Excited state quenching, normal

If flavin triplet excitation were involved in photosensory transduction of an organism, then substances that quench this excited state, and thus compete with the normal transduction sequence, should selectively inhibit photomovement in this organism. On the other hand, no effect on such an agent would be expected if the chromophore were a carotenoid. 

Fluorescence quenching is described in terms of two mechanisms that show different dependencies on quencher concentration. In dynamic quenching, the quencher can diffuse at least a few nanometers on the time scale of the excited state lifetime (nanoseconds). In static quenching, mass diffusion is suppressed. Only those dye molecules which are accidentally close to a quencher will be affected. Those far from a quencher will fluoresce normally, unaware of the presence of quenchers in the system. These processes are described below for the specific case of PMMA-Phe quenched by MEK. 

Under all conditions investigated the intensity of delayed fluorescence was small compared with that of normal fluorescence. Thus, when the exciting light is switched on we can ignore triplet-triplet quenching and consider the population of excited states (P, P2 ) to be maintained only by direct excitation from P to P. The rate of formation of P2 is then equal to its rate of disappearance, i.e.,... 

In some cases, simultaneously with the quenching of the normal fluorescence a new structureless emission band appeals at about 6000 cm-1 to the red side of the monomer fluorescence spectrum (Figure 6.4). This phenomenon was first observed in pyrene solution by Forster and was explained as due to transitory complex formation between the ground and the excited state molecules since the absorption spectrum was not modified by increase in concentration. Furthermore, cryoscopic experiments gave negative results for the presence of ground state dimers. These shortlived excited state dimers are called pxcimers to differentiate them from... 

A key observation that supports the notion of ILCT character is the decreased susceptibility of these complexes to exciplex quenching. This normally occurs by attack of Lewis bases at the vacant coordination sites of the metal, and is promoted for MLCT excited states by the increased electrophilic-ity of the metal in the transient, formally oxidized + 3 state, in effect by the hole generated on the metal [85]. Contribution of ILCT character serves to delocalize the hole away from the metal and onto the ligand. 

On excitation the phenolic group becomes more strongly acidic while the carboxylic acid group becomes a stronger base and proton exchange then occurs between the two. In confirmation of this explanation, methyl salicylate was found to behave similarly, but methyl 2-methoxybenzoate, with no transferable proton, showed a normal Stokes shift. Quenching experiments demonstrated that at room temperature the proton transfer reaction reached equilibrium within the lifetime of the excited state. 

Generation of excited triplet states is normally achieved as a result of (iv) above and, once formed, they may decay by processes analogous to (i)—(iv) with the obvious distinction that radiative decay of triplet states is termed phosphorescence, and that radiationless transition of excited triplet states, back to ground singlet states, involves intersystem crossing. Whilst there are very many mechanisms whereby so called quenching of excited states may occur (1,2), and a full discussion is outside the scope of this article, a large part of the review will be... 

---