Excited state quencher

In most cases two different additives are used, one acting as an internal filter or excited state quencher and the other one being a scavenger of free radicals. These two additives have a synergic effect, that is they are much more effective together than either of them separately. 

Complex-reactant, its excited state Quencher Complex-product Conditions, reaction parameters, notes Ref. 

Light Stabilizers Pigments (carbon black, iron oxides), UV absorbers (hydroxyphenones, benzotriazoles), excited-state quenchers (organic Ni complexes), free-radical scavengers Hindered amine light stabilizers [piperidines, hindered amine light stabilizers (HALS)]... 

Excited-state quenchers or sensitizers free in solution or linked as substituents R on 211 were found to affect the fatigue resistance of the spiropyran towardphotoexcitation. 

The enhancement in photoreaction rates of DDE and dichlorobenzc phenone on sorption to dissolved organic matter (DOM) seem to be incor sistent with other reports that the fluorescence of aromatic hydrocarbons strongly quenched by sorption to DOM (30, 33, 34). After all, fluorescence like photoreaction, is an excited-state process, and fluorescence quenchin indicates that locally high concentrations of excited-state quenchers are pre ent in the NOM. However, most NOM is fluorescent itself, a condition ii dicating that the quenchers in NOM cannot quench all excited states efl ciently (41). Possible explanations for the difference in the effects of sorptic... 

At temperatures of 40 C and above the chemiluminescence from a paper In a moist atmosphere was always observed to be smaller than In a dry atmosphere. Possibly the moisture allows the macro-molecular segments In the paper to slide past each other more easily, so that less bond scission occurs. Other explanations, however, are possible In terms of an effect of moisture on the efficiencies of excited state production, or an Increased mobility of excited state quenchers. Below 40 C, on the other hand, the maximum emission from samples In moist air was larger In many cases than In dry air, and we may be dealing with an effect Involving an Increased emission due to the change of atmosphere from the laboratory air to that In the humidified oven (cf. 

Although polycyclic azo N,N -dioxides of type were patented (224) as excited-state quenchers and radical Inhibitors for example, for stabilizing pigments and polymers—It Is not yet known If those compounds attained any practical utilization. Besides the l-02-quenchlng ability, nitrones such as can effectively trap R radicals (225), and cyclic azo dioxides similar to can photochemlcally expel NO (226), thus affording the cyclic nltroxldes In both cases. 

Excited state quenchers are agents that can absorb the excess energy from polymer species, and again dissipate it in a way that does not result in formation of active free radicals. Many of these agents also have other protective modes of behavior. For instance, benzotriazole compounds can act both as UV absorbers and as excited state quenchers. 

Metallic complexes that act as excited state quenchers are used to stabilize polymers, mainly polyolefins. They are nickel and cobalt compounds corresponding to the following structure ... 

Light stabilizers are used to protect plastics, particularly polyolefins, from discoloration, embrittlement, and eventual degradation by UV light. The three major classes of light stabilizers are UV absorbers, excited state quenchers, and free-radical terminators. Each class is named for the mechanism by which it prevents degradation. The major types included in each light stabilizer class may be categorized by their chemistries, as shown in Table 4.17. 

Several approaches to stabilisation can be used. Light stabilisers will be discussed under four broad categories light-absorbing fillers organic UV absorbing compounds excited state quenchers and photo-antioxidants. 

Hindered amine compounds are the most important category of commercial light stabilisers today. They are neither classical UV absorbers nor excited state quenchers. They have a characteristic molecular structure, in which the amine group is surrounded by a cluster of other substituent groups. 

Deactivating the excited species by energy transfer. These additives are known excited-state quenchers, typically nickel chelates of aromatic as well as aliphatic compounds. 

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