions to this rule, but there are many more mistaken reports of chemical reactions from short-lived upper excited states. Any such report... 

Many fluorescence kinetics fall into the ps time regime when non-radiative deactivation competes efficiently with the radiative transition. The natural... 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

The emission spectra in frozen butyronitrile (Fig. 33) exhibit broad and unstructured bands at Xmax = 710 nm for 77 and at Xmax = 715 nm for 78. These values compare favorably with other A-frame d8-d8 systems based on iridium and platinum metal centers.91-93 The Xmax and the non-radiative deactivation values of dinuclear complexes 77 and 78 are in the order 77 < 78, indicating that the nature of the excited states is influenced by the nature of the halide.94... 

In contrast, studies aiming at determining the exact nature of the non-radiative deactivation mechanism and the corresponding mean distance of interaction appear to be more in line with the Forster theory spirit and may be regarded as a fascinating attempt in this field. However, applications appear rather limited as far as the question of the hydration sphere determination is concerned. 

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